Modeling the phase equilibrium of multicomponent systems involving solids, supercritical fluids, and consolvents

Abstract

Carbon dioxide is one of the most commonly used supercritical fluids. However, its uses are limited due to its low solubility capacity for polar substances. Addition of cosolvents improves the solubility of polar and non-polar solutes in CO2. Many authors have used equations of state combined with mixing rules to develop mathematical models for the determination of solubilities of systems composed of a solute, a solvent, and a cosolvent. In this work, the Peng-Robinson equation of state with the van der Waals mixing rules were used to formulate a mathematical model for the solubility of solids in supercritical fluids with cosolvents, i.e., in ternary systems. All three binary interaction parameters involved were determined for nine ternary systems for which experimental data were available (for a total of 22 isotherms); these parameters were used to calculate the solubilities for those systems. Results obtained showed that the model fitted very well the experimental data for all the systems studied. Also, simulation runs were done varying the cosolvent concentration to evaluate the impact that it had in the systems under study. The practical use of this exercise is to know how much cosolvent to use in a given application. An increase of the cosolvent concentration improved significantly the solubility of the solutes in carbon dioxide except for 2-naphthol for which it was observed that, beyond a certain point, an increase in pressure or cosolvent concentration reduced the solubility of the solute in CO2.