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Reaction Products from the Oxidative Degradation of Monoethanolamine Andrew J. Sexton and Gary T. Rochelle* Department of Chemical Engineering, The UniVersity of Texas at Austin, 1 UniVersity Station, Mailcode C0400, Austin, Texas 78712, United States
Aqueous monoethanolamine (MEA) at concentrations of 5 and 7 M was degraded with 100 mL/min of 98% O2/2% CO2 (98 mL/min O2), with oxygen mass transfer achieved by vortexing in a low-gas-flow degradation apparatus. Degraded samples were analyzed by ion chromatography (IC) and high-pressure liquid chromatography (HPLC) with evaporative light scattering detection (ELSD) for oxidative degradation products. In a high-gas-flow apparatus, 7.5 L/min of 83% N2/15% O2/2% CO2 (1125 mL/min O2) was sparged through a mechanically agitated 5 M MEA solution. A Fourier transform infrared (FTIR) analyzer collected continuous gas-phase data. Formate (HCOO-), hydroxyethyl formamide (HEF), and hydroxyethyl imidazole (HEI) account for 92% of degraded carbon at low-gas-flow conditions and 18-59% of degraded carbon at high-gas-flow conditions. Oxalate (C2O42-), oxamide (C2H4N2O2), glycolate (HOCH2COO-), acetate (CH3COO-), carbon monoxide (CO), ethylene (C2H4), formaldehyde (CH2O), and acetaldehyde (CH3CHO) are oxidation products in lower concentrations. Ammonia (NH3), HEF, and HEI account for 84% of degraded nitrogen at low-gasflow conditions and 83-92% at high-gas-flow conditions. Nitrogen oxides (NOx), present in lower concentrations, are stripped from solution at high-gas-flow conditions and retained in the degraded solution at low-gas-flow conditions and oxidized to nitrite/nitrate (NO2-/NO3-). A comparison of product rates to MEA losses shows that 25-50% of products remain unaccounted for in unknown HPLC peaks. Oxygen consumption rates vary from 1 to 2 mM/h, whereas the overall oxygen stoichiometry is 0.75 mol of O2/mol of MEA degraded. Introduction Alkanolamines are used extensively in the gas processing industry to remove acid gases such as carbon dioxide and hydrogen sulfide. Aqueous monoethanolamine (MEA) is the solvent of choice for CO2 capture from flue gas because of its high capacity for CO2 absorption and fast reaction kinetics.1 Figure 1 shows the typical aqueous absorption/stripping process used for CO2 capture. A flue gas stream with 10% CO2 and 5% O2 enters the bottom of the absorber at 55 °C and 1 atm.2 Lean amine solution at a loading (R) of 0.2-0.4 mol of CO2/mol of MEA countercurrently contacts the flue gas. CO2 reacts reversibly with MEA to form MEA carbamate. The rich amine solution, with a CO2 loading of 0.45-0.5 mol of CO2/ mol of MEA, goes through a heat cross-exchanger, where it is preheated by the lean amine solution before entering the top of the stripper at 120 °C and 2 atm. In the stripper, heat is provided in the reboiler by steam, which is used to reverse the chemical equilibrium between the MEA and MEA carbamate, liberating the CO2 and some water. The vapor leaving the stripper is dehydrated and compressed before being pumped for sequestration. The hot lean amine solution is passed back through the cross-exchanger, where it is cooled and recycled back to the top of the absorber. A reclaimer off the bottom of the stripper takes a slip stream to remove heatstable salts and high-molecular-weight degradation products. Degradation of the solvent in this absorption/stripping system occurs by oxidation and carbamate polymerization.3 Carbamate polymerization occurs at stripper temperature with alkanolamines that form carbamates.4 Oxidative degradation can be significant in flue gas applications typically containing 3-15% O2. Because most gas treating processes using alkanolamines for CO2 removal have been performed in the absence of oxygen, * To whom correspondence should be addressed. E-mail: gtr@ che.utexas.edu.
