Derivation of a consistent modelling approach for unpromoted and enzyme-catalysed CO2 hydration in alkaline solutions

• Consistent concentration-based and activity-based kinetic rate values derived to describe CO 2 absorption in alkaline solutions. • Activity-based kinetic rate values for the CA-promoted system in the presence of 1 g CA l −1 derived. • Marginal differences in CO 2 capture efficiency when low ionic strength solutions were simulated for point source carbon capture. Increased deviations for the more concentrated solutions. • Differences were profound when modelling direct air capture (DAC) using a 1 M K 2 CO 3 solution due to modelling setup. • For higher ionic strength solutions, the elecNRTL model underpredicts the activity coefficient values but still provides a more accurate representation of the system compared to infinite dilution concentration kinetics. This study presents a consistent kinetic framework for modelling CO 2 absorption into alkaline solutions by comparing concentration-based and activity-based kinetic approaches for the unpromoted system, and by providing activity-based kinetic rate constant values for the carbonic anhydrase (CA)-promoted system in the presence of 1 g CA l −1 . Drawing data for the forward kinetic rate constants from published studies, reverse reaction rate constant values were obtained for the two CO 2 hydration pathway reactions (via water and via hydroxide). By employing these kinetic rate expressions, process simulations for point-source and direct air capture (DAC) applications were carried out using different solvent compositions with the results confirming the substantial increase in CO 2 absorption rate especially when 1 M K 2 CO 3 solutions were used as the base solvent. The comparison between concentration-based and activity-based kinetic rate expressions for the unpromoted point-source capture system revealed an absolute difference in CO 2 capture efficiency values of less than 3% in all cases while for the cross-flow air contactor design, simulations showed differences of up to 13% for the higher ionic strength 1 M K 2 CO 3 solution, thus highlighting the limitation of employing static, concentration-based infinite dilution kinetic rates