Deactivation and Regeneration of an Ionic Liquid during

Jan 10, 2012 - Room-temperature ionic liquids (ILs) are widely investigated to absorb SO2 from mixed gases or simulated flue gases and can capture a l...
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Deactivation and Regeneration of an Ionic Liquid during Desulfurization of Simulated Flue Gas Shuhang Ren,† Yucui Hou,‡ Shidong Tian,† Weize Wu,*,† and Weina Liu† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China

‡

ABSTRACT: 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, after many circulations of SO2 absorption from flue gas, the capacity of SO2 in ILs decreases. The main reason may be the oxidation of SO2, and the oxidized SO2 becomes sulfuric acid when water presents in flue gas. Sulfuric acid in the ILs influences the regeneration of ILs and further absorption of SO2 by the ILs. In this work, the effect of sulfuric acid in a task-specific IL, monoethanolammonium lactate ([MEA]L), on the absorption of SO2 and the regeneration of [MEA]L from a mixture containing sulfuric acid were studied. It was found that when sulfuric acid was added into the IL, the absorption capacity of SO2 in [MEA]L decreased greatly. However, when NaOH, CaO, or CaCO3 was added into the mixture of [MEA]L and sulfuric acid, [MEA]L could be regenerated and sulfuric acid was formed into its corresponding salts and precipitated for removal.

1. INTRODUCTION SO2 is one of the most prevalent pollutants in flue gas upon burning fossil fuel, and it causes environmental problems such as acid rain, polluted rivers, destroyed plants, and health concerns. However, SO2 is also an important and useful resource in chemical production, such as producing sulfur and sulfuric acid. Nowadays, the most effective way to control the emission of SO2 from flue gas by combustion of fossil fuels is flue-gas desulfurization (FGD). The traditional and commercial way is to capture SO2 by limestone. However, this method produces the waste byproduct (CaSO4), and SO2 cannot be reasonably recovered as a chemical resource. The pressure swing absorption (PSA) or temperature swing absorption (TSA) technologies are new technologies for SO2 capture. These methods can avoid byproducts, and SO2 can be collected for further use. Nevertheless, for practical use, it is difficult to find a material for reversible and selective absorption of SO2. Recently, room-temperature ionic liquids (ILs) have drawn much attention to the researchers.1−3 ILs have extremely low vapor pressure, high thermal and chemical stability, designable structure, and excellent solvent power, and they can absorb SO2 from flue gases with high capacity and easily be desorbed without any change. For years, researchers studied many kinds of ILs, especially task-specific ILs, to absorb SO2. 1,1,3,3Tetramethylguanidinium lactate ([TMG]L),4 which was the first kind of task-specific ILs used for SO2 absorption, could absorb nearly 1 mol of SO2 per mole of IL at 1 bar with 8% of SO2 in gas stream, and it could hardly absorb CO2. Due to the excellent capacity and selectivity of SO2, many TMG-based ILs are synthesized,5−7 such as 1,1,3,3-tetramethylguanidinium tetrafluoroborate ([TMG][BF4]) and 1,1,3,3-tetramethylguanidinium acrylate ([TMG]A). Hydroxyl ammonium ILs8,9 are another kinds of task-specific ILs for SO2 capture. These ILs can also have high capacity for SO2 at ambient pressure. For pure SO2, these ILs can absorb 0.60−1.04 mol of SO2 per mole of ILs at 25 °C. Compared with TMG-based ILs, the © 2012 American Chemical Society

