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Studies on catalytic pyrolysis of heavy oils: Reaction behaviors and mechanistic pathways
Applied Catalysis A: General 294 (2005) 168–176 www.elsevier.com/locate/apcata
Studies on catalytic pyrolysis of heavy oils: Reaction behaviors and mechanistic pathways Xianghai Meng *, Chunming Xu, Jinsen Gao, Li Li State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Received 25 February 2005; received in revised form 3 July 2005; accepted 6 July 2005
Abstract Catalytic pyrolysis of heavy oils on various catalysts was investigated in a confined fluidized bed reactor. As for catalytic pyrolysis of Chinese Daqing atmospheric residue (Daqing AR) on catalyst CEP-1, reaction temperature, residence time, weight ratios of catalyst-to-oil, steam-to-oil and feed properties have significant influence on product yields and product distribution. The optimal laboratory operating conditions are as follows: reaction temperature is within 650–680 8C, residence time within 2.0–4.0 s and catalyst-to-oil weight ratio within 13–18. The catalytic pyrolysis ability becomes better and the yields of light olefins become higher with the larger H/C mol ratio and the lower aromatic carbon content of feedstocks. After the cracking mechanisms of hydrocarbons are analyzed and the thermal pyrolysis of Daqing AR is investigated, a mechanism parameter RM is proposed to study the mechanistic pathways of heavy oil catalytic pyrolysis. As for the processes of Daqing AR catalytic pyrolysis on catalysts LCM-5 and CEP-1, the relative acting percentage of the free radical mechanism and that of the carbonium ion mechanism are obtained. # 2005 Elsevier B.V. All rights reserved. Keywords: Catalytic pyrolysis; Heavy oil; Reaction behavior; Mechanistic pathway; Ethylene; Propylene
1. Introduction Catalytic pyrolysis, a promising technology for the production of light olefins, is usually conducted at high temperature over special catalysts. Compared with conventional steam pyrolysis, catalytic pyrolysis can not only reduce reaction temperature and energy cost, but also allow one to flexibly adjust product distribution. Catalytic pyrolysis can also produce light olefins from a wide range of lower quality feedstocks, such as heavy oils. Studies of catalytic pyrolysis on heavy oil have been carried out since the 1960s, but these studies did not attract great interest until the 1980s. Up to now, many technologies of heavy oil catalytic pyrolysis have been studied and developed, together with pyrolyzing catalysts [1]. Experimental research shows that both product yields and the effects of operating conditions on product distribution vary * Corresponding author. E-mail address: [email protected] (X.H. Meng). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.07.033
greatly with both catalyst properties and feed properties, indicating that the processes of hydrocarbon catalytic pyrolysis are very complicated [2–4]. One of the most important aspects in understanding heavy oil catalytic pyrolysis is its mechanistic pathway. But unfortunately, there has not been a uniform viewpoint that could be accepted by all researchers so far. The majority of researchers think that hydrocarbon catalytic pyrolysis follows the free radical mechanism [5–7], while others consider that pyrolyzing reactions on acidic molecular sieve catalysts proceed by the carbonium ion mechanism [8,9], and still another group of researchers believes that pyrolyzing reactions on acidic molecular sieve catalysts (especially for the catalysts of catalytic pyrolysis process (CPP) technology) proceed by both the carbonium ion and the free radical mechanisms [10,11]. It is reported that the reaction mechanisms of catalytic pyrolysis vary with catalysts and technologies. Generally, catalytic pyrolysis involves catalytic cracking reactions and thermal cracking reactions, following both the carbonium
X. Meng et al. / Applied Catalysis A: General 294 (2005) 168–176
ion mechanism and the free radical mechanism. As for catalytic pyrolysis processes on metal oxide catalysts at high temperature, the free radical mechanism plays a leading role; as for those on acidic molecular sieve catalysts at low temperature, the carbonium ion mechanism plays a primary part; and as for those on zeolite molecular sieve catalysts with double acidic centers, both the free radial mechanism and the carbonium ion mechanism play important roles [1]. However, as far as a concrete catalytic pyrolysis process is concerned, the percentage contributions of the two reaction mechanisms are still unknown. In the present work, we investigate the reaction behaviors of heavy oil catalytic pyrolysis over catalyst CEP-1, and then introduce a mechanism parameter RM, which is used to study the mechanistic pathways of hydrocarbon catalytic pyrolysis.
169
Table 2 Properties of pyrolysis catalysts Catalysts CEP-1
LCM-5
AKZO
Micro-activity index Pore volume (cm3/g) Surface area (m2/g) Packing density (g/cm3) Particle density (g/cm3)
70 0.19 80 0.97 1.5
3 0.11 38 1.20 –
57 0.092 76.61 0.91 –
Particle size distribution (wt%) 0–20 (mm) 20–40 (mm) 40–80 (mm) >80 (mm)
1.2 13.4 55.9 29.5
2.0 18.8 56.1 24.1
1.4 10.6 49 39
a furnace to form steam, and then mixed with the feedstock pumped simultaneously by another pump at the outlet of a constant temperature box. The mixture was heated to approximately 500 8C in a pre-heater, and then entered into the reactor with an effective volume of about 580 ml.
2. Experimental 2.1. Feedstocks and catalysts In this study, Chinese Daqing AR, Chinese Daqing vacuum gas oil (Daqing VGO), Chinese Daqing vacuum residue (Daqing VR) and Chinese Huabei atmospheric residue (Huabei AR) were used as feedstocks; the main properties of each type are given in Table 1. The catalysts were CEP-1 (used for CPP technology), LCM-5 (used for heavy-oil contact cracking (HCC) technology) and AKZO (used for FCC technology), and their primary properties are listed in Table 2. 2.2. Apparatus In experiments of heavy oil catalytic pyrolysis, a confined fluidized bed reactor was used; the schematic diagram can be seen in references [12,13]. It is comprised of five sections: oil and steam input mechanisms, a reaction zone, temperature control system and a product separation and collection system. A variable amount of distilled water was pumped into
2.3. Operating conditions The operating conditions for the main catalytic pyrolysis tests are summarized in Table 3. 2.4. Analytical methods Catalytic pyrolysis products include pyrolysis gas, pyrolysis liquid and coke. An Agilent 6890 gas chromatograph with Chem Station software was used to measure the volume percentage of components in pyrolysis gas. The equation of state for ideal gases converts the data to mass percentages. The pyrolysis liquid was analyzed with a simulated distillation gas chromatogram to get the weight percentage of gasoline, diesel oil and heavy oil. Coke content on catalysts was measured with a coke analyzer.
