Masato Kida

Dissociation Termination of Methane–Ethane Hydrates in Temperature-Ramping Tests at Atmospheric Pressure below the Melting Point of Ice

Gas hydrates are a kind of clathrate compound in which hydrogen-bonded water (H2O) molecules create cages stabilized by gas molecules. Depending on the guest gas molecules and the pressure–temperature conditions of the formation, several crystallographic arrangements of cages are created. The von Stackelberg cubic structure type I (sI) is known to be formed by pure gases such as methane (CH4) and ethane (C2H6), while type II (sII) is formed by larger hydrocarbons such as propane (C3H8) or by a mixture of lighter hydrocarbons. Each type consists of two kinds of cages; the larger ones (tetrakaidecahedron for sI : 56, and hexakaidecahedron for sII : 56) are usually fully occupied and the smaller ones (dodecahedron for both types: 5) are filled from 0 to 100 % depending on the guest composition. For this reason, gas hydrates are considered to be an efficient storage medium that may capture, on average, approximately 170 m of gas in 1 m of solid under standard conditions. The stability of gas hydrates is provided by moderate to high pressures of the guest molecules and low temperatures. Outside the stability conditions, the gas hydrate dissociates to free gas and liquid water or ice. Usually, the dissociation rate of the gas hydrate depends on the driving force, such as the temperature difference over the dissociation condition at a fixed pressure. At temperatures just several degrees below the melting point of ice (Tm), gas hydrates have a unique property known as the self-preservation effect, 3] which is defined as a significant depression of the dissociation rate at ambient pressure. It allows a considerable amount of gas hydrates to survive outside of the thermodynamic stability field for weeks, even months. 5] Recently, several pure-gas hydrates were screened to identify self-preserved and non-self-preserved substances. In this classification, CH4 is a self-preserved guest whereas C2H6 is not, although both of these are major components of natural gases. In addition, the effect of crystal structures on the self-preservation was also of interest because a mixture of CH4 + C2H6 gases formed both type of structures depending on the guest composition. However, the investigations of the self-preservation effect for mixed-gas hydrates has been insufficient compared to those for pure-gas hydrates. For example, 0.82CH4 + 0.18C2H6 mixed-gas hydrates in the hydrate of sII [8] and 0.425CH4 + 0.575C2H6 mixed-gas hydrates in the hydrate of sII [9] were investigated, except for a pellet-type sample. CH4 + C2H6 mixed-gas hydrate is, therefore, a suitable material for understanding the self-preservation effect, and for considering the application of gas hydrates as a natural-gas carrier. Herein, we measured the dissociation gas volume to estimate the dissociation rate of CH4 + C2H6 mixed-gas hydrates with various compositions. The formed sample was characterized by X-ray diffraction, Raman spectroscopy, and gas chromatography. Systematic investigations revealed that CH4 + C2H6 mixed-gas hydrate exhibited self-preservation effects with any composition. A quantitative analysis suggested that initial sample conditions other than the C2H6 content also played important roles in the self-preservation effect. After sample preparation, a portion of the sample pieces was used for characterization experiments via spectroscopic analyses, gas chromatographic measurements, and mass-balance investigations. For example, the X-ray diffraction pattern and the Raman spectra of a fine gained CH4 + C2H6 mixed-gas hydrate formed from 0.528CH4 + 0.472C2H6 source gas are presented in Figures 1 and 2. The X-ray diffraction pattern (Figure 1) indicates that the sample was a mixture of ice and

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.

Crystallization of authigenic carbonates in mud volcanoes at Lake Baikal

This paper presents data on authigenic siderite first found in surface sediments from mud volcanoes in the Central (K-2) and Southern (Malen’kii) basins of Lake Baikal. Ca is the predominant cation, which substitutes Fe in the crystalline lattice of siderite. The enrichment of the carbonates in the 13C isotope (from +3.3 to +6.8‰ for the Malen’kii volcano and from +17.7 to +21.9‰ for K-2) results from the crystallization of the carbonates during methane generation via the bacterial destruction of organic matter (acetate). The overall depletion of the carbonates in 18O is mainly inherited from the isotopic composition of Baikal water.

Chemical Shift Changes and Line Narrowing in 13C NMR Spectra of Hydrocarbon Clathrate Hydrates

The solid-state (13)C NMR spectra of various guest hydrocarbons (methane, ethane, propane, adamantane) in clathrate hydrates were measured to elucidate the local structural environments around hydrocarbon molecules isolated in guest-host frameworks of clathrate hydrates. The results show that, depending on the cage environment, the trends in the (13)C chemical shift and line width change as a function of temperature. Shielding around the carbons of the guest normal alkanes in looser cage environments tends to decrease with increasing temperature, whereas shielding in tighter cage environments tends to increase continuously with increasing temperature. Furthermore, the (13)C NMR line widths suggest, because of the reorientation of the guest alkanes, that the local structures in structure II are more averaged than those in structure I. The differences between structures I and II tend to be very large in the lower temperature range examined in this study. The (13)C NMR spectra of adamantane guest molecules in structure H hydrate show that the local structures around adamantane guests trapped in structure H hydrate cages are averaged at the same level as in the α phase of solid adamantane.

