Gas Clathrate Hydrates Research Articles

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.

Historical perspectives on gas hydrates and citation impact analysis

AbstractGas hydrates, or clathrate hydrates, is a multidisciplinary field of research involving potential applications to the oil and gas industry, energy security, and innovative technological applications with literature referenced back to 1810. The field of gas hydrates or clathrate hydrates has progressed over the past several decades from academic curiosity‐driven research to industrially relevant research related to flow assurance and methane hydrates in nature as an energy resource. In the recent few decades, several innovative and sustainable applications have emerged with gas hydrate or clathrate hydrate as a technology enabler. In this work, I present a bibliometric analysis of the field of gas hydrates or clathrate hydrates for the period from 1901 to 2020 from the Web of Science core collection database of Clarivate Analytics. In total, 12 152 journal publications (review and original research articles) were analyzed from Web of Science core collection database spanning 121 years (1901–2020). Top countries, top cited review articles, and original research articles along with top source titles (journals) are identified and highlighted. In addition, the field classifications and citation rate trends have been analyzed and presented. Network visualization maps are presented for countries, sources, and organizations by analysing citations in VOSviewer. Co‐occurrence analysis is performed to identify the top keywords and their links through network visualization based on VOSviewer.

Molecular Dynamics Simulation Studies on the Stability and Dissociation of Clathrate Hydrates of Single and Double Greenhouse Gases

Comprehending and controlling the stability and dissociation of greenhouse gases hydrates are critical for a variety of hydrate-based industrial applications, such as greenhouse gas separation, sequestration, or utilization. Although the promotion effects of greenhouse F-gases (F-promoters) and new cyclic promoters on CO2 hydrates have been acknowledged, the involved molecular mechanisms are poorly understood. This work was therefore conducted to investigate the intermolecular mechanisms of the properties of CO2 and NF3 hydrates using molecular dynamics (MD) simulation to better understand their stability and dissociation and the effects of thermodynamic conditions as well as cage occupancy. In addition, the stability of CO2/CO2 + CH4 hydrates in the presence of seven thermodynamic hydrate promoters (THPs) from different molecular groups or substituents was evaluated. Results reveal that after the breakup of the hydrate, the propensity of NF3 to form nanobubbles is more than that of CO2 molecules. The relative concentration distribution of partially occupied hydrates was also found to be greater than that of completely filled by guest gases. MD simulation results of CO2 double and mixed hydrates also show that the type of large molecular guests in the large cages plays a major role in the stabilization of the clathrate hydrate network. The structural properties, however, indicate that the resistance against being dissociated for CO2 + promoter can be somewhat increased when half of the CO2 molecules in small cages is replaced by CH4. In addition, the existence of neopentyl alcohol in large cavities was found to facilitate the process of hydrate dissociation by making new hydrogen bonds between hydroxyl groups and water molecules. Among studied systems with THPs, cyclopentane, and cyclohexane in comparison with F-promoters seem to be more susceptible to maintaining the stability of CO2 clathrate hydrate.

Strengthening and weakening of methane hydrate by water vacancies

Gas clathrate hydrates show promising applications in sustainable technologies such as future energy resources, gas capture and storage. The stability of clathrate hydrates under external load is of great crucial to those important applications, but remains unknown. Water vacancy is a common structural defect in clathrate hydrates. Herein, the mechanical characteristics of sI methane hydrates containing three types of water vacancy are investigated by molecular dynamics simulations with four different water forcefields. Mechanical properties of methane hydrates such as tensile strength are dictated not only by the density but also the type of water vacancy. Surprisingly, the tensile strength of methane hydrates can be weakened or strengthened, depending on the adopted water model and water vacancy density. Strength enhancement mainly results from the formation of new water cages. This work provides critical insights into the mechanics and microstructural properties of methane clathrate hydrates under external load, which is of primary importance in the recovery of natural gas from methane hydrate reservoirs. Cited as: Lin, Y., Liu, Y., Xu, K., Li, T., Zhang, Z., Wu, J. Strengthening and weakening of methane hydrate by water vacancies. Advances in Geo-Energy Research, 2022, 6(1): 23-37. https://doi.org/10.46690/ager.2022.01.03

