Mechanism Of Hydrate Formation Research Articles

Water-in-oil emulsions as micro-reactors for achieving extremely rapid and high hydrogen uptakes into clathrate hydrates

As humanity transitions to a low-carbon future, hydrogen, with its high energy density and zero carbon emissions, is gaining prominence, and efficient hydrogen storage is thus a critical area of research. Storing hydrogen in a solid form by forming hydrates, ensuring straightforward formation and dissociation, environmental friendliness, and stability, is emerging as a new technology. In particular, the use of thermodynamic promoters such as tetrahydrofuran (THF) to form binary hydrogen hydrates enables hydrogen storage under much milder conditions compared to traditional compression and liquefaction processes. However, slow hydrate formation kinetics currently hinder commercialization, necessitating breakthroughs in this area. This study reports extremely rapid and high hydrogen uptake in binary hydrogen + THF hydrates by utilizing a water-in-oil emulsion. A hydrogen storage capacity of approximately 51.23 mmol H2/mol H2O and the hydrogen uptake rate of 91.71 mol min−1 m−3 water were achieved, significantly outperforming existing studies conducted at pressures below 15 MPa. This remarkable kinetics was achieved by controlling the distribution of THF within a ternary emulsion system composed of water, isooctane, and THF. Phase equilibrium measurements and a series of kinetic tests validated that using an excess of THF over stoichiometric amounts can lead to such high and rapid hydrogen uptake. Consequently, a hydrate formation mechanism in the emulsified system was suggested based on the results of a Raman analysis, water droplet size measurements, and 1H NMR analysis. These results will pave the way for further advances in practical hydrate-based hydrogen storage technology.

Intriguing phenomenon of hydrogen molecules occupancy in clathrate hydrate cages: Implications for hydrogen storage

The occupancy of hydrogen in hydrate cages plays a significant role in the hydrogen storage capacity of hydrate-based. Here, we have found two intriguing phenomena of multiple hydrogen molecules occupancy in hydrate cages under relatively mild conditions. We observed the double hydrogen molecule occupancies in the small cages with large-molecule guest substance (1,2-Epoxycyclopentan (ECP)) as the thermodynamic promoter. The discovery illustrates that the stereotypical idea that light guest molecules are always necessary for double hydrogen occupancy in the 512 cages needs to be reconsidered. And we also observed the coexistence of double hydrogen molecules occupancy in the 51264 cages, which is the prerequisite of high hydrogen storage capacity in clathrate hydrates. The discovery obtained in this study can also provide further insight into the mechanism of hydrate formation and significant advances in hydrate-based hydrogen storage technology.

Based on PVM image gray level data to analysis the effect of N-vinyl caprolactam on hydrate growth

The usage of hydrate inhibitors is an effective method to solve the problem of flow assurance in deep-water pipelines. However, the mechanism of hydrate formation and transformation in inhibitor-containing systems is not well understood, and the research methodology needs to be innovated. In this study, a possible mechanism of wearing away during hydrate particle transformation was proposed for the first time, based on the hydrate growth process in the system containing the kinetic inhibitor N-vinyl caprolactam monomer. A method to analysed the hydrate growth and transformation dynamics through the change of gray level in the image was proposed. And the hydrate growth process was divided into three different stages based on the trend analysis of the gray level data of the images,trends in the dynamics of hydrate particle growth within the bulk phase corresponding to changes in gray level were verified. On this basis, the effects of stirring rate, subcooling and the monomer concentration of the NVCap on the growth of hydrates were discussed, and it was suggested that there might be differences in the mechanisms exhibited by the inhibitor at different growth stages of hydrates. In this study, the process of analysing the internal growth dynamics of hydrate systems through image gray levels was reported for the first time, and its trend changes corresponding to hydrate growth dynamics were verified, which proved the feasibility of this method, and this study provides an innovative research methodology and idea for the study of the dynamics of hydrate generation and transformation.

