Ind. Eng. Chem. Res. 2010, 49, 89–93
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Kinetic Models for the Coke Combustion on Deactivated ZSM-5/MOR Derived from n-Heptane Cracking Ning Zhu, Yuan-yuan Liu, Yang Wang, Feng-qiu Chen,* and Xiao-li Zhan Department of Chemical and Biochemical Engineering, Zhejiang UniVersity, Hangzhou 310027, Zhejiang, China
The combustion behavior of coked H-ZSM-5/H-mordenite (ZSM-5/MOR) derived from catalytic cracking of n-heptane with and without water steam has been studied by thermogravimetric analysis (TGA). Catalytic cracking under vapor atmosphere with a water-oil ratio of 2 at 650 °C created only one type of coke deposit on the catalyst. On the other hand, two types of coke can be found on the coked catalyst deactivated by catalytic cracking without water steam. These results indicated different combustion kinetic models of different types of coke. A one-coke combustion model for the regeneration of coked catalyst obtained under water steam and a two-coke combustion model for the regeneration of coked catalyst derived without water steam were conducted by using TGA, respectively. Finally, the relationship between the C/H ratios and activation energies in the combustion kinetic models were investigated by ultimate analysis. 1. Introduction Catalytic cracking of naphtha over zeolite-based catalysts plays an important role in the refining and petrochemical industry.1 However, during cracking reactions catalyzed by acidic zeolites, the catalysts always suffer from strong deactivation due to the heavy deposition which causes the blockage of pores and coverage of active sites.2,3 Comparing with other zeolites with greater pore sizes, ZSM-5 is widely accepted as more resistant to carbonaceous deposition but less active in catalytic cracking.4-6 Because of the complexity of the feeds and products under industrial conditions, composite zeolites have been synthesized recently to fill the realistic needs of the petrochemical industry, such as ZSM-5/ZSM-11,7 MCM-22/ ZSM-35,8 and BEA/MOR.9 Recently, ZSM-5/MOR10,11 as a novel composite zeolite for catalytic cracking with promising activity and high stability was also reported. The effects of heavy depositions generated during catalytic cracking and the performance of regeneration are still very important factors affecting the catalytic performance of ZSM-5/MOR. These nondesorbed byproducts, called coke, with high molecular mass and C/H ratio can be removed by combustion under oxidative atmosphere. Coke deposition can be classified into many kinds by different C/H ratios. Coke species with a low C/H ratio, called light coke,12 can be easily removed. Coke with a high C/H ratio, called heavy coke,13 remains on the catalysts even at about 600 °C. In catalytic cracking of n-heptane over the novel ZSM-5/ MOR catalyst, the formation of coke is still the key factor which causes the deactivation of the catalyst. Therefore, the regeneration of the catalyst and the combustion behavior of removable coke are of great importance, because the study of the regeneration relates to the design of the regenerator and the improvement of catalysts. A number of studies have been investigated on the kinetic models of coke combustion over different zeolites following individual reactions by using many kinds of methods.14-17 In particular, thermogravimetric analysis (TGA) is widely accepted. This technique not only affords an easy way to identify different kinds of coke by different combustion temperatures, but also generates reliable data on the kinetics of coke combustion. Some investigations have been reported on the studies of the kinetics * To whom correspondence should be addressed. Tel.: +86-57187952728. E-mail:
[email protected].
for coke combustion by this kind of technique.16,18-21 In order to construct a proper kinetic model, the type of coke has to be identified accurately. Ren19 reported two types of coke with different C/H ratios derived at different temperatures and proposed a two-coke model to describe the combustion of these two types of coke. In addition to the different temperatures, some other reaction conditions can also result in different types of coke, such as reactions with and without water vapor, which are of great scientific interest. The present work focused on the effect of water steam on the types of coke generated during catalytic cracking by using TGA. A two-coke model and a onecoke model were proposed to describe the combustion of cokes derived from n-heptane cracking over ZSM-5/MOR zeolites with and without water steam, respectively. In addition, ultimate analysis was applied to the validation of the kinetic models. 2. Experimental Section 2.1. Catalysts. ZSM-5/MOR (Si/Al ) 20) was synthesized and provided by the Shanghai Research Institute of Petrochemical Technology. The diameters of the catalyst particles were from 0.03 to 0.05 mm. Two types of deactivated catalysts were obtained after catalytic cracking of n-heptane under a gas hourly space velocity of 1 h-1 at 650 °C with water steam (water-oil ratio ) 2) and without water steam, respectively, for 2 h. These two types of deactivated catalysts were denoted as Cat-n (for the catalyst derived without water) and Cat-w (for the catalyst derived with water), respectively. 2.2. Thermogravimetric (TG-DTG) Analysis. The TGDTG analysis was carried out using a thermogravimetric analyzer (METTLER TGA/SDTA 851e). In nonisothermal oxidation runs, a fresh coke sample of 30 mg was heated from room temperature to 900 °C under an oxidative atmosphere (N2 + O2) at a rate of 20 °C/min. In isothermal oxidation conditions, about 30 mg of coked sample was heated to the combustion temperature (500-700 °C) at a rate of 50 °C/min under a nitrogen flow of 100 mL/min, with a wait of 20 min for the removal of water and coke precursors. Then, the combustion process was carried out by switching from the nitrogen flow to oxidative flow (N2 + O2). The oxygen partial pressure was 8.5-21.3 kPa, and the total gas flow rate was 100 mL/min (0.2 m/s).
