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Available online at www.sciencedirect.com Energy Procedia Energy Procedia 4 (2011) 591–598 Energy Procedia 00 (2010) 000–000 www.elsevier.com/locate/procedia www.elsevier.com/locate/XXX
GHGT-10 Mechanism of Formation of Heat Stable Salts (HSSs) and their Roles in Further Degradation of Monoethanolamine during CO2 Capture from Flue Gas Streams Teeradet Supapa, Raphael Idem,1a,*, Paitoon Tontiwachwuthikula
a International Test Center for CO2 Capture, 3737 Wascana Parkway, Regina, Saskatchewan, Canada, S4S 0A2 Elsevier use only: Received date here; revised date here; accepted date here
Abstract
The roles of HSS induced acid products of MEA degradation were evaluated. The results show that formic and acetic acids, formed as a result of MEA oxidation, exist in 2 forms in equilibrium; namely, salts and amides. Specifically, these are formate and acetate, and N-(2-hydroxyethyl)formamide and N-(2-hydroxyethyl)acetamide, respectively. Glycolic acid, also formed due to MEA oxidation, is stable and exists mostly bonded with MEA to produce the glycolate HSS. Its amide is unstable, hydrolyzing back to glycolate and MEA. Oxalic acid was found as a reactive intermediate that mostly decomposed to formic acid, which in turn produced a stable N-(2- hydroxyethyl)formamide. Oxalic acid amide (N-(2-hydroxyethyl)oxamide) could also be formed but its formation was considered a minor route compared with the decomposition route. Succinic acid formed a stable imide (N-(2- hydroxyethyl)succinimide) through an intermediate amide (N-(2-hydroxyethyl)succinamide).
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1. Introduction
Oxygen (O2) induced degradation of amines used in carbon dioxide (CO2) capture process is a well known source of heat-stable salts (HSS). HSSs contain no value in absorbing CO2, and are also difficult to regenerate under normal regeneration conditions used in CO2 absorption unit. In case of high concentration build-up, HSSs also cause damage to plant construction materials due to their corrosive nature [1, 2]. One of the first oxidative pathways of amine degradation using monoethanolamine (MEA) was proposed by Jefferson Chemical [3]. The pathway showed
that the reaction of MEA and O2 produced oxalic acid through -amino acetaldehyde intermediate. Oxalic acid is also known to form a heat stable salt with MEA. Several years later, the formation routes to account for additional HSS-induced species specifically formic and acetic acids were proposed [4]. Lactate, formate and acetate were also proposed to be generated by radical-induced oxidation of diisopropanolamine (DIPA) and MEA, respectively [5, 6]. The mechanisms initially produced a peroxide radical which further reacted with O2 and DIPA or MEA forming
* Corresponding author. Tel.: +1-306-585-4470; fax: +1-306-337-3112. E-mail address: [email protected].
doi:10.1016/j.egypro.2011.01.093 592 T. Supap et al. / Energy Procedia 4 (2011) 591–598
2 Author name / Energy Procedia 00 (2010) 000–000
hydroperoxide. The compound later decomposed to form either lactic acid or acetic and formic acid. Although, the proposed mechanistic works were detailed, the existence of these radicals is yet to be verified. A mechanistic work on amine degradation carried out on the basis of a real amine plant sample analysis was also reported in 2003 [7]. This work employed various analytical techniques to identify the degradation products of MEA in plant samples in which pathways of some products were proposed including 2-oxazolidone, N-acetlyethanolamine, and 1- hydroxyethyl-2-piperazinone. The most recent work discussed MEA autoxidation by dioxygen (O2) as one of the three major amine degradation pathways [8]. The formation of glycine, an intermediate which reacted further to produce glycolic acid HSS was also proposed in this work. In the present study, we used MEA and major acidic degradation products often detected in CO2 capture process namely, formic acid, acetic acid, glycolic acid, oxalic acid, and succinic acid. These acids have been reported to form major HSSs with MEA. The objectives set for the experiments were to understand the mechanistic roles of these acids in the oxidation process after they are formed. The roles of MEA-HSSs in further degradation process were also investigated.
