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.