Catalytic Upgrading of Vacuum Residue Derived Cracking Gas-Oil for

Jul 26, 2019 - The article was devoted to investigate some fundamentals about the vacuum residue (VR) hierarchical gas-phase catalytic cracking proces...
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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Catalytic Upgrading of Vacuum Residue-Derived Cracking Gas-Oil for Maximum Light Olefin Production in a Combination of a Fluidized Bed and Fixed Bed Reactor Yuanyu Tian,†,‡ Yuanjun Che,*,† Minshen Chen,† Wen Feng,† Jinhong Zhang,† and Yingyun Qiao*,† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China Key Laboratory of Low Carbon Energy and Chemical Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China

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ABSTRACT: The article was devoted to investigate some fundamentals about the vacuum residue (VR) hierarchical gas-phase catalytic cracking process which contained the cracking of VR and shape-selective catalytic cracking of cracking gas-oil. First, three heat carriers were tested in the fluidized bed reactor, finding that cracking VR on the calcium aluminate catalyst obtained higher liquid yield and lower coke yield than that on the FCC catalyst and silica sand. Furthermore, gas chromatography/mass spectrometry analysis of three types of cracking oils (F-oil, S-oil, and C-oil) showed that compared with F-oil and S-oil, the C-oil contained more alkenes and alkanes but had less content of aromatics. Then, the major parametric influences (reaction temperature and catalyst filling amount) on the light olefin distribution were studied in the fixed bed reactor. The yield and selectivity of light olefins were 30.95 wt % and 77.05%, respectively, under the optimized operating conditions.

1. INTRODUCTION Light olefins such as ethylene, propylene, butene, and butadiene are very important basic chemical raw materials.1 In particular, ethylene production capacity is often regarded as a sign of the development level of petrochemicals in a country and region. Ethylene as a raw material has a significant impact on the production cost of downstream products, for example, in the production cost of polyethylene, the proportion of raw materials is as high as about 80%.2 Besides, polypropylene, acetone, propylene oxide, acrylonitrile, acrylic acid, and so forth are all synthesized by propylene. Nowadays, the strong demand for polypropylene in the industry makes the annual growth rate of propylene (5.4%) greater than ethylene (4.5%).3,4 Currently, approximately 95% of ethylene and 66% of propylene in the world are produced by steam thermal cracking process using light materials (such as natural gas liquids, naphtha, or light diesel).5 However, with the depletion of conventional crude oil resources, the quality of supplied crude oil has become heavy and inferior. The reserves of heavy and inferior petroleum resources are very abundant, and it is an important replacement resource for conventional petroleum. Heavy oil is rich in polycyclic aromatic hydrocarbons. It has the characteristics of high viscosity and density, high contents of sulfur, nitrogen, oxygen, and heavy metals, and high coking tendency.6 The contradiction between the limited light materials and ever-increasing demand for light olefins becomes prominent. Thus, how to lighten heavy oil and further produce light olefins is the cause of our attention. Conventional technologies for producing light olefins are steam cracking,7 catalytic cracking,8 methanol to olefins,9 and the dehydrogenation of propane.10 Steam-cracking technology is currently the primary way of producing light olefins. © XXXX American Chemical Society

However, because of the high energy consumption and the limited flexibility of steam cracking (the propylene to ethylene ratio is usually less than 0.6),11 the coproduction of propylene by steam cracking is far from meeting the market demand. Besides, the difficulty in treating the heavy oil has limited its application ranges greatly. Developed catalytic cracking in recent years, such as two-stage riser catalytic cracking (TMP) process,12,13 the deep catalytic cracking and catalytic pyrolysis process process,14 PetroFCC process,15 the high severity FCC process,16 the Indmax process,11 and the Maxofin process have received extensive attention. All these processes are carried out under relatively severe operating conditions (such high reaction temperature, large catalyst/oil ratio, and short reaction time). The developed catalytic cracking technology can extend the feedstock to the entire crude oil fraction. The yield of light olefins ranges from 30 to 50 wt %, far exceeding the level of existing refinery. However, it is difficult for these processes to handle inferior feedstocks because of the problem such as the poison of catalysts and poor atomization. Thus, the authors developed the vacuum residue (VR) hierarchical gas-phase catalytic cracking process to maximize the production of light olefins.17 It can be seen in Figure 1 that the process contains two stages: pretreatment stage (VR fast cracking) and shape-selective catalytic stages (product gas-oils direct catalytic cracking). VR is first fast cracked on the calcium aluminate catalyst and converts into product gas-oil and coke. Tang and co-workers18,19 found that cracking of VR on the base calcium aluminate catalyst could reduce the coke formation and thus maximize the production of gas-oil. Received: June 17, 2019 Revised: July 24, 2019 Published: July 26, 2019 A

