PVTx Measurements and Crossover Equation of State of Pure n-Hexane and Dilute Aqueous n-Hexane Solutions in the Critical and Supercritical Regions

Abstract

The PVTx relationship of aqueous n-hexane solutions (0.0201, 0.082, and 0.8500 mole fraction of n-hexane) has been measured in the near-critical and supercritical regions with a constant-volume piezometer. Measurements were made on the critical isotherm of pure water 647.1 K with pressures ranging from 8 to 33 MPa. The total uncertainties of density, pressure, temperature, and composition measurements are estimated to be less than 0.16%, 0.05%, 15 mK, and 0.001 mole fraction, respectively. The Krichevskii parameter was estimated (124.4 ± 20 MPa) from direct measurements of the P−x dependence along the critical isotherm-isochore of pure water. The measured PVTx data were used to calculate partial molar volumes at infinite dilution for n-hexane in near-critical water. The asymptotic behavior of the partial molar volume along the solvent's (pure water) critical isotherm-isobar was studied. The molar volume values for the dilute H2O + n-C6H14 mixture along the critical isotherm-isochore of pure water were also used to estimate the critical exponent of partial molar volume ∝ x-ε (ε = 0.795 ± 0.001). Using our new PVTx data together with data obtained in other studies, we developed a crossover Helmholtz free-energy model (CREOS) for dilute aqueous n-hexane solutions in wide temperature and pressure ranges around the vapor−liquid critical points. The CREOS model requires only the critical locus as an input and represents all available experimental PVTx data for a dilute H2O + n-C6H14 mixture with an average absolute deviation (AAD) of about 0.50−0.65% in the temperature and density ranges 0.98Tc(x) ≤ T ≤ 1.15Tc(x) and 0.35ρc(x) ≤ ρ ≤ 1.65ρc(x), respectively, and concentrations up to 0.05 mole fractions of n-hexane. The accuracy and predictive capability of the crossover model was confirmed by comprehensive comparison of present crossover EOS for the pure n-hexane with the available experimental data in the critical and supercritical regions.