Exploring High-Pressure Transformations in Low-Z (H2, Ne) Hydrates at Low Temperatures

The high pressure structural behavior of H2 and Ne clathrate hydrates with approximate composition H2/Ne·~4H2O and featuring cubic structure II (CS-II) was investigated by neutron powder diffraction using the deuterated analogues at ~95 K. CS-II hydrogen hydrate transforms gradually to isocompositional C1 phase (filled ice II) at around 1.1 GPa but may be metastably retained up to 2.2 GPa. Above 3 GPa a gradual decomposition into C2 phase (H2·H2O, filled ice Ic) and ice VIII’ takes place. Upon heating to 200 K the CS-II to C1 transition completes instantly whereas C1 decomposition appears sluggish also at 200 K. C1 was observed metastably up to 8 GPa. At 95 K C1 and C2 hydrogen hydrate can be retained below 1 GPa and yield ice II and ice Ic, respectively, upon complete release of pressure. In contrast, CS-II neon hydrate undergoes pressure-induced amorphization at 1.9 GPa, thus following the general trend for noble gas clathrate hydrates. Upon heating to 200 K amorphous Ne hydrate crystallizes as a mixture of previously unreported C2 hydrate and ice VIII’.

Theoretical investigation of methane hydrate nucleation kinetic from “water + gas” and “sea water + gas” mixtures

In this work we studied transition mechanics of a dynamic hydrogen bond network of water into a stable hydrogen bond network during gas clathrate hydrate formation process. For this purpose, we used Molecular Dynamic simulation methods and a bunch of home-made programs for determining structural properties during transition process. We have found that at certain conditions the network transformation process involves the whole system from the very beginning and could be considered as a collective effect, that results in high speed of hydrate formation.

Synergistic and antagonistic effects of amino acids in clathrate hydrates of greenhouse gases

Gas hydrates are the indispensable materials in curtailing greenhouse gas emissions, particularly given their larger volumetric compressibility. However, sluggish and inefficient conversions are notable hindrances, which can be surpassed by using certain additives. We used aqueous solutions with 0.5 wt % amino acids (l-phe, l-met, l-cys, l-val, and l-leu) as additives. Individually all are suitable promoters for CH4 hydrates, while the same is true for CO2 hydrates also excepting l-phe. However, a small amount of l-met or l-cys can assist the l-phe rich solution as a better promoter for CO2 hydrates. The solution mixture synergistically enhances the gas uptake by a factor of 10. The addition of l-val or l-leu shows an antagonistic effect, and the CO2 hydrate conversion is insignificant until the solution mixture became rich in l-val or l-leu. Ex-situ micro-Raman studies unambiguously confirm the solid phase as clathrate hydrate. The perturbations in the OH-stretching mode due to synergistic or antagonistic effects in amino acids mixed solutions are small but subtle.

Nitrogen Hydrate Cage Occupancy and Bulk Modulus Inferred from Density Functional Theory-Derived Cell Parameters

Gas clathrate hydrate solid materials, ubiquitous in nature as found either on the ocean floor, permafrost on the Earth, or in extraterrestrial planets and comets, are also technologically relevant...

