A batch unit, the HP-hybrid filter press is typically used for products with "specialty chemicals volumes" in the range of500-3000 gallon batches. Products are processed in an enclosed atmosphere without operator or environmental exposure. FDA requirements, and conformation to good manufacturing practices (GMP) requiring containment of product, are continual issues. Applications are replacement of plate and frame filter presses, mid products that are compressible and amorphous in nature with

Fermented products in the post-broth stage, where volumes are smaller, can be applications as production rates are limited in this design. Postcrys-tallization can also be an application if solids are found to be compressible.

Replacement of cartridge filter systems where high filter replacement costs occur as well as low volume waste treatment streams can also be processed.

Residual moistures will be reduced in comparison to standard plate and frame units and RVF's by up to one-third, due to the high driving force created by the hydraulic membrane of up to 375 psi. Particles can be retained to one micron, which can eliminate the need for a precoat and save on waste disposal. As the cake developed in the pressure chamber is relatively even, and the wash delivered is also consistent, washing efficiency is high. (See Fig. 12.)

Figure 12. HP hybrid filter press. (Courtesy ofHeinkel Filtering Systems, Inc.) 12.2 Operation

This unit is a fully automated, totally enclosed filter press. The core of the system is a pressure chamber. It can be connected to peripheral equipment, such as a dryer or bin, for a totally contained system.

The pressure chamber consists of a perforated candle filter. On top of the screen is a filter cloth and a membrane constructed of EPDM, BUNA, or

Viton. At present these are the only available materials of construction. The membrane pressure is achieved hydraulically with water. Charging the slurry and washing the cake take place as the vessel toggles 180 degrees to ensure an even cake and wash distribution. Vacuum pulls the membrane back to allow entry of the slurry. Pressing occurs after the feed, then washing (if required), repressing and finally, solids discharge. Pressure can be varied depending upon the product. Figure 13 depicts the operation and cross-section of the pressure chamber.

Inverting the membrane and reversing air flow through the cloth while slowly rotating the system 180° back and forth releases all cake from the cloth. No operator attention is required for discharge of the solids, as no residual heel is left on the cloth. Vapors and product are contained.

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