Thermal Degradation of Monoethanolamine in CO2 Capture with

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Thermal Degradation of Monoethanolamine in CO2 Capture with Acidic Impurities in Flue Gas Shan Zhou, Shujuan Wang,* and Changhe Chen Key Laboratory for Thermal Science and Power Engineering of Minister of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China

bS Supporting Information ABSTRACT: Acidic impurities in the flue gas, such as SO2 and NOx, are supposed to promote amine degradation in the amine scrubbing process for CO2 capture. This work investigates the effect of these acidic gases on thermal degradation of MEA (monoethanolamine) by preloading additives, which are Na2SO3, SO2, H2SO4, and HNO3, respectively. MEA concentration was 4.4 mol/kg. CO2 loading was 0.4 mol/(mol of MEA). Experiment temperature varied from 120 to 150 °C. Ion chromatography (IC) is the main analytical method. The results show that the MEA thermal degradation rate was not affected by these additives. Ammonium became an important product in the experiment with SO2 or HNO3 loading. Sulfate was stable in the experiment. In the experiment with SO2 loading, most of the sulfur remained dissolved in the solution after degradation but could not be detected by IC. A lower pH value will speed up sulfite degradation.

1. INTRODUCTION Amine scrubbing is probably the only available technology for postcombustion capture of CO2 from existing coal-fired power plants,1 although the cost is still too high for power plants to equip CO2 capture units at this moment. Echeverri et al.2 estimated that a carbon price below $40/ton is unlikely to produce investments in carbon capture for electric power. Amine degradation is an important source of operation cost of CO2 capture. Degradation products will cause a foaming problem and increase the corrosivity of solutions and the makeup cost. Rao and Rubin estimated that amine makeup requirements contribute about 10% to the cost of CO2 capture using MEA (monoethanolamine) solution.3 MEA is the most widely used amine in amine scrubbing technology. Chi and Rochelle defined three types of degradation of MEA in the CO2 capture process.4 Oxidative degradation with O2 and thermal degradation at higher temperature of stripper with CO2 are the two main degradation types. The main process of amine thermal degradation is carbamate polymerizing. Polderman et al. proposed the mechanism for thermal degradation of MEA for the first time.5 Talzi and Ignashin6,7 got mechanisms similar to that of Polderman. Davis8 screened several analogues of MEA in thermal degradation experiment and tried to complete the mechanism of MEA thermal degradation. Lepaumier et al.9 screened a series of ethanolamines and ethylenediamines and proposed the general pathway of thermal degradation of these amines. The proposed main reaction pathway of MEA thermal degradation is shown in Scheme 1. MEA reacts with CO2 and gives MEA carbamate in aqueous solution. 2-Oxazolidinone maintains a balance with the carbamate. Another MEA molecule would attack the α-carbon of the oxygen atom and give HEEDA (N-(2-hydroxyethyl)ethylenediamine). HEEDA combines with CO2 and forms a cyclic urea HEIA (N-(2-hydroxyethyl)imidazolidinone). Similar reactions can take place between MEA and the following polymer products. r 2011 American Chemical Society

HEEDA is known to thermally degrade more quickly.8,9 As a result, HEEDA cannot accumulate in the MEA thermal degradation process, and the concentration of HEEDA remians relatively steady. HEIA and another cyclic urea, whose molecular weight is 173, will accumulate in thermally degraded MEA solution. This cyclic urea is derived from MEA trimer. AEHEIA (N-(2-aminoethyl)N0 -(2-hydroxyethyl)imidazolidinone) and tri-HEIA (1-{2-[(2hydroxyethyl)amino]ethyl}-2-imidazolidinone) are the proposed structures for this molecule.8,10 SO2 and NOx are important pollutants from coal-fired power plants. The emission limits of both SO2 and NOx from new coalfired power plant are 100 mg/m3 in China.11 SO2 can be absorbed in the condition of CO2 absorption but is difficult to strip out under the conditions of CO2 stripping.12 Some researchers noted that amines degrade more quickly with SO2 impurity existing in pilot scale.13,14 The effect of the acidic gases on amine degradation should be a concern over a long running time. Idem et al.15,16 investigated the oxidative degradation of MEA at varied SO2 and O2 concentration and built semiempirical models in these conditions. SO2 has a higher order of reaction than O2 in these models. Chanchey et al.17 evaluated the effect of SO2 and NO2 on off-gas emission in terms of H2SO3 and HNO3 using normal conditions in the CO2 capture process. H2SO3 and HNO3 decreased ammonia emission into the gas phase, while promoting the formate formation in the oxidizing environment. Previous studies were mostly performed in oxidative degradation conditions. This paper will focus on the effect of the acidic impurities on MEA thermal degradation. Sulfite is known to be oxidized easily in the absorber. Pilot plant experiment results implied that sulfite could not be oxidized completely in the absorber and Received: September 27, 2011 Accepted: December 19, 2011 Revised: November 24, 2011 Published: December 19, 2011 2539

