Article pubs.acs.org/IECR
Understanding 2‑Ethanolamine Degradation in Postcombustion CO2 Capture Eirik F. da Silva,*,† Hélène Lepaumier,‡ Andreas Grimstvedt,† Solrun Johanne Vevelstad,‡ Aslak Einbu,† Kai Vernstad,† Hallvard F. Svendsen,‡ and Kolbjørn Zahlsen† †
SINTEF Materials and Chemistry, N-7465 Trondheim, Norway Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway
‡
S Supporting Information *
ABSTRACT: 2-Ethanolamine (MEA) degradation has been studied under varying conditions of relevance to postcombustion CO2 capture. Degradation experiments performed in the laboratory were chosen to be representative of the conditions in a CO2 capture plant facility. The thermal degradation of MEA was investigated in closed-batch experiments at 135 °C at different loadings. MEA degradation was also studied in oxidative conditions without additives or by adding FeSO4/fly ash. These experiments were compared with three MEA campaigns performed in pilot plants at Tiller (Norway), Esbjerg (Denmark), and Longannet (U.K.). The same analytical procedures were used to identify and quantify the main degradation compounds. Mechanisms are also proposed to account for the observed degradation products. For the Tiller campaign 99.7% of nitrogen containing compounds in the liquid at the end of the campaign was accounted for by the solvent and quantified degradation products.
1. INTRODUCTION The technology of amine-based postcombustion CO2 capture has continuously improved over the years. Until now, 2ethanolamine (MEA) has remained the benchmark solvent owing to its good properties toward CO2 (fast absorption rate, cheap, not volatile). However, a number of companies and research groups are also developing other solvent systems. In addition to the energy requirement of the process, solvent degradation and corrosivity are important issues for industrial application of this technology. In order to improve the MEA based process and develop better solvent systems, it is useful to understand the chemistry of MEA degradation. This entails identifying the degradation products formed and understanding their formation mechanisms. There has recently also been an increased focus on the need for monitoring emissions of solvent and degradation products from CO2 capture plants. Understanding of solvent degradation is an important input to emission monitoring and emission control. The study of amine degradation is, however, not an easy task. It is difficult to set up a lab-scale experiment representative of the dynamic cycling system of the solvent between the absorber and the stripper. In the real process, the solvent is subject to varying conditions (temperature, gas composition) which can lead to different kinds of degradation in the various parts. In fact, two main forms of degradation are expected to occur in a CO2 capture plant: degradation due to temperature with CO2 more likely to proceed in the stripper where the highest temperatures are encountered (around 120 °C for MEA); oxidative degradation, which is mostly expected in the absorber since this is where the O2 concentration is the highest. Amine degradation has been studied for a while: Nmethyldiethanolamine (MDEA) 1,2 and diethanolamine (DEA)3−8 were of interest due to their wide use in natural © 2012 American Chemical Society
gas treatment and MEA is the benchmark solvent for postcombustion application. The effect of temperature by itself was first reported by Chakma and Meisen,9 who studied the thermal stability of MDEA. A recent paper has shown that MEA cannot be considered completely stable at 140 °C.10 The thermal degradation in the presence of CO2 has been studied since the 1950s, first for MEA.11,12 The main degradation products have been identified as 2-oxazolidinone (OZD), N-(2-hydroxyethyl)ethylenediamine (HEEDA), and N-(2-hydroxyethyl)imidazolidinone (HEIA). N,N′-bis(2hydroxyethyl)urea was also reported.13 Regarding the degradation mechanisms, different explanations have been proposed especially regarding the order of appearance among these three compounds. The mechanism reported recently by Davis13 appears to be the most probable and was previously proposed by Kim and Sartori14 for DEA degradation. While thermal degradation seems to be reasonably well understood, there are still some knowledge gaps in the understanding of oxidative degradation. Numerous degradation products have been identified; among these are volatile compounds (ammonia, methylamine, formaldehyde, acetaldehyde), carboxylic acids (formic, acetic, glycolic, and oxalic), and more recently N-(2-hydroxyethyl)formamide (HEF) and N-(2hydroxyethyl)imidazole (HEI).15 Radical mechanisms are generally thought to play an important role in oxidative degradation (Rochelle and co-workers16−18 and Petryaev19). Rooney et al. suggested some reaction paths for the formation of carboxylic acids in the oxidation of MEA and other Received: Revised: Accepted: Published: 13329
