ARTICLE pubs.acs.org/EF
Cracking Performance of Gasoline and Diesel Fractions from Catalytic Pyrolysis of Heavy Gas Oil Derived from Canadian Synthetic Crude Oil Xianghai Meng, Chunming Xu, Li Li, and Jinsen Gao* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, China ABSTRACT: The secondary cracking performance of gasoline and diesel fractions from the catalytic pyrolysis of heavy gas oil derived from Canadian synthetic crude oil was investigated. Both diesel and gasoline fractions showed poor cracking performance. The feed conversion of the diesel fraction was below 51 wt %, and the yield of total light olefins was below 11 wt %. Meanwhile, the feed conversion of the gasoline fraction was below 30 wt % and the yield of total light olefins was below 7 wt %. The selectivity of total light olefins at 660 °C was only 22% for both fractions, and the selectivity of dry gas at 660 °C reached 25% for the diesel fraction and 34% for the gasoline fraction. The selectivity of coke at 660 °C reached 26% for the diesel fraction and 16% for the gasoline fraction. The diesel and gasoline fractions could partly crack to lighter components and could partly condense to heavier components. The condensation reaction played an important role in catalytic pyrolysis, and the selectivity of condensation products at 660 °C reached 42% for the cracking of the diesel fraction and 55% for the cracking of the gasoline fraction. The monomolecular cracking is the predominant cracking type, and both the free radical and the carbonium ion mechanisms play an important role under the experimental conditions.
1. INTRODUCTION Steam cracking is a traditional cracking process used to produce light olefins such as ethene, propene, and butene. Unlike other cracking processes, steam cracking should be performed at high temperature and with a short reaction time in the absence of a catalyst.1 In comparison to steam cracking, catalytic pyrolysis operates at a lower temperature owing to the presence of a catalyst. Moreover, catalytic pyrolysis also has more advantages, such as higher yield of light olefins, easier-to-control olefin distribution, lower energy consumption, and wider feed scope. These advantages have attracted many researchers to the process and technologies involved in catalytic pyrolysis. Reaction mechanisms and kinetics are important to a good understanding of catalytic pyrolysis. Gasoline and diesel are important and intermediate products for the catalytic pyrolysis of gas oils and heavy oils. The secondary cracking performance of gasoline and diesel fractions is important to the study of reaction mechanisms and kinetics. The feeds commonly used for catalytic pyrolysis include C4 hydrocarbons,24 paraffinic or olefinic hydrocarbons,58 naphtha,911 gas oil,12 heavy oil,13,14 bio-oil,15 and so on. It has been reported that catalytic cracking naphtha and coker naphtha shows good cracking performance, with the yield of total light olefins exceeding 30 wt %.11 The properties of gasoline and diesel fractions from the catalytic pyrolysis of gas oils were different from those of catalytic cracking naphtha and coker naphtha. Liu et al.16 investigated the secondary cracking performance of the liquid mixture (such as gasoline, diesel, and heavy oil) obtained from the catalytic pyrolysis of heavy oils. The mixture showed poor cracking performance, but no information was given on the cracking difference between gasoline and diesel fractions. Canadian oilsands bitumen, an unconventional petroleum resource, has become increasingly important; synthetic crude oil has always been a significant product of oilsands bitumen. r 2011 American Chemical Society
Heavy gas oil (HGO) derived from Canadian synthetic crude oil is a potential feed of catalytic pyrolysis. The main purpose of this research is to investigate the secondary cracking performance of gasoline and diesel fractions from the catalytic pyrolysis of HGO. This research will provide useful information on reaction mechanisms and lumping kinetics.
