CO2 absorption into catalyzed potassium carbonate–bicarbonate solutions: Kinetics and stability of the enzyme carbonic anhydrase as a biocatalyst

Abstract

The enzyme carbonic anhydrase (CA) has potential as an efficient biocatalyst to promote the absorption of CO2 into potassium carbonate–bicarbonate (K2CO3–KHCO3) solutions for CO2 capture from the flue gas of coal-fired power plants. Knowing the kinetics of the promoted absorption and the catalytic activity and stability of CA under typical operating conditions is essential for process design and techno-economic analysis. In this study, we investigated two specifically engineered CA enzymes and experimentally determined the first-order rate constant of CA (kCA) for catalyzing CO2 hydration into K2CO3–KHCO3 solutions by measuring the rate of CO2 absorption into the solutions in a stirred cell reactor and using the classic Danckwerts theory for data interpretation. The kCA values obtained were 4.02Lmg−1s−1(1.21×108M−1s−1) at 25°C, 4.25Lmg−1s−1(1.28×108M−1s−1) at 40°C, and 4.07Lmg−1s−1(1.22×108M−1s−1) at 50°C. These values are applicable in 20wt% K2CO3–KHCO3 solutions with CO2 loadings ranging from 10 to 40% carbonate-to-bicarbonate (CTB) conversion. The kCA value did not substantially depend on the reaction temperatures at 25–50°C, the composition of the 20wt% K2CO3–KHCO3 solutions with 10–40% CTB conversion, or the pH condition (10.1 to 11.0 pH). The rate of CO2 absorption was promoted by two to six times in the presence of 300mgL−1 of CA in 20wt% K2CO3–KHCO3 solutions at 40–60°C. The long-term thermal stability of CA was investigated at 40, 50, and 60°C in 20wt% K2CO3–KHCO3 solutions with 20 and 40% CTB conversion. The activity loss of CA, as indicated by a decrease in the enhancement factor (ECA) over time, was approximately 20% of its initial activity after 6 months at 40°C, was 50% after 2 months and 80% after 4 months at 50°C, and was 60% after 1 month and 80% after 2 months at 60°C. The CA showed comparable thermal stability in solutions with 20 and 40% CTB conversion, indicating that the CO2 loading of the solution was not an important factor in the thermal stability of the enzyme. The resistance of CA to flue gas impurities that may be transformed in the solvent, which are present in the form of SO42−, NO3−, and Cl− anions was examined at 50°C in the 20wt% solution with 20% CTB conversion in the presence of 0.4M K2SO4, 0.3M KCl, and 0.05M KNO3, either alone or in combination. The CA did not show any further appreciable loss of activity or long-term stability in the presence of the simulated impurities at the concentrations investigated. The results of this study provide valuable information and practical guidance for the potential application of CA as a biocatalyst to promote the absorption of CO2 into K2CO3–KHCO3 solutions for CO2 capture.