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alkanolamines.20 Finally, we are aware of only a single study in the open scientific literature dealing with degradation of MEA in a postcombustion CO2 capture plant.21 The present paper reports a study of MEA degradation both under laboratory conditions and real CO2 capture plant conditions. The thermal degradation of MEA in the presence of CO2 was investigated in closed-batch experiments at 135 °C at different loadings (where the loading is the ratio of moles of CO2 absorber to moles of solvent). It is also shown that there is no effect of previous degassing of the initial MEA solution, as well as of the presence of dissolved metals. The oxidative degradation experiment was performed in an open-batch system at 55 °C. The effect of addition of FeSO4 or fly ash was also studied. MEA degradation observed at the laboratory was compared with MEA campaigns at different pilot plant facilities: Tiller (Norway), Esbjerg (Denmark), and the mobile test unit (MTU) at Longannet (U.K.). During the campaigns, samples were taken in different parts of the process: from the lean solution (low in CO2 content), from rich MEA solutions (with high CO2 content), and from the water wash. These samples (from lab experiments and pilot plants) were analyzed by liquid chromatography−mass spectrometry (LC− MS), gas chromatography−mass spectrometry (GC−MS), and ion chromatography (IC). Further information was obtained by additional analyses (inductively coupled plasma (ICP)-MS, Kjeldahl analyses, heat stable salt quantification).
into the mixture (0.35 L/min air + 7.5 mL/min CO2) and heated to 55 °C. The gas blend was saturated with water before entering the reactor; this was done in order to maintain the water balance. The temperature in the reactor was chosen to be reasonably representative of absorber conditions (40−70 °C) while at the same time giving manageable conditions in an open-batch reactor. Regularly, some samples were taken from the liquid phase for analysis. The effect of iron or fly ash on oxidative degradation was also investigated separately by adding 1 mM FeSO4 or 3.4 g/kg solution of fly ash. The FeSO4 was added as iron(II) sulfate heptahydrate (FeSO4·7H2O, CAS No. 7782-63-0), obtained from Merck with a reported purity of 99.5%. The fly ash was obtained from the Longannet power station; its chemical composition was not determined. 2.2. MEA Campaigns in Pilot Plants. Tiller Pilot. The Tiller pilot plant has been constructed by SINTEF in Trondheim, Norway. The absorber has 19.5 m of packing and two water-wash sections with 1.7 m of packing each. It is accessible for sampling before and after each water-wash stage. The flue gas source is a propane burner intended to simulate the exhaust gas from a natural gas fired power plant. The flue gas contains 15% oxygen and 20 ppm NOx and has very low SOx levels. A 14 week campaign was carried out with 30 wt % MEA. The campaign was continuous and no reclaiming operation was carried out. Esbjerg Pilot Plant. The pilot plant at Esbjergværket (ESV) in Denmark operates on a slipstream of flue gas from a 400 MW pulverized-coal-fired power station. It is equipped with selective catalytic reduction (SCR) DeNOx, electrostatic precipitators (ESP), and flue gas desulfurization (FGD) processes to clean the flue gases before treatment. The flue gas has less than 10 ppm SO2 and less than 65 ppm NOx. In 2009, a 3360 h (20 weeks) test campaign was conducted at ESV using 30 wt % MEA. After 11 weeks of operation, liquid samples were taken in the different parts of the pilot plant: the lean amine in the absorber, the rich amine in the stripper, and the water washes (for both the absorber and stripper). In addition, the final lean MEA solution corresponding to the end of the campaign was analyzed. The campaign was continuous and no reclaiming operation was carried out. Longannet Pilot Plant. The mobile test unit (MTU) is a pilot plant designed and operated by Aker Clean Carbon. The MTU can be moved to different power plants in order to test CO2 capture on different exhaust gas sources. In 2009 the MTU operated on a slipstream of flue gas from the Longannet coal-fired power station (2400 MW) in Scotland (U.K.). A campaign with 30 wt % MEA was carried out for benchmarking purposes. The campaign was of approximately 6 months duration. A solvent reclaiming campaign was carried out after 3 months. The oxygen content was around 10%, and NOx levels were measured to be in the range 80−170 ppm. 2.3. Analyses. LC−MS based analysis is our preferred method for quantification of degradation products. LC−MS analysis can be used to detect most ionizable compounds with a certain mass. A limitation with LC−MS is that there is no library to match unknown degradation products with. GC−MS is not as sensitive as LC−MS, but libraries are available that can be utilized to identify unknown degradation products. Ion chromatography is chosen for detecting and quantifying ionic compounds for which we do not have an available LC−MS method.
