SO2 Absorption Performance Enhancement by Ionic Liquid Supported

Jan 5, 2015 - SO2 removal because of its high absorption capacity, simple synthesis process ... less than 1 μm.34 In addition, the leaching of ionic ...
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SO2 Absorption Performance Enhancement by Ionic Liquid Supported on Mesoporous Molecular Sieve Xiaoshan Li, Liqi Zhang,* Ying Zheng, and Chuguang Zheng State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, Hubei Province P.R. China ABSTRACT: TMGL (tetramethylguanidinium lactate) is believed to be a potential medium for SO2 removal. However, direct use of TMGL in SO2 absorption exhibits unsatisfactory absorption rate and desorption performance, mainly because of its hyperviscosity. In this work, mesoporous molecular sieve MCM-41 (mobil composition of matter no. 41) was employed as support material to prepare a novel absorbent (MCM-41-TMGL). The uptake of SO2 depends on TMGL, and the absorption rate depends on the porous feature of MCM-41-TMGL. Results show that MCM-41-TMGL dramatically improved the absorption kinetics and desorption efficiency, largely attributed to enhanced gas−liquid interface. Moreover, the absorbed SO2 in MCM-41-TMGL can be easily stripped out by heating at 90 °C under vacuum, allowing MCM-41-TMGL-10% to maintain 95% of the initial absorption capacity after 10 cycles of regeneration. In addition, the absorption performance of MCM-41-TMGL was investigated under different conditions, including absorption temperature, SO2 concentration, and vapor concentration of simulated flue gas.

1. INTRODUCTION Sulfur dioxide is one of the main air pollutants, leading to a series of environmental pollution problems such as acid rain and smog. The coal-fired power plant has been considered as a primary source of regional air pollution and ecosystem acidification, due to its huge emissions of acidic pollutants.1 Flue gas desulfurization (FGD) is proved to be one of the most effective technologies for SO2 control, which may be achieved through wet, semidry, and dry methods.2−4 However, these scrubbing processes suffer from various defects, such as large usage of water, wastewater treatment, and low-value byproducts disposal.5 Room-temperature ionic liquids (ILs) as environmentally benign solvents have gained increasing attention for SO2 capture due to their remarkable properties,6−8 such as high thermal and chemical stability, extremely low vapor pressure, designable structure, easy reuse, and the implementation of some of them is worth considering as a more cost-effective and environmentally friendly alternative to previous methods. Wu and co-workers9 first synthesized a new ionic liquid, 1,1,3,3-tetramethylguanidinium lactate (TMGL), for SO2 removal and showed high selectivity of SO2 over CO2. Since then, solubility and thermodynamic properties of SO2 in various ionic liquids have been intensively reported.10−22 TMGL is identified to be the most promising solvent for SO2 removal because of its high absorption capacity, simple synthesis process, and low cost compared with other ILs.9,20−22 However, TMGL displays a high viscosity, and the viscosity increases dramatically when combined with SO2.20 This may result in small gas−liquid interface and slow diffusion rate of gas molecules, which is a major operational drawback for usual gas−liquid contactors. Additionally, how to transport the viscous IL is another operational problem. Fortunately, some attractive approaches have been proposed to solve this problem, such as combining ILs with other organic solvents or water,23−27 Supported ionic liquid membrane (SILM)28−30 and supported ionic liquid-phase (SILP).31,32 However, ILs © XXXX American Chemical Society

