Iraqi Journal of Chemical and Petroleum Engineering Vol.15 No4 (December 2014) 25-35 ISSN: 1997-4884 Removal of SO2 over Modified Activated Carbon in Fixed Bed Reactor: I, Effect of Metal Oxide Loadings and Acid Treatment Neran K. Ibraheem 1 , Shahrazad R. Raouf 2 , Zainab A. Naser 3* 1, 2, 3 Chemical Engineering Department, University of Technology, Baghdad, Iraq Abstract The removal of SO2 from simulated gas stream (SO2 + air) in a fixed bed reactor using Modified Activated Carbon (MAC) catalysts was investigated. All the experiments were conducted at atmospheric pressure, initial SO2 concentration of 2500 ppm and bed temperature of 90 o C. MAC was prepared by loading a series of nickel and copper oxides 1, 3, 5, 7, and 10 wt% on AC. In some of the experimental runs, the original activated carbon was pretreated with two different concentrations of nitric acid 10 and 45 wt%. The results showed that the SO2 removal efficiency, breakthrough time (τ0.05) and sorption capacity increase with increasing metal oxides loadings up to a value of 7 wt% beyond which the desulfurization performance decreases. The pretreatment of the original AC with HNO3 enhanced the removal efficiency of SO2. The copper supported catalysts showed higher flue gas desulfurization activity as compared to the nickel supported catalysts. Keywords: SO2 removal, Modified Activated Carbon, Acid Treatment, Fixed Bed Reactor, SO2 Adsorption. Introduction Sulfur dioxide (SO2) is one of the gravest chemical threats to the global environment and considered both primary and secondary pollutant [1, 2]. It emits to the atmosphere from power plants, industry, refining of petroleum, smelting of ores and manufacture of sulfuric acid as primary pollutant [3, 4]. Some industrial sources emit H2S, which is oxidized to form H2O and secondary pollutant SO2, through the most important oxidizing reaction for H2S with ozone [2]. The famous method for reducing emissions of SO2 is flue gas desulfurization (FGD). A large number of FGD processes (regenerable and non-regenerable) have been developed and are expected to play an important role in reducing SO2 emission from power plants [5]. The SO2 removal by dry-regenerable methods provides an appropriate alternative to control the emissions of SO2 from oil-fired power plants [6]. Being relatively simple and of low cost, dry adsorption with activated carbon (AC) or other adsorbents for SO2 removal is being increasingly employed by most industries [7]. Pre- treatment of the AC or addition of promoters can significantly affect the dispersion of the active species [8, 9]. Wang and lu [10], examined the effects of acid treatments by HCL, HF and HNO3 on the properties of Ni Iraqi Journal of Chemical and Petroleum Engineering University of Baghdad College of Engineering Removal of SO2 over Modified Activated Carbon in Fixed Bed Reactor: I, Effect of Metal Oxide Loadings and Acid Treatment 26 IJCPE Vol.15 No.4 (December 2014) -Available online at: www.iasj.net supported AC catalyst. It was found that acid treatment significantly changed the surface chemical properties and enhanced the surface area of the AC. It was believed that the HNO3 as an oxidant can also oxidize some complexes producing acidic groups. The increase in surface acidity leads to more homogeneous distribution of nickel salt on AC. Tseng and Wey [11], found that the surface oxygen groups of the carbon support played a major role in determining the extent and strength of interaction with the metal precursor during catalyst preparation. The dispersion of supported metal is one of the main parameters related to its catalytic activity [12]. It is reported that the SO2 adsorption characteristics of active carbons are strongly influenced by the presence of certain transition metal derivatives [13]. Some authors have reported that Ca +2 and Mo +2 ions had the greatest effect on the increase of SO2 conversion [14]. It was established that heat treatment of modified AC (impregnated with aqueous solution of (Ni(NO3)2.6H2O) up to 400 o C leads to additional activation of the samples, which is accompanied by an increase in their adsorption characteristics (surface area). Destruction of the texture and decrease in the adsorption characteristics