decomposition temperatures of these ILs are little higher. Pyrimidine- and imidazole-based ILs10−14 were synthesized to absorb SO2 and can achieve high SO2 absorption, but these ILs are always used to absorb SO2 at high SO2 partial pressure. For example, at 25 °C, 0.245 MPa, 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIM][Tf2N]) can absorb 3.57 mol of SO2. Other kinds of ILs, such as caprolactam tetrabutylammonium bromide ILs ([CPL][TBAB])15,16 and azole-based ILs,17 can also be used to absorb SO2 with high capacity. To improve the absorption of SO2 by ILs, researchers tried membrane and polymer technologies,18−22 and these technologies can enhance the selectivity and capacity of this absorption. Most researchers studied the absorption of SO2 from pure SO2 or SO2 + N2 mixture. However, there are other compounds in real flue gas, such as O2, H2O, and ash. Our previous work23,24 investigated the effect of water and O2 on the absorption of SO2. It shows that IL can absorb H2O when H2O exists in the flue gas. With the presence of O2 in flue gas, the SO2 absorbed by ILs can be oxidized to a small extent, and the present ash can promote the oxidation. Although this oxidation is limited, it is inevitable. After long-term reuse of ILs, the oxidized SO2 becomes sulfuric acid when water exists in the flue gas, and the sulfuric acid in ILs cannot be separated during regeneration of SO2 by evaporation. In this work, we investigated the effect of sulfuric acid on the absorption of SO2 by monoethanolammonium lactate ([MEA]L) and tried to find a simple way to remove sulfuric acid from the IL solution. Received: Revised: Accepted: Published: 3425

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2. EXPERIMENTAL SECTION 2.1. Materials. SO2 (99.95%) and N2 (99.99%) were supplied from Beijing Haipu Gases. Monoethanolamine (≥99%) and calcium oxide (≥98%) were purchased from Tianjin Fuchen Chemical Co. (Tianjin, China); lactic acid (85% in aqueous solution) was from Tianjin Bodi Chemical Co. (Tianjin, China); sulfuric acid (98%), sodium hydroxide (96%), and calcium carbonate (≥99%) were from Beijing Chemical Plant. All reagents and solvents were analytical reagents. [MEA]L was synthesized and characterized following the procedure reported in the literature,8 and its structure is shown in Scheme 1. The IL was dried at 70 °C under vacuum

from a simulated flue gas, which contained N2 (70% in volume), O2 (20% in volume), H2O (2.3% in volume), CO2 (2.9% in volume), and SO2 (4.8% in volume), for many circulations of absorption and desorption. The apparatus, procedures, and analysis methods are identical with that in our previous work.23,24 It was found that after 10 circulations of the reuse of IL, the mole ratio of SO42− to [MEA]L could reach 0.035, and the absorption of SO2 by IL decreased from 0.158 for fresh IL to 0.085 in mass ratio of SO2 to IL. The oxidized SO2 became sulfuric acid, and the sulfuric acid in the IL could not be separated during recovery of SO2 by a simple evaporation. As a result, the sulfuric acid in the IL increased with the reuse circulations of IL. 3.2. Effect of Sulfuric Acid on Absorption of SO2. The oxidization of SO2 produces sulfuric acid in ILs, and sulfuric acid can influence the absorption of SO2 by ILs. Figure 1 shows