dx.doi.org/10.1021/ie202214y | Ind. Eng. Chem. Res. 2012, 51, 2539–2547

Industrial & Engineering Chemistry Research

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Scheme 1. Carbamate Polymerization Mechanism of MEA Thermal Degradation8,10

remained in the high-temperature areas.14 The effects of both sulfite and sulfate should be considered in amine thermal degradation. More than 90% of NOx is NO in the flue gas from a coal-fired power plant. NO is insoluble in water, while NO2 can react with H2O and give HNO3 and NO. As a result, most NOx will get away with the treated gas when the flue gas goes through the solution. HNO3 will accumulate in the liquid. Therefore, HNO3 was used to evaluate part of the effect of NOx in the flue gas on MEA thermal degradation. Previous work18 has reported the preliminary results of the effect of SO2. This paper will present the further results and analysis.

2. EXPERIMENTAL SECTION 2.1. Experimental Procedures. MEA (99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was dissolved in deionized water. Additives were loaded into the solutions before loading CO2 (99.5%, Beijing Huayuan Gas Chemical Industry Co., Ltd., Beijing, China), so that CO2 would not be released by the acidic additives. The initial MEA concentration is 4.4 mol/kg in all of the experiments. For unloaded condition, the MEA weight concentration is 30%, which is widely used in laboratory research and pilot plant application. Sulfur dioxide (Beijing Huayuan) was used to evaluate the effect of sulfite. The effect of sulfate was evaluated in independent experiments by preloading H2SO4 (Beijing Chemical Works, Beijing, China). HNO3 (Beijing Modern Eastern Fine Chemicals Co., Ltd., Beijing, China) was used to evaluate part of the effect of NO2. Considering a long running time condition, the initial sulfur concentration was set as 0.057 mol/(mol of MEA), while HNO3 concentration was set as 0.116 mol/(mol of MEA). For each series of experiments, the same solution was loaded into a set of 316 L stainless steel cylinders. The volume of each cylinder is about 4 mL. All of the cylinders were full of solution with little head space left, so that the oxygen in the head space in the cylinders could be neglected. Assuming O2 solubility in MEA solution is the same as that in water, the dissolved oxygen could also be neglected. The cylinders were sealed tightly and placed in forced convection ovens, which were maintained at constant

temperature. Each reactor with no leakage could provide one valid sample. 2.2. Analytical Method. Amine concentration was confirmed using the pH titration method. CO2 loading was measured using the total inorganic carbon (TIC) method or precipitation titration method.19 TIC method was performed using a Shimadzu TOC-V analyzer. A Metrohm 809 autotitrator was used for pH titration and precipitation titration. The ionic species in degraded samples were analyzed by ion chromatography (IC). A Dionex DX-120 system with IonPac column CG17/CS17 was used for cations. A Dionex ICS-1000 system with IonPac column AG15/AS15 was used for anions. Diluted formaldehyde was used as oxidation inhibitor for sulfite analysis. A Thermo MSQ mass spectrometer coupled with a Dionex ICS-2100 was used for mass determination of some of the ionic products. A Thermo IRIS Advantage ICP-AES (inductively coupled plasmaatomic emission spectrometry) system was used to analyze metals in the samples. A 5E-8SII sulfur analyzer from Changsha Kaiyuan Instruments Co. Ltd. was used to analyze the sulfur concentration. This instrument was designed to analyze the sulfur concentration in coal and oil through coulometry.

