Investigative studies on the stability of an amine blend in the presence of exhaust gas dust (metal oxide) impurities during an amine-based CO2 capture process

Abstract

This research work investigates the degradation kinetics of MEA/DMAE bi-blend solvent, with a focus on the influence of temperature, oxygen concentration, and type and amount of exhaust gas metal oxides. Utilizing a combination of experimental approaches and kinetic modeling, this study was used to provide a comprehensive analysis of the factors that affect MEA/DMAE stability and degradation rates. This research commenced first, by determining the solubility of various dominant iron and steel flue gas metal oxides, namely, Fe2O3, ZnO, MnO, and Al2O3. The oxides were dissolved in a 200 ml, 5M, and 0.30 mol CO2/mol bi-blend of MEA/DMAE solvent. In decreasing order, the solubility results for ZnO, Fe2O3, MnO, and Al2O3 were 387.51 ppm, 15.96 ppm, 4.57 ppm, and 3.43 ppm, respectively. By flowing oxygen at different concentrations (balance nitrogen) through a 200 ml volume filtrate of the generated metal oxide dissolved amine solvent in a three-necked flask exposed to different absorber temperatures, a continuous 21-day lab-scale degradation experiment was carried out. Fe2O3 had the greatest influence on the degradation of a CO2-loaded bi-blend of MEA/DMAE among the metal oxides taken into consideration, followed by ZnO all based on amine degradation results in mmol/hr, accumulated amount of ammonia emissions in ppmV, and ammonia emissions rate in ppmV/hr. Based solely on which one had the most degrading effect, Fe2O3 in the concentrations of 15.96 ppm, 11.97 ppm, and 7.98 ppm was selected and used against varying temperatures (in the range of 40 oC, 50 oC, and 60 oC) and oxygen concentrations (ranging from 6%, 12%, and 18%) to explore their effects on degradation rates and ammonia emission rates. A kinetic model was developed for the DMAE degradation rate and MEA degradation rate with activation energies of 50,495.13 J/mol, and 60,310.9 J/mol respectively. The order of reactions obtained from the kinetic analysis was 1.22 and 0.98 for DMAE and MEA respectively. The results showed that DMAE degraded at a faster rate than MEA. The studies also showed an increasing trend in the rate of MEA/DMAE degradation and the rate of ammonia emissions with increasing oxygen concentration and operating temperature. A high activation energy for MEA implies that more energy (temperature) was needed to degrade MEA relative to DMAE, which had a lower activation energy. A lower order of reaction for oxygen for MEA, also implies that the impact of oxygen on MEA degradation is less than its impact on DMAE degradation. Fe2O3 therefore has a higher catalytic effect on MEA/DMAE degradation implying that upon contact with the amine solvent, the amine has a high tendency to degrade at a faster rate, causing an increase in solvent losses and an increase in the cost of solvent replacement. Further implications include corrosion, clogging, and the degraded amine's fouling of columns and piping.