Phase equilibria and azeotropic behavior of C2F6 + N2 gas hydrates

Abstract

C2F6 (hexafluoroethane, R116) is a fluorinated gas (F-gas) widely used in semiconductor industries, which also has a high global warming potential and a long atmospheric lifetime. In this study, the thermodynamic and structural characteristics of the C2F6 +N2 gas hydrates were investigated for gas hydrate-based C2F6 separation from emission sources. This experiment measured the three-phase (hydrate, liquid water, and vapor [H-LW-V]) equilibria of ternary C2F6 (10, 20, 40, 60, and 80%)+N2 +H2O systems and indicated the possible existence of hydrate azeotropes at certain temperature ranges. Powder X-ray diffraction (PXRD) revealed that the ternary C2F6 +N2 +H2O systems form structure II (sII) hydrates (Fd3m) for all C2F6 concentrations considered in this study. The pressure-composition diagram obtained at two different temperatures (275.15K and 279.15K) demonstrated that C2F6 is highly enriched in the hydrate phase at 275.15K, whereas at 279.15K, the C2F6 +N2 +H2O systems have a hydrate azeotrope where the composition of the hydrate phase is the same as the composition of the vapor phase. The overall experimental results clearly indicate that hydrate-based C2F6 separation is thermodynamically feasible and the higher separation efficiency is achievable at lower temperature ranges.