Thermodynamic description of equilibria in mixed fluids (H2O-non-polar gas) over a wide range of temperature (25-700°C) and pressure (1-5000 bars)

A new method for computing complicated equilibria in hydrothermal mixed fluids, H2O-non-polar gas, is proposed. The computation algorithm is based on the electrostatic approach for the interaction between aqueous species and H2O. The approach uses the SUPCRT92 database and the HKF format and may be considered as an application of the revised HKF model for mixed H2O-non-polar gas fluids. Thermodynamic properties of dissolved gases at high temperatures and pressures are calculated using the Redlich-Kwong approach. Dielectric permittivity of the mixed solvent is estimated by the modified Kirkwood equation. The proposed approach is validated using available experimental data on the dissociation constants of H2O and NaCl and the solubility of both covalent and ion crystals (SiO2, AgCl, Ag2SO4, Ca(OH)(2), CaCO3) in H2O-non-polar component (dioxane, Ar, CO2) mixtures. Predicted and experimental data are in close agreement over a wide range of P-T-x,, conditions (up to 500 degrees C, 4 kbar and 0.25-0.3 mole fraction of non-polar gas). It is also shown how the computation method can be applied to estimate the Born parameters of aqueous species. The proposed approach enables not only examination of isolated reactions, but the study of equilibria of whole systems. Thus, it allows modelling of mixed natural fluids. Copyright (C) 1999 Elsevier Science Ltd.