Thermodynamic and structural features of chlorodifluoromethane (a sI-sII dual hydrate former) plus external guest (N-2 or CH4) hydrates and their significance for greenhouse gas separation

Abstract

In this study, a new sI-sII dual hydrate former [chlorodifluoromethane (CHClF2); an important greenhouse gas with a global warming potential of 1810], which forms sI hydrate by itself and forms sII hydrate in the presence of external help guests such as CH4 and N-2, was introduced and closely investigated for its potential significance in gas hydrate-based gas separation. The phase equilibria of CHClF2 hydrate, binary CHClF2 (5%) + N-2 (95%) hydrate, and binary CHClF2 (5%) + CH4 (95%) hydrate were measured to examine the formation conditions and thermodynamic stability regions of CHClF2 + external guest hydrates. Nuclear magnetic resonance and in situ Raman spectroscopic results confirmed the formation of sII hydrates for CHClF2 + external guest (N-2 or CH4) mixtures. Powder X-ray diffraction patterns clearly demonstrated a structural transition of sI to sII hydrates and a preferential incorporation of CHClF2 molecules in the hydrate phase when external guests (N-2 or CH4) were involved in CHClF2 hydrate formation. The measured dissociation enthalpy values of CHClF2 hydrate, binary CHClF2 (5%) + N-2 (95%) hydrate, and binary CHClF2 (5%) + CH4 (95%) hydrate using a high-pressure micro-differential scanning calorimeter also indicated preferential CHClF2 enclathration. The experimental results provide new insights into the thermodynamic and structural features of the CHClF2 (sI-sII dual hydrate former) + external guest hydrates for understanding and designing the hydrate-based CHClF2 separation process.