Dissociation behavior of methane--ethane mixed gas hydrate coexisting structures I and II.

Dissociation behavior of methane-ethane mixed gas hydrate coexisting structures I and II at constant temperatures less than 223 K was studied with use of powder X-ray diffraction and solid-state (13)C NMR techniques. The diffraction patterns at temperatures less than 203 K showed both structures I and II simultaneously convert to Ih during the dissociation, but the diffraction pattern at temperatures greater than 208 K showed different dissociation behavior between structures I and II. Although the diffraction peaks from structure II decreased during measurement at constant temperatures greater than 208 K, those from structure I increased at the initial step of dissociation and then disappeared. This anomalous behavior of the methane-ethane mixed gas hydrate coexisting structures I and II was examined by using the (13)C NMR technique. The (13)C NMR spectra revealed that the anomalous behavior results from the formation of ethane-rich structure I. The structure I hydrate formation was associated with the dissociation rate of the initial methane-ethane mixed gas hydrate.

Pressurization effects on methane hydrate dissociation

Elucidating the mechanisms of self-preservation and control of dissociation of gas hydrates is crucial for developing technologies facilitating high-stability and long-term storage of natural gas using gas hydrate crystals. Direct measurements of the dissociation behavior of pressurized and nonpressurized methane hydrate grains in a temperature ramping test were conducted. We revealed that a pressurized process is effective for enhancing self-preservation and that it is useful to store methane gas while maintaining a submillimeter-sized hydrate material under high temperature conditions.

Effective control of gas hydrate dissociation above the melting point of ice

Direct measurements of the dissociation behaviors of pure methane and ethane hydrates trapped in sintered tetrahydrofuran hydrate through a temperature ramping method showed that the tetrahydrofuran hydrate controls dissociation of the gas hydrates under thermodynamic instability at temperatures above the melting point of ice.

Contribution of water molecules to methane hydrate dissociation

In this report, we describe the effects of ice on the restriction of methane diffusion during the dissociation of pressurized methane hydrate grains using two deuterium-labelling approaches with D2O. Direct measurements of the dissociation behaviors of the methane hydrate samples labelled by a temperature ramping method at temperatures of 253.0–293.0 K were carried out. The deuterium-labelling approaches demonstrated that water molecules in the host framework of methane hydrate predominantly contribute to ice formation, which restricts methane release from the decomposing hydrate framework more than ice coexisting with methane hydrate. The shielding effect of ice in intimate contact with methane hydrate particles on methane diffusion during the decomposition of the hydrate framework depends on the ratio of preexisting ice in the methane hydrate sample.

Improvement of gas hydrate preservation by increasing compression pressure to simple hydrates of methane, ethane, and propane

In this report, we describe the dissociation behavior of gas hydrate grains pressed at 1 and 6 MPa. Certain simple gas hydrates in powder form show anomalous preservation phenomenon under their thermodynamic unstable condition. Investigation of simple hydrates of methane, ethane, and propane reveals that high pressure applied to the gas hydrate particles enhances their preservation effects. Application of high pressure increases the dissociation temperature of methane hydrate and has a restrictive effect against the dissociation of ethane and propane hydrate grains. These improvements of gas hydrate preservation by increasing pressure to the initial gas hydrate particles imply that appropriate pressure applied to gas hydrate particles enhances gas hydrate preservation effects.

FORMATION PROCESS OF STRUCTURE I AND II GAS HYDRATES DISCOVERED IN KUKUY, LAKE BAIKAL

Structure I and II gas hydrates were observed in the same sediment cores of a mud volcano in the Kukuy Canyon, Lake Baikal. The sII gas hydrate contained about 13-15% of ethane, whereas the sI gas hydrate contained about 1-5% of ethane and placed beneath the sII gas hydrate. We measured isotopic composition of dissociation gas from both type gas hydrates and dissolved gas in pore water. We found that ethane δD of sI gas hydrate (from -196 to -211 ‰) was larger than that of sII (from -215 to -220 ‰), whereas methane δC, methane δD and ethane δD in both hydrate structures were almost the same. δC of methane and ethane in gas hydrate seemed several permil smaller than those in pore water. These results support the following idea that the current gas in pore water is not the source of these gas hydrates of both structures. Isotopic data also provide useful information how the “double structure” gas hydrates formed.