of the samples were observed at 550 o C [15]. While, Jiaxiu et al. [16], found the characterization results showed that the Ni species on the catalyst (Ni/AC, Ni content of the catalyst was 0.92%) calcined at 400 o C is Ni2O3. After calcination at 550 o C, NiO species is formed on the AC. NiO and Ni coexist on the Ni/AC catalyst calcined at 800 o C and only pure Ni species is observed after calcination at 1000 o C. This suggested that Ni can form different chemical states on the Ni/AC catalyst calcined at different temperatures, which can lead to a different desulfurization performance. Ni/AC catalysts containing only NiO or NiO and Ni coexisting together exhibit good catalytic activity at low reaction temperature 90 o C. Also, it was found that the V2O5 plays little role in SO2 adsorption but a main role in the oxidation of SO2 [17]. Lόpez et al. [18], found that the adsorption profiles of AC, demineralized activated carbon (DAC), treatment of AC with HCl-HF-HNO3 solutions, and Cu supported DAC (Cu/DAC) show a similar behavior for adsorption of SO2 but, for the Cu/DAC sample the SO2 adsorption continues for more than 7h, which could indicate that there is a favorable effect of Cu which catalyzes the adsorption of SO2 on the carbonaceous material, this increases the breakthrough time and the adsorption capacity. The objective of the present study is to investigate the effect of the NiO and CuO loadings and the treatment of the original AC with nitric acid on the removal efficiency of SO2 from simulated gas stream. Experimental Work Catalyst Preparation The Modified Activated Carbon (MAC) was prepared by using industrial grade, granular activated carbon obtained from local markets, as a support. The chemical analysis (measured by atomic absorption technique), and the physical properties of the original AC (measured by automated surface area and porosity analyzer, (Micromeritics Company, USA)) are given in Tables 1 and 2 respectively. Prior to the impregnation process, AC was sieved to a size of 1 mm and dried in an oven at 110 o C for 4 hours, then it was impregnated with different concentrations of an aqueous solution of (Ni(NO3)2.6H2O) to give nickel oxide supported AC with different loadings of 1, 3, 5, 7 and 10 Neran K. Ibraheem, Shahrazad R. Raouf and Zainab A. Naser -Available online at: www.iasj.net IJCPE Vol.15 No.4 (December 2014) 27 wt%. The samples was dried at 110 o C in an oven for 4 hours and then calcined at 400 o C for 4 hours. These catalysts were labeled as 1Ni/AC, 3Ni/AC, 5Ni/AC, 7Ni/AC, and 10Ni/AC respectively. In some of the experimental runs, the original AC was pretreated with two different concentrations of nitric acid (10 and 45 wt%) and labeled as 10NAC and 45NAC respectively. Using the same method, copper oxide supported AC with different loadings of 1, 3, 5, 7 and 10 wt% were prepared by using an aqueous solution of (Cu(NO3)2.6H2O) and labeled as 1Cu/AC, 3Cu/AC, 5Cu/AC, 7Cu/AC, and 10Cu/AC respectively. All the details of the required quantities of chemicals required to prepare the different NiO and CuO loadings supported AC catalysts are presented in table 3 and 4. Table 1: Chemical Analysis of AC Material Wt % Material Wt % C 92.46 Zn +2 0.025 Ca +2 2.75 Mg +2 3.25 Co +2 0.075 Mn +2 0.1163 Cu +2 0.15 Pb +2 0.5125 Fe +2 0.0696 PO4 -3 0.0183 Fe +3 0.325 SO4 -2 0.0183 Others 0.23 Wt% Table 2: Physical Properties of AC Test Results Surface area (m 2 /g) 702 Pore volume (cm 3 /g) 0.59 Bulk density (g/cm 3 ) 0.72 Particle density (g/cm 3 ) 1.91 Table 3: Details of Loadings NiO on AC NiO (wt %) Symbol AC (g) Ni(NO3)2.6H2O (g / 100 cm 3 H2O) 1 1Ni/AC 95.05 4.95 3 3Ni/AC 85.14 14.86 5 5Ni/AC 75.23 24.77 7 7Ni/AC 65.32 34.68 10 10Ni/AC 50.45 49.55 Table 4: Details of Loadings CuO on AC CuO (wt %) Symbol AC (g) Cu(NO3)2.3H2O (g / 100 cm 3 H2O) 1 1Cu/AC 96.19 3.8 3 3Cu/AC 88.6 11.40 5 5Cu/AC 80.99 19.01 7 7Cu/AC 73.39 26.61 10 10Cu/AC 61.99 38.01 Chemicals for SO2 Generation and Detection For the generation and detection of SO2, Sodium sulfite, Na2SO3 was supplied from Merck, Germany. Sulfuric acid, H2SO4, iodine, I2, Iodine ampoule, 0.1 N, potassium iodide, KI, starch and Sodium thiosulfate ampoule, 0.1 N were analytical grade reagents from