3. RESULTS AND DISCUSSIONS 3.1. MEA Degradation Rate. To evaluate the effect of acid gases on MEA thermal degradation, a long running time condition was considered. All of the additives were concentrated in the solutions. All of the long time experiments for evaluation are listed in Table 1. Equation 1 defined the MEA remaining τ. MEA remaining data after 8 weeks in the experiments are compared in Figure 1. MEA thermal degradation data from Davis8 was also adapted and compared in Figure 1. As there are several parallel samples for some data points, the result ranges of all of the data points are marked in the figure.

τ¼ 2540

Wloaded, sample 100 Wloaded, initial

ð1Þ

dx.doi.org/10.1021/ie202214y |Ind. Eng. Chem. Res. 2012, 51, 2539–2547


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Table 1. Summary of 8 Weeks Thermal Degradation Experiment Conditions MEA concentration/

CO2 loading/

run

(mol/kg)

(mol/(mol of MEA))

baseline

4.4

0.4

1

4.4

2 3

additive concentration/ additive

(mol/(mol of MEA))

initial pH at 18 °C 9.88

135

0.4

H2SO4

0.057

9.35

120, 135, 150

4.4

0.4

SO2

0.057

9.47

120, 135, 150

4.4

0.4

HNO3

0.116

9.26

120, 135, 150

Figure 1. MEA remaining (τ) in thermal degradation after 8 weeks: dark gray bar, with H2SO4; medium gray bar, with SO2; black bar, with HNO3; light gray bar, from Davis8, and bar at far right in middle (135 °C) section, baseline.

where Wloaded, sample is the weight concentration of MEA in the sample; Wloaded, initial is the weight concentration of MEA in the initial solution. The MEA remaining decreased slightly with the acidic additives at 135 °C after 8 weeks. The MEA thermal degradation rate could be estimated on the basis of the assumption of pseudofirst-order reaction.20 The calculated thermal degradation rates with H2SO4 and SO2 are comparable with the degradation rate without acidic additives.18 Since the temperature of reboiler and stripper is not higher than 120 °C in practical application, the MEA thermal degradation rate will not change significantly with these acidic additives. 3.2. MEA Degradation Products. The reported main mechanism of MEA thermal degradation is carbamate polymerization as shown in Scheme 1. The produced 2-oxazolidinone and HEIA take about half of the degraded MEA.10 Ion chromatography, however, is not suitable for these products. Therefore, the mass of nitrogen could not get balanced in this work. By comparison of the cation chromatograms, all the cation products with acid additives can be found in the baseline experiment but with different distributions. The main polymerization products were identified by cation IC/MS method. Since there is no commercial source of AEHEIA or tri-HEIA, the concentration of AEHEIA or tri-HEIA cannot be quantified accurately using an external standard method. MEA and HEEDA have the same calibration curve on a molar basis using the analytical method in this work. The standard curve for MEA and HEEDA was used to estimate the concentration of the cyclic urea of MEA trimer (AEHEIA or tri-HEIA). Ammonium is another concentrated cation product in this work besides the MEA polymers and cyclic ureas. The percentages of nitrogen distribution in the solutions at 135 °C are compared in Figure 2. HEIA and 2-oxazolidinone, not quantified in this work, were part of the other degradation products in the figure. The nitrogen in other degradation products accounts for the largest