Gas hydrate technological applications: From energy recovery to carbon capture and storage

Hydrates are crystalline structures that trap small gas molecules inside hydrogen-bonded water cages. These structures form at high pressure and low temperature. In recent years, there has been a growing interest in gas hydrates for technological applications, specifically in energy recovery, as well as carbon dioxide capture and storage. In the CO2/CH4 exchange using gas hydrates, researchers have reported a large amount of natural gas trapped in the form of gas hydrates under the ocean seafloor and permafrost regions. This large amount of natural gas trapped inside hydrate cages in oceanic and permafrost deposits has driven interest in the energy sector to investigate the possibility of safely harvesting gas hydrates as one of energy resources. However, there are still unanswered fundamental questions including the mechanism of natural gas recovery from gas hydrates. Another gas hydrate-based technology that has a growing interest is the use of gas hydrates for separation including carbon dioxide capture and storage. Carbon dioxide molecules have been shown to be preferentially trapped in the hydrate phase, demonstrating the possibility of the usage for carbon capture technology. Similarly, there are still underlying concerns of these applications, such as thermodynamics and stability of gas hydrates in porous materials, and the crystallization kinetics and mechanism of hydrate formation. This paper provides an overview of laboratory investigations conducted to understand the mechanism and evaluate the feasibility of energy recovery as well as discussion on recent advances in laboratory investigations on gas hydrate-based technology.

Gas Hydrate Plugging Mechanisms during Transient Shut–In/Restart Operation in Fully Dispersed Systems

Gas hydrate formation poses a significant challenge in offshore oil and gas production, particularly during cold restarts after extended shut–ins, which can lead to pipeline blockages. Although steady–state models have traditionally been used to predict hydrate formation under continuous production conditions, these models are often inadequate for transient operations due to issues like near–zero fluid flow shear affecting the viscosity calculations of hydrate slurries. This study introduces novel conceptual models for dispersed water–in–crude oil systems specifically designed for cold restart scenarios. The models are supported by direct observations and various experimental approaches, including bottle tests, rheometer measurements, micromechanical force apparatus, and rocking cell studies, which elucidate the underlying mechanisms of hydrate formation. Additionally, this work introduces a modeling approach to represent conceptual pictures, incorporating particle settling and yield stress, to determine whether the system will plug or not upon restart. Validation is provided through transient large–scale flowloop tests, confirming the plugging mechanisms outlined. This comprehensive approach offers insights into conditions that may safely prevent or potentially lead to blockages in the fully dispersed system during field restarts, thereby enhancing the understanding and management of gas hydrate risks in offshore oil and gas operations.

Molecular insights into the impact of mineral pore size on methane hydrate formation

Natural gas hydrates are not only substantial energy sources but also have significant applications in the chemical industry and other fields. Although investigating hydrate formation in sediment minerals is crucial for their development and utilization, the underlying hydrate formation mechanism remains unclear. Here, molecular simulations were conducted in systems incorporating hydrophobic and hydrophilic pores of different sizes to investigate methane hydrate formation processes. The findings suggest that, as the hydrophobic slit size increases, there is a larger number of dissolved methane after the system reaches a metastable equilibrium state. The probability of cage formation indicates that hydrate cages readily form on hydrophobic surfaces or in the solution phase near the solution/gas interface. The larger slits are preferred for hydrate nucleation, regardless of whether the surface is hydrophobic, with most initial nuclei located near the liquid/methane interface. However, the interface perturbation can lead to the movement and growth of hydrate nuclei near the solution/methane interface into the bulk solution phase. Additionally, hydrate can nucleate and grow on the hydrophobic surface, facilitated by the adsorbed methane molecules and nonstandard cages. Pores hinder methane storage capacity in the hydrate phase due to the confinement effect and the amorphous nature of the hydrate formed. These molecular-level findings enhance our understanding of hydrate formation in sedimentary environments and porous materials, benefiting the development of natural gas hydrates and the use of porous materials for gas storage and transportation.

