Energy & Fuels 2008, 22, 237–242
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Potential for Catalyzed Coproduction of Hydrogen During Fluid Coking of Heavy Petroleum Feeds Edward Furimsky IMAF Group, 184 Marlborough AVenue, Ottawa, Ontario, Canada, K1N 8G4 ReceiVed September 14, 2007. ReVised Manuscript ReceiVed October 16, 2007
The coproduction of H2 during coking of petroleum residues was attracting attention, although it has not approached a near commercial stage. The sintering of catalysts, with a limited number of utilization cycles, was a drawback of the method. The features of pilot plants used for testing resembled those of Exxon’s fluid/ flexi-coking process. It was proposed that after some modifications, a high concentration H2 may be produced employing the latter process. Little modifications would be necessary in the case that Fe oxides used as catalysts were replaced by catalysts in which active metals (e.g., Fe, Ni, V, Co, etc.) are deposited on carbon particles whose properties approach those of the fluid/flexi coke. The thermodynamic analysis indicating the viability of such process was presented. This is supported by limited experimental data.
Introduction Steam reforming of natural gas is the most often used method for production of H2 on a large scale such as that required for petroleum refining. In recent years, growing interest in replacing natural gas with refinery residues as the source of H2 has been noted.1 In this case, refinery byproduct such as petroleum coke, distillation residues, visbreaking tar, and refinery sludge have been evaluated as potential feeds. The commercially established method involves gasification of the feed to synthesis gas which after cleaning and water-gas shift (WGS) only contains H2 and CO2. The latter is scrubbed to give a high concentration of H2. The byproduct of the WGS such as a high concentration CO2 can be used industrially, i.e., for enhanced oil recovery (EOR), if the upgrading plant is located in the proximity of the heavy feed producing site. Recent developments around the world indicate steadily increasing prices of natural gas. This improves the viability of other methods for the production of H2. It is, therefore, desirable that the methods which were not economical in the era of low natural gas prices are re-evaluated. Iron-catalyzed production of H2, combined with cracking of heavy petroleum feeds has been receiving attention for decades. The essential requirement of the process is the “red-ox” cycle during which Fe and/or FeO are oxidized by reacting with H2O to give H2. This has to be followed by a reduction of a higher oxidation state of Fe oxides such as Fe3O4 either to FeO or ideally to Fe. The IGT process employing Fe and FeO, developed in the 1950s,2,3 represents the first attempt to test such a process on a pilot-plant scale. Renewed interest in this method in Japan in the 1980s were noted.4–7 The investigated concept included three reactors, i.e., a coker, decoker, and H2 (1) Furimsky, E. ReV. Inst. Fr. Pet. 1999, 54, 597. (2) Gregory, D. P. Hydrogen Energy System, publication L. 21173; American Gas Association; Chicago, IL, 1972. (3) Chao, R. E. Ind. Eng. Chem. Prod. Res. DeV. 1974, 13, 94. (4) Suzuka, T.; Inoue, Y.; Aizawa, S.; Ozaki, H. J. Jpn. Pet. Inst. 1983, 26, 174. (5) Suzuka, T.; Inoue, Y.; Aizawa, S.; Ozaki, H. J. Jpn. Pet. Inst. 1983, 26, 181. (6) Suzuka, T.; Inoue, Y.; Aizawa, S.; Ozaki, H. J. Chem. Soc. Jpn. 1980, 6, 1920. (7) Fukase, S.; Suzuka, T. Appl. Catal. 1993, 100, 1.
