E. A. Bazaev

PVTx measurements for dilute water+n-hexane mixtures in the near-critical and supercritical regions

Abstract The PVTx relationship of four binary dilute (water+ n -hexane) mixtures (0.0021, 0.0050, 0.0850 and 0.0138 mole fraction of n -hexane) were measured with a constant-volume method. Measurements were made at five near-critical and supercritical temperatures of pure water: 643.05, 645.05, 647.05, 649.05 and 651.05 K. The range of pressures was from 8 to 35 MPa. The total uncertainty of density, pressure, concentration, and temperature measurements were estimated to be less than ±0.2%, ±5 kPa, 0.001 mole fraction, and 10 mK, respectively. The derived PVTx data have been differentiated to yield infinite dilution partial molar volumes V 2 ∞ . Asymptotic properties for V 2 ∞ along the critical isotherm–isobar of the pure solvent (water) are experimentally studied. We show that PVTx measurements for the dilute water+ n -hexane mixture confirm the non-classical power-law behavior of V 2 ∞ along the experimental path of constant T C – P C with a critical exponent of 0.79. The values of the Krichevskii parameter of 96.4 MPa for the dilute water+ n -hexane mixture was estimated from PVTx measurements and compared with values calculated from the initial slopes of the T C – x and P C – x critical lines and the slope of the vapor-pressure P S – T S curve at the pure solvent critical point. The excess molar volumes of the mixture near the critical point of pure water are calculated using measured PVTx properties for mixtures and pure components.

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.

The thermal properties of water-ethanol system in the near-critical and supercritical states

Experimental p, ρ, T, x-data are given for water-ethanol mixtures at temperatures up to 673.15 K, including the saturation curve and the critical and supercritical regions, and at pressures up to 50 MPa for ethanol concentrations of 0.2, 0.5, and 0.8 mole fraction. The data of p, ρ, T, x-measurements are used to determine the critical parameters of mixtures. The thermal decomposition of ethanol molecules is observed at a temperature above 623.15 K.

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.

An experimental investigation of the critical state of aqueous solutions of aliphatic alcohols

The ballast-free constant-volume piezometer technique is used for measuring the p, ρ, T dependences of aqueous solutions of aliphatic alcohols (methanol, ethanol, and 1-propanol) with 0.2, 0.5, and 0.8 mole fraction of alcohol in the temperature range from 373.15 to 623.15 K and at pressures up to 50 MPa for different values of density. The data of p, ρ, T measurements in the near-critical region are used to determine the parameters of critical state of solutions as a function of composition. Diagrams are constructed of projections of lines of critical points of thermodynamic surface in different planes.

Phase transformations in water-aliphatic alcohol binary systems

The parameters of liquid-vapor phase transitions ps, ρs, Ts and critical points pc, ρc, Tc were determined from the experimental data on the p, ρ, T, x-dependences of aqueous solutions of aliphatic alcohols (methanol, ethanol, n-propanol) that contain 0.2,0.5, and 0.8 mole fractions (x) of ethanol and correspond to single-phase (gas, liquid), two-phase, or subcritical areas. The dependence of the pressure of saturated vapor in solutions on the temperature and density was described by means of the expansion of the compressibility factor Z = p/RTρm in powers of the density and temperature along the coexistence curve away from the critical point. The temperature dependence of the density of solutions along the coexistence curve and inside the critical area was fitted using the power functions of parameters ω ∼

Thermodynamic Properties of Aqueous Solutions of Alkanes under Supercritical Conditions

\tau ^{\beta _i }

(p, v, T, x) Measurements of {(1 − x)H2O + xC2H5OH} mixtures in the near-critical and supercritical regions

, τ = (T − Tc) and ω = (ρ1,v − ρc)/ρc.

PVT measurements for pure methanol in the near-critical and supercritical regions

P–V–T–X relationships for the water–methane, water–n-pentane, water–n-hexane, water–n-heptane, and water–n-octane systems are derived by piezometry at a constant volume in the vicinity of the critical point of water (647.096 K) and up to 673.15 K at pressures up to 40 MPa and alkane mole fractions of 0–1.0. Conditions are determined under which real mixtures behave as an ideal gas or have a constant compressibility factor. It is demonstrated that the concentration dependence of the partial molar volumes of alkanes in the range of small concentrations is asymptotic if the isothermal isobar of the solvent is critical. The molecular parameters of the equation of state that is based on perturbed-hard-chain theory are determined and used in the calculation of thermodynamic parameters for this class of solutions.