3. Reaction behaviors of heavy oil catalytic pyrolysis Table 1 Properties of pyrolysis feedstocks Feedstocks
Density (20 8C) g/cm3 Viscosity (100 8C) mm2/s Carbon residue (wt%) Molecular weight Hydrogen (wt%) Carbon (wt%) H/C mol ratio Aromatic carbon (wt%) Group analysis (wt%) Saturates Aromatics Resin and asphaltene
Daqing AR
Daqing VR
Daqing VGO
Huabei AR
0.9069 28.9 4.3 577 13.11 86.52 1.82 10.90
0.9221 106 8.8 895 12.78 86.93 1.76 13.76
0.8011 7.2 0.05 426 13.58 86.36 1.89 6.84
0.9162 43.3 8.9 608 12.87 86.51 1.79 13.00
57.08 27.61 15.31
42.91 34.53 22.56
85.39 11.98 2.63
56.80 22.65 20.55
For heavy oil catalytic pyrolysis on CEP-1, the influences of operating conditions and feed properties on product yields and product distribution were investigated. Table 3 Operating conditions for main pyrolysis tests Item
Value
Temperature of reactor (8C) Temperature of steam furnace (8C) Temperature of pre-heater (8C) Water inflow (ml/min) Oil inflow (g/min) Catalyst loaded (g) Residence time (s) Catalyst-to-oil weight ratio Steam-to-oil weight ratio
600–716 300 500 1.0–6.0 2–10 20–80 1.5–4.5 6–27 0.2–1.6
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Table 4 Effect of reaction temperature on product distribution Reaction temperature (8C) 600
620
640
660
680
700
716
Yields of products (wt%) Dry gas LPG Gasoline Diesel oil Heavy oil Coke Feed conversion (%)
12.77 45.69 24.24 5.71 2.56 9.02 97.44
16.07 46.55 20.04 5.85 2.00 9.49 98.00
19.64 45.78 18.27 4.90 1.79 9.62 98.21
23.06 43.17 16.86 4.31 1.82 10.78 98.18
27.68 39.28 15.10 4.58 1.76 11.60 98.24
32.37 33.93 15.49 4.64 1.53 12.04 98.47
35.61 30.10 14.13 4.24 1.49 14.42 98.51
Yields of light olefins (wt%) Ethylene Propylene Butylene Total light olefins Selectivity (%)
8.65 18.15 10.81 37.61 38.60
10.76 21.29 11.16 43.21 44.09
12.42 22.50 11.23 46.14 46.98
13.75 22.58 10.65 46.98 47.86
15.64 22.08 9.65 47.37 48.22
17.32 19.91 8.24 45.47 46.18
18.25 17.95 6.79 42.99 43.64
3.1. Reaction behaviors of Daqing AR on CEP-1 The effect of reaction temperature, residence time, weight ratios of catalyst-to-oil and steam-to-oil on product distribution as well as light olefin yields was studied. The optimal laboratory operating conditions were obtained. 3.1.1. Effect of reaction temperature The effectiveness of reaction temperature was studied on feed conversion, selectivity of total light olefins and product distribution in the reaction temperature range of 600– 716 8C. The residence time (2.3 s), weight ratio of catalystto-oil (13.5) and steam-to-oil (0.70) was held constant. The experimental data is listed in Table 4. In this study, feed conversion is defined as the sum of the yields of dry gas, liquefied petroleum gas (LPG), gasoline, diesel oil and coke. The selectivity of light olefins is defined as the mass of light olefins formed per unit mass of feedstock reacted, that is, the ratio of yield of light olefins to feed conversion. In experimental tests, the feed conversion is above 97%, and goes up slightly with increasing reaction temperature. This shows that the catalytic pyrolysis ability of catalyst CEP-1 is very good. As the temperature goes up, the yields of dry gas and coke increase, while those of LPG, gasoline and diesel oil decrease. It is known that high reaction temperature means deep pyrolysis extent. Since dry gas and coke are end products of catalytic pyrolysis, their yields will increase with the enhancement of temperature. However, LPG, gasoline and diesel oil belong to intermediate products, and their yields will decrease after reaching the highest yields because of secondary reactions. According to the experimental data, ethylene yield increases monotonously with the increase of reaction temperature. The yields of propylene, butylene and overall light olefins pass through maxima in the range of 640–700 8C, and so does the selectivity of total light olefins. The yield of
total light olefins can reach 47.37 wt%, with yields of ethylene at 15.64 wt%, propylene at 22.08 wt% and butylene at 9.65 wt%. In practice, the catalytic pyrolysis process involves catalytic cracking reactions on catalyst surfaces and thermal cracking reactions both on catalyst surfaces and in the interspaces between catalyst particles. Propylene and butylene are mainly generated from catalytic cracking reactions following the carbonium ion mechanism, and they are intermediate products, which can undergo such secondary reactions as hydrogen transfer, aromatisation, cracking and polyreaction. Ethylene is primarily formed from thermal cracking reactions following the free radical mechanism, and it is close to an end product, so it is unlikely to undergo secondary reactions. As reaction temperature goes up, the reaction rates of both catalytic cracking and thermal cracking increase, causing pyrolysis to be more thorough, and therefore, the yields of light products increase. With further increase of temperature, the proportion of catalytic cracking reactions decreases and that of thermal cracking reactions increases. The ethylene yield then approaches the propylene yield, exceeding the propylene yield at a temperature above 716 8C. Since the increase of reaction temperature can accelerate the secondary reactions of intermediate products, the yields of propylene and butylene will consequently show maxima as the temperature goes up. For producers to achieve high yields of light olefins with low energy cost, the appropriate reaction temperature is in the range of 650–680 8C. Under such conditions, propylene yield is much higher than ethylene yield, and the butylene yield is lower than the ethylene yield. 3.1.2. Effect of residence time Here, residence time refers to the average time of oil vapor passing through the reactor. Keeping reaction temperature (650 8C), the weight ratios of catalyst-to-oil
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Table 5 Effect of residence time on product distribution Residence time (s) 1.59
1.79
1.92
2.04
2.53
3.46
4.39
Yields of products (wt%) Dry gas LPG Gasoline Diesel oil Heavy oil Coke Feed conversion (%)
20.51 45.53 15.97 3.47 1.32 13.19 98.68
21.38 45.57 15.39 3.39 1.17 13.10 98.83
20.89 44.10 16.84 3.45 1.50 13.22 98.50
21.47 45.37 16.50 3.55 1.57 11.54 98.43
22.65 43.76 16.46 3.89 1.18 12.05 98.82
24.24 41.93 16.94 3.88 1.33 11.68 98.67
23.60 41.52 16.85 3.99 1.57 12.47 98.43
Yields of light olefins (wt%) Ethylene Propylene Butylene Total light olefins Selectivity (%)
12.44 21.26 10.05 43.76 44.35
12.97 22.30 11.08 46.34 46.89
12.89 22.93 11.53 47.35 48.07
13.03 24.08 12.48 49.59 50.38
13.66 24.56 12.32 50.54 51.15
14.26 24.50 11.71 50.48 51.16
13.98 24.23 12.01 50.22 51.07
(17.6) and steam-to-oil (0.58) fixed, the effect of residence time on product distribution in the range of 1.5–4.5 s was studied, with the experimental results given in Table 5. Feed conversion is about 98.5% and remains relatively constant with residence time. As residence time prolongs, the yields of dry gas, gasoline and diesel oil increase slightly, while LPG yield decreases; meanwhile, the yields of light olefins first go up until a residence time of 2.0 s is reached, and then yields of light olefins remain relatively constant. The selectivity of overall light olefins shows the similar variation laws of the yield of total light olefins. A longer residence time indicates that there is more time for catalyticpyrolysisofhydrocarbons,andthereforethepyrolysis extent is more thorough. At our experimental conditions, the reaction rates of heavy oil catalytic pyrolysis are very fast. There is a large amount of aromatics in such intermediate products as gasoline and diesel oil, explaining why the secondary pyrolysis ability of these intermediate products is very poor. However, LPG can undergo secondary cracking reactions to produce dry gas, and can undergo polyreaction and aromatization to form gasoline and diesel oil. Accordingly, the yield of LPG decreases as residence time prolongs, while the yields of dry gas, gasoline and diesel oil increase. Prolonging residence time from 1.92 to 4.39 s, it is seen that the decreased value of LPG yield is almost equal to the increased one of the total yield of dry gas, gasoline and diesel oil. In addition, the increased value of gasoline yield is bigger than that of dry gas yield. The evidence points to the conclusion that the secondary reactions of LPG are mainly polyreaction and aromatization. With the general consideration of the influence of residence time on product yield and distribution, we obtain the optimal residence time range, 2.0–4.0 s. 3.1.3. Effect of catalyst-to-oil weight ratio The effect of catalyst-to-oil weight ratio was investigated in the range of 6–27 via changing the mass of catalyst loaded into the reactor. Reaction conditions were reaction
temperature of 650 8C, residence time of 2.6 s and steamto-oil weight ratio of 0.83. The yields of pyrolysis products, feed conversion and selectivity of total light olefins are found in Table 6. As the catalyst-to-oil weight ratio goes up, the feed conversion and the yields of dry gas and coke increase, gasoline and diesel oil decrease, while that of LPG shows a maximum at about 17. In experimental runs, the yields of light olefins vary a little with increasing catalyst-to-oil weight ratio, the ethylene yield goes up slightly, the butylene yield decreases slightly, the yields of propylene and total light olefins pass through maxima, and the selectivity of overall light olefins reaches its highest value of 49.42% at about 13. The value of the catalyst-to-oil weight ratio not only denotes the contact condition between oil vapor and catalysts, but also indicates the average activation of catalysts. As the catalyst-to-oil weight ratio increases, the contact opportunities between oil vapor and active centers also increase; however, the ratio of active centers contacted with oil gas to the overall active centers will decline. Correspondingly, less active centers on the surface per unit catalyst will be covered by coke. Large catalyst-to-oil weight ratio means that much energy can be transferred in the reaction process, which can accelerate thermal cracking reactions. To a certain extent, a high catalyst-to-oil weight ratio means a thorough pyrolysis. With the enhancement of the catalyst-to-oil weight ratio, the yields of gasoline and diesel oil decrease owing to secondary pyrolysis reactions; those of dry gas and coke increase, for they are end pyrolysis products; meanwhile, that of LPG reaches its highest mass at 40.07 wt% at a catalyst-to-oil weight ratio of 17.61, and then decreases because of secondary reactions. Enhancing the catalyst-to-oil weight ratio can increase the mean activation of catalysts and can increase pyrolysis, which can enhance the yields of light products to an extent. Also secondary reactions of light olefins will take place if
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Table 6 Effect of catalyst-to-oil weight ratio on product distribution Catalyst-to-oil weight ratio 6.35
9.68
13.29
17.61
21.96
26.58
Yields of products (wt%) Dry gas LPG Gasoline Diesel oil Heavy oil Coke Feed conversion (%)
27.14 38.47 20.05 5.48 2.03 6.83 97.97
28.47 38.83 16.18 4.88 2.09 9.55 97.91
29.36 39.26 14.85 3.54 1.28 11.71 98.72
29.34 40.07 14.92 2.85 0.86 11.97 99.14
30.01 39.44 13.48 2.51 0.72 13.83 99.28
32.03 37.14 12.65 2.11 0.62 15.44 99.38
Yields of light olefins (wt%) Ethylene Propylene Butylene Total light olefins Selectivity (%)
13.73 21.73 11.80 47.27 48.25
14.69 22.57 11.11 48.38 49.41
14.81 22.74 11.23 48.78 49.42
14.47 22.90 11.50 48.87 49.29
14.43 22.56 10.31 47.30 47.65
14.62 21.18 9.43 45.23 45.52
catalysts to maintain high average activation. These factors show that a large steam-to-oil weight ratio is beneficial to the production of light olefins. The steam-to-oil weight ratio cannot be enhanced unlimitedly, and it is restricted by the disposal capacity of units and the economic benefit. Analysis of the data shows that reaction temperature is the most important parameter among the operating conditions. From this investigation, the optimal operating conditions for Daqing AR catalytic pyrolysis on CEP-1 are as follows, reaction temperature within 650–680 8C, residence time within 2.0–4.0 s, catalyst-to-oil weight ratio within 13–18, and a large steam-to-oil weight ratio is good with the suitable allowance for the unit capacity and the economic benefit.
