Dissociation behavior of methane gas hydrate in silicone oil at the temperature condition of under 0°C

In this study, dissociation behavior of methane gas hydrate (MH) in silicone oil at the temperature condition of under 0°C was investigated experimentally. Pure methane hydrate powder was synthesized from ice and pure methane gas using " Ice-Gas Interface Method". With these powders, pellet shaped hydrate samples were prepared. These pellets were soaked in silicone oil in high-pressure optical vessel, and then dissociation reaction was induced by depressurizing the system pressure to atmospheric pressure. The dissociation rates were measured under several isothermal-atmospheric conditions, and the reaction surface was directly observed. During dissociation, hydrate pellet is covered with ice shell, because of the temperature condition. Since the ice shell can interfere with the diffusion of methane gas from hydrate phase to bulk phase, the dissociation rate is expected to become slow (self-preservation). A sequence of MH dissociation behavior in silicone oil under 0°C is obtained experimentally. Interestingly, whole shape of pellet did not change during dissociation of MH. Dissociation rates of MH at various temperature conditions were obtained. As a result, it become clear that the dissociation rate becomes slow as the temperature became low at the range of 6 to -15°C. But in case of -20°C, it is very interesting that the dissociation rate turn back to fast, showing nearly same rate of - 4°C and -6°C. Tentatively, an Arrhenius plot was given, assuming that the dissociation reaction is a first-order reaction. It suggests that the different mechanism controls the dissociation reaction between higher temperature range and lower temperature range. Finally, from the viewpoint of slurry transportation, it can be concluded that the optimal temperature condition exists in the range around -15 to -10°C.