Multiphase equilibrium calculations for binary, ternary, and quaternary hydrocarbon-water systems at high temperature were performed using the Schmidt-Wenzel equation of state. The solubility of water in the hydrocarbon-rich liquid phase and vapor phases is modelled using a constant binary interaction parameter between water and hydrocarbon, while the solubility of hydrocarbons in the aqueous phase is calculated using a temperature dependent binary interaction parameter. Two and three phase equilibrium calculations were performed using the method of successive substitution. A stability analysis using the tangent plane criterion was used to determine the correct number of phases present.

At high temperature the solubility of water in hydrocarbon liquids can be quite large. Using the procedure outlined above the solubility of water in the hydrocarbon-rich liquid phase was calculated accurately both for mixtures of water and pure hydrocarbon components, and for mixtures of water and petroleum fractions. The binary interaction parameters used in the hydrocarbon-rich liquid phase and the vapor phase were found to be dependent on the type of hydrocarbon. The interaction parameters for components in the same homologous group such as alkanes, aromatics, and alkenes were found to be almost the same.

The solubility of hydrocarbons in the aqueous phase was calculated reasonably accurately using temperature dependent interaction coefficients in the aqueous phase. Above 200 °F, the binary interaction parameters in the aqueous phase were linear functions of temperature. Binary interaction parameters from two phase binary data were found to be quite adequate to calculate three phase multicomponent equilibrium.

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