https://doi.org/10.1002/ghg.2059
“The chemical CO2 absorption capacity (CACchem; mol CO2·L−1 solution) and amine utilization (mol CO2·mol−1 amine) of the low- (0.1–0.5 M) and high- (15.0–30.0 wt%) concentration MDEA and TEA solutions are shown in Fig. 3a and b, respectively. CACchem indicates the amount of CO2 chemically absorbed according to Eqn (3), and it is calculated by subtracting the physical absorption capacity (CACphy) from CACtotal experimentally measured by the gas analyzer. The CACphy of each amine solution was calculated based on the water concentration and the total amount of CO2 absorbed of pure water, which were reported in our previous works.36
In Fig. 3a, CACchem of the MDEA system was larger than that of the TEA system for all the same mass concentration solutions. The increasing ratio of CACchem to the concentration (slope at Fig. 3a) was different at low- (0.1–0.5 M) and high- (15.0–30.0 wt%) concentration ranges, as summarized in Table 2.”
| Increasing ratio of CACchem (mmol CO2·L−1·g−1 amine) | ||
|---|---|---|
| System | Low-concentration range
0.1–0.5 M |
High-concentration range
15.0–30.0 wt%; 1.3–2.6 M MDEA 1.0–2.1 M TEA |
| MDEA | 8.71 | 5.15 |
| TEA | 5.83 | 3.20 |
”
The increasing ratio of CACchem in low- and high-concentration ranges of the MDEA system was 49 and 61% larger than that of the TEA system, respectively. This phenomenon is primarily ascribed to the different molecular weights of the two amines: with the same mass concentration of the two amines, the molar concentration of MDEA is larger than that of TEA because the molecular weight of MDEA is relatively low. Therefore, theoretical CACchem and the increasing ratio of CACchem according to mass concentration were larger in the MDEA system. The initial molar concentrations of the MDEA and TEA solutions (Cini) corresponding to their mass concentrations (X axis) are shown at the bottom of the X axis as X1 and X2 axis, respectively, in Fig. 3a. Therefore, CACchem and the increasing ratio of CACchem in the MDEA system should theoretically be 1.26 times larger than those of the same mass concentration solution points in the TEA system if Eqn (3) occurred completely. However, in the low- and high-concentration ranges, the real increasing ratio of CACchem in the MDEA system was 1.49 and 1.61 times larger than that in the TEA system, respectively, because the ratio of effective TEA molecules participating in absorption is further decreased than in the MDEA system. This point is detailed in Fig. 3b shown below.
Although the increasing ratio of CACchem (slope) of the two systems should be theoretically constant in the low- and high-concentration ranges because the absorption is carried out by Eqn (3) in the two systems, the real increasing ratio of CACchem in the low-concentration range was larger than that in the high-concentration range. This phenomenon is primarily because as amine concentration of the solution increases the interaction between the amine molecules strengthens and the water concentration decreased in the high-concentration solution. At this condition, that is, the distance between the amine molecules is further decreased, which increases the interaction between the molecules, which substantially decreased the amine molecules to participate in the absorption reaction. In addition, the decreased water concentration, which is one of the reactants, might reduce the extent of reaction, thereby decreasing the increasing ratio of CACchem at the high-concentration range. Cini point, which divides such low- and high-concentration ranges, could be obtained by crossing the two different regression lines of the increasing ratio of CACchem according to Cini in the two systems. The points of the MDEA and TEA systems were determined to be 18.1 and 13.5 wt%, respectively, as shown in Fig. 3a.
To quantify the ratio of effective amines participating in the reaction in both systems, the molar concentration of effective amines, which was stoichiometrically calculated using Eqn (3) with CACchem obtained in each solution, was divided by Cini. This factor was called the amine utilization (mol CO2·mol−1 amine) and is shown in Fig. 3b for all solutions in the two systems. The amine utilization does not theoretically exceed 1.0, based on Eqn (3). However, CO2 might be trapped in the free space present between amine and water molecules in a low-concentration tertiary amine solution because the amine molecule is larger, and its structure is more complex compared to primary and secondary amines. Therefore, amine utilization can be slightly over 1.0 in the low-concentration MDEA and TEA systems because free spaces can be filled with a small amount of additionally absorbed CO2.
In Fig. 3b, the MDEA utilization was higher than that of TEA in all solutions. However, the ratio of MDEA to TEA utilization remained almost constant-about 1.17 times higher in the low-concentration range below 0.5 M. This phenomenon is ascribed to the fact that amine utilization in the low-concentration solution is further dependent on the water concentration than that of amine in both systems. Meanwhile, although the TEA utilization over 0.5 M solutions considerably decreased lower than 1.0, the MDEA utilization was maintained higher than 1.0 even in the high-concentration solution of 1.90 M (22.5 wt%). This might be because the MDEA system has more spaces to trap CO2 between amine and water molecules in the high-concentration solutions because the MDEA molecule, which has two C2H4OH and one CH3 moieties, is more asymmetric and irregular in terms of molecular structure than TEA composed of three C2H4OH moieties; which might basically lead to the small different steric hindrance of MDEA and TEA systems.”
“Figure 3(a) Chemical CO2 absorption capacity and (b) amine utilization according to initial molar concentration of MDEA and TEA solutions.”