generator, all operating in fluidized bed mode. A continuous operation of the process was demonstrated; however, desirable yields of H2 were achieved at temperatures at which catalyst deactivation due to sintering could not be avoided. This and other facts prevented commercialization of the process. However, after closely examining these studies, it became evident that the selection of catalysts for these applications has not yet been optimized. For example, the resistance of catalysts to sintering may be enhanced when Fe is combined with supports such as solid carbons. Thus, the stability of carbon supported catalysts under more severe conditions has been known. Steam gasification of carbonaceous solids and steam reforming of hydrocarbons are essential reactions involved in H2 coproduction during coking of petroleum residues. The recent review published by Corella et al.8 confirmed the interest of many researchers in these reactions. This involved the effects of the inherent mineral matter and that of mineral matter added to the feedstock.9 Catalytic steam gasification of biomass in the presence of Ni and CeO2 catalysts was also reported.10 The studies in which gasification products passed either a WGS reactor or a fixed bed of CO2 scavenging solids placed downstream of the gasifier are of particular interest.11,12 Attempts were also made to develop models to simulate the steam gasification of biomass.13,14 In terms of experimental setup and conditions, this information is relevant to the steam gasification of petroleum coke, although it was obtained during the steam gasification of biomass.8–14 The significant catalytic effect of inherent mineral matter on the yield of H2 during steam gasification was also confirmed by comparing the reactivity of (8) Corella, J. M.; Toledo, J. M.; Molina, G. Ind. Eng. Chem. Res. 2006, 45, 6137. (9) Wei, L. G.; Xu, S. P.; Zhang, L.; Liu, C. H.; Zhu, H.; Liu, S. Q. Int. J. Hydrogen Energy 2007, 32, 24. (10) Tomishingo, K.; Kimura, T.; Nishikawa, J.; Miyazawa, T.; Kummari, K. Catal. Commun. 2007, 8, 1074. (11) Aznar, M. P.; Cabarello, M. A.; Corella, J.; Molina, G.; Toledo, J. M. Energy Fuels 2006, 20, 1305. (12) Orio, A.; Corella, J.; Narvaez, I. Ind. Eng. Chem. Res. 1997, 36, 3800. (13) Sanz, A.; Corella, J. Fuel Proc. Technol. 2006, 87, 247. (14) Corella, J.; Sanz, A. Fuel Proc. Technol. 2005, 86, 1021.
10.1021/ef700550j CCC: $40.75 2008 American Chemical Society Published on Web 11/15/2007
238 Energy & Fuels, Vol. 22, No. 1, 2008
a coal char before and after demineralization,15 as well as after the addition of fly ash and Fe to char.16,17 Petroleum coke was the feed in the study conducted by Trommer and Steinfeld18 on the kinetics and mechanism of steam gasification. A rather unique approach to the steam gasification of petroleum coke to H2 using concentrated solar power has been noted.19 The fluid/flexi-coking process developed by Exxon has some similar features as the concepts tested for the Fe-catalyzed H2 coproduction with coking of heavy petroleum feeds. In the fluid coking mode, this process consists of an assembly of a coker and burner. A heavy feed is injected into the fluidized bed of hot coke in the former. In this case, steam is used as the fluidizing medium. The gaseous and liquid products formed in the coker are withdrawn at the top while coke particles are transported to the burner where their temperature is increased by partial combustion before they are returned back to the coker as a carrier of heat required for coking. In the flexi-coking mode of the Exxon process, a burner is replaced by a heater and gasifier, whereas the coker has the same features as in the fluidcoking mode. As part of the modified flexi-coking process, the heater may operate under conditions approaching those in the burner of the fluid-coking process. The primary role of the burner, i.e., decoker, is partial decoking of coke particles to achieve a temperature, which is desirable for steam gasification in the H2 generator. More specifically, modifications would include the change in operating parameters of the burner (decoker) with the aim to increase the temperature of solid particles (to ∼800 °C) transferred from the coker. Subsequently, hot particles would be introduced into the H2 generator where they are contacted with steam. As part of the modified process, these particles would contain catalytically