A New Analytical Model for the Prediction of Vapor–Liquid Equilibrium Densities

A new simple, predictive model for estimating both the vapor and the liquid densities of fluids at the vapor–liquid equilibrium is presented. It is based on the symmetry of the derivatives of the two saturation densities with respect to the temperature, which is a consequence of applying the rectilinear diameter law. No adjustable coefficients are involved, and only two parameters—both with certain physical meaning—have to be calculated for each fluid. The method used for these calculations is straightforward, the required inputs being the critical temperature and density, and the value of the vapor and liquid densities at a reference temperature. The results show that the model is accurate for fluids of different kinds as long as the rectilinear diameter law holds, and that, in general, the accuracy is better than that of the most recent model with no adjustable coefficients.