Isobaric Heat Capacity Measurements of Liquid Methane, Ethane, and Propane by Differential Scanning Calorimetry at High Pressures and Low Temperatures

Abstract

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 x 1 = 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.

Thermodynamic properties of hydrofluoroolefin (R1234yf and R1234ze(E)) refrigerant mixtures: Density, vapour-liquid equilibrium, and heat capacity data and modelling

“…The temperature-controlled water bath was set to be 3 K lower than the DSC temperature and was controlled to follow the temperature profile during scanning with a −3 K offset. This minimised the heating required for the stem and, because liquid nitrogen was not being used, no cold spot part-way up the stem was present as was observed previously (Syed et al, 2012).…”

“…4. This instrument has previously been used for measurements of heat capacities, enthalpies of phase transitions and phase transition temperatures (Hughes et al, 2011;Syed et al, 2012;Syed et al, 2014;Oakley et al, 2017;Syed et al, 2017). The calibration of the DSC's heat flux sensor is discussed by Hughes et al (2011) and temperature calibration was most recently described by Oakley et al (2017).…”

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

“…Liquid densities for all the mixtures were calculated using the GERG 2008 2 equation of state as implemented in the software REFPROP 9.1. 3 From our previous work 1,6 we estimate the combined uncertainty at the 95 % confidence limit (k ≈ 2) in the heat capacity to be about 0.02•cp. The standard uncertainty in the temperature was estimated to be about 0.25 K above 220 K and 0.5 K below 220 K. The standard uncertainty in the pressure was estimated to be about 35 kPa.…”

“…For the five component mixed refrigerant system, the GERG 2008 predictions are in excellent agreement at the higher temperature, with relative deviations of less than 2 % in the range (168 to 148) K. At lower temperatures the deviations increase to a maximum of 19 % at 108 K. In contrast, the cp predictions of the Peng Robinson 4 EOS deviate from our results by an The similarity of the deviations between the models and the data of Furtado15 indicates that the trends observed for our results have a degree of generality. work demonstrates that the modifications made to the specialized, high-pressure DSC calorimeter reported by Syed et al1 were successful in enabling it to measure isobaric heat capacities of liquid alkane mixtures at cryogenic temperatures. It also shows that there is still significant room for improving equation of state predictions for the heat capacity of liquid alkane mixtures, including those of significant industrial importance such as the mixed refrigerants used in the LNG industry.…”

“…A commercial calorimeter Setaram BT2.15 Tian-Calvet type heat flow DSC with an operational temperature range from (77 to 473) K, modified to allow accurate isobaric heat capacity, cp, measurements of liquid methane, ethane, and propane between T = (108.15 to 258.15) K and pressure up to 6.4 MPa, has been described previously. 1 Figure 1 shows a schematic of the specialized calorimeter, and the modifications made in this work. The details of the calorimeter sensitivity, calibration and measurement method were described by Hughes et al 6 In this paper the additional procedures required to make measurements on mixtures are described in detail.…”