Efficient Enrichment Applications for Capacitive Deionization in Conventional Carbon Capture Processes
Authors: Landon James , Gao X. , Omosebi A. , Holubowitch Nicolas , Liu K. 
Published: February 2020

Liquid amine carbon capture systems have been in use for decades, but there remain certain drawbacks that need to be addressed before large-scale implementation of this technology will be more appreciably adopted. Many of these limitations are related to the energy cost of the capture process, but secondary emissions are a pervading limitation due to adverse health and environmental effects related to amine emissions and their degradation products. Secondary emissions from the CO2 absorber column (process unit shown in Figure 1) include vapors and aerosols of amine molecules as well as smaller amounts of their by-products, such as nitrosamines.(1) Currently, to limit the emission of these molecules, water wash sections are added to the top of the absorber that contact the exiting gas and reduce the concentrations of vapors, aerosols, and by-products being potentially emitted to the atmosphere. This system subsequently produces a liquid stream containing rather dilute concentrations of dissolved amine molecules (less than 1 wt%) and a small fraction of other contaminants. To reduce their accumulation in the water wash section for which emissions are controlled by their vapor-liquid equilibrium, they need to be removed using an efficient separation process capable of high water recovery rates.

Established methods to concentrate ionic content from aqueous streams, such as the amine molecules being adsorbed and concentrated here, include polymeric membranes, distillation, and ion exchange techniques. In the ideal concentration process, high water recovery and subsequently high concentrations of amine in the concentrate are desired with minimal energy input. While polymeric membranes can provide quite pure water streams, high pressures are typically required to obtain high water recoveries and resulting high concentrations in their concentrate streams. In addition, membranes have stability issues at high and low pH and suffer from limited operating temperature windows above 40 °C. Distillation techniques are dependent on high energy inputs, and corrosion persists as a notable problem in these units over longer periods of time. Therefore, a capacitive deionization (CDI) system is proposed here to concentrate and return amine molecules from the water wash section to the CO2 capture system. In fact, a similar capacitive mixing process has been shown previously for energy recovery from flue gas streams with dilute bicarbonate and monoethanolamine solutions.(2, 3) Due to CDI’s reported lower energy costs of separation, high water recoveries, and ability to concentrate ionic species at a range of temperatures, pressures, and pH levels, CDI could be an ideal candidate for this application.(4, 5)

Flow-through CDI and recently demonstrated inverted capacitive deionization (i-CDI) cells will be shown here to reversibly separate and concentrate CO2-loaded piperazine (PZ) molecules, an advanced carbon capture solvent currently under investigation, using cell potentials less than 1.5 V.(6) Longer-term performance characteristics including charge efficiency and salt adsorption capacity of the separation process will be evaluated for implementation at larger scales. Liquid chromatography-mass spectrometry (LC-MS) will be used to complement conductivity measurements and confirm piperazine concentrations for the adsorption and desorption profiles from these separation cells. Finally, energy assessments and water recoveries will be shown for this separation process and compared to conventional processes. Preliminary separation results using a batch-mode CDI system operated at 1.2/0 V for adsorption-desorption are shown in Figure 2.

References

1. J. G. Thompson, X. Li and K. Liu, Energy Procedia, 63, 951 (2014).

2. S. Porada, D. Weingarth, H. V. M. Hamelers, M. Bryjak, V. Presser and P. M. Biesheuvel, Journal of Materials Chemistry A, 2, 9313 (2014).

3. J. M. Paz-Garcia, J. E. Dykstra, P. M. Biesheuvel and H. V. M. Hamelers, Journal of Colloid and Interface Science, 442, 103 (2015).

4. M. A. Anderson, A. L. Cudero and J. Palma, Electrochimica Acta, 55, 3845 (2010).

5. S. Porada, R. Zhao, A. van der Wal, V. Presser and P. M. Biesheuvel, Progress in Materials Science, 58, 1388 (2013).

6. X. Gao, A. Omosebi, J. Landon and K. Liu, Energy & Environmental Science, 8, 897 (2015).

Acknowledgements

These authors are grateful to the U.S.-China Clean Energy Research Center, U.S. Department of Energy (DE-PI0000017) for project funding.

Figure 1