The Role of Methyl Diethanolamine (MDEA) in ... - ACS Publications

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The Role of Methyl Diethanolamine (MDEA) in Preventing the Oxidative Degradation of CO2 Loaded and Concentrated Aqueous Monoethanolamine (MEA)-MDEA Blends during CO2 Absorption from Flue Gases Olanike Lawal, Adeola Bello, and Raphael Idem* Process Systems Engineering Laboratory, Faculty of Engineering, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan S4S OA2, Canada

The products and pathway for the oxidative degradation of CO2-loaded and concentrated aqueous solution of monoethanolamine (MEA)/methyl diethanolamine (MDEA) mixture (i.e., MEAMDEA-H2O-CO2 system) were evaluated and compared with those for the MEA-H2O-CO2 system in a stirred cell reactor at temperatures in the range of 55-120 °C, overall amine concentration in the range of 5-9 mol/L, MDEA/MEA ratio of 0-0.4, CO2 loading in the range of 0-0.53 mol/mol of total amine, and O2 pressure of 250 kPa in order to determine the role of MDEA in preventing MEA degradation. The results showed that fewer degradation products were obtained for the MEA-H2O-O2 system for both the CO2-loaded and CO2-free cases as compared with the MEA-MDEA-H2O-O2 system. However, the addition of MDEA drastically reduced the extent of MEA degradation as well as the amount of nonenvironmentally benign degradation products. Our overall results indicate that, under our experimental conditions, MDEA is more prone to oxidative degradation and, when used in a mixture with MEA, is preferentially degraded to protect MEA. Our results further show that even in an initially O2free environment, O2 is produced as a byproduct of CO2-induced degradation, thereby eventually generating an oxidative degradation environment for the two systems. 1. Introduction MEA is the most widely studied solvent for the removal of CO2 from flue gases. Its rate of absorption of CO2 from flue gases is very high. However, it has a limitation that its maximum loading capacity of CO2, based on stoichiometry, is about 50%, in contrast to tertiary amines such as methyl diethanol amine (MDEA), which has a CO2 loading capacity that approaches 100%. Also, the heat of desorption of CO2 from MEA (during regeneration) is very high as compared with MDEA. Additionally, MEA degrades when CO2-loaded MEA comes in contact with oxygen (O2) and impurities such as sulfur dioxide (SO2) - a process catalyzed by multivariate cations (such as iron, copper, nickel, vanadium and chromium). Degradation leads to a loss of CO2 absorption capacity and may give rise to products that are environmentally difficult to dispose of. It is widely known that by blending a primary or secondary alkanolamine with a tertiary alkanolamine, bulk CO2 removal can easily be accomplished while minimizing cost for regeneration energy.1,2 In addition, another degree of freedom (the amine concentration) is gained. Thus, the amine concentration can be altered to achieve precisely the desired separation for a given process.3,4 There appears to be sufficient information in the literature concerning the CO2 loading3,4 and the rate of CO2 absorption into mixed amines.5-9 However, all these studies were not focused on CO2 induced degradation of the amines, and thus, no degradation products were considered or identified. As such, the CO2 and/or O2 * To whom correspondence should be addressed. Tel: (306) 585-4470. Fax: (306) 585-4855. E-mail: raphael.idem@ uregina.ca.

induced degradation characteristics of mixed alkanolamine systems are not well understood. Specifically, there appears to be limited information on the degradation characteristics of aqueous blended MEA-MDEA induced by CO2 (MEA-MDEA-H2O-CO2) or by O2 (MEA-MDEA-H2O-O2), or by O2 on CO2-loaded systems (MEA-MDEA-H2O-CO2-O2), despite the work of Dawodu and Meisen,4 Rooney et al.10 and Kindrick et al.11 The latter work showed the individual contributions of either CO2 or O2 toward degradation of amine blends whereas a typical flue gas stream contains both O2 and CO2. Furthermore, even the pathway for degradation of the MEA-H2O-CO2, MEA-H2O-O2 and MEA-H2O-O2CO2 systems is not completely understood at this time. For example, the work of Kohl and Nielsen12 was limited to conducting a comparative study on the oxidative degradation of alkanolamines. On the other hand, Blanc et al.,13 Lloyd and Taylor,14 Supap et al.15 and Hofmeyer et al.16 have identified products of MEA oxidative degradation but did not propose any mechanism for formation of the degradation products observed. Also, Rooney et al.10 studied MEA degradation in the presence of both CO2 and O2. Their report was limited to showing that primary amines were more vulnerable to oxidative degradation than secondary and tertiary amines. However, their results were in contradiction with earlier studies of Kindrick et al.11 In a recent study, Rooney et al.10 did not report an identification of all the degradation products formed from MEA degradation resulting in the fact that no overall mechanism was proposed to account for the degradation products. Only a modification of the mechanism proposed by Jefferson Chemicals10 to show the formation of some of the products not

10.1021/ie049261y CCC: $30.25 © 2005 American Chemical Society Published on Web 02/08/2005

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observed by Jefferson Chemicals was put forward. Also, there was no indication of products resulting from the reaction of MEA with CO2 and neither was there any mechanism proposed to account for the presence of both CO2 and O2.10 It can be inferred 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. Also, only one group has proposed a mechanism for the formation of only some of the products observed for combined CO2 and O2 induced degradation of MEA based on the analysis of the reclaimer waste from the IMC Chemicals Facility.17 This mechanistic work did not involve all of the possible products of reactions involving both O2 and CO2 exclusively with MEA. Also, 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 hydroxide (known to react with heat stable salts) is added in order to liberate the amine from the heat stable acid salts, and to minimize corrosion. Hence, other products, apart from the true degradation products, are also formed during this process. The current scenario for both MEA and MEA-MDEA systems makes it difficult to have a good understanding of the role played by MDEA in MEAMDEA system and to derive a degradation prevention strategy for the MEA-MDEA-O2-CO2 system. This work was undertaken with the aim to identify the products and elucidate the pathways for the oxidative degradation of CO2-loaded, concentrated aqueous MEA/ MDEA mixtures (i.e. MEA-MDEA-H2O-CO2 system) as well as to compare these results with those of the MEA-H2O-CO2 system in order to evaluate the role played by MDEA in a blend of MEA and MDEA. 2. Experimental Section 2.1. Equipment and Chemicals. The degradation experiments were conducted in a semi batch mode using a 600 mL stainless steel reactor (model 4560, Parr Instrument Co., Moline, Illinois) equipped with an insulating Jacket, a Bourdon-type pressure gauge, a variable speed stirrer and ports for introducing and withdrawing liquid and gas samples. The heat supplied to the reactor was controlled by a temperature indicatorcontroller system equipped with a J-type thermocouple. The temperature accuracy of the controller was within ( 0.1%. Analytical grade O2 and CO2 were used and were supplied by Praxair (Regina, SK). Concentrated MEA and MDEA (reagent grade, 99% purity) were obtained from Fisher scientific, Whitby, Ontario. These solvents were diluted with deionized water to the desired concentrations. 1 mol/L hydrochloric acid, also obtained from Fischer Scientific, was used to establish the exact MEA and MDEA concentrations. 2.2. Typical Experimental Run. For the CO2 or O2 induced degradation of aqueous MEA or MEA-MDEA, the water vapor pressures under various reaction conditions were determined earlier in the absence of O2 or CO2 pressures. For a typical experimental run, 450 mL of aqueous MEA or MEA/MDEA blends of the desired concentration was loaded into the reactor. The solvent was stirred at a speed of 500 rpm and heated to the desired temperature. O2 or CO2 was then fed into the

