Pyrolysis Performances of Catalytic Cracking Naphtha and Coker

Sep 10, 2009 - Telephone: 8610-8973-3392. ... The surface area and pore size of the inert carrier showed little effect on the pyrolysis performance...
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Energy Fuels 2009, 23, 5760–5764 Published on Web 09/10/2009

: DOI:10.1021/ef900645s

Pyrolysis Performances of Catalytic Cracking Naphtha and Coker Naphtha on Inert Carriers and an Active Catalyst Weikang Liu, Xianghai Meng, Xu Zhao, Gang Wang, Jinsen Gao, and Chunming Xu* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Received June 25, 2009. Revised Manuscript Received August 22, 2009

The pyrolysis performance of catalytic cracking naphtha and coker naphtha on different carriers and an active catalyst was investigated in a confined fluidized bed reactor. For the pyrolysis of catalytic cracking naphtha on quartz grain, the yield of total light olefins increased with the enhancement of temperature, while it varied slightly with increasing weight hourly space velocity (WHSV) and steam/oil weight ratio. The optimal reaction temperature, WHSV, and steam/oil weight ratio were 700 °C, 5.7 h-1, and 0.5, respectively. The selectivity of total light olefins of coker naphtha was better than that of catalytic cracking naphtha under the optimal reaction conditions. The surface area and pore size of the inert carrier showed little effect on the pyrolysis performance. The introduction of the active catalyst could improve the conversion and the yield of total light olefins.

to other naphthas, such as straight-run naphtha and hydrocracking naphtha.11 The product distribution can be influenced by the difference of the group compositions of feedstocks.12 Therefore, the results of treating them to obtain higher value light olefins will bring great help in determining the effect of feed properties on the catalytic pyrolysis process. In recent years, many kinds of catalysts and improved technologies on catalytic pyrolysis have been studied and developed, with the aim of enhancing the yield of light olefins.2,13,14 On the basis of these studies, one of the characteristics for catalytic pyrolysis is that the required reaction temperature is still high, which means that thermal pyrolysis also happens simultaneously.15 However, little information is available for the function of thermal pyrolysis in the catalytic pyrolysis process. Hence, it is necessary to have a study on the thermal pyrolysis performance for further explanation of the mechanism of catalytic pyrolysis. In the present work, the thermal pyrolysis performance of catalytic cracking naphtha on quartz grain was investigated. Then, the effect of feed properties and that of carrier properties on product distribution were discussed.

1. Introduction Catalytic pyrolysis is an effective way to produce light olefins, such as ethylene, propylene, and butylene, and has attracted widespread attention.1-3 Hydrocarbons, such as n-butane,4 heptane,5 naphtha,2,6 vacuum gas oil,7 and even low-value heavy oils,8 can be used as the feed of catalytic pyrolysis. Among these, naphtha has been used world wide for now. Catalytic cracking naphtha9 and coker naphtha10 are products of fluid catalytic cracking and delayed coking, respectively. Both of them have a high content of olefins compared *To whom correspondence should be addressed: State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Changping, Beijing 102249, China. Telephone: 8610-8973-3392. Fax: 86106972-4721. E-mail: [email protected]. (1) Meng, X. H.; Gao, J. S.; Li, L.; Xu, C. M. Advances in catalytic pyrolysis of hydrocarbons. Pet. Sci. Technol. 2004, 22 (9 and 10), 1327– 1341. (2) Jeong, S. M.; Chae, J. H.; Kang, J. H.; Lee, S. H.; Lee, W. H. Catalytic pyrolysis of naphtha on the KVO3-based catalyst. Catal. Today 2002, 74 (3 and 4), 257–264. (3) Gao, X. D.; Chen, B. Z.; He, X. R.; Qiu, T.; Li, J. C.; Wang, C. M.; Zhang, L. J. Multi-objective optimization for the periodic operation of the naphtha pyrolysis process using a new parallel hybrid algorithm combining NSGA-II with SQP. Comput. Chem. Eng. 2008, 32 (11), 2801–2811. (4) Zhu, X. X.; Liu, S. L.; Song, Y. Q.; Xie, S. J.; Xu, L. Y. Catalytic cracking of 1-butene to propene and ethene on MCM-22 zeolite. Appl. Catal., A 2005, 290 (1 and 2), 191–199. (5) Pant, K. K.; Kunzru, D. Catalytic pyrolysis of n-heptane on unpromoted and potassium promoted calcium aluminates. Chem. Eng. J. 2002, 87 (2), 219–225. (6) Basu, B.; Kunzru, D. Catalytic pyrolysis of naphtha. Ind. Eng. Chem. Res. 1992, 31 (1), 146–155. (7) Arandes, J. M.; Torre, I.; Castao, P.; Olazar, M.; Bilbao, J. Catalytic cracking of waxes produced by the fast pyrolysis of polyolefins. Energy Fuels 2007, 21 (2), 561–569. (8) Meng, X. H.; Xu, C. M.; Gao, J. S.; Li, L. Studies on catalytic pyrolysis of heavy oils: Reaction behaviors and mechanistic pathways. Appl. Catal., A 2005, 294 (2), 168–176. (9) Wang, G.; Xu, C. M.; Gao, J. S. Study of cracking FCC naphtha in a secondary riser of the FCC unit for maximum propylene production. Fuel Process. Technol. 2008, 89 (9), 864–873. (10) Torre, I.; Arandes, J. M.; Azkoiti, M. J.; Olazar, M.; Bilbao, J. Cracking of coker naphtha with gas-oil. Effect of HZSM-5 zeolite addition to the catalyst. Energy Fuels 2007, 21 (1), 11–18. r 2009 American Chemical Society