2. EXPERIMENTAL SECTION 2.1. Laboratory-Scale Experiments. Thermal Degradation without CO2 Loading. A 30 wt % aqueous solution of MEA was prepared by use of deionized water. After degassing with nitrogen for 15 min in order to strip any air or CO2 contamination, the solution was introduced in 316 stainless steel cylinders: in half of them, 7 mL of the solution was directly introduced, and in the second half, 3.5 mL of solution was first filled into a glass tube that was then introduced into the cylinder. The glass tube essentially worked as a liner inside the metal cylinders, preventing contact between metal and solvent. Later, the cylinders were put in a Memmert oven (Model 600) from GmBH+Co and heated for 5 weeks at 135 °C. This temperature was chosen as a compromise between classical stripper conditions (around 120 °C for MEA) and the desire to obtain quantifiable levels of degradation in an experiment of limited duration. At regular intervals (every week), one cylinder was taken and analyzed by LC−MS and GC−MS under the conditions described below. Potential leakages were checked by weight comparison of each cylinder before and after the experiments. Thermal Degradation with CO2 Loading. As previously, a 30 wt % amine solution was prepared with deionized water, degassed with N2, and loaded with 0.5 mol of CO2 per mole of amine. The solution was loaded by sparging CO2 gas into the solution and determining the uptake of CO2 by measuring the weight change; this was done prior to introducing the solution into the cylinders. Afterward, the same procedure as above was applied to the CO2-loaded amine solution. One sample was taken each week and analyzed. Oxidative Degradation. A 30 wt % MEA solution, previously loaded with CO2 (α = 0.4), was introduced in an open-batch reactor. The liquid volume in the reactor was 1 L. The reactor was equipped with two cooling coils to limit evaporative losses. A gas blend of air with 2% CO2 was sparged 13330
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Table 1. MEA Degradation under Different Conditions
1 Degree of confidence in identification. A, certain (checked with a commercial standard); B, likely; C, uncertain. 2Peak suspected to be this compound found in ion chromatography of a pilot plant sample. 3Samples not analyzed for this compound. 4IC column could not separate glycolate and acetic acid.
Liquid Chromatography−Mass Spectrometry (LC−MS). Analyses of the degraded samples were carried out on a LC− MS/MS system, a 6460 triple quadrupole mass spectrometer coupled with a 1290 Infinity LC chromatograph and an Infinity autosampler 1200 Series G4226A from the supplier Agilent
Technologies. The molecules were converted into ions by an electrospray ionization (ESI) source. The analytical column was an Ascentis Express RP-Amide HPLC column (15 cm × 4.6 mm, 2.7 μm, catalog no. 53931-U, Supelco Analytical, Bellefonte, USA). The eluent was 25 mM formic acid in 13331
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background conductivity was less than 1 μS, and the system pressure was around 1800 psi. The injection volume was 10 μL. All standards used for calibration were purchased from Sigma Aldrich with a purity of >97% with the exception of DL-lactic acid (90%). Samples were diluted by factors ranging from 1/50 to 1/2.
