Xe Hydrate Research Articles

Thermodynamic and computational approaches to the stability and structural transformation of Xe + large-molecule guest substance (LMGS) hydrates

In this study, the influence of different types of large-molecule liquid substances (LMGSs) and formation conditions on the stability and structural transformation of Xe + LMGS hydrates was investigated using phase equilibria, powder X-ray diffraction (PXRD), 129Xe NMR spectroscopy, and molecular dynamics (MD) simulations. The phase equilibria of Xe hydrates in the presence of three different LMGS types (neohexane [NH], methylcyclopentane [MCP], and methylcyclohexane [MCH]) were measured in the pressure range of 0.1–1.0 MPa. The equilibrium curves of Xe + MCH and Xe + NH hydrates shifted away from that of pure Xe hydrate under low pressure, whereas they coincided with that of pure Xe hydrate under high pressure. Interestingly, the equilibrium curve of Xe + MCP hydrate aligned with that of pure Xe hydrate throughout the pressure range. PXRD and 129Xe NMR analyses demonstrated that the equilibrium curve shift under low pressure was caused by the formation of sH (Xe + LMGS) hydrates, which was attributed to the inclusion of NH or MCH. However, MCP did not participate in sH hydrate formation with Xe. MD simulations revealed that MCP in the large cages did not contribute to the thermodynamic or dynamic stability of sH hydrates. The findings of this study provide valuable insights into the development of hydrate-based noble gas capture and storage.

A pore-scale study on microstructure and permeability evolution of hydrate-bearing sediment during dissociation by depressurization

Research data and visual evidence to understand the interaction between hydrate dissociation and micro-seepage in sediment are currently lacking. In this study, X-ray computed tomography (CT) is used to reveal the dynamic interaction process between sediment pore-scale structural changes and permeability during hydrate dissociation. After Xe hydrate formation in spherical quartz and then decomposed the hydrate by depressurization to obtain the real-time permeability and 3D structure of hydrate-bearing sediments (HBSs) with CT scans. The results show that the heterogeneity of hydrate spatial distribution does not change the power function relationship between permeability and hydrate saturation. The ball-and-stick model shows that the number and size of flow channels dominate the flow resistance of the fluid at high hydrate saturation, while the size of flow channels dominates it at low hydrate saturation. The saturation cut-off point is between 16.35% and 25.37%. The CT scan images show that the hydrate morphology changes from patchy to load-bearing, and then to cementing. The transition process from load-bearing to cementing has the greatest impact on sediment permeability. Further PNM simulations show that the size and number of flow channels and capillary forces are the main factors affecting the ability of gas–water two-phase flow in the sediment. Most importantly, a relative permeability model related to hydrate saturation parameters is proposed based on image processing technology. The good agreement between the model and available laboratory/field measurements proves its practicability. The method of proposing this model provides a new idea to establish new relative permeability models.

In situ inelastic neutron scattering of mixed CH4–CO2 hydrates

An abundant source of CH4 can be found in natural hydrate deposits. Recent demonstration of CH4 recovery from hydrates via CO2 exchange has revealed the potential as a fuel source that also provides a medium for carbon sequestration. It is vital to understand the structural and dynamic impacts of guest variation in CH4, CO2, and mixed hydrates and link the results to the stability of various deposits in nature, harvesting methane, and sequestering CO2. Molecular vibrations are examined in CH4, CO2, and mixed CH4-CO2 hydrates at 5 and 190 K and Xe hydrates for comparison. Inelastic neutron scattering (INS) is an ideal spectroscopy technique to observe the dynamic modes in the hydrate structure and enclathrated CH4, as it is extremely sensitive to 1H. The presence of CO2 in hydrates tightens the lattice. It introduces more active librational modes to the host lattice, while hindering the motion of CH4 in mixed CH4-CO2 hydrate at 5 K. At 190 K, a large broadening of the CH4 librational modes indicates disorder in the structure leading to dissociation.

