Hydrate-based gas separation (HBGS) has emerged as a promising technology for CO2 capture. However, conventional HBGS approaches, which rely on temperature-triggered phase transitions for gas capture and recovery, face challenges due to their irreversible nature and associated inefficiencies. An innovative approach was adopted in the present study to overcome these limitations, which involved preforming solid tetra-n-butylammonium chloride (TBAC) semiclathrates within porous silica gels to enable reversible CO2 encapsulation and release via pressure-driven gas diffusion. A series of analytical techniques confirmed the successful formation of TBAC semiclathrates and demonstrated their effectiveness in pre-combustion CO2 capture. Cryo-scanning electron microscopy provided direct visual evidence of the solid semiclathrate structures occupying the internal pores of the silica gels, as opposed to interstitial spaces. A differential scanning calorimetry analysis revealed no significant heat flow variations during gas adsorption and desorption, indicating the absence of phase transitions and supporting the diffusion-based mechanism. CO2 selectivity was demonstrated with a concentration of approximately 90% within the TBAC semiclathrates, highlighting the preferential occupation of CO2. In situ Raman spectroscopy further validated CO2 encapsulation, as distinct peaks appeared upon gas injection and disappeared following gas ejection. The reversibility of the process was confirmed through 10 consecutive cycles of gas adsorption and desorption; consistent performance was observed across all cycles, which underscored the stability of the semiclathrate structures. These findings offer new possibilities for advancements in HBGS and provide a practical foundation for next-generation CO2 capture solutions.

The management of CO2 emissions from fossil fuel-powered ships has been considered for complying with future enhancements in CO2 emission regulations in the shipbuilding industry. These regulations are (or will be) enforced using indexes such as the energy efficiency design index, energy efficiency existing index, energy efficiency operational indicator, and carbon intensity indicator. Hence, developing technology for reducing CO2 emissions from ships is necessary. This paper reports on the CO2 absorption performance of a pilot-scale CO2 capture system installed on a 1.075-MW HiMSEN 5H22CDF engine. The proposed technology is an exhaust gas CO2 absorption (EGCA) system. Our CO2 capture system is based on an exhaust gas cleaning system, which is already commercialized to reduce SOx emissions from ships that use heavy fuel oil. The CO2 absorption performance of the EGCA system was evaluated following the test procedure described in the NOx Technical Code 2008 (NTC 2008). Under the certified test procedure described in NTC 2008, the EGCA system exhibited 29/30 wt% (E2/E3 test mode) of CO2 absorption. On the basis of the test results, we expect that ships will be able to comply with future CO2 emission regulations by using the proposed EGCA system.

Octafluoropropane (C3F8) is a fluorinated gas (F-gas) known for its high global warming potential. In this study, the role of C3F8 as a novel gaseous structure H (sH) hydrate former was investigated. By employing a comprehensive methodology involving thermodynamic, crystallographic, and spectroscopic analyses, the successful encapsulation of C3F8 into the large cages of sH hydrates was unveiled for the first time. The incorporation of C3F8 into sH hydrates was inferred from a shift in the thermodynamic hydrate phase equilibria, and unequivocal evidence of successful C3F8 encapsulation was obtained using powder X-ray diffraction, 13C nuclear magnetic resonance, and Raman spectroscopy. The findings showed that C3F8 overcame the conventional limitations associated with sH hydrate formers, which are predominantly liquid and solid substances, thereby significantly expanding the scope of potential gaseous sH hydrate formers. This advancement not only enhances the fundamental understanding of gas hydrate formers but also opens up new avenues for hydrate-based technologies, particularly in the separation and recovery of large-size F-gases, which have substantial environmental implications.

Since the industrial revolution, which was accompanied with the use of fossil fuels as an energy source, the content of carbon dioxide (CO2) in the atmosphere has increased. To mitigate global warming, industries that utilize fossil fuels have continuously explored new approaches to reduce CO2 emissions and convert it to alternative fuels. The ocean is a vast source of absorbed CO2 on Earth, and various studies have been conducted on the use of the ocean to reduce global CO2. This study focused on reducing CO2 in the atmosphere by storing it as bicarbonate, a form of CO2 that exists in the ocean. The optimum condition for the conversion of CO2 into bicarbonate was investigated by considering the dissolved inorganic carbon (DIC; HCO3-, CO32-, H2CO3) concentration and pH. To confirm the biological impact of this conversion, biological impact experiments were conducted under various DIC concentrations using Skeletonema japonicum, a phytoplankton present in most areas of the sea. Based on the DIC concentration (2.09 mM) of the seawater, the DIC concentrations used in the Lab-scale experiment ranged from 2.5 mM to 18.75 mM, and the concentration with the highest conversion rate (< 6.38 mM) was applied in the pilot plant.Marine environmental impact modeling was performed to observe the effect of discharge to the ocean and its movement. The results revealed a slight growth inhibition of phytoplankton at DIC concentrations higher than the base concentration. Nevertheless, the change in the DIC concentration exerted no effect on the phytoplankton growth except at extremely high concentrations. Moreover, the high DIC concentration can be diluted by the ocean current flow rate, thus counterbalancing the growth inhibition effect. The results obtained in this study demonstrate the feasibility of CO2 storage in the form of DIC, and will be helpful for further development of CO2 mitigation.

