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Ind. Eng. Chem. Res. 2006, 45, 2646-2650
KINETICS, CATALYSIS, AND REACTION ENGINEERING FCC SOx Additives Deactivation Rodolfo Eugeˆ nio Roncolatto,†,‡ Mauri J. B. Cardoso,† Yiu Lau Lam,† and M. Schmal*,‡ CENPES/PETROBRAS, Ilha do Funda˜ o, Quadra 7, Rio de Janeiro, 21.949-900 Brazil and Federal UniVersity of Rio de Janeiro, PEQ/NUCAT/COPPE/UFRJ, Cidade UniVersita´ ria, Ilha do Funda˜ o, Caixa Postal 68502, CEP 21945-970, Rio de Janeiro, Brazil
The relative importance of possible factors leading to the deactivation of a commercial SOx emission reduction additive used in the fluid cracking process was evaluated individually. This additive was based on mixed oxides of Mg and Al and contained small amounts of Ce and V. The laboratory deactivation simulation was an extended hydrothermal treatment of the additive by steam with or without the presence of sources of possible poisons such as Si, V, and S. It was observed that a reduction of the area and sintering and migration of V from the catalyst to the additive did not affect the performance of the additive. However, migration of Si from a fresh catalyst or sulfate from an impregnated catalyst drastically reduced the SOx adsorption capacity of the additives. As the migration of Si was important mainly with fresh catalyst, which is present in the inventory of the unit in very small quantity compared to the large amount of equilibrium catalyst, the main factor leading to deactivation of the SOx additives in the industrial operation can be attributed to formation of stable sulfates. 1. Introduction FCC is the main process for production of gasoline and LPG in an oil refinery. In general, the catalyst, in the form of microspheres, contains Y zeolite in a matrix of SiO2 and Al2O3. The FCC unit is a converter constituted by a riser reactor and a regenerator. Gas oil, from the distillation units, is injected in the riser, where it is mixed with the FCC catalyst and cracked. Simultaneously, the catalyst is deactivated by coke and metals from the gas oil. After stripping with steam, the coked catalyst is directed to the regenerator. There, catalyst coke is burned and sulfur and nitrogen present, originating from the sulfur and nitrogen compounds in feed, are partially transformed to SOx and NOx. Regenerated catalyst is transferred again to the reactor to initiate new reaction-stripping-regeneration, passing this cycle hundreds of times. The regenerated catalyst is then termed an equilibrium catalyst, having the designation E-cat. SOx emission in the atmosphere brings a series of bad consequences to the surroundings and human beings. As the FCC unit, in an oil refinery, is the biggest individual SOx source,1 specific legislation exists applicable to this process. It is worthwhile to mention that the relative contribution of the refineries, in comparison with the total amount of SOx emitted to atmosphere by all kind of sources, is relatively small, in the range of 6-10%, but that in areas already saturated or in industrial complexes it can become very significant.2 Reduction of SOx emission in FCC units can be obtained by using lower S feed, SOx reduction additives, or a gas scrubber. The use of additives to the FCC catalyst is the most practical and economic way to reduce SOx emission. The generally accepted mechanism of the way SOx additives act consists of * To whom correspondence should be addressed. Tel.: 021-5902241. Fax: 5521 2562-8300. E-mail:
[email protected]. † CENPES/PETROBRAS. ‡ Federal University of Rio de Janeiro.
