Predicting the Phase Equilibria, Critical Phenomena, and Mixing Enthalpies of Binary Aqueous Systems Containing Alkanes, Cycloalkanes, Aromatics, Alkenes, and Gases (N2, CO2, H2S, H2) with the PPR78 Equation of State

The phase behavior of water/hydrocarbon mixtures in a wide range of concentrations, temperatures, and pressures is important in a variety of chemical engineering applications. For this reason, the physical understanding and mathematical modeling of these aqueous-organic mixtures constitute a challenging task, both for scientists and for applied engineers. In this work, mutual solubilities, critical loci, and mixing enthalpies of water + hydrocarbon, water + carbon dioxide, water + nitrogen, water + hydrogen sulfide, and water + hydrogen binary mixtures are predicted using the PPR78 cubic equation of state (EoS). The extremely nonideal behavior of these systems produces unusual and complex thermodynamic behavior. As an example, such mixtures often exhibit type III phase behavior in the classification scheme of Van Konynenburg and Scott and are characterized by a vapor-liquid critical line which first exhibits a temperature minimum and then extends to temperatures above the critical point of pure water. Such a behavior, called gas-gas equilibria of the second kind is a consequence of the large degree of immiscibility of the two components. The selected PPR78 model combines the Peng-Robinson cubic EoS and a group-contribution method aimed at predicting the temperature-dependent binary interaction parameters, k(ij)(T), involved in the Van der Waals one-fluid mixing rules. Although, it is acknowledged that cubic EoS with a constant k(ij) are not suitable to predict phase equilibria of such highly nonideal systems, the addition of the H2O group to the PPR78 model makes it possible to conclude that the use of temperature-dependent binary interaction parameters not only results in qualitatively accurate predictions over wide pressure and temperature ranges but also leads to quantitatively reasonable predictions for many of the studied systems.