Energy Fuels 2009, 23, 5308–5311 Published on Web 10/20/2009
: DOI:10.1021/ef9006164
Kinetic Model for Catalytic Cracking of Heavy Oil with a Zirconia-Alumina-Iron Oxide Catalyst in a Steam Atmosphere Eri Fumoto,* Akimitsu Matsumura, Shinya Sato, and Toshimasa Takanohashi Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 16-1, Onogawa, Tsukuba 305-8569, Japan Received June 18, 2009. Revised Manuscript Received September 28, 2009
A kinetic model was proposed to represent the catalytic cracking of heavy oil with a zirconia-alumina-iron oxide catalyst in a steam atmosphere. The model includes four lumps: heavy oil (boiling point above 350 °C), gas oil (boiling point of 250-350 °C), gasoline þ kerosene (boiling point less than 250 °C), and gas. In this reaction, heavy oil fractions reacted with lattice oxygen in iron oxide and the active oxygen species, which were incorporated from the steam into the iron oxide lattice. Hence, lighter fractions, such as gasoline, kerosene, and gas oil, and carbon dioxide were produced with almost no coke. Kinetic parameters were determined using a nonlinear least-squares regression of the experimental results obtained under the reaction conditions of 450-500 °C and a time factor, W/FR, of 3.8-28 h. The evaluated activation energy of heavy oil cracking was lower than those reported in the literature on the hydrocracking process. Accordingly, it is supposed that active oxygen species generated from steam and the lattice oxygen in iron oxide promoted the cracking of heavy oil.
to the lighter fuels. Zirconia-supporting iron oxide catalysts showed higher activity than iron oxide catalysts because zirconia promotes the production of these active species from steam.2 However, iron oxide had been changed from hematite to magnetite during the reaction, and the subsequent peeling of zirconia from the iron oxide caused deactivation of the zirconiasupporting iron oxide catalyst.3 To reduce this change, alumina was added to the iron oxide lattice and zirconia was highly dispersed in the iron oxide catalyst to produce a zirconia-alumina-iron oxide catalyst.4-6 This catalyst showed almost the same activity as the former catalyst, and its activity did not change after repeated reactions of residual oil.4 The main objective of this study was to develop a kinetic model for the cracking reaction of residual oil in a steam atmosphere. There are various kinetic models for the cracking of heavy oil. Weekman proposed a three-lump kinetic model for catalytic cracking of gas oil.7 The lumps were gas oil, gasoline, and gas þ coke. Oliveira and Biscaia improved Weekman’s model; their model omitted the reaction from heavy oil to coke.8 In the different approaches of hydrocracking models, gasoline, gas oil, and gas have been important lumps.9 In this paper, we propose four lumps: heavy feed oil, gas oil, gasoline þ kerosene, and gas. We developed a set of differential equations to represent mass balances of reaction products, and we evaluated the kinetic parameters in these equations by obtaining numerical solutions from experimental data.
1. Introduction The conversion of heavy oils, such as atmospheric and vacuum-distilled residual oils in the petroleum refining industry, into valuable lighter fuels is required. Various methods, such as thermal cracking, catalytic cracking, and hydrocracking, are used to produce lighter fuels from heavy oils.1 In these methods, coke formation in reactors and on catalysts leads to serious problems. Coke formation is usually reduced by increasing hydrogen consumption in the hydrocracking process, although hydrogen is expensive. Accordingly, the use of steam as alternative hydrogen resources for the production of lighter fuels from heavy oils could be a promising method. In previous papers, residual oils were decomposed with iron-based catalysts at 450-500 °C in a steam atmosphere.2-6 The yield of heavy oil was decreased with an increase in the temperature, and the largest amounts of lighter fuels were generated without any coke at 500 °C.5 In this reaction, heavy oils reacted with lattice oxygen in iron oxide and active oxygen species generated from steam were incorporated into the iron oxide lattice. Hence, heavy oil reacted with these oxygen species over the iron oxide catalyst to produce lighter fuels. Active hydrogen species were generated simultaneously from steam when the active oxygen species were generated from steam, and therefore, the active hydrogen species were added
2. Experimental Section
*To whom correspondence should be addressed. E-mail: e-fumoto@ aist.go.jp. (1) Rana, M. S.; S amano, V.; Anchyta, J.; Diaz, J. A. I. Fuel 2007, 86, 1216–1231. (2) Fumoto, E.; Tago, T.; Tsuji, T.; Masuda, T. Energy Fuels 2004, 18, 1770–1774. (3) Fumoto, E.; Tago, T.; Masuda, T. Energy Fuels 2006, 20, 1–6. (4) Fumoto, E.; Tago, T.; Masuda, T. Chem. Lett. 2006, 35, 998–999. (5) Fumoto, E.; Matsumura, A.; Sato, S.; Takanohashi, T. Energy Fuels 2009, 23, 1338–1341. (6) Funai, S.; Fumoto, E.; Tago, T.; Masuda, T. Chem. Eng. Sci. 2009, in press. r 2009 American Chemical Society
2.1. Catalysts and Feedstock. A zirconia-alumina-iron oxide catalyst was prepared by the co-precipitation method (7) Weekman, V. W. Ind. Eng. Chem. Process Des. Dev. 1969, 8, 385– 391. (8) Oliveira, L. L.; Biscaia, E. C., Jr. Ind. Eng. Chem. Res. 1989, 28, 264–271. (9) Ancheyta, J.; Sanchez, S.; Rodrı´ guez, M. A. Catal. Today 2005, 109, 76–92.
