Probing Vapor/Liquid Equilibria of Near-Critical Binary Gas Mixtures by Acoustic Measurements

Abstract

Phase behavior is a fundamental aspect of supercritical fluids. A simple acoustic method is described for investigating vapor/liquid equilibria. The method is used to investigate three different pure components (CO 2, C 2 H 6, and CF 3 CH 2 F, refrigerant R134a) and binary mixtures of CF 3 CH 2 F with CO 2 or C 2 H 6 . For the pure components, the reciprocal of the speed of sound was measured at selected temperatures around the critical point. The data obtained are in good agreement with literature values. The binary mixtures were investigated over the whole composition range (i.e., mole fraction of CF 3 CH 2 F from 0 to 1.0), and their critical curves were determined. The critical lines show the expected type I fluid phase behavior, but the critical line in C 2 H 6 + CF 3 CH 2 F shows an unusual pressure minimum and maximum. The resulting critical lines are discussed with respect to thermodynamics, the use of these mixtures for supercritical fluid chromatography and extractions, and the possibility of using CF 3 CH 2 F as a modifier.