temperature/°C

percentage of the total nitrogen in the solution shown in Figure 2. In all of the experiments, HEEDA and the cyclic urea of the MEA trimer are always concentrated degradation products. HEIA is believed to be concentrated according to chemical equilibrium, although without quantitative analysis. The main process of MEA thermal degradation with acidic additives is still carbamate polymerization. In run 2, ammonium became one of the most significant cation products at 135 and 150 °C. Ammonium is not the product in carbamate polymerization. The high ammonium concentration, therefore, implies a different but important reaction mechanism in this system. A similar phenomenon was observed in run 3, but not obvious at lower temperature. All of the ammonium concentration data are shown in Figure 3. The ammonium concentrations increase with experiment time at 120 and 135 °C. When the temperature rose to 150 °C, the ammonium concentration reached a steady level of about 400 mmol/kg at the end of the second week in run 2, and then the formation of ammonium seemed to be limited. In the baseline run, formate is the most concentrated salt on the anion chromatograms. In run 1, the formate concentration is much higher than that in other conditions, although cation products distribution was not affected at 135 °C. In run 3, the formate was almost cleaned. As a result, the degraded MEA solution in run 3 has the weakest corrosivity in this work. Concentrations of formate and metals in stainless steel in typical samples are compared in Table 2. 3.3. Additives Degradation Products. All of the additives were quantified using ion chromatography during the whole experiments. In run 1, sulfate concentration was kept constant. Nitrate concentration decreased slightly, but no new ionic products were detected on IC in run 3. All of the nitrate concentration data from IC analysis are shown in Table 3. Degradation of SO2 is more complex. About half of the loaded SO2 was converted to sulfite in the initial solution. The rest of the SO2 could not be detected by ion chromatography. Figure 4 is the anion chromatogram of MEA solution that degraded at 4 weeks with SO2 at 135 °C. Sulfate, thiosulfate, and unidentified product 1 (UP1) showed up in the first week. As the experiment went on, the concentration of unidentified product 2 (UP2) increased. The unidentified products in Figure 4 could not be detected in runs 1 and 3. Sulfite, sulfate, and thiosulfate were quantified using the IC method and plotted in Figure 5. Higher temperature promotes the degradation of sulfite. Sulfite was converted to other species quickly from the beginning. Considering the data at three temperatures, thiosulfate concentration reached a maximum level and then decreased until it disappeared completely. The sulfite concentration decreased for a time and then increased again. Sulfate was produced, while not concentrated in the whole experiment. 2541



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to be the intermediate. The overall SO2 disproportionation reaction is shown as 4SO3 2 þ H2 O a 2SO4 2 þ S2 O3 2 þ 2OH

ð2Þ

Guekezian et al.23 observed thiosulfate and sulfate in aged sulfite solution in the absence of air and amine, and suggested another two-step mechanism of sulfite disproportionation, shown as

Figure 2. Nitrogen distribution in the solutions at 135 °C: light gray, other degradation products; medium gray, ammonium; dark gray, HEEDA; black, cyclic urea of MEA trimer.

Rochelle21,22 proposed a two-step mechanism of thermal degradation of SO2 in acidic buffer solutions. S3O62 is considered

2SO3 2 þ 3H2 O þ 4e a S2 O3 2 þ 6OH

ð3Þ

SO4 2 þ H2 O þ 2e a SO3 2 þ 2OH

ð4Þ

In the overall SO2 disproportionation reaction (eq 2), for every 4 mol of sulfite loss, there will be a gain of 2 mol of sulfate and 1 mol of thiosulfate. In this work, however, the concentration ratio between sulfate and thiosulfate was about 2:1 only at the first week at 120 °C. The gain of sulfate and thiosulfate cannot cover the sulfite loss throughout the whole experiment period. Therefore, disproportionation could be part of the mechanisms of SO2 degradation in loaded MEA solution. Other reductants in the system could also provide electrons for eq 3 at the same time. There are also sulfurated degradation products derived through other pathways. There is always precipitation in the degraded MEA samples. The clear solutions on the top of several samples with SO2 loading were analyzed using a 5E-8SII sulfur analyzer. Figure 6 compares the total sulfur concentration from the sulfur analyzer and the sum of the sulfur concentration in sulfite, sulfate, and thiosulfate. The results imply that most sulfur element remains in the liquid phase. Part of the loaded SO2 is converted to organic species or zwitterions which cannot be detected by IC method. Sulfur can also be transferred into the unidentified products mentioned in the previous graphs. There is still up to 10% sulfur lost in the solution. This 10% loss may exist in the precipitation or escape into the gas phase. According to IC/MS analysis, m/z = 124/140 for UP1 and 193 for UP2. UP1 decreases with experiment time and could be condensation products of sulfite and MEA. N-(2-Hydroxyethyl)amidosulfurous acid is one possibility of UP1 and could be oxidized to N-(2-hydroxyethyl)sulfamic acid during analysis. UP2 increases with the experiment time and could be derived from MEA polymerization products. Possible structures of the unidentified products are shown in Figure 7. The concentrations of UP1 and UP2 were estimated using formate standard curve on a molar basis and shown in Table 4. 3.4. Exploration of Ammonium Formation and Sulfite Degradation. Sulfate has no effect on ammonium concentration at 135 °C, as shown in Figure 3. The ammonium concentration in run 1, therefore, is supposed to be the baseline condition at 150 °C. The ammonium formation was promoted in run 2 and run 3. In run 3, the ammonium concentration was increasing all the time, even when more than half of the MEA has been consumed in carbamate polymerization process at 150 °C. After 8 weeks at 150 °C, the ammonium concentration in run 3 is 140 mmol/kg higher than that in run 1, as in Figure 3. At the same time, the lost nitrate is only 70 mmol/kg. One possibility is nitrate is a catalyst, but not a reactant in the process generating ammonium. The ammonium concentration in run 2 is the highest among all runs with the same temperature. This value reached about 400 mmol/kg and stopped increasing at 150 °C when thiosulfate 2542



