Representation and Prediction of Vapor-Liquid Equilibrium Using the Peng-Robinson Equation of State and UNIQUAC Activity Coefficient Model

Many important processes in the oil and gas industry (e.g., distillation, absorption, extraction) involve contact between liquid and vapor phases. The reliable design of these industrial processes requires accurate thermodynamic models to describe the vapor-liquid equilibrium (VLE) of the mixtures of interest. Two common approaches, gamma-phi and phi-phi, are utilized to describe such VLE behavior. In this study, we present a comprehensive assessment of the representation and predictive capability of these two approaches, utilizing the UNIQUAC model to determine the activity coefficients and the Peng-Robinson (PR) equation of state to calculate the fugacity coefficients. The assessment was completed using a diverse binary VLE database consisting of 916 binary systems involving 140 compounds belonging to 31 chemical classes. Both the overall results and the results categorized for highly nonideal systems and for aqueous systems are presented within the context of internal and external consistency tests. Specifically, regressed and generalized parameters are utilized in internal and external consistency tests, respectively. Further, the phase behavior of sample systems was analyzed using Danner's molecular classification method based on the mNRTL1 parameter and G(E)/RT values. For the systems considered, the regression results show that the gamma-phi approach represents the VLE behavior more precisely compared to the phi-phi approach. The overall results using the gamma-phi approach exhibit an absolute average deviation (% AAD) of 1.6, 0.1, 4.5, and 5.7 for the pressure, temperature, mole fraction, and equilibrium constant (K), respectively. The phi-phi approach regression results are within 3 times the error of the gamma-phi approach. A similar trend was observed for the quantitative structure property relationship generalized predictions. The gamma-phi approach predicts the VLE behavior more accurately compared to the phi-phi approach. The overall results based on the gamma-phi approach exhibit % AADs of 5.1, 0.4, 5.9, and 8.1 for the pressure, temperature, mole fraction, and K, respectively. The phi-phi approach generalized predictions are within 2 times the error obtained from the gamma-phi approach. The results of Danner's molecular classification of the phase behavior indicated that systems with similar components are more likely to produce nearly ideal mixture behavior and systems involving dissimilar components are more likely to produce nonideal mixture behavior. Further, the quality of the representations for the phi-phi approach are generally good for most system classifications with the exception of adequate or poor fits observed for strongly polar-strongly polar and aqueous-strongly polar systems.