Clathrate hydrate formation by water spraying in a methane + ethane + propane gas mixture: Search for the rate-controlling mechanism of hydrate formation in the presence of methylcyclohexane

In a previous paper (Tsuji et al. Energy Fuels 2005, 19, 869-876), we reported that the rate of clathrate hydrate formation from a simulated natural gas - a mixture of methane, ethane, and propane in a 90:7:3 molar ratio - in the presence of methylcyclohexane (MCH) significantly changed with the repetition of hydrate-forming operations and that its asymptotic level substantially exceeds the rate available in the absence of MCH. Here, we report new results in which continual chromatographic analyses of the gas-phase composition inside the hydrate-forming spray chamber gave insight into the mechanism by which the presence of MCH influences the rate of hydrate formation. The observed evolution of the gas-phase composition during each hydrate-forming operation indicates that the hydrate formed over the major portion of the operation is in structure II, although MCH may provide guest molecules to fit into the 5¹²6⁸ cages of a structure-H hydrate. The difference in the rate of hydrate formation from operation to operation was found to be related to the difference in the initial gas-phase composition due to the preferential dissolution of ethane and propane from the gas phase into the liquid MCH phase before the inception of hydrate formation; the higher the initial concentrations of ethane and propane in the gas phase are, the higher the rate of successive hydrate formation is.