A.E. Ramazanova

P-V-T-x measurements of aqueous mixtures at supercritical conditions

We report P-V-T-x measurements for five binary systems: water+methane, water+n-hexane, water+n-octane, water+benzene, and water+nitrogen at supercritical conditions for several compositions. The experimental data were obtained along isotherms with a phase-equilibrium cell designed for accurate measurements at pressures up to 100 MPa. The uncertainties in temperature, pressure, density, and concentration are ±0.01 K, ±0.2%, ±0.2%, and ±0.002 mole fractions, respectively. The behavior of the second virial coefficient, the excess volume, and the excess Gibbs free energy is also discussed.

Excess, Partial, and Molar Volumes of n-Alkanes in Near-Critical and Supercritical Water

Densities of solutions of n-pentane, n-hexane, n-heptane, and n-octane in near-critical and supercritical water were measured at pressures between 4 and 38 MPa and temperatures from 643.15 to 648.15 K over the entire composition range. The measurements were performed at three isotherms: 643.15, 647.05, and 648.15 K. A constant-volume piezometer was used to measure the PVTx data. The overall accuracy of the pressure, density, temperature, and mole fraction data are ±0.15%. ±0.5%, ±10mK and ±0.0002, respectively. From these results, excess and partial molar volumes were determined. The uncertainties of the derived results are given. Analysis of the results for dilute water + n-alkane mixtures showed that partial molar volume of n-alkane (solute) and excess molar volume of the mixture near the critical point of pure water (solvent) exhibit remarkable anomalies. The experimental values of molar volumes are compared with predicted values based upon scaling theory. Analysis of the results confirms the prediction of scaling theory that along the critical temperature and pressure of water the limiting partial molar volume of alkane as mole fraction x → 0 is proportional to x−γ/βδ, where γ/βδ ≍ 0.79. Our results contribute to understanding of supercritical solubility in near-critical fluids.

Extrema of isochoric heat capacity of water and carbon dioxide

The experimental and predicted loci extrema behavior of the isochoric heat capacity Cv was examined for water and carbon dioxide along the subcritical and supercritical isotherms and along the liquid and vapor isochores. The studies were based on a nonanalytical Helmholtz energy-volume-temperature equation (AVT, fundamental equation of state), the IAPWS-95 formulation for water, and scaling-type crossover equations of state (CREOS). The measured isochoric heat capacity data for these fluids near the critical point were analyzed to study the behavior of loci of Cv maxima and to compare these with predictions by the equations of state. A CREOS was applied to study the behavior of the isochoric heat capacity maxima in the immediate vicinity of the critical point. Good agreement with the CREOS prediction and experimental isothermal Cv maxima loci was observed near the critical point. The basic characteristic points on the Cv extrema loci curves in the P-T and 𝜌-T planes were determined on the basis of detailed analysis of the experimental and prediction of Cv extrema loci behavior. Qualitative explanations are given for the nature of isochoric and isothermal Cv maxima-minima curves. The role of Cv extrema loci behavior in developing high-accuracy equations of state in the supercritical region and in the study of supercritical phase-transition phenomena are discussed.

Measurement of the PVTx properties of n-heptane in supercritical water

Abstract By means of a constant-volume piezometer, measurements have been made of the PVTx properties of water- n -heptane mixtures at supercritical conditions. The measurements cover the temperature range from 573 to 673 K and pressures from 2 to 30 MPa. Values of excess, partial, and apparent molar volumes were obtained from these measurements. Tests on the piezometer and consistency tests on the measurements suggest that the results are free from significant ‘dead volume’ error. The PVT data for the pure components (water and n -heptane) obtained using the piezometer are in excellent agreement with results obtained by other investigators. The overall accuracy of the pressure, density, temperature, and mole fraction are ± 0.15%, ± 0.5%, ± 10 mK and ± 0.002, respectively. Analysis of the results for dilute water- n -heptane mixtures show that the partial molar volume of n -heptane (solute) and the excess molar volume of the mixture near the critical point of pure water (solvent) exhibit remarkable anomalies. Our results contribute to the formulation of supercritical solubility in near-critical fluids.