Excess molar enthalpies of steam–n-hexane and steam–n-heptane up to 698.2 K and 12.6 MPa

Flow calorimetric measurements of the excess molar enthalpy, HEm, of [xH2O +(1–x)C6H14] and [xH2O +(1–x)C7H16] are reported. The measurements extend over the range 448.2–698.2 K at pressures up to 12.6 MPa. Most of the measurements are at x= 0.5 but the composition dependence was investigated under selected conditions. Whereas previous measurements of HEm for steam with C1–C5 n-alkanes at pressures up to 12 MPa could be fitted using the virial equation of state truncated after the third term, the new measurements can only be fitted at lower pressures. This is shown to be because the residual molar enthalpies of n-hexane and n-heptane are much larger than those of the fluids previously studied. Graphs of HEm against pressure of (0.5H2O + 0.5C7H16) exhibit maxima at pressures well below the saturation pressure. It is demonstrated that the maxima are due to the shape of the residual molar enthalpy curves of steam and n-heptane and not to any unusual molecular interaction. Residual molar enthalpies of (0.5H2O + 0.5C7H16) calculated form the HEm measurements lie on smooth monotonic curves. Comparison of the (0.5H2O + 0.5C7H16)HEm measurements with the Peng–Robinson and Patel–Teja equations of state show that neither cubic equation fits the measurements.