pyrolysis is too thorough. In addition, a higher catalyst-to-oil weight ratio will add production costs greatly. Therefore, the value of the catalyst-to-oil weight ratio cannot be too high, and it would be suitable in the range of 13–18. 3.1.4. Effect of steam-to-oil weight ratio Using a reaction temperature of 650 8C, residence time of 2.7 s and catalyst-to-oil weight ratio of 15.5, we investigated the effectiveness of steam-to-oil weight ratio in the range of 0.2–1.6 on product yields and product distribution. The experimental data is given in Table 7. As the steam-to-oil weight ratio goes up, the yields of dry gas, ethylene, propylene, butylene and overall light olefins increase monotonously to different extents; yields of gasoline and diesel oil decrease; meanwhile, the yield of LPG and the feed conversion change very slightly. Increasing the steam-to-oil weight ratio can reduce the partial pressure of oil vapor, which favors the cracking of hydrocarbons into low molecular weight products. Steam can restrain coking reactions on catalyst surface, which is good for
3.2. Reaction behaviors of different feedstocks on CEP-1 3.2.1. Effect of feed properties on product distribution The catalytic pyrolysis of four kinds of heavy oils over CEP-1 was investigated using reaction temperature of
Table 7 Effect of steam-to-oil weight ratio on product distribution Steam-to-oil weight ratio 0.21
0.39
0.66
0.84
1.08
1.23
1.58
Yields of products (wt%) Dry gas LPG Gasoline Diesel oil Heavy oil Coke Feed conversion (%)
21.45 44.24 18.27 3.87 1.73 10.44 98.27
21.83 43.09 18.42 4.31 1.90 10.44 98.10
22.74 43.28 17.71 3.52 1.16 11.59 98.84
22.71 43.57 17.60 3.54 1.30 11.29 98.70
23.79 44.84 16.46 3.70 1.18 10.03 98.82
23.94 44.71 16.31 3.93 1.25 9.87 98.75
24.68 44.95 16.10 3.15 1.30 9.83 98.70
Yields of light olefins (wt%) Ethylene Propylene Butylene Total light olefins Selectivity (%)
12.45 22.60 11.81 46.87 47.69
12.98 22.95 11.76 47.69 48.62
13.72 24.04 11.92 49.67 50.26
13.87 23.96 11.79 49.62 50.27
14.93 26.11 11.81 52.86 53.49
15.17 26.70 11.94 53.82 54.49
15.63 27.04 12.33 55.01 55.73
X. Meng et al. / Applied Catalysis A: General 294 (2005) 168–176
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Table 8 Effect of feed properties on product distribution Feedstocks
Daqing VGO
H/C mol ratio Aromatic carbon (wt%) Yields of products (wt%) Dry gas LPG Gasoline Diesel oil Heavy oil Coke Feed conversion (%)
Daqing AR
Huabei AR
Daqing VR
1.89 6.84
1.82 10.90
1.79 13.00
1.76 13.76
26.31 41.68 16.28 3.53 0.99 11.22 99.01
24.06 42.17 16.86 4.31 0.82 11.78 98.18
28.22 37.13 14.92 4.51 0.88 14.33 99.12
27.36 35.18 16.51 4.61 1.56 14.78 98.44
Yields of light olefins (wt%) Ethylene 13.53 Propylene 22.60 Butylene 11.94 Total light olefins 48.07 Selectivity (%) 48.55
Fig. 2. Propylene yield vs. reaction temperature. 13.75 22.58 10.65 46.98 47.86
12.21 19.27 10.43 41.92 42.29
12.14 19.93 8.41 40.48 41.12
660 8C, residence time of 2.2 s, catalyst-to-oil weight ratio of 15.5 and steam-to-oil weight ratio of 0.75. The experimental results are listed in Table 8. Feed conversion of the four kinds of heavy oils is very high, above 98%. With the increase of H/C mol ratio and the decrease of aromatic carbon content, the yields of dry gas, diesel oil and coke show an increasing trend, while the yields of LPG and light olefins together with the selectivity of overall light olefins show a decreasing trend. 3.2.2. Effect of feed properties on the yields of light olefins Keeping residence time, weight ratios of catalyst-to-oil and steam-to-oil at 2.2 s, 15.5 and 0.75, respectively, the variation laws of the yields of light olefins with feed properties at 600, 630, 660 and 700 8C were investigated. The results are presented in Figs. 1–3. For any kind of feedstock, ethylene yield goes up with increasing temperature; meanwhile, the yields of propylene and total light olefins show maxima, which is similar to the reaction behaviors of Daqing AR catalytic pyrolysis. When
Fig. 1. Ethylene yield vs. reaction temperature.
Daqing VGO and Daqing AR are used as feedstocks, the yields of propylene and total light olefin are much higher than when feedstocks of Huabei AR and Daqing VR are used, this is especially true at high temperature. Nevertheless, the difference of ethylene yields for different kinds of feedstocks is relatively small. The yields of light olefins go up with the increase of H/C mol ratio and the decrease of aromatic carbon content. H/C mol ratio and aromatic carbon content can be used to estimate the catalytic pyrolysis ability of feedstocks. The higher the H/C mol ratio and the lower the aromatic carbon content, the better the catalytic pyrolysis ability of heavy oils and the higher the yields of light olefins will be.
4. Mechanistic pathways of heavy oil catalytic pyrolysis 4.1. Analytical means of mechanistic pathway of heavy oil catalytic pyrolysis Following the carbonium ion mechanism, there is much iso-butane and iso-butylene, but less 1-butylene and 1.3butadiene in the pyrolysis gas. Following the free radical mechanism, there is much hydrogen, methane and C2
Fig. 3. Yield of total light olefins vs. reaction temperature.
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hydrocarbons, as well as 1-butylene and 1.3-butadiene, but less iso-butane and iso-butylene in the pyrolysis gas. There is a large difference between product distributions when the process follows different mechanistic pathways. The mechanistic pathway of heavy oil catalytic pyrolysis can be estimated according to the product distribution. In order to study the mechanistic pathways of heavy oil catalytic pyrolysis, experiments were conducted on quartz grain (74–150 mm), LCM-5, CEP-1 and AKZO, using Daqing AR as feed. Quartz grain seldom shows catalytic activation, and only operates as a heat carrier, consequently only thermal cracking reactions following the free radical mechanism take place. As for catalyst AKZO, thermal cracking reactions rarely occur at low temperature (500 8C), so it can be considered that only catalytic cracking reactions take place at low temperature following the carbonium ion mechanism. 4.2. Product distribution of Daqing AR thermal pyrolysis To study the mechanistic pathways of heavy oil catalytic pyrolysis, one important task is to investigate thermal pyrolysis reactions and the corresponding product distribution under the same conditions used with catalytic pyrolysis of heavy oils. With quartz grain used as the carrier of fluidization and heat transfer, thermal pyrolysis of Daqing AR was carried out at 600, 630, 660 and 700 8C, keeping residence time, and the weight ratios of steam-to-oil and quartz grain-to-oil constant at 3.0 s, 0.80 and 16, respectively. The yields of light products in pyrolysis gas are listed in Table 9. As the reaction temperature goes up, the butylene yield shows maximum at 660 8C and the butane yield changes slightly, while the yields of other light products increase monotonously, indicating that the reaction extent of Daqing AR thermal pyrolysis is increasing. As the reaction temperature increases from 600 to 700 8C, the yields of dry gas (260%), methane (480%), ethylene (250%) and Table 9 Yields of light products in pyrolysis gas (wt%) Reaction temperature (8C) 600
630
660
700
Dry gas LPG
10.29 9.53
19.06 15.64
27.73 21.85
36.49 23.13
Yields of light olefins Ethylene Propylene Butylene Total light olefins
5.58 4.94 4.12 14.64
10.94 8.51 6.42 25.87
15.90 12.56 8.41 36.87
19.61 14.08 8.16 41.84
Yields of light alkanes Hydrogen Methane Ethane Propane Butane Total light alkanes
0.18 1.72 1.13 0.27 0.20 3.50
0.22 4.22 2.70 0.50 0.21 7.85
0.28 6.78 3.99 0.68 0.21 11.94
0.43 10.02 4.60 0.71 0.19 15.95
Fig. 4. Yield of total light olefins at catalytic pyrolysis and thermal pyrolysis.
