Filtration driving forces

Diversity of Membrane Eiements and Configurations Currently, all membrane manufacturers offer their own design, size, and configuration of membrane elements and systems. The membrane systems differ by the type of filtration driving force (pressure versus vacuum), the size of the individual membrane elements, the size of the membrane vessels, the configuration of the membrane modules, the type of membrane element backwash, and the type of membrane integrity testing method and other factors. 

In vacuum filters, the driving force for filtration results from the appHcation of a suction on the filtrate side of the medium. Although the theoretical pressure drop available for vacuum filtration is 100 kPa, in practice it is often limited to 70 or 80 kPa. 

The scale-up of filtration centrifuges is usually done on an area basis, based on small-scale tests. Buchner funnel-type tests are not of much value here because the driving force for filtration is not only due to the static head but also due to the centrifugal forces on the Hquid in the cake. A test procedure has been described with a specially designed filter beaker to measure the intrinsic permeabiHty of the cake (7). The best test is, of course, with a small-scale model, using the actual suspension. Many manufacturers offer small laboratory models for such tests. The scale-up is most reHable if the basket diameter does not increase by a factor of more than 2.5 from the small scale. 

Membrane Filtration. Membrane filtration describes a number of weU-known processes including reverse osmosis, ultrafiltration, nanofiltration, microfiltration, and electro dialysis. The basic principle behind this technology is the use of a driving force (electricity or pressure) to filter... 

By operating cycle. Filtration may be intermittent (batch) or continuous. Batch filters may be operated with constant-pressure driving force, at constant rate, or in cycles that are variable with respect to both pressure and rate. Batch cycle can vary greatly, depending on filter area and sohds loading. 

Vacuum or Pressure The vast majority of all continuous filters use vacuum to provide the driving force for filtration. However, if the feed slurry contains a highly volatile hquid phase, or if it is hot, saturated, and/or near the atmospheric pressure boiling point, the use of pressure for the driving force may be required. Pressure filtration might also be used where the required cake moisture content is lower than that obtainable with vacuum. 

Rotary-Siphon Peeler Centrifuges In this type of centrifuge, a partial vacuum is drawn on the outer diameter or tne filter such that the filtrate flows through the cake under both centrifugal force as well as a positive pressure difference of about I atm or less. Thus, a higher rate of filtration takes place due to the increased driving force. 

Tneory of Centrifugal Filtration Theoretical prechc tions of the behavior of sohd-hquid mixtures in a filtering centrifuge are more (difficult compared to pressure and gravity filtration. The area of flow and driving force are both proportional to the radius, and the specific... 

Filtration Cross-flow filtration (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears earlier in this section) relies on the retention of particles by a membrane. The driving force for separation is pressure across a semipermeable membrane, while a tangential flow of the feed stream parallel to the membrane surface inhibits solids settling on and within the membrane matrix (Datar and Rosen, loc. cit.). 

The rate of the filtration process is directly proportional to the driving force and inversely propordonal to the resistance. 

The numerator of Equation 79 characterizes the cake resistance. The denominator contains information on the driving force of the operation. Constant K (sec/m ) characterizes tile intensity at which the filtration rate decreases as a function of increasing filtrate volume. 

The factors to consider in the selection of crossflow filtration include the flow configuration, tangential linear velocity, transmembrane pressure drop (driving force), separation characteristics of the membrane (permeability and pore size), size of particulates relative to the membrane pore dimensions, low protein-binding ability, and 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. 

SPAN module. It was mentioned at the beginning that the special polyacrylonitrile fibers of SPAN have a wall thickness of 30 gm, which is considerably thicker than the 8 gm wall thickness of the SMC modules [19]. As a consequence, the presence of stronger capillary effects from the special porous fiber material of the SPAN module would be a reasonable conclusion. Furthermore, the texture of the special polyacrylonitrile fibers is expected to have better surface properties, supporting the permeation of molecules as compared with synthetically modified cellulose. In conclusion, both convection and diffusion effectively contribute to the filtration efficiency in a SPAN module, whereas for the SMC membrane, diffusion is the driving force for molecular exchange, the efficiency of which is also considerable and benefits from the large surface-to-volume ratio. 

Batch filtration. Batch filtration involves the separation of suspended solids from a slurry of associated liquid. The required product could be either the solid particles or the liquid filtrate. In batch filtration, the filter medium presents an initial resistance to the fluid flow that will change as particles are deposited. The driving forces used in batch filtration are2 ... 

The model proposed above is analogous to a continuous, unsteady state filtration process, and therefore may be called "Filtration Model". In this model, the concentration of the filtrate, viz. the concentration of the solute remained in the treated solid is one s major concern. This is given by the rate of Step 3, which may be expressed by an equation similar to Pick s Law including a transmission coefficient D for the porous medium, viz. the P.S.Z. and the concentration difference Aw across the P.S.Z. as the driving force, and the thickness of the P.S.Z. as the distance Ax. 

DL-threonine and L-threonine crystals were supplied from Ajinomoto Co. Inc. and were used without further purification. Excess amounts of DL-threonine crystalline particles were dissolved in water kept at 55, 57, 58 or 60 C. After decantation and filtration each saturated solution was placed in the crystallizer maintained at 50 C. The difference between the saturation temperature and the crystallization temperature was defined as the initial supersaturation in terms of supercooling of the solution and was the driving force for the crystallization. 

Filtration separates components according to their size. Efficiency depends on the shape and compressibility of the particles, the viscosity of the liquid phase and the driving force, which is the pressure created by overpressure or by vacuum. Filtration can be performed either as dead-end filtration, where the feed stream flows perpendicular to the filter surface (Lee, 1989) or as tangential flow filtration, where the feed stream flows parallel to the filter and the filtrate diffuses across it. Examples of the former are the continuous rotaiy vacuum dram filter, where a rotaiy vacuum filter has a filter medium covering the surface of a rotating drum and the filtrate is drawn through the dram by an... 

Filtration is the separation of undissoived particulate solids from a mixture of fluid and solid. The separation is brought about by passage of the fluid thru a pervious septum (filter medium) in or on which the solids are retained. A driving force (gravity, vacuum, pressure, or centrifugal force) produces the flow. Filter aids may be added to the fluid before filtering to counterbalance the unfavorable characteristics of badly filtering materials... 

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