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Table 3. Nitrate Concentration in the Experiments with HNO3 (Run 3) nitrate concentration/(mmol/kg)

Figure 3. Ammonium concentration with acid additives at varied temperature: light gray, baseline; dark gray, with H2SO4 (run 1); medium gray, with SO2 (run 2); and black, with HNO3 (run 3).

Table 2. Formate and Metals at 135 °C after 4 Weeks formate/ run

Fe/

Cr/

Ni/

additives (mmol/kg) (mmol/kg) (mmol/kg) (mmol/kg)

baseline18 118

H2SO4

218 3

13 37

14.2 15.7

3.8 9.8

2.3 5.6

SO2

6

1.3

1.1

0.5

HNO3

1

1

0.3

0.2

decomposed completely. The maximum ammonium concentration difference between run 2 and run 1 at the same temperature is higher than 300 mmol/kg, while the initial concentration of loaded SO2 is less than 300 mmol/kg. Sulfite or some kind of sulfite degradation products could also be catalyst in the process generating ammonium. SO2 is more concentrated than NO2 in flue gas. Although HNO3 concentration used in run 3 is twice that of the SO2 concentration in run 2; the ammonium yield in run 3 is lower than that in run 2. SO2 promotes the ammonium formation greatly, and introduces more degradation products containing sulfur. Sulfite was paid more attention to in this work. Several 1

time/week

120 °C

135 °C

150 °C

0

504.5

504.5

504.5

1

481.7

477.7

504.8

2 4

442.8 493.1

495.5 485.9

494.1 490.9

8

492.5

486.2

440.8

week experiments were performed around sulfite to explore the ammonium formation mechanism. Amine thermal degradation is a slow process. Temperatures higher than practical value will speed up this process and have been chosen as experiment temperature in thermal degradation study of various amines.5,8,9,20,24,25 To accelerate the degradation reaction, all of the 1 week experiments were performed at 150 °C. To evaluate the sulfite effect on MEA thermal degradation in previous work,18 0.057 mol/(mol of MEA) Na2SO3 was added into 30 wt % MEA with 0.4 CO2 loading. Na2SO3 has behavior similar to that of SO2 at 135 °C. Na2SO3 was used as the sulfite source for convenience. The detailed experiment conditions are listed in Table 5. The analysis results of the final samples in these 1 week experiments are shown in Table 6. The concentrations of UP1 and UP2 were estimated using the formate standard curve. In expt 1, most MEA did not degrade. More than half of the sulfite also remained. As a primary amine, MEA exists as free amine, MEA carbamate, and protonated MEA in solution with CO2 loading. There was not significant ammonium formation with concentrated free MEA in expt 1. The MEA carbamate and protonated MEA were tested in the following experiments. 2-Oxazolidone is an important product in MEA thermal degradation. This species and MEA carbamate maintain a balance with each other in the condition of thermal degradation. 2-Oxazolidone solution could be considered as the MEA solution with 1.0 CO2 loading. MEA carbamate and 2-oxazolidinone would be more concentrated in expts 7 and 8 than that in 3 mol/(kg of MEA solution) with 0.4 CO2 loading. All of the species detected by IC in expts 7 and 8 could be found in the previous experiments. With comparison of the ion chromatograms of samples in expts 7 and 8 (Figure 8), sulfite has no effect on ammonium yield in 2-oxazolidone solution. Concerning sulfite degradation, thiosulfate was not detected, while only sulfate and UP2 were significant in expt 8. UP2 was supposed to derive from the MEA polymerization product in section 3.3. The reaction rate of MEA carbamate polymerization is higher with higher CO2 loading.8 Significant UP2 in expt 8 gave support for the previous consideration about the structure of UP2. It could also be concluded that ammonium formation and UP2 formation should be separated processes. H2SO4, instead of CO2, was used to evaluate the role of protonated MEA in the process producing ammonium. Thiosulfate and significant ammonium formation were detected in expt 2. This phenomenon suggested that ammonium could be promoted by MEA protonation and sulfite attendence. Ammonium was not reported as an important product in MEA thermal degradation, but significant in PZ (piperazine) thermal 2543