water, with a 0.6 mL/min flow rate. All LC−MS methods utilized in the present work were isocratic. For the quantification of the remaining amine, the sample was diluted to 1/10 000 in water before injection. A specific method (MS/MS) and addition of an internal standard, MEAd4 (HO−CD2−CD2−NH2) permitted a higher accuracy. With the use of an internal standard, drifts in the instrument and matrix effects can be corrected for. The product ion monitored for MEA had an m/z of 44.2. For quantification of degradation products, calibration curves were generated by injecting the commercial standard at different concentrations. Calibration curves are given in the Supporting Information (Figures S2−S11). It should be noted that both instrument drift and matrix effects are sources of uncertainty in the quantification of products. The ion transition ratios for the commercial standard were compared with those of the degradation product identified in the solvent. We required transition ratios to stay in a range of ±20% in order to accept it as a correct identification of the compound. For the detection of the degradation compounds (full scan mode), the initial sample was diluted in water before injection to 1/100 for the positive ions (M + H+) and 1/10 for the negative ions (M − H+). Gas Chromatography−Mass Spectrometry (GC−MS). GC−MS analyses were carried out on a 7890A gas chromatograph equipped with an autosampler 7693 and coupled with a mass spectrometer (inert XL EI/CI MSD with triple axis detector 5975C). The mass spectrometer allows having both scan and SIM modes and could be used in electron impact (EI) and positive and negative chemical ionizations (PCI and NCI). Helium was used as carrier gas and methane as reagent gas for chemical ionization (CI). The samples were first diluted in water to 1/10 to 1/100 depending on the type of analysis and on the level of degradation. Separation of the different analytes was performed on a CP-SIL-8 CB amines column (Varian) from the method described in the Supporting Information. Identification of the degradation compounds has been made possible by running in both EI mode (fragmentation of the molecule) and in CI mode (molecular weight). Quantification was performed in EI mode (SIM) by external calibration. Commercial standards were used to obtain calibration curves. The compound N-(2-aminoethyl)-N′-(2-hydroxyethyl)imidazolidinone (AEHEIA) was not commercially available; quantification was in this case an estimate based on a standard with a similar chemical structure. Ion Chromatography (IC). The IC analyses were carried out on a Dionex DX-500 system equipped with an autosampler (AS-50), gradient pump (GP50), column oven (TCC-3000), and electrochemical detector (ED40) with a conductivity cell with a DS3 detector stabilizer. The separation of the different analytes was performed on an IonPac AG11-HC guard column (4 × 50 mm) and AS11-HC analytical column (4 × 250 mm) with a self-regenerating suppressor (ASRS 300, 4 mm) and a trap column (ATC-3) which removed carbonate from the mobile phase, which was sodium hydroxide. The mobile phase, sodium hydroxide, was diluted in deionized water (18.2 MΩ) obtained from a Milli-QPlus water system from MilliPore: 1 mM NaOH for 18 min, then increasing to 15 mM over 12 min, and further increasing to 30 mM and then 60 mM each over 10 min, before reducing to 20 mM and 1 mM over 7 min with a flow rate of 1.5 mL/min. The
3. RESULTS In Table 1 is reported a summary of the main degradation compounds under the different conditions (thermal degradation, oxidative degradation, pilot plant campaigns). In Table 1 we have indicated the level of confidence in the identification of degradation products. We regard compounds that have been checked against commercial standards as certain. We use the category “likely” for compounds where we see a plausible mechanism for the formation of the degradation product and where the analytical data fit with the suggested compound, but where we have not been able to test the compound against a commercial standard. The category “uncertain” is used when the compound is our best guess for a mass seen in analysis, but where we have no validation against a standard and no mechanism to account for its formation. In Table 1 we have chosen not to report the pilot plants separately. The campaigns were carried out at different points in time, with different analytical methods available for the different campaigns. Differences in what degradation products have been identified in the different campaigns may therefore be due to differences in analytical work carried out, rather than any difference in what degradation products are formed in different plants. Nitrite and nitrate were also found as degradation products in MEA with iron and in the pilot plant samples. The nitrite and nitrate buildup in pilot samples is likely to be mainly a result of uptake of NOx from the flue gas, and the contribution from solvent degradation is likely to be relatively small. 3.1. Thermal Degradation. In Figure 1 is shown the effect of CO2 loading on the thermal degradation of MEA (30 wt %). Loadings were varied from 0 to 0.5. At 135 °C in the presence of CO2 (loading of 0.5), MEA degraded 57.6% after 5 weeks; the slope of the degradation rate was quite linear during the first 4 weeks, and then started to slow down. In Figure 2, the amine loss is plotted together with the formation of different degradation products. The plot
Figure 1. Effect of CO2 loading (a) on the thermal degradation of MEA (30 wt %, 135 °C). 13332
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Figure 2. Thermal degradation of MEA (30 wt %, CO2 loaded, α = 0.5, 135 °C).
