Excess molar enthalpies of steam–cyclohexane and steam–benzene from 448.2–698.2 K and up to 14 MPa

Flow calorimetric measurements of the excess molar enthalpy, HEm, of (0.5H2O + 0.5C6H12)(g) and (0.5H2O + 0.5C6H6)(g) are reported. The measurements were made at mole fraction x= 0.5 and extend over a temperature range 448.2–698.2 K and at pressures up to 14 MPa. The results were analysed using the cubic equation of state previously developed to fit the thermodynamic properties of mixtures containing steam. The equation of state and combining rules that fitted high pressure HEm measurements on mixtures of steam with C1–C8 alkanes fits both the high pressure HEm measurements on steam–cyclohexane and low pressure measurements at 0.1 MPa in the range 363.2–393.2 K, reported previously. The equation does not fit the measurements on steam–benzene but gives values of HEm that are too large. The difference between the experimental and calculated values of HEm has been attributed to weak association between steam and benzene, and an equilibrium constant K12 for this interaction has been incorporated into the combining rule. At 298.15 K, K12= 0.1794 MPa–1 and the enthalpy of association was found to be ΔH12=–12.3 kJ mol–1. With these parameters, the cubic equation is a good fit to both the high pressure HEm measurements and to the HEm measurements at 0.1 MPa, reported previously.