Experimental Investigation of Methane Separation from Low-Concentration Coal Mine Gas (CH₄/N₂/O₂) by Tetra-n-butyl Ammonium Bromide Semiclathrate Hydrate Crystallization

In this work, tetra- n -butyl ammonium bromide (TBAB) semiclathrate hydrate was employed in a batch operation to separate CH 4 from low-concentration coal mine methane (CMM) gas with a mole composition of 30% CH 4, 60% N 2, and 10% O 2 . TBAB semiclathrate hydrate formed from the low-concentration CMM gas has more favorable incipient equilibrium conditions than gas hydrates formed from the same gas mixture. The experiments were carried out at a fixed pressure of 4.0 MPa, three TBAB mole concentrations of 0.29, 0.62, and 1.38 %, and in the temperature range of 274.75–283.35 K. The effects of TBAB concentrations and subcoolings (Δ T sub ) on CH 4 separation from the low-concentration CMM gas were investigated. The results indicated that CH 4 was preferentially incorporated into the hydrate crystals rather than N 2 and O 2 in the presence of TBAB. The experimental conditions of the TBAB concentration of 1.38%, 4.0 MPa, and Δ T sub = 7.0 K were most favorable for TBAB semiclathrate hydrate to separate CH 4 from the low-concentration CMM gas. The CH 4 recovery obtained at these conditions was approximately 27%. Compared with CH 4 content in the low-concentration CMM gas, the mole fraction of CH 4 in TBAB semiclathrate hydrate was increased to 41%.