An equation of state for the CH4-CO2-H2O system: I. Pure systems from 0 to 1000°C and 0 to 8000 bar

An equation of state (EOS) for the CH4-CO2-H2O system covering a wide T-P range has been developed. In this article the new EOS is presented and applied to the pure endmembers. The equation is similar to that of Lee and Kesler (1975) and contains fifteen parameters. It is used with a mixing rule in the following article to provide a thermodynamic model for the mixed system. Though the parameters are evaluated from the PVT data in the temperature range from 0 to 450°C for CH4, from 0 to 1000°C for CO2 and H2O, and for pressures from 0 to 3500 bar, comparison of this EOS with a large amount of experimental data in the pure systems indicates that predictions for temperatures and pressures from 0 to 1000°C and 0 to 8000 bar (or slightly above) are very nearly within experimental uncertainty. The EOS can describe both the gas and the liquid phases of the endmember systems with similar accuracy. Fugacity coefficients are derived and compiled. In this paper mixing is considered using ideal mixing based on the endmember fugacities (Amagat's rule). It is shown that such an approach leads to quite accurate predictions for high temperatures and low pressures.