Mixing rules based on the local composition and one-fluid models have been developed for use in the mean-potential-model Modified Benedict-Webb-Rubbin equation of state to accurately predict the thermodynamic properties of fluid mixtures. These hybrid mixing rules require three adjustable parameters for strongly nonideal binary mixtures and two parameters for less nonideal mixtures. These hybrid mixing rules have been tested using a wide variety of binary mixtures: (1) nonpolar + polar systems, such as hydrocarbons with ketones, alcohols, and water; (2) polar + polar systems such as carbon dioxide + methanol, water + acetone and (3) nonpolar + nonpolar systems such as benzene + n-hexane, ethane + n-butane, and methane + n-decane. Test results show that the hybrid mixing rules can correlate vapor-liquid equilibrium and mixture density data, for these strongly nonideal solutions, better than the conformal solution model. The hybrid mixing rules with parameters obtained from the binaries have also been applied to multicomponent mixtures.

Activity coefficients and excess Gibbs free energy were calculated using the hybrid mixing rules. Results show that reasonable predictions for these properties can be obtained without their inclusion in the parameter determination process. The hybrid mixing rules with four adjustable parameters have been further applied to predict liquid-liquid equilibria for model coal compound + water systems. The prediction accuracy is close to the experimental uncertainty.