Body feed

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

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. 

Materials suitable as filter aids include diatomaceous earth, expanded perilitic rock, asbestos, ceUulose, nonactivated carbon, ashes, ground chalk, or mixtures of those materials. The amount of body feed is subject to optimisa tion, and the criterion for the optimisa tion depends on the purpose of the filtration. Maximum yield of filtrate per unit mass of filter aid is probably most common but longest cycle, fastest flow, or maximum utilisation of cake space are other criteria that requite a different rate of body feed addition. The tests to be carried out for such optimisation normally use laboratory or pilot-scale filters, and must include variation of the filtration parameters such as pressure or cake thickness in the optimisation. 

Filtration. Any type of clarification is foUowed by filtration through leaf-type vertical or horizontal pressure filters. Carbonatated Hquors, containing calcium carbonate, may require addition of diatomaceous earth as a filter precoat. Phosphatated Hquors are generally filtered with the addition of diatomaceous earth as precoat and body feed. 

Typically, a filter cake or precoat is built up on the filter septa to prevent blinding, short filter cycle times, and cosdy cleaning of the septa. Then diatomite is added as body feed to the Hquid to be filtered so that the permeabiHty of the filter cake may be maintained. Filler aid permeabiHty of diatomite ranges from 0.06 to 30 lnF. At the end of the filter cycle the filtrate is clear and the soHds are retained in the soHd or semisoHd diatomite filter cake. The type and amount of diatomite for precoat and body feed are normally deterrnined by pilot studies (18,19). 

Schweitzer, op. cit., sec. 4.2) and Hutto [Am. Tn.st. Chem. Eng. Symp. Ser, 73(171), 50 (1977)]. Figure 18-109 shows a flowsheet indicating arrangements for both precoat and body-feed applications. Most filter aid is used on a one-time basis, although some techniques have been demonstrated to reuse precoat filter aid on vertical-tube pressure filters. 

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.)... 

White wines that are to be aged may be put into oak barrels and tanks at this point or they may be chilled to 28° F (-2.22° C) for ten to fourteen days, rough filtered through nonasbestos pads with a diatomaceous earth body feed, then aged in oak containers. White wines that are to be bottled without aging are blended and finished at this point. 

Filtration. Filtration in Washington wineries is performed almost exclusively with plate-and-frame filter presses. In some cases, these filters have wide frames so that they are capable of performing diatomaceous earth filtrations. There is only one stainless steel, screened, diatomaceous earth, pressure-leaf filter in use in the Washington wine industry. Two major wineries have found they can utilize a paper septum over a medium pad in their plate-and-frame filters. This allows them to precoat and body feed with diatomaceous earth as the wine is filtered. This accomplishes both a coarse and medium filtration in one movement of the wine. 

The industry standard method for beer hltration is using diatomaceous earth (DE), a sand-like powder with a very high internal surface area and a hne porous strucmre, as a hlter aid in depth- and cake-type hltration. Conventional beer clarihcation employs hlter presses or pressure vessel hlters that are pre-coated with porous diatomaceous earth particles as hlter aids. In order to minimize clogging of the hltration media and maintain a porous cake, and thus extend the duration of the hltration mns, hlter aid is conhnuously metered into the unhltered beer as body feed [42]. 

For a given liquid, clarity of filtrate is governed principally by (a) grade and amount of filter aid for body feed (b) grade and amount of filter aid for precoat (c) length of cycle and (d) filtration rate. 

An additional recommendation is to use coagulant (alum or a suitable polymer) to coat the body feed to improve the removal rate for viruses, bacteria, and turbidity. Adding these chemicals to the coating does not improve Giardia removal rates. 

Continuous addition of filter aid (body feeding) is accomplished either by feeding filter aid as a slurry or by dry feeding. Slurry feeding is usually done with plunger or diaphragm pumps. If filtration is a batch process, the filter aid can be added directly to the batch. 

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. 

Fig. 5. Filtration cycle length affected by body feed.

The effects of excessive body feed are shown on Fig. 5. The sudden increase in pressure shown on curve 3 is the result of bridging of the cake between the leaves. This causes a sudden decrease in filter area. It can also result in severe damage to the leaves and loss of clarification. 

Short cycles can be caused by temporary or permanent stoppage of body feed addition, blinded septa, changes in characteristics of the liquid being filtered, entrapment of air in the filter, which will decrease the filter area, and too high a filtration rate. 

Preparation of filter body feed (diatomaceous earth) and precoat Verification of proper dosages Periodic backwashing Disposal of spent filter cake... 

In some cases, alum precoating of the diatomaceous earth can improve performance. Typical alum doses range from 1% to 2% hy weight (1-2 kg/100 kg of diatomaceous earth applied). Typical precoats of diatomaceous earth of 0.49-0.98 kg/m (0.1 to 0.2 Ih/ft ) are applied to prepare the filter. These filters also require a continuous supplemental body feed of diatomite because the filter is subject to cracking. If the filter has no body feed, there will be rapid increases in headloss due to buildup on the surface of the filter cake. Body feed rates must be adjusted for effective turbidity removal. Diatomaceous earth filters do not need a filter-to-waste cycle, because of the precoating process. 

Filter runs typically range from 2 to 4 d. The rate of the body feed and the diatomite media size are critical in determining the filter run length. Shorter filter runs will minimize... 

Tubular filters (Fig. 10) offer low cost construction and high hydraulic capacities. They are made with both rigid and flexible tubes. Celite is used on the rigid tube filters in the usual combination of precoat and body feed. With flexible tube filters, instead of body feed, an extra heavy precoat [25-30 lb (11.4-13.6 kg) filter aid per 100 ft (9.29 m ) filter area] is used. After filter pressure has reached a maximum, the precoat is bumped from the tubes, re-slurried, then re-deposited. This sequence is repeated until pressure is no longer reduced significantly, at which time the precoat is discarded and a new one applied. 

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 ... 

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

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. 

The waste residuals are easily dewatered and in some cases may be reclaimed for other uses, including soil conditioning and land reclamation. Research is under way to determine the feasibihty of reusing filter medium as body feed (7-10). 

Admix A substance that is added directly into the batch tank of prefilt to create a permeable filter cake. Usually used in place of body feed. 

Baffle A plate or deflector to provide flow distribution in a filter. Primary functions are to prevent erosion of precoat and setting of body feed in the filter tank. 

Body Feed A filter aid product, such as CELITE, that is continuously added to the filter while it is on-stream. Its purpose is to create a permeable filter cake. 

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