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The complex phase behaviors and structural coexistence of natural gas hydrates (NGHs) that contain large-molecule guest substances (LMGSs) were examined for their significance in the exploration an...

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 N2, was introduced and closely investigated for its potential significance in gas hydrate-based gas separation. The phase equilibria of CHClF2 hydrate, binary CHClF2 (5%) + N2 (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 (N2 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 (N2 or CH4) were involved in CHClF2 hydrate formation. The measured dissociation enthalpy values of CHClF2 hydrate, binary CHClF2 (5%) + N2 (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.

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 therm...

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 N2, was introduced and closely investigated for its potential significance in gas hydrate-based gas separation. The phase equilibria of CHClF2 hydrate, binary CHClF2 (5%) + N2 (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 (N2 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 (N2 or CH4) were involved in CHClF2 hydrate formation. The measured dissociation enthalpy values of CHClF2 hydrate, binary CHClF2 (5%) + N2 (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.

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 (275.15 K and 279.15 K) demonstrated that C2F6 is highly enriched in the hydrate phase at 275.15 K, whereas at 279.15 K, 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.

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 + N2 hydrates. The three-phase (hydrate (H)–liquid water (LW)–vapor (V)) equilibria of CHF3 (10%, 20%, 40%, 60%, and 80%) + N2 + water systems provided the thermodynamic stability conditions of CHF3 + N2 hydrates. Powder X-ray diffraction revealed that the structure of the CHF3 + N2 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 (51262) cages of the structure I (sI) hydrate during CHF3 + N2 hydrate formation. The o...

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In this study, gas hydrate-based fluorinated gas (F-gas) separation is proposed as a novel method to capture F-gases. This study investigates the thermodynamic, structural, and cage filling characteristics of the gas hydrates formed by two representative F-gases (CHF 3 and C 2 F 6 ) in order to verify the feasibility of the F-gas separation using gas hydrate formation. The three-phase (gas hydrate (H) – liquid water (L W ) – vapor (V)) equilibria of the pure CHF 3 and C 2 F 6 hydrates are measured in order to examine the hydrate formation conditions. The PXRD patterns reveal the structure of the CHF 3 hydrate and the C 2 F 6 hydrate as a cubic structure I (sI) and structure II (sII), respectively. The enclathration of CHF 3 and C 2 F 6 molecules in each pure CHF 3 and C 2 F 6 hydrate is confirmed through 13 C and 19 F NMR analyses. In-situ Raman measurements are used to monitor the growth process of pure CHF 3 hydrates, and they reveal the CHF 3 molecules trapped in the sI large (5 12 6 2 ) cages as well as in the sI small (5 12 ) cages. The computational study also demonstrates that CHF 3 is encaged in both small (5 12 ) and large (5 12 6 2 ) cages of the sI hydrate, whereas C 2 F 6 only occupies the large (5 12 6 4 ) cages of the sII hydrate.

The feasibility of hydrate-based sulfur hexafluoride (SF6) separation was investigated by primarily focusing on the thermodynamic, kinetic, and structural characteristics of SF6 + N2 hydrates, the separation efficiency, and the equilibrium recovery ratio. Three-phase (hydrate (H)–water (LW)–vapor (V)) equilibria of SF6 + N2 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 + N2 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 N2 in the SF6 + N2 hydrates. The dissociation enthalpy (ΔHd) of SF6 + N2 hydrates was measured using a high-pressure micro-differential scanning calorimeter (HP μ-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.

Sulfur hexafluoride (SF6), the most potent greenhouse gas, should be separated from gas mixtures for recycling and for mitigation of global warming. In this study, the formation and dissociation behaviors of SF6 hydrates in the presence of a surfactant (sodium dodecyl sulfate, SDS) and an antifoaming agent (antifoam A concentrate, AAC) were investigated, with a primary focus on kinetic, spectroscopic, and morphological analyses for hydrate-based SF6 separation. The optimum concentrations of SDS and AAC for SF6 hydrates were found to be 250 ppm and 1,500 ppm, respectively. The structure of SF6 hydrates in the presence of SDS and AAC was identified as structure II, indicating that SDS and AAC had no impact on hydrate structure. The formation behaviors of SF6 hydrates were thoroughly examined through gas uptake measurements, visual observation, and in-situ Raman spectroscopy. The addition of SDS 250 ppm significantly accelerated the formation rate of SF6 hydrate and the additional injection of AAC did not inhibit the promoting effect of SDS. Visual observation, temperature profiles, and volume of retrieved gas during the dissociation of SF6 hydrates clearly demonstrated that SDS also had a promoting effect on SF6 hydrate dissociation and its effect was slightly diminished with the addition of AAC, although AAC showed a powerful defoaming effect during the dissociation of SF6 hydrates. The experimental results obtained in this study will be very useful for accelerating the formation rate of SF6 hydrates using SDS and for solving the foaming problem using AAC in the design and operation of the gas hydrate-based SF6 separation process.

This study examined the synergistic inhibition effect of glycine (an amino acid) and [BMIM][BF4] (an ionic liquid) on the thermodynamic phase equilibria and growth behaviors of CH4 hydrates. Hydrate phase equilibria indicated that there was no thermodynamic synergism of inhibitor mixtures on CH4 hydrates. Powder X-ray diffraction (PXRD) patterns demonstrated that the presence of inhibitor mixtures did not affect the structural characteristics of CH4 hydrates. However, the glycine (1.5 mol%) + [BMIM][BF4] (1.5 mol%) system showed significantly less growth of CH4 hydrate, less final gas uptake, and less conversion of water into hydrate than a pure water system. Time-dependent Raman spectra revealed that [BMIM][BF4] inhibited CH4 molecules from occupying small 512 cages at the initial stage of hydrate formation, whereas glycine was effective in preventing CH4 molecules from entering large 51262 cages for the duration of hydrate formation. The cage-specific inhibition mechanism of the glycine and [BMIM][BF4] mixture had a synergistic effect, significantly reducing the growth of CH4 hydrate. The results of this study provide a better understanding of the inhibition mechanism and the synergistic potential of various inhibitors and could contribute to an expansion in the types of inhibitors that could be used for flow assurance in the pipelines of natural gas production and transportation.

This study examined the thermodynamic stability and guest gas inclusion of tetra-iso-amyl ammonium bromide (TiAAB) semiclathrates with CO2, N2, or CO2 (20%) + N2 (80%), with a primary focus on semiclathrate phase equilibria and Raman spectra. The three-phase (H-LW-V) equilibria of TiAAB semiclathrates with CO2, N2, or CO2 (20%) + N2 (80%) were measured at a stoichiometric concentration (TiAAB 3.7 mol%) using both a conventional isochoric method and a stepwise differential scanning calorimeter (DSC) method. The phase equilibria demonstrated that TiAAB (3.7 mol%) semiclathrates with CO2, N2, or CO2 (20%) + N2 (80%) were significantly stabilized compared with the corresponding CO2, N2, or CO2 (20%) + N2 (80%) gas hydrates. The enclathration of CO2 and N2 molecules in the cages of TiAAB semiclathrates was clearly confirmed via Raman spectroscopy. The experimental results indicate that TiAAB semiclathrates can incorporate CO2 and N2 into the cage lattices at elevated temperatures and lowered pressures and are potential materials for CO2 capture.