The influence of adsorption on PVT measurements in the gaseous phase

PVT measurements of 1,1,1,2-tetrafluoroethane (C2H2F4, HFC-134a) and its bl end with octofluoropropant (C3F8, FC-218) have been performed in the gas ph ase near the dew curve. The experimental data were obtained by variable-vol ume and vibrating tube methods. Discrepancies in the behavior of isotherms from their classical behavior were experimentally observed. It was Found th at the phase transition does not go to completion at a single point of the thermodynamic surface but extends over a limited range of conditions. Obtai ned results are in accordance with a concept of adsorption of the vapor sam ple on the surface of the experimental cell. An increase in adsorption unde r the conditions close to condensation is caused by capillary condensation of the sample at the walls of the cell that initiates an early phase transi tion. The ranges of diffuse phase transitions were determined for 1,1,1,2-t etrafluoroethane as well as for its mixture with octofluoropropane at diffe rent thermodynamic parameters. The influence of selective adsorption on the change in the conditions of phase transition of the 1,1,1,2-tetrafluoroeth ane/octofluoropropane mixture was also experimentally studied.