ASME - Citrus Waste Pumped Peel Systems

March 12, 1998

INTRODUCTION 

It was five years ago at this conference that Carlos Odio presented a paper on pumped peel systems in Brazil. This was the first time many of us had heard of the concept.

The key advantages claimed were a reduced initial capital investment, along with reduced maintenance and horsepower requirements. These resulted from elimination of many screw conveyors and the peel bin.

I remember many doubts being expressed in discussions following the presentation. The principal concern was in regards to handling the week-end shutdown typical of many Florida processors. Also, the absence of a peel bin meant that juice extraction would have to stop whenever anything went wrong in the feedmill.

Today four of the twenty-five citrus feedmills in Florida use pumped peel. One of these was a greenfield installation that gained the advantage of reduced capital investment. The other three converted from a traditional reaction conveyor system. At least two of these three made the change because they were facing major maintenance and rebuilding costs in aged facilities.

The first two, at Florida Juice and Caulkins Indiantown, were installed by Gumaco using the system described in Odio's presentation. The third system, designed and constructed by Cook Machinery, demonstrated remarkable innovation and advancement in the technology. The latest, at Tropicana in Ft. Pierce, includes further evolution of the pumped peel concept.

Another key development in pumped peel technology came from Cutrale in Araraquara. A paper describing this was presented by Daniel Marques at the 1995 ASME conference.

At that conference I also presented a paper on feedmill technology. In it I mentioned that Dan Vincent's 1940 patent covering peel liming said that three to five minutes reaction time was required. Later in the paper I said that normal practice is to use eighteen minutes. I was aware of this contradiction, so you can imagine my surprise when Marques gave his paper: He said that only four minutes was required and he had a feedmill to prove it.

All four of the pumped peel feedmills in Florida now operate with the short reaction time that Marques described. In fact, Florida Juice and Caulkins Indiantown converted after extended periods of operating difficulties associated with excessive reaction time.

ORIGINAL SYSTEM
To illustrate the technical changes that have occurred, we would like to start with the flow chart in Figure I. This system calls for using recirculating press liquor and molasses as a pumping medium. The presence of this press liquor and molasses, in the ratio of two parts liquid to one part peel, was felt necessary in order to fluidize the peel sufficiently to be pumped.

Note the metal separator tank. This was a relatively large tank that served to mix the peel, lime, press liquor, and molasses. It had a sloped bottom so that tramp metal would not enter the pumps.

The pumps used were progressive cavity pumps. It is interesting to note that today all four Florida plants use progressive cavity pumps built by either Geremia or Netzsch, the two manufacturers referred to in Odio's paper.

The reaction tank was exceptionally large, assuring a reaction time in excess of forty minutes. (The Florida Juice tank was sized for over 60 minutes!) The design called for the peel to be pumped into the bottom of the tank, overflowing from the top. This required operating with a full tank at all times, even during periods when peel flow to the feedmill was reduced.

A major departure from this system was described in Marques' paper. Since only four minutes of reaction time were required, the reaction could be completed in the pipeline running from the metal separator tank to the dewatering screens. This meant that a reaction tank was no longer required. Marques mentioned that the existing Cutrale reaction tank had been converted into a lime silo.

Note that the system requires separation of the pumping medium (press liquor and molasses) from the peel, ahead of the screw presses. The typical Brazilian system used static screens, although some preferred rotating drum screens or shaker tables. A key item is that the pumping medium, already containing d-limonene, flows into the hammermill.

COOK SYSTEM
Let us now take a look at the Cook system (Figure II). This design was installed at the new SunPure feedmill in Avon Park. It was also installed by FMC, under technical license from Cook, at the Parmalat feedmill in Sicily.

One very important change was that only non-recirculating molasses, with absolutely no press liquor, was used as a pumping medium. Also, despite the fact that citrus molasses is more viscous than press liquor, the ratio of molasses to peel was reduced to 1:1.

One fact about the use of molasses instead of press liquor is that improved oil recovery is made possible. This is because press liquor contains all of the d-limonene oil that is recovered in the feedmill. If recirculating liquor is used to pump the peel, there is opportunity for the oil that is present to be absorbed into the albedo of the peel. It is agreed that once oil is absorbed by albedo, it stays there; the action in the screw presses does not remove this oil.

The consequence is that oil absorbed by the albedo goes with the press cake into the peel dryer. Not only is this oil lost, but it is likely to be released, with the exhaust gasses, to the atmosphere. Thus the oil becomes a Volatile Organic Compound (VOC), a focus of the Clean Air Act.

No one disputes this theory. However there are many who do dispute the quantity of oil lost by pumping with recirculating liquor. Comparing oil recovery rates would seem to be the logical way to resolve the question. Unfortunately, conditions at feedmills vary so much that no clear answer has emerged.

There is another subtle, but important reason for pumping with molasses instead of press liquor. Molasses is warm because it comes from the Waste Heat Evaporator (WHE), while press liquor is at ambient temperature, the same as the oranges. As a result, a system pumping with molasses is noticeably warmer than one using press liquor. This difference can amount to 10º C, which does not sound like much.

However, increasing the temperature of a chemical mixture by 10º C cuts in half the time required for a reaction to occur. This is true in the case of the reaction between hydrated lime and citrus peel. Consequently a much faster and more complete reaction occurs, everything else being equal, in a system pumping with warm molasses.

The Cook system had a great many innovations, and, as can be expected with such a system, there were problems to be resolved during start up. One of these led to a change in the design of the mixing tank. It was found that the original tank at SunPure had warm spots and cold spots. These were traced to peel accumulating in the poorly agitated corners of the tank. This peel spoiled and eventually would break loose and get pumped into the screw presses. Screw presses do not operate well with old peel.

A replacement mixing tank was developed by SunPure personnel, and the fabricator, Keller Sales and Engineering, made significant contributions to the final design. (Figure III) Note that the dead spots were eliminated by going to a conical bottom. Internal baffling assured violent agitation, and a clever suction design keeps tramp metal out of the pumps.

There is an interesting detail applicable to any peel processing system. The metal separator used in the Cook plants is the simplest and most effective available. Figure IV shows this device, which is arranged so that molasses is added to the peel by pumping it into the bottom of a small tank through which the peel must pass. The peel enters at the top at one end and exits at the other. Since the molasses is flowing upward, the peel is fluidized as it passes through. The result is that heavy tramp material, including stainless steel, glass, rocks and sand, separates and falls to the bottom of the tank.

Devices similar to this have been installed, with great success, at Orange-co and Cargill Frostproof. Both of these plants have traditional reaction conveyor systems (not pumped peel) where molasses was added either ahead of the shredders or directly into the reaction conveyor. A simple piping change with the addition of a drop-out box has reduced damage to the shredder screens and screw presses at these plants.

Initially the two Cook pumped peel plants (SunPure and Parmalat) used rotary positive displacement pumps. There is a lesson in industrial marketing in what came about. At Parmalat the Italian OMAC pumps have worked fine and are likely to be used in future installations. However at SunPure there were repeated rotor failures. Not only was the pump manufacturer unable to help the situation, but high prices were charged for replacement parts. As a result the pumps were discarded after the first season and replaced with proven Brazilian progressive cavity pumps.

The Cook system uses a different reaction tank design. The peel is pumped into the side of the tank and drawn off at the bottom. This means that an extra set of pumps is required, as compared to the Brazilian system. However it also means that the tank can be operated at any desired level, which translates into any desired reaction time.

Operating at a low level in the reaction tank was found to be advantageous. Florida systems operating with full reaction tanks all had difficulty with screw press operation. After long periods of frustration, the operators at Florida Juice and Caulkins Indiantown traced the difficulty to fermentation that was occurring in the tanks. This seems to occur if there is excessive reaction time, even with good agitation. The result is that today these two plants entirely by-pass their reaction tanks; SunPure operates with a minimal 20% level.

At SunPure, the system was sized for 2,500 boxes per hour (significantly higher throughput can be achieved). At the 2,500 rating there is six minutes reaction time in the pipeline from the hammermill to the feedmill. Additional reaction time is available in the mixing tank and in the reaction tank.

The reaction tank does have an important function at SunPure. Even though the peel goes straight from extraction to the feedmill, without any delay in a peel bin, there are still periods of upsets. When these upsets occur the peel can instantly become un-pressable. At SunPure this condition is overcome by filling the reaction tank until a suitable, pressable, mixture is achieved. Thus the reaction tank is better described as a surge tank.

It is noteworthy that at SunPure there is a provision to add lime solution directly to the reaction/surge tank. This feature is useful when insufficiently limed peel reaches the feedmill.

The point needs to made that pumped peel systems are very sensitive. You will recall that our original worries were about weekly start-ups and having to shut down extraction if problems occurred in the feedmill. It is fair to say that it is the sensitivity to change that has caused far more problems to operators than the problems we anticipated.

By sensitive we mean that the presses stop working and operations become near impossible. There are many normal upsets that cause immediate problems in the pumped peel systems: a slug of CIP water, a load of cull fruit, trash water in the molasses, peel over a few hours old on a hot day, a jump in molasses pH, a skip in the limer. All of these lead to wet peel going into the dryer, which has resulted in multiple cases of smoldering peel in the dryer.

Returning to the SunPure system, another innovation is evident in how the peel and pumping medium are separated ahead of the screw presses. Instead of open screening devices, Cook chose to use closed pre-presses. These machines have limited compressive characteristics, so they perform a "soft" press on the peel. These have proven noticeably more effective than screens in separating free liquid from the peel. As a result, the screw presses that follow give better performance.

One feature of the SunPure system is the use of spent caustic as a replacement for lime. In the 1950's, working at Citrus World, R. W. Kilburn showed that spent caustic could be substituted, up to 50%, for lime. In oversimplified chemistry, in order to prepare peel so that it can be pressed it is necessary to break down the pectins and to raise the pH: the spent caustic will raise the pH, while it takes lime to break down the pectin. Cook chose to take advantage of this, using 10% to 20% caustic in the lime slurry. Lime and spent caustic are mixed, in batches, and then pumped to the hammermill.

One advantage of this is that, since less lime is used, there is less lime going to the Waste Heat Evaporator with the press liquor. As a result the formation of calcium citrate is reduced and the WHE works more efficiently for longer periods.

One aspect of this technology is difficulty of control. In a reaction conveyor system the addition of lime to the peel is set by matching the speed of the limer screw to the speed of the peel bin take-out screw. The operators adjust this system by measuring pH. In a pumped peel system it is more difficult to match the lime flow to the peel flow since the peel flow is not evident to the feedmill operators. Thus the operators are all the more dependent on pH as a means of controlling lime addition. Unfortunately, pH meters tend to be unreliable, leaving the operator uncertain about how much lime is required at any given moment.

One concern frequently voiced about pumped peel systems is that, since there is no peel bin, extraction will have to be shut down if something goes wrong in the feedmill. In fact, some of the Brazilian and most of the Florida pumped peel plants do have peel bins. The more practical solution is seen at SunPure where there is no peel bin as such. Instead as a first alternative they have provision to divert peel to a dump pad. From there the peel (both their own and peel from other processors that they run on occasion) can be put back into the feedmill flow with a front end loader. Their second alternative is to divert the peel to a small loading silo from which it can be trucked off-site. This system has worked well.

There are other innovations at the SunPure feedmill unrelated to pumped peel. The principal one is found in the WHE which is a five stage, three effect evaporator with advanced technology in the oil stripping area. This 80 WHE is matched to a 40 dryer which has resulted in excellent thermal efficiency.

TROPICANA SYSTEM
Figure V shows the system that went into operation at Tropicana Ft. Pierce last year. It can be described as a progression in pumped peel technology. It is characterized by gross simplification: the mixing tank is designed to provide surge capacity; the reaction tank is eliminated; and the pre-presses or dewatering screens are also eliminated.

A key to this simple system is pumping with a low ratio of liquid to peel. At Tropicana only one part molasses is added to three parts peel. To our surprise, the progressive cavity pump handles this thick mixture with no evident difficulty.

Flow through the pipeline is mostly laminar; there is little turbulence. Therefore the mixing of the lime with the peel must be thorough and complete before the peel enters the pump. At Tropicana the dry lime is added at the peel bin take-out screw. Molasses is added shortly afterwards, ahead of the hammermills. At times the mixing tank is operated at such a low level that very little mixing occurs in the tank. We were surprised that this relatively limited and simple mixing of peel and lime has proven quite adequate for achieving a proper reaction.

Operating with the peel from 3,000 boxes per hour, there is about one minute reaction time between the limer and the hammer mill. Six minutes are available if the mixing tank is full. Finally, there is one minute reaction time in the pipeline.

The mixing tank design is worthy of note. The Keller units at SunPure and Tropicana feature vertical internal baffles that work in conjunction with three tiers of center-mounted rotating paddles. Dead spots are minimal. The suction pipe requires the fluid to make a U-turn, assuring that tramp metal is effectively separated in the bottom, ahead of the pumps.

The original Tropicana system included the Vincent pre- presses shown in Figure VI. These were hard-piped directly from the peel pump. This means that the peel, from the time it enters the pump, is totally enclosed, completely preventing the emission of any VOC's.

As the ratio of molasses to peel was reduced, it was found that the pre-presses were not necessary. The main presses alone were found capable of removing the pumped fluid as well as doing their normal job of pressing the peel. The pre-presses were removed; the system as it is today is shown in Figure VII. Note that the peel is hard-piped to the presses.

It was found necessary to protect the hard-piped system from over-pressure. This has been achieved by the addition of a pressure relief valve, shown in Figure VIII.

CONTROL
Under steady full-load conditions, there are few problems controlling either traditional or pumped peel systems. However with pumped peel systems operators need to be more alert and learn new tricks to handle upsets and start-ups.

The upsets have been previously enumerated: a slug of CIP water, a load of cull fruit, trash water in the molasses, peel over a few hours old on a hot day, a drop in molasses pH, a skip in the limer.

In a traditional system these present a minor inconvenience for several reasons. There is usually the opportunity to mix some fresh peel from the peel bin with the bad peel. Peel flow is easily controllable by changing the speed of the peel bin take-out screw. Reacted peel color is readily observed. Lime flow is readily adjusted, sometimes by dumping bags directly into the reaction conveyor.

These are not characteristics of a pumped peel system. As a rule, the operator can observe only pH, Brix and the discharge cake from his peel presses. Thus the operator must learn to spot what is causing the upset and know what to do about it. His choices are largely limited to adjusting the lime flow and slowing his pumps so as to fill a surge tank. Filling the surge tank allows both mixing of new material with the material causing the problem as well as slowing the input to the presses.

One change to pumped peel systems that has been proposed is the addition of a weigh belt. A weigh belt would provide the operator with an indication of the tonnage of peel entering the system. This would be helpful in situations where extractor lines are started or stopped. It would give the operator knowledge of where his limer control should be set.

Originally there were serious concerns about start-up operations. Plants that shut down each weekend face a Monday start-up with water, sour press liquor, or cold molasses. However, many plants, both in Brazil and Florida, routinely get through their "Monday bad hair day." It takes careful attention by the operators, and efficiency is less than optimal, but the difficulties have proven readily surmountable.

EVALUATION
Despite the difficulties of start-up and control, there are strong reasons favoring the pumped peel system. The simplicity of the Tropicana system has obvious advantages of reduced maintenance, space requirement, and capital investment.

This same simplicity has important implications from the standpoint of VOC control. In a pumped peel system the peel is fully enclosed and captured from the inlet to the hammer mill all of the way through the screw presses. This eliminates a host of potential VOC emission points; the equation is simplified.

Older traditional feedmills experience a great deal of maintenance. This is reflected both in operating costs and, more importantly, downtime. Screw conveyors, and especially the reaction conveyor, are notorious for failing and interrupting feedmill operations. It is not uncommon for these situations to cause a halt in juice extraction operations. Because of this condition it becomes financially attractive to convert an older system to the pumped peel technology.

Nevertheless we should note that the presses, dryer, WHE, pellet mill, and pellet loading remain unchanged in a pumped peel feedmill. Consequently it cannot be said that fewer maintenance personnel and operators are characteristic of pumped peel feedmills.

SUMMARY
The technical progress in Florida, in pumped peel systems, has come as much from the citrus processing community as it has from the equipment suppliers. The early adaptors were the CEO's of their organizations: Ron Grigsby at Florida Juice and Roger Beret at Caulkins Indiantown. It is notable that both men were relative newcomers at the time the key decisions were made. It was their decisions that encouraged other processors to install pumped peel systems.

At SunPure it was Hadi Lashkajani who made the choice to go with the many innovations offered by Cook Machinery. Vice President Donald Dawson and Feedmill Manager Mac Greene stand out for having spent the innumerable hours required to make the system the success it is today.

Dave Van Etten, Plant Manager, had the vision that resulted in the Tropicana system. He heard Marques' presentation, went to Araraquara to see it, and came back convinced it offered many advantages in his plant. He is quick to give credit to Chris Sutherland, Instrumentation Manager, who had the understanding of what it would take to make it work on a day-to-day basis.

To summarize the technical progression that has occurred over the last five years: The Cutrale work with short reaction time allowed improvements at Florida Juice and Caulkins Indiantown, while it led into the innovation at SunPure and Tropicana. Ralph Cook first introduced concepts that will be fundamental for years to come: the avoidance of recirculating pumping fluid; bottom discharge reaction tanks; spent caustic as a lime substitute; using pre-presses instead of dewatering screens. At Tropicana the key new features are pumping with a limited amount of molasses, eliminating the reaction tank, and hard-piping to the presses. It remains to be seen what the next system will be like.

THE NEXT FIVE YEARS
A forecast of the future should take into consideration other technical innovations in feedmilling technology. Items of note include:

Work directed by Benedito Jorge at Citrosuco has led to the development of a single pre-press that will handle the pumped peel coming from fifty FMC extractors.

Vincent pre-presses have been added to the pumped peel systems at both Florida Juice and Caulkins Indiantown. These have improved the performance of the main presses at both plants.

At Cutrale's Leesburg feedmill, the use of twin screw superchargers, with overlapping flights, has led to improvements in press performance. This has important implications in double pressing operations. See Figure IX.

The Fiber Filter, shown in Figure X, is being developed to clean-up of press liquor. This will allow WHE's and pumps to operate with higher Brix molasses than was previously possible in many plants. This results in reduced WHE fouling and improved thermal efficiency.

The use of twin circuits in the WHE, to produce higher Brix molasses for second pressing, is likely to result in significant improvements in overall thermal efficiency. One Brazilian plant is using this principle with solid results.

Some Brazilian processors have found that eucalyptus stands are more effective for disposing of wastewater than orange groves. This can impact the demands made on the WHE.

The bagged peel system, described at the most recent Citrus Short Course, eliminates the feedmill dryer altogether. Air pollution regulations in California have led to the adoption of this bagging process. Either VOC regulations or the low price of pelleted citrus peel could bring it about in Florida.

Dreyfus in Winter Garden has now begun feedmill operations using a unique Vincent screw press. Their new press features a screw design originally developed for alcohol extraction in soybean plants. In addition, the press makes use of profile bar screens, as opposed to the standard 3/32" perforated screen material. Its performance on limed orange peel is being tested.

 Presented by Robert B. Johnston, P.E.

ASME - Screw Presses In Citrus Feedmills

March 23, 1995

Background 

A newcomer to the citrus industry can find papers and speeches of earlier decades that deal with pressing citrus peel. Many of these, including some by Vincent Corporation, speak of 60% and 62% press cake moisture. Yet, in practice, this was not being achieved on any consistent basis until relatively recently.

What is this percent moisture, and why is it important? The percent moisture is simply the amount of water that can be evaporated from a sample of peel, leaving the solids behind. Most generally it is measured by placing a 10 gram sample of homogeneous (blended) press cake on a scale. Heat is applied gradually, being careful not to burn off any solids. When the sample is bone dry, the solids remaining are weighed.

Since peel arrives at the dewatering press in a range of 80% to 84% moisture, then the moisture content of the press cake reveals how hard and how effectively the press is working. The moisture not removed from the peel by the press will have to be removed in the dryer. The amount of fuel consumed by the dryer is directly proportional to the amount of moisture being removed. Thus the effectiveness of the press has a direct bearing on the monthly fuel bill.

This thermal efficiency is illustrated in the Material Balances presented in Figures I through IV. These are very detailed, so they are included only for your later study. You will find that in the first the peel is pressed to 72% moisture, while in the second another extreme, 65%, is achieved. The fuel cost per ton is significantly lower with the 65% peel.

These Material Balances have the "givens" listed at the top of the page and the variable "Ratio Percents" selected in the third column. All of the rest of the figures calculate automatically when a spread sheet is set to reiterate to a common solution. These spread sheets are readily modified to analyze other conditions, such as (a) sale of part or all of the molasses, (b) addition of press liquor to the reaction conveyor, (c) recirculation of press cake, and (d) use of recirculated molasses for peel pumping purposes.

The Material Balances assume that the moisture in the press liquor from the press can be evaporated for free. And, generally it is. In a properly balanced system the waste (free) heat in the exhaust gasses coming from the dryer is sufficient to allow the waste heat evaporator (WHE) to remove the moisture from the press liquor.

Plants frequently find that they do not have enough WHE capacity to do the job. It is a problem of undersized and overloaded evaporators. This problem has been getting worse as more and more waste waters are being diverted to the feedmill for disposal. And the advent of higher performance presses is accentuating the problem.

This problem is put in perspective with a key figure on the Material Balance, the ratio of WHE load to dryer load. Most plants are designed for a 2:1 ratio. Notable exceptions are Winter Garden Citrus and Cambuhy of Brazil that have WHE's designed for 2.5:1. The higher the ratio, the more investment required in WHE surface.

The Material Balance also reflects pressing performance. In the 1940's and 1950's feedmill pressing technology evolved rapidly. Initially belt, roller and drum presses were installed. These fell from favor because of low capacity and weak pressing characteristics. The technology settled on the use of screw presses that featured an interrupted flight screw with an auto-adjusting discharge cone. This design has remained the norm until this day.

Figure V illustrates these features. The press is a dewatering machine with a conveying screw rotating inside a perforated cylinder. As incoming material is forced the length of the machine, liquid ("press liquor") is expelled through the cylinder wall. The remaining solids ("press cake") are discharged at the far end.

In operation, the peel enters through the inlet hopper where it is grabbed by the feeder portion of the screw. This pushes the peel into the tighter pitch compression stages of the screw. Notice that there is a gap between each stage of compression. Stationary resistor bars are mounted to the frame of the press, and they hold resistor teeth in the gaps between each succeeding screw flight. The main reason for having these gaps with stationary teeth is that it was found that they prevented the peel from turning with the screw (co-rotating) and not feeding through the press. Also, they cause the peel to be stirred within the press so that fresh material is exposed to the screen surface for improved dewatering.

The press cake encounters resistance at the discharge of the screen cylinder in the form of a cone mounted on a pneumatic ram. As the material reaches the discharge point, the cone exerts final pressing action to achieve maximum dewatering. More importantly, the design allows the cone to pinch tight against the screen discharge. This facilitates forming a "plug" when a press is first put in operation or when adverse conditions (distressed or dilute peel) are encountered. The plug is what starts the press working, and it will form automatically without need for operator attention.

This is the standard pressing technology. It is the same whether the press is mounted in the horizontal, inclined or vertical position.

New Technology
In the 1990's the citrus industry has seen the advent of new pressing technology.

One event has been the successful operation of presses that use continuous screws rather than the interrupted flight design. Both Tropicana and Procter and Gamble, as an industry newcomer, ran relatively successful tests with the Dupps screw press. These machines feature a single, continuous screw. These presses avoid plugging by having the capacity to exert high internal pressure. To achieve this they employ larger motors and feature much thicker metal with more robust gear boxes, frames and reinforcements. For example, the screens are made by drilling individual holes through steel plate, as compared to the use of rolled perforated sheetmetal in the interrupted flight machines.

Almost simultaneously, Caulkins Indiantown installed a pair of presses made by a Norwegian company, Stord. These machines are best known for pressing sugar beet pulp in the sugar industry. The presses feature not one, but two parallel continuous screws. As with the Dupps press, the Stord is a much heavier, higher powered machine.

What Stord achieved at Caulkins is remarkable. After severe start-up difficulties, they are able, throughout the season, to achieve 60% to 65% press cake moisture. When first installed, these presses generated far more press liquor than the existing waste heat evaporator could handle. The press cake was going into the dryer so dry that there was not enough exhaust gas to drive the WHE. For a fact, it was necessary to add a portion of the press liquor back to the press cake in order to keep the system balanced. (This condition has been improved by the recent installation of WHE and dryer capacity by Cook Machinery Company, plus the conversion of the previous WHE to steam operation.)

Another event that had an impact on pressing technology was the construction of the new Southern Gardens citrus processing plant. Stuart Salter's stated goal was for it to be the most energy efficient plant in the state, and this challenge was laid down to the screw press manufacturers.

At the same time, Cargill placed a Brazilian, Francisco Gomes, over their Frostproof plant. Mr. Gomes summarized some testing done in his Brazilian plant by stating, "The goal is 60% press cake moisture."

The success of Stord and challenges by Southern Gardens and Brazilian citrus processors led to a reexamination of the traditional interrupted flight design. The situation came to a head at Cargill Frostproof because a large Vincent VP-22 press was not meeting its guarantee conditions.

The press in question was originally built in 1978 for pressing alfalfa. It had a 100 hp DC drive at a time when 40 hp was standard on that model. In 1992 it was acquired by P&G who then owned the Frostproof plant. They had the press converted to pressing orange peel with a 75 hp motor and gear reducer.

In operation the horsepower drawn was under 40, and capacity was below expectations because an oversize screw shaft prevented sufficient peel from entering the screen area. Furthermore, press cake moisture was over 68%. A variety of speed changes had little impact on the operation.

Fortunately the press was fed by a (horizontal) variable speed feed screw with a 7-1/2 hp drive motor. One night during the 1992-93 season, in desperation, the cover to this feeder screw was bolted tight and the drive was over-sped. Peel and molasses squirted through every joint and the Supercharger was born.

By force feeding the press in this manner the horsepower went up to 50 and press cake moisture came down to 65%. At the same time throughput capacity increased measurably. The guarantee was met.

It was thought that something new had been invented - until someone pointed out a 1968 patent (Figure VI). Dan Vincent had the right idea, but it was ahead of its time.

During the break in the summer of 1993, Joe Wolfer and Tom Shier at Silver Springs Citrus decided to take a chance on the Supercharger technology. They already had a horizontal feeder screw in place on one of their presses, so the modifications were minor. One important step they took was to add a pressure sensing element in the inlet hopper of the press, downstream of the Supercharger, but ahead of the main screw. This senses the pressure, 0 to 10 psig, as the peel is forced into the press. The signal from this pressure transducer allowed controls to be calibrated so that the Supercharger automatically speeds up when the pressure in the inlet hopper drops, and vise versa. While the installation is far from ideal, it has meant that the press can be set to hold optimum pressing efficiency, without operator attention, as peel conditions vary through the day and season.

A notable impact of the Supercharger modification at Silver Springs Citrus was that press throughput capacity increased by as much as 30%. This is attributed to the facts that the Supercharger (a) forces out air that is entrained in the peel; (b) improves the slippage factor significantly; and (c) causes press liquor to be removed by the screen in the inlet hopper, prior to entering the main barrel of the press.

During that summer of 1993 a major Florida citrus processor purchased a VP-22 press complete with a vertical, factory built Supercharger. This Supercharger was supplied with a hydraulic drive that is controlled so as to supply constant torque (rather than speed). This feature has worked well in allowing the press to be set so that optimum performance is achieved, throughout the season, without operator attention.

Another feature of this press was that it had a new, high performance screw. Originally designed in conjunction with Southern Gardens, this screw incorporated many changes. Internal shaft and tube diameters and transitions were changed, along with increasing the pitch reduction, so as to achieve tighter pressing. Larger bearings and double seals had to be incorporated, along with a much stiffer structure.

Performance of the press exceeded expectations. During the 1993-94 season, in second pressing duty, the press consistently achieved 60% to 64% moisture in the press cake. Also, all time records for the daily tonnage of peel being processed were set.

Notably, the installation was approaching the low moisture performance of the Stord press at Caulkins Indiantown. At the same time the traditional strengths of the interrupted flight design had been retained: low initial cost, satisfactory operation under widely varying conditions, and low maintenance requirements.

While this was going on, still another pioneering effort was under way. Ron Grigsby at Florida Juice elected to build a new feedmill using the Brazilian pumped peel system. The technology of this system had been presented here, at this ASME conference, in 1993, by Carlos Odio of Gumaco.

This system presented new challenges to screw press designers. It was expected that the peel was going to be very soupy. It was also recognized that this condition would make it hard for the Supercharger screw conveyor to force the peel into the press, and is was expected that it would make it hard for the main screw of the press to get a "bite" on the peel. Also, the plug forming capability of the discharge cone was clearly going to be taxed. Despite these challenges, it was felt that 62% press cake moisture could be achieved.

Figures VII and VIII show the presses installed at Florida Juice. Note the vertical Superchargers installed above the inlet hoppers.

It was never imagined just how fluid the peel would be until it poured out. When the system overflowed during trials and the peel fell to the floor, it ran like very loose slurry. This condition was compounded by pH excursions, erratic operating levels, along with a dryer too large for the WHE capacity, all of which resulted in very weak molasses being produced. Frequently the molasses Brix were 30º or less.

A comment relative to molasses Brix is appropriate at this point. We know orange peel comes in at 82% moisture and we want to press it down to 62% moisture. Think of the reverse: the peel comes in at 18% solids and in pressing we need to get it up to 38% solids. Clearly, if we can add molasses at 50º Brix, which is 50% solids, to the 18% peel, then it is easy to raise the output to 38%. In fact, if you could add enough molasses, it would not be necessary to press the peel at all!

Thus, if we look only at the press operation, the higher the Brix of the molasses added to the peel, the lower the press cake moisture is apt to be. The limits are that the peel must be able to absorb and diffuse the molasses, and the dryer/WHE combination must be able to sustain production of sufficient molasses for steady level operation.

Returning to the Florida Juice feedmill... While operation last season was limited to only a few weeks, a very wide range of conditions were encountered and managed. In this process a great deal of technology was proven. During the trial period the feedmill operated only twelve hours each day, and one of two WHE's was not yet in service.

The rigidity of the press proved adequate to handle extreme forces. This was demonstrated during a period of very high pH peel. With high pH the pectins are hardened and the peel becomes very fibrous and tough. The press rumbled and the platform quivered. The press cake looked like ground wood, and the press almost tripped out on overload. But operations continued without interruption.

Machine rigidity has become more important with the advent of low moisture pressing. Tests were run in four different citrus plants using a transit to measure the deflection of various frame members. This lead to design changes and, in three cases, adding gussets to existing frames.

The discharge cone of the press is another important component. It was successful at Florida Juice in forming a plug under a wide range of conditions, performing marginally only when it was necessary to process spoiled and under-limed peel. On these occasions the press screens blinded with mat, and press throughput capacity fell off by at least 50%. Nevertheless, it was possible to continue operation, slowly delivering pressed peel to the dryer. (Both adding caustic wash and adding dried but not pelleted peel are remedies used in clearing mat from press screens.)