oxidation is a source of solvent degradation that has not been properly quantified. Oxidative degradation is important because it can impact the environment and process economics and decrease equipment life due to corrosion. Rao and Rubin5 estimated solvent degradation to be around 10% of the total cost of CO2 capture. Therefore, a comprehensive understanding of the fundamentals of degradation chemistry is important. The expected result of this effort is the identification and quantification of the liquid-phase and vapor-phase oxidative degradation products of monoethanolamine systems. Two mechanisms have been suggested for MEA oxidation: electron abstraction and hydrogen abstraction. Rosenblatt et al. focused on the oxidation of tertiary amines using chlorine dioxide and other single-electron oxidants.6-9 The validity of these mechanisms is supported by several molecular simulation studies.10-13 Fessenden and Fessenden14 established that aldehydes are very susceptible to autoxidation in the presence of oxygen.
Figure 1. Process flow diagram: Conventional MEA CO2 capture from flue gas.
10.1021/ie901053s 2011 American Chemical Society Published on Web 12/13/2010
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Several researchers have observed that amine oxidation is catalyzed by dissolved iron.15-17 The U.S. Navy supported early work on the oxidative degradation of alkanolamines used to remove CO2 from the air supply of submarines.18 This work tested the relative resistance to oxidative degradation of possible CO2 absorbents.19,20 The accelerated oxidation test involved contacting 100 mL/min of a 50% CO2/50% O2 gas mixture with 100 mL of 2.5 M amine solution at 80 °C and 0.5-1.2 mM Fe. Evolved ammonia was detected by passing the reaction gas through a weak acid solution to absorb the ammonia. The results showed that NH3 production occurred as follows: tertiary amines < primary amines < secondary amines. Blachly and Ravner21 measured the evolution of ammonia and the production of peroxides by sparging air at 1 mL of air/ mL of solution per minute through 4 M MEA at 55 °C for 3-13 days. Without CO2 in the air, they observed no perceptible degradation. Rooney22 measured the formation of carboxylic acids in loaded (R ) 0.25 mol of CO2/mol of amine) and unloaded solutions of 3.3 M MEA, 4.8 M diglycolamine (DGA), 2.9 M diethanolamine (DEA), and both 2.5 and 4.2 M methyldiethanolamine (MDEA). The solutions were degraded by bubbling air at 5.5 mL/min at 180 °F for 28 days. The study identified acetate, formate, glycolate, and oxalate as oxidation products in the degraded amines. These products were observed as well by Critchfield and Jenkins.23 Strazisar24 analyzed degraded MEA samples from the reclaimer bottoms at the IMC Chemicals Facility in Trona, California. A majority of the identified degradation products were created from high-temperature thermal degradation in the regenerator and the thermal reclaimer; however, the oxidation products ammonia, acetic acid, nitrite, and nitrate were quantified. Bello and Idem25 degraded MEA solutions at elevated temperatures and pressures in the presence of O2 and/or CO2; formic acid and N-(2-hydroxyethyl)acetamide were observed using gas chromatography/mass spectrometry (GC/MS). Chi26 and Chi and Rochelle27 decreased the time necessary to quantify amine degradation by instantaneously measuring evolved ammonia by Fourier transform infrared analysis. They found that dissolved iron (Fe2+) at 0.0001-3 mM catalyzed degradation rates from 0.12 to 1.10 mM/h NH3. Goff28 and Goff and Rochelle29,30 examined O2 mass-transfer effects and reaction kinetics by changing reaction conditions and showed that the rate of NH3 evolution with dissolved iron catalysis is controlled by the rate of O2 mass transfer into the amine, not by degradation kinetics. Experimental Methods Apparatus. Figure 2 depicts the oxidation experiment with low gas flow. Oxygen mass transfer was achieved by introducing 100 mL/min of 98% O2/2% CO2 (98 mL/min O2) gas into the vapor space above agitated 5 and 7 M MEA solutions in a temperature-controlled semibatch reactor. Agitation at 1400 rpm vortexed the reaction gas into the solution and transferred the oxygen needed to degrade the amine.29 Reaction gas, consisting of a mixture of CO2 and O2, was bubbled through water to presaturate the gas before it was introduced into the vapor space above the amine solution at atmospheric pressure. Two physical processes occur in this apparatus: The oxygen dissolves into the monoethanolamine solution and then reacts with MEA to form degradation products. Therefore, the overall rate of degradation might depend on the rate of oxygen mass transfer as well as the kinetics of the reaction of the amine with
Figure 2. Apparatus for degradation with low gas flow.
