Screening of Inhibitors for Amine Degradation
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- Institutt for kjemi 
Hindering the degradation of amines in the CO2-capturing process is important both for economical purposes when it comes to loss of solvent and impacts on the process, and to prevent emissions of volatile degradation compounds such as ammonia, nitrosamines and formaldehyde. To prevent the absorbent from degrading, either a non-degrading absorbent can be developed or a degradation inhibitor can be added to minimize the degradation. The degradation inhibitors tested in this thesis are meant to inhibit the oxidative degradation that mainly occurs in the absorber. The carbamate polymerization degradation due to CO2 and temperature has to be addressed on its own. The inhibitor screening apparatus was new, and a part of the assignment was testing this setup. The first experiment conducted on the inhibitor screening apparatus used a gas blend of 6% O2/2 % CO2 (N2 balance). This did not give enough degradation, which caused the need for rebuilding the rig. In the other experiments on the screening apparatus, a gas composition of 98 % O2/2 % CO2 was used to get sufficient amount of degradation for inhibitor screening. Inhibitor screening experiments were done using 150 mL of a 30 weight% (wt%) 2-aminoethanol (MEA) solution loaded with 0,4 mol CO2/mol MEA, at 55 °C with a gas flow of 10 mL/min. To test the stability of the inhibitors at higher temperature, thermal experiments with inhibitors were conducted. 7 mL solution was filled in stainless steel cylinders and heated at 135 °C, for a period of five weeks. The solution was 30 wt% MEA loaded with 0,5 mol CO2/mol MEA. Hydrazine was screened for inhibitory effect using a circulative closed loop apparatus because of the hazards related to this compound. The experiment was run with air, using a 30 wt% MEA solution loaded with 0,4 mol CO2/mol MEA, at 55 °C. Since experiments with both 6 % and 98 % oxygen were conducted, it was natural to compare the impact of oxygen concentration on the degradation products. Results indicated that 2-oxazolidinone (OZD) was preferred at the conditions with high oxygen, while N-(2-hydroxyethyl) glycine (HeGly) concentrations increased with decreasing oxygen content. The effect of metals on product composition was also investigated. The degradation compound N-(2-hydroxyethyl) imidazole (HEI) seems to be dependent on the metal concentrations, increasing in the presence of metals. For the inhibitors screened, the inhibition ranged from 23,59-67,81 %. Two compounds gave an increase in degradation. 1-hydroxyethane 1,1-diphosphonic acid (HEDP) was the only chelating agent stable at thermal conditions. The inhibitors did not appear to have a substantial effect on the carbamate polymerization. Quantification of degradation compounds in the samples was done using liquid chromatography-mass spectrometry (LC-MS) and anion chromatography-electrochemical detector (IC-EC). Amine loss and CO2-loading were determined using titration methods. Metal concentrations were determined using inductively coupled plasma-mass spectrometry (ICP-MS). Some analyses were done gravimetrically while others were done volumetrically. For comparison purposes, simple density measurements were done, and the data converted according to the amine loss in the sample.The initial intention was to use gas chromatography - mass spectrometry (GC-MS) to analyze the samples from the thermal experiments. The system was however not operable during the time available. ICP-MS analysis was not done in time for the last experiment. Ammonia analyses were not conducted in time for this thesis.