There was one more innovation of interest at Florida Juice. Provisions were made to recirculate approximately 20% of the press cake and blend it with the peel going into the presses. This variation was triggered by a paper entitled "Multiple Stage Pressing" that was presented to this group by Ralph W. Cook in 1983. Limited testing was performed, but it was clear that use of recirculated peel drove up the horsepower drawn by the press and presumably reduced the final moisture content of the peel going to the dryer. The concept is appropriate when a plant has excess press capacity, along with WHE capacity to take care of the additional press liquor that results. Further testing is being conducted during this present operating season.

(We should note that press cake recirculation, as we have just described, had been tried previously at Tropicana. The results were inconclusive.)

As if there were not enough irons already in the fire, Bryce Kelly and Joe Dalton at Cargill decided to push the technology one more step. They ordered a screw and set of resistor bars for their 75 hp press designed so as to achieve 62% or less moisture content with single pressing.

It was an all-or-nothing shot, so changes affecting seven different design parameters were made simultaneously. This included obvious things like tightening the pitch reduction, speeding up the Supercharger, and reducing the shaft diameter in the inlet hopper. It also went into more esoteric details of resistor tooth profile, helicoid contours, and diameter transitions. For example, one detail of resistor tooth cross section which had never been altered since the early 1950's was changed with successful results.

Operation with the Ultra High Performance screw took place sporadically in April through June of 1994. On one hand, it was a remarkable success: throughput was increased and press cake moisture (on Valencias) held in the range of 59% to 63%. The performance of the interrupted flight press design had finally matched the heavy duty continuous screw machine.

Despite many start-up problems, a very important mode of operation was proven out. Placing the Supercharger on manual control, the operator was able to select speed increases and watch resultant increases in the amperage of the press' main drive motor. It was possible to raise the motor loading up to the maximum rated 75 hp without reaching the highest possible output from the Supercharger. Thus it was possible to sit in the control room and dial in the amount of work to be done by the press.

This press was rebuilt last summer, primarily to repair

damaged drive components. While it has been performing without incident this season, press cake moisture on early fruit was considerably higher than expected. This was possibly related to the unusually high ratio fruit being harvested. Testing is continuing.

One phenomena worthy of mention is the appearance of frit in the press liquor. Frit is being defined as tiny pieces of citrus peel where the moisture and solids have not been separated. It looks like orange peel that has been grated, and it appears in the press liquor when extremely tight pressing is taking place. It can be produced in the laboratory by placing a sample of peel in a cylinder with a perforated screen at one end. When a high pressure hydraulic piston is used to extrude the peel through the screen, frit is produced. Both Caulkins Indiantown and Cargill have noted minor amounts of frit in their press liquor. There has been no problem screening out this material ahead of the WHE.

At the time that this paper is being presented, the technology to be watching is the new SunPure feedmill in Avon Park. This is a greenfield installation, designed and built by Cook Machinery Company. Because the peel must be transported five hundred feet from the processing operation, a pumped peel system is employed.

Peel Preparation
Pressing of citrus peel will never be successful without proper preparation of the peel. This must be kept in mind as changes and options in press design are studied.

Early theory held that for proper pressing to occur, prior to the addition of lime, the peel must first be ground and "sufficient liquids should be added until it is a freely flowing mass, thin enough to be handled by a centrifugal pump" (Dan Vincent US Patent 2,215,944, Food Product and Process of Making, 1940). This step was followed by addition of lime and agitation in a delay conveyor for three to five minutes. Later this was modified to first mixing peel, lime and water (or press liquor) prior to cutting the peel into thin slices. Quoting Dan Vincent's US Patent 22536240, Citrus Pulp Foodstuff, 1951, "The long shreds with cut smooth surfaces permit better contact with the liquid during the chemical treatment and present a more open mass to the gasses during the drying stages..." In any case, the idea was to expose the lime to the cells of the peel and allow it to break down the cell walls, thus making it easy to press out the moisture.

One important consideration at the time was that, since the peel was not pelleted, the dried peel was more desirable if it was bulky, with large pieces and a minimum of fines.

For a while thin bladed slicers were used. And the lime reaction was optimized by depositing the sliced peel and lime in a pit with water or press liquor.

This gradually gave way to the current practice. Thin bladed slicers were found to be very susceptible to damage by tramp iron, and the use of vertical shaft Rietz Disintegrators was adapted by several major processors. The Disintegrator has the advantage of being able to pass tramp metal with a minimum of damage to the screen, while still producing relatively well sliced peel.

At the same time, horizontal shaft hammermills became commonplace because of their lower capital cost. This led to the presence of larger pieces of peel, which extended the reaction time. At the same time liming pits were gradually replaced with continuous delay conveyors (also called reaction conveyors and pug mills).

Probably the biggest change was a result of the use of waste heat evaporators. Some plants felt that using press liquor to help the lime react the peel adversely affected d-limonene recovery. This is because, theoretically, mixing press liquor with fresh peel allows the essential oils a chance to be absorbed back into peel that may be destined for the dryer. Since essential oils are consumed in the dryer, it is desirable to send press liquor directly to the WHE. This led to the use of molasses as a medium for putting the lime in contact with the fresh shredded peel.

It is clear that the liming reaction is improved by the presence of a liquid to improve the contact between the lime and peel. Unfortunately it appears that this reaction process goes a little slower when molasses is used in place of water or press liquor. It is as if the acidity of the molasses acts to buffer the reaction. Little research has been done in this area.

These many changes have made it advisable to increase the time in the delay conveyor from the original three to five minutes up to eighteen minutes.

Another consideration in selecting a shredder has been that the horizontal hammermills mash the peel a little more than a Disintegrator. The mashed peel reacts with lime faster than the more uniformly shredded pieces from a vertical shaft machine. However these fines have a tendency to either burn in the dryer or to be blown through to the WHE. Their presence in the WHE leads to excessively dirty "black water". The advent of better cyclone dust collectors has reduced this black water problem. It is evident that there are many factors to consider.

In practice, most plants achieve satisfactory pressing most of the time. However there are occasions when poorly shredded peel and unduly short reaction times will cause pressing to become nearly impossible. It is a matter of which varieties of fruit are being run, the age of the peel, and even the weather conditions in the weeks prior to picking and the temperature at the time of pressing. The most common condition found is that the peel remains too slimy to press. A mat layer gets deposited on the screens of the press, effectively blinding the press. This reduces press capacity, and both dryer and WHE operations get out of balance.

Essential Oil Recovery
Essential oil recovery is an important part of the economic operation of a citrus processing plant. Roughly half is recovered in cold pressing, and the d-limonene half is recovered in the feedmilling operation. Without getting into the technology involved, it is worth defining the financial parameters.

d-Limonene recovery varies widely. With earlies and mids, it will range from .06 to .10 pounds per box, while with Valencias one can expect .18 to .30 pounds per box. It depends on the cold press operation, the loading on the WHE, and if a steam stripper is in use.

To keep this in perspective, it should be compared to the revenue stream arising from the sale of peel as animal feed. Normally nine and a half pounds of peel are sold per box of fruit. The price of d-limonene varies widely, but a representative recent figure is US$ 0.60 per pound, as compared to US$ 70.00 per ton of pelleted citrus peel. So if one considers a plant processing 6,000,000 boxes a season, the sale of pelleted peel will generate $2,000,000, while the sale of d-limonene will add some place in the neighborhood of $500,000.

These economics can assist in selecting the investments that are appropriate for feedmill operations.

Press Maintenance
At the same time that presses have been designed for improved pressing performance, there has been a continuing effort to improve the maintenance characteristics of these same screw presses.

Probably the most fundamental change has been the greater use of stainless steel. In the early 1960's a pound of carbon steel cost $0.10, while a pound of stainless cost $1.00. Today this 10:1 ratio has changed to 5:1. A pound of carbon steel costs $0.25, while stainless steel goes for $1.25 a pound. The result is most citrus peel presses are now being specified with all contact parts made of stainless steel.

Another change has been the greater use of catalog components in manufacturing peel presses. At one time press manufacturers made their own air cylinders and the use of specialty bearings was common. Today the use of standard OEM components has greatly reduced the cost of press maintenance.

The advent of tighter pressing has led to other changes that reduce maintenance. Double shaft seals are now standard. Split screens are made to be easily removed. Chain and sprocket drives have given way to in-line planetary gear drives which eliminate overhung shaft loads. Support and guiding of the discharge cone has been improved. And hardfacing of the compression flights on screws is becoming more common.

Many changes have been made to simplify disassembly of the press and accessibility to various components. One consequence of this is that the standard citrus press has grown from 18' to over 22' in length.

Double Pressing
No discussion of press technology would be complete without addressing the issue of double pressing vs: single pressing.

In single pressing, the peel is shredded and reacted with lime. The lime reaction, occurring in the delay conveyor, is improved by the addition of molasses. The pressing operation follows, and the press liquor is screened and pumped to the WHE, while the press cake, possibly with some surplus molasses added to it, goes to the dryer.

In double pressing this process is modified by adding a diffusion conveyor. In this conveyor some molasses is allowed to blend into the press cake from the first pressing. After a few minutes of diffusion time, the press cake is pressed once again. After this second pressing the press cake is sent to the dryer, while the additional press liquor is merely combined with the liquor from the first pressing.

The results of single and double pressing are seen in Material Balances Figures II and III. It has been generally accepted that about one and a half percentage points reduction in press cake moisture can be achieved, as shown.

An improvement in plant thermal efficiency results from this reduction in press cake moisture. This is reflected in the cost of fuel required per ton of pelleted feed that is being produced. This cost reduction is generally sufficient to make an attractive investment of the addition of a diffusion conveyor and additional pressing stage.

However, the fuel savings, in general, are not sufficient to justify the cost of additional WHE capacity. This need appears in the WHE load figures on the Material Balances.

The fact that double pressing increases WHE load is probably the main reason that several plants have switched from double pressing back to single pressing. As the amount of fruit being processed increased, a point was reached where the WHE could no longer handle the load. Switching back to single pressing reduces WHE load and allows a plant to process more fruit than before.

Another consideration is the advent of the high performance screw presses. If it is possible to achieve 60% to 62% press cake moisture with single pressing, why make the additional investment required for double pressing? The answer is similar to the issue of converting to a pumped peel system: In the case of a greenfield installation, the investment analysis of the alternatives is direct and easy. But in the case of an existing installation, analysis based on incremental investment can lead to an entirely different answer.

Overview
It is clear that a great many technical changes are taking place that affect feedmill operations. These changes allow the achievement of plant efficiencies that have not been possible in the past.

At the same time the interdependence of the main components (the press, the dryer, and the WHE) mean that what is best for one feedmill will not necessarily be best for another.

The following steps are suggested for analysis:

1. The biggest single investment in the feedmill is the WHE. Therefore the plant should be run so that every bit of available WHE capacity is being used.
2. The dryer must provide the quantity of high wet bulb gases that are needed to drive the WHE. This is a matter of matching the burner and dryer capacities to the WHE. (It has been noted that many dryers in Florida are operating with significant unburned hydrocarbons in the exhaust gasses; this represents a major opportunity to improve thermal efficiency.)
3. The liming, shredding, and reacting of the peel must each be adequate and proper so that the screw press can do its job.
4. The screw press should be set up to generate enough press liquor to keep the WHE loaded to its full capacity.

Presented by Robert B. Johnston, P.E.

Citrus Feedmill Thermal Efficiency

Citrus Feedmills 101

October 4, 2005                                                                                                                                                                                                   ISSUE #165

Last month Vincent Corporation gave a presentation at the Citrus Short Course (now called the International Citrus & Beverage Conference). Entitled "Citrus Feedmills 101", the presentation reviewed the efficiencies and economics of various feedmill concepts. The important points were as follows: 

1. Feedmill efficiency is measured by the therms of energy required to produce a ton of citrus animal feed. This was presented as the gallons of fuel oil required per ton of pellets. It was seen that a very efficient feedmill requires 29 gallons of fuel oil per ton of feed.

2. With the selling price of pellets being around $55 a ton, delivered to the port of Tampa, it is evident that feedmill operators face a serious cost problem. Thus variations of the California Feedmill #4 option, which does not use a dryer, are of particular interest.

3. The "dryer" ceases to be a dryer once a waste heat evaporator (WHE) is installed. Instead, it is a generator of nearly saturated, high wet bulb temperature, gasses. This point is overlooked by dryer companies, attempting to enter the market, without familiarity with the fundamentals of citrus feedmills and the WHE technology.

4. It was shown that the screw presses account for only 7% of the capital cost of a feedmill, and the dryer, 13%. The WHE accounts for 33% of the capital cost.

5. The WHE does wonders for feedmill thermal efficiency because it works under a vacuum. This allows it to evaporate water with very low heat input. Furthermore, the WHE makes its own vacuum by condensing the moisture in the gasses.

6. The Vincent presentation included a statistical industry report. It showed that the typical feedmill has an overall energy consumption of about 600 BTU's per pound of water evaporated. This is contrasted to the typical stand-alone dryer, which requires at least double that much. (A British Thermal Unit (BTU) was defined as an energy unit, 1000 of which are required to evaporate a pound of water at sea level atmospheric pressure.)

7. It was shown that it is of little use to press citrus waste to where it has less than 63% moisture. There are presses available that can do this. However it is of little value because the resulting volume of press liquor will be beyond the capability of the WHE.

8. Material balances were presented, although they are upper classman, not freshman, technology. These are expressed in simple "in equals out" equations and the concept of Brix. Reiteration of simultaneous equations in a spreadsheet gives the material balance.

Citrus Installations

Citrus KP-16

One of Vincent's more popular screw presses is the Model KP-16. Since its introduction in 1996, over one hundred have been built. Originally intended for applications with high freeness and capacity requirements, the presses have grown into high-torque machines being used in tough applications. At the same time, most of the economies of the original design have been retained.

Citrus Molasses

In the states of Florida and Sao Paulo in Brazil all citrus juice extraction plants have citrus peel feedmills that produce citrus molasses. This molasses is made from the press liquor that results from pressing orange peel. The molasses is produced in waste heat evaporators (WHE) that are driven (heated) by the exhaust gasses from the cake dryers. Typically the molasses are concentrated into the range of 40º to 50º Brix and they are either sold to a distillery or re- combined with the peel. If the molasses are to be stored for more than a few days, they are concentrated to 72º Brix as fermentation will not readily occur at this high a sugar content.

Citrus Pectin Peel Preparation

The process for producing pectin peel involves diffusing the sugars from the peel and then drying the peel at low temperature.  The dried peel is exported to plants that use an acid-alcohol precipitation process to extract the pectin.

This processing of citrus peel for sale to pectin extractor firms was an area of technical expertise of Vincent Corporation.  For many years we designed and manufactured the machinery that is required.  Today offer only our presses, shredders, and assistance.

Pectin peel is a by-product made from peel by citrus producers. It is generally made from lime or lemon peel, although it can also be made from grapefruit and orange peel.

The processed peel is sold to firms such as CP Kelco (formerly Copenhagen Pectin) in Denmark and Brazil,  Danisco in Mexico, and Cargill in Germany.  These firms, in turn, extract pectin from the peel and sell it as a food additive.  Contacts at these firms are found at the end of this report.

Pectin peel generally sells in the range of US$ 300 to $600 per short ton.  This can be compared to selling pelletized citrus peel for animal feed, where the market is in the range of US$ 50 to $100 per ton.

The process of producing pectin peel revolves around washing the peel in water so as to diffuse (leach) out the soluble sugars.

This contrasts to the process for making peel into animal feed in which the first step is to react the peel with hydrated lime.  Lime degrades the pectin, releases the bound juices, and thus permits efficient pressing and dehydrating.  In contrast to this, the production of pectin peel must preserve the pectin; therefore, it can not be limed.

As a consequence of this process the peel is a lot more slippery and a lot more difficult to press (dehydrate).  Accordingly we de-rate the capacity of our screw presses by 50% or more.

Further, pectin peel must be dried very carefully at low temperatures and with carefully controlled humidity.  The Vincent design triple pass rotating drum drier was the norm for the industry.  This dryer permitted recirculating some of the partially dried peel and mixing it with the material coming from the dewatering press.  It was a triple pass dryer with a stationary outer shell, which contrasts to the single pass rotating drum dryer most commonly used in the production of animal feed.  (Vincent withdrew from the dryer business in 2007.)

Most places that have tried to use the single pass dryer have failed to the point where the process was discontinued.  However there are some plants, notably at Decidersa in Murcia, Spain where operation is successful.

Processing plants where Vincent, over 50 years, installed pectin operations include Ci Pro Sicilia and Cesap in Italy; Laconia, Paco Hellas, Nikopolis (ex-Esperis) and Greek Juice Processing in Greece; Citrex and San Miguel in Argentina; Quimica Hercules, Productos Esenciales, Industrial Citricola, and Industriales Limonera in Mexico; Jn-Jacques and Moscoso in Haiti; Priman Canning and Yahkin in Israel; Avante in Brazil and Unipectin in Morocco.

The last two processors producing pectin peel in the United States were Ventura Coastal in California and Parman Kendall in Florida.  Both of these plants, now shut down, were engineered and built by Vincent Corporation.

Typically we look at peel coming from extraction in the range of four tons per hour on the low side and sixteen on the high side.  More recently flows of up to 50 MTPH have been of interest.

The following equipment is key to the process:

Vincent VS-180 Shredder.  This machine slices the peel in order to permit proper pressing without creating excessive fines.  It is of the thin, rigid blade design, as contrasted to the hammer mill concept.

Pulp Wash Conveyors or Tanks.  These are used for diffusing the sugar from the peel.  We recommend either three or four counterflow wash stages, either vertical or horizontal in configuration.

Pulp Wash Sumps.  Stainless steel sump tanks, possibly with progressive cavity peel transfer pumps.

Dewatering Between Stages.  We recommend the use of either Vincent Series KP "soft squeeze" presses or static screens for dewatering between wash stages.

Vincent Screw Press.  Traditionally the Series VP presses have been used to remove moisture from the peel prior to further dehydration in the dryer.  The more economical Series KP offer equal performance if run at low rpm.  The press separates the peel into (a) press liquor and (b) press cake.  The pectin peel is made from the cake.  It is a horizontal, all stainless machine featuring an interrupted flight design. The unit is equipped with a rotating air cushioned cone, complete with pneumatic controls.

Furnace.  Each dryer comes with its burner and refractory lined furnace for burning natural gas, fuel oil, or a combination of these fuels.  A very low gas temperature, in the neighborhood of 1200º F, must enter the dryer.  A low wet bulb temperature is important in the production of high quality pectin peel, whereas a feedmill dryer must produce high wet bulb temperature in order to drive the Waste Heat Evaporator.

Dryer Feeder.  This is a stainless steel screw conveyor and feeder with a companion flange matched to the dryer throat.  It has a variable speed drive.

Dryer Drum.  This is a triple pass dehydration unit with a stationary outer drum. The unit is equipped with recycle extractor conveyors so that partially dried material is extracted at the end of the second pass and mixed with the incoming press cake.  Also important is a 180º elbow between the Furnace and the inlet to the drum.  The selection of this dryer is made using the appropriate de-rating associated with (low temperature) pectin peel production.

Exhaust System.  This separation system features a cyclone separator.  This expansion chamber is complete with an air lock screw conveyor product discharge.

Exhaust Fan.  A radial blade fan complete with inlet elbow and exhaust stack.

Standard Instruments.  A solid state programmable controller modulates combustion through a sensor mounted at the inlet to the third pass of the dryer.  This is required for precise control of product quality.

Cooling Reel.  This is complete with a fan, dust collector, ductwork, supports, and electric motors.  This type of cooler is not used in modern feedmills because of the need for a pellet cooler (which is not used in pectin peel production).

Product Elevating Screw and Surge Hopper.  A carbon steel conveyor and hopper, leading to the bagging equipment.

Sewing Head and Bagging Scale.  A compact Fischbein sewing head and conveyor with a bagging scale are typically included.  CP Kelco plants use a proprietary baler.

Despite the length of this letter, there are a great many details that I have left out. We would be glad to work with you on specific requirements.

Robert B. Johnston, P.E.

P.S. The contacts at CP Kelco and Danisco are:

Renato Rodriguez <renato.rodriguez@danisco.com>

Danisco Cultor Mexicana, S.A. de C.V. 
Km. 37 Carr. Colima-Manzanillo 
28100 Tecoman, Col. MEXICO 
011-52-332-40940 or 42155 (Colima) 

For CP Kelco the switchboard number is 011-45-56-165616.
The director of peel buying operations is Paul van Wagenen 

CP KELCO, DK-4623 LILLE SKENSVED, DENMARK

Citrus Pressing

We recently sent out a newsletter, Four Kinds of Water, that helps explain why it is not possible to take potato waste down to 50% moisture in a screw press. The problem is that potato has a lot of water that is chemically bound to other molecules. The water cannot be released without the addition of heat, as occurs in a rotating drum dryer.

Citrus Waste Disposal

In was in the 1930's that citrus canneries first addressed the problem of disposing of waste. This waste consists mostly of peel, along with other elements such as seed, rag, core, pulp, and others. As early as 1931 Dan Vincent was operating a peel dryer in order to produce a feed for dairy cows.

The investment required for a modern citrus feedmill is in the millions of dollars. And, at today's low grain prices, it is a poor investment. As a result the citrus processors in small countries cannot justify erecting a feedmill.

At the same time environmental laws are coming into play that limit the options for disposing of citrus waste. Basically, there are the following alternatives to consider:

• In Sicily, Spain, Mexico and California the small plants give the peel to nearby farmers for livestock feed. The animals eat it fresh, within a couple days. No energy is required. The peel must be eaten before it starts to ferment.
• There are medium sized processors in Belize, Sicily and Mexico that have too much peel to give away, even for free. Their options are to either landspread or landfill the waste. This practice is being challenged on the basis of groundwater contamination.
• Many medium sized processors in Mexico have gone one step further. They react the peel with lime and then dry it in rotating drum dryers. The dry peel is then sold either as bulk dry feed or it is pelleted. About 1,500 BTU per pound of water evaporated are required to evaporate the moisture in the peel, so the fuel cost is apt to exceed the value of the livestock feed produced.
• In California some medium sized plants react the peel with lime and press it into press liquor and press cake. They concentrate the press liquor into citrus molasses (in steam evaporators) which is sold, either to distilleries or as a liquid animal feed. The press cake is sold as livestock feed which has a shelf life of a couple weeks. Some farmers store the peel press cake in gigantic plastic bags (10' diameter, up to 300' long) for extended storage. The only energy required is the steam for the molasses evaporator. This system is quite economic, but it is practical only in California where immense dairies and feedlots are located near citrus processors.
• In Florida and Brazil the citrus feedmills all have waste heat evaporators (WHE) in use. These greatly improve the thermal efficiency of the feedmill by driving the evaporator with the flue gasses from the dryer. The process is to react the peel with lime and press the peel into press liquor and press cake. The press liquor is made into molasses in the WHE, and this molasses is added back to the peel, usually in the reaction conveyor. The press cake is dried in a rotary drum dryer and subsequently pelleted. The process requires about 500 BTU per pound of water evaporated.
• The new option is to burn the peel as fuel. The heat value of the solids in the peel is excellent. The heat released by combustion is used to dry the peel and to generate steam. The steam can be run through a generator in order to supply more than enough electrical energy to run the entire citrus processing plant. In addition, the steam can be extracted from the turbine at 35 psi so as to supply the steam required by the juice evaporators.

Vincent Corporation gave a presentation at the recent 1998 Citrus Processing Short Course. This paper details the process of burning citrus waste in order to make the processing plant energy self-sufficient. A summary will be released in a future issue of Pressing News.

Double Pressing Basics

Many people assume that a second press is used because the first press fails to remove all the free moisture. This is not the case; presses are designed to remove almost all of the moisture that can be removed in a single pressing. However there are two applications where double pressing is technically sound.

Fiber Filter I

Vincent Corporation is introducing an exciting new product, the Fiber Filter. Featuring fine filtration of low consistency, high gpm throughputs, the Fiber Filter is a unique machine. There is nothing quite like it available in the market. It operates continuously with a fabric filter that is vibrated clean by the process flow, requiring only occasional back-flush cycles.

Fiber Filter II

After a year of testing, Vincent Corporation has gained a great deal of confidence in the Fiber Filter. It is a unique filtering machine that has broader market application than our traditional screw presses, dryers, and shredders. Once the newness is overcome, it is easy to understand.

The best place to test a Fiber Filter is where a centrifuge is being used. If the Fiber Filter works, it is a sure sale because its total cost is less than the routine maintenance of a centrifuge. There are applications in both fruits and meats where the performance of the Fiber Filter beats that of a centrifuge.

Another ready market for Fiber Filter is where the flow to be filtered tends to blind existing static or rotary drum screens. These traditional screens are very economical to acquire and operate. However they have been put into many applications where they are only marginal performers. The Fiber Filter not only features non-blinding characteristics but it also offers finer filtration.

Here is how a Fiber Filter works. A flow containing suspended solids is pumped into a fabric sleeve. The sleeves offered have hole sizes ranging from 0.001 to 0.006", which is finer than is available in metal screens. Normally a fabric with holes this small would rapidly become blinded by the solids being filtered from the liquid stream. This problem is overcome in the Fiber Filter by vibrating the fabric at high frequency. The vibration is induced by (a) tensioning the filter sleeves with a pair of springs and (b) a high speed rotor inside the sleeve induces pulse waves in the fluid being filtered. These pulses of liquid make the filter sleeve vibrate.

The whirling rotor also has ribbon flights to push the solids toward the sludge discharge end of the machine. At the same time the pulsing forces filtered liquid through the fabric.

Applications which promise success include:

• Cleaning press liquor from a screw press.
• Filtering waste water ahead of a treatment plant at vegetable, meat and fish processors.
• Concentrating a dilute flow of suspended solids so that the solids can be further dewatered in a screw press.
• Filtering a flow ahead of an evaporator so as to reduce evaporator fouling and to improve heat transfer efficiency.
• Removing water from spent grain at breweries, distilleries, and ethanol plants.
• Finishing fruit juices.
• Thickening good fiber by filtering out the ink, ash (clay) and water in a paper recycling operation.
• Removing fiber from black liquor ahead of the evaporators in a virgin fiber paper mill.

Because of the newness of this technology, we do not expect a firm to buy a Fiber Filter without first testing it. For that purpose a rental fleet is available. The small FF-6 rents for $200/week; the FF-12 for $350. The customer must also pay the freight to and from the test site.

Fiber Filter for Pectin Recovery

July 26, 2005                                                                                                                                                                                                       ISSUE #163

An interesting application for the Fiber Filter has evolved over the last few years. The project started as a rental at a plant whose wastewater treatment plant (WWTP) was causing excessive odor.

The customer's operation involves receiving fresh lemon peel as a raw material, washing the dissolved solids (sugars) from the peel, and then extracting pectin from the washed peel. The precipitated pectin is dried and sold, in powder form, as a food ingredient.

The WWTP load came principally from the dirty water from the peel washing system. This water contains both dissolved solids and suspended particles of lemon peel. The initial project involved using a Model FF-12, with coarse 190 micron sleeves, to filter this wastewater.

The project was a success. Odor from the WWTP noticeably decreased after filtering out the suspended solids. This is true even though most of the solids are dissolved, so they pass through the Fiber Filter.

At first, the solids sludge from the Fiber Filter were trucked to a remote site. Soon, however, it was determined that this sludge contained valuable pectin of good quality.

For the next season, a second Fiber Filter was added, and the sludge from the machines was pumped back to the peel wash system. In this manner the pectin was recovered.

Today, a third Fiber Filter has been added. The system has been further refined by pumping the sludge from the three machines directly to the acid/alcohol plant. There it is mixed with the washed peel, and the pectin is extracted.

IFT - Burning Citrus Waste

ADDENDUM
The two processing plant examples used in the September 17 presentation both are based on using existing rotating drum peel dryers. For the greenfield site, there exists a better investment. Instead of installing a dryer, the use of a screw press in combination with a steam driven evaporator is recommended. This will allow a significant improvement in efficiency over the use of a dryer. 

With a steam driven evaporator, the press liquor from the screw press can be made into 72º Brix or greater molasses. Diffusing the solids of this molasses into the citrus waste will allow the screw press to produce press cake with a moisture content below 60%. Such press cake can be burned directly in the fluid bed combustor. Blending the press cake with drier material is not required.

In order to minimize capital investment, a four effect steam evaporator has been selected for the calculations in Exhibit V. This evaporator will evaporate approximately three pounds of water for every pound of steam that it uses.

Exhibit V shows that the citrus processor can be entirely energy self-sufficient. A small turbine with 35 psi exhaust steam will be adequate to generate all of the process steam and electricity that is required. There is enough excess steam that the possibility of using an absorption refrigeration system should be considered.

From the standpoint of capital investment, the use of an induction generator is recommended. This is less efficient than alternative equipment; however, it does not require electrical switchgear to synchronize the electric current that is generated.

Addendum II, September 2010

Alex Andreassen of Danisco has pointed out that  burning citrus pellets will lead to slagging of the boiler tubes because of the alkali-metals (Na, K) content of the peel.  Thus applications would be limited to using pellets as dryer fuel.

1998 Citrus Processing Short Course
Energy Self-sufficiency: Using Citrus Peel as Fuel 
 

IFT - Citrus Shredder Performance

August 3, 1996, Presented by Robert B. Johnston, P.E.

The aim of this study was to find some generalizations in regards to citrus oil in feedmill press cake. It was hoped that both useful guidelines could be developed and focus areas for further study could be identified. Peel samples were taken at sixteen different Florida citrus feedmills late in May 1996. These samples were quick snapshots, taken randomly at the time our engineers arrived at the processing plant. 

No definitive conclusions should be drawn because of the limited nature of the sample taking. While this study points out several interesting conditions, its purpose is to encourage further research.

An observation that gave initial impetus to this paper was made at the Cargill feedmill in Frostproof in January 1995. It happened when separate samples of press liquor were taken from the inlet hopper screen, main screen, and discharge cone screen of their horizontal screw press. It was noted that as the peel progressed through the press, both the pH and Brix of the liquor became progressively lower. This hinted that, as the peel was progressively squeezed, more cells were broken open and, possibly, more oil was being expelled with the press liquor. This is illustrated graphically in Exhibit I which compares mason jars of press liquor taken from the main screen and discharge cone of a press. In the end we concluded that a condition such as this is indicative of incomplete reaction between the peel and the lime.

SAMPLING
It is uncertain which of the samples are representative of typical operation at each of the sixteen feedmills. There was room for error: upset conditions may have been occurring, especially because it was the end of the season and some plants were winding down; at least one plant was running sour peel; two plants were having press problems; molasses tank levels were not necessarily at equilibrium; some plants were running grapefruit along with their Valencias; etc. Rather than arbitrarily exclude data, we have attempted to present the full range of possible operating conditions.

The following are the tests that were conducted (Exhibit II):

1. Oil content of the peel at the peel bin.
2. Particle size distribution of the peel exiting the shredder.
3. Particle size distribution of press cake leaving the screw press.
4. Moisture content of press cake leaving the screw press (final pressing in plants with double pressing).
5. Oil content of press cake leaving the screw press.
6. Oil content of the press liquor.
7. Brix of the press liquor and, where applicable, molasses.

The peel and press liquor samples were kept refrigerated from the time they were gathered until the tests were conducted.

MATERIAL BALANCE
Moisture tests were run with two different moisture balances as well as with a laboratory oven and scale. Several tests were run on each sample to verify the results; we feel that the readings presented herein are fair for analysis purposes. These readings are key because they were used to determine the proportion of press cake and press liquor, as a percentage of inbound peel.

