Membrane plate, pressure filters

Membrane plate and tube presses are dealt with here belt and screw presses are included in the discussion of continuous pressure filters. 

These filters are used on slurries with small amounts of solids, usually less than 0.1%, and ganerally do not form any visible cake. Solids io be removed are usually very small panicles that may be trapped on the surface of the filter medium or within the medium. This typa of filter is usually used in a polishing application where excellent quality liquids are needed as in food or beverage, pharmaceutical, and electronic processing operations. The most common clarifying fillers are disk and plate presses, cartridge filters, precoal pressure filters, deep bed filters, and membrane filters. 

The principle of pressure filters differs from that of vacuum filters only in the fact that a positive rather than a negative pressure is used to force the filtrate through the filter membrane. On this account, pressures as high as feasible can be attained. Consequently, materials not filterable on vacuum filters may be handled by this means. Filter presses are of three general types those employing both plates and frames, those using recessed plates, and continuous rotary pressure filters. 

Plate-and-Frame (Conceptually the simplest, it is veiv much like a filter press. Once found in RO, UF, and IVIF, it is still the only module commonly used in electrodialysis (ED). A fevy applications in pressure-driven membrane separation remain (see Sec. 18 for a description of a plate-and-frarne filter press). 

We consider flow through a cake with the membrane located at a distance, x, from the filter plate. Neglecting all forces in the cake other than those created by drag and hydraulic pressure, a balance from x to L gives ... 

Ottewill and co-workers106,200 have used a compression method to measure the double-layer repulsion between the plate-like particles of sodium montmorillonite. This is a particularly suitable system for such studies, since the particles are sufficiently thin (c. 1 nm) for van der Waals forces to be unimportant and surface roughness is not a problem. The dispersion was confined between a semipermeable filter and an impermeable elastic membrane and an external pressure was applied via a hydraulic fluid so that the volume concentration of particles and, hence, the distance of separation between the particles could be measured as a function of applied pressure. 

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. 

While the designs of commercially available crystallizer cells have some variances, all are intended to cool and agitate the oil as a batch more or less in the same manner. There are, however, several types of filters in use based on the products and style of operation. These basic designs include plate and frame filters, continuous vacuum systems, membrane systems, and pressure leaf designs. 

A membrane filter press is shown in Figure 22.41. The impermeable flexible membranes, or diaphragms, are attached on the recessed plates. At the end of cake formation, the membranes can be inflated by air or pressured water to squeeze the cake for further cake deliquoring. This type of filter provides drier cakes compared with traditional plate and frame of recessed plate filter presses. 

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