Thermodynamic and Structural Investigation of Xe and Kr Hydrates Containing Tetrahydrofuran for Applications in Gas Capture and Storage

Noble gases, such as xenon (Xe) and krypton (Kr), can be captured in hydrate cages, and thus, gas hydrates can be used as capture and storage media for these gases. This study examined the phase equilibria, crystal structure, and guest distributions of Xe and Kr hydrates in the presence of tetrahydrofuran (THF). The three-phase [hydrate (H)–liquid (L)–vapor (V)] equilibria of the Xe + THF + water and Kr + THF + water systems were measured at two different concentrations of THF (1.0 and 5.6 mol %) to examine the thermodynamic promotion effect of THF on Xe and Kr hydrates. The inclusion of THF in the hydrate cages led to significant shifts in the equilibrium curves of the Xe and Kr hydrates to lower pressure and higher temperature regions. The powder X-ray diffraction patterns and the 129Xe and 13C nuclear magnetic resonance spectra revealed that THF was enclathrated in the large (51264) cages of structure II hydrates and that the small (512) cages were occupied by Xe (or Kr). In particular, at 1.0 mol % THF, the large 51264 cages were partly occupied by Xe (or Kr) as a result of tuning behavior. The experimental results expand the basic understanding of hydrate-based gas capture and storage for Xe and Kr.

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.

Lateral growth of xenon hydrate films on mica

<abstract> <p>In this paper, we report an <italic>in situ</italic> optical microscopy study of lateral growth of xenon (Xe) hydrate thin films on mica at sub-zero temperatures. The interactions between a solid surface and water molecules can strongly affect the alignment of water molecules and induce ice-like ordered structures within the water layer at the water-surface interface. Mica was chosen as a model surface to study the surface effect of hydrophilic sheet silicates on the lateral growth of Xe hydrate films. Under the experimental conditions, the lateral growth of Xe hydrate films was measured to be at an average rapid rate of ~200 μm/s and 400 μm/s under two different pressures of Xe. Mass transfer estimation of the Xe-water system revealed that the increasing trend of lateral film growth rates followed the increase in the net mass flux and aqueous solubility of Xe. However, as the supercooling temperature increased, the trend of lateral film growth rates attained a plateau region where little change in the rate was observed. This unique feature in the lateral film growth trend, the fast lateral growth kinetics, and the short induction time for hydrate film growth hinted at the assistance of the mica surface to aid the lateral growth process of Xe hydrate films at low Xe mass flux and at a low degree of subcooling. A mechanism based on the reported structured water layer at the interface on mica was proposed to rationalize a postulated surface-promotional effect of mica on the nucleation and lateral growth kinetics of Xe hydrate films.</p> </abstract>

Dynamic 3D imaging of gas hydrate kinetics using synchrotron computed tomography

The availability of natural gas hydrates and the continuing increase in energy demand, motivated researchers to consider gas hydrates as a future source of energy. Fundamental understanding of hydrate dissociation kinetics is essential to improve techniques of gas production from natural hydrates reservoirs. During hydrate dissociation, bonds between water (host molecules) and gas (guest molecules) break and free gas is released. This paper investigates the evolution of hydrate surface area, pore habit, and tortuosity using in-situ imaging of Xenon (Xe) hydrate formation and dissociation in porous media with dynamic three-dimensional synchrotron microcomputed tomography (SMT). Xe hydrate was formed inside a high- pressure, low-temperature cell and then dissociated by thermal stimulation. During formation and dissociation, full 3D SMT scans were acquired continuously and reconstructed into 3D volume images. Each scan took only 45 seconds to complete, and a total of 60 scans were acquired. Hydrate volume and surface area evolution were directly measured from the SMT scans. At low hydrate saturation, the predominant pore habit was surface coating, while the predominant pore habit at high hydrate saturation was pore filling. A second-degree polynomial can be used to predict variation of tortuosity with hydrate saturation with an R2 value of 0.997.

Structure and Density Comparison of Noble Gas Hydrates Encapsulating Xenon, Krypton and Argon.

