Pathways for the Formation of Products of the Oxidative Degradation

Ind. Eng. Chem. Res. 2005, 44, 945-969

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Pathways for the Formation of Products of the Oxidative Degradation of CO2-Loaded Concentrated Aqueous Monoethanolamine Solutions during CO2 Absorption from Flue Gases Adeola Bello and Raphael O. Idem*

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Process Systems Engineering Laboratory, Faculty of Engineering, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan, Canada S4S OA2

Oxidative degradation experiments involving CO2-loaded monoethanolamine (MEA) were performed in a stainless steel rotary-type autoclave with MEA concentrations of 5 and 7 mol/L, O2 pressures of 250 and 350 kPa, and a CO2 loading ranging from 0 to 0.44 mol of CO2/mol of MEA at temperatures of 55-120 °C in order to elucidate the pathways for the formation of degradation products under typical absorber and stripper conditions and to evaluate the effects of temperature, O2 pressure, MEA concentration, and CO2 loading on MEA degradation. The results showed that an increase in the temperature or O2 pressure resulted in an increase in degradation for both the MEA-H2O-O2 and MEA-H2O-O2-CO2 systems. However, an increase in the MEA concentration gave the opposite effect for all of the systems. The number of products and the extent of degradation decreased in the following order: MEA-H2O-O2 > MEA-H2OO2-CO2 > MEA-H2O-CO2. The gas chromatograph/mass spectrometer method was used to identify the degradation products, which were then used to propose pathways for their formation that are consistent with the products and the operating conditions. 1. Introduction Carbon dioxide (CO2) removal from flue gases can be implemented by employing several standard processes. These include methanation of CO2 (used primarily to remove small quantities of CO2 remaining in gas streams after bulk removal by other techniques), adsorption (a viable option for CO2 removal when the CO2 partial pressure in the feed is high, as in high-pressure gasification product gas streams), and membrane permeation (also applicable to the removal of CO2 from high-pressure gas streams), as well as absorption in physical and chemical alkaline solutions (suitable for treating high-volume gas streams containing CO2). Absorption in physical solvents is not economical when the CO2 partial pressure is low.1 Consequently, the most economical method for the separation of low-pressure dilute CO2 streams (e.g., flue gas from coal-fired power plants) is gas absorption using aqueous alkanolamines. The alkanolamine hydroxyl group (OH) contributes by reducing the vapor pressure of the alkanolamine monoethanolamine (MEA), thereby increasing the solubility of CO2 in water, whereas the alkalinity, which is responsible for the absorption of the acid gas, is produced by the amino group. This leads to accelerated absorption of CO2 from gas streams with low partial pressures, as is the case with power-plant-generated flue gas streams.2 Although there are different types of industrially utilized alkanolamines, MEA is the most widely used solvent for CO2 absorption because it has the highest alkalinity and, hence, reacts most rapidly with CO2. In addition, it can be reclaimed with ease from contaminated solutions.2 In general, MEA has the highest CO2 separation rate, which leads to relatively low overall * To whom correspondence should be addressed. Fax: (306) 585-4855. E-mail: [email protected].

costs. However, despite these advantages, MEA has the drawback that it requires a large amount of high-grade energy for regeneration, degrades most rapidly in the presence of oxygen (O2), has the highest corrosivity among commercially available amines, and has a substantially higher vapor pressure than other alkanolamines, resulting in significant vaporization and solvent losses.2-4 One of the other serious causes of MEA loss is degradation.2 In an ideal MEA-CO2 absorption system, the solvent is recycled and reused. However, some of the byproducts of MEA degradation can decrease the efficiency of CO2 capture and have also been implicated in the corrosion of machinery and toxicity to the environment. In addition, the degraded MEA has to be disposed of in an environmentally sound manner,5,6 which unfortunately leads to increased material and waste disposal costs. In addition, degradation and corrosivity have forced the use of low concentrations of MEA, leading to larger overall equipment size, higher solvent circulation rates, and therefore an increased energy requirement for CO2 regeneration.3 To effectively prevent MEA degradation, a degradation prevention strategy needs to be formulated, and this requires knowledge of the products, stoichiometry, and mechanism as well as the kinetics of the degradation process6,7 as a function of the various operating variables. Most of the earlier studies performed on the degradation of single alkanolamines have focused on the understanding of the natural gas sweetening processes where O2 is absent. However, degradation involving flue gases is more complicated because of the presence of O2 and other components, including CO2, CO, SOx, NOx, and fly ash.7,8 This suggests that, to carry out a proper study of MEA degradation during CO2 absorption from power plant flue gases, the effects of all possible components in the system must be considered in terms

10.1021/ie049329+ CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005

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Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005

Figure 1. Mechanism of oxidative degradation of MEA attributed to Jefferson Chemical Co.

Scheme 1

of identifying the products, elucidating the stoichiometry and mechanism for their formation, and evaluating the kinetics of the degradation process on the basis of a mechanistic approach in a systematic manner. The

Figure 2. Mechanism of the oxidative degradation of MEA proposed by Rooney et al.

present study is based on this approach for MEA degradation in the presence of both CO2 and O2 under typical CO2 absorption and stripping conditions. Polderman et al.9 first investigated the reaction of CO2 alone with MEA to form MEA degradation products and observed that MEA could be converted to two products [1-(2-hydroxyethyl)imidazolidone-2 and N-(2hydroxyethyl)ethylenediamine (HEED)] simply by heating MEA at the temperatures encountered in the gastreating plant (i.e., 40-140 °C).2,3 The mechanism of degradation due to CO2 alone, as proposed by Polderman et al.,9 is essentially identical with that latter proposed by Kim and Satori10 for diethanolamine (DEA) and Kim11 for 2,4-N-di-2-propanolamine. The reactions start with the formation of the carbamate ion, as in eq 1.2 Monoethanolamine carbamate then condenses to form oxazolidone-2, as in eq 2. Oxazolidone-2 then reacts with another molecule of MEA to form 1-(2-hydroxyethyl)imidazolidone-2, as in eq 3.2,3 The substituted imidazolidone then hydrolyzes to CO2 and HEED, also known as hydroxyethylenediamine, as in eq 4 (Scheme 1).11 It is also possible that HEED may continue to degrade in the presence of CO2 to form longer substituted diamines, which are characterized as polymeric materials.12 However, these polymeric materials have not yet been identified, and as such, their formation

6.9 13.8 25.4 49.1 93.1 86.2 74.6 50.9 1620 2340 5220 8100 a

100 100 100 100 0 15 30 45

Experimental conditions also included the use of degradation inhibitors and catalysts.

0.0 0.2 0.9 2.8 100 99.8 99.1 97.2 60 300 540 900

MEA content (mol % of fresh sample) time (min) % extent of degradation MEA content (mol % of fresh sample) time (min) time (min)

B. Extent of MEA Degradation with Time for a Run Conducted with 5 M MEA and 250 kPa of O2 Pressure

% extent of degradation MEA content (mol % of fresh sample)

not provided not provided none 0.25 21.3% 82.2 20 wt % 1

0.0 0.0 0.0 0.0

% extent of degradation

not provided

provided

scheme provided shows formation of some products not provided some some none 0 20 wt % 1 Rooney et

7-11 1 Supap et al.7

120, 140, and 160 82.2

21.3%

provided not provided not provided provided none 0

not provided not provided not provided not provided none 0.15-0.4 0.4 and 1% 2%

5 L/min air air alone and air + makeup CO2 350 and 250 kPa 55 55 2.5-12 7 this decade this decade Goff and Rochellea,20

55 2.5-12 this decade Chi and Rochellea,5,19

decade ago ref

al.12

provided provided

provided

scheme provided shows formation of some products not provided not provided some some none 0

kinetics O2 pressure temp (°C)

5 L/min air

overall mechanism product identification

stoichiometry

work done

corrosion inhibitor type operating conditions

CO2 pressure or loading MEA concn (M)