2. EXPERIMENTAL SECTION 2.1. Feeds and Catalyst. The gasoline and diesel fractions obtained from the catalytic pyrolysis of HGO were used as feedstocks. Catalytic pyrolysis of HGO was conducted in a pilot-scale catalytic pyrolysis unit. The properties of HGO and the introduction of the pilotscale unit were published in the literature.17 For the catalytic pyrolysis of HGO, the reaction temperature, catalyst-to-oil weight ratio, steam-to-oil weight ratio, and reaction time were 640 °C, 16.0, 0.5, and 2.04.0 s, respectively. A true boiling point distillation unit was used to separate the liquid fraction to gasoline fraction (C5200 °C) and diesel fraction (200350 °C). The main properties of gasoline and diesel fractions are listed in Table 1. The H/C atomic ratio of the gasoline fraction was only about 1.28, and the aromatics content reached up to 87.6 wt %. Meanwhile, the H/C atomic ratio of the diesel fraction was only about 1.17, while the content of aromatics reached up to 88.4 wt %. Both gasoline and diesel fractions had a low H/C atomic ratio and a high aromatics content, indicating unsatisfactory cracking performance of the feeds. In addition, the experiment involved a kind of ZSM-5 zeolite catalyst for catalytic pyrolysis of naphthenic hydrocarbons developed by China University of Petroleum.18 Its main properties are listed in Table 2. The microactivity index reached 72 and the surface area was 130 m2/g, and these indicate the good cracking ability of the catalyst. Received: March 21, 2011 Revised: June 3, 2011 Published: June 21, 2011 3382
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Table 1. Properties of the Gasoline and Diesel Fractions gasoline
diesel
density (20 °C), g/cm3
0.87
density (20 °C), g/cm3
0.99
H/C atomic ratio
1.28
H/C atomic ratio
1.17
group analysis, wt % paraffins
7.2
paraffins
3.6
olefins
4.0
naphthenes
8.0
naphthenes
1.2
single-ring naphthenes
3.1
aromatics
87.6
double-ring naphthenes
3.4
benzene
7.6
triple-ring naphthenes
1.5
toluene C8 aromatics
25.8 27.8
aromatics single-ring aromatics
88.4 27.1
C9 aromatics
13.4
double-ring aromatics
54.1
C10 aromatics
9.4
triple-ring aromatics
7.2
C11 aromatics
3.6
Table 2. Catalyst Properties property microactivity index surface area, m2/g
value 72 130
pore volume, cm3/g
0.21
packing density, g/cm3
0.88
particle size distribution, wt % 020 μm
4.8
2040 μm
17.2
4080 μm
47.8
>80 μm
30.2
into the reactor. Distilled water was pumped into a furnace to generate steam to fluidize the catalyst. The steam was mixed with the pumped feedstock at the same time as the feeding, and the mixture was heated to approximately 500 °C in a preheater before entering the reactor. Reactions took place immediately as the feed came in contact with the fluidized catalyst. The oil gas after reaction was allowed to cool down, and then the gas and liquid samples were separated using the product separation and collection system. Steam was used to strip the catalyst after the end of feeding with the stripping time of 15 min. The volume of the gas sample was measured by the salt-saturated water displacement method. The mass of the liquid sample was measured by an electronic balance. The spent catalyst was drawn out of the reactor by a vacuum pump. 2.3. Analytical and Calculation Methods. Several methods were used for the analysis. The gas sample was analyzed by an Agilent refinery gas analyzer (conform to ASTM D1945, D1946, and UOP 539 standard methods) to determine the volume percentage of the components. Data were converted to mass percentages with the state equation of ideal gases. The liquid sample was analyzed with simulated distillation gas chromatography [AC (Analytical Controls, Inc.) SIMDIS HT 750] to obtain the weight percentage of gasoline (C5200 °C), diesel (200350 °C), and heavy oil (>350 °C). The coke content on the spent catalyst was measured with a coke analyzer, which consisted of a combustion chamber, a thermal conductivity detector, and a signal transfer system. Coke was burned in an oxygen atmosphere in the combustion chamber, and carbon dioxide was detected by the thermal conductivity detector. Software was used to calculate the coke content according to the peak area of carbon dioxide and the mass of spent catalyst. The mass of gas components was calculated according to the volume of the gas sample and the weight percentage of gas components. The mass of gasoline, diesel, and heavy oil in the liquid sample was calculated according to the mass of the liquid sample and the weight percentage of the liquid products. The mass of coke was calculated according to the catalyst mass and the coke content on the spent catalyst.
