Oxidation of SO2 Absorbed by an Ionic Liquid during Desulfurization

Dec 1, 2010 - Shidong Tian , Yucui Hou , Weize Wu , Shuhang Ren , Chenxing Wang , Jianguo Qian. Journal of the Taiwan Institute of Chemical Engineers ...
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Ind. Eng. Chem. Res. 2011, 50, 998–1002

Oxidation of SO2 Absorbed by an Ionic Liquid during Desulfurization of Simulated Flue Gases Shuhang Ren,† Yucui Hou,‡ Weize Wu,*,† and Meijin Jin† State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China, and Department of Chemistry, Taiyuan Normal UniVersity, Taiyuan 030012, China

Room-temperature ionic liquids (ILs) are widely investigated to absorb SO2 from mixed gases or simulated flue gases and can capture a large amount of SO2, which can be recovered easily by heating and vacuum treatment. However, due to the existence of O2 in flue gases, the oxidation of SO2 may occur. This oxidation might influence the SO2 recovery from ILs and make ILs unable to reuse. This may limit further applications in large-scale desulfurization from mixed gases or flue gases by ILs. In this work, a task-specific IL, monoethanolaminium lactate ([MEA]L), was used to study the absorption of SO2 and oxidation of SO2 by O2 in simulated flue gases with and without ash and activated carbon in [MEA]L. It is found that the presence of O2 in the simulated flue gas does not influence the absorption of SO2 by [MEA]L, but it causes, to a very small extent, the oxidation of SO2. The increase of temperature, time, and the concentration of O2 can increase the oxidation of SO2. Adding in IL ash and activated carbon, which could be captured by IL from flue gases, can also increase the oxidation of SO2. For these problems, we have tried to find some ways to reduce the oxidation of SO2 absorbed by IL. Introduction SO2, a hazardous gas to our atmosphere, is mainly emitted from the burning of fossil fuels, and it causes environmental and health concerns, for instance, in acid rain, polluted rivers, and destroyed plants. In contrast, SO2 is also an important and useful source for many intermediates in chemical production. Up to now, flue-gas desulfurization (FGD) is one of the most effective ways to control emission of SO2 from combustion of fossil. The FGD ways, including wet FGD, semidry FGD, and dry FGD processes, have been adopted widely in commercial units. However, these processes have some disadvantages, such as the difficulty to recover SO2, the waste byproduct (CaSO4), and the secondary pollution. Because of these problems, the pressure swing absorption (PSA)1 or temperature swing absorption (TSA)2 technologies are studied as attractive approaches for SO2 gas separation, which could be energy saving as compared to FGD processes and avoid byproducts while allowing the SO2 to be used as a direct source for sulfuric acid production. However, for practical use, it is difficult to find a material for reversible and selective absorption of SO2. Recently, room-temperature ionic liquids (ILs) have been developed to absorb SO2 from mixed gases or flue gases with high absorption capacity, and the absorbed SO2 can be easily desorbed at high temperature or/and under a vacuum without any change. The desorbed SO2 can then be used as a potential SO2 resource. ILs are considered as a kind of environmentally benign solvents and have extremely low vapor pressure, high thermal and chemical stability, designable structure, and excellent solvent power. Up to now, many kinds of ILs, especially task-specific ILs, have been studied by several research groups to absorb SO2 with high absorption capacity. Han et al.3 synthesized a kind of task-specific IL, 1,1,3,3-tetramethylguanidinium lactate ([TMG]L), and found that the IL could absorb nearly 1 mol of SO2 per mol IL at 1 bar with 8% of SO2 in gas stream. Li et al.4 * To whom correspondence should be addressed. Tel./fax: +86 10 64427603. E-mail: [email protected]. † Beijing University of Chemical Technology. ‡ Taiyuan Normal University.

