The application of a shear mode piezoelectric sensor to monitoring the high-pressure phase behaviour of asymmetric binary systems

Abstract

We demonstrate for the first time that high-pressure phase diagrams can be measured in a simple, objective and fully automated manner for complex binary fluid mixtures. Supercritical CO 2 is becoming of increasing industrial importance. Many of the important binary systems with CO 2 are highly asymmetric, and exhibit a variety of the phase behaviour from liquid–liquid immiscibility to discontinuous critical loci. We investigate the possibility of mapping these different types of phase diagram using a single shear-mode piezoelectric sensor. We focus on two systems (1-octanol + CO 2 and 2-octanol + CO 2). We show that the existence of three phases (liquid–liquid–vapour) can be detected by the presence of two discontinuities when the impedance minimum of the sensor is measured as a function of fluid volume. This method is used to measure the three-phase coexistence curve quantitatively for the 1-octanol + CO 2 system and the result is in good agreement with literature data. By analyzing the impedance minimum along the p– T phase boundary, we have found that the vapour–liquid critical locus of 2-octanol + CO 2 is interrupted by the occurrence of a previously unobserved liquid–liquid immiscibility. For both systems, we have used the critical points, located either by the isochoric method or extrapolated from composition/ T/ p phase envelope to construct a p, T-projection with characteristic features which allow the particular mixture to be classified to its appropriate type (i.e., 1-octanol + CO 2, Type III; 2-octanol + CO 2, Type IV).