On the concentration dependence of the UNIQUAC/UNIFAC models

The UNIQUAC/UNIFAC models can not in general represent vapor-liquid and liquid-liquid equilibria simultaneously with satisfactory results. Recent modifications of the models take into account association and solvation effects by means of chemical theory. However, this approach leads to complicated expressions for the activity coefficients and the improvements are only moderate. A modification of the UNIQUAC/UNIFAC models is presented which takes into account solvation and association by allowing the interaction energies to vary with composition. This approach leads to explicit expressions for the activity coefficients. It is shown that the modified models in simultaneous correlations lead to quantitative representation of vapor-liquid equilibrium data and semi-quantitative representation of binary and ternary liquid-liquid equilibrium data.