The calculation of thermodynamic properties and phase equilibria using a new cubic equation of state

A new three constant equation of state for fluids and fluid mixtures has been proposed. The equation contains three constants and is a more general form of the Peng-Robinson equation. In addition to the critical temperature and the critical pressure, two parameters are required to characterize a particular fluid. These parameters have been evaluated by minimizing the deviation in the saturated liquid densities while simultaneously satisfying the equilibrium criteria (equality of fugacities) along the saturation curve. Thus, prediction of volumetric properties in the saturation region and other regions of the phase diagram is improved, while accuracy in the prediction of vapour-liquid equilibrium is maintained. Parameters for hydrocarbons and nonhydrocarbons of importance to the synthetic fuel industry are presented. The new equation is extended to mixtures by using mixing rules similar to those used by Peng and Robinson. Only one binary interaction coefficient is sufficient for the accurate prediction of vapour-liquid equilibria. Optimum values of the binary interaction coefficients for hydrocarbon-hydrocarbon and hydrocarbon-non-hydrocarbon systems have been obtained using the new equation, the Peng-Robinson equation and the Soave modification of the Redlich-Kwong equation. The criterion used for selecting the optimum interaction coefficient is the minimization of deviations in bubble point pressures. The new equation has been tested for the prediction of volumetric behaviour of pure fluids and the phase and volumetric behaviour of binary, ternary and multicomponent systems. Comparisons with conventional equations (P-R, S-R-K, R-K and B-W-R-S) are shown. The applicability of van-der-Waals orie fluid model to _ generalised equations of state is demonstrated. The equations of Han and Starling, Simonet and-Behar and Chaudron et al have been compared for volumetric predictions. The Han and Starling equation has also been used with two sets of mixing rules to predict vapour-liquid equilibrium. The van-der-Waals one fluid model has been shown to be a simple and effective method of applying the complicated equations of state to the prediction of thermodynamic properties of mixtures. A generalised form of the corresponding states principle using two non-spherical reference fluids is presented. This represents an alternative method of extending the equation of state approach to fluids and fluid mixtures when experimental data are not available.