Filter precoat filtration

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. 

Precoat Procedure Precoat filtration tests are run in exactly the same manner as bottom-feed tests except that the leaf must first be precoated with a bed of diatomaceous earth, perhte, or other shave-able inert sohds. Some trial and error is involved in selecting a grade of precoat material which will retain the filtered solids to be removed on the surface of the bed without any significant penetration. During this selection process, relatively thin precoat beds of I to 2 cm are satisfactory. After a grade has been selected, bench-scale tests should be... 

The depth of cut involved in precoat filtration is a veiy important economic factor. There is some disagreement as to the method required to accurately predic t the minimum permissible depth of cut. Some investigators maintain that the depth of cut can be evaluated only in a quah-tative manner during bench-scale tests by judging whether the process solids remain on the surface of the precoat beck This being so, they indicate that it is necessaiy to run a continuous pilot-plant test to determine the minimum permissible depth of cut. The use of a continuous pilot-plant filter is a veiy desirable approach and will provide accurate information under a variety of operating conditions. 

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.)... 

They are made by many companies, and they enjoy their widest use for filter-aid precoat filtration. 

A mixture of the hydrolyzate with diatomaceous earth, added as a filter aid, is then filtered on rotary, vacuum, precoated filters. The filtrate is a clear, slightly yellow, sweet-tasting liquor containing about 0.1% of protein and 0.2% of ash. The rest is carbohydrate, made up of 95.0% of D-glucose, 2.0% of maltose, 1.6% of isomaltose, and 1.4% of tri- and higher saccharides. 

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. 

Filtration Equipment Good filtration equipment and proper operation are essential if the filtration process is to be effective and economical. Modem-day bleaching earths should not, except in rare cases, require the use of filter aids such as diatomaceous earth. Rather, the clay is used as the precoat. According to Veldkamp (123), the two main objectives in precoat filtration are as follows ... 

Diatomaceous earth filtration (or precoat filtration), accomplishes particle removal by physically straining the solids out of the water. The thickness of the initial layer of diatomaceous earth (DE) filter medium is normally about 1/8-in. (3 mm), and the water... 

Diatomaceous earth filtration, also known as precoat filtration or diatomite filtration, relies on a layer of diatomaceous earth about 0.3-cm (1/8-in.) thick placed on a septum or filter element. The septums may be placed in pressure vessels or operated under vacuum in open vessels. A schematic diagram of a typical pressure system is shown in Fig. 2. 

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. 

Vertical filters are not convenient for the removal of dry cake, although they can be used in this service if they have a bottom that can be retracted to permit the cake to fall into a bin or hopper below. They are adapted rather to wet-solids discharge, a process that may be assisted by leaf vibration, air or steam sparging of a filter full of water, sluicing from fixed, oscillating, or traveling nozzles, and blowback. They are made by many companies, and they enjoy their widest use for filter-aid precoat filtration. 

After the enzymatic treatment the juice can be clarified. Flocculation aids and fining aids (gelatine, silica sol, etc.) help to coagulate the cloudy substances and facilitate their separation by settling and filtration. Bentonites can be used to eliminate proteins and other cloudy substances. Filtration (Kieselgur precoat filtration with plate and frame filters, rotating vacuum filters, and sheet filters as police filters ) is used to produce a crystal clear juice. 

FIGURE 22.34 A Sparkler leaf filter with leaves in horizontal position during precoat, filtration, washing, and drying and vertical position during the cleaning cycle. 

It is both convenient and reasonable in continuous filtration, except for precoat filters, to assume that the resistance of the filter clothplus filtrate drainage is neghgible compared to the resistance of the filter cake and to assume that both pressure drop and specific cake resistance remain constant throughout the filter cycle. Equation (18-51), integrated under these conditions, may then be manipulated to give the following relationships ... 

It is recommended that both precoats should be prepared with water or filtered wine. Filtration of the wine may start as soon as the filter precoat has been prepared. The outer surface of the filter layer is constantly renewed by continuous accretion, generally with the same adjuvant or mixture of adjuvants. This prevents rapid clogging of the precoat and increases the length of the filtration cycle. 

Untreated wine, before filtration Filtration through diatomaceous earth precoat Filtration throngh sterilizing filter sheets... 

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