Figure 3. Oxidative degradation of MEA (30 wt %, CO2 loaded, α = 0.4, sparged with air (0.35 L/min) + 2% CO2 (7.5 mL/min)).
Figure 4. Oxidative degradation of MEA with addition of 1 mM Fe (30 wt %, CO2 loaded, α = 0.4, sparged with air (0.35 L/min) + 2% CO2 (7.5 mL/min)). HSS is heat stable salts (in this case expected to be primarily organic acids).
one new compound, N-(2-aminoethyl)-N′-(2-hydroxyethyl)imidazolidinone (4, AEHEIA), whose structure was suggested by interpreting the mass spectra both in EI mode and in CI mode, was also identified. It can be seen from Figure 2 that the amount of the two imidazolidinones (HEIA and AEHEIA) increases with time, which is an indication of their relative stability. In contrast, the amounts of OZD and HEEDA are constant, which suggests that they are intermediate compounds
shows to what extent the lost amine is accounted for by the different degradation products (the percentage of formation takes into account the number of nitrogen atoms present into the different degradation products). The key degradation compounds that have been previously reported11 were also detected in this study: 2-oxazolidinone (1, OZD), N-(2-hydroxyethyl)ethylenediamine (2, HEEDA), and N-(2-hydroxyethyl)imidazolidinone (3, HEIA). In addition, 13333
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Figure 5. Oxidative degradation of MEA in the presence of fly ash (30 wt %, CO2 loaded, α = 0.4, sparged with air (0.35 L/min) + 2% CO2 (7.5 mL/min)).
Figure 6. Quantification of main degradation compounds observed in different samples from the MEA campaign at Esbjerg: A, lean MEA start-up; B and C, absorber and stripper water washes after 11 weeks; D and E, lean and rich MEA solutions after 11 weeks; F, lean MEA solution after 20 weeks.
Supporting Information) suggest that degassing had no significant effect on degradation rates. 3.2. Oxidative Degradation with CO2 Loading. The oxidative degradation of MEA has been studied under different conditions without additives (Figure 3), with FeSO4 (Figure 4), or with fly ash (Figure 5). The experiments have been performed under similar conditions: a wet gas blend of air with 2% CO2 is sparged for several days in a 30 wt % MEA solution. As in section 3.1, these plots show the percentage of formation of the main degradation compounds by taking into consideration the number of nitrogen atoms contained in the molecule. We have not quantified the uncertainty in the analytical methods, but the variation seen in amine loss over time in Figure 3 most likely reflects inaccuracy in the analysis. In Figure 3 the degradation rates are very low, and uncertainty in the analysis and sampling are more prominent. In Figure 5 there are a few points where the amine loss appears to be negative; this can be due to uncertainty in the analytical work but can also be a result of water evaporation (resulting in higher solvent concentration). The stability order of MEA in the conditions studied is the following: MEA > MEA + fly ash > MEA + FeSO4. The higher degradation rate in the presence of iron ions is expected, since metal ions are known to catalyze oxidative
undergoing further reactions. A similar trend was also observed in the recent work by Davis and Rochelle.13 The authors also detected N,N′-bis(2-hydroxyethyl)urea, which is not observed in the present study. This is most likely a result of our analytical methods not being appropriate for the detection of this type of compound. The thermal degradation experiments were carried out in stainless steel cylinders, and the buildup of metal ions in the liquid was monitored. Iron concentrations reached around 3000 μg/mL after 2 weeks before leveling off. A similar trend, but with somewhat lower concentrations, was seen for chromium, nickel, and molybdenum. Figure S1 in the Supporting Information shows metal ion levels. The effect of the presence of metals on thermal degradation was studied by comparing the degradation rates in a metal cylinder with the rate in a cylinder with a glass lining (where the solvent does not come in contact with the metal). The results (given in Table S2 in the Supporting Information) suggest that the presence of metals had no significant effect on degradation rates. This is an expected result, since the thermal degradation reactions are not expected to be susceptible to catalysis by metals. The effect of degassing of the solvent was studied in the same way. The results (given in Table S2 in the 13334
