Isochoric heat capacity of near- and supercritical benzene and derived thermodynamic properties

Abstract

As part of a continuing study of the critical and supercritical phenomena in fluids and fluid mixtures, one- (CV1) and two-phase (CV2) isochoric heat capacities, densities (ρS) and phase-transition temperatures (TS) of benzene were measured in the critical and supercritical regions. Measurements were made in the immediate vicinity of the liquid-gas phase transition and the critical points in order to accurately determine of the phase transition properties (TS,ρS,CV1, and CV2). The measurements cover the temperature range from (347 to 616) K for 10 liquid and vapor isochores between (265 and 653) kg·m−3 at pressure up to 7.5 MPa. The measurements were performed using a high-temperature, high-pressure, nearly constant-volume adiabatic calorimeter. The standard uncertainties of the density, temperature, and isochoric heat capacity, CV, measurements are estimated to be 0.1%, 0.02 K, and 1.5%, respectively. The measured one- (CV1) and two-phase (CV2) isochoric heat capacities along the critical isochore and the saturated liquid (ρS') and vapor (ρS") densities near the critical point were used to accurately estimate the theoretically meaningful asymptotic critical amplitudes (A0±and B0) and related amplitudes for other properties (Γ0+, D0, ξ0), and their universal relations, A0+/A0−, A0+Γ0+B02, αA0+Γ0+B0−2, D0Γ0+B0δ−1, ξ0αA0+vC1/3. Saturated liquid and vapor densities (ρS′,ρS″) together with measured two-phase CV2 data were used to estimate the values of asymmetric parameters a3 (complete scaling parameter) and b2 of the coexistence curve singular diameter. Experimentally determined asymptotical critical amplitudes (A0±and B0 fluid-specific parameters) were used to check and confirm the predictive capability of the universal correlation in terms of their dependence on the acentric factor ω based on the generalized corresponding states principle. The measured values of two-phase CV2 as a function of the specific volume V along various isotherms were used to calculate second temperature derivatives of vapor pressure d2PSdT2 and chemical potential d2μdT2 and to estimate the value of Yang-Yang anomaly strength parameter Rμ = − 0.683 for benzene. The contributions of the vapor pressure, CVP=VCTd2PSdT2, and the chemical potential, CVμ=−Td2μdT2, to the measured total two-phase CV2 were derived from the measurements.