[MIS17-P03] 安定同位体の異なるメタンを包接するクラスレートハイドレートの平衡圧
キーワード：ガスハイドレート、メタンハイドレート、平衡圧、安定同位体
Methane is composed of carbon and hydrogen. Because these components have isotopes, 12CH4 (98.9%), 13CH4 (1.1%), and CH3D (0.013%) exist in nature. Since their molecular weight differs with each other, physicochemical properties of them are also different with each other. Pure methane hydrate is "mixed-gas hydrate" of their isotopologues. Equilibrium pressure of pure methane hydrate was first reported more than a half century ago; however, those of 13CH4 and CH3D hydrate have not been studied yet. Fractionation of hydrogen isotope during the formation of methane hydrate has been reported by Hachikubo et al. (2007) that δD of hydrate-bound methane becomes several ‰ smaller than that of residual methane in the formation process. This result suggests that the equilibrium pressure of CH3D hydrate is larger than that of CH4 hydrate. In this study, we measured the equilibrium pressures of CH3D hydrate and confirmed the difference between them.
Methane hydrate samples were synthesized in small pressure cells (volume: 5mL). Fine ice powder (1g) was put in a pressure cell, and introduced CH3D (purity: 98%, Taiyo-Nissan). Clathrate hydrate was formed by melting the ice powder at the temperature of 273.2K under high pressure of methane. We also prepared normal methane (purity: 99.99% for methane, but 98.9% for 12CH4, Takachiho Chemical Industry Co. Ltd.) hydrate as a reference, using the same preparation method. These pressure cells were placed in a temperature-controlled liquid bath, and measured their equilibrium pressures from 273.2K to 277.2K.
The equilibrium pressure of CH3D hydrate was about 0.07MPa larger than that of CH4 hydrate, and the results agreed with the previous report by Hachikubo et al. (2007).
Reference
Hachikubo A, Kosaka T, Kida M, Krylov A, Sakagami H, Minami H, Takahashi N, Shoji H (2007) Isotopic fractionation of methane and ethane hydrates between gas and hydrate phases. Geophys Res Lett 34: L21502. doi:10.1029/2007GL030557
Methane hydrate samples were synthesized in small pressure cells (volume: 5mL). Fine ice powder (1g) was put in a pressure cell, and introduced CH3D (purity: 98%, Taiyo-Nissan). Clathrate hydrate was formed by melting the ice powder at the temperature of 273.2K under high pressure of methane. We also prepared normal methane (purity: 99.99% for methane, but 98.9% for 12CH4, Takachiho Chemical Industry Co. Ltd.) hydrate as a reference, using the same preparation method. These pressure cells were placed in a temperature-controlled liquid bath, and measured their equilibrium pressures from 273.2K to 277.2K.
The equilibrium pressure of CH3D hydrate was about 0.07MPa larger than that of CH4 hydrate, and the results agreed with the previous report by Hachikubo et al. (2007).
Reference
Hachikubo A, Kosaka T, Kida M, Krylov A, Sakagami H, Minami H, Takahashi N, Shoji H (2007) Isotopic fractionation of methane and ethane hydrates between gas and hydrate phases. Geophys Res Lett 34: L21502. doi:10.1029/2007GL030557