10.1021/ie900855y 2010 American Chemical Society Published on Web 11/16/2009
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2.3. Ultimate Analysis. A ThermoFinnigan elemental analyzer (Flash EA 1112) was used to analyze the C/H ratio and to comfirm the final kinetic models. 2.4. Model. The regeneration of a deactivated catalyst particle by combustion of the carbonaceous deposits involves both chemical reactions and transport processes. The effects of internal pore diffusion were mainly determined by the size of the catalyst particles. If the size of the catalyst particle is small enough, the effect of the internal diffusion on the observed combustion rate of coke deposits can be neglected. Kern and Jess22 reported that regeneration rates of the coked Al2O3 particles with a diameter of 2 mm were clearly diffusion limited. Tang et al.23 applied small catalyst particles (0.05-0.11 mm) and high gas velocity (0.22 m/s) to eliminate effects of internal and external diffusion. Keskitalo et al.24 reported that the internal diffusion is not a limiting factor when the average particle diameter of the ferrierite particles is 0.12 mm. In order to obtain the intrinsic kinetic parameters, the sizes of the catalyst particles adopted in this work were from 0.03 to 0.05 mm and the gas velocity was 0.2 m/s. The intrinsic kinetic model proposed here is the widely accepted one,18,19,25-27 which assumes that the coke combustion is a one-pseudocomponent oxidation reaction. The model can be described as r ) -dCC /dt ) kPOm2CnC
Figure 1. TG-DTG curves of Cat-n on the basis of ramped temperature experiments.
(1)
where r is the coke combustion rate, CC is the instantaneous concentration of coke, k is the rate constant, PO2 is the partial pressure of oxygen, m and n are the reaction orders with respect to the partial pressure of oxygen and the concentration of coke, respectively, and t is the reaction time. The reaction constant k is a function of temperature (T), which can be expressed as k ) A0 exp(-E/RT)
(2)
where A0 is the Arrhenius constant, E is the activation energy, and R is the gas constant. If the partial pressure of oxygen (PO2) is fixed, the nonlinear regressions of the TG-DTG curves at different temperatures can determine the parameter of the reaction order n. Subsequently, eqs 1 and 2 can be transformed into the form ln[-dCC /(CnC dt)] ) ln kPOm2 ) ln A0 + (-E/R)(1/T) + m ln POm2(3) A planar surface fit method under different temperatures and partial pressures of oxygen can be employed as a least-squares regression method to determine the parameters of A0, -E/R, and m. Finally, the kinetic model for the coke combustion can be obtained. 3. Results and Discussion 3.1. TG-DTG Characterization of the Coked Catalysts. To investigate the kinetics of the combustion of coked catalysts, we must figure out the combustion temperature and the types of coke. On the basis of ramped temperature experiments, the TG-DTG curves of the coked catalysts were obtained as shown in Figures 1 and 2. Besides the peak below 100 °C which is assigned to adsorbed water, two distinct negative peaks at about 400 and 610 °C can be observed in Figure 1. These two peaks indicate two types of coke on the catalyst derived from catalytic cracking without water steam (Cat-n), which is in agreement with the findings of Dechelette.28 The catalyst sample lost about 0.72% of weight during temperature between 350 and 550 °C.
Figure 2. TG-DTG curves of Cat-w on the basis of ramped temperature experiments.
