Associating models and mixing rules in equations of state for water/hydrocarbon mixtures

Abstract

Equations of state (EoS) for associating fluids calculate explicitly the effect of hydrogen bonding by using chemical theory, perturbation theory or quasi-chemical theory. In all cases, the number of hydrogen bonding sites per molecule is an input parameter to the model. Specifically for water, two-, three- or four-site models have been used in the past by different investigators. In this work, the associated-perturbed-anisotropic-chain-theory (APACT) and the statistical-associating-fluid-theory (SAFT) are applied to predict the phase equilibrium of water/hydrocarbon mixtures, with emphasis on liquid-liquid equilibria (LLE). The accuracy of the different models to describe association in water is investigated. Different mixing rules are examined for SAFT. The original mixing rules proposed based on the van der Waals one-fluid theory work well for vapor-liquid equilibria (VLE) predictions, but not for water/hydrocarbon LLE, which are primarily influenced by the large difference in intermolecular interactions between water and hydrocarbon. For water/hydrocarbon mixtures, the one-fluid theory fails to predict reliably the hydrocarbon solubility because the local composition is very different from the bulk composition. A more realistic mixing rule for these systems is proposed that is based on the asymmetric mixing rule introduced originally for the perturbed-hard-chain-theory (PHCT). This mixing rule is used here for SAFT predictions of water/hydrocarbon LLE resulting in an improvement of the hydrocarbon solubility predictions. Nevertheless, the agreement of all the models investigated with the experimental data is, in general, poor. For comparison, calculations with the Redlich-Kwong-Joffe-Zudkevitch (RKJZ) cubic EoS are also reported.