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Energy Fuels 2009, 23, 5308–5311
: DOI:10.1021/ef9006164
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Table 1. Properties of AR Derived from Middle East Crude density (g/cm3) C (wt %) H (wt %) N (wt %) S (wt %) H/C CCRa (wt %) V (ppm) Ni (ppm) Fe (ppm) a
0.944 85.7 11.6 0.16 2.48 1.63 5.8 10 4 10
Figure 2. Four-lump kinetic model proposed in this work.
Conradson carbon residue.
250 °C), gas oil (boiling point of 250-350 °C), and heavy oil (boiling point above 350 °C). The spent catalyst was calcined at 700 °C in an air stream after treatment at 700 °C in a nitrogen atmosphere using a thermogravimeter (TGA-50, Shimadzu Co. Ltd., Japan), and the weight of coke was measured. A detailed description of the experimental procedure was provided in our previous paper.5
3. Results and Discussion 3.1. Effect of the Time Factor, W/FR, on the Product Yield. Figure 1 shows the typical product yield after the reaction of AR with the catalyst. The composition of the original AR is shown for comparison. Lighter fuels, such as gasoline, kerosene, and gas oil, and carbon dioxide were produced by the oxidative reaction of heavy oil with lattice oxygen in the iron oxide catalyst and the active oxygen species generated from steam.5 The yield of these lighter fuels increased with an increase in the time factor. The higher the time factor, the larger the amount of carbon dioxide produced. There was no coke found at time factors between 9.1 and 27.6 h. These results indicate that the heavy oil reacted effectively with the large amounts of active oxygen species and the lattice oxygen in the iron oxide, producing large amounts of lighter fuels without any coke at higher time factors. Although a small amount of coke was generated when W/FR was 4 h at 450-500 °C because of insufficient amounts of active oxygen species, the coke yield was less than 3 mol % C. Heavy oil fractions were cracked, producing gasoline þ kerosene, gas oil, and gas. The yield of gas oil hardly changed as the time factor increased from 14.3 to 27.6 h. It is believed that the gas oil was subsequently cracked to yield gasoline þ kerosene and gas. The gasoline þ kerosene might have also been cracked to produce gas. 3.2. Kinetic Modeling. Figure 2 shows the proposed kinetic model, including four lumps: heavy oil, gas oil, gasoline þ kerosene, and gas. The mole balance equation can be represented as follows:
Figure 1. Product yield after the reaction of AR with zirconia-alumina-iron oxide catalysts at 475 °C in a steam atmosphere: (a) total yield and (b) gas yield.