BDH, England. Experimental Rig A schematic diagram of lab scale experimental rig designed for the adsorption of SO2 from simulated gas stream (SO2+Air) is shown in Figure 1. The rig consists of three sections: SO2 generation section, sorption section and the analysis section. For the generation of SO2, a three neck QVF 3 L-flask connected at its upper part to a glass burette 200 ml capacity which contains sulfuric acid was used. 2500 ppm of SO2 was generated by dropping specified concentration of sulfuric acid solution into the SO2 generation vessel that contained sodium sulfite. The inlet of the generation section is connected to a compressed air source. The test section is a QVF column, 2.54 cm inside diameter and 50 cm long, filled with catalyst to a height of 4 cm. The SO2 removal was carried out at 90 o C, 1 bar and 140 /h gas flow rate. The rig was insulated with glass wool to maintain the test section temperature nearly constant. Calibrated rotameters were used to measure the gas flow rate. The initial and final weights of the catalyst were measured to determine the total amount of SO2 adsorbed. The weight of SO2 Adsorbed was determined by measuring the residual amount by its quantitative reaction with iodine and titrating excess iodine with sodium thiosulfate. The number  Removal of SO2 over Modified Activated Carbon in Fixed Bed Reactor: I, Effect of Metal Oxide Loadings and Acid Treatment 28 IJCPE Vol.15 No.4 (December 2014) -Available online at: www.iasj.net of iodine equivalents is equal to the residual SO2 in the trap of the analysis section. Fig. 1: Schematic Diagram of the Experimental Apparatus. (1) Compressor (2) Needle Valve (3) Regulator (4&5) Wet and dry bulb thermocouple (6) Rotameter1 (7) Burette (8) SO2 generation vessel (9) Rotameter2 (10) U- tube manometer (11) Heater (12) Selector switch (13) AC (14) Variac (15) Temperature controller (16) Pressure drop (17) Absorption trap (18) Bed thermocouple (19) Reactor Data Analysis The total amount of SO2 adsorbed (mg/g) was calculated as the ratio of the difference between the initial weight of the catalyst (fresh bed) and the final weight of the catalyst (spent bed) to the initial weight of the catalyst, as shown in Equation 1. …(1) The removal efficiency of SO2 was calculated as the ratio of SO2 concentration that was removed by catalyst to the initial concentration of SO2 gas fed to the bed, as in Equation 2. ɳ % = *100 …(2) Full details of the experiment results and data analysis are given in [19]. Results and Discussion Effect of Metal Loadings The effect of nickel oxide loadings on SO2 sorption capacity and the dynamic SO2 removal efficiency before breakthrough point at gas rate of 140 /h, bed temperature of 90 height of 4cm are shown in Figures 2 and 3 respectively. The results indicate that the nickel oxide supported AC can significantly improve the desulfurization efficiency of the original AC. Figures 4 and 5 show the relation between the breakthrough time (τ0.05) and the sorption capacity with Ni/AC loadings respectively. It can be noticed from these figures that the breakthrough time and the SO2 sorption capacity increase as the Ni loading increases up to a value of 7 wt% beyond which the desulfurization performance decreases. For 7 wt% loaded AC, the breakthrough time and the sorption capacity increased to 165 minutes and 191 mg/g respectively as compared to 38 minutes and 120 mg/g for the original AC. 7Ni/AC catalyst exhibited the highest desulfurization activity, this behavior may be attributed to the uniform distribution of the active phase within the support and to the type and degree of interaction reached during the impregnation process. The decrease in desulfurization performance beyond 7 wt% Ni may be ascribed to the intimate interaction between nickel nitrate and AC in impregnation step which leads to the occlusion of small amounts of AC in Ni derivatives crystallites upon calcination [20].  