Forming Structure-H Hydrates Using Water Spraying in Methane Gas:  Effects of Chemical Species of Large-Molecule Guest Substances

The rate of structure-H hydrate formation has been studied experimentally to explore the possibility of storing and transporting natural gas in the form of structure-H hydrates. The experimental procedure for forming structure-H hydrates used in this study followed the one conceived in our previous study (Ohmura et al., Energy Fuels 2002, 16, 1141−1147)i.e., spraying liquid water down through methane gas onto a liquid LMGS layer lying on a pool of water, where LMGS means a large-molecule guest substance which provides guest molecules to fit into the 51268 cages of a structure-H hydrate. In addition to methylcyclohexane, the only LMGS used in our previous study, five other LMGS candidates were tested: 2,2-dimethylbutane (neohexane), tert-butyl methyl ether, 3-methyl-1-butanol (isoamyl alcohol), 3,3-dimethyl-2-butanone (pinacolone), and 2-methylcyclohexanone. On the basis of the experimental results obtained at a prescribed temperature−pressure condition (275 K, 2.9 MPa), we conclude that the rate of structure-H hydrate formation (evaluated as the rate of methane-gas consumption) depends strongly on the species of the LMGS used. Depending on the selection of the LMGS, the rate could exceed that for the structure-I methane hydrate that would form at a much higher pressure in the absence of any LMGS. The rates of hydrate formation observed with different LMGSs are not correlated uniformly with a simple thermodynamic driving force, such as the deviation of the temperature−pressure condition set in each experiment from the corresponding four-phase (methane/LMGS/water/hydrate) equilibrium condition. This paper gives a data-based discussion of what factors control the rate of structure-H hydrate formation in water-spraying-type hydrate reactors, and recommends tert-butyl methyl ether as a promising LMGS for practical hydrate-forming operations.