Ionic liquids (ILs) are receiving growing attention as liquid absorbents alternatives to aqueous amine solutions for the removal of CO2 from flue gas in post-combustion processes, due mainly to their thermal stability and nonvolatility. We have synthesized tetraalkylphosphonium based ionic liquids and have shown that the CO2 uptake capacity can be increased up to 1 mol of CO2 per mole of IL by appending the amine functionality to the anion¥. While the high CO2 capacity is desired, the relationship between the equilibrium and kinetics of the absorption reaction is important to understand when designing the absorber-stripper process.
Our preliminary studies based on water-miscible ILs show that they can have significantly faster reaction rates than aqueous amines. Second order reaction rate constants are estimated to be up to one order of magnitude higher than monoethanol amine. However not all functionalized ILs are miscible with water. Furthermore water is not desired as a reactive dilution medium, due to its high heat capacity. Therefore functionalized ILs are mixed with inert, low-viscosity and thermally stable solvents that have negligible vapor pressure at the measurement conditions. Physical properties such as the density and viscosity of the solutions as well as the solubility and diffusivity of CO2 in these solutions are required to study the kinetic properties. The density and viscosity of possible IL solutions are measured in the temperature range of 22 to 80 ⁰C and in the IL concentration range of 0.05 to 1 mol/L. The physical solubility of CO2 in these solutions is measured in-situ with an ATR-FTIR probe. Diffusivity of CO2 is most important for ILs when they are not diluted because the process can become mass-transfer limited owing to their high viscosities. For the IL solutions, diffusivities are estimated using a modified Stokes-Einstein relation. These physico-chemical properties and relationships enable the kinetic studies of CO2 absorption in ILs.
¥ B. E. Gurkan, J. C. de la Fuente, E. M. Mindrup, L. E. Ficke, B. F. Goodrich, E. A. Price, W. F. Schneider and J. F. Brennecke, Journal of the American Chemical Society, 2010, 132, 2116-+.
See more of this Group/Topical: Topical 1: Separation Needs for Energy Independence and Environmental Sustainability