Selective Adsorption of SO2 from Flue Gas on Triethanolamine

ARTICLE pubs.acs.org/IECR

Selective Adsorption of SO2 from Flue Gas on Triethanolamine-Modified Large Pore SBA-15 Yongting Zhi,† Yaping Zhou,† Wei Su,‡ Yan Sun,‡ and Li Zhou*,‡ †

Department of Chemistry, School of Science, and ‡School of Chemical Engineering and Technology, High Pressure Adsorption Laboratory, Tianjin University, Tianjin 300072, People's Republic of China ABSTRACT: Flue gas desulfurization technology is important for coal-fueled power plants, and adsorptive separation of SO2 is a potential method applicable in practice. Large pore SBA-15 with surface modified by TEA (triethanolamine) showed excellent performance for the selective adsorption of SO2 from flue gas. The SO2 capacity reached 177 mg/g at 80% TEA loading ratio, while the adsorption of CO2 was negligible. The adsorbent also showed good tolerance to moisture existing in flue gas, and the moisture showed a positive effect on the desulfurization. The saturated adsorbent was regenerated when heated to 120 °C, and the SO2 capacity was almost retained in consecutive adsorption/regeneration cycles.

1. INTRODUCTION The global economy is mainly supported by fossil fuels presently. A serious problem of burning fossil fuels is the emission of SO2 into atmosphere, among which 70% was emitted from power plants.1 About 104 tons of SO2 was emitted from a typical power plant each year.2 The SO2 in the air hurts humans’ health and causes acid rain, which has been a serious problem for the ecosystem. Therefore, legislation has been issued in many countries,3 and multiple desulfurization technologies (DSTs) have been proposed to solve the problem. A relatively sophisticated and widely applied DST is the lime/limestone process;4
6 however, the method bears some drawbacks, e.g., high investment cost, occupancy of large land area, plugging of equipment, and large consumption of fresh water. In addition, the gypsum generated in the process becomes a secondary pollutant. Therefore, adsorption,7
17 corona discharge18,19 and bioprocessing20 are considered to be alternative DSTs. The corona discharge method is simple in process/equipment without generating secondary pollution; however, the energy cost is high. Although the biomethod bears multiple advantages, practical utilization is limited by the cultivation of appropriate bacteria/microorganisms. The adsorption method features low investment and energy costs and simple equipment, and does not generate secondary pollutants; therefore, it attracts much research interest. Miscellaneous materials, such as oxides,7
9 activated carbon,10,11 silica gel,12 zeolites,13
15 and others16,17 have been tested in adsorptive removal of SO2. Most did not show satisfactory performance due to the small capacity and poor selectivity for SO2, as well as poor endurance to the moisture contained in flue gas, etc. For example, zeolite H
ZSM-5 (molar ratio of SiO2/Al2O3 = 26) possesses good thermal stability, and the adsorption capacity for SO2 reaches 80 mg/g at 50 °C; however, the capacity drops to 11 mg/g if the flue gas contains water vapor.13 The search continues for the adsorbent most suitable for the selective adsorption of SO2 from flue gas. Mesoporous materials that appeared in the late 1990s have found important applications in the catalysis, adsorption, separation, biomass, energy, and environmental areas.21,22 The orderly structured r 2011 American Chemical Society

channels of the materials provided an ideal space for loading active agent. For example, amines were loaded in the channel spaces of MCM-41, MCM-48, or other silicon materials to enhance the selectivity and capacity for CO2 or H2S.23
25 However, little research was reported in the literature for the selective adsorption of SO2 from flue gas, where the CO2 content was several orders higher than that of SO2 and the existence of moisture made the adsorption of SO2 more complicated. Therefore, the adsorption behavior of SO2 in the flue gas atmosphere on triethanolamine (TEA)-modified SBA-15 was studied. In order to load more amine, the pore space of SBA-15 was purposely expanded. TEA is a weak base with a high boiling point; therefore, the loaded TEA is stable and regeneration of the saturated adsorbent may be easier.

2. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of Adsorbent. The SBA-15 was synthesized from TEOS (tetraethyl orthosilicate), TMB (1,3,5-trimethylbenzene), and Pluronic P123 (MW = 5800, EO20PO70EO20) in acidic conditions. All chemicals used for the synthesis were purchased from Aldrich. P123 was used as the structure-directing agent, TEOS was used as the silica source, and TMB was used as the pore size expander. After 12.0 g of P123 was dissolved in 480 g of 1 M HCl solution, 18 g of TMB and 25 g of TEOS were added to the solution. The mixture was kept mixing for 24 h at 313 K, and then was put in an autoclave and there aged for 48 h at 393 K. After filtration and drying, the reaction product was calcined at 823 K for 6 h to remove template, and the SBA-15 sample was obtained. TEA was loaded on SBA-15 by a soaking method. SBA-15 was put into an acetone solution of TEA. The solution was kept agitating at 60 °C until acetone was totally drawn off. The quantity of TEA loaded on Received: March 8, 2011 Accepted: June 6, 2011 Revised: June 4, 2011 Published: June 06, 2011 8698

dx.doi.org/10.1021/ie2004658 | Ind. Eng. Chem. Res. 2011, 50, 8698–8702

Industrial & Engineering Chemistry Research

ARTICLE

Figure 1. Apparatus for breakthrough experiments. PR, reducing valve; V, valve; F, filter; MFC, mass flow rate controller; M, mixing room; PT, pressure transducer; T-V1, three-way valve; BPC, back-pressure valve; P1, vacuum pump.

adsorbent was indexed by the loading ratio, Rv, defined as the percentage occupation of pore volume by TEA: massðTEAÞ  100% Rv ¼ massðSBA-15Þ Vpore ðSBA-15Þ densityðTEAÞ ð1Þ The loading ratios tested in experiments were 0, 20, 40, 60, 80, and 100%. The characterization of adsorbents with and without TEA loading was based on the adsorption/desorption isotherm of N2 at 77 K collected on a Micromeritics ASAP 2020. The sample was dried in a vacuum dryer for 12 h at 393 K before the sorption measurement. The BET surface area26 was determined based on the sorption data for the relative pressure range 0.05
0.3, and the pore volume was estimated based on the adsorbed amount at relative pressure 0.99. Since the pore size falls in the mesopore range, the BJH method27 was applied for the determination of pore size distribution. To test the endurance of the TEA-modified SBA-15 at heating, TGA (thermogravimetric analysis) was carried out on a thermogravimetric instrument, Model NETZSCH STA 409 PC/PG. 2.2. Breakthrough Experiments. The desulfurization performance of adsorbents was examined in breakthrough experiments. The experiments were carried out at 25 °C and pressure 0.1 MPa using a simulated flue gas (SFG) composed of N2, 11.04% CO2, and 1340 ppm SO2. The SFG flows through a sorption bed of length 100 mm and inner diameter 4 mm at a rate of 312 cm3/ min. The experimental setup is schematically shown in Figure 1. The gas flow rate was controlled by mass flow controllers, Model SY9312 purchased from Beijing Shengye Sci. & Tech. Dev. Co. The SO2 content in the effluent stream was analyzed by an infrared gas analyzer, Model QGS-08C purchased from Beijing BAIF-Maihak Analytical Instrument Co. Ltd. A QMS Series gas analyzer purchased from Stanford Research Inc. was used to monitor the CO2 concentration. A gas was considered broken through the sorption bed when its content reached 2 ppm in the effluent stream. The sorption bed was considered saturated by a gas when the concentration of gas in effluent was the same as in the input stream. The breakthrough capacity of a sorbent was defined based on the breakthrough time: Q brk ¼

Mcvtb Vm

ð2Þ

Figure 2. Adsorption and desorption isotherms of N2 on adsorbent before and after TEA loading at 77 K. 1, Rv = 0; 2, Rv = 80%.

where Qbrk is the breakthrough capacity of adsorbent for SO2, mg/g; M is the molar weight of SO2, g/mol; c is the SO2 concentration in SFG, ppm; v is the flow rate of SFG, cm3/ min; V is the molar volume of gas at 25 °C and 0.1 MPa, dm3/ mol; tb is the breakthrough time of SO2, min; m is the mass of adsorbent filled in the sorption bed, g. 2.3. Regeneration Test. The sorption bed was heated to 120 °C by an outside wraparound heating belt while keeping a nitrogen stream flowing at a rate of 100 cm3/min for 30 min in order to regenerate the bed when it had been saturated by SO2.

