Effect of H2O on the Desulfurization of Simulated Flue Gas by an Ionic

Apr 14, 2009 - (6, 13-15) Because of their extremely low vapor pressure, stability, and designable structure, ILs can reasonably be used as a solvent ...
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SEPARATIONS Effect of H2O on the Desulfurization of Simulated Flue Gas by an Ionic Liquid Shuhang Ren,† Yucui Hou,†,‡ Weize Wu,*,† Xiaoting Chen,† Jinlong Fan,† and Jianwei Zhang† State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China, and Taiyuan Normal UniVersity, Taiyuan 030012, China

Functionalized ionic liquids (ILs) have been demonstrated to absorb SO2 from mixed gases or simulated flue gases efficiently. However, after absorbing a large amount of SO2, the viscosity of the ILs increases greatly, which might limit their eventual applications in large-scale desulfurization from mixed gases or flue gases. In this work, the effect of the presence of water in a simulated flue gas on the absorption of SO2 by a functionalized ionic liquid, 1,1,3,3-tetramethylguanidinium lactate, has been studied at different temperatures. It is found that the presence of water in the simulated flue gas can decrease the viscosity of the IL greatly, and it has no effect on the absorptivity of SO2 from the flue gas. The densities of the IL absorbing SO2 from the flue gas with or without water are also studied. They increase with the increase of the amount of SO2 absorbed from the flue gas in both cases. Introduction Air pollution has been a serious problem for years and has drawn increasing attention throughout the world. One of the poisonous pollutants is SO2, which is emitted from the burning of fossil fuels and results in acid rain and so on. At present, flue-gas desulfurization (FGD) is considered to be one of the most effective ways to control emissions of SO2 from the combustion of fossil fuels.1,2 Several processes, such as wet FGD, dry FGD, semidry FGD, pressure swing absorption (PAS), and temperature swing absorption (TSA), have been developed to clean flue gases.3-5 As we know, one of the most attractive approaches to separating a target compound from a gas mixture in a gas stream is selective absorption into a liquid.6,7 The liquid absorbents should endure the temperature of the gas stream and should have very low vapor pressure. Generally, amines are used to chemically trap the acidic gases, such as CO2, SO2, etc., by way of forming amine carbonate or sulfate. However, in the case of large-scale SO2 capture, the solution might evaporate into the gas stream, resulting in waste or environmental pollution. Room temperature ionic liquids (ILs) have extremely low vapor pressure, high thermal and chemical stability, designable structure, and excellent solvent power for organic and inorganic compounds. Therefore, ILs are considered to be environmentally benign solvents for a number of chemical processes, such as separation, hydrogenation, oxidation, etc.8-12 Recently, functionalized or task-specific ILs have been designed and synthesized for separation of mixed gases, such as CO2.6,13-15 Because of their extremely low vapor pressure, stability, and designable structure, ILs can reasonably be used as a solvent to scrub SO2 from flue gases. In the process, no volatile solvent can evaporate into the gas stream in the case of large-scale SO2 capture, compared with traditional basic organic solvent to absorb acidic gases. Therefore, several research groups reported that functionalized ILs can capture large-scale SO2 and can easily be recovered at higher temperature or/and under vacuum without * To whom correspondence should be addressed. E-mail: wzwu@ mail.buct.edu.cn. Tel./Fax: +86 10 64427603. † Beijing University of Chemical Technology. ‡ Taiyuan Normal University.

