A Comparative Study of the Perturbed-Chain Statistical Associating Fluid Theory Equation of State and Activity Coefficient Models in Phase Equilibria Calculations for Mixtures Containing Associating and Polar Components

Vapor-liquid equilibria (VLE), liquid-liquid equilibria (LLE), and vapor-liquid-liquid equilibria (VLLE) for systems involving highly nonideal components, namely, water, alcohols, alkanes, ketones, aldehydes, esters, and ethers, were investigated to evaluate the perturbed chain statistical associating fluid theory equation of state (PC-SAFT EOS) and two widely used activity coefficient models, that is, the universal quasichemical (UNIQUAC) and UNIQUAC functional-group activity coefficients (UNIFAC). Parameters used for the PC-SAFT EOS were taken from literature or estimated in this work, while those for UNIQUAC and UNIFAC were from commercial process simulator Aspen plus 8.4. It was found that all the three models yield reliable correlations/predictions for VLE calculations. However, UNIQUAC and UNIFAC were observed to be unreliable for LLE and VLLE calculations despite successful reproductions of experimental data in some cases. The calculated results deviate significantly from experimental data in many cases. Particularly, both models predict artificial liquid-liquid phase splitting for a number of miscible mixtures. Nonetheless, PC-SAFT EOS with the use of a single set of parameters reproduces experimental data quantitatively in most cases and provides reasonably accurate results in all other cases. This remarkable performance of PC-SAFT EOS potentially eliminates the need for various thermodynamic models and consequently the need for selecting a thermodynamic model when performing phase equilibria calculations using commercial software. This is important for practitioners, since (1) it remains unclear to select an appropriate model from the available models of a process simulator or thermodynamic package for a given phase equilibria calculation despite the presence of some type of rule of thumb and (2) it is also likely that none of the existing models is sufficiently accurate. In addition, it was shown that both pure component parameters and binary interaction parameters for the PC-SAFT EOS are well-behaved for a homologous series, which allows for parametrization for weakly characterized components by interpolation or extrapolation, and consequently, facilitates the development of a practical tool for phase equilibria calculations.