Tsutomu Uchida

Equilibrium conditions of CH4, CO2, and C3H8 hydrates confined in small pores of porous glass were determined. The dissociation temperature of each hydrate at a given pressure shifted lower than that for bulk hydrate; the largest shift for CH4 hydrate was −12.3 K ± 0.2 K for 4-nm-diameter pores and the shift decreased to only −0.5 K for 100-nm pores. CH4 hydrate experiments at temperatures lower than the quadruple point of 270.6 K in 30-nm porous glass showed no shift of the equilibrium line. Al

The dissociation conditions of methane hydrates in confined small pores were measured by the gradual temperature increase method. Significant downward shifts of the dissociation temperature were observed in porous glasses, which had small pores ranging from 100 to 500 Å in diameter, compared with that of the bulk hydrate at a given pressure. Systematic measurements revealed that the temperature offset was in inverse proportion to the pore diameter. The Arrhenius plot of the dissociation conditio

Decomposition conditions of methane hydrates in sediments were measured during formation‐decomposition cycles. As test sediments, we used silica sand, sandstone, and clays (kaoline and bentonite), which are typical natural materials known as hydrate bearing sediments, and the range of samples cover a range of water saturating abilities. To better understand the results, we also used uniformly sized glass beads. Pore effects on decomposition of these materials were investigated by analyzing the p

Abstract Hydration numbers of methane hydrates with various water/gas ratios of the sample were measured by Raman spectroscopy. Artificial methane hydrates were formed at temperatures ranging from 273.2 to 278.4 K, and pressures from 3.0 to 7.0 MPa, with a stirring rate of approximately 500 rpm. Under such nonequilibrium conditions, the obtained hydrate samples had a water/gas ratio ranging from 6.4 to 17.4, which was determined by mass measurements. The spectroscopic analysis revealed, however,

The formation of CH4-CO2 mixed gas hydrates was observed by measuring the change of vapor-phase composition using gas chromatography and Raman spectroscopy. Preferential consumption of carbon dioxide molecules was found during hydrate formation, which agreed well with thermodynamic calculations. Both Raman spectroscopic analysis and the thermodynamic calculation indicated that the kinetics of this mixed gas hydrate system was controlled by the competition of both molecules to be enclathrated int

Unique properties of micro- and nanobubbles (MNBs), such as a high adsorption of impurities on their surface, are difficult to verify because MNBs are too small to observe directly. We thus used a transmission electron microscope (TEM) with the freeze-fractured replica method to observe oxygen (O2) MNBs in solutions. MNBs in pure water and in 1% NaCl solutions were spherical or oval. Their size distribution estimated from TEM images close to that of the original solution is measured by light-sca

Gas-hydrate crystals have important roles in various energy and environmental issues and also have potential industrial applications. Yet their formation and dissociation mechanisms remain unclear. To accelerate their crystallization, one can use a thermal hysteresis process called the “memory effect”, which is recognized as a shortening of the induction time of gas hydrate nucleation. Although its mechanism is still under debate, submicron-sized bubbles, called “micro- and nanobubbles (MNBs)”,

Clathrate hydrates of methane−ethane mixed gases have two crystal structures depending on their composition. To study their compositions and cage occupancies and how their structure is determined, we synthesized hydrate samples from methane−ethane mixtures. Analysis of the samples using X-ray diffraction, Raman spectroscopy, and gas chromatography revealed their structures, compositions, and cage occupancies. Experimentally, hydrate structure II existed in samples formed when the gas equilibrate

Abstract Vapor compositions of methane and propane mixed gas in a batch‐type reactor were measured by gas chromatography during hydrate crystallization at 274 K with molar ratios of propane below 10 vol %. The volume ratio of propane in the vapor decreased as the hydrate crystallization progressed. When the initial propane concentration was between 4 and 8 vol %, rapid gas consumption occurred for about 1 h, causing an initial pressure drop, and after a temporary stabilization of the pressure, a

Micro- and nanobubbles (MNBs) are potentially useful for industrial applications such as the purification of wastewater and the promotion of physiological activities of living organisms. To develop such applications, we should understand their properties and behavior, such as their lifetime and their number density in solution. In the present study, we observed oxygen MNBs distributed in an electrolyte (NaCl) solution using a transmission electron microscope to analyze samples made with the free