Ind. Eng. Chem. Res. 2005, 44, 9973-9978
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Regenerable MgO-Based SOx Removal Sorbents Promoted with Cerium and Iron Oxide in RFCC Lee Soo Jae,† Hee Kwon Jun,† Suk Yong Jung,† Tae Jin Lee,‡ Chung Kul Ryu,§ and Jae Chang Kim*,† Department of Chemical Engineering, Kyungpook National University, Daegu, 702-701, Korea, Department of Chemical Engineering, Yeungnam University, Kyongsan, 712-749, Korea, and Korea Electric Power Research Institute, Daejon 305-380, Korea
The aims of this work were verifying the roles of various promoters in absorption and regeneration. In particular, we focus our attention on the oxidation properties of Ce, Co, and Fe and regeneration property of Fe of the MgO-based sorbents. The MgO-based SO2 sorbents promoted with Ce, Co, Fe, and Cu were developed for use in RFCC and FCC units. Their ability for SO2 absorption as well as regeneration was investigated in a fixed-bed reactor under RFCC and FCC reaction conditions. The promoters played an important role in transforming SO2 into SO3, which could be easily absorbed into MgO. Their oxidation ability increased in the following order Fe < Co < Ce. In addition, their reuse through the regeneration of sorbents with hydrogen in a FCC unit depended on these promoters. In particular, the MgO sorbent which was promoted simultaneously with Ce and Fe showed excellent characteristics in SO2 removal and regeneration in that this material satisfies the requirements of a large amount of SO2 absorption (ads. sulfur g/g sorbent) along with fast and complete regeneration under mild condition (1 atm, 530 °C). SOx (SO2 and SO3) is generated from coke burning in the catalyst regenerator
1. Introduction Sulfur dioxide (SO2) is emitted from many industrial processes that use sulfur-containing compounds such as fossil fuels. The discharge of waste gas streams with high levels of toxic compounds into the atmosphere is environmentally undesirable, which is frequently encountered in conventional operations. In residue fluid catalytic cracking (RFCC) and fluid catalytic cracking (FCC) units about 45-55% of the sulfur in the hydrocarbon feedstock is converted to hydrogen sulfide in the reactor units and about 35-45% remains as a liquid product. The rest of the sulfur (5-10%) is deposited on the FCC catalyst.1 It is well known that sulfur can promote the deactivation of cracking catalysts, while the catalytic cracking of hydrocarbons takes place in the reaction zone. Until now the deactivated catalyst must be regenerated in the presence of oxygen in the regeneration bed. The SOx emissions, 90% of SO2 and 10% of SO3, were usually produced from the regeneration units and should be removed before their emission enters the atmosphere in order to prevent environmental contamination. To reduce emissions one or more metal oxides capable of SOx added cracking catalyst (1-10 wt %). In the RFCC unit the function of this sorbent is absorption of SOx in the catalyst regeneration zone and transformation of SOx back to H2S in the cracking reaction zone, which can then be treated directly in a Claus plant. The mechanisms generally involve the following reactions. * To whom correspondence should be addressed. Tel.: 8253-950-5622. Fax: 82-53-950-6615. E-mail: kjchang@ bh.knu.ac.kr. † Kyungpook National University. ‡ Yeungnam University. § Korea Electric Power Research Institute.
