Investigation of theoretical maximum water yield and efficiency-optimized temperature for cyclopentane hydrate-based desalination.

Hydrate-based desalination (HBD) shows promise as a freshwater production technology for saline water. Liquid-phase hydrate formers, with their ability to facilitate hydrate formation at atmospheric pressure, have gained attention for their high energy efficiency in HBD. This study explored cyclopentane (CP) HBD by experimentally measuring the thermodynamic properties of CP hydrate in saline solutions and developing a theoretical framework to estimate the water yield of CP HBD under various operating conditions. The measured dissociation enthalpy of CP hydrate was found to be 12 % and 22 % lower compared to those of propane and R134a hydrates, respectively. The equilibrium dissociation temperatures of CP hydrate at different NaCl concentrations under atmospheric pressure were experimentally measured and then predicted using the Hu-Lee-Sum correlation. The theoretically achievable maximum salinity and water yield for CP HBD were calculated in the temperature range of 268-280 K and the initial salinity range of 0-8 wt.%. Additionally, the concept of HBD heat efficiency, representing the maximum amount of pure water producible per unit of heat, was introduced to identify an optimal operating condition for the HBD process. Efficiency-maximized temperatures, where the HBD heat efficiency reached its peaks, were determined for various initial salinities in the process, for example, 273.4 K for NaCl 3.5 wt.% solution. This novel approach provides invaluable guidance for determining the most energy-efficient operating conditions in the HBD process and establishes a solid foundation for further advancements in this field.

Abnormal structural transformation of tetra-n-butyl ammonium chloride + Xe clathrates and its significance for clathrate-based Xe capture and storage.

Tetra-n-butyl ammonium chloride (TBAC) is a semi-clathrate former that can be used for clathrate-based gas capture and storage since TBAC semi-clathrate has vacant small cages available for entrapping gas molecules under mild conditions. In this study, the phase equilibria and structural information of TBAC + Xe + water systems were experimentally investigated at two different TBAC concentrations (1.0 and 3.3 mol%). The slopes of the three-phase (clathrate [H] - liquid [L] - vapor [V]) equilibrium lines for the TBAC + Xe + water systems altered at one or two points as the pressure and temperature changed, which indicates that this slope change might be caused by the structural transformation of the clathrates that were formed. The powder X-ray diffraction (PXRD) patterns, in situ Raman spectra, and 129Xe nuclear magnetic resonance (NMR) spectra demonstrated that the clathrate structure of the TBAC + Xe + water systems changed from tetragonal (P42/m) or orthorhombic (Pmma) to cubic (Pm3̄n) as the pressure increased. Surprisingly, in the higher-pressure region, TBAC acted as a thermodynamic inhibitor without being enclathrated in the clathrate lattices. The thermodynamic and structural information of the TBAC + Xe clathrates will be helpful for conceptualizing and designing the clathrate-based noble gas or radioactive gas capture and storage process.

Exploring psychopathological and cognitive factors associated with help-seeking intentions among Korean high school students: A cross-sectional study.

Competitive college admissions and academic pressure have continuously increased the psychopathological burden of Korean high school students. Seeking help is one of the primary means of managing mental health, and more attention is required. This study aimed to explore the psychopathological and cognitive factors related to the help-seeking intentions of Korean high school students. This cross-sectional study was conducted between July and August 2020 using the General Help-Seeking Questionnaire, Symptom Checklist-90-R, and Mental Health Literacy Scale. Four hundred and twenty-one Korean high school students (275 males, 146 females; average age 17.44 years [standard deviation = 0.651]) completed self-report questionnaires. We performed analysis of variance, Spearman's correlation analysis, and stepwise regression analysis to explore the factors related to help-seeking intentions. The final model showed an explanatory power of 23.6% for the overall variance in help-seeking intentions. Somatization (ß = -0.200; P = .001) and hostility (ß = -0.203; P = .001) had a negative effect on help-seeking intentions. Further, knowledge of where to seek information (ß = 0.230; P < .001) and attitudes promoting recognition and help-seeking behavior (ß = 0.095; P = .030) had a positive effect. Students responded to society's negative awareness of mental illness by converting psychopathology into socially acceptable symptoms. Educational support can improve mental health literacy. This study is expected to help improve mental illness awareness and increase adolescents' access to public services.

