•Issues related to processes involving solid clathrate hydrates are discussed.•Current knowledge on the mechanism of hydrate formation is discussed.•Homo and heterogeneous nucleation of hydrates are ...both considered.•Proposed evidence for memory effects in hydrate reformation is discussed.•Directions to take in the future to understand these issues are outlined.
Among outstanding issues still to be understood regarding the clathrate hydrates are the mechanism of the processes involved in the formation and decomposition of clathrates: nucleation, decomposition, and the memory effect during reformation. The latter involves the shorter induction times required for solutions of decomposed hydrate to nucleate as compared to those for freshly prepared solutions. The formation of the clathrate hydrate phases of insoluble gases in water is accompanied by a ∼6000 fold concentration of the gas content in the solid phase compared to the aqueous phase from which it forms. The nucleation mechanism for the solid hydrate which allows the delivery of such high concentration of gas and water in one location has been the subject of much experimental and computational study. While these studies have improved our understanding of the nucleation process, many unknown aspects remain. These developments are described in this Opinion.
To study the effect of hydrogen isotope substitution on the uptake of water during formation of clathrate hydrates, the harmonic intermolecular librational modes of selected water clusters (X2O)n ...with n = 2–6 and hydrogen isotopes X = H, D, and T are studied. The effects of the quantum mechanical zero-point energy (ZPE) in each cluster on the binding energies of the H2O, D2O, and T2O clusters are determined, with ZPE leading to the smallest binding energies in the H2O clusters and the largest binding energies in the T2O clusters. Corrections for anharmonicity of the librational modes are considered, and these bring the frequency ranges of the calculated intermolecular librational modes in the clusters to the experimental ranges of the librational modes in the infrared spectra of H2O and D2O solid ice and clathrate hydrate phases, and liquid H2O water. These calculations show the expected ranges of the binding energy of tritiated water onto a solid ice and clathrate hydrate surface and can help quantify the isotopic enrichment on a growing clathrate hydrate phase from the solution.
We report a thermally induced phase transition of cubic structure II hydrates of tetrahydropyran (THP) and CO2 below about 140 K. The phase transition was characterized by powder X-ray diffraction ...measurements at variable temperatures. A dynamical ordering of the CO2 guests in small pentagonal dodecahedral 512 host water cages, not previously observed in the simple CO2 hydrate, occurs simultaneously with the symmetry lowering transition from a cubic structure II (space group Fd-3m with cell dimensions a = 17.3202(7) Å at 153 K) to a tetragonal (space group I41/amd with cell dimensions a = 17.484(4) Å and c = 12.145(1) Å at 138 K) unit cell. The effect of guest molecules on the phase transition at low temperatures is discussed, which demonstrates that the clathrate hydrate structures and thermodynamic properties can be modified by adjusting the size and chemical structure of larger and smaller guest molecules.
Display omitted
•Hydrate-based continuous fuel gas purification experiments.•Uses tetrahydropyran to moderate the operation pressure.•Long-term investigation on the time evolution in the gas and ...hydrate slurry phases.•CO2 mole fraction was maintained 1.00 only in a single stage.
Continuous hydrate-based CO2 separation is a compelling technology both from an environmental and economical perspective. We developed a method to continuously capture and separate CO2 from a gas mixture with H2 and determined the time-dependent changes of compositions in the gas and hydrate containing aqueous phases. Tetrahydropyran (THP), a representative water-soluble large molecular guest compound for structure II (sII) clathrate hydrate, was added in the reactor. THP forms hydrates under moderate conditions. The effects of THP mass fraction in the solution, wTHP, on the gas separation was simultaneously investigated by using wTHP = 0.11 and the stoichiometric value for sII hydrate, wTHP = 0.22. The results showed the gas phase hydrogen concentration reached steady state at 0.92 mol fraction from 0.60 in the feed gas. Also, the results demonstrated that sII hydrate has the highest CO2 loading capacity compared to ionic semi-clathrate hydrate phases. A continuous hydrate-based gas separation with sII hydrate has yet to be investigated. This study provides new experimental evidence which proves that the sII hydrate is the best structure for hydrate-based gas separation.
Position and orientation of water protons need to be specified when the molecular simulation studies are performed for clathrate hydrates. Positions of oxygen atoms in water are experimentally ...determined by X-ray diffraction analysis of clathrate hydrate structures, but positions of water hydrogen atoms in the lattice are disordered. This study reports a determination of the water proton coordinates in unit cell of structure I (sI), II (sII), and H (sH) clathrate hydrates that satisfy the ice rules, have the lowest potential energy configuration for the protons, and give a net zero dipole moment. Possible proton coordinates in the unit cell were chosen by analyzing the symmetry of protons on the hexagonal or pentagonal faces in the hydrate cages and generating all possible proton distributions which satisfy the ice rules. We found that in the sI and sII unit cells, proton distributions with small net dipole moments have fairly narrow potential energy spreads of about 1 kJ∕mol. The total Coulomb potential on a test unit charge placed in the cage center for the minimum energy∕minimum dipole unit cell configurations was calculated. In the sI small cages, the Coulomb potential energy spread in each class of cage is less than 0.1 kJ∕mol, while the potential energy spread increases to values up to 6 kJ∕mol in sH and 15 kJ∕mol in the sII cages. The guest environments inside the cages can therefore be substantially different in the sII case. Cartesian coordinates for oxygen and hydrogen atoms in the sI, sII, and sH unit cells are reported for reference.
