?????? ?????????????????? ?????? ???????????? ?????? ????????? ?????? ???????????? + ?????? ????????????????????? ????????????, ??????, ????????? ?????? ??????

Department of Urban and Environmental Engineering (Environmental Science and Engineering)To mitigate global warming, SF6, one of the most potent greenhouse gases, should not be emitted to the atmosphere and this study utilizes hydrate-based gas separation method (HBGS) to capture SF6 selectively and further recycle. For the hydrate-based SF6 separation process, this study examines fundamental characteristics of pure SF6 (Topic 1) and SF6 + N2 hydrate (Topic 2) with primary focus on thermodynamic, kinetic, and structural properties. Thermodynamic stability condition and dissociation enthalpy (??Hd) of SF6 and SF6 + N2 hydrates were measured via isochoric PVT method and ??-DSC. To observe kinetic behaviors during hydrate formation and dissociation process, time-dependent gas consumption, compositional change, and Raman peak intensity/area changes were observed using gas chromatography and in-situ Raman spectroscopy. Furthermore, hydrate structure and cage occupancy of SF6 and N2 in hydrate were identified through powder X-ray diffraction pattern analysis. Lastly, SF6 composition in vapor and hydrate phase were measured using gas chromatography to elucidate the separation efficiency. In topic 1, SF6 hydrate formation behaviors in the presence of the porous reaction media and surfactant, related to energy consumption for stirring process and reducing reaction time, were ed. The types of reaction media had no effect on thermodynamic stability of SF6 hydrates, but those had significant difference in hydrate formation behavior. The slow hydrate formation of SF6 hydrate was observed in stirred bulk water system and it can be resolved using porous silica gel with large surface area and adding surfactant. The unstirred porous silica gel promoted not only the amount but also the rate of SF6 hydrate formation and the surfactant accelerated SF6 hydrate formation rate significantly. Although SDS showed good kinetic promotion effect, it can cause foaming problem and the silicone based antifoaming agent was added to surfactant solution to reduce form generation effectively without hindering kinetic promotion effect of surfactant on SF6 hydrate formation. In topic 2, thermodynamic, kinetic, and structural characteristics of SF6 + N2 hydrate, separation efficiency, and equilibrium recovery ratio were investigated to examine the feasibility of hydrate-based SF6 separation process. In addition, the time-dependent SF6 selectivity during hydrate formation and dissociation was examined through experimental and computational methods to determine the optimal operational time. A significant thermodynamic stabilization of SF6 + N2 hydrates and a high SF6 composition in the hydrate phase demonstrated the preferential occupation of SF6 in hydrate lattices. The structures of SF6 and SF6 + N2 hydrates were identified as sII hydrate regardless of gas composition. The hydrate-liquefaction combined method was suggested to supplement the drawbacks of each method and conserve power consumption for pressurization. From the time-dependent SF6 selectivity observation, there was no kinetic selectivity of SF6 during hydrate formation and dissociation despite of superior thermodynamic selectivity of SF6. These overall results in this study provide valuable information of SF6 and SF6+N2 hydrates and thus it can contribute for designing and operating the hydrate-based SF6 separation process.ope

Kinetic Selectivity of SF6 during Formation and Dissociation of SF6 + N-2 Hydrates and Its Significance in Hydrate-Based Greenhouse Gas Separation

The time-dependent selectivity of SF6, the most potent global warming gas, in the hydrate-based gas separation process was investigated through both experimental and computational approaches. Guest-enclathrating and guest-releasing behaviors in SF6 + N-2 hydrate were observed via gas chromatography, in situ Raman spectroscopy, and molecular dynamics (MD) simulations. The increasing pattern of the normalized area ratio of the Raman peak for enclathrated SF6 molecules was similar to that for enclathrated N-2 during hydrate formation, and the composition of SF6 in the hydrate phase was almost constant throughout hydrate formation. MD simulations also showed that the captured SF6/N-2 ratio in the hydrate structure was nearly constant over time. These results evidenced no remarkable difference in kinetic selectivity between SF6 and N-2 during hydrate formation. The in situ Raman spectra and MD simulations examined during hydrate dissociation also demonstrated that SF6 was not kinetically selective in the guest-releasing process. The overall experimental and computational results indicated that none of the guest molecules in the SF6 + N-2 hydrate were kinetically selective during formation and dissociation despite the superior thermodynamic selectivity of SF6. The findings of this work provide the features of guest-filling and guest-liberating behaviors during the formation and dissociation of SF6 + N-2 hydrate. They will contribute to the determination of the optimal operation time for hydrate formation and thus to the development of the hydrate-based SF6 separation process

Phase equilibria and azeotropic behavior of C2F6 + N2 gas hydrates

C2F6 (hexafluoroethane, R116) is a fluorinated gas (F-gas) widely used in semiconductor industries, which also has a high global warming potential and a long atmospheric lifetime. In this study, the thermodynamic and structural characteristics of the C2F6 +N2 gas hydrates were investigated for gas hydrate-based C2F6 separation from emission sources. This experiment measured the three-phase (hydrate, liquid water, and vapor [H-LW-V]) equilibria of ternary C2F6 (10, 20, 40, 60, and 80%)+N2 +H2O systems and indicated the possible existence of hydrate azeotropes at certain temperature ranges. Powder X-ray diffraction (PXRD) revealed that the ternary C2F6 +N2 +H2O systems form structure II (sII) hydrates (Fd3m) for all C2F6 concentrations considered in this study. The pressure-composition diagram obtained at two different temperatures (275.15K and 279.15K) demonstrated that C2F6 is highly enriched in the hydrate phase at 275.15K, whereas at 279.15K, the C2F6 +N2 +H2O systems have a hydrate azeotrope where the composition of the hydrate phase is the same as the composition of the vapor phase. The overall experimental results clearly indicate that hydrate-based C2F6 separation is thermodynamically feasible and the higher separation efficiency is achievable at lower temperature ranges

