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Accurate predictions of phase equilibria of mixtures are essential for designing and optimizing separation processes and numerous other unit operations in the chemical industry. Although experiments provide accurate data, they are time consuming and expensive. A need exists for accurate, reliable models that can provide predictions of vapor-liquid equilibrium (VLE) of mixtures without the need for extensive experimentation. Cubic equations of state (CEOS), such as Soave-Redlich-Kwong (SRK) and Peng-Robinson (PR), are widely used in the chemical industry to perform reservoir simulations and process design calculations due to their inherent simplicity and efficiency. These equations have one or two empirical binary interaction parameters (BIPs) for each binary pair of a mixture. The BIPs cannot be predicted a priori from existing theory, but they can be regressed from VLE experimental data. The present study focused on developing reliable generalized models to estimate the BIPs for the PR EOS using a quantitative structure-property relationship (QSPR) modeling approach.For model development, a VLE database consisting of 916 low-pressure binary systems covering a wide range of functional-group interactions was utilized. Using this database, the BIPs for the PR EOS were determined by regressing the VLE experimental data. Then, the BIPs were generalized using QSPR modeling methodology. Results indicate that the generalized PR-QSPR model provides bubblepoint pressure predictions within two times the errors obtained through direct regressions of the BIPs. Further, the generalized model was validated by predicting bubble-point pressures of binary systems not used in model development and systems 3 that contained data at higher pressures. For 22 systems used for model validation, the developed PR-QSPR model provided generalized predictions comparable to those of data used for model development.

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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, γ−ϕ and ϕ−ϕ, 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 γ−ϕ approach represents the VLE behavior more precisely compared to the ϕ−ϕ approach. The overall results using the γ−ϕ 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 ϕ−ϕ approach regression results are within 3 times the error of the γ−ϕ approach. A similar trend was observed for the quantitative structure−property relationship generalized predictions. The γ−ϕ approach predicts the VLE behavior more accurately compared to the ϕ−ϕ approach. The overall results based on the γ−ϕ approach exhibit % AADs of 5.1, 0.4, 5.9, and 8.1 for the pressure, temperature, mole fraction, and K, respectively. The ϕ−ϕ approach generalized predictions are within 2 times the error obtained from the γ−ϕ 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 ϕ−ϕ 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.