3. RESULTS AND DISCUSSION
Figure 1. Diagram of the experimental setup (1, constant temperature box; 2, steam furnace; 3, feedstock; 4, electronic balance; 5, oil pump; 6, water tank; 7, water pump; 8, preheater; 9, reactor furnace; 10, thermocouple; 11, reactor; 12, inlet and outlet of catalysts; 13, filter; 14, condenser; 15, collecting bottle for liquid products; 16, gas collection vessel; 17, beaker; 18, gas sample bag).
2.2. Apparatus. Figure 1 illustrates the diagram of a confined fluidized bed reactor apparatus where the catalytic pyrolysis was conducted. The apparatus was composed of five sections, namely, an oil input mechanism, a steam input mechanism, a reaction zone, a temperature control system, and a product separation and collection system. A series of experiments was carried out in batches and the reactor had an effective volume of about 580 cm3. First, 50 g of catalyst was loaded
3.1. Cracking Performance of Diesel Fraction. The effect of reaction temperature, catalyst-to-oil weight ratio, steam-to-oil weight ratio, and weight hourly space velocity (WHSV) was investigated for the secondary cracking of the diesel fraction. Feed conversion is defined as the sum of the yields of dry gas, liquefied petroleum gas (LPG), gasoline, heavy oil, and coke. 3.1.1. Effect of Reaction Temperature. The effect of reaction temperature (600700 °C) on feed conversion and product distribution was investigated, and the catalyst-to-oil weight ratio, steam-to-oil weight ratio, and WHSV were maintained at 17, 0.6, and 9.5 h1, respectively. Table 3 shows the feed conversion and the product yields. As reaction temperature increased, feed conversion and the yields of dry gas, LPG, and coke increased, while that of gasoline varied slightly and that of heavy oil decreased. The selectivities of dry gas and coke increased with increasing reaction temperature. Meanwhile, those of LPG, gasoline, and heavy oil decreased and that of total light olefins showed a maximum at about 660 °C. Coke yield was the highest at a reaction temperature below 660 °C, and dry gas yield was the highest at a reaction temperature above 680 °C. LPG, gasoline, and heavy oil are intermediate products of catalytic pyrolysis, and they could undergo secondary reactions. Higher reaction temperature means deeper reaction extent. Therefore, their selectivities decreased with increasing reaction temperature. Dry gas and coke are end products; thus, their 3383
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Table 3. Effect of Reaction Temperature on Feed Conversion, Product Yield, and Selectivity for the Cracking of Diesel Fraction reaction temperature, °C conversion, wt % product yield, wt %
600 30.3
620 34.3
640 36.4
660 40.6
680 46.2
700 50.7
dry gas
5.0
6.3
8.1
10.0
14.4
16.7
LPG
4.5
5.2
5.1
6.0
6.1
6.1
gasoline
7.2
8.7
7.6
7.9
7.4
7.2
heavy oil
7.5
6.6
6.3
5.9
5.2
5.5
coke
6.1
7.5
9.3
10.8
13.1
15.2
dry gas LPG
16.5 14.7
18.4 15.1
22.3 14.0
24.7 14.7
31.1 13.3
33.0 11.9
gasoline
23.8
25.3
20.9
19.3
15.9
14.3
heavy oil
24.8
19.3
17.3
14.6
11.3
10.9
coke
20.2
21.9
25.5
26.7
28.4
29.9
total light olefins
19.3
20.3
20.4
22.2
22.0
21.3
product selectivity, %
Figure 2. Yield of light olefins as a function of reaction temperature for the cracking of diesel fraction.