active transition metals and/or metal oxides in their lower oxidation state. Catalytically active solids may include either naturally occurring Fe-containing ores or low cost solid byproduct disposed from industrial operations such as steel and aluminum production. The least modifications of the reactor features would be required in the case of the catalyst comprising either coke or activated carbon particles impregnated with catalytically active metals. Description of the Modified Process Simplified schematics of the process for the Fe-catalyzed coproduction of H2 during coking of heavy petroleum feeds resembles that shown in Figure 1. The process consists of three reactors, i.e., the coker, decoker, and H2 generator. Although each of the reactors operates in fluidized bed mode, the hydrodynamics of the reactors are different. The energy required for the process is supplied by a partial combustion of a coke layer deposited on the surface of catalyst particles during the last trip to the coker. In this case, a controlled supply of air to the decoker will remove the coke layer and increase the temperature of particles to above 800 °C. The hot particles are then transferred to the H2 generator where they are contacted with steam to produce H2. Here, the temperature of particles decreases to less than 700 °C. These particles are then transferred to the coker to supply the heat necessary for coking reactions. The off-gas from decoker containing H2, CO, CO2, SO2, and some hydrocarbons enters the boiler to produce a high temper(15) Kitsuka, T.; Bayarsaikhan, B.; Sonnoyama, N.; Hosokai, S.; Li, C. Z.; Norinaga, K. Energy Fuels 2007, 21, 387. (16) Palmer, A.; Furimsky, E. Fuel Sci. Technol. Int. 1986, 4, 433. (17) Yu, J. L.; Tian, F. J.; Chow, M. C.; MacKenzie, L. J.; Li, C. Z. Fuel 2006, 85, 127. (18) Trommer, D.; Steinfeld, A. Energy Fuels 2006, 20, 1250. (19) Trommer, D.; Noembrini, F.; Fasciana, A.; Rodriguez, D.; Morales, A.; Romero, M.; Steinfeld, A. Int. J. Hydrogen Energy 2005, 30, 605.
Furimsky
Figure 1. Modified flexi-coking process for catalyzed coprodcution of H2.
Figure 2. Schematic diagram of feed-coke interaction in the coker. (step 1) Atomization of feed droplets. (step 2) Impingment of coke particles and formation of initial granule of wet coke. (step 3) Breakup of ganules by hydrodynamic forces.8 Reprinted with permission from ref 21. Copyright 2002. Canadian Society for Chemical Engineering.
ature steam required for the H2 generator.20 If necessary, heating value of the off-gas can be increased by blending with a small amount of gas produced during coking. A more detailed account of the H2 generating reactions is given in following sections. Coker. The events occurring in the coker operating in fluidized bed mode using steam as the fluidizing medium are described by the schematic diagram (Figure 2) developed by Gray.21,22 In the first step, the droplet formed after atomization of the feed enters the fluid bed of coke. Subsequently, the impingement of the droplet on coke particles leads to the formation of a granule of wet coke. It has been established that in the fluid bed coking mode, the droplets of liquid feed coat different size particles with the same thickness of liquid.23 In the last step, the disintegration of the granule is caused by hydrodynamic forces in the bed and by vapor evolution due to cracking reactions, particularly those occurring in the liquid layer (20) Furimsky, E. Fuel Proc. Technol. 2000, 67, 205. (21) Gray, M. R. Can. J. Chem. Eng. 2002, 80, 393. (22) Gray, M. R. Upgrading Petroleum Residues and HeaVy Oils; Marcel Dekker: New York, 1994. (23) Dunlop, D. D.; Griffin, L. I.; Moser, F. F. Chem. Eng. Prog. 1958, 54, 39.
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Energy & Fuels, Vol. 22, No. 1, 2008 239
Table 1. Tentative Reactions Coker 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
C + H2O ) CO + H2 C + FeO ) CO + Fe Fe + H2O ) FeO + H2 3FeO + H2O ) Fe3O4 + H2 2Fe3O4 + H2O ) 3Fe2O3 + H2 Fe3O4 + C ) 3FeO + CO Fe + H2S ) FeS + H2 FeO + H2S ) FeS + H2O Fe3O4 + 2CH4 ) 3Fe + 2CO + 2H2O + 2H2 Fe3O4 + CH4 ) 3FeO + CO + 2H2 Fe3O4 + 2C2H4 ) 3Fe + 4CO + 4H2 2Fe3O4 + C2H4 ) 6FeO + 2CO + 2H2 Fe3O 4 + 4CH3 ) 3Fe + 4CO + 6H2
14. 15. 16. 17. 18. 19. 20.
C + 0.5O2 ) CO C + O2 ) CO2 Fe + 0.5O2 ) FeO 3FeO + 0.5O2 ) Fe3O4 2Fe3O4 + 0.5O2 ) 3Fe2O3 FeS + 1.5O2 ) FeO + SO2 FeO + SO2 + 3CO ) FeS + 3CO2
3. 4.