2. Experiments
2.1 Chemicals and Equipment
Monoethanolamine (MEA, 99%) was used to prepare an aqueous solution of desired concentrations. Formic acid (98 %), acetic acid (99.7 %), propanonic acid (99.5 %), butyric acid (99 %), glycolic acid, oxalic acid, and succinic acid were used by adding predetermined amounts in the aqueous MEA solution. These acids were also used as standard solutions for verification of the degradation products. Other standards used included N-(2- hydroxyethyl)acetamide, N-(2-hydroxyethyl)succinimide, N, N'-Bis(2-hydroxyethyl)oxamide, N, N'-Bis-(2- hydroxyethyl)formamide. A gas mixture consisting of 6% O2 with N2 balance was used as feed gas. All chemicals were reagent grade and were supplied by Sigma-Aldrich, Canada except the gas mixture which was provided by Praxair, Regina, Saskatchewan, Canada. The experiments were carried out in a 0.6 L-stainless steel semi batch reactor (model 4560, Parr Instrument Co., Moline, IL) equipped with an electrical heating jacket and a magnetic- drive stirrer. A controller (Model 4836, Parr Instrument Co., Moline, IL) of 0.1% accuracy was used to control and monitor the process temperature and the speed of the stirrer. MEA sample were analyzed using gas chromatograph-mass spectrometer instrument (GC-MS, model 6890-5073) with a method developed in our laboratory. Capillary electrophoresis with diode array detector (CE-DAD, model HP 3D CE) was used for HSSs analysis with a literature CE method [9]. GC-MS and CE were both supplied by Agilent Canada.
2.2 Experimental Procedures
A typical experiment was carried out in 0.6 L reactor using 0.45 L aqueous solution of either mixture of 1 kmol/m3 MEA and 0.1 kmol/m3 acids, or 0.1 kmol/m3 acid alone. The solution was heated to 393 K and stirred at a speed of 500 rpm respectively by electrical heating jacket and magnetic-drive stirrer. The solution was left to stabilize for a few minutes once the set temperature was reached. For some experiments which evaluated the O2 effect, the solutions were also pressurized to 303 kPa with 6% O2 feed gas. MEA samples were collected at predetermined time interval starting from 0 day (i.e. after set temperature stabilized or O2 feed into the solution) to the end of the test run which normally lasted for 1 week. Samples were analyzed by GC-MS for general degradation products while CE-DAD was used to analyze for specific HSSs.
3. Results and Discussion
3.1 MEA-Formic acid
3 3 The reactions of MEA and acids were performed at 393 K using 1 kmol/m MEA and 0.1 kmol/m acids. O2 was excluded from the experiments so that the interactions of the HSSs with MEA could be completely elucidated. For MEA-formic acid system, the reaction occurred instantly after mixing MEA and formic acid at room temperature and went beyond acid-base reaction converting the salt of MEA-formic acid into a new product as detected by GC-MS at 19 minutes. Based on mass spectrum analysis, this product was identified as N-(2- T. Supap et al. / Energy Procedia 4 (2011) 591–598 593
Author name / Energy Procedia 00 (2010) 000–000 3 hydroxyethyl)formamide consistent with the literature [7]. The mixture was subsequently run in the reactor with the condition previously described. Figures 1(a) - (d) show GC chromatograms of MEA-formic acid mixture at different reaction times. Based on GC peak area, used to determine the relative concentrations, the concentration of N-(2- hydroxyethyl)formamide labeled as peak 4 in Figure 1(a) was observed to increase with reaction time until it reached a maximum at about 4 days. Then, the peak started to drop off to stabilization at day 7 indicating the decomposition of N-(2-hydroxyethyl)formamide to MEA-formate salt. CE results also confirm this as it was used to measure formate in the mixture. Formate was found to initially decrease in peak area until approximately days 5 – 7 when its area started to increase again. The GC and CE results show that formic acid after being formed as a result of MEA oxidation, exists in 2 forms in equilibrium; (i) as formate HSS of MEA formed by the electrostatic force of MEA and formic acid, and (ii) as formamide which occurs due to covalent bonding between amino and carboxylic groups of MEA and formic acid. Our plant samples analyzed prior to the tests also showed both products in GC analysis. Figure 2 shows mechanistic roles of formic acid after MEA oxidation.
