Using Tetra-n-butyl Ammonium Chloride Semiclathrate Hydrate for Methane Separation from Low-concentration Coal Mine Gas

Abstract In this work, tetra- n -butyl ammonium chloride (TBAC) semiclathrate hydrate was formed for methane separation from the low-concentration coal mine gas containing 30 mol% CH 4 , 60 mol% N 2 , and 10 mol% O 2 . The phase equilibrium data of TBAC semiclathrate hydrate formed with the low-concentration coal mine gas were measured at three TBAC concentrations (0.49 mol%, 1.0 mol%, and 3.3 mol%) and were reported for the first time. The effects of TBAC concentration, subcooling (driving force), and surfactant (SDS) on hydrate formation kinetics were studied systematically. It was found that at a given temperature the phase equilibrium pressure for TBAC semiclathrate hydrate formed with the coal mine gas increases with the increase of TBAC concentration, and the enthalpy of hydrate dissociation at 3.3 mol% TBAC is higher than that obtained at 0.49 mol% and 1.0 mol% TBAC. 0.49 mol% TBAC was found to be the optimum concentration for CH 4 recovery among the three TBAC concentrations (0.49 mol%, 1.0 mol%, and 3.3 mol%) used. The subcooling of Δ T = 8 K was more favorable for hydrate-based CH 4 recovery as compared to the subcoolings of 7 K and 9 K. CH 4 recovery, separation factor, and CH 4 composition were increased when SDS concentration was increased from 300 ppm to 900 ppm. The hydrate-based CH 4 separation efficiency obtained in the presence of TBAC was comparable with that obtained in the presence of tetra- n -butyl ammonium bromide (TBAB).