A. Work Done in This Field in the Past 4 Decades

pathways are not fully understood. On the other hand, Aboudheir et al.13 has shown that there is substantial information concerning the kinetics of the CO2 absorption in aqueous MEA. However, all of these studies were not conducted in terms of CO2-induced degradation of MEA. As such, no degradation products were considered or identified. Alkanolamines are also subject to oxidative degradation by contact with free O2. According to the literature,2,3,6,12,14-23 products of the oxidative degradation of MEA have been identified to include carboxylic acids (e.g., formic acid, oxalic acid, and glycolic acid), glycine, ammonia, water, substituted amides6,14 [e.g., N-(2hydroxyethyl)acetamide and N-(2-hydroxyethyl)lactamide], pyridines (e.g., 3-methylpyridine), substituted alkanols [e.g., 2-(methylamino)ethanol], amines (e.g., ethylamine and 1-propylamine), substituted alkanones [e.g., 4-amino-2(1H)-pyridinone and DL-homoserine lactone], substituted azetidines [e.g., 2-methylazetidine, 5-(hydrazinocarbonyl)imidazole, uracil, and 5-ethyluracil], substituted aldehydes (e.g., 1-piperidinecarboxaldehyde), and high molecular weight polymers. Several oxidative degradation mechanisms have been identified to account for some of these products, with the principal one involving direct oxidation to organic acids.15 In addition, a mechanism attributed to Jefferson Chemical Co. to account for the reaction between MEA and O2 alone is shown in Figure 1.16 In addition, in the oxidative degradation work of Rooney et al.,12 a scheme (Figure 2) was proposed to account for the presence of some of the observed anions and those reported from past studies. It was concluded that, although the acid forms of acetic, formic, glycolic, glyoxalic, and oxalic acids were shown, each of these anions was completely ionized to the heat-stable amine salt such as oxalates and glyoxalates in strong base solutions.12 In a comparative studies, Kohl and Nielsen have indicated that primary amines such as MEA are more vulnerable to oxidation than secondary and tertiary amines,2 the result of which was confirmed by Hofmeyer et al.15,16 and Rooney et al.12 On the other hand, Asperger,17 an operator of many plants, reported that, as a general rule, methyldiethanolamine (MDEA) seems to be more sensitive than DEA to heat-stable salt formation and O2 contamination. MEA degradation in the presence of both CO2 and O2 by Rooney et al.12 involved a study of the rate of degradation (i.e., kinetics) of various alkanolamine mixtures in the presence of O2 and CO2 using 20 wt % MEA, 30 wt % DEA, and 30 and 50 wt % MDEA individually with 0.25 mol of CO2/mol of amine at 82.2 °C, with analysis performed using ion chromatography. It was observed that O2 resistance increased in the order 30 wt % DEA > 50 wt % DGA > 20 wt % MEA > 50 wt % MDEA > 30 wt % MDEA. This result is in contradiction with earlier studies of Kindrick et al.,18 who showed that MDEA had the best oxidative degradation resistance while DEA had the least resistance. In a comparison of oxidative degradation with and without CO2, it was observed12 that for 30 wt % DEA, 50 wt % DGA, and 20 wt % MEA without CO2 there was a larger total of moles of acetate, formate, glycolate, and oxalate as compared with the case with CO2, for which they concluded that the presence of CO2 lowered the O2 solubility (i.e., salting-out effect).19,20 The work of Rooney et al.12 did not identify all of the possible degradation products formed with the result that no overall mechanism was proposed to account for these degradation

Table 1. Work Done in This Field in the Past 4 Decades and Extent of MEA Degradation with Time for a Run Conducted with 5 M MEA and 250 kPa of O2 Pressure

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 947

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products. Only a modification of the mechanism proposed by Jefferson Chemical Co. (Figure 1) to show the formation of some of the oxidative degradation products not observed by Jefferson Chemical Co. was put forward (Figure 2). This implies that products included in the modified mechanism are only a few of the products that could be formed in the presence of O2. There was no indication of products resulting from the reaction of MEA with CO2. In addition, they did not propose a mechanism to account for degradation products obtained in the presence of CO2 alone as well as for the presence of both CO2 and O2. Table 1 summarizes all of the information on CO2induced MEA degradation as well as the O2-induced MEA degradation for the last 4 decades showing the operating conditions (i.e., temperature, MEA concentration, CO2 and/or O2 pressure, corrosion inhibitor type, etc.) under which the data were obtained. The table shows that there has been no pathway proposed for the reaction of CO2-loaded MEA with O2, though kinetic data have been provided. It is evident from the literature that mechanisms have been proposed for the formation of only some of the products of the reaction of MEA with O2 alone and CO2 alone. Other possible products were not reported probably because of difficulty in their identification. In addition, only one group has proposed a mechanism for the formation of only some of the products observed for the combined CO2- and O2induced degradation of MEA based on the analysis of the reclaimer waste from the IMC Chemicals Facility.6 This mechanistic work did not involve all of the possible products of a reaction involving both O2 and CO2 exclusively with MEA. In addition, the identified products cannot be considered to be representative of oxidative degradation products under absorber or stripper conditions because, in MEA reclaiming, sodium carbonate or sodium hydroxide (known to react with heatstable salts) is added in order to liberate the amine from the heat-stable acid salts. Hence, other products, apart from the true degradation products, are also formed during this process. Another factor is that, despite the array of research done in the field of oxidative degradation of MEA, there is the contradiction of whether oxidative degradation increases or decreases with CO2 loading.5,12,18 The present work will aim at resolving this contradiction as well as obtaining realistic pathways for the formation of all identifiable degradation products obtained as a result of the reaction of MEA with CO2 and/or O2 under various scenarios and conditions representative of a typical absorber and stripper. The results are presented and discussed in this paper. 2. Experiment 2.1. Equipment. The reactor used was a stainless steel rotary-type autoclave (model BC0030SS05AH; obtained from Autoclave Engineers, Erie, PA), which consisted of a baffle bar, an impeller, cooling coils, a gas feed port, an extra feed port, a gas product port, a liquid sampling valve, a thermowell, and a pressure gauge. The pressure gauge was fitted to the extra feed port and used to measure the total pressure in the reactor. A K-type thermocouple placed in the thermowell was used to measure the temperature of the reaction mixture, while an electric furnace, controlled using a proportional-integral-derivative temperature controller (model MTCPKS00; also from Autoclave Engineers, Erie, PA) and a K-type thermocouple, was used to

supply the heat. The accuracy of the temperature control was within (2 °C. 2.2. Chemicals. Concentrated MEA (research grade, 99% purity) was obtained from Fischer Scientific, Whitby, Ontario, Canada. For each experimental run, the MEA was diluted to the desired concentration and then standardized using 1 M hydrochloric acid (also obtained from Fischer Scientific). 2.3. Experimental Runs. 2.3.1. MEA-H2O-O2 System. The reaction was conducted using aqueous solutions of 5 and 7 mol/L MEA at temperatures of 55, 100, and 120 °C, as well as O2 pressures of 250 and 350 kPa. About 230 mL of aqueous MEA was loaded into the reactor and stirred at a rate of 300 rpm while heating to the desired temperature and simultaneously cooling the stirrer. When the reaction mixture reached the desired temperature, the pressure on the reactor pressure gauge was noted and added to the desired O2 pressure to obtain a final desired reactor pressure. The reactor O2 gas inlet valve was then opened and O2 fed into the reactor by opening the O2 cylinder and regulated to the desired reactor pressure. The gas inlet and O2 cylinder were left open, and the first sample (2.5 mL) was taken immediately. Because of the solubility of O2 in MEA, there was pressure depletion in the reactor. The reactor was boosted with extra O2 to maintain the desired pressure. This was also repeated each time a sample was taken. About 2.5-mL amounts of other samples were taken at predetermined intervals. The reaction in each sample was quenched by running icecold water over the sample vial for about 1 min. 2.3.2. MEA-H2O-CO2 System. The reaction was conducted using aqueous solutions of 5 and 7 mol/L MEA at temperatures of 100 and 120 °C as well as CO2 pressures of 250 and 350 kPa. Apart from using CO2 in place of O2, the procedure was the same as that in the MEA-H2O-O2 system. 2.3.3. MEA-H2O-O2-CO2 System. The reactor was also loaded with 230 mL of 5 or 7 mol/L MEA as desired. CO2 was preloaded into each solution by allowing 250 kPa of CO2 to contact the MEA in the reactor without heating while stirring at 300 rpm. After 24 h, a sample was taken and the CO2 loading was determined. The CO2-loaded MEA was then heated to the desired temperature, and the CO2 loading was again determined. In addition, the pressure on the reactor pressure gauge was noted and added to the desired O2 pressure to obtain the reactor pressure. Apart from the preloading of the MEA solution with CO2, the procedure was similar to the case for the MEA-H2O-O2 system. 2.4. Analysis of the Product. The samples were diluted with deionized water to 5 times their original concentration to prevent column overload. Each sample was analyzed using a gas chromatograph/mass spectrometer (GC/MS; model HP 6890/5073 supplied by Hewlett-Packard Canada Ltd., Montreal, Quebec, Canada). The GC/MS contained an HP Innowax column packed with cross-linked poly(ethylene glycol) used to separate the components. Sample injection into the GC/MS was done using an autosampler (model 7683 supplied by Hewlett-Packard Canada Ltd., Montreal, Quebec, Canada) to give better reproducibility. The components were identified through their mass spectra

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 949 Table 2. Summary of the Oxidative Degradation Products Obtained with 5 mol/L MEA at 120 °C and 135 h 250 kPa O2 S/N 1 2 3 4 5 6 7 8

product

350 kPa O2 molecular formula

N-(2-hydroxyethyl)acetamide 1-propanamine 4-(hydrazinocarbonyl)imidazole formic acid 5-(hydrazinocarbonyl)imidazole 3-cis-5-heptadien-1-oI N-(2-hydroxyethyl)succinimide 2-pyrrolidinone

C4H9NO2 C3H9N C4H6N4O CH2O2 C4H6N4O C7H12O C3H9NO3 C4H7 NO

9 10 11 12

3-methylpyridine 2-propanone oxime 2-(methylamino)ethanol ethylamine

C6H7N C3H7NO C3H9NO C2H7N

13 14 15 16 17 18 19

3-methylcyclopentanol 4-methylmorpholine 1H-imidazole trimethylhydrazine 1-methyl-2,4-imidazolidinedione 15-crown-5 1-(2-hydroxyethyl)-2-imidazolidinone