S(in coke) + O2 f SO2
(1)
SO2 + 1/2O2 f SO3
(2)
SOx is removed by the metal oxide sorbent
MeO + SO2 + 1/2O2 f MeSO4
(3)
MeO + SO3 f MeSO4
(4)
and regenerated in the catalyst reactor
MeSO4 + 4H2 f MeO + H2S + 3H2O
(5)
MeSO4 + 4H2 f MeS + 4H2O
(6)
MeS is hydrolyzed to form H2S and MeO in the stripper
MeS + H2O f MeO + H2S
(7)
This technique is inexpensive compared with the stackgas scrubbing or feed hydrodesulfurization techniques and is, from economical and technical viewpoints, a very practical and attractive technique.2-3 It is known that basic oxides such as magnesium oxide in the presence of an oxidant promoter like CeO2 have very high potential for SO2 removal following reactions 1 and 2. Different material promoters such as V, Ce, Co, and Pt have been claimed as SO2 oxidation promoters,4-7 but CeO2 is the most commonly used today.8-12 Numerous materials have been proposed for removing SOx. Al2O3based sorbents did not affect the thermal stability of aluminum sulfate in the regeneration conditions.6 CaObased sorbents have been the leading materials for several decades. They are not suitable in the RFCC and
10.1021/ie050607u CCC: $30.25 © 2005 American Chemical Society Published on Web 11/17/2005
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FCC units because the material requires a temperature in the range of 565-790 °C for removal of SOx, and their reuse is also impossible because the CaSO4 formed during SO2 absorption is very stable. MgO-based sorbents have been developed that can absorb SO2 even at a lower temperature similar to the FCC conditions. Many promoters were added to MgO in order to promote the SO2 transformation to SO3, which would be easily absorbed to MgO. Even in these cases, however, the MgSO4 formed during SO2 absorption is very stable and has not been completely regenerated under cracking conditions. Also, MgAl2O4 spinels can be used for SOx removal. It is limited in the amount of SOx that could be removed.13 Recently, hydrotalcite-type or magnesiumrich hydrotalcite materials promoted with various metal oxides such as Cu, Mn, Li, Fe, Pt, Ti, and Zn have been used.4,8-10,14-16 In this study MgO-based SO2 sorbents promoted with various functional metals such as Ce and transition metals were developed for use in RFCC and FCC units. Their ability for SO2 absorption in an oxidizing atmosphere and regeneration properties in a reducing atmosphere was investigated using SO2 breakthrough curves, XRD results, and temperature-programmed reduction (TPR). 2. Experiment 2.1. Preparation of Sorbent by the Coprecipitation Method. 2.1.1. Preparation of Ce-MgO, Co-MgO, Fe-MgO, and Cu-MgO. The metal nitrate solution, such as Ce(NO3)3‚6H2O, Co(NO3)3‚6H2O, Fe(NO3)3‚ 9H2O, and Cu(NO3)2‚2.5H2O, was added to 1.5 M Mg nitrate solution. After mixing for 10 min 1.5 M NaOH was added to the metals and Mg solution at a flow rate of 2 mL/min until pH ) 10. The resultant gel was aged 18 h at 80 °C, washed until pH ) 7, and then dried and calcined in air at 750 °C for 4 h. 2.1.2. Preparation of Ce-Fe-MgO. The two metal nitrate solutions of Ce (15 wt %) and Fe (5 wt %) were added to 1.5 M Mg nitrate solution. After mixing for 10 min 1.5 M NaOH was added to the metals and Mg solution at a flow rate of 2 mL/min until pH ) 10. The resultant gel was aged 18 h at 80 °C, washed until pH ) 7, and then dried and calcined in air at 750 °C for 4 h. 2.2. Apparatus and Procedure. Multiple cycles of sulfidation at 700 °C and regeneration at 530 °C were performed in a fixed-bed quartz reactor with a diameter of 1 cm in an electric furnace. Sorbent (1 or 0.25 g) was packed into the reactor, and the space velocity (SV) was maintained at 5000 h-1 to minimize severe pressure drops and channeling phenomena. All of the volumetric flows of gases were calculated at standard temperature and pressure (STP) conditions. The temperature of the inlet and outlet lines of the reactor was maintained above 80 °C to prevent condensation of water vapor in the sulfidation processes. The outlet SO2 and H2S gases from the reactor were automatically analyzed every 8 min by a gas chromatograph (thermal conductivity detector) equipped with an autosampler (Valco). Its detection limitation of SO2 and H2S is about 200 ppm. The column used in the analysis was a 1/8-in. Teflon tube packed with Chromosil 310. When the outlet gas SO2 concentration level reached 5000 ppm, the concentration of SO2 in the inlet stream of mixed gases, inert nitrogen gas without SO2, was introduced to purge the system until it reached the regeneration temperature. Sulfurized sorbents were regenerated by H2 gas until H2S was not detected.
