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[MIS26-08] Purity and specific surface area of methane hydrate synthesized from fine ice powder and gas
Keywords:gas hydrate, hydration number, methane, specific surface area
Since gas hydrates are considered for applications such as gas storage and transportation media, an efficient method of their formation has been investigated. There are two methods for generating gas hydrate: (1) agitating water and gas and (2) contacting ice and gas. In this study, we focused on the latter method, which does not require an agitation system and can be combined with storage system. The purity of gas hydrate is controlled by some factors, such as the contact area between the ice and gas, temperature, pressure, and reaction time. The effect of temperature on the purity of gas hydrate has not been investigated in detail below the ice point. We formed samples of methane hydrate by contacting fine ice powder and guest gas, and measured their specific surface area by a gas adsorption method.
Finely powdered ice was scraped with a microtome and put into a pressure cell at 253 K, and measured their specific surface area. The pressure cell was then evacuated at the temperature of liquid nitrogen and research grade methane was introduced. Temperature of the pressure cell was controlled from 225 K to 268 K in a cold room or thermostatic bath for a week. After that, the pressure cell was immersed in liquid nitrogen and the liquefied excess methane was evacuated to leave only methane hydrate and unreacted ice in the cell. The cell was weighed on an electronic balance to calculate the purity of methane hydrate. The hydration number of methane hydrate was supposed to be 6, because the effect of temperature and pressure on hydration number of methane hydrate is relatively small.
Original specific surface area of fine ice powder was around 254 [m2 kg-1], and it was almost uniform in the range of ±2%. The purity of methane hydrate formed at 225 K was low (about 70%), and showed a peak maximum (more than 95%) at around 245-255 K. At higher temperatures (higher than 255 K), the purity decreased. In the temperature range of 245-255 K, the purity was higher in the sample produced in the cold room than in the sample produced in the thermostatic bath. These differences might be due to the fluctuation of ambient temperature in the cold rooms compared to the liquid bath.
Finely powdered ice was scraped with a microtome and put into a pressure cell at 253 K, and measured their specific surface area. The pressure cell was then evacuated at the temperature of liquid nitrogen and research grade methane was introduced. Temperature of the pressure cell was controlled from 225 K to 268 K in a cold room or thermostatic bath for a week. After that, the pressure cell was immersed in liquid nitrogen and the liquefied excess methane was evacuated to leave only methane hydrate and unreacted ice in the cell. The cell was weighed on an electronic balance to calculate the purity of methane hydrate. The hydration number of methane hydrate was supposed to be 6, because the effect of temperature and pressure on hydration number of methane hydrate is relatively small.
Original specific surface area of fine ice powder was around 254 [m2 kg-1], and it was almost uniform in the range of ±2%. The purity of methane hydrate formed at 225 K was low (about 70%), and showed a peak maximum (more than 95%) at around 245-255 K. At higher temperatures (higher than 255 K), the purity decreased. In the temperature range of 245-255 K, the purity was higher in the sample produced in the cold room than in the sample produced in the thermostatic bath. These differences might be due to the fluctuation of ambient temperature in the cold rooms compared to the liquid bath.