Body feed systems

Body feed addition may be dry or in slurry form. Slurry concentrations can be as high as 18% but are best kept at 5-10% for lowest pump maintenance. Whatever system is used, it should have the following properties  

Body feed pumps work best if run continuously, recirculating back to body feed tank only when the filter is off-stream for cleaning. Piston pumps should have ceramic pistons, and flush-out connections on packings may be desirable. Outlets on single outlet diaphragm pumps should be at the bottom of the diaphragm chamber to prevent accumulation of filter aid in the chamber. 

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Considerations in design of the body feed system are quite similar to those above related to the precoat. The major difference is in the rate of utilization. Body feed is added continuously at low rates. Centrifugal pumps are not appropriate in most systems piston or diaphragm pumps are used instead. These should have variable capacity, which can be obtained by varying stroke length or speed. Filter aids also can be educted into the brine. 

In the precoat and body feed mode, filter aids allow appHcation of surface filtration to clarification of Hquids, ie, filtration of very dilute suspensions of less than 0.1% by volume, such as those normally treated by deep bed filters or centrifugal clarifiers. Filter aids are used in this mode with pressure filters. A precoat is first formed by passing a suspension of the filter aid through the filter. This is followed by filtration of the feed Hquid, which may have the filter aid mixed with it as body feed in order to improve the permeabiUty of the resulting cake. The proportion of the filter aid to be added as body feed is of the same order as the amount of contaminant soHds in the feed Hquid this limits the appHcation of such systems to low concentrations. Recovery and regeneration of filter aids from the cakes normally is not practiced except in a few very large installations where it might become economical. 

FIG. 18-109 Filter-aid filtration system for precoat or body feed. (Schweitzer, Handbook of Separation Techniques for Chemical Engineers, p. 4-12. Copy-light 1979 hy McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.)... 

In the operation of a filtration system, the filter is first precoated by circulating a mixture of filter aid and clear or filtered liquid from the precoat tank through the filter and back to the precoat tank. This is continued until all the filter aid is deposited on the filter. The body feed injection system is then started and the filter is changed over, with minimum fluctuations in pressure, from precoating to filtering. 

Have a means of regulating the rate of body feed addition while the system is mnning. Feed rate should be constant at each setting. 

During filtration, body feed, or the continuous addition of filter aid, can help prevenr blinding of the suspended solids on the precoat. The body feed surrounds the suspended solids to provide flow around them. The body feed slurry of filter aid and oil is injected into the system prior to the filter. The suspended solids are ridged or deformable and can elongate under pressure to extrude through the filter cake and slow or block the product flow. Body feed coats the deformed solids, allowing them to be retained on the filter cake. 

Not all filtration applications require body feed. If there is little to be gained from its use, it becomes counterproductive as it once again reduces the holding capacity of the filter for the precipitated solids. In some applications, body feed is added directly to the liquid to be filtered, removing the need for a tank and slurry delivery system. This approach has problems of its own, including extra wear on the main pump and sometimes inadequate dispersion. It is not common in the chlor-alkali industry. 

The end section of the fabric on the body of the truck and the attached cutting device are lifted after each automatically laid ply. These are driven by the sensor of a lifting device to avoid contact between preceding spreads and newly laid plies. During the spreading process, the fabric feed system automatically adjusts the feeding speed of the material and measure its length. 

Precoat filters in amine filtration. serviee use a diatomaceous earth or perlite filteraid pre-coat (typically about 10 pounds of filteraid per 100 square feet of filter surface area) and a body feed of 1 pound of filteraid per pound of solids removed. They usually filter a 10% slipstream, and depending on the type of filter aid, can remove particles as small as 1 micron. Precoat filters were widely used in large amine systems (greater than 1000 gpm circulation) in the 1070s. However, precoat filters require significant operator attention, and amine losses can be substantial unless they are carefully operated. For these reasons, precoat filters are no longer widely used for amine filtration. 

DE filters require slurry mixing tanks, injection pumps, and large quantities of body feed in addition to the filter vessel itself. Therefore, these systems are expensive to install and to operate, and they require much more space than do other filters. 

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