Filter aids cellulosic

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

Graded Adsorbents and Solvents. Materials used in columns for adsorption chromatography are grouped in Table 12 in an approximate order of effectiveness. Other adsorbents sometimes used include barium carbonate, calcium sulfate, calcium phosphate, charcoal (usually mixed with Kieselguhr or other form of diatomaceous earth, for example, the filter aid Celite) and cellulose. The alumina can be prepared in several grades of activity (see below). 

Powdered resins systems (80-600 mesh) typically operate at lower pressure drops, generate less waste water, take up less space, and cost much less when compared to deep-bed polishers. However, they operate at only 3 to 4 gpm sq ft (although the filter element area is large) and are designed to be disposable, so that the powdered resin must be replaced whenever the bed is reconditioned. Sometimes inert resins or cellulose-based fibers are used either in place of powdered resins or as a premix, where they function as both filter aids and absorbents. 

Another filtration approach concerns the addition of a filter aid after reaction, like powered cellulose, celite or powdered graphite [6a]. The solution is filtered and the catalytic behaviour of both components, solution and filter aid, is then tested and compared with the catalytic behaviour before the filter aid addition. This methodology was first described by Maitlis and co-workers for distinguishing a heterogeneous component in a starting... 

Apart from not requiring filtering aids, such as a-cellulose, membrane filters are very simple to automate, since they can be immediately brought on-line after back-pulsing. Registers that are suitable for retrofitting have been developed by Permas-cand and Akzo Nobel and are employed by many other plants, such as those at Bayer Uerdingen. Their operation at these plants has been successful. 

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. 

Choosing the filter medium to be used in a pressure filter is a very important consideration. The medium should be selected primarily for its ability to retain the contaminant to be separated, and to have an acceptable life in the filter environment. Filter media are manufactured from cotton, synthetic polymers, glass, cellulose, metal, carbon, refractories, and other porous or perforated solids, and sand and other particulate solids. The filter s effectiveness may be enhanced by the use of filter aids. 

Filtration—After an adsorbent has selectively captured the impurities, it must be removed from the oil before it becomes a catalyst for color development or other undesirable reactions. Filtration, the separation method most often used for spent bleaching media removal, is the process of passing a fluid through a permeable filter material to separate particles from the fluid. Examples of the filtration materials used are filter paper, filter cloth, filter screen, and membranes. Filter aid, such as diatomite, perlite, or cellulose, are usually used in conjunction with the permeable filters for surface protection. Traditionally, either plate and frame or pressure-leaf filters have been used for spent bleaching media removal. Currently, self-cleaning, closed filters that operate on an automated cycle are available. 

A major alternative to direct flow membrane filtration is depth filtration, in which particles are removed throughout the filtration matrix rather than just at the membrane surface, by various mechanisms such as size exclusion, electrostatic, and hydrophobic interactions. Depth filters are typically composed of a bed of cellulose or polypropylene fibers together with an inorganic filter aid such as diatomaceous earth and a binder to form a filter sheet. The filter aid imparts the matrix very high surface areas and plays an important role in increasing both retention and the capacity. Depth filters can also have an electrostatic charge usually associated with the binder polymer. 

The use of asbestos has been greatly diminished because of its identification with health hazards. There have been proposed replacement materials such as the Zeta Plus filter media from the AMF Cuno Division, consisting of a composite of cellulose and inorganic filter aids that have a positive charge and provide an electrokinetic attraction to hold colloids (usually negatively charged). These media therefore provide both mechanical straining and electrokinetic adsorption. 

The precipitates have very poor filtration behavior. Therefore, cellulose is often added as a filter aid. 

Commonly used filter aids include diatomaceous earth, perlite, asbestos, cellulose, agriculture fibers, etc. 

Other materials used as filter aids include asbestos, cellulose, rice hull ash, paper fibers, etc. 

Cellulose is widely used for filtration of products that cannot tolerate silica. The filterability of cellulose is not as good as DE or perlite, but cellulose can be incinerated. Calcined rice hull ash and fibers from used newspapers are newly available filter aids. They are used for wastewater sludge dewatering as well as other applications. 

Solka-Floc . [Mendell James River Grrfeo] Cellulose filter aids and fillers. 

In order to prevent peptization of the carbon that often occurs, Bender, Douglas, and Cuthbert40 developed a percolation unit. One part of cellulose pulp is intimately mixed with 1 to 3 parts of powdered activated carbon and a 2 to 4 inch layer of the mixture is placed in the filter unit. A 1 % pectin solution at 50° C is then percolated through the unit. The filtrate usually has no smoky appearance which would indicate the presence of fine carbon particles, but, should any carbon pass through, it is removed by filtration through a layer of filter aid. 

FILTER AIDS. Slimy or very fine solids that form a dense, impermeable cake quickly plug any filter medium that k fine enough to retain them. Practical filtration of such materials requires that the porosity of the cake be increased to permit passage of the liquor at a reasonable rate. This is done by adding a filter aid, such as diatomaceous silica, perlite, purified wood cellulose, or other inert porous solid, to the slurry before filtration. The filter aid may subsequently be separated from the filter cake by dissolving away the solids or by burning out the filter aid. If the solids have no value, they and the filter aid are discarded together. 

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