sI Hydrate Research Articles

Electric Field Influence on CO Clathrate Hydrates.

We consider carbon monoxide (CO) confined in the hydrogen-bonded building blocks of sI and sII clathrate hydrates, viz., (512, 51262, 51264) cages, within the density functional theory-based calculations. We study their response to the applied electric fields in terms of changes in the geometrical parameters, dipole moment, HOMO-LUMO gap, and vibrational frequency shift. We examine the stability of CO clathrate hydrate building block cages and identify a possible indication of the field-induced release of CO.

Natural Structural Transition of Gas Hydrates From sI to sII in the Deep Seafloor

AbstractThe evolution of gas hydrates influenced by the seawater environment is unknown. We present a model of structural transformation from sI hydrate to sII hydrate due to the influence of seawater environment and vent fluid in nature through in situ experiments of gas hydrate formation in the Haima cold seep area. The in situ experimental results indicate that gas hydrates preferentially form as sI hydrates even in cold seep environments where C2+ hydrocarbons are present. During subsequent evolution, the sI hydrates could restructured at the effect of seawater environment and vent fluid, causing transformation to sII hydrates under the influence of hydrate stability. The supply of gas and direct contact with seawater environment are critical factors for structural transformation. Such structural transformation is the result of gas hydrates seeking thermodynamic stability and may be common in active cold seep areas.

Promoting thermodynamic stability of hydrogen hydrates with gas-phase modulators for energy-efficient blue hydrogen storage

As the global climate crisis intensifies with rising carbon dioxide (CO2) concentrations, blue hydrogen (H2) produced through the integration of CO2 capture technologies is emerging as a promising alternative energy source and is challenging traditional fossil fuels in the de-fossilization paradigm. Gas hydrate could be one of the feasible storage medium for energy-efficient H2 storage, due to its potential for eco-friendly features and high H2 storage capacity. However, extreme thermodynamic formation conditions for pure H2 hydrate need to be mitigated for practical application. Here, we demonstrate a novel approach by blending a ‘gas-phase modulator’ with H2, specifically a C2H6 (70%) + C3H8 (30%), to enable hydrate-based H2 storage under moderate thermodynamic formation conditions. In this study, we systematically examined thermodynamic stability, crystallographic structure, guest occupation behavior, and the formation kinetics of the C2H6 + C3H8 + H2 hydrates by varying the H2 composition. The results revealed that the C2H6 + C3H8 mixture played a notable role in promoting thermodynamic formation condition of H2 hydrates. The mixed C2H6 + C3H8 + H2 hydrates were predominantly comprised of sII hydrate, with small amount of sI hydrate. Considering the small to large cage ratio of sI and sII hydrate, this observation indicates a significant advantage for H2 storage due to primarily occupation behavior of H2 into the small cages. In addition, as the H2 composition in the feed gas increased, we confirmed that not only that H2 storage in the hydrate was enhanced but also that the degree of the H2 enclathration ratio into the hydrate was elevated. Moreover, the maximum composition of H2 in the C2H6 (14%) + C3H8 (6%) + H2 (80%) hydrate was 44.37%. Hence, we believe that our findings can contribute valuable insights into the potential application of gas-phase modulators for hydrate-based H2 storage, we emphasize that C2H6 + C3H8 can act as a potential gas-phase modulator for energy-efficient H2 storage.

HTR+: a novel algorithm for identifying type and polycrystal of gas hydrates

In this work, the hierarchical topology ring (HTR+) algorithm, an extension of the HTR algorithm, was developed for identifying gas hydrate types, cage structures, and grain boundaries (GBs) within polycrystalline structures. Utilizing molecular dynamics trajectories of polycrystalline hydrates, the accuracy of the HTR+ algorithm is validated in identifying sI, sII and sH hydrate types, hydrate grains, and GBs in multi-hydrate polycrystals, as well as clathrate cages at GBs. Additionally, during the hydrate nucleation and growth processes, clathrate cages, hydrate type, hydrate grains and ice structures are accurately recognized. Significantly, this algorithm demonstrates high efficiency, particularly for large hydrate systems. HTR+ algorithm emerges a powerful tool for identifying micro/mesoscopic structures of gas hydrates, enabling an in-depth understanding of the formation mechanisms and properties of gas hydrates.

