https://doi.org/10.1016/j.cesx.2019.100032
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The pseudo-first order kinetic constant (kobs) for the blended amines was calculated as given in Eq. (12) using experimental data for viscosity, density and Henry’s law constant. Interpolated values were used at temperatures different from those measured, and the data used for the calculation are summarised in Appendix B. Fig. 11 presents the kobs as a function of temperature for the blended amines studied in this work along with the DEEA/MAPA solutions reported by Monteiro et al. (2015), 1 M MAPA reported by Monteiro et al. (2014) and 30 wt% MEA reported by Bernhardsen et al. (2019a). The figure shows that the kobs depend on the temperature, the molecular structure of the tertiary amine and the solution concentration. The kobs increased with increasing temperature, and for the blended amines with equal molarity and viscosity, the order of increasing kobs was DEEA/MAPA < 1-(2HE)PRLD/MAPA < 3DMA1P/MAPA at temperatures higher than 30 °C. This, difference was obtained despite the small difference detected in the pH value of the unloaded solutions. The pH of DEEA, 3DMA1P and 1-(2HE)PRLD blended with MAPA at 25 °C was measured to be 12.95, 12.96 and 12.81, respectively (repeatability ± 0.1 pH). Nevertheless, although the DEEA/MAPA solutions obtained lower kobs values, the slower kinetic was compensated by the higher CO2 solubility (smaller Henry’s law constant) which resulted into comparable Kov values as discussed above (Fig. 9).
Fig. 11. Pseudo-first order kinetic constant (kobs) as a function of temperature. *Monteiro et al. (2015); **Monteiro et al. (2014);***Bernhardsen et al. (2019a).
Further, when looking at the different 3DMA1P/MAPA solutions presented in Fig. 11, it can be seen that the kobs values increase with increasing concentration of 3DMA1P until a temperature of 47 °C. At higher temperatures, the 3 M 3DMA1P + 1 M MAPA solution shows higher kobs values than the most concentrated solution. However, as above, the advantage of higher kobs values is small when one considers the comparable Kov values. The comparable Kov values, when increasing the amine concentration, was due to the combination of increased viscosity (decreased CO2 diffusivity), increased physical solubility and increased reaction kinetics. Thus, the results illustrate that kobs alone cannot be used to identify fast CO2 absorbents as the absorption rate also depends on the diffusivity and physical solubility. This is also clear when looking at the kobs and Kov values for 1 M MAPA and 30 wt% MEA. The values for kobs were lower than the solvents studied in this work while the Kov was comparable mainly due to their low solvents viscosity (high CO2 diffusivity) (Amundsen et al., 2009, Monteiro et al., 2014).
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