Methane Hydration Research Articles

Geomechanical Properties of Deep-Sea Pore-Filled Methane Hydrate-Bearing Soils at Critical State Using DEM Analysis

The recognition of the geomechanical properties of methane hydrate-bearing soil (MHBS) is crucial to exploring energy resources. The paper presents the mechanical properties of a pore-filled MHBS at a critical state using the distinct element method (DEM). The pore-filled MHBS was simulated as cemented MH agglomerates to fill the soil pores at varying levels of methane hydration (MH) saturation. A group of triaxial compression (TC) tests were conducted, subjecting MHBS samples to varying effective confining pressures (ECPs). The mechanical behaviors of a pore-filled MHBS were analyzed, as it experienced significant strains leading to a critical state. The findings reveal that the proposed DEM successfully captures the qualitative geomechanical properties of MHBS. As MH saturation increases, the shear strength of MHBS generally rises. Moreover, higher ECPs result in increased shear strength and volumetric contraction. The peak shear strength of MHBS increases with rising MH saturation, while the residual deviator stress remains mainly unchanged at a critical state. There is a good correlation between fabric changes of the MHBS with variations in principal stresses and principal strains. With increasing axial strain, the coordination number (CN) and mechanical coordination number (MCN) increase to peak values as the values of MH saturation and ECPs increase, and reach a stable value at a larger axial strain.

Synergistic enhancement of coalbed methane hydration process by magnetic field and NiMnGa micro-nano fluid

Coalbed methane is an important renewable energy source. Gas hydration technology is a new method for enhancing the utilization of coalbed methane and reducing environmental pollution. Long induction periods, sluggish formation rates, low hydrate yields, and difficulty removing heat during hydrate formation are all issues with gas hydration technology. In this paper, 3 wt% NiMnGa (NMG) phase-change micro/nanoparticles and 0.05% sodium dodecyl sulphate (SDS) were compounded, and gas hydration experiments were conducted under various initial pressures and gas sample conditions to investigate. The findings revealed that NMG has efficient mass transfer properties as well as phase-change heat absorption properties, which significantly improved the kinetic process of the gas hydrate by mass and heat transfer, shortened the induction time, increased gas consumption, and increased the gas consumption rate during the rapid hydrate growth period. When the initial pressure was 6.2 MPa, the induction time was reduced by 89.26%, 92.48%, and 95.64%, and the maximum gas consumption rate was increased by 238.18%, 175.55%, and 113.60%, respectively, when using different concentrations of methane in the NMG-SDS system compared to the pure SDS system. The NMG used in this paper showed potential for future use in mixed gas hydration technology.

Experimental Study on the Kinetics of the Natural Gas Hydration Process with a NiMnGa Micro-/Nanofluid in a Static Suspension System

Natural gas is a resource-rich clean energy source, and natural gas hydration technology is a promising method for natural gas storage and transportation at present. To realize the rapid generation of hydrates with a high gas storage capacity, in this paper NiMnGa micro/nanoparticles (NMGs) with different mass fractions (0.1 wt%, 1 wt%, 2 wt%) were prepared with 0.05 wt% sodium dodecyl sulfate (SDS) and 1 wt% L-tryptophan to form static suspension solutions of gellan gum, and the methane hydration separation kinetics experiments were carried out under the condition of 6.2 MPa for the SDS-NMG-SNG (SNG) and L-tryptophan-NMG-LNG (LNG) systems. The results showed that the induction time of the systems with NMG micro-/nanoparticles was shortened to different degrees and the gas consumption rate was increased. The best effect was achieved in the SNG system with 1 wt% NMG, and the induction time was shortened by 73.6% compared with the SDS-gellan system (SG). The gas consumption rate of the system with L-tryptophan was better than that of the system with SDS, and the best effect was achieved in the system with 2 wt% NMG. The system with 2 wt% NMG had the best effect, and the problem of foam decomposition did not occur. The analysis concluded that NMG has strong mass transfer and phase-change heat absorption properties, which can significantly improve the kinetics of the natural gas hydrate generation process; L-tryptophan can weaken the diffusion resistance of methane molecules in the suspended static solution, further enhancing the mass transfer of the hydrate generation process. These findings will provide new perspectives regarding the application of phase-change micro-/nanoparticles in methane hydrate generation under static conditions.