vessel up to the desired reactor pressure (i.e. O2 or CO2 pressure + water vapor pressure) by opening the inlet valve of the O2 or CO2 cylinder tank set at a predetermined value. O2 and CO2 are soluble in aqueous amine blends. This resulted in an initial decline of O2 or CO2 pressure in the reaction chamber. However, to maintain a constant pressure, the reaction chamber was boosted with extra O2 or CO2 supply as appropriate. In all the test runs, it was ensured that the desired O2 or CO2 pressure was maintained throughout the duration of the reaction. In the case of O2 induced degradation of aqueous CO2loaded MEA or MEA-MDEA, CO2 was loaded into the liquid sample by feeding the CO2 at the desired pressure by opening the gas inlet valve of the CO2 cylinder tank set at a predetermined value. The gas inlet valve was left open for about 24 h and then closed, after which about 3 mL sample of the reaction mixture was withdrawn through the liquid sampling port of the reactor in order to determine the CO2 loading in the sample using a Chittick CO2 analyzer. The reactor content was then heated to the desired temperature and CO2 loading was again determined. The vapor pressures of CO2 and water was determined and O2 was then fed into the vessel up to the desired pressure (O2 pressure + water vapor pressure + CO2 vapor pressure) by opening the O2 gas inlet valve of the O2 cylinder tank set at a predetermined value. Apart from pre-loading the aqueous MEA or MEA/MDEA blend with CO2, the procedure was similar to the case for the MEA-H2O-O2 and MEA-MDEA-H2O-O2 systems. In each run, about 3 mL sample of the reaction mixture was removed from the reactor through the liquid sampling port at appropriate predetermined intervals up to a maximum of 530 h run time. Extra O2 or CO2 was also quickly added after each sampling to compensate for loss of pressure during the sampling process and to maintain the constant pressure of the system for the O2 and CO2 alone system, respectively. In the case of O2-CO2 system, only O2 was replenished whenever pressure drops after taking out samples. The reaction in each sample taken was quenched by quickly running cold water over the test tube containing the sample. Also, the reaction between aqueous MEA or MEA/MDEA blends and O2 or CO2 is exothermic. Thus, cooling water (flowing through the cooling coil) was employed to ensure reactor isothermality. Figure 1a is a schematic representation of the experimental setup. 2.3. Analysis of Products. Analysis of the samples was carried out using a gas chromatography-mass spectrometer (GC/MS model HP 6890/5073 supplied by Hewlett-Packard Canada Ltd., Montreal Quebec, Canada). An HP - Innowax column (length ) 30 m, internal diameter ) 250 µm, thickness ) 0.25 µm) packed with cross-linked poly(ethylene glycol) was used in the GC for the separation of components. These components were identified by their mass spectra. Prior to GC/MS analysis, each sample was diluted with deionized water to five times its original volume to avoid column overload and to improve separation of the components. The GC/MS conditions used are summarized as follows. An autoinjector (model 7683, supplied by Hewlett-Packard Ltd.) was used to automatically introduce samples into the GC column to give better reproducibility. A 10 µL syringe with an injection volume of 0.2 µL was used and a split mode was selected

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Figure 1. Schematic representation of an experimental run.

for the inlet with a split ratio of 10:1, split flow of 10.3 mL/min and 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 no hold time while the final temperature was 240 °C with a hold time of 10 min with an oven ramp of 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/sec. For the MS parameters, the interface, quadruple and source temperatures were 250, 150 and 230 °C, respectively and the electron multiplier (EM) voltage was 1200 V. The error of the GC/MS was estimated to be less than (3%. The overall schematic of the experimental setup is shown in Figure 1. Computer fitting of the mass spectrum to the mass spectra database was part of the strategy used to identify the samples. This was followed by the use of standards as well as elemental analysis in order to confirm the identification of the components in the samples. Some of the standards used were 2-methylamino ethanol, ethylenediamine and 18-crown-6. Typical results obtained for 2-methylamino-ethanol (Figure 2a-c) and 18-crown-6 (Figure 3a-c) indicate that the mass fragmentation patterns of these compounds in our samples matched those of the standards as well as those documented in the mass spectrometer database. In the case of 18-crown-6, Figure 3a shows additional mass fragments, which indicates that the particular GC peak contained other components in addition to 18-crown-6. This is usually the case if complete chromatographic separation is not achieved for some of the components. On the other hand, the other components (in the samples for which the standards were analyzed) showed complete chromatographic separation. To show that the samples we observed were indeed obtained as a result of degradation, we have included the chromatograms of a typical sample taken at the beginning (time ) 0 h) and at the end of the experiment (217 h) for MEAMDEA-H2O-O2 system using 7 mol/L (10% MDEA) total amine concentration. These are given in Figure 4, panels a and b, respectively. The figure shows the existence of MEA and MDEA only at the initial time of degradation. However, after 217 h of degradation time, a large number of degradation products are observed. Furthermore, most of the degradation products, includ-

ing the crown ethers have been observed for MEA degradation.18 3. Results and Discussion 3.1. MEA-H2O-O2 and MEA-MDEA-H2O-O2 Systems. 3.1.1. Products and Degradation Pathways. The oxidative degradation of the MEA-H2OO2 system was studied using 5 mol/L MEA with 250 kPa O2 partial pressure at 100 and 120 °C. Similar experiments were carried out for the MEA-MDEA-H2O-O2 system using 7 mol/L total amine concentration containing 10% MDEA. Tables 1 and 2 show the products obtained at these two temperatures. Although O2 is not expected to be present in the stripping column where the temperature goes beyond 100 °C, higher temperatures and O2 pressures were used in this study on O2 induced degradation to obtain accelerated results. Table 1 shows that at 100 °C, twelve products were observed for the MEA-H2O-O2 system for an experimental run time of 530 h whereas twenty products were obtained for the MEA-MDEA-H2O-O2 system for a run time of 480 h. Ethylamine (C2H7N) and 3-cis -5heptadiene-1-ol (C7H12O) were the only common products for the two systems. Proposed mechanisms showing the pathways for the formation of degradation products for both systems at 100 °C are given in Figure 5a,b. The proposed mechanisms as well as subsequent mechanisms are based on our proposed chemistry relating the feed (MEA, MDEA, CO2, O2 and H2O) to the product slate as a function of operating conditions. Figure 5a,b show that one of the common products, 3-cis-5-heptadiene-1-ol, has a formation pathway that is different in both systems. On the other hand, Figure 5b shows that MEA-MDEA-H2O-O2 system has one other formation pathway for ethylamine (the other common product) in addition to its common formation pathway in both systems. It can be inferred that the presence of MDEA in the mixed amine system resulted in an increase in the number of products and a change in type of products as well as changes in the formation pathways, even for the common products. Figure 5a,b also shows ammonia (NH3) is one of the byproducts of some of the reaction steps. This confirms the observation of Rochelle and Chi19 that NH3 is a product of the oxidative degradation of MEA. As mentioned earlier, one of the degradation


Figure 2. (a) Spectrum of 2-methylamino ethanol in degraded sample of MEA-MDEA-H2O-O2 system at 120 °C using 7 mol/L (10% MDEA) total amine concentration. (b) Spectrum of 2-methylamino ethanol in the database. (c) Spectrum of 2-methylamino-ethanol standard sample.

products is ethylamine for which its formation is explained as follows. There are a number of reaction steps involving MEA or its degradation products and O2 that lead to the formation of 1/2O2 (see Figures 5a and 6a,b). In the presence of 1/2O2 (atomic oxygen), MEA can reduce to form ethylamine by eliminating 1/2O2 (one atom of oxygen) from its molecule. The eliminated 1/2O2 combines with the 1/2O2 available from the previous reactions to form O2 implying that 1/2O2 acted as a seed for the formation of more O2. An

alternative pathway for formation of ethylamine would be the thermal decomposition of MEA as shown below:

2MEA f 2ethylamine + O2 However, according to Rochelle and Chi,19 this is not possible especially at 55 °C. Therefore, we are proposing that the reaction is as given in Figure 5a. Table 2 shows the results of similar experiments carried out at 120 °C for both the MEA-H2O-O2 (5

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Figure 3. (a) Spectrum of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) in degraded sample of MEA-H2O-CO2-O2 system at 100 °C using 7 mol/L (MEA) with CO2 loading of 0.4 mol of CO2/mol of MEA. (b) Spectrum of 18-crown-6 (1,4,7,10,13,16hexaoxacyclooctadecane) in the database. (c) Spectrum of 18-crown-6 standard sample.

