Takao Ebinuma

Tetra-N-butylammonium bromide-water (1/38)

Tetra-n-butylammonium bromide forms the title semi-clathrate hydrate crystal, C16H36N+.Br-.38H2O, under atmospheric pressure. The cation and anion lie at sites with mm symmetry and seven water molecules lie at sites with m symmetry in space group Pmma. Br- anions construct a cage structure with the water molecules. Tetra-n-butylammonium cations are disordered and are located at the centre of four cages, viz. two tetrakaidecahedra and two pentakaidecahedra in ideal cage structures, while all the dodecahedral cages are empty.

Separation of Gas Molecule Using Tetra-n-butyl Ammonium Bromide Semi-Clathrate Hydrate Crystals

Tetra-n-butyl ammonium bromide (TBAB) forms a semi-clathrate hydrate crystal with water molecules even under atmospheric pressure. We found that TBAB hydrate could encage methane molecules in mixtures of methane and propane or methane and ethane. Our preliminary result of single-crystal analysis using X-ray shows that TBAB hydrate has empty cages, all of which are small, dodecahedral cages. Therefore, TBAB hydrate crystals can be used to separate small gas molecules which fit in these dodecahedral cages. We concluded that these empty unisized cages of TBAB hydrate crystals function as a sieve for gas molecules.

Microscopic observations of formation processes of clathrate-hydrate films at an interface between water and carbon dioxide

The formation processes of clathrate-hydrate film are observed at the interface between water and carbon dioxide using microscopy. After a certain induction period, the hydrate film propagates from nucleation points along the interface. A second film propagation is sometimes observed immediately after the primary propagation, which illustrates the molecular distribution near the interface. The dendrite growth following the film coverage of the interface in both phases suggests that the hydrate film grow mainly in the water phase. The propagation rates are determined from video images and are found to depend on the supercooling. This result and the bubble formation on the hydrate film following film propagation indicate that the rate-determining process is mainly heat diffusion from the reaction sites. A film thickness of 0.13 μm is estimated using the heat diffusion model.

Gas Separation Method Using Tetra-n-butyl Ammonium Bromide Semi-Clathrate Hydrate

We used tetra-n-butyl ammonium bromide semi-clathrate hydrate, hereafter TBAB hydrate, as a tool for separating gases from binary mixed gas systems of methane+ethane, methane+propane, methane+hydrogen sulfide, methane+nitrogen, and carbon dioxide+hydrogen sulfide by growing the hydrate from 10 wt% TBAB aqueous solutions in a pressure vessel. TBAB hydrate has empty dodecahedral cages in the pure system. We found that small molecules such as methane, nitrogen, and hydrogen sulfide were selectively encaged during TBAB hydrate formation, probably because the dodecahedral cages are too small to incorporate large molecules such as ethane and propane. Furthermore, hydrogen sulfide was more readily encaged compared to the other small molecules; we argue that this was due to the very high solubility of hydrogen sulfide in water. We propose that gases with small molecular size and high solubility in water can be effectively separated using TBAB semi-clathrate hydrate.

Methane hydrate crystal growth in a porous medium filled with methane-saturated liquid water

The nucleation, growth and ageing of methane hydrate crystals were observed visually in a porous medium filled with liquid water presaturated with methane. The pore space dimensions of the porous medium were 1.0 × 102 µm. The pressure−temperature conditions at which hydrate formation was initiated corresponded to system subcoolings of 3.4 K, 6.7 K, 12.3 K and 14.1 K, respectively, where the system subcooling denotes the difference of the system temperature from the triple methane−hydrate−water equilibrium temperature under a given pressure. Faceted (skeletal) hydrate crystals grew and bridged the pore spaces without intervention of a liquid water layer when the subcoolings were equal or smaller than 6.7 K. The faceted crystals may form a physical bonding with the walls of the porous medium. At the higher subcoolings, the dispersive formation of dendritic crystals and subsequent morphological change into particulate crystals were observed. The bridging of the dendritic crystals is unlikely in the absence of a large amount of additional methane supply due to the dispersive spatial distribution of the dendritic crystals that have dimensions smaller than those of the pore spaces. As a result of the interpretation of the observed variation in the crystal morphology of the methane hydrate formed in liquid water, the dependence of the crystal morphology on the magnitude of the mass transfer of methane molecules in liquid water observed in the porous medium was consistent with that previously observed in a bulk methane–water system.