also used the molecular dynamics simulation to discuss this mechanism. Several new TMG-based ILs and some 1-butyl-3methylimidazolium (BMIM)-based ILs were synthesized by Huang et al.,5,6 such as tetramethylguanidinium tetrafluoroborate ([TMG][BF4]) and 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][BTA]). Only physical absorption was observed through their experiments. Hydroxyl ammonium ILs,7,8 pyrimidine, and imidazole ILs9-11 were used to absorb SO2 and could get a high SO2 absorption rate. Many new methods, such as membrane12,13 and polymer technology,14,15 have also been used to improve the absorption of SO2 by ILs. As we know, there are large amounts of O2, water, and a small amount of ash (including carbon) in flue gas by the burning of coal or oil with air. The O2 in flue gas can dissolve in ILs.16 In our previous work,17 the water in flue gas could be absorbed into IL during absorption of SO2 from flue gas by IL. It was reported that, for the adsorption of SO2 on activated carbon, SO2 could react with H2O and O2, catalyzed by the absorbent, activated carbon, to form sulfuric acid.18 During the absorption of SO2 from flue gas by ILs, ash in the flue gas may also be trapped. Therefore, the carbon in the ash may act as catalyst to convert SO2 to sulfuric acid in the presence of H2O and O2 in ILs. Whether the absorbent IL has any effect on the oxidation of SO2 has not been reported in the literature. If the absorbed SO2 in IL is converted into sulfate (SO42-), the anion SO42- will make impossible the recovery of SO2 from IL by simply heating and vacuum treatment. The difficulty to reuse ILs, caused by the oxidation of SO2 in ILs, may limit the eventual applications in large-scale flue gases desulfurization by ILs. However, up to now, there has been no research reported in the literature about the oxidation of absorbed SO2 with the presence of O2 in flue gases during the absorption of SO2 by ILs. In this Article, we studied the effect of O2 on the absorption of SO2 by the IL monoethanolaminium lactate ([MEA]L), the oxidation of SO2 during the absorption, and the effecting factors of this oxidation. Further-

10.1021/ie101126a  2011 American Chemical Society Published on Web 12/01/2010

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Figure 1. Schematic diagram of the apparatus for ILs to absorb SO2 with different gas mixtures: 1, cylinder filled with SO2 and N2 mixture with 1.90% (V/V) of SO2; 2, cylinder filled with pure N2; 3, cylinder filled with pure O2; 4, flow meters; 5, water baths; 6, temperature controller; 7, glass tube filled with IL; 8, glass bottle contained NaOH aqueous solution for absorbing the exhaust gases; 9, valves.

more, from these analyses we try to find a possible way to reduce the oxidation of SO2. Experimental Section Materials. SO2 (99.95%), N2 (99.99%), and O2 (99.95%) were supplied from Beijing Haipu Gases. Monoethanolamine was purchased from Tianjin Fuchen Chemical Co. (Tianjin, China); lactic acid was from Tianjin Bodi Chemical Co. (Tianjin, China). All reagents and solvents were analytical reagents. Activated carbon (AC) with the surface area of 897 m2/g was from Chengde Jibei Yanshan Activated Carbon Co. (Heibei, China). [MEA]L was synthesized and characterized following the procedure reported in the literature.7,8 Ash was collected from a head of sintering machine at the Tangshan Jiaheng Shiye Co. in China. The particle sizes of the ash and the AC are smaller than 0.45 mm. The water concentration of the task-specific IL, [MEA]L, after drying is measured by Karl Fischer analysis, and the water content is less than 0.1%. Apparatus and Procedures. The SO2/N2 gas mixture, with SO2 content of 1.90% by volume, was prepared by mixing SO2 and N2 in a high pressure cylinder of 40 L in volume. This absorption experiment was carried out at an ambient pressure. The schematic diagram of apparatus used is showed in Figure 1. It consisted mainly of a cylinder containing the SO2/N2 gas mixture, a cylinder containing pure N2 gas, a cylinder containing pure O2 gas, a glass tube with an inner diameter of 12 mm, and a length of 200 mm, three rotameters (Beijing Forth Automation Meter Factory, China) with an accuracy of (4.0%, and a constant temperature water bath. In a typical experiment, the SO2/N2 gas mixture and the pure O2 (or N2) went through the [MEA]L of about 3 g loaded in the glass tube. The flow rate of each gas was monitored by rotameters and calibrated by a soap film fluid meter. The compositions of simulated flue gas through the glass tube were calculated on the flow rate of each gas and their compositions. The glass tube was most partly immersed into the water bath, the temperature of which was maintained within (0.1 °C by using a temperature controller (Model A2, Changliu Co., Beijing). After a given time of absorption, about 0.05 g of IL mixture was sampled from the glass tube, and the content of absorbed SO2 in the IL mixture was analyzed. The samples were at intervals taken out until the absorption reaches equilibrium.