mixtures exhibit several major shortcomings, such as instability and the loss of the solvent, relatively higher viscosity compared to the organic solvents, a complex thermodynamic system, and the requirement of precise prediction models.33 To overcome the defects of ionic liquid with high viscosity as gas absorbent, supported ionic liquids (SILM and SILP) may prove a better choice in SO2 separation from flue gases. However, SILMs have some disadvantages in practice. For example, reported SILMs are quite thick, perhaps 150 μm or more, whereas highthroughput industrial applications will require selective layers of less than 1 μm.34 In addition, the leaching of ionic liquid through membrane pores is another intractable problem.35 In view of various defects of mixtures and SILMs, our investigation preliminarily focuses on the strategy of supported ionic liquidphase (SILP). The impregnation by an ionic liquid of a porous medium, for instance, as particles in packed bed columns, results in a SILP, where the gas−liquid interface is improved, increasing the rate of SO2 capture and lowering the required amount of IL at the same time. As a result, it lowers costs and improves efficiency. Thomassen and co-workers32 proposed a new concept of such SILP absorbers, and their use with selected ILs was found to significantly improve the ILs mass maximum capacity and absorption dynamics at low flue gas concentration. In addition, Zhang36 and Ren37 immobilized several amino-functionalized phosphonium amino acid ionic liquids into porous silica particles for CO2 sorption. Wang and co-workers38,39 used a porous poly(methyl methacrylate) microsphere (PMMA) as support to load amino acid functionalized ionic liquid for CO2 capture. Zhang and co-workers40 adopted porous silica (SiO2) particles as support material to prepare the TMGL-supported Received: April 30, 2014 Revised: January 3, 2015

A

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2.2. Preparation and Characterization of MCM-41TMGL. MCM-41-TMGL was prepared through the wet impregnation process. It is a physical method with a simple procedure and good yield. MCM-41 was calcined in a muffle furnace at 500 °C for 6 h. Then a certain weight of ionic liquid TMGL was completely dissolved in ethanol solvents. MCM-41 was impregnated in the dissolved liquids. The mixed liquid was stirred uniformly and placed in the vacuum drying oven to remove all the ethanol solvents. Several particles with different TMGL loadings were prepared, noted MCM-41-TMGL-X, where X stands for the mass percentage of TMGL in the whole adsorbent, namely, the TMGL loadings. m TMGL X= × 100% madsorbent (1)

particles for pure SO2 absorption. All the above results implied that it is an effective way of immobilizing ionic liquid into porous particles to improve the absorption rate and retain the high capacity of viscous ILs. The properties of the support material have a significant influence on the performance of supported ionic liquids. A perfect support material should have large specific surface area, pore volume, and porosity, which refer to the maximum mass loading. Appropriate pore diameter, sufficient mechanical strength, and good chemical and thermal stability are also required. Mesoporous molecular sieves containing adjustable pores with diameters between 2 and 50 nm may be a good option for support, because of the ordered nanostructure with large surface area and pore volume. MCM-41 (mobil composition of matter no. 41) has been extensively studied for gas separation and catalyst supports. Consequently, in this paper, MCM-41 was employed as support material to prepare a novel absorbent (MCM-41-TMGL), which combined the advantages of TMGL (high SO2 uptake) and MCM-41(high absorption rate). The effect of ionic liquid loadings on SO2 absorption and desorption performance were comprehensively investigated, and the quantitative analysis was conducted to describe the relationship between the absorption capacity and the pore characteristics. In addition, the absorption performance of MCM-41-TMGL was investigated under different conditions, including absorption temperature, SO2 concentration, and vapor concentration of simulated flue gas and the regeneration property was also studied.

mTMGL: weight of ionic liquid TMGL (g) madsorbent: weight of adsorbent MCM-41-TMGL (g) The theoretical maximum mass loading can be calculated from the pore volume of molecular sieve and density of ionic liquid. X max =

V: pore volume of molecular sieve (cm /g) ρ: density of ionic liquid (g/cm3) MCM-41-TMGL-X was characterized by Thermal Gravimetric Analyzer (TGA) and isothermal adsorption method (BET). A STA409 TGA instrument was used for analysis of the thermal stability of the adsorbents. The samples (10 ± 0.1 mg) were heated to 500 °C in 10 °C/min with N2 gas flow of 100 mL/min. The N2 adsorption and desorption experiment was carried out on a Micromeritics ASAP 2020 accelerated surface area and porosimetry system, from which the surface area, pore volume, and pore size distribution can be obtained. The data of surface area and pore size distribution was analyzed by applying the BET equation to the desorption data and BJH method, respectively. 2.3. Absorption and Desorption Experiment. The prepared particles with different TMGL loadings were used as absorbents for the desulfurization of simulated flue gas with different SO2 concentration. Dewatered SO2 was flowed through an absorption U-tube partly immersed in a thermostatic bath. The absorption experiment was performed at ambient pressure and at temperature ranging from 30 to 90 °C. A suitable flow rate (50 mL/min) was selected to ensure the complete gas−liquid interface. The exhaust gas was neutralized through a NaOH solution before being released to the atmosphere. The release of absorbed SO2 was achieved at 90 °C under vacuum. The amount of absorbed SO2 was found from the weight changes of the absorption tube at regular intervals of time through a Mettler Toledo balance with accuracy of ±0.0001 g. Each experiment was repeated more than three times with the experimental error at a level of ±5%. Regeneration properties were tested through 10 cycles of absorption and desorption. The absorption process was conducted at 30 °C for 180 min and desorption process was carried out at 90 °C under vacuum for 120 min.

