Chemical enhancement for CO2 absorption into dilute aqueous amine solutions

Chemical enhancement factors for CO2 absorption into dilute aqueous MEA/DEA solutions in a batch stirred-cell absorber were experimentally determined over a wide range of CO2 loadings. The experimental data are used to examin e the validity of two promotion mechanisms: the zwitterion mechanism, by wh ich the carbamate formation is the sole main reaction occurring in the liqu id, and the bulk-equilibrium mechanism, by which both the carbamate formati on and the carbamate hydrolysis are two fast main reactions to enable the c hemical equilibrium to prevail in the bulk-liquid. The results reveal that the zwitterion mechanism satisfactorily predicts the experimental data for CO2 loadings below 0.3, while the bulk-equilibrium mechanism is more compet ent in describing those for CO2 loadings larger than 0.5. A detailed rigoro us thermodynamic simulation shows that the equilibrium extent of the carbam ate hydrolysis, which generates free amine in the bulk and hence promotes a bsorption, increases with the decrease of the initial amine concentration a nd with the proceeding of CO2 loadings. The experimental re suits of absorp tion, therefore, indicate that the extent of the carbamate hydrolysis is in significant in systems with low CO2 loadings however becoming significant i n higher CO2 loading systems. The difference between the promotion mechanis ms for the absorption of CO2 by dilute aqueous NH3 solutions as previously reported by us and that by MEA/DEA solutions in this work is also discussed .