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Light products and H2-rich syngas over the bifunctional base catalyst derived from petroleum residue cracking gasification Ruiyuan Tang, Yuan-yu Tian, Yingyun Qiao, Guoming Zhao, and Haifeng Zhou Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00864 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 21, 2016
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Light products and H2-rich syngas over the bifunctional base catalyst derived from petroleum residue cracking gasification Ruiyuan Tanga, Yuanyu Tiana,b*, Yingyun Qiaoa*, Guoming Zhaob, Haifeng Zhoub State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East
a
China), Qingdao Shandong 266580, China. b
Key Laboratory of Low Carbon Energy and Chemical Engineering, Shandong
University of Science and Technology, Qingdao Shandong 266590, China.
*
Corresponding author. E-mail addresses:
[email protected] (Y Tian);
[email protected](Y Qiao).
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Abstract: Vacuum residue is utilized by a process involving the residue cracking and coke gasification regeneration. In this process, vacuum residue is first converted into the products of light olefins and light oils by catalytic cracking, and then the cracking -generated coke is gasified into H2-rich syngas by using a bifunctional base catalyst. Their cracking gasification effects of vacuum residue are studied in a dual fluidized bed reactor. The results show that the solid base catalysts could enhance light olefins yield (has high olefinicity) and inhibit the formation of coke in comparison with silica sand and a hydrothermal treatment zeolite catalyst (FCC catalyst). Furthermore, the catalyst prepared at a CaO/Al2O3 molar ratio of 12:7 displayed a better cracking effect than the one produced at the molar ratio of 1:1. The effects of the reaction temperature and the catalyst-to-oil ratio on the distribution of cracking liquid from vacuum residue solid base cracking are discussed. The results showed that the heavy oil conversion of more than 93.0%, the light oils yield about 81.0 wt%, the coke of ca. 5.2 wt%, and the C2–C3 olefinicity of higher than 53.0% are achieved by cracking at 700 oC with a catalyst-to-oil ratio of 7.0. The coke over solid base catalyst is well gasified at 800 oC in an atmosphere of steam-oxygen. The content of H2 is about 55.5 vol% and with the CH4 content of less than 0.2 vol% in comparison with 36.6 vol% and 2.4 vol% over the FCC catalyst, respectively. The cracking effects of solid base catalysts are stable via a few cycles process, although a decrease in catalytic effect is observed. Keywords: Bifunctional catalyst; Cracking; Light products; Gasification regeneration; H2-rich syngas
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1. Introduction Heavy oil occupies a higher proportion of the inferior heavy component in crude oil. The processing of the above-mentioned heavy oil feedstock results in the production of large amounts of petroleum residue (i.e., vacuum residue and atmospheric residue). Therefore, there is a considerable focus on research methods to transform the heavy oil feedstock into light. The current technologies proposed to process the residues include hydrocracking1, visbreaking2, residue fluid catalytic cracking3.4, coking (including flexi-coking, delayed coking, and fluid coking)5–7 as well as combined processing technologies8. The application of hydrocracking technology is limited because of high investment and operating costs. Coking technology is widely applied for processing heavy feedstock, given its advantages of wide feed adaptability and high operation reliability9.10. However, it generates a large amount of the petroleum coke and many contaminants (such as heavy metal elements, sulfur and nitrogen) may be enriched in the coke. In contrast, there is a high demand for hydrogen to preprocess low-grade oil and raw materials for several industrial products (i.e., ammonia synthesis and catalytic hydrocracking). In general, in the petroleum industry, hydrogen is mainly obtained via separation from the catalytic reformation of naphtha or from H2-rich off-gases9 (LPG and dry gas). The residues cracking and gasification process intended to convert the above-mentioned inferior feedstock into light products, and meanwhile co-producing hydrogen via gasifying the coke on the catalyst.