DOI: 10.1021/acs.energyfuels.9b01949 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Two stages of the VR hierarchical gas-phase catalytic cracking process. The bed materials used in the fluidized bed reactor are commercial FCC catalysts, silica sands, and calcium aluminate catalysts, and their properties are shown in Table 2. The catalyst used in the fixed bed

Meanwhile, the pretreatment stage can effectively remove the heteroatoms contained in the VR and increase the hydrogen content of product oil. Then, product gas-oil can undergo catalytic cracking on the ZSM-5 catalyst to produce light olefins. Because of the high shape-selectivity, the ZSM-5 catalyst has been widely applied in the catalytic cracking process to maximize the production of light olefins.20−22 Product gas-oil directly undergoes catalytic cracking without condensation, which can effectively save energy. Besides, catalytic cracking reaction occurs under gas-phase conditions to avoid the “cage effect” of the liquid-phase reaction.23 In this study, VR was fast cracked in a fluidized bed reactor, and the subsequent product gas-oil was passed through a fixed bed reactor. First, three kinds of different catalysts (including acidic FCC catalyst, silica sand, and base calcium aluminate catalyst) were used to study the fast cracking of VR in the fluidized bed reactor, and then, the effects of reaction temperature and loading amount on maximizing the light olefins yield during product gas-oil catalytic cracking over ZSM-5 were investigated. The experiments were carried out to obtain preliminary information about the product of the VR hierarchical gas-phase catalytic cracking process on a lab-scale fluidized bed−fixed bed dual reactor.

Table 2. Properties of Three Kinds of Bed Materials24 items FCC catalyst silica sand packing density (kg/m3) 892 1282 particle size (μm) 75−150 75−150 surface area (m2/g) 237 pore volume (cm3/g) 0.15 XRF analysis of the catalysts (wt %) Al2O3 SiO2 CaO MgO

47.09 42.51 0.88 0.72

99.5

calcium aluminate 1399 75−150 10.6 0.06 48.60 6.92 31.44 0.51

reactor was ZSM-5 zeolites (Si/Al = 40, Tiannuo Advanced Material Technology Co., Ltd., Jiangsu Province, China). The powdered ZSM5 catalyst was formed into strips by an extruder. The specific surface area and total pore volume are 342 m2/g and 0.17 cm3/g, respectively. 2.2. Equipment and Experimental Procedure. The schematic diagram of fluidized bed−fixed bed experimental apparatus is shown in Figure 2. It can be seen from Figure 2 that the apparatus consists of

2. EXPERIMENTAL SECTION 2.1. Materials. The VR used in this work is provided by Shida Changsheng Energy Technology Co., Ltd. (Shandong Province, China). Table 1 shows the physico-chemical properties of VR. The

Table 1. Properties of Shida Changsheng VR property

values −3

density (20 °C)/(kg·cm ) viscosity (100 °C)/(mm2 ·s−1) H/C C (wt %) H (wt %) S (wt %) N (wt %) O (wt %)

1020 3985 1.36 85.75 9.81 2.57 0.64 1.23

property

values

CCR (wt %) Ni (μg·g−1)

22.38 15.31

V (μg·g−1) SARA fraction (wt %) saturates aromatics resins asphaltenes

10.56 16.87 28.21 37.10 17.82

Figure 2. Schematic diagram of the fluidized bed-fixed bed reactor experimental apparatus. four parts, including an oil feeding system, a gas supplying system, a reaction system, and a product collection system. In the VR cracking experiment, about 240 g of the bed materials was first added to the reactor and heated to the desired temperature, while the VR and steam were both preheated to 300 °C. The feeding rate of VR in each experiment was 20 g/min for 1.5 min, while the water fed at 10 g/ min. The oil sample and water vapor were mixed in a mixing furnace and continuously supplied to the reactor through a double plunger

density and viscosity of the VR are 1020 kg/m3 and 3985 mm2/s, respectively. Thus, it must be heated above 130 °C to maintain fluidity for processing. Moreover, the Conradson carbon residue (CCR) of the VR is up to 22.38 wt %, which means that it has a high coking tendency. Furthermore, the contents of heteroatoms (i.e., Ni, V, S, and O) and asphaltene fraction in VR are considerably high. B