3. RESULTS AND DISCUSSION 3.1. Structural Property of Adsorbent. The adsorption and desorption isotherms of N2 on the adsorbent with and without TEA loading at 77 K are shown in Figure 2. The type IV feature of the isotherms is clearly shown, and the isotherms reflect mesoporous character even after TEA loading. The original BET surface area was 525 m2/g, and it dropped to 118 m2/g after TEA loading at Rv = 80%. The original pore volume was 2.47 cm3/g, and it dropped to 0.36 after TEA loading at Rv = 80%. The pore size was centered at 13 nm; however, the central location moved left to 11 nm after TEA loading at Rv = 80% as shown in Figure 3. The TGA diagrams of adsorbent with and without TEA loading are shown in Figure 4, indicating the stability below 200 °C. 3.2. Effect of Loading Ratios of TEA on SO2 Removal. The result of breakthrough experiments using SFG is shown in Figure 5. The maximum of breakthrough capacity appears at Rv = 75%, after which more loading of TEA leads to a smaller capacity for SO2. The highest capacity is about 25 times higher than that of SBA-15 without TEA loading. Too much loading of TEA leads to plugging of pore spaces and decreasing of the SO2 capacity. The adsorption selectivity of adsorbent for SO2 is overwhelming over CO2. CO2 broke through the sorption bed at 5 s; however, the breakthrough time of SO2 is 42 s at Rv = 0 and 3000 s at Rv = 80% as shown in Figure 6. Therefore, the CO2 contained in flue gas does not interfere with the SO2 capture. 3.3. Effect of Temperature on SO2 Removal. To observe the effect of temperature on the desulfurization performance, breakthrough experiments were carried out at 25, 45, 65, and 80 °C at 8699

dx.doi.org/10.1021/ie2004658 |Ind. Eng. Chem. Res. 2011, 50, 8698–8702

Industrial & Engineering Chemistry Research

ARTICLE

Figure 6. Comparison of selectivities for CO2 and SO2. 1, CO2; 2, SO2 at Rv = 0; 3, SO2 at Rv = 80%. Figure 3. Pore size distribution of adsorbent before and after TEA loading. 1, Rv = 0; 2, Rv = 80%.

Figure 7. Effect of temperature on breakthrough capacity. Figure 4. TGA diagram. 1, without TEA; 2, TEA loading with Rv = 80%.

Figure 5. Effect of loading ratio of TEA on breakthrough capacity at ambient temperature and pressure.

Figure 8. Effect of moisture contained in flue gas on breakthrough capacity of adsorbent for SO2.

Rv = 80%. The breakthrough capacity of adsorbent for SO2 at 80 °C was still fairly high (124 mg/g) as shown in Figure 7, though the capacity at 25 °C was 177 mg/g.

3.4. Effect of Water Vapor on SO2 Removal. To examine the effect of water vapor contained in flue gas on SO2 removal, the SFG stream bubbled in water at 25 °C before passing through the 8700

dx.doi.org/10.1021/ie2004658 |Ind. Eng. Chem. Res. 2011, 50, 8698–8702

Industrial & Engineering Chemistry Research

ARTICLE

’ REFERENCES

Figure 9. Breakthrough capacity of adsorbent for SO2 in consecutive adsorption and regeneration cycles.