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degradation.15 The chemical composition of the fly ash was not analyzed, but it may contain metals that may dissolve in solution and catalyze degradation in the same way as the iron ions. Five degradation compounds have been identified: HEF, HEA, HEPO (4-(2-hydroxyethyl)piperazin-2-one), OZD, and HEI. Volatile compounds such as ammonia and formaldehyde are also expected to form during oxidative degradation. We have not analyzed for these compounds, and that may be part of the reason why the nitrogen balance is not closed in this case. The only shared product with the thermal degradation is OZD (1), whose formation can be accounted for by the same mechanism, i.e., by carbamate ring closure. However, the two main degradation compounds are N-(2-hydroxyethyl)formamide (6, HEF) and N-(2-hydroxyethyl)imidazole (5, HEI).15 The former can be explained by the reaction of MEA with formic acid, while the mechanism of the formation of HEI is still unknown. 3.3. MEA Degradation in the Pilot Samples. In Figure 6 is shown the quantification of MEA degradation products from the Esbjerg campaign. It can be seen that HeGly (N-(2hydroxyethyl)glycine) and HEPO are the dominant degradation products formed, while there are also high concentrations of HEA, HEI, and HEF. In Figure 7 is shown the quantification of the ionic compounds from the Esbjerg campaign. It can be noted that
Figure 8. Quantification of main degradation compounds observed in different samples from the MEA campaign at Tiller. Values on the xaxis are number of hours into the campaign. All samples are lean amine.
significant concentrations. In general, there appears to be a steady accumulation of degradation products over time. For the Tiller campaign we also carried out an analysis of the total nitrogen balance in the liquid. The result of this analysis is shown in Figure 9 (values used in the plot are given in the
Figure 7. Quantification of ionic degradation compounds observed in different samples from the MEA campaign at Esbjerg: A, lean MEA start-up; B and C, absorber and stripper water washes after 11 weeks; D and E, lean and rich MEA solution after 11 weeks; F, lean MEA solution after 20 weeks.
Figure 9. Nitrogen balance for the MEA campaign at Tiller.
Supporting Information). The total nitrogen content was determined by the Kjeldahl method. We found that 99.7% of the nitrogen was accounted for by MEA, ammonia, and quantified degradation products. This means that in the Tiller campaign all major degradation products may have been identified and accounted for. It must, however, be noted that the Tiller campaign was of a relatively short duration and the conditions the solvent was exposed to were not particularly severe. Closing the liquid phase nitrogen balance for a more severely degraded solvent may be more challenging. At the time of the Longannet campaign only some of the methods for the quantification of degradation products were still not available. For this campaign only HEI, HEF, and OZD were quantified. HEF was found at a concentration as high as 8580 μg/mL, HEI was 160 μg/mL, and OZD was 22.9 μg/mL. The HEF level at Longannet was higher than seen at the other
we were not able to quantify acetic acid or glycolate in these samples. With the IC column utilized in the present work we were not able to separate glycolate from acetate; as a result we could not quantify these two compounds. The levels of nitrite or nitrate were too low to quantify with the IC setup utilized in the present work. In the analysis samples A and B were diluted by a factor of 1/10, sample C was diluted by a factor of 1/2, and samples D−F were diluted by a factor of 1/50. In Figure 8 is shown quantification of the same degradation products from the Tiller campaign. It can be seen that in this case there is more HEPO than HEGly formed. Again HEA, HEI, and HEF are the other degradation products found in 13335
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Figure 10. Proposed mechanism for the thermal degradation of MEA.
11 we show an overview of compounds we believe are formed in the first stage of oxidative degradation.
pilot plants, while the HEI and OZD levels were comparable to what was seen at other plants. Too little analytical data are available from Longannet to draw any conclusions on the causes of high HEF levels during this campaign. OZD and some other degradation products apparently do not build up to higher concentrations in pilot plants. This suggests that the degradation product is transient in nature, going on to react to another compound. The buildup of some components may also be limited by chemical equilibrium.
