Liquid filtration retention mechanisms

Cartridge-type hydrophobic membrane Filtration has largely replaced depth filtration as a means of sterilizing gases. Collection of panicles from a gas stream by membrane Filtration is, as with liquid filtration, a function of both sieving and other means of retention. Adsorption and electrostatic attraction are far more important to retention of particles in gas Filtration than in liquid Filtration. Because there are more mechanisms and interactions between pore surfaces and particles, removal of particles is more easily accomplished from gas streams than from liquids. Gases are quite satisfactorily sterilized using 0.45 pm pore size rated membranes. 

An item that we all see throughout our daily lives but do not realize that it is, in one of its forms, a filter medium, is paper. It is a very widely used medium, made by dispersing fibres into a suspension in water. This suspension is then filtered into a mat form, which is compressed and dried. By varying the process, different porosities are achieved. The size of the fibre is important. Cellulose fibres are relatively coarse while glass is finer. CeUu-lose-based papers have poorer retentive power and are widely used in industrial liquid filtration applications because of lower cost and good mechanical properties. Glass is typically used in laboratory applications. 

There are two ways to classify liquid chromatographic methods. The first and more common classification is based on the mechanism of retention, and from this the chromatographic modes discussed in Chapter 2 are derived. For example, the normal-phase mode can be performed by taking advantage of either the adsorption mechanism or the partition mechanism. The gel-filtration mode is performed using the mechanism of size exclusion. The second classification discussed below is based on the separation principle and is found mostly in the literature published before the 1990s. 

Under normal circumstances the flow rate of the liquid to he filtered in the sheet filter should not he too high, as otherwise the mechanical and physicochemical retention capacity is insufficient to guarantee effective clarification. The flow rate to he chosen depends on the configuration of the sheets. If the speed at which the liquid arrives is too great it hursts through the sheet, nullifying the filtration effect. The performance of a sheet filter depends on the size and number of the sheets and on the degree of turbidity or the nature of the sheets. 

Filtration separates cells from a fluid by forcing the fluid through a porous filter medium, which deposits solids as liquids pass through. Vacuum or positive-pressure equipment is used to create the driving force for filtration. The main advantages of filtration include high rates of separation, low cost, mechanical simplicity, and relative ease of maintenance. However, it can have a low retention or poor containment, and can require the addition of a filter aid to ensure good filtration when solids accumulate on the membrane. 

The filtration of particles in a gas stream can be quite different from the filtration of the same particles in a liquid stream. The three mechanisms of aerosol particle retention may be illustrated from the data of Spurny et al20 in Figures 2.22 and 2.23. The U-shaped curves are characteristic of the efficiency of aerosol particle collection as a function of particle size. However, "capillary-pore" membranes have a deeper minimum in the curves than do "tortuous-pore membranes."... 

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