using aqueous solutions containing iron(III) chloride, aluminum sulfate, and zirconium oxychloride (concentrations of FeCl3 3 6H2O = 140 mol/m3, Al2(SO4)3 3 14-18H2O = 9.0 mol/ m3, and ZrOCl2 3 8H2O=8.7 mol/m3). The catalyst was treated with steam at 600 °C for 1 h, pelletized without any binders, and crushed and sieved to yield particles 250-560 μm in diameter. The content of zirconia and alumina in the catalyst was 8.2 and 7.0 wt %, respectively. A detailed description of the catalyst preparation procedure has been reported elsewhere.5 We used atmospheric distilled residual oil (AR) derived from Middle East crude. Table 1 shows the properties of the AR. We diluted the AR with toluene to reduce its viscosity and used the resulting solution as feedstock. We confirmed that the activity of the catalyst caused negligible decomposition of the toluene.5 2.2. Catalytic Cracking. Catalytic cracking of AR with steam was carried out in a fixed-bed reactor loaded with 0.4-2.8 g of catalyst at a reaction temperature of 450-500 °C and an atmospheric pressure. The time factor W/FR ranged from 3.8 to 28 h, where FR is the flow rate of AR without toluene and W is the amount of catalyst. A mixture of steam and nitrogen was introduced into the reactor as a carrier gas at a flow rate of 4.4 10-3 m3/h steam and 3.0 10-4 m3/h nitrogen. After the reaction of AR for 2 h, the liquid and gas products were separated in an ice trap and analyzed by gas chromatographic distillation with a wide-bore capillary column (GC-17A, Shimadzu Co. Ltd., Japan) and gas chromatographs with PorapakQ and Unibeads 3S columns (GC-12A and GC-14A, Shimadzu Co. Ltd., Japan), respectively. Liquid products were defined as three fractions: gasoline þ kerosene (boiling point less than
dFi ¼ ri dW
ði ¼ A, B, C, and DÞ
ð1Þ
where Fi is the mole flow rate of the i lump (mol of C h-1), W is the mass of catalyst (kg), ri is the production rate of the i lump per unit mass of catalyst [mol of C (kg of catalyst)-1 h-1], and suffixes A, B, C, and D refer to heavy oil, gas oil, gasoline þ kerosene, and gas lumps, respectively. Bianco et al. reported that the experimental data of the residual oil cracking were well-adjusted to second-order anchez also reported that the cracking of residual kinetics.10 S oil was second-order kinetics, whereas that of light fractions, such as middle distillates, was expressed by first-order (10) Bianco, A. D.; Panariti, N.; Carlo, S. D.; Beltrame, P. L.; Carniti, P. Energy Fuels 1994, 8, 593–597.
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kinetics. Thus, the following differential equations were obtained using eq 1 based on the proposed model in Figure 2: dFA ¼ -ðkAB þ kAC þ kAD ÞCA 2 dW
ð2Þ
dFB ¼ kAB CA 2 - ðkBC þ kBD ÞCB dW
ð3Þ
dFC ¼ kAC CA 2 þ kBC CB þ kCD CC dW
ð4Þ
dFD ¼ kAD CA 2 þ kBD CB þ kCD CC dW
ð5Þ
Figure 3. Second-order test for the cracking of the heavy oil lump.
where Ci represents the mole concentration of the i lump (mol of C m-3). The total concentration in the reactor was assumed to be equal to that at the inlet of the reactor, because the flow rate of the carrier gas was much larger than that of residual oil. Thus, the concentration of the i lump, Ci, can be represented by ð6Þ Ci ¼ C0 fi ði ¼ A, B, C, and DÞ where C0 is the mole concentration of residual oil at the reactor inlet (mol of C m-3) and fi is the mole fraction of the i lump [mol of C (mol of C)-1]. The mole fraction, fi, is represented by Fi ði ¼ A, B, C, and DÞ ð7Þ fi ¼ φ FR where FR is the mass flow rate of residual oil (kg h-1) and φ is the mass of residual oil per unit mole of the oil [kg (mol of C)-1]. Equations 6 and 7 were substituted into eqs 2-5, yielding dfA ¼ -ðkAB þ kAC þ kAD ÞφC0 2 fA 2 ð8Þ dðW=FR Þ dfB ¼ kAB φC0 2 fA 2 -ðkBC þ kBD ÞφC0 fB dðW=FR Þ
ð9Þ
dfC ¼ kAC φC0 2 fA 2 þ kBC φC0 fB -kCD φC0 fC dðW=FR Þ
ð10Þ
dfD ¼ kAD φC0 2 fA 2 þ kBD φC0 fB þ kCD φC0 fC ð11Þ dðW=FR Þ
Figure 4. Kinetic runs performed at (a) 450 °C, (b) 475 °C, and (c) 500 °C. Plot, experimental value; line, calculated value.
