W. Shi, L. Hong, V.A. Kusuma, D.P. Hopkinson
National Energy Technology Laboratory,
Keywords: atomistic modeling, device-scale modeling, experiment, CO2 absorption, ionic liquid, reverse micelle
Summary:Highly viscous solvents for gas absorption will lead to large pumping power requirement and large absorption/desorption tower, resulting in high capital cost. Ionic liquids (ILs) represent such examples even though ILs have been extensively studied for gas separation due to their favorable properties, such as negligible volatility and large CO2 solubility. In additional to the high viscosity, ILs are typically hygroscopic due to strong anion-water interactions. This IL hygroscopicity implies that water must be removed before CO2 is captured. Both the high viscosity and hygroscopicity degrade the efficiency and economics of carbon capture by using ILs. In order to overcome the above two drawbacks, we have recently investigated IL reverse micelle (RM) for CO2 capture . The nano size and consequently the large specific surface area for the ILRM nanodroplet (IL core and surfactant) will significantly help gas mass transport. Additionally, a hydrophobic oil is used as the solvent phase to minimize water permeation through the ILRM system, which may alleviate problems related to water absorption in ILs. In this talk, I present both atomistic and device scale modeling to investigate CO2 absorption in ILRM. The ILRM consisted of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) IL as the micelle core, the benzylhexadecyldimethylammonium ([BHD]+) chloride ([Cl]−) was the cationic surfactant, and benzene as the continuous solvent phase. Both our atomistic simulations and experiments show that the IL inside the RM diffuses 5-26 times faster than the neat IL, which is partly due to the fast particle diffusion for the ILRM nanodroplet as a whole in the less viscous oil solvent. Simulations show that CO2 molecules are absorbed in four different regions of the ILRM system, that is, (I) in the IL inner core, (II) in the [BHD]+ surfactant cation layer, (III) at the interface between the [BHD]+ surfactant cation layer and benzene solvent, and (IV) in the benzene solvent. The CO2 solubility was found to decrease in the order II > III ~ IV > I, while the CO2 diffusivity and permeability decrease in the following order: IV > III > II > I. By combing the device-scale modeling with the unsteady-state CO2 absorption data obtained from our experimental Sievert’s apparatus, it was found that the apparent CO2 mass transport in the ILRM could be one magnitude faster compared with CO2 mass transport in the corresponding neat IL and surfactant. This is partly due to the large specific area for the ILRM nano particle. Both our simulations and experiments show that the ILRM is a promising material for CO2 capture applications.  Wei Shi, Lei Hong, Krishnan Damodaran, Hunaid B. Nulwala, and David R. Luebke,”Molecular Simulation and Experimental Study of CO2 Absorption in Ionic Liquid Reverse Micelle”, J. Phys. Chem. B 2014, 118, 13870-13881.