Yasushi Kamata

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.

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.

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.

Structure Analyses of Artificial Methane Hydrate Sediments by Microfocus X-ray Computed Tomography

The structure of natural gas hydrate sediments was characterized by microfocus X-ray computed tomography (CT). The obtained two-dimensional (2-D) and three-dimensional (3-D) images clearly showed the spatial distribution of the free-gas spaces, sand particles, and hydrates or ices. The estimated porosity from the X-ray CT data was consistent with the value that was obtained from the sample mass and volume. These results indicate that microfocus X-ray CT can be very useful for researching natural samples of hydrate sediments.

Distribution of Hydrate Saturation Ratios in Artificial Methane Hydrate Sediments Measured by High-Speed X-Ray Computerized Tomography

A high-speed X-ray computerized tomography (CT) system was developed for measuring the texture of methane hydrate sediment. The system enabled time dependent nondestructive observations of the hydrate sediment in a volume of 100×100 mm3 within 40 s. The spatial variation of mass density and the volume fraction of an artificial methane hydrate sediment that is either ice or hydrate, hereafter referred to as the hydrate saturation ratio within methane hydrate sediments, were measured using the X-ray CT system. The three-dimensional (3D) distribution change of density and hydrate saturation caused by the hydrate dissociation were also visualized on the basis of the measured gray scale values of the CT images taken before and after their dissociation. The technique allows us observations of the dissociation process of methane hydrate sediment under experimental conditions which are the same as conditions in the natural environment, and will allow the simulation of methane gas production from wells in both oceanic and permafrost areas.

13C Chemical Shifts of Propane Molecules Encaged in Structure II Clathrate Hydrate

Experimental NMR measurements for (13)C chemical shifts of propane molecules encaged in 16-hedral cages of structure II clathrate hydrate were conducted to investigate the effects of guest-host interaction of pure propane clathrate on the (13)C chemical shifts of propane guests. Experimental (13)C NMR measurements revealed that the clathrate hydration of propane reverses the (13)C chemical shifts of methyl and methylene carbons in propane guests to gaseous propane at room temperature and atmospheric pressure or isolated propane, suggesting a change in magnetic environment around the propane guest by the clathrate hydration. Inversion of the (13)C chemical shifts of propane clathrate suggests that the deshielding effect of the water cage on the methyl carbons of the propane molecule encaged in the 16-hedral cage is greater than that on its methylene carbon.

Water Permeability of Porous Media Containing Methane Hydrate as Controlled by the Methane-hydrate Growth Process

This chapter seeks to clarify the relation between fluid permeability and methane-hydrate saturation (Sh). The ultimate purpose is to estimate the theoretical expression by the equation K = K0(1 Sh)N (where K0 is the apparent permeability at Sh = 0 and N is a constant) for input into methane-hydrate numerical simulators. However, the permeability of hydrate-bearing sediment strongly depends on the hydrate saturation, grain-size distribution, porosity, pore-size distribution, hydrate formation method, and so on. To clarify the relation between the permeability and methane-hydrate saturation, we measured the water permeability of methane-hydrate-bearing sediments with different hydrate saturations for three contrasting methane-hydrate formation methods: (1) the connate water reaction method, (2) the gas diffusion method, and the (3) cementing method. The results demonstrate that the rate of decrease in the apparent water permeability (AWP) with increasing methane-hydrate saturation differs for each method of gas-hydrate formation. In addition, the values of K and N in the theoretical expression K = K0(1 Sh)N were estimated for each production method, and a different N value was obtained for each hydrate formation method. It is apparent that the method of gas-hydrate formation leads to a contrasting geometry of methane-hydrate growth at the pore scale and in turn affects the macroscopic AWP saturation relations.