sorbent bed. As shown in Figure 8, the breakthrough time increased more for wet than for dry flue gas, and the breakthrough capacity for SO2 increased to 207 mg/g; therefore, the existence of water vapor exerts a positive effect on the proposed desulfurization method. The water vapor condensed quickly on touching the surface of the adsorbent, and it might promote the chemical adsorption of SO2. 3.5. Regeneration of Adsorbent. The adsorbent was saturated by SO2 after a breakthrough experiment. The saturated sorption bed was purged with a hot (120 °C) nitrogen stream for 30 min at a rate of 100 cm3/min, and then cooled to ambient temperature and the next breakthrough experiment started. The initial breakthrough time for fresh adsorbent was 2994 s, and the breakthrough time decreased to 2853 s after regeneration. The breakthrough capacity decreased from 177 to 168 mg/g. However, it decreased only for 5%. The SO2 capacity in consecutive cycles is shown in Figure 9. It seems that the adsorbent at Rv = 80% is fairly stable for the desulfurization operation. There are several ways to deal with the removed SO2 in industry. Usually it is converted to a useful chemical or fertilizer, for example, with the desorbed gas washed by ammonia solution. It is also possible to convert it as a raw material in sulfuric acid production if the desorbed gas is washed by a solution of hydrogen peroxide.

4. CONCLUSION The SO2 in flue gas can be selectively adsorbed on TEA-modified SBA-15 with a breakthrough capacity of 177 mg/g at 25 °C. Compared to naked SBA-15, the capacity increased by 25 times. The saturated adsorbent was regenerated when heated to 120 °C by a nitrogen stream, and the SO2 capacity was almost maintained in consecutive regeneration cycles. The moisture of flue gas plays a positive role in the desulfurization method, and the massive amount of CO2 in flue gas does not interfere with the desulfurization process at all. ’ AUTHOR INFORMATION Corresponding Author

*Tel./fax: 86 22 87891466. E-mail: [email protected].

’ ACKNOWLEDGMENT Support of the National Natural Science Foundation of China (No. 20876114) and the Tianjin Natural Science Foundation (No. 10JCZDJC23900) is gratefully acknowledged.

