論文

This paper reports experimental values of the surface tension between CO2 gas and tetra-n-butylammonium bromide (TBAB) aqueous solution measured by the pendant drop method. The experimental conditions were pressures from 1.00 MPa to 4.00 MPa, temperatures from 288.15 K to 303.15 K and wTBAB from 0.10 to 0.40, where wTBAB denotes the mass fraction of TBAB in the aqueous solution. These pressure-temperature-composition conditions partially fall into the stability zone of (CO2 + TBAB) semiclathrate hydrate but all data were obtained without any hydrate in the system. Surface tension decreased with increasing wTBAB and pressure but temperature dependency was not observed. Compared to the air-TBAB aqueous solution system and CO2-water system, the values of the surface tension were smaller.

Experimental investigations to extinguish pool flames by CO2 hydrate powder and dry ice powder have been done. Methanol and 1-hexanol are used as a fuel of pool flames. The boiling point of the methanol, 67.4 °C, is lower than that of the water, while that of the 1-hexanol, 157 °C, is higher than that of the water. A variation in the flame shape to extinction is recorded by a high-speed video camera. Critical mass for extinction, which is defined as a minimum mass of fire extinguishing agent for extinction, extinction time, temperature variation in and above the fuel are measured. In the case of a methanol pool flame, more critical mass of CO2 hydrate is necessary compared to the dry ice to supply same mass of CO2 gas. This indicates that the supply of CO2 gas plays a key role to extinguish the methanol pool flame, which dilutes the methanol gas and weakens the mixing between methanol gas and air. In the case of 1-hexanol, less critical mass of CO2 hydrate is necessary compared to the dry ice. In this case, the water in CO2 hydrate dilutes and cools down the 1-hexanol which results in a decrease in the vaporization of the 1-hexanol.

Visual observations of CH4 + CO2 hydrate crystal growth formed at the gas/liquid interface and in liquid water presaturated with a mixed gas have been made. The compositions of the CH4 + CO2 gaseous mixture were 40:60 and 30:70 for the gas/liquid interface observations, 30:70 and 70:30 for water saturated with the guest gas. The feed gas compositions of the CH4 and CO2 gaseous mixture were 40:60 and 30:70 for the gas/liquid interface observations, or 30:70 and 70:30 for liquid water. The crystal morphology of the CH4 + CO2 hydrate observed in both feed gas compositions was similar. This may be ascribed to the fact that the molar ratios of CO2 to CH4 in the liquid phase ranged from 90:10 to 97:3 due to the greater solubility of CO2 in water. These results suggest that the crystal morphology of the CH4 + CO2 hydrate may be controlled by the guest composition in the liquid phase, not by the feed gas composition. As the system subcooling increased, the shape of the hydrate crystals changed from polygons to sword-like or dendrites. The implications for the process design of the hydrate-based technologies are discussed based on the observations.

The deuteration of the lattice molecules in clathrate hydrates is a widely used experimental technique to clearly separate the vibrational modes. However, the effect of the deuteration on the vibrational spectra and molecular motions is not fully understood. Since the guest-host coupling may change the vibrational spectra, a detailed analysis of the vibrational spectra of deuterated clathrate hydrate is significant in the understanding of the mechanism of the vibrational shift. In this study, the vibrational spectra of the deuterated methane hydrates were calculated by ab initio molecular dynamics simulation. The intramolecular vibrational frequency of the methane in D2O lattice and deuterated methane in H2O lattice was calculated and compared with the pure methane hydrate. The bending, rocking and overtone of the bending mode was also reported. The effect of coupling of the rattling motions of guest and host molecules on the vibrational spectra was revealed.

Clathrate hydrates are typically stabilized by suitably sized hydrophobic guest molecules. However, it has been experimentally reported that isomers of amyl-alcohol C5H11OH can be enclosed into the 51264 cages in structure II (sII) clathrate hydrates, even though the effective radii of the molecules are larger than the van der Waals radii of the cages. To reveal the mechanism of the anomalous enclathration of hydrophilic molecules, we performed ab initio and classical molecular dynamics simulations (MD) and analyzed the structure and dynamics of a guest-host hydrogen bond for sII 3-methyl-1-butanol and structure H (sH) 2-methyl-2-butanol clathrate hydrates. The simulations clearly showed the formation of guest-host hydrogen bonds and the incorporation of the O-H group of 3-methyl-1-butanol guest molecules into the framework of the sII 51264 cages, with the remaining hydrophobic part of the amyl-alcohol molecule well accommodated into the cages. The calculated vibrational spectra of alcohol O-H bonds showed large frequency shifts due to the strong guest-host hydrogen bonding. The 2-methyl-2-butanol guests form strong hydrogen bonds with the cage water molecules in the sH clathrate, but are not incorporated into the water framework. By comparing the structures of the alcohols in the hydrate phases, the effect of the location of O-H groups in the butyl chain of the guest molecules on the crystalline structure of the clathrate hydrates is indicated.