three steps: (A) oxidation of SO2 to SO3 in the regenerator, (B) SO3 uptake by the additive as sulfates in the regenerator, and (C) sulfates reduction to liberate S as H2S in the reactor. This way, additives convert SOx to H2S, which leaves the reactor together with the light cracked products and finally is transformed into solid sulfur in the sulfur recovering unit as a nontoxic and valuable product. In Scheme 1 the mechanism of SOx additives is illustrated. It is suggested that the sulfate reduction (step C) is the rate-determining step of the reaction.3,4 Most of the commercial additives consist of compounds based on MgO, Al2O3, and rare-earth elements, in particular Ce.1-4 MgO and Al2O3 have the function of taking up SO3, Al2O3 has the function of dispersing and equilibrating the alkalinity of MgO, and Ce has the function of promoting oxidation and reduction reactions of S compounds. Possible causes thought to be responsible for SOx additives deactivation include (a) reduction of the uptake area and sintering, which can reduce the oxidation activity,5,6 (b) Si migration from the catalyst to the additive, reducing the uptake sites,4-8 (c) V migration from the catalyst to the additive, eliminating adsorption surface sites, mainly for additives not containing V,5 and (d) formation of stable sulfate on the additive, eliminating adsorption.4,9-11 Most of these causes were demonstrated in conditions or tests that tried to simulate the regenerator environments. Hence, a couple of factors occurred together and their relative effects could not be isolated and compared directly. Independent of the cause, additives deactivation occurs progressively with circulation time in the unit and the repetition of the cycles of sulfur adsorption and desorption. Industrially, catalyst remains in the regenerator 15 min and in the reactor around 2 min. Hence, during 1 day about 85 complete cycles occur. To evaluate the relative importance of the causes of deactivation, a typical SOx additive was submitted to a drastic condition so that a possible accelerated deactivation could be
10.1021/ie0511985 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/09/2006
Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 2647 Scheme 1. Action Mechanism of SOx Additives
obtained in just one step. Treated additives were then characterized and tested such that possible effects of the accelerated deactivation into the physicochemical properties and performance were evaluated. 2. Experimental Section A commercial SOx additive constituted mainly of mixed oxides of Mg and Al containing Ce and V was tested. Separate deactivation causes were simulated according to the following procedures: (1) Sinteringsadditive was pretreated alone at 788 °C for 5 h with 100% steam pressure such that an accelerated hydrothermal deactivation could be obtained in only one step. (2) Si migrationsadditive was pretreated as in the above procedure in the presence of a typical virgin FCC catalyst in the proportion of 1 part of additive to 9 parts of catalyst. In the mixture the additive had a particle size smaller than 45 µm (325 mesh) and the catalyst a particle size greater than 53 µm (270 mesh). (3) V migrationsa similar procedure to that described for the effect of Si migration but the additive was mixed with a typical equilibrium catalyst (E-cat) containing 4442 ppm of V instead of the fresh catalyst. (4) S migrationsa similar procedure to that described for the effect of Si migration but the additive was mixed with a fresh catalyst containing 3.2 wt % of SO42- impregnated (measured by combustion method with a LECO 244 equipment). This catalyst is a special one containing zeolite, kaolin, and silica binder but without alumina to minimize the possible amount of irreversible adsorbed SO2. After treatments the additives were separated by sieving, when necessary, and characterized by X-ray diffraction (XRD) using a diffractometer from Phillips, model PW-1730, with a Cu anode and Cu KR radiation, textural measurements with Micromeritics Gemini 2140 equipment using N2 at temperature of the liquid N2 (-196 °C), and surface analysis by X-ray photoelectronic
spectroscopy (XPS) to determine surface composition in the last atomic layers (∼50 Å). XPS spectroscopy12 is a technique particularly useful for characterization of the additive once it possesses the capacity for directly identifying and quantifying elements present, such as Si, Al, Mg, S, V, Ce, O and C, and determining the chemical state of each of these species present in the surface of the additive. Additives were analyzed as received without any further pretreatment, preserving their original characteristics, fixed in the double face band on the sample holder of the spectrometer. XPS analysis was performed in a Escalab Mk II spectrometer. Data acquisition was done exciting samples with photons with an energy of 1486.6 eV of the transition KR of an Al anode operating at 10 kV and 10 mA. The hemispheric analyzer pass energy was maintained at 50 eV. A general spectrum and high-resolution spectrums in the interested region of each studied element were obtained. The elements Si, Al, Mg, S, V, Ce, O, and C were analyzed through the Si2p, Al2p, Mg2p, S2s, V2p, Ce3d, O1s, and C1s lines. From the high-resolution spectrums13 normalized areas of each of the photoelectron peaks were determined as well as their line width and binding energy. Binding energies were corrected through the Al2p line, whose binding energy was fixed in 74.5 eV. Quantitative analysis was done after subtraction of the baseline using the Shirley method, and normalization of the areas of the studied lines was through correction of the free medium path of the photoelectrons (KE1/2) of the transmission function of the spectrometer (KE-1/2) and through the cross section of the electrons calculated by Scofield. The atomic surface compositions were determined and atomic ratios between the elements calculated. Evaluation of the SOx adsorption capacity was determined in a fixed bed reactor