selectivities increased with reaction temperature. The cracking of the diesel fraction produced a large amount of light products, such as dry gas, LPG, and gasoline, as well as heavy products, such as heavy oil and coke. During catalytic pyrolysis, the diesel fraction partly cracked to gas and gasoline components and partly formed heavy oil and coke by condensation. The average selectivity of cracking products (58%) was higher than that of condensation products (42%). The average selectivity of dry gas was about 24%, while that of coke was about 25%. Light olefin and light alkane yields as a function of reaction temperature are illustrated in Figures 2 and 3, respectively. As reaction temperature increased, the yields of ethene, total light olefins, hydrogen, methane, and ethane increased, while those of butene, propane, and butane varied slightly and that of propene increased below 660 °C and then varied slightly. As reaction temperature increased from 600 to 700 °C, the yields of hydrogen, methane, ethane, and ethene increased by 3.1, 4.1, 2.1, and 2.0 times, respectively. However, the yields of propene and butene increased by only 0.5 and 0.1 times, respectively. Due to the short length of the side chains of the diesel fraction, the yield of total light olefins was only below 11 wt %. The optimal reaction temperature was 660 °C.
Figure 3. Yield of hydrogen and light alkanes as a function of reaction temperature for the cracking of diesel fraction.
Table 4. Effect of Catalyst-to-Oil Weight Ratio on Feed Conversion, Product Yield, and Selectivity for the Cracking of Diesel Fraction catalyst-to-oil weight ratio conversion, wt %
8.5 36.7
12 39.3
17 40.6
25 40.8
product yield, wt % dry gas
8.0
9.3
10.0
10.9
LPG
5.1
5.7
6.0
5.9
gasoline
7.7
7.9
7.9
7.3
heavy oil
6.4
5.9
5.9
5.7
coke
9.5
10.5
10.8
11.0
selectivity of total light olefins
20.6
21.8
22.2
22.9
Feed conversion and product distribution of catalytic pyrolysis of HGO diesel fraction was different from those of HGO. Compared with the catalytic pyrolysis of HGO,19 the feed conversion, yield, and selectivity of light olefin of the catalytic pyrolysis of HGO diesel fraction were much lower, while the selectivities of dry gas and coke were much higher. This explains the poor cracking performance of the diesel fraction. This result agreed well with the literature.20,21 3.1.2. Effect of Catalyst-to-Oil Weight Ratio. The effect of the catalyst-to-oil weight ratio (8.525) on feed conversion and product distribution was investigated, keeping reaction temperature, steam-to-oil weight ratio, and WHSV constant at 660 °C, 0.6, and 9.5 h1, respectively. Table 4 lists the feed conversion and the product yields. As the catalyst-to-oil weight ratio increased, feed conversion and the yields of dry gas and coke increased. Similarly, LPG yield increased at a catalyst-to-oil weight ratio below 12 and then varied slightly. Gasoline yield varied slightly at a catalyst-to-oil weight ratio below 17 and then decreased, while heavy oil yield decreased at a catalyst-to-oil weight ratio below 12 and then varied slightly. Large catalyst-to-oil weight ratio means high average activity of catalyst and deep reaction extent, which are favorable for the formation of gas components and coke. Light olefin and light alkane yields as a function of catalyst-tooil weight ratio are illustrated in Figures 4 and 5, respectively. As catalyst-to-oil weight ratio increased, the yields of ethene, propene, total light olefins, hydrogen, and methane increased and those of butene, ethane, propane, and butane varied slightly. Large catalyst-to-oil weight ratios favor the formation of light 3384
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Figure 4. Yield of light olefins as a function of catalyst-to-oil weight ratio for the cracking of diesel fraction.
Figure 6. Yield of light olefins as a function of steam-to-oil weight ratio for the cracking of diesel fraction.
Figure 7. Yield of hydrogen and light alkanes as a function of steamto-oil weight ratio for the cracking of diesel fraction. Figure 5. Yield of hydrogen and light alkanes as a function of catalystto-oil weight ratio for the cracking of diesel fraction.
