sH Hydrate Research Articles

Microscopic Equilibrium Determination for Structure-H (sH) Clathrate Hydrates at the Liquid–Liquid Interface: Krypton–Liquid Hydrocarbon–Water System

The dissociation temperature of clathrate hydrates in the krypton (Kr)–liquid hydrocarbon (methylcyclohexane, MCH and methylcyclopentane, MCP)–water systems was established in the temperature range of (273.2 to 285.6) K and pressure range of (0.5 to 2.4) MPa using optical scanning microscopy. From powder X-ray diffraction data, it was established that the clathrate hydrates formed in the Kr–MCH–water and Kr–MCP–water systems had the structure H (sH). The dissociation pressure–temperature (pT) conditions of the krypton–liquid hydrocarbon–water systems are milder than the pT conditions of Kr hydrate. Large guest molecules captured in the 51268 cage affect the phase stability of the sH hydrate when compared to the phase-equilibrium data in the Kr–neohexane–water system.

Cage occupancies in the high pressure structure H methane hydrate: A neutron diffraction study

A neutron diffraction study was performed on the CD(4) : D(2)O structure H clathrate hydrate to refine its CD(4) fractional cage occupancies. Samples of ice VII and hexagonal (sH) methane hydrate were produced in a Paris-Edinburgh press and in situ neutron diffraction data collected. The data were analyzed with the Rietveld method and yielded average cage occupancies of 3.1 CD(4) molecules in the large 20-hedron (5(12)6(8)) cages of the hydrate unit cell. Each of the pentagonal dodecahedron (5(12)) and 12-hedron (4(3)5(6)6(3)) cages in the sH unit cell are occupied with on average 0.89 and 0.90 CD(4) molecules, respectively. This experiment avoided the co-formation of Ice VI and sH hydrate, this mixture is more difficult to analyze due to the proclivity of ice VI to form highly textured crystals, and overlapping Bragg peaks of the two phases. These results provide essential information for the refinement of intermolecular potential parameters for the water-methane hydrophobic interaction in clathrate hydrates and related dense structures.

Phase Equilibrium Modeling of Structure H Clathrate Hydrates of Methane + Water “Insoluble” Hydrocarbon Promoter Using QSPR Molecular Approach

In this communication, the quantitative structure–property relationship (QSPR) strategy is applied to present two molecular models for determination of the structure H (sH) hydrate dissociation conditions with methane as help gas. Twenty-one water “insoluble” hydrocarbon promoters are examined. To propose reliable models, almost all of the available literature data are studied. Two mathematical methods including the genetic-algorithm-based multivariate linear regression (GA-MLR) and the least square support vector machines (LSSVM) are applied for determination (selection) of the model parameters. As a result, two reliable models are developed: (1) QSPR-GA-MLR linear model and (2) QSPR-LSSVM nonlinear model with satisfactory results quantified by the following statistical parameters: absolute average deviations (AAD) of the represented/predicted hydrate dissociation pressures from existing experimental values: about 9%, and squared correlation coefficient: 0.956 in the case of using the first model, and about 4% and 0.992 through applying the second model, respectively. These results demonstrate much better accuracy through the QSPR-LSSVM nonlinear model than applying the QSPR-MLR linear one.

Static Formation and Dissociation of Methane+Methylcyclohexane Hydrate for Gas Hydrate Production and Regasification

AbstractThe formation and decomposition of methane+methylcyclohexane (MCH) hydrate in a static batch reactor, which was also designed as a high‐pressure microwave reactor, were investigated. The addition of 300 ppm sodium dodecyl sulfate (SDS) provides continuous formation of CH4+MCH hydrate under static conditions. Increasing the initial pressure within the narrow range of 2.7 to 4.6 MPa at 274 K enhances the formation rate by even several times. The gas storage capacity can be largely improved with partial coexisting of sI CH4 hydrate. Unlike a stirred formation, an increase of nonaqueous MCH inhibits the static formation of sH hydrate. The following regasification by 2.45 GHz microwave heating indicates that the dissociation is rate‐controlled by the parallel connection of efficient internal heating and conventional external heating. The multiphase convection characterized by osmotic dehydration and driven by intensified regasification is considered as the dominant mechanism affecting the quiescent dissociation.

