Vapour-liquid equilibrium of water/ethanol/1-butanol/salt prediction and experimental verification

Experimental vapour-liquid equilibrium data of water/ethanol/1-butanol saturated with NaCl, KCl and NH4Cl compared well with those predicted by Tan-Wilson and Tan-NRTL models for multicomponent solvent-solute mixture. The Wilson and NRTL solvent-solvent interaction parameters were obtained by the regression of the experimental vapour-liquid equilibrium data of the salt-free solvent mixture. The solute-solvent interaction parameters were calculated from the experimental bubble points of the individual solvent components saturated with the respective salts. Water was significantly salted-into the liquid phase by all the three salts giving a vapour phase rich in organic solvent components and a liquid phase rich in water. Both NaCl and NH4Cl showed little or no preference in salting-out the two organic solvent components while KCl showed preferential salting-out of ethanol. These results are consistent with previous reports that solvent component i would be salted-in or out of the liquid phase relative to solvent component j depending on whether the ratio of the solute-solvent interaction parameter, A(sj)/A(si) (Tan-Wilson model) or exp(tau(is) - tau(js)) (Tan-NRTL model) is less than or greater than 1. These findings show that the two models describe reasonably well the effect of salts on the liquid-phase activity coefficient of the solvent components in a ternary solvent mixture. The criteria and the rapid screening method proposed by Tan on the selection of a suitable non-volatile soluble solute for the elimination of azeotropes and for the salt distillation of close-boiling and azeotropic binary solvent mixtures are therefore equally applicable to ternary solvent mixture.