ethane (300%) increase. Yields of LPG, propylene and propane increase by 143%, 190% and 160%, respectively. Compared with catalytic pyrolysis, the propylene yield of thermal pyrolysis is lower, while the ethylene yield is higher. The yield of total light olefins reaches 36 wt% at 660 8C and 41 wt% at 700 8C, which is very close to that of catalytic pyrolysis (as shown in Fig. 4). Such results indicate that the reaction extent of Daqing AR thermal pyrolysis in experimental runs is high, especially when the reaction temperature is above 660 8C. 4.3. Mechanistic pathway of heavy oil catalytic pyrolysis According to the mechanism analysis, for the distribution of C4 hydrocarbons in pyrolysis gas of heavy oil catalytic pyrolysis, many hydrocarbon isomers indicate that the reactions primarily follow the carbonium ion mechanism, but many straight chain hydrocarbons means that the reactions mainly follow the free radical mechanism. Therefore, the ratio (RM) of i-C4 yield to n-C4 yield can explain the relative percentages of the two mechanisms acting in catalytic pyrolysis processes. RM is a mechanism parameter of catalytic pyrolysis. A larger value of RM indicates that the carbonium ion mechanism plays a bigger function in catalytic pyrolysis processes. A smaller RM value indicates that the free radical mechanism operates predominantly. Experiments of Daqing AR catalytic pyrolysis on quartz grain, LCM-5, CEP-1 and AKZO were conducted and the RM value was calculated, as listed in Table 10. As for the Table 10 The RM value Reaction temperature (8C) 500 550 600 630 660 700
Quartz grain
AKZO
LCM-5
CEP-1
0.2153 0.1701 0.1587 0.1380
1.8740 1.3003 0.9425 0.8325 0.7251 0.6938
0.3321 0.2715 0.2299 0.2027
1.0177 0.8563 0.7826 0.6359
X. Meng et al. / Applied Catalysis A: General 294 (2005) 168–176 Table 11 Relative percentages of reaction mechanisms Reaction temperature (8C)
600 630 660 700
LCM-5 (%)
CEP-1 (%)
Free radical
Carbonium ion
Free radical
Carbonium ion
92.96 94.05 95.85 96.27
7.04 5.95 4.15 3.73
51.63 59.73 63.63 71.32
48.37 40.27 36.37 28.68
carbonium ion mechanism, the standard RM value can be calculated in terms of the data of catalytic cracking on AKZO at 500 8C. And for the free radical mechanism, the standard RM value varies with reaction temperature, and the value at different temperatures can be gained through the data of thermal pyrolysis on quartz grain. According to the standard RM value, together with the calculated RM value of Daqing AR catalytic pyrolysis on LCM-5 and CEP-1, the percentages of the two reaction mechanisms in the catalytic pyrolysis processes on catalysts LCM-5 and CEP-1 at different temperatures are available. The data are given in Table 11. The RM value differs greatly for different kinds of catalysts. For most catalysts, the RM value decreases as the reaction temperature goes up, indicating that the effect of the free radical mechanism in catalytic pyrolysis processes becomes more significant, and that of the carbonium ion mechanism becomes insignificant. In experimental tests, the RM value of Daqing AR catalytic pyrolysis on LCM-5 is very small, and the relative acting percentage of the free radical mechanism is above 90%, showing that the free radical mechanism plays a leading role in the process. Nevertheless, as for catalyst CEP-1, as reaction temperature goes up from 600 to 700 8C, the RM value decreases, and the relative acting percentage of the free radical mechanism increases from 51.63% to 71.32%, while that of the carbonium ion mechanism decreases from 48.37% to 28.68%, indicating that both the reaction mechanisms play important parts in the process. LCM-5 is a metal oxide pyrolysis catalyst, and there are rarely proton acidic centers on its surface for the microactivity index is only 3. Carbonium ion reactions seldom take place in this catalytic pyrolysis process with LCM-5. The data in Tables 10 and 11 further prove that it is primarily the free radical reactions that have occurred on catalyst LCM-5. However, CEP-1 is a kind of molecule sieve catalyst, and there are not only proton acidic centers on catalyst surfaces, but also non-proton acidic centers, which can promote the reactions of both carbonium ions and free radicals. As for catalytic pyrolysis on CEP-1, the free radical mechanism plays the most important part, and the carbonium ion mechanism also plays a significant role. On the basis of the above research, we can concluded that, in catalytic pyrolysis processes, the free radical mechanism will play a primary role if the RM value is
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less than 0.5, the carbonium ion mechanism will play a leading role if the RM value is above 1.5, and both the two mechanisms will play a significant part if the RM value is in the range of 0.5–1.5.
5. Conclusions The experiments of heavy oil catalytic pyrolysis were carried out in a confined fluidized bed reactor. The results show that catalyst CEP-1 is good at converting heavy hydrocarbons to light olefins, and that the catalytic pyrolysis of Daqing VGO and Daqing AR is better than that of Huabei AR and Daqing VR. For catalytic pyrolysis of Daqing AR on CEP-1, such operating conditions as reaction temperature, residence time, weight ratios of catalyst-to-oil and steam-to-oil have great effect on product distribution. The yield and the selectivity of total light olefins show maxima with the increase of reaction temperature; residence time and the catalyst-to-oil weight ratio, while, the yield goes up monotonously with increase of steam-to-oil weight ratio. Among the operating conditions, reaction temperature is the most important parameter. The optimal operating conditions are as follows: reaction temperature is within 650–680 8C, residence time is within 2.0–4.0 s, and the catalyst-to-oil weight ratio is within 13–18, while a large steam-to-oil weight ratio is good with the suitable allowance for the handling capacity and the economic benefit. Feed properties also have significant influence on catalytic pyrolysis of heavy oils. The catalytic pyrolysis ability becomes better and the yields of light olefins become higher with the higher H/C mol ratio and the lower aromatic carbon content of heavy oils. At high temperature (above 660 8C), Daqing AR shows good thermal pyrolysis ability. Compared with catalytic pyrolysis on CEP-1, the propylene yield of thermal pyrolysis is lower, but the ethylene yield is higher, and the yield of total light olefins is close to that of catalytic pyrolysis on CEP-1. Product distribution of Daqing AR catalytic pyrolysis on different kinds of catalysts differs greatly. On the basis of the mechanism analysis of hydrocarbon cracking, a mechanism parameter RM is proposed to study the mechanistic pathways of heavy oil catalytic pyrolysis. Research results show that, for the process of heavy oil catalytic pyrolysis on catalyst CEP-1, both the free radical mechanism and the carbonium ion mechanism play important parts, but only the free radical mechanism plays a leading role in the process of heavy oil catalytic pyrolysis on catalyst LCM-5.
References [1] X.H. Meng, J.S. Gao, C.M. Xu, Petrol. Sci. Technol. 22 (9–10) (2004) 1327.
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[2] C.G. Xie, Petrochem. Technol. 26 (12) (1997) 825. [3] Y.X. Sha, Z.Q. Cui, M.D. Wang, G.L. Wang, J. Zhang, Petrochem. Technol. 28 (9) (1999) 618. [4] Z.G. Zhang, C.G. Xie, Z.C. Shi, Y.M. Wang, Petrol. Process. Petrochem. 32 (5) (2001) 21. [5] V.I. Erofeev, L.V. Adyaeva, Y.V. Ryabov, Russ. J. Appl. Chem. 74 (2) (2001) 235. [6] Z.L. Zhang, S.X. Zhou, H.P. Wang, W.Z. Lin, J. Univ. Petrol. Chin. 22 (1) (1998) 60.
