Solution of Problems Involving Single Columns Used to Effect Azeotropic and Extractive Distillations

6-2 SOLUTION OF PROBLEMS INVOLVING SINGLE COLUMNS USED TO EFFECT AZEOTROPIC AND EXTRACTIVE DISTILLATIONS 

Prior to the treatment of systems of azeotropic and extractive distillation columns, two other topics are considered, namely, the quantitative description of the behavior of solvents and the solution of problems involving columns in which three phases exist in the accumulator. 

The behavior of solvents in the vapor and liquid phases which form nonideal solutions are described by the same set of expressions as those given by Eq. (5-1). Further insight into the behavior of solvents in azeotropic and extractive distillation may be gained by the reconsideration of the first expression of Eq. (5-1), which may be restated in the following form 

If a mixture behaves as an ideal solution in the vapor phase y = 1 for all /, and if it also behaves as an ideal solution in the liquid phase yf = 1 for all /, then Eq. (6-1) reduces to 

Azeotropic and extractive distillation are made possible by the unique variations of the activity coefficients when a solvent is added to the mixture. A measure of the effect of a given solvent on two components may be expressed in terms of their volatility ratio. Let the volatility of component i be defined by y./Xj. Then the volatility of component / relative to any component b (or the relative volatility a, of component i relative to component b) is given by 

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