The use of saline extractive agents in extractive distillation processes has been studied in the past few years. Among these agents, potassium acetate should be highlighted (Sanchez and Cardona, 2005). As the salts are nonvolatile compo­nents, the distillate obtained in distillation columns is much easier to separate. Therefore, the energy costs are lower compared to traditional extractive distilla­tion. The salt effect consists of the preferential solvation of the ions formed from the salt dissociation with the less volatile component that leads to the increase of the relative volatility of the most volatile component (ethanol).

Two possible configurations have been simulated for ethanol dehydration using potassium acetate (Ligero and Ravagnani, 2003). In the first case, the starting point is a diluted ethanol solution that is fed to the saline extractive distillation column followed by a multistage evaporation and spray drying for salt recovery. In the column distillate, the anhydrous ethanol is obtained due to the breaking of the azeotrope induced by the salt. The second case has a previous distillation column to concentrate the ethanol solution, which is fed to the saline extractive distillation column with the subsequent salt recovery through spray drying. This second option presents best results from the viewpoint of energy costs. In addi­tion, the salt used is not toxic, which indicates that this process can replace the traditional azeotropic distillation that uses such toxic entrainers as benzene.

Rigorous models for saline extractive distillation for ethanol-water-CaCl2 system have been developed. These models take into account the contribution of the salt to the liquid phase enthalpy (Llano-Restrepo and Aguilar-Arias, 2003). This type of model allows simulating the phase equilibrium and thermody­namic properties of solvent-electrolyte mixed systems having a strong nonlin­ear character that becomes an obstacle for their study. In this way and starting from theoretical considerations, the production of water-free ethanol using CaCl2 as the separation agent was predicted. These rigorous models can make possible the utilization of this type of process again by the industry considering that saline extractive distillation was discarded in the past due to, among other reasons, the difficulty in its design, which was caused by the complexity of the nonideal behavior of the phase equilibrium and the intricate modeling of the thermodynamic properties of these systems. This process was also discarded because of the technical problems derived from the dissolution and subsequent crystallization of the salt, as well as the need to use materials resistant to the corrosion (Pinto et al., 2000).

Besides inorganic salts, the possibility of employing hyperbranched poly­mers like polyesteramides and hyperbranched polyesters during the extractive distillation of ethanol-water mixtures has also been studied. These polymers exhibit a remarkable separation efficiency and selectivity, and their physical — chemical properties can be tailored depending on the required application. The recovery of these polymers can be carried out by washing, evaporation, drying, or crystallization (Sanchez and Cardona, 2005; Seiler et al., 2003).