Stability of amines for CO2 capture

Abstract

Despite of being a well-tested and highly attractive technology for capture of carbon dioxide (CO2), amine scrubbing encounters economic and operational challenges originating from degradation of the amine solvent, in particular oxidative degradation. By finding means of tackling of solvent degradation, associated problems like corrosion, emissions, and environmental concerns can also be significantly reduced. Degradation mechanisms have been studied for more than two decades and have yet to be fully understood within the whole process. This, mainly experimental work aims to contribute to further understanding of amine degradation in the CO2 capture process and how it can be avoided. Measurement of oxygen solubility is a central part of the study, as oxygen plays a vital role in the degradation reactions. It was found that all studied amines seem to have comparable oxygen solubility to water in the absence of CO2. The parameters that influence oxygen solubility the most are the presence of CO2, temperature, and mass transfer limitations due to rapidly occurring degradation reactions in unstable amines like ethanolamine (MEA). Experiments where aqueous amine solvents were subjected to sparging with 98% oxygen gas at 60 °C, with addition of ferrous (Fe2+) and under constant stirring were used to assess oxidative amine degradation. The oxidative stability of 18 different amines was studied, and seen in context of thermal, and biological stability. Low thermal, and oxidative degradability are highly desirable properties, while a low biodegradability is undesirable. It was found that, despite of an overarching trend of correlation, there are amine solvents that have high biodegradability but also low degradability under oxidative conditions. No correlation between literature data for thermal degradation and oxidative stability in this work was found. The oxidative degradation experiments show the correlations between structural features of the amine and stability, where MEA is the least stable of alle those studied. Tertiary and sterically hindered amines are the most stable under oxidative conditions, and only naturally occurring amines were found to be readily biodegradable. The most thoroughly studied amine in this work was MEA, because of its relatively fast degradation rate compared to most other amines. It was found that CO2-free MEA hardly degrades under oxidative conditions, that Fe2+ and Cu2+ have similar catalytic abilities on oxidative MEA degradation, and potassium iodide (KI) was identified as an inhibitor for MEA degradation under laboratory-scale oxidative degradation experiments.