Isobaric heat capacities of a methane (1) + propane (2) mixture by differential scanning calorimetry at near-critical and supercritical conditions

Isobaric heat capacity data are needed to test and improve thermodynamic models of natural gas over wide ranges of temperature and pressure. Measurements are reported here at temperatures between (184 and 421) K and pressures between (5 and 32) MPa for supercritical mixtures of methane (1) + propane (2) at x₁ = 0.950 (±0.005). Estimated relative uncertainties in the measured heat capacities range from (1.8 to 4.5) %. In addition, measurements at temperatures of (184, 190, 197, 203 and 209) K were performed at pressures (1.67 to 2.39) MPa higher than saturation conditions to estimate the heat capacity of the mixture at the bubble point. The binary mixture data were compared with the predictions of three models: the Groupe Européen de Recherches Gazières (GERG) 2008 multi-parameter equation of state (EOS), the Peng-Robinson (PR) EOS used widely by chemical engineers, and the Statistical Associating Fluid Theory (SAFT)-γ Mie EOS incorporating group contributions. Among the three models, the PR EOS was found to describe the heat capacity values best. A brief investigation indicated that the Joback and Reid method that the SAFT-γ Mie EOS is based on fails to accurately predict the ideal gas heat capacity of methane at cryogenic temperatures, while the inaccuracy of the GERG-2008 EOS stems from the residual part of the methane (1) + propane (2) mixing functions.