Fe + H2O ) FeO + H2 3FeO + H2O ) Fe3O4 + H2
Decoker
H2 Generator
Table 2. Effect of Temperatures on the Free Energy of Formation (kJ/mol) for the Coker temperature, K reaction
600
800
1000
1200
1. 2. 3. 3.a 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
12.3 11.4 -4.0 -5.4 -6.6 12.6 11.8 -13.9 -9.9 19 4 14 4 -59
5.3 6.7 -3.7 -5.1 -2.8 16.9 2.6 -13.9 -10.4 -7 -12 -19 -17 -107
1.7 -0.8 -2.3 -4.6 -1.5 21.3 -7.4 -13.8 -10.5
-8.5 -6.7 -2.8 -4.0 0.7 54.0 -16.8 -12.8 -10.7
a
-55 -46 -143
Normalized to one Fe.
coating the particles. Particles covered with the last layer of coke are withdrawn from the coker before entering the burner. Compared with coke particles, for the particles of Fe-containing inorganic solids, hydrodynamic forces in bed may change because of different specific gravity. Tentative reactions involving steam occurring in the coker are shown in Table 1. At typical temperatures employed, i.e., 550-600 °C, the steam reforming of hydrocarbons will be unimportant. As the free energy of formation values in Table 2 show, reactions 1 and 2 will be very slow as well, i.e., the smaller the ∆G value, the greater the driving force for the corresponding reaction. These reactions depict the conversion of coke which deposited on the surface of catalyst particles during coking. Thus, a sufficient driving force for these reactions requires temperatures higher than 700 °C. However, at the temperature of coking, there may be sufficient driving force for reaction 6 in which the FeO is regenerated from Fe3O4. The former is catalytically active for H2O decomposition. It was postulated that by using Fe-containing solids as fluid bed particles, the H2 generating reactions involving steam may occur in the coker simultaneously with coking reactions.3–7
Indeed, at the coking temperatures, the free energy values indicate sufficient driving forces, i.e., favorable conditions for such reactions in the presence of Fe and FeO (reactions 3 and 4), whereas there is little probability for generation of H2 via Fe3O4 (reaction 5). Reaction 4 generates H2; however, it converts a catalytically active species into an inactive one. This suggests that Fe3O4 would have to be regenerated at least to FeO in order to sustain catalytic generation of H2. It is important to note that the back reactions (reduction of the Fe oxides with H2) are not favorable. This suggests that the consumption of H2 in these reactions may be negligible. Moreover, there might be some kinetic limitations due to rapid coke deposition on the surface of the Fe-containing particle. It was indicated above that there is little probability for the removal of this coke via reactions with H2O and Fe oxides under coking conditions. In the presence of H2S, the unwanted reactions 7 and 8 cannot be ruled out. The partial pressure of H2S depends on the sulfur content in heavy feed. These reactions represent the catalyst deactivation because FeS is catalytically inactive. It can be estimated that, at 600 K, the equilibrium constants for reactions 7 and 8 are more than 3 orders of magnitude greater than those for reactions 3 and 4. The difference in favor of the former reactions is further increasing with a temperature increase. This suggests that even at very low concentrations of H2S, i.e., ∼0.01 vol %, the catalyst poisoning may not be completely avoided. However, in reacting with Fe and FeO (reactions 7 and 8), H2S must compete with H2O (reactions 3 and 4). It should be noted that the latter is in significant excess compared with H2S. It is, therefore, believed that poisoning by H2S may not be a serious drawback for the Fe catalyzed decomposition of H2O to H2 under typical conditions employed in the coker. Indeed, Kasaoka et al.24 reported that steam gasification of carbon catalyzed by Fe and Ni was not inhibited by H2S. Apparently, reactions 9-13 are the most important reactions occurring in coker. They are used to illustrate the involvement of hydrocarbons and radicals during regeneration of the inactive Fe3O4 to active Fe and FeO simultaneously with the production of H2. It is believed that, under coking conditions, a wide range of hydrocarbons and alkyl radicals are produced near and/or on the catalyst surface and as such are readily available for these reactions. Reactions 9-13 occurring in the coker may be the most important reactions for sustained generation of H2 in the H2 generator after a partial removal of the coke layer in the decoker. Decoker. The Fe-containing particles deposited with coke are treated in the decoker under limited supply of O2. In this case, it is desirable that the temperature reaches at least 800 °C. This depends on the amount of deposited coke and the amount of air supplied to the decoker. The objective is the limited removal of coke from the outer surface of particles to improve the access of H2O to catalytically active species. Ideally, the combustion of volatiles produced by the decomposition of the coke layer would be the main source of heat leaving a portion of the coke layer on the surface of particles. This would completely eliminate the formation of tar in the H2 generator. At the same time, none or only a partial oxidation of Fe should occur. However, there are significant driving forces for reactions 14-19 (Table 1) as it is indicated by the free energies of formation shown in Table 3. Nevertheless, the unwanted reactions (e.g., 16 and 17) could be avoided by maximizing the rate of reaction 6 with the aid of the deposited coke providing that the supply of air is carefully monitored. The rather large (24) Kasaoka, S.; Sakata, Y.; Shimada, M. Int. Chem. Eng. 1986, 26, 705.
