Measuring Isobaric Heat Capacity of Fluids in the Critical Region by Continuous-Flow Adiabatic Calorimetry Method

Considering the importance of refining the fundamental equations of state for hydrocarbons, the authors discuss the methodological and design features of experimental measurement of isobaric heat capacity in the critical region by using the continuous-flow adiabatic calorimetry method. The pressure measurement system was improved by introducing a differential pressure gauge into the measuring circuit, which made it possible to not only increase the accuracy of the pressure determination, but also implement a universal scheme of the calorimetric experiment. The use of such universal scheme of the calorimetric experiment makes it possible to determine two values of isobaric heat capacity at pressures that differ by the value of pressure drop inside the calorimeter. Such approach in the critical region is important, since it provides a quite simple and reliable way to determine the value of the derivative of heat capacity over pressure, which is used to estimate not only the error of referencing the value of heat capacity to a certain pressure, but also the equilibrium conditions of the experiment in a continuous-flow calorimeter. The authors review the procedure for determining and correcting for inhomogeneity of the lead wires of the differential thermocouple, and for throttling the substance flow inside the calorimeter. Proper relationships were obtained for determining the average experimental temperature when utilizing various methods of measuring temperature and temperature difference in a continuous-flow calorimeter. The results of experimental measurements of isobaric heat capacity of n-pentane in the critical region are presented, which were obtained by using a universal scheme of the calorimetric experiment. For n-pentane, the measurements were performed at 3.400 MPa (critical pressure – 3.355 MPa), which is the closest to the critical point in the practice of flow calorimetry.