Maxima of the Specific Heat at Constant Pressure of the n-Alkanes C7-C9 in the Supercritical Region

Abstract

In this work, the specific heat at constant pressure, C p , was determined using an adiabatic flow calorimeter equipped with a calorimetric flowmeter [1], through which a substance under investigation is sequentially pumped. The main (heat-measuring) calorimeter operates under the experimental conditions, namely, at T 1 up to 650 K and P 1 up to 60 MPa, while the calorimetric flowmeter works at the experimental pressure P 1 and room temperature T 2 . The C p value at the parameters of the calorimetric flowmeter is known, which allows one to determine the flow rate G . Currently, this method is considered to be the most accurate. The investigations were carried out over a wide range of state parameters in the immediate vicinity of the liquid-vapor boundary curve in the supercritical region in the neighborhood of the critical point.For the experiments in the vicinity of the supercritical point (in the range of sharp maxima of C p ), where we expected to observed the effect of trace impurities on the specific heat at constant pressure, samples of n -alkanes of high purity grade were used. The samples were subjected to additional purification on a laboratory pilot fine distillation column with regular stack packing [2]. After the additional purification, the content of the main component was 99.9 wt % for n -ë 7 , 99.8 wt % for n -ë 8 , and 99.8 wt % for n -ë 9 .In the vicinity of the critical point, where C p sharply increases, there are certain difficulties in measuring the temperature difference ∆ T between the inlet and the outlet of the main calorimeter because of the decrease in temperature to 0.1 K. This was the reason why we decided to replace the gold/platinum differential thermocouple (with a low sensitivity of 8 µ V/K and a very high thermal conductivity (for gold, λ = 308 W/(m K))) with a Chromel/Copel thermocouple of similar design (with a sensitivity of 70-90 µ V/K). Taking into account published data [3,4] and the results of our calibration tests, we found that thermocouples made from base metals can be used for precision thermophysical measurements, provided that certain conditions are met; namely, the thermocouple wires should be selected from the same batch, cut in a definite order, annealed, and individually calibrated.The experimental isobars of C p for n -heptane in the range of the maxima are shown in Figs. 1 and 2. For the other members of the homologous series, the isobars are similar and are arranged correspondingly on the thermodynamic surface. In the range of the maxima of the specific heat, C p depends on the state parameters much more strongly. Therefore, to reliably interpolate the data and accurately determine the coordinates of the maxima on the thermodynamic surface, we carried out a considerable scope of measurements (Figs. 1, 2). The temperature of the maximum on the isobar was determined by the rectilinear diameter method in the C p -T coordinates. The table presents the results of determining the maxima of C p on the isobars and the corresponding temperatures (here and hereafter, ...