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Comparison of Thermal Cracking Processes for Athabasca Oil Sand Bitumen: Relationship between Conversion and Yield Masato Morimoto, Yoshikazu Sugimoto, Shinya Sato, and Toshimasa Takanohashi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef501610d • Publication Date (Web): 30 Sep 2014 Downloaded from http://pubs.acs.org on October 2, 2014
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Energy & Fuels
Comparison of Thermal Cracking Processes for Athabasca Oil Sand Bitumen: Relationship between Conversion and Yield
Masato Morimoto*, Yoshikazu Sugimoto, Shinya Sato, Toshimasa Takanohashi
Advanced Fuel Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8569, Japan
＊
Corresponding author.
E-mail:
[email protected] ACS Paragon Plus Environment
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Abstract This study compared various thermal cracking processes for Athabasca oil sand bitumen according to the relationship between vacuum residue (VR) conversion and the product yield for each process, using reported data. The conversion was defined as the fraction of VR which was converted to lighter products. The conventional processes examined were visbreaking, delayed coking, and fluid coking, and the developing processes were high conversion soaker cracking (HSC), heavy to light (HTL), IyQ, Eureka, and supercritical water cracking (SCWC). HSC and SCWC were higher severity visbreaking-type processes with conversions of 0.49 and 0.39–0.50, respectively. HTL and IyQ_(recycle) were lower severity fluid coking-type processes with a conversion of 0.52. IyQ (once through), and Eureka showed the highest conversions (0.62–0.68). Supercritical water (SCW) upgrading was operated experimentally at higher severity, with a conversion of 0.64, and showed the highest yield of distillate product (DP) among all thermal cracking processes investigated. Analysis of the conversion–yield relationship revealed the thermal cracking behavior of Athabasca bitumen. The key for achieving higher conversions with lower coke yield was considered to be efficient mass transfer of the volatile fraction stripped away from condensed phase which is liquid fraction at the experimental condition. The condensed-phase decomposition showed an upper limit of conversion of 0.55, and that with high mass-transfer system exceeded 0.65.
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Keywords: Bitumen, Upgrading, Cracking, Visbreaking, Coking, Supercritical water 1. Introduction Oil sand bitumen in Alberta, Canada, is an important energy resource because of its abundant recoverable reserves. The proven reserve is estimated to be about 170 billion barrels, and the Athabasca deposit in Alberta shows the largest cumulative production.1 Because of the low fluidity of bitumen, a thermal upgrading process is essential, unless it is diluted for transportation purposes. Several thermal cracking processes that do not involve the use of catalysts and hydrogen are available for Athabasca bitumen: visbreaking, delayed coking, and fluid coking are conventional methods2, and heavy to light (HTL)3-6, IyQ7, 8, high conversion soaker cracking (HSC)
9, 10
, Eureka11-13, and supercritical water cracking (SCWC)
14
have been
evaluated for potential use. The design and concept of each process have been described in detail elsewhere.2-14 Briefly, visbreaking is operated under less severe conditions because the main purpose is to simply reduce the viscosity without coke formation. On the other hand, delayed coking and fluid coking are high-severity processes, targeting light oil production with coke as the by-product. Fluid coking burns a certain amount of coke in the process to supply heat energy to the cracking reaction, resulting in a decrease of discharged coke. HSC is a soaker-type visbreaking process with a steam stripping system. HTL is a fluid coking process with a rapid quenching system for cracked products using a fluid catalytic cracking (FCC) reactor with sand as the bed material. IyQ is a type of fluid coking that uses a cross-flow fluidized bed reactor. Eureka is short-duration
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delayed coking by high-temperature steam stripping. SCWC is a combined process of visbreaking and deasphalting using supercritical water (SCW), as SCW was inert medium affording no chemical effects on the upgrading reaction of bitumen.15-17 The developing processes mentioned above each have their own advantages over conventional processes. However, how the developing processes affect the thermal cracking behavior of bitumen is not clear. Herein, we compared the conventional and developing processes with lab-scale experiments based on thermal cracking behavior by investigating the relationship between conversion and yield for Athabasca bitumen.
2. Comparison Method The properties of typical Athabasca bitumens are as follows. 18-21 API gravity: 7.7-9.0°. Elemental composition: C 83-84, H 10-11, N 0.4-0.9, S 4.4-5.4, O 0.7-1.4, Ni 0.0069-0.0085, V 0.0081-0.0218 wt%. Asphaltene (pentane-insoluble fraction) content: 16-25 wt%. Microcarbon residue (MCR) and Conradson carbon residue (CCR): ~12-19 wt%. We summarized 12 sets of reported operational conditions and product yields for the thermal cracking of Athabasca bitumen. The eight processes investigated were visbreaking22, delayed coking23, fluid coking8, HSC10, HTL4-6, IyQ8, Eureka12,
13
, and SCWC22. The four
experiments were as follows: (1) batch experiments in a nitrogen atmosphere at low pressure (N2-Batch_lowP)24, (2) batch experiments in a high-pressurized nitrogen at 23–25 MPa (N2-Batch_highP)15, (3) batch experiments in SCW at 27–30 MPa (SCW-Batch)15, and (4)
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experiments in SCW using a continuous stirred tank reactor (CSTR) at 440°C and 25 or 30 MPa (SCW-CSTR)17. The latter three experiments were conducted by us. In the reaction, gases, distillate product (DP), and coke were produced from the original VR. The conversion (C) is defined as the fraction of VR which is converted to lighter product, calculated by Eq. (1):
C = 1−
YVR + YCoke , YVROrig
(1)
where Y represents the yield (wt%) and subscripts VROrig, VR, and Coke are original vacuum residue (VR), recovered VR, and product coke (toluene-insoluble fraction), respectively. The DP was defined as the liquid fraction obtained by vacuum distillation. The cutpoint for VR and DP separation depended on the report, either 525 or 540°C. The C was utilized to clarify the thermal cracking behavior of each process. It can be noted that the economic value of unconverted vacuum residue in a liquid blend is much higher than coke for an industry that sells liquid volume.
3. Results and Discussion 3.1 Product yield and conversion Table 1 summarizes the reaction conditions, conversion, and product yields of each process. The reported fraction of VR in the feed bitumen ranged from 50 to 54 wt%, depending on each cutpoint. The operation temperature for each process was as follows: visbreaking 470-495°C,
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delayed coking 480-565°C, HSC 440-460°C for heater and 400-420°C for soaker, HTL 530°C, IyQ 500-529°C, Eureka 495°C for heater, 400-440°C for reactor, and 600-685°C for steam, SCWC 430°C, N2-Batch_lowP 430°C, N2-Batch_highP 420-450°C, SCW-Batch 420-450°C, and SCW-CSTR 440°C. The coke yields for HSC, Eureka, and SCWC were not reported, and those listed in the table for fluid coking and HTL represent values without coke combustion, even though those processes burn some fraction of the coke produced (about 30 and 50%, respectively). As a result, the value of coke yield (wt%) for each process was as follows: visbreaking 2, delayed coking 34, fluid coking 28, HTL 21, IyQ (recycle) 26, IyQ (once through) 6–7, N2-Batch_lowP 0, N2-Batch_highP 2–23, SCW-Batch 4–14, and SCW-CSTR 12 at 25 MPa and 10 at 30 MPa. The conversion value for each process was as follows: visbreaking 0.32, delayed coking 0.36, fluid coking 0.47, HSC 0.49, HTL 0.52, IyQ (recycle) 0.52, IyQ (once through) 0.62–0.68, Eureka 0.65, SCWC 0.39–0.50, N2-Batch_lowP
