[M-IS21] Gas hydrate in environmental-resource sciences

[MIS21-01] Equilibrium pressure of clathrate hydrates encaged ¹³CH₄

*Yuki Kikuchi¹, Akihiro Hachikubo¹, Takahiro Ozeki¹, Satoshi Takeya² (1.Kitami Institute of Technology, 2.National Institute of Advanced Industrial Science and Technology (AIST))

Keywords:gas hydrate, methane hydrate, equilibrium pressure, stable isotope

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