Fluorinated gases (F-gases), such as CHF3 and C2F6, which are used in the semiconductor industry and have considerable global warming potential, can be recovered after use through a gas hydrate-based separation method to prevent their release into the atmosphere. In this study, the guest distributions and dissociation enthalpy (ΔHd) of F-gas (CHF3 or C2F6)+N2 hydrates with different F-gas concentrations (CHF3: 20%, 80%, and 100% and C2F6: 20%, 60%, 80%, and 100%) were experimentally investigated using a powder X-ray diffractometer and a high-pressure micro-differential scanning calorimeter, respectively. At high N2 concentrations in the feed gas, the occupancy of N2 in the small (512) cages of the F-gas+N2 hydrates increased significantly. As a result, the F-gas+N2 hydrates exhibited reduced hydration numbers at high N2 concentration. The ΔHd values (in kJ/mol gas) of the F-gas (CHF3 or C2F6)+N2 hydrates decreased with increased N2 concentration. The overall experimental results provide useful insights into the design and operation of gas hydrate-based F-gas separation processes.

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.

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.

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 + N2 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 N2 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/N2 ratio in the hydrate structure was nearly constant over time. These results evidenced no remarkable difference in kinetic selectivity between SF6 and N2 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 + N2 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 + N2 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.

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 and exploitation of NGHs as well as natural gas storage and transportation. Methylcyclopentane (MCP) and 2,2-dimethylbutane [neohexane (NH)] were chosen as LMGSs, and the simplest composition of NGHs and natural gas was simulated by a gas mixture of CH4 (90%) + C2H6 (10%). The coexistence of structure II (sII) and structure H (sH) hydrates in the CH4 + C2H6 + LMGS + water mixtures was revealed by C-13 nuclear magnetic resonance (NMR) and powder X-ray diffraction (PXRD). CH4 and LMGSs were captured in sH, whereas CH4 and C2H6 were enclathrated in sII. Endothermic heat flow curves, which were obtained by a differential scanning calorimeter (DSC), and pressure (P)-temperature (T) traces exhibited two-step dissociation of formed gas hydrates (sH dissociation followed by sII dissociation). The experimental results for the PXRD patterns, C-13 NMR spectra, phase equilibria, DSC heating curves, and P-T traces clearly demonstrated that the CH4 (90%) + C2H6 (10%) + LMGS + water mixtures formed both sII hydrates and sH hydrates and that the final hydrate structure at equilibrium dissociation points was sII.

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 5(12) cages at the initial stage of hydrate formation, whereas glycine was effective in preventing CH4 molecules from entering large 5(12)6(2) 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.

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 thermodynamic stability and guest gas inclusion of tetra-iso-amyl ammonium bromide (TiAAB) semiclathrates with CO2, N-2, or CO2 (20%) + N-2 (80%), with a primary focus on semi-clathrate phase equilibria and Raman spectra. The three-phase (H-L-W-V) equilibria of TiAAB semi-clathrates with CO2, N-2, or CO2 (20%) + N-2 (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, N-2, or CO2 (20%) + N-2 (80%) were significantly stabilized compared with the corresponding CO2, N-2, or CO2 (20%) + N-2 (80%) gas hydrates. The enclathration of CO2 and N-2 molecules in the cages of TiAAB semiclathrates was clearly confirmed via Raman spectroscopy. The experimental results indicate that TiAAB semiclathrates can incorporate CO2 and N-2 into the cage lattices at elevated temperatures and lowered pressures and are potential materials for CO2 capture. (C) 2019 Elsevier Ltd.

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-LW-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 (ΔHd) 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.

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.

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 + N-2 gas hydrates were investigated for gas hydratebased C2F6 separation from emission sources. This experiment measured the three-phase (hydrate, liquid water, and vapor [H-L-W-V]) equilibria of ternary C2F6 (10, 20, 40, 60, and 80%) + N-2 + H2O systems and indicated the possible existence of hydrate azeotropes at certain temperature ranges. Powder X-ray diffraction (PXRD) revealed that the ternary C2F6 + N-2 + 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 + N-2 + 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. (C) 2017 Elsevier Ltd.

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 (CHF3 and C2F6) in order to verify the feasibility of the F-gas separation using gas hydrate formation. The three-phase (gas hydrate (H) – liquid water (LW) – vapor (V)) equilibria of the pure CHF3 and C2F6 hydrates are measured in order to examine the hydrate formation conditions. The PXRD patterns reveal the structure of the CHF3 hydrate and the C2F6 hydrate as a cubic structure I (sI) and structure II (sII), respectively. The enclathration of CHF3 and C2F6 molecules in each pure CHF3 and C2F6 hydrate is confirmed through 13C and 19F NMR analyses. In-situ Raman measurements are used to monitor the growth process of pure CHF3 hydrates, and they reveal the CHF3 molecules trapped in the sI large (51262) cages as well as in the sI small (512) cages. The computational study also demonstrates that CHF3 is encaged in both small (512) and large (51262) cages of the sI hydrate, whereas C2F6 only occupies the large (51264) cages of the sII hydrate.