C6H12O C5H11NO C3H4N2 C3H10N2 C4H6N2O2 C10H20O5 C5H10 N2O2

area %

S/N

product

Major Products 1.02 1. N-(2-hydroxyethyl)acetamide 2.94 2. formic acid 1.06 3. 1H-imidazole 2.10 4. N-formyl-N-methylformamide 3.46 5. 1,3-dioxane 33.85 6. uracil 0.51 7. 4-(hydrazinocarbonyl)imidazole 0.96 8. 5-(hydrazinocarbonyl)imidazole 9. 2,2-dimethyl-3(2H)-furanone 10. N-(2-hydroxyethyl)succinimide 11. 1-piperazineethanol 12. diisopropanolamine 13. (dimethylamino)ethylidenetert-butylamine Minor Products 0.14 14. 2-(2-aminoethoxy)ethanol 0.20 15. 2-methylpropanenitrile 0.30 16. ethoxyethene 0.40 17. 3-methyl-4-amino-4,5(1H)-dihydro1,2,4-triazol-5-one 0.08 18. 4-ethylquinoline 0.41 19. 4-methylmorpholine 0.29 20. 2,6-dioxatricyclo[3.3.2.0-(3,7)]dec-9-ene 0.24 21. 2,3-dihydro-1-methyl-4(1H)-pyridinone 0.08 22. 12-crown-4 0.14 23. 3-methylpyridine 0.24 24. 2-quinazoliamine 0.14 25. acetamide 0.20 26. nitrosomethane 0.30 27. 1-butyl-5-methyl-2-pyrazoline 0.40 28. 2-(methylamino)ethanol 0.08 29. acetic acid 30. 1-methylazetidine 31 2-methyl-1H-imidazole

by carrying out a NIST library search available on the GC/MS. The GC/MS conditions used are summarized as follows. A 10-µL syringe with injection volume of 0.2 µL was used, and a split mode was selected for the inlet with a split ratio of 10:1, a split flow of 10.3 mL/min, and a total flow of 13.9 mL/min. The inlet temperature and pressure were 70 °C and 9.18 psi, respectively. The initial temperature of the oven was 100 °C with a hold time of 10 min, while the final temperature was 240 °C with a hold time of 10 min. The oven was ramped at 10 °C/min for a total run time of 27 min. The column flow rate was 1 mL/min, while the pressure and average velocity were 9.18 psi and 37 cm/s. The sample reproducibility (volume) was better than 3%. For the MS parameters, the interface, quadruple, and source temperatures were 250, 150, and 230 °C, respectively, and the electron multiplier voltage was 1200 V. The error of the GC/MS analysis was estimated to be better than (3%.

molecular formula

area %

C4H9NO2 CH2O2 C3H4N2 C3H5NO2 C4H8O2 C4H4N2O2 C4H6N4O C4H6N4O C6H8O2 C3H9NO3 C6H14N2O C6H15NO2 C7H16N2

2.83 3.57 1.55 10.79 1.28 16.31 0.79 1.81 31.46 2.35 0.48 0.87 1.82

C4H11NO2 C4H7N C4H8O C3H6N4O

0.17 0.10 0.14 0.15

C11H11N C5H11NO C8H10O2 C6H9NO C8H16O4 C6H7N C8H7N C2H5NO CH3NO C8H16N2O C3H9NO C2H4O2 C4H9N C4H6N2

0.20 0.29 0.19 0.23 0.29 0.04 0.08 0.07 0.05 0.37 0.44 0.70 0.17 0.16

and those for 250 kPa of O2 pressure at 55 °C are also given in Table 3. The products obtained at 120 °C for 7 mol/L MEA are given in Table 4. Formic acid and acetic acid were two of the products obtained at 120 °C (Table 3). This is consistent with the work of Hofmeyer et al.,15 Blanc et al.,22 Rooney et al.12 and Lloyd and Taylor,16 the latter of which reported additional products latter

3. Results and Discussion This study was focused on the identification of the oxidative degradation products of MEA in both the presence and absence of CO2. It also involved the determination of the extent of degradation as well as the postulation of possible mechanisms for the formation of the degradation products as functions of the MEA concentration, O2 pressure, and degradation temperature. 3.1. MEA-H2O-O2 System. Test runs were conducted using 5 and 7 mol/L aqueous MEA solutions with O2 pressures of 250 and 350 kPa at temperatures of 55, 100, and 120 °C. The degradation products obtained with 5 mol/L MEA for 250 and 350 kPa of O2 pressure are given in Table 2 for 120 °C and Table 3 for 100 °C,

Figure 3. Mechanism of MEA oxidation by single electrons and by oxygen.

950


Table 3. Summary of the Oxidative Degradation Products Obtained with 5 mol/L MEA 250 kPa of O2 at 100 °C, 530 h

350 kPa of O2 at 100 °C, 732 h

S/N

product

molecular formula

1 2 3

N-formyl-N-methylformamide 1,2,3,6-tetrahydro-1-nitrosopyridine 3-methyl-1,2-cyclopentanedione

C3H5NO2 C5H8 N2O C6H8O2

4 5 6 7

3,4-dehydro-DL-proline 4-ethylcyclohexene 1,2-ethanediol 3-cis-5-heptadien-1-oI

C5H7NO2 C8H14 C2H6O2 C7H12O

Major Products 0.93 1 1,3,4-dehydroproline 9.33 2 ethylamine 5.16 3 1H-imidazole-4-carboxylic acid methyl ester 0.54 4 3-cis-5-heptadien-1-oI 1.33 0.76 1.08

nitrosomethane ethylamine 2,2-dimethyl-3-(2H)-furanone uracil 1H-imidazole-4-carboxylic acid methyl ester

CH3NO C2H7N C6H8O2 C4H4N2O2 C4H6N4O

Minor Products 0.11 5 0.31 6 0.19 7 0.43 8 0.13

8 9 10 11 12

area %

S/N

product

3,3-(1,2-ethanediyl)bis(sydnone) 1-methyl-1H-imidazole-2-methanol 2,4-dihydro-4,5(3H)-pyrazol-3-one 6-hydroxy-4(1H)-pyrimidinone

molecular formula

area %

C5H7NO2 C2H7N C4H6 N4O

1.08 0.54 1.16

C7H12O

C6H6N4O4 C5H8N2O C5H8N2O C4H4N2O2

26.48

0.11 0.28 0.07 0.03

250 kPa of O2 at 55 °C, 341h S/N

product

molecular formula

area %

Major Products 1 2

12-crown-4 2-(2-ethoxyethoxy)ethanol

C8H16O4 C6H14O3

2.10 1.82

3 4

Minor Products 15-crown-5 1,4,7,10,13,16-hexaoxacyclooctadecane

C10H20O5 C12H24O6

0.18 0.05

Table 4. Summary of the Oxidative Degradation Products Obtained with 7 mol/L MEA at 120 °C 250 kPa of O2 at 135 h

350 kPa of O2 at 164 h

S/N

product

molecular formula

area %

1

3,3-(1,2-ethanediyl)bis(sydnone)

C6H6N4O4

0.88

2

methylpyrazine

C5H6N2

0.07

molecular formula

area %

Major Products 1 6-hydroxy-4-(1H)-pyrimidinone 2 1H-imidazole-4-carboxylic acid methyl ester 3 1H-imidazole 4 1,3,5-triazine

C4H4N2O2 C5H6N2O2 C3H4N2 C3H3N3

0.59 0.62 0.71 13.42

Minor Products 5 methylpyrazine 6 2-propanone oxime 7 ethylamine 8 N-formyl-N-methylformamide 9 1-amino-2-propanol 10 2,4-dihydro-4,5(3H)-pyrazol-3-one 11 5-(hydrazinocarbonyl)imidazole

C5H6N2 C3H7NO C2H7N C3H5NO2 C3H9NO C5H8N2O C4H6N4O

0.21 0.14 0.21 0.28 0.03 0.08 0.21

S/N

analyzed to be ammonia, water, and amides. This observation of these three products is also consistent with our results. The observation of amides in our studies is also supported by the mechanism proposed by Rochelle and Chi, as shown in Figure 3,21 as well as the results of Polderman et al.,9 Rooney et al.,12 and Jefferson Chemical Co.23 In addition, most of the major degradation products reported by Supap et al.7,14 for the reaction of MEA with O2 were also observed in our work except 4-amino-2(1H)-pyridinone, 5-ethyluracil, 1-propylamine, and 1-piperidinecarboxaldehyde. This could be attributed to high-temperature experiments (i.e., 140 and 160 °C) performed by Supap et al.,7,14 whereas the temperature used in this study was limited to 120 °C. This is an illustration of the impact of the temperature on MEA degradation. It is to be noted that the residence time of the absorbent in one cycle through the absorber and stripper is much shorter than the experiment time used in this study. However, we carried out the degradation experiments for long periods of time. The reason was to evaluate the long-term effect of prolonged use of MEA. This is based on the fact that the absorption process goes through a large number of cycles, which could add

product

up to about the same time that we used for our experiments or even longer. In addition, each cycle introduces some degradation products, thereby giving rise to further degradation of MEA. The effect of time on the extent of degradation of MEA for a run conducted with 5 M MEA and 250 kPa of O2 pressure at 120 °C is shown in Table 1B. 3.1.1. Effect of the O2 Pressure. The effect of the O2 pressure was evaluated in experimental runs with 5 mol/L MEA at 120 °C by comparing the results obtained using 250 kPa of O2 pressure with those using 350 kPa of O2 pressure. The high O2 pressure used in this study was done in order to obtain accelerated oxidative degradation results even though much lower O2 pressures are applicable in actual CO2 absorption processes. The results are given in Table 2, which shows that common products such as 3-methylpyridine, 2-(methylamino)ethanol, formic acid, 4-methylmorpholine, 1H-imidazole, N-(2-hydroxyethyl)acetamide, 4-(hydrazinocarbonyl)imidazole, 5-(hydrazinocarbonyl)imidazole, and N-(2-hydroxyethyl)succinimide were obtained for both O2 pressures. Proposed pathways for the formation of these products are given in eqs 5-12 (Scheme 2). These pathways were proposed on the basis that they

