Low-temperature high-pressure crystallization and fluid-phase equilibria of the binary systems nitrogen + trifluoromethane and argon + trifluoromethane between 110 and 230 K and at pressure up to 200 MPa

Abstract

In this study the phase behaviour of binary systems of trifluoromethane was investigated with the gases nitrogen or argon as the non-polar second component. The experiments were carried out in the temperature range from 110 K to 230 K and at pressures up to 200 MPa. The accent was on the experimental determination of the crystallization and fluid phase equilibria. Three-phase equilibria solid-fluid-fluid were also determined. All measurements were performed in a high-pressure low-temperature apparatus according to the synthetic method, i.e. pressure and/or temperature were varied slowly until crystallization or demixing of the homogenous mixtures under test is detected. The mixtures were analysed by online gas-chromatography. The phase behavior of N 2 + trifluoromethane corresponds to type III according to van Konynenburg and Scott's classification. From the upper critical end point (120K, 88.6 MPa) the critical curve f=f extends to higher temperatures and lower pressures with a positive curvature. The three-phase line solid-fluid displays a temperature minimum at 116.8 K and 28 MPa and ends at a critical end point. The system argon + trifluoromethane shows a gas-gas equilibrium of the second type. Above the temperature minimum (154.4 K, 100 MPa) the critical curve extends steeply to higher pressures with increasing temperature. The phase behaviour of the investigated systems is represented in a p(T) diagram and discussed in comparison with other nitrogen, trifluoromethane and argon systems.