Figure 3. Apparatus for degradation with high gas flow.
oxygen. The presence of dissolved metal catalyst ensures fast reaction kinetics, so that the bulk concentration of dissolved oxygen is near zero and the rate of degradation is controlled by oxygen mass transfer.28 Oxygen mass transfer is promoted by vigorous agitation in the apparatus. In addition, the use of pure oxygen ensures an excess of oxygen in the system without increasing the total pressure (and temperature) of the experimental system, which could introduce the effects of carbamate polymerization.28 Oxidation with high gas flow was achieved by sparging gas through a 5 M agitated MEA solution in a separate temperaturecontrolled semibatch reactor, shown in Figure 3.28 A mixture of house air, nitrogen (N2), and CO2 was bubbled through water at approximately 7.5 L/min to presaturate the gas before it was sparged through the amine solution; the oxygen flow rate through the solution was approximately 1125 mL/min. The combination of vigorous agitation and sparging of the high gas flow through the bottom increased the oxygen mass transfer into the solution. Temperature was continuously recorded with a thermocouple in the agitated solution. To maintain 55 °C in the jacketed reactor, the temperature bath and presaturator bath were set at approximately 63 °C. Analytical Methods. Anionic species formed from the oxidative degradation of amines were quantified using a Dionex IonPac AS15 analytical column and AG15 guard column housed in a Dionex ICS-3000 IC system. The mobile phase was 2 mM aqueous potassium hydroxide (KOH) at a flow rate of 1.6 mL/ min from 0 to 17 min, ramped to 45 mM from 17 to 26 min, and held at 45 mM from 26 to 40 min.
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Table 1. Analysis Regions and Absorbance Limits for Compounds Studied range 1 cm-1
compound water CO2 CO N2O NO NO2 NH3 formaldehyde acetaldehyde MEA methylamine
1883 980 2007 2107 1760 2550 910 988 1034 980 980
range 2 cm-1
abs limit 2161 1130 2207 2246 1868 2933 964 1111 1243 1119 1303
1.0 1.0 0.5 0.5 0.8 0.5 1.0 1.0 1.0 1.0 1.0
range 3
3142 1999 2624 2647 1869
3319 2208 2750 2900 1991
0.5 0.5 1.0 0.5 0.8
980 2450 2638 2624 2450
1196 2600 2916 3150 2650
0.5 0.6 1.0 1.0 1.0
The AG15 and AS15 columns, housed in an oven kept at 40 °C, are designed specifically for the separation of low-molecularweight anions.31 The KOH mobile phase flushes the anionic species off the column and carries them to the ASRS 4-mm suppressor. Amides were quantified by adding 1 mL of 5 M NaOH to 1 mL of experimental sample and allowing 24 h for hydrolysis of any amides. Samples were then run using the method for anionic species to detect carboxylic acids released by the hydrolysis. Nonionic species (amino acids, aldehydes, polymeric products) are not retained on the columns and will not be measured by the ionic conductivity detector. Cationic species (positively charged degradation products and amines) were quantified using two IonPac CS17 columns in series housed in a chromatography oven at 30 °C in a Dionex ICS-2500 IC system. A CSRS 4-mm suppressor removed any negatively charged ions and pumped them to waste, leaving a weakly ionized solution of positively charged cations in water. The mobile phase was 5 mM methanesulfonic acid (MSA) at 1.2 mL/min from 0 to 7 min, jumped to 11 mM at 7 min, and then ramped from 11 mM to 39 mM from 12 to 17 min. The mobile phase remained at 39 mM until 20 min. HPLC analysis of nonionic species was performed with evaporative light scattering detection (ELSD). Nonpolar degradation products were separated with a Waters T3 C18 column housed in the Dionex ICS-3000 apparatus at 40 °C. The nebulizer and evaporator were both set at 50 °C, with a N2 flow rate of 1.6 standard L/min and a light source intensity of 85%. The method started with 98% H2O/2% acetonitrile (ACN) by volume at a rate of 1.0 mL/min from 0-3 min, ramped to 80% H2O/20% ACN from 3 to 15 min, and held there for an additional 5 min. The PL-ELS 2100 evaporative light scattering detector is a unique and highly sensitive detector for semivolatile and nonvolatile solutes in a liquid stream. The heated solvent stream containing the solute material is nebulized and carried with heated nitrogen through an evaporation chamber. The solvent is volatilized, leaving a mist of solute particles that scatter light to a photosensitive device. When highly volatile compounds (including the solvent) are