Dew Point, Liquid Volume, and Dielectric Constant Measurements in a Vapor Mixture of Methane + Propane Using a Microwave Apparatus

An apparatus based on a microwave resonant cavity has been used to measure dew points and liquid volume fractions in a zC3H8+(1−z)CH4 mixture with z=0.250±0.001 mole fraction. The microwave cavity is optimized for the measurement of small liquid volume fractions in lean natural gases. Argon and carbon dioxide were used to calibrate the resonator for dielectric constant and liquid volume measurements in mixtures. Estimated uncertainties are 1×10−4 for dielectric constants and (0.05 K, 0.05 MPa) for dew points. The novel use of multiple cavity modes, each sensitive to different liquid volume regimes, substantially improves the reliability of liquid volume measurements. Liquid volume fractions can be resolved to better than 0.01%. Densities inferred from (P, T, e) measurements agree within 0.6% of equation of state (EOS) densities with an estimated uncertainty of 0.1%. Liquid volume fractions measured with the microwave apparatus compare well with values determined using a conventional PVT cell. Fourteen dew points were measured at ten different temperatures. From these data, the mixture cricondentherm is estimated to be (293.45±0.05) K, which is 0.15 K higher than the value predicted using the Peng–Robinson equation of state.