Molecular simulation study of methane hydrate formation mechanism in NaCl solutions with different concentrations

The effects of sodium chloride on hydrate formation depend on its concentrations. NaCl solutions can promote hydrate formation at low concentrations and inhibit it at high concentrations. Molecular dynamics simulations were performed to study the effect of different concentrations of NaCl on hydrate formation. Results implied that the induction time of hydrate in 1 wt% NaCl solution is shorter than that in pure water. The induction phase of hydrate formation in NaCl solution with a mass concentration greater than 1% is longer than that in pure water. The induction time of hydrate prolongs with increasing salt concentration due to the NaCl reducing tetrahedral water molecules in the methane hydration layers and inhibiting methane dissolution. During the hydrate growth phase, some ions are involved in the formation of unstable hydrate cages, while disturbing and destroying the nearby cage structures, retarding the hydrate growth. Besides, ions destroy unstable structures in amorphous hydrates, thereby reducing the empty cage rate. It was found that 1 wt% NaCl promotes hydrate formation for the local accumulation effect of ions on methane. Furthermore, analysis of the methane hydration layers around chloride ion shows that chloride ions facilitate the transformation of methane hydration layers to cage structures, which also have a positive effect on hydrate formation.

Molecular dynamics simulation of the effects of different salts on methane hydrate formation: an analysis of NaCl, KCl and CaCl2

The formation of hydrates in seawater is of great significance for hydrate prevention and control and seawater desalination. Molecular dynamics simulations were performed to investigate the spontaneous methane hydrate formation in 3.5wt % (seawater salinity) NaCl, 3.5wt % KCl and 3.5wt % CaCl2 solution, respectively. Results indicated that salt inhibited the formation of hydrates, and the strength of inhibition followed the sequence of 3.5wt % CaCl2 > 3.5wt % NaCl > 3.5wt % KCl. Salt inhibited nucleation and growth of hydrates by suppressing methane dissolution and reducing the number of tetrahedral water molecules in the methane hydration layer. It was observed that Na+, K+ and Cl- can replace the water molecules in the hydrate cage to participate in the formation of the cage. Compared with the pure water system and KCl system with a fast hydrate growth rate, the randomness of hydrate crystallization was smaller in the NaCl system and CaCl2 system with slow hydrate growth rate.

Cyclodextrins as eco-friendly nucleation promoters for methane hydrate

• HPS-ALTA used to rigorously characterise effects of cyclodextrins (CDs) on methane hydrate formation. • CDs can increase the hydrate nucleation rate by up to 6 times at a fixed temperature. • The highest and lowest promotion and H-bonding disruption were observed for α-CD and β-CD, respectively. • No measurable changes to the initial growth rate under stirred conditions caused by CDs. Gas hydrates offer potential methods for the storage and transportation of gases. However, factors such as slow kinetics can hinder storage applications. Here we demonstrate the use of cyclodextrins (CDs) as green chemicals for accelerating methane hydration formation. A second generation high pressure, stirred, automated lag time apparatus (HPS-ALTA) was used to determine the hydrate formation probability as a function of CD concentration, subcooling, and time. Nucleation rates were increased for all three studied (α, β, and γ) CDs, as determined from formation measurements under ramped and fixed temperatures. Analysis of the ramped temperature experiments suggests that the nucleation promotion is related to a CD-induced reduction in the thermodynamic nucleation parameter which provides a measure of nucleation work. Hydrate formation in the presence of CDs was also confirmed to be consistent with the mononuclear nucleation process predicted by the classical nucleation theory. Rates of initial growth were also measured but showed no CD dependence. At the optimum CD loadings found for nucleation, the highest and lowest levels of nucleation promotion were observed for α- and β-CD, respectively. These CDs also generated the highest and lowest levels of H-bonding disruption at the gas-water interface (as measured by sum frequency generation spectroscopy), respectively, suggesting that such disruption may play a role in the hydrate nucleation promotion in CD-containing systems.

Unraveling nucleation pathway in methane clathrate formation

Methane clathrates are widespread on the ocean floor of the Earth. A better understanding of methane clathrate formation has important implications for natural-gas exploitation, storage, and transportation. A key step toward understanding clathrate formation is hydrate nucleation, which has been suggested to involve multiple evolution pathways. Herein, a unique nucleation/growth pathway for methane clathrate formation has been identified by analyzing the trajectories of large-scale molecular dynamics (MD) simulations. In particular, ternary water-ring aggregations (TWRAs) have been identified as fundamental structures for characterizing the nucleation pathway. Based on this nucleation pathway, the critical nucleus size and nucleation timescale can be quantitatively determined. Specifically, a methane hydration layer compression/shedding process is observed to be the critical step in (and driving) the nucleation/growth pathway, which is manifested through overlapping/compression of the surrounding hydration layers of the methane molecules, followed by detachment (shedding) of the hydration layer. As such, an effective way to control methane hydrate nucleation is to alter the hydration layer compression/shedding process during the course of nucleation.