mol/L MEA) system and the MEA-MDEA-H2O-O2 (7 mol/L total amine concentration with 10% MDEA) system. Nineteen degradation products were observed for the MEA-H2O-O2 system after 135 h of experimental run time. On the other hand, the MEA-MDEAH2O-O2 system produced 43 products after an experimental run time of 217 h. There were 10 common products for the two systems at 120 °C. They included 3-methylpyridine (C6H7N), 2-(methylamino)-ethanol (C3H9NO), formic acid (CH2O2), ethylamine (C2H7N), 1-proN-(2-hydroxyethyl)-acetamide (C4H9NO2), panamine (C3H9N), 4-hydrazinocarbonyl-imidazole

(C4H6N4O), 5-hydrazinocarbonyl-imidazole (C4H6N4O), 15-crown-5 (C10H20O5) and 1-(2-hydroxyethyl)-2-imidazolidinone (C5H10N2O2). Mechanisms showing the pathways for the formation of degradation products for the two systems at this temperature are given in Figure 6a,b. Among the common products, ethylamine had a formation pathway that is the same for both systems. However, the MEAMDEA-H2O-O2 system provided one additional pathway for its formation just as was the case at 100 °C. This also applied to 1-propanamine. In contrast, the formation pathways for other common products were


Figure 4. (a) Chromatogram of MEA-MDEA-H2O-O2 system at zero hour using 7 mol/L (10% MDEA) total amine concentration at 120 °C. (b) Chromatogram of MEA-MDEA-H2O-O2 system at the end of the experiment (217 h) using 7 mol/L (10% MDEA) total amine concentration at 120 °C.

different in both systems. Therefore, just as at 100 °C, the addition of MDEA to MEA-H2O-O2 resulted in an increase in the number of products, a change in the type of products and changes in formation pathways even for the common products. To obtain a clearer understanding of the role of MDEA in a MEA-MDEA blend, experiments were conducted at 55 and 120 °C using 5 and 7 mol/L MEA for the MEA-H2O-O2 system, with an addition of 2 mol/L MDEA in each of the two cases resulting in a total amine concentration of 7 and 9 mol/L for the MEAMDEA-H2O-O2 system. Experimental results are presented in Tables 3 and 4 for runs at 55 and 120 °C, respectively. Table 3 shows that the MEA-H2O-O2 system produced four products after 341 h experimental run time while the MEA-MDEA-H2O-O2 system produced seven products after 312 h experimental run time. 15-crown-5 and 2-(2-ethoxyethoxy)-ethanol were the only observed common products at 55 °C. Separate degradation mechanisms showing the reaction pathways that are consistent with the respective product slates and operating conditions have been proposed for

the two systems. These are shown in Figure 7a,b. Products from MEA alone, MDEA alone and reactions involving products of both MEA and MDEA are 5, 1 and 6, respectively. The proposed pathways for the formation of the common products were different in the two systems. Ethylamine was one of the products specific to the MEA-MDEA-H2O-O2 system whose formation was through reactions of either MEA or MDEA. 1,3-propanediamine, 1-amino-2-propanol, 2-(methylamino)ethanol and 2-propanamine were produced from either MEA alone or reactions involving both MEA and MDEA while 15-crown-5 and 2-(2-ethoxyethoxy)-ethanol were formed from reactions involving both MEA and MDEA as shown in Figure 7b. These results show that the addition of MDEA to MEA-H2O-O2 system resulted in a change in the type of products as well as an increase in the number of products. These in turn, affected the formation pathways of the common products. The number of products from MEA alone in the MEAMDEA blend was higher than the number of products from MEA-H2O-O2 system by one. Therefore, at a low

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Table 1. Oxygen-Induced Degradation Products at 100 °C and 250 kPa O2 Pressure MEA-MDEA-H2O-O2 system (7 mol/L total amine concentration with 10% MDEA)

MEA-H2O-O2 system (5 mol/L MEA) product

molecular formula

product

molecular formula

N-formyl-N-methyl-formamide 1,2,3,6-tetrahydro-1-nitroso-pyridine 3-methyl-1,2-cyclopentanedione 3,4 dehydro-DL-proline 4-ethyl-cyclohexene ethylamine 1H-imidazole-4-carboxylic acid,methylester 3,cis-5-heptanedien-1-ol 1,2-ethanediol uracil 2,2-dimethyl-3-(2H)-furanone nitrosomethane

C3H5NO2 C5H8N2O C6H8O2 C5H7NO2 C8H14 C2H7N C5H6N2O2 C7H12O C2H6O2 C4H4N2O2 C6H8O2 CH3NO

12-crown-4 1-propanamine 3,cis-5-heptadien-1-ol 2-(methylamino)ethanol 2-amino-1,3-propanediol 15-crown-5 N-(2-hydroxyethyl)-acetamide ethylamine diethanolamine DL-alanine 5-hydrazinocarbonyl-imidazole 2-methyl-1H-imidazole formic acid 1,2-ethanediol, monoformate 1,3-propanediamine bicine 1-methyl-3-pyrrolidinol ethoxy-ethene 4-carbhydrazino-imidazole homoserine

C8H16O4 C3H9N C7H12O C3H9NO C3H9NO2 C10H20O5 C4H9NO2 C2H7N C4H11NO2 C3H7NO2 C4H6N4O C4H6N2 CH2O2 C3H6O3 C3H10N2 C6H13NO4 C5H11NO C4H8O C4H6N4O C4H9NO3

Table 2. Oxygen-Induced Degradation Products at 120 °C and 250 kPa O2 Pressure MEA-H2O-O2 system (5 mol/L MEA)

MEA-MDEA-H2O-O2 system (7 mol/L total amine concentration with 10% MDEA)

product

molecular formula

product

molecular formula

3-methylpyridine 2-propanone,oxime 2-methyl amino ethanol formic acid ethylamine 3-methylcyclopentanol 4- methyl morpholine 1H-imidazole N-2-hydroxyethyl acetamide 1-propanamine trimethyl-hydrazine 1-methyl-2,4 imodazolidinedione 4-hydrazinocarbonyl-imidazole 5-hydrazinocarbonyl-imidazole 3,cis-5-heptanedien-1-ol N-2-hydroxyethylsuccinimide 15-crown-5 1-(2hydroxyethyl)-2-imidazolidinone 2-pyrrolidinone

C6H7N C3H7NO C3H9NO CH2O2 C2H7N C6H12O C5H11NO C3H4N2 C4H9NO2 C3H9N C3H10N2 C4H6N2O2 C4H6N4O C4H6N4O C7H12O C3H9NO3 C10H20O5 C5H10N2O2 C4H7NO

ethyl-urea N-(2-hydroxyethyl)-acetamide 1-propanamine 2-(methylamino)-ethanol 3,cis-5-heptadien-1-ol 1,2-dimethyl-hydrazine formaldehyde, dimethylhydrazone 2-amino-2-methyl-1,3-propanediol DL-homoserine lactone 2,2-di-methyl-3-pentanol 4-hydrazinocarbonyl-imidazole 5-hydrazinocarbonyl-imidazole bicine N-(2-hydroxyethyl)lactamide triethylamine 1-methyl-aziridine 1-piperazinecarboxylic acid-4-nitrosoethylester 4-(oxiranylmethyl)-morpholine 1-methyl-5-imidazolic hydrazide ethylamine formic acid 2-ethoxyethylamine 3-methyl-1H-pyrole 4-acetyl-morpholine N-ethyl-N-methyl-1-butanamine 1,3-dimethyl-2-imidazolidinone methyl-pyrazine 3-methyl-pyridine 2-[(2-aminoethyl)amino]-ethanol 1-methyl-1H-imidazole 1H-pyrazole N,N′-diethyl-urea 4-morpholinepropanamine (1,2,3-trimethyl-cyclopent-2-enyl)-methanol 2-amino-4,6-dihydroxypyrimidine 4-methoxy-1-butanol N-ethyl-N-nitroso-urea N-methoxy-N-methylformamide 1-(1-propenyl)-pyrolidine 1-(2-hydroxyethyl)-2-imidazolidino 2,4-dimethyl-1H-imidazole 15-crown-5 carbamic acid, ethylnitroso-, ethylester

C3H8N2O C4H9NO2 C3H9N C3H9NO C7H12O C2H8N2 C3H8N2 C4H11NO2 C4H7NO2 C7H16O C4H6N4O C4H6N4O C6H13NO4 C5H11NO3 C6H15N C3H7N C7H13N3O3 C7H13NO2 C5H8N4O C2H7N CH2O2 C4H11NO C5H7N C6H11NO2 C7H17N C5H10N2O C5H6N2 C6H7N C4H12N2O C4H6N2 C3H4N2 C5H12N2O C7H16N2O C9H16O C4H5N3O2 C5H12O2 C3H7N3O2 C4H9NO2 C7H13N C5H10N2O2 C5H8N2 C10H20O5 C5H10N2O3

temperature, the presence of MDEA in the blend actually leads to a slight increase in the number of MEA derived degradation products. NH3 was also one of the

byproducts of some of the reaction steps in these systems as shown in Figure 7a,b. No heat stable amine salt was observed in the two systems at 55 °C.