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 951 Scheme 2

accounted for all of the products and were consistent with the operating conditions. It should be noted that all species with the symbol *, such as C2H6NO2* formed in eq 8, are considered as active intermediates that are capable of reacting further to give stable products, such as those in eq 9. (C2H6NO2* in eq 10 is derived from eq 9, whereas CH3NH2* in eq 11 is derived from eq 6.) The products that were exclusive to 250 kPa of O2 pressure included 1-propanamine, 3-cis-5-heptadien-1-

ol, 2-propanone oxime, ethylamine, 3-methylcyclopentanol, trimethylhydrazine, 1-methyl-2,4-imidazolidinedione, 15-crown-5, 1-(2-hydroxyethyl)-2-imidazolidinone, and 2-pyrrolidinone. The proposed pathways for the formation of these products are given in eqs 16-24 (Scheme 3). The products that were obtained exclusively at 350 kPa of O2 pressure included acetamide, nitrosomethane, 1-methylazetidine, 2-methyl-1H-imidazole, 2-methylpropanenitrile, ethoxyethene, 2-(2-aminoethoxy)ethanol,

952


Scheme 3


1,3-dioxane, 3-methyl-4-amino-4,5(1H)-dihydro-1,2,4triazol-5-one, 4-ethylquinoline, 2-quinazolamine, 2,2dimethyl-3(2H)-furanone, 2,6-dioxatricyclo[3.3.2.0(3,7)]dec-9-ene, 2,3-dihydro-1-methyl-4(1H)-pyridinone, 1piperazineethanol, di-2-propanolamine, (dimethylamino)ethylidene-tert-butylamine, 12-crown-4, uracil, Nformyl-N-methylformamide, and acetic acid. It is postulated that the pathways for the formation of these products are as given in eqs 25-43 (Scheme 4). (The CH3NH2* formed in eq 6 could also react further, as shown in eq 40.) The effect of the O2 pressure was also evaluated in experimental runs with 5 mol/L MEA at 100 °C by comparing the results obtained using 250 kPa of O2 pressure with those using 350 kPa of O2 pressure. The results are shown in Table 3, which shows that the common products for both O2 pressures included ethylamine, 1H-imidazole-4-carboxylic acid methyl ester, and 3-cis-5-heptadien-1-ol. Apart from ethylamine, whose pathway for formation was already shown in eqs 16 and 17, the mechanism of formation of the common products is shown in eqs 44-46 (Scheme 5). In 350 kPa of O2 pressure, C5H7NO2 is present as 1,3,4-dehydroproline, which is an isomer of 3,4-dehydro-DL-proline found in the 250 kPa of O2 pressure system; hence, the proposed pathway in eq 44 is representative of both O2 pressures. In addition, in the 350 kPa of O2 pressure system, 3-cis5-heptadien-1-ol can be formed as shown in eq 45, while in the 250 kPa of O2 pressure system, it can be formed as shown in eq 46. The products exclusive to 250 kPa of O2 pressure included hexanal, 3,4-dehydro-DL-proline, N-formyl-N-methylformamide, 3-methyl-1,2-cyclopentanedione, 1,2,3,6-tetrahydro-1-nitrosopyridine, 4-ethylcyclohexene, uracil, 1,2-ethanediol, nitrosomethane, and 2,2-dimethyl-3(2H)-furanone. Apart from nitrosomethane, whose formation pathway was shown in eq 39, and that of uracil, shown in eq 42, the pathways for the formation of the other products are shown in eqs 47-54 (Scheme 6). CH3NO* formed in eq 45 can react to form 3-methyl-1,2-cyclopentanedione. The products that were formed exclusively with 350 kPa of O2 pressure are 6-hydroxyl-4(1H)-pyrimidinone, 2,4-dihydro-4,5(3H)-pyrazol-3-one, 1,3,4-dehydroproline, 1-methyl-1H-imidazole-2-methanol, and 3,3-(1,2-ethanediyl)bis(sydnone). These products can be formed based on the proposed pathways shown in eqs 54-67 (Scheme 7). 1,3,4-Dehydroproline is an isomer of 3,4-dehydroproline, whose formation was shown in eqs 51 and 52.As such, eqs 51 and 52 could be used to represent the formation of 1,3,4-dehydroproline. In addition, 1,3,4dehydroproline could also be formed as shown in eq 61 (where C3H8O* is derived from eq 65) and also as shown in eq 62. The effect of the O2 pressure was also

evaluated in experimental runs with 7 mol/L MEA at 120 °C by comparing the results obtained using 250 kPa of O2 pressure with those using 350 kPa of O2 pressure. The results are shown in Table 4, which shows that methylpyrazine was the only common product for both O2 pressures. The possible pathway for the formation of methylpyrazine is shown in eq 69. The product formed exclusively with 250 kPa of O2 pressure is 3,3-(1,2-ethanediyl)bis(sydnone), whose pathway for formation was earlier shown in eq 56. The products exclusive to 350 kPa of O2 pressure included 1H-imidazole, N-formyl-N-methylformamide, 1-amino-2-propanol, 5-(hydrazinocarbonyl)imidazole, 1Himidazole-4-carboxylic acid methyl ester, 1,3,5-triazine, 6-hydroxy-4(1H)-pyrimidinone, and 2,4-dihydro-4,5(3H)pyrazol-3-one. The proposed pathways for their formation are as given in eqs 70-91 (Scheme 8). CH3NH2* in eq 73 is obtained from eq 65. It should also be noted that the CH3NH2* formed in eq 60 can oxidize to form CH3NO* as shown in eq 80, which can then react to form N-formyl-N-methylformamide, as was already shown in eq 50. So far, the proposed pathways for the formation of degradation products show that degradation occurred as a result of four possible types of reactions, namely, reactions between products (e.g., eq 18), reactions between products and MEA (e.g., eq 21), reactions between MEA and O2 (e.g., eq 16), and reactions between products and O2 (e.g., eq 29). With 5 mol/L MEA at 120 °C, it is evident that some of the products exclusive to 350 kPa of O2 pressure were obtained as a result of further oxidation of products that were equally formed at 250 kPa of O2 pressure (eqs 28, 29, 37, 38, and 40). In contrast, most of the products formed at 250 kPa of O2 pressure were obtained as a result of the reaction of some of the products with each other because of the higher contents of products plus MEA relative to the O2 content in the 250 kPa of O2 pressure system. Thus, at 250 kPa of O2 pressure, O2 acted as the limiting reactant. A similar observation was made with 5 mol/L MEA at 100 °C and 7 mol/L MEA at 120 °C. It is also important to note that with 5 mol/L MEA at 120 °C it was observed that, after 135 h, 19 products were obtained at 250 kPa of O2 pressure, whereas at 350 kPa of O2 pressure, 31 products were obtained. In addition, with 7 mol/L MEA at 120 °C, 2 products were obtained at 250 kPa of O2 pressure, whereas at 350 kPa of O2 pressure, 11 products were obtained. Because a higher O2 pressure implies a higher O2 content in the liquid phase,20 the results imply that the higher O2 concentration resulted in an increase in the number of degradation products produced. Except in the case of 5

954


Scheme 4


956


Scheme 5

mol/L MEA at 100 °C, in which 12 products were obtained at 250 kPa of O2 pressure while 8 products were obtained with 350 kPa of O2 pressure. 3.1.2. Effect of the Temperature. The effect of the temperature was directly verified using experimental runs with 5 mol/L MEA and O2 pressures of 250 and 350 kPa at 55, 100, and 120 °C. The results are shown in Tables 2 and 3. These tables confirm that the number of degradation products formed increases with temperature. The tables also show that, at an O2 pressure of 250 kPa, there are only two common products (ethylamine and 3-cis-5-heptadien-1-ol) for the temperatures of 100 and 120 °C, whereas there was one common product (15-crown-5) between 55 and 120 °C and none between 55 and 100 °C. At 350 kPa of O2 pressure, there are no common products between the products formed at 100 and 120 °C, implying that the mechanism of the degradation process changes with temperature. This is further confirmed by the fact that, even though 3-cis5-heptadien-1-ol is obtained at both temperatures (100 and 120 °C) at an O2 pressure of 250 kPa, the pathways for its formation are not the same at both temperatures, as illustrated in eqs 45, 46, and 66. The products formed at 55 °C are 12-crown-4, 1,4,7,10,13,16-hexaoxacyclooctadecane, and 2-(2-ethoxyethoxy)ethanol. The pathways of the formation of these products are shown in eqs 8386 (Scheme 9). (The C4H8O2* formed in eq 85 then reacts with MEA, as shown in eq 86.) 3.1.3. Effect of the MEA Concentration. The effect of the MEA concentration was evaluated at 250 and 350 kPa of O2 pressure at 120 °C by comparing the results of 5 mol/L MEA with those of 7 mol/L MEA. The results are given in Tables 2 and 4, which show that 31 products were obtained at 350 kPa of O2 pressure with 5 mol/L MEA as compared with only 11 products obtained with 7 mol/L MEA, even though the latter experimental run was allowed to go on for 29 h longer. In addition, the common products at both concentrations were 1H-imidazole and 5-(hydrazinocarbonyl)imidazole. The proposed pathways for the formation of these products with 5 mol/L MEA are as already given in eqs 5, 8, and 38, respectively, whereas with 7 mol/L MEA, the proposed pathways for the formation of these products are as given previously in eqs 78 and/or 79, 75, and 73, respectively.

