The CO 2-H 2O system. II. calculated thermodynamic mixing properties for 400°C, 0–400 MPa

Abstract

An excess molar volume ( V ex)-explicit virial equation, and two empirical V ex expressions developed from experimentally determined densities, were used to calculate excess Gibbs free energies ( G ex) and activity-composition ( a- X) relations for CO 2-H 2O fluids at 400°C, 0–400 MPa. Excess Gibbs free energies are continuously positive and asymmetric toward H 2O at all pressures up to 400 MPa, rising to peak values of approximately 1300, 1800, 2000 and 2100 J · mol −1 at 50, 100, 200 and 400 MPa, respectively. Calculated activities for H 2O and CO 2 vary correspondingly, increasing substantially from 0 to 100 MPa, moderately from 100 to 200 MPa, and slightly from 200 to 400 MPa. In addition, because G ex is asymmetric toward H 2O, a- X relations for H 2O are distinctly different from those for CO 2. These results indicate that CO 2-H 2O fluids are strongly nonideal at 400°C and all pressures above ∼30 MPa, despite the fact that peak values for V ex decrease from ∼50 cm 3 · mol −1 at 30 MPa to ∼1 cm 3 · mol −1 at 200 MPa, and remain small to pressures at least as high as 500 MPa. Excess Gibbs free energies and a- X relations for CO 2-H 2O fluids at 400°C and pressures to 400 MPa calculated from modified Redlich-Kwong and Lee-Kesler equations of state generally suggest significantly smaller positive deviations from ideality.