Experimental investigation on the methane storage by forming sH hydrate

Methane storage via clathrate hydrates is a promising method, the structure H (sH) hydrate can be formed at more moderate conditions compared to structure I (sI) hydrate and has a larger storage capacity than structure II (sII) hydrate. In this work, CH4 storage by forming sH hydrate in the tert-butyl methyl ether (TBME) solution was explored at an initial pressure of 5.0 MPa and within the temperature range of 274.15– 278.15 K. The CH4/TBME hydrate phase equilibrium condition was first determined with the isochoric pressure searching method, and the results revealed that the measured equilibrium data of CH4/TBME hydrate at different TBME concentrations were almost consistent. The effect of TBME concentration (0– 4.29 mol%), promoter type and sub-cooling on the hydrate formation kinetics and gas uptake was systematically investigated. The results showed that the hydrate formation rate increased with the increase of TBME concentration. The CH4 uptake achieved 117.7 ± 3.6 v/vw (volume ratio of gas to water at standard condition) in the 4.29 mol% TBME system, which was significantly increased than that of pure water (44.1 ± 5.0 v/vw). The cocamidopropyl dimethylamine (CDA) further improved the CH4/TBME hydrate formation kinetics and gas uptake, the CH4 storage capacity attained 145.6 ± 1.7 v/vw, which was slightly higher than that of the sodium dodecyl sulfate (SDS) system (143.6 ± 3.0 v/vw). As the sub-cooling increased, the formation kinetic profiles of CH4/TBME hydrate changed from a two-step to a one-step drop, indicating that the shift of mixed sH and sI hydrates to single sH hydrate. Additionally, the time taken for hydrate growth in the presence of TBME was significantly decreased than the pure water system, and the adding of CDA and SDS further shorten it. The CH4 recovery in each test was above 95.0 % and no foam generated during hydrate dissociation for the CDA-contained system. The stability test of CH4/TBME hydrate indicated that around 86 % CH4 was still enclosed in the hydrate phase after one week. This study would contribute to the application of the hydrate-based methane storage technology in future.

Natural self-assembled supramolecules as green inhibitors against methane hydrate formation

Environmentally friendly hydrate inhibitors are the most promising method to solve the environmental pollution of inhibitors, among which Safranine O (SO) with antifreeze protein properties shows excellent application prospects. SO will self-assemble into macromolecules under hydrogen bonding and electrostatic action in an aqueous solution. The assembled macromolecules' hydrophobic and hydrophilic functional groups are regularly arranged with the properties of an antifreeze protein. In the crystal growth inhibition (CGI) test, the supercooling degree of the complete inhibition region (CIR) of 1.0 wt% SO solution for SI hydrate was significantly higher than that of 1.0 wt% PVP solution. Further, the acidic environment did not affect the inhibitory effect of SO on hydrate. The complete inhibition range of 1.0 wt% SO solution on SI hydrate at pH 1.4 and 7.1 was 7.41 °C and 8.35 °C, respectively. These results suggest that SO-based KHIs may provide new ideas for developing highly efficient acid-resistant hydrate inhibitors.

Adsorption behaviors for clathrate hydrates of CO2 with mixed gases

Here, the preference and adsorption capacities of guest molecules in the sI and sII hydrate of water cages when CO2 + H2S, CO2 + SO2 and CO2 + N2O gas mixture form hydrates are simulated by using grand canonical Monte Carlo and molecular dynamics hybrid simulations method. The results indicate that SO2 prefers to occupy large cages, H2S prefers small cages for CO2 + H2S and CO2 + SO2 and N2O exhibits little preference between small or large or cages of CO2 + N2O sI hydrates, respectively. The abundance ratio shows hydrate can effectively achieve the separation of flue gases. Moreover, the thermodynamics calculation results show that the impurity gases H2S, SO2 and N2O favors the formation of CO2 hydrate. The stability CO2 + H2S, CO2 + SO2 and CO2 + N2O sI hydrates is higher compared to the stability of sII hydrates. The phase equilibrium analysis provides clear evidence that enclathration of H2S and SO2 molecules into the water cages of the hydrate structure plays a crucial role in stabilizing the hydrate lattices. The findings of this study suggest that removal of certain flue gas impurities is not required during the replacement of CH4 in hydrate form with the CO2 gas. The current investigation provides theoretical background for executing replacement of CH4 in hydrate form with the CO2 gas.