Neran K. Ibraheem, Shahrazad R. Raouf and Zainab A. Naser -Available online at: www.iasj.net IJCPE Vol.15 No.4 (December 2014) 29 Fig. 2: Effect of NiO Loadings on SO2 Sorption Breakthrough Fig. 3: Dynamic Change of SO2 Removal Efficiency for Different NiO Loadings Fig. 4: Effect of Ni O Loadings on Breakthrough Time Fig. 5: Sorption Capacity of SO2 for Different NiO Loadings Effect of Acid Treatment The breakthrough curves and the dynamic SO2 removal efficiency of 7Ni/AC and 7Ni/NAC catalysts are presented in Figures 6 and 7 respectively. The results indicate that the acid treatment significantly enhanced the SO2 removal efficiency, the desulfurization activity of the catalysts is in the following sequence from poor to excellent: 7Ni/AC<7Ni/10NAC< 7Ni/45NAC, indicating that the increase in concentration of acid treatment results in a more homogeneous distribution of nickel oxide on AC and increase the surface area of catalyst.Surface oxygen on carbon consists of inorganic and Organic matters. Acid treatment mainly removes the inorganic oxides and increases the organic oxygen- containing materials, this removal leaves sites on the carbon surface which can chemisorb oxygen in air and this would result in more oxygen surface complexes which are more acidic [21]. The results that are presented in precedent figures are in agreement with the result obtained by Leon and Radovic [22], and Tseng et al. [23] who proposed that the interaction between metallic precursor and carbon support depends on the amphoteric character of carbon material. At pH > pHslurry the acidic Removal of SO2 over Modified Activated Carbon in Fixed Bed Reactor: I, Effect of Metal Oxide Loadings and Acid Treatment 30 IJCPE Vol.15 No.4 (December 2014) -Available online at: www.iasj.net groups on the carbon surface will dissociate; the negatively charged surface then attracts and adsorbs cations from the solution; at pH < pH slurry it will attract anions. In other words, if the pH of the impregnation solution is higher than the pH of the aqueous slurry of the support (pHslurry), then the adsorption of cations is favored. In this sense, these surface oxygen groups can be considered as the anchoring centers for the metal precursor and its presence should favor higher dispersions. Fig. 6: Dynamic Change in SO2 effluent concentration for 7Ni/NAC Catalysts Fig. 7: Effect of HNO3 Concentration on SO2 Breakthrough time for 7Ni/NAC Catalysts Figures 8 and 9 indicate that as the acidity of the AC increased the breakthrough time and the sorption capacity increased. The breakthrough time increased from 210 minutes to 285 minutes and the sorption capacity increased from 197 mg/g to 208 mg/g as the concentration of HNO3 increased from 10 to 45% for 7Ni/NAC catalyst. This may be attributed to the increase in surface area as given in Table 5. The results also indicate that the surface area of 7Ni/45NAC is lower than that of 45NAC this is mainly due to metal loadings. The better performance of 7Ni/45NAC as compared to 45NAC may be due to the role of Ni derivatives on catalytic activity. Fig. 8: Effect of HNO3 Concentration on Breakthrough Time for 7Ni/NAC Catalyst Fig. 9: SO2 Sorption Capacity for Different HNO3 Concentration for 7Ni/NAC Catalyst Neran K. Ibraheem, Shahrazad R. Raouf and Zainab A. Naser -Available online at: www.iasj.net IJCPE Vol.15 No.4 (December 2014) 31 Table 5: Surface Area (m 2 /g) of AC Before and After Acid Treatment Sample Surface area AC 702 10NAC 816 45NAC 1010 7Ni/10NAC 767 7Ni/45NAC 895 Acidic oxygenated surface groups (carboxylic, lactone, carbonyle, phenolic) were determined by Fourier transform infrared (FTIR, Bruker Company), the results are shown in Figures 10-13. Lisovskii et al. [24] clarified that the carbon which contains more oxygenated groups and in which the acidity is higher, is characterized by a larger SO2 adsorption. This indicates that there is an interaction between the SO2 and the acidic oxygenated surface groups produced during oxidation and the presence of acid groups improved the extractability of the acid. Such oxygenated structures are also responsible for catalytic activity of carbon in SO2 oxidation. Fig. 10: FTIR Spectra of 7Ni/AC Catalyst in the 600-2200 cm -1 region The chemical state of 7Ni/45NAC was characterized using X-ray Diffraction (XRD, Shimadzu Company) and the crystalline phases were identified by comparison with reference data, International Center for Diffraction Data, (ICDD). The results as shown in Figure 14 indicate that NiO phase shows major peaks but minor