Japan Geoscience Union Meeting 2019

Presentation information

[J] Poster

M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

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

Tue. May 28, 2019 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall8, Makuhari Messe)

convener:Hitoshi Tomaru(Department of Earth Sciences, Chiba University), Akihiro Hachikubo(Kitami Institute of Technology), Atsushi Tani(Graduate School of Human Development and Environment, Kobe University), Shusaku Goto(Institute for Geo-Resources and Environment National Institute of Advanced Industrial Science and Technology)

[MIS21-P03] Validation of self-preservation effect of methane and ethane mixed-gas hydrate by Raman spectroscopy

*Yunosuke Hase1, Jumpei Matsuda1, Akihiro Hachikubo1, Satoshi Takeya2 (1.Kitami Institute of Technology, 2.National Institute of Advanced Industrial Science and Technology (AIST))

Keywords:gas hydrate, self preservation effect, methane, ethane

Methane hydrate can be stored long time under atmospheric pressure below 273.2K, even though their P-T conditions is unstable, namely "self-preservation effect". On the contrary, Takeya and Ripmeester (2008) reported that ethane hydrate shows no self-preservation. In natural condition, methane and ethane mixed-gas hydrates have been found at Lake Baikal, however, there is no information for the mixed-gas system. In the last JpGU meeting we report self-preservation effect for mixed-gas hydrate, but the conditions of samples are not uniform. Because the size of crystals affect rate of survival at higher temperature, we controlled the specific surface of area of gas hydrate samples in this study.

We put 0.7 g of fine ice balls (diameter: 0.108-0.180 mm) into a pressure cell (volume: 30 mL) and mixed-gas of methane and ethane was introduced. Hydrate crystals formed at 273.2 K. The gas hydrate sample was degased and recovered at the temperature of liquid nitrogen. We controlled the ethane composition of hydrate-bund gas from 0% to 100%, covering the area of methane-rich structure I, ethane-rich structure I, and their intermediate structure II. We measured compositions of methane and ethane by a gas chromatograph (GC-2014, Shimadzu Corporation). Gas hydrate samples were placed on a temperature-controlled device and kept at 80 K. Raman spectra of hydrate samples were obtained using a Raman spectrometer (RMP-210, JASCO Corporation). We checked the Raman spectra of C-H stretching mode at 83 K, and then increased the sample temperature every 10 K until complete dissociation. We calculated the Raman peak area for C-H stretching mode of methane and ethane around 2900 cm-1 by a peak fitting method and obtained the survival rate of gas hydrate samples with temperature.

Pure methane hydrate survived well, more than 20% of hydrate still remained at 263 K, whereas pure ethane hydrate completely dissociated at around 200 K. These results agree well with our previous report. As for mixed-gas hydrate composed of methane and ethane, the rates of survival of structure I were lower than that of pure methane hydrate, but the crystals remained till 253 K, suggesting self-preservation. That of structure II well remained at 83-183 K, but quickly disappeared at 213 K. Therefore, it is possible that the survival rate of mixed-gas hydrate composed of methane and ethane depends on not only gas composition but also crystallographic structure.

Takeya S, Ripmeester JA (2008) Dissociation behavior of clathrate hydrates to ice and dependence on guest molecules. Angew Chem Ed 47: 1276-1279