Effects of Tetrabutyl-(ammonium/phosphonium) Salts on Clathrate Hydrate Capture of CO₂ from Simulated Flue Gas

Quaternary salts can form semi-clathrate hydrates, caging gas molecules in the empty small cages, which have the potential for the separation of mixtures, such as the simulated flue gas [CO2 (17 mol %)/N2 mixtures]. To enhance the CO2 separation from CO2/N2 binary mixtures, three quaternary salts, tetra-n-butylammonium bromide (TBAB), tetra-n-butylphosphonium bromide (TBPB), and tetra-n-butylammonium nitrate (TBANO3), are investigated at different operating conditions by a one-stage hydrate separation process. The results indicate that the induction time for each quaternary salt system can be shortened to less than 5 min under the optimal operating condition. Meanwhile, each quaternary salt can significantly promote the CO2 separation under its optimal condition. TBANO3 displays the strongest capability in terms of gas consumption and CO2 separation with the pressure drop of 0.72 MPa and the highest split fraction of 67% and separation factor of 15.54 compared to the other two salts. Besides, CO2 can be further removed from 17 to 7 mol % in the presence of TBANO3. TBPB also has a potential effect on CO2 separation with the pressure drop of 0.57 MPa and the separation factor of 14.06. The result demonstrates that TBANO3 and TBPB are two better additives for efficient hydrate capture of CO2.