Figure 1. SO2 breakthrough curves of Mg-based sorbents promoted with Fe, Co, Cu, and Ce. Table 1. Experimental Conditions for MgO-based Sorbents T (°C) P (atm) flow rate (mL/min) gas composition
SO2 absorption
regeneration
700 1 50 SO2 5000 ppm O2 5.2 vol % N2 balance
530 1 50 H2 50 or 100 vol % N2 balance
Table 2. Composition and Surface Area for the MgO-based Sorbents Promoted with Fe, Co, Cu, and Ce composition (wt %) Fe-MgO Co-MgO Cu-MgO Ce-MgO Ce-Fe-MgO
Fe: Co: Cu: Ce: Ce:
20, MgO: 80 20, MgO: 80 20, MgO: 80 20, MgO 80 15, Fe: 5, MgO: 80
surface area (m2/g) 46 48 49 50 40
Table 3. Comparison of Theoretical Sulfur Removal Capacities and Sulfur Removal Capacities of Promoted Metal Oxide
promoted metal oxide
theoretical sulfur removal capacity (abs sulfur g/g absorbent × 100)
total sulfur removal capacity (abs sulfur g/g absorbent × 100)
CeO2 Co3O4 CuO Fe3O4
18.6 39.9 40.0 41.5
8.4 3.7 2.1 2.8
2.3. Characterization of Sorbent. X-ray diffraction (XRD) was performed to identify crystalline phases in the mixed oxides. A Philips XPERT instrument, using Cu KR radiation, was used. The regeneration property was determined by the TPR method in a quartz reactor and a thermal conductivity detector. Sorbent (0.2 g) was heated to 400 °C at a rate of 1 °C/min (400-700 °C) and 50 mL/min of H2 50 vol % and N2 balance was fed into the reactor. 3. Results and Discussion 3.1. SO2 Absorption Performance and the Role of Promoters. Figure 1 shows the SO2 breakthrough curves of various Mg-based sorbents at 700 °C. MgO alone showed a very short breakthrough time, while Mgbased sorbents promoted with Fe, Co, Cu, or Ce required 280, 960, 1000, and 1380 min for complete absorption under the same experimental conditions, respectively.
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Figure 2. (a) XRD patterns of Mg-based sorbents before sulfidation. (b) XRD patterns of Mg-based sorbents after sulfidation.
The amount of sulfur absorbed per gram of sorbent (abs sulfur g/g absorbent × 100) was calculated from these SO2 breakthroughs, and the results were 27, 47, 48, and 54 (abs sulfur g/g absorbent × 100) for Mg-based sorbents promoted with Fe, Co, Cu, and Ce, respectively. The Fe additive contributed less to the SO2 absorption than other promoters such as Co, Cu, and Ce. To study the sulfur removal capacity of the promoted metals the metal oxide of Fe, Co, Cu, and Ce (Fe3O4, Co3O4, CuO,
CeO2) alone absorbed SO2 and then the amount of g of sulfur/g of absorbent and the theoretical SO2 uptakes were calculated. The results are shown in Table 3. The theoretical sulfur removal capacity of promoters was calculated by assuming that these promoters participated in SO2. Sorbent and 1 mol of promoter reacted with 1 mol of SO2. The sulfur removal capacity was very small compared with the amounts of the theoretical sulfur removal capacity. Figure 2 shows the XRD results
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of these MgO-based sorbents before and after SO2 absorption. As shown in Figure 2a the peak from the MgO-based sorbents promoted with Ce, Cu, and Fe showed a separate single metal oxide peak for each promoter in addition to the MgO peak, while the MgObased sorbent promoted with Co shows only the MgO phase without the separate Co peak. This result suggests that Co is intercalated within the MgO lattice. In the case of XRD patterns of various MgO-based sorbents after SO2 absorption, as shown in Figure 2b, peaks from MgSO4 and the metal oxide added as promoter are observed. This indicates that not all promoters added to MgO directly participate in SO2 absorption. When considering the results