Figure 5. cont’d

Table 4 gives the products from MEA-H2O-O2 (7 mol/L MEA) and MEA-MDEA-H2O-O2 system (9 mol/L total amine concentration with 2 mol/L MDEA) at 120 °C. The table shows that for the MEA-H2O-O2 system, 2 products were obtained for 96 h of degradation time whereas 14 products were obtained for the MEAMDEA-H2O-O2 system for 96 h of degradation time. Table 4 also indicates that there was one common product: methyl-pyrazine. Figure 8, panels a and b, respectively, shows mechanisms indicating the proposed pathways for formation of the degradation products for MEA-H2O-O2 and MEA-MDEA-H2O-O2 systems at 120 °C for these amine concentrations. The mechanism proposed for the MEA-MDEA-H2O-O2 system (Figure 8b) shows that the number of products possible from MDEA alone, MEA alone, and reactions involving both MEA and MDEA degradation products were 11, 7 and 9, respectively. These results show that the addition of MDEA to MEA increases the number of MEA derived

degradation products from 2 (in the MEA alone system) to 7 (in the MEA-MDEA system). Products such as 3-methyl-1H-pyrole and 2-(2-aminoethoxy)-ethanol were directly or indirectly derived from MDEA exclusively. Possible products from either MDEA or MEA in the MEA-MDEA-H2O-O2 system were ethylamine and dimethylamine whereas N-propyl1-butanamine, methylpyrazine and N-hydrohycarbamic acid-2-propoxycarbonylamino-ethylester were produced from MDEA alone or reactions involving both MEA and MDEA. Products such as 2-methylamino-ethanol, N,Ndimethyl-1,2-ethanediamine, N-methyl-ethanamine and N-ethyl-1,2-ethanediamine were produced from either MEA alone, MDEA alone or further reactions of degradation products of both MEA and MDEA. 3,cis-5Heptadien-1-ol and 2- propanol, 1,3-bis (dimethylamino), were formed from reactions involving both MEA and MDEA oxidative degradation products. 1,2-propanediamine was produced from MEA alone. It can be

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Figure 5. Pathways for formation of oxygen-induced degradation products at 100 °C and 250 kPa O2 pressure, (a) MEA-H2O-O2 system (5 mol/L MEA), (b) MEA-MDEA-H2O-O2 system (7 M total amine concentration with 10% MDEA).

inferred from these results that the presence of MDEA in the MEA-MDEA-H2O-O2 accounted for the increase in the number of products obtained during the oxidative degradation of the system. Also, the presence of MDEA resulted in a drastic modification in the degradation pathways for MEA alone in the MEAMDEA-H2O-O2 system as compared with the solely MEA-H2O-O2 system. This explains why N,N dimethyl,1,2-ethanediamine and 2-methylaminoethanol reported as products of MEA degradation for the MEAMDEA-H2O-O2 system (Figure 8b) were totally absent in the MEA-H2O-O2 system (Figure 8a). Considering the larger number of degradation products from MEAMDEA-H2O-O2 system as compared to MEA-H2OO2 system, it can be inferred that the MEA-MDEAH2O-O2 system is more prone to oxidative degradation than the MEA-H2O-O2 system.

It is interesting to note that the products obtained from these systems at all the operating conditions are consistent with the literature. For example products such as 2-methylaminoethanol and diethanolamine were identified by Chakma and Meisen20 as products of CO2 induced degradation of partially degraded MDEA, although the temperature and CO2 partial pressure were not stated. Triethylamine (Table 2) was also one of our products for the MEA-MDEA-H2O-O2 system, which we considered to be analogous to trimethylamine observed by Chakma and Meisen.20 Also, 4-methylmopholine and N,N-dimethylethanamine observed by the same researchers20 using 4.8 mol/L MDEA, 2.59 MPa CO2 partial pressure at 180 °C are also analogous to 4-acetylmorpholine (Table 2), and N-methyl-ethanamine (Table 4) that we observed in the MEAMDEA-H2O-O2 system. Diethanolamine, methylami-


Figure 6. cont’d

noethanol and hydroxyethyl-imidazolidone observed by Dawodu and Meisen4 from CO2 induced degradation of MDEA, MEA+DEA, MDEA+MEA using 4.2 mol/L total amine with 2.58 MPa CO2 partial pressure at temperatures ranging from 120 to 180 °C are also some of the products we observed (Tables 1-4) for the MEA-MDEAH2O-O2 system. These researchers also observed ethyleneglycol (ethanediol) at the stated conditions whereas we observed its formate salt (ethanediolmonoformate or glycolmonoformate as shown in Table 1) in the MEAMDEA-H2O-O2 system. Also, some of the products observed by Straizer et al.17 under CO2 induced degradation of MEA are similar to some of our products in the MEA-MDEA-H2O-O2 system. Examples include N-formylethanolamine (isomer of DL-alanine (C3H7NO2) as given in Table 1) and N-(2-hydroxyethyl-lanthamide (isomer of N-2-hydroxyethyllactamide (C5H11NO3) given in Table 2). They also

observed butyric acid and propionic acid whereas we observed DL-2-amino-4-hydroxybutyric acid (i.e. homoserine as given in Table 1) and DL-2-aminopropionic acid (i.e. DL-alanine as given in Table 1). The mechanism of oxidative degradation of MEA reported by Rooney et al.10 as attributed to Jefferson chemicals also showed that glycine was one of the products of oxidative degradation of MEA. Therefore, the presence of N,Ndihydroxyethylglycine (i.e. bicine as given in Tables 1 and 2), which is a substituted glycine provides the confirmation of the consistency of our result with the literature. In another MEA oxidative degradation pathway given by Rooney et al.,10 formic acid was also one of the products of oxidative degradation of MEA.10,21 We also observed formic acid as a degradation product as given in Tables 1 and 2. 3.1.2. Extent of Degradation. A semiquantitative estimation of the degradation of both MEA and MDEA

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Figure 6. cont’d

induced by O2 was made for the MEA-H2O-O2 and MEA-MDEA-H2O-O2 systems. The estimation is as given in eq 1.

% degradation ) 100 ×

C0 - C1 C0

(1)

where C0 ) concentration of fresh amine and C1 ) concentration of amine after degradation. Table 5 gives the extent of degradation observed for 5 mol/L MEA for the MEA-H2O-O2 system and 7 mol/L total amine concentration with 2 mol/L MDEA for the MEA-MDEA-H2O-O2 system both with 250 kPa O2 at 55 and 120 °C. The result showed that the extent of degradation of MEA at 55 °C was 3.4% while that for MDEA was 2% after 312 h for a run conducted using 7

mol/L total amine concentration (2 mol/L MDEA) for the MEA-MDEA-H2O-O2 system. For the MEA-H2OO2 system, MEA degradation was 0.8% after 341 h. This result shows that at 55 °C the extent of MEA degradation for MEA-H2O-O2 system was actually less than that for the MEA-MDEA-H2O-O2 system, thereby supporting our previous observation of a larger number of MEA derived products for the blended amine. A higher temperature was then used to obtain accelerated results. However at the higher temperature (120 °C) the presence of O2 is very rare (e.g. stripping column) except from some reactions such as shown in Figure 10a,b. In this case we observed that the degradation of MEA in the MEA-H2O-O2 system was 49.2% after 135 h, whereas in the MEA-MDEA-H2O-O2 system, MEA degradation was 22.2% while that of MDEA was 27.9%


