Greenhouse Gas (CHF3) Separation by Gas Hydrate Formation

In this study, the feasibility of gas hydrate-based greenhouse gas (CHF3) separation was investigated with a primary focus on thermodynamic, structural, and cage-filling characteristics of CHF3 + N-2 hydrates. The three-phase (hydrate (H)-liquid water (L-w)-vapor (V)) equilibria of CHF3 (10%, 20%, 40%, 60%, and 80%) + N-2 + water systems provided the thermodynamic stability conditions of CHF3 + N-2 hydrates. Powder X-ray diffraction revealed that the structure of the CHF3 + N-2 hydrates was identified as sI (Pm3n) for all the CHF3 concentration ranges considered in this study. A pressure composition diagram obtained at two different temperature conditions (279.15 and 283.15 K) demonstrated that 40% CHF3 could be enriched to 88% CHF3 by only one step of hydrate formation and that separation efficiency was higher at the lower temperature. Furthermore, Raman spectroscopy revealed that CHF3 molecules preferentially occupy large (5(12)6(2)) cages of the structure I (sI) hydrate during CHF3 + N-2 hydrate formation. The overall experimental results clearly demonstrated that the hydrate-based separation process can offer highly concentrated CHF3 and would be more effective for recovering CHF3 from exhaust gas when it constitutes a hybrid system with existing separation methods.