A thermodynamic analysis of three-phase equilibria in binary and ternary systems for applications in rapid expansion of a supercritical solution (RESS), particles from gas-saturated solutions (PGSS), and supercritical antisolvent (SAS)

Supercritical fluids are being increasingly used as media for fine particles formation: the most important techniques are the rapid expansion of a supercritical solution process (RESS), the particles from gas-saturated solutions process (PGSS), and the supercritical antisolvent recrystallization process (SAS). To verify the feasibility of such processes, and to optimize the choice of operative variables, it is important to understand the phase behavior of the systems involved. To perform a thermodynamic analysis, an equation of state has to be used to take into account the effects of pressure; moreover, a solid phase is involved, and the heavy component is usually a high molecular weight and poorly characterized compound. In this work the Peng-Robinson equation of state with classical mixing rules and one or two binary interaction parameters are used. The fugacity of the heavy component in the solid phase is calculated by means of a subcooled liquid reference state: only heat, of fusion and melting temperature of the heavy component are needed. The aim of this work is to develop a thermodynamic model which allows to calculate solid-liquid-vapor (S-L-V) equilibria of binary (RESS and PGSS) and ternary (SAS) systems. In regard to binary systems, the knowledge of P-UCEP and T-UCEP allows the calculation of binary interaction parameters: then the P-T trace of S-L-V equilibrium and the solubility of the heavy component in the light supercritical fluid can be reasonably well predicted. For ternary systems available S-L-V and S-L1-L2-V equilibrium data are well correlated, so that an analysis on the effect of operating variables (P and T) on the SAS process can be performed.