Figure 6. Pathways for formation of oxygen-induced degradation products at 120 °C and 250 kPa O2 pressure, (a) MEA-H2O-O2 system (5 mol/L MEA), (b,c) MEA-MDEA-H2O-O2 system (7 M total amine concentration with 10% MDEA). Table 3. Oxygen-Induced Degradation Products at 55 °C and 250 kPa O2 Pressure MEA-H2O-O2 system (5 mol/L MEA) product

molecular formula

12-crown-4 2-(2-ethoxyethoxy)ethanol 15-crown-5 1,4,7,10,13,16-hexa oxacyclooctadecane

C8H16O4 C6H14O3 C10H20O5 C12H24O6

after 312 h. Since the degradation of MEA in the MEAH2O-O2 system was large relative to MEA in the MEA-MDEA-H2O-O2 system at 120 °C, it implies that MDEA preferentially degrades especially at high temperatures to protect MEA. In actual commercial situation, this may pose little problem because the

MEA-MDEA-H2O-O2 system (7 M total amine concentration with 2 mol/L MDEA) product 2-(methylamino)-ethanol 1,3-propanediamine ethylamine 2-propanamine 1-amino-2-propanol 2-(2-ethoxyethoxy)-ethanol 15-crown-5

molecular formula C3H9NO C3H10N2 C2H7N C3H9N C3H9NO C6H14O3 C10H20O5

amount of O2 produced from these reactions is much smaller than what we used in this experiment, which was done to obtain accelerated results. Also, it is important to remember that the amine in both the absorption and the stripping columns will always have some quantity of dissolved CO2. As such, this will lead

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Table 4. Oxygen-Induced Degradation Products at 120 °C and 250 kPa O2 Pressure MEA-H2O-O2 system (7 mol/L MEA)

MEA-MDEA-H2O-O2 system (9 M total amine concentration with 2 mol/L MDEA)

product

molecular formula

product

molecular formula

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

C5H6N2 C6H6N4O4

methyl pyrazine ethylamine 1,2-propanediamine 2-(methylamino)-ethanol N-ethyl-1,2-ethanediamine 2-(2-aminoethoxy)-ethanol dimethylamine 3,cis-5-Heptadien-1-ol N,N′-dimethyl-1,2-ethanediamine N-propyl-1-butanamine 3-methyl-1H-pyrole 2-propanol, 1,3-bis(dimethylamino) N-methyl-ethanamine N-hydrohycarbamic acid, 2-(propoxycarbonylamino) ethylester

C5H6N2 C2H7N C3H10N2 C3H9NO C4H12N2 C4H11NO2 C2H7N C7H12O C4H10N2 C7H17N C5H7N C7H18N2O C3H9N C7H14N2O5

to a reduction in O2 induced degradation because the CO2 presence in the solvent reduces O2 solubility. Even though a typical real life absorption contact time with flue gases is minimum of five minutes and maximum of 6 h, our degradation experiments were conducted for longer period in order to evaluate the longterm effect of prolonged usage of MEA and MEAMDEA. This is based on the fact that the absorption process goes through a larger number of cycles, which add up to about the same time as we used in our experiments or even longer. Figure 9 shows how the percentage degradation varies with time for MEA-H2O-O2 and MEA-MDEA-H2OO2 systems using 5 mol/L MEA and 7 mol/L (2 mol/L MDEA) total amine concentration respectively at 120 °C. It shows that MEA alone degrades more than MEA in MEA-MDEA blend. This figure also shows the rate of degradation for the two systems as a function of time. 3.1.3. Heat Stable Amine Salt (HSAS). HSAS include formates, acetates, propionates, glycolates, sulfates, oxalates thiosulfates, etc. These HSAS are difficult to regenerate at stripper conditions, and will therefore reduce the system CO2 absorption capacity. They also lead to increased corrosion, viscosity and foaming tendency of the solvent.22 1H-imidazole-4-carboxylic acid, methylester was observed as the HSAS for the MEAH2O-O2 system (5 mol/L MEA) at 100 °C. 1,2-ethanediolmonoformate (ethyleneglycol-monoformate) was the only HSAS observed in the MEA-MDEA-H2O-O2 system (7 mol/L total amine with 10% MDEA) at this temperature. The proposed pathway (Figure 5b) showed that it could be formed from either MEA alone, MDEA alone or reactions involving both MEA and MDEA. Thus, the presence of MDEA can result in a change of the type of HSAS formed from MEA in a MEA-MDEA blend as compared to the MEA alone system. No HSAS was observed in the MEA-H2O-O2 system (5 mol/L MEA) at 120 °C (probably due to a higher temperature) unlike in the case at 100 °C whereas 1-piperazinecarboxylic acid-4-nitrosoethylester and carbamic acid, ethylnitroso-ethylester were the two HSAS observed in the MEA-MDEA-H2O-O2 system (7 mol/L total amine with 10% MDEA) at 120 °C. The proposed mechanism (Figure 6b) shows that the two HSAS could be produced from either MEA alone, MDEA alone or reactions involving both MEA and MDEA. The MEA-H2O-O2 system at 7 mol/L MEA and 120 °C did not produce any heat stable amine salts. In

contrast, (N-hydroxycarbamic acid, 2-propoxycarboxylamino) ethyl ester, was observed as the HSAS in the MEA-MDEA-H2O-O2 system at 120 °C (9 mol/L total amine concentration, 2 mol/L MDEA). The proposed pathways (Figure 8b) showed that it could be produced from either MDEA or reactions involving both MEA and MDEA. Therefore, its formation is not likely to reduce the availability of free MEA for CO2 absorption. 3.1.4. Environmentally Hazardous Products. Environmentally hazardous products that could arise from the degradation of MEA-H2O-O2 and MEA-MDEAH2O-O2 systems were also determined based on Material safety data sheet (MSDS) information. These are products that would make it difficult to dispose of used solvent in an environmentally acceptable manner. Table 6 lists these products for 5 mol/L MEA at 55 °C (typical absorber temperature) after 341 h in the MEA-H2OO2 system and 7 mol/L total amine with 2 mol/L MDEA at 55 °C after 312 h in the MEA-MDEA-H2O-O2 system according to the alkanolamine responsible for their formation. For the MEA-MDEA-H2O-O2 system, 5 hazardous products were formed directly or indirectly form MEA alone, 1 was formed from MDEA alone and 6 were produced from a combination of reactions involving both MEA and MDEA. In the case of MEA-H2O-O2 system, 4 hazardous products were produced. Again, the addition of MDEA increased the total number of hazardous products from the MEAMDEA-H2O-O2 system as compared to the MEAH2O-O2 system. It also led to a slight increase in the MEA alone derived hazardous products. However, Table 6 also shows that 0.8% of the initial MEA in the single amine system was degraded into hazardous products after 341 h whereas 3.4% of the initial total amine in the mixed amine was converted after 312 h. Therefore, the addition of MDEA to the MEA-H2O-O2 system resulted in an increase in the amount of total amine being converted into degradation products that are not environmentally benign. 3.2. MEA-H2O-CO2 and MEA-MDEA-H2OCO2 Systems. 3.2.1. Degradation Products and Pathways. Degradation products obtained from the MEA-H2O-CO2 system (7 mol/L MEA) were also compared with those from the MEA-MDEA-H2O-CO2 system (9 mol/L total amine with 2 mol/L MDEA) at 120 °C with 250 kPa CO2 partial pressure in order to evaluate the role of MDEA in the later system. For an experiment carried out for up to 490 h in the MEA-


Figure 7. cont’d

H2O-CO2, five degradation products were obtained as shown in Table 7. With the addition of 2 mol/L MDEA, only four products were obtained for experiments carried out up to 312 h as shown in Table 7. Also, the product slates for these two systems were completely different. Mechanisms showing the pathways for formation of CO2-induced degradation products for the two systems have been proposed and are given in Figure 10a,b. In the case of MEA-MDEA-H2O-CO2 system, the proposed mechanism (Figure 10b) shows that two products could be formed from MEA alone (ethylamine and 3-amino-1-propanol), four products could be formed from MDEA alone (ethylamine, 3-amino-1-propanol, N,N-dimethyl-1,2-ethanediamine, and N,N-dimethylurea) and three could be produced from reactions involving both MEA and MDEA (3-amino-1-propanol,