being nebulized, only the vapor passes through the light path, and the amount of light scattered is minimal. When a nonvolatile solute is present, a particle cloud passes through the light path, causing light to be scattered. The signal is amplified, and a voltage output results from the concentration of solute particles passing through the light. Hydroxyethyl formamide (HEF) and hydroxyethyl imidazole (HEI) were identified by injecting known standards using the stated method and matching their retention time to the retention times of the unknown peaks. The unknown peaks were then spiked with HEF and HEI to determine whether the known spike enhanced the unknown peak or created a new one. The presence
cm-1
abs limit
abs limit
2450
2650
0.5
2550
2650
1.0
3219 2650
3396 3211
0.5 0.5
2800
3450
1.0
no. of refs 13 3 8 5 4 4 3 3 1 3
of these peaks was also confirmed at an independent laboratory using HPLC-MS. Dissolved iron was added to the reactor as iron(II) sulfate heptahydrate (FeSO4 · 7H2O). Sulfate (SO42-) is not consumed in any of the degradation reactions, so it was assumed that sulfate was conserved. Sulfate concentration was quantified using anion chromatography and used as an internal standard. If the water content in the reactor deviated from its initial value at any point during the degradation experiment, the sulfate concentration would change. Any increase in water content would result in a smaller sulfate peak area, whereas any decrease in water content would produce a larger sulfate peak area. Solution level in the reactor was monitored daily, and water was added to offset any evaporative losses. Sulfate area can also be affected by the response of the ICS3000 conductivity detector, which can vary by (1% from day to day. If any deviation in sulfate concentration occurred, then all other amine and degradation product concentrations were adjusted accordingly. Therefore, the concentration numbers reported for each oxidative degradation experiment are on the same water basis. Experimentally degraded samples were placed in the refrigerator, but were not kept under a nitrogen pad. Therefore, oxygen was present in any vapor space above the solution. Based on the length of time required to develop the analytical methods and the availability of analytical equipment, it was not possible to analyze all samples as soon as they were removed from the reactor. Every effort was made to avoid the effects of sample aging. With high gas flow, a hot gas FTIR analyzer (a Temet Gasmet Dx-4000) provided simultaneous analysis of up to 50 components. The sample pump and sample cell were controlled at a temperature of 180 °C to provide sample measurement without having to dry or dilute the gas stream. To properly resolve multiple components, different analysis areas (wavenumber regions) can be set for each component. The Calcmet software allows for up to three analysis areas to be set for each compound, each with a different absorbance maximum. If the absorbance of the sample spectrum goes above the set maximum, the software will no longer use the analysis region for that compound. The analysis regions are also determined by choosing regions where absorption peaks for multiple compounds do not overlap or interfere with one another. Table 1 lists the regions used for each compound in the highgas-flow experiments and the number of references used for each component. Total nitrogen content in degraded amine solution was determined using EPA Method 351.4.32 Total Kjeldahl nitrogen was determined potentiometrically using an ion-selective elec-
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Table 2. Oxygen Stoichiometry for Important Liquid- and Gas-Phase Oxidative Degradation Products of MEA product
stoichiometry (ν)
NH3 formaldehyde formic acid hydroxyethyl imidazole hydroxyethyl formamide NO CO2 HNO2 NO2 N2 O oxalic acid HNO3
0.0 0.25 0.75 0.625 0.75 1.25 1.25 1.5 1.75 2.0 2.0 2.0