Rate enhancement of methane hydration in slurry of ice by phase change of water-in-oil emulsions

The paraffin, decane, octane and hexane, were chosen to make emulsions of the water-in-oil in terms of their lower fusion points and higher methane solubilities to study the methane hydration rate. The ice particles from supercooled emulsions provided the nucleation center and removed the hydration heat promptly through their reversed phase change. Meanwhile the solubility differences of methane in the paraffins and water enlarged the driving force of mass transfer. Finally, the hydration rates of methane were significantly enhanced in a batch autoclave at initial conditions of temperature from −1 to −5.7 °C, pressure 5.90 MPa, water-cuts 10–30 vol.% and stirrer speed of 700 rpm. The highest hydration rate reached 158.2 kmol h−1 m−3, which is obviously larger than those in the slurry of phase change material in water and the other achievements.

Water hydrogen degrees of freedom and the hydrophobic effect.

Hydrogen bonds are the key interaction that establishes the liquid and solvent properties of water. Nevertheless, it is possible to construct an accurate molecular model of water which does not include hydrogens or any orientational interactions. Using this model, we calculate the structural and thermodynamic properties for the hydration of methane and ethane. The addition of the hydrophobic solute leads to changes in structure, as can be seen in slightly enhanced tetrahedral geometries and slightly reduced Voronoi volumes of water near the solute. The entropy of hydration from the model is about half the experimental value, suggesting that what is left out of the model-the orientational or hydrogen response-contributes to about half the entropy. For the hydrophobic association of two methane molecules in water, the hydrogen degrees of freedom do not seem to play an important role and the entropy of association is similar to all-atom models.

Promotion effects of mung starch on methane hydrate formation equilibria/rate and gas storage capacity

The effects of mung starch on methane hydrate formation equilibria/rate and gas storage capacity were investigated in this work. Mung starch at three concentrations of 100, 500, and 800 ppm were tested comparing with the pure water. The results show that mung starch has slight thermodynamic promotion effect on methane hydrate formation, because it decreases the gas-liquid-hydrate three-phase equilibria pressure at the same temperature. The formation rate and gas storage capacity of methane hydrate in the presence of mung starch were studied at 8.0 MPa and 275.15 K–281.15 K. The results demonstrate that mung starch significantly accelerates methane hydrate formation rate. It could shorten the induction time and reaction time of methane hydration process. In addition, the gas storage capacity of methane hydrate is also increased greatly. The solubility data of methane indicate that mung starch indeed plays a role of solubility enhancement. The green and environmental friendly characteristics of mung starch could be helpful to promote the application of hydrate-based technology.

The inhibition of methane hydrate formation by water alignment underneath surface adsorption of surfactants

Sodium dodecyl sulfate (SDS) has been widely shown to strongly promote the formation of methane hydrate. Here we show that SDS displays an extraordinary inhibition effect on methane hydrate formation when the surfactant is used in sub-millimolar concentration (around 0.3mM). We have also employed Sum Frequency Generation vibrational spectroscopy (SFG) and molecular dynamics simulation (MDS) to elucidate the molecular mechanism of this inhibition. The SFG and MDS results revealed a strong alignment of water molecules underneath surface adsorption of SDS in its sub-millimolar solution. Interestingly, both the alignment of water and the inhibition effect (in 0.3mM SDS solution) went vanishing when an oppositely-charged surfactant (tetra-n-butylammonium bromide, TBAB) was suitably added to produce a mixed solution of 0.3mM SDS and 3.6mM TBAB. Combining structural and kinetic results, we pointed out that the alignment of water underneath surface adsorption of dodecyl sulfate (DS−) anions gave rise to the unexpected inhibition of methane hydration formation in sub-millimolar solution of SDS. The adoption of TBAB mitigated the SDS-induced electrostatic field at the solution’s surface and, therefore, weakened the alignment of interfacial water which, in turn, erased the inhibition effect. We discussed this finding using the concept of activation energy of the interfacial formation of gas hydrate. The main finding of this work is to reveal the interplay of interfacial water in governing gas hydrate formation which sheds light on a universal molecular-scale understanding of the influence of surfactants on gas hydrate formation.

