A procedure for measuring high-pressure phase equilibria by means of a double-chamber recirculation apparatus

An experimental method for measuring high-pressure phase equilibria of systems composed of a supercritical and a liquid phase is described. Measurements are carried out by means of a recirculation apparatus composed of two chambers: the “equilibrium cell”, where a liquid phase coexists with a supercritical phase, and the “gas cell”, which contains the supercritical phase only. The supercritical phase is continuously recirculated between the two chambers by means of a piston pump in order to reduce time for equilibration. Before sampling, the gas cell is separated from the equilibrium cell by closing valves, in order to avoid any disturbances of the equilibrium when reducing pressure during the withdrawal of the supercritical phase. In this way, large samples of the supercritical phase can be withdrawn. This is very important for low-solubility systems, which are common in supercritical processes. Since sampling of the phases is a crucial part of the experiments, a detailed description of the preliminary runs that allowed to fix the optimal procedure is reported. The apparatus and the proposed method have been tested on the well-known binary system Carbon dioxide + Limonene. Furthermore, some experimental data on the multicomponent system Carbon dioxide + Lemon essential oil are reported.