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Figure 2. Absorption of SO2 by [MEA]L as a function of time with 1.68% of SO2 (V/V) at different temperatures: 0, 40.0 °C without O2; 9, 40.0 °C with 11.5% of O2; O, 50.0 °C without O2; b, 50.0 °C O2 with 11.5% of O2; 4, 60.0 °C without O2; 2, 60.0 °C with 11.5% of O2.

After the IL was saturated by SO2, another sample was taken out, and the oxidized SO2 was measured. Determination of the Absorbed SO2. In this Article, the standard iodimetry was used to measure the content of SO2 in the IL during the absorption. Both the preparation of the reagents and the measuration were followed by HJ/T 56-2000 (the standard of State Environmental Protection Administration of China). The reproducibility of the measurements was better than (1%, and it was estimated that the data were accurate to (2%. Determination of the Oxidized SO2. The content of oxidized SO2 (SO3 or SO42-) in the IL absorbent was measured by a spectrophotometer (TU-1901, Beijing Puxi Tongyong Instrument Co. Ltd., China) and followed the reported procedure.19,20 Briefly, the sample was dissolved in water. Polyvinyl alcohol, which served as the protect reagent, was then added into the aqueous solution. Hydrochloric acid and barium chloride were slowly added into the aqueous solution, and mixed by shaking. After 10 min, the absorbance of the aqueous mixture was measured by the spectrophotometer under 422.5 nm, and the content of SO42- was calculated. The reproducibility of the measurements was better than (2.5%, and it was estimated that the data of oxidized SO2 contents were accurate to (5%. Results and Discussion Effect of O2 on the Absorption of SO2. In this work, we studied the effect of O2 on the absorption of SO2, and the results are shown in Figures 2 and 3. We found that the presence of O2 in the simulated flue gases does not influence the absorption of SO2. Figure 2 shows that the absorption of SO2 by [MEA]L as a function of time at three temperatures with or without the presence of 11.5% O2 in the simulated flue gases containing 1.68% of SO2. The absorption curves with the presence of O2 in the simulated flue gas are almost the same as that in the absence of O2 at each temperature. For example, with 11.5% O2 in the simulated flue gas, the absorption capacity of SO2 is 0.084 g/g IL at 60.0 °C, while without O2 in the gas, the mass ratio of SO2 to IL is 0.083. This result suggests that the presence of 11.5% O2 in flue gas has no change on the absorption of SO2 at each temperature. Figure 2 also shows that low temperature is in favor of high absorption of SO2 with the presence of O2 in the simulated flue gas. When the temperature is changed from 60.0 to 40.0 °C, the absorption capacity of SO2 rises from 0.084 to 0.13 g/g IL

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Figure 3. Absorption of SO2 by [MEA]L as a function of time at 50.0 °C with different concentrations of O2: 9, 6.50%; b, 11.5%; 2, 16.0%.