[7] A.A. Lemonidou, I.A. Vasalos, E.J. Hirschberg, Ind. Eng. Chem. Res. 28 (1989) 524. [8] Z.T. Li, Chin. Eng. Sci. 1 (2) (1999) 67. [9] X.Q. Wang, F.K. Jiang, Petrol. Process. Petrochem. 25 (1) (1994) 1. [10] C.G. Xie, Petrol. Process. Petrochem. 31 (7) (2000) 40. [11] C.G. Xie, R.N. Pan, Process. Petrochem. 25 (6) (1994) 30. [12] X.H.Meng,C.M.Xu,L.Li,J.S.Gao,Ind.Eng.Chem.Res.42(2003)6012. [13] X.H. Meng, C.M. Xu, Q. Zhang, J.S. Gao, Chin. J. Chem. Eng. 12 (1) (2004) 152.
Studies on catalytic pyrolysis of heavy oils: Reaction behaviors and mechanistic pathways Xianghai Meng *, Chunming Xu, Jinsen Gao, Li Li State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Received 25 February 2005; received in revised form 3 July 2005; accepted 6 July 2005
Abstract Catalytic pyrolysis of heavy oils on various catalysts was investigated in a confined fluidized bed reactor. As for catalytic pyrolysis of Chinese Daqing atmospheric residue (Daqing AR) on catalyst CEP-1, reaction temperature, residence time, weight ratios of catalyst-to-oil, steam-to-oil and feed properties have significant influence on product yields and product distribution. The optimal laboratory operating conditions are as follows: reaction temperature is within 650–680 8C, residence time within 2.0–4.0 s and catalyst-to-oil weight ratio within 13–18. The catalytic pyrolysis ability becomes better and the yields of light olefins become higher with the larger H/C mol ratio and the lower aromatic carbon content of feedstocks. After the cracking mechanisms of hydrocarbons are analyzed and the thermal pyrolysis of Daqing AR is investigated, a mechanism parameter RM is proposed to study the mechanistic pathways of heavy oil catalytic pyrolysis. As for the processes of Daqing AR catalytic pyrolysis on catalysts LCM-5 and CEP-1, the relative acting percentage of the free radical mechanism and that of the carbonium ion mechanism are obtained. # 2005 Elsevier B.V. All rights reserved. Keywords: Catalytic pyrolysis; Heavy oil; Reaction behavior; Mechanistic pathway; Ethylene; Propylene
1. Introduction Catalytic pyrolysis, a promising technology for the production of light olefins, is usually conducted at high temperature over special catalysts. Compared with conventional steam pyrolysis, catalytic pyrolysis can not only reduce reaction temperature and energy cost, but also allow one to flexibly adjust product distribution. Catalytic pyrolysis can also produce light olefins from a wide range of lower quality feedstocks, such as heavy oils. Studies of catalytic pyrolysis on heavy oil have been carried out since the 1960s, but these studies did not attract great interest until the 1980s. Up to now, many technologies of heavy oil catalytic pyrolysis have been studied and developed, together with pyrolyzing catalysts [1]. Experimental research shows that both product yields and the effects of operating conditions on product distribution vary * Corresponding author. E-mail address: [email protected] (X.H. Meng). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.07.033
greatly with both catalyst properties and feed properties, indicating that the processes of hydrocarbon catalytic pyrolysis are very complicated [2–4]. One of the most important aspects in understanding heavy oil catalytic pyrolysis is its mechanistic pathway. But unfortunately, there has not been a uniform viewpoint that could be accepted by all researchers so far. The majority of researchers think that hydrocarbon catalytic pyrolysis follows the free radical mechanism [5–7], while others consider that pyrolyzing reactions on acidic molecular sieve catalysts proceed by the carbonium ion mechanism [8,9], and still another group of researchers believes that pyrolyzing reactions on acidic molecular sieve catalysts (especially for the catalysts of catalytic pyrolysis process (CPP) technology) proceed by both the carbonium ion and the free radical mechanisms [10,11]. It is reported that the reaction mechanisms of catalytic pyrolysis vary with catalysts and technologies. Generally, catalytic pyrolysis involves catalytic cracking reactions and thermal cracking reactions, following both the carbonium
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ion mechanism and the free radical mechanism. As for catalytic pyrolysis processes on metal oxide catalysts at high temperature, the free radical mechanism plays a leading role; as for those on acidic molecular sieve catalysts at low temperature, the carbonium ion mechanism plays a primary part; and as for those on zeolite molecular sieve catalysts with double acidic centers, both the free radial mechanism and the carbonium ion mechanism play important roles [1]. However, as far as a concrete catalytic pyrolysis process is concerned, the percentage contributions of the two reaction mechanisms are still unknown. In the present work, we investigate the reaction behaviors of heavy oil catalytic pyrolysis over catalyst CEP-1, and then introduce a mechanism parameter RM, which is used to study the mechanistic pathways of hydrocarbon catalytic pyrolysis.
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Table 2 Properties of pyrolysis catalysts Catalysts CEP-1
LCM-5
AKZO
Micro-activity index Pore volume (cm3/g) Surface area (m2/g) Packing density (g/cm3) Particle density (g/cm3)
70 0.19 80 0.97 1.5
3 0.11 38 1.20 –
57 0.092 76.61 0.91 –
Particle size distribution (wt%) 0–20 (mm) 20–40 (mm) 40–80 (mm) >80 (mm)
1.2 13.4 55.9 29.5
2.0 18.8 56.1 24.1
1.4 10.6 49 39
a furnace to form steam, and then mixed with the feedstock pumped simultaneously by another pump at the outlet of a constant temperature box. The mixture was heated to approximately 500 8C in a pre-heater, and then entered into the reactor with an effective volume of about 580 ml.
2. Experimental 2.1. Feedstocks and catalysts In this study, Chinese Daqing AR, Chinese Daqing vacuum gas oil (Daqing VGO), Chinese Daqing vacuum residue (Daqing VR) and Chinese Huabei atmospheric residue (Huabei AR) were used as feedstocks; the main properties of each type are given in Table 1. The catalysts were CEP-1 (used for CPP technology), LCM-5 (used for heavy-oil contact cracking (HCC) technology) and AKZO (used for FCC technology), and their primary properties are listed in Table 2. 2.2. Apparatus In experiments of heavy oil catalytic pyrolysis, a confined fluidized bed reactor was used; the schematic diagram can be seen in references [12,13]. It is comprised of five sections: oil and steam input mechanisms, a reaction zone, temperature control system and a product separation and collection system. A variable amount of distilled water was pumped into
2.3. Operating conditions The operating conditions for the main catalytic pyrolysis tests are summarized in Table 3. 2.4. Analytical methods Catalytic pyrolysis products include pyrolysis gas, pyrolysis liquid and coke. An Agilent 6890 gas chromatograph with Chem Station software was used to measure the volume percentage of components in pyrolysis gas. The equation of state for ideal gases converts the data to mass percentages. The pyrolysis liquid was analyzed with a simulated distillation gas chromatogram to get the weight percentage of gasoline, diesel oil and heavy oil. Coke content on catalysts was measured with a coke analyzer.