Scheme 1. Structure of [MEA]L

treatment for 2 days. The water concentration of [MEA]L after drying was measured by Karl Fischer analysis, and water contents was less than 0.1% in mass fraction. 2.2. Apparatus and Procedures. The SO2/N2 gas mixture, with SO2 content of 2.1% by volume, was prepared by mixing SO2 and N2 in a high-pressure cylinder of 40 L in volume. The absorption experiment consisted mainly of the cylinder containing the SO2/N2 gas mixture, a test tube with an inner diameter of 12 mm and a length of 200 mm, a rotameter (Beijing Forth Automation Meter Factory), and a constant temperature water bath. [MEA]L and 1.84 mol/L H2SO4 aqueous solution with a desired ratio were mixed together to obtain an IL mixture. To regenerate the H2SO4-containing IL mixture, NaOH, CaO, or CaCO3 with a molar amount equal to that of H2SO4 was added into the IL mixture, a solid sulfate salt was observed to form and to precipitate onto the bottom of the test tube, and the precipitated solid was separated from the IL solvent by filtration. Before each absorption experiment, the IL or IL mixture was purified by a sweeping method,25 until no obvious loss was observed. In a typical experiment, the SO2/N2 gas mixture went through the [MEA]L loaded in the test tube, and the flow rate was monitored by the rotameter and calibrated by a soap film fluid meter. The test 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, China). After a given time for absorption, the weight of the test tube was measured via an electrical balance (BS224S Sartorius) with an accuracy of ±0.0001 g, and the content of absorbed SO2 in the IL was calculated by the weight difference. The reproducibility of the measurements was better than ±2.5%, and it was estimated that the data were accurate to ±5%. Before and after the absorption of SO2, about 1 g of IL was sampled, and 1.75 g of water was added into the IL. The pH value of the mixture was measured by a pH meter (Shanghai Precision & Scientific Instrument Co. Ltd.). The IL, after absorbing SO2, was desorbed at 80 °C under vacuum treatment.

Figure 1. The absorption of SO2 in [MEA]L via time at 60 °C with different mole ratios of H2SO4 to [MEA]L: ■, 0.00; ●, 0.05; ▲, 0.10; ▼, 0.20; ◀, 0.30; ▶, 0.40.

the trend of the SO2 absorption amount in [MEA]L with different mole ratios of H2SO4 to [MEA]L as a function of time at 60 °C, and Figure 2 shows the equilibrium absorption

Figure 2. The equilibrium absorption of SO2 in [MEA]L at 60 °C with different mole ratios of H2SO4 to [MEA]L.

amount of SO2 in [MEA]L with different mole ratios of H2SO4 to [MEA]L at 60 °C. It can be seen that, with increasing the mole ratio of H2SO4 to [MEA]L, the mass ratio of SO2 to [MEA]L decreases sharply. When H2SO4 was not added into [MEA]L, the mass ratio of SO2 to [MEA]L could reach 0.109.

3. RESULTS AND DISCUSSION 3.1. Deactivation of [MEA]L during Desulfurization from Simulated Flue Gas. [MEA]L was used to absorb SO2 3426

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Scheme 2. Proposed Mechanism of the Absorption of SO2 by [MEA]L