with the present of O2 in the simulated flue gas. This phenomenon is similar to that without O2 in gas; that is, the absorption capacity of SO2 is from 0.083 to 0.13 g/g IL. Figure 3 shows the effect of different concentrations of O2 present in the flue gas on the absorption of SO2 at 50.0 °C. It clearly indicates that different concentrations of O2 have almost not any influence on the absorption of SO2. For example, with 6.50% of O2 in the simulated gas, the absorption of SO2 is 0.11 g SO2/g IL, and with 16.0% of O2, the mass ratio of SO2 to IL is also 0.11 g SO2/g IL. Figures 2 and 3 indicate that O2 plays a role as an inert gas like N2. The solubility of O2 in ILs has been studied by several research groups,16,21,22 and the Henry’s constant is very high. Yuan et al.21 studied the solubility of O2 in IL, 1,1,3,3tetramethylguanidinium lactate, and the Henry’s constant is 242.26 MPa at 55 °C. On the basis of Henry’s constant and the maximum O2 partial pressure we investigated in this work (shown in Table 1), the corresponding mole fraction of O2 in the IL [MEA]L is 6.6 × 10-5, and the mass ratio of O2 to [MEA]L is just 1.4 × 10-5. This can be ignored, as compared to the absorption of SO2 in the IL with mass ratios of SO2 to [MEA]L from 0.083 to 0.13. It is reasonably concluded that O2 in the flue gas has no effect on the absorption of SO2 by [MEA]L. Effect of Temperature on the Oxidation of SO2. Table 1 shows the oxidation of SO2 absorbed by [MEA]L at different conditions. First, for comparison, we measured the oxidation of SO2 absorbed at three temperatures of 40.0, 50.0, and 60.0 °C without O2 in the flue gas, and there is no oxidized SO2 (SO3 or SO42-) measured in the IL solution, as shown in entries 2, 4, and 6 in Table 1, which suggests that there is no oxidation happening during absorption. However, when there is 11.5% of O2 present in the flue gas, a small amount of oxidized SO2 can be measured as shown in entries 1, 3, and 5 in Table 1 with oxidized SO2 from 0.00080 to 0.0031. This result indicates that with O2 in the flue gas, the oxidation of SO2 absorbed by [MEA]L happens, but its content is very limited. With the presence of O2, the oxidation of SO2 increases with the increase of temperature, as shown in entries 1, 3, and 5 in Table 1. For example, the mass ratio of oxidized SO2 to total absorbed SO2 is 0.00080 at 40.0 °C; while when the temperature is increased to 60.0 °C, the mass ratio of oxidized SO2 to total absorbed SO2 reaches 0.0031, which is nearly as 4 times that at 40.0 °C. The effect of temperature on the reaction rate of oxidation SO2 to SO3 over vanadium oxide has been studied by Paˆrvulescu et al.23,24 It indicates that the activation energy of the oxidation

of SO2 over vanadium oxide is about 20 kcal/mol, and the increase of the oxidation temperature is in favor to the oxidation SO2. Yet for the absorption of SO2 by IL, there is no oxidation catalyst, such as activated carbon. The IL, [MEA]L, possibly acts as catalyst, because the IL may form an unstable state (transition state) with SO2,25 which possibly activates SO2 and increases the oxidation of SO2. These data in Table 1 indicate that the temperature of the absorption is an important effect on the oxidation of SO2. The lower is the temperature of absorption, the less is the oxidation of SO2 in the absorbent IL. From Figure 2, it can be concluded that the absorption of SO2 increases with the decrease of temperature. These results indicate that both high absorption of SO2 and low oxidation of SO2 can be obtained through decreasing absorption temperature. Effect of O2 Concentration on the Oxidation of SO2 Absorbed by IL. From entries 3, 7, and 8 in Table 1, it can be seen that the increase of the O2 concentration enhances the oxidation of SO2 at 50.0 °C. For instance, when the concentration of O2 is 6.50% (V/V) in the simulated flue gas, the mass ratio of oxidized SO2 to total absorbed SO2 is 0.0016; while when the concentration reaches 16.0% (V/V), the mass ratio of oxidized SO2 to total absorbed SO2 becomes 0.0027. Anthony et al.22 studied the solubility of O2 in five ILs and reported the concentrations of O2 in ILs increased with O2 pressure at temperatures of 10-50 °C. Therefore, the increase of O2 concentration in flue gas at ambient pressure will increase the concentration of O2 in the IL [MEA]L, and further increase the oxidation rate of SO2 in the IL. Effect of Time on the Oxidation of SO2. The effect of time on the oxidation of SO2 absorbed by [MEA]L is shown in entry 3 in Table 1. When the absorption of SO2 reaches saturation at 50.0 °C after 4 h of absorption, the mass ratio of oxidized SO2 to total absorbed SO2 is 0.0020. The SO2 contained IL solution was then conserved in the glass tube with a cover. After 30 days, the mass ratio of oxidized SO2 to total absorbed SO2 was measured to be 0.011; after 45 days, the mass ratio of oxidized SO2 to total absorbed SO2 was further increased to 0.012. This result indicates that after the absorption, the SO2 absorbed in the IL is still being oxidized into SO3 or SO42-, because there remains O2 in the IL and the vapor above the IL in the glass tube, and the oxidation reaction still occurs in the IL. To reduce the oxidation of SO2 in the absorption process in desulfurization from flue gases, the time of the absorption and the remaining in IL before desorption should be shortened. Also, after the absorption, the absorbed SO2 should be desorbed immediately to avoid further oxidation of the absorbed SO2. Effect of Ash and Carbon on the Absorption and Oxidation of SO2. As we know, ash and carbon cannot be avoided in real flue gas. During the absorption of SO2 from flue gas, they can be captured by IL absorbents, so it is necessary to study the effect of ash and carbon on the absorption and oxidation of SO2. In this work, we used these two kinds of solid, ash and AC, to simulate the real process. Each of these solids was added into [MEA]L before the absorption, and the content of solid in the IL was 1% in weight. Figure 4 shows the effect of added different solids in the IL on absorption of SO2 with 1.68% of SO2 (V/V) and 11.5% of O2 (V/V) in the simulated flue gas at 50.0 °C. The figure indicates that the added AC or ash has no effect on the absorption of SO2 as compared to no added solid in the IL. For example, with ash in the IL, the mass ratio of SO2 to [MEA]L is 0.11, and without any solid in the IL, the mass ratio of SO2