of Table 3 and Figure 2 it was clear that these promoters did not directly participate in SO2 absorption and that the main active species for SOx removal was the MgO phase because it is well known that in order for sulfate absorption to occur SO2 must be oxidized to SO3, which is then absorbed as sulfate. It is believed that the promoters played an important role in promoting the catalytic oxidation of SO2 to SO3 (reactions 1 and 2), which can be easily absorbed to MgO, even though the role of promoter has not been exactly verified. Figure 3 shows the amount of SO2 absorbed for the MgO sorbents with various loadings of promoters. As shown in Figure 3a the Ce-MgO sorbent with 0.5 wt % loading absorbed a very small amount of SO2. When more than 1 wt % of Ce was loaded to MgO the sulfur removal capacity increased to 49-52 (abs sulfur g/g sorbent × 100), indicating that addition of 1 wt % of Ce was enough to transform MgO to MgSO4. It was also found that more than 5 wt % of Co should be added to MgO. In the case of MgO-based sorbents promoted with Fe, the sulfur removal capacity gradually increased with the loading amount of Fe. From these results it was confirmed that the oxidizing ability increased in the following order: Fe < Co < Ce. 3.2. Regeneration Performance. The regeneration performance of spent MgO-based sorbents was investigated in a fixed bed at 530 °C as shown in Figure 4. Pure hydrogen was used to regenerate the spent MgObased sorbents. Metal sulfates formed during SO2 absorption were transformed into the metal oxides, and H2S was formed under highly reducing regeneration conditions. The MgO-based sorbents promoted with Co, Ce, and Fe were almost completely regenerated, while MgO promoted with Cu sorbent did not release SO2 as well as H2S during regeneration. The Mg-based sorbents promoted with Co or Ce needed a very long regeneration time (2-3 h), while the sorbent promoted with Fe needed only 1 h. This result indicates that the regeneration rate of Mg-based sorbent promoted with Fe was much faster than those promoted with Co or Ce with our regeneration temperature. It is known that Fe promoter plays an important role in the reduction of sulfate.8,17 Figure 5 shows the XRD patterns of these MgO-based sorbents after regeneration using pure hydrogen at 530 °C. It was found that MgSO4 of the CuMgO sorbent was not transformed to MgO, while other MgO-based sorbents promoted with Co, Ce, and Fe were completely regenerated and the MgSO4 peak was not found. Figure 6 shows the results of the temperatureprogrammed reduction of the spent sorbents using a 50 vol % of H2 stream while the temperature was gradually increased 1 °C/min from 530 to 700 °C. The metal
Figure 3. Sulfur removal capacities of Mg-based sorbents with various loadings of promoters: (a) Ce, (b) Co, and (c) Fe.
Figure 4. Regeneration performance for Mg-based sorbents promoted with Fe, Ce, Co, and Cu at 530 °C.
sulfate of the Fe-MgO sorbent was transformed into the metal oxide at a temperature below 550 °C, while those of other Mg-based sorbents promoted with Co or Ce were transformed at temperatures above 600 °C. In particular, the regeneration temperature of the Mgbased sorbents promoted with Cu was higher than 650 °C. These results indicate that promoters such as Co, Ce, Cu, and Fe added to MgO play an important role in the regeneration performance of spent MgO-based sor-
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Figure 5. XRD patterns of Mg-based sorbents after regeneration.
Figure 6. TPR (temperature-programmed reduction) analysis of various Mg-based sorbents using 50 vol % of H2 carrier gas.