The integration of eq 8 gives 1 1 ¼ φC0 2 ðkAB þ kAC þ kAD ÞðW=FR Þ fA fA0
parameters were evaluated by a nonlinear least-squares regression of the experimental data. This analysis was carried out based on a quasi-Newton method. Product distributions were numerically calculated from the experimental data using eqs 8-11 and are represented by curves in Figure 4. Plots in this figure show experimental data. The product yield was predicted well with an average absolute error of less than 3%, indicating that the proposed model adequately predicts the cracking of heavy oil with steam over the zirconia-alumina-iron oxide catalyst. The yield of heavy oil decreased with an increase in the time factor at each temperature, and larger amounts of lighter fractions were produced at higher time factors. However, the yield of gas oil was decreased when the time factor
ð12Þ
Figure 3 shows the plots of 1/fA-1/fA0 against the time factor W/FR; these were obtained at a constant C0 of 1.8 mol of C m-3 and φ of 0.014 kg (mol of C)-1. The value of 1/fA-1/fA0 was well-correlated to the time factor, suggesting that cracking of heavy oil was second-order. 3.3. Estimation of Kinetic Parameters. Equations 8-11 were solved with the Runge-Kutta method, and the kinetic (11) S anchez, S.; Ancheyta, J. Energy Fuels 2007, 21, 653–661.
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: DOI:10.1021/ef9006164
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and the lattice oxygen in the iron oxide promoted the cracking of heavy oil. Activation energies for the reaction of gas oil and gasoline þ kerosene (kBC and kCD) were larger than those of the reaction of heavy oil (kAB, kAC, and kAD). In this reaction, the heavy oil fraction was oxidatively cracked, yielding gas oil, gasoline, kerosene, and carbon dioxide. We suppose that subsequent cracking of gas oil, gasoline, and kerosene generated from heavy oil was non-oxidative cracking, and therefore, the activation energy of this non-oxidative cracking was relatively high. 4. Conclusion Catalytic cracking of AR with steam was conducted using a zirconia-alumina-iron oxide catalyst at 450-500 °C under a time factor, W/FR, ranging from 3.8-28 h. A four-lump kinetic model for the catalytic cracking of AR was proposed, in which heavy oil, gas oil, gasoline þ kerosene, and gas were taken into account. Kinetic parameters in the proposed model were evaluated. The evaluated activation energy of heavy oil cracking was lower than those reported in the literature on the hydrocracking process. Accordingly, it is concluded that the active oxygen species generated from steam and the lattice oxygen in iron oxide promoted the cracking of heavy oil.
Figure 5. Arrhenius plots of kinetic parameters: (a) second-order and (b) first-order rate constants.
Acknowledgment. This work was supported by the Industrial Technology Research Grant Program in 2008, 08B36001c, from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
Table 2. Activation Energies of the Proposed Model rate constant kAB kAC kAD kBC kCD
E (kJ mol-1) 115 139 41.6 297 233
Nomenclature Ci =mole concentration of the i lump (mol of C m-3) C0 =mole concentration of residual oil at the reactor inlet (mol of C m-3) E=activation energy (kJ mol-1) fi =mole fraction of the i lump Fi =mole flow rate of the i lump (mol of C h-1) FR =mass flow rate of residual oil (kg h-1) kAB, kAC, and kAD = intrinsic second-order rate constant [m6 (mol of C)-1 (kg of catalyst)-1 h-1] kBC, kBD, and kCD =intrinsic first-order rate constant [m3 (kg of catalyst)-1 h-1] ri=production reaction rate of the i lump [mol of C (kg of catalyst)-1 h-1] W=mass of catalyst (kg of catalyst) φ=mass of residual oil per unit mole of the oil [kg (mol of C)-1]
was above 20 h at 500 °C, indicating that gas oil might be subsequently cracked. Figure 5 shows Arrhenius plots for the evaluated kinetic parameters. The data, excluding kBD, lay on a straight line for each kinetic parameter. The values of kBD at each temperature were null. This means that gas production by the cracking of gas oil hardly proceeds at the temperature range of this reaction, 450-500 °C. The yield of gas oil decreased, while the gasoline þ kerosene and gas yield increased, when the time factor was above 20 h at 500 °C. Therefore, we suppose that heavier fractions were converted to lighter hydrocarbons: gas oil f gasoline þ kerosene f gaseous hydrocarbons, whereas carbon dioxide was generated by oxidative cracking of heavy oil. The slopes of the lines in Figure 5 give the activation energies, which are summarized in Table 2. The activation energies for the cracking of heavy oil (kAB, kAC, and kAD) were lower than those reported in the literature on the hydrocracking process.9 Especially, the activation energy for the reaction of gas formation from heavy oil (kAD) was less than half of those for other reactions. These results suggest that the active oxygen species generated from steam
Subscripts A=heavy oil lump B=gas oil lump C=gasoline þ kerosene lump D=gas lump i=i lump 0=reactor inlet
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