any change. Han et al.16 used an IL, 1,1,3,3-tetramethylguanidinium lactate (TMGL), to absorb SO2. A 1 mol portion of the IL could absorb nearly 1 mol of SO2 at 1 bar with 8% SO2 in the flue gas stream. They proposed that this process had both physical and chemical absorption. Riisager et al.17,18 synthesized several new TMG-based ILs, such as TMG tetrafluoroborate ([TMG][BF4]) and TMG bis(trifluoromethanesulfonyl) amide ([TMG][BTA]). They reported that both TMG-based and 1-butyl-3-methylimidazole(BMIM)-based ILs could reach high absorption (1.1-1.6 mol SO2/mol IL) at 1 bar with pure SO2. And, they suggested that the ILs could absorb large amounts of SO2 gas by physical interaction. The mechanism of SO2 absorption by TMG-based ILs was also discussed from the molecular dynamics simulation, which shows strong organization of SO2 around the TMG cation and the lactate anion and may explain the selectivity of the TMGL toward SO2.19 Zhang et al.20 and Brennecke et al.21 studied the solubility of SO2 in several ILs, such as hydroxyl ammonium ILs, pyrimidine and imidazole ILs, at different conditions, indicating that large amounts of SO2 can dissolve in the ILs. Functionalized ILs were also supported on membrane22 or were constructed to polymer23,24 to enhance the absorption efficiency of SO2. Although ionic liquids can absorb SO2 excellently, the water in flue gases and the high viscosity of ILs during absorption have not been studied in the literature to the best of our knowledge. The high viscosities of ILs and SO2 absorbed ILs are unfavorable to their transportation, which may limit the industrial application of ILs. The presence of H2O in the flue gas has been studied by several research groups, and the results show that it is beneficial to the SO2 capture in the lowtemperature dry and semidry FGD processes.25,26 In this work, we studied the effect of H2O on the absorption of SO2 from a simulated flue gas and found that the H2O present in the flue gas is also absorbed into ILs, but does not decrease the absorption efficiency. And importantly, the viscosity of ILs decreases greatly. Experimental Section Materials. SO2 (99.95%) and N2 (99.95%) were purchased from Beijing Haipu Gases. 1,1,3,3-Tetramethylguanidine was

10.1021/ie9000844 CCC: $40.75  2009 American Chemical Society Published on Web 04/14/2009

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Figure 1. Schematic diagram of the apparatus for TMGL to absorb SO2 from a flue gas stream with or without H2O: 1 cylinder filled with a simulated flue gas, SO2 and N2; 2 flow meters; 3 water baths; 4 temperature controllers; 5 glass bottle to supply moisture to the flue gas steam; 6 heating belt and temperature controller; 7 glass tube filled with IL; 8 glass bottle containing NaOH aqueous solution for absorbing the remaining SO2 in the vent gas; 9 valves.

purchased from Baigui Chemical Company (Shijiazhuang, China), which was used after fresh distillation. All acids and solvents were A.R. grade and produced by Beijing Chemical Reagent Plant (Beijing, China). TMGL was synthesized and characterizedfollowingtheproceduresreportedintheliteratures.16,27 The IL was dried at 80 °C under vacuum for 48 h. The water concentration in the IL after drying was 0.1-0.14 wt %, as measured by Karl Fischer analysis. Apparatus and Procedures. The simulated flue gas, SO2 content of 2% by volume, was prepared by mixing SO2 and N2 in a high pressure cylinder of 40 L in volume. The absorption experiment was carried out at an ambient pressure. The schematic diagram of apparatus employed is shown in Figure 1. It consisted mainly of a cylinder containing the simulated flue gas (N2 + SO2), a test tube with an inner diameter of 12 mm and a length of 200 mm, a moisture-producing water bottle of 100 mL in volume, two rotameters (Beijing Forth Automation Meter Factory, Beijing, China), and two constant temperature water baths. In a typical experiment, the gas mixture was bubbled through the water bottle and then went through the IL loaded in the test tube, and the flow rate was set to 50 mL · min-1, which was monitored by the rotameter and calibrated by a soap-film flow meter. Both the test tube and the water bottle were partly immersed into the water bathes, the temperatures of which were maintained within (0.1 °C by using temperature controllers (A2, Changliu Company, Beijing). The temperature of the water bottle was controlled at 40.0 °C in order to supply water vapor into the simulated flue gas stream. The saturated water pressure in the gas stream was 7.38 kPa at 40.0 °C (calculated from the WebBook of NIST); thus, the content of water in the gas stream was 7.3% in volume. After a given time for absorption, a small amount of IL was sampled from the test tube, and the content of absorbed SO2 in the IL was analyzed. The procedure for absorption without water in the flue gas is similar to that mentioned above, and the difference was that the flue gas from the rotameter directly went into the test tube. Analysis of SO2 Content in IL. The content of SO2 in the sampled IL was measured following the standard iodimetry (HJ/T 56-2000, a standard method of State Environmental Protection Administration of China). The detailed procedure was also reported in the literature.28 Briefly, the sample was dissolved in an aqueous solution, and it was mixed with a small amount of fresh starch solution as an indicator. Then, the aqueous