The impact of the abnormal salinity enrichment in pore water on the thermodynamic stability of marine natural gas hydrates in the Arctic region.

In this study, the thermodynamic and structural characteristics of natural gas hydrates (NGHs) retrieved from gas hydrate mounds (ARAON Mound 03 (AM03) and ARAON Mound 06 (AM06)) in the Chukchi Sea in the Arctic region were investigated. The gas compositions, crystalline structure, and cage occupancy of the NGHs at AM03 and AM06 were experimentally measured using gas chromatography (GC), 13C nuclear magnetic resonance (NMR), Raman spectroscopy, and powder X-ray diffraction (PXRD). In the NGHs from AM03 and AM06, a significantly large fraction of CH4 (> 99%) and a very small amount of H2S were enclathrated in small (512) and large (51262) cages of sI hydrate. The NGHs from AM03 and AM06 were almost identical in composition, guest distributions, and existing environment to each other. The salinity of the residual pore water in the hydrate-bearing sediment (AM06) was measured to be 50.32‰, which was much higher than that of seawater (34.88‰). This abnormal salinity enrichment in the pore water of the low-permeability sediment might induce the dissociation of NGHs at a lower temperature than expected. The saturation changes in the NGHs that corresponded with an increase in the seawater temperature were also predicted on the basis of the salinity changes in the pore water. The experimental and predicted results of this study would be helpful for understanding the thermodynamic stability of NGHs and potential CH4-releasing phenomena in the Arctic region.

Thermodynamic and structural features of chlorodifluoromethane (a sI-sII dual hydrate former) + external guest (N2 or CH4) hydrates and their significance for greenhouse gas separation.

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.

Evaluation of kinetic salt-enrichment behavior and separation performance of HFC-152a hydrate-based desalination using an experimental measurement and a thermodynamic correlation.

Hydrate-based desalination (HBD), a type of freezing-based desalination, can concentrate salts of saline water and produce fresh water via hydrate crystal formation. In this study, the thermodynamic stability, crystallographic information, and kinetic growth behavior of HFC-152a hydrate were investigated to estimate the desalination efficiency of HBD. The phase equilibria revealed that the HFC-152a hydrate could be formed at a higher temperature in the presence of NaCl (0 wt%, 3.5 wt%, and 8.0 wt%) than the HFC-134a hydrate at 0.3 MPa. The hydration number of the HFC-152a hydrate (sI) was found to be 7.74 through the Rietveld refinement of the powder X-ray diffraction patterns, and it was also used to determine the dissociation enthalpy of the HFC-152a hydrate. The Hu-Lee-Sum correlation was employed to predict the equilibrium shift and hydrate depression temperature of both HFC-152a and HFC-134a hydrates in the presence of NaCl. Faster hydrate growth kinetics and higher hydrate conversion were observed for the HFC-152a hydrate in saline solutions despite the smaller initial driving force at 0.3 MPa and the subcooling temperature of 3 K. Additionally, to quantify the desalination efficiency of the HFC-152a HBD, the maximum achievable salinity and maximum water yield were examined using the HLS correlation. The salt-enrichment efficiency decreased with an increase in the initial salinity and increased with increasing the subcooling. The overall results indicate that HFC-152a is, potentially, a superior candidate for HBD. The novel approach examined in this study will be useful for assessing the desalination efficiency of the HBD process.

Connectivity-informed drainage network generation using deep convolution generative adversarial networks.

