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 constantvolume 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 V h 2 ∞ 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 H 2 O + n-C 6 H 14 mixture along the critical isotherm-isochore of pure water were also used to estimate the critical exponent of partial molar volume V h 2 ∞ ∝ x -( ) 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 H 2 O + n-C 6 H 14 mixture with an average absolute deviation (AAD) of about 0.50-0.65% in the temperature and density ranges 0.98T c (x) e T e 1.15T c (x) and 0.35F c (x) e F e 1.65F c (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.

PVT Measurements for Pure Ethanol in the Near-Critical and Supercritical Regions

“…The apparatus used for P V T measurements of ethanol is similar to that used in previous papers [49][50][51][52][53][54][55][56][57][58][59] to measure P V T properties of pure methanol, n-alkanes, and their aqueous solutions in the critical and supercritical regions. Detailed descriptions of the apparatus and the experimental procedure, and uncertainty assessment have been described in our previous publications [49][50][51][52][53][54][55][56][57][58][59].…”

“…Detailed descriptions of the apparatus and the experimental procedure, and uncertainty assessment have been described in our previous publications [49][50][51][52][53][54][55][56][57][58][59]. The measurements were made using the constant-volume method, with an extraction of the sample from the piezometer under isothermal conditions.…”

“…This work is a part of a continuing program on volumetric (PVT) and caloric (C V V T ) property measurements of alcohols and their aqueous solutions in the critical and supercritical regions. The present PVT apparatus has been previously used for accurate measurements on other pure fluids and aqueous solutions in the critical and supercritical regions (Abdulagatov et al [49][50][51][52][53], Bazaev et al [54][55][56][57][58], and Rabezkii et al [59]). …”

Thermodynamic Properties of Methanol in the Critical and Supercritical Regions

“…For this purpose, we have used the parametric crossover model CREOS-97 developed for pure fluids and fluid mixtures by Kiselev [110], as modified subsequently by Kiselev and Rainwater [111,112]. The Helmholtz free-energy density of a single-component fluid in this model can be written in the parametric form [110][111][112][113][114],…”

Experimental study of the one-, two-, and three-phase isochoric heat capacities of n-hexane+water mixtures near the lower critical line. Part II. Krichevskii parameter and thermodynamic and structural properties

“…The derived values are 124.4 MPa (from PVTx data, 119.54 MPa from crossover EOS, and 119.0 MPa from upper critical curve data [48]) and also positive. Thus, the Krichevskii parameter for n-hexane + water mixture (where n-hexane solvent) almost 10 times lower than the Krichevskii parameter for water + n-hexane mixture (where water solvent) because the slope of the upper critical line for water + nhexane mixture considerable (8 times for the slope of the critical temperatures curve, dT C dx x¼0 = − 344.85 K, and 3 times for the critical pressure curve, dP C dx x¼0 = 26.51 MPa) higher than for nhexane + water mixture.…”