P−V−T−x and Vapor−Liquid Equilibrium Properties of Pentafluoroethane (R125) + 1,1,1,3,3,3-Hexafluoroethane (R236fa) and 1,1,1,2-Tetrafluoroethane (R134a) + R236fa Systems Derived from Isochoric Measurements

Abstract

P−V−T−x measurements were performed using a constant-volume apparatus for the pentafluoroethane (R125) + 1,1,1,3,3,3-hexafluoropropane (R236fa) and 1,1,1,2-tetrafluoroethane (R134a) + 1,1,1,3,3,3-hexafluoropropane (R236fa) systems, in a temperature range of (253−372) K and a pressure range of (113−2360) kPa. Experimental data were collected on three different compositions for each of the systems. Eight expansion series with a total of 121 data points and nine expansion series with 110 data points were performed, respectively, for the R125 + R236fa and R134a + R236fa systems, both within the vapor−liquid equilibrium (VLE) boundary and in the superheated vapor region. The data in the superheated vapor region were interpreted using tried and tested correlating methods for the second and third virial coefficients. The VLE parameters were derived by applying two methods: (1) The flash method enabled VLE parameters to be derived directly from P−V−T−z data within the VLE boundary by applying the Carnahan−Starling−De Santis (CSD) equation of state (EOS). (2) The dew-point method was based on dew-point evaluation by interpolating experimental P−T sequences. The resulting dew-point value was then used to derive VLE parameters, again using the CSD EOS. A comparison of the results emerging from these two methods, followed by the interpretation of the volumetric properties of the superheated vapor, confirmed their internal consistency.