Understanding the effect of guest species on the host framework is important for the development of structure-based properties of inclusion compounds. Herein, the crystal structures of the noble gas hydrates encapsulating Xe, Kr, and Ar were studied by powder X-ray diffraction measurements. The crystal structures and hydration numbers of these noble gas hydrates were solved by Rietveld refinements using optimized models with the direct-space technique. It was revealed that host cage size of these hydrates changed depending on the type of guest species even though their unit-cell parameters were the same. Based on the structure models obtained, the densities of Xe, Kr, and Ar gas hydrates were also determined to be 1.837, 1.445 and 1.097 g/cm3 at 93 K, respectively. Our findings, from a crystallographic point of view, may give insight into further understanding the thermodynamic stability and physical properties of gas hydrates encapsulating small guests.

Four-Phase Equilibrium Relations Including Clathrate Hydrate Phase in a Ternary System of Xenon, Benzene, and Water

Thermodynamic stability boundaries of xenon (Xe) + benzene mixed hydrate system were measured in a temperature range of (274.1 to 284.4) K and pressure range up to 0.5 MPa. The four-phase equilibrium curve of structure-II hydrate (HsII), aqueous (Lw), C6H6-rich liquid (LHC), and gas (G) phases in the Xe+benzene mixed hydrate system intersects with the three-phase equilibrium curve (HsI+Lw+G) in the structure-I simple Xe hydrate system. The slope of HsII+Lw+LHC+G in the Xe+benzene mixed hydrate system changes around (283.00 ± 0.05) K and (0.420 ± 0.005) MPa. At temperatures above 283.00 K, the structure-II Xe+benzene mixed hydrate collapses and instead, the structure-I simple Xe hydrate is formed.

Theoretical performance analysis of hydrate-based heat engine system suitable for low-temperature driven power generation

We analyzed a heat engine using clathrate hydrate as its working media and evaluate the performance of this system operated with high and low temperature reservoirs of 295 K and 280 K “OTEC (Ocean Thermal Energy Conversion)” may be a prospective example of the technologies utilizing the small-temperature difference for power generation. This heat engine generates mechanical power through the cycle of following processes: hydrate formation at low temperature, pumping of hydrate, isobaric heating of hydrate, hydrate dissociation and adiabatic expansions of dissociated gas and water. The thermal efficiency for Kr, Xe, CH3F, CH2F2 and CH4 hydrates were evaluated. The analysis showed the dominant properties were the enthalpy difference of the working media in the adiabatic expansions, the pressure range in the whole process and the dissociation heat. The thermal efficiency is 2.20% for Kr hydrate and 2.89% for Xe hydrate. While these are slightly inferior to those of Rankine cycle: 3.30% for C2H3F3 and 3.34% for C3H8, Kr and Xe hydrates are greatly favorable in terms of environmental friendliness. These results indicate the prospects of the hydrate heat engine for the power generation utilizing a small temperature difference as an environment-friendly technology.

Crystal Phase Boundaries of Structure-H (sH) Clathrate Hydrates with Rare Gas (Krypton and Xenon) and Bromide Large Molecule Guest Substances

Phase equilibrium pressure–temperature (pT) boundaries of structure-H clathrate hydrates (sH hydrates) with rare gas (Kr and Xe)-bromide large molecule guest substances (LMGSs: bromocyclohexane, BrCH and bromocyclopentane, BrCP) were measured. The phase boundaries for the sH hydrates in the Kr–LMGS–water systems shifted to lower pressures than those for the pure Kr hydrate in the temperature range of (273.2 to 279.3) K. In this study, sH hydrate formation was not confirmed in the Xe–BrCP–water system, but sH hydrates were found in the Xe–BrCH–water system. At temperatures below 277 K, equilibrium conditions were observed at lower pressures for the Xe–BrCH–water system than for the pure Xe hydrate. However, the equilibrium pT curve for the Xe–BrCH–water system crossed over the equilibrium pT curve for the Xe hydrate at around 277 K. Intersections between the equilibrium pT curves for the Xe hydrates and the sH hydrates (Xe + LMGS) have also been found in Xe–methylcyclohexane–water systems. Using the Kr–and Xe–bromide LMGS–water systems showed that the sH hydrate phase stabilities are strongly related to the encaptured LMGS.