The tables also show that, with 250 kPa of O2 pressure, there were common products between both concentrations. Tables 2 and 4 further show that, for 250 kPa of O2 pressure at 120 °C, 19 products were obtained with 5 mol/L MEA as compared with only 2 products with 7 mol/L MEA. The lower number of degradation products at higher MEA concentration could be explained on the basis of lower O2 solubility at the higher MEA concentrations. Consequently, at the higher concentration (7 mol/L MEA), a larger number of MEA molecules are competing for a relatively smaller quantity of O2 present in the system as opposed to that for 5 mol/L MEA. This is further confirmed by the fact that at both temperatures, even though common products were observed, most of the products formed with 5 mol/L MEA involved an oxidation of MEA or other products (e.g., eqs 5, 8, 13, 16, 17, 20, and 37), whereas most of those formed with 7 mol/L MEA involved reactions between products other than with O2 (e.g., eqs 78, 79, 84, and 85). It thus appears that a higher MEA concentration could be used to reduce the number of degradation products produced. By comparison of the effects of MEA and O2 concentrations, our results show that the effect of the MEA concentration is therefore completely opposite to that of O2 pressure. 3.2. MEA-H2O-CO2 System. 3.2.1. Effect of the Concentration. The effect of the MEA concentration was evaluated by comparing the results of 5 mol/L MEA with those of 7 mol/L MEA for 250 kPa of CO2 pressure at 120 °C. The results are given in Table 5, which shows that the common products at both MEA concentrations are 12-crown-4 and 1,4,7,10,13,16-hexaoxacyclooctadecane. The pathway for the formation of 12-crown-4 is the same as that shown earlier in eq 25, while that of 1,4,7,10,13,16-hexaoxacyclooctadecane is represented in eq 88 (Scheme 10) for 5 mol/L MEA and as previously shown in eq 84 for 7 mol/L MEA. The products formed exclusively with 7 mol/L MEA were 15-crown-5, 2-propanone oxime, and 2-(2-ethoxyethoxy)ethanol. The proposed pathway for the formation of 2-(2-ethoxyethoxy)ethanol was earlier shown in eq 86. The proposed pathways for the formation of the remaining products are shown in eqs 89-92 (Scheme 11). It is seen that eqs 89 and 90 illustrate two probable pathways for the formation of 2-propanone oxime. Equation


89 simply involves a reaction between MEA and CO2 to form a carbamate, followed by protonation of the carbamate,2,9 whereas eq 90 involves the hydrolysis of aminocyanoacetic acid, a reactive intermediate product that disappeared after 45 min of the experimental run. The product formed exclusively with 5 mol/L MEA is 2-butanamine. The proposed pathway for the formation of this product is illustrated in eq 93 (Scheme 12). In some of the proposed pathways (eqs 75, 89, and 90), it is seen that, even though O2 was not fed into the reactor, it could be produced as a byproduct during the MEA degradation process. This is further confirmed by the observation that, although Polderman et al.10 studied CO2-induced degradation, one of the degradation products obtained by them [i.e., (1,2-dihydroxyethyl)-2imidazolidone] was obtained in the O2 system (see eqs 3 and 21). In addition, Table 5 shows that, with 5 mol/L MEA, three products were observed as compared with five products for 7 mol/L MEA. It thus appears that a

higher MEA concentration cannot be used to reduce the total number of degradation products produced, unlike the case for the O2 system. 3.2.2. Comparison of the MEA-H2O-CO2 System with the Corresponding MEA-H2O-O2 System. A comparison between the MEA-H2O-CO2 system and the corresponding MEA-H2O-O2 system was made with 5 and 7 mol/L MEA at 120 °C using 250 kPa of pressure of either O2 or CO2. Table 4 shows that when 7 mol/L MEA was used for the MEA-H2O-O2 system, two products were obtained versus the five products that were obtained in the MEA-H2O-CO2 system (Table 5). It appears that the shorter degradation time used for the CO2 system for the case of 7 mol/L MEA is responsible for the smaller number of degradation products as compared with the case of the O2 system. In addition, Table 2 shows that when 5 mol/L MEA was used for the MEA-H2O-O2 system, 19 products were obtained versus the 3 products that were obtained in the MEA-H2O-CO2 system. This latter observation is

958


Scheme 7


very significant because the MEA-H2O-O2 system ran for only 135 h, whereas the MEA-H2O-CO2 system ran for 490 h. It is thus consistent with the literature, which shows that MEA is more prone to degradation in the presence of O2 as compared to the presence of CO2. 3.3. MEA-H2O-O2-CO2 System. Typically, it is not expected that O2 will be in the rich CO2-loaded amine system under stripper conditions (greater than 100 °C). However, the mechanism proposed for the MEA-H2OCO2 system showed that there was the possibility of having O2 in this system even at the higher temperature normally used in the stripping column because of the occurrence of some reactions (eqs 25, 89, and 90). It is therefore essential to know what would be the prolonged effect of the presence of O2 in the MEA-H2O-CO2 system under stripper conditions. High O2 pressures (250 and 350 kPa) at 100 and 120 °C are used in order to obtain accelerated results, which will also provide some indication of its effect in this system at a lower temperature in the absorbing column (O2 is one of the components of flue gas). The basis of this is that if there is some possibility of causing degradation at higher temperatures (stripper temperatures), there is the likelihood that degradation will also occur at a lower temperature, though at a much lower rate. Test runs were conducted for this system using 5 and 7 mol/L aqueous MEA solutions. 3.3.1. Effect of the Temperature. The effect of the temperature on the product distribution of this system was evaluated with 5 mol/L MEA for 350 kPa of O2 pressure at 100 and 120 °C (Table 6) and with 7 mol/L MEA for 250 kPa of O2 pressure at 100 and 120 °C (Table 7). The tables show that, with 5 mol/L MEA and an O2 pressure of 350 kPa, five products were obtained at 100 °C, while at 120 °C, two products were obtained. In addition, it is seen that, for 5 mol/L MEA and an O2 pressure of 350 kPa, there is no common product between the products formed at 100 and 120 °C. This suggests that the mechanism of the reaction changes with the temperature just as in the case of the MEA-H2O-O2 system. The results show that the products observed exclusively at 120 °C for 5 mol/L MEA

with 350 kPa of O2 pressure are ethylenediamine and 3,3-(1,2-ethanediyl)bis(sydnone). The pathways for the formation of 3,3-(1,2-ethanediyl)bis(sydnone) and ethylenediamine are shown in eqs 94 and 95, respectively. The results also show that the products observed exclusively at 100 °C are 1,2-ethanediol, 1,3,5-triazine, N-butylformamide, 1,4,7,10,13,16-hexaoxacyclooctadecane, and 1,2,3,6-tetrahydro-1-nitropyridine. The pathway for the formation of 1,4,7,10,13,16-hexaoxacyclooctadecane was shown earlier in eq 84, which shows that it was formed from 15-crown-5, an intermediate product that was last observed at 16 h. On the other hand, the pathways for the formation of the other products are as given in eqs 96-100 (Scheme 13). [The carbamate (HOCHOHCH2NHCO2-) in eq 104 is obtained from eq 103.] In addition, 7 mol/L MEA was exposed to 250 kPa of O2 pressure at both 100 and 120 °C (Table 7). The results show that, at 100 °C, only two products were observed even though the experiment was allowed to go on for 254 h, whereas five products were observed at 120 °C, even though the experiment was allowed to run for only 198 h. There was only one product common at both temperatures, which was ethylamine. The pathway for the formation of this product was given previously in eq 93. The products that were formed exclusively at 120 °C were methylpyrazine (C5H6N2), 7-oxabicyclo[2.2.1]hept5-en-2-one (C6H6O2), 1-propanamine (C3H9N), ethylamine, 1,3,5-triazine, and 3,3-(1,2-ethanediyl)bis(syndone). The pathways for the formation of ethylamine and 1,3,5-triazine are as given in eqs 93 and 97,

