EQUATIONS OF STATE USING AN EXTENDED TWU-COON MIXING RULE INCORPORATING UNIFAC FOR HIGH-TEMPERATURE AND HIGH-PRESSURE PHASE-EQUILIBRIUM PREDICTIONS

A mixing rule recently developed by Twu and Coon [C.H. Twu, J.E. Coon, CEOS/A(E) mixing rules constrained by the vdw mixing rule and the sec ond virial coefficient, AIChE J. 42 (1996) 3212-3222] is extended to i ncorporate the UNIFAC group contribution method into an equation of st ate for the prediction of phase behavior of highly non-ideal systems o ver wide ranges of temperature and pressure. The mixing rule developed by Twu and Goon reduces to the van der Waals mixing rule. The Helmhol tz excess free energy function with respect to a van der Waals fluid a t infinite pressure has been related rigorously to the Helmholtz exces s free energy at zero pressure. This extension of the Twu-Coon Mixing Rule goes gracefully from the classical van der Waals one-fluid mixing rule for non-polar fluids needed in the refining and gas processing i ndustries to a mixing rule combining excess free energy models at low pressure with equations of state for the strongly polar systems found in the chemical industries. When the UNIFAC group contribution method is incorporated, the mixing rule becomes totally predictive, The compl etely predictive equation of state is shown to give accurate results f or systems for which the UNIFAC model is in agreement with the experim ental activity coefficients at low pressure. The UNIFAC-incorporated m ixing rule provides a simple way to extend the UNIFAC group contributi on method to high temperatures and pressures. (C) 1997 Elsevier Scienc e B.V.