Process and Material Design for Micro-Encapsulated Ionic Liquids in Post-Combustion CO2 Capture

Aprotic Heterocyclic Anion (AHA) Ionic Liquids (ILs) have been identified as promising new solvents for post-combustion carbon capture due to their high CO₂ uptake and the high tunability^1,2 of their binding energy with CO₂. Some of these compounds change phase (solid to liquid) on absorption of CO₂; these Phase Change ILs (PCILs)³ offer the additional advantage that part of the heat needed to desorb the CO₂ from the absorbent is provided by the heat of fusion as the PCIL solidifies upon release of CO₂. However, the relatively high viscosity of AHA ILs and the occurrence of a phase change in PCILs present challenges for conventional absorption equipment. To overcome these challenges we are pursuing the use of new technology to micro-encapsulate the AHA ILs and PCILs. Our partners at Lawrence Livermore National Laboratory have successfully demonstrated this technology in the application of post-combustion carbon capture with sodium and potassium carbonate solutions,⁴ and have recently shown the feasibility of micro-encapsulation of an AHA IL for carbon capture.⁵ The large effective surface area and high CO₂ permeability of the micro-capsules is expected to offset the drawback of the high IL viscosity and to provide for a more efficient and cost-effective mass transfer operation involving AHA ILs and PCILs.

These opportunities, however, present us with both process and materials design questions. For example, what is the target CO₂ absorption strength (enthalpy of chemical absorption) for the tunable AHA IL? What is the target for micro-capsule diameter in order to obtain a high mass transfer rate and good fluidization performance? What are the appropriate temperatures and pressures for the absorber and stripper? In order to address these and other questions, we have developed a rate-based model of a post-combustion CO₂ capture process using micro-encapsulated ILs. As a performance baseline, we have also developed a rate-based model of a standard packed bed absorber using an un-encapsulated AHA IL absorbent. Using such models we can determine optimal CO₂ capture performance and investigate the sensitivity of the optimum with respect to the key thermo-physical and transport properties of the IL (e.g., CO₂ binding energy, viscosity, etc.) and the micro-capsules (e.g. diameter, CO₂ permeability, etc.). Results of these process and material design studies will be presented, and the performance of this novel micro-encapsulation technology will be assessed.

References

(1) Gurkan, B.; Goodrich, B. F.; Mindrup, E. M.; Ficke, L. E.; Massel, M.; Seo, S.; Senftle, T. P.; Wu, H.; Glaser, M. F.; Shah, J. K.; Maginn, E. J.; Brennecke, J. F.; Schneider, W. F. Molecular Design of High Capacity, Low Viscosity, Chemically Tunable Ionic Liquids for CO₂ Capture. J. Phys. Chem. Lett. 2010, 1, 3494-3499.

(2) Seo, S.; Quiroz-Guzman, M.; DeSilva, M. A.; Lee, T. B.; Huang, Y.; Goodrich, B. F.; Schneider, W. F.; Brennecke, J. F. Chemically Tunable Ionic Liquids with Aprotic Heterocyclic Anion (AHA) for CO2 Capture. J. Phys. Chem. B 2014, 118, 5740-5751.

(3) Seo, S.; Simoni, L. D.; Ma, M.; DeSilva, M. A.; Huang Y.; Stadtherr, M. A.; Brennecke, J. F. Phase-Change Ionic Liquids for Postcombustion CO₂ Capture. Energy Fuels, 2014, 28, 5968-5977.

(4) Vericella, J. J.; Baker, S. E.; Stolaroff, J. K.; Duoss, E. B.; Hardin, J. O.; Lewicki, J.; Glogowski, E.; Floyd, W. C.; Valdez, C. A.; Smith, W. L.; Satcher, J. H.; Bourcier, W. L.; Spadaccini, C. M.; Lewis, J. A.; Aines, R. D. Encapsulated Liquid Sorbents for Carbon Dioxide Capture, Nature Commun., 2015, 6, 6124.

(5) Stolaroff, J. K.; Ye, C.; Oakdale, J. O.; Baker, S. E.; Smith, W. L.; Nguyen, D. T.; Spadaccini, C. M.; Aines, R. D. Microencapsulation of Advanced Solvents for CO₂ Capture. Faraday Discuss., 2016, in press, DOI: 10.1039/C6FD00049E.