Measurements of the heat capacity at constant volume of H 2O+Na 2SO 4 in near-critical and supercritical water

Abstract

The heat capacity at constant volume C VX of aqueous solutions of Na 2SO 4 has been measured from 460 to 669 K along 11 isochores from 250 to 926 kg m −3 for three compositions, namely: 0.0012798; 0.0066328, and 0.0139002 mole fraction of Na 2SO 4. Measurements of the isochoric heat capacity C V for pure water were also made at the critical isochore ( ρ C=317.9 kg m −3) and at six near-critical isochores, namely: 309.598, 335.570, 354.480, 368.310, 370.370 and 373.340 kg m −3, and the coexistence curve. A total of 368 C VX data for H 2O+Na 2SO 4 and 273 C V data for pure water were measured along 17 isochores in the one- and two-phase regions. The measurements were made in a high-temperature and high-pressure adiabatic nearly constant-volume calorimeter. The uncertainty of the isochoric heat capacity measurements is estimated to be within ±2%. One- ( C V′) and two-phase ( C V″) heat capacities, temperatures ( T S), and densities ( ρ S) at saturation were obtained by the method of quasi-static thermograms. The experimental C VX results have been compared with Pitzer's classical equation of state. The relative average deviations for H 2O+Na 2SO 4 solutions were about ±2%, except for the critical region where differences reached 15–20%.