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Energy & Fuels Table 3. Product Yields of VR Cracking over Three Catalysts FCC catalyst gas yield (wt %) dry gas LPG liquid yield (wt %) gasoline diesel VGO heavy oil coke (wt %) conversion rate (%)

silica sand

calcium aluminate

550 °C

600 °C

600 °C

650 °C

550 °C

600 °C

650 °C

20.59 12.40 8.19 59.39 22.94 22.67 10.82 2.95 20.02 98.27

25.51 15.79 9.71 54.24 18.81 23.05 10.29 2.09 20.25 98.86

16.37 9.70 6.67 71.00 11.48 19.46 20.52 19.55 12.63 86.12

23.59 15.21 8.38 62.47 14.01 19.08 16.34 13.04 13.93 91.85

10.26 6.24 4.02 80.01 11.55 19.91 26.47 22.08 9.72 82.34

20.57 12.83 7.74 68.86 13.98 21.87 21.47 11.53 10.57 92.06

29.83 18.75 11.07 58.79 14.26 19.81 17.90 6.82 11.38 95.99

pump. The fluidized bed reactor had a total length of 700 mm, including a 350 mm long expanded portion of 51 mm in radius. The atomized oil droplets quickly dispersed on the surface of bed materials with high temperature and underwent thermal/catalytic cracking reaction to form product gas-oil and coke. The generated gas-oil and coking catalyst are separated by the steel filter on the flange. In the product gas-oil direct catalytic cracking experiment, a certain amount of ZSM-5 catalyst was packed in the fixed bed and heated to the desired temperature. The fixed bed reactor was made of stainless steel and its total length and diameter were 400 and 20 mm, respectively. Uncondensed product gas-oil passed through the catalyst bed and led to shape-selective catalytic cracking. In order to prevent condensation of product gas-oil, the transfer line between the fluidized bed and the fixed bed was heated to 200 °C. A two-step condensation process was used to condense the cracking gas. The collected liquid products were weighed to determine the liquid yield, while the noncondensable gas was measured by a gas flow meter. The coke yield is obtained by measuring the carbon content on the coking catalyst. After cracking was completed, the coked catalyst was stripped for half an hour. In this study, each test was repeated twice to ensure the accuracy of the experiment. 2.3. Characterization and Analysis. The compositions of cracking gas were analyzed by SCION gas chromatography (GC) that has both thermal conductivity and hydrogen flame ionization detectors. The gas product contained dry gas (H2, CO, CO2, and C1− C2 hydrocarbons) and liquefied petroleum gas (LPG, C1−C4 hydrocarbons). The compositions of liquid oil were analyzed by another SCION GC, according to the ASTM-D2887 standard test method. According to the boiling point, the liquid oil was broadly divided into four fractions, including gasoline (500 °C). The sum yield of dry gas, LPG, gasoline, diesel, VGO, and coke was defined as the VR conversion. The chemical composition of the liquid oil was analyzed by gas chromatography/ mass spectrometry (GC/MS, Techcomp). The oil sample was diluted with carbon disulfide and then injected into GC/MS. The detail analysis methods were reported in author’s previous literature.6 The HX-HW8 infrared carbon sulfur analyzer and the HX2-GP2 highfrequency induction furnace were used to accurately determine the coke content of the catalyst surface.

