ASSOCIATING MODELS AND MIXING RULES IN EQUATIONS OF STATE FOR WATER HYDROCARBON MIXTURES/

Equations of state (EoS) for associating fluids calculate explicitly t he effect of hydrogen bonding by using chemical theory, perturbation t heory or quasi-chemical theory. In all cases, the number of hydrogen b onding sites per molecule is an input parameter to the model. Specific ally for water, two-, three- or four-site models have been used in the past by different investigators. In this work, the associated-perturb ed-anisotropic-chain-theory (APACT) and the statistical-associating-fl uid-theory (SAFT) are applied to predict the phase equilibrium of wate r/hydrocarbon mixtures, with emphasis on liquid-liquid equilibria (LLE ). The accuracy of the different models to describe association in wat er is investigated. Different mixing rules are examined for SAFT. The original mixing rules proposed based on the van der Waals one-fluid th eory 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 hydrocarb on. For water/hydrocarbon mixtures, the one-fluid theory fails to pred ict reliably the hydrocarbon solubility because the local composition is very different from the bulk composition. A more realistic mixing r ule for these systems is proposed that is based on the asymmetric mixi ng rule introduced originally for the perturbed-hard-chain-theory (PHC T). This mixing rule is used here for SAFT predictions of water/hydroc arbon LLE resulting in an improvement of the hydrocarbon solubility pr edictions. Nevertheless, the agreement of all the models investigated with the experimental data is, in general, poor. For comparison, calcu lations with the Redlich-Kwong-Joffe-Zudkevitch (RKJZ) cubic EoS are a lso reported. Copyright (C) 1997 Elsevier Science Ltd.