Morphology and kinetic investigation of TBAB/TBPB semiclathrate hydrates formed with a CO2 + CH4 gas mixture

This work presents a morphological and kinetic investigation of tetra-n-butyl ammonium bromide (TBAB) or tetra-n-butyl phosphonium bromide (TBPB) semiclathrate hydrates formed with a CO2 + CH4 gas mixture at the stoichiometric concentration (2.57 mol%). A 3-stage procedure was proposed and employed in the experiments. All experiments were performed at 2.8 MPa and a subcooling of 12 K was used as driving force for hydrate nucleation and growth. It was found that the shapes of hydrate crystals formed in TBPB and TBAB solutions were quite different. Besides, the TBPB semiclathrate hydrate grew laterally at gas/liquid interface and quickly covered the interface while the TBAB semiclathrate hydrate initially formed around the gas/liquid interface and then sank to the reactor bottom. As a consequence, gas uptake and the rate of hydrate growth obtained in TBAB solutions were higher than that in TBPB solutions. It was also found that CO2 content in hydrate phase, CO2 recovery, and separation factor obtained in TBAB solutions were higher than those obtained in TBPB solutions. Therefore, TBAB semiclathrate hydrate formed at the stoichiometric concentration and quiescent conditions will be a promising approach to improve the hydrate based CO2 separation from the CO2 + CH4 gas mixture.