Density Data of Two (H₂ + CO₂) Mixtures and a (H₂ + CO₂ + CH₄) Mixture by a Modified Burnett Method at Temperature 673 K and Pressures up to 25 MPa

The PVT properties of the (H₂ + CO₂) mixtures and the (H₂ + CO₂ + CH₄) mixtures in the supercritical region of water are significant for making full use of the gas mixtures in the coal and supercritical water gasification. We report PVT measurements of the (0.6005 H₂ + 0.3995 CO₂), (0.6992 H₂ + 0.3008 CO₂), and (0.6026 H₂ + 0.1948 CO₂ + 0.2026 CH₄) mixtures (all in mole fractions) at temperature 673 K and pressures of (0.5 to 25) MPa, measured by a modified Burnett method. The upper bounds of the expanded uncertainties at 95% confidence are 0.2 K for temperature, 0.015·p for pressure, and 0.008·ρ for density. The density data were compared with those predicted using the GERG-2008 equation of state. The deviations between the calculated and experimental data are not more than 0.021·ρ. Furthermore, the densities were found in accordance with the weighted average of the components' existing equations of state using their mole fractions within 0.0076·ρ for all of the mixtures in this work. The second and third virial coefficients were determined at temperature 673 K for hydrogen and all of the mixtures investigated.