PVTx measurements and partial molar volumes for water-hydrocarbon mixtures in the near-critical and supercritical conditions

By means of a constant-volume piezometer, measurements have been made of the PVTx properties of water + hydrocarbon (n-heptane, n-octane, and benzene) mixtures at near-critical and supercritical conditions. The measurements cover the pressure range from 5 to 40 MPa and densities between 20 and 450 kg m(-3) for six compositions from 0 to 0.614 mole fraction of hydrocarbon. Tests on the piezometer and consistency tests on the measurements suggest that the results are free from significant 'dead volume' error. The experimental errors of the present PVTx measurements are estimated to be within +/- 0.01 K in temperature, +/- 0.15% in density, +/- 2 kPa in pressure, and +/- 0.002 mole fraction in composition. Good agreement was obtained with values reported in the literature for pure components (water, n-heptane, n-octane, and benzene), measured using other techniques. Values of partial molar volumes and the Krichevskii parameter have been obtained from these measurements. Analysis of the results for dilute water-hydrocarbon mixtures shows that partial molar volumes of the hydrocarbon (solute) near the critical point of pure water (solvent) exhibit remarkable anomalies. The experimental results have been interpreted in terms of modern molecular thermodynamic models for the partial molar volumes of the solute near the solvent critical point and Kirkwood-Buff fluctuation theory of solutions in terms of direct correlation function integral. (C) 1998 Elsevier Science B.V. All rights reserved.