Table 5. Effect of Steam-to-Oil Weight Ratio on Feed Conversion, Product Yield, and Selectivity for the Cracking of Diesel Fraction steam-to-oil weight ratio conversion, wt %
0.3 43.0
0.6 40.5
1.0 39.9
dry gas LPG
10.3 5.9
9.8 6.0
9.6 6.3
gasoline
8.0
7.9
7.4
heavy oil
5.6
5.9
6.3
coke
13.2
10.9
10.3
selectivity of total light olefins
20.6
22.3
23.6
product yield, wt %
olefins but also increase catalyst attrition, energy consumption, and operating difficulty. The optimal catalyst-to-oil weight ratio was 17. 3.1.3. Effect of Steam-to-Oil Weight Ratio. The effect of steam-to-oil weight ratio (0.31.0) on feed conversion and product distribution was also investigated, keeping reaction temperature, catalyst-to-oil weight ratio, and WHSV constant at 660 °C, 17, and 9.5 h1, respectively. Table 5 shows the feed conversion and product yields. As steam-to-oil weight ratio increased, feed conversion and the yields of dry gas, gasoline, and coke
decreased. In contrast, the yields of LPG and heavy oil and the selectivity of total light olefins increased. Light olefin and light alkane yields as a function of steam-to-oil weight ratio are illustrated in Figures 6 and 7, respectively. As steam-to-oil weight ratio increased, the yields of ethene, propene, butene, and total light olefins increased, while those of ethane and propane decreased and those of hydrogen, methane, and butane varied slightly. Large steam-to-oil weight ratios favor the formation of light olefins, but result in low reactor efficiency and high energy consumption. The optimal steam-to-oil weight ratio was 0.6. 3.1.4. Effect of WHSV. The effect of WHSV (6.514 h1) on feed conversion and product distribution was investigated, while reaction temperature, catalyst-to-oil weight ratio, and steam-tooil weight ratio were kept constant at 660 °C, 17, and 0.6, respectively. Table 6 lists the feed conversion and product yields. As WHSV increased, feed conversion and the yields of dry gas and coke decreased. The yields of LPG and heavy oil and the selectivity of total light olefins increased, while gasoline yield varied slightly. Larger WHSV means shorter reaction time and lower reaction extent. Therefore, feed conversion and the yields of end products such as dry gas and coke decreased with increasing WHSV. LPG yield increased due to low secondary reaction extent. Light olefin and light alkane yields as a function of WHSV are illustrated in Figures 8 and 9, respectively. As WHSV increased, the yields of ethene, hydrogen, and methane decreased, while those of propene, butene, and total light olefins increased and those of ethane, propane, and butane varied slightly. The optimal WHSV was 14 h1. 3385
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Table 6. Effect of Weigh Hourly Space Velocity on Feed Conversion, Product Yield, and Selectivity for the Cracking of Diesel Fraction
Table 7. Effect of Reaction Temperature on Feed Conversion, Product Yield, and Selectivity for the Cracking of Gasoline Fraction
weigh hourly space velocity, h1 conversion, wt % product yield, wt %
6.5 42.4
9.5 40.6
14 39.8
reaction temperature, °C conversion, wt % product yield, wt %
dry gas
11.5
10.0
9.6
dry gas
6.2
7.1
LPG
5.6
6.0
6.4
LPG
2.7
2.6
gasoline
7.7
7.9
7.8
diesel
10.0
9.8
2.5
620 21.4
640 22.6
660 24.2
680 27.0
700 28.3
8.2
9.6
10.8
2.5
2.9
2.8
9.7
10.1
10.0
3.1
3.8
4.4
4.7
heavy oil
5.3
5.9
6.2
coke
coke
12.3
10.8
9.8
product selectivity, %
selectivity of total light olefins
20.8
22.2
23.6
dry gas
28.8
31.4
34.0
35.4
38.0
LPG diesel
12.7 46.8
11.5 43.4
10.5 39.9
10.7 37.5
10.0 35.4
coke
11.7
13.7
15.6
16.4
16.6
total light olefins
23.6
22.9
22.2
22.4
21.9
Figure 8. Yield of light olefins as a function of weight hourly space velocity for the cracking of the diesel fraction.