trode. This method is favored because interference from metals is eliminated with the addition of sodium iodide (NaI). Total organic carbon (TOC) was measured using a Shimadzu 5050A TOC analyzer. The Shimadzu TOC analyzer can be used to measure both total inorganic carbon (TIC) and total carbon (TC). TOC was calculated from the difference between the two measurements. For the TIC measurement, 2.5 M phosphoric acid (H3PO4) was used to evolve the CO2 gas from the solvent. The stream of CO2 was analyzed with a nondispersive infrared (NDIR) detector. For total organic carbon analysis, a precisely metered slipstream of the sample was combusted over platinum catalyst at 680 °C with ultrapure air. The resulting CO2 was measured with the NDIR detector. A 10 mM IC standard was prepared from a mixture of sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), and deionized (DI) water. Amine solutions were loaded with CO2 by sparging with pure CO2 in 1000 mL gas washing bottles. CO2 loading was verified by TIC analysis using a model 525 analyzer from Oceanography International Corporation. All experimental procedures were explained in greater detail by Sexton.33 Oxygen Stoichiometry. Goff28 concluded that the rate of evolution of NH3 is controlled by the rate of oxygen absorption when catalyzed by dissolved metals under experimental and industrial conditions. Goff assumed that 1 mol of MEA degraded to 1 mol of ammonia. Ammonia evolution rates increased with agitation rate and increased linearly with oxygen concentration. Goff28 proposed that MEA reacts with oxygen to form ammonia and other carbon-containing degradation products, which are listed in Table 2. Each of the major degradation products has a specific oxygen stoichiometry, also given in Table 2. NH2CH2CH2OH + νO2 f NH3 + product Hydroxyethyl formamide (HEF) can be formed by the reaction of formaldehyde and MEA, followed by oxidation of the imine structure to HEF. Although metal ion catalysis might not be necessary to facilitate this reaction path, it likely enhances the reaction rate.
Figure 4. 1-(2-Hydroxyethyl)imidazole (HEI).
With the exception of the partial amide of MEA and oxalic acid, MEA amides do not have anionic character and thus go undetected using anion chromatography analysis. Koike et al.34 showed that N-formyldiethanolamine (FORMYDEA) in degraded solutions of diethanolamine could be determined as formate by hydrolyzing unknown samples with 6 M sodium hydroxide (NaOH). In addition to HEF, HPLC analysis revealed 1-(2-hydroxyethyl)imidazole (HEI, Figure 4) present in significant concentration. According to Arduengo et al.,35 HEI can be produced from ammonia, formaldehyde, glyoxal (C2H2O2), and MEA, all of which could be present in degraded solution. Determination of Product Rates. Oxidation experiments were performed at high and low gas rates in the presence of dissolved iron added as iron(II) sulfate heptahydrate. Liquidphase product rates were calculated by dividing the final concentration of each individual component by the total experiment time. For each volatile component, the continuous production rate was integrated over the entire experimental run and reported as an average rate (mM/h). MEA volatility was calculated and quantified in the same manner. Amides were determined by both HPLC (HEF only) and anion IC (acetamide, glycolamide, formamide, and oxamide) after hydrolysis to the carboxylic acid. Because the HPLC analysis does not alter the chemistry of the degraded samples, it was used in all material balances. Concentrations for unknown peaks from HPLC were estimated using the calibration curve for HEI. Total MEA loss was calculated from initial and final MEA concentrations as determined by cation IC. Loss rates of less than 0.3 mM/h are too small to detect using this method. For the high-gas-flow experiments, overall MEA loss was calculated using cation chromatography, and volatile MEA loss was calculated using FTIR spectroscopy. The difference between these two rates gave an MEA degradation loss rate. The total carbon and nitrogen in products was calculated without including formamide by IC or unknowns by HPLC; formamide concentration was incorporated from HPLC analysis. Nitrogen in solution was determined using Kjeldahl analysis; total organic carbon in solution was calculated using a Shimadzu TOC analyzer. The nitrogen imbalance represents nitrogen that remained unaccounted for after MEA nitrogen and product nitrogen concentrations were subtracted from total nitrogen in solution; the carbon imbalance was calculated in a similar manner. Total oxygen consumption was determined by calculating the sum of the oxygen stoichiometry coefficient (ν) for each degradation product multiplied by the respective degradation rate of each product. Results Products with Low Gas Flow. Figure 5 illustrates the typical accumulation of liquid-phase degradation products with low gas flow. HEF, HEI, and formate were the most concentrated oxidation products.
