Inhibitors of Monoethanolamine Oxidation in CO₂ Capture Processes

Aqueous monoethanolamine (MEA) is a good solvent for postcombustion CO₂ capture; however, it is prone to oxidative degradation. The initial test conditions mimicked those found in the absorber of a commercial system: MEA was degraded in the presence of CO₂, with high oxygen mass transfer, and with dissolved metals, at absorber temperatures. In later experiments, high temperature cycling was incorporated into the apparatus to better represent a real system. These cycling results suggest that the unadditized 7 <i>m</i> MEA in a real system contacted with 5% oxygen in the absorber and with the stripper operated at 120 °C would experience close to 5% amine loss per week. Inhibitors such as inhibitor A (Inh A), 2,5-dimercapto-1,3,4-thiadiazole (DMcT), diethylenetriamine pentaacetic acid (DTPA), hydroxyethylidenediphosphonic acid (HEDP), and methyldiethanolamine (MDEA) reduced MEA oxidation at low temperature by more than 90%. Results of the screening study also confirmed the reliability of ammonia production as an accurate indicator of MEA oxidation under all test conditions, allowing for rapid and accurate screening of new additives. Unfortunately, none of the additives screened at low temperature significantly reduced oxidation with high temperature cycling; thus they are not recommended at this point for use in a commercial CO₂ capture system