The Separation Factor a Azeotropes

A convenient and consistent measure of separation is provided by the so-called separation factor or relative volatility a, which is composed of the product of two mole fraction ratios. These ratios are defined in such a way as to minimize changes in a with composition and are represented by the following expression  

Here x and y refer to the more volatile component (i.e., the constituent with the higher pure-component vapor pressure or the lower boiling point). In general, the higher the value of a above unity, the greater the degree of separation or enrichment. 

Note that any decrease in y/x toward 1 as y and x approach unity is neatly offset by the compensating ratio (1 - x)/(l - y). Thus, if y = 0.99 and x = 0.98, for example, a will still be considerably above rmity  

Consequently, the use of a predicts, correctly, that there is still substantial separation to be obtained even when the mole fractions are near unity. 

Separation factors greater than unity result in x-y curves that lie entirely above the 45° diagonal. The higher the value of a, the greater the distance is between the two lines. This implies that separation becomes easier as the x-y curve bulges out and away from the 

Azeotropic behavior is a common occurrence in vapor-liquid equilibria (VLE). Approximately one-third of all systems listed in standard VLE handbooks exhibit azeotropes. In general, the more dissimilar the two components, the greater the likelihood that such mixtures will be formed. Combinations of polar-nonpolar substances and those with widely differing structural features are particularly prone to azeotropic behavior. Table 6.7, which lists some of the more conventional azeotropic pairs, conveys a sense of the degree of dissimilarity that leads to the formation of azeotropes. 

Azeotropic mixtures require special methods for their separation, which usually consist of adding a third component that has the ability to break the azeotrope. Perhaps the most famous case is that of ethanol-water, which has an azeotropic mole fraction in ethanol of 0.8943 at atmospheric pressure (Table 6.8). Here the added component is benzene and results, on distillation, in the recovery of pure ethanol and a ternary azeotrope containing benzene. That mixture, on condensation, results in two immiscible aqueous and organic layers, which are separated and further processed by distillation. 

We now turn to the question of the behavior of a for ideal liquid solutions. Here the Raoult s law equations (Equation 6.15a and Equation 6.15b) provide a quick answer. Dividing the two expressions, we obtain in the first instance 

Azeotropic systems (a) boiling-point diagram (b) vapor-pressure diagram (c) x-y diagram. 

Ethanol (l)-Ethyl acetate (2) Ethanol (l)-Chloroform (2) Ethanol (l)-Carbon tetrachloride (2) 0.48 Yes Yes 72.1 

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