A Theoretical Study of the Hydration of Methane, from the Aqueous Solution to the sI Hydrate-Liquid Water-Gas Coexistence.

Monte Carlo and molecular dynamics simulations were done with three recent water models TIP4P/2005 (Transferable Intermolecular Potential with 4 Points/2005), TIP4P/Ice (Transferable Intermolecular Potential with 4 Points/ Ice) and TIP4Q (Transferable Intermolecular Potential with 4 charges) combined with two models for methane: an all-atom one OPLS-AA (Optimal Parametrization for the Liquid State) and a united-atom one (UA); a correction for the C–O interaction was applied to the latter and used in a third set of simulations. The models were validated by comparison to experimental values of the free energy of hydration at 280, 300, 330 and 370 K, all under a pressure of 1 bar, and to the experimental radial distribution functions at 277, 283 and 291 K, under a pressure of 145 bar. Regardless of the combination rules used for σC,O, good agreement was found, except when the correction to the UA model was applied. Thus, further simulations of the sI hydrate were performed with the united-atom model to compare the thermal expansivity to the experiment. A final set of simulations was done with the UA methane model and the three water models, to study the sI hydrate-liquid water-gas coexistence at 80, 230 and 400 bar. The melting temperatures were compared to the experimental values. The results show the need to perform simulations with various different models to attain a reliable and robust molecular image of the systems of interest.

The dual effect of sodium halides on the formation of methane gas hydrate

Inorganic salts are known to inhibit the formation of gas hydrates. Here we show the duality of sodium halides of submolar concentration in affecting the formation of methane gas hydrates. Sodium halides, especially NaI, at low concentration effectively promote methane hydrate formation while they all turn to be an inhibitor at high concentration. Maximum gas consumption, growth rate and induction time were experimentally determined as a function of salt type and concentration. We explain the dual effect of salts by the hydrophobic hydration. The promoting effect of dilute sodium halides is due to the fact that large and polarizable anions (e.g. iodide) behave as hydrophobic entities and interact with surrounding water molecules to form hydrophobic hydration shells whose water structure is similar to that of hydrophobic hydration shells of methane. Since hydrophobic hydration of methane in neat water is thermodynamically unfavourable because it associates with a negative entropy change and a partial loss in the hydrogen-bonded network, the structurally similar shells of halide ions facilitate the process of entropy change and, therefore, facilitate gas hydrate nucleation. Our proposal also explains the decrease in the promoting capability of salts in the order from iodide to fluoride because of the decrease in hydrophobicity of the halide ions. The inhibition effect of salts at high concentration is explained by the advantageous competition of the halide ions with methane gas molecules to gain water for hydration as well as their radical effect on distorting the water structure. Our hypothesis is experimentally supported by the difference in the salt recovery into hydrates and the hydrophobicity (measured by contact angle) of halide ions. Further research is required to obtain a fuller insight of the influence of salts and additives on gas hydrate formation.

An X-ray investigation of the adsorption of methane, water, and their mixtures in carbon micropores

The adsorption of methane and water, and their coexisting state in carbon micropores were investigated by X-ray scattering measurements. The mechanisms of single-component adsorption of water and methane in the carbon micropores proceeded by cluster formation and micropore filling, respectively. The X-ray scattering of the coexisting state reveals that a mesoscopic phase separation occurred in the carbon micropores at 111K because of the weak interaction between water and methane. At this low temperature, water molecules could not reform hydrogen bonds even with further adsorption of methane molecules. The X-ray scattering measurements provided detailed information on the coexisting state and can be used to study the temperature dependence of the phase behavior of the methane–water system during methane hydration formation in nanospaces.

Hydrostatic pressure effect on hydrophobic hydration and pairwise hydrophobic interaction of methane

At room temperature, the Ben-Naim standard hydration Gibbs energy of methane is a positive quantity that increases markedly with hydrostatic pressure [M. S. Moghaddam and H. S. Chan, J. Chem. Phys. 126, 114507 (2007)]. This finding is rationalized by showing that the magnitude of the reversible work to create a suitable cavity in water increases with pressure due to both the increase in the volume packing density of water and the contribution of the pressure-volume work. According to the present approach, at room temperature, the Gibbs energy of the contact-minimum configuration of two methane molecules is a negative quantity that increases in magnitude with hydrostatic pressure. This result is not in line with the results of several computer simulation studies [T. Ghosh, A. E. Garcia, and S. Garde, J. Am. Chem. Soc. 123, 10997-11003 (2001)], and emerges because pairwise association causes a decrease in solvent-excluded volume that produces a gain of configurational/translational entropy of water molecules, whose magnitude increases with the volume packing density of the liquid phase.