Material Balances were found necessary to determine the proportion figures. These were run for each of the sixteen plants, taking into account single pressing, double pressing or pumped peel. It was during this step that something very interesting in regards to molasses diffusion was observed.

MOLASSES DIFFUSION
Almost half of the plants were running with press liquor Brix of 20º or higher. Despite many hours of computer time it was impossible to fit this high a Brix into a reasonable material balance for most of the plants. It possibly could be achieved by drawing down the molasses tank (using more molasses than was being produced), but this was not logically occurring at so many feedmills. Finally it was recognized that the most reasonable explanation for high press liquor Brix is incomplete diffusion of molasses into the peel.

This theory tied to the oft-repeated question, "Why add molasses in a delay conveyor between first and second pressing if it is just going to be pressed out again?" Our conclusion is that this is definitely what is taking place at many feedmills. Many years ago Dan Vincent did testing at Lykes Pasco that showed that it takes ten to twenty minutes for complete diffusion between peel and molasses. Yet most feedmills have only one to three minutes of delay between first and second pressing, thus explaining our data.

We feel that even a short diffusion period is better than none. Diffusing molasses into peel that is about to be pressed does improve the pressing action and ultimate thermal efficiency of a feedmill. We are anxious to see the results in the 1996-97 season at Southern Gardens: there a reaction conveyor has been relegated into service as a delay conveyor between first and second pressing. The size of the conveyor should allow around eight minutes for diffusion of the molasses. It will be interesting to see the extent to which pressing action is enhanced.

The press cake moisture readings of the samples taken for this presentation almost all read quite a bit higher than was expected for late season Valencias. We feel that this can be related to the fact that the Brix readings of peel coming from extraction were unusually low: our readings were almost all in the range of 7º to 9º, whereas normal readings are 10º to 11º Brix.

Oil analyses were made using the Scott method.

SHREDDER MODELS
Peel samples were taken from four different models of shredders. The two most common were the Rietz RD-18 Disintegrator manufactured by Hosokawa Bepex and the 18" horizontal shredder manufactured by the former Gulf Machinery Company. Both models are 75 hp machines.

The other two brands are relatively new to Florida citrus: the Jacobson and Gumaco (Brazil) hammermills. These are larger machines, ranging from 150 to 300 hp. Exhibit III has photos of these various machines.

All four shredders can be operated with a variety of screen sizes. In fact, some were being operated with no discharge screens, and, being the end of the season, some were being operated with damaged screens and worn hammers.

PARTICLE SIZE DISTRIBUTION
An interesting technique was used to measure the particle size distribution of the shredded peel. The required equipment was loaned to us by CSC Scientific of Fairfax, Virginia. Their laboratory air lift dryer was used to gently dry the peel before vibrating it through a stack of sieves.

Samples from the shredder frequently were quite wet with molasses, and they took 25 minutes to dry. In contrast, samples of press cake dried out in 15 minutes. Appendix I shows and describes the apparatus that was used.

The photos in Exhibit IV show some typical peel samples before and after they were air dried and sieved. In the tests, sieve screen sizes ranging from 9.5 mm down to 0.212 mm were used. The photos illustrate the separations that were achieved.

The bar charts in Exhibit V illustrate the particle size distribution by weight. In general, one of two distributions was found: ether a bell shaped curve or a curve skewered to the larger sized particles. It is interesting to note that the same shredder will produce different particle profiles depending on the type of juice extractor that is used.

The results in the bar charts are in remarkably close agreement with measurements published by Dr. Bob Braddock in 1978 (slide).

SHREDDER PERFORMANCE FACTORS
Our preconception was that the best shredding resulted in (a) a minimum of fines and (b) a minimum of large pieces. The fines are undesirable because they are likely to either burn in the dryer or to be carried into the waste heat evaporator (WHE) where they result in problematic black water. At the same time the large pieces were thought to be undesirable because they seemed apt to contain oil that is not expressed from the peel in the pressing operation. (This proved to be not necessarily true.)

PEEL FINES
The reason that fines are likely to burn in the dryer is explained by the concept of latent heat of evaporation. As long as a particle of peel in the dryer contains moisture, it is cooled by evaporation and will not go much above 212º F. A small particle has much more surface area in proportion to its volume; evaporation is proportional to the surface area, so small particles become bone dry before the larger particles. Once bone dry, the particle increases in temperature until a point is reached where combustion occurs.

Fines leaving the dryer are very low in moisture, and some of these escape past the cyclone dust separators. Such particles either go to the WHE or they are recirculated back to the burner area. Being dry to start with, they are likely to burn upon being re-admitted to the hottest portion of the dryer.

In the study it was found that the percentage by weight of fine particles in the peel (the two smaller sieve sizes) varied significantly. The range was 1/2% to 6% fines in samples of peel coming from the shredder. If these are lost in the dryer, they can have a measurable effect in peel recovery (as measured by pounds of citrus pellets produced per box of fruit).

The percentage of fines could not be tied to any particular style of shredder or hammermill. As one would expect, the peel from FMC extractors did average somewhat more fines than that of Brown extractors, but this was not enough to explain the range of values that was measured. Possibly the disposition of extraction pulp would explain the differences we observed. I wish I could say more, but more study will be required to explain the wide range in the percentage of fines.

The percentage of fines did increase during the reaction and pressing operations. This increase of about 2-1/2 percentage points is shown in Exhibit VI.

The increase in the amount of fines was compared between the vertical and horizontal presses. They averaged virtually the same increase, right at 2-1/2 percentage points. Furthermore, the fines could not be tied to the moisture content (tightness of pressing) of the press cake from these presses.

It is noteworthy that in 1949 Dan Vincent was awarded US Patent 2,490,564 covering a "Vegetable Pulp Shredder Screen Having Cutter Blades". This patent dealt with using thin 1/16" blades to both reduce fines and improve the peel reaction. These machines were used in citrus for many years, and they gave good results compared to units with 1/2" and wider hammers. However, the design ultimately proved impractical due to its vulnerability to tramp metal.

PRESS CAKE OIL
We did not measure oil recovery in this study. Clearly the oil recovery systems used in both the juice extraction operation and in the WHE will govern oil recovery. Instead, we looked at the oil (mostly d-limonene) carried into the drier with the press cake. This oil is almost entirely evaporated in the dryer and released to the atmosphere. It exhausts as an unburned hydrocarbon.

LARGE PIECES OF PEEL
Seeking a correlation between large pieces of peel and the oil going into the dryer with the press cake proved interesting. To begin with there are two key measurements: (1) the pounds of oil going into the dryer per ton of peel entering the feedmill, and (2) the percentage of oil going into the dryer relative to the quantity of oil in the peel entering the feedmill.

Note that the raw measurement of percent oil in the press cake is only part of the equation. A tricky part of the analysis is to recognize that the percentage of oil in the press cake must be adjusted for the pounds of press cake per one hundred pounds of peel. Because of this, a feedmill that presses very tight will have a little less oil going into the dryer. This is true even though the percentage of oil in the press cake may be higher than what is found in more moist press cake.

In general, as expected, the plants running Brown extractors had a higher proportion of large pieces of peel after the shredding operation than did those plants with FMC extractors. We were interested to see if the presence of large pieces of peel led either to increased oil in the press cake or to higher press cake moisture. Therefore a comparison was made separating samples from the extractor manufacturers. Each of these two groups were further split so that comparison could be made between shredders that were producing predominantly large pieces of peel and those that had a lesser proportion of large pieces.

The results were surprising, as shown in Exhibit VII. The pressing operation was definitely able to achieve dryer press cake when there were fewer large pieces of peel. The average was about 2-1/2 percentage points lower moisture content.

On the other hand, the pounds of oil in the press cake per ton of inbound peel went down only slightly. In the spreadsheet it is seen that generally the presses that pressed tighter had only a little less oil in the press cake than the pressing operations characterized by high press cake moisture.

In other words, shredding to reduce the fraction of large pieces of peel will reduce press cake moisture (and therefore improve thermal efficiency). However, the quantity of oil going into the dryer does not change appreciably.

Looking at it from another perspective, contrary to our expectations, we cannot say that press cake oil was significantly higher in samples that had a higher percentage of large pieces. Thus the data supports the postulate that fine shredding allows oil to be absorbed into the albedo, and that this oil does not press out of the peel.

The oil analysis brought attention to another condition. On the average, there was a noticeably higher oil content in peel from plants using Brown extractors as compared to FMC extractors. On an approximate basis, raw Brown peel had 1.0% oil, while FMC had 0.5%. The surprising thing is that about one third of the oil in the Brown peel was measured going into the dryer, as compared to two thirds of the oil in the FMC peel. The end result is that almost equal pounds of oil per one hundred pounds of peel were found in the press cake, regardless of which juice extraction system is employed!

Unfortunately our study of shredding and press cake had to ignore some very important considerations. Brown plants that made more use of the BOE (Brown Oil Extractor) did better than those that did not. Similarly, the FMC oil recovery system employed undoubtedly governed the results of the FMC plants. Other important factors which could have distorted our analysis are (1) the sufficiency of the peel reaction system and (2) the oil stripping characteristics of the WHE.

SUNPURE FEEDMILL
Before concluding this presentation I want to make mention of the new feedmill at SunPure. This processor takes pride in the high level of citrus oil recovery that they achieve. The day that we took the first samples there was an upset that led to a second set of samples being gathered.

Both sets showed a high level of oil recovery. In fact, the inclusion of the SunPure results are enough to distort the averages shown in Exhibit VII.

The minimal amount of oil going to the dryer at SunPure is helped by the fact that they are able to press the peel to the same low level of moisture content as the best feedmills in the State. However, we suspect that the outstanding low levels of press cake oil can be tied to the Cook Machinery Company technology used in the feedmill. This technology involves a combination of (1) improvements on the Brazilian pumped peel flow schematics, (2) using available heat to accelerate the peel reaction, and (3) WHE technology.

Recently a number of modifications have been made at the SunPure feedmill, so we are anxious to measure performance once again in the 1996-1997 season.

SUMMARY

To summarize this presentation, let's look at citrus oil recovery in a broader sense. Clearly the two most important considerations are the oil recovery systems used (1) at extraction and (2) in the WHE. This paper has not examined either of these. Rather, we have focused narrowly on the shredding and pressing operations. It should be obvious that differences in extraction and WHE systems may have distorted our analysis.

At the same time some interesting points can be made:

• A decline of press liquor pH between the inlet and outlet of the screw press is indicative of under-reacted peel.
• High Brix press liquor is frequently an indication of incomplete diffusion of molasses into the peel.
• Plants with Brown juice extraction systems will tend to have a higher proportion of larger pieces in their shredded peel. They should be sure their shredding equipment is doing a good job in order to achieve the best thermal efficiency.
• There can be a significant fraction of peel fines going into the dryer, more so at plants using the FMC juice extraction system. Therefore dryer dust separation equipment is of importance.
• Shredding the large pieces of peel into smaller pieces does not significantly improve oil recovery in the feedmill. However, it does improve pressing action and, consequently, thermal efficiency.

Let me conclude by warning against blindly accepting the results of this study. Our intent has not been to give the final word, but rather to point the way and to encourage additional and more thorough investigation.

ACKNOWLEDGMENTS
This paper could not have been prepared without the assistance of a great many individuals and their firms. We want to extend our appreciation to the following:

• CSC Scientific Company, 8315 Lee Highway, Fairfax, Virginia. Telephone 1-800-458-2558. The loan of their Test Sieves, High Performance Compact Shaker, and Fluid Bed Dryer proved invaluable.
• Automatic Machinery and Electronics, Inc., 333 Avenue M, N.W., Winter Haven, Florida. Telephone 941-299-2111. Their laboratory took on the task of running oil content analysis on almost eighty samples of peel and liquor.
• Lala Produce Inc., 1402 25th Street, Tampa, Florida, 33605. The generous use of their cold storage facilities was most helpful in controlling close to one thousand pounds of peel samples. 
  
  Participating Citrus Processors
  
  • Alcoma Packing Company
  • Cargill Citro America Inc.
  • Citrus World, Inc.
  • Coca-Cola Foods, Auburndale
  • Coca-Cola Foods, Leesburg
  • Florida Juice Partners, Ltd.
  • Golden Gem Growers Inc.
  • Holly Hill Fruit Products
  • Indian River Foods Inc.
  • Orange-co of Florida Inc.
  • Peace River Citrus Products
  • Silver Springs Citrus Co-op.
  • Southern Gardens
  • SunPure, Ltd.
  • Tropicana Products, Inc., Bradenton
  • Tropicana Products, Inc., Ft. Pierce

  Technical Review
  

  • Dr. Robert J. Braddock, University of Florida, Lake Alfred
  • Messrs. John and Ralph Cook, Cook Machinery Company, Dunedin
  • Mr. Don Kimball, retired, Coca-Cola Foods, Winter Haven
  • Dr. Ashley Vincent, Savant-Vincent, Tampa

Material Balance

January 14, 2006                                                                                                                                                                                                  ISSUE #169

A material balance is a set of equations that express the flows of materials. The mathematics is all based on the simple premise that what comes in must equal what comes out. Variables are easily changed in order to gauge the impact on a system. Material balance is key to analyzing, and understanding, the flow of material in a processing plant. 

The concept of Brix is vital in a material balance done for fruit and vegetable processing facilities. Brix is much like a percentage, denoting the amount of sugar dissolved in water. It is measured in degrees, using an instrument called a refractometer. The equation for Brix is as follows: Bx = (Ds x 100)/(Ds + w), where Ds is the weight of dissolved solids and w is weight of water.

Note that suspended (or, insoluble) solids, which are almost always present, do not enter into the equation.

The beauty of Brix is that it holds constant as a flow of material is divided. That is, if a flow at 7° Bx is pumped into a screw press, the press liquor will have 7° Bx. Furthermore, if a drop of water is squeezed from the press cake, then it, too, will measure 7° Bx.

For example, if orange peel with 11° Bx and 9% suspended solids is fed into a screw press, both the press liquor and press cake will measure 11° Bx. (Naturally, the amount of suspended solids will be higher in the press cake than in the press liquor.)

Another important characteristic of dissolved sugar is that if flows of different Brix are mixed, diffusion occurs until a balance is achieved. Thus, if a pound of 50° Bx citrus molasses is added to two pounds of orange peel with 80 percent moisture and 11° Bx, the result will be three pounds of material with a moisture content of 70% and 25° Bx. When this material is pressed, the cake moisture content will be significantly lower than if the straight orange peel were pressed. This is simply because of the greater dissolved solids content in the water contained in the press cake.

Vincent has available a large number of material balances. These reflect single and double pressing, counter-flow diffusion, recirculation of press liquor, addition of oil house water, and many other options. These are transmitted by e-mail, in Excel.

The Excel spreadsheets contain a large number of simultaneous equations. Thus, Excel must be set on "reiterate" in order to find the common mathematical solution. Excel will freeze up if an illogical number is entered in error, so work must be saved frequently. In the event that a spreadsheet freezes, it must be closed without saving it.

Moisture Content

Everyone knows that screw presses can be used to reduce the moisture content of a material. Less appreciated are the limits that exist. Each material has a natural limit below which the moisture content cannot be reduced in a screw press.

New Citrus Feedmills

Two greenfield citrus feedmills have recently been commissioned in the Valencia area of eastern Spain.  Both turnkey projects were designed and constructed by FOMESA AGROINDUSTRIAL, a firm long based in Valencia.

ZUVAMESA with a 50 WHE was €8M, while CITROTECNO was €12M, each plus 18% VAT. The CITROTECNO feedmill with a 40 WHE was €7-7-1/2M; the rest was for the ethanol plant."

The CITROTECNO project is rated for 25-30 MTPH of citrus waste.  The waste comes from several citrus processors in the area as well as packing houses.  An ethanol plant at the site, using de-oiled and pasteurized press liquor, is entering the start-up stage.

Both local and national government entities have given financial support to the operation. 

The ZUVAMESA project is rated for 50 MTPH.  The waste comes from a juice extraction plant at the same site.  The company is owned by several hundred citrus growers in the area.  The local government has provided financial support.

The two feedmills are quite similar.  Both feature the use of a Waste Heat Evaporator (WHE) to achieve high thermal efficiency.  These WHE's are a three effect, vertical tube, falling fill design.  In the construction 304 stainless is used for vapor and water components while product contact is in 316L.  Durco pumps, imported from the States, were selected for their known reliability.

Single pass rotary drum dryers, standard of the citrus industry, are employed.  The CITROTECNO unit, rated at 30,000 pph of water evaporation, is paired with a 40,000 pph WHE.  At ZUVAMESA at 40,000 pph dryer is paired with a 50,000 pph WHE.

The dryers and WHE's were designed and constructed by FOMESA.

Both feedmills feature double pressing with Vincent screw presses with 24" diameter screws.  First pressing is done with a low torque Series KP press, while second pressing is done with a high torque Series VP press.  All presses use 50 hp motors.

The flexibility offered by double pressing has been advantageous where spoiled fruit is being processed.  This is quite common at CITROTECNO.

During start up a great deal of difficulty was encountered with the high viscosity and insoluble fiber content of the press liquor.  There is something different, possibly in the firmness, about this Mediterranean fruit as compared to fruit in Brazil and Florida.  As at FMC's Parmalat job in Sicily, it was necessary to add centrifuges to remove fiber from the press liquor.  Only after this was done was satisfactory WHE performance achieved. 

Both WHE's produce 45 Brix molasses on a consistent basis.  A 24 hour cleaning cycle is used.  An Alfa Laval centrifuge is used at ZUVAMESA while the unit at CITROTECNO is a GEA WESTFALIA.

The shredders feature hammers made of Duplex stainless steel, which is proving exceptionally durable.  The inlet housings are square, an innovation first introduced by Tegreene in Florida.  The blades are 180 degrees apart, and innovation introduced by CORENCO.  The discharge screens are exceptionally thick, 5/8", with square round holes.

The reaction conveyors are sized for 15 minutes reaction time, with 0.5% hydrated lime addition.  Inclined units are used to elevate the peel from the peel bin up to above the screw presses.  These use slow turning 24" diameter screw conveyors with half pitch flighting.

Pellet mills supplied by Van Aarsen are used at both feedmills.  These produce excellent pellets with a minimum of difficulty.

ADDENDUM: The CITROTECNO ethanol plant made 14,000 l of 90% ethanol the first season. They run on press liquor which is pasteurized at 90 C. They pump it to a flash tank to remove the d-limonene. When they had no press liquor, they kept the alcohol plant running by diluting molasses to 15-20 Bx."

Vincent Corporation is proud to have played an important role in the success of these feedmills.

The photos and layout drawing show how Zuvamesa does first pressing with two Model KP-24 presses, and then second pressing with two VP-24's. After that the press cake goes to the dryer. In the photos the two KP-24's are on the right hand side, and the dryer is off to the left. You can see that they used screw conveyors to lift the cake up into a horizontal screw conveyor which runs from right to left. The peel comes into this conveyor on the right hand side. The peel can then fall into the two KP-24's, or it can by-pass to the two VP-24's, or it can by-pass all four presses and go to the dryer.

The cake from the two KP-24's is elevated in a screw conveyor which has a circular (cylindrical) trough; it is mounted at about 60 degrees. Similarly, the cake from the two VP-24's is elevated in a screw conveyor which has a circular (cylindrical) trough; it is mounted at about 60 degrees. The cake coming up in these two elevating conveyors drops into the long horizontal conveyor which runs from the far right to the dryer at the far left.

This is a very effective system of double pressing.

Orange Peel as Fuel

In September 1998 Vincent Corporation gave a presentation to a group of over five hundred citrus processors. The subject was using citrus waste as a boiler fuel.

Pumped Citrus Peel

Last month Vincent presented a paper at the Citrus Engineering Conference of the ASME. This paper, Pumped Peel - Five Years Later, discusses advances in citrus peel processing technology.

Pumped Peel

Traditionally screw conveyors have been used to convey citrus waste from the juice extraction building through the feedmill. The waste is mostly orange peel, and it is dehydrated and pelleted into animal feed in the feedmill.

Small Scale Citrus Feedmill

Citrus plants around the world are facing the same problem. The problem faced by these juicing plants is the disposal of orange peel. They are finding increased costs associated with landfill or with finding farmers willing to take the wet peel for animal feed. This is occurring at the same time as increased environmental regulations are being applied.

These plants are very small compared to the large scale operations in Florida and Brasil. That is, they generate a few tons per hour of peel, generally running only eight hours per day, as compared to 1,000 tons per day at the larger plants.

Because of the vast difference in scale the smaller plants cannot justify the investment associated with a highly efficient, full blown feedmill.

There is one solution that was popular among Florida's smaller processors in the 1960's. It is a "beginners" feedmill plant. It minimizes the capital expenditure at the expense of requiring more energy.

To understand this "beginners" plant, it is important to first understand how a large plant operates. In the large scale plants, peel moisture is removed in three independent operations:

1. Firstly the peel is pressed to separate it into press cake and press liquor. This is a very energy efficient operation as the press horsepower is relatively low.
2. The press liquor is evaporated in a waste heat evaporator. This heat exchanger evaporates water out of the liquor and leaves behind dissolved solids in a solution commonly called molasses. This water removal process is very economical: it requires no fuel because its energy source is the waste heat in the exhaust gasses leaving the dryer. The molasseses produced are then added back to the press cake.
3. The press cake, with the added-back molasses, is dried in a rotating drum dryer. This is the least efficient of the three water removal devices. It requires about 1,600 BTU in the form of fuel oil or natural gas per pound of water removed from the peel.

In the process just described citrus peel, which starts at about 82% moisture, is dried down to 10 to 12% moisture. (This means that for every 100 pounds peel entering the peel bin, about 20 pounds of finished animal feed will result). As a final step the dried peel is pelletized in order to reduce its bulk. This is done in order to minimize transportation and storage costs.

The previously mentioned "beginners" process that may be economical for smaller processors calls for using only a dryer to remove all the moisture. This eliminates the need for investment in a dewatering press and a waste heat evaporator. In such a "beginners" plant we also recommend not investing in a pelleting mill and its associated pellet cooler.

To dry peel in this manner will require approximately 85 U.S. gallons of heavy fuel oil per short ton of feed produced. This compares to a range of 30 to 45 in Florida citrus plants. Thus we can see that the "beginners" plant has its lower capital investment being offset by higher operating costs.

The dried peel produced in this process is an excellent, palatable animal feed for dairy or cattle. Because of its low moisture content, it can be stored for prolonged periods with minimal spoilage.

The enclosed diagram shows the equipment required for drying the peel in this manner. One notable item is the reaction conveyor. It is necessary to add lime to the peel prior to drying; this is done in the reaction conveyor. The lime attacks the cells and releases the moisture so that it can rapidly and efficiently be evaporated in the dryer.

A frequent query has to do with incorporating a press into the cycle. The difficulty that arises involves what to do with the press liquor. The authorities will not accept it in the sewer system, and it can be used for irrigation purposes only if it is mixed with a great deal of water. (In heavy applications it kills the soil where land spreading is performed.)

Sometimes alcohol producers will buy press liquor at 8 to 12 Brix. It ferments readily and makes an excellent citrus alcohol. But how many small citrus processors have a nearby distillery?

Generally the only practical thing to do with press liquor is to run it through a steam or waste heat evaporator. A waste heat evaporator system generally costs almost as much as all of the rest of the feedmill put together. This investment becomes difficult to justify until all alternative methods of disposing of citrus peel have been exhausted.

CITRUS PEEL DRYING SYSTEM

Vincent Drawing C-91310 shows a citrus peel drying plant based on a Model 150 Dryer. The plant will dry approximately 19,000 pounds per hour of 82% moisture citrus peel, without the benefit of pressing or the use of a waste heat evaporator. The plant will produce approximately 3,800 pounds per hour of citrus peel dried to 10 to 12% moisture content.

The principal items, in their sequence in the production cycle, are as follows:

1. Peel Bin. This vertical front peel bin, with hydraulically operated doors, is of carbon steel construction. A caged ladder to the top and a catwalk across the top are included. The unit is prefabricated and knocked down for shipment, to be welded together at the job site.
2. Peel Bin Discharge Conveyor. Vincent will supply an ultra heavy duty conveyor, including three-bolt drilling, Gatke hanger bearings and a variable speed electric drive. This conveyor is of stainless steel construction, with a metering orifice plate. The peel bin is constructed with sufficient elevation such that the Discharge Conveyor can feed directly into the Peel Shredder.
3. Liming System. A Vincent VL-450 Hydrated Lime Proportioning System, mounted on a fabricated steel base, is included. The lime hopper, sized to hold one and a half bags of lime, is installed adjacent to the shredder. An auger from this hopper adds approximately 1/2% by weight of hydrated lime to the peel as it leaves the peel bin.
4. Peel Shredder. A Vincent VS-180 Shredder is included. This horizontal rotor machine reduces the peel mostly to a range of 1/4" to 3/4", with a minimum of fine material. It is of the thin, rigid blade design, as contrasted to the hammer mill concept. All contact parts are of stainless steel. The blades, which are fixed, cut the material before it is discharged through the perforated screen. The shredder housing is hinged so as to allow ease of washing, inspection, and changing the screens. In operation the housing fits folded onto the chute that feeds the shredder, assuring a tight fit. The rotor turns in only one direction; however, the blades can be reversed to give double life.
5. Reaction Conveyor. Shredded peel drops directly into a slightly inclined Reaction Conveyor. This conveyor is sized to allow approximately 10 to 12 minutes dwell time. It is of carbon steel construction and features a notched blade screw. The chemical reaction between the lime and the peel that occurs in this conveyor is required in order to break down the cell structure of the peel so that moisture can be better removed in the drying operation.
6. Elevating Conveyor. Limed peel from the Reaction Conveyor is elevated to the Dryer Feeder by this stainless steel conveyor. Also, recycled material from the second pass of the dryer is mixed with the limed peel in this conveyor. The design is such that, if required, material can be dropped back into the inlet of the reaction conveyor.
7. Dryer Feeder. This screw conveyor receives peel from the Elevating Conveyor and feeds it into the Dryer. This is a stainless steel screw feeder fitted with a companion flange matched to the dryer throat. It has a variable speed drive to control the process feed rate.
8. Burner. The burner will require up to 150 U.S. gallons per hour of fuel oil. The burner package is suppled with dampers and valves and a steam heater for the oil. The burner is capable of burning optional lighter fuel oils, or natural gas. It comes with the required combustion air blower.
9. Furnace. Vincent will supply a Model VF-150 refractory lined furnace consisting of a carbon steel shell mounted on a fabricated steel base. It is designed to receive and mix recirculated exhaust gasses from the Dryer discharge in order to control the gas temperature entering the Dryer. The firebrick lining is supplied and installed by the customer.
10. Return Elbow. There is a 180º Elbow between the Furnace and the Dryer so as to minimize the possibility of overheating peel in the Dryer.
11. Controls. Vincent supplies a solid state programmable controller that modulates combustion and monitors the flame. Control of the combustion rate is through a sensor mounted at the inlet to the third pass of the dryer. This is required for precise control of product quality.
12. Model 150 Dryer. This is a Vincent triple pass dehydration unit with an insulated, stationary outer drum. The unit includes recycle conveyors so that partially dried material can be extracted at the end of the second pass and mixed with the incoming material. This is especially important for the proper drying of unpressed citrus peel. The drum is carried on machined steel tires, mounted on an expansion type steel base with cast steel machined trunnions. The rotor is driven by a chain and sprocket system with a speed reducer. The base frame can be bolted to a 6" concrete slab without any special foundations; this saves installation costs.
13. Separation System. The Vincent low level entry cyclone separator, ductwork, dampers, and stack are supplied. This system assures gentle handling of the dried peel. A motorized air lock and carbon steel screw conveyor are used to move the peel from the separation chamber to the cooling reel and bagger. A radial blade exhaust fan with its drive are also included.
14. Cooling Reel. The Vincent Model 525 Cooling Reel gently cools the dried peel with an action similar to that of a large diameter clothes drier. Ambient air is drawn counter- current through the unit to achieve an evaporative cooling effect. Cooling is required in order to prevent the phenomena of re-heating of the stored peel. In the cooling process, evaporation results in a further reduction of moisture content of about 1%. The unit comes complete with a fan, dust collector, duct work, supports, and electric motors.
15. Bagging System. This system includes an surge hopper and a semi-automatic weighing and bagging unit. This unit consists of a hold/weighing bin with an adjustable discharge, a weight indicator, a bag holder, and a compact Fischbein sewing head. The take-away belt, belt trays and motor and drive are included.

Steam Injection

July 5, 2002                                                                                                                                                                                                         ISSUE #129

Up until the Oil Embargo of the early 1970's it was common practice to inject steam into screw presses. The steam was injected directly, through hollow resistor teeth, into the material being pressed. This was done to reduce the moisture content of orange peel that was being made into cattle feed. The practice seems to have been abandoned for two reasons: the high cost of steam energy, and the fact that most citrus feedmills lacked the Waste Heat Evaporator capacity to dispose of all the press liquor being produced.

Driven by the need to reduce VOC (Volatile Organic Compound) emissions, interest has resumed in pressing as much liquid as possible from citrus waste. One solution has been to use high pressure, high horsepower screw presses. To evaluate an alternative, Vincent Corporation acquired a boiler and conducted tests with live steam injection.

Initially we were cool to the idea of using steam. We reasoned that it would be more efficient to evaporate moisture with a direct fired rotary drum dryer. However a California research firm, Altex Technologies, brought to our attention that steam addition in a press will drive out liquid water, whereas a drum dryer removes the moisture in the form of water vapor.

The difference is very important: by pressing out liquid water, there is a savings of about 1,000 BTU's per pound of water. This is because the drum dryer requires this energy to convert the water from the liquid to vapor state.

To prove the concept, a basic test was conducted. Drums of peel were brought to the Tampa works from two local feedmills. This peel had been reacted with lime and single pressed. The peel was collected at the inlets to rotary drum dryers. The samples from both feedmills had a respectable 18o Brix. However the moisture contents of both samples were high, 71% to 73%, because of special conditions existing when the peel was collected.

This cake was second pressed in a laboratory screw press in Tampa. Probably because of a delay of a few hours that occurred after the samples were collected, the resulting press cake was reduced (consistently) to about 59% moisture. Part of this moisture reduction was due to using the press electrical drive to heat the peel: cake temperature increased by 10° F to 15° F while passing though the press.

Addition of steam, at various flow rates and at pressures up to 45 psi, significantly improved the press cake moisture that could be achieved. Final moisture contents of 55% to 57% resulted. In the process the temperature of the discharge cake and liquor was increased to about 160° F.

Most importantly, the extra moisture separated by the screw press came out as a liquid, not as vapor. Therefore the old steam injection technology appears worthy of further investigation. We hope to participate in full scale testing during the next citrus season.

Twin Screw Press

October 24, 2000                                                                                                                                                                                                 ISSUE #110

Following field tests during the last six months, Vincent is introducing the Twin Screws Press. Designated the Series TSP, five models are being offered.

These presses have very positive feeding characteristics, making them well suited for slippery materials. The strong pressing action of overlapping screws, plus the use of seven stages of compression, result in the best dewatering action we have ever seen in a Vincent screw press.

The Twin Screw Press is well suited to many traditional markets: citrus peel, fruit and vegetable juicing, fish waste, spent brewers' grain, shrimp shells, and alfalfa. We do not anticipate application in the pulp and paper industry, but we think that sugar beet pulp should work well.

