THEORETICAL MODELLING OF AN ABSORPTION UNIT FOR CO2 CAPTURE ASSISTED BY IMMOBILIZED ENZYME

The biomimetic CO2 capture strategy is based on the enhancement of CO2 absorption rate into aqueous solvents catalyzed by the enzyme Carbonic Anhydrase (CA). This strategy may provide an environmental friendly alternative to the amine-based CO2 capture process. Key issues for the development of biomimetic CCS process include the development of the biocatalysts to be confined into the absorption unit, immobilizing the enzyme is the most promising option. This issue is a mandatory requirement in the case of both solvent regeneration (aimed at CO2 storage) and of solvent processing (aimed at CO2 utilization). The design of the absorption unit must ensure absorption enhancement by the heterogeneous biocatalyst. The immobilization of CA on fines and the occurrence of biocatalysis at the gas-liquid interface may provide absorption rate enhancement [1, 2]. The present contribution reports the theoretical study of a slurry absorption unit for CO2 capture into K2CO3 aqueous solvent assisted by CA. Main assumptions were: i) the absorption unit described according to the tanks-in-series model; ii) counter-current flow of gas and liquid streams; iii) CO2 chemical absorption described according to the two films theory; iv) slurry phase modelled according to the pseudo-homogeneous approach [3]. The model was based on: a) mass balances on the dissolved species CO2, HCO3 - and CO3 2-; b) charge balance and solvent dissociation equilibrium condition extended to both the liquid boundary layer and the liquid bulk; c) the overall mass balance on carbon. For fine dispersed biocatalysts, the model included an analytical relation between the reactant concentrations, the Thiele modulus and the effectiveness factor of the biocatalyst particles . A staged bubble column was selected as potential configuration for counter-current gas-liquid equipment able to handle slurry phase. The mass transfer coefficient were retrieved from the literature [4]. Model computation was performed using the commercial software COMSOL Multiphysics. Results pointed out an increase of the CO2 capture in the presence of dissolved CA. The concentration profiles for CO2 in the liquid boundary layer highlighted the beneficial effect of enzyme catalysis on CO2 absorption rate even though the advantages were strongly limited by the enzyme solubility (about 100 mg/L). Simulations carried out for CA immobilized on fines showed that absorption rate enhancement increased with respect to that obtained by homogeneous enzyme catalysis. The sensitivity analysis to the CA turnover number was carried out to take into account its spectrum depending on the primary CA form and on the immobilization technique.