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Table 4. Summary of Oxidative Degradation Product Rates (mM/h) for High-Gas-Flow Experimentsa first experiment
second experiment
Results (mM/h) MEA loss C in products N in products O2 consumption
5.8 1.5 2.0 0.9
3.8 1.1 2.0 1.1
HPLC (mM/h) HEF HEI unknown peaks
0.00 0.00 0.54
0.00 0.00 0.50
Anion IC (mM/h) Figure 5. Oxidative degradation of 5 M MEA. Conditions: 1 mM Fe2+, 0.40 mol CO2/mol of MEA, 55 °C, 1400 rpm, 95-105 mL/min 98% O2/ 2% CO2, sulfate correction included. Table 3. Summary of Oxidative Degradation Product Rates (mM/h) for Low-Gas-Flow Experimentsa MEA concentration (M): iron concentration (mM):
5 0.6
7 0.1
5 1.0
N/A 0.5 0.5 1.2
3.8 6.3 2.5 2.1
N/A N/A N/A
0.66 0.77 2.28
0.41 N/A 0.46 0.05 N/A 0.02 0.00 0.03
0.29 0.35 0.21 0.09 0.09 0.02 0.00 0.00
Results (mM/h) MEA loss carbon in products nitrogen in products O2 consumption
N/A 0.7 0.4 1.2 HPLC (mM/h)
HEI HEF unknown peaks
N/A N/A N/A Anion IC (mM/h)
formate formamide nitrite nitrate oxamide oxalate glycolate acetate
0.40 N/A 0.24 0.16 N/A 0.04 0.10 0.02
a Conditions: 55 °C, 95-105 mL/min 98% O2/2% CO2, R ) 0.40, 1400 rpm.
In Table 3, analysis shows that the NaOH method underpredicted the HEF concentration by approximately 55%. HPLC gave a HEF rate of 0.77 mM/h, whereas NaOH addition coupled with anion IC gave a calculated formamide rate of 0.35 mM/h. It is possible that the reaction of concentrated NaOH with the degraded MEA sample did not completely reverse the reaction back to MEA and formate, or that the amide broke down into other products. Another explanation is that the products formed from the hydrolysis of the HEF reacted with other degradation products in solution to disguise the formate as another substance. Results from the formation of degradation products listed in Table 3 showed some variance among the low-gas-flow degradation experiments. Formate and nitrite/nitrate products both varied by approximately 25% between the two low-gasflow experiments conducted in the presence of 5 M MEA. This variance can be explained in terms of sample aging: not all samples were analyzed directly after they were withdrawn from the reactor. Dissolved Fe2+ varied from 0.1 to 1 mM, yet product formation rates were all on the same order of magnitude. It appears that there is a minimum concentration of iron (less than or equal to 0.1 mM) that provides sufficient activity to ensure that degradation is controlled by oxygen mass transfer.
formate formamide oxamide
0.10 0.16 0.01
0.18 0.49 0.10
FTIR Spectroscopy (mM/h) NH3 CO CH4 N2 O NO NO2 C2 H4 formaldehyde acetaldehyde methylamine MEA volatile loss
1.83 0.30 0.00 0.00 0.12 0.01 0.24 0.09 0.16 0.00 2.5
1.69 0.00 0.03 0.16 0.12 0.00 0.00 0.02 0.06 0.01 3.2
a Conditions: 5 M MEA, 7.25-7.75 L/min 15% O2/2% CO2, R ) 0.40, 1 mM Fe+2, 1400 rpm.
