Studies On Hydrate Separation Of N₂/CH₄ In Ordered Nano-space

Coal-bed methane(CBM) is a kind of natural gas which is buried in coal seams. Unlike ordinary natural gas, it does not contain heavy hydrocarbon components like C2+. According to estimates, there is about 260×1012 m3 of coal-bed methane throughout the world. Most of it has yet to be exploited. When coal is excavated, huge amounts of coal-bed methane are produced. However, less than 50% of CBM is utilized because of its low methane concentration of 30-50%. In addition, methane is a greenhouse gas and its greenhouse effect is 21 times stronger than that of CO2. It is crucial from both energy and environmental standpoints to develop a flexible methane enrichment technology for low quality CBM. The main components in pre-purified CBM are nitrogen and methane. Both N2 and CH4 are supercritical temperature gases at normal temperatures and it is very difficult to separate them because they are both non-condensable and have similar physical and chemical properties. The core issue in CBM enrichment is the separation of N2 and CH4 and this is also the most difficult problem.With the development of the subject of hydrate, people have been attempt to use the technology of hydrate in chemical areas.The methane and nitrogen can both form hydrates in suit conditions. But the pressures when they form hydrates are different. At 273.15 K, the hydrate formation pressures of CH4 and N2 are 2.56 and 14.3 MPa respectively. Thus methane locked in the cage of hydrate at low pressures whereas nitrogen does not. So, by controlling the experimental pressure, methane can be enriched and the goal of separating maethane and nitrogen can be achieved. The equilibrium isotherms of pure N2 and CH4 in CMK-3, a mesoporous carbon material, in the presence of water were measured in order to determine the optimal separation parameters. Tetrahydrofuran(THF) was added to the water to reduce the formation pressures of the gas hydrates. The THF concentration of 5 mmol/g and the temperature of 2? is the most convenient condition. Through the calculation of amount of hydrate and the enthply change at the turning point, we can conclude that portion of CH4 hydrate change into s I hydrate from sII hydrate along with the increasing pressure. Carrying on the PSA craft base on the difference between the formation pressures of CH4 and N2, can make the feed gas with different CH4 concentrations 30%, 40%, 50% increase by 20% when the pressures are different. Moreover, ten separation-regeneration cycles indicate that the sorbent is stable and reusable. This method has a good guidance and reference effect on the separation of gas mixture with different hydrate formation pressures such as carbon dioxide, ethane, hydrogen sulfide, sulfur dioxide, hydrogen, etc.