Electrofiltration

There is an additional pressure drop across the cake, developed by electroosmosis, which leads to increased flow rates through the cake and further dewatering at the end of the filtration cycle. The filtration theory proposed for electrofiltration assumes the simple superposition of electroosmotic pressure on the hydraulic pressure drop. 

Dewatering of high value products and particle systems sensitive to high pressure drops are the most likely candidates for electrofiltration. The Dorr-OHver Electrofilter is a commercial example of a vacuum filter adapted for electrofiltration. 

Process Concept The application of a direct elecdric field of appropriate polarity when filtering should cause a net charged-particle migration relative to the filter medium (electrophoresis). The same direct electric field can also be used to cause a net fluid flow relative to the pores in a fixed filter cake or filter medium (electroosmosis). The exploitation of one or both of these phenomena form the basis of conventional electrofiltration. 

The resistances, when incorporated into equations descriptive of cross-flow filtration, yield the general expression for the permeate flux for particulate suspensions in cross-flow-electrofiltration systems. 

FIG. 22-28 Regimes of operation of cross-flow-electrofiltration (a) voltage less than critical, (h) voltage equal to the critical voltage, (c) voltage greater than critical. 

General Reference R Krishnaswamy and P. Klinkowski, Electrokinetics and Electrofiltration, in Advances in Solid-Liquid Separation, H. S. Murali-dhara (ed.), Battelle Press, Columbus, OH, 1986. 

Cross-flow—electrofiltration (CF-EF) is the multifunctional separation process which combines the electrophoretic migration present in electrofiltration with the particle diffusion and radial-migration forces present in cross-flow filtration (CFF) (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears later in this section) in order to reduce further the formation of filter cake. Cross-flow—electrofiltration can even eliminate the formation of filter cake entirely. This process should find apphcation in the filtration of suspensions when there are charged particles as well as a relatively low conductivity in the continuous phase. Low conductivity in the continuous phase is necessary in order to minimize the amount of electrical power necessary to sustain the electric field. Low-ionic-strength aqueous media and nonaqueous suspending media fulfill this requirement. 

FIG. 2G26 Alternative electrode configurations for cross-flow-electrofiltration. 

Theory Cross-flow-electrofiltration can theoretically be treated as if it were cross-flow filtration with superimposed electrical effects. These electrical effects include electroosmosis in the filter medium and cake and electrophoresis of the particles in the slurry. The addition of the applied electric field can, nowever, result in some qualitative differences in permeate-flux-parameter dependences. 

Figure 49. Flux as a function of Reynolds number for crossflow electrofiltration of oil-water emulsions at several voltages (23)...

F(=) Elutriation Countercurrent electrophoresis Filtration Ultrafiltration Reverse osmosis Pressure dialysis Zone melting Electrofiltration... 

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