Figure 7. Regeneration performance of Fe(20)-MgO sorbent with different concentrations of H2 at 550 °C.
bents as well as in the oxidation of SO2 to SO3 during SO2 absorption. It should be noted that verification of the roles of these additives during regeneration needs further study. The effect of the H2 concentration level on the regeneration of Fe-MgO was tested. Figure 7 shows the H2S emission of the sorbents at different H2 concentrations at 550 °C. As expected, the regeneration
Figure 8. Sulfur removal capacity of three-component Mg-based sorbents: Co(15)-Fe(5)-MgO, Cu(15)-Fe(5)-MgO, and Ce(15)Fe(5)-MgO.
rate gradually increased with H2 concentration and MgSO4 was almost completely transformed to MgO even at a H2 concentration lower than 10%. In the Mg-based sorbent promoted with Co, Ce, and Cu, no H2S was observed at concentrations lower than 100% hydrogen, further indicating that the Fe additive can transform the stable MgSO4 into MgO very effectively under our reducing conditions. 3.3. SO2 Absorption and Regeneration Performance of Three-Component Sorbent. Co, Ce, and Cu play an important catalytic role in transforming SO2 to SO3, and Fe additive improved the regeneration ability of MgO-based sorbents even at low H2 concentrations and temperatures. In the current study Ce-FeMgO, Co-Fe-MgO, and Cu-Fe-MgO were prepared by coprecipitation. Their SO2 absorption and regeneration performances were investigated. As shown in Figure 8 the sulfur removal capacity of the Ce-Fe-MgO sorbent was 44 (abs sulfur g/g sorbent × 100), while those of Cu-Fe-MgO and Co-Fe-MgO showed values below 5 (abs sulfur g/g sorbent × 100). The high sulfur removal capacity of Ce-Fe-MgO (abs sulfur g/g sorbent
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& Sequestration Research Center, one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of the Korean government. Literature Cited
Figure 9. Regeneration performance of the Ce(20)-MgO and Ce(15)-Fe(5)-MgO sorbents with 50 vol % of H2 at 530 °C.
× 100 ) 44) was maintained without deactivation even after three sulfidation and regeneration cycles. Fe-MgO and Co-MgO showed better capacities than Co-FeMgO and Cu-Fe-MgO. The reasons for these results are not yet clear. Figure 9 shows the regeneration properties of the spent Ce-Fe-MgO and Ce-MgO sorbents. By adding a small amount of Fe (5 wt %) to the Ce-MgO sorbent a dramatic increase in regeneration ability was observed and most metal sulfates were transformed into their initial phases as metal oxides. It seems that the CeFe-MgO sorbent is an excellent material for SO2 removal in the regeneration bed of the RFCC and FCC systems in that it satisfies the requirements of SO2 absorption and it exhibits excellent regeneration performance. 4. Conclusions MgO-based SO2 absorption sorbents promoted with Ce, Co, Fe, and Cu for use in RFCC and FCC units were investigated. These promoters played an important role in transforming SO2 into SO3, which could be easily absorbed into MgO. Their oxidation ability increased in the following order: Fe < Co < Ce. In addition, their reuse through regeneration in RFCC and FCC depended on these promoters. It has been known that the MgObased sorbents have not been regenerated, but when Fe is added to the MgO-based sorbents this problem is solved. Regeneration of the sorbent promoted with Fe was faster than that promoted with other metal (Co, Ce, Cu, etc.) at regeneration conditions. In particular, the MgO sorbent, which was promoted simultaneously with Ce and Fe, satisfied the requirements of SO2 absorption and exhibited an excellent regeneration performance. Acknowledgment This research was supported by the Kyungpook National University Research Team Fund, 2003 and by a grant (DA2-202) from the Carbon Dioxide Reduction
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Received for review May 23, 2005 Revised manuscript received August 14, 2005 Accepted September 29, 2005 IE050607U