Abundance
Abundance TIC: MecFormic00011.D\ data.ms 2 TIC: MecFormic00004.D\ data.ms 1.15e+07
1e+07 1.1e+07
9500000 1.05e+07
9000000 1e+07
9500000 8500000 9000000 8000000 8500000 7500000 8000000 7000000 1 7500000 6500000 7000000
6000000 6500000
5500000 6000000
5000000 5500000 4500000 5000000 4500000 4000000 4000000 3500000 GC Abundance 3500000 GC Abundance 3 3000000 3000000 2500000 2500000 2000000 4 2000000 1500000 1500000
1000000 1000000
500000 500000
5.00 10.00 15.00 20.00 25.00 30.00 5.00 10.00 15.00 20.00 25.00 30.00 Time--> Time--> 5 10 15 20 25 30 5 10 15 20 25 30 Time (min) Time (min) (a) 0 day (b) 2 days
Abundance Abundance
TIC: MecFormic00016.D\ data.ms TIC: Mecformic00026.D\ data.ms 1.15e+07 1.1e+07 1.1e+07 1.05e+07 1.05e+07 1e+07 1e+07 9500000 9500000 9000000 9000000 8500000 8500000 8000000 8000000 7500000 7500000 7000000 7000000 6500000 6500000 6000000 6000000 5500000 5500000 5000000 5000000 4500000 4500000 4000000 4000000GCAbundance
3500000 3500000GC Abundance
3000000 3000000
2500000 2500000
2000000 2000000
1500000 1500000 1000000 1000000 500000 500000
5.00 10.00 15.00 20.00 25.00 30.00 5.00 10.00 15.00 20.00 25.00 30.00 Time--> 5 10 15 20 25 30 Time--> 5 10 15 20 25 30 Time (min) Time (min) (c) 4 days (d) 7 days
Figure 1 GC Chromatograms of MEA-formic acid mixture at different reaction times (1 kmol/m3 MEA, 0.1 kmol/m3 formic acid, 393 K; 1 = water; 2 =MEA; 3 = formic acid; 4 = N-(2-hydroxyethyl)formamide) O O -H O O H2N 2 + - + H OH OH H O MEA H NH OH
Figure 2 Role of formic acid in MEA degradation
3.2 MEA-Acetic acid
Systems of MEA-acetic acid behaved in a similar manner as that of MEA-formic acid. Figures 3(a) – (d) show GC chromatograms of MEA-acetic acid samples collected at different reaction times. N-(2-hydroxyethyl)acetamide labeled as peak 4 in Figure 3(a) was the only product identified. This amide also existed in real plant samples. Its peak area increased until a maximum and started to decrease at day 6 until it leveled off. On the other hand, CE analysis shows that acetate increased at the same time. Similar to formic acid, acetic acid existed in the system as both acetate salt of MEA and acetamide, and their reactions are given in Figure 4.