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Figure 4. Anion chromatogram of degraded MEA with CO2 and SO2 (run 2) at 135 °C, 4 weeks.

Figure 7. Possible structures of unidentified products in anion chromatogram.

degradation.20,24,25 Freeman25 proposed the SN2 substitution reactions pathway for PZ thermal degradation. Protonated PZ is the key species in this mechanism. In the first step, H+PZ could be attacked at the α-carbon by another PZ and give AEAEPZ (1-[2-[(2-aminoethyl)amino]ethyl]PZ). Other SN2 substitution reactions can go on between PZ and its degradation products. As the amino group of PZ plays the role of nucleophile, all of the SN2 substitution reactions give amine products. One of the reactions that can produce NH4+ is

Figure 5. Concentration of sulfite, sulfate, and thiosulfate in the experiment with SO2 (run 2): 9, SO32; (, SO42; 2, S2O32.

Lepaumier at al.9 also proposed a substitution mechanism for demethylation/methylation reactions. A nucleophilic substitution involves methyl group migration from ammonium salt to amine. For primary amine and secondary amine, the demethylation/methylation reaction is

where R1 and/or R2 = H. For MEA, eq 6 can be modified and simplified to

Figure 6. Sulfur concentration in MEA solution loaded SO2: gray, total sulfur; black, sulfur in sulfite, sulfate, and thiosulfate.

SN2 substitution could be the mechanism of ammonium formation in MEA thermal degradation. However, ammonium is the only obvious degradation product on the cation chromatogram 2544



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of expt 2. More degradation products need to be detected to prove this mechanism. Thorn26 used DETA (diethylenetriamine) buffer solution for desulfurization. The degraded solution was analyzed by gas/ liquid chromatography with FID (flame ionization detector). EDA (ethylenediamine) was the only detectable product on the chromatographs. Thorn suggested that sulfite could attack the α-carbon of the secondary amine and give EDA and sulfethylamine, as eq 8. Since the range of the degraded DETA buffer solution is from pH 3.5 to pH 5, most of DETA in Thorn’s work should also be protonated.

Ammonium could also be a degradation product in Thorn’s work on the basis of a similar mechanism, such as eq 9, while the Table 4. Estimated Concentration of UP1 and UP2 in the Experiment with SO2 (Run 2) UP1/(mmol/kg)

UP2/(mmol/kg)

time/week

120 °C

135 °C

150 °C

120 °C

135 °C

150 °C

1

3.9

8.7

7.9

0.8

0.9

8.5

2

5.4

8.3

7.0

1.0

2.9

18.6

4 8

5.9 5.3

7.6 6.4

4.8 2.9

1.0 2.9

8.1 19.0

21.4 23.7

Table 5. Summary of 1 Week Experiments for Ammonium Formation Mechanism initial concentration/(mol/kg) expt

MEA

HEEDA

2-oxa

CO2

1

4.7

0

0

0

0

0.27

2

2.1

0

0

0

0.58

0.11

3 4

0 0

2.8 3.1

0 0

0 0

0 0

0 0.16

5

0

2.7

0

1.1

0

0.15

6

0

2.4

0

0

1.06

0.14

7

0

0

3.0

0

0

0

8

0

0

2.9

0

0

0.18

H2SO4

Na2SO3

corresponding gas ammonia has a poor response on FID in the gas/liquid chromatography method.