2. Experimental Section 2.1. Feedstocks and Carriers. Catalytic cracking naphtha, taken from Fushun Petrochemical Company, China National Petroleum Corporation, and coker naphtha, taken from Tianjin (11) Erofeev, V. I.; Adyaeva, L. V.; Ryabov, Y. V. Pyrolysis of straight-run naphtha on ZSM-5 zeolites modified with alkaline-earth metal cations. Russ. J. Appl. Chem. 2001, 74 (2), 235–237. (12) Meng, X. H.; Xu, C. M.; Gao, J. S.; Liu, Z. C. Influence of feed properties and reaction conditions on catalytic pyrolysis of gas oils and heavy oils. Fuel 2008, 87 (12), 2463–2468. (13) Lemonidou, A. A.; Vasalos, I. A.; Hirschberg, E. J.; Bertolacini, R. J. Catalyst evaluation and kinetic study for ethylene production. Ind. Eng. Chem. Res. 1989, 28 (5), 524–530. (14) Kolesov, S. V.; Tsadkin, M. A. Mixed metal chloride catalysts for pyrolysis of naphtha cuts. Chem. Technol. Fuels Oils 2003, 39 (6), 354– 357. (15) Xie, C. G. Studies on catalytic pyrolysis process for ethylene production and its reaction mechanism. Pet. Process. Petrochem. 2000, 31 (7), 40–44.

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Table 1. Group Compositions of Feedstocks (wt %)

Table 3. Operation Conditions for Main Pyrolysis Tests

item

catalytic cracking naphtha

coker naphtha

parameter

value

n-paraffins iso-paraffins olefins naphthenes aromatics

5.46 23.99 37.13 7.84 25.58

31.84 21.24 28.33 6.57 12.02

reaction temperature (°C) steam/oil weight ratio loading of carrier or catalyst (g) WHSV (h-1)

600-700 0.3-0.9 60 5-11.3

Table 4. Effect of the Reaction Temperature on Product Distribution

Table 2. Physical Properties of Inert Carriers and Catalyst item surface area (m2/g) pore volume (cm3/g) average pore diameter (nm) packing density (g/cm3) particle size distribution (wt %) 0-20 μm 20-40 μm 40-80 μm 80-120 μm >120 μm

quartz grain

silica gel

catalyst

0.9 0.002 10 1.68

317 1.0 12 0.78

133 0.2 0.86

2.24 43.02 48.26 6.48

4.41 39.52 50.23 5.84

6.07 16.31 46.29 30.11 1.22

reaction temperature (°C) yield of product (wt %) dry gas LPG liquid product coke feed conversion (wt %) selectivity of total light olefins (%)

