Fugacity coefficients of H 2, CO 2, CH 4, H 2O and of H 2O- CO 2-CH 4 mixtures: A virial equation treatment for moderate pressures and temperatures applicable to calculations of hydrothermal boiling
Equations derived from a quadratic virial equation in pressure, with virial coefficients expressed as a function of temperature, are fitted to published P- V- T and solubility data to yield values of second and third virial coefficients for pure and mixed gases. These coefficients are not virial coefficients sensu stricto and are used to compute fugacity coefficients of pure H 2, H 2O, CO 2 and CH 4, and of mixed H 2O, CO 2 and CH 4, and to estimate enthalpies for these gases. For H 2, the P-T range of application is from 25 to 600 C and up to 3000 bars, and for CH 4, from 16 to 350 C and up to 500 bais. For H 2O and CO 2, two P-T ranges are considered: below 350 C, up to 500 bars, and from 450 to 1000 C, up to 1000 bars. The method presented here is limited to the P-T range of the fitted experimental data, and cannot represent accurately P-V-T data close to the critical region. This virial equation treatment yields simple analytical expressions that are suitable for multicomponent equilibrium calculations. Examples of equilibrium calculations between aqueous and gas phases show that ideal mixing of real gases is a sufficient approximation for modeling boiling in geothermal and epithermal systems. However, non-ideal mixing has to be considered for aqueous-gas systems at pressures much higher than the saturation pressure of pure water.