3.3 MEA-Glycolic acid
Glycolic acid was also tested similar to tests for formic and acetic acids. Figure 5 (a) – (d) shows GC chromatograms of MEA-glycolic acid samples at different times. Based on the analysis of the mass spectrum of peak 4 in Figure 5(a), the reaction product of glycolic acid and MEA was structurally analogous to amides of formic acid and acetic acid with MEA, and thus, believed to be N-(2-hydroxyethyl)glycolamide. However, N-(2- hydroxyethyl)glycolamide was much less stable than the observed N-(2-hydroxyethyl)formamide and N-(2- hydroxyethyl)acetamide because the peak almost disappeared completely after 4 days. Unlike formic and acetic acids, glycolic acid mostly took a more favorable form of glycolate salt of MEA in the system. This could explain the nonexistence of N-(2-hydroxyethyl)glycolamide in real plant samples. Additional tests also suggested high 3 stability of glycolic acid for the reaction performed using 0.1 kmol/m glycolic acid and 303 kPa of 6% O2 at 393 K O2. Most of the glycolic acid remained intact. Only trace amount was converted to formic acid after several days of 594 T. Supap et al. / Energy Procedia 4 (2011) 591–598
4 Author name / Energy Procedia 00 (2010) 000–000
reaction. Decomposition of glycolic acid to formic acid could be through the formation of oxalic acid. The role of glycolic acid in MEA system is shown in Figure 6.
Abundance Abundance 2 TIC: Mecacetic00004.D\data.ms TIC: Mecacetic00003.D\data.ms 1.2e+07
1.15e+07 1.15e+07
1.1e+07 1.1e+07 1.05e+07 1.05e+07 1e+07 1e+07
9500000 9500000
9000000 9000000
8500000 8500000
8000000 8000000 7500000 1 7500000 7000000 3 7000000 6500000 6500000
6000000 6000000
5500000 5500000
5000000 5000000
4500000 4500000 GC Abundance GC Abundance 4000000 4000000 3500000 3500000 3000000 3000000
2500000 2500000
2000000 2000000
1500000 1500000 1000000 4 1000000 500000 500000 5.00 10.00 15.00 20.00 25.00 30.00 5.00 10.00 15.00 20.00 25.00 30.00 Time--> 5 10 15 20 25 30 Time--> 5 10 15 20 25 30 Time (min) Time (min) a) 0 day b) 1 day
Abundance Abundance TIC: Mecacetic00006.D\ data.ms 1.15e+07 TIC: Mecacetic00008.D\ data.ms 1.1e+07 1e+07 1.05e+07 9500000
1e+07 9000000
9500000 8500000 9000000 8000000 8500000 7500000 8000000 7000000 7500000 6500000 7000000 6000000 6500000
6000000 5500000
5500000 5000000 5000000 4500000 4500000 4000000 4000000 3500000 GC Abundance 3500000 GC Abundance 3000000 3000000 2500000 2500000 2000000 2000000 1500000 1500000 1000000 1000000
500000 500000
5.00 10.00 15.00 20.00 25.00 30.00 5.00 10.00 15.00 20.00 25.00 30.00 Time--> 5 10 15 20 25 30 Time--> 5 10 15 20 25 30 Time (min) Time (min) c) 4 days d) 6 days Figure 3 GC Chromatograms of MEA-acetic acid mixture at different reaction times (1 kmol/m3 MEA, 0.1 kmol/m3 acetic acid, 393 K; 1 = water; 2 =MEA; 3 = acetic acid; 4 = N-(2-hydroxyethyl)acetamide)
O O -H O O H2N 2 + - + H3C OH OH H C O MEA H C NH 3 3 OH
Figure 4 Role of acetic acid in MEA degradation
Abundance 2 Abundance TIC: Mecglycolic00004.D\data.ms 1.05e+07 TIC: Mecglycolic00006.D\data.ms 1e+07 8000000 9500000 7500000 9000000 7000000 8500000 6500000 8000000 7500000 1 6000000 7000000 5500000 6500000 5000000 6000000 4500000 5500000 5000000 4000000 4500000 3500000 4000000 3000000 GCAbundance
3500000 GCAbundance
3000000 2500000 2500000 2000000 2000000 1500000 1500000 4 3 1000000 1000000 500000 500000 5.00 10.00 15.00 20.00 25.00 30.00 5.00 10.00 15.00 20.00 25.00 30.00 Time--> 5 10 15 20 25 30 5 10 15 20 25 30 Time (min) Time--> Time (min) a) 0 day b) 2 days
