Mechanisms of CO2 Capture into Monoethanolamine Solution with Different CO2 Loading during the Absorption/Desorption Processes

Abstract

Though the mechanism of MEA-CO₂ system has been widely studied, there is few literature on the detailed mechanism of CO₂ capture into MEA solution with different CO₂ loading during absorption/desorption processes. To get a clear picture of the process mechanism, ¹³C nuclear magnetic resonance (NMR) was used to analyze the reaction intermediates under different CO₂ loadings and detailed mechanism on CO₂ absorption and desorption in MEA was evaluated in this work. The results demonstrated that the CO₂ absorption in MEA started with the formation of carbamate according to the zwitterion mechanism, followed by the hydration of CO₂ to form HCO₃^–/CO₃^2–, and accompanied by the hydrolysis of carbamate. It is interesting to find that the existence of carbamate will be influenced by CO₂ loading and that it is rather unstable at high CO₂ loading. At low CO₂ loading, carbamate is formed fast by the reaction between CO₂ and MEA. At high CO₂ loading, it is formed by the reaction of CO₃^–/CO₃^2– with MEA, and the formed carbamate can be easily hydrolyzed by H⁺. Moreover, CO₂ desorption from the CO₂-saturated MEA solution was proved to be a reverse process of absorption. Initially, some HCO₃^– were heated to release CO₂ and other HCO₃^– were reacted with carbamic acid (MEAH⁺) to form carbamate, and the carbamate was then decomposed to MEA and CO₂.