Solid-supercritical fluid phase equilibria of binary and ternary mixtures.

Abstract

Experimental investigation of phase equilibria at supercritical fluid conditions was carried out for four binary mixtures and two ternary mixtures consisting of supercritical carbon dioxide and aromatic compounds (naphthalene, biphenyl, m-terphenyl and phenanthrene). A new technique, the first freezing point method, was developed in this study to determine the pressure-temperature (P-T) projection of the solid-liquid-gas (S-L-G) three-phase coexistence curves for binary and ternary mixtures at supercritical fluid conditions. In addition, the equilibrium liquid compositions along the three-phase coexistence curves were also determined. A temperature minimum in the P-T projection of the three-phase coexistence curve was observed for each of the binary mixtures. The liquid-gas (L = G) critical loci of two binary mixtures consisting of super-critical carbon dioxide and a solid (naphthalene or biphenyl) were determined. The bubble-point pressures along three isotherms as well as the solubilities of carbon dioxide in liquid naphthalene and biphenyl were also measured. By means of the intersection method, the upper critical end points (UCEP) were established to be 333.4 K, 25.9 MPa and 0.16 mole fraction of naphthalene for naphthalene-carbon dioxide mixture and 328.5 K, 48.5 MPa and 0.18 mole fraction of biphenyl for biphenyl-carbon dioxide mixture. A "crossover region" was found in the study of isothermal solubilities of super-critical CO$\sb2$ in liquid biphenyl at a pressure of about 36 MPa. Below the crossover region pressure, an increase in temperature caused a decrease in solubility of carbon dioxide in the liquid phase, while above the crossover region pressure the opposite effect occurs. A rational explanation was given. The P-T projection of the solid 1-solid 2-liquid gas (S$\sb1$-S$\sb2$-L-G) four-phase coexistence curve of two ternary mixtures--naphthalene-biphenyl-carbon dioxide and naphthalene-phenanthrene-carbon dioxide--were determined. The results indicate that the assumption of an unchanged eutectic composition of the solids with pressure may lead to a not negligible error in the measurements. The freezing point depression of the solid under the pressure of a supercritical solvent and the solubility behaviour in the vicinity of the lower critical end point (LCEP) and the UCEP were explored and discussed. The slopes of the depression curves at the triple points of the solids were predicted. Two different approaches, based respectively on the compressed gas model (equation of state) and the expanded liquid model (activity coefficient model), were developed to describe the S-L-G three-phase equilibria and the solubilities of supercritical carbon dioxide in the melted solids. Using the Peng-Robinson equation of state with the modified correction factors, $\alpha$, together with the composition-dependent mixing rules, the correlations of the experimental results were accomplished with satisfactory accuracy. The merits of these two approaches in the representation of the S-L-G three-phase equilibria were compared.