Accurate description of gas hydrates of carbon dioxide and hydrogen for storage and transportation

This paper reports theoretical study of stability zones and composition of “H2 + CO2” clathrate hydrates of cubic structure I (sI) and II (sII) in dependence on pressure and temperature for different gas phase composition using extended van der Waals and Platteeuw statistical thermodynamic model. It was found that rising of CO2 content in H2 + CO2 gas mixture phase makes formation of sI hydrate more thermodynamically preferable rather than sII hydrate, and in the same time CO2 content increasing reduces formation pressure of the resulting hydrate. Observed difference between gas phase composition and gas molecules trapped in hydrate phase allows to consider these hydrates as a gas separation medium. We believe that presented results could be applicable for improvements of hydrate technology for CO2 utilization as a greenhouse gas, and hydrogen obtained from natural gas refinement transportation.

Dielectric and optical properties of gas hydrates and ice: <i>ab initio</i> simulation results

The dielectric tensors εik(ω), optical absorption spectra R(ω), reflection spectra a(ω) and the density of electronic states N(E) have been calculated for the lattices of gas hydrates with the structure sI and sH, as well as for ice Ih and III. The dependence of the dielectric permittivity on the type of guest molecules in the sI hydrate is considered.

Influence of structural transformation on guest exchange behavior in the sII hydrate – (CO2 + N2) replacement for energy recovery and CO2 sequestration

In this study, the guest exchange behavior in the sII (CH4 + C3H8) hydrate – (CO2 + N2) replacement at various CO2 concentrations was experimentally investigated to elucidate the influence of the structural transformation of the initial sII hydrate on the replacement efficiency at different feed gas compositions and injecting pressures. The extent of replacement, structural identification, and cage occupancy of guest molecules in the replaced hydrates were determined by a combination of gas chromatography (GC), powder X-ray diffraction (PXRD), and 13C NMR spectroscopy. The experimental results demonstrated that injecting feed gas with higher CO2 concentrations (%CO2) at a higher partial pressure of CO2 (PCO2) resulted in a higher weight fraction of sI hydrates (a greater degree of structural transformation from sII to sI) after replacement and consequently, higher replacement efficiency. The newly formed sI hydrates after replacement were primarily composed of CO2 and N2. The guest-inclusion behavior in the small (512) cages of the sII hydrates after replacement, as revealed by Rietveld refinement of the PXRD patterns, had a dominant influence on the total CO2/N2 ratios in the replaced hydrates. The findings of this study offer valuable insights into the guest exchange mechanism occurring in sII hydrates during flue gas injection and can aid in estimating the optimal compositions of flue gas for energy recovery and CO2 sequestration through guest replacement in natural gas hydrates.

Experimental investigation on the synergistic influence of tetra-n-butyl ammonium bromide(TBAB) and cyclopentane(CP) in hydrate-based gas separation

To enhance the CO2 separation efficiency of hydrate-based gas separation (HBGS) technology, the kinetic process of hydrate formation from synthesized CH4/CO2/N2 gas mixtures in the presence of cyclopentane (CP) and tetra-n-butyl ammonium bromide (TBAB) and gas separation efficiency were studied at an initial pressure of 7.0 MPa and temperatures ranging from 275.1 to 290.1 K. The crystallographic properties of the mixed hydrates were also evaluated by Raman, powder X-ray diffraction(XRD) and cryo-scanning electron microscop(cryo-SEM). Results showed that the combination of CP and TBAB tended to inhibit the separation efficiency compared with pure CP. The maximum CH4 recovery was 1.37 and the CO2 split ratio was 0.791 when using CP and TBAB solution. The order of CO2 split ratio in the different systems followed the order SDS > CP > TBAB/CP > CP/SDS > TBAB. Meanwhile, the adsorption capacity and gas consumption also decreased with an increase in TBAB concentration. Raman and XRD tests revealed that sI, sII, and semi-clathrate hydrates were found to coexist after hydrate formation and no other types of hydrates was formed. CH4 could only enter the small cages (512) of the hydrate. SEM images showed that the hydrate crystals were ellipsoid-shaped with different sizes and stacked together to form a globular structure, suggesting that the specific surface area of the hydrate crystals enhanced with the increasing of TBAB concentration. These findings can be used to evaluate the feasibility of applying TBAB and CP in HBGS technology.

