Separating agent extractive distillation using dissolved salts

Figure 1. Flowsheet for extractive distillation using a dissolved salt as the separating agent...

The topic covered in the 10 papers of the first section is commonly referred to as salt effect in vapor-liquid equilibrium and is potentially of great industrial importance. This salt effect leads to extractive distillation processes in which a dissolved salt replaces a liquid additive as the separating agent the replacement often results in a greatly improved separating ability and reduced energy requirements. Two papers in this volume, those by Sloan and by Vaillancourt, illustrate the use of such processing to concentrate nitric acid from its aqueous azeotrope. Nevertheless, the effect has not been exploited by industry to nearly the extent that would seem to be merited by its scientific promise. 

The use of a dissolved salt in place of a liquid component as the separating agent in extractive distillation has strong advantages in certain systems with respect to both increased separation efficiency and reduced energy requirements. A principal reason why such a technique has not undergone more intensive development or seen more than specialized industrial use is that the solution thermodynamics of salt effect in vapor-liquid equilibrium are complex, and are still not well understood. However, even small amounts of certain salts present in the liquid phase of certain systems can exert profound effects on equilibrium vapor composition, hence on relative volatility, and on azeotropic behavior. Also extractive and azeotropic distillation is not the only important application for the effects of salts on vapor-liquid equilibrium while used as examples, other potential applications of equal importance exist as well. 

A salt dissolved in a mixed solvent is capable, through such effects on the structure of the liquid phase as preferential association and others, of altering the composition of the equilibrium vapor. Hence salt effect on vapor-liquid equilibrium relationships provides a potential technique of extractive distillation. A review is presented of the use of dissolved salts, rather than liquid solvents, as separating agents for extractive distillation. 

In certain systems where solubility considerations permit, it is possible to use a salt dissolved into the liquid phase as the separating agent in place of the normal liquid. The attraction of the salt-effect distillation technique lies in its potential for greatly reduced energy requirements compared with conventional extractive and azeotropic distillation processes. 

Another method used in industry, which can be categorized as extractive distillation, is to use a dissolved salt (e.g., calcium chloride) as a separating agent to altCT the relative volatility of the two components in the mixture. This behavior, known as salt effect, is due to the preferential salvation of the ions (formed when the salt dissociates in solution) by the less volatile component of the solvent mixture. The bottom product of this saline extractive distillation column is the mixture of the salt and the less volatile component (e.g., water). The separation of this bottom stream may require an evaporative crystallizer and a spray dryer for recovery of the anhydrous salt. A sahne extractive distillation method is usually more expensive than conventional extractive distillation. 

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