960


Scheme 8


respectively, whereas the pathways for the formation of the remaining products are as given in eqs 101-104 (Scheme 14), where CH3CH2OH* is obtained from eq 99. 3.3.2. Effect of the O2 Pressure. The effect of the O2 pressure was evaluated with 7 mol/L MEA (CO2 loading of 0.27 mol of CO2/mol of MEA) and 7 mol/L MEA (CO2 loading of 0.33 mol of CO2/mol of MEA) at 120 °C by comparing the results obtained with 250 kPa of O2 pressure with those obtained with 350 kPa of O2 pressure. The results are given in Table 7, which shows Scheme 9

that 5 products were obtained at 250 kPa of O2 pressure whereas, at 350 kPa of O2 pressure, 12 products were obtained. This suggests that the higher the O2 pressure, the higher the number of degradation products obtained, just as was the case in the MEA-H2O-O2 system. The table also shows that there is only one common product for both O2 pressures, which is ethylamine. The pathway for the formation of this product was already given in eq 93. The products that were observed exclusively for 250 kPa of O2 pressure are methylpyrazine, 7-oxabicyclo[2.2.1]hept-5-en-2-one, 1-propanamine, and 1,3,5-triazine. It is important to note that all of these products were earlier reported in the MEA-H2O-O2 system. Also provided were pathways for their formation such as 1-propanamine (eq 15), 1,3,5-triazine (eq 97), 7-oxabicyclo[2.2.1]hept-5-en-2-one (eq 101), and methylpyrazine (eq 104). The products that were formed exclusively at 350 kPa of O2 pressure included 2-propanone oxime, 2-ethenyl1H-imidazole, [(aminocarbonyl)amino]acetic acid, 3amino-1-propanone, nitrosomethane, 2-(2-aminoethoxy)ethanol,N-butylformamide,3-cis-5-heptadien-1-ol,ethoxyethylamine, 2-amino-1-propanol, and N-diethylurea. The pathways for the formation of these products are shown in eqs 105-114 [Scheme 15). [The carbamate (HOCHOHCH2NHCO2-) in eq 111 is obtained from eq 98.] The pathways for the formation of degradation products obtained with 7 mol/L MEA (CO2 loading of 0.27

962


Scheme 10

Scheme 11

mol of CO2/mol of MEA) and 7 mol/L MEA (CO2 loading of 0.33 mol of CO2/mol of MEA) at 120 °C indicate that the common products to both systems underwent further oxidation (eqs 105 and 111) at 350 kPa of O2 pressure to form products exclusive to the 350 kPa of O2 pressure. This confirms that the abundance of O2 in the system is responsible for the observation of products obtained exclusively at 350 kPa of O2 pressure, just as was the case in the MEA-H2O-O2 system. 3.3.3. Effect of the Concentration. The effect of the MEA concentration was evaluated by comparing the

results of 5 mol/L MEA (CO2 loading of 0.44 mol of CO2/mol of MEA) with that of 7 mol/L MEA (CO2 loading of 0.33 mol of CO2/mol of MEA) for 350 kPa of O2 pressure at 120 °C. The resulting products are given in Tables 6 and 7, which show that, for 5 mol/L MEA (CO2 loading of 0.44 mol of CO2/mol of MEA) at 350 kPa O2 pressure, 2 products were obtained after 159 h, whereas for 7 mol/L MEA (CO2 loading of 0.33 mol of CO2/mol of MEA), 12 final products were obtained after 282 h. It is to be mentioned that when the 7 mol/L MEA (CO2 loading of 0.33) was run with 350 kPa of O2


Table 5. Summary of the CO2-Induced Degradation Products of MEA at 120 °C 7 mol/L, 250 kPa of CO2

5 mol/L MEA 250 kPa of CO2 molecular formula

area %

S/N

product

S/N

product

1 2 3

12-crown-4 1,4,7,10,13,16-hexaoxacyclooctadecane 2-(2-ethoxyethoxy)ethanol

Major Products, 490 h C8H16O4 1.55 1. 12-crown-4 0.52 C12H24O6 C6H14O3 0.61

4 5

2-propanone oxime 15-crown-5

C3H7NO C10H20O5

Minor Products 0.14 2. 1,4,7,10,13,16-hexaoxacyclooctadecane 0.15 3. 2-butanamine

molecular formula

area %

C8H16O4

1.36

C12H24O6 C3H7NO

0.03 0.03

Table 6. Summary of the Oxidative Degradation Products of CO2-Loaded 5 mol/L MEA at 350 KPa O2 Pressure 0.41 mol of CO2 /mol of MEA at 100 °C, 159 h S/N

product

0.44 mol of CO2/mol of MEA at 120 °C, 165 h

molecular formula

area %

S/N

1 2 3 4

1,2-ethanediol N-butylformamide 1,2,3,6-tetrahydro-1-nitrosopyridine 1,3,5-triazine

Major Products C2H6O2 1.14 C5H11NO 0.82 C5H8N2O 1.02 C3H3N3 3.33

5

1,4,7,10,13,16-hexaoxacyclooctadecane

C12H24O6

Minor Products 0.20 1 2

pressure after 169 h, two products were observed. This implies that the number of degradation products increased with the concentration, unlike the case for the MEA-H2O-O2 system. It appears that the shorter time and higher CO2 loading in the case of 5 mol/L MEA is responsible for the smaller number of degradation products, as compared with that of 7 mol/L MEA. The common products between these runs included 2-propanone oxime and ethylamine. The pathways for the formation of these products have already been given in eqs 89 and 93, respectively. The product formed exclusively with 5 mol/L MEA was 3,3-(1,2-ethanediyl)bis(sydnone). The pathway for the formation this product is the same as that shown earlier in eq 103. The products formed exclusively with 7 mol/L MEA include N-butylformamide, ethoxyethylamine, 2-(2-aminoethoxy)ethanol, 1-amino-2-propanol, 3-amino-1-propanol, 2-ethenyl-1H-imidazole, N,N-diethylurea, 3-cis5-heptadien-1-ol, [(aminocarbonyl)amino]acetic acid, and nitrosomethane. The pathways for the formation these products are also the same as those shown earlier in eqs 105-111, respectively.

product

molecular formula

area %

ethylenediamine 3,3-(1,2-ethanediyl)bis(sydnone)

C2H8N2 C6H6N4O4

0.24 0.30

3.3.4. Effect of the Presence of CO2. The effect of the presence of CO2 was evaluated by comparing the results of 5 mol/L MEA at 100 °C and 350 kPa of O2 pressure with that of 5 mol/L MEA (CO2 loading of 0.41 mol of CO2/mol of MEA) also at 100 °C and 350 kPa of O2 pressure. The results are shown in Table 3 for the MEA-H2O-O2 system and in Table 6 for

964


Scheme 13

Scheme 14

the MEA-H2O-O2-CO2 system. The results show that, in the presence of CO2, five products were formed, while in its absence, eight products were formed. There was only one common product, 1,3,5-triazine. The pathways for the formation of this product were already given in eq 64 for the MEA-H2O-O2 system and in eq 96 for the MEA-H2O-O2-CO2 system. The effect of the presence of CO2 was also evaluated by comparing the results of 5 mol/L MEA at 120 °C and 350 kPa of O2 pressure with that of 5 mol/L MEA (CO2 loading of 0.44 mol of CO2/mol of MEA) also at 120 °C and 350 kPa of O2 pressure. The results are given in Table 2 for the MEA-H2O-O2 system and in Table 6 for the MEA-H2O-O2-CO2 system. The results show that, in the presence of CO2, 2 products were formed, while 31 products were formed in the absence of CO2.

In addition, there was no common product between the two systems. The implication is that the presence of CO2 alters the mechanism for the oxidative degradation of the MEA-H2O system. The result is also consistent with that of Rooney et al.,12 who concluded that the addition of CO2 lowers the O2 solubility.12,20 3.3.5. Comparison of the MEA-H2O-CO2 System with the Corresponding MEA-H2O-O2-CO2 System. A comparison of the MEA-H2O-O2-CO2 system with the corresponding MEA-H2O-CO2 system was made in order to obtain an understanding of the effect of the presence or absence of O2 in a CO2-loaded system. This was done by using CO2-loaded 7 mol/L MEA at 120 °C with and without 250 kPa of O2 pressure. An O2 pressure of 250 kPa was used in the


former case. The results are shown in Tables 5 and 7, which show that, for the MEA-H2O-O2-CO2 system (i.e., 7 mol/L MEA with CO2 loading of 0.27 mol of CO2/ mol of MEA and 250 kPa of O2 pressure at 120 °C), five products were formed, just as in the MEA-H2O-CO2 system despite the fact that the MEA-H2O-O2-CO2 system was allowed to run for only 198 h whereas the

latter system (MEA-H2O-CO2 system) was allowed to run for 490 h. This is an indication that if the two systems were allowed to run for the same length of time, the number of degradation products formed due to oxidative degradation of CO2-loaded MEA would be higher than the number of degradation products obtained from a purely CO2-induced degradation.

966


Table 7. Summary of the Oxidative Degradation Products Obtained with CO2-Loaded 7 mol/L MEA 0.27 mol of CO2/mol of MEA, 250 kPa of O2 at 120 °C and 198 h

0.33 mol of CO2/mol of MEA, 350 kPa of O2 at 120 °C and 282 h

S/N

product

molecular formula

1 2 3 4

7-oxabicyclo[2.2.1]hept-5-en-2-one 1-propaneamine 1,3,5-triazine ethylamine

C6H6O2 C3H9N C3H3N3 C2H7N

5

methylpyrazine

C5H6N2

area %

S/N

product

Major Products 0.47 1 3-cis-5-heptadien-1-ol 0.85 2 nitrosomethane 14.49 0.52 Minor Products 0.04 3 4 5 6 7 8 9 10 11 12.