Stochastic network modeling is often limited by high computational costs to generate a large number of networks enough for meaningful statistical evaluation. In this study, Deep Convolutional Generative Adversarial Networks (DCGANs) were applied to quickly reproduce drainage networks from the already generated network samples without repetitive long modeling of the stochastic network model, Gibb's model. In particular, we developed a novel connectivity-informed method that converts the drainage network images to the directional information of flow on each node of the drainage network, and then transforms it into multiple binary layers where the connectivity constraints between nodes in the drainage network are stored. DCGANs trained with three different types of training samples were compared; (1) original drainage network images, (2) their corresponding directional information only, and (3) the connectivity-informed directional information. A comparison of generated images demonstrated that the novel connectivity-informed method outperformed the other two methods by training DCGANs more effectively and better reproducing accurate drainage networks due to its compact representation of the network complexity and connectivity. This work highlights that DCGANs can be applicable for high contrast images common in earth and material sciences where the network, fractures, and other high contrast features are important.

Influence of Competitive Inclusion of CO2 and N2 on sII Hydrate-Flue Gas Replacement for Energy Recovery and CO2 Sequestration.

This study investigated the structural transformation, guest distributions, and the extent of replacement in CH4 + C3H8-flue gas replacement occurring in sII hydrates via gas chromatography, NMR spectroscopy, and powder X-ray diffraction (PXRD). Simulated flue gas (CO2 (20%) + N2 (80%)) was injected into an sII CH4 (90%) + C3H8 (10%) hydrate for guest exchange. The extent of replacement occurring in CH4 + C3H8-flue gas replacement was much lower than that of CH4 + C3H8-CO2 replacement. Furthermore, 13C NMR spectra and PXRD patterns revealed that unlike CH4 + C3H8-CO2 replacement, CH4 + C3H8-flue gas replacement did not undergo any structural transformation during the replacement (i.e., iso-structural replacement in the sII hydrate). Rietveld refinement of PXRD patterns of gas hydrates after replacement using flue gas injection demonstrated that CO2 molecules occupied both the small (512) and large (51264) cages, whereas N2 molecules occupied only the small (512) cages. CO2 and N2 were not complementary but competitive in replacing CH4 in the small (512) cages, which contributed to the maintenance of the cage stability of the initial sII hydrate and thus, resulted in a lower extent of replacement. The experimental results obtained in this study provide valuable insights on the accurate replacement mechanism and cage-specific guest exchange behavior of sII hydrates using flue gas injection for energy recovery and CO2 sequestration.

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-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.

Spatial and temporal variations of volatile organic compounds using passive air samplers in the multi-industrial city of Ulsan, Korea.

The source-receptor relationship of volatile organic compounds (VOCs) is an important environmental concern, particularly in large industrial cities; however, only a few studies have identified VOC sources using high spatial resolution data. In this study, 28 VOCs were monitored in Ulsan, the biggest multi-industrial city in Korea. Passive air samplers were seasonally deployed at eight urban and six industrial sites. The target compounds were detected at all sites. No significant seasonal variations of VOCs were observed probably due to the continuous emissions from major industrial facilities. Benzene, toluene, ethylbenzene, xylenes, and styrene accounted for 66-86% of the concentration of Σ28 VOCs. The spatial distribution of the individual VOCs clearly indicated that petrochemical, automobile, non-ferrous, and shipbuilding industries were major VOC sources. Seasonal wind patterns were found to play a role in the spatial distribution of VOCs. Diagnostic ratios also confirmed that the industrial complexes were the dominant VOC sources. The results of principal component analysis and correlation analyses identified the influence of specific compounds from each industrial complex on individual sites. To the best of our knowledge, this is the first comprehensive report on the seasonal distribution of VOCs with high spatial resolution in a metropolitan industrial city in Korea.

Enhanced CH4 Recovery Induced via Structural Transformation in the CH4/CO2 Replacement That Occurs in sH Hydrates.

