Thermodynamic anomalies near the critical point of steam

Abstract

Ideas about the character of critical anomalies obtained from the lattice gas model are tested on the thermodynamic properties of steam, using methods developed for nonpolar gases. The critical anomalies in steam are shown to be nonclassical and very similar to those of simple gases. Specifically, the exponents obtained by power-law analysis of the coexistence curve, specific heat C v and compressibility K T are β = 0.347 ±0.005, α = 0.1 ±0.05, γ = 1.20 ±0.05, implying δ = 4.45. Since the critical point is thus a point of nonanalyticity in the thermodynamic behavior, the scaled equation of state may be an appropriate means to describe the critical region. Some results of such a scaled analysis of the PVTdata are presented and shown to be consistent with independently measured vapor pressure and specific heat data and with the exponents quoted. The best value obtained for the critical temperature by scaling is 373.9° C ± 0.05°, values between 0.322 and 0.327 g/cm 3 are obtained for the critical density depending on the property analyzed.