NMR transverse relaxation times and phase equilibria of methane hydrate in mesoporous alumina

The processes of formation (and decomposition) of methane hydrate from water adsorbed in the pores of spherical granules of mesoporous alumina (Al2O3) have been investigated using the low-field NMR spin–spin relaxation time (T2) and DSC methods. Analysis of the obtained data showed that changes observed in the relaxation time spectra represent a strong case in favor of the model envisaging hydrate growth in pore spaces without conspicuous water transfer through the volume content of the sample with mesoporous structure. As the supercooling strength of the liquid phase enhances, the size of the pores in which hydrate formation takes place decreases. At this, the size of the hydrate particles previously formed in larger pores tends to increase. Hydrate nucleation was shown to be followed by intensive and rapid hydrate formation in some parts of the alumina granules in the sample. The “skipping” mechanism of hydrate formation between granules remains unclear.

Promotion mechanism of carbon dioxide hydrate formation by l-Methionine and its competitive effects with NaCl

The growth kinetics and macroscopic morphology evolution of CO2 hydrate promoted by l-Methionine (L-Met) in NaCl aqueous solutions in a wide range of mass fractions were systematically investigated. 0.05 wt% was the threshold mass fraction of L-Met to have prominent promotion effects in a batch mode. The addition of NaCl could suppress its promotion performance, while it could be regained by further addition of L-Met to 1.0 wt%. Porous feature and wall-climbing phenomena of CO2 hydrate were obvious in the presence of L-Met while it could be suppressed by NaCl. The tangential (lateral) growth rate of the hydrate film on planar pure water surface was the highest while both L-Met and NaCl could decrease it. For L-Met, it was the highest at 0.05 wt%. Special ball-like, mosaic-like and sword-like appearances of CO2 hydrates were found. Both the adsorption-capillary effects of L-Met and the hydration effects of L-Met and NaCl were thought to account for all the phenomena and a detailed scenario was proposed and discussed. A macroscopic growth model based on the tangential growth rates of hydrate films was proposed and evaluated.

Gas solubility enhancement and hydrogen bond recombination regulated by terahertz electromagnetic field for rapid formation of gas hydrates

The low solubility of methane in water and the formation of hydrate film at methane-water interface restrict interfacial mass transfer between methane and water, thereby impeding the rapid formation of hydrate. Herein, THz electromagnetic field (TEMF) is introduced to destroy hydrogen bonds through resonance with water molecules for decreasing methane-water interfacial tension (IFT) and improving methane solubility in water. Thereafter, TEMF is removed to restore hydrogen bonding interaction for hydrate nucleation within bulk-phase water rather than at methane-water interface, thereby accelerating hydrate formation. The results show that a TEMF at 15.8 THz can increase methane solubility by 50-fold and provide more nucleation sites for hydrate formation by lowering the IFT by 15-fold. Furthermore, the hydrate nucleation follows “blob” nucleation at low solubility and Local Structuring Mechanism nucleation at high solubility. The higher solubility of methane shortens the induction period of hydrate nucleation form 58.5 ns to 29 ns. Particularly, small 512 cages form preferentially regardless of nucleation pathway. The underlying mechanism for TEMF-induced the acceleration of hydrate formation is attributed to the transition of water phase from a disordered to an ordered structure. This transition arises from the resonance of the TEMF with water molecules, disrupting the hydrogen bond between water molecules and providing sufficient driving force for their rearrangement after removing the TEMF. In rearrangement process, weakly hydrogen-bond interactions correspondingly are converted into strongly hydrogen-bond ones. These findings are expected to improve the understanding of hydrate formation mechanism and promote the applications of terahertz technology in gas hydrates.