4. DISCUSSION 4.1. Thermal Degradation. The mechanism for the thermal degradation of MEA has been the subject of some discussion. It seems accepted that the first step is oxazolidinone formation by carbamate ring closure. Then the opinions are divided concerning the order of formation between HEEDA (2) and HEIA (3). In 1984, Kim and Sartori studied the thermal degradation of DEA and suggested that the dimer would be obtained by opening of the corresponding oxazolidinone with another DEA molecule.14 By analogy, Davis and Rochelle came to the same conclusions regarding the degradation mechanism of MEA: HEEDA (2) would be obtained by OZD (1) opening and would react again with CO2 to give HEIA (3).13 This last reaction can also be supported by a recent study which showed the complete instability of HEEDA in the presence of CO2 with HEIA as the major degradation compound.10 Concerning AEHEIA (4), its formation could be explained in the same way: HEEDA reacts with the oxazolidinone to give the trimer, which, in the presence of CO2, can give this imidazolidinone. The general pathway for the thermal degradation of MEA is represented in Figure 10. 4.2. Oxidative Degradation. For oxidative degradation it is generally believed that the initial degradation steps involve radical reactions.16,18,22 This initial degradation phase may involve the formation of short-lived species that we have yet to quantify or observe. From this initial degradation phase we expect degradation products that are either fragments of the MEA molecule or oxidized fragments of the MEA molecule. Ammonia is an example of a compound that can be regarded as a fragment of the MEA molecule. The organic acids can be regarded as oxidized fragments of the MEA molecule. In Figure
Figure 11. First stage of oxidative degradation.
The degradation products formed in the first stage of oxidative degradation are, however, reactive chemical species. These species may react with the solvent or other degradation products. We believe it is very likely that degradation products with acid functionality can react with MEA (or any other compound with a primary amine functionality). In Figure 12 we show the general scheme for such a reaction. We see that a number of the degradation products we observe are consistent with this reaction scheme. HeGly also appears to be the result of the reaction between a compound formed in the first stage of oxidation and MEA. It is not, however, clear what the precursor is, and we were also
Figure 12. Reaction between MEA and organic acids. 13336
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Figure 13. Mechanism for the formation of HEPO.
varied set of degradation products (HEPO being a specific example of this). 4.4. General Trends in Degradation. Looking at the main degradation products observed in MEA, we see that many have maintained the two carbon atom chain of the MEA molecule, while some such as HEF have a single carbon atom. We see very little evidence of carbon−carbon bond formation during MEA degradation. While there are probably a number of degradation products present in the system, many of these may be short-lived or present in very small quantities. The degradation products that build up over time are likely to be compounds that are thermodynamically stable and cannot readily undergo further reactions with the solvent or other degradation products. 4.5. Comparison with Literature Data. The only previous study of solvent degradation in a CO2 capture plant running on MEA that we are aware of is the work of Strazisar et al.21 We do see broad agreement between the degradation products reported in the present work and those reported by Strazisar et al. There are, however, some degradation products reported by Strazisar that we have not found our own analysis of degraded MEA samples: N-(hydroxyethyl)succinimide, N(2-hydroxyethyl)lanthamide, 1-hydroxyethyl-3-homopiperazine, 3-hydroxyethylamino-N-hydroxyethyl propanamide, 2,6dimethyl-4-pyridinamine, 2-imidazolecarboxaldehyde, and 1methyl-2-imidazolecarboxaldehyde. Further work should in our view be carried out to clarify if these compounds are significant degradation products in MEA. 4.6. Nitrosamines and Nitramines. In recent years there has been some focus on the potential formation of nitrosamines and nitramines in postcombustion CO2 capture plants.24,25 Such compounds are often carcinogenic, and may be of concern even if formed in small concentrations. The analysis of such compounds has not been a topic of the present study, but we will note that SINTEF has quantified nitrosamines and nitramines in degraded MEA.26 The nitrosamines quantified were N-nitrosodiethanolamine (NDELA), N-nitrosomorpholine (NMOR), and N-nitrosodimethylamine. NDELA was found in concentrations of around 2000 ng/mL, while the other compounds were present in lower concentrations. The nitramine form of MEA, 2-(nitroamino)ethanol, was found in concentrations of around 2100 ng/mL.