Petroleum residue comprises many large molecules and contaminants. Desired cracking oil and gas products comprise mainly of gasoline and diesel fractions, has 3
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high olefinicity and H2-rich syngas. Thus, the residues conversion has to involve two types of reactions. Namely, the residues are firstly converted into light products (light oils and light olefins) by catalytic cracking, and meanwhile removed the contaminants. Subsequently, the coke is gasified into the H2-rich syngas in the atmosphere of steam or oxygen-steam. For integrating these two types of reactions, a dual fluidized bed system is used and the catalyst particles could alternately operate between the cracking and gasification reactions. The catalyst regeneration process could provide catalysis and exothermic heat for the residue cracking reaction. Besides, it is expected that the used catalyst could enhance the coke gasification reaction to a certain degree and not deteriorate the used catalyst in the gasification condition.
The catalyst applied in this cycle process is crucial. It is necessary for the catalyst to possess a high degree of stability against contaminants. The catalyst is also required to moderate reasonable activity during heavy oil cracking attribute to high activity would lead to deep cracking and thereby a lower oil yield and a higher gas yield. Besides, for regeneration purposes, the catalyst should have high hydrothermal stability to gasify the coke in steam condition. At present, the zeolite catalyst is used in the cracking and gasification processes. However, this catalyst has an adverse effect on reducing coking formation9.10, irrespective of whether a fresh catalyst or a hydrothermal treatment catalyst is used. Thus, this could increase both regeneration time and operation cost during practical applications. Furthermore, the zeolite catalyst can lead to excessive cracking of the residues and coke formation at a high reaction temperature. Previous researches indicated that the solid base catalyst possess many advantages, such as high 4
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cracking stability, good hydrothermal properties, high olefin yield, and an anti-coking property11.12. Few studies have used solid base catalysts for the residues cracking13. However, solid base catalysts are widely used in catalytic transesterification reactions to produce biodiesel14.15 and sorption fields contribute to advantages including good selectivity, high activity, and mild reaction conditions16.17. Moreover, the base catalyst (alkali metal and alkali earth metal) is used in the gasification reaction because of its high reactivity18–20 and product selectivity. Wu et al.21 revealed that the petroleum coke could be utilized via the potassium catalytic steam gasification to produce the H2-rich syngas and virtually no CH4. Li et al.22 observed that the petroleum coke gasification rate is improved with the addition of base catalysts. These studies demonstrated that the solid base catalyst performed well with respect to residual oil catalytic cracking23.24, and the coke gasification reaction.
In this study, calcium aluminate is used as the bifunctional solid base catalyst because of its simple preparation method, lower cost, and higher stability. The study focused on investigating the cracking vacuum residue over solid base catalyst with different test conditions (such as different reaction temperature and the catalyst-to-oil ratio). The study also discussed spent catalyst gasification and catalyst stability. In this study, four catalysts (silica sand, a hydrothermal treatment zeolite catalyst, and two solid base catalysts with different total base number) are used. Silica sand is used as a reference to study the thermal cracking effect of vacuum residue. The hydrothermal treatment zeolite catalyst and two solid base catalysts are used to study the cracking effects of vacuum residue. 5
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2. Experimental Section 2.1 Materials and Reagents. Al2O3 used in this study is supplied by the Zibo Catalyst Factory, China, and calcined in air at 200 oC for 1 h before further use. Calcium oxide and silica sand are purchased from Tianjin Kemiou Chemical Reagent Co., Ltd, China, and used without further purification. Carbon black obtained from Alfa Aesar is dried at 120 oC before further use. Vacuum residue is provided by Shenghua refining, China and it properties are listed in Table 1. The used hydrothermal treatment zeolite catalyst (HZSM-5 with the Si/Al ratio of 25, denoted as the FCC catalyst) is supplied by Zibo catalyst Factory, China, and its XRF analysis are shown in Table 2. Table 1. Properties of vacuum residue Properties o