Table 1. Viscosity of TMGL and Other Common ILs molecular formula

molar mass (g/mol)

viscosity (cP)

EmimAC N2222AC BmimL EmimSCN N2222L MEAL TMGL

C8H14N2O2 C10H23NO2 C11H20N2O3 C6H12N2O4S C11H23NO3 C5H13NO4 C8H19N3O3

170.21 189.30 228.29 208.24 219.32 151.26 205.25

63 83 209 356 202 656 6910

(2) 3

2. EXPERIMENTAL SECTION 2.1. Materials. SO2-containing mixture gas was supplied by Minghui Gas Technology Co., Ltd., China. Four mixture gases contain 8%, 5%, 1%, 0.2% vol of SO2, respectively, and the left is the inactive gas N2. CaCl2 particles were used as desiccant in the dryer to remove vapor in mixed gas. MCM-41 was obtained from Nanjing JiCang Co., Ltd., China. 1,1,3,3-tetramethylguanidinium lactate (TMGL, 205.25 g/mol) was obtained from Lanzhou Greenchem ILS, LICP, CAS, China. TMGL was dried at 90 °C under vacuum for 72 h before use. The content of water in TMGL was measured to be 1.49 wt % by Karl Fischer titration. The viscosity of TMGL and other common ILs was measured through American Brookfield DV-C Viscometer, shown in Table 1. There is a circulation constant temperature

ILs

V×ρ V×ρ+1

water bath connected to the viscometer. During the viscosity measurement, the temperature is controlled at 30 °C. Compared with the viscosity of water (0.8 cP) at the same condition, these ionic liquids show pretty high viscosity. TMGL exhibits 10−100 times more viscous than other common ILs, indicating that the interionic interactions are stronger in TMGL.

3. RESULTS AND DISCUSSION 3.1. Pore Structure Characteristics. Generally, the absorption performance is closely related to the pore structure characteristics in a gas−solid absorption system. Figure1 and B

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3.2. FTIR Characterization. Figure 3 shows FTIR spectrum of support material MCM-41, ionic liquid TMGL

Figure 1. Pore size distribution of MCM-41-TMGL.

Figure 2 present the pore structure characteristics of MCM-41TMGL with different TMGL loadings. As shown in Figure 1,

Figure 3. FTIR spectra of prepared ionic liquids.

and supported ionic liquid MCM-41-TMGL-40%. A strong characteristic peak at 1080 cm−1 (Si−O) was observed in FTIR spectra of MCM-41. As expected, compared with MCM-41, the spectrum of MCM-41-TMGL showed some functional groups of TMGL, such as N−H (1620 cm−1), CO in COO− (1722 cm−1) and C−N (1069 cm−1). Some new functional groups of TMGL at around 1080 cm−1 was absent in FTIR spectra of MCM-41-TMGL due to the interference of MCM-41. 3.3. Thermal Stabilities. The thermal stabilities of the supported particles, MCM-41, and neat TMGL were investigated through TGA, shown in Figure 4. After it was

Figure 2. Specific surface area and pore volume of MCM-41-TMGL.