HEI, HEF/formamide, and formate were the major oxidation products at low gas flow. Carboxylic acids, nitrate, and nitrite were observed in lower concentrations. Products with High Gas Flow. Major products at high gas rate include ammonia, formamide, and formate (Table 4). HEF and HEI were not detected at high gas rates. However, unknown peaks in the HPLC data were approximately equal to the amount of formate released by NaOH hydrolysis and reported as formamide. The absence of HEF and HEI at high gas rates could be attributed to the stripping of ammonia and/or aldehydes needed for HEI synthesis. Likewise, NOx/N2O concentrations from the last experiment at high gas flow (0.28 mM/h) were similar to nitrite/nitrate concentrations in the last experiment at low gas flow (0.30 mM/ h). Nitrite/nitrate was not observed with high gas flow because the intermediates necessary for their formation have been stripped out of solution. The MEA loss was 32% in the first high-gas-flow experiment and 16% in the second. Volatile MEA losses match data from Hilliard.36 Other volatile degradation products included CO, C2H4, formaldehyde, and acetaldehyde. In one high-gas-flow experiment, these combined product concentrations comprised 43% of total ammonia production. In the other high-gas-flow experiment, they made up 7% of the total ammonia production. The elevated combined concentration of formate, formaldehyde, and CO in the first high-gas-flow experiment was similar to the formate and formaldehyde concentration in the second experiment. Formate is the reaction product of CO and water, whereas formaldehyde is an intermediate in formate production. Likewise, the combinations of gas- and liquid-phase products containing two carbons from the two experiments were similar in total concentration. The data suggest that the earlier highgas-flow experiment stripped more of the degradation products out of the apparatus.
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Kjeldahl and TC analysis were run on the second high-gasflow experiment to verify the FTIR measurements. The FTIR results showed that 13% of total nitrogen in solution was lost to either volatility or gas-phase degradation, whereas the total nitrogen analysis showed 29% nitrogen losses. FTIR showed 3% carbon losses to the gas phase, whereas total carbon analysis showed 5% carbon losses. Material Balance. The oxygen consumption rate calculated for each degradation experiment reveals that, despite the differences in the two degradation experiments, the mass-transfer capabilities were quite similar. Oxygen consumption rates ranged from 1.15 to 1.93 mM/h in the low-gas-flow apparatus; in the high-gas-flow apparatus, the oxygen consumption ranged from 0.93 to 1.15 mM/h. An average oxygen stoichiometry for each experiment in which HPLC analysis had been performed was calculated. An approximate MEA loss was calculated for each experiment using the total C and N in the products; for example, in the second low-gas-flow experiment, the MEA loss from detected products was 2.8 mM/h. A formation rate of 2.5 mM/h of N in the products correlates to 2.5 mM/h of degraded MEA, whereas 6.3 mM/h of C in the products correlates to 3.15 mM/h of degraded MEA; the mathematical average equates to 2.8 mM/ h. Because the oxygen consumption rate was 2.1 mM/h, the average oxygen stoichiometry is 0.75 mol of O2/mol of MEA degraded at low gas rate. The average ν value ranged from 0.65 to 0.85 at high gas flow rate. Despite the high gas-phase product concentrations and inexplicably high MEA losses in the first high-gas-flow experiment, the overall total carbon and nitrogen productions were similar for the two experiments. If the material balance for each experiment were to close 100%, then the MEA degradation rate (in mM/h) would equal the nitrogen formation rate and twice the carbon formation rate. For the high-gas-flow experiments, only 13-23% of the degraded MEA carbon has been accounted for in noted degradation products. In the case of degraded MEA nitrogen, 37-100% was accounted for in degradation products. For the fully analyzed low-gas-flow experiment, 65% of the degraded nitrogen and 83% of the degraded carbon were accounted for. HPLC analysis of the degraded MEA