OS6A-6:SEPARATION OF CH4 AND N2 VIA HYDRATES FORMATION: EQUILIBRIUM EXPERIMENTS AND CALCULATIONS

Qiang SUN, Xuqiang GUO
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, CHINA

    Coalbed methane (CBM), whose main component is CH4, is commonly considered as a clean and efficient source of energy. China possesses abundant CBM resources ranking the 3rd in the world. However, on account of the limitations of exploitation technology and reservoir-forming conditions, most extracted CBM in China is mixed with air and released to the air directly. The annual emission of CBM from China is up to 19.4 billion m3, which ranks the 1st in the world. It results in serious resources waste and greenhouse effect. Therefore, it is necessary to study the recovery of CH4 from air-mixed CBM. Air-mixed CBM could be treated as a mixture of CH4 and N2, because the concentration of O2 in air-mixed CBM is far less than that of CH4 and N2, and O2 hydrate formation condition is close to N2 hydrate. In this work, we separated various CH4 and N2 gas mixtures with different compositions via hydrates formation in tetrahydrofuran (THF) solution under constant temperature and pressure, and calculated CH4 concentrations in different phases after equilibrium separation using Chen-Guo hydrate model, P-T equation of state and Wilson activity coefficient model. The results demonstrate that gas-liquid-solid (hydrate) three-phase equilibrium model should be used to calculate the equilibrium separation of gas mixtures with low concentration of CH4, because free water will coexists with gas mixtures at the experimental conditions. However, gas-solid two-phase equilibrium model is suitable for the calculation of equilibrium separation of gas mixtures with high concentration of CH4. The average relative error between the experimental results and calculation values (27 sets of comparisons in total) is 7.56 %. It indicates that the phase equilibrium calculation model in this work could well predict the separation results of the experimental system, and Chen-Guo hydrate model is appropriate for the phase equilibrium calculation of THF-containing systems. The results also show that CH4 can be effectively recovered from CH4/N2 mixture in the presence of THF. Hydrate separation method is an effective way to separate the air-mixed CBM.