Laboratory to bench-scale evaluation of an integrated CO2 capture system using a thermostable carbonic anhydrase promoted K2CO3 solvent with low temperature vacuum stripping

Abstract

An advanced post-combustion CO2 capture process with combined attributes of a thermostable carbonic anhydrase (CA) enzyme catalyst, low-enthalpy K2CO3 solvent, and vacuum stripping utilizing low exergy steam was evaluated from laboratory concept to application performance testing in an integrated 30 standard liters per minute gas flow bench-scale system operated for 500 h. Laboratory concept studies were performed using a wetted wall column to characterize solvent CO2 absorption kinetics and using a recirculating temperature loop to evaluate CA thermo-stability. Wetted wall column tests showed a dramatic 5-fold increase in CO2 mass transfer coefficient when combining 2 g/L CA with aqueous 23.5 wt% K2CO3 solvent. Further increasing the CA concentration resulted in a gradual increase in mass transfer coefficient until a performance plateau was observed beyond a 4 g/L CA dose. Operating temperature had limited impact on CO2 capture over the range 30–50 °C. Surface tension measurements of 23.5 wt% K2CO3 solvent exhibited a gradual decrease with increasing CA concentration. Thermo-stability tests in a temperature cycling loop designed to mimic the temperature swings between absorption and desorption showed that CA longevity could be extended by decreasing the total cycle time spent at high temperature. Parametric tests in the bench-scale unit resulted in a CO2 capture efficiency increase of 4.6-fold when increasing the CA concentration from zero to 2.5 g/L. Capture efficiency increased with higher reboiler duty (i.e. reboiler temperature) and lower absorber temperature. Tests with a 30 °C absorber temperature delivered >90% capture. Variation in solvent flow rate had little impact on capture efficiency because the reaction closely reached equilibrium at the top of the absorber. The integrated bench-scale system operated successfully for an accumulated 500 h under conditions of 40 °C absorber temperature and stripper at 35 kPa pressure with an approximate 77 °C stripper bottom temperature, delivering an average 84% CO2 capture with 23.5 wt% K2CO3-based solvent containing 2.5 g/L CA. Dissolved CA replenishment and conventional process controls were demonstrated as straightforward approaches to maintain system performance of this benign, low-temperature, CA-promoted process for CO2 capture.