https://doi.org/10.1016/j.cesx.2021.100096
“The addition of both NaCl and KI reduce the oxidative degradability of aqueous 30 %wt. MEA significantly, as shown in Fig. 3. Of the two, KI is the strongest inhibitor of oxidative degradation. Whereas 30 %wt. MEA (aq.) experiences a loss of 40 ± 4% alkalinity after three weeks under the conditions applied in this work (assumed to directly correlate to the concentration of MEA), the addition of NaCl to the solvent results in a loss of alkalinity of only 24 ± 5%, and the addition of KI reduces this loss to as little as 4 ± 1%. This shows that the increase of salinity of the solvent has a positive effect in terms of degradability, making MEA less degradable under oxidative conditions. As no significant difference in stability was seen neither between the 2.0 %wt. and 7.5 %wt. NaCl SAS , nor between the 1.0 and 2.0 %wt. KI SAS, it can be inferred that, if the salt employed possesses certain properties, its concentration is of less importance. Moreover, a SAS with 15 %wt. NaCl was tested in the same setup. Although oxidative stability was in the same range as the two other NaCl SAS, the reproducibility of the parallels was low, and that data is not reported in this study. An important factor behind these effects might be that we are approaching a saturation limit of the NaCl in solution, especially if water is lost or degradation compounds are formed, thus influencing the solubility of the salt. A reduction in KI concentration to 0.2 %wt. gave much more oxidative degradation than the higher concentrations of KI as can be seen in Fig. 4.”
“Fig. 3. Amine conservation during the oxidative degradation experiments in Setup 1 with 30 wt%. MEA (aq.), with and without salt addition. Absolute amine concentrations back-calculated to CO2-free solution, measured and corrected by titration and TIC analysis. Error bars represent standard error of the 2 or 3 parallel experiments.”
“Fig. 4. Oxidative degradation of 30 %wt. MEA (aq.) in Setup 1 with different concentrations of KI added. Absolute amine concentrations back-calculated to CO2-free solution, measured and corrected by titration and TIC analysis. Error bars represent standard error of the 2 or 3 parallel experiments.”
“As Fig. 4 shows, the degradation inhibition is equally strong with 1.0 %wt. KI as it is with 2.0 %wt. Upon decreasing the concentration to 0.2 %wt. KI, there is a slight degradation inhibition observed, similar to that of the NaCl SAS shown in Fig. 3. It should be noted that also for the 0.2 %wt. KI SAS there is no plateau in the degradation curve and the rate of degradation is steady throughout the experiment. To investigate whether the inhibition effect would wear off with the 1.0 %wt. KI case, if the KI is indeed consumed by the reaction, the 1.0 %wt. KI experiment was run twice as long as the other experiments. After six weeks, no significant amine loss was observed.
In the case of all the KI SAS, it was observed that the typical yellow/orange coloration, characteristic of iron-containing solutions faded away in the course of the three weeks of experiment, while a red precipitate accumulated on the reactor walls. This effect was less visible in the 0.2 %wt. KI SAS than in 2.0 and 1.0 %wt., but still evident. We therefore hypothesize that KI reduces the solubility of iron oxide in 30 %wt. MEA (aq.), promoting its precipitation. For this reason, 2.0 %wt. KI SAS was also studied in another oxidative degradation setup and compared to literature data for pure 30 %wt. MEA (aq., without iron) from Vevelstad et al (2016). The results and comparison are given in Fig. 5. As the literature shows significant degradation of MEA (~25% loss after three weeks) even in the absence of iron, the fact that no significant amine loss was seen in the 2.0 %wt. KI SAS confirms that salting out of metal from the solution is not the only effect causing the solvent to be more stable under oxidative conditions.”
“Fig. 5. Validation of oxidative degradation of 2.0 %wt. KI SAS with iron against 30 %wt. MEA (aq.) without iron from Vevelstad et al. (2016) in Setup 2.”
Additionally, anion exchange chromatography showed no loss of iodide through the duration of the 21 days of oxidative degradation experiments in the 2 %wt KI case, which indicates that iodide is not being consumed while it inhibits the degradation reactions. The results of this analysis can be viewed in the Appendix.
Since both oxidative degradation setups used in these experiments use continuous agitation of the liquid, both by magnetic stirring (~200 rpm) and bubbling the gas into the solution, mass transfer from gas to liquid phase should not be the limiting factor for whether degradation takes place or not. Setup 2 has the advantage of recycling the gas phase and thereby enhancing the total gas flow into the liquid phase, making mass transfer of oxygen as high as possible throughout the experiment.
We would like to address the hypothesis that the salts might shield the amine from being degraded by being themselves oxidized instead, thus being “sacrificed” for the amine. If the salts are being consumed during the oxidation process, this will decrease their value as inhibitors because of the subsequent need for replenishing. This hypothesis is particularly interesting in elucidating the good performance of potassium iodide, which is known for being readily oxidized (Altshuller et al., 1959).”