the pore diameter of MCM-41 was 3.37 nm. As TMGL was loaded into the pore channel of MCM-41, the pore diameter of MCM-41-TMGL decreased evidently. The declining trend also occurred at the specific surface area and pore volume of MCM41-TMGL with different TMGL loadings, as shown in Figure 2. The MCM-41 support had a specific surface area and pore volume of 900 m2/g and 0.94 cm3/g, respectively, much larger than the widely used silica36,37,40 and PMMA.38,39 With the increasing loading of TMGL, both of them decreased largely. Changes in pore structure indicated that TMGL was indeed loaded into the pore channels of MCM-41. It is worth noting that the pore diameter of MCM-41TMGL-50% and MCM-41-TMGL-70% cannot be obtained from the N2 adsorption and desorption isotherms. The reason is that when the loading is 50% or above 50%, the pore channels of MCM-41 were completely blocked with TMGL so that N2 was prevented from getting into the pore channels. The results were exactly consistent with the calculated theoretical maximum loading of 50.5%, which was obtained from the density of TMGL (1.085 g/cm3) and pore volume of MCM-41 (0.94 cm3/g). The excess loading of TMGL indicated that they were no longer porous materials and could lead to lower absorption rate, as discussed below.

Figure 4. TGA curves of the supported particles, MCM-41, and neat TMGL.

calcined, MCM-41 had little weight loss with the temperature increasing to 500 °C. Neat TMGL started to decompose at 120 °C and completely decomposed at 350 °C. Neat TMGL, MCM-41-TMGL-50%, and MCM-41-TMGL-70% had around 3% weight loss at 100 °C, which can be considered to be the desorption of moisture. With the increasing loading of TMGL, MCM-41-TMGL showed different TGA curves and decomposition process. When the temperature increased to 200 °C, neat TMGL had around 20% loss of original mass, whereas the C

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small specific interface area, low liquid-side diffusion and mass transfer coefficient, which led to low absorption rates. It took a long time to reach stable SO2 capture for about 360 min. As shown in Figure 7, desorption performance of absorbedTMGL was unsatisfactory. After 60 min, only 48% of the

MCM-41-TMGL particles started to decompose or had minimal weight losses. The onset decomposition temperatures of the supported particles, MCM-41, and neat TMGL were determined from their TGA curves, as presented in Figure 5. It

Figure 5. Onset decomposition temperatures of the supported particles and neat TMGL.

Figure 7. Desorption performance of neat TMGL for SO2.

can be seen that the onset decomposition temperature decreased with the increasing TMGL loadings. Furthermore, all the supported particles with different loadings exhibited higher onset decomposition temperature than neat TMGL. As a result, compared with neat TMGL, the supported particles had better thermal stabilities and could be employed at relatively high temperature. 3.4. SO2 Absorption/Desorption Performance of Neat Ionic Liquid TMGL. The absorption performance of neat ionic liquids for SO2 was measured at 30 °C and ambient pressure. As shown in Figure 6, neat TMGL can capture a large amount of SO2 reaching 0.226 g SO2/g IL (0.725 mol SO2/mol IL). However, gas flowed through the liquid very slowly with big bubbles. Ionic liquids with high viscosity lead to high mass transport resistance and have negative impact on the absorption and desorption dynamics. Due to the high viscosity of 6910 cP, the transfer of SO2 molecules in viscous TMGL was poor

absorbed SO2 was stripped. The reason may be that there are both chemical and physical absorption reactions between TMGL and SO2, among which chemical absorption was the dominant factor. Wu22 had quantitatively distinguished the chemical and physical absorption by neat ionic liquids with the aid of the modified RK Eos. According to their calculation, the chemical absorption capacity of TMGL is around 0.5 mol SO2/ mol IL and the rest belongs to physical absorption. During the desorption process, physical absorbed-SO2 can be stripped quickly and completely at high temperature, although the chemical absorbed-SO2 had difficulty in desorption because of the strong chemical interactions between SO2−lactate complexes resulting in high absorption enthalpy.21 These results are in accordance with the existing research reports.9,10,41 3.5. SO2 Absorption/Desorption Performance of MCM-41-TMGL and Comparisons. 3.5.1. Effect of Ionic Liquids Loadings on SO2 Absorption. From the results of pore structure characteristics, it can be found that TMGL loading has great influence on the internal pore channel structure of supported ionic liquid particles, where mass transfer between SO2 and TMGL takes place. Consequently, TMGL loading is one of the most critical influencing factors on SO2 absorption performance. SO2 absorption performance of MCM-41-TMGL with TMGL loadings of 10%−60% was investigated at 30 °C, shown in Figure 8. MCM-41 support displayed a low absorption capacity for SO2 which could be ignored, compared with the large value of ILs. There was only some weak physical absorption reaction between SO2 and MCM-41 so that the absorbed SO2 could be released completely and easily at high temperature or reduced pressure. The effect of N2 was also neglected in this work because ILs could hardly absorb nitrogen.42 Supported ionic liquids with the loading of 10% showed the largest SO2 absorption capacity of 0.223 g SO2/g IL (0.716 mol SO2/mol IL). MCM-41-TMGL with excess loadings of 50% and 60% absorbed less SO2, especially MCM-41-TMGL-50%, with the lowest absorption capacity. In addition, it can be seen

