Methane Separation from Coal Mine Methane Gas by Tetra-n-butyl Ammonium Bromide Semiclathrate Hydrate Formation

This work presents new data to further develop the hydrate-based process for the separation of CH4 from the low-concentration coal mine methane gas (30 mol % CH4/N-2) through the formation of tetra-n-butyl ammonium bromide (TBAB) semiclathrate hydrate. The TBAB semiclathrate hydrate formed from the 30 mol % CH4/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 % CH4/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 CH4 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 CH4 from the 30 mol % CH4/N-2 gas mixture. The CH4 recovery was found to be approximately 25% at 0.29 mol % TBAB concentration in the semibatch operation and the corresponding CH4 concentration released from the TBAB hydrate was nearly 43 mol %. A CH4-rich stream (70 mol % CH4/N-2) was obtained after two stages of TBAB semiclathrate hydrate formation.