Morphological Change in Structure H Clathrates of Methane and Liquid Hydrocarbon at the Liquid–Liquid Interface

The crystal morphology of a structure H (sH) hydrate in a methane (CH4)–liquid hydrocarbon (methylcyclohexane and neohexane)–water system in the temperature range 273.4–277.3 K was investigated in situ by optical scanning microscopy. Near the phase equilibrium condition, the sH hydrate showed a hexagonal shape with respect to its crystalline space group (P6/mmm). As the pressure increased under constant temperature, the edges of the hexagonal sH hydrate grew, and then the sH hydrate changed from a hexagonal to three-dimensional six-branched shape. We found that the branching growth of the sH hydrate appears at a specific degree of subcooling, ΔT > 4 K. This indicates that the sH hydrate with a snowflakelike crystal shape can be obtained by controlling ΔT.

13C NMR Studies of Hydrocarbon Guests in Synthetic Structure H Gas Hydrates: Experiment and Computation

(13)C NMR chemical shifts were measured for pure (neat) liquids and synthetic binary hydrate samples (with methane help gas) for 2-methylbutane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, methylcyclopentane, and methylcyclohexane and ternary structure H (sH) clathrate hydrates of n-pentane and n-hexane with methane and 2,2-dimethylbutane, all of which form sH hydrates. The (13)C chemical shifts of the guest atoms in the hydrate are different from those in the free form, with some carbon atoms shifting specifically upfield. Such changes can be attributed to conformational changes upon fitting the large guest molecules in hydrate cages and/or interactions between the guests and the water molecules of the hydrate cages. In addition, powder X-ray diffraction measurements revealed that for the hexagonal unit cell, the lattice parameter along the a-axis changes with guest hydrate former molecule size and shape (in the range of 0.1 Å) but a much smaller change in the c-axis (in the range of 0.01 Å) is observed. The (13)C NMR chemical shifts for the pure hydrocarbons and all conformers were calculated using the gauge invariant atomic orbital method at the MP2/6-311+G(2d,p) level of theory to quantify the variation of the chemical shifts with the dihedral angles of the guest molecules. Calculated and measured chemical shifts are compared to determine the relative contribution of changes in the conformation and guest-water interactions to the change in chemical shift of the guest upon clathrate hydrate formation. Understanding factors that affect experimental chemical shifts for the enclathrated hydrocarbons will help in assigning spectra for complex hydrates recovered from natural sites.

New silica clathrate minerals that are isostructural with natural gas hydrates

Silica clathrate compounds (clathrasils) and clathrate hydrates are structurally analogous because both materials have framework structures with cage-like voids occupied by guest species. The following three structural types of clathrate hydrates are recognized in nature: cubic structure I (sI); cubic structure II (sII); and hexagonal structure H (sH). In contrast, only one naturally occurring silica clathrate mineral, melanophlogite (sI-type framework), has been found to date. Here, we report the discovery of two new silica clathrate minerals that are isostructural with sII and sH hydrates and contain hydrocarbon gases. Geological and mineralogical observations show that these silica clathrate minerals are traces of low-temperature hydrothermal systems at convergent plate margins, which are the sources of thermogenic natural gas hydrates. Given the widespread occurrence of submarine hydrocarbon seeps, silica clathrate minerals are likely to be found in a wide range of marine sediments.

Modeling of phase transformation sII–sH in Ar hydrates at high pressure

Dynamical and thermodynamical properties of Ar hydrates sII and sH with multiple filling of the large cages are studied within the framework of the lattice dynamics approach in the quasiharmonic approximation. The approach is based on previously suggested generalization of the van der Waals and Platteeuw theory of clathrate hydrates. The existence of structural transition from sII to sH structure in Ar hydrates at elevated pressure is shown. The calculation of Ar structure sH hydrate with single occupancy of medium and small cages and occupancies from 1 to 5 of large cages shows thermodynamic stability of these phases and agrees with the experimental data.

Computational approach to study hydrogen storage in clathrate hydrates

This work presents collectively, analyzes and compares the results from previous Monte Carlo studies performed towards the evaluation of the hydrogen-storage capacity of clathrate hydrates. Clathrate hydrates can be regarded as a special type of nanoporous material that consists of hydrogen-bonded water molecules that form cavities where gas molecules can be entrapped. Three different hydrates structures (sI, sII, and sH) are examined where hydrogen is either the single guest component or a second substance (promoter) coexists. The storage of hydrogen in hydrates can be simulated as a process of gas adsorption in a solid material and, consequently, the Grand Canonical Monte Carlo approach seems to be the most appropriate approach. These simulations are themselves consistent and, most importantly, they are in good agreement with the available experimental data. This fact illustrates the efficiency of the specific computational approach in the study of hydrates. The comparison between the three hydrate types shows that sH hydrates can store larger amounts of hydrogen than sI or sII hydrates. In particular, the maximum hydrogen content achieved is 3.6 wt% with pure hydrogen, and 1.4 wt% with the use of a promoter (at 274 K and 500 MPa).