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Figure 2. Absorption of SO2 by TMGL as a function of time at different temperatures: (0, 9) 20.0; (O, b) 40.0; (∆, 2) 60.0 °C; with 7.3% of water (open marks) and without water (filled marks) in the flue gas stream.

mixture was analyzed by titration of iodine aqueous solution, the concentration of which was determined by sodium thiosulfate. Viscosity Determination. The viscosity was determined by a traditional technique (Ubbelohde viscometer, according to Poiseuiele Law). The viscometer was calibrated using standard oils of different viscosities provided by National Standard Bureau of China. The reproducibility of the measurements was better than (1%, and it was estimated that the data were accurate to (3%. Density Determination. The density was determined via a pycnometer of 5 mL in volume. The volume of the pycnometer was calibrated by pure water at 20.0 °C. The reproducibility of the measurements was better than (0.5%, and it was estimated that the data were accurate to (1%. Results and Discussion Effect of H2O and Temperature on the Absorptivity of SO2 by IL. Figure 2 shows the absorption of SO2 by TMGL as a function of time at different conditions and under ambient pressure. Obviously, SO2 content in the IL increases with the increase of time at the beginning, then it reaches equilibrium, indicating that the IL absorption is starting to approach its maximum, time-independent capacity for SO2. For example, at a temperature of 40.0 °C, the mass ratio of SO2 to IL increases within 400 min, then it reaches a high level ratio of about 0.16. This indicates that the IL is able to absorb SO2 from the flue gas efficiently, which is in agreement with the results reported in the literature.16-18 However, our absorption amount of SO2, 0.16 in mass ratio to IL, is lower than the value of 0.305 at the same temperature reported by Han et al.16 This is because the 2% SO2 content in the flue gas is less than 8% used by the authors. As we know, temperature has great influence on the solubility of gases in liquid solution. We investigated three temperatures, 20.0, 40.0, and 60.0 °C on the absorption efficiency, and the results are shown in Figure 2. The IL absorption behaviors at 20.0 and 60.0 °C are similar to that at 40.0 °C. The ratio of SO2 to IL decreases with an increase of the absorption temperature, especially when absorption reaches equilibrium. The mass ratios of SO2 to TMGL at 20.0, 40.0, and 60.0 °C are 0.190, 0.157, and 0.123, respectively. Therefore, a decrease of temperature is in favor of SO2 absorption, which accords with normal gases solubilities in liquids. It is well known that flue gases contain a large amount of water. We investigated the effect of H2O with 7.3% in flue gas on SO2 absorption by TMGL, the results of which are

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Figure 3. Absorption of H2O in TMGL with 7.3% H2O in the flue gas stream at different temperatures: (9) 20.0; (b) 40.0; (2) 60.0 °C.