600

620

640

660

680

700

8.12 4.31 87.32 0.25 12.68 46.61

10.14 6.42 83.23 0.21 16.77 54.32

10.51 7.83 81.48 0.18 18.52 62.47

18.64 11.62 69.53 0.21 30.47 58.06

23.54 14.33 61.95 0.18 38.05 58.63

25.61 16.36 57.84 0.19 42.16 60.67

catalyst in the reactor. The feedstock was pumped with a certain flow rate and mixed with the steam. The mixture entered a preheater, where it was heated to approximately 350 °C, and then entered the reactor. Reactions took place as the feed contacted with the fluidized carrier or catalyst. The reaction product was cooled and separated into liquid and gas samples by the product separation and collection system. The inert carrier or catalyst after the reaction was drawn from the reactor by a vacuum pump. The operation conditions for the main pyrolysis tests are summarized in Table 3. Here, weight hourly space velocity (WHSV) was defined as the ratio of the flow rate of the feed to the mass of the inert carrier or catalyst loaded into the reactor. 2.3. Analysis Methods. Pyrolysis products include pyrolysis gas, pyrolysis liquid, and coke. An Agilent 6890 gas chromatograph with ChemStation software was used to measure the volume fractions of components in pyrolysis gas. The ideal gas state equation was used to convert the data to mass basis. The pyrolysis liquid was analyzed with a SP3420 gas chromatograph to obtain the weight percentage of paraffins, olefins, naphthenes, and aromatics. The coke content deposited on the inert carrier or catalyst was measured with a HV-4B coke analyzer.

Figure 1. Diagram of the confined fluid bed reactor unit (1, constant temperature box; 2, steam furnaces; 3, feedstock; 4, electronic scales; 5, oil pump; 6, water tank; 7, water pump; 8, reheater; 9, reactor furnaces; 10, thermocouple; 11, reactor; 12, catalyst inlet and outlet; 13, filter; 14, condenser; 15, liquid product sampler; 16, gas-collection vessels; 17, beaker; 18, gas sample bag).

3. Results and Discussion

Petrochemical Company, SINOPEC Corporation, were used as the feedstocks. The group compositions of these feeds are given in Table 1. Quartz grain, silica gel and a molecular sieve catalyst developed by China University of Petroleum for catalytic cracking16 were used as the inert carrier and the catalyst in the experiment, respectively. The physical properties of the carrier and the catalyst are listed in Table 2. The surface area and pore volume of the carrier and catalyst were determined by gas adsorption using the Brunauer-Emmett-Teller (BET) method.17 2.2. Apparatus and Operating Conditions. A confined fluidized-bed reactor unit was used in experiments for the pyrolysis, and its diagram is shown in Figure 1. It is comprised of five sections, oil and steam input mechanisms, a reaction zone, temperature control system, and a product separation and collection system. Experiments were conducted in a batch mode. For each experiment, 60 g of the inert carrier or catalyst were loaded into the reactor with an effective volume of about 580 cm3. Distilled water was pumped with a certain flow rate into a furnace to generate steam, which was used to fluidize the inert carrier or

Feed conversion was defined as the sum of the yields of dry gas, liquefied petroleum gas (LPG), and coke. The selectivity of total light olefins (ethylene, propylene, and butylene) was defined as the ratio of the yield of total light olefins to feed conversion. 3.1. Reaction Performance of Catalytic Cracking Naphtha. 3.1.1. Effect of the Reaction Temperature. The effect of the reaction temperature on quartz grain was investigated in the range of 600-700 °C, keeping WHSV and the weight ratio of steam/oil constant at about 9 h-1 and 0.5, respectively. Table 4 gives the product distribution at various reaction temperatures. As the reaction temperature increased, the yields of dry gas and LPG increased monotonously and that of the liquid product decreased obviously. The coke yield varied slightly. Because dry gas is the final product of the thermal reactions, its yield increased with the enhancement of the temperature. A high reaction temperature means deep pyrolysis extent; therefore, feed conversion increased with an increasing reaction temperature. Figure 2 shows the yields of light olefins as a function of the reaction temperature. In the range of 600-700 °C, the yields of ethylene, propylene, and butylene increased monotonously with an increasing reaction temperature. The yield

(16) Shen, B. J.; Gao, J. S.; Xu, C. M.; Zhao, L.; Li, X. F.; Wu, P. Catalyst composition for heavy oil feed process, 2006; CN101032694A. (17) Chen, J. W. Technology and Engineering of Catalytic Cracking, 2nd ed.; China Petrochemical Press: Beijing, China, 2005; pp 280-301.

