Filtration pore-size characteristics

From the above description of the basic geo-filtration mechanism, it becomes evident that the performance of geotextiles as filter media depends largely on two properties the material s cross-plane permeability and pore-size characteristics. 

High surface areas and small pore sizes characteristic of inorganic gels are properties unattainable by conventional ceramic processing methods. These unique properties may be exploited in applications such as filtration, separations, catalysis, and chromatography. (See Table 12.) The use of porous gels in separations and nitrations was reviewed by Klein [243]. 

The factors to consider in the selection of cross-flow filtration include the cross-flow velocity, the driving pressure, the separation characteristics of the membrane (permeability and pore size), size of particulates relative to the membrane pore dimensions, and the hydrodynamic conditions within the flow module. Again, since particle-particle and particle-membrane interactions are key, broth conditioning (ionic strength, pH, etc.) may be necessary to optimize performance. 

The optimum filter aid should have maximum pore size and ensure a prespecified filtrate clarity. Desirable properties characteristics for the optimum filter aid include ... 

A second way to achieve collection of coarse- and fine-mode particles onto filters is through tandem filtration through the stacked filter unit (SFU) (4, 5, 6). In these devices, the convenient filtration characteristics of filters (Nuclepore) allow a 2.5- xm cut point on the basis of pore size and the face velocity of the airstream. Such devices are very compact and inexpensive and have been heavily used in remote-area networks (7, 8, 9). Again, however, the limitations of the method limit the number and sharpness of the size cuts so that almost all units are operated at 2.5 xm and give coarse and fine fractions very much like those of the virtual impactor. Examples of the 2.5- xm cut points of VI, SFU, and impactors are shown in Figure 2. However, cyclones, virtual impactors, and stacked filter units cannot give the sharp, multiple cut points of impactors as shown in Figure 1. 

In addition to the pore size-particle size retention relationship problems mentioned above, other factors can influence a filter medium s retention characteristics. Absorptive retention can be influenced by the organism size, organism population, pore size of the medium, pH of the filtrate, ionic strength, surface tension, and organic content. Operational parameters can also influence retention, such as flow rate, salt concentration, viscosity, temperature, filtration duration, filtration pressure, membrane thickness, organism type, and filter medium area [52,53]. 

The diameter or the radius of the pores is one of the most important geometric characteristic of porous solids. In terms of lUPAC nomenclature, we can have macropores (mean pore size greater than 5 x 10 m), mesopores (between 5 x 10 and 2 x 10 m) and micropores (less than 2 x 10 m). The analysis of species transport inside the porous structure is very important for the detailed description of many unit operations or applications among them we can mention suspension filtration, solid drying and humidification, membrane processes (dialysis, osmosis, gaseous permeation. ), flow in catalytic beds, ion exchange, adsorp-... 

The normal method of separation is filtration. The division between dissolved and particulate thus becomes a function of pore size and filtering characteristics of the particular filter chosen. The pore size... 

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