Double Pressing Basics

February 23, 1999
Rev. 2008

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.

In the production of juices for human consumption, double or even triple pressing is common. This is because the moisture in the press cake from first pressing contains many of the dissolved solids which are the essence of the juice. To capture these solids, water is added to the press cake. The dissolved solids in the cake diffuse into the water. The water is separated in the second pressing. This water carries with it the dissolved solids from the previous press cake. These are valuable solids which would otherwise be lost. Production of apples juice and coconut cream are two applications where double pressing is recommended.

The second application where double pressing is sound is in the production of animal feed from citrus waste.

In 1970 Dan Vincent was awarded a patent covering this double pressing concept. The system described pressing citrus waste in two consecutive presses, positioned in series. The idea was to reduce the moisture content of the press cake going into the peel dryer, thus reducing the amount of fuel required to dry the peel.

The key to this double pressing is to diffuse dissolved solids into the peel. This is done in a diffusion conveyor located between the first and second presses. High Brix (solids) molasses is added to the press cake from the first pressing. After a couple minutes of stirring in the diffusion conveyor, some of the solids in the molasses diffuse into the moisture in the press cake. Equilibrium is reached at a medium dissolved solids content, a point which is between the low Brix press cake and the high Brix molasses. When this cake is run through a second pressing, the cake resulting will have a higher Brix and, consequently, lower moisture content.

With a lower moisture content it takes less fuel energy to dry the cake into cattle feed. The result is a lower fuel cost per ton of pellets produced.

A material balance shows us how this works. We start with peel at 80% moisture and 10º Bx. In a 100# pound sample this is 20# total solids and 80# water. The 80# of water, at 10º Bx, has 8.9# of dissolved solids (mostly sugars). The rest of the solids, 11.1#, are suspended (insoluble) solids.

If we press this peel, the cake will still have 10º Bx. By diffusing 50º Bx molasses into the cake, the Brix will equalize at around 20º Bx. When this mass is run through the second pressing, the resultant cake will still have 20º Bx. In effect water in the first press cake has been displaced with dissolved solids from the molasses.

Some people argue against double pressing because "the second pressing only squeezes out the molasses added in the diffusion conveyor." This is not an accurate evaluation. Solids from the molasses will have been diffused into the peel. This is seen in the fact that the press liquor from the second pressing will have a lower Brix than the molasses that was added in the diffusion conveyor.

Copies of the 1970 patent are available upon request.

BRIX DEFINITION: DISSOLVED SOLIDS DIVIDED BY THE SUM OF THE DISSOLVED SOLIDS PLUS WATER, x 100.

NOTE THAT SUSPENDED SOLIDS DO NOT ENTER INTO THE EQUATION.

Issue 91