Practical equation of state and activity coefficient models based on the exp-6 fluid

Abstract

Dodd L.R. and Sandler S.I., 1991. Practical equation of state and activity coefficient models based on the exp-6 fluid. Fluid Phase Equilibria, 69: 99-139. In this paper we test a partition function model recently developed for the exp-6 fluid mixture against experimental data for real fluids. This model leads to a unique size and density-dependent mixing rule providing non-quadratic density-dependence which satisfies the low-density and pure fluid boundary conditions. The equation of state calculations with this model gave good results, especially for highly asymmetric systems. Applications of a similar mixing rule to cubic engineering equations of state for real fluids yielded better predictions than the van der Waals one-fluid mixing rule. However, except in the near critical region, any clear advantage of the new density-dependent mixing rule for real fluids is lost when adjustable parameters are added. We also show that an activity coefficient model based on this exp-6 model leads to good correlations for low pressure vapor-liquid equilibria of both simple and very complex mixtures, indeed mixtures that could not easily be described by other models with the same number of parameters.