Permeability, diffusivity and solubility of carbon dioxide in fluoropolymers: an experimental and modeling study

Carbon dioxide has successfully been used as an alternative refrigerant in many applications, replacing chlorofluoro- and hydrofluorocarbons (CFCs and HFCs), due to its negligible ozone depletion and significantly lower global warming potential. However, the use of carbon dioxide as a refrigerant requires a refrigeration cycle with greater extremes of pressure, placing greater demands on the plant’s seals and packings. The integrity of the refrigeration system (it should release as little refrigerant as possible to the atmosphere) depends crucially on the material used for the seals and packings. Using a high-pressure permeation cell, the permeability and diffusivity of carbon dioxide were measured in several polymers used as packing and sealing materials. These were the fluoropolymers PTFE, FKM and TFM, both pure and containing glass, graphite, Ekonol and polysulfone as additives. The solubility coefficient of carbon dioxide in these polymers was then obtained as the ratio of the measured permeability and diffusivity data. Measurements were carried out over a pressure range of 45–50 bar and a temperature range of 40–80 °C. The solubility of carbon dioxide in the polymers is then modeled with the Simplified Perturbed Chain – Statistical Associating Fluid Theory (sPC-SAFT) equation of state. Pure component parameters were determined using an extrapolation method based on the lower molecular weight monomer and available density data for the polymers. In the case of copolymers, mixing rules were used to determine parameters. Carbon dioxide solubility can be accurately correlated to the measured data with the sPC-SAFT equation of state using a temperature–independent binary interaction parameter.