https://doi.org/10.1021/acs.iecr.2c02344
“MEA is the most studied amine in terms of degradation, where a large number of degradation compounds are identified, in some cases quantified, and pathways of formation have been suggested. A general overview of amines investigated combined with degradation compounds identified for different amines and formation pathways has been summarized by Gouedard et al. (126) Morken et al. (127) published a comprehensive data set related to MEA degradation on a pilot scale. Lepaumier et al. (12,14,15) suggested degradation compounds and pathways for a set of alkanolamines, ethylenediamine, and some other polyamines both thermal and oxidative degradation, as did Gouedard, (65,110) particularly for MEA. Vevelstad et al. (16,17,54) suggested pathways and degradation compounds for oxidative degradation of a set of alkanolamines, Eide-Haugmo (62) suggested schemes for the thermal degradation of over 40 amines. Davis and Rochelle, (28) Voice et al., (24) Freeman et al. (22) (Pz), and Wang et al. (19−21) also add valuable contributions to amine degradation mechanisms. MEA is the most studied amine in terms of degradation, where a large number of degradation compounds are identified and, in some cases, quantified, and pathways of formation have been suggested. A general overview of amines investigated combined with degradation compounds identified for different amines and formation pathways has been summarized by Gouedard et al. (126) Morken et al. (127) published a comprehensive data set related to MEA degradation on a pilot scale. Lepaumier et al. (12,14,15) suggested degradation compounds and pathways for a set of alkanolamines, ethylenediamine, and some other polyamines, both thermal and oxidative degradation. Gouedard et al. (65,110) suggested pathways and degradation compounds, particularly for MEA. Vevelstad et al. (16,17) studied oxidative degradation of several alkanolamines, while Eide-Haugmo (62) focused on the thermal degradation of over 40 amines. Davis and Rochelle, (28) Voice et al., (24) Freeman et al., (22) and Wang et al. (19−21) also add valuable contributions to amine degradation mechanisms.
For oxidative degradation products, more than one pathway is needed to explain the identified compounds. Thus, MEA oxidative degradation is divided into different pathways for different degradation compounds. The compounds described here are also present in pilot samples. In most cases, several pathways have been suggested for the same compound, and several suggested degradation compounds are still not identified in pilot samples. It is not expected that any of them are major degradation compounds since, in several cases, the nitrogen balance has been closed, meaning that nearly all the nitrogen contained in the initial MEA solution is recovered in MEA and its known degradation compounds after degradation. The initial reactions in the absorber are believed to be radical reactions which will give oxidative fragmentation of the amine either by electron or hydrogen abstraction. Both mechanisms occur for various amines depending on their molecular structure. It has been suggested that the electron abstraction mechanisms dominate for tertiary amines, while hydrogen abstraction mechanisms are more important going from secondary to primary amines.
(128) Both mechanisms give similar degradation products, e.g., aldehydes (formaldehyde, acetaldehyde), carboxylic acids (formic, acetic, oxalic, glycolic, glyoxylic acid), ammonia, alkylamine (methylamine, dimethylamine etc.), and variations of the same intermediates, e.g., imine (either as iminium or as imine radical) either resulting in splitting between C–C or C–N. A thorough overview of several of these mechanisms/pathways is given in the review by Gouedard et al.
(126) Typically, one sees a buildup of formic, oxalic, and, in some cases, acetic acid in addition to ammonia. In some cases, alkylamines, depending on the molecular structure of the solvent amine, are formed.
A summary of some of the suggested reaction schemes to form a range of the most significant degradation compounds of MEA is given in
Scheme 2 and
Scheme 3. For MEA, 4-(2-hydroxyethyl)-2-piperazinone (4HEPO/HEPO) and
N-(2-hydroxyethyl)-glycine (HEGly) are major degradation compounds in pilots plants (SINTEF Tiller CO
2 plant, Esbjerg, TCM) as well in cyclic degradation equipment.
(46,104,127) These degradation compounds contribute heavily to the solvent sample nitrogen balance. There are no standard methods for analyzing solvent samples and no regulations regarding which components need to be quantified in the solvent samples from pilot plants. Different components are often reported from different campaigns due to availability and limited analytical capabilities, and different components being reported does not necessarily mean that these components are not formed. The result is large variations in degradation compounds reported in various experiments and pilots.”
“Scheme 2. Suggested Pathways for the Formation of HEGly, HEHEAA, 1HEPO, and 4HEPO
(15,17,35,46,65)“
“Scheme 3. Suggested Pathway of Formation for the Amides HEF, HEA, 2-Hydroxy-
N-(2-hydroxyethyl)-acetamide (HHEA), HEO/HEOX,
N1,
N2-Bis(2-hydroxyethyl)-ethanediamide (BHEOX), and Oxazolines, 2-oxazoline and 2-methyloxazoline.
