An accurate direct technique for parameterizing cubic equations of state: Part II. Specializing models for predicting vapor pressures and phase densities

Abstract

A direct approach for specializing cubic equations of state to the prediction of the vapor–liquid equilibrium envelope of pure fluids is presented. The method is based on an asymptotic approximation of the equilibrium condition at low pressure, which forces a four-parameter van der Waals model to exactly reproduce the critical point and the saturation properties of a single reference state (usually, the normal boiling point). The required input data are the critical properties (temperature, pressure and volume) together with the boiling temperature, the local geometry of the vapor pressure curve and the volume of the liquid phase at the reference point. The proposed method, applied to a large database of pure fluids, is able to predict accurate vapor pressures and reasonably accurate liquid volumes from the reference point up to the critical range. In addition, an extension of the present method is outlined for improving further the interpolation of liquid volumes in the sub-critical range. For this latter purpose, the critical volume predicted by the equation of state is scaled from the experimental value by introducing a fluid-dependent factor, which may be estimated from experimental thermal expansion data of the liquid phase. The so-parameterized model yields reasonable results when applied to the prediction of enthalpies and to the interpolation of vapor–liquid equilibrium data of binary mixtures.