Highly-selective xenon–krypton separation using hydrate-based technology

Xenon (Xe) and krypton (Kr) are used in many important industries such as lighting, lasers, double glazing for insulation, and medicine. As Xe and Kr are found in extremely low concentrations in the atmosphere, only small quantities of the mixture of these gases are obtained as byproducts from air separation processes. As the further separation of Kr from Xe is achieved via a costly energy-intensive cryogenic distillation method, the development of a new cost-effective approach for Xe–Kr separation is essential. In this study, we demonstrated that clathrate hydrates can be used as a promising Xe trap to separate a Xe–Kr mixture and produce Kr-rich vapors. The Xe and Kr guest distributions in mixed clathrate hydrates were experimentally identified using powder X-ray diffraction, solid-state 129Xe NMR spectroscopy, and gas chromatography. The mixed Xe + Kr hydrates have a Pm3¯n structure, termed structure I (sI), regardless of the concentration of Xe in the feed vapor in the range of 3% to 100%, and the sI hydrate selectively captures Xe from the feed mixture. The phase equilibria of the mixed Xe + Kr hydrates were also investigated, and it was confirmed that the hydrate-based Xe–Kr separation process requires milder temperature conditions than the cryogenic distillation process. Overall, the findings in this work suggest that the hydrate-based Xe–Kr separation process is an excellent alternative to the energy-intensive cryogenic distillation method.

Hydrogen solubility in gas hydrates with various auxiliary guests. Methane hydrate case study and comparison with the literature data

The experimental data on hydrogen solubility in methane gas hydrate and new data on phase equilibria in the hydrogen - methane - water system are presented in this work. The solubility data were obtained for hydrogen partial pressures from 0.8 to 5.4 MPa. It was found that hydrogen solubility in methane and carbon dioxide hydrates is almost the same and the dependence of hydrogen content on hydrogen partial pressure can be fitted with the equation ln(CH2) = −4.332(54) + 0.919(62)-ln(PH2), where CH2 is the hydrogen content in the hydrate (wt%) and PH2 is the hydrogen partial pressure (MPa). The data obtained in this work were compared with available data on hydrogen solubility in sI and sII hydrates. Analysis of data on the solubility of hydrogen in sII double hydrates with different thermodynamic promoters (tetrahydrofuran, cyclopentane, propane, tetrathiophene, furan) taken from various literature sources also shows a relatively small scatter with respect to the straight line approximating them. In general, the fraction of free small 512 cages filled with hydrogen decreases in the following order: sI methane hydrate > sI CO2 hydrate > sII hydrates.

Coupling Amino Acid with THF for the Synergistic Promotion of CO2 Hydrate Micro Kinetics: Implication for Hydrate-Based CO2 Sequestration

CO2 capture and sequestration are broadly recognized as the most effective technology for reducing CO2 emissions and mitigating global climate change. In this regard, hydrate-based CO2 sequestration (HBCS) has been proposed as an effective method for long-term safe and stable CO2 sequestration. It is a promising technology for reducing the concentration of CO2 in the atmosphere and achieving carbon neutrality. However, one major challenge related to HBCS technology is the lack of understanding of how to improve the formation kinetics of CO2 hydrate effectively. Herein, in this study, we identified the synergistic promotion effect on CO2 hydrate formation when a low-dose thermodynamic promoter (tetrahydrofuran, THF) is coupled with an environment-friendly kinetic promoter (l-methionine, l-Met). Based on the kinetic experiments and the morphological observation, 0.6 mol % THF coupled with 0.1 wt %l-Met yields the optimal CO2 gas uptake. In situ Raman spectra reveal the mechanism of the observed two-step growth behavior at the molecular level, where low-dose THF promotes the initial CO2-THF sII hydrate nucleation and growth in the solution and l-Met promotes the subsequent CO2 sI hydrate growth along the surface of the reactor. Our study unveils the underlying synergistic mechanism when thermodynamic and kinetic promoters are coupled for CO2 hydrate formation. The results provide guidance for the identification and use of novel bio-friendly thermodynamic and kinetic promoters and are beneficial to a series of sustainable hydrate-based technologies when fast CO2 formation is required.

