Methane Separation from Coal Mine Methane Gas by Tetra-n-butyl Ammonium Bromide Semiclathrate Hydrate Formation Dongliang Zhong* ,† and Peter Englezos ‡ † Key Laboratory of Low-Grade Energy Utilization Technologies and Systems of Ministry of Education, and College of Power Engineering, Chongqing University, Chongqing 400044, China ‡ Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada ABSTRACT: This work presents new data to further develop the hydrate-based process for the separation of CH 4 from the low- concentration coal mine methane gas (30 mol % CH 4 /N 2 ) through the formation of tetra-n-butyl ammonium bromide (TBAB) semiclathrate hydrate. The TBAB semiclathrate hydrate formed from the 30 mol % CH 4 /N 2 gas mixture has more favorable equilibrium conditions than the gas hydrate formed from the same gas mixture. The incipient equilibrium conditions at 0.17, 0.29, and 0.62 mol % TBAB were experimentally determined and reported. The experiments of forming TBAB semiclathrate hydrate from the 30 mol % CH 4 /N 2 gas mixture were performed in both semibatch and batch operation under a fixed driving force of 3.5 MPa. The results indicate that CH 4 is preferentially encaged into the TBAB hydrate. The use of a 0.29 mol % TBAB solution is preferred over the 0.17 and 0.62 mol % TBAB solutions. The semibatch operation is more effective than the batch operation for the separation of CH 4 from the 30 mol % CH 4 /N 2 gas mixture. The CH 4 recovery was found to be approximately 25% at 0.29 mol % TBAB concentration in the semibatch operation and the corresponding CH 4 concentration released from the TBAB hydrate was nearly 43 mol %. A CH 4 -rich stream (70 mol % CH 4 /N 2 ) was obtained after two stages of TBAB semiclathrate hydrate formation. 1. INTRODUCTION Coal mine methane (CMM) refers to methane (CH 4 ) gas that is a constituent of coal mine gas. 1 For safety reasons, CMM should be diluted and removed from coal mines through the ventilation system. When the released CMM is mixed with air, the concentration of CH 4 in the mixture is approximately in 30-50 mol %, that of O 2 is around 10 mol %, and the balance is N 2 . However, emission of methane to the atmosphere is prohibited because it has a global warming potential (GWP) that is 21 times greater than that of CO 2 . 2,3 The CMM gas is currently utilized as a low-energy-fuel gas by power plants located near coal mines. An alternative way of utilizing the CMM gas is to convert it into a methane-rich gas. Pressure swing adsorption (PSA), cryogenic liquefaction, and membrane separation are among the methods to separate CH 4 from the CH 4 /N 2 /O 2 mixture. 4-6 However, the operating cost is significant, and hence, research is carried out to find a lower cost separation method. Gas hydrate crystallization is a potentially low cost gas separation process. 7 Gas hydrates are ice-like crystalline com- pounds formed when small-sized gas molecules (e.g., N 2 , CH 4 , and CO 2 ) are encaged in the cavities constructed by hydrogen- bonded water molecules. 8 The fact that the concentration of a gas component in the hydrate crystal is different than that in the original gas mixture forms the basis for using gas hydrate formation as a gas separation method. CO 2 separation from flue gas (CO 2 /N 2 /O 2 ), fuel gas (CO 2 /H 2 ), and sulfur hexafluoride (SF 6 ) from the (SF 6 /N 2 ) mixture can be accomplished with hydrates. 9-12 Since the equilibrium formation pressure of CH 4 hydrate is much lower than that of N 2 and O 2 hydrates at the same temperature, it is likely that CH 4 enters the hydrate phase preferentially and therefore can be recovered from the CH 4 /N 2 /O 2 gas mixture after decomposing the gas hydrate crystals. The feasibility of this idea for methane separation from the CMM gas has been confirmed by Zhang et al. 13 in a low- concentration CMM system. The CMM gas is defined as a low- concentration CMM gas when its methane content is below 30 mol %. The incipient hydrate formation pressure at 274.15 K for a low-concentration CMM gas (30 mol % CH 4 /60 mol % N 2 /10 mol % O 2 ) is 7.73 MPa. 14 This means that a separation method based on hydrates will be expensive because of the compression required to bring the CMM gas to the necessary hydrate formation pressure. The quaternary ammonium salt of tetra-n-butyl ammonium bromide (TBAB) is a chemical that may be used to reduce the hydrae phase equilibrium pressure at a given temperature. 15 TBAB forms semiclatharate hydrate with water at atmospheric pressure. 16,17 In TBAB hydrate crystals, the hydrophilic anions (Br - ) are hydrogen bonded with water molecules and build cavities that are occupied by the hydrophobic tetra-n-butyl ammonium cations (TBA). 16,18 The empty dodecahedral cavities (5 12 ) can encage small gas molecules (e.g., N 2 , CH 4 , and CO 2 ). 15,19 Phase equilibrium data for TBAB semiclathrate hydrate formed with low-concentration CMM gas (29.95 mol % CH 4 / 60.0 mol % N 2 /10.05 mol % O 2 ) were reported. 20 It was also reported that methane is preferentially encaged in the TBAB semiclathrate hydrate. Thus, it is of interest to exploit the TBAB hydrate as a means to concentrate the CMM gas toward higher methane content. Sun et al. 21 formed TBAB semiclathrate hydrate Received: December 24, 2011 Revised: March 9, 2012 Published: March 9, 2012 Article pubs.acs.org/EF © 2012 American Chemical Society 2098 dx.doi.org/10.1021/ef202007x | Energy Fuels 2012, 26, 2098-2106