A Novel Approach to Phase Equilibria Predictions Using <i>Ab Initio</i> Methods

Abstract

Molecular orbital ab initio calculations have been used to compute interaction energies between pairs of molecules in a large molecular cluster. These energies are then used as the interaction energy parameters in the widely used Wilson and UNIQUAC activity coefficient models. Low-pressure vapor−liquid equilibria predictions based on the calculated parameters have been computed for binary systems of water with methanol, ethanol, 1-propanol, 2-propanol, formic acid, acetic acid, acetone, acetonitrile, acetaldehyde, and m-methylformamide. Excellent predictions are obtained with the UNIQUAC model, whereas poor results are found with the Wilson model. In several cases, our predictions are also superior to those obtained from UNIFAC. In addition, using the same parameters and the UNIQUAC model, high-pressure vapor−liquid equilibria predictions were made using the Peng−Robinson−Stryjek−Vera equation of state and the Wong−Sandler mixing rule for methanol, ethanol, 2-propanol, and acetone separately with water. The low- and high-pressure results demonstrate that this unique approach can lead to accurate vapor−liquid equilibrium predictions for hydrogen-bonding mixtures based only on pure-component properties and the structure of the molecules.