10:45 〜 11:00
[MIS21-01] 13CH4を包接するメタン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 molecule is composed of carbon and hydrogen, and three kinds of isotopologues, 12CH4 (98.9%), 13CH4 (1.1%), and CH3D (0.013%) exist in nature. We often measure stable carbon isotope (13C/12C) of methane to understand its gas origin. Since their weight of isotopologues differ with each other, physicochemical properties of them are also different. Pure methane hydrate must be "mixed-gas hydrate" of their isotopologues. Ozeki et al. (2018) measured an equilibrium pressure of CH3D hydrate, but that of 13CH4 has not been studied yet. Fractionation of hydrogen isotope of methane 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. This result suggests that the equilibrium pressure of CH3D hydrate is larger than that of CH4 hydrate, and Ozeki et al. (2018) demonstrated the difference between these equilibrium pressures. Since Hachikubo et al. (2007) showed no isotopic fractionation in methane δ13C, the equilibrium pressures of 12CH4 and 13CH4 hydrates are thought to be almost the same. In this study, we measured the equilibrium pressures of 13CH4 hydrate to check the difference from that of 12CH4 hydrate.
Methane hydrate samples were synthesized in small pressure cells (volume: 5 mL). Fine ice powder (1g) was put in a pressure cell, and introduced 13CH4 (purity: 99.5%, 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 269.5K to 277.9K.
The difference in equilibrium pressure between 13CH4 and normal methane (mainly 12CH4) hydrates was smaller than the measurement error. This results agree with the previous report by Hachikubo et al. (2007).
References
Ozeki T, Kikuchi Y, Takeya S, Hachikubo A (2018) Phase equilibrium of isotopologue methane hydrates enclathrated CH3D and CD4. J Chem Eng Data 63(6): 2266-2270, doi: 0.1021/acs.jced.8b00203
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: 5 mL). Fine ice powder (1g) was put in a pressure cell, and introduced 13CH4 (purity: 99.5%, 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 269.5K to 277.9K.
The difference in equilibrium pressure between 13CH4 and normal methane (mainly 12CH4) hydrates was smaller than the measurement error. This results agree with the previous report by Hachikubo et al. (2007).
References
Ozeki T, Kikuchi Y, Takeya S, Hachikubo A (2018) Phase equilibrium of isotopologue methane hydrates enclathrated CH3D and CD4. J Chem Eng Data 63(6): 2266-2270, doi: 0.1021/acs.jced.8b00203
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