Abstract
PVTx properties for the binary systems of supercritical carbon dioxide with methanol, ethanol and 1-propanol have been measured at temperatures of 313.15, 323.15, and 333.15 K and in the pressure range from 7.6 to 25 MPa. Solute concentrations were varied from 0 to 0.33 mol%. A circulation-type apparatus with a vibrating-tube densimeter was used in the experiments to measure the density of the mixtures. The change in solute concentration gave only a small change in density. However, partial molar volumes of the solutes calculated from the measured densities showed large negative values around the critical point of carbon dioxide, indicating the formation of a solvation structure in supercritical carbon dioxide. The excess number of solvating carbon dioxide molecules was evaluated from the measured PVTx properties utilizing the Kirkwood-Buff solution theory to investigate the effect of temperature, pressure and solute type on the solution structure. It was found that the excess number of solvating molecules showed a maximum near the critical point of carbon dioxide, and the three alcohols gave almost the same excess number of solvating molecules. Prediction of these properties was also carried out with a combined model of the SRK equation of state, an excess Gibbs energy mixing rule and volume transition methods. The volume transition method proposed by Chou and Prausnitz predicted partial molar volumes and excess number of solvating molecules quantitatively, while the method proposed in this work gave a better result for PVTx properties.