The CH4/CO2 replacement that occurs in sH hydrates is investigated, with a primary focus on the enhanced CH4 recovery induced via structural transformation with a CO2 injection. In this study, neohexane (NH) is used as a liquid hydrocarbon guest in the sH hydrates. Direct thermodynamic measurements and spectroscopic identification are investigated to reveal the replacement process for recovering CH4 and simultaneously sequestering CO2 in the sH (CH4 + NH) hydrate. The hydrate phase behavior and the (13)C NMR and Raman spectroscopy results of the CH4 + CO2 + NH systems demonstrate that CO2 functions as a coguest of sH hydrates in CH4-rich conditions, and that the structural transition of sH to sI hydrates occurs in CO2-rich conditions. CO2 molecules are found to preferentially occupy the medium 4(3)5(6)6(3) cages of sH hydrates or the large 5(12)6(2) cages of sI hydrates during the replacement. Due to the favorable structural transition and resulting re-establishment of guest distributions, approximately 88% of the CH4 is recoverable from sH (CH4 + NH) hydrates with a CO2 injection. The hydrate dissociation and subsequent reformation caused by the structural transformation of sH to sI is also confirmed using a high-pressure microdifferential scanning calorimeter through the detection of the significant heat flows generated during the replacement.

Incorporation of ammonium fluoride into clathrate hydrate lattices and its significance in inhibiting hydrate formation.

The stability of hydrate frameworks is influenced by guest molecules capable of hydrogen bonding with surrounding water molecules. Four remarkable features from the ammonium fluoride incorporation into a crystalline hydrate matrix provide important information on the thermodynamic stability, formation kinetics, structural characteristics, and molecular behavior in clathrate hydrate systems.

Experimental verification of methane-carbon dioxide replacement in natural gas hydrates using a differential scanning calorimeter.

The methane (CH4) - carbon dioxide (CO2) swapping phenomenon in naturally occurring gas hydrates is regarded as an attractive method of CO2 sequestration and CH4 recovery. In this study, a high pressure microdifferential scanning calorimeter (HP µ-DSC) was used to monitor and quantify the CH4 - CO2 replacement in the gas hydrate structure. The HP µ-DSC provided reliable measurements of the hydrate dissociation equilibrium and hydrate heat of dissociation for the pure and mixed gas hydrates. The hydrate dissociation equilibrium data obtained from the endothermic thermograms of the replaced gas hydrates indicate that at least 60% of CH4 is recoverable after reaction with CO2, which is consistent with the result obtained via direct dissociation of the replaced gas hydrates. The heat of dissociation values of the CH4 + CO2 hydrates were between that of the pure CH4 hydrate and that of the pure CO2 hydrate, and the values increased as the CO2 compositions in the hydrate phase increased. By monitoring the heat flows from the HP µ-DSC, it was found that the noticeable dissociation or formation of a gas hydrate was not detected during the CH4 - CO2 replacement process, which indicates that a substantial portion of CH4 hydrate does not dissociate into liquid water or ice and then forms the CH4 + CO2 hydrate. This study provides the first experimental evidence using a DSC to reveal that the conversion of the CH4 hydrate to the CH4 + CO2 hydrate occurs without significant hydrate dissociation.

CO2 capture from simulated fuel gas mixtures using semiclathrate hydrates formed by quaternary ammonium salts.

In order to investigate the feasibility of semiclathrate hydrate-based precombustion CO2 capture, thermodynamic, kinetic, and spectroscopic studies were undertaken on the semiclathrate hydrates formed from a fuel gas mixture of H2 (60%) + CO2 (40%) in the presence of quaternary ammonium salts (QASs) such as tetra-n-butylammonium bromide (TBAB) and fluoride (TBAF). The inclusion of QASs demonstrated significantly stabilized hydrate dissociation conditions. This effect was greater for TBAF than TBAB. However, due to the presence of dodecahedral cages that are partially filled with water molecules, TBAF showed a relatively lower gas uptake than TBAB. From the stability condition measurements and compositional analyses, it was found that with only one step of semiclathrate hydrate formation with the fuel gas mixture from the IGCC plants, 95% CO2 can be enriched in the semiclathrate hydrate phase at room temperature. The enclathration of both CO2 and H2 in the cages of the QAS semiclathrate hydrates and the structural transition that results from the inclusion of QASs were confirmed through Raman and (1)H NMR measurements. The experimental results obtained in this study provide the physicochemical background required for understanding selective partitioning and distributions of guest gases in the QAS semiclathrate hydrates and for investigating the feasibility of a semiclathrate hydrate-based precombustion CO2 capture process.

2-Propanol as a co-guest of structure II hydrates in the presence of help gases.