solutions revealed unknown peaks other than HEF and HEI. Identification and quantification of the unknown peaks using high-pressure liquid chromatography with mass spectrometry (HPLC/MS) will be necessary to close the carbon and nitrogen material balances. We attempted to quantify the aldehyde concentration using HPLC per methods developed by Nascimento et al.,37 but found aldehydes in only trace concentrations in bulk degraded solution. Formaldehyde, acetaldehyde, hydroxyacetaldehyde (C2H4O2), and glyoxal should be present in measurable quantities, either in pure form or tied up with MEA as imine or amide complexes. It is possible that the unknown HPLC peaks are these highermolecular-weight amide degradation products. The carbon-to-nitrogen ratio of the products should be 2:1, but at high gas flow rate, it ranged from 0.45:1 to 0.69:1. On the other hand, at low gas flow rate (where gas-phase products were not analyzed and were probably insignificant), the carbonto-nitrogen ratio ranged from 1:1 to 2.5:1. In the first two lowgas-flow experiments, sulfate was not used to account for any changes in water content. For these experiments, the error due to water content is upward of 20%, compared to 2% for experiments conducted with sulfate analysis. According to TIC analysis, an unloaded 5 M MEA experiment run in the presence of 1 mM of iron(II) and 0.5 M potassium
Figure 6. MEA concentration losses during degradation experiments.
formate (KCOOH) in the high-gas-flow apparatus produced CO2 at a rate of 0.10 mM/h. This rate of CO2 production is not enough to close the carbon gap in the material balance. It is very likely that the gap in the material balance lies with the unidentified peaks that appear using evaporative light scattering detection. Figure 6 illustrates the MEA loss for three experiments. All MEA disappearance rates appear to be fairly linear. One highgas-flow experiment produced a significantly higher MEA disappearance rate than the other two experiments; MEA entrainment was problematic in this experiment and migh account for this enhanced loss rate. At approximately 200 h, MEA losses varied by 35% between the other two experiments. Conclusions Formate, hydroxyethyl formamide (HEF), and hydroxyethyl imidazole (HEI) accounted for 92% of the degraded carbon that has been quantified in the low-gas-flow apparatus; they accounted for 18-59% of the degraded carbon in the high-gasflow apparatus. Oxalate (and its MEA amide), glycolate, and acetate were also present at much lower concentrations. The ratio of MEA formamide to formate varied from 1.2:1 to 2.7:1, whereas the MEA oxamide-to-oxalate ratio varied from 4.5:1 to 10:1. Formate and its hydroxyethyl formamide were approximately 6 times more abundant than oxalate in the highgas-flow apparatus and 10 times more in the low-gas-flow apparatus. We believe that the formation of these amides is reversible, especially at stripper conditions. The reversibility of the formation of HEI is unknown. Ammonia, HEF, and HEI were found to be the dominant nitrogen-containing degradation products; they accounted for 84% of the degraded nitrogen in the low-gas-flow apparatus and 83-92% in the high-gas-flow apparatus. At high gas flow rate, NOx was produced and stripped from the solution. At low gas flow rate, NOx was retained in the solution and oxidized to nitrite and nitrate. At high gas flow rate, NOx/N2O production occurred at approximately 15% of the rate of ammonia production. At low gas flow rate, nitrite/nitrate production occurred at the same rate as NOx/N2O production at high gas rate. For the three experiments in which amides were accounted for, the ratio of NOx/N2O/nitrite/nitrate to formate/formamide was 30-45%. Acknowledgment This work was supported by the Luminant Carbon Management Program at The University of Texas at Austin. Mark Nelson of Air Quality Analytical helped develop methods for the FTIR experiments. Jason Davis at The University of Texas at Austin assisted in developing the HPLC-ELSD method, and Robert Grigsby of Huntsman aided in positively identifying the
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ReceiVed for reView June 30, 2009 ReVised manuscript receiVed November 16, 2010 Accepted November 17, 2010 IE901053S
