Department of Urban and Environmental Engineering (Environmental Science and Engineering )This study investigated thermodynamic and microscopic characteristics of various F-gas hydrates to examine the feasibility of gas hydrate-based F-gas separation process and to demonstrate the newly discovered F-gas hydrates. Thermodynamic phase equilibria were measured to determine the thermodynamically stable region of each F-gas hydrate, while powder X-ray diffraction was conducted to identify the gas hydrate crystal structure and lattice parameter. In addition, 13C & 19F NMR and insitu Raman spectroscopy were utilized to confirm the hydrate structure and observe cage-filling guestmolecular behavior. Lastly, the gas and hydrate phase compositions were analyzed via gas chromatography to examine the separation efficiency by gas hydrate formation process. From the experimental results, the thermodynamic stability range of pure CHF3 and CHF3 + N2 gas hydrates demonstrated that CHF3 can be captured in hydrate phase with high separation efficiency, while they form sI hydrate regardless of CHF3 concentrations used in this study. On the other hand, pure C2F6 and C2F6 + N2 gas mixture formed sII hydrates, and since C2F6 + N2 + water system showed an azeotropic behavior at high temperature range, restricting the gas hydrate-based separation process only applicable at specific temperature and pressure range. Lastly, the fundamental thermodynamic and spectroscopic properties of pure NF3 hydrate were obtained to estimate the feasibility for gas hydrate-based separation process. This study also made important discoveries on two F-gases (c-C4F8 and C3F8) which form sH hydrate in presence of suitable guest molecules. Since C3F8 and c-C4F8 molecules have large molecular sizes, those molecules have not been expected to be enclathrated in sI or sII hydrate cages. However, this study discovered that c-C4F8 molecules can be enclathrated in sH large (51268) cages in presence of CH4 as help gas, which was demonstrated through PXRD and 13C NMR spectroscopy. In addition, C3F8 was found to act as a dual hydrate former between sH and sII hydrates according to help gas molecules. Via 13C NMR and Raman spectroscopy, C3F8 was confirmed to form sH hydrate with CH4, while forming sII hydrate in presence of SF6. The discovery of c-C4F8 and C3F8 as sH hydrate former is very meaningful, since there have been no gas-phase sH hydrate former investigated until present. The overall results obtained in this study provide invaluable information of various properties of F-gas hydrates, and are expected to be useful sources for gas hydrate application fields in the future.clos