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Figure 2. Effect of the reaction temperature on the yields of light olefins.

Figure 3. Effect of the WHSV on the yields of light olefins.

Table 5. Effect of the WHSV on Product Distribution WHSV (h-1)

5.0

5.7

7.5

9.0

11.3

yield of product (wt %) dry gas LPG liquid product coke feed conversion (wt %) selectivity of total light olefins (%)

36.46 20.83 42.54 0.17 57.46 62.29

35.02 19.01 45.79 0.18 54.21 65.25

32.05 17.82 49.92 0.21 50.08 64.02

29.54 15.75 54.52 0.19 45.48 60.88

29.14 15.11 55.53 0.22 44.47 61.07

of total light olefins can reach 26.4 wt %, with ethylene yield of 9.98 wt % and propylene yield of 9.06 wt % at 700 °C. The reaction in the pyrolysis process abides by the free radical mechanisms, which mainly reacts on the carrier surface and interspaces between carrier particles. The optimal reaction temperature was 700 °C for the production of light olefins. 3.1.2. Effect of the WHSV. The effect of the WHSV on quartz grain was investigated in the range of 5-11.3 h-1 via changing the feeding rates of naphtha and water. The reaction temperature and the steam/oil weight ratio were kept constant at about 700 °C and 0.5, respectively. The yields of pyrolysis products and feed conversion are given in Table 5. As WHSV increased, the yield of dry gas decreased, while that of the liquid product increased. The yields of LPG and coke varied slightly. The feed conversion decreased with an increasing WHSV. A large WHSV indicates that there is a shorter time for the pyrolysis of hydrocarbons, and therefore, the pyrolysis extent is less thorough. Figure 3 illustrates the yields of light olefins as a function of the WHSV. As WHSV increased from 5 to 11.3 h-1, the yields of ethylene, propylene, and total light olefins decreased. As WHSV increased, the reaction time and the reaction extent decreased and resultantly, the yields of light olefins showed decreasing trends. The optimal WHSV was about 5.7 h-1 with the consideration of the yield of light olefins. 3.1.3. Effect of the Steam/Oil Weight Ratio. The effect of the steam/oil weight ratio on quartz grain was investigated in the range of 0.3-0.9 via changing the feeding rate of naphtha. The reaction temperature and WHSV were kept constant at about 700 °C and 5.7 h-1, respectively. The experimental results are given in Table 6. As the steam/oil weight ratio increased from 0.3 to 0.7, the yield of dry gas, LPG, and the feed conversion varied slightly, while those were a bit lower at a steam/oil weight ratio of 0.9. The yield of coke almost held the line with an increasing steam/oil weight

Figure 4. Effect of the steam/oil weight ratio on the yields of light olefins. Table 6. Effect of the Steam/Oil Weight Ratio on Product Distribution steam/oil weight ratio

0.3

0.5

0.7

0.9

yield of product (wt %) dry gas LPG liquid product coke feed conversion (wt %) selectivity of total light olefins (%)

29.04 15.32 55.46 0.18 44.54 63.16

29.02 15.48 55.31 0.19 44.69 64.56

29.49 15.49 54.82 0.20 45.18 65.85

28.49 13.32 58.00 0.19 42.00 62.93

ratio. Steam could reduce the partial pressure of oil vapor, which could further restrain condensation or polymerization reactions. Therefore, the increase of the steam/oil weight ratio could prevent coking deposition on the inert carrier surface and could reduce the yield of coke. However, the coke deposition was very low, and it was hard to obtain obvious variation of coke yield as a function of the steam/oil weight ratio. Figure 4 shows the effect of the steam/oil weight ratio on the yield of light olefins. As the steam/oil weight ratio increased from 0.3 to 0.9, the yield of total light olefins had no distinct variation. Increasing the steam/oil weight ratio can reduce the partial pressure of oil vapor, which favors the cracking of hydrocarbons into light products. A large steam/ oil weight ratio is good with the suitable allowance for the handling capacity and the economic benefit. The recommended steam/oil weight ratio is about 0.5. 5762

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: DOI:10.1021/ef900645s

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Table 7. Pyrolysis Product Distribution of Catalytic Cracking and Coker Naphtha (Reaction Temperature, 700 °C; Steam/Oil Weight Ratio, 0.5; WHSV, 5.7 h-1) feeds

catalytic cracking naphtha

coker naphtha

yield of product (wt %) dry gas LPG liquid product coke feed conversion (wt %) selectivity of total light olefins (%)