Pressure-induced amorphization of noble gas clathrate hydrates

The high-pressure structural behavior of the noble gas (Ng) clathrate hydrates $\mathrm{Ar}\ifmmode\cdot\else\textperiodcentered\fi{}6.5\phantom{\rule{0.16em}{0ex}}{\mathrm{H}}_{2}\mathrm{O}$ and $\mathrm{Xe}\ifmmode\cdot\else\textperiodcentered\fi{}7.2\phantom{\rule{0.16em}{0ex}}{\mathrm{H}}_{2}\mathrm{O}$ featuring cubic structures II and I, respectively, was investigated by neutron powder diffraction (using the deuterated analogues) at 95 K. Both hydrates undergo pressure-induced amorphization (PIA), indicated by the disappearance of Bragg diffraction peaks, but at rather different pressures, at 1.4 and above 4.0 GPa, respectively. Amorphous Ar hydrate can be recovered to ambient pressure when annealed at $g1.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and 170 K and is thermally stable up to 120 K. In contrast, it was impossible to retain amorphous Xe hydrate at pressures below 3 GPa. Molecular dynamics (MD) simulations were used to obtain general insight into PIA of Ng hydrates, from Ne to Xe. Without a guest species, both cubic clathrate structures amorphize at 1.2 GPa, which is very similar to hexagonal ice. Filling of large-sized H cages does not provide stability toward amorphization for structure II, whereas filled small-sized dodecahedral D cages shift PIA successively to higher pressures with increasing size of the Ng guest. For structure I, filling of both kinds of cages, large-sized T and small-sized D, acts to stabilize in a cooperative fashion. Xe hydrate represents a special case. In MD, disordering of the guest hydration structure is already seen at around 2.5 GPa. However, the different coordination numbers of the two types of guests in the crystalline cage structure are preserved, and the state is shown to produce a Bragg diffraction pattern. The experimentally observed diffraction up to 4 GPa is attributed to this semicrystalline state.

Stability and NMR Chemical Shift of Amorphous Precursors of Methane Hydrate: Insights from Dispersion-Corrected Density Functional Theory Calculations Combined with Machine Learning.

Clathrate hydrates of natural gases are important backup energy sources. It is thus of great significance to explore the nucleation process of hydrates. Hydrate clusters are building blocks of crystalline hydrates and represent the initial stage of hydrate nucleation. Using dispersion-corrected density functional theory (DFT-D) combined with machine learning, herein, we systematically investigate the evolution of stabilities and nuclear magnetic resonance (NMR) chemical shifts of amorphous precursors from monocage clusters CH4(H2O)n (n = 16-24) to decacage clusters (CH4)10(H2O)n (n = 121-125). Compared with planelike configurations, the close-packed structures formed by the water-cage clusters are energetically favorable. The 512 cages are dominant, and the emerging amorphous precursors may be part of sII hydrates at the initial stage of nucleation. Based on our data set, the possible initial fusion pathways for water-cage clusters are proposed. In addition, the 13C NMR chemical shifts for encapsulated methane molecules also showed regular changes during the fusion of water-cage clusters. Machine learning can reproduce the DFT-D results well, providing a structure-energy-property landscape that could be used to predict the energy and NMR chemical shifts of such multicages with more water molecules. These theoretical results present vital insights into the hydrate nucleation from a unique perspective.

Dependence of the Kinetic Hydrate Inhibition Effect of Poly(N-vinylpyrrolidone) upon the Molecular Weight Is Influenced by Water Mobility in Millisecond Dynamics

Explosive formation of a natural gas clathrate hydrate in production lines has been a technical bottleneck in ocean field natural gas drilling and gas transport. To mitigate this problem, polymer i...

Controlling water dynamics for kinetic inhibition of clathrate hydrate

Subsea natural gas drilling and gas transport is challenged by the catastrophic formation of gas clathrate hydrate in production lines. Polymer inhibitors are vital to mitigate this technical problem in subsea gas drilling operation. However, physico- chemical factors affecting their performance are not yet completely known. Herein, we report that the activities of synthetic kinetic inhibitors of clathrate hydrates are correlated to the surrounding water dynamics. Two types of linear polyamide inhibitors are found to be capable of producing immobilized nonfreezable water molecules with fast transverse relaxation in sub-millisecond to millisecond time-scale as revealed by NMR relaxometry. Such a unique state of water can be construed as a form of nonfreezable bound water measured by differential scanning calorimetry. The quantity of such water species can be tuned by the size of the hydrophobic groups introduced to the polyamides and result in longer inhibition time before the rapid growth of the hydrate crystals. Addition of a single hydroxyl side group in the polymer structural unit can also affect the dependence of the kinetic inhibition time on the amount of the supercooled water. Both the hydrophobic and hydrophilic groups in the polymer inhibitors exert coordinated influence on the dynamic properties of the bound water, which are responsible for the early recognition and inhibition of the clathrate hydrate clusters.