2-ethenyl-1H-imidazole N-butylformamide ethylamine N,N-diethylurea [(aminocarbonyl) amino]acetic acid 3-amino-1-propanol 2-(2-aminoethoxy)ethanol ethoxyethylamine 1-amino-2-propanol 2-propane oxime

molecular formula

area %

C7H12O CH3NO

1.57 0.49

C5H6N2 C5H11NO C2H7N C5H12N2O C3H4N2O4 C3H9NO C4H11NO2 C4H11NO C3H9NO C3H7NO

0.17 0.11 0.16 0.05 0.05 0.05 0.08 0.06 0.02 0.12

0.40 mol of CO2/mol of MEA, 250 kPa O2 at 100 °C S/N

product

1

12-crown-4

molecular formula

area %

C8H16O4

1.32

Major Products, 254 h Table 8. Extent of Degradation for All Systems Investigated initial MEA final MEA concn concn area concn area concn % (M) % (M)

experimental run operating conditions O2 temp pressure % time (°C) (kPa) convn (h)

MEA-H2O-O2 System 3.1 120 1.0 120 4.6 100 4.0 100 4.8 100 3.6 100 6.2 120 5.8 120 5.0 55

99.41 99.29 99.87 99.87 99.51 99.51 99.74 99.74 92.57

5.03 5.03 5.03 5.03 5.03 5.03 6.97 6.97 5.03

50.58 19.08 91.15 79.71 95.91 70.26 89.35 83.09 91.26

250 350 250 250 350 350 350 350 250

49.1 80.8 8.7 20.2 3.6 29.4 10.4 16.7 1.4

135 135 357 530 324 732 135 164 341

97.67 99.08

5.03 7.03

MEA-H2O-CO2 System 95.42 4.9 120 250 97.02 6.9 120 250

2.3 2.1

490 490

99.42 99.33 99.76 99.76 97.57

5.03 5.03 7.03 7.03 7.03

MEA-H2O-CO2-O2 System 96.48 4.881 255 281 100 350 99.71 5.049 242 928 120 350 88.52 6.237 927 025 120 250 83.81 5.906 017 442 120 250 96.31 6.939 215 948 100 250

3.0 ≈0.0 11.3 16.0 1.3

159 165 128 198 230

3.4. Extents of Degradation. The extent of degradation for each set of conditions was evaluated as shown in eq 116,

% extent of degradation )

A0 - AT × 100% (116) A0

where A0 ) initial MEA concentration ) initial GC MEA peak area % × concentration of MEA loaded and AT ) degraded MEA concentration ) GC peak area % of degraded MEA × concentration of MEA loaded. The evaluation was done for the MEA-H2O-O2, MEAH2O-CO2, and MEA-H2O-O2-CO2 systems, as a function of the O2 pressure, MEA concentration, and temperature. The results obtained for all of the systems are given in Table 8. 3.4.1. MEA-H2O-O2 System. 3.4.1.1. Effect of the O2 Pressure. The effect of the O2 pressure on the extent of degradation was investigated with 5 mol/L MEA at 120 °C using both 250 and 350 kPa of O2 pressure. The extent of degradation was calculated after 135 h of experimental run time. At 250 kPa of O2 pressure, the extent of MEA degradation was 49.1%, while at 350 kPa of O2 pressure, the extent of MEA degradation was 80.8%, showing that a higher O2 pressure results in a higher extent of MEA degradation. Considering that a higher O2 pressure means a higher O2 solubility12 and hence a higher O2 content in the liquid phase, it is imperative to limit the O2 content in order to minimize MEA degradation. 3.4.1.2. Effect of the Temperature. The effect of the temperature on the extent of MEA degradation was investigated with 5 mol/L MEA using both 250 and 350 kPa of O2 pressure at 55, 100, and 120 °C. In the case of 250 kPa of O2 pressure, MEA degradation at 55 °C after 341 h was 1.4%. On the other hand, at 100 °C, the extent of degradation after 357 h was 8.7%, whereas it was 49.1% after only 135 h. In the case of 350 kPa of O2 pressure, MEA degradation at 100 °C was 29.4% after 732 h, while at 120 °C, it was 80.8% after only 135 h. Hence, the overall results show that not only does a higher temperature lead to the formation of a higher number of degradation products, but it also leads to a larger extent of degradation. 3.4.1.3. Effect of the Concentration. The effect of the MEA concentration on the extent of MEA degradation was investigated at 250 kPa of O2 pressure and 120 °C with both 5 and 7 mol/L MEA with the extent of


degradation calculated after 135 h. It was observed (Table 8) that with 5 mol/L MEA the extent of MEA degradation was 49.1%, while with 7 mol/L MEA, it was 10.4%. These results show that not only does a higher MEA concentration lead to fewer degradation products, but it also leads to a lower extent of MEA degradation. 3.4.2. MEA-H2O-CO2 System. 3.4.2.1. Effect of the Concentration. The effect of the MEA concentration on the extent of degradation was investigated at 250 kPa of CO2 pressure and 120 °C with both 5 and 7 mol/L MEA, with the extent of degradation calculated after 490 h. It was observed (Table 8) that with 5 mol/L MEA the extent of degradation was 2.3%, while with 7 mol/L MEA, it reduced marginally to 2.1%. Hence, even though the difference is marginal, the results indicate that a higher MEA concentration leads to the formation of fewer degradation products; it also leads to a lower extent of MEA degradation, just like the case for the O2 system even though the difference in the extent of degradation for the CO2 system was not as high as that for the O2 system. 3.4.3. MEA-H2O-O2-CO2 System. 3.4.3.1. Effect of the Temperature. The effect of the temperature on the extent of MEA degradation was investigated with CO2-loaded 7 mol/L MEA (0.27 mol of CO2/mol of MEA) at both 100 and 120 °C with 250 kPa of O2 pressure. The results (Table 8) show that, at 120 °C, the extent of degradation was 16.0%, while at 100 °C, it was just 1.3% even after 230 h. Hence, the results show once again that not only does a higher temperature lead to the formation of a higher number of degradation products, but it also leads to a higher extent of degradation. 3.4.3.2. Effect of the Concentration. The effect of the MEA concentration on the extent of degradation was investigated at 250 kPa of O2 pressure and 120 °C with 5 mol/L (0.41 mol of CO2/mol of MEA) and 7 mol/L MEA (0.40 mol of CO2/mol of MEA). It was observed (Table 8) that, with 5 mol/L MEA, the extent of MEA degradation was 3.0% after 159 h, while with 7 mol/L MEA, the extent of degradation was 1.3% after 230 h. The results show that a higher MEA concentration leads to both the formation of a lower number of degradation products as well as a smaller extent of MEA degradation. 3.4.3.3. Effect of the O2 Pressure. The effect of the O2 pressure on the extent of degradation was investigated at 120 °C with CO2-loaded 7 mol/L MEA (0.27 mol of CO2/mol of MEA) at 250 kPa of O2 pressure and CO2loaded 7 mol/L MEA (0.33 mol of CO2/mol of MEA) at 350 kPa of O2 pressure. At 250 kPa of O2 pressure, the extent of MEA degradation was 2.4% after 282 h, while at 350 kPa of O2 pressure, it was 0.18% after 169 h. Though in the O2 system a higher O2 pressure results in a higher extent of MEA degradation, the higher CO2 loading at the higher O2 pressure and the shorter run time for the CO2-O2 system could have caused the anomalous result. 3.4.4. Overall Comparison of the Extents of Degradation of the Systems. A comparative extent of degradation was made for the three systems. First, the MEA-H2O-O2 system was compared with the MEA-H2O-O2-CO2 (0.33 mol of CO2/mol of MEA) system using 7 mol/L MEA with 350 kPa of O2 pressure at 120 °C (Table 8). The result showed that the extent of degradation of the MEA-H2O-O2 system (10.4%) was greater than that of the MEA-H2O-O2-CO2 (0.33 mol of CO2/mol of MEA) system (2.4%). Second, the MEA-H2O-O2-CO2 (0.27 mol of CO2/mol of MEA; 250

kPa of O2 pressure) system was compared with the MEA-H2O-CO2 (250 kPa of CO2 pressure) system using 7 mol/L MEA at 120 °C. The results (Table 8) showed that the extent of degradation of the MEAH2O-O2-CO2 (0.27 mol of CO2/mol of MEA; 250 kPa of O2 pressure) system (11.3%) was greater than that of the MEA-H2O-CO2 (250 kPa CO2 pressure) system (2.1%). Thus, it can be concluded that the extent of degradation for the three systems decreased in the order MEA-H2O-O2 > MEA-H2O-O2-CO2 > MEA-H2OCO2. 3.5. Oxidative Degradation Product Hazard Information. Some of the MEA degradation products may be environmentally hazardous, in which case they would have to be disposed of in environmentally acceptable ways.27 However, no work has been reported on the nature of the potential of producing hazardous products from the degradation process. This work investigates these products. 3.5.1. MEA-H2O-O2 System. Approximate estimates of hazardous products from a typical run (5 mol/L MEA, 120 °C, 250 kPa of O2 pressure, 135 h) were made based on the total GC peak area percent of the hazardous products formed. This was found to be 36.51%. In addition, information was obtained from material safety data sheets (MSDSs) on the oxidative degradation products. About 24 out of the 41 degradation products obtained with 5 mol/L MEA at 120 °C were found to be hazardous from their MSDSs, while at 100 °C, 9 out of the 18 products obtained were hazardous. With 7 mol/L MEA at 120 °C, 15 out of the 19 degradation products obtained were hazardous. Examples of these hazardous products are polymeric substances (e.g., 15-crown-5), carboxylic acids (e.g., acetic acid), amides and substituted amides (e.g., acetamide and N-(2-hydroxyethyl)acetamide), amines (e.g., ethylamine), ketones (e.g., 2-pyrrolidinone), substituted alkanols (e.g., 2-(methylamino)ethanol), imidazoles and substituted imidazoles (e.g., 1H-imidazole and 2-methyl-1H-imidazole), aromatics (e.g., 4-ethylquinoline), oxygenated alkanes and cycloalkanes (e.g., 1,3-dioxane and ethoxyethane), and substituted nitriles (e.g., 2-methylpropanenitrile). 3.5.2. MEA-H2O-CO2 and MEA-H2O-O2-CO2 Systems. All of the products obtained except aminocyanoacetic acid, whose MSDS was not found, were found to be hazardous. 3.6. Heat-Stable Amine Salts Formed. Heat-stable amine salts are unregenerable salts that are formed by acids stronger than CO2 and H2S reacting with alkanolamines. These salts are difficult to regenerate thermally to yield the alkanolamine. Consequently, this reduces the amount of active amine available in the system.2 The formation of these salts in the systems studied is discussed below: 3.6.1. MEA-H2O-O2 System. Only one heat-stable salt (1H-imidazole-4-carboxylic acid methyl ester) was formed. It was observed with 5 mol/L MEA with both 250 and 350 kPa of O2 pressure at 100 °C as well as with 7 mol/L MEA at 350 kPa of O2 pressure at 120 °C. No heat-stable salts were observed with 5 mol/L MEA at both 250 and 350 kPa of O2 pressure at 120 °C. This could be explained on the basis that the run in both cases lasted for only 135 h, as opposed to those at 100 °C, which lasted for at least 530 h. It was interesting to note that, in the case with 350 kPa of O2 pressure, the heat-stable salt was not observed until the experiment had proceeded for 324 h, and when 250 kPa of O2