Abundance Abundance TIC: Mecglycolic00010.D\ data.ms TIC: Mecglycolic00008.D\ data.ms
6500000 6000000
6000000 5500000
5500000 5000000
5000000
4500000
4500000 4000000 4000000
3500000 3500000
3000000 3000000 2500000 2500000 GCAbundance GCAbundance 2000000 2000000
1500000 1500000 1000000 1000000
500000 500000
5.00 10.00 15.00 20.00 25.00 30.00 5.00 10.00 15.00 20.00 25.00 30.00 Time--> 5 10 15 20 25 30 Time--> 5 10 15 20 25 30 Time (min) Time (min) c) 4 days d) 6 days
Figure 5 GC Chromatograms of MEA-glycolic acid mixture at different reaction times (1 kmol/m3 MEA, 0.1 kmol/m3 glycolic acid, 393 K; 1 = water; 2 =MEA; 3 = glycolic acid; 4 = N-(2-hydroxyethyl)glycolamide)
O O
HO H2N HO - + OH + OH O MEA
1/2O2 -H2O
O O
HO H OH NH OH
Figure 6 Role of glycolic acid in MEA degradation T. Supap et al. / Energy Procedia 4 (2011) 591–598 595
Author name / Energy Procedia 00 (2010) 000–000 5
3.4 MEA-Oxalic acid
Tests were also carried out for a mixture of oxalic acid and MEA. As shown in Figure 7(a) labeled as peak 3, 4, and 5, respectively, the mixture produced formic acid, N-(2-hydroxyethyl)formamide, and N, N'-Bis(2- hydroxyethyl)oxamide instantly. This indicates that oxalic acid is highly reactive, and so, it could be an intermediate in MEA oxidation. Once it is generated by MEA oxidation, one mole of oxalic acid reacts right away with 2 moles of MEA to form N, N'-Bis(2-hydroxyethyl)oxamide. The decomposition of oxalic acid also occurs simultaneously producing formic acid which in turn reacts with MEA to form N-(2-hydroxyethyl)formamide. After 1 day, N, N'- Bis(2-hydroxyethyl)oxamide peak disappeared but a new peak labeled as peak 6 emerged as shown in Figure 6(b). A quick disappearance of N, N'-Bis(2-hydroxyethyl)oxamide indicates a high instability of this amide. The decomposition of N, N'-Bis(2-hydroxyethyl)oxamide is believed to occur by hydrolysis similar to formamide, acetamide, and glycolamide. However, N, N'-Bis(2-hydroxyethyl)oxamide was not completely hydrolyzed to regenerate MEA and oxalic acid back to the system. Instead, it lost only one molecule of MEA on one side and produced a more stable N-(2-hydroxyethyl)oxamide believed to be peak 6 in Figure 7(b). The conversion of oxalic acid to its amides and formic acid was also evident as shown in the CE results given in Figure 8. Oxalate peak clearly reduced with time while a new peak at 6 min appeared after one day, and this increased in peak size until it stabilized at day 5 to 7. This peak was believed to be N-(2-hydroxyethyl)oxamide which its remaining of carboxyl group made it detectable by CE technique specifically developed for anion analysis. N, N'-Bis(2- hydroxyethyl)oxamide was not detected at 0 day simply due to its inability in acquiring a charge in CE analysis. Figure 8 also shows formate peak after one day which remained detected throughout the test period. Since oxalic acid could behave as an intermediate of formic acid, the role of O2 in oxalic-formic conversion was also undertaken 3 using 0.1 kmol/m oxalic acid, 303 kPa of 6% O2, and 393 K. Figure 9 shows clearly that in the presence of O2, oxalic acid quickly degraded to formic acid. Practically, this reaction could be dominant as opposed to oxalic acid- oxamide conversion as our plant sample analysis always contained formate and N-(2-hydroxyethyl)formamide as major degradation products. On the other hand, oxalate was found in much less concentration with oxamide rarely detected, thus its formation was considered minor. The role of oxalic acid can be summarized in Figure 10.
