Membrane processes have been applied to a wide variety of industrial separation processes for process schemes requiring either concentration or purification of aqueous streams ranging from clarification of juices, removal of products from fermentation broths and desalination of brackish waters, etc. Some of the processes described above have been demonstrated on an industrial scale. This technology has been possible due to the development of new membranes which exhibit a high degree of hydraulic permeability coupled with ability to retain small molecules.
The membrane filtration process is, however, adversely affected by two phenomena, namely, concentration polarization and fouling. Concentration polarization is caused by the accumulation of solute molecules at the upstream surface of the membrane. This phenomena causes a reduction in the efficiency and of rate of the membrane filtration process. Concentration polarization is generally a hydrodynamic and diffusion phenomena.
Fouling also occurs with a variety of feeds including proteins and colloids. Fouling can be the result of an insoluble precipitate/ particle or gel layer within the pores (plugging) or the result from build-up on the surface of the membrane itself. It has been reported in the literature that up to 100 micron thick gel layers can be formed over a period of 24 hours as a result of the denaturation of proteins. The presence of fat globules can also be responsible for formation of gel layers.
Such gel layers and plugging are responsible for decreases in membrane flux and, hence, affect product throughput and economics. Various methods of treating fouling and concentration polarization have been reported in the literature but have only had limited success in minimizing the problem. Some of these methods are heat treatment with pH adjustment, immobilized protease treatment and membrane scouring.
Some typical industrial applications of membrane separation in the food and beverage industry are removal of casein, fats and lactose from whey; clarification of liqueur and vodkas; sterilization of liquids, e.g. beer, wine; continuous microfiltration of vinegar; and concentration and demineralization of cheese, whey, soy whey and vegetable extracts. Other applications in the wastewater treatment industry include removal of cyanides from electroplating wastewaters; reuse of wastewater from ammunition manufacture; recovery and recycling of sewage effluent; and recovery of starch and proteins from starch factory waste, wood pulp and paper processing. Further applications are listed in an article by Minh S. Le et al, The Chemical Engineer, pp. 48-53, July/August 1985.
Previous work in this area includes the following U.S. Pat. Nos.: 2,571,247; 3,980,541; 4,032,454; 4,057479; 4,100,068; 4,224,135; 4,269,681; 4,276,146; 4,343,690; 4,421,579; and 4,617,128.
Membrane separation techniques typical of the prior art include those that employ classical crossflow electrofiltration. Several good review articles that give an excellent background discussion include: Electrokinetic Membrane Processes, Milan Bier, Membrane Processes in Industry and Biomedicine, Milan Bier--Editor. Plenum Press, N.Y.-London, (1971) pp. 233-266; A Solid/Liquid Separation Process Based on Cross Flow and Electrofiltration, J. D. Henry, Jr., et al, AlChE Journal, Vol. 23, No. 6. November 1977, pp. 851-859; Membrane Fouling Prevention in Crossflow Microfiltration by the Use of Electric Fields, R. J. Wakeman et al, Chemical Engineering Science, Vol. 42, No. 4, pp. 829-842, 1987. In classical crossflow electrofiltration the material to be filtered passes between a first electrode and a membrane or filter with a second electrode positioned on the other side of the filter. The electrodes are energized to pull suspended particles from the material to be filtered toward the first electrode and away from the filter. Thus the suspended particles cannot deposit on the filter or membrane.
U.S. Pat. No. 4,604,174 to Bollinger et al teaches an improvement of the prior art crossflow: however, the general design follows FIGS. 1 and 2 herein that illustrate typical prior art crossflow arrangements. In Bollinger, one electrode assembly forms a wall of the crossflow chamber and the second electrode is located in the filtrate chamber. This arrangement causes the slurry material passing through the crossflow chamber to flow between the two electrodes see FIG. 1 of Bollinger and the discussion in the Summary of the Invention, Column 2, line 57 to Column 3, line 32.
Objects of the present invention include the enhancement of crossflow and dead-end electrofiltration by reducing energy requirements, improving filtration efficiency, reducing filtration time, reducing costly filter cleaning steps, and reducing filter fouling.