This chapter provides a summary of the governing expressions describing conventional pressure-induced filtration and a description of major equipment. Standard filtration practices refers to the most common or classical method of filtration, sometimes referred to as cake filtration.
This type of filtration relies on the use of a porous bed, or more accurately - a porous media which can be cloth. With high-solids-concentration suspensions, even relatively small particles (in comparison to the pore size) will not pass through the medium, but tend to remain on the filter surface, forming "bridges" over individual openings in the filter medium. The filtrate flows through the filter medium and cake because of an applied pressure on one side of the media bed, the magnitude of which is controlled proportionally to the filtration resistance. This resistance results from the frictional drag on the liquid as it passes through the filter and cake. The hydrostatic pressure varies from a maximum at the point where suspension enters the cake, to zero where liquid is expelled from the medium; consequently, at any point in the cake the two are complementary. That is, the sum of the hydrostatic and compression pressures on the solids always equals the total hydrostatic pressure at the face of the cake. Thus, the compression pressure acting on the solids varies from zero at the face of the cake to a maximum at the filter medium.
When solid particles undergo separation from the mother suspension, they are captured both on the surface of the filter medium and within the inner pore passages. The penetration of solid particles into the filter medium increases the flow resistance until the filtration cycle can no longer continue at economical throughput rates, at which time the medium itself must either be replaced or thoroughly cleaned.
At the end of this chapter you will find three annexes. The first of these is a list of nomenclature used in the chapter. There are quite a few design equations that are summarized in the foregoing sections and, hence, you will need to refer to this annex from time to time. The second annex is a list of recommended references that I have relied on over the years, plus some interesting Web sites for you to visit for vendor-specific information, as well as supplemental design and equipment sizing information. The final annex is the Questions for Thinking and Discussing. This annex is a collection of problem-solving questions, many of which are based on actual design cases developed over the years and also developed for seminar programs that my father (Paul N. Cheremisinoff, P.E.) and an old colleague of mine (Dr. David S. Azbel) taught to process engineers. Some of these problems are quite challenging and I recommend you work on these with your colleagues. You will find that you may have to refer to some of the recommended references in addition to the design formulae presented in this chapter to answer all of these questions. But, if you can successfully get through these questions, you should have a pretty good command of the subject and be able to confidently select and develop preliminary equipment sizing for different applications. Note that I have taken a very generalized approach to the subject in that it does not restrict us to water treatment applications alone. Indeed many of the filtration machines discussed in this chapter are not necessarily the proper or best choices for water treatment applications, but they can be depending on the unique industry application. Having said this, what this chapter attempts to provide you with is a very solid foundation on sizing and equipment selection for this important class of filtration practices. Remember to refer to the Glossary at the end of the book, if you run across any terms that are unfamiliar to you.
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