Due to the high reactivity with CO2, monoethanolamine (MEA) has been used in industrial processes to capture CO2 for many years. The mechanism of CO2 absorption into MEA solution has been widely studied, but it is still in dispute. There is few literature reported regarding to the detailed reaction processes of CO2 absorption into MEA solution with high CO2 loading and desorption from the CO2-saturated MEA solution.
To get a clear picture of the process mechanism, 13C nuclear magnetic resonance (NMR) was used to analyze the species of MEA solution with different CO2 loading, and the detailed absorption and desorption mechanism of MEA-CO2 system was evaluated in this work. The results demonstrated that the absorption reaction was started with chemical reactions of MEA with CO2 to form carbamate according the zwitterion mechanism, followed by the hydration of CO2 to form HCO3-/CO32-, and accompanied with the hydrolysis of carbamate. Base on our results by 13C and experimental observations, the reaction was found to vary under different CO2 loading. At low CO2 loading, CO2 absortion into MEA was an exothermic reaction and the reaction rate was fast. In this stage, the absorption capacity of the solvent was found to be 0.40 mol CO2/ mol MEA, which was comparatively comformable to the zwitterion mechanism. At high CO2 loading, the hydration of CO2into solution was the main reaction and was an endothermic process.
It is interesting to find that the existence of carbamate will be influenced by CO2 loading, and it is rather unstable at high CO2 loading. At low CO2 loading, carbamate is formed fast by the reaction of CO2 and MEA. But at high CO2 loading, it will be formed by the reaction of CO3-/CO32- with MEA, and be easily hydrolyzed by H+. The Hydrolysis rate was faster than the formation rate, and the concentration of carbamate decreased as the absorption carried out at high CO2loading.
Moreover, CO2 desorption from the CO2-saturated MEA solution was proved to be a reverse process of the absorption. Initially, part of HCO3- were heated to release CO2 and part of HCO3- reacted with carbamic acid (MEAH+) to form carbamate, then the carbamate decomposed to MEA and CO2.
The results indicated that the reaction of CO2 capture into MEA solution varied with different CO2loading, and the process mechanisms would more accurate reflect the reaction.