Molecular dynamics simulation study of the properties of doubly occupied N2 clathrate hydrates

Self Cite

Binary structure H (sH) hydrogen and methyl-tert-butylether (MTBE) clathrate hydrates are studied with molecular dynamics simulations. Simulations on a 3 x 3 x 3 sH unit cell with up to 4.7 mass % hydrogen gas are run at pressures of 100 bars and 2 kbars at 100 and 273 K. For the small and medium cages of the sH unit cell, H2 guest molecule occupancies of 0, 1 (single occupancy), and 2 (double occupancy) are considered with the MTBE molecule occupying all of the large cages. An increase of the small and medium cage occupancies from 1 to 2 leads to a jump in the unit cell volume and configurational energy. Calculations are also set up with 13, 23, and 89 of the MTBE molecules in the large cages replaced by sets of three to six H2 molecules, and the effects on the configurational energy and volume of the simulation cell are determined. As MTBE molecules are replaced with sets of H2 guests in the large cages, the configurational energy of the unit cell increases. At the lower temperature, the energy and volume of the clathrate are not sensitive to the number of hydrogen guests in the large cages; however, at higher temperatures the repulsions among the H2 guest molecules in the large cages cause an increase in the system energy and volume.

Section: Methodsmentioning

confidence: 99%

Molecular dynamics simulations of binary structure H hydrogen and methyl-tert-butylether clathrate hydrates

Alavi

Ripmeester

Klug

2006

Self Cite

“…The initial input structures for the clathrate oxygen atoms correspond to those in the experimental sI clathrate hydrate 23 with the positions of the configurationally disordered water hydrogen atoms chosen by a Monte Carlo procedure which minimizes the dipole moment of the unit cell 24 in a manner consistent with the Bernal-Fowler ice rules. 25 The extended simple point charge ͑SPC/E͒ model 26 is used for water.…”

Section: Molecular Dynamics Methodsmentioning

confidence: 99%

Nonequilibrium adiabatic molecular dynamics simulations of methane clathrate hydrate decomposition

Alavi

Ripmeester

2010

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.

“…5 These results support the double occupation determined by the above neutron diffrac-tion measurements. Klaveren et al also calculated the vibrational frequencies for sII nitrogen hydrate, 6,7 in which they showed three N-N stretching peak frequencies for the single occupied S cages, the double occupied L cages, and the single occupied L cages. 6 Although a number of Raman experiments for the N-N stretching vibrations in natural or synthetic air hydrates have been performed, [8][9][10][11][12][13][14][15] there is no evidence for the double occupancy.…”

Section: Introductionmentioning

confidence: 99%

Microscopic observation and in situ Raman scattering studies on high-pressure phase transformations of a synthetic nitrogen hydrate

Sasaki

Hori

Kume

et al. 2003

Visual observations through a microscope and in situ Raman scattering measurements of a synthesized nitrogen hydrate have been performed at pressures up to 6 GPa and 296 K. High-pressure transformations have been found at 0.85 and 1.45 GPa. The cubic structure II (sII) nitrogen hydrate initially transforms to the hexagonal structure (sH) at 0.85 GPa and finally forms the orthorhombic dihydrate (sO) above 1.45 GPa. The sO phase of nitrogen hydrate exists up to at least 6 GPa. A variety of Raman spectra composed of three peaks have been sometimes observed in sII phase below 0.50 GPa, which implies that the guest nitrogen molecules doubly occupy the large hexakaidecahedron cages. Two Raman bands of the guest nitrogen vibrations with nearly equal intensities appearing in sH phase suggest that five nitrogen molecules are filling in extra large icosahedron cages.