Excess enthalpies and excess volumes of (0.5CO2+ 0.5C2H6) in the supercritical region

Abstract

A continuous flow-mixing calorimeter connected in series with a vibrating-tube densimeter has been used to measure the excess molar enthalpy H^E_m and the excess molar volume V^E_m of (0.5CO₂+ 0.5C₂H₆) at 308.40, 310.15, 312.16, 316.15 and 323.15 K at pressures from 4.0 to 12.5 MPa. At each temperature H^E_m(x= 0.5) and V^E_m(x= 0.5) were measured over a range of pressure. The measurements at 308.40 K are at approximately 3 K above the critical temperature of both pure components. The estimated uncertainty on the H^E_m measurements is ±2%, while that on the V^E_m measurements is ±4%. Graphs of H^E_m(x= 0.5) plotted against pressure show double maxima which are sharp at 308.40 K and which fade as the temperature is increased. Graphs of V^E_m(x= 0.5) plotted against pressure show similar behaviour, though the second maximum is much smaller than the first. The shape of the H^E_m and V^E_m graphs is well reproduced by the Patel–Teja equation of state using k₁₂= 0.1325 in the combing rule a₁₂=(1 –k₁₂)(a₁₁a₂₂)^½. It is shown that the double maxima are a consequence of the way the excess functions are related to the residual functions of the pure components and of the mixture.