Abundance Abundance 2 TIC: Mecoxalic00001.D\ data.ms TIC: Mecoxalic00003.D\ data.ms 9000000 8000000 8000000
7000000 7000000 6000000 6000000 1
5000000 5000000 4 4000000 4000000
3000000 3000000 GC Abundance GC Abundance 2000000 2000000 3 5 6 1000000 1000000 5.00510.00 10 15 15.00 20 25 30 35 4020.00 25.00 30.00 35.00 40.00 5.005 10 15 10.00 15.00 20 25 30 35 420.00 25.00 30.00 35.00 40.000 Time--> Time (min) Time--> Time (min) a) 0 day a) 1 day
Abundance Abundance
TIC: Mecoxalic00005.D\ data.ms TIC: Mecoxalic00008.D\ data.ms 9000000 9000000
8000000 8000000
7000000 7000000
6000000 6000000
5000000 5000000
4000000 4000000
3000000 3000000 GC Abundance GC Abundance
2000000 2000000
1000000 1000000
5.005 10 15 10.00 15.00 20 25 30 35 420.00 25.00 30.00 35.00 40.000 5.005 10 15 20 25 30 35 410.00 15.00 20.00 25.00 30.00 35.00 40.000 Time--> Time (min) Time--> Time (min) a) 3 days d) 6 days
Figure 7 GC Chromatograms of MEA-oxalic acid mixture at different reaction times (1 kmol/m3 MEA, 0.1 kmol/m3 oxalic acid, 393 K; 1 = water; 2 =MEA; 3 = formic acid; 4 = N-(2-hydroxyethyl)formamide, 5 = N, N'-Bis(2- hydroxyethyl)oxamide, 6 = N-(2-hydroxyethyl)oxamide)
596 T. Supap et al. / Energy Procedia 4 (2011) 591–598
6 Author name / Energy Procedia 00 (2010) 000–000
DAD1 A, Sig=350,80 Ref=240,10 (TEERA\MECOXALIC000002.D) ᳀ᲔᲨ Oxalate 100 50 0 day 0 CE Abundance 1 2 3 4 5 6 1 2 3 4 5 6 7 7 8 9 8 9 ᳀᳁ DAD1 A, Sig=350,80 Ref=240,10 (TEERA\MECOXALIC000003.D) ᳀ᲔᲨ N (2 hydroxyethyl)oxamide 100 75 Formate 50 25 1 day 0 CE Abundance 1 2 3 4 5 6 7 8 9 ᳀᳁ DAD1 A, Sig=350,801 2 3 4 5 6 Ref=240,10 (TEERA\MECOXALIC000005.D) 7 8 9 ᳀ᲔᲨ 100 75 50 3 days 25 0 CE Abundance 1 2 3 4 5 6 1 2 3 4 5 6 7 7 8 9 8 9 ᳀᳁ DAD1 A, Sig=350,80 Ref=240,10 (TEERA\MECOXALIC000007.D) ᳀ᲔᲨ 100 75 50 25 5 days 0 CE Abundance 1 2 3 4 5 6 1 2 3 4 5 6 7 7 8 9 8 9 ᳀᳁ DAD1 A, Sig=350,80 Ref=240,10 (TEERA\MECOXALIC000009.D) ᳀ᲔᲨ
75 50 25 7 days 0 CE Abundance 1 2 3 4 5 6 1 2 3 4 5 6 7 7 8 9 8 9 ᳀᳁ Time (min)
Figure 8 CE Electrophoregram of MEA-oxalic acid mixture at different reaction times (1 kmol/m3 MEA, 0.1 kmol/m3 oxalic acid, 393 K)