Separation efficiency and equilibrium recovery ratio of SF6 in hydrate-based greenhouse gas separation

The feasibility of hydrate-based sulfur hexafluoride (SF6) separation was investigated by primarily focusing on the thermodynamic, kinetic, and structural characteristics of SF6 + N-2 hydrates, the separation efficiency, and the equilibrium recovery ratio. Three-phase (hydrate (H)-water (LW)-vapor (V)) equilibria of SF6 + N-2 hydrates were measured to examine the effect of guest occupation on their thermodynamic stability. A pressure-composition diagram, which was obtained at 275.15 K, was constructed to elucidate the separation efficiency. The final SF6 compositions in the vapor phase during hydrate formation in isochoric and isobaric conditions showed agreement with the corresponding equilibrium compositions. SF6 + N-2 hydrates were identified as sII via powder X-ray diffraction (PXRD). The Rietveld refinement of the PXRD patterns offered quantitative cage occupancy of SF6 and N-2 in the SF6 + N-2 hydrates. The dissociation enthalpy (Delta H-d) of SF6 + N-2 hydrates was measured using a high-pressure micro-differential scanning calorimeter (HP mu-DSC). The overall experimental results clearly demonstrated that SF6 was selectively captured in the hydrate phase. The hydrate-based method required a lower initial SF6 concentration and pressure to attain a specified recovery ratio of SF6 compared with the liquefaction method; however, it offered lower SF6 purity. Therefore, the hydrate-liquefaction combined method is suggested to supplement the drawbacks of each method and conserve power consumption for pressurization

Structure Identification and Thermodynamic Stability of Mixed CHF3 + N2 Gas Hydrates

MEA??? CO2 ?????? ????????? ????????? ???????????? MEA carbamate??? ????????????, ????????? ?????? ????????? CO2??? ???????????? ???????????? ????????????. ?????????, ?????? ??????????????? MEA carbamate??? ???????????? ??????????????? ????????????. ?????? MEA??? CO2 ??????????????? ???????????????, ????????? ???????????????, ????????? ????????? ?????????, ????????? ???????????? CO2 ??????????????? ????????? ??? ??????. MEA carbamate??? ????????? ????????? ??? ?????? ??? ?????? ????????? oxazolidone, cyclic urea of trimer, HEIA(1-(2-hydroxyethyl)-2-imidazolidone), HEEDA(N-(2-hydroxyethyl)-ethylenediamine) ?????? ??????. ?????? ??????????????? MEA carbamate??? ????????? ????????? ?????? ????????? MEA??? HEEDA??? ????????? ????????? ???????????????????????? ????????? ??????????????????. CO2 loading??? ??=0.406, ??=0.6??? MEA carbamate ???????????? pipe reactor??? ?????? 130??C, 150??C??? ?????? 1, 2, 4, 6, 8??? ?????? ?????? ?????? ????????? ???????????? ?????????. ????????? ???????????????????????? ????????? MEA??? ????????? ????????? ???????????? ???????????? ??????, CO2 loading??? ????????? ????????? ???????????? MEA??? ?????? ????????? ??? ??? ?????? ????????? ??? ?????????. HEEDA??? ????????? ????????? ???????????? ??????????????????, ??=0.406, 150??C ??? ??? ?????? ????????? ?????????. ????????? CO2 ?????????????????? ???????????? ????????? ????????? ASTM ?????? ????????? ????????? CO2 loading, N2 ??????, ?????? ?????? ????????? ????????? ???????????????. ?????? ????????? ?????? ????????? ??? ??? ????????? amine??? ???????????? CO2 ?????? ????????? ????????? ???????????? ????????????

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 method

SF6 Hydrate Formation in Various Reaction Media: A Preliminary Study on Hydrate-Based Greenhouse Gas Separation

SF6 hydrate formation behaviors in various reaction media, such as bulk water, porous silica gel, and hollow silica, were investigated for hydrate-based SF6 separation with a primary focus on thermodynamic stability and formation kinetics. The measured three-phase (H-L-w-V) equilibria demonstrated that the types of reaction media used in this study had no effect on the thermodynamic stability of SF6 hydrates. The dissociation enthalpy (Delta H-d) of SF6 hydrate was measured using a high-pressure micro-differential scanning calorimeter, and it corresponded well with estimates from the Clausius-Clapeyron equation. The unstirred porous silica gel system showed a larger gas uptake and a higher growth rate at the early stage of SF6 hydrate formation. However, the gas uptake and growth rate of SF6 hydrates in stirred bulk water and unstirred hollow silica were significantly increased at a larger temperature driving force or in the presence of sodium dodecyl sulfate. The experimental results obtained in this study will be very helpful for a better understanding of the thermodynamic and kinetic characteristics of SF6 hydrate formed in various reaction media and in surfactant-added solution, and are expected to contribute to further development of the hydrate-based SF6 separation process