Separation Process of Nonpolar Gas Hydrate in Food Solution under High Pressure Apparatus

Separation process of nonpolar gas hydrate formation in liquid food was experimentally studied under high pressure container. Xenon (Xe) gas was selected as hydrate forming gas and coffee solution was used as a sample of liquid food. The high-pressure stainless steel container having the inner diameter of 60 mm and the volume of 700 mL with a U-shaped stirrer was designed to carry out this experiment. A temperature of 9.0°C and Xe partial pressure of 0.9 MPa were set as a given condition. The experiment was designed to examine the effect of steel screen size, formation rate, temperature condition, and amount of Xe gas dissolving in the solution on the separation process which was indicated by concentration efficiency. Screen size of 200 and 280 mesh resulted in higher concentration efficiency than that of 100 mesh. The higher stirring rate caused the higher formation rate of Xe hydrate and created the smaller Xe hydrate crystals. At the condition giving the same solubility in water, temperature of 14.8°C resulted in lower concentration efficiency than 9.0°C. The increase in the amount of Xe gas dissolving in coffee solution caused the concentration efficiency to decrease; however, the concentration ratio between the final and initial concentration of the solution increased.

Low-Frequency Raman Scattering in a Xe Hydrate

The physics of gas hydrates are rich in interesting phenomena such as anomalies for thermal conductivity, self-preservation effects for decomposition, and others. Some of these phenomena are presumably attributed to the resonance interaction of the rattling motions of guest molecules or atoms with the lattice modes. This can be expected to induce some specific features in the low-frequency (THz) vibrational response. Here we present results for low-frequency Raman scattering in a Xe hydrate, supported by numerical calculations of vibrational density of states. A number of narrow lines, located in the range from 18 to 90 cm(-1), were found in the Raman spectrum. Numerical calculations confirm that these lines correspond to resonance modes of the Xe hydrate. Also, low-frequency Raman scattering was studied during gas hydrate decomposition, and two scenarios were observed. The first one is the direct decomposition of the Xe hydrate to water and gas. The second one is the hydrate decomposition to ice and gas with subsequent melting of ice. In the latter case, a transient low-frequency Raman band is observed, which is associated with low-frequency bands (e.g., boson peak) of disordered solids.

Phase Equilibrium Conditions for Clathrate Hydrates of Tetra-n-butylammonium Bromide (TBAB) and Xenon

Phase equilibrium pressure–temperature (pT) conditions for the xenon (Xe)–tetra-n-butylammonium bromide (TBAB)–water system were characterized by an isochoric method in the pressure range from (0.05 to 0.3) MPa using TBAB solutions with mole fractions ranging from (0.0029 to 0.0137). The phase equilibrium pT conditions in the system appeared at a lower pressure and higher temperature than in the pure Xe hydrate. Furthermore, under atmospheric pressure, the dissociation temperature in the Xe–TBAB–water system shifted to a higher region than in the pure TBAB hydrate. In the experimental TBAB concentration range, the powder X-ray diffraction patterns of the Xe–TBAB–water system revealed that the TBAB clathrate hydrate is TBAB·38H2O.

Neutron diffraction study on the Xe behavior in clathrate hydrate analyzed by Rietveld/maximum entropy method

We analyzed the neutron scattering length density distributions of xenon deuterohydrate by Rietveld and maximum entropy method analyses to clarify the Xe motion behaviour in the clathrate cages. The crystal structure of Xe hydrate was refined as a structure I type clathrate hydrate which consists of 46 ice molecules in a framework of two dodecahedral cages (small cages) and six tetrakaidecahedral cages (large cages). In the small cage, a spherical nuclear density attributed to the Xe atom was observed at the center of the cage. On the other hand, an ellipsoid shaped nuclear density distribution was observed at the center of the large cage; the major axis of the ellipsoid nuclear density was directed toward the center of a hexagonal facet of the large cage. The distributions phenomena of the Xe atom in the both cages were independent of temperature.