3. Reaction behaviors of heavy oil catalytic pyrolysis Table 1 Properties of pyrolysis feedstocks Feedstocks
Density (20 8C) g/cm3 Viscosity (100 8C) mm2/s Carbon residue (wt%) Molecular weight Hydrogen (wt%) Carbon (wt%) H/C mol ratio Aromatic carbon (wt%) Group analysis (wt%) Saturates Aromatics Resin and asphaltene
Daqing AR
Daqing VR
Daqing VGO
Huabei AR
0.9069 28.9 4.3 577 13.11 86.52 1.82 10.90
0.9221 106 8.8 895 12.78 86.93 1.76 13.76
0.8011 7.2 0.05 426 13.58 86.36 1.89 6.84
0.9162 43.3 8.9 608 12.87 86.51 1.79 13.00
57.08 27.61 15.31
42.91 34.53 22.56
85.39 11.98 2.63
56.80 22.65 20.55
For heavy oil catalytic pyrolysis on CEP-1, the influences of operating conditions and feed properties on product yields and product distribution were investigated. Table 3 Operating conditions for main pyrolysis tests Item
Value
Temperature of reactor (8C) Temperature of steam furnace (8C) Temperature of pre-heater (8C) Water inflow (ml/min) Oil inflow (g/min) Catalyst loaded (g) Residence time (s) Catalyst-to-oil weight ratio Steam-to-oil weight ratio
600–716 300 500 1.0–6.0 2–10 20–80 1.5–4.5 6–27 0.2–1.6
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Table 4 Effect of reaction temperature on product distribution Reaction temperature (8C) 600
620
640
660
680
700
716
Yields of products (wt%) Dry gas LPG Gasoline Diesel oil Heavy oil Coke Feed conversion (%)
12.77 45.69 24.24 5.71 2.56 9.02 97.44
16.07 46.55 20.04 5.85 2.00 9.49 98.00
19.64 45.78 18.27 4.90 1.79 9.62 98.21
23.06 43.17 16.86 4.31 1.82 10.78 98.18
27.68 39.28 15.10 4.58 1.76 11.60 98.24
32.37 33.93 15.49 4.64 1.53 12.04 98.47
35.61 30.10 14.13 4.24 1.49 14.42 98.51
Yields of light olefins (wt%) Ethylene Propylene Butylene Total light olefins Selectivity (%)
8.65 18.15 10.81 37.61 38.60
10.76 21.29 11.16 43.21 44.09
12.42 22.50 11.23 46.14 46.98
13.75 22.58 10.65 46.98 47.86
15.64 22.08 9.65 47.37 48.22
17.32 19.91 8.24 45.47 46.18
18.25 17.95 6.79 42.99 43.64
3.1. Reaction behaviors of Daqing AR on CEP-1 The effect of reaction temperature, residence time, weight ratios of catalyst-to-oil and steam-to-oil on product distribution as well as light olefin yields was studied. The optimal laboratory operating conditions were obtained. 3.1.1. Effect of reaction temperature The effectiveness of reaction temperature was studied on feed conversion, selectivity of total light olefins and product distribution in the reaction temperature range of 600– 716 8C. The residence time (2.3 s), weight ratio of catalystto-oil (13.5) and steam-to-oil (0.70) was held constant. The experimental data is listed in Table 4. In this study, feed conversion is defined as the sum of the yields of dry gas, liquefied petroleum gas (LPG), gasoline, diesel oil and coke. The selectivity of light olefins is defined as the mass of light olefins formed per unit mass of feedstock reacted, that is, the ratio of yield of light olefins to feed conversion. In experimental tests, the feed conversion is above 97%, and goes up slightly with increasing reaction temperature. This shows that the catalytic pyrolysis ability of catalyst CEP-1 is very good. As the temperature goes up, the yields of dry gas and coke increase, while those of LPG, gasoline and diesel oil decrease. It is known that high reaction temperature means deep pyrolysis extent. Since dry gas and coke are end products of catalytic pyrolysis, their yields will increase with the enhancement of temperature. However, LPG, gasoline and diesel oil belong to intermediate products, and their yields will decrease after reaching the highest yields because of secondary reactions. According to the experimental data, ethylene yield increases monotonously with the increase of reaction temperature. The yields of propylene, butylene and overall light olefins pass through maxima in the range of 640–700 8C, and so does the selectivity of total light olefins. The yield of
total light olefins can reach 47.37 wt%, with yields of ethylene at 15.64 wt%, propylene at 22.08 wt% and butylene at 9.65 wt%. In practice, the catalytic pyrolysis process involves catalytic cracking reactions on catalyst surfaces and thermal cracking reactions both on catalyst surfaces and in the interspaces between catalyst particles. Propylene and butylene are mainly generated from catalytic cracking reactions following the carbonium ion mechanism, and they are intermediate products, which can undergo such secondary reactions as hydrogen transfer, aromatisation, cracking and polyreaction. Ethylene is primarily formed from thermal cracking reactions following the free radical mechanism, and it is close to an end product, so it is unlikely to undergo secondary reactions. As reaction temperature goes up, the reaction rates of both catalytic cracking and thermal cracking increase, causing pyrolysis to be more thorough, and therefore, the yields of light products increase. With further increase of temperature, the proportion of catalytic cracking reactions decreases and that of thermal cracking reactions increases. The ethylene yield then approaches the propylene yield, exceeding the propylene yield at a temperature above 716 8C. Since the increase of reaction temperature can accelerate the secondary reactions of intermediate products, the yields of propylene and butylene will consequently show maxima as the temperature goes up. For producers to achieve high yields of light olefins with low energy cost, the appropriate reaction temperature is in the range of 650–680 8C. Under such conditions, propylene yield is much higher than ethylene yield, and the butylene yield is lower than the ethylene yield. 3.1.2. Effect of residence time Here, residence time refers to the average time of oil vapor passing through the reactor. Keeping reaction temperature (650 8C), the weight ratios of catalyst-to-oil
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Table 5 Effect of residence time on product distribution Residence time (s) 1.59
1.79
1.92
2.04
2.53
3.46
4.39
Yields of products (wt%) Dry gas LPG Gasoline Diesel oil Heavy oil Coke Feed conversion (%)
20.51 45.53 15.97 3.47 1.32 13.19 98.68
21.38 45.57 15.39 3.39 1.17 13.10 98.83
20.89 44.10 16.84 3.45 1.50 13.22 98.50
21.47 45.37 16.50 3.55 1.57 11.54 98.43
22.65 43.76 16.46 3.89 1.18 12.05 98.82
24.24 41.93 16.94 3.88 1.33 11.68 98.67
23.60 41.52 16.85 3.99 1.57 12.47 98.43
Yields of light olefins (wt%) Ethylene Propylene Butylene Total light olefins Selectivity (%)
12.44 21.26 10.05 43.76 44.35
12.97 22.30 11.08 46.34 46.89
12.89 22.93 11.53 47.35 48.07
13.03 24.08 12.48 49.59 50.38
13.66 24.56 12.32 50.54 51.15
14.26 24.50 11.71 50.48 51.16
13.98 24.23 12.01 50.22 51.07
(17.6) and steam-to-oil (0.58) fixed, the effect of residence time on product distribution in the range of 1.5–4.5 s was studied, with the experimental results given in Table 5. Feed conversion is about 98.5% and remains relatively constant with residence time. As residence time prolongs, the yields of dry gas, gasoline and diesel oil increase slightly, while LPG yield decreases; meanwhile, the yields of light olefins first go up until a residence time of 2.0 s is reached, and then yields of light olefins remain relatively constant. The selectivity of overall light olefins shows the similar variation laws of the yield of total light olefins. A longer residence time indicates that there is more time for catalyticpyrolysisofhydrocarbons,andthereforethepyrolysis extent is more thorough. At our experimental conditions, the reaction rates of heavy oil catalytic pyrolysis are very fast. There is a large amount of aromatics in such intermediate products as gasoline and diesel oil, explaining why the secondary pyrolysis ability of these intermediate products is very poor. However, LPG can undergo secondary cracking reactions to produce dry gas, and can undergo polyreaction and aromatization to form gasoline and diesel oil. Accordingly, the yield of LPG decreases as residence time prolongs, while the yields of dry gas, gasoline and diesel oil increase. Prolonging residence time from 1.92 to 4.39 s, it is seen that the decreased value of LPG yield is almost equal to the increased one of the total yield of dry gas, gasoline and diesel oil. In addition, the increased value of gasoline yield is bigger than that of dry gas yield. The evidence points to the conclusion that the secondary reactions of LPG are mainly polyreaction and aromatization. With the general consideration of the influence of residence time on product yield and distribution, we obtain the optimal residence time range, 2.0–4.0 s. 3.1.3. Effect of catalyst-to-oil weight ratio The effect of catalyst-to-oil weight ratio was investigated in the range of 6–27 via changing the mass of catalyst loaded into the reactor. Reaction conditions were reaction
temperature of 650 8C, residence time of 2.6 s and steamto-oil weight ratio of 0.83. The yields of pyrolysis products, feed conversion and selectivity of total light olefins are found in Table 6. As the catalyst-to-oil weight ratio goes up, the feed conversion and the yields of dry gas and coke increase, gasoline and diesel oil decrease, while that of LPG shows a maximum at about 17. In experimental runs, the yields of light olefins vary a little with increasing catalyst-to-oil weight ratio, the ethylene yield goes up slightly, the butylene yield decreases slightly, the yields of propylene and total light olefins pass through maxima, and the selectivity of overall light olefins reaches its highest value of 49.42% at about 13. The value of the catalyst-to-oil weight ratio not only denotes the contact condition between oil vapor and catalysts, but also indicates the average activation of catalysts. As the catalyst-to-oil weight ratio increases, the contact opportunities between oil vapor and active centers also increase; however, the ratio of active centers contacted with oil gas to the overall active centers will decline. Correspondingly, less active centers on the surface per unit catalyst will be covered by coke. Large catalyst-to-oil weight ratio means that much energy can be transferred in the reaction process, which can accelerate thermal cracking reactions. To a certain extent, a high catalyst-to-oil weight ratio means a thorough pyrolysis. With the enhancement of the catalyst-to-oil weight ratio, the yields of gasoline and diesel oil decrease owing to secondary pyrolysis reactions; those of dry gas and coke increase, for they are end pyrolysis products; meanwhile, that of LPG reaches its highest mass at 40.07 wt% at a catalyst-to-oil weight ratio of 17.61, and then decreases because of secondary reactions. Enhancing the catalyst-to-oil weight ratio can increase the mean activation of catalysts and can increase pyrolysis, which can enhance the yields of light products to an extent. Also secondary reactions of light olefins will take place if
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Table 6 Effect of catalyst-to-oil weight ratio on product distribution Catalyst-to-oil weight ratio 6.35
9.68
13.29
17.61
21.96
26.58
Yields of products (wt%) Dry gas LPG Gasoline Diesel oil Heavy oil Coke Feed conversion (%)
27.14 38.47 20.05 5.48 2.03 6.83 97.97
28.47 38.83 16.18 4.88 2.09 9.55 97.91
29.36 39.26 14.85 3.54 1.28 11.71 98.72
29.34 40.07 14.92 2.85 0.86 11.97 99.14
30.01 39.44 13.48 2.51 0.72 13.83 99.28
32.03 37.14 12.65 2.11 0.62 15.44 99.38
Yields of light olefins (wt%) Ethylene Propylene Butylene Total light olefins Selectivity (%)
13.73 21.73 11.80 47.27 48.25
14.69 22.57 11.11 48.38 49.41
14.81 22.74 11.23 48.78 49.42
14.47 22.90 11.50 48.87 49.29
14.43 22.56 10.31 47.30 47.65
14.62 21.18 9.43 45.23 45.52
catalysts to maintain high average activation. These factors show that a large steam-to-oil weight ratio is beneficial to the production of light olefins. The steam-to-oil weight ratio cannot be enhanced unlimitedly, and it is restricted by the disposal capacity of units and the economic benefit. Analysis of the data shows that reaction temperature is the most important parameter among the operating conditions. From this investigation, the optimal operating conditions for Daqing AR catalytic pyrolysis on CEP-1 are as follows, reaction temperature within 650–680 8C, residence time within 2.0–4.0 s, catalyst-to-oil weight ratio within 13–18, and a large steam-to-oil weight ratio is good with the suitable allowance for the unit capacity and the economic benefit.