Effects of multi-walled carbon nanotubes on microstructure transformation of water before carbon dioxide hydrate formation

There is no consensus on the micro mechanism of gas hydrate formation. The essence of water conversion into hydrates is the transformation of disorderly arranged water into the water with specific structure under a certain condition. This study used different multi-walled carbon nanotubes (MWCNTs) as solid promoters for gas hydrate formation. By comparing the effects of nanotubes modified with hydroxyl, carboxyl and amidogen on the microstructure transformation of water before CO2 hydrate formation, the micro mechanism and influence rules of gas hydrate formation were systematically study. The results indicated that the conversion of water to hydrates mainly manifested as the mutual conversion of strong/weak hydrogen bonding water. Due to the Brownian motion and dispersion of MWCNTs, the hydrogen bonds of the weak hydrogen-bonded water weakened (the corresponding peak blue shifts), and the distance between water molecules (dO−O) expanded, forming relatively loose hydrate, which was conducive to gas diffusion in the solid hydrate phase and increased the final gas consumption. Meanwhile, the strengthening of strong hydrogen bonded water increased the rate of hydrate formation.

Kinetic analysis of CO2 hydrate formation in the aqueous solutions of transition metal chlorides

AbstractCO2 hydrate technology can be applied to seawater desalination. However, the kinetics of CO2 hydrate formation were inhibited in the aqueous solution with inorganic salts, and the kinetic mechanism of CO2 hydrate formation for inorganic salts with different metal cations and anions was still unclear. In this work, CO2 hydrate nucleation and growth were studied in aqueous solutions of metal chlorides. Instead of Na+ and K+ ions, CO2 hydrate nucleation was promoted in the presence of Ni2+, Mn2+, Zn2+ and Fe3+ ions due to the co‐ordination bonds between transition metal ions and water molecules to enhance the formation of the critical crystal nuclei. The induction time was increased by 61.1% in aqueous solution with 0.32 mol/L NaCl, while it was shortened by 55.6% in FeCl3 aqueous solution at the same concentration of Cl− anions. In the process of CO2 hydrate growth, Cl− ions played a more important role than the metal ions in affecting the stability of CO2 hydrate cages. The gas storage capacity was reduced by 10.3% in the presence of NaCl, and was lower than that of other metal chlorides. Cl− anions were absorbed on the hydrate surface and involved in hydrate cages to inhibit the hydrate growth due to the hydrogen bonds between the Cl− ions and water molecules of the hydrate cages. © 2024 Society of Chemical Industry and John Wiley & Sons, Ltd.

Adsorption enhances CH4 transport-driven hydrate formation in size-varied ZIF-8: Micro-mechanism for CH4 storage by adsorption-hydration hybrid technology

The adsorption-hydration hybrid technology holds significant potential for future CH4 storage applications. However, the transport of CH4/H2O and the mechanisms governing hydrate formation within metal-organic framework (MOF) systems remain unclear. In this study, the mechanism of CH4 adsorption, transport, and hydrate formation within ZIF-8 pores of different sizes was delineated utilizing the magnetic resonance imaging (MRI) technique. Primarily, CH4 undergoes physisorption within the inner cavity of ZIF-8, facilitating diffusion through the pores, which preferentially accumulate in larger pore spaces. Due to favorable interactions between CH4 and pre-absorbed water on the ZIF-8 surface, ZIF-8–01 and ZIF-8–02 preferentially facilitate substantial hydrate formation within larger pore spaces. The continuous hydrate growth depletes CH4 in the pore space, enabling two-way migration of adsorbed CH4 to support ongoing hydrate formation. This limited availability of adsorption sites within ZIF-8 induces the diffusion of surrounding CH4 into smaller pore spaces, facilitating the conversion of bound water. The size of the particles considerably influences the transport of physically adsorbed CH4 and impacts hydrate formation. Similar patterns are observed in the adsorption mass transfer coefficients of ZIF-8–01 and ZIF-8–02, which range from 0.0038 to 0.0044 and 0.0031 to 0.0124, respectively. The presence of more adsorption vacancies in the early stage leads to rapid diffusion adsorption of CH4 owing to strong adsorption effects. As adsorption within ZIF-8 pore cavities approaches saturation, increased water transport elevates internal gas-liquid two-phase resistance, slowing the kinetics of late-stage CH4 adsorption. At 6 MPa, CH4 with a larger particle size preferentially overcomes the gas-liquid energy barrier, expediting transport and promoting hydrate formation. Furthermore, the optimal particle sizes of ZIF-8 substantially influence physisorption and significantly improve the kinetics of hydrate formation.

