Isobaric Heat Capacity Measurements of Liquid Methane + Propane, Methane + Butane, and a Mixed Refrigerant by Differential Scanning Calorimetry at High Pressures and Low Temperatures

Isobaric heat capacity c p measurements are reported at temperatures between (108 and 168) K for liquid mixtures of CH4 (1) + C3H8 (3) at x 1 ≈ 0.8 and p ≈ 5 MPa; CH4 (1) + C4H10 (4) at x 1 = 0.96, 0.88, and 0.60 and p ≈ 5 MPa; and a five-component mixture that is similar in composition to a commercial mixed refrigerant at p = (1.0 and 5.0) MPa. Measurements were made, with an estimated uncertainty of about 0.02·c p , using a specialized calorimeter further modified to prevent preferential condensation of the less volatile component and minimize possible stratification of the mixture during sample loading. Our results for CH4 + C3H8 agreed within 0.02·c p of the values predicted using the GERG 2008 equation of state (EOS), whereas literature c p data over the same temperature range agree within 0.03·c p . In contrast the c p predicted for this mixture with the Peng–Robinson EOS (PR-HYSYS) shows an offset of about 0.02·c p from the average of the literature data at 165 K, which increases to 0.045·c p at 120 K. For CH4 (1) + C4H10 (4) our data for x 1 = 0.96 and 0.88 are within 0.05·c p of the GERG 2008 predictions; however, at x 1 = 0.6, the deviations increase from 0.02·c p at 168 K to nearly 1.1·c p at 118 K. The PR-HYSYS predictions for this binary mixture deviate from the measured c p data by 0.11·c p at 168 K, which increases to 0.21·c p at 118 K. For the mixed refrigerant (mole fraction composition 0.247 CH4 + 0.333 C2H6 + 0.258 C3H8 + 0.076 C4H10 + 0.059 N2), the deviations of the measured c p from GERG 2008 are less than 0.02·c p in the range (168 to 148) K, increasing to 0.19·c p at 108 K. The deviations from the predictions of PR-HYSYS range from 0.07·c p at 168 K to 0.15·c p at 108 K. Similar differences from the various EOS are observed for the data of Furtado, who measured the c p of a mixture with a mole fraction composition of 0.359 CH4 + 0.314 C2H6 + 0.327 C3H8 at similar temperatures.