The water molecule arrangement over the side chain of some aliphatic amino acids: A quantum chemical and bottom‐up investigation

AbstractThe geometry of surrounding water molecules on the side chain of glycine, alanine, α‐aminoisobutyric acid, α‐aminobutyric acid, valine, and related hydrocarbons has been analyzed combining bottom‐up and quantum chemical methodologies. To minimize the cavity size and to prevent water‐water hydrogen bonding loss, the water molecules adopt a shape, resembling the one found in crystal structure of gas clathrate hydrates, with water molecules tangentially oriented to the surface of hydrophobic side chain. The cage is directly hydrogen bonded to the backbone's polar groups, thus hydration shells around hydrophobic and hydrophilic groups are folded together in amphiphilic molecules. The hydrophobe enclathration implies a substantial freedom degree reduction which makes it entropically disfavored. This disadvantageous entropic contribution is partially compensated by the favorable van der Waals interactions with guest in stabilizing clathrate hydrate formation. The water shell around the side chain relates intimately with the side‐chain rotational isomerism. Present data are correlated with the experimental determined populations of the three rotamers, yielding promising results for both α‐aminobutyric acid and valine.

Study the effect of organic promoters on thermodynamics behaviour for refrigerant gas clathrate hydrate

The present work consists of an experimental and theoretical investigation dealing with hydrate formation for the binary system (water+ refrigerant gas) and ternary systems (water+ refrigerant gas+ promoter) at constant initial conditions (pressure and temperature). Isochoric method (constant volume) search was used in current study to measure the formation data (pressure, temperature) for hydrate. Refrigerant R134a gas and different organic promoters such as benzene and cyclohexane were used in this study. For ternary systems at the different concentrations of promoters shifted hydrate formation data when benzene and cyclohexane decree pressure and increasing temperature. Many objective functions were obtained from kinetic models such as the amount of gas consumed (Δn), the growth rate (r (t)), and conversion of the water to hydrate. The gas consumed (Δn) of the binary system increased lead to increase in hydrate formation, also the hydrate growth rate (r(t)) and increase water conversion to hydrate. The gas consumed in the presence of cyclohexane was higher than at using benzene as a promoter. The experimental results show that the presence of different promoters in the ternary systems have an effect on thermodynamic promotion of refrigerant hydrates formation. The promotion effect increased with increasing the concentration of promoter in the select range. Also, the results manifested that benzene and cyclohexane had a clear effect in reducing the pressure and raising the temperature of the hydrate formation. The storage capacity was investigated. It has been noticed that the existence of the promoters in the hydrate formation system led to a slight increase in the storage capacity with maximum value about (78.66) V/V.

Revealing CO-Preferential Encapsulation in the Mixed CO–N2 Clathrate Hydrate

In addition to their various applications in the environment, energy and technology fields, gas clathrate hydrates, called gas hydrates (inclusion compounds composed of a H-bonded water network for...

Grand Canonical Monte Carlo Simulations on Phase Equilibria of Methane, Carbon Dioxide, and Their Mixture Hydrates

The cage occupancy plays a crucial role in the thermodynamic stability of clathrate hydrates and is an important quantity for understanding the CO2-CH4 replacement phenomenon. In this work, the occupancy isotherms of pure CH4, pure CO2, and their mixture in sI and sII hydrates are studied by GCMC + MD simulations. The adsorption of CH4 and CO2 + CH4 in the sI and sII hydrates can be categorized as the one-site Langmuir type. The calculated occupancy ratio θL/θS and the abundance ratio of CO2 to CH4 vary with the temperature and pressure, which provide the prerequisite information for the prediction of CH4 recovery yield at different conditions in the CO2-CH4 gas exchange process. The phase equilibria of clathrate hydrates of pure gases and mixtures are explored and the corresponding heat of dissociation and hydration numbers are determined. The current investigation provides new perspectives to understand the mechanism behind the gas adsorption behavior of clathrate hydrates.

On the phase behaviors of hydrocarbon and noble gas clathrate hydrates: Dissociation pressures, phase diagram, occupancies, and equilibrium with aqueous solution.

