Degradation of amines during gas treating process is a complex phenomenon that leads to a number of different organo-chemical products that may not yet have been fully characterized and quantified. Although, the degradation process was resulted from combined effect of oxidative catalyzed by CO₂ and thermal degradation, thermal degradation in the absence of oxidative agents was suspected to have little influence especially at temperature below 120°C. Moreover, this research examines the impact of thermal degradations on the performance of the CO₂ absorption by benchmarking the performance of the CO₂ absorption at a temperature and pressure chamber with various amines such as MEA, MDEA and some amine blends. Since, the overall objective is to understand the environmental impact of amino solvents when implemented for post combustion CO₂ capture at large scale from low pressure flue gas stream, the emphasis in this report has been given to those amino solvents that are being used at present either commercially or at the technology demonstration scale for such an application.

Four different amines or amine blends have been used for the experiment. MEA was purchased from Sigma-Aldrich (Singapore) with purity of > 99% while MDEA was purchased from Dow Chemical with purity of > 99%. In order to obtain similar industrial application, two hybrid amine blends have also been employed, ie. Blend-1 (Piperazine (5%-wt)/MDEA (95%-wt)) and Blend-2 (Piperazine (5%-wt)/Sulfolane (25%-wt)/MDEA (45%-wt)/Water (25%-wt)). In common gas processing application, Blend-1 is frequently mentioned as activated MDEA while Blend-2 is applied for absorbing gas that contains impurities, such as Benzene-Toluene-Xylene (BTX) and mercaptanes. For pure MEA and MDEA, the CO₂ absorption was conducted at two different concentrations, those of 20%-wt mixed with water and pure (100%-wt) amine solvent.

Following the decreasing rate of CO₂ absorption or activity, rate of amine lost could be calculated. Order of amine strength from least to most degradation follows Blend-1 < MDEA < MEA. In Blend-1, the presence of piperazine (PZ) showed to improve the stability of amines. This result is in accordance to the result of Closmann that found the similar experimental result. PZ is very stable and starts to degrade at very high temperature > 150^oC. MDEA, classified as a ternary amine, tend to be more stable than MEA. The rate of MEA degradation was estimated to be 3.9 × 10^-1 mmol/hr while MDEA showed to be smaller at 3.2 × 10^-1 mmol/hr. The slowest degradation rate was found for MDEA/PZ with rate of 3 × 10^-1 mmol/hr. Compared to degradation due to the presence of oxidative agent, this value is much lower. This could support the finding of Chakma & Meisen that thermal degradation in the absence of oxidative agent is negligible.

As shown in Figure 1, the degradation of MDEA occurred in two stages through the formation of (2) complex of N-methylethanolamine (C₃H₉NO) and vinyl alcohol (CH₂=CH-OH) and (2) complex of N-methylethanolamine (C₃H₉NO) and acetaldehyde (CH₄-C=O). However, to proceed the formation of first complex (C₃H₉NO + CH₂=CH-OH), the Gibbs barrier (ΔG^‡) that has to overcome was ca. 60.7 kcal/mol (or 59.6 kcal/mol at 120^oC). This will result a calculated degradation rate of 5.7 × 10^-21 s^-1 that considers to be very small. The experimental data supported the simulation result that the degradation of MDEA at 120^oC in the absence of oxidative compounds was only 3.2 × 10^-1 mmol/h. We have explored the possibility of presence of H atom (both in radical/ionic form) to lower the barrier of the reaction energy. However, it shows that H atom attack to the N-C bond has more or less similar energy barrier (ΔG^‡ = 55 kcal/mol) compared to the H attack from ethanol group of MDEA (ΔG^‡ = 59.6 kcal/mol). In general, the reaction was initiated bond breaking between methanol group and nitrogen atom. This reaction required a lot energy to surpass the energy barrier. In the presence of oxidative compounds, the barrier energy of the bond breaking could reduce significantly. Later on, H atom transfer from methanol group into nitrogen to form N-H bond occurred and simultaneously form double bond of C to C in ethanol group producing vinyl alcohol as the product. H rearrangement will convert (tautomerize) vinyl alcohol into acetaldehyde as acetaldehyde. The tautomerization reaction was preferable since the product (N-methylethanolamine + acetaldehyde) has lower energy (-12.9 kcal/mol of ΔG) compared to N-methylethanolamine + vinyl alcohol. This reaction was confirmed by GC-MS spectra that detected the presence of acetaldehyde at residence time of 6.2 minutes although the peak shows very small.

Figure 1. Energy profile of MDEA degraded products.

Then, the reaction will be finalized by substituting a methyl group that bond to nitrogen atom with H atom from acetaldehyde to form acetone. Unfortunately, MEA as the couple product acetone could not be separated with MDEA peak as it has the same retention time but the presence of acetone could be enough to confirm the mechanism. The energy profile of MDEA degradation into acetone is shown in Figure 1.