Structural Stabilization of D- and T-Cages of the sI Hydrate by Gas Molecules

Structural Stabilization of D- and T-Cages of the sI Hydrate by Gas Molecules

Effects of X (X = H2S, SO2 and N2O) mole fractions on adsorption behavior and phase equilibrium properties of CO2 + X mixed gas hydrate

As a remedy for air pollution caused by flue gas, Hydrate Based Gas Separation (HBGS) technology is under development. Among many issues in HBGS, the prediction for the adsorption and dissociation conditions of hydrate and the impact from mixed gas on the formation of carbon dioxide hydrate are crucial to understand the sequestration and separation of CO2. In this work, the occupancy isotherms of the mixed systems of CO2 + H2S, CO2 + SO2 and CO2 + N2O with different mole fractions of H2S, SO2 and N2O in sI and sII hydrates are examined. The results show that X (H2S, SO2 and N2O) in flue gases can increase the absorption of mixed gas; CO2 + H2S in the sI and sII hydrates can be categorized as the one-site Langmuir type. The calculated abundance ratio of CO2 to X vary with the mole fraction of impurity gas and pressure, which provide the prerequisite information for prediction of gas recovery yield at different conditions during the CO2 + X gas separate process. The phase equilibria of clathrate hydrates of mixed gases are predicted and the hydration numbers are determined. The results show that the dissociation pressure of mixed hydrates decreases as the impurity gas mole fraction increases, indicating that SO2, H2S and N2O gases can all promote the formation of CO2 hydrate. Additionally, inspection indicates that the mixed flue gas can be efficiently separate by clathrate hydrate.

Thermodynamics, Kinetics, Morphology, and Raman studies for sH Hydrate of Methane and Cyclooctane

Natural gas is expected to be the major energy source in the near future, and storing it in the form of gas hydrate is a safe, clean, and economical approach. However, required thermodynamic conditions and slow kinetics are the key challenges that need to address for process viability. This study involves an experimental investigation of methane and cyclooctane sH hydrate formation for possible applications in gas storage using thermodynamics, kinetics, morphology, and Raman analysis. The hydrate formation is carried out at such thermodynamic conditions where only sH hydrate would form. The four-phase (Lw-LHC-H-V) sH hydrate equilibrium is studied for the methane and cyclooctane system via dissociation along the phase boundary method which is a robust method as it delivers a greater number of equilibrium data points in a single experimental run compared to other available methods. The sH hydrate formation helps in lowering the equilibrium conditions compared with sI hydrate formation. The slow sH hydrate formation kinetics can be improved by using low tryptophan concentrations. In this work, 0.1 wt % is the optimum tryptophan concentration as the gas uptake, and the hydrate formation rate is found to be the highest compared to 0.01, 0.05, and 1 wt % tryptophan concentrations. Here, we also visually investigate the sH hydrate formation and observed that the hydrate formation occurs below the interface for the system with no tryptophan; however, hydrate formation occurrence above the interface increases with an increase in the tryptophan concentration. The increase in the hydrate formation could be dedicated to the increased gas uptake due to the increasingly porous nature of hydrate formation. The Raman analysis confirmed the presence of methane and cyclooctane in sH hydrate cages. The higher intensity of the peaks using tryptophan additionally confirms the higher hydrate formation compared to the system with no tryptophan.

Carbon dioxide sequestration in natural gas hydrates - effect of flue and noble gases.

