Adsorption equilibria and separation of pentafluoroethane and hexafluoroethane on NaX zeolite

Abstract

Removing pentafluoroethane (R125) impurities from hexafluoroethane (R116) is important for industrial applications, especially in the semiconductor field. The separation of R125 and R116 on NaX zeolite was analyzed using their adsorption isotherms at 273–303 K (with pressures up to 110 kPa), and the Langmuir, Freundlich, Sips, and Dubinin–Radushkevich isotherm models were applied to the experimental data, among which the Sips model best fitted the data. The thermodynamic parameters were examined to elucidate the adsorption behavior. The negative standard enthalpy changes of R125 and R116 indicated an exothermic adsorption process in which both gases were spontaneously adsorbed on NaX. Fixed-bed adsorption experiments of the binary gas mixtures on NaX showed that this adsorbent exhibited high selectivity for R125 adsorption owing to the different interaction forces between the NaX adsorbent and each gas. At 303 K and 100 kPa, the selectivity of NaX for R125, in a binary gas mixture of R125 and R116 with a molar ratio of 10:1, reached 1195, consistent with that (1158) predicted by the ideal adsorption solution theory (IAST). The IAST results indicated a negative correlation between R125 adsorption selectivity and temperature, and a positive correlation between pressure and mole fraction.