Combining Different Additives with TBAB on CO2 Capture and CH4 Purification from Simulated Biogas Using Hydration Method

CH4 purification and CO2 removal by hydrate-based separation method from the simulated biogas (64 mol % CH4/CO2) were investigated in tetrabutylammonium bromide (TBAB) solution. Three different hydrate additives were combined with TBAB to study the synergistic influence on thermodynamics and kinetics of hydrate formation. These additives were 1-butyl-3-methylimidazolium hexafluorophosphate ([EMIM]BF4), sodium dodecyl benzene sulfonate (SDBS), and propylene carbonate (PC). The thermodynamic experiment results showed that [EMIM]BF4 and PC have an inhibition effect on CO2 and CO2/CH4 hydrate formation. SDBS has a little influence on phase equilibrium. As for the results of hydrate kinetic and biogas hydrate separation, CH4 mole fraction in the remaining gas could reach 79.20 mol % from the initial 64.0 mol %, and CH4 recovery was 94.21% in TBAB + [EMIM]BF4 solution when the operation conditions were 3.5 MPa and 277.15 K. CO2 mole fraction in the hydrate phase could reach 50 mol % or even higher from the initial 36.0 mol %. The gas storage in the hydrate phase did not change dramatically. Basically, the gas storage in the hydrate phase was 0.07 mol on account of addition of TBAB. The CO2 hydrate ratio was about 55%, and it did not change widely. On the whole, the overall effect of biogas hydrate separation in TBAB solution was [EMIM]BF4 > PC > SDBS.