Counterflow Wash Systems
March 29, 2015
Many industrial processes involve washing dissolved solids – usually sugars – from organic material or fiber. These systems are based on the principle of diffusion: if two masses, both containing water and dissolved solids, are mixed, the dissolved solids will flow (diffuse) from the higher concentration into the lower concentration mass until equilibrium is reached. Commercially these go by the names of either counterflow or countercurrent extraction systems.
Dissolved solids are measured in degrees Brix. This number is much like the percentage concentration of dissolved solids in a sample of organic material.
Lemon peel typically has 7º Bx. If the need were to wash the dissolved solids from a ton of this lemon peel, the peel could be mixed with water in a tank. Water could be added slowly, while the material was being agitated, until the liquid entrained in the peel came down to 4º Bx. Then the entire tankfull could be run through a screw press. The water separated would have dissolved solids measuring 4º Bx, and the moisture remaining in the lemon peel (now "press cake") would also have 4º Bx worth of dissolved solids.
To further reduce the sugars in this press cake, the cake could be put into a second tank. Water could be added until the Brix in the tank measured 1º Bx. Then that tankfull could be run through a screw press. The resultant press cake would have remaining moisture which would measure 1º Bx. The dissolved solids have been diffused from the lemon peel. The peel has been washed, and a product named pectin peel has been produced.
The disadvantage of such a system is that it requires a lot of water. In many industrial processes the water in the "press liquor" must be evaporated in order to produce a syrup or molasses. The less water there is to be evaporated, the lower is the energy consumption at the facility.
A significant reduction in water usage can be achieved by counterflow washing. If the peel washing were being done with a continuous flow of peel, the fresh water would be added only to the second tank, which is the tank with the peel with the least dissolved solids. Then, when the flow out of the second tank is pressed, the press liquor is pumped to the first tank. This press liquor will have a lower Brix than the incoming fresh peel, so diffusion will still bring down the Brix of the peel.
Thus it is seen that the fresh water goes into the second tank and flows backwards to the first tank, while the fresh peel flows from the first tank to the second tank. This is the basis of a countercurrent wash system.
Generally three wash stages are used, although Vincent has worked with systems which use as many as five stages. Applications where Vincent screw presses are used include:
- Washing lime or lemon peel in the preparation of dried peel from which pectin is produced.
- Washing tobacco stems and dust to extract the solubles ahead of making paper which is used as cigar wrapper or cigarette ingredient.
- Washing soybean white flake with alcohol in the production of soybean protein concentrate.
- Washing corn stover, switch grass, sugar cane bagasse, and other biomass in the production of material which can be fermented for the production of cellulosic ethanol.
- Washing shredded coconut meat in the production of cream of coconut.
- Washing juice pulp in the production of orange juice.
- Washing crushed apples and other fruits in the production of juice.
Another type of countercurrent washing involves admitting a fibrous material at one end of a drum or ribbon flight conveyor, with the clean water coming in at the other end. In the case of sugar beets and cranberries, the water flows downhill, gradually gaining concentration of sugars, while the organic fiber is conveyed gradually upwards, until it reaches the discharge where it has a final bathing in fresh water. The CONTEX unit offered by GEA Niro is an example, http://www.gea-pe.fi/nfi/cmsdoc.nsf/webdoc/ndkw73gh4q
Yet another system uses horizontal tray conveyors to carry the material, through compartments, from one end to the other. The best known is the Extractor produced by Crown Iron Works. Fresh solvent is sprayed in the final compartment of the machine. This liquid percolates, typically through oil seeds, and then it is pumped to be sprayed into the next to last compartment. Finally the hexane or alcohol with the greatest concentration of oil is washed through the fresh seeds entering the first compartment of the Extractor.
Similarly, Casa Cuervo uses a traveling tray extractor to wash sugars from shredded agave. In a step which follows, the sugars are fermented in the production of tequila.
The disadvantage of the drum, conveyor, and tray diffusers is that, without intermediate squeezing/pressing, they need to be very long in order to achieve high extraction/washing efficiency.
Vincent's interest in all these systems is that screw presses are used to separate the wash liquid.