There are three principal advantages to the Vincent design:
Automatic Control. The use of an air cylinder operated discharge cone allows the press to work well with changing feed material conditions and throughput capacities. The press has above average turn-down characteristics.

Low Horsepower. To improve dewatering, material is sliced in the press. This is a characteristic of the interrupted screw with fixed resistor teeth that can be seen in the brochure photo. The press requires less horsepower than a continuous screw press where the action is more like mashing than slicing. Also, the result of this particle size reduction is that, if the press cake goes to a dryer, better dryer performance is achieved.

OEM Gearbox. We have carefully selected major OEM gearboxes for use with our press. A choice of Sumitomo and Foote- Jones/Illinois Gear drives are available.

Overall, Vincent is delighted with the Twin Screw Press. It marks a significant advance in screw press design because the pressing performance is equal to or better than anything achieved in the past. In financial terms, it is possible to guarantee a machine with double the capacity of a single screw press, but at less cost than two single screw presses of the same diameter.

Bagasse Paper Mill

Productora de Papeles (PROPAL), based in Cali, Colombia, is a group of two paper mills. Originally International Paper operations, they are now owned by the Carvajal family group. Both mills produce paper using sugar cane bagasse as furnish. There are many sugar cane mills in the area, so a year-round supply of raw material is assured.

Boiler Ash

October 16, 1997                                                                                                                                                                                                       ISSUE #68

Recently our sales rep in North Western Ontario, Process Flow Systems, took us to visit the one of the largest paper mills in North America. There are kraft, newsprint, and recycle mills all on the same property.

We were asked to demonstrate our small CP-4 with the sludge that comes from a scrubber on their hog fuel boiler. The water and particulate coming from the spray chamber were a filthy jet of boiler ash.

We did not expect the test to work. As expected, the material just oozed through the press with no dewatering. However it looked like there was potential because some polymer had been added. With the polymer, clear water could be seen forming in the nearby puddles and streams.

By chance nearby there was a pile of paper mill waste fiber. This was added to the ash and water solution, and, bingo! It was like magic. With the fiber as a press aid, the press started producing cake that measured 60% solids.

The application was found to work well with 10% by volume reject fiber. By weight the percentage would have been much lower, as is normal with the use of a press aid.

On-site testing is planned with a larger scale rental press.

Boiler Fuel

We frequently are asked about the fuel value of waste materials from pulp and paper mills. This question arises because, when screen rejects and knots & shives are pressed, the dry press cake that is formed can be burned instead of landfilled. The combustion can be either in a coal fired boiler or in a bark burner. Tipping fees are an important part of the analysis. A high rate, like $30/ton, would make press cake disposal more important.

Clarifier Sludge

August 28, 1998                                                                                                                                                                                                    ISSUE #82

In 1997 Putney Paper in Putney, Vermont purchased a Vincent VP-16 screw press. Its function was to further dewater sludge from the primary clarifier in their wastewater treatment plant. The sludge was pre-thickened with a belt press ahead of the screw press.

The mill both recycled OCC (Old Corrugated Container) and did deinking of other waste paper. The OCC operation put fiber in the clarifier, which improved the subsequent dewatering. Deinking, on the other hand, results in a higher proportion of clay and other materials that are hard to dewater in a screw press.

Testing with a rental machine and initial operation with the VP-16 gave excellent results. Press cake solids of 55% were obtained. This addressed problems associated with landfill hauling and fees.

A year later the press would not work. Capacity had fallen off and dewatering was minimal. Examination revealed that the sludge no longer had the fiber content that had existed previously. This had occured as a result of improved fiber recovery. Nothing could be done with the mud-consistency material coming from the belt press. The mill stopped using the screw press.

Trials elsewhere prompted Vincent to make this proposal to mill management: if they would substitute the existing 1800 rpm drive motor with a 900 rpm model, Vincent would supply the belts and sheaves to cut the screw speed in half. This combination cut the speed of the press from the normal 13 rpm down to about 3 rpm.

The results were a dramatic improvement in dewatering capability. The 35% solids cake from the belt press was further dewatered to a respectable 45% solids in the Vincent press.

Unfortunately capacity of the screw press fell off with the speed reduction. At times this was a problem for the mill, so the 1800 rpm motor was re-installed. With this change, press cake solids fell to 40%, but capacity was up. Later, a frequency inverter was installed so that the press could be run at speeds from 3 to 7 rpm. The speed was set according to the nature of the sludge being pressed at any given time.

Clarifier Underflow - A

An excellent installation was made in 1999 at a Weyerhaeuser paper mill. The mill installed a Model VP-16 screw press, along with a pair of sidehill screens. This equipment is used to dewater the underflow from their clarifier. Photos of the installation are available.

Contract Wastewater Treatment

December 4, 2001                                                                                                                                                                                                   ISSUE #123

It was at a Tappi Environmental show that Vincent met representatives of Integrated Technical Services of Baton Rouge, Louisiana. This firm specializes in handling waste disposal on a contract basis. Pulp and paper is but one of many industries served by ITS.

Working with Vincent, ITS secured a three year contract to handle wastewater disposal at a Georgia-Pacific mill. The mill, in Crossett, Arkansas, was once the largest mill in the United States. It is rated at 1,600 tons per day, producing softwood Kraft pulp.

The combined wastewater flows of the mill are pumped a few miles off-site to an in-the-ground clarifier. The flow to the clarifier averages fourteen million gallons per day. It includes the undercooked chips, shives, screen rejects, and wash water from all sources.

Underflow from this clarifier is pumped to sidehill screens mounted above Vincent Model VP-16 screw presses. The arrangement has a pair of 3' sidehills over the inlet of each of two presses. These face each other so that any splash flow falls into the press.

The flow to the sidehill screens averages 1% to 2% solids. Even under adverse conditions this is concentrated sufficiently by the screens for the screw presses to operate reliably. Only under very difficult conditions is any polymer used in the treatment.

Press capacity is extremely high. This is due to the effectiveness of the sidehill screens and the high freeness of the Kraft fiber coming from the mill. Throughputs in excess of 70 TPD of air-dry solids per press have been measured. This is an outdoors installation. The only building is a trailer used by an operator. Currently the cake from the presses is landfilled, although there are plans to use the cake as boiler fuel. 

Deinking

July 13, 1999                                                                                                                                                                                                         ISSUE #96

Deinking Mills are a major specialty among paper recycling mills. The principal raw material categories for these mills are newsprint, magazines, telephone directories, and mixed office waste. There are many sub-specialties. For example, some mills will recycle only newsprint, while others will mix magazines with newsprint. This adds fiber strength and brightness, but it requires the addition of equipment to screen ash from the furnish. (Ash is mostly the kaolin clay coating used in glossy magazine paper.) MOW (Mixed Office Waste) mills use the least expensive material; however, it requires special equipment for removing a broad range of contaminants such as Styrofoam and stickies (contact cement).

The common feature of deinking mills is that they must separate printers' ink from the fiber. This is made difficult because the objective of ink formulators is to produce a product that stays bound to the paper fibers. Obviously there are many types of ink: water based, latex, those that work on ground wood paper, those that work for glossy coated paper, etc. One of the worst is laser printer ink, because it is heat bound to the fibers.

Once the ink and ash have been separated from the fibers, these contaminants must be screened from the flow. Traditional technology is to use a machine similar to a belt press for that purpose. The performance of these deinking machines is principally measured by (a) the brightness of the good fiber that is accepted, and (b) the amount of good fiber going in the reject steam.

Last year a Fiber Filter machine was tested for deinking capability. This was done in the laboratory of Thermo Black Clawson, the premier manufacturer of recycle paper mill machinery. The results were extremely encouraging. Compared to the conventional equipment, the Fiber Filter scored higher in brightness and, simultaneously, higher in fiber recovery. This means that more ink went through the fabric sleeve while at the same time that less good fiber was getting through the same sleeve. Ash separation was also excellent.

Feed consistencies ranging from 0.25% to 2.0% were tested. Accept consistencies were in the range of 7.5% to 14% solids, with flow rates of 150 to 250 gpm in the Model FF-12.

Kneader Prethickener

In 1997 Vincent was awarded a contract for a Model VP-16 screw press that was used in an important pulp & paper application. The order was received from Thermo Black Clawson, the leading American supplier of paper mill machinery. The press is for Papelera Aragua, their client in Cagua, Venezuela.

The application was at a deinking mill that recycled ONP (old news print). Once hydrapulped, the newspaper must be scrubbed to remove ink from the cellulose fibers. This is achieved in a Thermo Black Clawson machine known as a Kneader. In the Kneader the fibers are rubbed against one another in order to separate the ink.

The Kneader requires a furnish with 25% to 30% solids. This is pre-thickened from a flow with 2% concentration by the Vincent screw press. The installation processes 30 MTPD (metric tons of air dry solids per day). The freeness varies considerably, from 150 to 500 csf (Canadian standard freeness).

The Vincent interrupted screw negates the need for a variable frequency drive. This press design uses interruptions in the screw flights with stationary resistor teeth inserted in the gaps. The resulting agitation allows the press to operate consistetly, without co-rotation, over wide ranges of both throughput and feed consistency. The use of an air cylinder actuated discharge cone allows production of press cake of steady solids content.

This job is important to Vincent because it is a step into yet another pulp & paper application: thickening pulp for both wet storage and baling purposes. Frequently there is a need for storage between pulping operations and the paper making machine. Water must be removed, without damaging the fibers, to reduce the storage bulk to a manageable volume. We have one small press operating in this capacity at a Georgia-Pacific mill, so we have confidence our larger presses can do the job.

Knots & Shives - B

Vincent has supplied screw presses to several Kraft mills for use on knots and shives. These rejects are material that is not made into paper pulp. The knots come from knotters following the digester, while shives are bundles of fiber, high in lignin, that are separated by pressure screens.

Knots & Shives

In 1994 Vincent presses were used for on-site testing at two paper mills, a James River plant and Buckeye Florida in Perry, FL. The tests at James River were arranged by the Atlanta office of Sandwell Inc., a consulting firm, while the tests at Buckeye were initiated by Agri-Products, a landscaping materials firm. Since then both mills have bought VP-16 presses.

Both plants produce paper pulp, which starts with wood chips that are digested. The mass discharged from the digesters contains pulp fiber, spent chemicals, and waste materials. The liquid is very black in color, hence its name, black liquor. This liquid gives pulp mills their characteristic odor. Conventional practice is to screen out the usable pulp fiber. The screen rejects are waste materials which, along with some black liquor, are sent to landfill.

These screen rejects consist of knots, shives, as well as some good pulp fiber. The shives are little bundles of cellulose fiber that are still bound by lignin that failed to get dissolved in the cooking process. At James River the knots were fairly evident, about the size of quarters, while at Buckeye they rarely appeared. This was a function of the species, as well as age and condition, of the trees being harvested.

The amount of good fiber present in the waste stream varied considerably; at Buckeye it was low, around 25%, while at James River, on the softwood side, it was running unusually high, probably around 80%.

Moisture content of the material coming into the presses from the vibratory screens varied widely. Readings ranged from 76% to 91% moisture. Successful operation is also achieved with some flows in the 96% to 99% moisture range.

Excellent results are evident pressing the knots and shives at both plants. Black liquor, with its chemicals, is a financially important recovery at James River. At this hardwood mill, the rejects are unwashed, coming directly from a continuous digester. At Buckeye the chemicals are less concentrated because the rejects come from the brown stock wash system.

Solids content of the press cake at both mills averages 40% to 50%. Some material can be taken up to 55%. The James River material can be pressed so that it is suitable for a boiler fuel.

Buckeye is entertaining a proposal involving mixing the pressed material with cypress mulch for landscaping application. Another potential use at James River is as a filler in the production of tar paper. In any case landfill problems of contamination and availability are avoided.

The most commanding performance feature of the press is the ability, without operator attention, to handle a wide variety of conditions. Flow rates can be varied from "off" to surge to normal. Tests were run with only plant water going into the press, followed by thin and thick flows. The press rarely wants to purge. There is a tendency to overload and jam on dry material, so reinforced profile bar screens and the heaviest available drives are used for knots and shives.

One characteristic that catches everyone's eye is that knots are disintegrated into small pieces in the Vincent press. This opens the possibility of additional fiber recovery.

In anticipation of future pollution regulations, presses for black liquor applications are offered with vapor barrier construction.

Non-Pressable Sludge

Very frequently we are asked if some material can be dewatered in a Vincent press. There is a very simple test that indicates if a screw press will work: First, put a small amount of material in the palm of your hand. Next, close your fingers gently around the mass of material. Work the material with your palm and fingers so that something is squeezed out between your fingers. If a liquid comes out between your fingers and if, in the end, there is some solid material left in your palm, then a screw press might succeed.

Paper Mill Prethickening Options

In some pulp & paper screw press applications there is no need to prethicken the flow to the press. This is the case both with rejects from pressure screens and knots from shaker tables. Similarly, most clarifier underflow has 2% to 3% solids consistency, which, especially in virgin fiber mills, is adequate for dewatering in a screw press.

Paper Recycle Mills

Paper recycling is not new: Vincent presses have been installed at two mills that are over fifty years old. At the same time there has been a large construction boom of paper recycling mills in the 1990's. The mills are scattered across the country, generally being located close to population centers. There are seventeen in Los Angeles alone.

Pressable Sludge

Probably the most frequent inquiry received at Vincent involves using our screw press to dewater sludge. The reason for this is that screw presses have been promoted as being able to press sludge even though there are many cases where they cannot actually do so. At the same time, there is a great demand from plant operators. They know a screw press operates relatively unattended and requires a minimum of maintenance, while belt presses do not share this reputation.

Pressing Cake from a Belt Press

Energy costs have caused several mills to look at adding a screw press to their wastewater treatment plant. This is done so that the sludge from the facility can be used as boiler fuel. Typically the screw press is fed the cake produced by an existing belt press.

Pulp & Paper Installations

PULP & PAPER USER'S LIST
JULY, 2010
 

1994 Smurfit-Stone Container, Wabash, Indiana (now Paprworks Industries)
SCREEN REJECTS AT RECYCLE MILL
Two VP-16's doing screen rejects at a recycle boxboard mill. Tom Manley,
513-746-6493, was the plant engineer; we co-authored a TAPPI paper. We
arrived at this, our first paper mill, in June 1994.

1995 Domtar (formerly Georgia-Pacific), Port Edwards, Wisconsin
STOCK THICKENING (FOR STORAGE)
They bought a CP-6 to thicken screen rejects which are stored and later added to
special runs of bond paper. [Mill closed.]

1995 Coastal Paper, Mississippi
REJECT BLEACHED PULP JUST AHEAD OF PAPER MACHINE
This is a paper maker that buys market pulp and converts it into specialty papers.
The tiny CP-4 works perfectly for their screen rejects.

1995 Bowater (formerly Halla Paper), Mokpo, Korea
WASTEWATER SLUDGE AFTER IT IS THICKENED WITH A BELT PRESS d
This is a deinking mill. The furnish is 95% old newspapers and 5% groundwood a
fiber. They have four CP-12's, which are beautiful machines. The presses dewater 3
screen rejects and some primary clarifier sludge, all coming from belt presses.
The cake, at 55% solids, goes to a fluidized sand bed boiler.

1996 Georgia-Pacific (formerly Fort James), Pennington, Alabama
KNOTS AND SHIVES (VIRGIN FIBER MILL)
The VP-16 for dewatering knots and shives at this large virgin fiber mill is
working well. Pat Braud, 205-459-1737, is the cognizant engineer.

1996 CANFOR (Formerly FIBRECO), Taylor, British Columbia
WASTEWATER SLUDGE, PRE-THICKENED WITH A BELT PRESS
This CTMP mill has purchased a VP-16 for pressing primary sludge with some
fraction of secondary sludge blended in. The sludge comes from a belt press.
They produce and sell market pulp made primarily from wood chips

1996 Evergreen Pulp (Formerly Samoa Pacific Cellulose) , Samoa, California
1. REJECT BLEACHED PULP JUST AHEAD OF PAPER MACHINE
2. SHIVES
The last virgin fiber mill in California. They have a pair of VP-10's plus a rental
CP-10. One is used to dewater bleached rejects from just ahead of a paper
machine. Another is for shives. It enables them to haul the rejects to landfill
without environmental problems. [Mill closed.]
1996 Celotex, Quincy, Illinois
SCREEN REJECTS AND WASTEWATER AT RECYCLE MILL
They had a CP-10 rented medium term and have since commissioned a VP-
16. They are a recycle mill that makes the outer paper layer for plasterboard.
The press is used to dewater screen rejects after the hydrapulper. They solve
a landfilling cost and problem by dewatering the rejects. [Mill shut in 2002.]

1996 Liberty Paper, Becker, Minnesota
SCREEN REJECTS AND WASTEWATER AT RECYCLE MILL
This is an OCC recycle mill pressing tertiary screen rejects plus DAF sludge.
Following successful tests, they purchased a pair of VP-16's. Steve McPherson,
Production Manager, 763-261-6120.

1996 Durango Paper (formerly Gilman), St. Marys, Georgia
KNOTS & SHIVES FROM QUATERNARY SCREEN
This firm has a pair of VP-12 presses, all-stainless construction (including the
frames), along with sidehill pre-thickening screens. Quaternary rejects are pressed
prior to burning. [Mill shut in 2002.]

1996 Georgia-Pacific (formerly Fort James), Clatskanie, Oregon
SAWDUST AT PAPER MILL THAT USES SAWDUST DIGESTERS
Two VP-30 presses were intended to take rain water out of sawdust. With 400 hp
drives, these are 35' long and weigh 80,000 pounds each. The units are now idle.

1996 APC Paper Company, Claremont, New Hampshire
QUATERNARY SCREEN REJECTS & WASTEWATER AT RECYCLE MILL
This firm has a VP-10 to dewater OCC waste sludge prior to landfill.
Environmental problems associated with an unstable landfill led to the testing that
led to the purchase. They pre-thicken the flow to the press with a sidehill screen.

1996 Millar Western, Meadow Lake, Saskatchewan
DEWATERING CAKE FROM A BELT PRESS AT A CTMP MILL
A Model VP-16 dewatering screw press is used to increase solids in cake from a
belt press. This zero effluent mill burns the cake in a hog fuel boiler. The primary
sludge solids are increased from 30% or less to 40% or more with the screw press.

1996 Simpson Paper, Gilman, Vermont (Dirigo Paper)
WASTEWATER SLUDGE AFTER IT IS THICKENED WITH A CLARIFIER
This mill uses market pulp to make high-grade specialty paper. Reject material
from a large clarifier is fed to Somat presses. The sludge from the Somat's, at 10%
to 15% consistency, is fed to a Vincent Model CP-6 press. The Vincent takes it up
to 35% or 40% solids. [Mill closed in 2007.]

1997 Ponderay Newsprint (AbitibiBowater) Usk, Washington
SCREEN REJECTS AND WASTEWATER AT RECYCLE MILL
This newsprint mill is 80% virgin fiber, 20% recycle. They are pressing a wide
range of screen rejects from both operations. Following a long rental period, they
bought a VP-16 in T-316 stainless. Their order included sidehill screens for use
ahead of the press.

1997 Buckeye (formerly Merfin), Delta, British Columbia
WASTEWATER SLUDGE AFTER IT IS THICKENED WITH A CLARIFIER
This mill makes sanitary napkins. A rental CP-6 worked well in competition
with a FAN press. They purchased a CP-10 for more capacity. They are pressing
reject pulp, which is collected in a Krofta clarifier.

1997 Louisiana-Pacific, Chetland, British Colombia
This mill acquired a CP-10 for sludge dewatering shortly before the mill was
closed.
1997 Buckeye (formerly Merfin Europe Ltd.), County Cork, Ireland
REJECT PULP OFF THE PAPER MACHINE
This sister plant has purchased a VP-10 for dewatering reject pulp from a system
that converts market pulp into paper product on a dry basis (Air Laid).

1997 Kadant Black Clawson, Mason, Ohio
THICKENING PULP AHEAD OF A DEINKING KNEADER
A VP-16 press for use in a secondary fiber mill in Venezuela. The press,
positioned head of a deinking kneader, is used to thicken 35 TPD of stock to the
range of 30 to 35% solids. The user is Papelera Aragua.

1997 Putney Paper, Putney, Vermont
DEWATERING CLARIFIER UNDERFLOW
This firm purchased a VP-16 for dewatering clarifier underflow. An improved
fiber recovery system seriously reduced the capacity of the press. [Mill closed.]

1997 Buckeye Florida, Perry, Florida
KNOTS AND SHIVES (VIRGIN FIBER MILL)
After on-site testing, this 1,200 TPD softwood Kraft mill purchased a VP-16
for dewatering knots and shives. This relieves landfill load and groundwater
contamination, while creating a saleable by-product. A sidehill screen thickens
the flow ahead of the press.

1998 St. Anne-Nackawic, Nackawic, New Brunswick
TERTIARY/QUATERNARY REJECTS PRE-THICKENED WITH A SIDEHILL
This mill rented a VP-16 for two summers before purchasing the machine. It is
used to dewater reject fiber that was otherwise being taken to landfill in a dripping
wet condition. After pressing the cake burns extremely well in their hog fuel
boiler.

1998 Kerwin Paper, Appleton, Wisconsin
SCREEN REJECTS FROM HYCOR DISCOSTRAINER
This recycle paper mill produces colored construction paper. The VP-10 press
dewaters rejects that arise when a color change is made. The press cake is re-used
in black construction paper.

1998 Terrace Bay Pulp, Terrace Bay, Ontario
(Formerly Neenah Paper, Kimberly-Clark)
WASTEWATER SLUDGE AFTER IT IS THICKENED WITH A CLARIFIER
This large virgin fiber mill ran tests using a rental CP-10, dewatering primary
sludge that was pumped directly from the clarifier. They ordered a pair of VP-
16's and sidehill screens, and they are very pleased with these. The press cake
is burned in their boiler. In 2001 a third VP-16 was ordered to handle a mill
expansion, and all the presses were relocated from the WWTP to the boiler house.

1999 ALBERTA RESEARCH COUNCIL, Edmonton, Alberta
A Model CP-10 press is used for laboratory and pilot testing at the Alberta
Research Council.

1999 International Paper, Oswego, New York
SCREEN REJECTS AND WASTEWATER AT RECYCLE MILL
This mill had a CP-6 rented for half a year. While we were switching up to a
CP-10, they tried a press by Press Technology, a Springfield, Ohio firm. That
press failed due to wear which was related to high screw rpm. [The mill is shut.]

1999 Crown Vantage, Milford, New Jersey
DEWATERING CAKE FROM A BELT PRESS
This mill produces food grade wrapping paper from market pulp. Rated at 200
tons per day, the mill was sending 4 to 8 tons of clarifier underflow sludge to
landfill daily. The purchase of a CP-10 screw press was justified by reduced
landfill charges. The screw press increases the solids of cake from a Komline-
Sanderson belt press from 20-27% to in excess of 50%.

1999 Corrugated Services, Forney, Texas
This recycle mill uses a small KP-6 for light dewatering of solids screened from
the wastewater flow. "The best capital investment this company ever made."

1999 Weyerhaeuser Paper, New Bern, North Carolina
PRIMARY CLARIFIER SLUDGE, PRE-THICKENED WITH A SIDEHILL
This Kraft mill replaced a worn belt press with a Model VP-16 screw press and a
pair of Vincent sidehills. Final press cake solids were improved from the 30's to
45%; the cake is now sold as a fuel to a cogen power plant. No polymer is used in
the clarifier.

1999 Nevamar (formerly International Paper), Hampton, South Carolina
DEWATERING FIBER DUST FROM A SPRAY CHAMBER
This mill is using a Model CP-12 screw press to dewater fiber and phenolic dust.
The dust comes from the spray chamber dust collector. The mill specializes in the
manufacture of hard board used in decorative door facings, counter tops and screw
conveyor hanger bearings.

1999 Celulosa Arauco (Alto Parana), Misiones, Argentina
DEWATERING SCREEN REJECTS, 200 GPM FEED AT 1% CONSISTENCY
This Chilean paper company procured a Vincent Model CP-12 screw press for
dewatering rejects from a brown stock tertiary screening stage. The installation is
at a softwood Kraft mill in Argentina. The objective is to recover the chemicals in
the black liquor filtrate.

1999 FiberMark, Brattleboro, Vermont
DEWATERING CAKE FROM A COIL FILTER
This specialty mill produces coated or embossed pressboard, using a furnish of
30% virgin and 70% recycled market pulp. They have a Vincent Model CP-12
screw press that is used to further dewater primary clarifier sludge that is first
prethickened to 22% solids with a vacuum coil filter. The Vincent press increases
the consistency to the 45% range, significantly reducing hauling and landfill
charges.

1999 Minas Basin Pulp, Hantsport, Nova Scotia
DEWATERING SCREEN REJECTS, PRE-THICKENED WITH A SIDEHILL
This mill has a CP-12 to dewater OCC waste sludge and tertiary/quaternary rejects
prior to hauling them to landfill. Press cake is in the range of 50% solids, and
alternatives such as burning are being considered. Pre-thickening is done with a
sidehill screen.

2000 Georgia-Pacific, Crossett, Arkansas
DEWATERING MILL WASTE STREAM
Integrated Technical Services of Baton Rouge, LA bought a pair of VP-16's, each
with a pair of sidehills for prethickening, for dewatering the clarifier flow at this
1,600 TPD hardwood and softwood mill. Later, LARCO purchased another VP-
16 for use under a similar contract at the mill.

2001 Gulf States Paper, Demopolis, Alabama
KNOTS AND SHIVES
This mill used a VP-16 from our rental fleet while a new machine was built to
their specifications. The load is estimated at 41 TPD, dry solids.

2001 Rock-Tenn, Sheldon Springs, Vermont
BLACK CLAWSON ULTRA SORTER REJECTS
A small CP-6 is used to dewater 7 TPD of rejects. They feed from the Kadant
Black Clawson Ultra Sorter at 25% solids. The press cake, at 45% solids, is sent
to landfill.

2001 Caraustar Mill Group (formerly Smurfit-Stone), Lafayette, Indiana
DEWATERING RECYCLE MILL REJECTS
A rental CP-10 was used dewatering rejects from screens at this recycle paper
mill. Mark Lindstrom, 765-423-5631, is the cognizant engineer.

2001 Durango-McKinley Paper, Prewitt, New Mexico.
MIXTURE OF MILL WASTE STREAMS
Furnish for this 550 TPD linerboard mill arrives by train and trailer. Four waste
streams, with varying flow rates, including one that is biological, are fed to an
Andritz belt press. The 35% solids cake from the belt press is further dewatered
to 37% to 45% solids (depending on the biological component) with a Model VP-
16 turning 5 rpm. This rental press was used until another press at the mill could
be rebuilt. Ed Tom, 505-876-2171, supervises operations. Rented another unit in
2006.

2002 Georgia-Pacific, Cedar Springs, Georgia
CLARIFIER UNDERFLOW
All wastewater solids from this 2,300 tons per day mill are dewatered in a pair
of VP-16 presses. Pairs of sidehill screens are mounted over the inlet of each
of screw press. The WWTP is operated and managed by Integrated Technical
Services of Baton Rouge, Louisiana.

2002 Rock-Tenn, Aurora, Illinois
DEWATERING PRESSURE SCREEN REJECTS
A rental CP-10 was purchased for dewatering screen rejects in this book binders
board mill. John Goll, Assistant General Manager, 630-898-4231.

2002 Noss AB, Sweden
LABORATORY PRESS
This paper industry equipment supplier has purchased a Model CP-4 laboratory
press for use in pulp thickening trials.

2002 Ohio Pulp Mills, Cincinnati, Ohio
REJECTS DEWATERING
This mill recovers secondary fiber from gable-top cartons, ice cream cartons, and
meat pads. A great deal of plastic film is present in the rejects. A custom Model
KP-10 press, cut short for low headroom, dewaters their waste flows ahead of the
dumpster.

2002 Buckeye, Mt. Holly, North Carolina
DAF SLUDGE AND SIDEHILL SCREEN REJECTS
This conversion mill has air laid paper machines making diaper products. Excess
pulp from the paper machines and floor drains feeds to a DAF or sidehill, then is
pumped to a CP-10. The Vincent press replaced a plate and frame press. Contact
is Tim Kistemaker, 704-822-6400.

2002 Linpac Paper, Cowpens, South Carolina
REJECTS DEWATERING AT RECYCLE MILL
Rejects at this 450 TPD OCC mill include unusually high quantities of plastic.
A flow of 200 gpm is fed directly to a Model CP-12 screw press, without
prethickening. Bill Johnson, 864-463-9090, is the Mill Engineer.

2003 P. H. Glatfelter Co., Spring Grove, Pennsylvania
DEWATERING A KNOTTER REJECT STREAM
A simple, robust Model CP-10 screw press was purchased in 2003. An enlarged
headbox was included to handle surges. Trials were run with a rental CP-6 prior
to the purchase. Since the feed flow can reach 100 gpm, a 3/32" perforated screen
was selected. Reclaimed liquor is returned to process.

2003 Flakeboard Company, St. Stephen, New Brunswick
OIL RECOVERY
A small CP-4 press was purchased to recover valuable oil from waste in this board
manufacturing facility.

2003 AV Cell, Atholville, New Brunswick
SCREEN REJECTS DEWATERING
This mill has a Model VP-16 press on extended rental. (A new 316 stainless press
was purchased in 2006.)

2003 Inland Paper, Orange, Texas
DEWATERING CLARIFIER UNDERFLOW
A VP-16 was purchased by Bruce Brothers Dredging under a contract to dewater
WWTP sludge at this virgin fiber mill. Clarifier underflow is prethickened with
sidehill screens.

2003 US Gypsum, Gypsum, Ohio
PRIMARY CLARIFIER UNDERFLOW
This mill makes liner for gypsum wallboard. They use a Model VP-16 press for
sludge dewatering. An IPEC rotary drum screen is used to prethicken the flow to
the press.

2004 Blue Ridge Paper, Canton, North Carolina
DEWATERING SCREEN REJECTS
This virgin fiber mill purchased CP-10 presses for dewatering screen rejects in
both their softwood and hardwood mills. The reclaimed liquor is returned to the
system. The press cake is burned.

2004 Sindihan Paper (UEM), Egypt
DEWATERING CLARIFIER SLUDGE
A Model CP-10 press was sold for dewatering clarifier sludge at this Egyptian
paper mill. In 2006 they inquired about purchasing another identical press for a
new project.

2004 Georgia-Pacific, Green Bay, Wisconsin
STYROFOAM AND OTHER FLOATING CLARIFIER WASTE
The KP-16 press at this 1000 TPD recycle mill is in an unusual application.
Floating wastes, mostly Styrofoam and woodchips, are skimmed at about 200 gpm
from the WWTP clarifiers. The press dewaters this waste stream. This is our first
installation of a Model KP-16 press in the pulp and paper industry.

2005 International Paper, Pensacola, Florida
EXPERIMENTAL TESTING
Two screw presses, a VP-16 and a KP-16 were used in series and parallel to
thicken stock for a specialty paper.

2005 Eurocan, Kitimat, British Columbia
DEWATERING CLARIFIER POND SLUDGE
A Model VP-22 is used by Jose's Excavating, under contract with the mill,
to dewater primary clarifier sludge. The sludge is loaded to the press after
draining in a pond. The press cake is landfilled. [Mill closed in 2009.]