conversion rate was higher than 98% at all tested temperatures, indicating that VR was almost completely converted. Because of the acid active sites and the high specific surface area (Table 2), the FCC catalyst had high catalytic activity. The activation energy of the catalytic cracking reaction of VR was significantly lower than that of the thermal cracking reaction, thus, the cleavage of the alkane and the alkyl side chain, the ring opening of the cycloalkane ring, and the secondary cracking of the olefin were promoted. Besides, the strong acidity of the FCC catalyst led to the excessive cracking of gasoline and diesel fractions.25 Therefore, compared to other two heat carriers, the liquid yield over the FCC catalyst was low, whereas the gas yield was high. The VR mainly underwent catalytic cracking reaction over the FCC catalyst, but when the reaction temperature was 600 °C, the thermal cracking reaction played an important role in the distribution of cracking products.26 On the one hand, as the temperature increased, the secondary cracking of the gasoline fraction was aggravated, thereby generating more gas products. On the other hand, with the increase of temperature, the thermal cracking reaction in the catalytic cracking of VR was intensified, which promoted the decomposition of small molecular hydrocarbons. In addition, the high temperature could suppress the hydrogen transfer reaction which was an exothermic reaction and reduced the consumption of C2H4. Thus, as the temperature increased to 600 °C, the yield of gasoline fraction decreased remarkably, while the dry gas yield increased. It could also be seen from Table 3 that there were more LPG components over the FCC catalyst than that over silica sand at the same temperature. This was attributed to the fact that the reaction mechanism of VR on different heat carriers was different. The cracking of VR over FCC catalyst followed the carbocation mechanism, thus the cracking gas mainly contained C3 and C4 hydrocarbons.26 Due to the fact that a considerable portion of VR (especially heavy resins and asphaltenes) could not be vaporized under the reaction conditions, the condensed aromatic hydrocarbon in the heavy fraction was easily adsorbed on the surface of the FCC, and occurred dehydrogenation condensation reaction to form a coke precursor or coke. In addition, the hydrogen transfer and aromatization reaction were significant in the process of catalytic cracking of VR, resulting in higher content of aromatic hydrocarbon products as coke precursors. Therefore, the yield of coke obtained by catalytic cracking of VR on the FCC catalyst was significantly higher than that on silica sand and calcium aluminate catalyst. When silica sand was used, the liquid yield was as high as 71 wt % at 600 °C, whereas its conversion was only 86.12%. The yields of gasoline and diesel oil only accounted for about 36 wt

3. RESULTS AND DISCUSSION 3.1. Fast Cracking of VR on a Fluidized Bed Reactor. To understand the effect of different bed materials on the conversion of VR, a comparative test was carried out using the FCC catalyst, silica sand, and calcium aluminate catalyst. All experiments were carried out in a fluidized bed reactor, and the catalyst-to-oil weight ratio and steam-to-oil weight ratio were about 8 and 0.5, respectively. 3.1.1. Product Yields for Various Bed Materials. The product yields of VR cracking over different bed materials are shown in Table 3. With respect to the FCC catalyst, the C

DOI: 10.1021/acs.energyfuels.9b01949 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. (a−c) GC/MS total ion chromatogram of cracking oil obtained by cracking VR on different heat carriers.

At the same reaction temperature, the calcium aluminate had a higher cracking activity than that the silica sand, which promoted the cracking of the heavy fraction. Therefore, the light oil fraction obtained by cracking the VR on the calcium aluminate catalyst had a high yield of the light oil fraction. Calcium aluminate had the ability to promote the dehydrogenation of hydrocarbons,28 which increases the amount of hydrogen radicals during the reaction. The hydrogen radical could inhibit the polycondensation reaction of the aryl radical, thereby reducing the formation of the coke precursor. Besides, due to the fact that the calcium aluminate catalyst had no acidity, the hydrogen transfer reaction could be inhibited. All these indicated that the calcium aluminate catalyst could suppress the formation of coke. The yield of light oil (gasoline and diesel fractions) over calcium aluminate catalyst increased from 31.46 to 35.85 wt % and then sharply dropped to 24.07 wt % when the reaction temperature increased from 550 to 650 °C, whereas the yield of VGO and heavy oil fraction decreased gradually with the increase of reaction temperature. These were because the cracking process of VR was a strongly endothermic process. Increasing the reaction temperature could enhance the cracking reaction of the VR and the secondary cracking of the heavy fraction in the liquid product to form light fractions. However, at the high temperature of 650 °C, the yield of dry gas was more than 18 wt %. This was attributed to the severe cracking of VR molecules. The reaction of VR on calcium

% while the yield of VGO and heavy reached up to 35 wt %, which indicated that the cracking performance of the VR on silica sand was poor, and the noncatalytic cracking could not convert the VR sufficiently. Due to the fact that the silica sand had almost no pore structure and surface area, and there is no catalytic active center on the surface, the thermal cracking was the main way to convert VR over silica sand.25 The macromolecular compound in the VR was broken into small molecule products by the radical reaction mechanism. With the reaction temperature raising from 600 to 650 °C, the coke and gas yields (especially, the yield of dry gas increased by 5.51 wt %) increased while the liquid yield decreased. This was because the higher temperature made the molecule crack deeper. Calcium aluminate was mainly composed of alumina, calcium oxide, and other relatively small amounts of metal oxides (Table 2), and it had the advantages of a good crystal phase, cracking performance, no acidity, and good stability.27 The conversion rate of VR on the calcium aluminate catalyst was significantly higher than that of silica sand. Tang et al.18 believed that the catalytic activity of the calcium aluminate catalyst was attributed to the presence of base active sites, which promoted the conversion of VR. Compared with the other two bed materials at the reaction temperature of 600 °C, the total yield of gasoline, diesel, and VGO fractions on calcium aluminate catalyst was the highest (reaching 57.33 wt %), whereas the coke yield was the lowest (only 10.57 wt %). D