Figure 9. Yield of hydrogen and light alkanes as a function of weight hourly space velocity for the cracking of diesel fraction.
Overall, the cracking performance of the diesel fraction was poor. The optimal reaction temperature, catalyst-to-oil weight ratio, steam-to-oil weight ratio, and WHSV were 660 °C, 17, 0.6, and 14 h1, respectively. 3.2. Cracking Performance of Gasoline Fraction. Gasoline fraction from the catalytic pyrolysis of HGO contained a large amount of aromatic hydrocarbons with a short side chain; thus, the cracking performance of the gasoline fraction would be poor. The effect of reaction temperature (620700 °C) on feed conversion and product distribution was investigated, keeping catalyst-to-oil weight ratio, steam-to-oil weight ratio, and WHSV
constant at 17, 0.6, and 9.5 h1, respectively. Feed conversion in this section is defined as the sum of the yields of dry gas, LPG, diesel, and coke. Table 7 lists the feed conversion and product yields. As the reaction temperature increased, feed conversion and the yields of dry gas and coke increased, while the yields of LPG and diesel varied slightly. The conversion ranged from 21.4 to 28.3 wt %, which was much lower than the conversion of the diesel fraction. The content of aromatic hydrocarbons in the gasoline fraction (87.6 wt %) was close to that in the diesel fraction (88.4 wt %), but the aromatic hydrocarbons were mainly C7C9 aromatic hydrocarbons, with the carbon number of side chains ranging from one to three. This resulted in the low conversion of the gasoline fraction. The selectivities of dry gas and coke increased with increasing reaction temperature, but those of LPG, diesel, and total light olefins decreased. The average selectivity of dry gas reached 34%, that of diesel reached 41%, and that of total light olefins was only about 23%. During catalytic pyrolysis, the gasoline fraction partly cracked to gas components and partly formed diesel and coke by condensation. The average selectivity of condensation products (55%) was higher than that of cracking products (45%). Compared with the product distribution of the diesel fraction, the selectivity of dry gas of the gasoline fraction was much higher, while that of coke was much lower. The yields of light olefin and light alkane as a function of reaction temperature are illustrated in Figures 10 and 11, respectively. As reaction temperature increased, the yields of ethene, total light olefins, hydrogen, and methane increased, while those of propene, butene, ethane, propane, and butane varied slightly. The yield of total light olefins was below 7 wt %, which was even lower than that of the catalytic pyrolysis of the diesel fraction. The conversion and the yield of total light olefins of the gasoline fraction were lower than those of the diesel fraction. The results indicate that the cracking performance of the gasoline fraction was worse than that of the diesel fraction. The cracking performance of both gasoline and diesel fractions was much worse than that of coker naphtha and catalytic cracking naphtha.11 The primary reason is that the aromatic content in the gasoline and diesel fractions is very high. The poor cracking performance of the gasoline and diesel fractions indicates the deep reaction extent of catalytic pyrolysis of HGO. A high coke yield was obtained during secondary 3386
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Figure 10. Yield of light olefins as a function of reaction temperature for the cracking of the gasoline fraction.
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Figure 12. The CMR value as a function of reaction temperature.
Figure 13. The RM value as a function of reaction temperature. Figure 11. Yield of hydrogen and light alkanes as a function of reaction temperature for the cracking of the gasoline fraction.