2005 Rayonier, Fernandina Beach, Florida
KNOTS & SHIVES
A Model VP-22 was purchased for dewatering knots. The press was built in
all-316 stainless, including the base frame, because of adverse environmental
conditions. The press cake is burned in a mill boiler.

2005 Evergreen Packaging (formerly International Paper), Pine Bluff, Arkansas
PRIMARY CLARIFIER UNDERFLOW
This mill is renting two Model VP-16 presses for dewatering a combination of
clarifier sludge and knots/rejects. Vincent Sidehill screens are used to prethicken
the flow to the presses. The cake is burned.

2006 Elite Kraft, Thailand
DEWATERING SLUDGE AT WASTEWATER TREATMENT PLANT
A Model CP-12 press is used to dewater sludge that is prethickened with a tray
belt prethickener.

2006 AV Cell, Atholville, New Brunswick
SCREEN REJECTS DEWATERING
This mill purchased a Model VP-16 press built in 316 stainless construction. This
followed extended use of a similar model in 304 stainless.

2006 Inland Paper, Orange, Texas
DEWATERING CLARIFIER UNDERFLOW
A KP-16 was rented by Bruce Brothers Dredging under a contract to dewater
WWTP sludge at this virgin fiber mill. Clarifier underflow, at very low
consistency is pumped directly into the press. The cake produced is quite suitable
for landfill stacking.

2006 Evergreen Packaging (formerly International Paper) Pine Bluff, Arkansas
DEWATERING WHITEWATER REJECTS
This mill rented (since purchased) a Model KP-16 press for dewatering small
reject fibers after bleaching. A feed of 200+ gpm at 0.5 - 1.3% consistency
results in press cake at 38% solids. The load is about 7 TPD dry solids. This is
a very successful application for a Series KP press. The cake is sold to a roofing
company.

2007 Elite Kraft, Thailand
DEWATERING COMBISORTER REJECTS
A Model KP-16S press has been ordered to dewater small pieces of plastic film,
Styrofoam and similar Combisorter or Serraplast rejects.

2007 First Quality Tissue, Lock Haven, Pennsylvania
DEWATERING CLARIFIER UNDERFLOW
A pair of CP-12 presses, each with paired sidehill screens for pre-thickening, is
used to dewater clarifier underflow.

2007 Pactiv Corporation, Plattsburgh, NY
PULP & PAPER
A Model CP-6 press was purchased for de-watering tertiary rejects.

2007 PROPAL (Productora de Papeles), Cali, Colombia
DEWATERING CLARIFIER UNDERFLOW
This bagasse paper mill purchased a Model VP-24 for dewatering clarifier
underflow. No polymer was required. The cake was to be blended with coal
for use as boiler fuel. This job was a failure; however, it presents an interesting
history.

2007 Sappi Fine Papers, Cloquet, MN
DEWATERING KNOTS AND SHIVES
A Model VP-24 has been purchased for dewatering knots. The load is
approximately 50 TPD of dry solids. The liquor will be recycled and the crushed
knots returned to the digester.

2007 Flambeau River Papers, Park Falls, Wisconsin
BEECH KNOTS
A Model CP-10 has been used on a rental basis for an extended period. With a
3600 rpm motor, this press achieves outstanding capacity.

2007 Inland Paper, Orange, Texas
DEWATERING CLARIFIER UNDERFLOW
A KP-16 was rented long term by Bruce Brothers Dredging under a contract
to dewater WWTP sludge at this virgin fiber mill. Clarifier underflow is
prethickened with sidehill screens.

2008 Rayonier, Jesup, GA
SCREEN REJECTS
A model KP-16, owned by Absorption Corp, is in service at the mill. The
dewatered fiber is used in the production of litter for domestic pets.

2008 UPPC (United Pulp & Paper Company), Philippines
DEWATERING WWTP SLUDGE
A Model CP-12 is giving excellent results dewatering cake from belt presses at the
wastewater treatment plant.

2008 Blue Heron Paper, Oregon City, OR
SCREEN REJECTS
This recycle mill has rented a Model KP-10 press for an extended period. The
waste includes a considerable amount of Styrofoam and plastic.

2008 Hartford City Paper, Hartford, Indiana
DEWATERING SCREEN REJECTS
Based on a project carried out by AMEC consulting engineers, this mill has
purchased a Model KP-10 press for dewatering WWTP sludge. The flow comes
from a Poseidon clarifier and an Andritz sidehill screen. The sidehill, with 0.005"
slots, is PLC controlled.

2008 SAICA, Venizel, France
DEWATERING REJECTS
This large recycle mill uses a Model KP-16 to dewater sludge ahead of landfill.
The press was selected after trials with German equipment.

2008 Georgia-Pacific, Crossett, Arkansas
DEWATERING MILL WASTE STREAM
Larco, a contractor operating the WWTP, bought a Model KP-16 for assisting in
dewatering the clarifier flow at this 1,600 TPD combined hardwood and softwood
mill.

2008 Fiber Solutions, Pine Bluff, AR
DEWATERING FIBER FROM WHITEWATER
This contractor has rented a Model KP-16 for over a year. It is being used to
recover fiber from whitewater. The press cake is sold for the manufacture of
shingles and other by-products.

2008 Georgia-Pacific, Palatka, FL
PRESSING KNOTS
This mill rented a Model KP-16 press was used on knots and shives. The
installation was temporary while other mill machinery was being rebuilt.

2008 Sonoco, Menasha, WI
DEWATERING SLUDGE
This secondary fiber mill installed a Model KP-16 to de-water sludge and screen
rejects. It replaces a screw press associated with excessive maintenance expense.

2008 NGC (National Gypsum Company), Pryor, OK
DEWATERING SLDUGE AT A WALLBOARD FACTORY
This plant uses a Model KP-16 press to dewater screen rejects and clarifier sludge.

2008 SAICA, Zaragoza, Spain
SLUDGE DEWATERING AT A SECONDARY FIBER MILL

2008 Alabama Rivera Pulp, Perdue Hill, AL
DEWATERING KNOTS
A ModelVP-16 screw press is used to dewater knots at this pine mill. The press
cake is conveyed to the chip yard.

2009 Norampac, Niagara Falls, NY
DEWATERING WWTP SLUDGE
This mill has rented a Model KP-16 de-watering sludge.

2009 University of British Colombia, British Colombia, Canada
LABORATORY USAGE
The University has purchased a Model CP-4 press for laboratory and pilot
operations at the Pulp & Paper Centre.

2009 PCA (Packaging Corporation of America), Filer City, MI
DEWATERING COMBINED TERTIARY SCREEN REJECTS AND PRIMARY
CLARIFIER UNDERFLOW
This mill has installed a pair of Model VP-16 presses. An initial objective was to
reduce landfill expense. Permitting was sought to allow the cake to be sold for a
biofuel project. Additional savings were realized by the elimination of polymer
usage. A biological waste stream is to be blended into the feed to the press.

2009 Hutamaki, Sacramento, CA
WWTP OPERATION
This secondary fiber mill is using a Vincent Fiber Filter to pre-thicken a dilute
flow of extractor pressate ahead of a Model KP-6 screw press. This has resolved
problems with a minor waste stream and provides a saleable by-product which is
shipped to a plastics recycler.

2009 Tama Paperboard (Caraustar Industries), Tama, IA
DEWATERING CLARIFIER SLUDGE
This mill is using a rental Model KP-10 to de-water sludge and Poseidon rejects.

2009 Kimberly-Clark, New Milford, CT
FILTERING CLARIFIER UNDERLFOW
This mill purchased a Fiber Filter FF-6 to separate plastic flakes and beards and
crumbs from a clarifier overflow.

2009 Kimberly-Clark, Beech Island, SC
FLOW FILTRATION
This mill is using a rental Fiber Filter FF-6 to clean up short fibers and stickies
from a stream. The sludge from the Fiber Filter is then de-watered in a Model CP-
4 screw press.

2009 Lydall Filtration/Separation, Rochester, NH
DEWATERING SLUDGE
This mill is using a rental KP-10 for dewatering sludge. This unit is replacing a
PT&M press.

2009 Glatfelter, Spring Grove, PA
DEWATERING LIME GRIT
This mill has a specially modified CP-4 which is used to de-water grit produced
when they slake their lime.

2009 PCA (Packaging Corporation of America), Counce, TN
DEWATERING REJECTS
A rental KP-16 has seen extended service dewatering screen rejects at a recycle
mill.

2009 Weidmann Electrical Technology, St. Johnsbury, VT
DEWATERING SLUDGE
This mill purchased a CP-12 press for dewatering sludge. The mill produces paper
insulation for electrical transformers.

2010 Bataan 2020, Philippines
DEWATERING WWTP SLUDGE
A Model CP-12 is giving excellent results dewatering cake from belt presses at the
wastewater treatment plant. The flow is pre-thickened with a sidehill screen.

 

Pressing Dilute Flows

November 30, 2005                                                                                                                                                                                              ISSUE #167

Occasionally questions arise about feeding a screw press with extremely dilute flows. We have one good set of data that involves this condition. They come from an OCC (old corrugated container) recycle mill, Liberty Paper, in Becker, MN.

They fed a flow of 260 gpm into a Vincent Model VP-16 press. This flow was mill effluent with a solids consistency of only 0.7% (7,000 ppm). This represents a feed of 11 tons per day of dry solids going into the screw press. The result was that 4 TPD,DS were captured and came out as press cake with 45% to 50% solids. The press liquor (effluent) from the press had about 4,500 ppm of solids. Thus the capture rate of the press was about 35%.

This operating condition occurred only during mill shut-downs because normally the feed to the press was in the range of 2% to 3%. (The city objected to the 4,500 ppm discharge periods, so at the time Liberty had to add sidehill screens ahead of the screw press, just to cover down periods.)

Thus it is seen that the press does not become inoperable due to extremely low consistency feed. Even with straight water going into the press, water will not purge from the solids discharge end of the press. On the other hand, the capture rate does go down significantly.

The Smurfit mill in Wabash, Indiana tested this to the limit in 1994. They ran the press normally for a while and then replaced feed flow to the press with a firewater hose. The press was run this way in order to confirm that a plug of fiber at the solids discharge would hold, preventing any water from coming out the solids discharge end of the press. All of the fire water came out through the screen. Normal press operation resumed automatically when the normal flow was re-admitted to the press.

(The other extreme of this same test was to have the press in normal operation and then switch the flow into press to cake from the press. That is, the press was fed only cake with 50% solids. This material passed through the press without the press tripping out on overload or damaging itself. Negligible press liquor came through the screens when operating in this manner.) 

Sidehill Prethickening

July 22, 2004                                                                                                                                                                                                      ISSUE #151

Many flows require prethickening before they can be dewatered in a screw press. If the feed to a screw press is too dilute, two problems are likely. The obvious problem is that the press may not have sufficient open screen area to pass all the water. The press can become limited by its hydraulic capacity rather than the solids discharge capacity. This can be avoided with either a larger screw press or some form of prethickening. The prethickening option almost always costs less.

A more subtle problem associated with dilute flow in a screw press is that the solids capture rate will drop. The reduced capture of solids occurs because the velocity of water passing through the screen increases to where the solids particles are swept through the screen. This is apparent in manure dewatering applications: the solids capture rate with scraped manure barns is 50%, while it drops to only 25% at flush barn installations.

In general, the least expensive prethickening device is a sidehill screen. (These are also known as gravity, sloped, and static screens.) Sidehills are inexpensive, and they have the advantage of having no moving parts. However, sidehills get a bad rap.

Engineers and operators will frequently voice objections to sidehills because they are not consistent. It is common for them to flood when the flow goes up or when the screen surface becomes dirty and blinded. This changes the consistency of the discharge sludge, which may cause problems in downstream equipment.

If the downstream equipment is a screw press of the continuous flighting design, yes, there are apt to be problems. A reduction in feed consistency to a screw press of this design can cause the press to produce wet cake, or even purge, if its speed is not reduced.

In contrast, the Vincent screw press, because of its interrupted flighting design, works very well with sidehill prethickeners. The gaps in flighting between each of the compression stages of the screw must fill with solids of some consistency before material will move toward the discharge of the press. Consequently, even if the feed consistency decreases due to flooding of the sidehill, the Vincent screw press will continue to produce discharge cake with a steady solids content. No adjustment or change in speed is required.

This advantage of the interrupted screw design has been demonstrated most vividly in the pulp and paper industry. A significant number of installations, in both virgin fiber and recycle mills, have now been operating for enough years to offer convincing evidence.

Pulp & Paper I

It was in 1994 that Sales Representative Gary Gann came to us with a chance to test in a paper mill. His customer, a major recycle paper mill, had a severe problem because their primary and secondary sewage treatment plants were no longer able to carry the load. Would we be able to filter the waste stream?

Pulp & Paper II

It was in June of 1994 that Vincent was invited to their first on-site testing in a paper mill. This has led to a major change in the Company: today more than half the presses being built are going to the pulp and paper industry.

Paper mills fall into three main categories:

• The virgin fiber mills where digesters cook wood chips to make paper pulp
• Recycle mills where waste paper is pulped by agitating it in a water bath
• Mills that buy market pulp from other mills and convert it into their own specialty papers.

Vincent has screw presses operating in all three of these industry segments. In most cases our press is being used to dewater waste streams.

The waste streams generally have consistencies in the range of 1% to 12% solids. They are dewatered so that the solids become a bulky material that can be readily handled. At the same time the pressate liquid frequently becomes clear enough that it is acceptable for re-use in the mill.

The waste streams that can be dewatered in a Vincent press consist of various forms of screen rejects. In a virgin fiber mill rejects come both before and after bleaching. Ahead of bleaching we deal with brown stock rejects: knots from the knotters, and shives (along with a fair percentage of good fiber) from the tertiary or quatenary pressure screens.

In a recycle mill a variety of screenings are used to separate unusable fiber along with dirt, ink, plastic, etc. All of the waste streams will contain some measure of good fiber.

The press cake will typically be in the range of 40% to 50% solids. The lower solids content is adequate for easy handling and transport, while the higher solids material might be used for boiler fuel. (In the boiler fuel application the press cake is combined with 80% to 90% coal.) Frequently the press cake is landfilled. However occasionally the press cake is sold to mills capable of using the material in the production of lower grade pulp products. Other uses include filler for tar paper and shingles as well as landscaping mulch.

This is where we find the market.

The most common inquiry from the pulp and paper industry involves pressing clarifier sludge at the wastewater treatment plant. We have found that our press will usually work well on this material. This is especially true if there is little or no biological (secondary treatment) sludge present. Our competitors in this market are the immense continuous screw presses offered by Andritz and FKC.

With our press design we have found that the best performance is achieved if the clarifier sludge is prethickened with a gravity table or a belt press. To address this opportunity we have established a relationship with Phoenix Process Equipment Company, a major supplier of belt presses to the pulp and paper industry.

Future issues of Pressing News will detail our applications in both virgin fiber and recycle mills.

Sawdust

Largely because of action by environmental groups, the cost and availability of timber in the northwest has become a problem for paper mills. Paper makers now make paper that contains fiber extracted from lumber mill sawdust. Because lumber mills normally burn their sawdust, it represents an economic source of raw material for a pulp mill.

In 1996 this situation led to pressing trials using sawdust. The tests were conducted in Tampa with engineers from Fort James trying out three different Vincent screw presses.

The thin, high speed saw blades used at the lumber mills are cooled with water. This process increases the moisture content of the sawdust slightly from the normal 52% found in green lumber. Rainfall adds even greater moisture content. The samples tested in Tampa had 65% moisture (35% solids).

The normal press configuration was found to produce press cake with 42% solids. The horsepower required was in the high range for Vincent presses. Since the Fort James specification called for 45% to 48% solids, a special screw design was used in subsequent testing.

It is noteworthy that the pressing action is only removing free water. We are not breaking open the cells of the wood. A competitive press requiring 3,500 hp could achieve solids closer to 60%. However this broke down the cellulose fibers, reducing the quality of the paper being produced.

Therefore, the final contract called for two VP-30 presses, each with 300 horsepower drives. These subsequently were modified to 400 horsepower. The bulk density of the sawdust usually is low, about 22 pounds per cubic foot. This is compressed dramatically, to the point where water is expelled, by changing the screw shaft diameter just ahead of the first resistor tooth. The pair of presses can handle approximately 100,000 pounds per hour of sawdust.

The presses were supplied with an unusual feature: The resistor teeth were drilled for the possible addition of steam directly into the sawdust as it is being pressed. This technique was used many years ago in both fish meal and citrus peel to achieve lower cake moisture. To date it has not been used at the paper mill.

Unfortunately, the need at the Fort James Clatskanie mill is unique. They have an overloaded sawdust digester. The arrangement does not allow for the addition of sufficient steam when the sawdust is wet. Thus the capacity of the digester drops from 300 TPD in the summer to 240 TPD in the winter rainy season.

Another problem is that James River in Clatskanie has a limited amount of white liquor generating capacity. The excess water in the sawdust requires extra white liquor to overcome a dilution effect.

On the other hand, improvements in the material handling system appear to have resulted in increased digester capacity. Simply being able to feed more tonnage of the damp sawdust into the digester may have been all that was required.

The installation of the VP-30 screw presses never quite succeeded, primarily due to stick-slip vibration that occurs. This vibration occurs when tight pressing is attempted. A variety of mechanical failures have resulted from the vibration, in spite of numerous modifications.

Shower Water Filtration

October 25, 2001                                                                                                                                                                                                 ISSUE #122

Tests were recently conducted using a Vincent Fiber Filter on shower water at Atlantic Packaging in Scarboro, Ontario. The Fiber Filter was selected because of its small footprint and attractive capital cost compared to conventional filters.

This recycle paper mill has a combination OCC (Old Corrugated Container) and tissue operations. The most important use of shower water is on the felt of paper machines. Other applications are on trommel and rotary drum screens.

The primary source of shower water comes from disc strainers used at the end of the stock prep process. The disc strainers raise the paper pulp from 1.2% to 12% consistency. There are three effluents: cloudy, clear, and super clear. The cloudy effluent is pumped back to the start of the process at the hydrapulpers. The clear effluent is re-used half way back in the stock prep process, with the excess going to sewer.

The disc filter itself uses some of the super clear; the rest is filtered prior to use as shower water. The existing super clear water filters are Sweco machines that feature a vertical axis with a spinning drum screen. The drums have fabric panels backed up with coarse mesh steel screen. The drums are about 4' in diameter.

These Sweco's are very old and have served their purpose; maintenance expense has become excessive. Thus our Fiber Filter is being evaluated as a potential replacement.

The very low solids consistency of the super clear water resulted in high capacity in the Fiber Filter even with very fine fabric sleeves. With a feed consistency of 70 ppm, using a 31 micron sleeve, we were able to produce filtrate with 20 to 25 ppm. This was done with the Model FF-12 being fed about 200 gpm and producing about 1 gpm of fiber-sludge discharge.

A goal of the project is to eliminate the need for addition of city water make-up to the shower water tank.

There is a rule-of-thumb used in selecting filters used on flows ahead of spray nozzles: The filter basket should have openings one tenth the size of the openings of the shower nozzles. This is achieved with the 20 and 31 microns sleeves available for use with the Fiber Filter machines.
 

Spray Chamber Dust

Everyone is familiar with Formica counter tops. This material is made from ground wood dust that is combined with phenolic resin. The material is heated and pressed to produce a rigid, homogeneous, void-free material. In addition to counter tops, the material is used for wall paneling (Masonite board) and furniture. Vincent Corporation buys rods and tubes of this material, called Ryertex, to manufacture certain bushings and screw conveyor hanger bearings.

Stock Thickening

The best Pulp & Paper application for Vincent presses has been to squeeze as much water as possible from reject fiber. Typically we are working with screen rejects, knots & shives, and clarifier underflow. If the waste solids are to be landfilled, they are squeezed to only 35% to 40% solids, minimizing abrasive wear within the press. If the waste is to be used as boiler fuel, then the discharge pressure of the same press is increased and cake with 50% solids is achieved.

TAPPI - Pressing Knots & Shives

Tappi Journal, Vol. 82, No. 2, February 1999

ADVERTISING PAYS!
The project described in this article all started with an ad in The Tappi Journal. The ad was brought to the attention of Joe Lukasik, an engineer at the Atlanta office of Sandwell Inc. At the time Sandwell was working on a fiber recovery project at the James River Naheola Mill in Pennington, Alabama. Through Sandwell, arrangements were made for on- site testing. The success of the trials ultimately led to the four installations described in this article.

An unusual project with many advantages has recently come on stream at the Gilman Paper Mill in St. Marys, Georgia. The installation features a pair of screw presses that are used to dewater quaternary screen rejects. This technology parallels a similar project at Fort James Naheola Mill, with interesting differences.

The Gilman mill, rated at 1,200 tons, has been in service since 1941. It has both hardwood and softwood kraft operations employing thirteen batch digesters. About half of the tonnage goes to bleached and unbleached kraft multiwall specialty products, with the other half going to bleached board, coated and uncoated.

Similarly, the Naheola Mill, rated at 1,100 tons, has both hardwood and softwood kraft operations. About half of the pulp produced in converted on-site to tissue and towels, while the rest is used for packaging board.

At both mills the screw press project was part of a new fiber preparation system that features Thermo Black Clawson pressure screens. At Gilman, the principal benefit of the new system is improved stock quality to the #1 paper machine, while at the same time converting waste materials into useful boiler fuel. In contrast, at Fort James the principal benefit is increased chemical and fiber recovery, while at the same time reducing loading at the wastewater treatment plant.

At Gilman prior to the installation of the new pressure screens and screw presses, rejects from secondary screens were refined and pumped to the blow tank. This was undesirable because it resulted in poor stock quality to one of the paper machines.

In contrast at Fort James, prior to the installation, insufficient quaternary screening was available. Rejects from vibratory (secondary) knotters were accumulated at ground level prior to hauling to landfill, and rejects (mostly shives) from the quaternary pressure screen were diverted to the wastewater treatment plant. This system had poor yield and excessive loss of digester chemicals.

With the new systems, both mills use Vincent screw presses to dewater combined flows of knots, shives, and other rejects.

At Gilman the new system uses a Vincent sidehill screen to thicken the 160 gpm 2.3% consistency flow ahead of each of the screw presses. The screen tailings are funnelled directly into the 12" presses. At Fort James, in contrast, the flow of quaternary rejects and knots from the knotters flow directly to a 16" press without benefit of a sidehill screen. Surge flows of up to 400 gpm go to the press.

Both mills use a hot stock screening system. The rejects from the unwashed stock, at 190º F, are what go to the screw press. The press liquor, containing chemicals and some usable fiber, is returned either to the rejects tank supplying the pine side secondary screen or to the chemical and fiber recovery flow.

The horizontal screw press has two screening sections: at the inlet hopper and over the compression stage. An initial knot dewatering application supplied by Vincent Corporation to Fort James Naheola mill had profile bar screens (baskets) with nominal 0.020" (one half millimeter) slots. To optimize chemical and fiber recovery these were subsequently changed to 3/32" perforated screens. Based on this experience, the Gilman presses had press screens with 3/32" perforations from the start.

It was found at Fort James that in normal operation the inlet hopper screen of the press allows free liquor to drop out, which consists of up to 40% of the total pressate flow. In addition an unexpected source of liquor recovery was found to arise from pressing the knots. Testing showed that 20% by weight of knotter rejects is converted into press liquor in a Vincent screw press. It is estimated that this liquid flow amounts to 10% of the total chemical recovery, as well as some of the fiber recovery.

The screw presses were designed specifically for the pulp and paper industry. All contact parts are made of stainless steel (316 at Gilman; 304 at Fort James), and weld applied hardfacing is used on the wear areas of the screw and discharge cone. Heavy duty drive and screw flighting allow the press to operate under conditions of a "hard cook" when material similar to ground wood enters the press. Vibration was minimized in the Gilman presses by specifying that the drive motor be mounted in line with the gearbox. Since this eliminates V-belts from the drive train, a variable frequency drive (VFD) is available. Gasketing compatible with H2S was employed, and all bronze materials (nuts and bushings) were eliminated.

Gasketed covers were supplied with provision for modification for vacuum recovery of vapors. This is in anticipation of future requirements for collection of Total Reduced Sulphur (TRS) emissions.

In passing through the press, the knots are broken into small bundles of fiber. This occurs because the press design is based on an interrupted screw flight with stationary resistor teeth. The agitation and shear caused by these members break down the knots. Some fiber recovery improvement is achieved as a result of this action. It is difficult to spot the difference between the knots and the shives in the press cake.

The press cake moisture can be controlled by adjusting the air pressure actuating the discharge cone (also called a stopper or plug). With this devise press cake moisture in the range of 45% to 55% solids is maintained. In mills where the cake is landfilled, lower cone pressures and higher cake moistures are typical.

At Gilman the press cake is made of elements that previously were refined and recirculated, while at James River they were sent either to landfill or wastewater treatment. With the new installations the rejects are used as boiler fuel at both mills.

Similar installations exist at two other paper mills. At these the press cake is being sold as landscaping mulch and as raw material for an asphalt shingle manufacturing operation. Vincent has supplied screw presses for use on shives to Louisiana-Pacific, Samoa, California and for knots and shives to Buckeye Cellulose, Perry, Florida. The motivating factors at these mills were to facilitate off-site hauling, to avoid landfill problems, and to extend landfill life.

The success of these installations has generated interest in pressing knots and shives. The projects are relatively small and simple. Incremental improvements in pulp making and abatement of environmental pressures are the benefits.

Ross is Chief Engineer, Gilman Paper, 1000 Osborne St., St. Marys, Georgia 31558; and Johnston is Professional Engineer, Vincent Corp., 2810 East 5th Avenue, Tampa, FL 33605.

The flow of hot black liquor being squeezed from the quaternary rejects is seen dramatically when the screw press covers are removed.

One of a pair of Vincent Model VP-12 presses. Tailings from sidehill screens on the floor above drop through the vertical chute into the press.

Press cake drops to a concrete bunker at ground level prior to being transported to the hog fuel boilers.

This press cake typically has 50% solids and has excellent combustion characteristics.

TAPPI - Pulp & Paper Waste Dewatering

Tappi Journal, Vol.78, No. 12, December 1995.

Prepared by Thomas H. Manley, Plant Engineer, Boxboard Mill Division, Jefferson Smurfit Corporation, Wabash, Indiana; and Robert B. Johnston, P.E., Vincent Corporation, Tampa, Florida.

Screw presses installed at the Jefferson Smurfit boxboard mill in Wabash, IN have significantly decreased the load on the wastewater treatment facility and facilitated the capture and disposal of fines in the primary clarifier sludge.

These benefits were achieved by reversing the position in which screw presses are normally used. In the typical installation, the screw press goes at the end of the cycle, receiving the sludge from clarifiers and/or DAF systems. At Jefferson Smurfit the presses were instead placed to receive reject material flows ahead of the clarifiers.

Higher than anticipated reject rates from the mill's cleaning systems had increased the load on the existing wastewater treatment facilities. Conditions reached a level where, during upsets, unacceptable discharges could occur. Resolution of this problem was necessary to ensure continued compliance with the mill's NPDES permit.

Screw presses offered key advantages. They operate continuously through wide swings in flow rate and solids concentration; they operate unattended; and they require minimal maintenance.

Background

Established as a recycle mill in 1892, today this plant specializes in producing high quality boxboard. Typical end uses include breakfast cereal boxes.

The nominal mill capacity is 365 TPD. Basic machinery includes six Hydrapulpers and two paper machines: a 96" ten cylinder (400 fpm) Multiply and a 120" eight unit (500 fpm) Ultraformer. Both machine coated and uncoated combination boxboard is produced.

Wastewater Sources

There are a great many point sources of wastewater in the plant. Important ones include pulper detrashing screens, pressure screen rejects, unclaimed cooling water, and tank overflows. Rejects from the pulp cleaning system are the focus of this paper. These rejects are pumped directly to a screw press. They include forward cleaner rejects as well as fine and coarse screen rejects.

Large trash from the Hydrapulpers is removed continuously by a continuous scavenger system. This bulky material is moved by conveyor to a dump hopper.

Wastewater Treatment

Primary wastewater treatment is performed in an Infilco clarifier.

The secondary treatment plant is an activated sludge system. It is physically located on an adjoining property. Originally it was operated by the City of Wabash, treating both mill and municipal wastewater. It consists of three rectangular aeration basins, two rectangular digesters and three final settling tanks. The water is discharged into the Wabash River in accordance with an NPDES permit.

Polymer is added to sludge that is pumped from the Digesters. This sludge is then dewatered on a belt press. The belt press requires an operator on each shift, and it is generally regarded as a high maintenance machine.

To minimize the tonnage or cubic feet going to landfill, 40% or higher solids is desirable in the press cake. The belt press used at the secondary treatment plant can achieve only 30% (approximately). Although screw press material, due to its characteristics, can be spread with solids up to 50%, the belt press cake material cannot be spread at consistencies above 30% solids. This is due to the operation of the feeders on the trucks that are used to landspread the press cake.

There are various practical and theoretical means of disposing of sludge from the belt press. One of the most economic is land application: farmers accept the material without charge because of its benefits to the soil, and the farm acreage in the immediate area of the plant currently supports these operations.

Placing the press cake in a landfill was very economic in the past. However with the decline in landfill sites in the immediate area, plus regulations applied to landfill operations, this disposal option has lost favor.

Additional potential future disposal means are under review. The sale to other business operations is especially attractive. Potential buyers include paper recyclers capable of using the fiber that is rejected at Wabash because of stringent product specifications. Also, it is recognized that material that is dewatered to approximately 50% moisture might be used as a boiler fuel by blending with coal. A final option under review involves coal mine reclamation activities.

Press Application

Most waste water streams from the mill are combined ahead of wastewater treatment. The combined flow is pumped across a bank of inclined screens to remove long fiber prior to entering the primary treatment. The long fiber is returned to the mill for re-processing.

Reject streams from forward cleaners and pressure screens do not pass over the sidehills. Instead they are fed directly into a pair of screw presses. Filtrate water from the screw presses flows to the primary clarifier. Excess clarified water then overflows to the secondary treatment plant.

The screw presses generate 7 to 21 dry tons per day of press cake at up to 50% moisture.

It is important to note that the wastewater treatment facility does not have to handle this tonnage of solids. By capturing the solids with a screw press, a significant reduction of load on the wastewater treatment plant is achieved.

Capture of clarifier sludge in a screw press is difficult. There is a tendency for the fines (clay or ash) to blind the screens of the press, which results in drastically reduced press throughput capacity.

The operation results in screw press filtrate water with 500 to 1000 ppm solids. This range of solids is within an acceptable range for treatment and capture in the secondary treatment plant.

Selection of a Screw Press

Six presses by four different manufacturers were tested on-site with varying results. In the end a design manufactured by Vincent Corporation was selected. The design is a modified version of their standard citrus press, a machine used in converting orange peel into cattle feed. The modifications were required because, while wet fiber dewaters much more readily than citrus peel, it is much less compressible once the free water is removed.

During the trial operation, efforts were made to develop a set of specifications for the screw presses. This effort began with a focus on normal technical details such as gpm capacity, horsepower requirements, press cake moisture and screw diameter. This proved unsatisfactory because of the very wide range of flow rates and solids concentrations that were encountered. The varying nature of the inbound flow (easy to press fiber as compared to difficult to press sludge) made the specifications difficult to write.

