Framework for In Situ Measurements of Vapor-Liquid Equilibrium Using a Microwave Cavity Resonator

The ability to accurately predict the behavior of multiphase fluid mixtures underpins a broad range of industrial and scientific activity. Expanding the scope and improving the performance of predictive thermodynamic models relies on the availability of accurate experimental data for the complete phase behavior of the corresponding fluid mixtures. Here, we present a novel approach to in situ measurements of heterogeneous two-phase behavior in binary fluid mixtures using a single apparatus. A modified microwave re-entrant cavity apparatus is employed to simultaneously measure the dielectric properties of the liquid and vapor as well as the quality of each phase, based on the frequency shifts caused by a heterogeneous fluid for three independent resonant modes. We report a so far unique mathematical framework to further characterize the thermophysical properties of each phase along tie lines, determining the compositions of the coexisting vapor and liquid phase as well as the vapor and liquid phase densities within the two-phase region based on the Clausius-Mossotti relation between phase dielectric properties, density, and molar polarizability. The framework was validated by comparison of the measured and predicted properties of a (0.35 propane + 0.65 carbon dioxide) mixture throughout the two-phase region along an isothermal pathway at T = 280 K. These proof-of-concept results demonstrate for the first time that thermophysical properties of a binary mixture with a known overall composition can be determined from experiments with a microwave cavity using a synthetic approach.