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a

Table 1. Oxidation of SO2 Absorbed by [MEA]L at Different Conditions entry

temp (°C)

SO2/N2 flux (mL/min)

O2 flux (mL/min)

1 2 3 4 5 6 7 8 9 10

60.0 60.0 50.0 50.0 40.0 40.0 50.0 50.0 50.0 50.0

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

13.0

N2 flux (mL/min) 13.0

13.0 13.0 13.0 13.0 7.0 19.0 13.0 13.0

CSO2 (%)

CO2 (%)

1.68 1.68 1.68 1.68 1.68 1.68 1.78 1.60 1.68 1.68

11.5 0 11.5 0 11.5 0 6.50 16.0 11.5 11.5

add.

RSO2/IL

RS2/ST

AC ash

0.084 0.083 0.11 0.11 0.13 0.13 0.11 0.11 0.11 0.11

0.0031 0 0.0020 0 0.00080 0 0.0016 0.0027 0.0045 0.0071

30 days RS2/ST

45 days RS2/ST

0.011

0.012

a

The concentration of SO2 in SO2/N2 flux is 1.90% (V/V). CSO2 stands for the concentration of SO2 in the simulated gas. CO2 stands for the concentration of O2 in the simulated gas. RSO2/IL means mass ratio of SO2 to [MEA]L. RS2/ST means mass ratio of oxidized SO2 to total absorbed SO2 (containing the oxidized SO2). The 30 days RS2/ST and 45 days RS2/ST mean mass ratios of oxidized SO2 to total absorbed SO2 after 30 and 45 days, respectively. Add. means additive in IL.

Figure 4. Absorption of SO2 by [MEA]L as a function of time with 1.68% of SO2 and 11.5% of O2 at 50.0 °C in [MEA]L solution filled with 1 wt % of different additives: 9, activated carbon; b, ash; 2, no additive.