Figure 6. Absorption performance of neat TMGL for SO2. D

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absorption by MCM-41-TMGL is regarded as the sum of the chemical and physical contributions of the support and the ionic liquid. The total absorption amount was divided into two parts, seen as follows. The g/g unit was used to distinguish the absorption amount of ionic liquid and the support MCM-41. m Total = mMCM ‐ 41 + mIL

(3)

For each loading MCM-41-TMGL-x, (x = 0.1−0.4) m Total ‐ x = mIL ‐ x +

⎛1 − x ⎞ ⎜ ⎟m ⎝ x ⎠ MCM ‐ 41 ‐ x

mIL − x = k1·mIL − (x + 0.1) , k1 < 1, mIL − x < mIL − bulk

(5)

vMCM − 41 − TMGL − x vMCM − 41

mMCM − 41 − x = k 2· mMCM − 41 − (x + 0.1), k 2 ∝ , mMCM − 41 − x < mMCM − 41

Figure 8. Effect of different TMGL loadings on SO2 absorption performance.

(4)

(6)

where k1 refers to the relationship of the absorption capacity of ionic liquid with different loadings and k2 refers to the relationship of the physical absorption capacity of MCM-41 with different loadings. Because MCM-41-TMGL with the excess loading of 50% and 60% were no longer porous particles, the physical absorption of the support was neglected. The chemical absorption capacity by the ionic liquids depends on the chemical structure of the ionic liquids, such as the alkaline functional groups and follows the chemical equilibrium. Because the ionic liquid was loaded into the pore channel of the support, k1 is related to the confined effect. Because the confined ionic liquid tends to recover its bulk properties with the IL increasing loading, the absorption performance of confined ionic liquids would approach to that of bulk IL. The absorption capacity of ionic liquids with low loadings should be lower due to the strong confined effect (k1 < 1). The physical absorption capacity by the MCM-41 support could be affected by the different pore characteristics. In the calculation of Garcia,45 the CO2 saturation capacity in zeolites roughly follows the order of the pore volumes. Yan46 also found that the adsorption capacity linearly increased with the total pore volume of the support (R2 > 0.94). Therefore, k2 should be related to the pore volume. Therefore, the quantitative calculation results with the loadings under the theoretical maximum value were obtained in Figure 9. For bulk TMGL, the SO2 absorption capacity is measured, 0.725 mol/mol (0.226 g/g). First, we assumed that mIL increases linearly with the loadings (Figure 9a), and the maximum value is less than 0.226.

that MCM-41-TMGL-40% had the higher capacity of SO2 than MCM-41-TMGL-30%, followed by MCM-41-TMGL-20%. The effect of the loadings on the absorption capacity can be explained by the different pore characteristics. Coasne43 and Monk44 reported the effect of ionic liquids loadings on the structural and dynamical properties of the supported ionic liquids through molecular dynamics simulations. The results indicate that as the loading increases, the dimension of the system increases from a 2D-like structure (a single layer coating the pore surface) to a 3D-like structure (the pore with the IL), and the ionic liquid fills the pore center and tends to recover its bulk properties. Because the bulk TMGL or neat TMGL showed larger absorption capacity than the supported ionic liquids, MCM-41-TMGL-40% with bulk-like properties had the higher capacity of SO2 than MCM-41-TMGL-20% and MCM41-TMGL-30%. It is worth noting that the reason why MCM-41-TMGL-10% showed the highest capacity of SO2 was that despite the lower absorption capacity by TMGL, the physical absorption by porous particles resulted in larger amounts of SO2 absorption. As shown in Figure 2, the pore volume and specific surface area of supported ionic liquids decreased with the increasing loadings. The specific surface area and pore volume were still high for MCM-41-TMGL-10% (688 m2/g and 0.67 cm3/g), which implied that particles with small amount of TMGL loading presented a very porous structure or a 2D-like structure. Appreciable porosity was still preserved, and this may be beneficial for the physical adsorption. As a consequence, not only chemical reactions between SO2 and TMGL but also physical absorption by the porous support contributed to the ability for MCM-41-TMGL-10% to absorb large amounts of SO2. 3.5.2. Quantitative Analysis on the Relationship between the Adsorption Capacity and the Pore Characteristics. Although it was well accepted that the effect of the loadings on the absorption capacity can be explained by the different pore characteristics, little quantitative information has been published to describe this behavior. A suitable numerical or analytical model can help to make this point clear. Thus, a rough calculation was conducted to quantitatively describe the relationship between the absorption capacity and the pore characteristics and distinguish the physical absorption and chemical absorption of the absorption system. The SO2

