Lessons Learned from the Use of Unconventional Materials for CO 2 Capture
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Lessons Learned from the Use of Unconventional Materials for CO2 Capture Jason E. Bara, Max S. Mittenthal, Brian Flowers, Wesley F. Taylor, Alex H. Jenkins, David A. Wallace, J. David Roveda University of Alabama, Department of Chemical & Biological Engineering, Tuscaloosa, AL 35487-0203 USA ABSTRACT Having worked on several approaches to CO2 capture over the past decade, we have studied a great number of physical and chemical solvents as well as polymer and composite membranes. Initially, most of these materials were based upon ionic liquids (ILs), however due to challenges encountered in applying ILs to meet the demanding requirements in CO2 separation processes, there is a need to reconsider what role (if any) ILs might play in CO2 capture technologies. Ultimately, more promising and robust materials will not come from ILs themselves, but from retrosynthetic analysis and a reconsideration of which structural variables and properties are (and are not) important. The hybridization of the constituent parts into entirely new, yet seemingly familiar substances, can yield greatly improved properties and economics. This manuscript highlights recent work from our group based on lessons learned from ILs that have spurred the development of new amine solvents and polymer materials to better address the demanding process conditions and requirements of CO2 capture and related separations. INTRODUCTION Ionic liquids (ILs) have been touted as “green” replacements for conventional aqueous amine absorbents for CO2 capture, natural gas sweetening and other industrial gas treating applications.[1-7] However, in recent years there has been only incremental progress in furthering ILs for these applications, and we are unaware of any significant gas treating pilot projects that have utilized ILs as the working fluid. This lack of scale-up may be attributable to one or more limitations associated with nearly all ILs: low CO2 capacity, relatively large viscosities, chemical stability and/or cost.[8] Efforts to rectify one of these issues typically exacerbate one or more of the other aforementioned issues. Since most well-known ILs have no active sites available to chemically react with CO2, the primary approaches taken to improve CO2 capacity in ILs have focused on the inclusion of functionalities such as amines, either through direct functionalization of the cation and/or anion[9] or through blending small amines (e.g. monoethanolamine (MEA)) with the ILs themselves.[10] Such studies have certainly shown significant improvements in the amount of CO2 that can be absorbed by a given volume of the IL or IL-amine solution. However, absorption of CO2 by amines forms carbamate and ammonium ions, which when introduced into an already charged solvent (the IL itself), result in dramatic increases in overall solvent viscosity.[11] Furthermore, depending on the nature of the IL, the presence of amines may chemically degrade the IL itself and render the amines inactive.[12] ILs are already much more expensive than traditional solvents, and if/whe
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