The result indicates that one type of coke can be removed below 550 °C. This type of coke, denoted as C1 hereafter, is the light coke. The peak at 610 °C in Figure 1 is assigned to the heavy coke, which can only be removed at temperatures above 585 °C. This type of coke is denoted as C2. The heavy coke is about 1.52% of weight in the catalyst. On the other hand, only one peak at 610 °C can be observed in Figure 2, which suggests that there is only one type of coke deposits on the deactivated catalyst after catalytic cracking with water steam (Cat-w). This type of coke, denoted as C3, is about 0.18% of the catalyst. These results suggest different combustion models should be proposed between Cat-n and Cat-w. 3.2. Combustion Model of Coke on Cat-n. 3.2.1. Determination of the Reaction Order n. TG curves at different temperatures under the oxygen partial pressure of 8.5 kPa are shown in Figure 3. The combustion rate increased with the reaction temperature. Apparently, the light coke C1 is reactive below 550 °C. On the contrary, the heavy coke C2 is inactive until the temperature is higher than 585 °C. Nonlinear regression was performed on determining the reaction order n by using a trial-and-error method. In this study, the numerical simulation was carried out by simulation software. The regression results are presented in Table 1. As shown in Figure 4, the two-coke model can be applied to the TG curves derived between 585 and 650 °C. On the other
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Figure 3. Weight fraction of Cat-n at different temperatures under the same oxygen partial pressure of 8.5 kPa.
Figure 4. Nonlinear regressions of experimental data during combustion of coke on Cat-n at different temperatures; oxygen partial pressure, 8.5 kPa. Table 1. Results of the Reaction Orders with Respect to Coke Concentrations at Various Temperatures, PO2 ) 8.5 kPa C1
C2
temp/°C
n1
k1POm2
500 550 585 600 625 650
1.03 0.97 0.99 0.95 0.93 0.91
0.097 0.362 0.592 0.806 0.944 1.697
n2
k2POm2
0.68 0.63 0.66 0.74
0.0113 0.0161 0.0271 0.0460
hand, the one-coke model fit the curves well at 500 and 550 °C. The light coke was burnt out prior to the combustion of the heavy coke. This implies the much lower activation energy of the light coke. Moreover, the reaction order n determined by nonlinear regression is about 1 when the content of coke is greater than 1.52%, and about 0.7 when the coke concentration is lower than that. The regression results are given in Table 1, which indicates that the reaction rate of light coke C1 is a little more sensitive to the content of coke. 3.2.2. Determination of m, E, and A0. As mentioned above, only the light coke participated in the combustion reactions at
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Figure 5. Isothermal TG curves at various oxygen partial pressures at 550 °C on Cat-n.
Figure 6. Isothermal TG curves at various oxygen partial pressures at 650 °C on Cat-n.
temperatures under 550 °C, and when the content of coke was less than 1.52%, only heavy coke combusted above 585 °C. The typical TG curves of light coke combustion and heavy coke combustion under different partial pressures of oxygen are shown in Figures 5 and 6, respectively. In addition to the temperatures of 550 and 650 °C, the TG curves of the coke combustion at other different temperatures (500, 585, 600, 625 °C) can be obtained as well. The plots of ln(kPOm2) vs ln PO2 vs ln(1000/T) at different temperatures are shown in Figures 7 and 8. According to eq 3, the planar surface fit of these data points can determine the oxygen partial pressure order m, activation energy E, and Arrhenius constant A0 of C1 (Figure 7) and C2 (Figure 8). The regression parameters are presented in Table 2. The oxygen partial pressure orders m1 and m2 are about 0.7 and 0.5, respectively. The activation energy (E1) of the light coke is 112 kJ · mol-1, much lower than that of the heavy coke, which is about 142 kJ · mol-1. This result proves that the heavy coke is more resistant to combustion than the light coke. Based on the above results, two types of coke with different C/H ratios existed on the deactivated ZSM-5/MOR obtained by catalytic cracking of n-heptane without water steam. A twocoke kinetic model can be deduced as follows:
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kinetic model of light coke C1: dCC1 112000 0.7 ) 1.15 × 106 exp PO2 CC1 (CC g 0.0072) dt RT (4)
(
)
kinetic model of heavy coke C2: dCC2 142000 0.5 0.7 ) 3.25 × 106 exp PO2 CC2 (CC < 0.0072) dt RT (5)
(
)
The average relative error for the calculated combustion rate between this model and experimental data is 3.7%.