pyrolysis is too thorough. In addition, a higher catalyst-to-oil weight ratio will add production costs greatly. Therefore, the value of the catalyst-to-oil weight ratio cannot be too high, and it would be suitable in the range of 13–18. 3.1.4. Effect of steam-to-oil weight ratio Using a reaction temperature of 650 8C, residence time of 2.7 s and catalyst-to-oil weight ratio of 15.5, we investigated the effectiveness of steam-to-oil weight ratio in the range of 0.2–1.6 on product yields and product distribution. The experimental data is given in Table 7. As the steam-to-oil weight ratio goes up, the yields of dry gas, ethylene, propylene, butylene and overall light olefins increase monotonously to different extents; yields of gasoline and diesel oil decrease; meanwhile, the yield of LPG and the feed conversion change very slightly. Increasing the steam-to-oil weight ratio can reduce the partial pressure of oil vapor, which favors the cracking of hydrocarbons into low molecular weight products. Steam can restrain coking reactions on catalyst surface, which is good for
3.2. Reaction behaviors of different feedstocks on CEP-1 3.2.1. Effect of feed properties on product distribution The catalytic pyrolysis of four kinds of heavy oils over CEP-1 was investigated using reaction temperature of
Table 7 Effect of steam-to-oil weight ratio on product distribution Steam-to-oil weight ratio 0.21
0.39
0.66
0.84
1.08
1.23
1.58
Yields of products (wt%) Dry gas LPG Gasoline Diesel oil Heavy oil Coke Feed conversion (%)
21.45 44.24 18.27 3.87 1.73 10.44 98.27
21.83 43.09 18.42 4.31 1.90 10.44 98.10
22.74 43.28 17.71 3.52 1.16 11.59 98.84
22.71 43.57 17.60 3.54 1.30 11.29 98.70
23.79 44.84 16.46 3.70 1.18 10.03 98.82
23.94 44.71 16.31 3.93 1.25 9.87 98.75
24.68 44.95 16.10 3.15 1.30 9.83 98.70
Yields of light olefins (wt%) Ethylene Propylene Butylene Total light olefins Selectivity (%)
12.45 22.60 11.81 46.87 47.69
12.98 22.95 11.76 47.69 48.62
13.72 24.04 11.92 49.67 50.26
13.87 23.96 11.79 49.62 50.27
14.93 26.11 11.81 52.86 53.49
15.17 26.70 11.94 53.82 54.49
15.63 27.04 12.33 55.01 55.73
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Table 8 Effect of feed properties on product distribution Feedstocks
Daqing VGO
H/C mol ratio Aromatic carbon (wt%) Yields of products (wt%) Dry gas LPG Gasoline Diesel oil Heavy oil Coke Feed conversion (%)
Daqing AR
Huabei AR
Daqing VR
1.89 6.84
1.82 10.90
1.79 13.00
1.76 13.76
26.31 41.68 16.28 3.53 0.99 11.22 99.01
24.06 42.17 16.86 4.31 0.82 11.78 98.18
28.22 37.13 14.92 4.51 0.88 14.33 99.12
27.36 35.18 16.51 4.61 1.56 14.78 98.44
Yields of light olefins (wt%) Ethylene 13.53 Propylene 22.60 Butylene 11.94 Total light olefins 48.07 Selectivity (%) 48.55
Fig. 2. Propylene yield vs. reaction temperature. 13.75 22.58 10.65 46.98 47.86
12.21 19.27 10.43 41.92 42.29
12.14 19.93 8.41 40.48 41.12
660 8C, residence time of 2.2 s, catalyst-to-oil weight ratio of 15.5 and steam-to-oil weight ratio of 0.75. The experimental results are listed in Table 8. Feed conversion of the four kinds of heavy oils is very high, above 98%. With the increase of H/C mol ratio and the decrease of aromatic carbon content, the yields of dry gas, diesel oil and coke show an increasing trend, while the yields of LPG and light olefins together with the selectivity of overall light olefins show a decreasing trend. 3.2.2. Effect of feed properties on the yields of light olefins Keeping residence time, weight ratios of catalyst-to-oil and steam-to-oil at 2.2 s, 15.5 and 0.75, respectively, the variation laws of the yields of light olefins with feed properties at 600, 630, 660 and 700 8C were investigated. The results are presented in Figs. 1–3. For any kind of feedstock, ethylene yield goes up with increasing temperature; meanwhile, the yields of propylene and total light olefins show maxima, which is similar to the reaction behaviors of Daqing AR catalytic pyrolysis. When
Fig. 1. Ethylene yield vs. reaction temperature.
Daqing VGO and Daqing AR are used as feedstocks, the yields of propylene and total light olefin are much higher than when feedstocks of Huabei AR and Daqing VR are used, this is especially true at high temperature. Nevertheless, the difference of ethylene yields for different kinds of feedstocks is relatively small. The yields of light olefins go up with the increase of H/C mol ratio and the decrease of aromatic carbon content. H/C mol ratio and aromatic carbon content can be used to estimate the catalytic pyrolysis ability of feedstocks. The higher the H/C mol ratio and the lower the aromatic carbon content, the better the catalytic pyrolysis ability of heavy oils and the higher the yields of light olefins will be.
4. Mechanistic pathways of heavy oil catalytic pyrolysis 4.1. Analytical means of mechanistic pathway of heavy oil catalytic pyrolysis Following the carbonium ion mechanism, there is much iso-butane and iso-butylene, but less 1-butylene and 1.3butadiene in the pyrolysis gas. Following the free radical mechanism, there is much hydrogen, methane and C2
Fig. 3. Yield of total light olefins vs. reaction temperature.
