Mass transfer operations

It is useful to classify the operations and to cite examples of each, in order to indicate the scope of this book and to provide a vehicle for some definitions of terms which are commonly used. 

This category is by far the most important of all and includes the bulk of the mass-transfer operations. Here we take advantage of the fact that in a two-phase system of several components at equilibrium, with few exceptions the compositions of the phases are different. The various components, in other words, are differently distributed between the phases. 

In some instances, the separation thus afforded leads immediately to a pure substance because one of the phases at equilibrium contains only one constituent. For example, the equilibrium vapor in contact with a liquid aqueous salt solution contains no salt regardless of the concentration of the liquid. Similarly the equilibrium solid in contact with such a liquid salt solution is either pure water or pure salt, depending upon which side of the eutectic composition the liquid happens to be. Starting with the liquid solution, one can then obtain a complete separation by boiling off the water. Alternatively, pure salt or pure water can be produced by partly freezing the solution or, in principle at least, both can be obtained pure by complete solidification followed by mechanical separation of the eutectic mixture of crystals. In cases like these, when the two phases are first formed, they are immediately at their final equilibrium compositions and the establishment of equilibrium is not a time-dependent process. Such separations, with one exception, are not normally considered to be among the mass-transfer operations. 

In the mass-transfer operations, neither equilibrium phase consists of only one component. Consequently when the two phases are initially contacted, they will not (except fortuitously) be of equilibrium compositions. The system then attempts to reach equilibrium by a relatively slow diffusive movement of the constituents, which transfer in part between the phases in the process. Separations are therefore never complete, although, as will be shown, they can be brought as near completion as desired (but not totally) by appropriate manipulations. 

The three states of aggregation, gas, liquid, and solid, permit six possibilities of phase contact. 

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R. Krishna and R. Taylor iu N. P. Chemetisiuoff, ed.. Handbook for Heat and Mass Transfer Operations, Vol. 2, Gulf Publishing Corporation, Houston, Tex., 1986. 

R. E. Treybal, Mass Transfer Operations, 3rd ed. McGraw Hill, New York, 1980. 

Taylor and Krishna, Multicomponent Mass Transfer, Wiley, 1993. Toumie, Laguerie, and Couderc, Chem. Engr. Sd., 34, 1247 (1979). Treybal, Mass Transfer Operations, 3d ed., McGraw-Hill, 1980. 

All these processes are, in common, liquid-gas mass-transfer operations and thus require similar treatment from the aspects of phase equilibrium and kinetics of mass transfer. The fluid-dynamic analysis ofthe eqmpment utihzed for the transfer also is similar for many types of liquid-gas process systems. 

To design deep-bed contactors for mass-transfer operations, one must have, in general, predictive methods for the following design parameters ... 

The power for agitation of two-phase mixtures in vessels such as these is given by the cuiwes in Fig. 15-23. At low levels of power input, the dispersed phase holdup in the vessel ((j)/ ) can be less than the value in the feed (( )df) it will approach the value in the feed as the agitation is increased. Treybal Mass Transfer Operations, 3d ed., McGraw-HiU, New York, 1980) gives the following correlations for estimation of the dispersed phase holdup based on power and physical properties for disc flat-blade turbines ... 

FK . 15-22 Uqiiid agitation by a disc flat blade turbine in the presence of a gas-liquid interface a) without wall baffles, (h) with wall baffles, and (c) in full vessels without a gas-bqiiid interface (continuous flow) and without baffles. [Couitesy Treyhal, Mass Transfer Operations, 3rd ed., p. 148, McGraw-Hill, NY,... 

SOURCE Treyhal, Mass Transfer Operations, 3ded., p. 152, McGraw-Hill, NY, 1963. 

In any mass transfer operation, the compositions of the liquid and vapor phases are assumed to follow the relationship illustrated by the column operating line. This line represents the overall calculated profile down the column however, the composition on each individual square foot of a particular column cross-section may vary from that represented by the operating line. These variations are the result of deviations in the hydraulic flow rates of the vapor and liquid phases, as well as incomplete mixing of the phases across the entire column. 

Mass transfer operations (in which a material is transferred across a phase boundary or interface)... 

Sherwood, T. K., Pigford, R. L., and Wilke, C. R., Mass Transfer. McGraw-Hill, New York, 1 975. Sissom, L. E., and Pitts, D. R., Elements of Transport Phenomena. McGraw-Hill New York, 1972. Treybal, R. E., Mass Transfer Operations. McGraw-Hill, New York, 1968. 

As for other mass transfer operations in ehemieal engineering, several authors have proposed equations for the ealeulation of heat and mass balanees used for the estimation of erystal yield, heat load, and evaporation duty in bateh erystal-lizations, e.g. (Mullin, 2001)... 

Discuss the differences between the various mass transfer operations. 

Most packed towers are used for mass transfer operations such as absorption, distillation, and stripping however, there are other uses such as heat transfer quenching and entrainment knockout. 

Treyball, R E. Mass Transfer Operations, McGraw-Hill Book Co., New York, N.Y. (1955). 

When solid particles are subject to noncatalytic reactions, the effects of the reaction on individual particles are derived and then the results are averaged to determine overall properties. The general techniques for this averaging are called population balance methods. They are important in mass transfer operations such as crystallization, drop coagulation, and drop breakup. Chapter 15 uses these methods to analyze the distribution of residence times in flow systems. The following example shows how the methods can be applied to a collection of solid particles undergoing a consumptive surface reaction. 

The techniques of process integration have been expanded for use in optimising mass transfer operations, and have been applied in waste reduction, water conservation, and pollution control, see Dunn and El-Halwagi (2003). 

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