In the end, the unique purchase specifications were as follows: The primary performance criteria for satisfactory operation of each press are (1) it must not plug or jam and (2) it must not pass large quantities of unpressed liquid into the flow of press cake. The press must operate like a pump: reliably, unattended, and with very infrequent maintenance.

The presses that were purchased have many unique features. For example, it was found that the use of wedgewire screens, as opposed to perforated metal, not only increased physical strength but also reduced the concentration of suspended particles in the press filtrate. Wedgewire appeared to be more self-cleaning than perforated metal.

Accommodating the absolute peak flow under conditions of maximum blinding would have required an excessively large screw press. Rather than purchasing such a large machine, provisions were made to allow the incoming flow to overflow the inlet hopper during the unusual peaks. This overflow is directed back into the treatment system. It is estimated that this overflow provision is used less than 5% of the time.

A pneumatically adjustable cone at the press discharge allows the press to operate satisfactorily over a wide range of flow rates and solids concentrations. If the inbound solids are low, the cone pinches off the discharge to prevent liquid from purging into the press cake discharge. The design of this cone mechanism negated the need for a variable speed drive on the screw press, which represented a significant capital savings.

The presses also feature an interrupted screw flight design, as opposed to a continuous screw. Because the screw is discontinuous, fixed resistor teeth can be mounted to the press frame, protruding into the flow of material inside the screen. This design reduces co-rotation, the condition where material rotates with the screw and nothing either enters or leaves the press. The stirring action by the teeth allows for a shorter machine that requires less horsepower to operate.

To assist in un-blinding the filter screen, presses were acquired that have a wiper-brush mounted on the screw auger. This clears blinding material from the screen surface. The feature assists operation during periods of high sludge content.

Maintenance requirements also guided the press selection process. Presses were purchased with all contact parts made of T-304 stainless steel, which specification will minimize maintenance requirements over many years. Similarly, presses in a horizontal configuration were selected because of the ease of disassembly in the event of screw, screen, drive, or cone maintenance. Finally, the presses selected make use of standard OEM gear boxes, bearings, seals, etc., which further reduces maintenance expense over the long run.

Results

The principal result of the installation of the screw presses has been to relieve solids loading on the wastewater treatment facilities. A recent expansion of the mill cleaning system had resulted in serious overloading of reject material going to the wastewater plant. With the addition of the screw presses this condition has been resolved.

One side benefit of removing such large quantities of solids ahead of the treatment plant has been a reduction in the amount of sludge to be belt pressed. The sludge from this source has been reduced from 1,000 to 600 dry tons per month.

Results from when the wastewater treatment plant was at times overloaded to conditions following the installation of the first screw press have been compared. The analysis shows that suspended solids were reduced from an average of 75 mg/l (or ppm) to 25 mg/l.

BOX INSERT

TROUBLE SHOOTING GUIDE

During an extended period of trials at Jefferson Smurfit, numerous problems were encountered. These included:

1. Overload and Trip-Out. This was apt to occur when pressing too tight. Typically it was a consequence of feeding thick stock to the press during upset conditions. Solutions included providing for dilution water at times of high amperage draw and oversizing the press.
2. Purge of Liquid in the Press Cake. Some presses had difficulty when insufficient fiber was present to form a press cake. Pre-thickening can relieve this problem. The design of the press discharge cone is very important.
3. Screen Blinding. The screens of most presses tend to blind on clarifier sludge. It appears that platelets of clay in the sludge bridge the openings in the screen and prevent the flow of liquid through the openings. This problem is even more pronounced if biological (secondary treatment) sludge is present. At times the addition of fiber waste will act as a press aid and wipe the screen clear. Also, pre- thickening with a belt press can help. Alternatively, the use of a low rpm continuous screw press will address this problem.
4. Excessive Solids in Press Filtrate. With certain screen configurations, the parts per million of suspended solids in the press filtrate were found to be ten times greater than the acceptable range. The use of wedgewire screens resulted in the best performance.
5. Interrupted Operations. Some presses required operator attention before satisfactory operation was achieved on a re-start. This occurred under conditions such as a period of no incoming flow followed by resumption of mill operations. Dilute flows during flushing and wash-down can also require operation attention to the screw press. The machines at the Wabash boxboard mill were selected to be able to handle these swings without adjustment.
6. Screw Wear. Excessive abrasive wear was noted on press components such as the screw flights and discharge cone. This is addressed with the addition of hardfacing in the wear areas.

Venezuelan Paper Mills

In December Vincent conducted a screw press workshop and presented a paper at a Pulp and Paper conference in Maracay, Venezuela. The trip offered a chance to study the pulp and paper industry typical of many small countries.

Virgin Fiber Mills

Paper mills that process chipped logs and sawmill waste into paper are located in the woodlands of the northern and southeastern United States.

Wax Coated Box Recycling

Recently Thermo Black Clawson of Middletown, Ohio has started promoting a new, patented recycling process. The system, trade named Xtrax, is used to recycle wax-coated corrugated containers. Inland Paperboard and Packaging Inc. is a partner with Thermo Black Clawson in the development.

Chilli Peppers

Some plants and factories pay a municipal sewer surcharge based on the quantity of suspended solids in the wastewater from the plant. We have worked with food processors, plastic recyclers and even a paper recycle mill that operate on this basis.

Deinking

Deinking Mills are a major specialty among paper recycling mills. The principal raw material categories for these mills are newsprint, magazines, telephone directories, and mixed office waste. There are many sub-specialties. For example, some mills will recycle only newsprint, while others will mix magazines with newsprint. This adds fiber strength and brightness, but it requires the addition of equipment to screen ash from the furnish. (Ash is mostly the kaolin clay coating used in glossy magazine paper.) MOW (Mixed Office Waste) mills use the least expensive material; however, it requires special equipment for removing a broad range of contaminants such as Styrofoam and stickies (contact cement).

Dilute Flow Filtration

Recently we tested the Fiber Filter at Liberty Paper in Becker, Minnesota. This is a modern OCC (Old Corrugated Container) recycle mill. Rated at 300 tons per day, the mill specializes in producing linerboard. The test machine was a FF-6, the smaller version of the FF-12 machine that would be required to satisfy the mill loading.

We were working with dilute feed flows, .02% to 0.20% solids consistency, and remarkable success was experienced. The results broaden the market for Fiber Filters from deinking to include most recycle paper mills.

The applications tested were (a) filtering press liquor from Vincent screw presses that are used on the mill wastewater stream; (b) filtering the flow of wastewater ahead of the clarifier; and (c) filtering the final wastewater flow that goes into the city sewer system.

Feed:  Press Liquor from Vincent VP-16's screw presses.  These are used on the mill wastewater stream after it has been across sidehill screens.
          Feed:          .06% consistency      620 ppm TSS *
          Filtrate:      .02% consistency      330 ppm TSS *
          Cake/Sludge:  3.0% consistency

Clarifier Feed Chest
          This includes sidehill screen filtrate, press liquor, and other waste streams.
          Feed:          .15% consistency   1,970 ppm TSS *
          Filtrate:      .04% consistency     460 ppm TSS *
          Cake/Sludge:  4.1% consistency

These two tests were run with an 86 micron sleeve in the FF-6, very high elevation adjustment, 30 to 60 gpm, no VFD.

* The differences between percent solids consistency and parts per million of total suspended solids arise from using two different laboratory procedures for measuring the same thing.

We also ran tests on the city sewer flow. The results were surprising:

City Sewer Water:

• Feed: 204 ppm
• Filtrate: 148 ppm

This test was run with a 43 micron sleeve, very high elevation adjustment, 90 to 120 gpm. Their normal flow is 400 gpm, so a FF-12 may carry it all with a 31 micron screen. The cake/sludge we collected had sand and grit, with some fiber.

The cake/sludge from the Fiber Filter would be fed back into the screw presses.

Two alternate projects are being reviewed for potential financial justification:

• Filtration of clarifier feed so as to reduce polymer consumption in the clarifier.
• Filtration of final flow to the city sewer so as to reduce sewer surcharges.

Fiber Filter Operating Hints

• Volume of inbound flow
• Angle of inclination of the rotor
• Cleanliness of the fabric sleeves
• Mesh of the fabric sleeves
• Rotational speed of the rotor

Fiber Filter Patent

Here at Vincent we are proud of the award on September 12, 2000 of United States Patent number 6,117,321. It describes an EXTERNAL TENSION ADJUSTMENT DEVICE FOR A FILTERING SLEEVE IN A FILTERING MACHINE. This came as a result of our work over the last three years with the Fiber Filter.

Fiber Filter for Pectin Recovery

An interesting application for the Fiber Filter has evolved over the last few years. The project started as a rental at a plant whose wastewater treatment plant (WWTP) was causing excessive odor.

Maximizing Capture Rate

The unique fine filtration and capture rate characteristics of the Vincent Fiber Filter have resulted in successful operation in pineapple juice and paper mill shower water applications. The same principle is used in both these two very different installations.

Meat Flavor Extracts

Ariaki U.S.A., a Virginia firm, has been running a new oil recovery system. The elements of the installation include a Model KP-16 screw press, the press liquor from which flows directly into a Model FF-6 Fiber Filter. Waste materials from the plant are run through the system in order to recover oils.

Peach Cannery

A lead from the 1999 IEFP (International Exposition of Food Equipment) show led to the placement of a Model FF-12 Fiber Filter at the Del Monte peach cannery in Kingsburg, California. This eighty-year-old cannery operates during a three month season each year.

Shower Water Filtration

Tests were recently conducted using a Vincent Fiber Filter on shower water at Atlantic Packaging in Scarboro, Ontario. The Fiber Filter was selected because of its small footprint and attractive capital cost compared to conventional filters.

Bagged Feed Storage

One of the largest citrus processors in California uses an interesting technology to dispose of their orange peel. Like their Florida counterparts, they run the citrus peel through screw presses, getting the peel from 82% down to 70% press cake moisture. However, they have no dryer or WHE (waste heat evaporator) for processing their peel.

Chitosan (Shrimp)

Chitosan has been receiving considerable attention as a natural diet additive. It absorbs fat and oil, so it is deemed to be good at reducing cholesterol levels and helping a person to control weight. It is also used commercially in such products as swimming pool cleaners.

Chitosan is found in the exoskeleton of insects and crustaceans. The commercial product is made from shrimp and crab shells. Vincent has worked in plants in Washington and Iceland, and the material is now being produced in India.

Chitosan is produced from shrimp waste by a multistage process. First the salt is washed out and the material is shredded. Then the protein, which makes up 30% of the waste, is removed in a hot bath. The calcium, 50%, is removed with acid. After caustic neutralizing the remaining material, chitin, is first pressed and then dried in a rotary drum dryer. Another process is used to convert the chitin into chitosan. This process, too, involves pressing water from the material and then drying it.

Vincent screw presses are used in three steps of the extraction process. First, the raw shrimp waste is dewatered in a Series KP "soft squeeze" press. The other two applications are ahead of the chitin and chitosan dryers. Here the tighter squeezing Series CP or VP presses are needed for maximum dewatering.

Pressing ahead of the dryers is difficult. The material is very slippery and it has a high propensity to bridge, even inside a screw press. At the same time it can be over-pressed so that it jams the press. (In trials with a Stord twin screw press, the material jammed so tight that both screws broke in half!)

Recently we assisted pre-start-up trials at a plant in Iceland where our presses are used ahead of the dryers. Many modifications were made in order to improve the press operation. Changes included:

• Cutting the resistor teeth in half.
• Removing the resistor teeth on one side of the press.
• Switching between perforated screen and profile bar screens.
• Both standard and Sterile Butterfly screws were tested.
• Sharpening the leading edge of the flights.
• Beveling the resistor teeth.
• Adding a stripper in the inlet hopper.

We tried hard piping the flow and forcing material into the press with a progressive cavity pump. The material jammed so tight the pressure went to 80 psi and all the joints started to squirt.

Both thickening and thinning the material entering the press were tried. A static screen was used for the thickening, and a water jet wash used in the inlet hopper for thinning.

A VFD drive allowed testing over a wide range of press speeds.

The feeder flighting was notched at the point where it leaves the inlet hopper, and the stripper was extended downward, like an additional resistor tooth.

A Series KP press was flown in so as to test rounded teeth and only three stages of compression. At this writing Teflon sheet material is being bonded in the inlet hopper.

In some instances it has been found that the effectiveness of the press is tied to the pH of the shrimp material.

One thing has never failed: if the press stops processing material, it is always possible to resume operation by reversing the direction of rotation for a few seconds before proceeding.

Despite the difficulties implied by this number of modifications, we now have three satisfied customers.

2001 Addendum: The Twin Screw Series TSP presses have overcome the above described co-rotation problems.

Coffee

Over the years Vincent has had many inquiries in regards to using a screw press with a coffee waste product.

The Vincent press is well suited for dewatering waste streams in the coffee processing industry. This is most commonly done with chaff and with spent grounds in soluble coffee (instant and decaf) operations. The objective is to remove the excess water so as to produce a press cake suitable either for incineration or landfill.

One local coffee bean processor has a Jones vertical press in daily use on chaff off the bean. The press cake goes to a fertilizer company that uses it as a filler.

A large coffee bean processor in Texas is getting 50% to 55% moisture content in the press cake with either a Davenport or a Rietz V-press. These robust machines come in two sizes: 3' and 5'. They consist of two rotating cones with perforated screen on the faces of the cones. The shafts are mounted at an angle to each other so that there is a large gap on one side and a small gap on the other. Material is fed into the large gap and as the cones rotate the material is compressed by brute force until it is allowed to fall out at the small gap: The pressing is very gentle as there is absolutely no agitation. These machines are expensive because of their heavy construction, and they are of low capacity as compared to screw presses. Their maintenance cost is regarded as excessive by most users we have talked to.

These Davenport presses will handle from 4,000 to 10,000 pounds per hour of inbound material. Their press cake will consistently be in the range of 50% to 55% moisture, which makes the press cake suitable for incineration. Some Folgers and Maxwell plants were originally set up for incineration, but most seem to have abandoned the practice or been shut down over the years.

In 1986 Vincent ran tests for Westreco (Nestles) with inbound material that contained 70% to 74% moisture. This was for a waste disposal project involving canned iced tea and a coffee drink. Moisture contents in the press cake of 58% to 63% were achieved with our press. This was regarded as good as was being achieved elsewhere with vertical presses with the same material, but no better. Thus they could not justify buying our press even though they liked its performance.

Recently we received an order for a CP-10 that will be used to dewater coffee grounds for a venture with Starbucks coffee. The press will reduce the moisture content of the grounds to the point where they can be conveyed in a pneumatic vacuum system.

In summary, we would not recommend a Vincent press where the material is to be incinerated. However, we have excellent technology for the dewatering required to make spent grounds suitable for either landfilling or use as by-product.

There is another interesting application of Vincent equipment in the coffee industry. It has to do with one of the first steps in the production of coffee, near the site where it is grown. Coffee growers must dispose of the red pulp that they remove form the coffee bean. This pulp can be converted into a material to be blended with animal feedstuffs. This is achieved by first pressing and then further drying the material.

November 2001 Addendum: Vincent has been selling about one Series CP press per year for dewatering spent coffee.

Coffee Pulp

A layer of soft pulp surrounds coffee beans when they are harvested. This pulp is removed with abrasive rollers and a flow of water. The bean that remains is dried and, later, roasted.

Dole KP Test

The commercial production of salads has grown exponentially for several years. The original large market was fast food restaurants such as McDonalds and Taco Bell. This has now spread to bagged assortments in the public supermarket.

The factories producing these salads produce huge tonnages of waste material. As much as 30% of the as-received lettuce is scrapped. Most factories sluice the waste with water in floor drains. It goes into pits from which it is pumped with chopper pumps. The flow is run across screens, and the tailings are usually hauled to landfill.

Since these tailings are sopping wet, their disposal presents nasty problems of draining in the parking lot, freezing in the dumpster, and acceptance at landfill. The obvious solution is to run them through a Model KP screw press. However, it has been difficult to close a sale because the press liquor from the press is so loaded with vegetable material that the BOD (Biological Oxygen Demand) is unacceptable to most sewer systems.

In recent months a series of tests have been run at the Soledad, California factory of Dole Fresh Vegetables. Unlike most salad factories, Dole conveys their waste material in a pneumatic system. This greatly reduces the amount of water present.

Testing was done with and without shredder ahead of the presses. Two presses were used: a KP-16 with .095" diameter perforated screen, and a VP-6 with 0.030" perforated screen and a rotating cone option.

The results of BOD measurement are important to note. Despite the wide differences in hole size, only 5% more BOD was measured with the KP. In fact, changes in discharge cone pressure had noticeably more effect on BOD than the hole size.

Physical volume reduction is a goal with Dole. The following results were achieved on cabbage leaves, lettuce and carrot peel:

BOD levels were 20,000 to 22,000 mg/l on the press liquor from shredded material, as compared to 16,000 to 17,000 mg/l without the shredder. These are very high levels, but it must be kept in mind that there was very little wastewater present.

Initially the Dennis Jones Group of Salt Lake City, consulting for Dole, was pleased with these results. It was felt that the BOD would be reduced to an acceptable level by dilution into the plant wastewater stream. However, calculations revealed that this would not be the case. The alternative of hauling the press liquor in tanker trucks for land application also proved uneconomical.

If we are to satisfy the market demand for produce and food disposal equipment, a practical means must be found for disposing of press liquor.

Revision May 1998. Dole Fresh Vegetables is very pleased with the results of the KP-16 at their Springfield, Ohio facility. It is operated with a minimum air pressure on the discharge plate so as to minimize solids in the wastewater. It is felt that the loads of waste material are reduced to one quarter of what they would be without the use of the press.

Later Visit: The plant had raised the air pressure so that fully one half of the waste load was going to the WWTP. This cut the hauls per week in half from what it would be otherwise.

Dryer For Mango Waste

Vincent is currently manufacturing a small rotary drum dryer that is destined for the interior of Venezuela. It will be used to convert mango waste into a dry animal feedstuff. This will be done at a small plant that produces aseptic drums of mango puree from a plantation of 15,000 trees.

Egg Shells

Egg crackers make up an industry that not many people are aware of. Their business is to process truckloads of eggs from layer farms into basic food ingredients: egg whites and yolks. These two bulk commodities are sold to food processors who use them as food ingredients. They ship their products in refrigerated tanker trucks containing approximately 5,000 gallons each.

One member of the industry is Daybreak Foods of Minnesota. We visited one of their plants where they have six egg cracking machines. These are contained within the "sanitary" half of their small factory. They crack about 3,000,000 eggs per day.

Banks of suction cups lift the eggs from the shipping cartons. Some cracked and broken eggs are culled by inspectors. Next the eggs are candled: a bright light is shined through the egg to detect internal impurities. This is done to eliminate any fertile eggs containing blood spots (embryos). Following an automatic wash cycle the eggs are conveyed into the room with the egg cracking machines. These are Lazy Susan type machines that automatically crack the eggs and separate the egg whites (albumen) and yolks into two separate flows.

This factory generates about 18 tons of waste material per day. This waste is mostly eggs shells, but it also includes the cull eggs, the no-cracks from the egg crackers, and other "bad" eggs. This material is sent to a landfill.

As an experiment we tried pressing this waste material. One objective was to crush the shells so as to increase the bulk density; this was aimed at reducing the number of truckloads being hauled to landfill.

Another objective, with a much better payoff potential, was to separate the yolk and egg white from the shells. The yolk and egg white "juice" is classified as non-edible since it is unfit for human consumption. However pet food companies will pay $0.045 per pound for the material.

Our press definitely ground up the egg shells. The testing was done with a KP-6 press, and the press was easily overloaded. Consequently we would recommend a Series CP press for the application because it has the screw shaft supported at both ends of the press. [In 2004 this is no longer so.]

In one test we ran 56 pounds shells and got 43 pounds of fine ground shell (press cake) along with 12 pounds of "juice" (press liquor). Unfortunately the juice was (1) very foamy and (2) full of egg shell chips. But it did represent 20% yield by weight (including chips) salvaged for sale to the pet food companies.

In another test we pressed whole cull eggs. The press liquor yield was 85% of the inbound flow.

The foam was not a real problem, but it was felt that the shell bits would have to somehow be screened out.

In the end the use of a press could not be justified by Daybreak. They have installed a drain pan system at the back of the waste truck. It allows the juice to drain into a tank while the truck is being filled. The juice is rather clear, and they screen it through a 3/8" perf sheet followed by 3/32" perf on its way into the tank. The tank holds about 25 gallons.

We did get measurements of flow rate and bulk density improvement. Ten gallons of egg shells from the truck came down to seven gallons of press cake. Flow of this material was 2,000 pph in the KP-6 at 30 rpm. On another flow test results were even a little better!

2011 UPDATE: Be sure to see the EGG SHELL UPDATE newsletter. This market has really taken off!

Egg Shells Update

A lot has happened since we wrote a rather negative Pressing News in 1998 about egg shells.  For one, the plant where we did our testing, Daybreak Eggs, finally bought a screw press from us in 2010.

Literally dozens of KP-6, KP-10 and KP-16 presses have been sold for this application.  In fact, a new model, the KP-12, is being developed as a specialty for egg shells.

The egg shells come from at least four basic sources:  egg inoculators, poultry hatcheries, egg graders, and egg breakers.  The inoculator market rejects infertile eggs, while the graders and breakers reject fertile eggs:  our presses do the same job either way.

The market has been given a boost by new regulations involving inedible egg material.  For example, infertile eggs destined for a hatchery can no longer be sold for human consumption.  A bigger impetus to the market has been regulations limiting the transport of inedible egg material.

The albumen recovered from the egg shells is a high value protein which is used for animal feeds.  Because of its value, it is used in specialty applications such as mink feed, pet foods, and piglet starter feed.  A significant amount is exported from the United States.

As a rule, the crushed egg shells are land applied to farm land where they improve soil properties.  Also, they can be used in specialty applications like litter in horse barns and riding arenas.

The alternative to using a screw press to separate albumen from egg waste is a centrifuge.  These are unpopular due to heavy maintenance requirements.  In addition, they do not crush the egg shells to the extent that occurs in a screw press.

For the last two years Vincent has had a booth at the International Poultry Expo held in January in Atlanta.

MODEL KP-6 FOR EGG SHELL	PERFORATED SCREEN	NOTE PRESS CAKE

Issue 232

Feather Meal

November 9, 1995                                                                                                                                                                                                  ISSUE #35

Modern broiler plants will process 200,000 to 1,000,000 birds per day. With average live bird weights running from four to six pounds, the tonnage of waste material is enormous. All of this waste material (feathers, offal, trims, blood, and DOA's) goes to the By-Products operation. The by-products are mostly animal feed, which is produced by rendering (cooking) the waste materials.

A most important by-product at a rendering plant is feather meal. Feathers constitute 9% by weight of the live bird, and they are rich in protein content. This protein is in a form, like hair, that is not digestible. To make it digestible it must first be converted through a hydrolysis process. Hydrolyzation is accomplished by cooking the feathers with steam. This was previously done in batch cookers; the current technology is to use continuous hydrolyzers.

Hydrolyzed feathers are produced with moisture contents in the range of 45% to 65%. This moisture must be reduced to 8% for the product to have adequate shelf life as an animal feed. Vincent rotating drum dehydrators are used to remove this excess moisture.

In the Spring of 1995 we worked with a rendering plant that had reached the capacity limit of their feather meal dryer. Immediate relief was required because of the economic opportunity being lost.

Trials were run and it was found that their continuous hydrolyzers were producing feather meal at 55% to 56% moisture. This contrasts to another renderer with whom we have worked where the meal leaves their homemade continuous hydrolyzer at 47% to 48% moisture. There is a third western firm with whom we are in contact where the meal has 60% moisture prior to drying.

A series of tests were run using a rental VP-16 screw press. Running the press at 60 Hz corresponds to the standard 13 rpm screw speed. At this speed the press did very little dewatering. It reduced the moisture content of the feather meal by only one or two percentage points.

However when the press speed was reduced to half speed performance improved. Running at 30 Hz, readings of 49% to 50% press cake moisture were obtained. This does not sound like much, but the impact on dryer capacity is surprisingly large.

Here is how we calculate the impact: If we put 2,000 pounds of feather meal at 56% moisture into the dryer, and dry it down to 10% moisture, we end up with 978 pounds of finished meal. This is calculated by taking 44% of 2,000 (which is the pounds of dry solids in the 2,000 pounds), and dividing those solids by 0.9 (which is 1.0 minus the final 10% moisture). To check that answer we can multiply the 978 figure by 90% to see if that is indeed all of the solids in the 2,000 pounds we started with.

We can see that to produce 978 pounds of meal, it was necessary for the dryer to evaporate (2000 - 978) or 1,022 pounds of water.

For the alternative case, let us assume the screw press takes the moisture down to 50%. If we put 2,000 pounds of that material into the dryer, we will be putting in 1000 pounds of dry solids. If this is dried down to the same 10% moisture, we end up with 1,111 pounds of finished feather meal. To produce this meal the dryer had to remove (2000 - 1111) or 889 pounds of water.

We can compare these two results to determine the factor by which dryer capacity is affected. In the first case the dryer must remove 1,022 pounds of water to produce 978 pounds of meal. This calculates to removing 2,090 pounds of water per ton of 10% feather meal.

In contrast, in the second case the dryer needs to remove only 889 pounds of water to produce 1,111 pounds of meal. This works out to evaporating 1,600 pounds of water to produce one ton of 10% feather meal.

Since the dryer is running flat out, it will evaporate the same number of pounds of water per hour in both cases. Thus we can calculate that with the press in operation the production of feather meal increases by 30%!

This is figured by dividing the capacity of the dryer (in pounds of water evaporation per hour) by 2,090. Since the dryer in question has been evaporating 14,100 pounds of water per hour, the production calculates at 6.7 tons of 10% meal per hour. Under the second operating condition, 14,100 divided by 1,600 gives 8.8 tons per hour. This is a production increase of 30% with the same dryer.

Fish

There are two types of fish processing plants where Vincent finds applications. (1) Fish Meal plants which convert trash fish and fish wastes into fish meal, a valuable animal feedstuff, and (2) Plants which process fish such as menhaden and anchovy to recover fish oil as well as produce fish meal.

Once a mainstay of Vincent Corporation, the industry has almost entirely moved offshore in recent decades. The industry decline has been due to a combination of both fish migration to other areas and environmental problems associated with odor and wastewater.

A popular combination in the fish plants consisted of a Vincent screw press, flash (or steam) evaporator and rotating drum dryer. The presses were of carbon steel construction because the fish oil prevented rusting. The flash evaporator was inexpensive, but relatively inefficient and odorous; few of these remain in operation today, none in the United States.

Oil Recovery plants process anchovies and menhaden, among other fish. These are caught in purse seines that are a quarter mile long. The usual catch is 5 to 30 tons per set of nets. A full boat will have 500 to 1,500 tons in its refrigerated hold.

At the docks the fish are pumped into a storage "raw box" and then metered to a live steam cooker. The cooked fish are pressed in screw presses (double pressing has been used successfully).

The press liquor, which has about 10% solids, goes to a centrifuge to separate oil. After the oil is removed the press liquor, which is called "stick water", goes to a steam evaporator. The concentrated stick water is called "solubles" and is sold (at 50% moisture) as an animal feed ingredient.

The press cake is made into fish meal in a dryer without recycle capability. Extensive (triple) cooling is required for the meal coming out of this dryer. Either an antioxidant must be added or it must be shifted from pile to pile because the high protein is prone to spontaneous combustion. It is sold as fish meal, a very valuable ingredient for animal feedstuffs, especially chicken feed.

Oil recovery from menhaden can be 7 to 15% oil. Menhaden is known as the fish the pilgrims planted with their corn seeds; they did this with it because the fish was too oily to eat.

Fish meal is produced as a by-product in the second type of fish processing plant. The facility works with waste materials such as scrap fish from shrimp boats, crab shells, or trim materials and viscera from a larger fish processing plant. The raw fish (whole or pieces) are coarse ground at the start of the process. Small plants run the ground material directly to a rotating drum dryer, while the larger plants will first run the fish through a screw press, followed by a dryer. The fish meal comes out of the dryer at around 10% moisture.

Dryers used to produce this fish meal require the capability to recycle the fish meal; commonly they are small 10,000 #/hr systems. The rotating drum dryer is ideally suited for preparing fish meal because it very efficiently removes the moisture. The key design feature for making whole fish meal is that partially dried material is extracted from the triple pass dryer and mixed with the incoming material. This recirculation assures both that the product will not be overheated and burned, and that the dryer will not plug with sticky substances.

Fish meal has 60% protein or more, with only 5% fat, so it is a valuable ingredient for animal feedstuffs. The final product is fine ground (not pelleted) before being loaded out to a feedmill. At the feedmill, typically 5% is blended into broiler feed.

Garbage

August 20, 2003, Revised November, 2010                                                                                                                                                              ISSUE #141

GARBAGE
(ANIMAL FEED AND BIOFUEL)

At least once a year we are approached by entrepreneurs who conceive a system for converting garbage into animal feed. The garbage in question is frequently restaurant waste, although it may also be institutional or even residential waste.

Invariably these projects collapse under harsh realities.

The request is frequently made for a screw press that will dewater the waste to some unrealistically low percent moisture, like 50% or less. In reality, garbage is 85% to 95%
moisture, and, after pressing, the press cake will still have over 80% moisture content. At that point there is no more free liquid to squeeze out of it.

Specifying a machine to remove more moisture means nothing if a machine cannot do it. Pressure in a screw press will remove only so much moisture. After that, the solids will extrude through the screen like thick milkshake. To remove additional water, a change,caused by chemical reaction or adding heat, will be required.

In order to prevent spoilage, the moisture content of an organic material must be reduced to a range of 10% to 15%. This dehydration must be done in a rotary drum dryer, and the fuel costs will exceed the value of the dry garbage. That is, to produce a ton of 12% moisture material from press cake with 80% moisture, the dryer will require over one hundred therms of energy. That is more than $50 worth of fuel alone to produce one ton of animal feed.

Besides this, pressing garbage to remove the moisture creates an impossible sewage disposal problem. The press liquor from pressing garbage is a thick goo of suspended
solids. The BOD loading of this liquid will overwhelm most wastewater treatment systems.

In addition, the labor costs to remove contaminants are significant. Restaurant waste typically has plastics (shrink wrap and Styrofoam), metal (silverware and coins), string
and cord, not to mention broken chinaware, glass and pop cans. Farmers are reluctant to buy feed that might contain these items, even at discounted prices.

None of this seems to faze our would-be garbage millionaires. What should bring them down to earth are two facts: (1) the opportunity they see has been obvious in America for at least a century, and no one has made a go of it, and (2) nobody has made a go of this business potential in third world countries where labor is cheaper, farmers are less picky, feed is more expensive, and businesses are less encumbered by environmental regulations.

November, 2010. More recently garbage inquiries have focused on biofuel applications. A key bit of logic is as follows: before something can be burned, all the water must first
be removed. In this water evaporation process, a point is reached at which there is only 10% moisture left. If we freeze the system at that point, an analysis of value will show that the material at 10% moisture has greater market value as animal feed than it does as boiler fuel. The only exception we have found to this is in Europe where use of biofuel is being mandated by government regulations. Pity their taxpayers.

Hake

Not many people have heard of a fish called "hake", even under its proper name of Pacific Whiting. Part of the reason is that hake have not been harvested commercially until recently. This was because the fish has a built-in enzyme that takes action soon after the fish dies. Only recently has an inhibitor been developed to prevent this spoilage.

