Filtration Applications

Filters have been extensively used in households and industry for removing particles from air or liquid. In the case of enviromental protection filters are used to remove pollutants from air or water. In military, filters are used in uniform garments and isolating bags to decontaminate aerosol dusts, bacteria, while maintaining permeability to moisture vapor for comfort. A respirator is another example requiring an efficient filtration function. The same properties are also needed for some fabrics used in the medical field.  

For a fiber-based filter, removal of particles is determined by different mechanisms. Large particles are blocked on the filter surface due to the sieve effect. Particles that are smaller than the surface pores will penetrate into the filter, which could still be collected by the fibers, via either interception or impaction or static electrical attraction.  

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Polymer Membranes These are used in filtration applications for fine-particle separations such as microfiltration and ultrafiltration (clarification involving the removal of l- Im and smaller particles). The membranes are made from a variety of materials, the commonest being cellulose acetates and polyamides. Membrane filtration, discussed in Sec. 22, has been well covered by Porter (in Schweitzer, op. cit., sec. 2.1). 

One of the differences between polishing and cake filtration is the space between the plates. For polishing applications the clearances are about 20 mm as opposed to cake filtration applications where, depending on the percentage of solids and cake build-up properties, clearances may reach 100 mm. Hence, polishing filters accommodate more plates than cake filters so for the same vessel size more effective area is available with polishers. 

A flow diagram of a simple cross-flow system is shown in Figure 16.12. This is the system likely to be used for batch processing or development rigs it is in essence a basic pump recirculation loop. The process feed is concentrated by pumping it from the tank and across the membrane in the module at an appropriate velocity. The partly concentrated retentate is recycled into the tank for further processing while the permeate is stored or discarded as required. In cross-flow filtration applications, product washing is frequently necessary and... 

Figure 15.2(a). The membrane impedes further penetration of even smaller particles through the porous filter media. In many filtration applications, this filtration mechanism is valid for an axial velocity greater than about 4 to 6 m/s. 

A PET microfiber is loosely defined as one with a decitex (see Section 5 earlier) per filament less than one. This translates to a fiber diameter of 10 xm or less. In fabric form, such fibers provide a very soft hand and a non-shiny appearance. They can also make moisture-repellant fabrics without sacrificing comfort or air porosity, ideal for sportswear. The larger fiber surface area also can be useful for filtration applications. 

The earliest commercially available filters were manufactured in two pore sizes 0.45 and 0.8 pm. The 0.45 pm-rated membranes were considered to be sterilizing-grade filters and were successfully used in the sterile filtration of pharmaceuticals and parenterals. The membrane filters were qualified using Serratia marcescens, a standard bacterium, having dimensions of 0.6 x 1 pm. However, in the late 1960s it became apparent that the matrix of the 0.45 pm-rated filters could be penetrated by some pseudomonad-like organisms (1). For sterile filtration applications in the 1990s, 0.2 pm-rated membranes are the industry standard in the manufacture of sterile parenterals and pharmaceuticals. 

A typical in-line cartridge filtration application is illustrated in Figure 7.13. A pump forces liquid through the filter, and the pressure across the filter is measured by a pressure gauge. Initially, the pressure difference measured by the gauge is small, but as retained particles block the filter, the pressure difference increases until a predetermined limiting pressure is reached, and the filter is changed. 

Graded-density submicron depth filters (Fig. 18.13) are suitable for broader PSD slurry global distribution loop filtration. These filters with large surface area and low face velocity are suitable for high flow POU and POD filtration applications, are typically disposable in nature, and may have nominal ratings of 0.2, 0.3, 0.5, and 1.0 pm. 

Due to their greater chemical and thermal stabilities and narrower pore size distributions compared to polymer membranes, ceramic membranes are attractive in a number of filtration applications related to the fermentation broths. They can be used for either upstream or downsueam processing. 

Coal gasification, fluidized-bed coal combustion, waste incineration Separation of metal dust, fiu-idized-bed coal combustion, waste incineration Catalyst recovery, coal-fired boilers, metal smelting, soot filtration Applications up to 1500 C Coal gasification, fluidized-bed coal combustion, waste incineration, soot filtration... 

Alvin M.A., Lippen TE. and Lane J.A., Assessment of porous ceramic materials for hot gas filtration applications. Ceramic Bulletin 70 1491 (1991). 

The separation of suspensions is the selective removal of suspended solids, say, by the ordinary processes of filtration. Application can also made to the separation of colloidal suspensions of minute or microscopic solid particles, and even of emulsions, the suspension of minute immiscible liquid droplets within another liquid phase. A distinguishing feature of ordinary filtration is usually that the discharged liquid phase does not form a continuum on the downflow or reject side of the membrane, or filter, and more or less exists at atmospheric pressure. If otherwise, if a contiunuum is formed, the process is more that of reverse osmosis, also called hyperfiltration. In common use, notably for the upgrading or desalination of salt water or brackish water, reverse osmosis is a subject for special consideration. 

Vims filtration membranes are made of various polymers such as hydrophilized PVDF, hydrophilized PES, and cellulose. They are available in pleated cartridge and hollow-fiber formats. At present almost all vims filtration applications are performed in the direct filtration mode. 

Cherry, G.B. McCann, E.P. Parker, A. The removal of bacteria from air by filtration application to industrial-scale fermentations. J. Appl. Chem. 1951, I, S103. 

Precoat Filtration Applications, Advantages, and Disadvantages Summary... 

Filter Aid Any material that assists in the separation of solids from liquids. Usually used on difficult filtration applications. 

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