Greenhouse Gas (CHF3) Separation by Gas Hydrate Formation

In this study, the feasibility of gas hydrate-based greenhouse gas (CHF 3 ) separation was investigated with a primary focus on thermodynamic, structural, and cage-filling characteristics of CHF 3 + N 2 hydrates. The three-phase (hydrate (H)–liquid water (L W )–vapor (V)) equilibria of CHF 3 (10%, 20%, 40%, 60%, and 80%) + N 2 + water systems provided the thermodynamic stability conditions of CHF 3 + N 2 hydrates. Powder X-ray diffraction revealed that the structure of the CHF 3 + N 2 hydrates was identified as sI ( Pm 3 n ) for all the CHF 3 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% CHF 3 could be enriched to 88% CHF 3 by only one step of hydrate formation and that separation efficiency was higher at the lower temperature. Furthermore, Raman spectroscopy revealed that CHF 3 molecules preferentially occupy large (5 12 6 2 ) cages of the structure I (sI) hydrate during CHF 3 + N 2 hydrate formation. The overall experimental results clearly demonstrated that the hydrate-based separation process can offer highly concentrated CHF 3 and would be more effective for recovering CHF 3 from exhaust gas when it constitutes a hybrid system with existing separation methods.