Investigation of hydrogen-propane hydrate formation mechanism and optimal pressure range via hydrate-based hydrogen storage

Hydrogen-propane hydrate is a promising practical hydrogen storage medium. However, the optimal conditions for the hydrogen-propane hydrate remain unclear. In this study, the thermodynamic and kinetic characteristics of hydrogen-propane gas (molar ratio 0.67/0.33) hydrate formation were investigated using experiments and molecular dynamics simulation. Moreover, the effect of liquid propane on hydrogen-propane hydrate formation in the pressure range of 0.5–2.8 MPa was investigated. The characteristic peaks of the heat flow curve during the experiment showed that under the pressure range of 2.0–2.8 MPa, propane hydrate and the ice-liquid propane mixture formed but not hydrogen-propane hydrate. Raman spectroscopy verified that when the initial pressure was higher than 2.0 MPa, only propane hydrate formed. Molecular dynamics simulation results showed that if liquefied propane exists, propane molecules will aggregate, preventing water molecules from contacting crystal nuclei and inhibiting hydrogen bonding, further hindering the growth of hydrogen-propane hydrate. Based on the experimental and theoretical analysis, the pressure range of 0.8–1.8 MPa is optimal for hydrogen-propane hydrate formation, specifically corresponding to the initial molar ratio of hydrogen to propane at 0.67/0.33. This study aids in determining the optimal operating conditions for hydrate-based hydrogen storage.

Combined gas well hydrate prevention and control technology and its application

The high pressure in some gas wells, such as those in the Xushen gas field in Daqing, China, makes them susceptible to freezing and hydrate blockages. Downhole throttling technology is widely used to reduce costs during well construction, however, due to the limitations of temperature, pressure and depth structure, this technology is sometime applied independently in some gas wells in which freezing and blockages are a frequent problem that can seriously affect production capacity. Moreover, artificial alcohol injection of ‘passive plugging’ to prevent hydrate formation not only consumes significant amounts of methanol but its efficiency is also dependent on factors such as weather, personnel and equipment, so it is not a continuous solution. In order to solve the above problems, the mechanism of hydrate formation was analyzed in this study, from which a combined mechanical and chemical hydrate control process was developed. OLGA software was used to design the process parameters of the novel mechanical and chemical inhibition technology for hydrate prevention and control, and also to simulate and analyze the wellhead temperature, pressure and hydrate generation once the process was implemented. Based on the results of the parameters calculation, the downhole throttle and hydrate inhibitor automatic filling device are used to realize the functions of downhole throttle depressurization and hydrate inhibitor continuous filling, reduce the wellhead pressure and hydrate generation temperature, and ensure the continuous production of gas well. This novel combination process was subsequently tested in three wells in the Daqing gas oilfield. Measurements showed that the average daily gas increase from a single well was 0.5×104m3, methanol consumption was reduced from the original maximum daily amount of 1750 kg to just 60 kg, the manual maintenance workload was reduced by 80%, and the rate of the well openings was increased from 45% to 100%. These results proved that this technology is feasible and efficient for applications in gas wells with high downhole pressure and low wellhead temperature, and, thus, provides important technical support for the prevention of gas hydrate and improvement of gas well production.

Metastable structure II methane hydrate: A bridge to obtain high methane uptake in the presence of thermodynamic promoter

The addition of thermodynamic promoters is a promising and perhaps the only effective method to reduce hydrate formation conditions. However, this is achieved at the cost of the gas storage capacity reduction since the thermodynamic promoter will occupy some hydrate cages. To date, there is no effective method that can simultaneously limit this negative effect and obtain high water-to-hydrate conversion. In this work, we determine the methane (CH4) hydrate formation mechanism in the presence of a thermodynamics promoter, Cyclopentane (CP), and find that structure II (Str.II) CH4 hydrate, as a metastable intermediate structure of the reaction, plays a quite significant role in encapsulating CH4 molecules in some 51264 cages (Str.II hydrates) and facilitating structure I CH4 hydrate formation, thereby increasing gas uptake significantly. A CH4 uptake, as high as 141.21 mmol/molH2O, is obtained from this work. Relative to the acknowledged limit of CH4 uptake of Str.II hydrates formed commonly in the presence of same mole fractions of thermodynamics promoter (8(Promoter)·16(CH4)·136H2O)), the highest increase in CH4 uptake is up to 232.75 %. These results would stimulate the development of the hydrate reaction theory and provide a new direction for promoter design.

