WATER-SALT PHASE-EQUILIBRIA AT ELEVATED-TEMPERATURES AND PRESSURES - MODEL DEVELOPMENT AND MIXTURE PREDICTIONS

A model is developed to predict the water-salt phase equilibria at elevated temperatures and pressures based on the associated-perturbed-anisotropic-chain theory (APACT). At high temperatures, salts in aqueous solutions exist in the form of ion pairs, unlike at ambient temperatures, where they strongly dissociate. Therefore, a molecular approach can be used to model their thermodynamic behavior. APACT accounts explicitly for the strong dipole-dipole interactions between water and salt molecules. Molecular salts have a dipole moment that is on average 5 times higher than the water dipole moment. In addition, a chemical equilibrium model is used to describe the hydration of salt molecules by water molecules and the water self-association. The salt parameters for APACT are obtained on the basis of molecular properties (crystal ionic radius, molecular polarizability, enthalpy and entropy of hydration of corresponding ions). The model is applied to predict the vapor-liquid equilibria of binary water-salt mixtures for 10 alkali halide salts with very good accuracy in the temperature range 150-500 degrees C. In addition, it predicts accurately the equilibrium pressure of the solid-liquid-vapor equilibria of three binary water-salt mixtures in the temperature range 200-450 degrees C. In general, the model is shown to be most accurate in the temperature range 300-420 degrees C. Below 200 degrees C, when salt dissociation becomes dominant, the model seems to be less accurate.