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

This study presents a consistent kinetic framework for modelling CO₂ 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 CO₂ 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 CO₂ absorption rate especially when 1 M K₂CO₃ 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 CO₂ 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 K₂CO₃ solution, thus highlighting the limitation of employing static, concentration-based infinite dilution kinetic rates.