Brief discussion of various amine solvents for CO2 absorption

https://doi.org/10.1016/j.egyr.2021.07.056

“Typical reactive solvents used for capturing CO₂ are amines such as monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), N-methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), and piperazine (PZ). Among these amines, MEA has been industrially used for several decades because it reacts rapidly with CO₂ and possesses moderate absorption capacity. However, its major drawback is a high energy requirement for solvent regeneration. TEA and MDEA, on another hand, show high absorption capacity and low energy requirement for solvent regeneration, but have slow reaction with CO₂ [5]. AMP, a sterically hindered amine, shows a better absorption capacity and regeneration performance than the conventional primary MEA. Due to its sterically hindered structure, AMP forms an unstable AMP carbamate, which can be easily hydrolyzed to free AMP and carbonate. The free AMP then again reacts with CO₂, thus absorption capacity of AMP is then higher than that of MEA. Additionally, carbonate, which is formed by a hydrolysis of AMP carbamate, requires much lower energy for releasing CO₂ than MEA carbamate. Interestingly, PZ, a cyclical diamine, possesses very high absorption capacity and reactivity with CO₂. However, AMP and PZ precipitate at high concentration. Thus, their concentration ranges for capturing CO₂ are limited [6]. As can be seen that each conventional amine has advantages and disadvantages, numbers of scientists are now searching novel amines that can be used in substitution of the conventional amines. [7] and [8] reported that N-methyl-4-piperidinol (MPDL), a cyclical tertiary amine (presented in Table 1), shows a promising CO₂ absorption and regeneration performance in that MPDL has higher absorption capacity, faster reaction rate, and lower energy requirement for solvent regeneration than conventional tertiary TEA and MDEA. They suggested that MPDL should be used in substitution of conventional tertiary amine. However, MPDL cannot be used as a single solvent because of its considerably slow reaction kinetics comparing with MEA, AMP, and PZ. To improve the overall reaction kinetics of MPDL, it is suggested that MPDL should be blended with highly reactive amines (such as AMP and PZ). Our previous work confirms that an addition of AMP into MPDL can enhance the overall CO₂ absorption kinetics of the blended solvent [9]. Since PZ is much faster than AMP, it can be deduced that an addition of PZ may result in an enhancement of overall reaction kinetics of the blended PZ-MPDL solvent.”