For example, when the mole ratio of H2SO4 to [MEA]L reached 0.05, the pH value decreased to 4.88 from 7.69. And when 0.5 mol of H2SO4 was added into 1 mol of [MEA]L, the pH value became 1.37 and the IL loses its ability to absorb SO2. It should be noted that the IL with pH values from 2.37 to 4.88 also can absorb SO2. This result may further demonstrate that the SO2 absorption mechanism is the replacement of lactate anion with sulfite anion because there still exists an amount of [MEA]L at the mole ratios of H2SO4 to [MEA]L. When the mole ratio of H2SO4 to [MEA]L reached 0.5, all [MEA]L is replaced by [MEA]2SO4, and the IL lost its ability to absorb SO2. Therefore, when H2SO4 was added into [MEA]L, lactic acid was produced and the pH value decreased. After the absorption of SO2, the pH value of [MEA]L also decreased with the increase of the mole ratio of H2SO4 to [MEA]. For instance, the pH value of pure [MEA]L decreased to 3.87 from 7.69 after absorption of 0.109 mol of SO2 per mol IL. When the mole ratio of H2SO4 to [MEA]L is 0.05, the pH value of the absorbent becomes 3.73. And it decreases with further increasing the mole ratio of H2SO4 to [MEA]L up to 0.5. It can be noted that the pH values of the IL after SO2 absorption are lower than those of the IL before SO2 absorption. Because there is no absorption of SO2 with 0.5 mol ratio of H2SO4 to [MEA]L, the pH value does not change. On the basis of the pH values of different mole ratios of H2SO4 to [MEA]L in Figure 3, we can predict the content of sulfuric acid in IL before and after the desorption, and it can help us to know when we should remove H2SO4 from IL to maintain the high SO2 absorption capacity of IL. 3.4. Regeneration of [MEA]L. H2SO4, produced during the SO2 absorption, has a high boiling point and a low volatility. Hence, H2SO4 in the IL cannot be removed during the desorption of SO2 from IL by the traditional method of heating the IL at high temperatures and/or under vacuum. Importantly, it can react with [MEA]L, decreasing the IL capacity of SO2 absorption. As a result, after many circulations of the absorption, H2SO4 levels became greater and greater in [MEA]L, resulting in the absorption capacity of SO2 in IL decreasing dramatically. This may limit the eventual use of [MEA]L in flue gas desulfurization. In this work, we tried to use some base materials to regenerate [MEA]L from a mixture containing H2SO4. When NaOH was added into a [MEA]L and H2SO4 mixture, solid Na2SO4 was produced and the IL was regenerated after filtration of the solid. Figure 4 shows the equilibrium absorption of SO2 in [MEA]L with different additives. When equivalent NaOH was added into [MEA]L with 0.3 mol of H2SO4 per mole of IL and the pH value recovered to 7.69, the absorption capacity of SO2 in the IL could almost be recovered. The pure [MEA]L can absorb 0.108 g of SO2/g of IL, and the regenerated [MEA]L can reach to 0.097 g of SO2/g of IL. The effect of Na2SO4 on the absorption of SO2 by IL was also investigated. No obvious loss of absorption capacity was found during the absorption when the mass fraction of NaSO4 was 0.01 (see Figure 4). CaO and CaCO3 with water were also used to regenerate the IL from a mixture containing H2SO4. The SO2 capacity of the regenerated IL is almost the same as the original one.

When the mole ratio of H2SO4 to [MEA]L increased to 0.1, the mass ratio of SO2 to [MEA]L decreased to 0.059, which is almost half of the absorption capacity of pure IL. However, when 0.5 mol of H2SO4 added into 1 mol of [MEA]L, there is no absorption of SO2 observed. This trend means that the lactate anion may play an important role in the absorption of SO2, and it shows a good agreement with previous research.26−28 Wang et al.26,27 used quantum chemical calculations and molecular dynamics simulation to investigate the interaction between SO 2 molecules and ions of [TMG]L. They found that a strong organization of SO2 with the [TMG]L components, especially the lactate anion, and the oxygen atoms of the lactic acid anion are the main active sites for the absorption of SO2. When H2SO4 was added into [MEA]L, [MEA]L reacted with H2SO4, and [MEA]2SO4 was produced. Then the content of [MEA]L decreased. Because the strong organization was formed by the lactate anion in ILs and SO2, the absorption of SO2 in the mixture of H2SO4 and [MEA]L decreased following the increase of H2SO4 content in [MEA]L. In our previous work,28 we found that due to the lower acidity of lactic acid than that of sulfurous acid, [MEA]L and [TMG]L can chemically absorb SO2 by replacing lactate anion with sulfite anion, as shown in Scheme 2. When H2SO4 was added into [MEA]L, due to the strong acidity of H2SO4, [MEA]2SO4 cannot react with SO2 because the acidity of H2SO3 is weaker than that of H2SO4 and it is stronger than that of lactic acid. The reason why the equilibrium absorption of SO2 in [MEA]L at 60 °C with different mole ratios of H2SO4 to [MEA]L was not linear to the mole ratio of H2SO4 to [MEA]L was that the existence of lactic acid may restrain the reversible reaction of [MEA]L with SO2, which produces lactic acid.28 3.3. Effect of Sulfuric Acid on the pH Value of [MEA]L before and after Absorption of SO2. The pH values of [MEA]L before and after the absorption of SO2 are shown in Figure 3. Before the absorption of SO2, the pH value of pure

Figure 3. The pH value of aqueous [MEA]L with different mole ratio of H2SO4 to [MEA]L: ■, before the absorption of SO2; ●, after the absorption of SO2.

