Impact of Operational Parameters on the CO2 Absorption Rate in Ca(OH)2 Aqueous Carbonation─Implications for Process Efficiency

The impact of operational parameters on the CO2 absorption rate (η(t)) during Ca(OH)2 aqueous carbonation has been investigated, including the reagent concentration, CO2 volumetric flow rate, temperature, Na- and K-salt impurity concentration, ionic strength, and mixing system. The carbonation mechanisms were numerically investigated with the modeling tool Phreeqc. Positive correlations were predicted and confirmed between η(t) and Ca(OH)2 initial concentrations, reduced volumetric flow rate, elevated temperature, and less obvious parameters like salt impurities and increased ionic strength. The present results indicate a modest impact (a few percent increase) on the absorption rate when varying Ca(OH)2 concentration from 1 to 10 wt %. Increasing the temperature from 283 to 363 K enhances the η(t) of about 37%. The average seawater NaCl concentration (3.5 wt %) enhances η(t) values by as much as 75%. Na2SO4 and K2SO4 in place of NaCl and KCl, respectively, suggest that sulfate ions promote the dissolution of CO2 in aqueous solutions more efficiently than chloride ions. We demonstrate that the CO2–water interfacial surface area plays a crucial role, causing a cascade of kinetic acceleration of dissolution. Utilizing a static mixer (sparger and porous stone diffusor) significantly accelerates dissolution–precipitation, leading to enhanced absorption rates and reducing energy consumption associated with mixing. This approach, coupled with 3.5 wt % NaCl concentration, achieves a CO2 absorption rate of up to 90% for a 2 L/min CO2 flow. The proven enhancements pave the way for more efficient reactor design tailored for ex situ carbon capture both "at the smokestack" and potentially for direct air capture.