25.61 16.36 57.84 0.19 42.16 60.67

36.55 20.16 43.11 0.18 56.89 67.39

Table 8. Group Compositions of the Liquid Products, wt % (Reaction Temperature, 700 °C; Steam/Oil Weight Ratio, 0.5; WHSV, 5.7 h-1) feed

cracking naphtha

coker naphtha

n-paraffins iso-paraffins olefins naphthenes aromatic

2.64 12.39 18.36 2.68 63.93

16.76 11.77 19.70 3.89 47.88

Figure 5. Effect of the reaction temperature on group compositions of the liquid product of catalytic cracking naphtha.

3.2. Effect of the Naphtha Properties on Pyrolysis Performance. The relationship between feed properties and the yields of light olefins was investigated. Table 1 shows the differences of the group compositions between catalytic cracking naphtha and coker naphtha. The content of paraffins of coker naphtha was 53.08 wt %, and that of catalytic cracking naphtha was 29.45 wt %. The content of olefins of catalytic cracking naphtha was 37.13 wt %, and that of coker naphtha was 28.33 wt %. The pyrolysis product distribution of catalytic cracking naphtha and coker naphtha was investigated under the optimal operating conditions (reaction temperature of 700 °C, WHSV of 5.7 h-1, and steam/oil weight ratio of 0.5). The results are listed in Table 7. The yield of gas and the selectivity of total light olefins for the pyrolysis of coker naphtha were higher than those of catalytic cracking naphtha. Because the sum content of paraffins and olefins in coker naphtha was higher than that in catalytic cracking naphtha, we presumed that it was the most important factor for the pyrolysis of naphtha. The larger the sum content of paraffins and olefins in the feed, the better its pyrolysis performance and the higher the selectivity of the total light olefins. The group compositions of the liquid product are shown in Table 8. The group compositions of the liquid product of the pyrolysis of both catalytic cracking naphtha and coker naphtha as a function of the reaction temperature were further studied, with the results shown in Figures 5 and 6, respectively. The content of olefins in the liquid product decreased, while that of aromatics increased obviously. The contents of paraffins and naphthenes in the liquid product decreased slightly. 3.3. Effect of the Carrier Properties on Pyrolysis Performance. 3.3.1. Thermal Pyrolysis on Quartz Grain and Silica Gel. The pyrolysis of coker naphtha on quartz grain and silica gel was investigated at a reaction temperature of 700 °C, WHSV of 5.7 h-1, and steam/oil weight ratio of 0.5. Product yields and distribution of thermal pyrolysis of coker naphtha on different carriers are shown in Table 9. The pyrolysis product distribution and the yields of light olefins of silica gel were similar to those of quartz grain, although there were big differences on surface area and pore size of both inert carriers. This shows that surface area and pore size of the inert carrier have little effect on the pyrolysis of coker

Figure 6. Effect of the reaction temperature on group compositions of the liquid product of coker naphtha. Table 9. Pyrolysis Product Yields of Coker Naphtha on Different Inert Carriers item

silica gel

quartz grain

yield of product (wt %) dry gas LPG liquid product coke ethylene propylene butylene total light olefins feed conversion (wt %) selectivity of total light olefins (%)

34.28 20.03 45.57 0.12 16.21 12.98 8.43 37.61 54.43 69.98

36.55 20.16 43.11 0.18 15.59 11.72 7.48 34.79 56.89 67.39

naphtha. These results agree with the report of Towfighi et al.,18 who consider that the surface area of packed bed materials has no considerable effect on the product yields. 3.3.2. Catalytic Pyrolysis on Catalyst and Thermal Pyrolysis on Quartz Grain. The catalytic pyrolysis of catalytic naphtha on the active catalyst and the thermal pyrolysis on quartz grain were investigated at a reaction temperature of 600 °C, WHSV of 9 h-1, steam/oil weight ratio of 0.5, and carrier/oil weight ratio of 6. Table 10 lists the conversion, product yields, and selectivity of catalytic and thermal pyrolysis. In comparison to the thermal pyrolysis, the conversion of catalytic pyrolysis increased from 12.68 to (18) Towfighi, J.; Zimmermann, H.; Karimzadeh, R.; Akbarnejad, M. M. Steam cracking of naphtha in packed bed reactors. Ind. Eng. Chem. Res. 2002, 41 (6), 1419–1424.