240 Energy & Fuels, Vol. 22, No. 1, 2008
Furimsky
Table 3. Effect of Temperature (K) on the Free Energy of Formation (kJ/mol) in the Decoker
a
reaction
800
1000
1200
14. 15. 16. 17. 17.a 18. 18.a 19. 20.
-43.7 -23.0 -52.4 -46.2 -15.4 -31.8 - 5.4 -98.9 -53.6
-47.9 -21.5 -49.5 -40.5 -13.5 -24.9 - 4.2 -94.9 -45.2
-52.1 -19.8 -46.4 -34.8 -11.6 -18.3 - 3.1 -90.0 -36.6
Normalized to one Fe.
driving force for reaction 20 should be noted. This reaction depletes active FeO by converting it to inactive FeS. However, this may be kinetically limited because it requires the presence of five reactants among which SO2 and CO are continuously removed from the system under conditions of fluid bed oxidation. Apparently, there are similarities between the conditions in decoker and those in the burner of the fluid coking process. In the latter case, the amount of air supplied to the burner allows only about 1% conversion of the carbon inventory in the burner.20 This involves the outer thin layer (about 5 µm) of coke deposited during the last trip to the coker. Practical experience, i.e., only about 1 wt % of oxygen in fluid coke, confirms that a controlled oxidation of the most outer layer of coke in the burner can be readily established. However, hydrodynamics in the burner would be different compared with the decoker because of the significant difference in specific gravity between the coke particles and Fe-containing catalyst particles. Therefore, a modification of the burner would be necessary unless the difference in specific gravity is minimized, e.g., by using a catalyst consisting of carbon particles impregnated with active metals such as Fe, Ni, Co, etc. H2 Generator. Partially oxidized (decoked) particles exiting the decoker at temperatures exceeding 800 °C may be used for the production of H2 according to reactions 3 and 4. Presumably, this could be achieved by contacting catalyst particles with H2O in an H2 generator operating in fluidized mode. By selecting a nonreactive carbon support, reaction 1 may only involve a portion of the layer of coke deposited during the last trip to coker which was unconsumed in the decoker. On the basis of the driving forces in Table 2, the probability for the occurrence of reaction 2 is much lower than that of reactions 3 and 4. Particles exiting the H2 generator at about 700 °C are still too hot to be transferred to the coker as the carrier of heat required for coking reactions. The temperature of the exiting particles may be adjusted by the operating conditions in the H2 generator to achieve desirable conversions in the coker.It was indicated that, at about 800 °C, sintering of the Fe-containing particles could not be avoided. This applies to both the decoker and the H2 generator. This would affect the sustained production of H2 in the latter. It is believed that the catalyst sintering can be minimized by fine-tuning of the operating parameters, temperature, and residence time in particular. Also, sintering resistence may be improved by catalyst preparation procedures. It is expected that the Fe-containing inorganic solids are more prone to sintering than the Fe catalysts supported on carbon solids. For example, automotive catalysts which are supported on
