Solution distillation and special methods

In the selective processes so far described an alteration in the phase equilibrium is attained either by the use of a carrier vapour or by the addition of other liquids (azeotropic and extractive distillation). 

Another possibility is to dissolve salts in the mixture to be separated, so as to bring about a change in the ratio of partial pressures of the components. This method of solvtion distillation has been employed mainly for mixtures of which one comjK)-nent is water. The mixture water-acetie acid, for example, at atmospheric jiressure has an equUibriuin curve approaching the diagonal very closely above 96% wt. acid, and it is consequently very difficult to eliminate the water entirely. From an industrial point of view this distillation also possesses the economic drawback that water (the distillate) has nearly five times the heat of evaporation of acetic acid this becomes important in the concentration of dilute acid. It would therefore be preferable if the acetic acid could be obtained as overhead product and the water as residue. This can 

Similarly, the azeotropic mixture methanol-acetone can be enriched beyond 

Furter [91] has analyzed the state of the art from the point of view of employing the salt effect in industrial processes, especially in extractive distillation. In addition, he ha.s made up a list of references covering the years 1966 to 1977 [91 a]. Schubert et al. [92] investigated the effect of some metal chlorides and other salts on the isothermal = 60°C) phase equilibrium behaviour of the systems n-propanol-water, n-butanol-water and methanol-water. Using CH30H/H20/NaBr as an example, the method of predicting salt effects for vapour-liquid equilibria as developed by Schuberth has been extended to uusaturated solutions [92a]. 

Jaques and Furter [95] derived an equation for the salt effect on the water-vapour equilibrium in binary mixtures which correlates the temperature and the liquid concentration of the three components ethanol, water and salt. The equation has 6 constants. The theory of the salt effect has been discussed by Furter and Meranda [96]. On the basis of simplifying assumptions Sada et al. [97] have established a relation for the calculation of vapour-liquid equilibria for non-aqueous binary systems in which the salt is dissolved only in one component e.g., benzene-ethanol with lithium or calcium chloride). 

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