also shown in Figure 2. Interestingly, water in flue gas has almost no effect on the SO2 absorption by the IL. For example, at 40.0 °C, the absorption curves, mass ratio of SO2 to IL vs time with 7.3% water in flue gas, are almost the same as that without water. When the absorption reaches equilibrium, the absorption mass ratio of SO2 to TMGL with 7.3% H2O in flue gas are 0.191, 0.153, and 0.122 at 20.0, 40.0, and 60.0 °C, respectively, which are very similar to the mass ratios of SO2 to TMGL without water in the flue gas stream, 0.190, 0.157, and 0.123 at 20.0, 40.0, and 60.0 °C, respectively. These results show that the presence of H2O in the flue gas has little effect on the absorptivity of SO2 by the IL and the rates of SO2 absorption hardly change with and without H2O present in the flue gas. The absorbed SO2 could be removed under vacuum at 90-100 °C for 1-2 h. After removal of SO2 absorbed at 40 °C, the mass ratio of SO2 to IL in the IL became 0.011. The regenerated IL was reused to absorb SO2 from the flue gas at 40.0 °C, and the absorption mass ratio of SO2 to TMGL was 0.151. And for the third reuse, the absorption mass ratio of SO2 to TMGL was 0.150. These results show that the IL can be recovered and reused several times without any obvious change. Absorption of H2O by IL. During the absorption of SO2 from the flue gas with 7.3% water, we observed that the volume of the IL increased with an increase of time. Therefore, the water contents in the IL were also analyzed and they are shown in Figure 3. As seen in Figure 3, water content in the IL increases with an increase of time at the three temperatures investigated. TMGL is hydrophilic, and it can be miscible with water at room temperature.27 So, the interaction between water and TMGL is strong, and TMGL can easily absorb H2O from the gas stream. Figure 3 also shows that the content of water in TMGL is influenced greatly by the temperature. As we know, a decrease of temperature is favorable to gas solubilities in liquids. As can be seen in Figure 3, the mass ratios of H2O to TMGL decrease from 0.192 to 0.029 with increasing temperature from 20.0 to 60.0 °C, when the absorption of SO2 by TMGL reaches equilibrium. Furthermore, Figures 2 and 3 also indicate that TMGL can absorb SO2 and water from flue gas simultaneously, and they are independent. The results also demonstrate that the low temperature can get a better absorption of both SO2 and H2O. Johnstone et al.28 studied the solubility of SO2 in water. At 40 °C and 2% SO2 in N2 stream under ambient pressure, the solubility of SO2 in water is about 0.002 g/g water (interpolated). From Figures 2 and 3, we can see that when the SO2 absorption reaches equilibrium, the IL absorbs about 0.1 g water/g IL. The

Figure 4. Viscosity of TMGL as a function of mass ratio of SO2 to TMGL at 40.0 °C: (9) without H2O in the flue gas stream; (0) with 7.3% of H2O in the flue gas stream.

water of 0.1 g can only absorb 0.0002 g of SO2. Compared with the 0.16 g of SO2 absorbed by 1 g of the IL, water’s contribution to SO2 absorption can be ignored. It further demonstrates that the IL remains the high efficiency of SO2 absorption with water present in the flue gas. Effect of H2O on the Viscosity and Density of IL Absorbing SO2. The viscosity of TMGL absorbing SO2 at 40.0 °C is shown in Figure 4. As seen in the figure, the viscosity of the TMGL increases greatly with the content of absorbed SO2. The viscosity of fresh TMGL is 663 mPa · s, and when the mass ratio of SO2 to IL reaches 0.1495, the viscosity of the IL is 8797 mPa · s. The mechanism of absorption of SO2 by TMGL might be that SO2 is absorbed by NH2 group on the cation and formed sulfamic acid, while O on SdO probably forms inner molecular hydrogen bonding with H of the amine.16 The absorbed SO2 forms a semistable compound and increases the interaction of ILs, which may lead to the high viscosity of IL. Davis et al.6 designed and synthesized a functionalized IL, 1-(3-aminopropyl)-3-nbutylimidozolium tetrafluoroborate, and when the IL absorbed ca. 50 mol % of CO2, its viscosity also increased greatly. They thought that an ammonium carbamate salt formed when the IL absorbed CO2, which increases the high viscosity. However, the mechanism for the viscosity increase in the base-functionalized ILs has not been thoroughly investigated, although strong evidence exists for the formation of salt bridges.29 The high viscosity of the IL may influence its large-scale gas scrubbing applications. Interestingly, when water exists in the flue gas, the viscosity of the IL stays low. The effect of water in the flue gas on the viscosity of TMGL absorbing SO2 at 40.0 °C is also shown in Figure 4. With 7.3% of H2O in the flue gas, the viscosity of TMGL increases slightly from 663 to 1020 mPa · s within 60 min, corresponding to 0.03 of mass ratio of SO2 to TMGL. Then the viscosity of the TMGL decreases with an increase of time and the ratio of SO2 to TMGL. When the ratio of SO2 to TMGL reaches 0.154, the viscosity lowers and keeps at 254 mPa · s. As seen in Figures 3 and 4, the IL absorbs a large amount of water, 0.10 g/g IL. Sedden et al.30 studied the effect of water content on the viscosity of [C4mim][BF4], and the results show that the presence of water in the IL reduces notably the viscosity of the IL. The presence of the water molecules in IL can reduce the electrostatic attractions between the ions. Thus, the overall cohesive energy of the system is lowered, and hence, its viscosity is lowered too. Although as mentioned above, the presence of SO2 in TMGL