On the basis of the above discussion, sulfite could be one of the nucleophilic substitutions in the ammonium formation process. However, sulfethylamine was not confirmed in Thorn’s work. The stoichiometric ratios between sulfite and EDA, sulfite and NH3 are 1:1 in eqs 8 and 9. As discussed in previous graphs, the maximum differences of ammonium yield between run 2 and run 1, as well as run 3 and run 1, at 150 °C are greater than the lost additives obviously. UP2 is the most concentrated sulfurated product in expt 8, as shown in Figure 8, while the UP2 formation process consumed sulfite without ammonium formation. The additives, therefore, could be only catalysts for the ammonium formation process. Other nucleophiles, besides sulfite, must have been involved in the systems with acidic additives. The protonated amino group of both MEA and MEA degradation products could be the source of ammonium. HEEDA is an important product in MEA thermal degradation and also an analogue of MEA. In expts 3 and 4, amine degradation and thiosulfate were not observed. HEEDA loss was significant only in expt 5, while ammonium and EDA were concentrated in both expts 5 and 6. MEA was also detected. The substitution reactions are promoted by HEEDA protonation when sulfite attends. In summary, sulfite is relatively stable in expts 1 and 4, while decreasing quickly in the solutions with CO2 or H2SO4 loading. UP2 could not be detected in the experiments without CO2 loading, which also supports the consideration about UP2 structure. Thiosulfate was not detected in expts 1, 4, and 8, where neither CO2 nor H2SO4 was loaded in the initial solution. This phenomenon suggested lower pH would promote thiosulfate formation through eq 3. Thiosulfate is not stable in acidic environment.27 Various conditions could lead to different products. According to the total sulfur analysis for run 2, 90% of the sulfur is still dissolved in the solution. Thiosulfate decomposed completely when sulfite concentration began to increase again, which means eq 3 could be reversible. The lost sulfur in this work may precipitate as a simple substance or escape as H2S through the gas phase. Furthermore, thiosulfate could also degrade to trithionate, tetrathionate, and S2, which are difficult to separate from the AS15 column.

Table 6. Results of 1 week Experiments for Ammonium Formation Mechanism concentration of reagents and products/(mmol/kg) expt

HEEDA

EDA

ammonium

sulfite

sulfate

thiosulfate

UP1

UP2

total Sa

formate 1.4

1

4660

31

0

38

188

8

0

3.6

0

223

2

2007

7

4

51

37

613

34

3.3

0

688

3

0

2752

31

3

4

a

MEA

2.1

1 2

2994

22

3

116

4

0

0

0

128

2.4

5

67

1153

55

123

37

9

22

0

2.8

129

8.3

6 7

96 971

2315 49

89 4

102 23

1

1085

46

0

0

1161

5.7 17.4

8

826

48

3

29

0

6

0

0

82.3

152

33.2

Total S was analyzed using the 5E-8SII sulfur analyzer. The other species were analyzed by IC. 2545



Figure 8. Ion chromatograms of degraded 2-oxazolidinone: , with Na2SO3 (expt 8); --, without Na2SO3 (expt 7).

4. CONCLUSIONS This paper evaluated the effect of acidic impurities in the flue gas on MEA thermal degradation in amine scrubbing technology for CO2 capture, using H2SO4, SO2, and HNO3 as additives. The additives in this work will not affect the thermal degradation rate significantly in the practical conditions in the stripper but will influence the degradation products distribution. Ammonium becomes an important degradation product in the system with SO2 or HNO3 loading. SN2 substitution reaction could be the mechanism for ammonium formation. The additives should play a role of catalyst in the substitution mechanism. Sulfite disproportionation is one of sulfite degradation pathways. Lower pH will speed up sulfite disproportionation. Most sulfur remained in the liquid phase and may be combined with MEA carbamate polymerization products. ’ ASSOCIATED CONTENT

bS

Supporting Information. Text describing gas loading, cylinder management, ion chromatography system configuration, analysis method for sulfite. This information is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the National Natural Science Foundation Project of China (Grant 50876051) is gratefully acknowledged. ’ REFERENCES (1) Rochelle, G. Amine Scrubbing for CO2 Capture. Science. 2009, 325 (25), 1652–1654. (2) Echeverri, D.; Jayapt, B.; Chen, C. Should a Coal-Fired Power Plant be Replaced or Retrofitted? Environ Sci. Technol. 2007, 41, 7980–7986.

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