Hydroponic Tomatos

Recently a Model KP-6 screw press was tested on cull tomatos at Vine Ripe in Owatonna, MN. This firm serves the Twin Cities with premium quality tomatos. They have four acres of greenhouses, to which they are in the process of adding three more. Dutch firms dominate in the growing technology, so the greenhouses are imported, knocked down in cargo containers, from Holland. Dutch crews erect the houses, with only the glass windows being supplied locally.

The greenhouses use glass as it is far more durable than plastic film and it does a better job of transmitting light.

The vines are rooted at ground level in rock wool batting, roughly three inches thick by twelve inches wide. There is no dirt used in the production process. Food for the plant is supplied by a drip irrigation system that feeds nutrients and water directly onto the rock wool.

A tomato plant will yield fruit several years. However, because yield tends to fall off, the plants are replaced once a year in Holland. In Minnesota, because of the adverse and varying weather conditions, the plants are replaced twice a year.

The tomatos are harvested continuously, except during the months of January and February. These months are skipped, even though November and December are the coldest months, because window frost in January and February reduces the sunlight that reaches the plants.

The greenhouses are equipped with rails between each row of plants. These guide electric carts that are used for weekly maintenance and harvesting of the plants.

Weekly maintenance consists of attaching plastic clips to hold the vines to strings that are suspended from the ceiling beams of the greenhouse. The vines grow to be thirty-five feet long. The clips are about a foot apart, roughly the length grown each week. Also, arched plastic supports, called trusses, are attached to the vine where there is a branch with tomatos. This allows the tomatos to grow without dragging down the stem to which they are attached.

Bumble bees are kept in the greenhouses to provide pollination. Without pollination, the tomato does not have seeds and it grows to a dwarf size. The bumblebees eat some pollen as a source of protein. Sugar water is kept in their hives so as to provide other nutrients. The bumblebees do not make honey as the tomato flower does not have sufficient nectar.

Pest control is critical. One factor that add difficulty is that special insecticides must be selected to prevent killing the bumblebees.

Large hydroponic operations are found in states such as Arizona and Texas where up to 300 days of sunlight are available per year. A typical large operation would have 40 acres of greenhouses.

The flow rate through the Vincent KP press was excellent. In pressing the cull tomatos we found it easy to achieve a separation of 50/50 between press liquor and press cake. The cake is suitable for composting, and it was hoped that the liquor could be sewered. Most of the seeds ended up with the press cake.

The reason for pressing the cull tomatos is to reduce the charges for hauling the tomatos to a disposal site.

Meat Plant Waste

Ed Miniat Inc. is a large specialty meat processor in the Chicago area. They produce meats commonly found in delicatessens. Boneless Cooked Seasoned Beef Ribs would be a typical product.

Mushrooms

Our most recent serious testing was with samples of mushroom bedding, before and after composting.  These were supplied by Monterey Mushroom in Zellwood, Florida.

Monterey grows the mushrooms in 4' x 8' trays, 8" deep.  The trays are filled with wheat straw, chicken manure, and cotton seed meal, along with 2" of peat moss (rich soil).  (Sussex Mushroom in England said they use 80% straw.)  Monterey incubates the mushroom spores in rye or millet grain. 

Each week Monterey produces three compost batches of 28,000 cubic feet.  This is neutralized to a pH of 6 to 7-1/2 by adding sugar beet lime (possibly gypsum or alum?). 

Monterey wanted to know about the nitrogen in the press liquor, because of fertilizer applications.  Pressing this compost was just like pressing peat:  only single cell biological mud, no free liquid at all, came through the screen.  That happened with materials from both before and after composting.  Worse, both materials just sat in the press and co-rotated. 

We could not get cake to come out of the presses.  We tried two CP-4's and one KP-6, and achieved the same bad results with all of them.  Consideration was given to trying a 1/2 pitch screw, or a perforated screen, but it was clear that would not make enough improvement to justify the effort. 

The "as received" non-composted material was 67% moisture; the composted, 64%.  The press cake of the composted material came out 61%.  There was no free water in this material; not a drop could be squeezed from a fist full.

Most mushrooms come from Pennsylvania; mushrooms are Pennsylvania's biggest export.

Okara

May 25, 2009                                                                                                                                                                                                       ISSUE #211
                                                                                                             OKARA

In one month we had four different inquiries dealing with pressing okara. One, with White Wave, reached the point of bringing two 55-gallons drums of okara to Tampa for testing.

Okara is a waste product resulting from the production of tofu. The process for making tofu starts with soaking soybeans overnight, and then grinding them. The beans are about the size of baked beans. After soaking they are fairly soft and crumbly. Following grinding, the material is heated by being pumped through a heat exchanger.

A leading producer of tofu in the States is White Wave. Years ago we tested a Fiber Filter at their factory in Boulder, Colorado. At that time, the heated flow was pumped to 3 hp "centrifuges"; these were more like clothes dryers. The hot okara dropped to a narrow belt conveyor and was sent to farmers, for animal feed. When squeezed, not much water came through your fingers, and the moisture which squeezed out soaked right back in.

The hot soy milk was pumped into pails, along with some coagulant, and agitated a little. After a short dwell time the pails were dumped onto a 1 meter belt press, with fabric on the bottom and with metal slats forming the top belt. The filtrate liquid went to the drain, and the tofu cake that come out was about 1-1/2" thick. It was sliced into bricks that were dumped in a tub of cold water. After cooling the bricks were put in packages, water was added, and then the packages were sealed. Next they were pasteurized.

The plant waste water is what we were running through the Fiber Filter. The particles were too fine for the machine to work. We wrote it off as an another unsuccessful trial.

Today there are likely both specialty screw presses and proper centrifuges which replace some of the equipment such as the belt conveyor.

The purpose of our recent testing was to remove additional moisture from the okara. It had 86% moisture, which is way too high for economical production of animal feed in a dryer.

The okara we tested in Tampa, measuring 86% moisture, had come from a centrifuge, so there was no water left which a screw press could remove. Therefore we tried various press aids which are acceptable in animal feed, such as lime, gypsum and alum, along with cellulose fiber. None of these worked at all. The screw press removed negligible water. We have written it off as yet another unsuccessful trial.

Onion Waste

Vincent screw presses are used to dewater the waste from a wide range of fruits and vegetables. We have had outstanding success in applications such as (1) lettuce and cabbage from prepared salad factories, (2) carrot waste from facilities that produced baby carrots, (3) peel from plants that use steam and brush peelers on potatoes and carrots, (4) corn cob, husk, and silk at sweet corn canneries.

Pea Waste

April 23, 2009                                                                                                                                                                                                         ISSUE #210

                                                                                                           PEA WASTE

Pea cannery operations generate a lot of waste which is screened from their wastewater. Generally the flow is pumped over static (sidehill) screens, which separate a sopping wet mass, mostly peas. These screen tailings must be trucked from the site.

Because the tailings hold so much water, the tonnage is significant. More importantly, free water jostles free in transport, leaving a trail of water behind the waste hauler.

Dewatering this waste is a good application for an inexpensive Series KP screw press. When the screen tailings are fed to the press, depending on the effectiveness of the static screen, from 60% to over 80% of the weight is separated as wastewater.

This press liquor does not contain an excessive amount of suspended solids. Regardless, it should be directed to the wastewater flow which goes to the static screens.

In one installation the cannery also has a flow of waste which is exhausted from the factory with a fan system. The discharge of the fan is blown to a chicken wire cage built over the inlet to the screw press. This waste, along with screen tailings, falls into the press.

The peas are so soft and squishy that low discharge cone pressure is used. The rotating cone feature is very important to successful operation. Also, the new two-thirds pitch screw has proven especially effective. With these options, high press screw press capacity is achieved without pushing excessive solids into the press liquor stream.

In one application a 10" press worked fine on peas. However a larger 16" press was required, due to increased load, when the plant switched to canning corn.

Pickle Waste

We recently had a chance to visit the Claussen pickle factory. The plant runs year around by bringing in cucumbers from all across the United States, especially Florida, plus Costa Rica, Honduras, Mexico and other countries.

Pineapple Waste

September 22, 2009                                                                                                                                                                                             ISSUE #215
                                                                                                    PINEAPPLE WASTE

In 1975 Vincent Corporation sold equipment for a feedmill that would process pineapple waste. The project, sold by FMC Corporation, was for Dole Pineapple in Mindanao, in the Philippine islands. Now long abandoned, the project involved dewatering waste from pineapple juicing so that it could be sold as animal feed. Because of the current high value of animal feed, several inquiries have been received recently about the process.

Dave Kalashian's trip report from Mindanao mentions that the cake was coming out of the press at no more than 78% moisture content, that being acceptable. We also found a material balance prepared by FMC in which they worked with 76% press cake moisture.

This leaves too much water to be evaporated in a dryer. The cost of the fuel required would exceed the value of the animal feed produced. Therefore recently tests were undertaken in Tampa.

Pineapples were shredded and pressed to remove the juice. The waste from this process was tested with three different press aids. We tried varying percentages of hydrated lime, gypsum, and alum. The gypsum and alum had no effect. However, an excellent reaction with lime was achieved.

The waste with lime pressed to much lower values, in the range of 60% to 70% moisture content. A relatively high amount of lime was required, 1% to 3% by weight. This is similar to what we have found with other wastes such as potato peel, onion peel, and tomato waste. It compares to 0.5% which is typically used on orange peel.

Pressing material with this much lime is a high torque operation. The cake becomes very hard and dry. Lime mixed with water makes cement, as used for concrete. It is likely that to some extent this reaction is occurring in the press. Nevertheless, the pressing operation is within the range of normal screw presses.

We were not able to test with steam injection, due to the small scale of our trials. However, we know that direct injection of steam into the pineapple waste will reduce the moisture content of the final press cake. In the case of orange peel, moisture content is reduced by up to four percentage points with steam injection (Pressing News #129 of July, 2002).

In the pineapple waste feedmill, the press cake would be dried to 10% moisture in a rotary drum dryer. This would be pelleted and sold. Reportedly the value is higher than that of dried citrus waste because the nutritional value is greater.

Pineapple waste has a little more Brix than citrus waste. This means that it is practical to use a conventional Waste Heat Evaporator (WHE) to make pineapple molasses out of the press liquor. Unfortunately, most likely candidates for a pineapple feedmill are too small to afford the investment required for a WHE. (The use of a WHE would reduce fuel consumption per ton of finished product by as much as two thirds.)

This means that the press liquor will be sent to the wastewater treatment plant (WWTP). This will present a severe load on the WWTP as about two thirds of the solids in the pineapple waste, in the form of dissolved sugars, will be lost with the press liquor. If a lagoon system is used, odor problems are likely.

One alternative under consideration is to use the press liquor as a feed to an anaerobic digester. The biogas produced therein would be an excellent boiler fuel. And the odor problem would be addressed.

Pistachio Nuts

Pistachio processors in California operate during a short harvest season, from September into October. The green berries, as harvested, are about the size of an olive. At the processing plant, an exterior layer of pulp is removed. This is done in abrasive peelers, with water. Once the rejects (immature and hollow seeds) are removed, the pistachios are dried, usually in GSI dryers. The product that comes out is the snack we are familiar with: a white shell, cracked opened by the heat, containing a delicious nut.

Potato Peel

We have a four or five inquiries a year about pressing potato peel. This peel is a waste product that comes from peeling machines used by potato processors. Three types of peeling equipment are used: abrasive rollers, caustic, and, most commonly, steam peeling.

Potato Peel, Improved

A Model KP-10 screw press was recently put into service dewatering wastewater at a plant processing sweet potatoes and Russet (white) potatoes. All of the waste peel, starch, and water collects in a pit and then is pumped to the wastewater treatment area.

Potatoes & Carrots

For several decades, Vincent has attempted to use a screw press to dewater potato peel. Our 1994 Pressing News described the best results that could be obtained. These results were relatively marginal, limiting the market for screw presses. After all, it is hard to justify a screw press if all you are going to do is put half the waste into the sewer and reduce the moisture content of the remaining solids to 82%.

Produce Waste

April 5, 1993, Revised September 1996                                                                                                                                                                       ISSUE #2

Bush Brothers, a leading canning company in Tennessee, is making excellent use of a Vincent VP-16 dewatering press. They have integrated the press into several facets of their business so as to achieve outstanding synergy.

The three principal products being processed are cabbage, green beans, and potatoes. In the processing a considerable load of waste products such as culls, stems, trims, leaves, and peels are generated. The press was acquired to reduce the bulk of the waste materials and to improve the quality of these materials as a cattle feed.

In operation it has been found that high concentrations of low fiber materials, such as whole potatoes and dried beans, press poorly. That is, they become mush: the cake is not firm and solids pass into the expelled liquid. To avoid this condition the plant operators make a point of mixing in fibrous material, such as cabbage leaves, as needed.

The cake produced by the press is fed into a trench silo where it is conveyed to the company's cattle feeding area. The cake is referred to as silage, and it is fed directly to the cattle.

The press liquor produced in the press is drained to the company's waste water pit. This pit is of considerable capacity as the canning factory generates up to one million gallons per day of waste water. The water from the pit is used to irrigate the company's pasture grounds. Thus the nutrients in the press liquor are ultimately used to fertilize the fields.

In summary, the synergy of the Bush Brothers operation is self evident. A wide range of waste materials are processed through a single press that converts them into useful by- products for a minimal cost.

September 1996 update: Few canners can afford a VP model press for waste disposal service. For that reason the new Series KP presses have been developed. These models are currently undergoing field trials.

Rice Wine

Two Vincent CP-10 presses are being commissioned in Taichung, the third largest city in Taiwan. These units are at the new rice winery of the Taiwan Tobacco & Wine Monopoly Bureau.

Series KP Presses

Vincent Corporation has introduced a new series of screw presses. These have been designed to serve two applications where light dewatering is required. In both applications free water is removed more thoroughly than can be achieved with conventional screening devices.

The broadest market for the KP presses is waste from canneries and food processing plants. In these facilities waste is normally sluiced to a collection pit from which it is pumped with a chopper pump. Static and vibratory screens are generally used to strain the waste water from the solids. The solids are then transported to landfills, or given to farmers for animal feed or landspreading.

The problem with this system is that water continues to drain from the solids after the screening. This results in dripping in the parking lot, citations for leaking wastewater on the highway, and loads that are rejected at the landfill because of excessive moisture content.

A conventional screw press is not suitable for dewatering this waste both because the cost of the press is excessive and because the press loses capacity and forces excessive solids into the wastewater stream.

The KP press, with three stages of compression instead of five, addresses these problems. It dewaters far better than a screen, yet it drives less suspended solids into the press liquor than a conventional press. Waste streams that have been successfully tested to date include: cull tomatoes, potato peel from peelers, egg shells at an egg breaker, spent brewers grain, trim material at a facility producing TV dinners, and out-of-date produce at a vegetable and fruit warehouse. One unusual application in this category involves pressing dairy manure to reduce load on the waste treatment lagoon.

The second market for the KP presses is in place of a screen ahead of a conventional screw press. This screening allows conventional presses to press tighter, with higher throughput capacities. It is expected that the KP press will be a significant improvement in this process.

Two applications where the KP press is used ahead of conventional presses are being tested: (1) thickening pumped corn waste materials at wet corn milling plants, and (2) thickening shredded citrus peel in plants that pump the peel to the feedmill. In describing the application we are referring to the KP as a pre-press, suitable for pre-thickening. In essence we are offering double pressing at a bargain capital cost.

The cost of manufacturing a KP press is approximately half of that of a comparable Model VP or CP press. Costs were reduced through a combination of several unique features:

The inlet hopper was simplified, eliminating the inlet screen.

The thrust bearing was eliminated by selecting gearboxes with suitable thrust carrying capacity.

The discharge cone was replaced with a simple discharge plate actuated by a 4-bar mechanism.

The screen covers, spreader bar, and collection pan were replaced with either a single piece of pipe or a pan and cover.

The press does not have a base frame; prior to operating the press the customer must anchor it to steelwork or other suitable foundation.

The value of the KP presses must not be underestimated. The construction is entirely of stainless steel; drive motors have half again the anticipated horsepower requirement, and the gearboxes have been selected for a twenty-year life expectancy.

Shrimp Waste

For almost a year we have been averaging one inquiry per month in regards to dewatering shrimp and crab waste. The material to be pressed includes shrimp heads; the shells from the shrimp tail; and both hardshell and softshell crab waste.

Shrimp Waste, Cooked

Historically, Vincent has had trouble pressing shrimp shells. A "strawberry milkshake" would ooze through the screen of the screw press, and, since the shells would co-rotate with the screw, capacity was dreadfully low. To combat these difficulties, a VFD would be programmed to auto-reverse periodically; this proved to be an acceptable solution.

Spent Coffee

Producers of soluble coffee generate waste that is an excellent boiler fuel. As described in the February 20, 1997 Pressing News #56, "Coffee", traditional technology has been to press this waste, spent coffee grounds, as tight as possible and then burn it. This material, at 55% moisture, was burned in stoker grate furnaces.

Spent Grain

Firms that produce alcoholic beverages from grain are an important market for dewatering screw presses. The industry firms consist mostly of beer brewers and distilleries. Their raw materials are barley, corn, wheat, rice, and other grains. They all produce a common by-product: spent grain.

Carpet Fiber

Vincent Corporation recently shipped a VP-6 press for an unusual application. This press will be used to wring wash water from carpet fibers in a recycling operation.

Plastic Recyclers

Because of the increasing prices of virgin pellets [in 1995], plastic recyclers are at long last enjoying a measure of prosperity. This cyclical upswing has coincided with the refinement of Vincent products for the recycling industry.

Plastics Recycler Users

USERS LIST – PLASTICS RECYCLERS
 

Mr. Doug Brooks
A.E.R.T., Inc.
P. O. Box 1237
Springdale, AR 72765

Mr. Joe Warren
Dart Container Corp.
500 Hogsback Road
Mason, MI 48854

Mr. Ed Farley
Wellman, Inc.
P. O. Drawer 188
Johnsonville, SC 29555-0188

Mr. Steve Adams
Brigadoon/USPL
1909 NE 25TH Avenue
Ocala, FL 34470

Mr. Ed Nummer
Cadillac Products, Inc.
P. O. Box 1345
Sterling Heights, MI 48311

Mr. Francisco Morales
Eagle Brook Plastics
2600 West Roosevelt Road
Chicago, IL 60608

Mr. Franco Previd
Enviro Plastics
P.O. Box 363
Auburn, MA 01501

Mr. Doug Meredith
Image Industries Inc.
106 John Bankson Drive
Summerville, GA 30747

Mr. Tom Hart
Waste Alternatives
(Out of business)
Ocala, FL

Mr. Bill Anderson
Merlin Plastics
Unit 109 - 917 Cliveden Avenue
Delta, BC CANADA V3M 5R6

Mr. Greg Davis
Orion Pacific
P. O. Box 4148
Odessa, TX 79760-4148

Mr. Ernie Maroschak
Plastic Tubing
P.O. Box 878
Roseboro, NC 28282

Mr. Adam Marciciszyn
Plastics Forming Ent.
850 E. Industrial Park, Unit 1
Manchester, NH 03109

Mr. Gary Fish
Premier Midwest Plastics
201 W. Plymouth
Jefferson, WI 53549

Mr. Ajit Perera
Talco Plastics, Inc.
3270 East 70th Street
North Long Beach, CA 90805

Mr. Jerry White
Envision Plastics
(FCR Plastics; Resource Recycling)
606 Walters Street
Reidsville, NC 27320

Mr. Walter Aristondo
Envision Plastic
14312 Central Avenue
Chino, CA 91710

Plastics Recycling

June 3, 1993                                                                                                                                                                                                           ISSUE #4

The industry that recycles post consumer plastics is divided into three main segments. These are the recycling of bottles, film, and polystyrene. The bottle segment is the largest, and it has several specialists: PET (two liter Coke bottles); clear high density polyethylene (HDPE) (one gallon milk containers); and colored HDPE (laundry detergent, cooking oil, hair shampoo). The film comes mostly from industrial wrapping and supermarket bags. And we are all familiar with the sources of polystyrene foam.

The Vincent dewatering screw press has found a common application in the factories of all of these segments. That application is the dewatering of wash tank sludge.

The recyclers of post consumer plastics all grind (shred) their incoming waste material after it has been sorted. Next it must be washed in order to assure purity of their end products. This washing is done in tanks, and sludge with a high fiber content accumulates in the process.

The need is to dewater the sludge arises because all the recyclers send the sludge to landfill. Without dewatering the sludge drips water, and highway officials will not permit hauling material that drips on the roads. Further, landfill tipping fees are less for dewatered material.

A question that invariably comes up is how to feed the sludge into the Vincent press. We find that in practice plastic recyclers do this in a variety of ways. Some pump the dilute sludge directly from the wash tank to the press. Other recyclers pump the wash water over a screen and feed the tailings into the press. The recyclers frequently have more than one source of sludge, so they might end up using a combination of these methods with one or more presses.

The screens being used ahead of our presses are both the static sloped screen with wedge wire and the round Sweco vibrating variety. In at least one case the tailings drop directly into the press, but most commonly the tailings are fed to the press with a screw conveyor.

In all cases the objective is simply to turn the sludge into a cake that will not run or drip. The volume typically is such that the smallest capacity presses are used.

Capture Rate

June 28, 2001                                                                                                                                                                                                     ISSUE #M17

Engineers designing manure handling systems frequently ask about capture rate. They need to know the percent of the solids in the manure that will be captured in a manure separator. 

The captured solids are those that come out in the press cake. The solids not captured are those that flow in the liquid stream draining from the screen of the separator. The capture rate is calculated by dividing the solids in the press cake by the total solids pumped to the screw press.

Late last year capture rate tests were run at the University of Tennessee Dairy Experiment Station in Lewisburg, Tennessee. These tests confirmed a very significant difference in capture rate between flush and scrape barn systems.

The data show that when manure with 7% solids, typical of a scrape barn, are pumped to a screw press, the capture rate runs about 50%. In contrast manure with 2% solids, typical of a flush barn, had a capture rate of only 25%. (These figures are from a graph prepared by Dr. Robert Burns.)

The problem with a very dilute inbound flow can be explained as follows: The freeness tends to be high and a very large amount of water rushes through the screen. It goes through the screen so fast that what fiber is present tends to wash through the perforations or slots of the screen. Thus the capture rate is low with very dilute flows.

With thicker flows the longer particles tangle together, acting as a press aid. This forms a mat that entraps the finer particles. Thus the capture rate is higher with pre-thickened feed to a screw press.

It is noteworthy that solids in manure can be classified as either suspended solids or dissolved solids. The dissolved solids are like sugar in water: they cannot be captured with a mechanical filter. Since most of the manure flow to a screw press leaves as liquid, most of the dissolved solids pass through the press with this liquid. This places a definite limit on capture rate.

Cushman Farm Digester

October 1, 2001                                                                                                                                                                                                      ISSUE #M19

The Cushman Dairy in Franklin, Connecticut is a show case farm. The electrical energy used at this farm comes from a manure digester system developed by CST Industries (better known as Harvestore - Slurrystore). The original installation was made in 1998. 

The farm has 700 head, utilizing coarse sawdust as bedding. A scraped barn system is used to collect the manure, which is pumped to the digester.

The digester is an insulated tank made of the traditional Harvestore glass porcelainized steel panels. Manure is pumped in at the two-thirds level. Agitation is achieved with a centrifugal pump that takes its suction off the bottom of the tank. Digested manure sludge floats over a weir at the top and goes to a Vincent screw press for dewatering.

The biogas collected from the top of the digester flows to a positive displacement blower and then to a Cummins internal combustion engine. The engine is coupled to an induction generator that fills the farm's electrical needs. Radiator cooling water goes through a tubular heat exchanger that heats water used to maintain the digester temperature.

Digested manure flows to a Vincent Model KP-10 manure separator that was installed in 1998. Typically this produces 20 gpm of press liquor which flows to the manure pond. Press cake production normally runs 600 pph, with the press operating about ten hours per day.

Dairy Manure

The then-new KP screw press was first shown at the 1996 World Dairy Exhibition in Madison, Wisconsin. Rather than a sales effort, the objective of out booth was to determine the marketability of the product and to find suitable channels of distribution.

We knew that dairy farmers, especially large-scale operations, had been processing their manure through screw presses. With 68,000 attendees, the show gave us ample opportunity to find out why.

Keeping bedding and manure out of the treatment pond extends lagoon life. This reduces the frequency necessary to dredge the lagoon. This cost deferral is a financial benefit.

Environmental regulators are also a driving force. By keeping the fibrous solids out of the lagoon, odor generation is significantly reduced. Also, by pressing the manure ahead of the waste lagoon, a farmer can extend the capacity of existing lagoons. This avoids a number of permitting difficulties.

Farmers in northern climates must store their manure until the spring thaw. Pressing it to remove the solids can greatly facilitate this operation.

Pressing the manure separates it into two flows: dirty liquid and damp fibrous solids. Weather permitting, the liquid can be drained to the waste lagoon, or it can be pumped for irrigation purposes. (Full strength manure is too thick to pump very far, and it plugs irrigation spray nozzles and cakes the soil).

The manure solids from the screw press are composted for approximately ten days. This produces a rich fertilizer that is excellent for landspreading or tilling into a field. Some dairy farmers are selling this compost in bulk to nurseries and municipalities, while others are bagging the material for retail sale to home gardeners.

One increasingly common use of the compost is as bedding for dairy barns. The farmers we interviewed were using materials such as sand, sawdust, and rice hulls, which go either on the concrete floor or over a foam pad. (In fact, there were at least four exhibitors showing rubber or plastic "cow mattresses".)

University studies have shown that pressed manure has just the right moisture content (70%) for composting. This raises the manure temperature sufficiently to kill bacteria, thus making the compost safe for use as bedding. However, most of the farmers we talked to were reluctant to use the material for fear of infection (principally mastitis) spreading to the herd. (Since this original writing, it has become common to bed with manure solids straight from the screw press, without composting.)

Three different "manure separating" devices were exhibited at the 1996 show: sidehill screens; drag flight conveyors with slotted bottom troughs; squeeze rolls; and a screw press. Besides being large and mechanically complicated, the drag flight conveyors had limited, if any, squeezing action, so the dewatering that they achieved was marginal. The squeeze rolls dewatered a little better, but they were maintenance prone and costly.

The screw press being displayed was made by FAN of Germany. The principal complaint we heard were that it is expensive to maintain. Maintenance concerns centered on the availability and cost of spare parts such as the gearbox, screen and screw.

These areas have been addressed in our KP series of presses. The screw of the press is supported at both ends, rather than using a cantilevered support. This prevents the screw from hitting the screen once the machine loosens up. Also, the gearbox is separated from the inlet hopper by a drop-out gap; this saves the gearbox upon failure of the shaft seal in the inlet hopper. Our use of standard NEMA (American) motors also reduces maintenance costs.

A marketing problem was evident in that Vincent's industrial sales representatives do not call on dairy farms. To address this Vincent signed an agreement with A.O. Smith Harvestore. This firm, a leader in farm feed storage systems, will work with Vincent to develop models suitable for not only dairy, but also swine and cattle. This development contract is expected to lead to a distribution agreement. (This relationship failed to develop, and Vincent now sells through dairy equipment dealers.)

1977 TEST DATA:

The KP-10 press on test at the University of Wisconsin will handle 50 gpm of manure and bedding, with only milking parlor water being added. The press achieves a separation of 43 gpm of liquid. The inbound solids split evenly, with 50% in the press liquid and 50% in the press cake. (This split is low because of the dissolved solids in the manure; a screw press cannot capture dissolved solids.) Press cake moisture content measures 72%. They process all the manure and bedding from 500-confined head in three and a quarter hours each day.

Fairgrove Farms Digester

Almost twenty years ago John Pueschel, Dave Pueschel, and Larry Kelly, owners of Fairgrove Farms in Sturgis, Michigan, installed a system to generate electricity from cow manure.

Fighting Cocks

We have a customer who, with an extended farm south of Guadalajara, Mexico, purchased a KP-6 screw press. They refer to it as "the recovery machine". It is used to separate undigested food solids from manure at the pig farm. Originally, this press failed to adequately dewater these solids. Both 0.030" perforated and 0.020" wedgewire screens were tried. It was only with the addition of a small 18" static screen, to pre-thicken the flow, that successful operation was achieved. The press cake is used as a feed supplement in the cattle grow-out operation.

Gearbox Protection

June 26, 2000                                                                                                                                                                                                           ISSUE #M8

Vincent has never had a gearbox failure in a Series KP screw press. This record is attributable to two design features of these machines.

A critical observer will note that the construction used to mount the gearbox on the press is relatively expensive. The gearbox is mounted on a plate with four legs that are welded to the inlet hopper (A-plate) of the press. It would cost a lot less to simply bolt the box to the A-plate.

The reason for the separation is to provide for leakage at the shaft seal. There are a pair of Johns Manville seals located in a seal housing that is bolted to the A-plate. The four legs are used to make a large physical separation between this seal and the gearbox. That way, when the seals eventually fail, leakage will be to the structure below the press.

Without this provision the leakage would be against the gearbox shaft seal. Gearbox seals are designed to keep oil in, not to keep manure out. It is common for competitors to have bearing and gearbox failures due to manure contamination since provision is not made to drain away leakage. It is all but impossible for this to happen with a Vincent press.

Inclined Discharge

ISSUE #M24

Screw presses used to dewater manure are notorious for "purging" or "blowing the plug". The condition is one where all the manure being pumped to the press simply flows out through the cake discharge, with no liquid being separated through the screen. Whether it is 20 gpm or 600 gpm does not matter: if it starts at midnight and no one notices until dawn, the mess is terrible.

Improving the screw press design to address this problem has been difficult because a press will operate for weeks or months without an occurrence. Gradually a number of different factors have been observed that contribute to blowing the plug.

1. In the winter the cake being discharged can freeze and jam the discharge door open.
2. When the screw becomes worn, manure can flow between the screw and the screen, wetting the plug and allowing it to wash away.
3. When the manure pit is near empty, the flow from a centrifugal pump can go down by 75% compared to when the pit is full. The plug will tend to blow when the pit is low.
4. A change in feed or bedding (like going from sawdust to manure) results in reduced dewatering and wetter cake.

The latest improvements in press design that address the problem are (a) Extending the screw shaft through the discharge so that the cake door is breaking up a donut, rather than a solid plug, of manure, and (b) Double flighting the screw over the first half of the screen where the manure is wettest. This produces drier cake, earlier in the dewatering process, which is less likely to purge.

Another useful remedy is to elevate the discharge end of the press. This has been found to be particularly effective when the pit is low and the flow of solids to the press is reduced. At this time the cake discharge becomes so slow that water wicks (soaks) into the cake on the lower side of the screw, allowing part of the plug to become soft and to blow out. Without sufficient solids being fed into the press, the plug does not re-form. A 5º incline to the press can make a world of difference, and it can cause no harm.

The photo at the top right hand corner of our brochure shows an installation with an elevated discharge. This was retrofit, during start-up, in a successful effort to eliminate blowing the plug.

Installation Quick Tips

May 9, 2001, Revised April 2003                                                                                                                                                                             ISSUE #M16

There are a number of details relating to the installation of screw presses as manure separators that should be noted:

Unless a piston pump is used, it is necessary to have a line that allows manure to recirculate from the press inlet back to the manure pit. This will prevent pressurizing the inlet of the press, which can be a cause for purging.

