Thermodynamic Properties and Phase Equilibria of Aqueous Systems

(partially funded through NATO Collaborative Research Grants, 1996 - 1998)

Dr. Ioannis G. Economou, Dr. Georgios K. Boulougouris

Accurate prediction of thermodynamic properties of aqueous systems is a challenging problem due to the complexity of interactions. Because of its unique thermodynamic properties and low cost, water is used widely for several industrial processes. Recently developed chemical engineering processes utilize water at extreme conditions, approaching the critical point or even higher. As a result, there is an increasing need for accurate thermodynamic models. Macroscopic models, in terms of equations of state (EoS), fail to describe accurately the phase behavior of aqueous mixtures with non-polar components such as hydrocarbons.

Molecular simulation provides an appealing alternative for these calculations. Several molecular models were proposed over the last two decades and used for structure, thermodynamic and transport properties, mostly at ambient conditions. However, these models become less reliable at elevated temperature and pressure. The need for molecular models applicable over a broad range of temperature and pressure is, thus, profound. In this project, different molecular models were used to calculate the pure water phase equilibria (coexisting saturated densities, vapor pressure, heat of vaporization, and second virial coefficient) from relatively low temperature (around 50oC) up to very close to the critical point. Limitations of the different models for specific properties were identified. A simple scaling method was introduced that allowed re-evaluation of model parameters for a given model in order that agreement with experimental data improves. A new simple molecular model was developed that predicts the coexisting liquid-vapor densities and vapor pressure with good accuracy over a wide temperature range. It is based on the simple point charge (SPC) model that assumes a Lennard-Jones site and a positive partial charge at the oxygen and two positive partial charges at the hydrogens.

The new and existing molecular models were applied to predict the Henry's law constant of small normal hydrocarbons (such as methane and ethane) in water at different temperatures using the Widom test particle insertion method. Furthermore, the mutual solubilities at higher pressure up to 3000 bar and at conditions approaching the pure water critical point were calculated. In all cases, reasonable agreement with experimental data was obtained. Furthermore, new methodologies are developed for the phase equilibrium calculation of water with higher normal alkanes (such as n-butane and n-hexane) that combine Widom method with Expanded Ensemble techniques.