Extended Flexibility for GE Models and Simultaneous Description of Vapor-Liquid Equilibrium and Liquid-Liquid Equilibrium Using a Nonlinear Transformation of the Concentration Dependence

A method to increase the flexibility of the composition dependence of G^E models or equations of state is proposed. The formalism is based on a nonlinear transformation of the concentration space and can be applied to any mixture model without re-deriving the model equations. Any number of additional binary parameters can be introduced, which show nearly no intercorrelation with the original model parameters. The reason is that the additional parameters only effect the form but not the size and symmetry of, for example, G^E. The mathematical formalism for the introduction into G^E models was derived and applied to the UNIQUAC model. The modified model (FlexQUAC) was then used to regress a large number of vapor-liquid equilibrium (VLE) data sets (approximately 4000), whereby significant improvements were observed for systems with medium to large positive deviation from Raoultʹs law. As a result of the very small intercorrelation with the original model parameters it was assumed that the additional parameters have no negative effect on the prediction of multicomponent behavior from binary data. This was verified for 13 carefully selected systems. The motivation for this development was not primarily the improvement of the regression capability but the simultaneous description of VLE and liquid-liquid equilibrium (LLE) data. Typically models such as NRTL or UNIQUAC predict immiscible regions that are way too large from VLE data or VLE separation factors that are too low from LLE. This shortcoming of the original models is corrected by the concentration space transformation. Examples for the simultaneous description of VLE and LLE for binary and ternary systems are presented and compared to the corresponding results of the UNIQUAC equation.