Molecular analysis of hydrogen-propane hydrate formation mechanism and its influencing factors for hydrogen storage

Combining hydrogen and propane, which acts as a thermodynamic additive, to form hydrates is a promising hydrogen storage method. However, the mechanism of hydrate formation, which is the core of hydrate-based hydrogen storage, remains obscure. In this study, the effects of gas composition, pressure, and temperature on hydrogen–propane hydrate formation kinetics were investigated through molecular dynamics simulations. A hydrogen fraction (Xh) in the range of 0.47–0.67 was optimal for forming hydrate cages, whereas for Xh ≤ 0.37 and Xh ≥ 0.77, propane or hydrogen bubbles formed, which inhibited hydrate formation. Within the simulated pressure range, the number of hydrate cages increased with the increasing pressure. However, the analyzed self-diffusion coefficients suggested that the effect of pressure on the hydrate formation kinetics gradually weakened as the pressure increased up to a certain value. Within the simulated temperature range, the number of hydrate cages increased with temperature up to a point below the hydrogen–propane hydrate phase-equilibrium temperature. This result can be attributed to the increase in the self-diffusion coefficients of hydrogen and propane with the increasing temperature. These results provide valuable microscopic insights into the hydrogen–propane hydrate formation mechanism for hydrate-based hydrogen storage and will be useful in experimental research.

RATIONALE FOR CHOOSING CORRELATIONS FOR GAS HYDRATE FORMATION TEMPERATURE COMPUTATION FOR GASES OF VARIOUS COMPOSITIONS

Gas hydrates are the object of continuous research in the oil and gas industry. Gas hydrates can form in gas production systems: in the bottomhole zone, in the wellbores, in plumes and infield reservoirs, in gas treatment systems, as well as in main gas transport systems, causing serious problems associated with the disruption of these processes. However, recently, the gas industry has found new uses for hydrates (for example, energy recovery, separation, storage, and transportation of gas) prompting scientists to conduct new and more detailed studies of the hydrate formation process.
 Understanding the conditions and mechanisms of hydrate formation makes an engineer able to control this process. Therefore, in practice, simplified models are often required to predict hydrate formation. This paper presents various correlations that are widely used in the oil and gas industry to determine the temperature of hydrate formation. The accuracy of the presented models is compared. The influence of the features of the gas composition on the conditions for the hydrates formation is studied.
 It is shown that for the considered gases, the most accurate results can be obtained using the G.V. Ponomarev correlation. When the relative density of gases is greater than 0.6, it is possible to use the Towler and Bahadori correlations. Correlations proposed by Hammerschmidt, Mottie and Berge show the worst results.
 Since the temperature of hydrate formation in the considered correlations does not depend on the gas composition, but only on the relative density, the influence of the content of non-hydrocarbon components on the temperature of hydrate formation was studied in this work. It was found that the content of hydrogen sulfide has the greatest influence. Moreover, at high contents of hydrogen sulfide in the gas composition, the temperature of hydrate formation shifts towards higher values. The content of nitrogen and carbon dioxide to a lesser extent affect the value of the temperature of hydrate formation.

