Introduction

Suspended solid matter in a fluid media represents a heterogeneous system that from a very general viewpoint is a fundamental fluid dynamics problem that virtually all chemical engineers are familiar with. It is so frequently encountered in numerous industry applications that considering only the problem of water treatment simply does not do the subject justice. Having noted this, the principles covered in this chapter are general enough to be applied to the class of unit operations aimed at separation, handling or processing heterogeneous or two-phase systems, including those dealing with gas-solid suspensions. Examples of operations where heterogeneous systems are encountered include sedimentation of dust in chambers and cyclone separators, separation of suspensions in settlers, separation of liquid mixtures by settling and centrifuging, hydraulic and pneumatic transport, hydraulic and air classification, flotation, mixing by air and others. Each of these operations involves the simultaneous flow of gas and solid, or liquid and solid phases. The widespread and successful application of these hydrodynamic processes to a large number of industrial problems is based on our ability to take advantage of one or a combination of five primary forces: gravity, centrifugal, buoyancy, pressure, and electric. Gravity is the controlling force for separations achieved in settlers; centrifugal force is applied to cyclone separators, dryers and mixers; and pressure forces are employed in sprayers, pneumatic transport and filters. Electrical forces are employed in special techniques, such as precipitators. Buoyancy is related to gravity and takes advantage of density differences.

You will find a very fundamental and in part, empirical approach to this subject. My experience has been for the majority of water treatment assignments over the years that a "first-principles" approach to sedimentation and clarification techniques generally tends to get the job done. The techniques and methodologies covered have been around for quite a few years and still seem to be applied with a high degree of success in preliminary and even detailed equipment sizing and overall plant designs. There are certainly more sophisticate methods, including numerical modeling that enables one to develop reliable designs, but our intent is to look at rather simple calculation methods that enable us to define the type of equipment needed and just enough information so that we can work with an equipment vendor to define the hardware in greater detail. More sophisticated approaches will be

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 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. Remember to refer to the Glossary at the end of the book if you run across any

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