Filter polymer membranes

Filter-medium selection embraces many types of construction fabrics of woven fibers, felts, and nonwoven fibers, porous or sintered solids, polymer membranes, or particulate solids in the form of a permeable bed. Media of all types are available in a wide choice of materials. 

It is recommended that all solutions be filtered through membrane filters to remove lint, polymer gel, and other materials likely to obstruct the columns and other system components. 

In tangential filtration, membranes are used as filter media. Membranes are defined as barriers of reduced thickness, across which physical and/or chemical gradients are established to facilitate the preferential migration of one or more components from a given mixture, promoting their separation (Klein, 1991). They are usually made of polymers or inorganic materials, such as ceramic or sintered steel. In the biopharmaceutical industry, membranes find various applications, such as production of water for injection (WFI), sterilization of culture media, buffer solutions and gases, separation of cells and cell debris, and purification and concentration of proteins. 

In micro- and ultrafiltrations, the mode of separation is by sieving through line pores, where microfiltration membranes filter colloidal particles and bacteria from 0.1 to 10 mm, and ultrafiltration membranes filter dissolved macromolecules. Usually, a polymer membrane, for example, cellulose nitrate, polyacrilonytrile, polysulfone, polycarbonate, polyethylene, polypropylene, poly-tretrafhioroethylene, polyamide, and polyvinylchloride, permits the passage of specific constituents of a feed stream as a permeate flow through its pores, while other, usually larger components of the feed stream are rejected by the membrane from the permeate flow and incorporated in the retentate flow [10,148,149],... 

Fig. 18.6. AC resistometric sensing apparatus as described by Piletsky et al. [92]. 1. Electrochemical cell, 2. filter with polymer membrane, 3. platinum electrodes, 4. AC generator, 5. voltmeter, 6. 1 kOhm resistance.

There are many filter configurations within the industry, such as sheet or modular depth filter types for prefiltration purposes, flat filter membranes mainly for microbial detection and, specifications, and, most commonly, filter cartridges containing either depth filter fleeces or membrane filters. Such membrane filters are available in a large variety of membrane polymers for different applications. These materials are discussed later in the chapter. 

In cross-flow flltration, the wastewater flows under pressure at a fairly high velocity tangentially or across the filter medium. A thin layer of solids form on the surface of the medium, but the high liquid velocity keeps the layer from building up. At the same time, the liquid permeates the membrane producing a clear filtrate. Filter media may be ceramic, metal (e.g., sintered stainless steel or porous alumina), or a polymer membrane (cellulose acetate, polyamide, and polyacrylonitrile) with pores small enough to exclude most suspended particles. Examples of cross filtration are microfiltration with pore sizes ranging from 0.1 to 5 pm and ultrafiltration with pore sizes from 1 pm down to about 0,001 pm. 

Figure 7. Size scaling of the relative depression ATx/T of the X point of ( He) , in finite systems, according to Eqs. (38a) and (39). o ( He) clusters of radius Ro (Ref. 65) He in vicor glass, pore radius Rq = 35 A (Ref. 159) O He in porous gold, pore radius Ro = 120 A (Ref. 160) A He confined in cylindrical pores (radius do = 400 A) in polymer membrane (Ref. 159) V He in cyhndrical pores (radius do = 150A-1000A) in nucleopore filters (Ref. 192). The confining dimension is L = Ro for spherical clusters or pores, or do for cylindrical pores. The sohd fine corresponds to the size scaling with = 1.7 A and v = 2/3.

The symmetric, microporous polymer membranes made by phase inversion are widely used for separations on a laboratory and industrial scale.22 Typical applications range from the clarification of turbid solutions to the removal of bacteria or enzymes, the detection of pathological components, and the detoxification of blood in an artificial kidney. The separation mechanism is that of a typical depth filter which traps the particles somewhere within the structure. In addition to the simple "sieving" effect, microporous phase inversion membranes often show a high tendency of adsorption because of their extremely large internal surface. They are, therefore, particularly well suited when a complete re-... 

The spinning of asymmetric hollow fibers with the skin on the inside closely resembles the procedure used in casting flat-sheet membranes. Figure 3.1510 is a schematic diagram of a spinneret used to spin these fibers. The degassed and filtered polymer solution is forced under pressure into a coaxial tube spinneret. The liquid is extruded through an annular orifice and the hollow fiber (still liquid) is stabilized and precipitated by an internal coagulating fluid (usually water) which flows out the center tube. 

The microstructure of an SiC-filter made from silicon infiltrated polymer foam is shown in Fig. 19. Cell size, cell geometry, and cell anisotropy is controllable during processing [283]. The structural variability of this material reaches from tube-like anisotropic to isotropic pore nets with several pore and bridge averages. This porous SiC material cannot only be applied to filters and membrane supports it can also be used as catalyst support or heat exchanger [284]. 

Further applications of metal-polymer nanocomposites are in the fields ofultrahigh and ultralow refractive index materials [Weibel et al., 1991 Zimmerman et al., 1992, 1993], dichroic color filters [Dirix et al., 1999a,b], nonlinear optical filters [Qu, S. et al., 2002], and catalytic polymer membranes [FritschandPeinemann, 1995 Troger etal., 1997]. 

The most popular nominal pore size in microflltration is 0.2 pm, with many polymer membranes and some ceramic types available at this size. These fibers are believed to provide a sterile liquid because bacteria are retained in the retentate and the permeate is bacteria flee. In order to ensure sterile conditions a 0.1 pm membrane is sometimes used. This is also, therefore, a popular commercial microfiltration membrane pore size. For details of the standard biological test see Section 6.6.1. Other cellular material of a biological origin, such as beer and wine yeasts, can be filtered by similar pore dze membranes. Coarser pore sizes are available 0.45, 0.8,1.0, 1.2,2.0, 3.0, 5.0 and 10 pm. 

In some instances, however, the use of a finer pore-sized membrane provides hi er stable fluxes over longer periods than is found with coarser membranes. This is particularly true when the suspended sohds particle size is close enough to the membrane pore size for internal filter clogging to occur. Figure 10.10 illustrates an experiment conducted with two polymer membranes and a very low concentration latex su ension, with particle size in the range 0,2-2 pm. 

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