Amine oxidation in CO₂ capture processes
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Aqueous amine solutions were batch loaded into 500 mL glass jacketed reactors and subjected to oxidative degradation at both low and high gas rates. Solutions at low gas were degraded with 100 mL/min of 98%O2/2%CO2 with mass transfer achieved by vortexing. Samples were drawn from the reactor during the course of the experiment and analyzed for degradation using ion chromatography and HPLC with evaporative light scattering detecion. In a parallel apparatus 7.5 L/min of 15%O2/2%CO2 was sparged through 350 mL of solution; additional mass transfer was achieved by vortexing. A Fourier Transform Infrared Analyzer collected continuous gas-phase data on amine volatility and volatile degradation products. Hydroxyethyl-formamide (HEF), hydroxyethylimidazole (HEI) and formate are the major carbon containing monoethanolamine (MEA) oxidation products; HEF, HEI and ammonia are the major nitrogen containing products. In terms of catalyst oxidation potential, Cu > Cr/Ni (combined) > Fe > V. The oxygen stoichiometry (ν) ranges from 1.5 mol MEA degraded/mol O2 consumed for Cu and Fe catalyzed systems to 1.0 for V catalyzed systems. Estimation of rates from an industrial absorber show degradation costs to range from $1.17 / metric ton (MT) CO2 captured for a system controlled by the solubility of O2 to $2.22 / MT CO2 for a mass transfer controlled system. Inhibitors A and B (reaction mechanism inhibitors) and EDTA (a chelating agent) were established as effective MEA oxidation inhibitors. EDTA and Inhibitor A were successful inhibitors at 100 mM, while 7.5 mM Inhibitor B successfully inhibited degradation. Sodium sulfite and reaction intermediates formaldehyde and formate (expected oxygen scavengers) were unsuccessful at inhibiting MEA oxidation. Cu catalyzes concentrated PZ oxidation, while Fe has no effect on PZ oxidation even at high catalyst concentration. MEA/PZ blends were more susceptible to oxidation than any other amine system investigated. It is believed that free radicals formed in the MEA oxidation process serve to accelerate the degradation of the PZ structure. All MEA analogs (glycine, ethylenediamine and ethylene glycol) and secondary/hindered amines (diethanolamine, diglycolamine and 2-amino-2-methyl-1-propanol) were resistant to oxidation in the presence of Fe or Cu, except for diethanolamine.