N,N-dimethylurea and N,N-dimethyl 1,2-ethanediamine). Therefore, the addition of MDEA to the system reduced the number of degradation products. A major observation in the proposed schemes (Figure 10a,b) for the two systems is the presence of O2 as a byproduct of some of the reaction steps and its further reaction with either MEA, MDEA or some of the products (see Figure 10a,b). These reaction steps are responsible for the common degradation products obtained earlier for the MEA-H2O-O2 and MEA-MDEAH2O-O2 systems, and explain why some of the products in the CO2-induced degradation systems reported in this work and in the literature4,20 are common to those of O2-induced degradation system of Figure 8b. It also implies that even in an initially O2-free system, O2 is generated as a byproduct thereby eventually generating

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Figure 7. Pathways for formation of oxygen-induced degradation products at 55 °C and 250 kPa O2 pressure, (a) MEA-H2O-O2 system (5 mol/L MEA), (b) MEA-MDEA-H2O-O2 system (7 M total amine concentration with 2 mol/L MDEA).

an oxidative degradation environment for both the MEA-H2O-CO2 and MEA-MDEA-H2O-CO2 systems. For example, the similarities between some of the products we observed for the MEA-MDEA-H2O-O2 system and some products (2-(methylamino)-ethanol, diethanolamine, 4-methylmorpholine and N,N-dimethyl ethanamine) identified by Chakma and Meisen20 from CO2 induced degradation of partially degraded aqueous MDEA is also an indication that there is the possibility of O2 induced degradation in the MEA-MDEA-H2OCO2 system. 3-amino-1-propanol that we observed in the MDEA-H2O-CO2 system is also an isomer of 2-methylaminoethanol observed by these researchers.20 The presence of some products (methylaminoethanol, diethanolamine and hydroxyethyl-imidazolidone or 1,2(hydroxyethyl)-2-imidazolidinone) from experiments of Dawodu and Meisen4 is also a further indication of the possibility of O2 induced degradation in the CO2 alone system at a high temperature. It was interesting that

for the run time used in our studies, no HSAS was observed for both the MEA-H2O-CO2 and MEAMDEA-H2O-CO2 systems, which means that the presence of MDEA does not induce any HSAS for CO2 induced degradation at a high temperature, which is usually found in the stripping column. 3.2.2. Extent of Degradation. The extent of degradation in the MEA-H2O-CO2 and MEA-MDEAH2O-CO2 systems was evaluated by using the results of experimental runs of 7 mol/L MEA with 250 kPa CO2 at 120 °C for 490 h (MEA-H2O-CO2 system) and 9 mol/L total amine concentration (2 mol/L MDEA) at the same temperature and CO2 pressure for 312 h run time (MEA-MDEA-H2O-CO2 system). The percentage degradation of the amine in each system was calculated using eq 1 and the results are given in Table 5. The table shows that the percentage degradation of MEA in the MEA-H2O-CO2 system was 1.7% after 490 h whereas in the case of MEA-MDEA-H2O-CO2 system, the MEA degradation was approximately zero while


Figure 8. cont’d

that of MDEA was 2.5% after 312 h. Based on these results, it is reasonable to conclude that MDEA is preferentially degraded to protect MEA in the MEAMDEA-H2O-CO2 system as MEA degradation in the latter system has been pratically eliminated thereby making all of the MEA available for use in CO2 absorption as compared with the corresponding single amine system. 3.2.3. Environmentally Hazardous Products. Table 8 shows the result of the evaluation of hazardous products from MEA-H2O-CO2 and MEA-MDEAH2O-CO2 systems. The table indicates that five products were observed from MEA for the MEA-H2O-CO2 system. In the case of MEA-MDEA-H2O-CO2 system, two products were observed from MEA alone, four from MDEA alone and three from reactions involving both MEA and MDEA. Thus, by comparing the number of MEA derived hazardous products of the MEA-H2OCO2 system with that of MEA-MDEA-H2O-CO2 system, it is clear that MDEA degrades to protect MEA. Also, about 3.0% of the MEA charged in the MEA alone system degraded into hazardous products after 490 h whereas 2.6% of the total amine charged in the

MEA-MDEA system degraded into these products after 312 h. Thus, the presence of MDEA led to slight reduction in the total amine being degraded into non environmentally benign degradation products. 3.3. MEA-H2O-CO2-O2 and MEA-MDEA-H2OCO2-O2 Systems. 3.3.1. Degradation Products and Pathways. Typically, it is not expected that O2 will be present in the rich CO2-loaded single or mixed amine system under stripper conditions (i.e. temperatures greater than 100 °C). However, the mechanisms proposed for the MEA-H2O-CO2 and MEA-MDEAH2O-CO2 systems (Figure 10a,b) showed that there was the possibility of having O2 in these systems even at the higher temperatures normally used in the stripping column due to the occurrence of some reactions as shown in Figure 10a,b. It is therefore essential to know what would be the prolonged effect of the presence of O2 in the MEA-MDEA-H2O-CO2 system under stripper conditions. This accelerated condition will also provide some indication as to its effect in this system at a lower temperature that is usually applicable in the absorption column knowing that O2 is one of the components of flue gas in this column. The basis of this is that if there is

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Figure 8. Pathways for formation of oxygen-induced degradation products at 120 °C and 250 kPa O2 pressure, (a) MEA-H2O-O2 system (7 mol/L MEA), (b) MEA-MDEA-H2O-O2 system (9 M total amine concentration with 2 mol/L MDEA). Table 5. Extent of Degradation initial concn (mol/L)

final concn (mol/L)

amount degraded

% degradation

run temp time (°C) (h) MEA MDEA MEA MDEA MEA MDEA MEA MDEA a a

MEA-H2O-O2 System 4.96 a 0.04 2.54 a 2.46

55 120

341 135

5 5

0.8 49.2

a a

55 120

312 312

5 5

MEA-MDEA-H2O-O2 System 2 4.83 1.96 0.17 0.04 2 3.97 1.4 1.03 0.6

3.4 22.2

2.0 27.9

120

490

7.0

MEA-H2O-CO2 System 6.88 a 0.12 a

120

312

MEA-MDEA-H2O-CO2 System 7.0 2.0 7.0 1.95 0.0 0.05

1.7

a

120

MEA-H2O-CO2-O2 System (0.27 mol/mol of CO2) 198 7.0 a 5.9 a 1.1 a 15.7

0.0

2.5

a

a a

a

MEA-MDEA-H2O-CO2-O2 System (0.4 mol/mol of CO2) 120 288 7.0 2.0 6.6 1.9 0.4 0.12 5.8 6.0 a

Not applicable.

the possibility of 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.

The role of MDEA in the MEA-MDEA-H2O-CO2O2 system was studied by comparing the products obtained from the MEA-H2O-CO2-O2 system (7 mol/L MEA and CO2 loading of 0.3 mol/mol MEA) with those obtained from the MEA-MDEA-H2O-CO2-O2 system (9 mol/L total amine concentration and CO2 loading of 0.4 mol/mol total amine) both at 120 °C with O2 partial pressure of 250 kPa. Table 9 presents the products for the two systems and it shows that the MEA alone system produced five degradation products whereas; the mixed amine system produced 10 degradation products. The number of products from MEA alone, MDEA alone and reactions involving products from both MEA and MDEA in the MEA-MDEA-H2O-CO2-O2 system were 6, 8 and 8, respectively. The number of MEA derived degradation products from MEA-MDEA-H2OCO2-O2 system was higher than that from MEA-H2OO2-CO2 system by one. Therefore the presence of MDEA led to a slight increase in the number of MEA derived products. In addition to products listed in Table 10, ethanol, 2-(ethenyoxy) is a product of either MEA or MDEA while (N-hydroxycarbamic acid, 2-(propoxycarbonylamino) ethylester) is a product from reactions involving both MEA and MDEA degradation products. There is no common product for the two systems