using 0.2 g of a mixture containing 97% of the FCC equilibrium catalyst and 3% of the additive, which was previously submitted to different treatments. Samples were heated from room temperature to 550 °C at a heating rate of 10 °C/min with a flow of 50 mL/min of He. Then, FCC reactor conditions were simulated by flowing 50 mL/min of a stream with 10% H2 diluted in He with the sample at 530 °C for 30 min. The temperature was then increased in about 20 min to 720 °C with flow of He of 50 mL/min. At 720 °C, He was substituted by the flow of 250 mL/min of a stream containing around 655 vppm of SO2 and 1.3 v% of O2 in He, simulating the regenerator condition. Products were quantified in a combustion gas analyzer from Testo, model 360, that functions with electrochemical cells to dose O2, CO, SO2, and NO. 3. Results and Discussion 3.1. Characterization. Effect of Sintering and Steam Treatment. Semiquantitative X-ray fluorescence (XRF) analysis indicated that the commercial SOx additive is constituted of 65 wt % of Al2O3, 24 wt % of MgO, 8 wt % of CeO2, and 3 wt % of V2O5, that is, it has an atomic ratio of Mg to Al of around 0.5. X-ray diffraction (XRD) analysis results of the fresh additive and after the procedure to evaluate the effect of sintering are shown in Figures 1 and 2. As can be observed, the more prominent crystalline products are CeO2 [1] (2θ ) 28°, 33°, 47°, and 56°) and the spinel MgAl2O4 [2] (2θ ) 37°, 45°, and 65°). There are also characteristic peaks of hydrotalcite (HTC) Mg6Al2CO3(OH)16 [3] (2θ ) 11° and 23°). After the severe hydrothermal treatment there was a small reduction of the HTC peaks and an increase in the intensity of the peaks for CeO2 and the spinel, already present. The half width of the CeO2 peak localized in 2θ equal to 28.6° had a
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Figure 1. Diffractogram of the fresh SOx additive containing CeO2 [1], spinel MgAl2O4 [2], and hydrotalcite [3] phases.
Figure 2. Diffractogram of SOx additive treated alone showing CeO2 [1], spinel MgAl2O4 [2], and hydrotalcite [3] phases.
slight narrowing, from 0.549 to 0.417 Å, indicating a small increase of the crystallite size. In Table 1 are shown the specific surface area and the surface composition measured by XPS of the fresh additive and those after being pretreated at 788 °C for 5 h with 100% steam under various deactivation conditions. As can be noted, the additive treatment at high temperature and steam pressure causes a
profound reduction in the specific surface area, although the X-ray diffractograms show only small differences. Surface Analysis of AdditiVes and EVidence of Poison Migration. XPS surface analysis shows that Mg is concentrated on the surface of the additive, once the bulk ratio of Al/Mg is around 2. The promoters of oxidation and reduction, Ce and V, are not detected on the surface of the fresh additive, probably
Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 2649 Table 1. Properties of Fresh Additive and Submitted To Different Pretreatments
Table 2. Performance of Fresh Additive and Those Submitted To Different Pretreatments
treated treated treated with with cat. treated with cat. fresh alone fresh cat. with V impregnated SO42-
additive specific area (m2/g) XPS atomic ratio O/Mg Al/Mg Si/Mg S/Mg V/Mg Ce/Mg
180
67
59
69
37
2.8 0.64 0.10 0 0 0
2.4 0.74 0.20 0 0.027 0.019
2.4 0.45 0.76 0.021 0.013 0
1.9 0.21 0.40 0 0.036 0
2.4 0.32 0.40 0.25 0.021 0
a
due to the employed preparation conditions. On the other hand, some Si, probably due to impurities present in the raw materials of the additive, was also detected. When the additive was pretreated alone there was some increase of Si, V, and Ce on the surface. When the pretreatment happens in the presence of the FCC catalyst it occurs, in some cases, with Si, V, and/or S transference from catalyst to the additive. Silica Migration. It was observed that after pretreatment with the three catalysts, Si transference occurs from the catalyst to the additive once that the surface ratio of Si/Mg increases. Although Si(OH)4 is not a volatile compound at low temperatures and polymerizes rapidly when heated, at high temperature and in the presence of water it can exist in equilibrium in the gaseous phase in the steam.14 According to Blanton7 the silica that migrates is amorphous and does not come from the zeolite present in the catalyst. The reaction that follows illustrates the case where the additive contains MgO, causing fosterite formation
2MgO(s) + Si(OH)4(g) f Mg2SiO4 + 2H2O(g)
(1)
Transference is more pronounced from fresh catalyst, once silica species are more reactive, considering that the catalyst was not submitted to high temperatures previously. Vanadium Migration. There was additional V on the surface of the additive treated together with the equilibrium catalyst (E-cat). At high temperature and water pressure vanadium compounds are transformed into vanadic acid, which tends to combine with alkaline compounds. In the case of MgO in the additive the reaction would be
2H3VO4(g) + 3MgO(s) f Mg3(VO4)2(s) + 3H2O (2) which forms magnesium vanadate with high stability. In this case the transference of V would occur despite V already existing in the additive. Sulfur Migration. From the catalyst containing impregnated SO42- there is also S migration, which tends to form sulfite and sulfate on the surface and interior of the additive containing Mg,11 according to the reaction
SO3(g) + MgO(s) f MgSO4(s)
additive initial conv. (%) saturation time (min)
(3)
In all cases of transferences, Si, V, or S, there is an Al/Mg ratio reduction on the surface of the additives. In summary, the characterizations proved that the pretreatments modified the additive according to what was planned, in both the sintering and the deposition of possible chemical poisons. 3.2. Evaluation of Adsorption Capacity. Results showing initial conversion and saturation time are presented in Table 2. Three samples were tested in duplicate. Very similar results were obtained (not shown).
