SOME CHARACTERISTICS OF PURE FLUID PROPERTIES THAT CHALLENGE EQUATION-OF-STATE MODELS

Precise descriptions of the thermodynamic properties of pure fluids re quire accurate vapor pressures and phase volumes as well as residual v olumes, enthalpies and entropies. There is also the desirability of ob taining the density extrema in isothermal variations of the isochoric heat capacity, extrema of the isothermal compressibility and speed of sound, and densities where the reduced bulk modulus and isobaric expan sivity are essentially independent of temperature (or have very weak m axima). While carefully fitted multiparameter equation of state models (EOS) show all of these qualities, cubic and other EOS based on 2- or 3-parameter corresponding states principles (CSP) usually do not, Til e common modifications to the attraction parameter and covolume depend ence in generalized van der Waals models for improving vapor pressures and phase volumes do not improve descriptions of thew extrema for the derivative properties. This paper describes characteristics of the de rivative propel-ties for methane from a highly accurate equation and c ompares them with results from several common EOS models. To obtain th e extrema at all, the EOS covolume parameter must be at least temperat ure dependent, and most common models require density dependence. Accu rate description is not, possible with such models unless the covolume has a complex dependence on both temperature and density.