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

Abstract

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% N2, and 10% O2. 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 (ΔTsub) 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 N2 and O2 in the presence of TBAB. The experimental conditions of the TBAB concentration of 1.38%, 4.0 MPa, and ΔTsub = 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%.