A simplified CEOS/ AE mixing rule and its applications for vapor–liquid and liquid–liquid equilibrium predictions

Abstract

The new CEOS/ A E mixing rule recently developed by Twu et al. [Fluid Phase Equilib. 158–160 (1999) 271] is simplified and tested for high-pressure vapor–liquid equilibrium (VLE) and liquid–liquid equilibrium (LLE) predictions. This mixing rule has some significant features such as: (1) it is density dependent, (2) it is based on no reference-pressure, and (3) it automatically reduces to van der Waals mixing rule for normal mixtures. The mixing rule is used with any G E activity coefficient model such as NRTL, UNIQUAC, and UNIFAC. This approach can employ the available binary interaction parameters from those activity coefficient models without requiring any additional parameters or causing any thermodynamic inconsistency. It extends the predictive capability of the cubic type equation of state (CEOS) to highly nonideal mixtures, which have traditionally been handled using an activity coefficient approach. This work has tested various binary VLE/LLE systems including nonpolar, polar, and associating compounds. Overall, the accuracy of the mixing rule with a CEOS (with a modified alpha-function for each component) is comparable with that of the activity coefficient approach. For LLE, the mixing rule can predict the three different types of phase diagrams (with UCST, LCST, or both). This work also shows that the CEOS/ A E mixing rule can be applied to high-pressure systems for which the activity coefficient models have limited success.