carbon are known to exhibit a high performance during longterm exposure to temperatures exceeding 1000 °C. Selection of Catalysts The objective is to find a catalyst which would require little modification of the fluid/flexi-coking process. In an ideal case, the specific gravity and particle size distribution of the catalyst would be in the range of that of the fluid coke particles. The information on such catalysts in the literature is rather scarce compared with inorganic solids containing Fe.3–7 Although the primary focus is on the Fe-containing catalysts, other transition metals (e.g., Ni, Co, etc.) may exhibit desirable activity as well. Such metals and oxides may be identified on the basis of the free energy of formation for their reaction with H2O to give H225 similarly as reactions 3 and 4 (Tables 1 and 2) for Fecontaining solids. It is, however, unlikely that such metal oxides can be as readily available and/or be cost competitive with Fecontaining solids. The essential requirement of the process is the red-ox cycle which ensures regeneration of the catalytically inactive Fe3O4 to FeO and ideally to Fe. This cycle involves reactions 3 and 4 generating H2 as well as reaction 6 and reactions 9-13, in which inactive Fe3O4 is reduced to active Fe and FeO in the coker. Reactions 2 and 6 are solid–solid reactions involving carbon and an Fe oxide. For Fe catalysts supported on a carbon solid, these reactions are favored by a significant excess of carbon compared with Fe. For example, for catalysts containing about 5 wt % Fe, the molar surface concentration of carbon could be almost 2 orders of magnitude greater than that of Fe. It is unlikely that, in the coker, any of the gas–solid reactions in Table 1 involving steam could occur to a great extent because of the rapid coke deposition on the surface of catalyst particles. On the other hand, hydrocarbons and radicals which are efficient reducing agents of Fe3O4 (reactions 9-13) are formed in situ by decomposition of the liquid layer deposited on the catalyst surface. It is believed that a better control over reaction 6 may be achieved in the decoker, i.e., under oxygen starving combustion conditions. The active Fe-containing species are then reoxidized in the H2 generator via reactions 3 and 4. Because of the rapid coke deposition and H2S present in the coker, reactions 3 and 4 are expected to proceed at a much greater rate in the H2 generator than in the coker. In fact, their contribution to the H2 generation in coker may be negligible compared with reactions 9-13. Therefore, to ensure a desirable performance of the process, the catalyst would have to withstand repeated exposure to temperatures exceeding 800 °C in the decoker and H2 generator. Inorganic Solids. Fukase and Suzuka7 used the naturally occurring Fe-containing ore such as laterite (∼83 wt % of Fe2O3) as a potential catalyst for coproduction of H2 during coking of heavy feeds. In this case, the H2 yield decreased with the increasing number of cycles and leveled off at about 20% of the initial yield after three cycles. This resulted from the sintering of the active phase. This was indicated by the gradual decline in surface area and the increase in bulk density of the catalyst with an increasing number of redox cycles. Furthermore, the increasing content of unconsumed FeO with repeating red-ox cycles confirmed that it is the active FeO which was sintered and as such became less available for the H2 generating reactions. These observations may suggest that the merit of the process can be improved in the case of a throw-away once(25) Barin, I.; Knacke, O. Thermochemical properties of inorganic substances; Springer-Verlag: Berlin, 1973.