Ni, Ni(OH)2 and Ni2O3 peaks are detected. Fig. 11: FTIR Spectra of 7Ni/AC Catalyst in the 2400-4000 cm -1 region Fig. 12: FTIR Spectra of 7Ni/45NAC catalyst in the 600-2200 cm -1 region Fig. 13: FTIR Spectra of 7Ni/45NAC catalyst in the 2400-4000 cm -1 region Removal of SO2 over Modified Activated Carbon in Fixed Bed Reactor: I, Effect of Metal Oxide Loadings and Acid Treatment 32 IJCPE Vol.15 No.4 (December 2014) -Available online at: www.iasj.net Fig. 14: XRD Patterns of 7Ni/45NAC This is attributed to that all the catalysts were calcined at 400 o C, and Ni is rarely formed at this temperature [15]. The coexistence of Ni and NiO species on the activated carbon favors the flue gas desulfurization. Comparison between the Performance of NiO and CuO Supported AC Catalysts The desulfurization performance of Cu/AC catalysts is shown in Figures 15-18. Compared with AC, Cu/AC catalysts exhibit better performance. The SO2 effluent concentration decreased from 60 ppm to zero ppm and the SO2 removal efficiency increased from 97.5% to 100% at time duration of 15 minutes. This may be expected due to modifications of physical and chemical properties of AC. The same behavior was obtained by other investigators [14, 25, 26]. The change in sorption breakthrough and removal efficiency with time for different CuO loadings is shown in Figures 15 and 16. The results indicate that the removal efficiency increases as the metal loading increases. This behavior is to be expected due to that low loading offer few active sites of Cu and thus show lower activity; higher loading above 7wt% resulted in active sites aggregation and consequent reduction of catalytic activity. Similar behavior was also obtained with Ni/AC catalysts. The breakthrough time and sorption capacity of Cu/AC were higher than those of Ni/AC. The breakthrough time (τ0.05) for 7Cu/AC compared to 7Ni/AC increased from 3h to 4h and the sorption capacity increased from 191 mg/g to 200 mg/g as shown in Figures 17 and 18. This may be attributed to that Cu and its derivatives favoring more transformation of the adsorbed SO2 and oxidizing SO2 into other more stable SO3 and H2SO4. The present findings are in agreement with the results obtained by Tseng et al. [25] and Lόpez et al. [18] but differ from Ma et al. [17] who showed that metals play little role in SO2 adsorption but a main role in oxidation of SO2. Fig. 15: Effect of CuO Loadings on SO2 Sorption Breakthrough Neran K. Ibraheem, Shahrazad R. Raouf and Zainab A. Naser -Available online at: www.iasj.net IJCPE Vol.15 No.4 (December 2014) 33 Fig. 16: Dynamic Change in SO2 Removal Efficiency for Different Copper Oxide Loadings Fig. 17: Breakthrough Time of Ni/AC and Cu/AC Catalysts with Different Metal Loadings Fig. 18: Sorption Capacity of Ni/AC and Cu/AC Catalysts with Different Metal Loadings Conclusions 1. For both nickel and copper Oxides supported AC catalysts, 7wt% metal loading gave the optimum desulfurization efficiency, sorption capacity and breakthrough time. 2. The chemical state of the 7Ni/45NAC catalyst as characterized using XRD, shows major peaks for NiO phase but minor Ni, Ni(OH)2 and Ni2O3 peaks were detected. 3. Fourier Transform Infra-Red (FTIR) and surface area tests showed that the oxygen containing functional groups and the surface area of AC increase with increasing the concentration of nitric acid. 4. Compared with nickel supported AC catalysts, the copper supported AC catalysts showed similar behavior in SO2 removal. However the breakthrough time and sorption capacity of Cu/AC catalysts were higher than those for Ni/AC. The breakthrough time and sorption capacity for 7Cu/AC were 4hours and 200mg/g as compared to 3hours and 191mg/g for Ni/AC respectively. Notation Symbol Description Unit C Exit SO2 Concentration ppm Co Initial SO2 Concentration ppm Wf Mass of fresh catalyst g Ws Mass of spent catalyst g τ0.05 Breakthrough time min AC Activated Carbon -- MAC Modified Activated Carbon -- References 1- Lee, K., T., Christopher, K., C., Fernando, W., J., Bhatia, S., and Mahamed, A., "Modeling and Simulation of Flue Gas Desulfurization Using CaO/Coal Fly Ash Sorbent", J. of Chem. Eng. Of Japan, Vol. (38), No.(6), PP. 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