https://doi.org/10.1021/acs.energyfuels.0c00880
“To better understand the role of N-methyl-2-pyrrolidone in depressing the CO2 absorption capacity of solvents based on diisopropylamine and DEEA, we have assessed the VLE and heat of absorption for water-lean mixtures with different water–NMP proportions plus 10 wt % of each of the amines. This procedure allows for a smooth evaluation of the shift from aqueous to nonaqueous amine. Solvents with 60 wt % water to 30 wt % NMP and 30 wt % water to 60 wt % NMP (or 2:1 and 1:2 water-to-NMP mass ratios) were investigated at 40 °C, and the resulting data can be seen in the Supporting Information and in Figure 10.”
“Figure 10. Vapor–liquid equilibrium data for solvents containing different proportions of water and N-methyl-2-pyrrolidone plus 10 wt % amine at 40 °C.”
“In Figure 10, one can see that the shift from aqueous to nonaqueous DEEA 10 wt % is indeed gradual and monotonical. The highest CO2 solubility is observed for the aqueous solvent (pink △). Then, as more water is exchanged by NMP, the CO2 solubility steadily decreases while the VLE curve experiences what in Figure 10 can be translated as a “shift to the left”. In this case, the solvent with 2:1 water–NMP mass ratio (purple △) shows a VLE very similar to the aqueous amine, while the solvent with 1:2 water–NMP mass ratio (dark blue △) shows a huge decrease in capacity. This time, and differently from our previous studies, (19,32) nothing indicates that the VLE curves are somewhat parallel—indeed, they visibly are not. The culmination of this shift to the left process is that, in nonaqueous DEEA in NMP, the behavior of the solvent is similar to that of a physical absorbent.”