Determination of Heat Capacities at Constant Volume from Experimental (P-Rho-T) Data

Abstract

This work examines the uncertainty in the determination of the heat capacity at constant volume from experimental volumetric data. The proposed methodology uses the experimental (P-ρ-T) data for a ternary mixture of methane, ethane and propane measured over a range of temperatures from 140 to 500 K at pressures up to 200 MPa. The resulting approach determines truly isochoric data and calculates isochoric densities with a maximum deviation of ± 0.5 % from the predictions of GERG 2008 EoS. Values for the first and second derivative of the pressure with respect to temperature at constant density needed for determining the caloric properties come from a fit of the (P-ρ-T) data using a rational equation and also from numerical techniques. By transformation of the data, it is possible to calculate the residual and absolute heat capacities at constant volume with a maximum uncertainty of 1.5% that results from comparing the differences in the predictions using analytical and numerical approaches, which reflects the uncertainty of the experimental (P-ρ-T) data.