Understandings of structure-based properties of porous materials, such as gas storage and gas separation performance, are important. Here, the crystal structures of the canonical structure II (sII) ...clathrate hydrates encapsulating cyclic molecules (tetrahydrofuran, cyclopentane, furan, and tetrahydropyran) are studied. To understand the effect of guest molecules on the host water framework, we performed powder X-ray diffraction measurements where the hydrate structures and guest distribution within 51264 cages were obtained by the direct-space technique followed by the Rietveld refinement. It was shown that the sizes of the 512 and 51264 cages of sII hydrates expand, as its unit-cell size is enlarged by the guest. In this process, it is revealed that the shape of 51264 cages with larger guest molecules became more spherical and volume ratio of empty small 512 cages in the unit cell decreases. Our findings from crystallographic point of view may give insights into better understanding of the thermodynamic stability and higher gas storage capacity of binary clathrate hydrates.
We use molecular dynamics simulations to study the structure, dynamics, and transport properties of nano-confined water between parallel graphite plates with separation distances (H) from 7 to 20 Å ...at different water densities with an emphasis on anisotropies generated by confinement. The behavior of the confined water phase is compared to non-confined bulk water under similar pressure and temperature conditions. Our simulations show anisotropic structure and dynamics of the confined water phase in directions parallel and perpendicular to the graphite plate. The magnitude of these anisotropies depends on the slit width H. Confined water shows "solid-like" structure and slow dynamics for the water layers near the plates. The mean square displacements (MSDs) and velocity autocorrelation functions (VACFs) for directions parallel and perpendicular to the graphite plates are calculated. By increasing the confinement distance from H = 7 Å to H = 20 Å, the MSD increases and the behavior of the VACF indicates that the confined water changes from solid-like to liquid-like dynamics. If the initial density of the water phase is set up using geometric criteria (i.e., distance between the graphite plates), large pressures (in the order of ~10 katm), and large pressure anisotropies are established within the water. By decreasing the density of the water between the confined plates to about 0.9 g cm(-3), bubble formation and restructuring of the water layers are observed.
Nonequilibrium, constant energy, constant volume (NVE) molecular dynamics simulations are used to study the decomposition of methane clathrate hydrate in contact with water. Under adiabatic ...conditions, the rate of methane clathrate decomposition is affected by heat and mass transfer arising from the breakup of the clathrate hydrate framework and release of the methane gas at the solid-liquid interface and diffusion of methane through water. We observe that temperature gradients are established between the clathrate and solution phases as a result of the endothermic clathrate decomposition process and this factor must be considered when modeling the decomposition process. Additionally we observe that clathrate decomposition does not occur gradually with breakup of individual cages, but rather in a concerted fashion with rows of structure I cages parallel to the interface decomposing simultaneously. Due to the concerted breakup of layers of the hydrate, large amounts of methane gas are released near the surface which can form bubbles that will greatly affect the rate of mass transfer near the surface of the clathrate phase. The effects of these phenomena on the rate of methane hydrate decomposition are determined and implications on hydrate dissociation in natural methane hydrate reservoirs are discussed.
•Hydrate-based continuous CO2 separation experiments were permormed in the H2 + CO2 + CP + water system.•The results of the experiments show that continuous separations of CO2 was successfully ...implemented in H2 + CO2 + CP + H2O system.•H2 was incorporated into the hydrate in the H2 + CO2 + CP + water system.•The molecular diameter of the guest molecules need be considered when selecting guest compound of hydrates.
CO2 separation from H2 + CO2 gas mixture is a necessary technology for a stable supply of blue hydrogen. Among CO2 separation technology, hydrate-based CO2 separation is an environmentally and economically superior technology. Cyclopentane (CP) was selected as guest compound to enable hydrate-based CO2 separation under thermodynamic conditions closer to normal pressure and temperature. Hydrate-based continuous CO2 separation experiments are conducted in the H2 + CO2 + CP + H2O system at 284 K. The results of these experiments showed that mole fraction of H2 in the gas phase reached 0.9 at the steady state in the H2 + CO2 + CP + H2O system after the CO2 was extracted via hydrate formation. At the same time, more H2 was incorporated into the mixed hydrate during this process as the concentration of H2 in the gas phase increases post hydrate formation. Therefore, it was demonstrated that hydrate-based CO2 separation was successful in H2 + CO2 + CP + H2O system at 284 K but there is also increase of energy costs caused by H2 loss. The PXRD results identified H2 + CO2 + CP hydrate as structure II hydrate. Although H2 + CO2 + Tetrahydropyran (THP) hydrate also forms structure II hydrate, more H2 is encapsulated in the H2 + CO2 + CP mixed hydrate than in the H2 + CO2 + THP mixed hydrate. This difference may be caused by the difference in the lattice constants of CP and THP hydrates, which arises from the difference in molecular size of CP and THP, from the differences between the large guest–host interactions, and from differences arising from hydrogen bonding of THP with the host water framework which can decrease the H2 holding capacity. In this study, it was revealed that the molecular diameter of the guest molecules need also to be considered in addition to the phase equilibrium conditions of the hydrates when selecting guest compound of hydrates.
PDF