surprised that this compound was formed in such high concentrations. The high concentration of HeGly would suggest that the precursor is present in high concentrations and reacts readily with MEA. DEA might seem to be a potential precursor to HeGly, but it is present in too low concentrations to account for the large amounts of HeGly observed. In Figure 13 we propose a reaction mechanism for the formation of HEPO from MEA and HeGly. All our experimental results suggest that relatively high temperature is required for HEPO to form. In pure oxidative experiments where the solvent is not exposed to high temperatures, we see very little HEPO. The formation of HEPO can be seen as an example of advanced interplay between oxidative and thermal degradation: the degradation proceeds in a number of steps with both oxygen and high temperature playing a role. We suspect that the different ratios between HeGly and HEPO in different pilot plants are a result of differences in how the stripper is operated (determining the extent to which the solvent is exposed to high temperatures). For the formation of N-(2-hydroxyethyl)imidazole (HEI) we have not a detailed mechanism, but a general suggestion of how it is formed. Ben23 reports that HEI can be synthesized from MEA, ammonia, formaldehyde, and glyoxal. Ammonia and formaldehyde are known to be present in degraded MEA, while glyoxal is a plausible degradation product (its presence in MEA, however, to our knowledge has not been confirmed). It is possible that these compounds react to form HEI in degraded MEA. 4.3. MEA Degradation in Real CO2 Capture Plant Conditions. Comparing the pilot plant data with the lab-scale experiments, it is clear that a greater variation of degradation products is formed in the pilot plants. This is not particularly surprising since the conditions encountered by the solvent are more varied in the pilot plants. In the pilot plants the solvent shifts between being exposed to high levels of oxygen and being exposed to high temperatures, while the lab-scale experiments give constant exposure to one set of conditions. The nature of the degradation products formed, however, does seem to be similar in pilot plant samples and laboratory experiments. Overall, the degraded solvent from the pilot plant bears the most resemblance to the oxidative degradation experiments. Almost none of the degradation products found in thermal degradation experiments were encountered in the pilot plant samples, while all major degradation products from the oxidative experiments were encountered in pilot plants. This suggests that oxidative degradation dominates in the pilot plants. In the pilot plant the solvent is, however, exposed to more varied conditions, and this probably gives rise to a more
5. CONCLUSION The present study reports work on MEA degradation in pilot plants and laboratory experiments. There is significant overlap between degradation products found in pilot plants and in 13337
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Industrial & Engineering Chemistry Research
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oxidative degradation experiments; however, very few degradation products from thermal degradation experiments were found in pilot plant samples. This suggests that oxidative degradation dominates in pilot plants. HEPO (4-(2hydroxyethyl)piperazin-2-one and HeGly (N-(2-hydroxyethyl)glycine) are found to be the dominant degradation products in the pilot plants. Other significant degradation products found in pilot samples are HEA (N-(2-hydroxyethyl)acetamide), HEI (N-(2-hyd roxyeth yl)imidazole), and HEF (N-(2hydroxyethyl)formamide). A broader range of degradation products is found in the pilot plants than in the laboratory experiments; this is likely to be due to the solvent being exposed to more varied conditions in the pilot plants.
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ASSOCIATED CONTENT
S Supporting Information *
Data are given on GC−MS operation, LC−MS calibration, thermal degradation experiments, and quantification of degradation products at the Tiller pilot plant (underlying data for Figure 9). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +47 93059441. Fax: +47 73596995. E-mail: Eirik.Silva@ sintef.no. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the REACT and SOLVit projects for supporting the lab experiments performed to understand the thermal and oxidative degradation of MEA. The authors acknowledge the partners in REACTStatoil, Shell, and the Research Council of Norwayand in SOLVitAker Clean Carbon, Gassnova, EON, Scottish Power, Statkraft, and the Research Council of Norwayfor their support. We also acknowledge the CESAR project for providing samples obtained during the MEA campaign at the Esbjerg pilot plant and the SOLVit project for providing samples from the MEA campaign at Longannet.
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REFERENCES
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