Clathrate hydrate crystal growth in liquid water saturated with a hydrate-forming substance: Variations in crystal morphology

This paper reports on our interpretation of our visual observations of the variations in macroscopic morphology of hydrate crystals growing in liquid water saturated with a guest substance prior to the hydrate formation. The observations were made in a high-pressure cell charged with liquid water and gaseous CO2. They revealed distinct variations in the morphology of hydrate crystals depending on the system subcooling ΔT sub, the temperature deficiency inside the cell from the triple CO2–hydrate–water equilibrium temperature under a given pressure. When ΔT sub ≳ 3 K, a hydrate film first grew along the CO2–water interface; then hydrate crystals with dendritic morphology grew in large numbers into the liquid-water phase from that hydrate film. When ΔT sub ≲ 2 K, the dendritic crystals were replaced by skeletal or polyhedral crystals. We present a non-dimensional index for such variations in hydrate crystal morphology. This is based on the idea that this morphology depends on the growth rate of hydrate crystals, and their growth rate is controlled by the mass transfer of the hydrate–guest substance (CO2 in the present experiments), dissolved in the bulk of liquid water, to the hydrate crystal surfaces. The morphology variations observed in the present and previous studies are related to this index.

Formation, growth and ageing of clathrate hydrate crystals in a porous medium

An experimental study was performed to visually observe the driving force dependence of hydrate growth in a porous medium filled with either liquid water and dissolved CO2 or liquid water and gaseous CO2. The given system subcooling, ΔT sub, i.e. the deficiency of the system temperature from the triple CO2−hydrate−water equilibrium temperature under a given pressure, ranged from 1.7 K to 7.3 K. The fine dendrites initially formed at ΔT sub = 7.3 K changed quickly into particulate crystals. For ΔT sub = 1.7 K, faceted hydrate crystals grew and the subsequent morphological change was hardly identified for an eight-day observation period. These results indicate that the physical bonding between hydrate crystals and skeletal materials becomes stronger with decreasing driving force, suggesting that the fluid dynamic and mechanical properties of hydrate-bearing sediments vary depending on the hydrate crystal growth process.

Observations of CO2-hydrate decomposition and reformation processes

We observed CO2 hydrate decomposition and reformation (re-growth) through temperature and pressure changes using a microscope. At pressures above CO2 vapor–liquid equilibrium , decomposition from increasing temperatures left small liquid CO2 drops in the solution phase. Conversely, below , decomposition from increasing temperatures was more rapid due to the release of CO2 gas that mechanically broke the hydrate apart. Similarly, hydrate decomposition by a pressure decrease also released CO2 gas that broke the hydrate apart. Reformation occurred more readily only by cooling, not by a pressure increase. A barrier to CO2 nucleation can explain this memory effect by allowing a greater concentration of CO2 to be left in the aqueous solution after hydrate decomposition.

Structural investigation of methane hydrate sediments by microfocus X-ray computed tomography technique under high-pressure conditions

The structure of natural gas hydrate sediments was observed by microfocus X-ray computed tomography (CT). A newly developed high-pressure vessel for the microfocus X-ray CT system was applied to observe the sediments at a temperature above 273 K and under high-pressure conditions. The obtained two-dimensional CT images clearly showed the spatial distribution of the free-gas pore, sand particles, water, and hydrates. These results demonstrated that microfocus X-ray CT can be effective for studying natural gas hydrate sediment samples.

Visual observation of dissociation of methane hydrate crystals in a glass micro model: Production and transfer of methane

Dissociation of methane hydrate crystals in pore space caused by temperature increase or depressurization, which is one of the main concerns in developing techniques to exploit natural gases in clathrate hydrate in sediments, was observed by using a glass mimic of sediments. Hydrate crystals were formed at a system temperature of T=274.0 K and system pressure of p=12.5 MPa or T=279.6 K and p=10.1 MPa and then decomposed by increasing the system temperature to 289 or 290 K, respectively. Two experimental runs of depressurization were also conducted. Hydrate crystals formed at T=282.9 K and p=10.1 MPa or T=274.0 K and p=12.5 MPa were decomposed by depressurization to 1.7 or 0.5 MPa at a system temperature of 285.5 or 274.3 K. The glass mimic of sediments (glass micromodel) was composed of a quartz glass plate on which straight microchannels were carved and another quartz glass plate covering the microchannels. The microchannels were approximately rectangular in cross section with a top width of 1.0×102 μm a...