to [MEA]L is also 0.11. This suggests that the solid in the IL has no effect on the absorption of SO2 from the simulated flue gas in the presence of O2. However, it is shown in entries 9 and 10 in Table 1 that with solids, ash, and AC in the IL and 11.5% of O2 in flue gas, the oxidation of SO2 occurs. As compared to no solid in the IL as shown in entry 3 in Table 1, the oxidation of SO2 with solids in the IL is enhanced. When the AC is added into the IL, the mass ratio of oxidized SO2 to total absorbed SO2 is enhanced to 0.0045; when ash is in the IL, the mass ratio is 0.0071, which is increased from 0.0020 of the mass ratio with no added solid in the IL. Our results indicate that both the ash and the AC can enhance the oxidation of SO2 in the IL. AC has been studied as adsorbent to adsorb SO2 from flue gas, and it also shows an oxidation catalyst for oxidation of SO2 to SO3 in the presence of O2 and water in flue gas at room temperatures.26,27 Therefore, when AC is added in the IL, it shows an ability as catalyst to promote the oxidation of SO2. Yet there is only a very small amount of SO2 oxidized, as shown in entry 9 in Table 1. The reasons are that when AC is soaked in IL, its activity centers may be covered by IL molecules, they have strong relation with IL molecules, and hence AC loses its strong catalysis ability. Furthermore, there are some kinds of metal oxide in the ash, such as iron oxide, alumina oxide, cobaltous oxide, vanadium oxide, and so on. Both vanadium oxide23,24,28 and cobaltous oxide29 can be used as catalysts for the oxidation of SO2. Therefore, a small amount of ash can enhance the oxidation of SO2. Yet, because the temperature used for absorption of SO2 is just not more than 60 °C, the mole ratio of O2 in the IL is very low, and these

oxides are trace components in ash, the oxides in ash have much lower activity for the oxidation of SO2 at the condition we used than the industrial catalysts, vanadium oxide. In a word, in the presence of O2, the existence of solid ash or carbon, which may be captured by IL from flue gases, can enhance the oxidation of SO2, and they do not influence the absorption of SO2. Solutions To Prevent the Oxidation of SO2. The above results show that the presence of O2 in the simulated flue gases does not influence the absorption of SO2, but the absorbed SO2 can be oxidized by O2 to a very small extent. For reuse of IL and recovery of SO2, it is necessary to reduce the oxidation of SO2. Several ways are suggested to reduce the oxidation during the desulfurization from flue gases: (1) to reduce absorption temperature, (2) to shorten the absorption time, and the absorbent should be desorbed as soon as possible after the absorption, (3) to remove solid ash from flue gas before the absorption by IL, and (4) to remove the solid ash from IL in time during desulfurization circulation of IL. Conclusions In summary, the presence of O2 in the simulated flue gases does not influence the absorption of SO2 from flue gas by IL, [MEA]L, but a very small amount of the absorbed SO2 can be oxidized. Increases of absorption temperature, O2 concentration in flue gas, and remaining time are favorable to the oxidation of SO2. Ash and activated carbon added in the IL can promote the oxidation of SO2. According to the results, several ways are suggested to reduce the oxidation of SO2 for reuse of IL and recovery of SO2. Acknowledgment We thank Prof. Chengyue Li, Prof. Zhenyu Liu, and Dr. Qingya Liu for their help, and the Natural Science Foundation of China (20746001 and 20776004), Program for New Century Excellent Talents in University (NCET-08-0710), and the Scientific Research Foundation of Graduate School of Beijing University of Chemical and Technology (09Ch007) for financial support. Literature Cited (1) Kikkinides, E. S.; Yang, R. T.; Cho, S. H. Concentration and Recovery of Carbon Dioxide from Flue Gas by Pressure Swing Adsorption. Ind. Eng. Chem. Res. 1993, 32, 2714–2720. (2) Machida, M.; Yoshii, A.; Kijima, T. Temperature Swing Adsorption of NOx over ZrO2-based Oxiders. Int. J. Inorg. Mater. 2000, 2, 413–417. (3) Wu, W. Z.; Han, B. X.; Gao, H. X.; Liu, Z. M.; Jiang, T.; Huang, J. Desulfurization of Flue Gas: SO2 Absorption by an Ionic Liquid. Angew. Chem., Int. Ed. 2004, 43, 2415–2417.

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ReceiVed for reView May 18, 2010 ReVised manuscript receiVed October 28, 2010 Accepted November 8, 2010 IE101126A