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hydrocarbons, as well as 1-butylene and 1.3-butadiene, but less iso-butane and iso-butylene in the pyrolysis gas. There is a large difference between product distributions when the process follows different mechanistic pathways. The mechanistic pathway of heavy oil catalytic pyrolysis can be estimated according to the product distribution. In order to study the mechanistic pathways of heavy oil catalytic pyrolysis, experiments were conducted on quartz grain (74–150 mm), LCM-5, CEP-1 and AKZO, using Daqing AR as feed. Quartz grain seldom shows catalytic activation, and only operates as a heat carrier, consequently only thermal cracking reactions following the free radical mechanism take place. As for catalyst AKZO, thermal cracking reactions rarely occur at low temperature (500 8C), so it can be considered that only catalytic cracking reactions take place at low temperature following the carbonium ion mechanism. 4.2. Product distribution of Daqing AR thermal pyrolysis To study the mechanistic pathways of heavy oil catalytic pyrolysis, one important task is to investigate thermal pyrolysis reactions and the corresponding product distribution under the same conditions used with catalytic pyrolysis of heavy oils. With quartz grain used as the carrier of fluidization and heat transfer, thermal pyrolysis of Daqing AR was carried out at 600, 630, 660 and 700 8C, keeping residence time, and the weight ratios of steam-to-oil and quartz grain-to-oil constant at 3.0 s, 0.80 and 16, respectively. The yields of light products in pyrolysis gas are listed in Table 9. As the reaction temperature goes up, the butylene yield shows maximum at 660 8C and the butane yield changes slightly, while the yields of other light products increase monotonously, indicating that the reaction extent of Daqing AR thermal pyrolysis is increasing. As the reaction temperature increases from 600 to 700 8C, the yields of dry gas (260%), methane (480%), ethylene (250%) and Table 9 Yields of light products in pyrolysis gas (wt%) Reaction temperature (8C) 600
630
660
700
Dry gas LPG
10.29 9.53
19.06 15.64
27.73 21.85
36.49 23.13
Yields of light olefins Ethylene Propylene Butylene Total light olefins
5.58 4.94 4.12 14.64
10.94 8.51 6.42 25.87
15.90 12.56 8.41 36.87
19.61 14.08 8.16 41.84
Yields of light alkanes Hydrogen Methane Ethane Propane Butane Total light alkanes
0.18 1.72 1.13 0.27 0.20 3.50
0.22 4.22 2.70 0.50 0.21 7.85
0.28 6.78 3.99 0.68 0.21 11.94
0.43 10.02 4.60 0.71 0.19 15.95
Fig. 4. Yield of total light olefins at catalytic pyrolysis and thermal pyrolysis.
ethane (300%) increase. Yields of LPG, propylene and propane increase by 143%, 190% and 160%, respectively. Compared with catalytic pyrolysis, the propylene yield of thermal pyrolysis is lower, while the ethylene yield is higher. The yield of total light olefins reaches 36 wt% at 660 8C and 41 wt% at 700 8C, which is very close to that of catalytic pyrolysis (as shown in Fig. 4). Such results indicate that the reaction extent of Daqing AR thermal pyrolysis in experimental runs is high, especially when the reaction temperature is above 660 8C. 4.3. Mechanistic pathway of heavy oil catalytic pyrolysis According to the mechanism analysis, for the distribution of C4 hydrocarbons in pyrolysis gas of heavy oil catalytic pyrolysis, many hydrocarbon isomers indicate that the reactions primarily follow the carbonium ion mechanism, but many straight chain hydrocarbons means that the reactions mainly follow the free radical mechanism. Therefore, the ratio (RM) of i-C4 yield to n-C4 yield can explain the relative percentages of the two mechanisms acting in catalytic pyrolysis processes. RM is a mechanism parameter of catalytic pyrolysis. A larger value of RM indicates that the carbonium ion mechanism plays a bigger function in catalytic pyrolysis processes. A smaller RM value indicates that the free radical mechanism operates predominantly. Experiments of Daqing AR catalytic pyrolysis on quartz grain, LCM-5, CEP-1 and AKZO were conducted and the RM value was calculated, as listed in Table 10. As for the Table 10 The RM value Reaction temperature (8C) 500 550 600 630 660 700
Quartz grain
AKZO
LCM-5
CEP-1
0.2153 0.1701 0.1587 0.1380
1.8740 1.3003 0.9425 0.8325 0.7251 0.6938
0.3321 0.2715 0.2299 0.2027
1.0177 0.8563 0.7826 0.6359
X. Meng et al. / Applied Catalysis A: General 294 (2005) 168–176 Table 11 Relative percentages of reaction mechanisms Reaction temperature (8C)
600 630 660 700
LCM-5 (%)
CEP-1 (%)
Free radical
Carbonium ion
Free radical
Carbonium ion
92.96 94.05 95.85 96.27
7.04 5.95 4.15 3.73
51.63 59.73 63.63 71.32
48.37 40.27 36.37 28.68
carbonium ion mechanism, the standard RM value can be calculated in terms of the data of catalytic cracking on AKZO at 500 8C. And for the free radical mechanism, the standard RM value varies with reaction temperature, and the value at different temperatures can be gained through the data of thermal pyrolysis on quartz grain. According to the standard RM value, together with the calculated RM value of Daqing AR catalytic pyrolysis on LCM-5 and CEP-1, the percentages of the two reaction mechanisms in the catalytic pyrolysis processes on catalysts LCM-5 and CEP-1 at different temperatures are available. The data are given in Table 11. The RM value differs greatly for different kinds of catalysts. For most catalysts, the RM value decreases as the reaction temperature goes up, indicating that the effect of the free radical mechanism in catalytic pyrolysis processes becomes more significant, and that of the carbonium ion mechanism becomes insignificant. In experimental tests, the RM value of Daqing AR catalytic pyrolysis on LCM-5 is very small, and the relative acting percentage of the free radical mechanism is above 90%, showing that the free radical mechanism plays a leading role in the process. Nevertheless, as for catalyst CEP-1, as reaction temperature goes up from 600 to 700 8C, the RM value decreases, and the relative acting percentage of the free radical mechanism increases from 51.63% to 71.32%, while that of the carbonium ion mechanism decreases from 48.37% to 28.68%, indicating that both the reaction mechanisms play important parts in the process. LCM-5 is a metal oxide pyrolysis catalyst, and there are rarely proton acidic centers on its surface for the microactivity index is only 3. Carbonium ion reactions seldom take place in this catalytic pyrolysis process with LCM-5. The data in Tables 10 and 11 further prove that it is primarily the free radical reactions that have occurred on catalyst LCM-5. However, CEP-1 is a kind of molecule sieve catalyst, and there are not only proton acidic centers on catalyst surfaces, but also non-proton acidic centers, which can promote the reactions of both carbonium ions and free radicals. As for catalytic pyrolysis on CEP-1, the free radical mechanism plays the most important part, and the carbonium ion mechanism also plays a significant role. On the basis of the above research, we can concluded that, in catalytic pyrolysis processes, the free radical mechanism will play a primary role if the RM value is
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less than 0.5, the carbonium ion mechanism will play a leading role if the RM value is above 1.5, and both the two mechanisms will play a significant part if the RM value is in the range of 0.5–1.5.
5. Conclusions The experiments of heavy oil catalytic pyrolysis were carried out in a confined fluidized bed reactor. The results show that catalyst CEP-1 is good at converting heavy hydrocarbons to light olefins, and that the catalytic pyrolysis of Daqing VGO and Daqing AR is better than that of Huabei AR and Daqing VR. For catalytic pyrolysis of Daqing AR on CEP-1, such operating conditions as reaction temperature, residence time, weight ratios of catalyst-to-oil and steam-to-oil have great effect on product distribution. The yield and the selectivity of total light olefins show maxima with the increase of reaction temperature; residence time and the catalyst-to-oil weight ratio, while, the yield goes up monotonously with increase of steam-to-oil weight ratio. Among the operating conditions, reaction temperature is the most important parameter. The optimal operating conditions are as follows: reaction temperature is within 650–680 8C, residence time is within 2.0–4.0 s, and the catalyst-to-oil weight ratio is within 13–18, while a large steam-to-oil weight ratio is good with the suitable allowance for the handling capacity and the economic benefit. Feed properties also have significant influence on catalytic pyrolysis of heavy oils. The catalytic pyrolysis ability becomes better and the yields of light olefins become higher with the higher H/C mol ratio and the lower aromatic carbon content of heavy oils. At high temperature (above 660 8C), Daqing AR shows good thermal pyrolysis ability. Compared with catalytic pyrolysis on CEP-1, the propylene yield of thermal pyrolysis is lower, but the ethylene yield is higher, and the yield of total light olefins is close to that of catalytic pyrolysis on CEP-1. Product distribution of Daqing AR catalytic pyrolysis on different kinds of catalysts differs greatly. On the basis of the mechanism analysis of hydrocarbon cracking, a mechanism parameter RM is proposed to study the mechanistic pathways of heavy oil catalytic pyrolysis. Research results show that, for the process of heavy oil catalytic pyrolysis on catalyst CEP-1, both the free radical mechanism and the carbonium ion mechanism play important parts, but only the free radical mechanism plays a leading role in the process of heavy oil catalytic pyrolysis on catalyst LCM-5.
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