Filter aid precoating

External-Cake Tubular Filters Several filter designs are available with vertical tubes supported by a filtrate-chamber tube sheet in a vertical cylindrical vessel (Fig. 18-115). The tubes may be made of wire cloth porous ceramic, carbon, plastic, or metal or closely wound wire. The tubes may have a filter cloth on the outside. Frequently a filter-aid precoat will be applied to the tubes. The prefilt slurry is fed near the bottom of the vertical vessel. The filtrate passes from the outside to the inside of the tubes and into a filtrate chamber at the top or the bottom of the vessel. The sohds form a cake on the outside ofthe tubes with the filter area actually increasing as the cake builds up, partially compensating for the increased flow resistance of the thicker cake. The filtration cycle continues until the differential pressure reaches a specified level, or until about 25 mm (1 in) of cake thickness is obtainea... 

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

Use of selected fibrous or granular filter aids (precoating), permitting the high-efficiency collection of submicrometer smokes and gaseous contaminants... 

Diatomaceous earths may resemble the forms of the charcoals. The earths are primarily filter aids, precoats or adsorbents, the hmction of the filter medium being secondary. Fuller s earth and clays are used for decoloring applications diatomaceous earths are used for clarification. The adsorbtivity of diatomaceous earth works in the same fashion as activated carbon, but isotherms (affinity) for many chemical species like the hydrocarbons is weaker. For this reason, activated charcoal or carbon is much preferred in wastewater treatment applications expecially when taste and odor issues are priorities. 

For smaller boiler plants, various sieves, screens, cyclones, and strainers may sometimes be used for filtering MU and FW. Also, bag, ceramic membrane, cartridge, and candle filters, either with a filter aid (precoat filters) or without (naked filters), all of which are typically available in single- and multiple-filter component vessel configurations. 

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. 

Clarification Ne gible cake Cartridges Granular beds Filter aids, precoat filters... 

Diatomaceous earths may resemble the forms of the charcoals. The earths are primarily filter aids, precoats or adsorbents, the function of the filter medium being secondary. Fuller s earth and clays are used for decoloring applications diatomaceous earths are used for clarification. 

Some general industry guidelines for filter aid precoating as applied to batch filters with a flat filter medium operating under pressure are given below. Examples of specific filter types that fall into this category are filter presses and leaf filters. 

In certain cases, however, the natural or piDcessh induced form of a material, for example, tea leaves, expeller calms, and mkrooiganisms, will dictate the use of a size outside the aforementioned range. In su cases it nu be necessary to use flocculatii agoits, pelletizing, filter aids, precoat filteis, or centri-fiiges to facilitate solid-Iiqnid separation or prevem flow difficulties. 

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