July 20, 2016
Carrageenan is a hydrocolloid which is a very popular food ingredient because of its gelling, thickening, and stabilizing properties. It is used in the production of toothpaste, salad dressing, and ice cream. Also, it is added to marinades and brines which are injected into meats, holding liquid for a juicier end product.
In a major test with carrageenan gel, we were working with the final product which had already had water separated from it with a belt press. That is the last step ahead of final drying. Vincent's goal in these tests was to see if a screw press could further dewater the cake from belt press.
Extensive testing was performed in the Tampa pilot facility. The entire operation can be seen on a YouTube video which averages 160 hits a month: https://www.youtube.com/watch?v=VY4oQWzeoJ0. Given that it is 27 minutes long, probably not many people watch the whole thing.
The material used for testing in these trials had 16% solids content. The best result achieved was press cake with 22-24% solids. It seems probable that the current design twin screw presses could achieve this with a reasonable throughput capacity. The savings in dryer fuel consumption would be very large. The increase from 16% to 22-24% solids represents removing 30% of the dryer's load.
[In another test with carrageenan from a different customer, our press cake came out with only 50% moisture. This had to be a different carrageenan, probably involving an alcohol process.]
Our first efforts were with a 6" twin screw press, the TSP-6. The carrageenan gel passed through the press with minimal water separation. So we piped in a McMaster Carr pressure multiplier. That took the air pressure on the discharge cone up from 5 to 10 bar. The press then forced more gel into the press liquor, but dewatering was still inadequate.
In an effort to clear the screen we tried an automatic reversing VFD program. The press ran continuously forward and backwards, hopefully breaking up the gel on the reverse turns. It didn't work.
Our second trial was with a Model KP-6 press with a perforated screen. It worked for only a while, and we saw that excessive fines were coming through the screen of the press.
Next we equipped that press with a fabric screen with openings of only 31 microns. This was achieved by fitting the fabric against a reinforcing layer of perforated stainless screen with 800 micron (0.033") holes. To prevent the fabric from being snagged by the flights of the screw, an inner layer of 800 micron perf was fit on the inside. While it worked, clear press liquor (water) was produced. However the layers of screening material soon plugged. We have never been able to overcome this problem despite a number of succeeding efforts with other materials.
Our third trial was with a laboratory Model CP-4 press. The press jammed almost immediately due to co-rotation of the material within the press. To prevent co-rotation, the inlet hopper of the press was pressurized with a 1,650 pound dead weight. This forced enough material through the press to show us how dry the carrageenan could be dewatered in a screw press. The video is almost comical at that point.
Our final test was back with the double screw Model TSP-6. The twin overlapping screws overcame the co-rotation problem. A characteristic of twin screw presses is that when material wants to co-rotate with one screw, the other screw forces the material to keep moving through the press.
Unfortunately little moisture removal was achieved. The material just could not be squeezed hard enough. Today we manufacture our twin screw presses with the screws further apart. This allows the shafts of the screws to be made in a conical form. The material being pressed is forced outward against the screen. Even at the time the video was made it was recognized that this would allow greater dewatering to be achieved.
Most of the presses used in these trials had wedgewire screens with slots which were 400 to 500 microns wide. Excessive fines came through these screens. However, when a slot width of 175 microns was used, the problem was solved.
Vincent would welcome the opportunity to demonstrate what can be done with a TSP press from our rental fleet.