DOI: 10.1021/acs.energyfuels.9b01949 Energy Fuels XXXX, XXX, XXX−XXX

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generation of free radical. Subsequently, the free radical continued to undergo cleavage of the C−H bond under the action of another active oxygen to form an olefin molecule. In addition, because there was no acidic site on calcium aluminate, the hydrogen transfer reaction could be suppressed. Thus, cracking of VR on the calcium aluminate catalyst could enhance the content of alkenes in product oil. 3.1.2.2. Alkanes and Alkene/Alkane. For the F-oil, the relative content of alkanes was higher than that of alkenes. The reactive activity of alkanes was lower than that of alkenes, attributing to a more difficult formation of carbenium ions. For S-oil and C-oil, the alkene/alkane ranged from about 2.3 to 2.7. The VR underwent thermal cracking on the silica sand and calcium aluminate catalyst. Research29,30 found that the thermal cracking of hydrocarbons in the vapor phase produced more alkenes than that in liquid phase. Due to the fact that the VR was introduced as fine droplets into the fluidized bed reactor for enhancing the vapor phase cracking, thus, for S-oil and C-oil, the relative content of alkanes was lower than that of alkenes. 3.1.2.3. Aromatics. The data presented in Table 4 showed that the relative contents of aromatic in three types of product oil were in the order F-oil > S-oil > C-oil. The relative content of aromatics in F-oil was significantly higher than that of C-oil and S-oil. This was because the ways for hydrocarbons reacting on the FCC catalyst to form aromatics were numerous, such as the cleavage reaction of alkyl side chain attached to aromatics, the dehydrogenation reaction of the cycloalkanes, and especially the hydrogen transfer reaction of alkenes.31 Thermal cracking of VR on silica sand also obtained a large amount of aromatics. This was due to incomplete cracking of VR on quartz sand, which contains a large amount of VGO and heavy oil fractions. Furthermore, the dehydro-aromatization of alkenes and alkadienes and the dehydrogenation of cycloalkenes could also form the aromatics.32 Due to the fact that the alkenes could form carbenium ions easily during catalytic cracking, the alkenes had high reactivity. However, the aromatics could not enter the pore size of the ZSM-5 zeolites and easily formed coke on the surface of the catalyst to cause the deactivation of the catalyst. In summary, the calcium aluminate catalyst was more suitable as the bed material for the fluidized bed reactor than FCC catalysts and silica sand. 3.2. Gas-Phase Catalytic Cracking of the Product GasOil. To maximize the production of light olefins, the product oil required further processing. Therefore, the gas-oil cracked by the VR in the fluidized bed was directly subjected to typeselective catalytic cracking through a fixed bed containing the ZSM-5 catalyst. As the results in Section 3.1, the bed material in the fluidized bed reactor was calcium aluminate catalysts, and the reaction temperature, catalyst-to-oil weight ratio, and steam-to-oil weight ratio were 600 °C, 8, and 0.5, respectively. 3.2.1. Effect of Reaction Temperature on the Yield and Distribution of the Product. The product gas-oil mainly underwent catalytic cracking reaction in the fixed bed reactor. There were many factors affecting the catalytic cracking reaction, and temperature was one of the most important factors. In order to investigate the effect of temperature on product distribution, the catalyst filling amount was set as 15 g. Table 5 showed the corresponding product distributions for product gas-oil catalytic reaction at 600, 625, 650, 675, and 700 °C, respectively. Comparing the Tables 3 and 5, it could be found that the yield of gas product in the presence of ZSM-

