Bridging filter media

Surface Filters. In surface filters (Fig. 2), the goal is to achieve separation on the upstream side of a relatively thin filter medium. The particles to be separated must be larger than the pores in the medium, ie, in strainers, membrane filters, etc, or the particles must approach the pores in large numbers and bridge over the pores, as in cake filters. 

An additional benefit of prethickening is reduction in cake resistance. If the feed concentration is low, there is a general tendency of particles to pack together more tightly, thus leading to higher specific resistances. If, however, many particles approach the filter medium at the same time, they may bridge over the pores this reduces penetration into the cloth or the cake underneath and more permeable cakes are thus formed. 

The blocking of the pores of the filter medium by particles is a complex phenomenon, partly because of the complicated nature of the surface structure of the usual types of filter media, and partly because the lines of movement of the particles are not well defined. At the start of filtration, the manner in which the cake forms will lie between two extremes — the penetration of the pores by particles and the shielding of the entry to the pores by the particles forming bridges. Heertjes(11) considered a number of idealised cases in which suspensions of specified pore size distributions were filtered on a cloth with a regular pore distribution. First, it was assumed that an individual particle was capable on its own of blocking a single pore, then, as filtration proceeded, successive pores would be blocked, so that the apparent value of the specific resistance of the filter cake would depend on the amount of solids deposited. 

Cake filtration this is undoubtedly the most widely encountered mechanism in industrial filtration and involves the accnmulation of particles that bridge together in a porous structure on the surface of the fabric. It follows from this that, once formed, the cake effectively becomes the filter medium, with the fabric thereafter acting simply as a support. In cases where it is difficult for the particles to form a naturally porous cake, the use of a special precoat or body feed may be employed to assist in this task. 

Problems are usually encountered when pressure systems designed to operate at reasonable levels of particulate concentration are fed with dilute suspensions at the start of filtration. Low concentrations of sohds prevent the bridging effect which ensues when concentrated swarms of sohds are directed towards the pores in the filter medium. This effect is discussed quantitatively elsewhere in this Chapter. Failure to bridge the cloth pores will lead to deposition of particles inside the fabric. 

Along with particle concentration, the filtration velocity has an influence on the bridging process [Rushton Hassan, 1980], Simultaneous arrival of sufficient particles to form a bridge will be influenced by the fluid drag on the particles, as fliey approach the filter medium Ei erimental measurements have been r orted in terms of the particle flux (j) calculated fi om the product of the particle concentration and approach velocity ... 

Cotton cellulose is the filtermass used in brewing, but -wood cellulose is also available. It is sometimes used as underlay for diatomite or perlite or as a blend with diatomite. Generally the fibres are about 20 pm diameter and 50-100 pm long, which win bridge wide openings in the filter medium Cellulose is very pure, is combustible and soluble in solvents so that inert suspended solids may be recovered by either of these methods. 

Vertical vessds can accommodate up to 80 m of vertical plate larger areas, up to 300 m, are attainable with vertical leaves in horizontal vessels. Plate spacing must be considered, in relation to the volume of cake filtered per batch. Adequate space must be allowed between the leaves for cake deposition and subsequent discharge. Bridging of solids between plates can lead to les thy discharge times and, in some circumstances, damage to the filter medium... 

Figure 10. Suspension flow downward onto a filter medium. An initial accumulation ofsoUds occurs around the pott entrance followed by particle bridging.

To understand the primary photochemistry of 6F-PIs in air, Hoyle et al. [I72-I74] used V-phenylphthalimide as a model compound, since this model probably reflects the structural units in 6F-PIs where conjugation between the diimide groups is prevented by the C(CF3)3 bridge. Upon irradiation with a Pyrex filtered medium pressure mercury lamp, this model in an air-saturated acetonitrile solution rapidly decomposes to form phthalic anhydride (quantum yield major product, phthalimide (0 = 2X and a trace amount of nitrobenzene (this originates probably from... 

Bridging effect refers to the interaction of blood cells and fibre, when blood cell particles that stick together are stopped by the filter medium forming a bridge across a pore. 

Cartridges are available in a very wide range of types, and in almost every type of filter medium. However they can be broadly classed into two types, according to their construction, which can easily be distinguished, although even here there are formats that bridge this classification. The first uses an integral piece of porous... 

The capsules are separated from the process liquid by filtering (Figure 11-8). The filter uses a medium a flat material with little holes. Surprisingly the holes are usually chosen larger than the particles (capsules) to be retained. If the pores are not too large, particles form bridges... 

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