Value −3
Density 20 C/g·cm Viscosity 80 oC/mm2·s−1
0.98 900
H/C ratio
1.67
Carbon residue /wt%
13.5
Elemental analysis /wt%
Group composition /wt%
C
87.0
Saturates
38.6
H
12.0
Aromatics
33.5
S
0.3
Resins
26.8
N
0.4
Asphaltenes
1.1
O (by difference)
0.3
Proximate analysis /wt% MVR
AVR
VVR
FCVR
–
0.02
88.88
11.1
Table 2. XRF analysis of FCC catalyst Components FCC
Al2O3 /wt% 52.2
SiO2 /wt%
Na2O /wt%
35.1
Re2O3 /wt%
0.2
8.1
2.2 Solid Base Catalyst Preparation. Calcium aluminate catalyst is prepared by the solid phase method. In this method, CaO and Al2O3 are mixed with a molar ratio of 6
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12:7 or 1:1, followed with addition of 15.0 wt% Ca12Al14O33 as the crystal nucleus and of 4.0 wt% carbon black as hard template. These mixtures are triturated in the grinding miller for 3 min (3000 r·min−1). These mixtures are calcinated at 1300 oC in Ar for 2 h, and carbon removal by 100% steam at 800 oC for 1 h (heating rate=20 oC·min−1). The catalysts are denoted as CA1 (n(CaO)/n(Al2O3)=12:7) and CA2 (n(CaO)/n(Al2O3)= 1:1). The spent catalysts are gasified by different gasification reagents (pure steam or steam-oxygen) at 800 oC for 25 min. Besides, the regenerated catalysts are denoted as CA1R1 and CA2R1 after the first times gasification regeneration, CA1R2 (CA1R3) and CA2R2 (CA2R3) after the second (three) times gasification regeneration.
2.3 Cracking Gasification Experiment. Vacuum residue cracking and gasification is performed in a dual fluidized bed reactor. The schematic diagram of the test device is exhibited in Figure 1. It is seen that the test device consists the oil feeding system, the temperature controlling system, the reaction system, as well as the separation and analysis system. The reactor is made up of stainless steel with the total length of 800 mm and the inner diameter of 25 mm. But the expanded section is with the length of 200 mm and the inner diameter of 90 mm. A self-designed stainless steel distributor is used to ensure the fluidization of the catalyst particles. Steam is adopted to atomize vacuum residue and fluidize the catalyst particles, which is also part of the reagent gas for the coke gasification. Besides, a certain amount of oxygen is introduced into the reagent gas for the coke gasification and combusted the remaining coke. Nitrogen is used as the purging gas and the protecting gas for cracking gasification process.
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Figure 1. Schematic diagram of vacuum residue cracking gasification process (red line represented electric tracing band).
For each residue cracking run, it is heated up to the preset temperature in N2. Vacuum residue and water is preheated to about 100 oC and 200 oC, respectively. Steam is firstly fed into the reactor by a plunger oil pump to fluidize the catalyst particles. And then vacuum residue is fed into the reactor and mix in the preheated section. The steam-oil mixture is atomized into tiny oil drop and transported into the middle of the fluidized catalyst bed (such as silica sand or solid base catalysts) above the stainless steel distributor, where the tiny oil droplets are cracked via interacting with the catalyst particles in the cracking section. The gaseous product is obtained via the oil and gas separation system. The liquid product is cooled by a water-cooled tube to collect the heavier oil in the first collector, while the lighter oil is collected in second collector with cooling water of 1 oC. The non-condensable gas contents are measured by the wet gas meter and analysis by the gas chromatography.
The spent catalyst is also regenerated in this reactor. Firstly, turning off the feed of 8
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vacuum residue and steam, and switched to nitrogen to purge the whole reaction system, and heated the reactor to the preset gasification temperature (heating rate=10 o
C·min−1) with nitrogen. Then nitrogen is switched into the gasification reagent either
pure steam or steam-oxygen. The produced gas products are first condensing with a cooling water of 1 oC, and then measure and analysis by the wet gas meter and GC, respectively. When vacuum residue cracking and coke gasification is not performed, the coke content of the catalyst could be measured by a coke analyzer. Each cycle process is repeated at least twice and the material balance is over 95.0% with the relatively error of the measurement is below 5.0%.
2.4 Characterization and Analysis. The catalyst composition is determined using X-ray fluorescence (XRF) spectrometry (AXIOS, PANalytical). The sample (
