Permeability precoat filters

PRECOAT FILTRATION - DIATOMACEOUS EARTH FILTRATION. The mOSt economical method of filtering fruit juices with a high suspended solids content is with precoat filters. In these, the filter sheet is created by precoating a liquid-permeable filtering element with the filter aid. In addition to diatomaceous earth, perlite and cellulose are also used as filter aids in fruit juice production. 

Figure 2 illustrates the operation of a precoat filter with cake in place. Point A represents the entrance of the cake into the unfiltered suspension. Point B Fepresents the point of emergence and C is the point at which the cake, together with its accumulated suspension particles, reaches minimum permeability. Maximum filtration... 

Precoat filtration can be incorporated within a wide range of pressure filters including leaf, multi-element and plate and frame types (see Section 1.4.2). Up to 700 g m of precoat is typically filtered onto the filter medium prior to introduction of the feed suspension. The feed, which may also contain a significant addition of filter aid to improve cake permeability, is filtered until the filtrate flow rate is sufficiently low to warrant cake discharge in the normal way. It is not economical to recover the feed solids from the precoat, and it follows that washing of the solids is not practised. Moreover, the filter aid tends to abrade the pumps used to promote the filtration. Precoat pressure filtration is most often used for the removal of finer particles from dilute suspension where other potential processes would require too high an investment. 

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. 

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. 

Conventional filter systems are bound by one immutable constraint, namely, that clay particles must be separated from the clay/oil slurry by passing through a precoat of clay that is tight enough to retain all the suspended solids from the liquid to be filtered, but still permeable enough to produce economical rates of oil production. If particles become too small, they cause the filter rate to decrease so much... 

Any substances that can enhance the filtration efficiency are termed a filter aid. Diatomaceous earth is the most common filter aid for the precoat filtration system. An efficient, economical filter aid must (a) have rigid, intricately shaped, porous, individual particles (b) form a highly permeable, stable, incompressible filter cake (c) remove even the finest solids at high rates of flow and (d) he as chemically inert and essentially insoluble in the Uquid being filtered. Commercial diatomaceous earth, such as Celite diatomite, meets these requirements due to the wide variety of intricately shaped particles and inert composition that makes it practically insoluble in all hut a few liquids. 

It is beyond the scope of this entry to review the basic principles governing filtration. However, it is interesting to note that filtration produces a more concentrated and dewatered cell sludge (20-35% w/v) or cell solids (>40% w/v) than settling. A variety of filter media, membranes, and equipment are commercially available. In the case where the deposited cake is compressible with low permeability and thereby adds more resistance to filtration, filter aids or precoats often alleviate the problem. Two of the most widely used filter aids are diatomaceous earth and perlite. 

Dead-ended filtration of fermentation broth is complicated by the problem of low porosity and compressibility of the accumulated solids, which results in gradually decreasing permeability during the filtration cycle. With compressible solids, an increase in the differential pressure across the membrane can actually lead to a reduced permeation rate. This problem can be reduced by the use of fitter aids added to the broth and onto the filter paper as a precoat. The two most widely used filter aids are the diatomaceous earths and the perlites. 

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... 

A two-layer precoat must be prepared on the filter rack prior to starting filtration. The second layer activates the filtration cycle. The first, mechanical, layer is made using a coarse adjuvant (permeability above 1 Darcy), with the possible addition of 10% of a cellulose-based product. The quantities... 

Since the precipitates we deal with typically are compressible and do not form permeable cakes with desirable characteristics, it is standard practice to add a filter aid. Materials such as the flocculants that are used in brine treatment are sometimes referred to as filter aids because they improve the filterability of the particles that are to be removed. In our usage, however, a filter aid is a solid material added to the process to improve the characteristics of a filter cake. It should provide a noncompressible surface that does not quickly lose its porosity as solids accumiflate. Without such a material, the surface in a pressure filter would soon become blinded. The filter aid is applied to the filtering surface before flow of brine begins in order to provide a good foundation. A slurry is circulated from a tank through the filter until enough filter aid, or precoat, has deposited. 

Filter aids can improve the permeability and sometimes porosity of a filter cake, improve filtrate clarity and help to prevent filter medium blinding. They comprise relatively porous particles such as diatomite, perlite and activated carbon and are either filtered as a precoat onto the medium or mixed as body feed with the suspension during a pretreatment stage the latter beneficially improves the porosity of a subsequently formed filter cake. Both the cost of filter aid and the need to remove filter aid from the processed solids can present problems however, the use of filter aids on rotary drum filters and in the filtration of dilute feeds (such as those found in the brewing industries) can bring undoubted benefits. More detail on filter aids is provided in Chapter 2. 

Surface filters are usually used for suspensions with higher concentrations of solids, say above 1 % by volume, because of the blinding of the medium (or of the precoat) that occurs in the filtration of dilute suspensions. This can, however, sometimes be avoided by an artificial increase of the input concentration, in particular by adding a filter aid as a body feed as filter aids are very porous their presence in the cake improves permeability and often makes cake filtration of dilute and generally difficult slurries possible. 

Cellulosie fiber (ground wool pulp) is applied to eover metallie eloths. This filter aid forms a much more compressible cake with good permeability but displays a smaller particle retentivity than diatomite or perlite. The cost of cellulose is higher than that of diatomite or perlite. Thus this and other filter aids are only applied to special cases (such as precoat stabilization or chemical resistance). 

After the precoat, the water is introduced and filtration begins. A filter aid such as DE and cellulose fiber must be mixed with the water to promote an even build-up of filter cake and to maintain the filter cake s permeability. This combination is called "body feed." The weight of body feed should be roughly equal to the weight of the solids to be filtered. When the pressme drop reaches the high limit, usually between 25 and 35 psig (170 and 240 kPa), the filter cake must be back-washed from the leaves and the process started over with the precoat. 

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