mIL − x = 0.252x + 0.098

(7)

Then the residue was considered as physical absorption capacity by the support. The relationship between mMCM‑41‑x and the pore volume was fitted linearly. It can be seen that the absorption capacity of the support roughly follows the order of the pore volumes (Figure 9b). mMCM − 41 − x = 0.0168V − 0.0031

(8)

On the basis of the experimental data (pore characteristics and the absorption capacity of ionic liquids and the support), the relationship between the total absorption amount and the pore characteristics could be obtained. Because pore volume is linearly related to the loading according to the accessible BET results (Figure 9c), the relationship between mMCM‑41 and the E

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the IL loadings increases, the physical absorption capacity of the support was reduced due to the porous pore characteristics, and the chemical absorption capacity of the ionic liquid was enhanced due to the confined property. 3.5.3. Comparisons between Neat TMGL and MCM-41TMGL. Generally, the absorption rate is also an important aspect to assess the absorption performance besides absorption capacity. Large absorption capacity and short equilibrium time are in favor of excellent absorption performance. Chen and coworkers47 proposed two parameters including initial absorption rate (r10) and the degree of difficulty to reach phase equilibrium (t0.9) as the standards to evaluate the absorption kinetics. r10 is expressed as SO2 absorption capacity during the first 10 min, whereas t0.9 refers to the time when 90% of the CO2 absorption capacity is reached. The two parameters were applied in this work, listed in Table 3. r10 decreased and t0.9 increased with the increasing TMGL loadings, indicating a drop in absorption rate. This agrees well with the changes in pore structure of MCM41-TMGL with different loadings. Among them, MCM-41TMGL-10% demonstrated the highest uptake of SO2 in initial 10 min and the shortest time to reach 90% of SO2 uptake capacity due to its largest surface area and pore volume. MCM41-TMGL with excess loadings of 50% and 60% had much smaller value of r10 and larger value of t0.9 than other loadings of 10−40%. The reason for such an unsatisfactory absorption rate

(9)

V = 0.898 − 1.688x mMCM − 41 − x = 0.012 − 0.03x

(10)

The relationship between the total absorption amount and the loadings could be obtained using eqs 7−10. m Total − x =

0.012 + 0.282x + 0.056 x

(11)

Thus, the calculated and the experimental total absorption amount of different loadings are listed in Table 2, and the deviation of the two values is within 8.5%. Table 2. Calculated and the Experimental Total Absorption Amount loading x

mtotal(exp)

mtotal(cal)

deviation (%)

0.1 0.2 0.3 0.4

0.223 0.163 0.182 0.201

0.204 0.172 0.181 0.199

8.5% 5.5% 0.5% 1.0%

The physical and chemical contributions of the support and the ionic liquid were distinguished, presented in Figure 9d. As

Figure 9. Quantitative analysis results: (a) assumed linear relationship between mIL and the loadings; (b) relationship between mMCM‑41 and the pore volume; (c) the pore volume vs the loadings; (d) mMCM‑41 and mIL vs the loadings. F