DAD1 A, Sig=350,80 Ref=240,10 (TEERA\MECOXALICOXY001.D) ᳀ᲔᲨ 150 Oxalate 100 50 0 day 0 CE Abundance 0 1 2 3 4 1 2 3 4 5 5 6 6 ᳀᳁ DAD1 A, Sig=350,80 Ref=240,10 (TEERA\MECOXALICOXY002.D) ᳀ᲔᲨ 150 100 Formate 50 1 day 0 CEAbundance 0 1 2 3 4 5 6 ᳀᳁ DAD1 A, Sig=350,80 Ref=240,101 2 3 4 (TEERA\MECOXALICOXY005.D) 5 6 ᳀ᲔᲨ 150 100 50 3 days 0 CEAbundance 0 1 2 3 4 5 6 ᳀᳁ DAD1 A, Sig=350,80 Ref=240,101 2 3 4 (TEERA\MECOXALICOXY006.D) 5 6 ᳀ᲔᲨ 150 100 50 5 days 0 CEAbundance 0 1 2 3 4 5 6 ᳀᳁ DAD1 A, Sig=350,80 Ref=240,101 2 3 4 (TEERA\MECOXALICOXY007.D) 5 6 ᳀ᲔᲨ 150 100 50 7 days 0 CEAbundance 0 1 2 3 4 1 2 3 4 5 5 6 6 ᳀᳁ Time (min)
Figure 9 CE Electrophoregram of Oxalic acid-O2 system at different reaction times (1 kmol/m3 MEA, 0.1 kmol/m3 oxalic acid, 393 K)
O HO O 2 H2N + - OH + MEA O OH - + O O MEA O /O2 -2H2O O O HO O NH -MEA H O + CO2 + 2 NH HO H OH OH NH O OH MEA O
O O -H2O - + H O MEA H NH OH
Figure 10 Role of oxalic acid in MEA degradation
T. Supap et al. / Energy Procedia 4 (2011) 591–598 597
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3.5 MEA-Succinic acid
The reaction of succinic acid and MEA was carried out in a similar manner to all acids described earlier. Figures 11(a) – (d) show GC results at various reaction times. In Figure 11(a), the mixture produced a product right away labeled as peak 3, which was verified with a standard as N-(2-hydroxyethyl)succinimide. This was an imide in addition to N-(2-hydroxyethyl)formamide and N-(2-hydroxyethyl)acetamide detected in our plant samples. This imide peak reached its maximum height and started to fall just similar to N-(2-hydroxyethyl)formamide and N-(2- hydroxyethyl)acetamide. The CE analysis of the initial reaction times (i.e. 0 – 3 days) shown in Figure 12 also corresponded to the results of GC when succinate peak initially reduced as succinimide was formed. However, succinate peak still decreased further though GC peak as the succinimide decreased. Further decrease of N-(2- hydroxyethyl)succinimide peak could be explained by formation of a new CE peak at about 7 min. Although, this peak could not be identified due to commercial unavailability, we believed it was N-(2-hydroxyethyl)succinamide. It is well known that amide is an intermediate of imide formation. Thus, this is also the case for N-(2- hydroxyethyl)succinimide which was formed through N-(2-hydroxyethyl)succinamide intermediate. The reactions showing the role of succinic acid is given in Figure 13.