Figure 9. Extent of degradation vs. time for MEA-H2O-O2 and MEA-MDEA-H2O-O2 systems at 120 °C using 5 mol/L MEA and 7 mol/L (2 mol/L MDEA) total amine concentration. Table 6. Environmentally Unfriendly Products for MEA-H2O-O2 and MEA-MDEA-O2 Systems at 55 °C and 250 kPa O2 Partial Pressure MEA-H2O-O2 System (5 mol/L MEA) products from MEA 12-crown-4 2-(2-ethoxyethoxy)ethanol 15-crown-5 1,4,7,10,13,16-hexaoxacyclooctadecane area % ) 0.8 MEA-MDEA-H2O-O2 System (7 mol/L total amine, 2 mol/L MDEA) products from MEA

products from MDEA

products from MEA-MDEA blends

2-(methylamino)-ethanol 1,3-propanediamine ethylamine 2-propanamine 1-amino-2-propanol

ethylamine

2-(methylamino)-ethanol 1,3-propanediamine 2-propanamine 1-amino-2-propanol 2-(2-ethoxyethoxy)ethanol 15-crown-5

area % ) 3.4%

Table 7. Carbon Dioxide-Induced Degradation Products at 120 °C and 250 kPa CO2 Pressure MEA-H2O-CO2 system (7 mol/L MEA)

MEA-MDEA-H2O-CO2 system (9 M total amine concentration with 2 mol/L MDEA)

product

molecular formula

product

molecular formula

2-propanone,oxime 12-crown-4 1,4,7,10,13,16-hexaoxacyclooctadecane 2-(2-ethoxyethoxy)ethanol 15-crown-5

C3H7NO C8H16O4 C12H24O6 C6H14O3 C10H20O5

N,N-dimethyl-1,2-ethanediamine 3-amino, 1-propanol N,N-dimethyl-urea ethylamine

C4H12N2 C3H9NO C3H8N2O C2H7N

Different product slates therefore suggests that the presence of MDEA in MEA causes a change in the production pathway as well as a change in the type of product slates. 3.3.2. Extent of Degradation. The role of MDEA on the extent of degradation in MEA-H2O-CO2-O2 and MEA-MDEA-H2O-CO2-O2 was investigated using 7 mol/L MEA (CO2 loading ) 0.3 mol/mol MEA) and 9 mol/L total amine (2 mol/L MDEA, CO2 loading ) 0.4 mol/mol total amine) for the respective systems at 120 °C with 250 kPa O2 partial pressure. The single amine system experimental run time was 198 h while that of the mixed amine system was 288 h. Equation 1 was also

used in the calculation of the percentage degradation of MEA or MDEA after the experiments. The results of our investigation are presented in Table 5. The table shows that 15.7% of MEA has been degraded after 198 h in the MEA-H2O-CO2-O2 system whereas only 5.8% MEA was degraded after 288 h in the MEA-MDEAH2O system. The percentage degradation of MDEA was 6.0%. From these results, it can be concluded that the presence of MDEA in the mixed amine system drastically reduced the degradation of MEA in the MEAMDEA system. 3.3.3. Heat Stable Amine Salt. It is also important to note the presence of HSAS (N-hydroxycarbamic acid,

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Figure 10. cont’d

2-(propoxycarbonylamino) ethylester) in the MEAMDEA-H2O-CO2-O2 system and not in the MEAH2O-CO2-O2 system. The proposed scheme (Figure 11b) showed that the HSAS could be produced from reactions involving both MEA and MDEA. Thus, its formation is not likely to reduce the availability of free MEA for CO2 absorption. 3.3.4. Environmentally Hazardous Products. Table 10 gives the list of environmentally hazardous products for MEA-H2O-O2-CO2 and MEA-MDEA-H2O-CO2O2 systems for experimental runs with 7 mol/L MEA (CO2 loading ) 0.3 mol/mol of MEA) and 9 mol/L total amine concentration (2 mol/L MDEA, CO2 loading ) 0.4 mol/mol of total amine) at 120 °C and 250 kPa O2 partial pressure. Four hazardous products were observed from the MEA-H2O-O2-CO2 system. In the case of MEAMDEA-H2O-CO2-O2 system, 5 hazardous products

were observed from MEA alone, 7 from MDEA and 7 were observed from reactions involving both MEA and MDEA. By comparing the hazardous products in the two systems, it is clear that the presence of MDEA increases the MEA derived hazardous products. Also, for the MEA-H2O-O2-CO2 system, 16% of the initial MEA charged degraded into hazardous products after 198 h whereas 4.4% of the initial total amine charged in the MEA-MDEA-H2O-O2-CO2 system degraded into these products after 288 h. Therefore, MDEA addition led to a reduction in the amount of total amine converted into non environmentally benign degradation products. The overall results for the mixed amine system show that in general, more hazardous degradation products seem to originate from MDEA than either MEA alone or reaction steps involving both MEA and MDEA. Based on our results on extent of degradation we observed that


Figure 10. Pathways for formation of carbon dioxide-induced degradation products at 120 °C and 250 kPa CO2 pressure, (a) MEAH2O-CO2 system (7 mol/L MEA), (b) MEA-MDEA-H2O-CO2 system (9 M total amine concentration with 2 mol/L MDEA). Table 8. Environmentally Unfriendly Products for MEA-H2O-CO2 and MEA-MDEA-CO2 Systems at 120 °C and 250 kPa CO2 Partial Pressure MEA-H2O-CO2 System (7 mol/L MEA) products 2-propanone,oxime 12-crown-4 1,4,7,10,13,16-hexaoxacyclooctadecane 2-(2-ethoxyethoxy)ethanol 15-crown-5 area % ) 3.0% MEA-MDEA-H2O-CO2 System (9 mol/L Total Amine Concentration with 2 mol/L MDEA) products from MEA

products from MDEA

products from MEA-MDEA blends

3-amino-1-propanol ethylamine

3-amino-1-propanol N,N-dimethyl-1,2-ethanediamine ethylamine N,N-dimethyl-urea area % ) 2.6%

3-amino-1-propanol N,N-dimethyl-1,2-ethanediamine N,N-dimethyl-urea

Table 9. Oxygen- and Carbon Dioxide-Induced Degradation Products at 120 °C and 250 kPa O2 Pressure MEA-H2O-O2-CO2 system (7 mol/L MEA, 0.27 mol CO2/mol of MEA)

MEA-MDEA-O2-H2O-CO2 system (9 M total amine with 2 mol/L MDEA, 0.43 mol CO2/mol of total amine)

product

molecular formula

product

molecular formula

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

C5H6N2 C6H6O2 C2H7N C3H9N C3H3N3

3-methyl pyridine 1,2-propanediamine 2-butanamine 1-amino-2-propanol) N-hydrcabaminicacid, 2-(propoxycarbonylamino) ethyl ester dimethylamine 15-crown-5 ethanol, 2-(ethenyoxy)ethyl-urea 1,2-ethanediamine, N,N-dimethyl

C6H7N C3H10N2 C4H11N C3H9NO C7H14N2O5 C2H7N C10H20O5 C4H8O2 C3H8N2O C4H12N2

in addition to the already known advantages of mixed amine system,1-4 the presence of MDEA in the system also leads to a reduction in the degradation of MEA.

This information provides the necessary knowledge for a cost benefit analysis to be performed to determine the best combination of MEA and MDEA as well as the

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Table 10. Environmentally Unfriendly Products from MEA-H2O-CO2-O2 and MEA-MDEA-O2-CO2 Systems at 120 °C and 250 kPa O2 Partial Pressure MEA-H2O-CO2-O2 System (7 mol/L MEA, CO2 Loading ) 0.3 mol/mol of MEA) products methyl-pyrazine ethylamine 1-propaneamine 1,3,5-triazine area % ) 16% MEA-MDEA-H2O-CO2-O2 System (9 mol/L total amine, 2 mol/L MDEA, CO2 Loading ) 0.43 mol/mol of Total Amine) products from MEA

products from MDEA

products from both MEA and MDEA

dimethylamine 2-butanamine 1-amino-2-propanol ethylurea N,N-dimethyl-1,2-ethanediamine

dimethylamine 1,2-propanediamine 3-methylpyridine 2-butanamine 1-amino-2-propanol ethylurea N,N-dimethyl-1,2-ethanediamine area % ) 4.4%

