Clathrate Hydrate Formation from a Hydrocarbon Gas Mixture: Evolution of Gas-Phase Composition in a Hydrate-Forming Reactor

The clathrate hydrate formation from a model natural gas, i.e., a mixture of methane, ethane, and propane in a 90:7:3 molar ratio, under constant pressure was experimentally investigated, focusing on the evolution of the gas-phase composition inside the hydrate-forming reactor during each semi-batch hydrate-forming operation. In a previous study (Kobayashi, T.; Imura, N.; Ohmura, R.; Mori, Y. H. Energy Fuels 2007, 21, 545-553), we measured the evolution of the gas-phase composition during the initial period (up to similar to 3 h) following the inception of hydrate formation, in which the total amount of gas uptake in the formed hydrates was limited to 0.8 times the amount of gas initially contained in the reactor. This study extended the previous study, such that we could observe the evolution of the gas-phase composition for a much longer time (up to similar to 50 h) until a quasi-steady state was established inside a newly constructed gas-bubbling-type reactor equipped with a gas-phase-mixing stirrer that enabled the gas sampling from the well-mixed gas at specific times. The gas-chromatographic analyses of the continually sampled gas revealed a monotonic increase in the methane fraction and decreases in the ethane and propane fractions until a quasi-steady state was established when the total amount of gas uptake in the formed hydrates approached the amount of gas initially contained in the reactor. These experimental results are in reasonably good agreement with predictions based on the thermodynamic simulation scheme that we recently reported (Ogawa, H.; Imura, N.; Miyoshi, T.; Ohmura, R.; Mori, Y. H. Energy Fuels 2009, 23, 849-856).