An Introduction to Mass-Transfer Operations

The importance of mass-transfer operations in chemical engineering is profound. There is scarcely any chemical process which does not require either a preliminary purification of raw materials or a final separation of products from by-products, and for these, mass-transfer operations are commonly used. Frequently, the separations constitute the major part of the costs of a process. 

The chemical engineer faced with the problem of separating the components of a solution must ordinarily choose from several possible methods. While the choice is usually limited by the specific physical characteristics of the materials to be handled, the necessity for making a decision nevertheless almost, always exists. The principle basis for choice in any situation is cost that method which costs the least is usually the one to be used. Occasionally, other factors also influence the decision, however. The simplest operation, while it may not be the least costly, is sometimes 

The choice of the most suitable separation method for a given separation depends on the mixture at hand and the required purity specifications. Although this is the most important decision in the design and operation of a separation process, it will therefore not be considered in this chapter. 

In the following, the principles of mass-transfer separation processes will be outlined first. Details of mass-transfer calculations will be introduced next and examples will be given of both equilibrium-stage processes and diffusional rate processes. The chapter will then conclude with a detailed discussion of the two single most applied mass-transfer processes in the chemical industries, namely distillation and absorption. 

In laboratory analysis, the extent of separation is of major importance, i.e. is it thermodynamically possible to separate the components, 

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Volumes 1, 2 and 3 form an integrated series with the fundamentals of fluid flow, heat transfer and mass transfer in the first volume, the physical operations of chemical engineering in this, the second volume, and in the third volume, the basis of chemical and biochemical reactor design, some of the physical operations which are now gaining in importance and the underlying theory of both process control and computation. The solutions to the problems listed in Volumes 1 and 2 are now available as Volumes 4 and 5 respectively. Furthermore, an additional volume in the series is in course of preparation and will provide an introduction to chemical engineering design and indicate how the principles enunciated in the earlier volumes can be translated into chemical plant. 

Chapter 16 presents an introduction to the subject of mass transfer as it applies to separation operations with particular emphasis on the design and operation of packed columns. The concluding chapter deals with the important topic of energy conservation in distillation and includes a method for computing thermodynamic eflSciency so that alternative separation processes can be compared. 

Included also in this chapter is a qualitative description of separations based on intraphase mass transfer (dialysis, permeation, electrodialysis, etc.) and discussions of the physical property criteria on which the choice of separation operations rests, the economic factors pertinent to equipment design, and an introduction to the synthesis of process flowsheets. 

Section 12-2 deals with liquid-liquid dispersion, while in Section 12-3 we discuss coalescence. Section 12-4 gives an introduction to the methods used for population balance models, along with references for further reading. In Section 12-5 we describe more concentrated dispersed phase systems, including phase inversion. Section 12-6 deals with other considerations, such as suspension, mass transfer, and other complexities. Section 12-7 with equipment used in liquid-liquid operations. Section 12-8 with scale-up, and Section 12-9 provides industrial examples. Nomenclature and references then follow. Although every attempt has been made to make this a stand-alone chapter, space limitations occasionally make it necessary to refer to other chapters in the book. 

The development of in situ electrolytic methods by Allied Chemical resulted in a novel unit, the electropulse column, in which mass transfer and electrolytic reduction are carried out simultaneously (25). The basic feature of the electropulse column, (Fig. 3), is the dual function of the horizontal perforated plates, acting as cathodes as well as pulse plates, and the introduction of vertical anode screens contained in porous ceramic sleeves. This design was found particularly suitable for Pu-U partitioning, since it permits operation with an aqueous-continuous phase, which is needed to maintain adequate electrical conductivity, while the organic to aqueous flow ratio is kept quite large to obtain a high plutonium concentration in the exiting aqueous stream. 

The introduction of higher temperatures and pressures leads to a potential problem. The control schema for bioreactors is generally tied to the gas flow rate. The idea is that the gas flow rate would impact gas holdup, which would, in turn, control gas-liquid mass transfer. Significant temperature changes, however, decouple gas-liquid mass transfer from gas holdup and add new operational variables. Most correlations do not include temperature and pressure effects directly, but instead attempt to quantify these effects by altering or introducing effective liquid properties, which are much harder to cost-effectively measure in an industrial setting. 

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