treated with treated treated with treated with cat.a impregnated fresha alone fresh cat.a cat. with V SO4238 4
49 3
0 -
36 3
0 -
Tested in duplicate.
Figure 3. SO2 concentration in the inlet of the reactor (time lower than zero) and at the outlet of the reactor at different times.
As can be seen, pretreatment of the additive alone or with an equilibrium catalyst (E-cat), containing a high amount of V, did not affect significantly the additive activity. This means that specific surface area reduction and CeO2 crystallite size increase did not deactivate the additive, and the catalytic performance observed was similar to the fresh additive. The literature presents different results related to the hydrothermal deactivation of SOx additives. This is not surprising as different additive compositions and steaming procedures were used. Byrne et al.15 showed a reduction of SOx adsorption of 40% for the additive X-2094 (undeclared composition) after steaming for 24 h at 766 °C with 1 atm of steam in a fluidized bed. Yoo et al. 4 observed that the steamed solid solution spinel MgO‚MgAl2O4 exhibited a higher SOx activity than those of the corresponding calcined counterparts. They suggested that the enhanced SOx activity might be due to finely dispersed MgO exsolved on the surface during the steaming process. However, they observed that when the atomic ratio of Mg/Al is higher than 1, simple calcination produced two phases, the stoichiometric spinel, MgAl2O4, and MgO, and steaming showed a significant deactivation in the De-SOx activity. This way, as in our case, the atomic ratio of Mg/Al is around 0.5; a reduction of the additive activity was not expected due to steaming. Similar to this work, the literature reported that steaming with V migration did not affect significantly the additive performance. Rheaume and Ritter6 observed the same for the additive R, from W. R. Grace and Co. In their study they impregnated Ni and V to a mixture of 90% FCC catalyst and 10% additive R and then deactivated the mixture at 766 °C for 8 h with 100% steam at 15 psig. At a level of 5000 ppm of total metal (Ni:V ) 1:2) there was no effect on the SOx activity. For the total metal level of 10 000 ppm, only 9% of the SOx reduction capability was observed. In Figure 3 are shown the performances of the equilibrium catalyst (E-cat) without or with the pretreated additives that presented activity. It is shown that the SO2 concentration in the step stimulates the regenerator with flow of SO2 and O2. The concentration of SO2 in the entrance of the reactor is represented by times lower than zero. Then the SO2 concentration in the exit of the reactor was shown after different times.
