https://doi.org/10.3390/en14206822
“When analyzing the CO2 loading behavior as a function of time (see Figure 3c), it was observed that the kinetic mechanism involved two steps: a rapid absorption step, and a slow diffusion step (Figure 5). This kinetic mechanism is similar to that reported for CO2 adsorption processes [55]. Da Silva and Svendsen [56] commented that the two-step mechanism applies to the reaction between CO2 and primary and secondary amines. Furthermore, Wai et al. [57] performed a kinetic and thermodynamic analysis of CO2 capture for combustion gases using AMP–DETA (2-amino-2-methyl-1-propanol–diethylenetriamine) mixtures, and the trend was similar to what was observed in this work.”
“Equation (7) is the mathematical expression for the kinetic mechanism proposal. The first term corresponds to the CO2 loading at equilibrium. The parameters (A1,k1,A2,k2) of the time-dependent terms were determined by the least squares regression from the experimental data (CO2 loading vs. time, Figure 3c).
“Moreover, Figure 6a shows the data on CO2 loading for other amines (MEA, DEA) taken from [46]. Similarly, there was a linear relationship between the CO2 loading and the amine concentration, as obtained for EDA in this work. Bernhardsen and Knuutila [60] reviewed potential amine solvents for the CO2 absorption process, showing the linear dependence of CO2 loading at equilibrium on the MEA concentration. MEA has a greater absorption capacity than DEA, but EDA surpasses both in the concentration range studied. EDA has two amino groups that promote affinity and reactivity towards CO2; however, this trend is not consistent with the results reported by Gomes et al. [61], where DEA and MEA achieved greater absorption capacity compared to EDA, possibly because the equilibrium conditions were not reached under their experimental setup.
| EDA Conc. (wt.%) | Kinetic Data | ||||
|---|---|---|---|---|---|
| A1 | k1×103 | A2 | k2×103 | AAD (%) | |
| 0 | 8.158 | 12.764 | 7.083 | 226.615 | 0.586 |
| 5 | 7.687 | 7.380 | 13.737 | 29.199 | 0.191 |
| 10 | 10.657 | 6.776 | 17.311 | 27.009 | 0.196 |
| 20 | 18.088 | 4.821 | 25.356 | 20.363 | 0.321 |
“Finally, the damping-film theory model was applied to investigate the apparent absorption rate performance of an aqueous EDA solution using Equation (4) at the rapid absorption step. Slopes of the constant rate (k) of aqueous solutions of EDA are shown in Figure 6c. The apparent absorption rate constant of aqueous EDA solutions was much higher than that of pure water, with a value of 0.0019 min−1. It was also observed that EDA significantly intensified the CO2 absorption performance of aqueous solutions, resulting in k values of 0.0040 min−1, 0.0049 min−1, and 0.0060 min−1 for EDA concentrations of 5, 10, and 20 wt.%, respectively. Note that in the initial minutes (<10 min), the CO2 absorption rate was higher for 10 wt.% EDA solutions, consistent with the initial CO2 capture rate values (see dn/dt data in Table 2). The addition of EDA accelerated the absorption performance of the CO2-trapping chemical solvent within the investigated timeframe. The increased absorption performance in aqueous EDA solutions was due to the chemical absorption of CO2 into aqueous solutions of diamine at high pressure taking less time to absorb more CO2 molecules than pure water.”