In order to reduce the emission of coal bed methane mixed with air (can be regarded as a CH4 and N2 mixture), which will be helpful to adequately utilize the natural resources and protect environment, the separation of CH4 and N2 via hydrate formation in tetra-n-butylammonium bromide (TBAB) solution was systematically studied in this work. The CH4−N2 hydrate formation conditions were determined in TBAB solution first, and then the separation experiments were carried out in TBAB and TBAB−sodium dodecyl sulfate (SDS) solution, respectively. The experimental results show that CH4 and N2 form a hydrate much easier after adding TBAB to water. The composition of CH4 in the hydrate after single-stage equilibrium separation in TBAB solution can be increased from 46.25 mol % to 67.86 mol %. At the same conditions, the composition of CH4 after separation in TBAB−SDS solution is 68.66 mol % and the reaction time shortens greatly. Besides, the recovery of CH4 is more than 47%, and the gas storage capacity of hydrate is 19−21 m3/m3. Higher composition and recovery of CH4 are expected to be obtained if multistage separation is applied. It indicates that CH4 can be concentrated effectively from CH4 and N2 via hydrate formation in TBAB solution. Since the hydrate separation technology can substantially avoid the explosion problem caused by CH4 and on the basis of the results obtained in this work, we may say that this technology is quite suitable for the separation of coal bed methane mixed with air and has broad prospects for industrial applications.

Because of the technology limitation and safety requirement, a large amount of air-mixed coal bed methane is released directly to the atmosphere. It leads to severe waste of resources and exacerbates the greenhouse effect. This work tries to alleviate this problem by separating CH 4 from a mixture of CH 4 and N 2 via hydrate separation method. Several experiments were systematically carried out in 6 mol % tetrahydrofuran (THF) solution to separate CH 4 /N 2 gas mixtures containing 4.90 to 71.23 mol % CH 4 . The results that THF can significantly reduce the formation pressure of CH 4 −N 2 hydrate materialize the possibility of separating CH 4 and N 2 using hydrate separation technology in industrial scale. CH 4 can be separated effectively from CH 4 /N 2 mixture and concentrated in hydrate phase with the presence of THF. The recovery of CH 4 ranges from 34.06% to 58.16% and the separation factor is between 2.29 and 5.17. A two-stage separation process of 46.28 mol % CH 4 /53.72 mol % N 2 with recycle is designed to increase the recovery of CH 4 . The calculated results indicate that CH 4 concentration in hydrate gas could reach 82.61 mol % with a recovery yield of 47.28%. Hydrate separation technology is an effective way to process and utilize the airmixed coal bed methane. Multistage separation process does have wide application prospects.

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Tetra-n-butyl ammonium bromide (TBAB) is most-widely used as a hydrate promoter in the separation of flue gases and CO 2 sequestration. Because of the complicacy of semiclathrate hydrate formed by TBAB aqueous solution and gases, few researchers have investigated the thermodynamic modeling of phase equilibria of semiclathrate hydrates. In this work, the CH 4 −N 2 hydrate dissociation conditions were determined in TBAB solution first, and then a thermodynamic model for gas hydrate proposed by Chen and Guo was extended for semiclathrate hydrates of gas mixtures in TBAB aqueous solution. The Patel-Teja equation of state (PT EoS) was used for calculation of the fugacity of the gas phase. To evaluate the activity of water and activity coefficients of TBAB in TBAB aqueous solution, a correlation on the basis of existing osmotic coefficient and activity coefficient values is employed. For modeling the hydrate phase, parameters for sII hydrates proposed in the Chen−Guo hydrate model were used to calculate the fugacity of gases in hydrate phase, and parameters of TBAB were obtained from the correlation with the literature dissociation data for the pure gas + TBAB + water system. Then the presented model was used to predict dissociation data for the gas mixtures + TBAB + water system, and the results show acceptable agreement with other literature data and the experimental data for CH 4 −N 2 in this work.

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