Hydrogen Storage in sH Clathrate Hydrates: Thermodynamic Model

A thermodynamic model of equilibrium states of structure sH clathrate hydrates of hydrogen with methyl tert-butyl ether, methyl cylohexane, or 1,1-dimethyl cyclohexane is presented. The model uses the van der Waals-Platteeuw statistical-thermodynamical model to represent the hydrates and the cubic-plus-association equation of state to represent the fluid phases. Good agreement between experimental and calculated phase equilibrium data has been obtained, with average absolute pressure deviations between experiments and calculations ranging from 1.2 to 2.1% depending on the promoter. The model has also been used to estimate the occupancy of the cavities of the hydrate by hydrogen and the promoter, as well as the hydrogen storage capacity of the hydrate. This capacity has been found to vary between 0.85 and 1.05% of hydrogen by weight at the conditions of formation of the sH hydrates (270-280 K and 60-100 MPa).

THERMODYNAMICS AND HYDROGEN STORAGE ABILITY OF BINARY HYDROGEN + HELP GAS CLATHRATE HYDRATE

Storage of hydrogen as hydrogen hydrate is a promising alternative technology to liquefied hydrogen at cryogenic temperatures or compressed hydrogen at high pressures. In this paper, computer simulation is performed based on the solid solution theory of clathrates of van der Waals and Platteeuw with some modifications that include in particular the account of multiple cage occupancies and host relaxation. The quasiharmonic lattice dynamics method employed here gives the free energy of clathrate hydrate to first order in the anharmonicity of intermolecular potential and permits to take into account quantum zero-point vibration of host lattice and hydrogen in the cages. It is employed to study the thermodynamic functions of binary (mixed) H 2– CH 4 hydrates of cubic structure II (sII) and hexagonal structure H (sH). It is shown that at divariant equilibrium "gas phase–gas hydrate" with increasing pressure the filling of large cavities by hydrogen proceeds gradually from single filling to the maximal number of hydrogen molecules in clusters included in large cages (four in sII and five in sH) preserving stability of the hydrogen–methane hydrates sII and sH. The results show that mass fraction of hydrogen in the mixed sH hydrate is significantly lower than in the mixed sII hydrate. Pressure of monovariant equilibrium " IceI h–gas phase–mixed sII hydrate" with increasing methane concentration in the gas phase lowers in comparison with the pressure of pure hydrogen hydrate formation. For the mixed hydrogen + methane sH hydrates, it was demonstrated that thermodynamic stability depends on the filling degree of small cavities by methane molecules and stability area shifts to lower pressure with increasing filling.

Hydrogen Storage in sH Hydrates: A Monte Carlo Study

Grand canonical Monte Carlo simulations are performed to evaluate the hydrogen-storage capacity of the recently discovered hydrogen hydrates of the sH type, at 274 K and up to 500 MPa. First, the pure H2 hydrate is investigated in order to determine the upper limit of H 2 content in sH hydrates. It is found that the storage capacity of the hypothetical pure H2 hydrate could reach 3.6 wt % at 500 MPa. Depending on pressure, the large cavity of this hydrate can accommodate up to eight H2 molecules, while the small and medium ones are singly occupied even at pressures as high as 500 MPa. Next, the binary H2-methylcyclohexane sH hydrate is examined. In this case, the small and medium cavities are again singly occupied, resulting in a maximum H2 uptake of 1.4 wt %. Finally, the results from simulations on pure H2 and binary hydrates are utilized to investigate the potential of H2 storage in sH hydrates where the promoter molecules occupy the medium instead of the large cavities.

Monte Carlo study of sII and sH argon hydrates with multiple occupancy of cages

A series of grand canonical Monte Carlo simulations are performed to investigate the argon uptake inside sII and sH argon hydrates at 274 K and pressures up to 700 MPa. Our results indicate that the small cavities of the sII argon hydrate, as well as the small and medium cavities of the sH hydrate are singly occupied without the observation of any multiple occupancy. On the other hand, the large cavities of the sII hydrate are found to be doubly occupied, and the large cavities of the sH hydrate are found to be filled with up to five argon molecules. The results from our simulations are in very good agreement with the available experimental data concerning cage occupancies and with similar results from other types of molecular simulations.

