Precoat filtration applications

Precoat Filtration Applications, Advantages, and Disadvantages Summary... 

Economics Microfiltratiou may be the triumph of the Lilliputians nonetheless, there are a few large-industrial applications. Dextrose plants are veiy large, and as membrane filtration displaces the precoat filters now standard in the industry, very large membrane microfiltratiou equipment will be built. 

Filter aids may be applied in one of two ways. The first method involves the use of a precoat filter aid, which can be applied as a thin layer over the filter before the suspension is pumped to the apparatus. A precoat prevents fine suspension particles from becoming so entangled in the filter medium that its resistance becomes exces-sive. In addition it facilitates the removal of filter cake at the end of the filtration cycle. The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its compressibility, and reduces the resistance of the cake. In some cases the filter aid displays an adsorption action, which results in particle separation of sizes down to 0.1 /i. The adsorption ability of certain filter aids, such as bleached earth and activated charcoals, is manifest by a decoloring of the suspension s liquid phase. This practice is widely used for treating fats and oils. The properties of these additives are determined by the characteristics... 

The ability of an admix to be retained on the filter medium depends on both the suspension s concentration and the filtration rate during this initial precoat stage. The same relationships for porosity and the specific resistance of the cake as functions of suspension concentration and filtration rate apply equally to filter aid applications. 

In precoating, the prime objective is to prevent the filter medium from fouling. The volume of initial precoat normally applied should be 25 to 50 times greater than that necessary to fill the filter and connecting lines. This amounts to about 5-10 lb/100 fF of filter area, which typically results in a 1/16-in. to 1/8-in. precoat layer over the outer surface of the filter medium. An exception to this rule is in the precoating of continuous rotary drum filters where a 2-in. to 4-in. cake is deposited before filtration. The recommended application method is to mix the precoat material with clear liquor (which may consist of a portion of the filtrate). This mixture should be recycled until all the precoat has been deposited onto the filter medium. The... 

Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the plates and the filtrate flows to further processing. When the solids are fine and slow to filter a body-aid is added to the feed slurry in order to enhance cake permeability. However, it should be kept in mind that the addition of body-aid increases the solids concentration in the feed so it occupies additional volume between the plates and increases the amount of cake for disposal. Likewise, for all those applications when the cake is the product, precoat and filter-aid may not be used since they mix and discharge together with the cake. 

Body-aid (i.e., the filter-aid) and precoating are often mentioned in connection with pressure filtration and the difference in their application is (1) Body aid is used when the slurry is low in solids content with fine and slimy particles that are difficult to filter. To enhance filtration coarse solids with large surface area are added to the slurry and serve as a body-aid that captures and traps in its interstices... 

Figures 4-65, 4-66, and 4-67 show several units of the bag. The bags may be of cotton, wool, synthetic fiber, and glass or asbestos with temperature limits on such use as 180°F, 200°F, 275°F, 650°F respectively, except for unusual rnaterials. (See Table 4-12A and B.) These units are used exclusively on dry solid particles in a gas stream, not being suitable for wet or moist applications. The gases pass through the woven filter cloth, depositing the dust on the surface. At intervals the unit is subject to a de-dust-ing action such as mechanical scraping, shaking or back-flow of clean air or gas to remove the dust from the cloth. The dust settles to the lower section of the unit and is removed. The separation efficiency may be 99%-i-, but is dependent upon the system and nature of the particles. For extremely fine particles a precoat of dry dust similar to that used in some wet filtrations may be required before re-establishing the pi ocess gas-dust flow.

Disc filters are similar in principle to rotary filters, but consist of several thin discs mounted on a shaft, in place of the drum. This gives a larger effective filtering area on a given floor area, and vacuum disc filters are used in preference to drum filters where space is restricted. At sizes above approximately 25 m2 filtration area, disc filters are cheaper but their applications are more restricted, as they are not as suitable for the application of wash water, or precoating. 

With the exception of the precoat applications, RVF s do not usually yield absolutely clear filtrate. Although still widely used, rotary vacuum filters are, in some cases, being replaced by membrane separation technology as the method of choice for clarification of fermentation broths and concentrating cell mass. Membranes can yield more complete filtration clarification, but often a wetter cell paste. 

Diatomaceous earth of varying permeability, as well as mixtures of diatomaceous earth and cellulose, make filtration through precoats suitable for a wide range of applications. Table 11.2 shows the clarification of a turbid white wine filtered through three different types of earth. Filtration behavior may be predicted by laboratory tests (Section 11.6.2). This type of filtration is generally restricted to untreated wines, as one of the first stages in clarification. However, currently available fine earths may also be used to prepare... 

Tangential microfiltration has been used in many wine treatment applications over the past 10 years. Suitable membranes are now available for clarifying must or untreated wines, as well as the final clarification of prefiltered wines. This technique may also provide an alternative to filtration through diatomaceous earth precoats, especially in cases where waste discharges could lead to excessive pollution. Tangential microfiltration is still. 

Filter aids have been considered already in Chapter 5, and filtration process equipment will be discussed in Chapter 11. Precoat fihration is the application of a filter aid on a mainly conventional filter under clean conditions ie. in the absence of suspended solids... 

A more specialized application is the filtration of mercury-cell liquor to remove mercury from the product. We consider two types of filter here, the candle filter using carbon tubes and the pressure-leaf filter precoated with activated carbon. The operation of candle filters also has been described in Section 7.S.4.2. 

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