aluminate followed the mechanism of free radical reaction mechanism.24 Thus, there was a large amount of C1 and C2 low molecular hydrocarbons in the cracking gas when the large molecular hydrocarbons were decomposed. 3.1.2. Chemical Characterization of the Oil Product. As mentioned above, the product gas-oil would be directly subjected to catalytic cracking for producing light olefins. Therefore, it was crucial to pay attention to the composition of product oil. In this section, the chemical composition of the oil product was investigated by GC/MS. For convenience of description, the oil product obtained by reacting VR on the FCC catalyst, silica sand, and calcium aluminate catalyst was recorded as F-oil, S-oil, and C-oil, respectively. Figure 3 is the chromatogram of the oil product obtained by cracking VR on different heat carriers. It could be seen from the Figure 3 that there were significant differences in the compounds in the three types of oil products. This indicated that the reaction paths of the VR on different bed materials were distinct. GC/ MS analysis detected more than 200 different compounds in the oil product. Generally, the types of compounds in cracking oil were divided into alkenes (1-alkenes, iso-alkenes and cycloalkenes), alkanes (n-alkanes, iso-alkanes and cycloalkanes), aromatics, alkadienes, and sulfur compounds. The relative content of compounds in product oil was determined based on peak area normalization method, as shown in Table 4. Table 4. Relative Content of Compounds in Three Types of Oils alkenes (%) 1-alkenes iso-alkenes cycloalkenes alkanes (%) n-alkanes iso-alkanes cycloalkanes aromatics (%) alkadienes (%) S-compound (%)

F-oil

S-oil

C-oil

15.08 5.42 7.24 2.42 20.59 16.20 3.45 0.94 56.70 2.33 5.30

30.34 22.89 2.19 5.26 14.15 12.50 1.20 0.45 45.59 4.31 5.61

35.05 29.08 3.16 2.81 15.25 14.16 0.86 0.23 41.86 3.38 4.46

3.1.2.1. Alkenes. It could be seen from Table 4 that the relative contents of alkenes in three different types of product oil were in the order C-oil > S-oil > F-oil. The relative content of alkenes in F-oil was lowest compared to C-oil and S-oil. Besides, the alkenes in F-oil were dominated by iso-alkenes. On one hand, under catalytic cracking conditions, 1-alkenes were prone to undergo double-bond shift reaction and skeletal isomerization reaction to produce iso-alkenes. On the other hand, due to the high hydrogen transfer activity of the FCC catalyst, the saturation of the alkenes was increased, resulting in a low content of alkenes in the product oil. In contrast, the relative content of alkenes in C-oil was higher than that of F-oil and S-oil. The cracking of hydrocarbons on calcium aluminate catalysts followed the free radical reaction mechanism, but it was different from noncatalytic free radical reaction. The hydrogen atom on the C−H bond in the hydrocarbons was chemically adsorbed on the alkali center (lattice oxygen) of the calcium aluminate surface,28 and then, the interaction between the active oxygen and the adsorbed hydrogen atom weakened the energy required to break the C−H bond and promoted the E

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Generally, the reaction pathway could be divided into two major directions:26 one direction was the cracking reaction in which the molecules were gradually reduced and finally the gas was generated; the other direction was the condensation reaction in which the molecules gradually increased, and the hydrogen−carbon ratio became smaller, and finally the coke was formed. The data in Table 5 presented that with the reaction temperature increasing, the yield of gasoline fraction changed from an increase to a decrease while the yields of diesel, VGO, and heavy oil decreased gradually. As the reaction temperature increased, the heavy fractions (VGO and heavy oil) contained in the cracking oil would be further cracked. On the one hand, VGO and heavy oil were converted into light oils (gasoline and diesel oil), while gasoline and diesel as intermediate products could undergo secondary reaction to produce cracking gas eventually; on the other hand, due to the fact that VGO and heavy oil contained a large amount of condensed aromatic hydrocarbons, these fractions could undergo the condensation reaction to promote the formation of coke. This was the reason for the increase in coke yield with increasing temperature. It could be seen from Figure 5 that the yields of ethylene, propylene, and butene exhibited different trends. The yield of

Table 5. Yields and Distribution of Products by Catalytic Cracking of Product Gas-Oil at Different Reaction Temperatures temperature (°C) gas yield (wt %) dry gas LPG liquid yield (wt %) gasoline diesel VGO heavy oil coke (wt %) conversion rate (%)