Clean energy is one of the immediate requirements all over the world to tackle the global energy demands. Natural gas hydrates (NGHs) are one of the proposed alternatives that could be used to extract methane as clean energy and simultaneously sequestrate carbon dioxide. However, the formation of CH4-CO2 mixed hydrates and the first hydrate layer besides the interface reduces the rate of CO2 sequestration and methane extraction in NGHs, and thus, multistep extraction of methane is one of the proposed solutions. We report the atomic level factors that could enhance CO2 sequestration in the newly formed first hydrate layer besides the interface in the presence of flue and noble gases using DFT calculations and molecular dynamics simulations at 250 K and 0.15 kbar. The simulations show the formation of stable dual cages (large-large or small-large) that lead to the formation of a four-caged, Y-shaped cluster (growth synthon) which leads to the formation of a hydrate unit cell in heterogeneous medium. Among the flue and noble gases, only argon forms energetically favorable dual cages with itself and CO2 due to which enhanced CO2 sequestration is observed at different concentrations of Ar and CO2 where the CO2 : Ar (2.5 : 1.5) system shows the best CO2 sequestration in the first layer besides the interface. The results also provide understanding into the previously reported concentration dependent CO2 selectivity in sI hydrates in the presence of third gases (N2 and H2S).

Molecular guest exchange and subsequent structural transformation in CH4 – CO2 replacement occurring in sH hydrates as revealed by 13C NMR spectroscopy and molecular dynamic simulations

This study adopted experimental and computational approaches to investigate CH4 – CO2 replacement in the structure H (sH) CH4 + methylcyclopentane (MCP) hydrate for its dual functions of CH4 recovery and CO2 sequestration. Hydrate phase equilibria, 13C NMR spectra, and molecular dynamics (MD) simulations of CH4 + MCP – CO2 replacement were examined and compared with those of CH4 + neohexane (NH) – CO2 replacement to elucidate the molecular guest exchange behaviors in both systems. The structure I (sI) hydrates were thermodynamically favored in CO2-rich gas mixtures (CH4 + CO2 + MCP systems), and a structural transformation from sH to sI occurred when CO2 composition in the feed gas was higher than 20 %. The 13C NMR spectra indicated that the CO2 molecules preferred to occupy the large (51262) cages of sI and the medium (435663) cages of sH compared to the small (512) cages of both sI and sH during replacement. Following CO2 injection into the sH CH4 + MCP hydrate, the initial sH hydrate transformed to the sI hydrate with CH4 recovery of approximately 78 %. The MD simulations also demonstrated that structural transformation in the CH4 + MCP – CO2 replacement would occur at the lower CO2 enclathration ratio compared to the CH4 + NH – CO2 replacement, thereby leading to a lower CO2 concentration in the newly formed sI hydrate. This is the first study to provide both experimental and computational evidence of guest-dependent structural transformation in sH CH4 + liquid hydrocarbons – CO2 replacement.

Dynamic Dissociation Behaviors of sII Hydrates in Liquid Water by Heating: A Molecular Dynamics Simulation Approach.

An understanding of the dynamic behavior of subtle hydrate dissociation in the liquid water phase is fundamental for gas production from marine hydrate reservoirs. Molecular dynamics simulations are performed in this study to investigate the dissociation kinetics of pure propane and binary propane + methane sII hydrates in a liquid water environment. The results show that faster hydrate dissociation rates are observed at higher initial temperatures. The hydrate phase dissociates from the cluster surface to the inside in a layer-by-layer manner under the simulation temperature conditions, which is similar to the behavior of sI hydrates and is independent of the hydrate crystal type. Compared to the binary sII hydrate, the pure sII hydrate dissociates more easily under the same initial temperature conditions, which can be attributed to the stabilizing effect of guest molecules in the hydrate cages. The empty cages collapse in one step, in contrast to the two-step pathway induced by the guest-host interaction. In addition, a hydrocarbon phase forms in the binary hydrate dissociation system instead of nanobubbles. These results can provide molecular-level insights into the dynamic mechanism of hydrate dissociation and theoretical guidance for gas recovery by thermal injection from marine hydrate reservoirs.