https://doi.org/10.3390/su14127095
“During CO2 absorption into an amine absorbent in the presence of water, hydrogen bonds form during the CO2-amine-water reaction. The formation of weak hydrogen bonds leads to a reversible reaction mechanism. Therefore, the reversible reaction in which CO2 is absorbed into the blended PZ + AMP solution in the presence of water is governed as follows [40]:
Base-catalysed hydration reaction:
CO2+AMP+H2O⇌AMPH++HCO−3
Monocarbamate formation:
CO2+PZ+H2O⇌PZCOO−+H3O+
Monocarbamate formation by PZ/AMP:
CO2+AMP+PZ⇌PZCOO−+AMPH+
Dicarbamate formation reaction:
CO2+PZCOO−+H2O⇌PZ(COO−)2+H3O+
Dicarbamate formation reaction by PZCOO−/AMP:
CO2+AMP+PZCOO−⇌PZ(COO−)2+AMPH+
Bicarbonate formation reaction:
CO2+2H2O⇌HCO−3+H3O+
Formation of carbonate:
HCO−3+H2O⇌CO2−3+H3O+
Protonation of PZ:
PZ+H3O+⇌PZH++H2O
Protonation of monocarbamate:
PZCOO−+H3O+⇌H+PZCOO−+H2O
Protonation of AMP:
AMPH++H2O⇌AMP+H3O+
Dissociation of the water molecule:
2H2O⇌H3O++OH−
Considering the reaction between CO2 and AMP during the absorption process, AMP-catalysed CO2 hydration was also considered [41,42]. The CO2-AMP-H2O reaction did not lead to carbamate formation because AMP showed a low stability constant [43]. Thus, bicarbonate formation was considered, as represented by Equation (1). Additionally, the subsequent reactions between CO2 and PZ (including the CO2-PZCOO− reaction) with AMP are represented by Equations (2)–(5). The CO2-PZ reaction involved a two-step reaction: zwitterion formation followed by zwitterion deprotonation by PZ, AMP, and PZCOO−, which produced PZ-carbamate, PZ-dicarbamate, and a protonated base [44]. The concentration of each base defined its contribution to the reduced concentrations of H2O and OH− during zwitterion deprotonation. For instance, if the AMP concentration is significantly higher than the PZ concentration, AMP appears to catalyse the reaction between CO2 and PZ to form carbamate, leading to AMP deprotonation. Based on Equation (6), CO2 hydration was considered, but the reaction was extremely slow and was usually neglected [45]. As represented by Equations (6)–(11), the instant and reversible proton transfer reaction was considered in the column according to mass transfer and the process being at equilibrium [8].
Based on the reaction mechanism, AMP-carbamate is not stable and would form bicarbonate upon being hydrolysed and subsequently release free amines for an extra reaction [46]. Thus, the reaction between CO2 and free AMP molecules would increase the overall CO2 loading capacity up 1 mol of CO2 per 1 mol of AMP. Besides, the stoichiometry of the PZ-CO2 reaction allows the loading of 2 mol of CO2 per 1 mol of PZ [47]. Thus, in terms of reaction rate and CO2 loading capacity, the addition of PZ is a good alternative to enhance the low reaction rate of AMP. Theoretically, a blended PZ + AMP solution has a potential to replace the industrial amine, MEA which react with CO2 to form stable carbamates and the CO2 loading is limited to 0.5 mol of CO2 reacting with 1 mol of MEA. Hence, this study aims to examine the CO2 removal performance using a blended PZ + AMP solution in a packed column at CO2 partial pressures ranging from 20 to 110 kPa.”