It is best to have a vent line at the inlet to the press. A 1" line, 5' tall, is adequate for breaking a suction in the recirculation line. Such a suction can lead to reduced press capacity.

The drain line from the press should go below the surface of the pit or pond into which it drains. If this line is relatively small in diameter and has a steady downward slope, a vacuum will be induced around the screen of the screw press. This will increase press capacity.

The amount of vacuum is a function of the elevation between the press and the drain pond. Where convenient presses should be mounted on 20' or higher stands.

The control panel for the manure pump and the press should have a timer. This timer should be set to have the press run for two minutes after the pump shuts off. This will partially clear the press so that it will not trip out on overload when it is re-started.

Minimize the time that the press is run with no material being fed into it. Running dry will allow abrasive rubbing. The last manure admitted to the press will dry to an abrasive powder.

When bolting the frame of a manure separator to its platform, look into the cake discharge end. Note if the screw is being pulled into the screen. This is an indication that the frame is racking. If this occurs, shims should be used so that the mounting bolts tighten evenly. The screw must be kept centered in the screen.

Elevate the discharge end of the press a few inches to minimize any tendency for the press to purge. This has been found to be particularly effective when the pit is low and the flow of solids to the press is reduced. At this time the cake discharge becomes so slow that water wicks (soaks) into the cake on the lower side of the screw, allowing part of the plug to become soft and to blow out. Without sufficient solids being fed into the press, the plug does not re-form. A 5º incline to the press can make a world of difference, and it can cause no harm.

Where the manure is very thin, it may be necessary to pre-thicken the flow to the screw press. This is best done with a sidehill (inclined) screen. Without pre-thickening, some dilute flows will flush the solids through the screen. This makes it difficult to form a plug, and the capture rate is reduced excessively.

Iowa State Digester

DECEMBER 5, 2001, Revised September 2010                                                                                                                                                          ISSUE #M21

The Dairy Foundation in cooperation with the Iowa State University has started up a new dairy manure digester system.  The installation is at the model farm located at Northeast Iowa Community College in Calmer, Iowa.

The school has opened a new campus with a herd of 170 cows.  The manure and bedding from one hundred of these cows are disposed of in a digester.  The biogas generated is used to produce hot water used on the farm.

The manure digester is the plug flow type.  This uses a concrete tank 10' deep by 13' wide by 68' long, holding a nominal 50,000 gallons.  Foam insulation helps keep the liquid at 95o F, which is well suited for producing methane.

Biogas from the digester is burned in a boiler system designed and manufactured by Perennial Energy of West Plains, Missouri (417-256-2002).  Rated at 150,000 BTU/hour, this heats water that is used to maintain the digester at temperature and to supply floor heat and hot wash water for the milk pipeline.

The manure sludge from the digester is pumped to a Vincent Model KP-6 screw press.  The cake from this press is composted and used for bedding, while the press liquor flows to a treatment pond.  Normal cake production is in the range of 150 to 250 pounds per hour, and press liquor flow of 34 gpm was measured recently.

Daniel Meyer of Fayette IA (319-425-3331) has been actively involved the $200,000 project since its inception.  He favors the plug-flow type digester for dairy manure.  He has also noted that scrape barn manure works better than flush barn for digesters because of the greater concentration of solids.

Mr. Meyer describes biogas production as a two stage process.  In the first stage the solids are broken down with acid forming bacteria.  In the second stage methane forming bacteria cause gasification.  The pH must be kept about neutral during the process.  The average electric production is one kilowatt of generator capacity per 10 cows.

Ray Crammond of Ankeny IA (515-965-8301) was a consultant on the design of the system.

TEN YEARS LATER
September 2010
Like most manure digester projects, this one has been shut down. 
When the digested plugged with solids (sand?) which settled out, a group of students were drafted into digging it out.  The system was altered so that the manure was pressed ahead of the digester.  Only the press liquor was put into the digester.  This should have worked.
 

Manure Digester Bio-Gas

SEPTEMBER 6, 2001                                                                                                                                                                                             ISSUE #M18

Due to renewed energy awareness there has been a surge in interest in manure digesters.  In these digesters the manure decomposes anaerobically (without Oxygen), releasing bio-gas. This bio-gas is collected and burned to release its energy.

These dairy systems have environmentally attractive features.  Energy is produced from a waste source.  Also, the digester action treats the manure, which addresses other problems:  run-off is controlled and odor from the dairy is reduced.

The bio-gas (methane with CO2, SO2 and other gasses) can be burned in a boiler to produce either steam or hot water for the farm.  Alternatively, this low BTU gas can be burned in a Waukesha or Caterpillar internal combustion engine.  These engines are coupled to induction generators that more than fill the farm's electricity requirements.

Besides internal combustion engines, mini gas turbines are being offered for use on biogas.  The metallurgy of the engine and other components is critical because of corrosive gases.

Two types of digesters predominate.  In both the gas is collected off the top.  The most common, a plug flow digester, is a long pit where the manure enters at one end and progresses slowly to the discharge.  Our presses are used with this type of digester at Fairgrove Farms in Michigan, High Plains Dairy in Kansas, and Iowa State University.

Mixed flow digesters using tanks are a second type of digester where extensive development has been done.  A.O. Smith Harvestore leads in this technology, and Cushman Farms in Franklin, Connecticut is a good installation to visit.  Manure solids flow off the top of the digester into our Model KP-10. 

Another type of mixed flow digester uses a pit instead of a tank.  A good example is u8nder construction at Matlink Dairy in Clymer, New York.  This 660,000 gallon pit digester has propeller agitators on two sides.  Digested manure will flow over a weir to be pumped to a Vincent press.

The manure can be pressed in a screw press ahead of the digester, with the press liquor being directed into the digester.  However the more common arrangement has the press dewatering the sludge that comes from the digester.  The press cake, from either raw or digested manure, has about 70% moisture.  This is ideal for composting and producing a rich soil amendment, or for immediate use as barn bedding.

TEN YEARS LATER
It is surprising how accurate this report is.  The trend has been toward mixed flow instead of plug flow, and gas generation and recovery technology has improved.  However politically incorrect unstated facts from ten years ago remain true:  No digester project goes ahead without a government grant because the investments do not pencil out.  And, most digester projects get abandoned.  For example, all five showcase digesters mentioned in this Pressing News have been shut down.
 

Manure Incineration

Van Der Geest Dairy, located near Wausau, Wisconsin, has developed a unique technology for manure disposal. The farm has 3,800 cows and uses a flush-barn manure collection system.

Bedding, in the form of dried manure, is produced by burning about half of the manure generated by the herd. To do this, the manure from the dairy is first dried to a low moisture content so that it can either be incinerated or used as bedding. The heat released in the incineration process is what is used to dry the manure to low moisture content.

Initial dewatering of the manure is achieved by pumping the manure across two sidehill screens, one curved surface Houle and one flat panel Agpro. Both have 0.080" slot widths. The narrower, but taller, Agpro is preferred.

Prior to selecting the sidehills, the farm operated both Integrity roller drum machines and the Accent internally fed rotary drum screen. Sidehills won out because of their high capacity and simplicity.

Solids from the sidehills fall into a pair of KP-16 screw presses. The Vincent press was selected, after a nine-month testing program, over FAN, Manure Monster, and Boldt screw presses. Both Vincent and FAN modified their presses numerous times. Efforts by both companies failed to adequately press the manure without prethickening.

Key features that led to the selection of the Vincent press are as follows:

• The KP-16, with its larger screw diameter, has greater capacity.
• There is ample separation between the gearbox and the inlet hopper seal, assuring long gearbox life.
• Vincent presses use standard NEMA motors which are readily available throughout North America.
• The Vincent press has removable covers over the screen, making it convenient to pressure and acid wash the screen.
• The screw of the Vincent press is supported at both ends, at the gearbox and at the cake discharge, which assures the screw-screen alignment that is necessary for long screen life.

The cake from the screw presses is conveyed to a system developed by Energy Unlimited. The key component is a Heil triple pass rotary drum dryer. Here the manure press cake, at 70% moisture content, is dried down to 10%.

Next, the dried manure is separated from the gas stream in a cyclone separator. The solids are blown into a storage bin, from which they a blown into a vertical combustion chamber. The solids are burned in the combustion chamber without the need for auxiliary fuel. The products of combustion (hot gasses) are directed into the dryer drum, where they dry the manure press cake.

An induced draft fan draws the gasses through the dryer and combustion chamber. From the dryer, the gasses go to a covered trench. Filtrate from the sidehill screens and press liquor from the two screw presses flows through this 450' long trench. Particulate and gas pollutants are captured and oxidized, and aeration is achieved. Another induced draft fan draws the gasses through the trench and pushes them to a smoke stack. A white plume of water vapor is generally visible from the stack.

Water from the trench is directed into a treatment pond. The treated water is used for barn flushing.

About half of the dried manure is used as bedding for the cows, with the other half being used as fuel for the combustion chamber.

This unique system for manure disposal is receiving a great deal of attention from environmental specialists.

Manure Installations

Ohio State University in Wooster, Ohio is running at 40 gpm press liquor from their KP-10. This is the machine in the video that Harvestore made; we have a copy if you want it. The new barn manager is Kevin Miller, 330-263-3924, Dairy Research Center Manager, Krauss Dairy Center. They have 120 cows, scrape barn, feeding our press. They run only 16 hours a week, making bedding. They put the press cake fresh into the stalls, without composting.

The original KP-10 at the University of Wisconsin goes great. We are not sure who the farm manager is, so the best contact would be Dr. Dick Koegel ("Cagle") at 608-264-5149 or 608- 264-5355. He is at the U.S. Dairy Forage Research Center. The KP-10 processes 50 gpm of dairy manure, with a separation of 43 gpm of liquid. The inbound solids were split evenly, with 50% in the press liquid and 50% in the press cake. Press cake moisture content measured 72%. Scraped barn.

One we have not heard from for a year is Warren Hatcher, mobile 423-309-8108, farm 423-338-2780 at Riverside Dairy Farm in Benton, Tennessee. They have a KP-16 on 800 head, flush barn. I saw a good 800 gpm going through the machine.

Manure Separator Inquiry

April 18, 2001, Revised October 2002                                                                                                                                                                      ISSUE #M15

Recently an inquiry was received that highlighted the need for information in quoting a manure separator. The e-mailed inquiry read: "Please quote the price for a manure separator to handle 100,000 liters of manure." 

Here are questions that we asked in order to select the proper size machine, with the right screen and drive motor:

What animals does the manure come from? Cows or pigs? 
We avoid poultry layer manure, although broiler grow-out house litter can be pressed.

By chance, is the manure actually sludge from a bio-gas digester?
Capacity goes down significantly on such sludge.

Assuming it is dairy cow manure, how many cows are on the farm? Are they confined (100% of the manure is captured), or is the manure only from a milking parlor or feeding barn?
Is a flush barn or scraped barn system used to collect the manure? This will tell us the dilution to expect.

What is the gpm capacity of the manure pump? 
How many hours a day is the manure separator expected to run? Some farms want to run only four hours a day, while others run up to twenty four hours a day.

What kind of bedding is used? 
Sawdust will require more horsepower than straw. Sand must be separated before the manure is pumped to the separator.

What is the power supply at the farm, single phase or three phase? What voltage? 
Most small farms are single phase 220 volt, while some large farms have 440 volt three phase power.

Is compressed air available for use with the manure separator? 
The large Model KP-16 has a discharge door that is actuated by an air cylinder. This feature is an option with the smaller 6" and 10" machines.

Is the manure currently being separated? If so, how? What is the cake used for? 
Reviewing these questions will result in a much more satisfactory machine selection.

Manure Separator Screens

May 24, 2000                                                                                                                                                                                                        ISSUE #M7

All categories of manure separators use metal screens made of stainless steel. These screens all fall into one of two categories: either perforated metal or wedgewire.

The perforated metal screens are made of medium gauge stainless sheet. The smaller the hole, the thinner the sheet. The most popular size is 3/32" perf (0.093" or 2-1/4 millimeter holes). This is generally made of 14 gauge metal, which is 0.075" thick. Another popular size, used in the small KP-6 separators, is 0.050" perf (1-1/4 mm); it is made from 24 gauge sheet which is 0.024" thick.

Wedgewire is the name given to a screen that is made from wires that are roll formed into a truncated pyramid cross section (a triangle with the top cut off). These wires are welded into flat panels with a fixed distance between each wire. By putting the broad base of the triangle against the liquid, a self-relieving passage is formed that has a minimal tendency to plug. Any solids getting through the narrow entrance between two wires will be swept away with the filtrate.

The flat panels thus produced can be used at the bottom of drag flight separators. Or, the panels may be curved to a gentle radius to make a sidehill (gravity) screen. The panels can be rolled to a cylindrical shape for use in a screw press, but they are expensive and tend to lack durability and burst strength.

The opening between adjacent wires is referred to as a slot. Typical slot widths used on manure range from 0.020" (1/2 mm) to 0.040" (1 mm).

The relative amounts of free area are of interest. For comparison purposes, 3/32" perf has 23% open area, while .030" wedgewire has only 15% open area.

Extensive testing has shown that changing the size of the opening has little impact on the solids capture rate. Oversimplifying a little, manure seems to be split between big particles and little particles. That is, almost all the little particles are small enough to get through any opening 0.010" or larger, while most big particles will be caught by an opening as long as it is under 3/32" inch (2-1/4 mm). (We tested 5/32" perf, and abandoned the effort.)

Manure Variations

September 22, 2000                                                                                                                                                                                               ISSUE M11

It is the goal of Vincent Corporation to have one basic press design that will work in all manure applications. We feel that it is reasonable to expect to separate all manure types with a single screw configuration, one screen selection, and one discharge plate mechanism. It is because of this goal that each production run of Series KP presses shows some improvement over its predecessors.

A wide variety of manure is being run through KP presses. The machines have proven themselves in four broad areas: cow, pig, biogas digester sludge, and slaughterhouse pauch manure. The varieties within these categories are surprising.

Cow manure general is split into scrape and flush barn systems. Obviously one has thicker solids than the other, and consistent agitation is required for consistent feed. However an equally important variable is the type of bedding that is in use. After all, the bedding can be a major component going into the pit. Straw and shredded paper make a great press aid and facilitate operation of the separator. Sawdust will press well, but it will draw more horsepower. (Of course, sawdust can range from sawdust to wood shavings or sanding dust; they behave differently in the press.) When manure is the only bedding used, we must count on the screen to pass the tiny digested solids into the press liquor flow; otherwise they tend to blind a screen.

Pig manure and sludge from a biogas manure digester have an important characteristic in common. Both have a high percentage of micron size particles that pass through the screen of the press. Thus, the solids capture rate is lower than when there is a preponderance of large particles of undigested food.

Definite variations exist in pig manure. For example, in Alberta, barley is fed to pigs. This is a coarse grain that puts fiber in the manure that facilitates press operation. On the other hand, in the States the feed is milled to 750 micron average particle size. This makes a manure whose tiny solids are hard to capture.

The absence of bedding in pig manure is normal, as is a lack of agitation. Flow starts with thick solids, material that has floated to the top. Then there is a prolonged flow of highly clarified water, followed by bottom sludge. To handle this range of conditions requires careful press design.

Manure as Bedding

October 20, 2001                                                                                                                                                                                                 ISSUE #M20

For many years leading agricultural universities have recommended dewatered manure as a dairy bedding material. Until relatively recently the recommendation has been that the manure be composted prior to use in the stalls.

The dewatering requirement gave rise to a number of pieces of equipment that are commonly called manure separators. They separate free water from the manure.

The reason for the composting requirement was twofold. Firstly, some manure separators leave a visible amount of moisture with the manure solids. This material is too soupy or slushy for use as bedding. Composting allows removal of the excess moisture.

The second reason for composting had to do with sanitation and disease control. It seemed obvious that fresh manure was full of bacteria that could cause mastitis in the herd. And it was shown that the heat generated in composting killed off the great majority of these bacteria.

Composting is no longer being regarded as an absolute requirement. Two observations have led to this. One observation was that cows in southern states, like Florida, wallow in ponds in order to keep cool. These ponds contain a great deal of manure, and yet mastitis is a manageable problem.

Another observation was that the few bacteria left after composting multiply exponentially. Composted manure ends up in the barn with the same bacteria count as the original material. Despite this, mastitis was shown to have low incidence in dairies using composted manure as bedding.

This has led to the use as bedding of material straight from the manure separator. This practice has been used at the Ohio State farm in Wooster, Ohio. Press cake from a Vincent Model KP-10 press has been used immediately as bedding for over a year.

The excellent dewatering characteristic of screw presses is recognized. This is resulting in increasing popularity of these machines on the farm.

Matlink Digester (Continued)

MAY 28, 2002, Revised September 2010                                                                                                                                                                 ISSUE #M23

Three months ago we wrote about an extremely smooth start-up that had taken place at the Matlink Farm in Clymer, New York.  This involved a manure digester system designed by RCM of Berkeley, California, 510-658-4466. 

The Vincent screw press worked very well dewatering the digester sludge.  However this only lasted for a few months during which sawdust was used as bedding.  When the farm switched over to using composted press cake, troubles were encountered.  The gpm capacity of the press went down by about 50%. 

A worse problem was that the press would occasionally go into a purge condition.  This condition is commonly referred to as having the press "blow the plug" at the cake discharge.  At this time unpressed manure flows through the press without any dewatering, flooding the area where press cake normally forms a pile.

It was observed that the purge condition occurred when firm, solid press cake held open one side of the discharge door.  This reduced the pressure against the cake on the other side of the door.  The cake would become more and more moist, eventually reaching a condition where it flowed, without being pressed, past the door.

After two visits and testing a number of experimental doors, a very satisfactory solution was developed.  The shaft of the screw was extended an additional 14".  This makes the screw extend well past the cake discharge of the press.  A door with a hole in the center, to allow for the extended screw length, was required.  The result was absolute:  there is no longer sufficient door area for cake to hold the door open.  The purging characteristic has been totally eliminated.

Retrofit kits are being produced to modify manure presses that have had problems with purging.  The kit consists of an extension that is welded to the end of the screw shaft along with a new door and yoke style actuator arms.  The kits can be installed without the need to disassemble the machine. 

At the same time a screw design has been developed that drastically increases the capacity of the press on digester sludge.  This design, operating at Matlink, includes double flighting in the inlet portion of the press where the manure is thin.  This design has also been found to significantly improve screw press capacity on flush barn systems where press cake is used for bedding.

TEN YEARS LATER
September 2010
The farm entered bankruptcy and the digester is no longer being operated.  The Vincent press was purchased by Noblehurst Farms.  Today we would eliminate the purging problem by using half pitch (not double pitch) flighting and the rotating cone feature.

Matlink Digester

February 13, 2002                                                                                                                                                                                                    ISSUE #M22

An extremely smooth start-up has taken place at the Matlink Farm in Clymer, New York. Owned and operated by Ted Mathews, the farm has installed a manure digester and electricity generating system designed by RCM (Resource Conservation Management, Berkeley, California, 510-658-4466). The system employs a 660,000 gallon stirred pit digester. Manure from the 1,200 head dairy is scraped and pumped to the concrete pit.

Digested manure floats over a weir and into a secondary pit. The manure in this secondary pit is pumped to a Vincent Model KP-10 screw press. In normal operation the press dewaters the manure at the rate of 120 cows per hour.

Methane from the digester is burned in a Waukesha internal combustion engine. This engine is coupled to an induction type electric generator. The rating of this generator is 130 kilowatts (175 hp), at which rate the system is currently operating.

Waste heat from the motor that drives the generator is used to heat the digester. Cooling water from the radiators is circulated through the pit so as to maintain a temperature of approximately 100° F.

Circulation is maintained in the digester with a pair of sidethruster propeller pumps. These are positioned diagonally opposite in the sides of the pit. 

Pig Manure

Trials running pig manure through our KP presses have been run in the States and Canada, and KP-6 presses have been sold to pig farmers in Costa Rica and Sri Lanka. In all cases the press has done its job of separating solids and liquids, but machine sales did not result in the States or Canada.

Sand Bedding

October 24, 2000, Revised April 2003                                                                                                                                                                      ISSUE #M12

Questions about sand bedding as it relates to manure separators came up many times at this year's World Dairy Show. It is clear that sand bedding is very popular, and dairy farmers are well aware of the abrasion it causes.

Earlier this year a Vincent KP-10 was tested at the Everett Williams 400 head dairy in Pennington, Georgia. Good results were expected because of minimal wear experienced at McArthur Farms in Okeechobee, Florida. McArthur's has a great deal of coral sand in their manure, their press has held up very well over a period of years. (Although McArthur's is currently adding settling basins.)

The opposite occurred at Williams' farm. In only a few weeks the Georgia sand wore out a screw. The screw was replaced with one using improved technology: the screw flights were made of abrasion resistant plate, with two layers of MIG applied hardsurfacing followed by TIG applied Stellite or Colmonoy. This was a notable improvement and minimal wear ensued.

However in no time at all the screen was worn through. (The latest Vincent manure presses use sleeve insert screens that are inexpensive to replace.)

It was concluded that sand bedding and screw presses do not mix.

This same knowledge applies to other manure separators exhibited at the World Dairy Show. A drag flight conveyor separator is vulnerable because of chain and sprockets that must operate in the sand/manure environment. The same goes for the wringer/roller type machines.

Our advise to farmers who use sand bedding was to either have good separation of the sand and manure ahead of the separator, or to avoid manure separation. Sand can be separated by flowing the manure through a trench with velocity such that the sand settles out and the manure stays in the flow stream.

If this separation is not possible, then the farmer is better off draining the manureand sand into a pond, which will have to dredged periodically. If they buy a mechanical separator based on savings of avoided dredging costs, they are likely to face repair and parts expenses before the payback period is complete.

Screen Patching

It is not uncommon for the screen of a screw press to wear out. Abrasion from dry manure is a cause. Also, operating with a worn screw puts increased loading on the machine. This can cause the screw to shift sideways, leading to rubbing between the screw and the screen. 

When a screw is tight against a screen, the screen material bows outward, minimizing wear. Consequently wear tends to occur where there are reenforcing rings on the screen that prevent it from bowing away from the screw.

Screen Patch Kits are available for all the Series KP presses. The patches supplied by Vincent are made of 11 gauge steel (1/8" thick) with 3/8" hole perforations. This assures plenty of draining surface even when extensive patching is required.

The patches can be welded in place without having to remove a screen from a press.

The kits can be used only if the screen is still relatively intact. If it has been torn into two or more pieces or if it has been beer-canned, it is likely beyond repair.

The kits consist of a series of pairs of half-cylinder sleeves that fit around the outside of the screen. Each sleeve is about 180º so that the two pieces of a pair can be wrapped around a worn or torn part of a screen.

Once the sleeves are in position they should be TIG welded in place. (Stick welding can be made to work.) The width of the patch sleeves is such that they fit between two adjacent reinforcing rings of the original screen. The patches are welded to these rings and to each other.

The patches, like the original screens, are made of stainless steel.

In a bind, patches can be made of any sheet metal. Because of the excess of open area in the original screens, patches need not be made of perforated material.

Screw Removal and Replacement

Removing and replacing the screw of a Series KP press is a matter of pushing or pulling against an internal snap ring. This snap ring is located in the hollow shaft of the gearbox. A large washer, almost as large in diameter as the hollow shaft of the gearbox, is seated on one side or the other of the snap ring. All-thread rod and a nut are used to push or pull against this washer in order to move the screw in the desired direction.

Complete tool kits and drawings of these kits are available upon request. The Operating Hints section of the O&M manual has a number of helpful suggestions:

The snap ring, washer, and all-thread rod combination must be used to remove the screw from the press. Using a wheel puller can destroy the gearbox.

It is common that heavy force may be required in either removing or reinstalling a screw.

Be sure to lubricate the screen when reinstalling a screw, and be sure to apply Never Seize to the portion of the screw shaft that goes into the gearbox.

Heavy industrial snap ring pliers will be required for use with the internal snap ring.

A special nut with a lug that catches in the keyway is required when pushing the screw out of the press.

When pushing a screw out of the press a steel disc should be placed over the end of the all-thread rod in order to prevent it from screwing into the threaded hole in the end of the screw.

When reinstalling a screw, the screw must be pulled in until the step in the shaft seats against the thrust bearing of the gearbox. Be careful when guiding this step in the shaft through the shaft seals. It is best to remove the four bolts holding the seal housing during the screw installation.

When pulling a screw into the press the all-thread rod must be screwed into the end of the screw. This can be done before starting to put the screw into the press.

Types of Manure Separation

March 8, 2000                                                                                                                                                                                                        ISSUE #M4

There are four different types of machines commonly seen separating dairy manure: sidehill screens, drum screens, drag flight conveyors, and screw presses.

Sidehill screens are the least expensive and require the least maintenance. These are simple sloped screens (alternatively called static and gravity screens) with a weir box at the top. The manure is pumped to flow over the dam of the weir box and down the screen. The filtered liquid goes through the screen, while the solids (tailings) fall off at the bottom.

The disadvantage of a sidehill screen is that even under the best of operating conditions the solids come off as a wet sludge. This sludge is almost too wet to compost, and it is difficult to handle for spreading on the field. Additionally, sidehills tend to blind over, requiring a farm hand to wash down the screen. A sidehill will keep a lot of solids out of the pond, but it is definitely not a strong manure separator.

Drum screens use a drum made of perforated metal to separate the solids from the liquid. Alfa Laval previously offered one, and Houle currently has a unit in their line. These will produce drier cake than a sidehill. However they are rarely popular. Maintenance is frequent because of all the moving parts and adjustment; screens can be damaged by large solid tramp materials; and they are exceptionally dirty.

Drag flight conveyor separators come in two varieties. The Blossom and Agpro machines are single pass: the lower end is submerged in the manure pit, and the solids are dragged up perforated or wedgewire screens to where they fall into a small screw squeezer. The Albers unit is a double pass design. The manure starts by being dragged down a sidehill. It makes a U-turn in a sump and is dragged back up to where it falls into a roller squeezer.

Drag flight separators have the disadvantage of many moving parts: chain, sprockets, drag flights, bushings, besides an optional squeezer at the discharge. Leakage and large size are negatives. The manure cake is dryer than achieved with a sidehill screen, but not up to that produced by a screw press.

The screw press, which produces the driest cake, is the fourth category of manure separator. These are designed to operate intermittently, unattended, with a minimum of maintenance. They will be further described in later Pressing News issues.

Vacuum Testing

March 1, 2001, Revised August 2001                                                                                                                                                                           ISSUE #M14

In February tests were run to test vacuum on the outside of the screens of a screw press. The testing was done with a Model KP-6 press operating at the Lewisburg dairy of the University of Tennessee. Manure is pumped to the press from the pit into which it is scraped. There is a recirculation line from the inlet of the press back the pit, which assures that the inlet hopper will not become pressurized. The press liquor drains though a hose, going down 12' to a point where the drain line could be either open-ended or sealed under liquid in a 5-gallon pail.

The tests were facilitated by the fact that the screen of the KP-6 is surrounded by an integral, airtight, combination cover and press liquor collection pan. A hole was drilled through this cover, and a manometer was inserted to measure the vacuum surrounding the screen.

We were in for a number of surprises. While there are several unanswered questions, some facts were established and best-performance conditions were observed. The facts are that a vacuum can be formed on the outside of the screen, and the throughput capacity of the press easily went up 25% to 100% with vacuum conditions. The vacuum ranged from 8" to 42" water column (28" w.c. equals one psi). The best results where when we had the vent pipe open and the end of the drain hose submerged.

Dave DeWaard at DariTech in Washington is insistent that there must be an open vent line on the inlet hopper of the press in order obtain maximum capacity. Previously we thought the vent line was needed to let air out of the press on start-up. However it is now hypothesized that press capacity goes up with the vent line open because the recirculation line carrying manure back to the pit draws a vacuum in the inlet hopper, counteracting the vacuum in the drain line. An intermittent gurgling sound like a flushing toilet was heard from the press during our tests with an open vent line.

Why Separate Dairy Manure?

April 5, 2000                                                                                                                                                                                                          ISSUE #M5

The use of manure separators is practically unknown in many areas, while in other regions no dairy is without a device of some sort. There are reasons why this is true, and it is equally true that the trend is toward the expanded use of manure separators.

There are different justifications for the use of manure separators. Probably the strongest reason is to keep solids out of the manure ponds and to get the manure solids into a form that can be land applied without building a cake layer on the pasture or field. If un-separated manure is sent to the pond, tank or Slurrystore structure, settling can become a problem. This is especially true in flush barn ponds where a source of clarified water is requisite. The repeated cost of dredging a pond can be sufficient to justify installation of a separator.

Spraying the raw manure on a field or pasture is not a foolproof alternative. Flush barn farms usually need the recycled water. Besides, the wet manure can build up a mat layer so dense that grass will not grow through it. In contrast, composted manure solids are readily tilled into the soil, with strong benefits from fertilizer and organic addition.

There are areas where an altogether other justification exists for using a manure separator. Many farmers, especially in the north central States, use the composted manure as bedding for their cows. This represents a direct savings of cash that would be spent on sawdust, sand, ground paper, straw, or other bedding. (Farmers in other areas fear that, because of the climate, disease outbreak may occur with the use of manure bedding, so the practice is not used among them.)

Many other reasons are given for using a manure separator. In every county there is the story of the farmer who is getting rich raising worms in the compost, selling potting soil to nurseries, or selling bags of compost for $4.00 each. There is less discussion of the practice of blending press cake (which is mostly undigested feed, 50% protein) in the ratio of 5% for heifer and non-lactating cow ration.

Odor reduction is probably the greatest fringe benefit of using a manure press. However, environmental regulations have not reached the point of mandating the use of these machines.

Wooble-Stick Switch

October 24, 2002                                                                                                                                                                                                 ISSUE #M25

The screws of screw presses used in manure are subject to abrasive wear. This wear does not occur in normal operation nearly as much as it does in a condition of "running dry". If a manure press is allowed to continue running when there is little or no cake being produced, the cake remaining in the machine becomes bone dry. This dry material, even without sand being present, is extremely abrasive.

Because of this it is recommended that a wobble-stick switch be incorporated in the electrical control system. The switch is mounted someplace below the cake discharge so that the wobble stick is in the path of the falling press cake. This is illustrated in the sketch below.

The switch should be connected to a timer such that, if the switch does not detect activity for a while, like three minutes, the manure pump and press are shut down.

Shutting down the system when cake is not being produced is advantageous for several reasons:

1. Even though the press is not producing cake, as long as the screw is turning the flights are being worn down. In this stalled condition, the cake at the discharge remains stationary, tends to get very dry and even hot, and the amount of abrasion occurring to the circumference of the flights increases substantially.

2. If cake is not being produced, it is likely that the pit has been drawn down to where the capacity of the manure pump has been seriously reduced. This comes about because of the increased height that the manure must be pumped. (The pumped height is from the level of the pit to the inlet of the separator, not from the pump suction to the inlet of the separator.) It is time to shut down the system.

3. If very little cake is being produced, water on the inlet end of the screen will soak into the dry cake at the discharge. In this manner the lower portion of the plug at the discharge becomes sopping wet to the point where it breaks loose and flows out. The discharge door is held open by the dry material on the upper portion of the plug, and purging will begin. Shutting down the system before this happens can prevent the mess that comes with blowing the plug.

Square D offers wobble-stick switches and timers, as do many other companies. They can be bought through McMaster-Carr, Grainger, and Gulf Controls among others.