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TMGL. SO2 uptakes in the first 10 min by MCM-41-TMGL (10% ∼ 40%) were nearly 2−5 times higher than that by neat TMGL. On the other hand, MCM-41-TMGL (10−40%) showed much better desorption performance than neat TMGL seen in Figure 10. To sum up, MCM-41-TMGL could solve the problems of neat ionic liquid with large viscosity effectively. They can greatly improve the absorption rate as well as desorption efficiency and maintain the adsorption capacity at the same time. After loading TMGL on mesoporous molecular sieve, the large uptake of SO2 depends on TMGL, whereas the absorption rate depends on the porous feature of MCM-41, namely, the supported ionic liquids combining the advantages of both ionic liquids and porous materials. It should be noted that it is significant to seek for an optimal TMGL loading to balance the absorption and desorption performance for MCM41-TMGL. According to the above results, it is believed that an optimal TMGL loading of 10% exhibits the most excellent SO2 absorption and desorption performance. 3.6. Effect of Temperature and Water Vapor on SO2 Absorption. Generally, temperature is a critical influence factor during absorption process. It is known that the temperature of real flue gas is above 100 °C. However, almost all of the researchers investigated SO2 absorption by ILs at low temperature between 20 and 60 °C and desorption process at high temperature above 90 °C and/or under reduced pressure. Therefore, it is significant and valuable to investigate the absorption capacity of ILs at high temperatures. Furthermore, it is suggested by several researchers that the presence of water in flue gas is conducive to SO2 absorption during the FGD processes.48 However, few studies have considered the effect of H2O in flue gas on absorption by ionic liquids or supported ionic liquids. In this work, SO2 absorption of MCM-41-TMGL-40% was measured at various temperatures from 30 to 90 °C with 12.2% of water and without water in flue gas. The experiment apparatus was similar to that in the literature.20 The water vapor in the gas stream was supplied by the deionized water in a glass bottle of 100 mL in volume. The bottle was immersed into the water baths, the temperature of which was controlled at 50 °C. Because the saturation pressure of water vapor at 50 °C is 0.01226 MPa (calculated by NIST Refprop software), the water vapor in gas stream is 12.2% in volume. SO 2 concentration in outlet gas stream was analyzed by gas chromatography (GC, FL9070), and the absorption capacity was determined from the difference in the SO2 concentrations between the inlet and outlet integrated over time. The results were presented in Figure 11. At the beginning of the absorption process, large amount of SO2 was absorbed by MCM-41TMGL so the outlet SO2 concentration decreased. For example, at 30 °C, the outlet SO2 concentration decreased to the minimum value of 1.5% in 10 min. As the absorption became slow and reached equilibrium, the outlet SO 2 concentration increased and returned to the initial SO2 concentration. When 12.2% of water was added in SO2/N2 gas mixtures, it is interesting to see that the absorption curve with 12.2% of H2O slightly differed from that without water at each temperature. It can be found that H2O had little influence on the SO2 absorption by MCM-41-TMGL-40%, especially at high temperature. The absorption capacity at 50 and 70 °C has slightly decreased by 6.8% and 2.7%. Ren and his co-workers suggested that the presence of water in the flue gas has little

Table 3. SO2 Absorption Capacity and Rate of Neat TMGL and MCM-41-TMGL absorbents

AC (mol/mol)

r10

t0.9 (min)

MCM-41-TMGL-10% MCM-41-TMGL-20% MCM-41-TMGL-30% MCM-41-TMGL-40% MCM-41-TMGL-50% MCM-41-TMGL-60% neat TMGL

0.716 0.523 0.585 0.644 0.513 0.584 0.725

0.189 0.099 0.092 0.085 0.033 0.033 0.039

30 50 55 55 125 110 280

was that the pore channel was blocked by ILs, which made SO2 molecular more difficult to diffuse into ILs. Besides, the two parameters of MCM-41-TMGL with excess loadings of 50% and 60% were nearly the same and close to that of neat TMGL, which implied that excess loadings were not appropriate for SO2 absorption performance enhancement. As a result, the TMGL loading prefers to be smaller than the calculated maximum loading. Desorption performance of MCM-41-TMGL was investigated at 90 °C under vacuum, shown in Figure 10. Obviously,

Figure 10. Adsorption/desorption performance of MCM-41-TMGL and neat TMGL.