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In comparison to the selectivity of small molecular products, the selectivity of ethane of thermal pyrolysis was 4.35 times that of catalytic pyrolysis. However, the selectivity of ethylene of thermal pyrolysis was close to that of catalytic pyrolysis. This shows that the inhibition of the catalyst to ethane was higher than that to ethylene. Catalytic pyrolysis was carried out at high react temperatures; therefore, the tert-butyl carbonium ions could absorb the energy of 10-25 kJ/mol to form n-butyl carbonium ions, which had β position of C-C bonds, and then cracked to form ethylene. For the sec-butyl carbonium ions, the β position of C-C bonds were at the end position; they could also isomerize to n-butyl carbonium ions, in which the β-position energy of the C-C bond was lower. The inhibition of the catalyst to ethylene was low, which means that part of the ethylene in the products was the product of the carbonium ion mechanism.

Table 10. Product Yields and Selectivity of Thermal and Catalytic Pyrolysis at 600 °C yield of product (wt %) product hydrogen methane ethane dry gas LPG coke ethylene propylene butylene total light olefins feed conversion

selectivity of product (%)

thermal pyrolysis

catalytic pyrolysis

thermal pyrolysis

catalytic pyrolysis

0.17 1.73 1.08 8.12 4.31 0.25 1.85 2.08 1.98 5.91

0.08 0.82 0.62 6.67 21.5 3.5 4.57 13.41 6.39 24.37

1.34 13.64 8.52 64.04 33.99 1.97 14.59 16.4 15.62 46.61

0.25 2.59 1.96 21.06 67.89 11.05 14.43 42.34 20.18 76.95

16.17

31.67

4. Conclusions

31.67 wt %. The yields of ethylene, propylene, butylene, and total light olefins of catalytic pyrolysis were 2.47, 6.45, 3.23, and 4.12 times those of thermal pyrolysis, respectively. The yields of small molecular products of thermal pyrolysis were much higher than those of catalytic pyrolysis. The yields of hydrogen, methane, and ethane of thermal pyrolysis were 2.13, 2.11, and 1.74 times those of catalytic pyrolysis, respectively. The addition of the catalyst could enhance both the conversion and the yields of total light olefins. The selectivity of hydrogen, methane, ethane, and dry gas of thermal pyrolysis were much higher than those of catalytic pyrolysis. The selectivity of ethylene of thermal pyrolysis was close to that of catalytic pyrolysis, while that of LPG, propylene, butylene, and total light olefins were lower. Thermal pyrolysis on quartz grain follows the free-radical mechanism, which results in the high proportion of hydrogen, methane, ethane, and ethylene. For the catalytic pyrolysis, both the carbonium ion and free-radical mechanisms play important roles.8 The product distribution was greatly changed by the intervention of the carbonium ion mechanism, which promoted the formation of propylene and butylene while restrained that of hydrogen, methane, ethane, and ethylene.

(1) The reaction temperature, WHSV, and steam/oil ratio showed various effects on the thermal pyrolysis of catalytic cracking naphtha. Among the operating conditions, the reaction temperature was the most important parameter. The optimal reaction temperature, WHSV, and steam/oil weight ratio were 700 °C, 5.7 h-1, and 0.5, respectively. (2) The sum content of paraffins and olefins in the feed was the most important factor for pyrolysis of naphthas. The selectivity of total light olefins and the pyrolysis performance of coker naphtha were better than those of catalytic cracking naphtha. (3) The pyrolysis performance of naphtha on silica gel was similar to that on quartz grain. The surface area and pore size of the inert carrier showed little effect on the pyrolysis of naphtha. For the catalytic pyrolysis, both the carbonium ion and free-radical mechanisms play important roles. The introduction of the catalyst can improve the conversion and the yield of total light olefins; part of ethylene was the product of the carbonium ion mechanism. Acknowledgment. Financial support was provided by the National Science Fund for Distinguished Young Scholars of China (20525621 and 20725620), the Major Research Plan of the Ministry of Education of China (307008), and the National Basic Research Program of China (973 Program, No. 2004CB217803).

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