Comparing MEA and PZ + AMP in Terms of CO2 Absorption

The removal behaviour of the absorbents can be further elaborated based on the trendlines observed in the CO₂ removal profiles from along the column. Referring to the trendline of AMP, as shown in Figure 1, a steady increase in CO₂ removal can be observed from 0 to 2.04 m of the column height, and this absorbent exited the column without reaching a plateau. The CO₂ absorption achieved by 30 wt.% AMP contributed to the lowest CO₂ removal among all of the absorbents, achieving a 52% removal. As expected, the CO₂ absorption performance of MEA surpassed the performance of AMP. The MEA solution was 24% better at absorbing CO₂ than the AMP solution. The results show that AMP absorbed CO₂ at a slower absorption rate, while MEA was faster. The different performance observed was due to the reaction constant of AMP being 810.4 m³/kmol·s compared to the higher reaction constant of MEA at 4090 m³/kmol·s [49]. Koronaki et al. [50] conducted an experimental study to compare the absorption performance between MEA and AMP. The same conclusion was reported in their studies, with MEA capturing 97% of CO₂ from the gas stream, which was 42% higher than the amount absorbed by AMP.

In this study, at a CO₂ partial pressure of 80 kPa, an increased CO₂ removal was observed when the PZ concentration was gradually increased in the AMP solution in 2 wt.% increment, with 3, 5, 7, and 9 wt.% additions of PZ. This observation is in line with the observations made by Khan et al. [39], which conducted their study at a low CO₂ partial pressure (10–15 kPa). The experimental result similarly showed an increased CO₂ removal performance at higher PZ concentrations in an AMP solution. This behaviour can be explained by the presence of two nitrogen atoms in the molecular structure of PZ, and these nitrogen atoms are beneficial for direct reactions with CO₂ molecules [29]. The enhanced removal performance could also be due to the increasing formation of stable PZ-dicarbamate molecules, leading to a higher percentage of CO₂ being absorbed [51].

Based on Figure 1, the most reactive section of the column for the different blended PZ + AMP solutions was at the top of the column (1.7 to 2.04 m from the bottom). For instance, a solution with a 9 wt.% of PZ + 21 wt.% of AMP was able to remove 45% of the CO₂ in this section. The highest CO₂-amine reaction was achieved in this section due to the interaction between CO₂ and the liquid, which has a CO₂ loading capacity of zero, since CO₂ was introduced at the top of the column. As the liquid travelled downwards, the CO₂ loading in the liquid phase gradually increased, limiting the CO₂-amine reactions. It was also observed that, from 0 to 1.7 m of the column height, the blended solution with 9 wt.% PZ + 21 wt.% AMP was only able to remove 42% of the CO₂ in the process.

The process performances for the mass transfer coefficients are presented in Figure 2. As shown in Figure 1 (previously), the benchmarking absorbent, MEA, has the ability to remove 76% of CO₂ under these operation conditions. As expected, the performance of AMP was lower than that MEA, with only 52% CO₂ removal and a KGav¯¯¯¯¯¯¯¯of0.041 kmol/m³∙h∙kPa. This phenomenon is reasonable because of AMP’s reaction rate is lower than that of the primary amine, MEA. Therefore, PZ was added to the AMP solution at different ratios to enhance its performance as a potential blended solution. The added PZ acted as a reaction rate accelerator during the process. The increased absorption performance at higher PZ concentrations was due to the increased formation of PZ-carbamate and PZ-dicarbamate, which consequently enhanced the reaction rate [51]. The experimental results show that the CO₂ removal performance gradually increased from 55% to 88% when the PZ concentration in the solution increased from 3 to 9 wt.%. The KGav¯¯¯¯¯¯¯¯ values also showed a steady improvement of approximately 2.46 times, increasing from 0.046 to 0.113 kmol/m³∙h∙kPa.”