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Energy & Fuels, Vol. 22, No. 1, 2008 241
Table 4. Metal Oxides in Coke and Lignite Ash (wt %) Fe2O3 NiO V2O5 CaO MgO BaO
coke ash
lignite ash
9.1 1.2 4.4 5.0 2.0 0.1
7.0 tr tr 10.7 2.0 0.3
Table 6. Driving Force for Reactions of Fe3C ∆G at temperature, K Fe3C + H2O ) 3Fe + CO + H2 Fe3C + 4H2O ) 3FeO + CO + 4H2 3Fe + C ) Fe3C Fe3C + CO2 ) 3Fe + CO Fe3C + 4O2 ) 3FeO + 5CO
Table 5. Yield of Products (mol) from Steam Gasification feed
temperature, °C
H2
CO
CO2
H2S
coke coke + ash coke coke + ash
830 830 930 930
0.22 0.22 0.38 0.62
0.10 0.06 0.10 0.13
0.07 0.10 0.15 0.22
nd nd 0.003 0.005
through (perhaps twice) material. Low cost, throw-away Fecontaining solids are byproducts of several industrial processes, e.g., steel and aluminum production. In this case, such material would have to be available in sufficient quantities in a close proximity to the heavy feeds upgrading plant to make the process modifications viable unless some improvements in the catalyst stability can be achieved. In this regard, the study of Urasaki et al.26 showed that the activity and stability of FeO could be enhanced by doping with a small amount (0.23 mol %) of either Pd or Zr. In this case, both metals enhanced the rate of the redox cycle with Pd increasing the rate of reduction to metallic Fe, whereas Zr increased the rate of the H2 generating reactions. However, these findings would have to be confirmed on a larger scale during repeated utilization-regeneration cycles. Carbon-Based Catalysts. The significant difference in specific gravity between Fe-containing solids and hot coke used as a carrier of heat to the coker indicates the different reactor hydrodynamics in the system shown in Figure 1. For the former, this would require a redesign of the reactors and solid transport lines used in the fluid/flexi-coking process. This can be overcome using a catalyst comprising active metals supported on a carbon solid. In an ideal case, the catalyst may consist of a fluid coke impregnated with the salts of catalytically active metals. The content of catalytically active metal oxides in the ash from fluid coke obtained from Syncrude operation is shown in Table 4. In a real situation, most of the metals in coke are in a reduced form rather than in the form of oxides. Thus, the results reported in Table 4 reflect the method used for the ash analysis. It is noted that the content of ash in the coke approached about 6 wt %. Alkali and alkali earth metal oxides are also included as it has been generally established that they are active for carbon gasification, although the mechanism of their involvement differs from that of transition metals.27–29 This coke was used to test its activity during steam gasification with and without the addition of lignite ash. For this purpose, fine particles of the ash were mechanically mixed with coke particles. This, of course, gives a much less efficient contact between the ash and coke than an impregnation using water soluble salts. In spite of this, ash significantly enhanced H2 formation at 930 °C although its effect at 830 °C was not evident. These results are shown in Table 5. At 830 °C, the yield of H2 approached that during identical treatment when H2O was replaced with N2. A simple calculation showed that more than 30 m3 H2/tonne (26) Urasaki, K.; Tanimoto, N.; Hayashi, T.; Sakine, Y.; Kikuchi, E.; Matsukata, M. Appl. Catal. 2005, 288, 143. (27) Furimsky, E. Can. J. Chem. Eng. 1986, 64, 293. (28) Otto, K.; Bartosiewicz, L.; Shelef, M. Fuel 1979, 58, 565. (29) Radovic, L. R., Jr.; Jenkins, R. G. Fuel 1983, 62, 849.
900
1000
1100
1.7 -9.5 1.1 21 -3.9
-2.1 -10.0 0.4 -2.8 -10.8
-5.3 -12.6 0.2 -5.4 -15.0
fluid coke can be obtained by thermal treatment at 830 °C.30 Part of this H2 is released from coke and subsequently combusted in the decoker. The enhanced production of H2 at 930 °C in the presence of ash shown in Table 5 can, almost certainly, be attributed to the improved contact of the catalytically active species with carbon. This resulted from the so-called “wetting” phenomenon caused by the softening of mineral matter occurring above 900 °C.31 It has been generally established that the impregnation of carbon with inorganic salts ensured the efficient distribution of metals which are nearly an atomic size.32 The work of Asami et al.33 showed that the maximum of gasification rate was decreased from ∼1000 to ∼750 °C after the addition of about 5 wt % Fe to char by impregnation. At this temperature, a carbon supported catalyst with well-distributed Fe species should exhibit better activity than any inorganic Fe-containing solid. After impregnation, the predominant portion of Fe is in pores rather than on the external surface of particles. Under these conditions, sintering of catalytically active metals at high temperatures is significantly diminished. Moreover, the more intimate contact of Fe with carbon may favor the formation of the Fe-C bonds which may have a