Excess molar enthalpy of steam–n-octane up to 648.2 K and 10.0 MPa

Abstract

Measurements of the excess molar enthalpy, H^E_m, of mixtures of steam–n-octane made at temperatures up to 648.2 K and pressures up to 10.0 MPa are reported. Most of the measurements were made at x= 0.5, but measurements were also made over a range of composition at six selected conditions. The overall accuracy of the measurements is estimated to be ±2%. The measurements are compared with a truncated virial equation of state and with a cubic equation of state developed to fit the properties of mixtures containing steam. In the virial equation the cross-term interaction energy ε₁₂ was calculated from ε₁₂=ξ(ε₁₁ε₂₂)^1/2, where ξ= 2(σ³₁₁σ³₂₂)^1/2σ^–3₁₂(l₁l₂)^1/2(l₁+l₂)^–1, and in the cubic equation the pseudo-critical temperature T_c12=ξ(T_c1T_c2)^1/2, where ξ= 2(V_c1V_c2)^1/2V^–1_c12(l₁l₂)^1/2(l₁+l₂)^–1. At pressures up to 3 MPa the measurements are consistent with both the virial and cubic equations of state using values of ξ calculated from these combining rules. At higher pressures the virial equation is inadequate and the cubic equation is better, though it does not fit the measurements on steam–octane as well as it fits measurements on steam–C₁ to C₆ n-alkanes. This is attributed to the uncertainty in the residual enthalpy of n-octane, which is known with less accuracy than the residual enthalpy of C₁ to C₆ n-alkanes. The high-pressure H^E_m measurements can, however, be fitted fairly well using the cubic equation by adjusting ξ from the calculated value (0.82) to 0.95. With this value the composition dependence of H^E_m is reproduced almost within the uncertainty on the measurements.