[MEA]L is 7.69, and it is alkalescent. As expected, when H2SO4 was added into [MEA]L, the pH value of [MEA]L decreased. 3427

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Figure 4. Absorption of SO2 in [MEA]L at 60 °C with different additives in the IL. H2SO4 was added at 0.3 mol per mole of IL and then an equivalent amount of NaOH, CaO, or CaCO3 was added to regenerate the IL. Na2SO4 was added to the pure IL for comparison.

Figure 5. Three cycles of SO2 absorption using [MEA]L after regeneration by NaOH: ■, first circle; ●, second circle; ▲, third circle.

Although, H2SO4 can dramatically decrease the absorption capacity of SO2 in IL, when NaOH, CaO, or CaCO3 is added into the IL and H2SO4 mixture, H2SO4 can form precipitated Na2SO4 or CaSO4 and can be separated from the IL. The effect of H2SO4 on the absorption of SO2 can be removed.

When NaOH was added into the [MEA]L and H2SO4 mixture, Na+, Lac−, MEA+, and SO42−existed in the mixture. As the Na2SO4 is a solid and can hardly dissolve in the IL solvents, it can separate out from the mixture, as shown in eq 1.

4. CONCLUSIONS In this work, we studied the effect of sulfuric acid in [MEA]L on the absorption of SO2 and the regeneration of [MEA]L. It was found that the sulfuric acid could decrease the absorption of SO2 in [MEA]L greatly and influence the reuse of the IL. However, when NaOH, CaO, or CaCO3 was added into the mixture of [MEA]L and H2SO4, sulfuric acid was changed into the corresponding salts, precipitated, and separated by filtration, and then [MEA]L could be regenerated.

2Na+ + MEA+ + Lac− + SO4 2 − → MEA+ + Lac− + Na2SO4 ↓

(1)

As we know, H2O always exists in flue gas and it is also absorbed by IL. So when water is present in IL, Na2SO4 can dissolve in the IL + H2O mixture and cannot precipitate for easy separation. As H2SO4 in the IL is neutralized by NaOH, it is possible to use CaO to neutralize H2SO4 in IL instead of NaOH. When CaO was added, the solid CaSO4 was produced, as shown in eq 2. The solubility of CaSO4 in IL or/and water is much less than that of Na2SO4, and it is easy to separate CaSO4 from the IL mixture by filtration.

■

*Tel./Fax: +86 10 64427603. E-mail: [email protected]. Funding

The project is financially supported by the Natural Science Foundation of China (21176020 and 20746001) and Program for New Century Excellent Talents in University (NCET-08− 0710).

Ca 2 + + MEA+ + Lac− + SO4 2 − → MEA+ + Lac− + CaSO4 ↓

AUTHOR INFORMATION

Corresponding Author

(2)

■

And interestingly, when CaCO3 was added into the IL with H2SO4, CO2 was emitted from the mixture at first, and then solid CaSO4 was found to precipitate. This means that the SO42− in the mixture is removed and the IL is regenerated. The ILs regenerated by CaO and CaCO3 were used to absorb SO2, and the absorption amounts are almost identical with that of the fresh IL. Figure 4 indicates that the use of NaOH, CaO, or CaCO3 is a plausible way to remove H2SO4 in IL. During the regeneration process, a small amount of sulfate salt may be left in IL. Na2SO4 was added to pure IL for comparison, and the SO2 absorption amount is almost the same as that of the fresh IL, which suggests that sulfate salt residue in IL will not influence the next SO2 absorption by the IL. Figure 5 shows three circulations of SO2 absorption by [MEA]L after regeneration of the mixture of [MEA]L and H2SO4 by NaOH. There is a drop in absorption capacity after the first circulation, which may result from the IL not being desorbed completely after the first cycle. The figure indicates that the regenerated [MEA]L could be reused three times with few changes.

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