Investigating the significance of structural transition in chlorodifluoromethane (R22) + N2 hydrates for hydrate-based greenhouse gas separation

Abstract

Chlorodifluoromethane (CHClF2 or R22) is known for its high global warming potential and ozone-depleting characteristics. Due to its significant environmental impact, R22 is scheduled for a complete ban by 2030 under the Montreal Protocol. To prevent the release of R22 into the atmosphere, it is imperative to efficiently separate and recover R22 from gas mixtures. Thus, the aim of this study was to explore the influence of gas hydrate structure on the efficiency of hydrate-based gas separation (HBGS) in separating R22 from R22 + N2 mixtures. This was achieved by examining a pressure-composition diagram and the in situ Raman spectra and powder X-ray diffraction (PXRD) patterns of R22 + N2 hydrates with various R22 compositions. In situ Raman spectroscopic and PXRD pattern analyses revealed that as R22 composition decreased, the R22 + N2 hydrates underwent a structural transition from structure I (sI) to a coexistence of structures I and II, ultimately adopting structure II (sII). The pressure-composition analysis suggested that the cusp point observed in the hydrate composition curve was caused by the structural transition of the R22 + N2 hydrates. It was noted that the structural transition from sI to sII led to a significant decrease in separation efficiency. The experimental results demonstrated that the gas hydrate structure plays a crucial role in determining the gas separation efficiency of HBGS and designing the HBGS process.