600 31.23 15.39 15.84 57.31 16.88 19.86 17.05 3.52 11.46 97.98

625 33.86 16.69 17.17 54.99 19.07 18.40 14.56 2.96 11.15 98.37

650 40.17 19.83 20.34 48.21 18.22 15.81 11.93 2.25 11.62 98.91

675 42.37 22.26 20.11 45.32 17.96 14.72 10.89 1.75 12.30 99.20

700 45.24 25.47 19.77 41.91 17.01 14.19 9.51 1.19 12.86 99.50

5 catalyst was higher than that in the absence of the ZSM-5, whereas the yield of liquid product was lower than that in the absence of the ZSM-5. The product gas-oil formed through the cracking of VR generated the carbenium ions on the ZSM-5 catalyst, and then the carbenium ions cracked via β-scission to produce low molecular hydrocarbons. Thus, the catalytic cracking of product oil by ZSM-5 could increase the yield of gas products and the conversion rate of VR. It could be seen from Table 5 that the gas yield in the product increased gradually as the reaction temperature increases. The yield of dry gas in the gas product increased gradually, while the yield of LPG increased first and then decreased, and obtained the maximum at 650 °C. As the reaction temperature increased, the depth of catalytic cracking increased, thus the yields of dry gas and LPG increase gradually. When a higher reaction temperature was used, the product gas-oil typically underwent thermal cracking and catalytic cracking reaction simultaneously. From the aspect of reaction activation energy, the activation energy of thermal cracking reaction (210−290 kJ·mol−1) was significantly higher than catalytic cracking (42−125 kJ·mol−1).26 This indicated that the thermal cracking reaction was more sensitive to the reaction temperature than the catalytic cracking reaction. Therefore, the higher reaction temperature enhanced the thermal cracking reaction (the thermal cracking reaction accounted for the increased share of the entire reaction), resulting in further cracking of hydrocarbons in the LPG to produce smaller molecular products (such as methane, ethane, ethylene, etc.). Increasing reaction temperature from 600 to 700 °C, the yield of liquid oil decreased significantly (decreased from 57.31 to 41.91 wt %), whereas the conversion of VR and coke yield increased. The catalytic cracking reaction of product gas-oil was a complex parallel−tandem reaction, a model of which is shown in Figure 4. It could be seen from Figure 4 that the reaction pathway was not single and had several directions.

Figure 5. Effect of reaction temperature on yield and selectivity of light olefins.

ethylene gradually increased with the increase of the reaction temperature. The ethylene yield was only 8.96 wt % at 600 °C, whereas the ethylene yield increased to 14.40 wt % when the reaction temperature raised to 700 °C. The yields of propylene and butene showed a trend of increasing first and then decreasing, reaching a maximum at 650 °C, and yields of propylene and butene were 15.01 and 4.32 wt %, respectively. The yield of total light olefins increased first and then stayed stabilized. When the temperature exceeded 650 °C, the yield of total olefins did not change much. Increasing the reaction temperature would accelerate the reaction rate of catalytic cracking, that is, as the temperature increases, the reaction rate of cracking of gasoline and diesel into gas increased. In terms of the reaction types, the increase of reaction temperature was beneficial to the cracking reaction and was not conducive to hydrogen transfer reaction. Therefore, the increase in the reaction temperature was advantageous for the formation of light olefins. At low reaction temperatures, the cracking reaction of product gas-oil was dominated by catalytic cracking, whereas the cracking reaction temperature was higher than 650 °C, the proportion of thermal cracking reaction was gradually increased. Therefore, propylene and

Figure 4. Parallel tandem reaction model of catalytic cracking of product gas-oil. F

DOI: 10.1021/acs.energyfuels.9b01949 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels butene would undergo secondary reaction to form ethylene and methane. At this point, the selectivity of the light olefins began to decrease. In summary, the yield and selectivity of light olefins had a maximum at 650 °C, which were 30.95 wt % and 77.05%, respectively. 3.2.2. Effect of Catalyst Filling Amount on Enhancing the Production of Light Olefins. This section investigated the effect of catalyst filling amount on the catalytic cracking of product gas-oil to produce light olefins, and the reaction temperature was 650 °C. Table 6 shows the distributions and yields of the product after catalytic cracking of product gas-oil in the fixed bed which loading different amount of ZSM-5. Table 6. Yields and Distribution of Products by Catalytic Cracking of Product Gas-Oil at Different Catalyst-Filling Amounts catalyst inventory (g) gas yield (wt %) dry gas LPG liquid yield (wt %) gasoline diesel oil VGO heavy oil coke (wt %) conversion rate (%)

5 25.16 12.29 12.87 66.93 20.37 24.44 17.62 4.50 7.91 96.99

10 32.26 16.16 17.76 56.16 17.94 18.90 16.18 3.66 11.45 97.92

15 40.17 19.83 20.34 48.21 18.22 15.81 11.93 2.25 11.62 98.92

20 40.68 20.20 20.48 48.10 18.50 16.20 11.49 1.91 11.74 99.08

Figure 6. Effect of the catalyst-filling amount on yield and selectivity of light olefins.

