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, 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 GE/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.