Coal mine gas separation of methane via clathrate hydrate process aided by tetrahydrofuran and amino acids

Coal bed methane (CBM) is an unconventional natural gas resource generated and stored in coal seams. It is currently not commercially viable in many regions due to the high concentration of non-flammable gases such as nitrogen. The CBM separation is crucial for both economic benefit and methane emission reduction. The biggest challenge lies in the separation of low-concentration CBM. Gas separation via gas hydrate is gaining great attraction due to its safe, clean, and economically efficient features. We examined the effects of two environmentally benign amino acids L-tryptophan and L-leucine on hydrate formation from 30% CH4/70% N2 mixture in the presence of tetrahydrofuran (THF, 5.56 mol%). Experiments were carried out at 283.2 K and 3.0 MPa with different solution volumes to study the impact of gas/liquid volume ratio (calculated at standard pressure and temperature conditions). The lower gas/liquid volume ratio improved the CH4 recovery ratio and separation factor but lowered the kinetics. The effect of amino acid is concentration-dependent. In synergy with 5.56 mol% THF solution (gas/liquid volume ratio of 67/1), L-tryptophan (5000 ppm) enhanced the CH4 recovery ratio from 58.3% to 71.4% and boosted the initial hydrate growth rate by 130%. On the other hand, L-leucine (5000 ppm) showed no obvious impact on the kinetics but achieved the highest CH4 enrichment in the hydrate phase (from 30.0 mol% in the feed gas to 56.8 mol% in the hydrates). The high CH4 recovery ratio, enhanced kinetics, and simplicity of this process demonstrate the potential of developing an efficient and economical recovery method for low-concentration CBM.