Analysis of the market of rare-earth elements (REEs) and REE catalysts

The market for rare-earth elements (REEs) and REE catalysts is analyzed. Essential data on the REE deposits in Russia and the world are given. The use of REEs is analyzed with emphasis on the production of magnets and catalysts. Some data on the fields of the application of REE catalysts, e.g., the purification of vehicle exhaust gases, catalytic cracking, hydrogenation, dehydrogenation, hydration, dehydration, steam, carbon dioxide, and oxidative conversion of methane, waste water treatment, are given. It is concluded that the development of the production of REE catalysts could lead to breakthroughs in the field of creating low-temperature, energy-saving catalytic processes.

Characterization of the Methane-Graphene Hydrophobic Interaction in Aqueous Solution from Ab Initio Simulations.

In this article, the interaction between a methane molecule and a graphene plane in liquid water has been characterized employing DFT-based free energy Molecular Dynamics calculations. This system represents a good model to understand the generic interaction between a small hydrophobic solute (methane molecule) and an extense hydrophobic surface (graphene plane). The structural and dynamical properties of graphene and methane hydration water are analyzed and found to be closely related to the main features of the potential of mean force. The results could be used in coarse-grained models to take into account the effect of the hydrophobic interaction in realistic systems relevant to experiment.

Reversible methane storage in porous hydrogel supported clathrates

Methane gas hydrates are a promising alternative for storage and transport of natural gas. In this paper, porous poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(2-hydroxyethyl methacrylate-co-methacrylic acid) (PHEMA-co-MAA) hydrogel microspheres were synthesised and examined for their application as a reusable scaffold for methane storage in their hydrated form. The hydration kinetics, methane storage capacity of the hydrogel microspheres, and a mixed colloidal system made of hydrogel particles and dry-water droplets were investigated in a 300cm3 steel vessel at 273.2K and varying pressures. Hydration of methane in the mixed colloidal system is high in capacity and exceedingly reversible. Higher pressure and smaller size of hydrogel microspheres result in improved capacity and kinetics, however reduce the recyclability of hydration. The porous hydrogel particles alone are too soft for reuse and need to be improved for practical application.

Structure and stability of an amorphous water–methane mixture produced by cold compression of methane hydrate

In a process similar to the formation of high-density amorphous ice, amorphous methane hydrate was formed by the pressure-induced collapse of crystalline methane clathrate at 32 kbar and 100 K. This produced a regular array of methane embedded within cavities in an amorphous water network. In situ annealing results in further densification into a more disordered and higher density amorphous state before crystallization into ice VIII and hexagonal clathrate. Whereas the very high-density amorphous form of pure water undergoes a glass transition near 130 K at low pressure and crystallizes over a broad pressure range at \ensuremath{\sim}160 K, the amorphous network of the water--methane binary system exhibits a region of metastability that persists up to 220 K between 15 and 40 kbar. It appears that in this system the connectivity of the water network exhibits sufficiently low mobility so as to inhibit the diffusion and phase separation of the constituents until 220 K, where the high-pressure ice VIII crystallization can occur. The structure of the amorphous form is studied by neutron diffraction and the hydration of methane in this disordered water network is more closely related to the small clathrate cage than the large.

A Simple QM/MM Approach for Capturing Polarization Effects in Protein−Ligand Binding Free Energy Calculations

We present a molecular simulation protocol to compute free energies of binding, which combines a QM/MM correction term with rigorous classical free energy techniques, thereby accounting for electronic polarization effects. Relative free energies of binding are first computed using classical force fields, Monte Carlo sampling, and replica exchange thermodynamic integration. Snapshots of the configurations at the end points of the perturbation are then subjected to DFT-QM/MM single-point calculations using the B3LYP functional and a range of basis sets. The resulting quantum mechanical energies are then processed using the Zwanzig equation to give free energies incorporating electronic polarization. Our approach is conceptually simple and does not require tightly coupled QM and MM software. The method has been validated by calculating the relative free energies of hydration of methane and water and the relative free energy of binding of two inhibitors of cyclooxygenase-2. Closed thermodynamic cycles are obtained across different pathways, demonstrating the correctness of the technique, although significantly more sampling is required for the protein-ligand system. Our method offers a simple and effective way to incorporate quantum mechanical effects into computed free energies of binding.