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and No. 20533010) and Program for New Century Excellent Talents in University for financial support. Literature Cited

Figure 5. Density of TMGL as a function of mass ratio of SO2 to TMGL at 40.0 °C: (9) without H2O in the flue gas stream; (0) with 7.3% H2O in the flue gas stream.

can increase the viscosity of the IL, the effect of water on the reduction of the IL’s viscosity dominates the property of the IL mixture. So the presence of water in TMGL cannot only reduce the high viscosity of IL by absorbing SO2 but can also maintain the high SO2 absorptivity. The density of the TMGL at different conditions is also studied. First, the density of the pure TMGL that we synthesized was measured at 40.0 °C. The density of the IL is 1.085 g · cm-3, which is in good agreement with the 1.07 g cm-3 of the IL’s density at 45 °C reported by Han et al.27 The effect of absorbed SO2 on the density of TMGL is shown in Figure 5. The density of the IL increases with the increase of the amount of SO2 absorbed by the IL. When water is present in the flue gas, TMGL absorbs both SO2 and H2O and, also, the density of the IL still increases with the absorption of H2O and SO2, as shown in Figure 5. When the ratio of SO2 to IL does not exceed 0.1, there is no obvious difference between the densities of TMGL with or without water present in the flue gas. When the ratio exceed 0.1, the density of TMGL without water present in flue gas is greater than that with water present in flue gas. The reason for this may be that the density of water is lower than that of TMGL. Thus, following the increase of the amount of water, the density of the absorbent with water becomes lower than that without water. Conclusions TMGL can absorb SO2 from a simulated flue gas with high absorptivity. The effect of temperature on the absorption of SO2 shows that the mass ratio of absorbed SO2 to the IL increases from 0.123 to 0.190 with a decrease of temperatures from 60.0 to 20.0 °C when the absorption reaches equilibrium. The viscosity of the IL increases greatly with an increase of the amount of absorbed SO2. When 7.3% water is present in the flue gas, the IL absorbs both SO2 and water. However, the absorption of water by the IL does not decrease the absorptivity of SO2 and, importantly, it reduces greatly the viscosity of the IL. The densities of the IL absorbing SO2 increase with an increase of the amount of SO2 absorbed from the flue gas with and without water. Acknowledgment The authors thank Prof. Chengyue Li, Prof. Zhenyu Liu, and Dr. Qingya Liu for their helpful discussion and suggestions, and the Natural Science Foundation of China (No. 20746001

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ReceiVed for reView January 18, 2009 ReVised manuscript receiVed March 13, 2009 Accepted March 25, 2009 IE9000844