We apply a statistical mechanical theory on clathrate hydrates to an exploration of the phase behaviors of hydrocarbon and noble gas clathrate hydrates. Two- and three-phase coexisting conditions in the whole space of thermodynamic variables (temperature, pressure, and composition) are evaluated only from intermolecular interactions for water and guest species. The occupancy of guest molecules in various types of cages is also calculated. We find that a small difference in the guest size gives rise to a rich variety of phase behaviors, notably for the shape of the two-phase boundary and the occupancy. Ethane clathrate hydrate is found to exhibit the most drastic and intriguing features in various properties arising from its non-stoichiometry. We investigate the phase behaviors of clathrate hydrate in terms of the partial molar quantities derived from the chemical potentials of guest and water. Our method also allows exploring the aqueous solution of an apolar guest molecule in the low temperature and high pressure regime coexisting with the corresponding clathrate hydrate for which the free guest fluid phase is substituted at high temperatures. It is shown that the temperature dependence of methane solubility in liquid water in the presence of clathrate hydrate is opposite to that being in equilibrium with the methane fluid without clathrate hydrate. This phenomenon is elucidated by a substantial decrease in the chemical potential of methane from the hydrate/guest boundary to the hydrate/water.

Antiagglomerants Affect Gas Hydrate Growth.

In gas clathrate hydrates, inclusion gas molecules stabilize crystalline water structures. In addition to being fundamentally interesting, gas hydrates attract significant practical attention because of their possible application in various high-tech technologies. However, gas hydrates pose health, safety, and environmental risks when they form within oil and gas pipelines, as well as within hydrocarbon-producing and treatment facilities. Among available strategies to control and sometimes prevent hydrate plug formation is the use of surface-active low-molecular-weight compounds, known as antiagglomerants (AAs). AAs prevent the agglomeration of small hydrate particles into large plugs. It is not clear whether AAs promote or frustrate hydrate growth. We present two molecular mechanisms by which AAs promote and frustrate, respectively, hydrate growth. Our results could lead to innovative methodologies for managing hydrates in high-tech applications, as well as for securing the safety of oil and gas operations.

Clathrate Hydrates of Greenhouse Gases in the Presence of Natural Amino Acids: Storage, Transportation and Separation Applications

Storage of greenhouse gases in the form of gas hydrates is attractive and is being pursued rigorously in recent times. However, slow formation rate and inefficient water to hydrate conversion are the main hindering factors. In this report, we examine the role of two amino acids (0.5 wt%), l-methionine (l-met) and l-phenylalanine (l-phe) on the formation of gas hydrates using methane (CH4), carbon dioxide (CO2) and their mixtures as guest molecules. Experiments are conducted under non-stirred and isochoric configurations. The hydrate conversion efficiency of both amino acids is identical for hydrates formed with CH4 and mixture of (CO2+CH4). However, the hydrate conversion is significantly less in CO2 hydrates in l-phe system. Addition of amino acids to the water dramatically improved the kinetics of hydrate formation and 90% of maximum gas uptake in hydrate phase occurred in less than an hour. The water to hydrate conversion is also very efficient (>85%) in the presence of amino acids. Therefore, the amino acids containing systems are suitable for storing both CH4 and CO2 gases. The gas hydrates were characterised using powder x-ray diffraction (XRD) and Raman spectroscopic measurements. These measurements indicate the formation of sI hydrates and encasing of gas molecules as guests.

Guest Partitioning and Metastability of the Nitrogen Gas Hydrate

Gas clathrate hydrates or gas hydrates are made of H-bonded water molecules forming cages, within which gaseous (guest) molecules are encapsulated. The formed clathrate structures, which may be metastable, depend on the nature and on the partitioning of the guest molecules in the water cage. This work focuses on the structural and vibrational properties of nitrogen hydrate in its two clathrate forms (namely, SI and SII) in the thermodynamic ranges 50–200 bar and 150–270 K, together with a comprehensive analysis of the transformation from SI to SII of this gas hydrate. The thermal expansion of both structures has been measured at 1 bar, and the melting of the nitrogen hydrate has been measured at ca. 210 K at 1 bar. Moreover, the SI structure is metastable in the studied pressure region: from time-dependent neutron powder diffraction analysis, it is shown that the SI structure transforms over time to the SII structure with a rate of (1.37 ± 0.17) × 105 s–1 at 100 K and at 1 bar. The transformation is also ...