Volumetric properties of steam–carbon dioxide mixtures derived from excess molar enthalpy measurements

Flow calorimetric measurements of the excess molar enthalpy, HEm, of [0.5H2O–0.5CO2](g), which extend over the temperature range 448.2 to 698.2 K and at pressures up to 20 MPa, were recently reported. In the initial analysis of these measurements, the residual molar enthalpy of carbon dioxide was calculated from the equation of state proposed by Kubic, the residual molar enthalpy of steam was calculated from a modification of Kubic's equation proposed by Lancaster and Wormald, and simple one-fluid mixing rules were used to calculate the residual molar enthalpy of the mixture. The binary interaction constant used to secure agreement between experimental and calculated HEm values was ξ= 1.33. This value is larger than that for steam–alkane interactions, and was attributed to a weak specific interaction between steam and carbon dioxide molecules in the vapour phase. The HEm measurements have now been reanalysed using mixing rules which include terms to account for the association between the unlike molecules. This results in a value of the equilibrium constant K12(298.15 K)= 0.0954 MPa–1, and a value of ΔH12=–11.7 kJ mol–1 for the enthalpy of the specific interaction between steam and carbon dioxide. The use of the association model slightly improves the fit to the HEm measurements and gives good agreement with experimental values of volumetric properties, particularly at low temperatures.