2024 AIChE Annual Meeting

Membrane Crossover Modeling for Electrochemical CO2 Separations

Adequate reduction of atmospheric carbon dioxide (CO2) necessitates efficient and low cost carbon capture processes. Electrochemical CO2 separation systems may provide a lower energy alternative to traditional thermochemical capture processes, but ongoing techno-economic evaluations are required to clarify further research directions and technology readiness. Key operating envelopes and cell designs relevant to these systems have yet to be studied in depth, creating the opportunity for preliminary modeling of these systems to define design specifications. While prior work in the Brushett group has investigated the performance and costs associated with a four-stage capture system, these models have yet to account for losses associated with crossover of redox-active species through the separator/membrane.1 Electrochemical CO2 capture systems may be able to withstand higher rates of crossover than other electrochemical systems (i.e., redox flow batteries) because the same redox-active species exist on either side of the separating membrane, resulting in no unrecoverable losses in the redox-active species itself. To verify this, cell modeling allows for understanding of how performance is impacted by crossover in the cell but also clarifying how lower costs can be achieved as these systems may tolerate more conductive but less selective separators/membranes.

The existing four-stage capture model includes three process units: an electrochemical cell for redox-active species activation and deactivation, an absorber for CO2 binding, and a flash tank for release of concentrated CO2. Using a CSTRs-in-series or “cascade” approach may allow for simplification of mathematical solving, perhaps reducing 2D concentration gradients into segmented 1D concentration gradients.2 While we expect system efficiencies to decrease when accounting for crossover, the active species itself does not decay and thus will likely not contribute to shorter operating lifetimes. Ultimately, this electrochemical cell model can integrate into a previously-developed economic model to elucidate the cost trade-offs between conductivity and selectivity in separator/membrane selection in order to improve technology readiness.

(1) Clarke, L. E.; Leonard, M. E.; Hatton, T. A.; Brushett, F. R. Thermodynamic Modeling of CO2 Separation Systems with Soluble, Redox-Active Capture Species. Ind. Eng. Chem. Res. 2022, 61 (29), 10531–10546. https://doi.org/10.1021/acs.iecr.1c04185.

(2) Walsh, F.; Trinidad, P.; Gilroy, D. Conversion Expressions for Electrochemical Reactors Which Operate under Mass Transport Controlled Reaction Conditions - Part II: Batch Recycle, Cascade and Recycle Loop Reactors. IJEE 2005, 21 (5), 981–992.