Assessment of the Relationship between Degradation and Emission Activities of Carbon Capture Amines Based on their Chemical Structures

Abstract

In this work, 16 alkanolamines were investigated to determine the effect of their chemical structural positioning of different functional groups on the rates of amine degradation and ammonia (NH3) emissions as well as to determine if there were inter-relationships between degradation and emission based on their structures. The oxidative degradation and emission experiments were carried out continuously for 2 weeks at the temperature of 60 C using 99.5% O2 gas set at a flow rate of 200±2 mL/min. The amine degradation rate was determined by evaluating the rate of decrease of the amine concentration with time. The NH3 emissions was investigated by collecting the NH3 released from the amine solution in an impinger system set up in accordance with the relevant EPA method. The effect of the alkyl chain length in between amino and hydroxyl groups was investigated by considering groups of primary and tertiary alkanolamines. The degradation rate was observed to decrease with increasing alkyl chain length which helped to strengthen the N-C bond due to the electron donating properties of the alkyl group. The degradation rate was found to decrease in the order: MEA  3A1P > 5A1P and DMAE > 3DMA1P > 4DMA1B for primary and tertiary alkanolamines, respectively. In contrast however, the opposite trend was observed for NH3 emissions in the two amine groups. In the case of end alkyl chain length of secondary alkyl-alkanolamines, it was observed that a longer chain length led to a decrease in the degradation and NH3 emission rates. Their order was: MAE  EAE > BAE. The longer chain length of end alkyl group contributed to both electronic and steric hindrance effects that increase the stability of N-C bond which reduced the formation free radicals thereby decreasing the degradation and NH3 emission rates. For tertiary alkyl-alkanolamines, the degradation rate noticeably decreased in the order of MDEA > EDEA > BDEA. The reasons are similar to those previously given for the same group in secondary alkyl-alkanolamine. NH3 emission rates obtained by this amine group was extremely low with the actual trend unable to be clearly determined. The presence of hydroxyl group known as an electron withdrawing group enhanced the degradation rate as a result its ability to weaken the N-C bond. This facilitated the occurrence of amine degradation. This trend was obtained for secondary alkanolamines (less steric effect) whose degradation and NH3 emission rates of DEA containing two hydroxyl groups were higher than those for EAE containing one hydroxyl group. Steric hindrance effect derived from the presence of the hydroxyl group was also there specifically for tertiary alkanolamines and sterically hindered primary amines comparisons. Having more hydroxyl group in the amine molecule will enhance the oxidative degradation reaction, but at the same time, will promote the steric hindrance effect. The latter effect is more dominant, and thus, increased hydroxyl group number has the net effect of obstructing the occurrence of oxidative degradation reaction thereby resulting in the decrease of the degradation rate. On the other hand, we did not observe any significant difference in NH3 emission rate from tertiary and sterically hindered primary amines due to slow rate of degradation as a result of the steric effect introduced by their structure. Since the net degradation rate decreased with increasing number of hydroxyl groups, it can be concluded that the steric hindrance effect is more dominant in the rate of amine degradation and NH3 emission than the electronic effect.