Swapping Phenomena Occurring in Deep-Sea Gas Hydrates

On the basis of crystallographic analysis results, a recent study reported that structure H (sH) hydrate exists in the natural environment, providing direct evidence from hydrate samples recovered ...

Interactions between Structure H Hydrate Formers and Water Molecules

Studies on methane storage in hexagonal structure H (sH) hydrates reveal that hydrate stability conditions, formation rates, and cage occupancies differ for the three large molecule guest substances (LMGSs) studied: tert-butyl methyl ether (TBME), neo-hexane (NH), and methyl-cyclohexane (MCH). In this work, the structures of aqueous solutions with the LMGS are examined with molecular dynamics to gain insights into the interactions between water and LMGS. Water molecules form solvation cavities that follow the contours of the LMGS molecules and have diameters between 4 and 7 Å. The cavity size increases from TBME < NH < MCH, and the nearest water molecules to the LMGS have a local density that is higher by a factor of 2 than that of the bulk water phase. Water molecules interact strongly with the ether oxygen of TBME but not with other atoms in the LMGS molecules. This strong attraction gives TBME a greater solubility in water and better wetting on ice than NH or MCH. Moreover, TBME also has a larger diffusivity than NH in water. The greater contact between TBME and the host water molecules may cause the faster rate of initial hydrate growth from solid ice particles (fixed bed) or liquid water with mixing for this guest species. However, the strong attraction of the TBME molecule with water may also slow down the subsequent crystal growth and may perhaps limit the occupancy of the other guests (methane) in the sH clathrate. The volume change due to solvation is negative for all three LMGSs. This suggests a more compact hydrogen bond network in the hydration shell that may facilitate the formation of clathrates when a hydrate seed is available and when mass and heat transfer resistances are negligible. For example, this may occur during hydrate formation from a fixed ice bed with temperature ramping. Hence, the clathrate growth with hydrophobic guests (NH and MCH) proceeds rapidly toward completion as the temperature is increased above the ice melting point. On the other hand, the formation of hydrate cages for the TBME system is disrupted because of the strong water−solute interactions. Consequently, hydrate formation from ice with thermal ramping with TBME is slower.

Molecular dynamics study of structure H clathrate hydrates of methane and large guest molecules

Methane storage in structure H (sH) clathrate hydrates is attractive due to the relatively higher stability of sH as compared to structure I methane hydrate. The additional stability is gained without losing a significant amount of gas storage density as happens in the case of structure II (sII) methane clathrate. Our previous work has showed that the selection of a specific large molecule guest substance (LMGS) as the sH hydrate former is critical in obtaining the optimum conditions for crystallization kinetics, hydrate stability, and methane content. In this work, molecular dynamics simulations are employed to provide further insight regarding the dependence of methane occupancy on the type of the LMGS and pressure. Moreover, the preference of methane molecules to occupy the small (5(12)) or medium (4(3)5(6)6(3)) cages and the minimum cage occupancy required to maintain sH clathrate mechanical stability are examined. We found that thermodynamically, methane occupancy depends on pressure but not on the nature of the LMGS. The experimentally observed differences in methane occupancy for different LMGS may be attributed to the differences in crystallization kinetics and/or the nonequilibrium conditions during the formation. It is also predicted that full methane occupancies in both small and medium clathrate cages are preferred at higher pressures but these cages are not fully occupied at lower pressures. It was found that both small and medium cages are equally favored for occupancy by methane guests and at the same methane content, the system suffers a free energy penalty if only one type of cage is occupied. The simulations confirm the instability of the hydrate when the small and medium cages are empty. Hydrate decomposition was observed when less than 40% of the small and medium cages are occupied.

Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation

Storage and transportation of natural gas as gas hydrate (“gas-to-solids technology”) is a promising alternative to the established liquefied natural gas (LNG) or compressed natural gas (CNG) technologies. Gas hydrates offer a relatively high gas storage capacity and mild temperature and pressure conditions for formation. Simulations based on the van der Waals–Platteeuw model and molecular dynamics (MD) are employed in this study to relate the methane gas content/occupancy in different hydrate systems with the hydrate stability conditions including temperature, pressure, and secondary clathrate stabilizing guests. Methane is chosen as a model system for natural gas. It was found that the addition of about 1% propane suffices to increase the structure II (sII) methane hydrate stability without excessively compromising methane storage capacity in hydrate. When tetrahydrofuran (THF) is used as the stabilizing agent in sII hydrate at concentration between 1% and 3%, a reasonably high methane content in hydrate can be maintained (∼85–100, v/v) without dealing with pressures more than 5 MPa and close to room temperature. The free energy calculations from MD suggest that the methane storage capacity may be increased when methane molecules are placed inside the large cage of structure H (sH) hydrate. However, methane storage in pure methane sH clathrates with multiple methane occupancies are only of theoretical interest as the required pressure is about 0.5 GPa or higher. It has also been demonstrated that methane may replace tert-butyl methyl ether (TBME) as the large guest species in the large cage.

Gauche conformation of acyclic guest molecules appearing in the large cages of structure-H clathrate hydrates

In the present study, measurements and analyses were made of the High-Power Decoupling (HPDEC) solid-state 13C NMR spectra of structure-H (sH) methane hydrates with isopentane, one of the simplest and smallest acyclic large guest molecules, and methylcyclohexane (MCH), a commonly used cyclic guest molecule that is larger than isopentane. From the spectroscopic information, clear and definite evidence for the molecular conformation of acyclic guest molecules that are sufficiently small so as to be entrapped into the structure-H large cage (sH-L) was expected. The 13C NMR chemical shift change was additionally checked through the use of a hydrogen-hydrogen steric perturbation model. From the overall results, we concluded that one of the smallest acyclic guest molecules, isopentane, participating in the formation of a structure-H clathrate hydrate is encaged, confirming the gauche conformation in large cavities. The present results strongly suggest that the guest position and structure in hydrate cages are greatly influenced by both short-range interactions between guest molecules and cage frameworks and long-range interactions between small and large guests. Accordingly, cage dynamics must be carefully considered when a specific sH hydrate is designed and synthesized for the purpose of tuning material properties.

Methane conversion rate into structure H hydrate crystals from ice

AbstractThe methane uptake and conversion rate to structure H (sH) hydrates was measured and compared to crystallization kinetics models. Three large molecule guest substances (LMGS) were used as sH hydrate formers: neohexane (NH), methylcyclohexane (MCH), and tert‐butyl methyl ether ( TBME). The initial crystallization occurred quickly at the LMGS liquid‐ice interface until ∼20–30% of ice was converted into hydrate (hydrate growth stage I). Slower hydrate crystal growth was observed after a hydrate film covered the ice surface at a rate of 3–400 nm2/h (hydrate growth stage II). The TBME system showed the fastest kinetics at the beginning of the reaction followed by NH and MCH system. However the trend changed when the temperature was increased (“reaction” stage III). Surprisingly, the conversion rate achieved with the TBME system upon melting the ice was the smallest. This was attributed to the strong interaction of TBME with water molecules that increased the energy barrier for water molecules to form hydrate cages. The conversion rates were well correlated with the Avrami equation and the shrinking core model. Finally, NH was found to be the best LMGS in this study to obtain full conversion within a short reaction time and achieving high methane gas storage in the hydrate. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Characterization of gas hydrates with PXRD, DSC, NMR, and Raman spectroscopy

Structure I (sI) and H (sH) hydrates containing methane were synthesized and characterized with PXRD, DSC, NMR, and Raman spectroscopy. Three well-known large molecule guest substances (LMGSs) were selected as sH hydrate formers: 2,2-dimetylbutane (NH), methylcyclohexane (MCH), and tert-butyl methyl ether (TBME). The solid phase analysis confirmed the presence of sH hydrate whenever a LMGS was present. The presence of a non-hydrate former (n-heptane) did not affect the methane hydrate structure or cage occupancies. Ice to hydrate conversion was limited when the LMGS amount was less than stoichiometric and synthesized at low methane pressure, but nearly complete conversion was achieved with temperature ramping and excess LMGS. The methane occupancies were found to depend on the type of LMGS and increased with pressure. The hydrate with TBME was found to have the smallest methane content followed by the hydrates with NH and MCH. Both NMR and Raman identified methane and LMGS signals from the hydrate phase, however, the cage occupancy values of sH hydrate can only be obtained from NMR spectroscopy. The hydrate structures, ice to hydrate conversion, gas content in hydrate and cage occupancy from the various measurements are consistent with each other.