Figure 9. Nonlinear regressions of experimental data during combustion of coke on Cat-w at different temperatures; oxygen partial pressure, 8.5 kPa. Table 3. Parameters of the Kinetic Model for Coke Combustion on ZSM-5/MOR Deactivated from Catalytic Cracking with Water Steam
Figure 7. Planar surface fit results of the experimental data of the combustion of C1 at different temperatures and different oxygen partial pressures on Cat-n.
n
m
E/ kJ · mol-1
A0/(105 Pa)-mi · min-1
R2
0.91
0.51
131
1.27 × 106
0.99
3.3. Combustion Model of Coke on Cat-w. As discussed above, only one type of coke C3 existed on the coked ZSM-5/ MOR derived from the catalytic cracking of n-heptane under the vapor atmosphere. Similarly, the reaction order n of the kinetic model on Cat-w can also be determined by the nonlinear regression of the TG curves. Plots of the nonlinear regression results are shown in Figure 9. The simulation results fit the experimental data well. This result indicates that the one-coke kinetic model of coke combustion can be deduced. The total coke content on this deactivated sample is about 0.18%, much lower than that of Catn. The final parameters obtained by the regression of the experimental data are presented in Table 3. The regression results gave an activation energy (E) of 129 kJ · mol-1, which also indicates the medium C/H ratio of C3 between C1 and C2. Based on the above results, the one-coke kinetic model of C3 combustion over the ZSM-5/MOR deactivated by catalytic cracking with water steam can be described as -
dC 131000 0.5 0.9 ) 1.27 × 106 exp PO2 CC dt RT
(
)
(6)
The average relative error for calculated combustion rate between this model and experimental data is 1.1%. 3.4. Ultimate Analysis of Deactivated Catalysts. The characterizations of ultimate analysis on Cat-n and Cat-w indicated that Cat-n possessed two types of coke with two different C/H ratios. The C/H ratio of the light coke was about 1.06, and the C/H ratio of the heavy coke was 2.07. Cat-w had only one type of coke with an almost identical C/H ratio of 1.48. These results are in good agreement with the values of activation energies in the kinetic models as shown in Figure 10. The higher the C/H ratio, the harder the coke can be removed. Therefore, the kinetic models established above are of good reliability. Figure 8. Planar surface fit results of the experimental data of the combustion of C2 at different temperatures and different oxygen partial pressures on Cat-n. Table 2. Results of Oxygen Partial Pressure Order, Activation Energy, and Arrhenius Constant coke type C1 C2
m 0.72 0.54
E/kJ · mol-1
A0/(105 Pa)-mi · min-1
R2
112 142
1.15 × 10 3.25 × 106
0.99 0.99
6
4. Conclusion In general, the present work studied the regeneration kinetics over the novel ZSM-5/MOR catalyst coked by catalytic cracking with and without water steam. The TG analysis implied that two types of coke with different C/H ratios existed on the catalyst deactivated by coking without water steam. On the other hand, only one type of coke with an identical C/H ratio was deposited on the catalyst deactivated by coking with water steam. In addition,
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Figure 10. Comparison between the C/H ratios (derived from ultimate analysis) and activation energies of the models.
an intrinsic kinetic model was employed to describe the combustion behavior of the coke in the catalysts. A two-coke model and a one-coke model were established over different deactivated catalysts which were obtained by coking with and without water steam, respectively. The parameters were determined by the nonlinear fit and planar surface fit methods according to TG curves. Finally, the results of ultimate analysis proved two types of coke on the catalyst deactivated by coking without water steam and one type of coke on the catalyst deactivated by coking with water steam. Moreover, the order of the activation energies was coincident with the order of C/H ratios, which confirmed the validity of the proposed model. Acknowledgment This work was financially supported by NSFC (National Natural Science Foundation of China)-Sinopec Joint Fund of Research (20736011). Nomenclature r ) coke combustion rate, mg · mg-1 · min-1 CC ) instantaneous concentration of coke, mg · mg-1 · cat. m ) reaction order with respect to partial pressure of oxygen n ) reaction order with respect to concentration of coke E ) activity energy, kJ · mol-1 R ) gas constant, 8.314 J · K-1 · mol-1 PO2 ) partial pressure of oxygen, kPa T ) reaction temperature, K k ) reaction rate constant, (105 Pa)-mi · min-1 t ) reaction time, min A0 ) Arrhenius constant, (105 Pa)-mi · min-1 R2 ) regression coefficient
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ReceiVed for reView May 25, 2009 ReVised manuscript receiVed October 25, 2009 Accepted November 4, 2009 IE900855Y
