Equation of State and Activity Coefficient Models for Improved Engineering Practice

Amadeu K. Sum and Stanley I. Sandler, University of Delaware

Research Objectives

This project investigates the application of ab initio molecular orbital theory to determine parameters often encountered in applied thermodynamics models, such as equations of state and activity coefficient models used in phase equilibria predictions.

Computational Approach

Energy minimizations of molecule clusters were used to determine the interaction energies between pairs of molecules computed using ab initio methods, in particular Hartree-Fock theory. Calculations were performed with the Gaussian 94 computational chemistry program. Most of the calculations were done on the Cray J90 machines.

Accomplishments

A collection of binary aqueous systems was studied to determine the interaction energy between pairs of molecules. At first, clusters of as large as eight molecules were optimized by minimizing the energy of the system, using Hartree-Fock theory. Interaction energies were then calculated for the like and unlike pairs of molecules (also using Hartree-Fock theory) which subsequently were used as parameters in the widely used Wilson and UNIQUAC activity coefficient models for phase equilibria calculations. This method resulted in excellent phase behavior predictions using the UNIQUAC model.

Significance

Our current research focuses on using microscopic information available from computational chemistry to determine parameters in engineering models predicting the phase behavior of fluids and their mixtures. We have successfully applied this approach to two engineering equations of state, and are now expanding the scope of our project to include liquid solution (activity coefficient) models. By being able to determine these parameters in engineering models from first-principle calculations, we are developing a new generation of predictive models-the tools engineers need for making predictions about thermodynamic properties and phase behavior.

Vapor-liquid equilibria for ethanol (1) + water (2) using parameters determined from ab initio methods.

Publications

A. K. Sum and S. I. Sandler, "Use of ab initio methods to make phase equilibria predictions using activity coefficient models," in Proceedings of Eighth International Conference on Properties and Phase Equilibria for Product and Process Design (Netherlands, in press, 1998).

A. K. Sum and S. I. Sandler, "A novel approach to phase equilibria predictions using ab initio methods," presented at the AIChE Annual Meeting, Miami Beach, FL, November 15-20, 1998.