Investigation on methane recovery from low-concentration coal mine gas by tetra-n-butyl ammonium chloride semiclathrate hydrate formation

This work presents an investigation of using tetra-n-butyl ammonium chloride (TBAC) semiclathrate hydrate to separate CH4 from a low-concentration coal mine gas containing 30 mol% CH4, 60 mol% N-2, and 10 mol% O-2. Phase equilibrium data of TBAC semiclathrate hydrate formed in the presence of low-concentration coal mine gas with TBAC concentration ranging from 0.49 mol% to 3.3 mol% were measured and reported. A high pressure micro-differential scanning calorimeter (HP mu-DSC) was employed to characterize the phase behaviors of TBAC semiclathrate hydrate formed in the presence of coal mine gas with TBAC concentration fixed at 3.3 mol% (the stoichiometric concentration of TBAC semiclathrate). It was found that pure TBAC semiclathrate hydrate coexists with the TBAC-coal mine gas mixed hydrate in the pressure range of (1.4-6.2 MPa). In addition, kinetic experiments were performed to elucidate the effects of TBAC concentration, subcooling, and sodium dodecyl sulfate (SDS) concentration on CH4 separation efficiency and an optimum experimental condition for semiclathrate-based CH4 recovery was screened. The highest CH4 recovery and separation factor were obtained at the conditions of 0.49 mol% TBAC, Delta T = 8 K, and 900 ppm SDS. It was found that CH4 recovery obtained at 0.49 mol% TBAC is comparable with that obtained at 0.29 mol% TBAB, whereas CH4 concentration of the gas mixture released from the TBAC hydrate (49.6 mol%) is increased substantially comparing to that obtained at 0.29 mol% TBAB (42.5 mol%).