REPRESENTATION OF VAPOR-LIQUID-EQUILIBRIA IN WATER-ALCOHOL ELECTROLYTE MIXTURES WITH A MODIFIED UNIFAC GROUP-CONTRIBUTION METHOD

A new method for the correlation and prediction of salt effects on vapour-liquid equilibria of mixed solvent systems is proposed. The model combines a term of the Debye-Huckel type with a modified UNIFAC equation and is based on the solvation concept. The model parameters are ion-specific and no ternary parameters are required. Group interaction parameters have been estimated between ions (Na+, K+, Li+, Mg2+, Ca2+, Co2+, Ba2+, Sr2+, NH4+, Cl-, Br-, F-, NO3-, SO42-) and solvent groups (CH3OH, CH2, OH), while previously published group interaction parameters between solvent groups and between water and ionic species have been maintained. Application of this model to alcohol-water-salt mixtures shows that the change in vapour phase composition on adding salt to the liquid phase can be represented satisfactorily. The average expected accuracy of the total pressure or temperature is about 4% and of vapour phase mole fractions better than 4%. Since it is a group-contribution method, it has a much broader range of applicability than classical approaches based on local composition models such as NRTL or UNIQUAC.