30 43.16 22.61 20.55 44.69 18.63 15.95 9.14 0.97 12.19 99.58

degree. However, at the same time, propylene and butene could undergo hydrogen transfer reaction to form aromatics or further thermal cracking into small molecular hydrocarbons.35 Thus, when the catalyst-filling amount was higher than 15 g, the yield of gasoline fraction showed a slight increase (Table 6), and the yield of propylene and butane varied little.

4. CONCLUSIONS The VR hierarchical gas-phase catalytic cracking process was investigated by the fluidized bed−fixed bed dual reactor in this study. The main conclusions were as follows: By comparing the yields and distributions of cracking products on the FCC catalyst, silica sand, and calcium aluminate catalyst, it was found that the calcium aluminate catalyst has moderate catalytic cracking activity, high liquid yield, and high heavy fraction conversion. When the reaction temperature was 600 °C, the conversion rate of VR could reach 96.94 wt %, the liquid yield was 69.93 wt %, and while the coke yield was only 9.26 wt %. GC/MS analysis of liquid oil indicated that compared with F-oil and S-oil, the contents of alkenes and alkanes in C-oil were high while those of aromatics was low. The effect of gas phase catalytic reaction temperature and catalyst-filling amount on product distributions was studied by using the ZSM-5 catalyst on a fixed bed. The optimal reaction temperature and catalyst filling amount were 650 °C and 15 g, respectively. Under these conditions, the yield and selectivity of light olefins were 30.95 wt % and 77.05%, respectively.

It could be seen from Table 6 that with the amount of catalyst loading in the fixed bed increasing, the gas yield continued to increase while the liquid yield decreased gradually. As the catalyst filling amount increasing, the ratio of catalyst to cracking oil became larger. With the ratio increasing, the contact chance between the product gas-oil and the active center of the catalyst also increased.33 Besides, the large catalyst to oil ratio meant that a large amount of heat could be transferred during the reaction, which accelerated the thermal cracking reaction and enhanced the proportion of thermal cracking. On the other hand, as the amount of catalyst loading increasing, the residence time of catalytic cracking of product gas-oil became longer. Longer residence time indicated more time for the catalytic decomposition of hydrocarbons and thus enhancing the degree of catalytic cracking. Because of the deactivation of the catalyst, a longer residence time also indicated an increase in the thermal cracking reaction.34 Therefore, as the catalyst-filling amount is increasing, the conversion ratio increased and the liquid yield decreased; whereas the yield of gas product as the final products gradually increased. It could be seen from Figure 6 that as the catalyst filling amount increased, the overall yield of the light olefins first increased and then remained relatively constant. However, the selectivity of the light olefins first increased and then decreased, reaching a maximum at the catalyst filling amount of 15 g. As mentioned above, as the amount of catalyst loading is increasing, the thermal cracking reaction would be accelerated, thus, the yield of C1 and C2 hydrocarbons increased. This was also the reason for the decline of the selectivity of the light olefins at the larger catalyst-filling amount. In addition, the larger catalyst-filling amount increased secondary cracking reaction of hydrocarbons. This promoted the formation of propylene and butene to a certain

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.C.). *E-mail: [email protected]. Phone: +86-532-86981711. Fax: +86-532-86981711 (Y.Q.). ORCID

Yuanyu Tian: 0000-0003-3326-7484 Yuanjun Che: 0000-0003-4765-3650 Jinhong Zhang: 0000-0002-0212-0503 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support provided by the National Natural Science Foundation of China (21576293, 21706287 and 21878335), the Natural Science Foundation of G

DOI: 10.1021/acs.energyfuels.9b01949 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

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Shandong Province (ZR2018QB008), and the Fundamental Research Funds for the Central Universities (18CX02117A).

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DOI: 10.1021/acs.energyfuels.9b01949 Energy Fuels XXXX, XXX, XXX−XXX