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Four-phase (ice + structure-H hydrate + large-molecule guest substance liquid + methane-rich vapor) equilibrium pressure−temperature conditions were measured at temperatures from 252 K to 272 K in systems of water + methane + LMGS, where LMGS is a large-molecule guest substance for a structure-H hydrate. The tested LMGSs were 2,2-dimethylbutane (neohexane), methylcyclohexane, and tert-butyl methyl ether. The results obtained in the 2,2-dimethylbutane system agree with the corresponding data reported by Makogon et al., which were the only data set on the structure-H hydrate phase equilibrium at temperatures below the freezing point of water. Among the three systems studied, the lowest equilibrium pressure at a given temperature was observed in the 2,2-dimethylbutane system, and the highest, in the tert-butyl methyl ether system.

The pressure and temperature conditions for the four-phase equilibrium in systems that include structure-H hydrate, methane gas, liquid water, and either 3,3-dimethyl-2-butanone (pinacolone) or 3,3-dimethyl-2-butanol (pinacolyl alcohol) liquid have been measured over the temperature range T = 273 K to T = 281 K. At a given temperature, the equilibrium pressures of the systems with pinacolone and pinacolyl alcohol are lower by 1.8 MPa and 1.3 MPa, respectively, than those of the structure-I hydrate-forming methane + water system. The equilibrium pressures in the system with pinacolone are roughly equal to those for the system with neohexane, whereas the pressures for the system with pinacolyl alcohol are higher by 0.5 MPa than those with neohexane. This suggests that the hydroxyl functional groups on the large-molecule guest substances can increase the equilibrium pressure in structure-H hydrate-forming systems, whereas the ketone groups can have little influence on the pressure.

The viscosity of water containing dissolved CO2 at pressures up to 5 MPa was measured by a new method based on dynamic light scattering. The accuracy of this method was checked by measuring the temperature dependence of the viscosity of pure water at pressures up to 5 MPa and comparing the results to values in the literature that were determined by other methods. The discrepancy of the measured viscosity of water from the literature was ±6%, which roughly equals the composite uncertainty of this method. The effect of dissolved CO2 on the viscosity was determined as functions of time, temperature, and pressure. The results indicated that the viscosity of the solution increased with increasing CO2 concentration. High viscosities were observed under conditions in which CO2 hydrates can form.

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 equilibrated with hydrates was approximately 2% C2H6 (molar fraction) whereas both structures I (sI) and II (sII) coexisted for 12 to 22% C2H6. The structures below 2% and above 22% of C2H6 existed only as sI hydrates. Volume ratios of both structures were obtained from the ratio of the peak intensities of the C−C stretching peaks in the Raman spectra. In the transition zone containing both structures, the volume ratio of the sII structure gradually decreased with increasing C2H6 concentration. Cage occupancies of guest molecules in the hydrate cages were determined by the relative intensity ratio of Raman spectra. C2H6 molecules occupied only large cages in both structures, whereas CH4 molecules occupied the remaining cages. The experiments agree with the structure and molecular distributions that were predicted by minimizing the Gibbs free energy of the sample. This model calculation provides insight into the structural transition mechanism.

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. All temperature shifts were fitted by the Gibbs-Thomson equation; the best fits for CH4, CO2, and C3H8 hydrates predicted hydrate−water interfacial energies of 1.7(3) × 10^-2 J/m², 1.4(3) × 10^-2 J/m², and 2.5(1) × 10^-2 J/m², respectively. Both type-I hydrates of CH4 and CO2 had interfacial energies within 20% of each other but significantly smaller than the type-II hydrate of C3H8. Ice formation in the same porous glass fit the Gibbs−Thomson relation with an interfacial energy of 2.9(6) × 10^-2 J/m², which is in good agreement with established values. The estimated interfacial tensions between gas hydrates and water were found to be only weakly affected by the kinds of gas. This indicated that the pore effect on the phase equilibrium was mainly due to the water activity change. The wide range of experiments on pore size, temperature, and the kind of gas allowed us to evaluate the validity of previous model predictions for pore effects on gas hydrate stability.

In situ observations of CH4 hydrate dissociation using X-ray diffraction were carried out at atmospheric pressure and at both 168 and 189 K. Dissociation rates of the hydrate and the rate of transformation into hexagonal ice were measured using time-resolved energy-dispersive X-ray diffraction. The dissociation of CH4 hydrate had an initially fast regime followed by slower dissociation. Thus, the data support a previously suggested two-step process. In addition, we observed dynamic behavior of the X-ray diffraction intensities of ice Ih, which implies a transient crystal structure at the beginning of the dissociation. Our analyses indicates that the first step, which lasted several tens of minutes, was the formation of an ice Ih layer around the CH4 hydrate, and the second step was relatively slow because the CH4 had to diffuse through the thickening ice layer. This second step determined the hydrate lifetime. The resulting diffusion coefficients were estimated at 2.2 × 10^-11 m²/s at 189 K and 9.6 × 10^-12 m²/s at 168 K.

We compared time-resolved Raman spectroscopic measurements of CH4-saturated aqueous solution with CH4hydrate crystal formed from the same solution. At 287.1 K and 6.6 MPa the C-H stretching mode for CH4molecules in the solution (?sol 2911.7 cm?1) was intermediate between that in the large cage (?L 2905.1 cm?1) and that in the small cage (?S 2914.1 cm?1), thus indicating an intermediate molecular environment. Comparison of these results with previous work on the CO2-H2O system indicated that water molecules in the type-I small-cage-like structure surrounded CH4molecules in the solution. Furthermore, the free energy barrier for CH4hydrate nucleation is likely to be formation of the large cage structure, which is probably larger than that for CO2hydrate formation.

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 conditions suggests that the heat of methane-hydrate dissociation tended to be small compared to that of bulk hydrates in pores smaller than 300 Å in diameter. Applying the Gibbs−Thomson effect to the quantitative analysis of the phenomenon indicated that the dissociation condition of methane hydrates in small pores shifted because of changes in the water activity. The apparent interfacial free energy between methane hydrates and water in the confined condition was estimated to be approximately 3.9 × 10^-2 J m^-2, which is comparable to that between ice and water in the similar condition.

This paper reports confirmation of structure II hydrate formation in a methane−2-propanol−water system, which was previously suggested by Østergaard et al. (Ind. Eng. Chem. Res. 2002, 41, 2064−2068) based on a comparison of the phase-equilibrium data with corresponding statistical-thermodynamics predictions. A hydrate crystal sample was prepared with a 16.4 mass % aqueous solution of 2-propanol pressurized with methane and then subjected to a powder X-ray diffraction analysis. The X-ray diffraction pattern thus obtained from the sample indicated that the crystallographic structure of the hydrate was structure II. The pressure−temperature data for aqueous liquid−hydrate−methane-rich vapor three-phase equilibrium in a temperature range from T = 273 to 283 K are also reported.