1,2-propanediamine 3-methylpyridine 2-butanamine 1-amino-2-propanol ethylurea 15-crown-5 N,N-dimethyl-1,2-ethanediamine

optimum operating conditions that will provide the optimum package for CO2 absorption. 4. Effect of Other Components of Flue Gas on Amine Solvent Degradation Apart from O2 and CO2, other components of flue gas include nitrogen (N2), flyash and trace contaminants such as sulfur oxides (primarily SO2) and nitrogen oxides (primarily NO2). Most of these contaminants are undesirable to amine treating units.23 Fly ash is the fine solid components in flue gas typically consisting of inorganic oxides such as SiO2, Al2O2, Fe2O3, CaO, MgO, Na2O, K2O and P2O5.23 The presence of fly ash in flue gas stream can lead to a number of operational prob-

Figure 11. cont’d

lems. For example, it can deposit on process equipment, blocking flow, damaging pumps and seals, and clogging equipment and instrumentation. SO2 and NO2 are the combustion products of sulfur and nitrogen compounds in the coal. If these gases enter the CO2 absorber, they will degrade the amine solvent forming heat stable salts and depleting the absorption capacity of the solvent.23 This increases the consumption of amine. It can also result in other operational problems such as foaming and corrosion.23 5. Conclusions 1. The presence of MDEA in a blend of aqueous MEA and MDEA results in smaller number of CO2, induced


Figure 11. Pathways for formation of carbon dioxide and oxygen-induced degradation products at 120 °C and 250 kPa CO2 pressure, (a) MEA-H2O-O2-CO2 System (7 mol/L MEA, 0.27 mol/mol CO2 loading), (b) MEA-MDEA-H2O-O2-CO2 System (9 M total amine concentration with 2 mol/L MDEA, 0.43 mol/mol CO2 loading).

degradation products from MEA alone as compared to the MEA-H2O system. However, there is a general larger overall number of products from the blended MEA-MDEA system than the MEA system. 2. The presence of MDEA in a blend of MEA and MDEA drastically modifies the degradation pathway as compared with the MEA alone system. 3. The overall results show that at temperatures g 100 °C, MDEA is more prone to oxidative degradation than MEA and when used in a blend with MEA is preferentially degraded to protect MEA. At 55 °C, the addition of MDEA to MEA resulted in an increase in MEA derived degradation products. 4. The extent of degradation of MEA in all the systems at temperatures g 100 °C also shows that the MEA degradation is drastically reduced when MDEA is present in the system confirming that MDEA is pref-

erentially degraded at a higher temperature to protect MEA in the MEA-MDEA system as compared to the MEA system. At 55 °C, the exact opposite is the case. 5. Except at 55 °C, the addition of MDEA to the MEA-H2O system reduced the amount of nonenvironmentally benign oxidative degradation products. 6. Even in an initially O2-free environment, O2 is produced as a byproduct of CO2-induced degradation, thereby eventually generating an oxidative degradation environment for both the MEA-H2O and MEA-MDEAH2O systems. Acknowledgment The financial support provided by the Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged.

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Literature Cited (1) Charkravarty, T.; Phukan, U.K.; Weiland, R. H. Reaction of acid gases with mixtures of amines. Chem. Eng. Prog. 1985, 81, 32-36. (2) Campbell, S. W.; Weiland, R. H. Modeling of CO2 removal by amine blends. Paper presented at the AICHE spring National meeting, Houston, TX, 1989, April 2-6. (3) Austgen, D. M.; Rochelle, G. T.; Chen, C. C. A model of vapour liquid equilibria for aqueous acid gas alkanolamine systems II representative of H2S and CO2 solubility in aqueous MDEA and CO2 solubility in aqueous mixture of MDEA and MEA or DEA. Ind. Eng. Chem. Res. 1991, 30 (3), 543-555. (4) Dawodu, O. F.; Meisen, A. Degradation of alkanolamine blends by carbon dioxide. J. Chem. Eng. 1996, 74, 960-962. (5) Critchfield, J.; Rochelle, G. T. 1987. CO2 absorption into aqueous MDEA and MDEA/MEA solutions. Paper No. 43e, presented at A.I.Ch.E National Meeting, Houston, TX, March 30, 1987. (6) Versteeg, G. F.; Kuipers, J. A. M.; van Beckum, F. P. H.; van Swaaij, W. P. M. Mass transfer with complex reversible chemical reactions -II. Parallel reversible. Chem. Eng. Sci. 1990, 45, 183-197. (7) Rangwala, H. A.; Morrell, L. B. R.; Mather, A. E.; Otto, F. D. Absorption of CO2 into aqueous tertiary amine and/MEA solutions. Can. J. Chem. Eng. 1992, 70, 482-500. (8) Hagewiesche, D. P.; Ashour, S. S.; Al-ghewas, A. H.; Sandall, O. C. Absorption of carbon dioxide into aqueous blends of monoethanolamine and N-methyldiethanol amine. Chem. Eng. Sci. 1995, 50 (7), 1071-1079. (9) Liao, C. H.; Li, M. H. Kinetics of absorption of carbon dioxide into aqueous solutions of monoethanolamine reactions. Chem. Eng. Sci. 2002, 45, 183-197. (10) Rooney, P. C.; Dupart, M. S.; Bacon, T. R. Oxygen’s role in alkanolamine degradation. Hydrocarbon Processing, 1998, July, 109-113. (11) Kindrick, R. C.; Atwood, K.; Arnold, M. R. The relative resistance to oxidation of commercially available amines. Girdler Report No. T2.15-1-30, in Report: Carbon dioxide absorbents, Contract No. Nobs-50023, by Girdler Corp, Gas Processes Division, Louisville, KY, for The Navy Department, Bureau of Ships, Washington 25, DC (Code 649P), 1950, June 1. (12) Kohl, A. L.; Nielson, R. B. Gas Purification, 5th ed.; Gulf Publishing: Houston, Texas, 1997.

(13) Blanc, C.; Grall, M.; Demarais, G. The part played by degradation products in the corrosion of gas sweetening plants using DEA and MDEA. Proc. Gas Conditioning Conference, University of Oklahoma, 1982, 32 (3). (14) Lloyd, W. G.; Taylor, F. T., Jr. Corrosion by deterioration of glycol and glycol-amine. Ind. Eng. Chem. 1954, 46, 2407-2416. (15) Supap, T.; Idem, R.; Veawab, A.; Aroonwilas, A.; Tontiwachwuthikul, P.; Chakma, A.; Kybett, B. Kinetics of the oxidative degradation of aqueous MEA in a flue gas treating unit. Ind. Eng. Chem. Res. 2001, 40, 3445-3450. (16) 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, Texas, 1956, April 8-13. (17) Straizisar, B. R.; Anderson, R. R.; White, C. M. Degradation pathways for monoethanolamine in a CO2 capture facility. Energy Fuel. 2003, 17, 1034-1039. (18) Supap, T. Kinetic study of oxidative degradation of MEA in gas treating unit using aqueous MEA solution. MASc Thesis, University of Regina, Canada, 1999. (19) Rochelle, G. T.; Chi, S. Oxidative degradation of Aqueous MEA in capture systems under absorber conditions. Ind. Eng. Chem. Res. 2002, 41, 4178-4186. (20) Chakma, A.; Meisen, A. Identification of methyldiethanolamine degradation products by gas chromatography and gas chromatography-mass spectroscopy. J. Chromatogr. 1988, 457, 287-297. (21) Barnes, D. R. Reduction of heat stable salts formation in a Monoethanolamine (MEA) CO2 removal system. 49th Annual Laurence Reid Gas Conditioning Conference, Norman, Oklahoma: proceedings, 1999, February 21-24. (22) Rooney P. C.; Bacon, T. R.; Dupart, M. S. Effect of heat stable salts on MDEA solution corrosivity. Hydrocarbon Processing Int. Ed. 1996, 75 (3), 95-96. (23) Wilson, M.; Tontiwachwuthikul, P.; Chakma, A.; Idem, R.; Veawab, A.; Aroonwilas, A.; Gelowitz, D.; Barrie, J.; Mariz, C. Test results from CO2 extraction pilot plant at boundary dam coal-fired power station. Energy 2004, 29, 1259-1267.

Received for review August 13, 2004 Revised manuscript received December 8, 2004 Accepted December 17, 2004 IE049261Y

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