(1) Gr€ubler, A. A review of global and regional sulfur emission scenarios. Mitigation Adapt. Strategies Global Change 1998, 3, 383. (2) Environmental impacts of coal power: air pollution. http:// www.ucsusa.org/clean_energy/coalvswind/c02c.html. (3) Hower, J. C.; Robl, T. L.; Thomas, G. A. Changes in the quality of coal combustion by-products produced by Kentucky power plants, 1978 to 1997: Consequences of Clean Air Act Directives. Fuel 1999, 78, 701. (4) Srivastava, R. K.; Jozewicz, W.; Singer, C. SO2 Scrubbing Technologies: A Review. Environ. Prog. 2001, 20, 219. (5) Srivastava, R.,K.; Jozewicz, W. Flue Gas Desulfurization: The State of the Art. Air Waste Manage. Assoc. 2001, 51, 1676. (6) Henzel, D. S.; Laseke, B. A.; Smith, E. O.; Swenson, D. O. Handbook for Flue Gas Desulfurization Scrubbing with Limestone; Noyes Data Corp.: Park Ridge, NJ, USA, 1982. (7) Brandt, C.; Eldik, R. V. Transition Metal-Catalyzed Oxidation of Sulfur (lV) Oxides. Atmospheric-Relevant Processes and Mechanisms. Chem. Rev. 1995, 95, 119. (8) Xiang, J.; Zhao, Q. S.; Hu, S.; Sun, L. S.; Su, S.; Fu, P.; Zhang, A. C.; Qiu, J. R.; Chen, H. P.; Xu, M. H. Experimental research and characteristics analysis of alumina-supported copper oxide sorbent for flue gas desulfurization. Asia-Pac. J. Chem. Eng. 2007, 2, 182. (9) Grapain, A. R.; Alatorre, J. A.; Cortes, A. G.; Diaz, G. Catalytic properties of a CuO-CeO2 sorbent-catalyst for de-SOx reaction. Catal. Today 2005, 107
108, 168. (10) Lisovski, R. S.; Aharoni, C. Adsorption of sulfur dioxide by active carbon treated by nitric acid: Effect of the treatment on adsorption of SO2 and extractability of the acid formed. Carbon 1997, 35, 1639. (11) Sumathi, S.; Bhatia, S.; Lee, K. T.; Mohamed, A. R. Selection of best impregnated palm shell activated carbon (PSAC) for simultaneous removal of SO2 and NOx. J. Hazard. Mater. 2010, 176, 1093. (12) Kopac, T.; Kocabas, S. Adsorption equilibrium and breakthrough analysis for sulfur dioxide adsorption on silica gel. Chem. Eng. Process. 2002, 41, 223. (13) Tantet, J.; Eic, M.; Desai, R. Breakthrough study of the adsorption and separation of sufur dioxide from wet gas using hydrophobic zeolites. Gas. Sep. Purif. 1995, 9, 213. (14) Gupta, A.; Gaur, V.; Verma, N. Breakthrough analysis for adsorption of sulfur-dioxide over zeolites. Chem. Eng. Process. 2004, 43, 9. (15) Allen, S. J.; Ivanova, E.; Koumanova, B. Adsorption of sulfur dioxide on chemically modified natural clinoptilolite. Acid modification. Chem. Eng. J. 2009, 152, 389. (16) Zhao, B. Y.; Jiang, D. E.; Xie, Y. C. Dispersion of Na2CO3 on γAl2O3 and the threshold effect in flue-gas desulfurization. Fuel 2002, 811, 565. (17) Dahlan, I.; Lee, K. T.; Kamaruddin, A. H.; Mohamed, A. R. Evaluation of various additives on the preparation of rice husk ash (RHA)/CaO-based sorbent for flue gas desulfurization (FGD) at low temperature. J. Hazard. Mater. 2009, 161, 570. (18) Xu, F.; Luo, Z. Y.; Cao, W.; Wang, P.; Wei, B.; Gao, X.; Fang, M. X.; Cen, K. F. Simultaneous oxidation of NO, SO2 and HgO from flue gas by pulsed corona discharge. J. Environ. Sci. 2009, 21, 328. (19) Wang, Z. W.; Zeng, H. C.; Guo, J.; Zhou, J.; Liu, N. Sulfur dioxide (SO2) gas transfer process enhanced by corona discharge. J. Electrost. 2007, 65, 485. (20) Philip, L.; Deshusses, M. Sulfur Dioxide Treatment from Flue Gases Using a Biotrickling Filter-Bioreactor System. Environ. Sci. Technol. 2003, 37, 1978. (21) Lehmann, T.; Wolff, T.; Zahn, V. M.; Veit, P.; Hamel, C.; Morgenstern, A. S. Preparation of Ni-MCM-41 by equilibrium adsorption— Catalytic evaluation for the direct conversion of ethene to propene. Catal. Commun. 2011, 12, 368. (22) Chen, A. B.; Zhang, W. P.; Liu, Y.; Han, X. W.; Bao, X. H. In-situ Incorporation of Dispersed Metallic Ag Nanoparticles into the Channels of Mesoporous Carbon CMK-3. Chem. Lett. 2007, 18, 1017. (23) Belmabkhout, Y.; Guerrero, R. S.; Sayari, A. Adsorption of CO2-Containing Gas Mixtures over Amine-Bearing Pore-Expanded 8701

dx.doi.org/10.1021/ie2004658 |Ind. Eng. Chem. Res. 2011, 50, 8698–8702

Industrial & Engineering Chemistry Research

ARTICLE

MCM-41 Silica: Application for Gas Purification. Ind. Eng. Chem. Res. 2010, 49, 359. (24) Huang, H. Y.; Yang, R. T. Amine-Grafted MCM-48 and Silica Xerogel as Superior Sorbents for Acidic Gas Removal from Natural Gas. Ind. Eng. Chem. Res. 2003, 42, 2427. (25) Ma, X. L.; Wang, X. X.; Song, C. S. “Molecular Basket” Sorbents for Separation of CO2 and H2S from Various Gas Streams. J. Am. Chem. Soc. 2009, 131, 5777. (26) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309. (27) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms. J. Am. Chem. Soc. 1951, 73, 373.

8702

dx.doi.org/10.1021/ie2004658 |Ind. Eng. Chem. Res. 2011, 50, 8698–8702