3.1 Screen Filtration

Before the mid 1960's, draft beer was sold in large containers, called kegs, under refrigerated conditions. Draft beer, which had not gone through a high temperature pasteurization step to destroy the yeast and any bacteria, had to be consumed within a short time of its brewing and had to be kept refrigerated prior to consumption, in order to avoid problems with further fermentation. Purists liked the draft beer because they claimed that the pasteurization step, which was necessary for beer sold in small containers at room temperature, would destroy some of the delicate flavors of the beer. In the 1960's, the beer manufacturers found that one could substitute an extremely fine filtration process for the pasteurization step, since all the yeast and bacteria could be simply filtered from the beer. The relatively small molecular species producing the delicate flavors could pass through the very tiny filter holes. The resulting beer could be packaged under sterile conditions and could then be sold in cans or bottles stored at room temperature. This is an example of very fine screen filtration, where the requirements for filtration of the yeast and bacteria could be carefully determined. Of course, the yeast do tend to pile up on the filter screen and blind it periodically, requiring replacement of the filter screen.

Screen filtration can be accomplished under batch or continuous operation. With batch filtration, as described above for the beer example, the operation can be conducted under a) constant pressure, in which the volumetric flowrate decreases with time or b) constant flowrate, in which the pressure drop increases with time, as the filter becomes loaded with particles. Commercially, one frequently encounters operations requiring the filtration of particles which are rigid and incompressible, unlike bacteria. With these liquid/particle systems, the particles build up on the screen, forming a filter cake. If the resulting filter cake is indeed incompressible, equations (from Momentum, Heat, and Mass Transfer, 3rd ed., by C. O. Bennett and J. E. Myers, McGraw-Hill, New York, (1982), p. 233) for the filtration process are:



where f is the filtration time, Vf is the volume of filtrate, P is the pressure difference (usually upstream gage pressure for atmospheric filtration) and K1 and K2 are characteristics of the filtration, suspension concentration, and particle size and shape. K1 and K2 would normally be determined by experimentation, although Bennett and Myers do provide equations to relate the values to such variables as specific cake resistance, concentration, and viscosity of fluid.

For filtration at a constant volumetric rate q0, the equation is:



where K1 and K2 are the same as for constant pressure filtration. Thus, one can predict constant flowrate filtration performance from constant pressure data and vice versa. All of this is predicated on incompressible filter beds, however, and at least half, if not more, commercial slurries exhibit compressible bed performance. Although one can attempt to account for this by a dependence of specific cake resistance on pressure, the final design normally requires close experimental verification anyway.