Isobaric heat capacity measurements of liquid methane, ethane, and propane by differential scanning calorimetry at high pressures and low temperatures

A commercial differential scanning calorimeter (DSC) was adapted to allow accurate isobaric heat capacity, c_p, measurements of cryogenic, high-pressure liquids. At (subcritical) temperatures between (108.15 and 258.15) K and pressures between (1.1 and 6.35) MPa, the standard deviation in the measured c_p values was 0.005·c_p for methane, 0.01·c_p for ethane, and 0.015·c_p for propane, which is comparable to both the scatter of c_p data for these liquids measured using other techniques and with the scatter of those data sets about the reference equation of state (EOS) values. Three key modifications to the commercial DSC were required to enable these accurate cryogenic, high-pressure liquid c_p measurements. First, methods of loading and removing the liquid from the calorimeter without moving the sample cell were developed and tested with high boiling temperature liquids; this modification improved the measurement repeatability. Second, a ballast volume containing a high-pressure vapor phase at constant temperature was connected to the DSC cell so that the liquid sample's thermal expansion did not cause significant changes in pressure. A third modification was required because the boil-off vapor from the liquid nitrogen used to cool the calorimeter resulted in a temperature inversion, and hence convection, along the tubing connecting the DSC's sample cell to the ballast volume, that lead to an unstable calorimetric signal at T > 130 K. The modifications to the specialized DSC were tested by measuring c_p values for heptane, methylbenzene, and a heptane (1) + methylbenzene (2) mixture with x₁ = 0.38 at atmospheric pressure and temperatures of (228.15, 238.15, 303.15, and 313.15) K. The measured values had relative deviations from those measured adiabatically by Holzhauer and Ziegler (J. Phys. Chem. 1975, 79, 590-604) of less than 0.006·c_p, indicating that the specialized DSC could be used for liquids and liquid mixtures at conditions relevant to liquefied natural gas production.