there was a decline tendency for the desorption efficiency with the increasing TMGL loadings. The absorbed SO2 by MCM41-TMGL-10% was easily released, reaching 97.5%. TMGL in the porous channels of the MCM-41-TMGL adsorbent had different desorption behavior from that on the external surface. When the TMGL loading was 10% ∼ 40%, all the TMGL was in the porous channels with huge surface area, which was beneficial for SO2 release. However, when the TMGL loading was excessive, a certain amount of TMGL was coated on the external surface of the molecular sieve. Losing the advantage of large surface area, this part of TMGL had the SO2 desorption behavior like neat TMGL, slowly and incompletely. As a result, these particles with excessive loadings had a lower desorption efficiency than particles of all the TMGL in the porous channels. Comparisons between neat TMGL and MCM-41-TMGL were also made on the SO2 absorption and desorption performance. On one hand, two absorption kinetics parameters indicated that the absorption rate of supported ionic liquids was much faster than that of neat TMGL. As seen in Table 3, it only took about 30−55 min for MCM-41-TMGL (10−40%) to reach 90% of SO2 uptake capacity compared to 280 min of neat G

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Energy & Fuels

Figure 11. SO2 absorption by MCM-41-TMGL-40% at different temperatures: (■,□)30 °C; (●,○)50 °C; (▲,Δ) 70 °C; (▼,∇) 90 °C with 12.2% of H2O (open marks) and without H2O (filled marks) in flue gas. (a) SO2 concentration vs time. (b) SO2 absorption capacity vs time. (c) SO2 absorption capacity vs temperature.

effect on SO2 absorption capacity and rate by neat TMGL.20 Neat TMGL can absorb SO2 and H2O from the flue gas simultaneously and independently. However, in our cases of gas−solid absorption system, the absorption behavior is closely related to the pore structure characteristics. Stevens49 studied the effect of H2O on CO2 adsorption onto microporous 13X through FTIR, with the results indicating that H2O and CO2 competed for the same adsorption sites on the zeolite surface. H2O can block the adsorption sites of the zeolite and displace adsorbed CO2 species from the surface, even while in a CO2rich environment, they competed for the same adsorption sites on the zeolite surface. In the investigation of Sjostrom and Krutka,50 water significantly decreased the capacity of zeolites to adsorb CO2. Stenger51 also found that the SO2 absorption capacity of mordenite presented a drastic decrease in the presence of water. Therefore, in our investigation, competitive adsorption of SO2 and H2O may occur in the pore channel, and the hydrogen bond interaction between TMGL and water lead to less SO2 absorption active sites. As expected, temperature had a remarkable effect on the absorption process. Absorption capacity declined with increasing temperatures, suggesting an exothermic process. This indicated that low temperature is in favor of absorption and the SO2 absorbed supported ionic liquid is reversible at high temperature, which is consistent with normal gases absorption by neat ionic liquids. 3.7. Kinetics of SO2 Absorption by MCM-41-TMGL. Various kinetic models have been adopted to fit the sorption dynamic curves. A series of exponential models were founded

to well describe the sorption process. Therefore, the absorption curves without H2O at different temperatures were fitted into the double exponential model: AC = A1exp( −k1t ) + A 2 exp( −k 2t ) + AC0

(12)

where k1 and k2 are rate constants, A1 and A2 are parameters, and AC0 are the saturated absorption capacity. The fitted results were obtained in Table 4. k1 and k2 refer to two different Table 4. Fitted Kinetic Parameters of SO2 Absorption by MCM-41-TMGL-40% at Different Temperatures T (°C)

k1 (s)

k2 (s)

AC0

R2

30 50 70 90

0.002701 0.003262 0.004398 0.007342

0.000754 0.001135 0.001742 0.002225

0.699 0.464 0.301 0.201

0.9978 0.9977 0.9974 0.9997

sorption processes: SO2 directly absorption on the surface and SO2 absorption kinetically controlled by diffusion processes. At each temperature, the value of k1 was higher than that of k2, indicating that SO2 absorption was very fast on the surface and SO2 diffusion into the absorbent was the rate-determining step. With the increasing temperature, the rate constant k1 and k2 of the two sorption processes increased, which meant that it was easier and faster to reach absorption equilibrium at high temperatures. Considering the low absorption capacity at high temperatures, MCM-41-TMGL absorbent was more suitable for low temperature (