CO2 capture by membrane absorption coupling process: Application of ionic liquids

An experiment system made up of a membrane absorption unit and a membrane vacuum regeneration unit was set up. Ionic liquids (ILs), 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4]) and 1-(3-aminopropyl)-3-methyl-imidazolium tetrafluoroborate ([apmim][BF4]) as two absorbents for CO2 capture, were applied to the experiment system. Coupling processes of membrane contactor-IL absorbents and membrane vacuum-IL regenerations were evaluated by both the membrane absorption unit and the membrane vacuum regeneration unit. Water content in the ILs, CO2 loading capacity, operation conditions and modes, and regeneration factors were investigated. Results show that lower water content in [apmim][BF4] and higher water content in [bmim][BF4] were favorable for the coupling process. The aqueous [apmim][BF4] could give a high CO2 loading capacity at almost atmospheric pressure and its absorption capability was much larger than that of [bmim][BF4] in the coupling process. CO2 partial pressure had a certain influence on the membrane fluxes of both the ILs. The liquid flowrate evidently affected the membrane flux of the coupling process with aqueous [apmim][BF4]. The aqueous [apmim][BF4] could maintain a higher initial membrane flux and give a much larger instantaneous membrane flux compared with the aqueous [bmim][BF4]. The aqueous [bmim][BF4] could be completely regenerated even at low vacuum degree, while the aqueous [apmim][BF4] was difficult to be regenerated comparatively even at high vacuum degree.