The CO₂-H₂O system. II. Calculated thermodynamic mixing properties for 400°C, 0-400 MPa

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 CO₂-H₂O fluids at 400°C, 0-400 MPa. Excess Gibbs free energies are continuously positive and asymmetric toward H₂O 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 H₂O and CO₂ 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 H₂O, a-X relations for H₂O are distinctly different from those for CO₂. These results indicate that CO₂-H₂O 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 cm³ · mol^-1 at 30 MPa to ~1 cm³ · 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 CO₂-H₂O 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.