(456g) Understanding Mechanisms of CO2 Capture into Monoethanolamine Solution with Different CO2 Loading

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^-/CO₃^2-, 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^-/CO₃^2- 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.