Mixing behaviour of a mixture of equal amounts of substance of 1,1,1,2-tetrafluoroethane and 1,1-difluoroethane .2. Representation of thermal properties by equations of state

In a preceding investigation, the heat capacity at constant pressure and the Joule-Thomson coefficient of the refrigerants R134a (CF3CH2F), R152a (CHF2CH3) and of (0.5CF(3)CH(2)F + 0.5CHF(2)CH(3)) have been measured in the gaseous phase for temperatures from 298 K to 423 K at pressures up to 2.5 MPa. In the present work, the Joule-Thomson coefficient and residual parts of the heat capacity and enthalpy have been calculated with a fundamental equation of state, developed previously by Tillner-Roth on the basis of a large number (3433) of experimental data points, and with a generalized Bender equation of state, the parameters of which are based only on experimental values of the vapour pressure and critical temperature. Both equations describe the dependence of the thermal properties on temperature and pressure rather well, although the deviations from the experimental data are somewhat larger than the experimental uncertainty. In spite of the much smaller experimental basis of the generalized Bender equation, the deviations from the measurements are, on average, only twice as large as with the fundamental equation. Also, the calculated values of the excess molar heat capacity and of the excess molar enthalpy, which have the opposite sign to most other gaseous mixtures, are in reasonable agreement with the experimental results. (C) 1997 Academic Press Limited.