Observation of Xe Hydrate Growth at Gas−Ice Interface by Microfocus X-ray Computed Tomography

The growth of Xe clathrate hydrate in a Xe−ice system was observed in situ by microfocus X-ray computed tomography (CT). In the initial stage of pressurization with Xe gas, quasi-two-dimensional hydrate growth on the ice surface was observed at the Xe−ice interface. With time, Xe hydrate grain grew not only along the ice surface (cover growth), but also toward the gas phase (outer growth), and inside the ice phase (inner growth). A lens-shaped Xe hydrate was formed at the initial interface between the gas phase and ice. Anisotropic hydrate growth resulted in a lens-shaped Xe hydrate. Xe hydrate growth at the gas−ice interface is strongly influenced by the supply of H2O molecules. The inner growth in the lens-shaped Xe hydrate changed in shape from spherical to elliptical. Variations in the contrast in cross-sectional CT images show that the inside of the Xe hydrate region became condensed as time elapsed. Inner growth would result from the formation of a condensed hydrate region, which may lead to a change in the Xe gas supply through the hydrate layer. Observations of Xe hydrate growth by microfocus X-ray CT can be applied to a discussion of the hydrate growth process in a gas−ice system.

Heat transfer in crystalline clathrate hydrates at low temperatures

The experimental results on the thermal conductivity κ(T) of crystalline Xe, CH4, and THF clathrate hydrates are analyzed. In a wide region of temperatures above 2K, κ(T) exhibits a behavior typical of disordered solids, which depends weakly on their chemical composition, crystalline structure, and microstructure. The results are discussed in the context of phenomenological models of phonon scattering by local modes. It is found that the Xe clathrate has a feature unusual for glasses, namely, κ(T) decreases almost two-fold as the temperature increases from 50 to 100K. The behavior of κ(T) is presumably determined mainly by the strong phonon scattering on water molecules.

Molecular dynamics study on the structure I xenon hydrate

A 368 water molecule structure I gas hydrate encased with 22 xenon molecules have been calculated by molecular dynamical simulations. The potential TIP4P was used for water interactions and Lennard-Jones for Xe-Xe and Xe-water interactions. There is a flat region and a local mininum between 75.0 and 100 K in the potential energy curve. Glassy phase transition for Xe hydrate is predicted in this region by analyzing anomalous heat capacity, phonon density of states, and radial distribution functions.

Thermodynamic and Raman Spectroscopic Studies of Xe and Kr Hydrates

The thermodynamic stability boundaries for two noble gas (Xe and Kr) hydrates have been investigated in a pressure range up to 445 MPa and a temperature range of (324.4 to 345.3) K for the Xe hydrate system and (274.4 to 320.0) K for the Kr hydrate system. The Raman spectrum of the intermolecular O−O stretching vibration mode has been evaluated for each hydrate single crystal along with the three-phase coexisting curve. The slope change on the boundary curve and the pressure dependence on the Raman shift for the Kr hydrate crystal reveal that a structural phase-transition point exists at 414 MPa and 319.20 K, whereas the Xe hydrate crystal of structure I does not exhibit any phase transition over the region studied.

Using a quartz crystal microbalance to probe formation of Xe hydrate in thin ice films

We have demonstrated the ability of the quartz crystal microbalance to successfully measure the formation and dissociation of thin films of Xe hydrate formed from ice films in the presence of Xe gas or from a cooled mixture of water vapor and Xe gas. By monitoring the uptake of mass, we have measured the formation of Xe hydrates on ice films in the temperature range 180<T<251K. Analysis of the mass uptake reveals a final sample that has 60% by weight of Xe, in good agreement with a fully hydrated sample. The formation and dissociation conditions suggest that there is a critical temperature T0∼255K beyond which thin films of Xe hydrate are not stable.