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[MIS26-10] Concentration of helium at the formation of methane and helium mixed-gas hydrate
Keywords:gas hydrate, helium, source gas
It is reported that helium can be enclathrated in cages composed of water molecules, i.e. gas hydrates (Londono et al., 1992; Maekawa, 2003). For example, Maekawa (2003) reported that equilibrium pressure of methane and helium mixed-gas hydrate increased with composition of helium. Because helium is one of the components of natural gas, it is possible that small amount of helium molecules are enclathrated in natural gas hydrates. In the last JpGU-AGU Joint Meeting, We reported preliminary results of synthetic methane and helium mixed-gas hydrate (Kimura et al., 2020). Although the composition of helium in the hydrate-bound gas was up to 6% in the last report, it must be much lower composition in natural condition. In this report, we formed methane and helium mixed-gas hydrate in the condition of lower helium concentration, and measured composition of helium in hydrate-bound and residual gases.
Experimental method is almost the same as the last report (Kimura et al., 2020). Samples of mixed-gas hydrate composed of methane and helium were synthesized in a pressure cell. Gas hydrate formed at around 273 K, and residual gas in the cell was sampled and the rest of residual gas was evacuated at the temperature of liquid nitrogen. After completion of evacuation, hydrate remained in the pressure cell was dissociated at a room temperature. Samples of hydrate-bound and residual gases were stored in vials (volume: 100 mL) and their molecular composition were measured using a portable gas chromatograph (CP-4900, Varian) in the same day.
The concentration of helium in hydrate-bound gas ranged from 0.001% to 0.1%, corresponded to those in residual gas ranged from 0.1% to 10%. Therefore, concentration of helium in the hydrate-bound gas was about two orders smaller than that of residual gas, suggesting that methane preferred to be encaged in hydrate phase rather than helium. Concentration of helium in hydrate increased with the partial pressure of helium in the initial gas. We also checked a possibility that helium escaped from the hydrate phase at the vacuum process, however, concentration of helium did not change under various vacuum time.
References
Kimura H, Shirakura S, Hachikubo A, Takeya S (2020) Formation of gas hydrates composed of methane and helium. JpGU-AGU Joint Meeting 2020, MIS32-P11.
Londono D, Finney JL, Kuhs WF, et al. (1992) Formation, stability, and structure of helium hydrate at high pressure. J Chem Phys 97: 547–552.
Maekawa T (2003) Gas hydrate formation for mixtures of methane + helium and ethane + helium. J Chem Eng Data 48: 1283–1285, doi:10.1021/je0301592
Experimental method is almost the same as the last report (Kimura et al., 2020). Samples of mixed-gas hydrate composed of methane and helium were synthesized in a pressure cell. Gas hydrate formed at around 273 K, and residual gas in the cell was sampled and the rest of residual gas was evacuated at the temperature of liquid nitrogen. After completion of evacuation, hydrate remained in the pressure cell was dissociated at a room temperature. Samples of hydrate-bound and residual gases were stored in vials (volume: 100 mL) and their molecular composition were measured using a portable gas chromatograph (CP-4900, Varian) in the same day.
The concentration of helium in hydrate-bound gas ranged from 0.001% to 0.1%, corresponded to those in residual gas ranged from 0.1% to 10%. Therefore, concentration of helium in the hydrate-bound gas was about two orders smaller than that of residual gas, suggesting that methane preferred to be encaged in hydrate phase rather than helium. Concentration of helium in hydrate increased with the partial pressure of helium in the initial gas. We also checked a possibility that helium escaped from the hydrate phase at the vacuum process, however, concentration of helium did not change under various vacuum time.
References
Kimura H, Shirakura S, Hachikubo A, Takeya S (2020) Formation of gas hydrates composed of methane and helium. JpGU-AGU Joint Meeting 2020, MIS32-P11.
Londono D, Finney JL, Kuhs WF, et al. (1992) Formation, stability, and structure of helium hydrate at high pressure. J Chem Phys 97: 547–552.
Maekawa T (2003) Gas hydrate formation for mixtures of methane + helium and ethane + helium. J Chem Eng Data 48: 1283–1285, doi:10.1021/je0301592