968


pressure was used, the heat-stable salt was not formed until the experiment had proceeded for 530 h. This implies that a higher concentration of O2 leads to a faster formation of heat-stable salts. No heat-stable salts were observed with 7 mol/L MEA at both 250 and 350 kPa of O2 pressure at 120 °C. Again this could be explained on the basis that the experiments were allowed to run for only 135 h. 3.6.2. MEA-H2O-CO2 System. There were no heatstable salts obtained in this system for both 5 and 7 mol/L MEA with 250 kPa of O2 pressure at 120 °C, even though the reaction was allowed to run for 490 h in both cases. 3.6.3. MEA-H2O-O2-CO2 System. No heat-stable salt was observed in this system probably because of the short duration of the experiments. 3.7. Effects of Degradation Products on the Absorption Ability of MEA. We had mentioned earlier that heat-stable salts are unregenerable salts, the presence of which reduces the amount of active MEA available in the system for CO2 absorption.2 For the systems investigated, only one heat-stable salt (1Himidazole-4-carboxylic acid methyl ester) was observed. Its presence would lead to a reduction in the absorption capacity of MEA. In addition, the proposed mechanism of formation shows that some of the degradation products formed reacted further with MEA (as shown in eqs 5, 9, 13, 15, 22, 27, 35, 38, and 48-51) to form other products, hence using up some of the MEA available for CO2 capture. However, it is also important to note that, even though the other products are not heat-stable salts, the various reactions that involve MEA for the formation of these products compete with the absorption of CO2 by MEA. On the other hand, some of the degradation products such as 2-pyrrolidinone and di-2-propanolamine could be used to increase the rate of CO2 absorption in MEA because their corresponding substituted members, N-methyl-2-pyrrolidinone and 2,4-N-di-2-propanolamine, are known to be good activators of MEA for CO2 capture. 4. Conclusion 1. For both the MEA-H2O-O2 and MEA-H2OCO2-O2 systems, higher O2 pressure and temperature not only resulted in a higher number of degradation products but also led to a larger extent of MEA degradation. 2. The mechanism of degradation changes with the temperature for all of the systems. 3. For all of the systems, fewer degradation products were formed at a higher MEA concentration. In the case of both the MEA-H2O-O2 and MEA-H2O-CO2-O2 systems, this was attributed to lower O2 solubility at higher MEA concentration. 4. The proposed pathways show that, in the MEAH2O-CO2 system, O2 is produced as a degradation product, implying that, even if O2 was not initially present in the feed gas stream, an oxidative degradation environment could still be created. 5. The mechanisms for the formation of degradation products for the MEA-H2O-O2, MEA-H2O-O2-CO2, and MEA-H2O-CO2 systems were completely different from each other. 6. The extent of degradation decreased in the order MEA-H2O-O2 > MEA-H2O-O2-CO2 > MEA-H2OCO2.

Acknowledgment The financial support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) is greatly appreciated. Literature Cited (1) Aboudheir, A. Kinetics, modeling and simulation of Carbon dioxide absorption into highly concentrated and loaded MEA solutions. Ph.D. Thesis, Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada, 2002. (2) Kohl, A. L.; Nielson, R. B. Gas Purification; Gulf Publishing: Houston, TX, 1997. (3) Maddox, R. N. Gas Conditioning and Liquid Sweetening; Campbell Petroleum Series: Norman, OK, 1974. (4) Shrikar, C.; Guptha, A.; Humek, B. Advanced technology for the capture of CO2 from flue gases. First National Conference on Carbon Sequestration, Washington, DC, May 15-17, 2001; pp 1-11. (5) Rochelle, G. T.; Chi, S. Oxidative degradation of Aqueous MEA in capture systems under absorber conditions. Ind. Eng. Chem. Res. 2002, 41, 4178-4186. (6) Straizisar, B. R.; Anderson, R. R.; White, C. M. Degradation of MEA used in CO2 capture from flue gas of a coal-fired electric power generating station; Clean Air Technology Division, National Energy Laboratory, U.S. Department of Energy: Pittsburgh, PA, 2003; pp 1-80. (7) Supap, T.; Idem, R.; Veawab, A.; Aroonwilas, A.; Tontiwachwuthikul, P.; Chakma, A.; Kybett, B. D. Kinetics of the Oxidative degradation of aqueous MEA in a flue gas treating unit. Ind. Eng. Chem. Res. 2001, 40, 3445-3450. (8) Howard, H. An Introduction into CO2 Separation and Capture Technologies; MIT Energy Laboratory: Cambridge, MA, Aug 1999. (9) Polderman, L. D.; Dillon, C. P.; Steele, A. B. Why MEA solution breaks down in gas treating service. Oil Gas J. 1950, 53, 69-71. (10) Kim, C. J.; Satori, G. Kinetics and mechanism of diethanolamine degradation in aqueous solutions containing carbondioxide. Int. J. Chem. Kinet. 1984, 16, 1257-1266. (11) Kim, C. J. Degradation of alkanolamines in gas treating solutions: kinetics of di-2-propanolamine degradation is aqueous solutions containing carbondioxide. Ind. Eng. Chem. Res. 1988, 27 (1), 1-3. (12) Rooney, P. C.; Dupart, M. S.; Bacon, T. R. Oxygen’s role in alkanolamine degradation. Hydrocarbon Process., Int. Ed. 1998, 109-113. (13) Aboudheir, A.; Tontiwachwuthikul, P.; Chakma, A.; Idem, R. Kinetics of the reactive absorption of carbon dioxide in high CO2-loaded, concentrated aqueous monoethanolamine solutions. Chem. Eng. Sci. 2003, 58, 5195-5210. (14) Supap, T. Kinetic study of Oxidative degradation of MEA in gas treating unit using aqueous MEA solution. Masters Thesis, Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada, 1999. (15) Hofmeyer, B. G.; Scholton, H. G.; Lloyd, W. G. Contamination and corrosion in monoethanolamine gas treating solutions. Presented at the National Meeting of the American Chemical Society, Dallas, TX, Apr 8-13, 1956. (16) Lloyd, W. G.; Taylor, F. T., Jr. Corrosion by deterioration of glycol and glycol-amine. Ind. Eng. Chem. 1954, 46, 2407-2416. (17) Asperger, R. G. New corrosion issues in gas sweetening plants. Proceedings of the 73rd Annual GPA Convention, New Orleans, LA, Mar 1994. (18) Kindrick, R. C.; Atwood, K.; Arnold, M. R. The relative resistance to oxidation of commercially available amines; Girdler Report No. T2.15-1-30, Carbon dioxide absorbents, Contract No. Nobs-50023, by Girdler Corp., Gas Processes Division, Louisville, KY, for The Navy Department, Bureau of Ships, Washington, DC (Code 649P), June 1, 1950. (19) Dawodu, O. F.; Meisen, A. Degradation of alkanolamine blends by CO2. Can. J. Chem. Eng. 1996, 74, 960. (20) Rooney, P. C.; Daniels, D. D. Oxygen Solubility in various alkanolamine/water systems. J. Pet. Sci. Technol. 1998, 3 (1), 97.

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 969 (21) Rochelle, G. T.; Chi, S. Oxidative degradation of Aqueous MEA in capture systems under absorber conditions. Ind. Eng. Chem. Res. 2002, 41, 4178-4186.

(23) Blankly, C. H.; Ravner, H. Stabilization of monoethanolamine solutions in CO2 Scrubbers. U.S. Naval Research Laboratory Report 6189; Dec 1964.

(22) Blanc, C.; Grall, M.; Demarais, G. The part played by degradation products in the corrosion of gas sweetening plants using DEA and MDEA. Proceedings of the Gas Conditioning Conference; University of Oklahoma, Norman, OK, 1982; Vol. 32; Issue 3.

Received for review July 28, 2004 Revised manuscript received November 18, 2004 Accepted November 24, 2004 IE049329+

Pathways for the Formation of Products of the Oxidative Degradation

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