(55,65,109,110) The Amides Could Also Be in Their Salt Form, as Described by Ref
(117).”
”
In
Scheme 2, it is clear that some of the degradation components both are products and intermediates. This is the case for HEGly,
N-(2-hydroxyethyl)-2-[(2-hydroxyethyl)amino]-acetamide (HEHEAA), 1-(2-hydroxyethyl)-2-piperazinone (1HEPO), and 4HEPO/HEPO. Thus, isolating their behavior will be hard. HEEDA and OZD are suggested intermediates in the formation of respectively 4HEPO and 1HEPO or only 4HEPO. The formation of 1HEPO and 4HEPO also requires first-generation degradation products like aldehydes or their corresponding acids, e.g., glyoxal, glyoxylic acid, and glycolic acids. Currently, for the mentioned carboxylic acids, only quantitative data for glycolic acid has been reported from MEA samples.
(15,26,127) Glyoxal forms oligomers in water and has only been indirectly identified as an expected intermediate product and as part of the oxidation pathway.
(26,54,65) We have quantified small amounts of glyoxylic acid in pilot samples. However, formic, acetic, and glycolic acids are major acidic components in these samples (glyoxylic acid less than 1% of formic acid concentration). HEGly and 4HEPO are found in the largest amounts in SINTEF solvent degradation rig as well as other pilot samples. In the few cases where 1HEPO has been quantified, it is found in small amounts.
(31) The concentrations of 1HEPO, OZD, HEEDA, and HEHEAA are typically less than 1 mmol/L.
Degradation compounds in
Scheme 3 have all been identified and, in most cases, quantified in MEA samples in samples from SINTEF solvent degradation rig and various pilots. 2-[(2-Hydroxyethyl)amino]-2-oxoacetic acid (HEO/HEOX) is only qualitatively identified.
(54,109,129) Among the products in
Scheme 3,
N-(2-hydroxyethyl)-formamide (HEF),
N-(2-hydroxyethyl)-acetamide (HEA), and 2-oxazoline are the most prominent components.
Carbamate polymerization reactions are described thoroughly in the dissertations of Davis and Eide-Haugmo,
(62,64) and an overview is given in
Scheme 4. The reaction scheme shows several suggested routes to the same products. The pathways in black have been more accepted in the past decade, while those in gray are less commonly accepted. For the carbamate polymerization reactions, the initial step is the formation of OZD from MEA and CO
2 (a cyclization reaction). Then a ring opening occurs, resulting in a diamine (AEEA/HEEDA) or a urea compound (
N,
N′-bis(2-hydroxyethyl)-urea(MEA-urea/BHEU)). The diamine can either react with OZD and form triamine or undergo another cyclization reaction to imidazolidinone (1-(2-hydroxyethyl)-2-imidazolidinone (HEIA)). Also, the triamine could form several imidazolidinones (1-[2-[(2-hydroxyethyl)amino]ethyl]- 2-imidazolidinone (TriHEIA) or 1-(2-aminoethyl)-3-(2-hydroxyethyl)-2-imidazolidinone (AEHEIA)). Quantitative data (based on commercial standards) for OZD, AEEA/HEEDA, and HEIA are available from thermal degradation experiments, e.g.,
(28,46,55) SINTEF solvent degradation rig,
(104) and from the MEA campaign at Technology Centre Mongstad (TCM).
(127) Additionally, MEA-urea data is available from thermal degradation experiments and from SINTEF’s solvent degradation rig.
(28,104) MEA-trimer and TriHEIA were identified by Davis,
(64) and AEHEIA was quantified using a compound of similar structure da Lepaumier et al. and Silva et al.
(46,55) In thermal degradation experiments, HEIA is often a significant component. It is a far less significant degradation component in cycled degradation setups and pilot plants. In the SINTEF solvent degradation rig, however, MEA-urea was one of the major degradation compounds. Additionally, HEEDA was present at a relatively high concentration in this setup compared to samples from TCM.”
“aThe black pathways are the most accepted mechanism the last ten years, while those shown in grey are less commonly accepted.
OZD is another important intermediate for both the carbamate polymerization route and the formation of 4HEPO, and an overview of the formation and consumption pathways for OZD is shown in
Scheme 5. Currently, quantitative data for 4HEPO and HEEDA are available, while the rest are suggested components.”
“Scheme 5. Some of the Suggested Pathways for the Formation of OZD and OZD Reaction Pathways to Form Other Compounds (28,54,55,65)a
aSome of these compounds are not positively identified in pilot samples, such as piperazinone, epoxide, and substituted OZD component.”