The enclathration of 2-propanol (2-PrOH) as a co-guest of structure II (sII) hydrates in the presence of CH4 and CO2 was experimentally verified with a focus on macroscopic phase behaviors and microscopic analytical methods such as powder X-ray diffraction (PXRD) and NMR spectroscopy. 2-PrOH functioned as a hydrate promoter in the CH4 + 2-PrOH systems, whereas it functioned as an apparent hydrate inhibitor in the CO2 + 2-PrOH systems despite the inclusion of 2-PrOH in the hydrate lattices. From the PXRD patterns, both double CH4 + 2-PrOH and double CO2 + 2-PrOH hydrates were identified to be cubic (Fd3m) sII hydrates. From the (13)C NMR spectra, it was found that, at a lower 2-PrOH concentration, the small 5(12) cages of the sII hydrate were occupied by CH4 molecules only, whereas the large 5(12)6(4) cages were shared by CH4 and 2-PrOH molecules. However, at a stoichiometric concentration, the large cages were occupied by 2-PrOH molecules only, and the corresponding chemical formula for this concentration is 1.50CH4·0.98 2-PrOH·17H2O.

Structural transformation of isopropylamine semiclathrate hydrates in the presence of methane as a coguest.

Guest-induced structural transformation in amine semiclathrate hydrates is a unique pattern caused by modifying the hydrophobic-hydrophilic balance, and thus, it can be applied to potential gas storage and transportation areas. The experimental results of the structural transformation of isopropylamine (IPA) semiclathrate hydrates in the presence of methane (CH(4)) as a coguest are presented with a focus on the macroscopic phase behavior and microscopic analytical methods such as powder X-ray diffraction (PXRD) and NMR spectroscopy. The introduction of CH(4) molecules as coguests changed the structure of the IPA·8.0H(2)O semiclathrate hydrates (hexagonal, P6(3)/mmc) to sII gas hydrates (cubic, Fd3m). The microscopic analysis results indicate that the guest gas distribution and the clathrate hydrate composition can be altered with adjustment of the IPA concentration. The overall experimental results are valuable for increased understanding of the stability conditions, structural details, and guest-host interactions in hydrophobic guest gas + IPA clathrate hydrates.

Thermodynamic and spectroscopic identification of guest gas enclathration in the double tetra-n-butylammonium fluoride semiclathrates.

The precise nature and unique pattern of the double tetra-n-butylammonium fluoride (TBAF) semiclathrates with a guest gas (CH(4) or CO(2)) was closely investigated through thermodynamic and spectroscopic analyses. The three-phase equilibria of semiclathrate (H), liquid water (L(W)), and vapor (V) for the ternary CH(4) + TBAF + water and CO(2) + TBAF + water mixtures with various TBAF concentrations were experimentally measured in order to determine the stability conditions of the double TBAF semiclathrates. The double CH(4) (or CO(2)) + TBAF semiclathrates showed remarkably enhanced thermal stability when compared with pure CH(4) (or CO(2)) hydrate. The highest stabilization effect was observed at the stoichiometric concentration of pure TBAF semiclathrate, which is 3.3 mol %. Gas uptake measurements were undertaken in order to estimate the amount of gas consumed during double semiclathrate formation. CH(4) was found to be a relatively more favorable guest for the 5(12) cages of the double TBAF semiclathrate than CO(2). From the results of the NMR and Raman spectroscopic analyses it was identified that the guest gas molecules (CH(4) or CO(2)) were enclathrated in the 5(12) cages of the double TBAF semiclathrates. The overall results given in this study are useful for understanding the fundamental guest gas enclathration behavior in the double semiclathrates.

Guest gas enclathration in semiclathrates of tetra-n-butylammonium bromide: stability condition and spectroscopic analysis.

