Other Mass Transfer Operations

Although distillation may be the most common mass transfer operation, it is also the most difficult to assimilate. Indeed, the separation between components is noticeably obscure, because they occupy the same phase. Other mass transfer operations involve separation or combination of different phases  

Because of this distinction, one of the exit streams in ea( h of the above is either pure, as the vapor from an evaporator, or in an equilibrium state independent of material-balance considerations. Although material-balance control can be enforced in each of these mass transfer operations, the separation between phases generally simplifies its formulation by 

The final controlled variable in every case is composition, requiring some sort of an analytical measurement . For most of these applications, a nonspecific determination, such as density, is sufficient. But occasionally, as in a drying operation, even nonspecific analyses are not available, so other variables must be found to provide some degree of regulation. 

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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)... 

Like all the mass transfer operations, liquid extraction is a means of separating the components of a solution, and it is accomplished by bringing the solution into contact with another insoluble phase. The unequal distribution of the components of the solution between the two phases which then results provides the separation. In the case of liquid extraction, of course, the two phases in question are both liquids, but, just as in the other mass transfer operations, intimacy of contact and large interfacial area between the phases are required for rapid diffusional transfer of substance from one phase to the other. 

In chemical process control the variable that is most commonly inferred from secondary measurements is composition. This is due to the lack of reliable, rapid, and economical measuring devices for a wide spectrum of chemical systems. Thus inferential control may be used for the control of chemical reactors, distillation columns, and other mass transfer operations such as driers and absorbers. Temperature is the most common secondary measurement used to infer the unmeasured composition. 

Drying is similar in many ways to other mass transfer operations, particularly to humidification. If the surface of a solid is completely covered with liquid, the rate of its evaporation is controlled by the same mechanism as humidification. In order for equilibrium to exist under this condition, the gas must be 100 percent saturated with moisture, because the solid is. If the surface is free of this unbound moisture, however, the moisture content will vary with the relative humidity of the surrounding gas, at equilibrium. Figure 12.13 shows an equilibrium curve for a typical solid. 

In competition with other mass-transfer operations Here relative costs are important. Distillation and evaporation are direct separation methods, the products of which are composed of essentially pure substances. Liquid extraction, on the other hand, produces new solutions which must in turn be separated, often by distillation or evaporation. Thus, for example, acetic acid can be separated from dilute solution with water, with difficulty by distillation or with relative ease by extraction into a suitable solvent followed by distillation of the extract. For the more dilute solutions particularly, where water must be vaporized in distillation, extraction is more economical, especially since the heat of vaporization of most organic solvents is substantially less than that of water. Extraction may also be attractive as an alternative to distillation under high vacuum at very low temperatures to avoid thermal decomposition. For example, long-chain fatty acids can be separated from vegetable oils by high-vacuum distillation but more economically by extraction with liquid propane. Tantalum and niobium can be separated by very tedious fractional crystallization of the double fluorides with potassium but with relative ease by liquid extraction of the hydrofluoric acid solutions with methyl isobutyl ketone. 

In the general case, it will be easiest to make calculations graphically, as in other mass-transfer operations, and this will require graphical representation of... 

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