CRITICAL EVALUATION OF CALCULATED VAPOR-LIQUID EQUILIBRIA FOR CO2-H2O FLUIDS AT 300 AND 350°C*

Excess molar volumes (V^ex) and vapor-liquid equilibria (VLE) for CO₂-H₂O fluids at 300 and 350°C, 7.5-100 MPa--measured precisely and accurately in experiments performed with a unique, vibrating U-tube apparatus--were used to assess corresponding CO₂-H₂O V^ex and VLE calculated from the thermodynamic models of Holloway (1977, as corrected by Flowers, 1979), Kerrick and Jacobs (1981), and Duan et al. (1992a,b). The Duan et al. (1992a,b) model provides fairly accurate CO₂-H₂O VLE for 350°C, but results for 300°C are spurious, indicating erroneously that critical conditions are not achieved at that temperature. The CO₂-H₂O VLE for 300 and 350°C supplied by the Holloway (1977) and Kerrick and Jacobs (1981) modified Redlich-Kwong equations are too errant to be of any practical use in geochemistry and petrology. The inaccuracies in the CO₂-H₂O VLE for 300 and 350°C calculated from the three thermodynamic models are attributable to poor predictions of: (i) the saturation vapor pressure of pure H₂O at 300 and 350°C; and (ii) the P-V-T properties of CO₂-H₂O mixtures at 300 and 350°C, 0-100 MPa. It is concluded that additional accurate volumetric and VLE data, and more rigorous thermodynamic modeling techniques, are required to develop a comprehensive, theoretically robust, and computationally reliable equation of state for CO₂-H₂O fluids.

*Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR22725, Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.