Dependence of the Overall Mass Transfer Coefficient on CO2 Loading in an Aqueous Ammonia-Based CO2 Absorption

Abstract This study investigates the mass transfer behavior of aqueous ammonia during CO2 absorption in a packed-bed absorber, under continuous CO2 loading. The overall mass transfer coefficient of CO2 absorption by aqueous ammonia was investigated based on the free ammonia concentration or CO2 loading in the solution during a recycling process in a packed-bed absorber. Experimental conditions covered a wide range of gas flow rates (76.4–367 m3 · m–2 · h–1), liquid flow rates (4.8–20.8 m3 · m–2 · h–1), and ammonia concentrations (2–10 wt %). The volumetric overall mass transfer coefficient (KGav) was evaluated, revealing that higher liquid flow rates and ammonia concentrations significantly enhance KGav. Gas flow rate was also found to play a significant role. Initially, KGav increased with increasing gas flow rate; however, at higher gas flow rates, KGav decreased. The KGav values for CO2-loaded ammonium solutions were compared with those obtained using fresh ammonia solutions. Increasing CO2 loading reduced KGav because of the decline in free NH3 concentration. Furthermore, KGav was found to exhibit an approximately linear dependence on the free ammonia concentration. This near-linear dependence, stronger than the square root dependence on free ammonia concentration (WNH30.5) predicted by pseudo-first-order theory, was attributed to operation in the intermediate reaction regime at high ammonia concentrations and CO2 partial pressures. An empirical correlation has been proposed between the volumetric overall mass transfer coefficient and the investigated experimental parameters for the present system as follows:KGav∝WNH3L0.3G0.33 where L and G correspond to liquid flow rate and gas flow rate, respectively, m3 · m–2 · h–1.