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 CH4 from low concentration coal mine methane (CMM) gas with a mole composition of 30% CH4, 60% N-2, and 10% O-2 TBAB semiclathrate hydrate formed from the low concentration CM M 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 (Delta T-sub) on CH4 separation from the low-concentration CMM gas were investigated. The results indicated that CH4 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 138%, 4.0 MPa, and Delta T-sub = 7.0 K were most favorable for TBAB semiclathrate hydrate to separate CH4 from the low concentration CMM gas. The CH4 recovery obtained at these conditions was approximately 27%. Compared with CH4 content in the low concentration CMM gas, the mole fraction of CH4 in TBAB semiclathrate hydrate was increased to 41%.