Hydrate Growth over a Sessile Drop of Water in Cyclopentane

Liquid cyclopentane is frequently used in hydrate formation studies as an analogue of natural gas because cyclopentane hydrates are stable above the ice melting point at ambient pressure. In this study, hydrate growth was established on a sessile water drop of 11 mm in diameter and 4.5 mm in height (volume of 0.25 mL) immersed in liquid cyclopentane. The hydrate formation mechanism and growth processes were observed optically over an extended range of subcooling temperatures from 5.1 to 15.2 °C, with the cyclopentane bulk temperature maintained in different experimental runs between 2.6 and −7.5 °C. Qualitative and quantitative comparisons were performed to confirm the absence of ice freezing during hydrate formation, and thus, the lack of contamination of the latter from the former in the experiments. Different transformations in the hydrate film morphology were registered from macroscopic observation over the considered range of subcooling temperatures, with the hydrate crystals composing the film taking the form of polyhedral, dendritic, or spherulitic structures. It was also found that the hydrate growth rate varied depending on the subcooling temperature, with the variation of the growth rate as a function of this temperature changing from a power to an (approximately) linear law with an increase in the degree of subcooling. We postulate that hydrate film growth can be governed by different mechanisms, whose roles change over the range of explored subcooling temperatures.

Microscopic study on the key process and influence of efficient synthesis of natural gas hydrate by in situ Raman analysis of water microstructure in different systems with temperature drop

Gas hydrate technology has considerable potential in many fields. However, due to the lack of understanding of the micro mechanism of hydrate formation, it has not been commercially applied so far. Gas hydrate formation is essentially a gas-liquid-solid phase transition of water and gas molecules at a certain temperature and pressure. The key to the hydrate formation is the transformation of water molecule from disordered arrangement to ordered arrangement. In this process, weakly hydrogen bonded water will be correspondingly converted to strongly hydrogen bonded water. Through in situ Raman analysis and experiments, the position change of the corresponding peaks of the strongly hydrogen bonded water and the weakly hydrogen bonded water was compared in this work, and the key microscopic process and influence of gas hydrate formation in different systems were comprehensively studied and summarized. It is found that, with the decrease of temperature, the OH of the weakly hydrogen bonded water remains unchanged when the temperature drops to a certain value, which is the key to the transformation of water into cage hydrate rather than ice. The conversion from the weakly hydrogen bonded water to the strongly hydrogen bonded water is closely related to the gas-liquid interface force, the hydrophilicity/hydrophobicity of the promoter, the ionization degree of liquid, and the electrostatic field of the system. Among the four most common promoters, tetrahydrofuran (THF) has the highest efficiency in promoting methane (CH4) hydrate formation. Therefore, this study provides a scientific direction and basis for the development of high efficient hydrate formation promoters, which can effectively weaken the hydrogen bond of weakly hydrogen bonded water and promote the conversion of weakly hydrogen bonded water to strongly hydrogen bonded water.

Clay nanoflakes and organic molecules synergistically promoting CO2 hydrate formation

Carbon dioxide (CO2) reduction is an urgent challenge worldwide due to the dramatically increased CO2 concentration and concomitant environmental problems. Geological CO2 storage in gas hydrate in marine sediment is a promising and attractive way to mitigate CO2 emissions owning to its huge storage capability and safety. However, the sluggish kinetics and unclear enhancing mechanisms of CO2 hydrate formation limit the practical application of hydrate-based CO2 storage technologies. Here, we used vermiculite nanoflakes (VMNs) and methionine (Met) to investigate the synergistic promotion of natural clay surface and organic matter on CO2 hydrate formation kinetics. Induction time and t90 in VMNs dispersion with Met were shorter by one to two orders of magnitude than Met solution and VMNs dispersion. Besides, CO2 hydrate formation kinetics showed significant concentration-dependence on both Met and VMNs. The side chains of Met can promote CO2 hydrate formation by inducing water molecules to form a clathrate-like structure. However, when Met concentration exceeded 3.0 mg/mL, the critical amount of ammonium ions from dissociated Met distorted the ordered structure of water molecules, inhibiting CO2 hydrate formation. Negatively charged VMNs can attenuate this inhibition by adsorbing ammonium ions in VMNs dispersion. This work sheds light on the formation mechanism of CO2 hydrate in the presence of clay and organic matter which are the indispensable constituents of marine sediments, also contributes to the practical application of hydrate-based CO2 storage technologies.