Description of SF6 behavior within the state range from the triple point to supercritical fluid

Abstract

P-ρ-T data of SF6 were approximated by the combined equation of state within the ranges of reduced densities (0 < ρ/ρ c < 2.5) and temperatures (225 < T< 340 K). The equation of state, which has the form of explicit dependence of pressure P on ρ and T, includes a new regular part for approximation of P-ρ-T data in liquid an gaseous ranges of states beyond the critical region, nonparametrical scaling equation of state valid near the critical point of evaporation, and new crossover function for combination of these equations. The regular part of combined equation of state includes 8 simple terms of equation with eight adjustable parameters, three of which are determined by the conditions at the critical point. The total number of system-dependent constants is 14, including the critical values of P,ρ, and T. For the scaling part of state equation, the critical indices of 3D Ising model are used. At approximation of high-accuracy P-ρ-T data of SF6 by the new combined equation their description with the pressure error below ± 0.35 % was obtained in the whole range of gas and liquid states. Heat capacity C v was calculated on the critical isochore using constants of combined equation. Calculation results coincide with the known experimental data within their error limit. The behavior of C v on isotherms was predicted in a wide range of densities, and these data were compared with predictions of other modern equations of state.