In this study, guest gas enclathration behavior in semiclathrates of tetra-n-butylammonium bromide (TBAB) was closely investigated through phase equilibrium measurement and spectroscopic analysis. The three-phase equilibria of semiclathrate (H), liquid water (L(W)), and vapor (V) for the ternary CH(4) + TBAB + water and CO(2) + TBAB + water mixtures with various TBAB concentrations were experimentally measured to determine the stability conditions of the double TBAB semiclathrates. Equilibrium dissociation temperatures for pure TBAB semiclathrate were also measured at the same concentrations under atmospheric conditions. The dissociation temperature and dissociation enthalpy of pure TBAB semiclathrate were confirmed by differential scanning calorimetry. The experimental results showed that the double CH(4) (or CO(2)) + TBAB semiclathrates yielded greatly enhanced thermal stability when compared with pure CH(4) (or CO(2)) hydrate. The highest stabilization effect was observed at the stoichiometric concentration of pure TBAB semiclathrate, which is 3.7 mol%. From the NMR and Raman spectroscopic studies, it was found that the guest gases (CH(4) and CO(2)) were enclathrated in the double semiclathrates. In particular, from the cage-dependent (13)C NMR chemical shift, it was confirmed that CH(4) molecules were captured in the 5(12) cages of the double semiclathrates.

Phase behavior and 13C NMR spectroscopic analysis of the mixed methane + ethane + propane hydrates in mesoporous silica gels.

In this study, the phase behavior and quantitative determination of hydrate composition and cage occupancy for the mixed CH(4) + C(2)H(6) + C(3)H(8) hydrates were closely investigated through the experimental measurement of three-phase hydrate (H)-water-rich liquid (L(W))-vapor (V) equilibria and (13)C NMR spectra. To examine the effect of pore size and salinity, we measured hydrate phase equilibria for the quaternary CH(4) (90%) + C(2)H(6) (7%) + C(3)H(8) (3%) + water mixtures in silica gel pores of nominal diameters of 6.0, 15.0, and 30.0 nm and for the quinary CH(4) (90%) + C(2)H(6) (7%) + C(3)H(8) (3%) + NaCl + water mixtures of two different NaCl concentrations (3 and 10 wt %) in silica gel pores of a nominal 30.0 nm diameter. The value of hydrate-water interfacial tension for the CH(4) (90%) + C(2)H(6) (7%) + C(3)H(8) (3%) hydrate was found to be 47 ± 4 mJ/m(2) from the relation of the dissociation temperature depression with the pore size of silica gels at a given pressure. At a specified temperature, three-phase H-L(W)-V equilibrium curves of pore hydrates were shifted to higher pressure regions depending on pore sizes and NaCl concentrations. From the cage-dependent (13)C NMR chemical shifts of enclathrated guest molecules, the mixed CH(4) (90%) + C(2)H(6) (7%) + C(3)H(8) (3%) gas hydrate was confirmed to be structure II. The cage occupancies of each guest molecule and the hydration number of the mixed gas hydrates were also estimated from the (13)C NMR spectra.

Separation of SF6 from gas mixtures using gas hydrate formation.

This study aims to examine the thermodynamic feasibility of separating sulfur hexafluoride (SF(6)), which is widely used in various industrial fields and is one of the most potent greenhouse gases, from gas mixtures using gas hydrate formation. The key process variables of hydrate phase equilibria, pressure-composition diagram, formation kinetics, and structure identification of the mixed gas hydrates, were closely investigated to verify the overall concept of this hydrate-based SF(6) separation process. The three-phase equilibria of hydrate (H), liquid water (L(W)), and vapor (V) for the binary SF(6) + water mixture and for the ternary N(2) + SF(6) + water mixtures with various SF(6) vapor compositions (10, 30, 50, and 70%) were experimentally measured to determine the stability regions and formation conditions of pure and mixed hydrates. The pressure-composition diagram at two different temperatures of 276.15 and 281.15 K was obtained to investigate the actual SF(6) separation efficiency. The vapor phase composition change was monitored during gas hydrate formation to confirm the formation pattern and time needed to reach a state of equilibrium. Furthermore, the structure of the mixed N(2) + SF(6) hydrate was confirmed to be structure II via Raman spectroscopy. Through close examination of the overall experimental results, it was clearly verified that highly concentrated SF(6) can be separated from gas mixtures at mild temperatures and low pressure conditions.