filtration(redirected from filtration of x-ray photons)
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the process of filtering, that is, of separating suspensions or aerosols by means of a filter medium that permits the liquid or gas to pass through but retains the solid particles. Filtration is carried out in special devices called filters. The most important type of filtration—the separation of suspensions—is discussed below.
During the filtration of a suspension, the solid particles separated from the fluid almost always form a layer of moist sediment, called a filter cake, on the filter medium. Solid particles from very viscous suspensions or from finely dispersed suspensions of low concentration may penetrate the pores of the filter medium and become trapped in the medium without forming a cake. In some filtrations, the solid particles may simultaneously penetrate the pores and form a cake. Filter aids are used to prevent or inhibit the clogging of the pores, which is called blinding. Filter aids are substances, such as diatomaceous earth, perlite, asbestos, and cellulose, that are either used to coat the filter medium or are added to the suspension. Such substances form microscopic protective domelike structures over the pores. The fluid that passes through the filter medium is called the filtrate.
As the filtered fluid passes through the cake and the filter medium, it encounters resistance due to fluid friction. To overcome the resistance, a pressure drop must be established by producing a vacuum below the filter medium or excess pressure above the medium. With a constant pressure drop, the filtration rate decreases as the cake thickens and, consequently, as the resistance due to fluid friction increases. If the suspension is fed to the filter medium by a reciprocating pump, filtration occurs at a constant rate with a continuous increase in the pressure drop. If the suspension is fed by a centrifugal pump, both the pressure drop and the filtration rate change continuously. As the temperature increases, the filtration rate also increases, because the suspension becomes less viscous.
The following three types of filtration are distinguished: (1) the separation of a suspension into solid particles, which are trapped in the filter medium, and a large amount of fluid, which passes through the medium; (2) the thickening of a suspension, that is, increasing the concentration of the solid particles in the suspension by removing some of the fluid by means of a filter medium; and (3) the clarification of a liquid by removing the small amount of suspended fine particles from the liquid.
A filter cake may be either compressible or incompressible. The porosity of an incompressible cake remains constant during filtration; the porosity of a compressible cake decreases during filtration. In the case of an incompressible cake—for example, grains of sand or calcium carbonate crystals—the fluid flow through the filter medium is laminar, and the filtration rate is proportional to the pressure drop and the thickness of the cake. In the case of a compressible cake, for example, the hydroxide of a metal, the relationship is more complex and is unique for each suspension. The compression of a filter cake increases the resistance due to fluid friction and decreases the filtration rate. Coagulants and flocculating agents, which facilitate the aggregation of small particles and increase the porosity of a cake, are added to finely dispersed suspensions in order to prevent compression.
Filtration is widely used in laboratories and in the chemical, food-processing, petroleum-refining, and mining industries.
REFERENCESZhuzhikov, V. A. Fil’trovanie: Teoriia i praktika razdeleniia suspenzii, 2nd ed. Moscow, 1968.
Malinovskaia, T. A. Razdelenie suspenzii v promyshlennosti organicheskogo sinteza. Moscow, 1971.
Kasatkin, A. G. Osnovnye protsessy i apparaty khtmicheskoi tekhnologii, 8th ed. Moscow, 1971.
N. I. GEL’PERIN
The separation of solid particles from a fluidsolids suspension of which they are a part by passage of most of the fluid through a septum or membrane that retains most of the solids on or within itself. The septum is called a filter medium, and the equipment assembly that holds the medium and provides space for the accumulated solids is called a filter. The fluid may be a gas or a liquid. The solid particles may be coarse or very fine, and their concentration in the suspension may be extremely low (a few parts per million) or quite high (>50%).
The object of filtration may be to purify the fluid by clarification or to recover clean, fluid-free particles, or both. In most filtrations the solids-fluid separation is not perfect. In general, the closer the approach to perfection, the more costly the filtration; thus the operator of the process cannot justify a more thorough separation than is required.
Gas filtration involves removal of solids (called dust) from a gas-solids mixture because: (1) the dust is a contaminant rendering the gas unsafe or unfit for its intended use; (2) the dust particles will ultimately separate themselves from the suspension and create a nuisance; or (3) the solids are themselves a valuable product that in the course of its manufacture has been mixed with the gas.
Three kinds of gas filters are in common use. Granular-bed separators consist of beds of sand, carbon, or other particles which will trap the solids in a gas suspension that is passed through the bed. Bag fitters are bags of woven fabric, felt, or paper through which the gas is forced; the solids are deposited on the wall of the bag. Air filters are light webs of fibers, often coated with a viscous liquid, through which air containing a low concentration of dust can be passed to cause entrapment of the dust particles. See Air filter, Dust and mist collection
Liquid filtration is used for liquid-solids separations in the manufacture of chemicals, polymer products, medicinals, beverages, and foods; in mineral processing; in water purification; in sewage disposal; in the chemistry laboratory; and in the operation of machines such as internal combustion engines.
Liquid filters are of two major classes, cake filters and clarifying filters. The former are so called because they separate slurries carrying relatively large amounts of solids. They build up on the filter medium as a visible, removable cake which normally is discharged “dry” (that is, as a moist mass), frequently after being washed in the filter. It is on the surface of this cake that filtration takes place after the first layer is formed on the medium. The feed to cake filters normally contains at least 1% solids. Clarifying filters, on the other hand, normally receive suspensions containing less than 0.1% solids, which they remove by entrapment on or within the filter medium without any visible formation of cake. The solids are normally discharged by backwash or by being discarded with the medium when it is replaced. See Clarification