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It can be observed that the E-cat (linear baseline) without additive has no SOx activity once the SO2 concentration remains constant after passing through the reactor. The initial SOx concentration projected was to be 655 ppm, but the analysis varied between 630 and 720 ppm. It was also observed that initially additives adsorbed SO2 and its concentration at the exit of the reactor was reduced but soon got saturated and the concentration of SO2 returned to the base level. This is a typical behavior of this kind of additive. The capturing of S compounds is in the atmosphere of the regenerator, but the releasing is only in the atmosphere of the reactor, where the additive becomes active again. As it can be observed in Table 2, the additive did not adsorb SOx after pretreatment with fresh catalyst. That is, Si, which migrates from the catalyst to the additive, as evidenced by the increase of the atomic ratio of Si/Mg from 0.2 to 0.76, deactivated the Mg sites, mainly responsible for SOx adsorption. The literature has already documented well the effect of silica poisoning; however, in most of the examples other factors were also present. For example, Byrne et al.15 and Sigan et al.8 both observed the deactivation of additives alone under an atmosphere of steam. The activities of the additives were reduced to 4060% of their original value. When mixed with a FCC catalyst, under the corresponding conditions, further reductions of their activities to only 10% or their original values were observed. This was attributed to silica poisoning. These superimposed effects were totally consistent with the individual effects shown in the present work. It was also observed that the additive did not adsorb SOx after pretreatment with catalyst containing a high amount of impregnated S. That is, S that migrates from the catalyst to the additive, as evidenced by the increase of the atomic ratio of S/Mg from 0 to 0.25, deactivated the Mg sites. Most likely, formation of stable sulfates of Mg, not reduced in the reactor atmosphere due to low contact time or temperature, is the main cause of additives deactivation during their use in the FCC units,9 and the sulfate reduction is believed to be the ratedetermining step in the SOx reduction process in FCC.4 4. Conclusions The causes of deactivation of a commercial SOx reduction additive used in the FCC process, based on mixed oxides of Mg and Al and containing Ce and V, were individually
evaluated. These include the effects of surface area reduction and sintering, Si and V migration from the catalyst, and formation of stable sulfates. The loss of performance of the additive was only observed due to Si migration and formation of stable sulfates. However, Si migration from the catalyst occurs when the additive is submitted to the regenerator conditions mainly in the presence of a fresh catalyst, where the Si species present are more reactive. Yet, as the greatest catalyst fraction of the industrial inventory is constituted by equilibrium catalyst (E-cat), deactivation by Si migration, in practice, must be less significant. In this way, it is proposed that stable sulfates formation is the main cause of SOx additives deactivation during their use in the FCC units. Literature Cited (1) Dishman, K. L.; Doolin, P. K.; Tullock, L. D. Ind. Eng. Chem. Res. 1998, 37, 4631. (2) Wen, B.; He, M.; Costello, C. Energy Fuels 2002, 16, 1048. (3) Yoo, J. S.; Bhattacharyya, A. A.; Radlowski, C. A.; Karch, J. A. Ind. Eng. Chem. Res. 1992, 31, 1252. (4) Yoo, J. S.; Bhattacharyya, A. A.; Radlowski, C. A.; Karch, J. A. Proc. 10th Int. Congr. Catal.-Hungary 1993, 1391. (5) Cheng, W. C.; Kim, G.; Peters, W.; Zhao, X.; Rajagopalan, K. Catal. ReV.sSci. Eng. 1998, 40, 39. (6) Rheaume, I.; Ritter, R. E. ACS Symp. Ser. 1988, 375, 146. (7) Blanton, J.; William, A. U.S. Patent 4243556, 1981. (8) Sigan, J. A.; Kelly, P. A.; Lane, W. S.; Letzsch, W. S.; Powell, J. W.; AIChE 1990. (9) Wang, J. A.; Chen, L. F.; Limas-Ballesteros, R.; Montoya, A.; Dominguez, J. M. J. Mol. Catal. A: Chem. 2003, 194, 181. (10) Wang, Y.; Li, C. Appl. Surf. Sci. 2000, 161, 406. (11) Waqif, M.; Saur, O.; Lavalley, J. C.; Wang; Y., Morrow, B. A. Appl. Catal. 1991, 71, 319. (12) Briggs, D.; Seah, M. P. Practical Surface Analysis; John Wiley & Sons: New York, 1996; Vol. 1, 2a. (13) Cardoso, M. J. B. In Caracterizac¸ a˜ o de Catalisadores por XPS; 2 Curso Ibero-Americano sobre Caracterizac¸ a˜ o de Catalisadores e AdsorVentes, Sa˜ o Carlos, SP.; Apostila Cyted/CNPq/UFSCar; Cardoso, D., Jorda˜o, M. H., Machado, F., Eds.; 2001, pp 170-200. (14) Iler, R. K. The Chemistry of Silica, John Wiley & Sons: New York, 1979; p 12. (15) Byrne, J. W.; Speronello, B. K.; Leuenberger, E. L. Oil Gas J. 1984, Oct 15, 101.
ReceiVed for reView October 27, 2005 ReVised manuscript receiVed December 26, 2005 Accepted February 2, 2006 IE0511985