stabilizing effect on catalytically active sites. Under the O2 starving conditions in the decoker, the external coke layer deposited on the surface of particles during the last trip to the coker may play an important role in minimizing consumption of the support’s carbon. In this regard, the origin of the carbon support may be an important factor. For example, if an activated carbon is used as support, its consumption in gasification reactions in the decoker would be low because of its rather low reactivity compared with that of the coke layer formed during the last trip to the coker. Thus, general practice suggests that activated carbons are made during prolonged contact of carbonaceous solids with oxidation medium at or above 850 °C. The oxidation medium may comprise CO2, steam, or a low concentration O2-containing gas such as diluted air (e.g., less than 5% O2). Asami et al.33 determined Fe species in a char loaded with Fe and tempered at 850 °C. Almost 80% of the Fe was in the form of Fe3C with R-Fe, γ-Fe, and Fe-C containing moieties accounting for the rest. After 10 min of gasification in CO2 at this temperature, most of the Fe3C was converted to R-Fe and γ-Fe. A small amount of the inactive Fe3O4 appeared after almost 30 min. This suggests that catalytically active species are rather stable at temperatures employed in the H2 generator. Table 6 shows that, in the presence of H2O, Fe3C can be readily converted either to Fe or FeO and H2. The driving forces for these reactions are in the same range as those for the corresponding reactions involving CO2. It is unlikely that Fe3C can reappear during the subsequent trip to the coker and decoker (30) Furimsky, E. Fuel Proc. Technol. 1998, 56, 263. (31) Huttinger, K.; Adler, T.; Hermann, T. In Carbon and Coal Gasification; Figueiredo, J. L., Moulijn, J.A., Eds.; NATO ASI Series E, Applied Science; NATO ASI: Dordrecht, The Netherlands, 1986; No. 105. (32) Furimsky, E. Catalysis,in press. (33) Asami, K.; Sears, P.; Furimsky, E.; Ohtsuka, Y. Fuel Proc. Technol. 1996, 47, 139.
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suggesting that it would play a certain role in steam decomposition only during the early stages. During coking, metals in heavy feeds (e.g., V, Ni, Fe, etc.) deposit on the external surface of catalyst particles. After removing the coke layer (from the last trip to the coker) in the decoker, these metals remain on the catalyst. Fortunately, they are all catalytically active for steam gasification.34 At least after the first few trips to the coker, the incremental catalytic effect by these metals may be small. Thus considering the analysis in Table 4 and an ash content of about 6 wt %, Fe would be in significant excess for a catalyst containing 5 wt % Fe. The contribution of the metals from feed to steam decomposition should gradually increase with the increasing number of trips of catalyst particles to the coker. In an ideal case, a modified fluid/flexi-coking process may comprise a catalystssupported on a carbon solid the properties of which, i.e., specific gravity and particle size distribution, approach those of the fluid/flexi coke. In the decoker, oxidation of the coke layer deposited during the last trip to the coker would increase the temperature of catalyst particles to that required for steam decomposition in the H2 generator. It is essential that the reactivity of the last coke layer in the decoker is significantly greater than that of the carbon support. As a result of thermal effects, the most reactive portion of this layer is pyrolyzed. This is followed by combustion of the released volatile components. (34) McKee, D. W. Chem. Phys. Carbon 1981, 16, 1.
Furimsky
This eliminates the unwanted formation of tar in the H2 generator. The desirable reactivity difference between the last layer of coke and the carbon support may be established using a specially prepared activated carbon. The coke layer consumed in the decoker and H2 generator could be replenished in the coker by fine-tuning the operation of the three reactors. This would ensure that the consumption of the carbon support would be minimized. Under such conditions, coke stockpiling practiced on the site of some heavy feeds upgrading plants could be avoided. Future Perspectives The potential for coproduction of H2 during fluid bed coking of heavy petroleum feeds has not yet been fully explored. At this stage, a sufficient experimental database for excluding this method from further consideration has not been established. Therefore, this method deserves another look with a focus on catalyst development. In this regard, catalysts in which active metals are supported on carbon supports may possess a higher catalytic activity and adequate resistence to sintering. For this purpose, carbon supports varying widely in properties have been made commercially. Among these supports, activated carbon possesses surface properties desirable for impregnation with catalytically active metals. EF700550J