cracking of the gasoline and diesel fractions, and this explains that the condensation reactions play an important role in the reaction network of catalytic pyrolysis of gas oils. This provides useful information to the study of reaction kinetics and mechanisms. 3.3. Reaction Pathway Analysis. The thermal cracking activity during the fluid catalytic cracking process is almost negligible at reaction temperatures below 500 °C, since the contact time of the feed and the catalyst is short.22 However, the reaction temperature of catalytic pyrolysis ranged from 600 to 700 °C. In addition to catalytic cracking, thermal cracking might also play a role. Light products are formed by cracking reactions, which include monomolecular and bimolecular types of cracking. In this paper, cracking mechanism ratio (CMR)23 and ratio of mechanisms (RM)13 were used to evaluate the reaction pathway. These two indexes were defined as CMR ¼
RM ¼
C1 þ C2 i-C04
i-C4 n-C4
Where, C1, C2, and i-C40 were mole concentrations of methane, ethane plus ethene, and isobutane in the gas product, while i-C4
was the yield of isobutane plus isobutene and n-C4 was the yield of n-butane plus n-butene. For the gas components, C1 (methane) and C2 (ethane and ethene) are typical products from monomolecular protolytic cracking, while isobutane is a typical product formed by βscission of branched components. A high value of CMR means a relatively high contribution of the monomolecular cracking, and a low value of CMR points to a relatively high contribution of the bimolecular cracking.23 Figure 12 illustrates the CMR value at various reactions temperatures. The CMR value increased with increasing reaction temperature, and this indicates that the relative contribution of the monomolecular cracking increases, while that of the bimolecular cracking decreases. The increasing speed of the CMR value also increased with the increase of reaction temperature, indicating that the cracking of monomolecular type becomes more and more important with increasing temperature. For both the gasoline and the diesel fraction, the CMR value was above 95, indicating that the monomolecular cracking plays a predominant role under the experimental conditions. A large amount of normal components in the gas product is formed by thermal cracking followed by the free radical mechanism, while lots of isomery components are formed by catalytic cracking followed by the carbonium ion mechanism. Therefore, a large value of RM means a high contribution of the carbonium ion mechanism, and a low value of RM means a high contribution of the free radical mechanism.13 Figure 13 illustrates the RM value at 3387
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4. CONCLUSIONS (1) The cracking performance of the diesel fraction in this study was poor, with feed conversion below 51 wt % and total light olefins yield below 11 wt %. Specifically, the selectivities of dry gas, coke, and total light olefins at 660 °C were 25%, 26%, and 22%, respectively. Total light olefins yield increased with increasing reaction temperature, catalyst-to-oil weight ratio, steam-to-oil weight ratio, and WHSV. (2) The cracking performance of the gasoline fraction in this study was poor, with feed conversion below 30 wt % and total light olefins yield below 7 wt %. The selectivities of dry gas, diesel, coke, and total light olefins at 660 °C were 34%, 40%, 16%, and 22%, respectively. (3) During catalytic pyrolysis, the diesel and gasoline fractions partly cracked to lighter components and partly condensed to heavier components. For the catalytic pyrolysis of the diesel fraction at 660 °C, the selectivities of cracking and condensation products were 58% and 42%, respectively. For the catalytic pyrolysis of the gasoline fraction at 660 °C, the selectivities of cracking and condensation products were 45% and 55%, respectively. (4) The reaction pathway was analyzed for the cracking of the gasoline and the diesel fractions. The monomolecular cracking is the predominant cracking type, and both the free radical and the carbonium ion mechanisms play an important role under the experimental conditions. ’ AUTHOR INFORMATION Corresponding Author
*Tel: 86-10-8973-3993 (office). Fax: 86-10-6972-4721. E-mail:
[email protected].
’ ACKNOWLEDGMENT Financial support was provided by the National Science Fund for Distinguished Young Scholars of China (Grant 20725620) and the Major Research Plan of the Ministry of Education of China (Grant 307008). ’ REFERENCES (1) Picciotti, M. Novel ethylene technologies developing, but steam cracking remains king. Oil Gas J. 1997, 95 (25), 53–58. (2) Jiang, G. Y.; Zhang, L.; Zhao, Z.; Zhou, X. Y.; Duan, A. J.; Xu, C. M.; Gao, J. S. Highly effective P-modified HZSM-5 catalyst for the
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dx.doi.org/10.1021/ef200427a |Energy Fuels 2011, 25, 3382–3388
