Thermal degradation of aqueous amines used for carbon dioxide capture
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Aqueous amine solutions loaded with CO2 were degraded in stainless steel sealed containers in forced convection ovens. Amine loss and degradation products were measured as a function of time by cation chromatography (IC), HPLC, and IC/mass spectrometry. A full kinetic model was developed for 15-40 wt% MEA (monoethanolamine) with 0.2 – 0.5 mol CO2/mol MEA at 100°C to 150°C. Experiments using amines blended with MEA demonstrate that oxazolidone formation is the rate-limiting step in the carbamate polymerization pathway. With 30 wt% MEA at 0.4 mol CO2/mol MEA and 120°C for 16 weeks there is a 29% loss of MEA with 13% as hydroxyethylimidazolidone (HEIA), 9% as hydroxyethylethylenediamine (HEEDA), 4% as the cyclic urea of the MEA trimer, 1-[2-[(2-hydroxyethyl)amino]ethyl]-2-imidazolidone, 3% as the MEA trimer, 1-(2-hydroxyethyl)diethylenetriamine, and less than 1% as larger polymeric products. In the isothermal experiments, thermal degradation was slightly more than first order with amine concentration and first order with CO2 concentration with an activation energy of 33 kcal/mol. In a modeled isobaric system, the amount of thermal degradation increased with stripper pressure, but decreased with an increase in amine concentration and CO2 concentration due to a reduction in reboiler temperature from the changing partial pressure of CO2. Three-fourths of thermal degradation in the stripper occurred in the reboiler due to the elevated temperature and long residence time which offset the decrease in CO2 concentration compared to the packing. The amount of degradation for other amines tested starting with the least degraded include; cyclic amines with no side chains < long chain alkanolamines < alkanolamines with steric hindrance < tertiary amines < MEA < straight chain di- and triamines. Piperazine and morpholine had no measurable thermal degradation under the conditions of this experiment and were the most resistant to thermal degradation. Diethyelenetriamine and HEEDA had the largest amount of degradation with over 90% loss at 135°C for 8 weeks.