Abundance Abundance 2 TIC: Mecsuccinic00002.D\data.ms TIC: Mecsuccinic00003.D\ data.ms 7000000 7000000 6500000 6500000 6000000 6000000 5500000 1 5500000 5000000 5000000 4500000 4500000 4000000 4000000 3500000 3500000 3000000 3000000
2500000 2500000 GC Abundance GC Abundance 2000000 3 2000000
1500000 1500000
1000000 1000000
500000 500000
5.00510.00 10 15 20 25 3015.00 20.00 25.00 30.00 5.00510.00 10 15 20 25 3015.00 20.00 25.00 30.00 Time--> Time--> Time (min) Time (min)
a) 0 day b) 1 day
Abundance Abundance
TIC: Mecsuccinic00005.D\data.ms TIC: Mecsuccinic00008.D\ data.ms 7000000 7000000 6500000 6500000 6000000 6000000 5500000 5500000 5000000 5000000 4500000 4500000 4000000 4000000 3500000 3500000 3000000 3000000 2500000 2500000 GC Abundance GC Abundance 2000000 2000000 1500000 1500000 1000000 1000000
500000 500000
5.00510.00 10 15 20 25 3015.00 20.00 25.00 30.00 5.00510.00 10 15 20 25 3015.00 20.00 25.00 30.00 Time--> Time--> Time (min) Time (min) c) 3 days d) 6 days Figure 11 GC Chromatograms of MEA-succinic acid mixture at different reaction times (1 kmol/m3 MEA, 0.1 kmol/m3 succinic acid, 393 K; 1 = water; 2 =MEA; 3 N-(2-hydroxyethyl)succinimide)
DAD1 A, Sig=350,80 Ref=240,10 (TEERA\MECSUCCINIC0007.D) ᳀ᲔᲨ 400 Succinate 300 200 0 day 100 0 CEAbundance 1 2 3 4 5 6 7 8 9 ᳀᳁ DAD1 A, Sig=350,801 2 3 4 5 6 Ref=240,10 (TEERA\MECSUCCINIC0009.D) 7 8 9 ᳀ᲔᲨ 400 N (2 hydroxyethyl)succinamide 300 200 1 day 100 0
CEAbundance 1 2 3 4 5 6 7 8 9 ᳀᳁ DAD1 A, Sig=350,801 2 3 4 5 6 Ref=240,10 (TEERA\MECSUCCINIC0018.D) 7 8 9 ᳀ᲔᲨ 400 300 200 3 days 100 0 CEAbundance 1 2 3 4 5 6 7 8 9 ᳀᳁ DAD1 A, Sig=350,801 2 3 4 5 6 Ref=240,10 (TEERA\MECSUCCINIC0021.D) 7 8 9 ᳀ᲔᲨ 400 300 5 days 200 100 0 CEAbundance 1 2 3 4 5 6 1 2 3 4 5 6 7 7 8 9 8 9 ᳀᳁ Time (min)
Figure 12 CE Electrophoregram of succinic acid-O2 system at different reaction times (1 kmol/m3 MEA, 0.1 kmol/m3 succinic acid, 393 K)
598 T. Supap et al. / Energy Procedia 4 (2011) 591–598
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O O HO + H2N HO OH - + OH O MEA O O
-H O O 2 O -H2O N OH HO NH O OH O
Figure 13 Role of succinic acid in MEA degradation
4. Conclusions
4.1 Formic and acetic acids exist in 2 forms in equilibrium; formate and acetate HSSs of MEA and N-(2- hydroxyethyl)formamide and N-(2-hydroxyethyl)acetamide, respectively. 4.2 Glycolic acid was found to favor HSSs formation with MEA. Its amide was found to be unstable, and mostly hydrolyzed back to glycolate and MEA. 4.3 Oxalic acid was determined to be a reactive intermediate mostly decomposing to formic acid, which in turn, produced a stable N-(2-hydroxyethyl)formamide. N-(2-hydroxyethyl)oxamide could also be formed but its formation was much less prominent. 4.4 Succinic acid formed a stable N-(2-hydroxyethyl)succinimide through an intermediate of N-(2- hydroxyethyl)succinamide.
Acknowledgements
The authors would like to acknowledge the financial assistance from the Natural Sciences and Engineering Research Council of Canada (NSERC), and the International Test Center for CO2 Capture (ITC), University of Regina, Regina, Saskatchewan, Canada.
References
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