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Energy & Fuels 1997, 11, 596-601
Coke Formation in the Fluid Catalytic Cracking Process by Combined Analytical Techniques Kuangnan Qian,*,† Douglas C. Tomczak,† Edward F. Rakiewicz,† Robert H. Harding,‡ George Yaluris,† Wu-Cheng Cheng,‡ Xinjin Zhao,‡ and Alan W. Peters‡ Washington Research Center, W.R. Grace Co.-Conn, 7500 Grace Drive, Columbia, Maryland 21044 Received November 12, 1996X
A combination of analytical techniques, including X-ray photoelectron spectroscopy (XPS), solid state 13C nuclear magnetic resonance (NMR) spectroscopy, and supercritical fluid extraction/ mass spectrometry (SFE/MS), were used to characterize the detailed composition and structure of coke formed on catalyst in the fluid catalytic cracking (FCC) process. By characterizing coke samples from a series of designed FCC experiments, the effects of conversion on coke composition were systematically studied. SFE is shown to be an effective technique for removing low molecular weight coke molecules from the catalyst. When combined with mass spectrometry, the technique provided molecular level information of the extracted coke species. The coked catalysts were directly analyzed by XPS and NMR to obtain information relevant to surface and bulk coke structures, respectively. The study revealed the presence of two types of nitrogen-based coke and showed that N distributions were strongly affected by FCC conversion level. The study also suggests that most nitrogen-containing coke is formed in the earlier stages of cracking while hydrocarbons are the primary contributors to coke yield in the later stages of cracking. The aromaticity of coke remains fairly constant at high conversions.
Introduction In the fluid catalytic cracking (FCC) process, the reversible deactivation of FCC catalysts has been mainly associated with the carbonaceous deposits or coke formed on the catalyst surface.1,2 Coke yield is an important measurement of FCC catalyst performance and one of the key criteria in catalyst selection and catalyst performance optimization. A detailed understanding of coke composition and the impact of catalyst properties, feed composition, and feed-catalyst interactions on coke formation is critical to the development of FCC catalysts and commercial FCC operation. In addition, since a majority of the coke is combusted in the FCC regenerator and released as flue gases, a better understanding of coke composition, particularly the heteroatomic coke components, may have significant implications for FCC environmental technologies, e.g., SOx, NOx, and CO controls. It has been generally recognized that coke is formed in an FCC process via reactions of feedstock molecules on acid sites of the FCC catalyst.2 Both feedstock composition and catalyst formulation have significant impact on coke yield.3-5 Hydrocarbon coking on zeolites have been extensively studied, where coking rate and †
W.R. Grace Co-Conn. Grace Davison. Abstract published in Advance ACS Abstracts, April 1, 1997. (1) Appleby, W. G.; Gibson, J. W.; Good, G. M. Ind. Eng. Chem. Process Des. Dev. 1962, 1, 102-110. (2) Wojciechowski, B. W.; Corma, A. Catalytic Cracking; Marcel Dekker, Inc.: New York, 1996. (3) Harding, R. H.; Zhao, X.; Qian, K.; Rajagopalan, K.; Cheng, W-C. Ind. Eng. Chem. Res. 1996, 35, 2561-2569. (4) Hughes, R.; Hutchings, G.; Koon, C. L.; McGhee, B.; Snape, C. E. Stud. Surf. Sci. Catal. 1996, 100, 313-322. (5) Ross, J. L.; Johnson, A. R.; Saraf, A. Proc. Int. Conf. Pet. Refin. Petrochem. Process. 1991, 317-323. ‡
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selectivity were found to be closely related to the zeolite types and pore structures.6-9 Coke can be formed from a wide range of hydrocarbons, including small molecules such as propene, via a series of polymerization and dehydrogenation reactions.6,10 Polynuclear aromatics (PNA) and heteroatomic molecules in feedstocks are the most important precursors of FCC coke.1,11,12 Nitrogen molecules, particularly basic nitrogen compounds (pyridinic compounds), have the greatest impact on coke yield.13 Despite the progress in analytical techniques, characterization of coke composition has remained a challenge due to the fact that (a) coke molecules are largely nonvolatile and very complex in composition, (b) coke molecules are strongly absorbed on the catalyst surface and are difficult to remove by conventional extraction methods, and (c) coke concentration on catalyst can be low (2% coke yield). As conversion level increases, nonpolarized N coke species are gradually converted into polarized N coke species, indicating stronger interactions between coke and catalyst surface at the high FCC conversions. The aromaticity of coke molecules initially increases, and then stabilizes at roughly 90% aromatic carbon level, suggesting that coke dealkylation is important only in the earlier stages of FCC cracking. Coke extraction efficiency by SFE decreases significantly as conversion level increases, suggesting that coke molecules become more aromatic, bigger, and more tightly bounded to the catalyst surface as conversion level increases. The increase in hydrocarbon coke was found to be mainly due to the increase of core aromatic structures. These results support the view that naphthenic cleavage and dehydrogenation and aromatic condensations are the main reactions in coke formation, particularly at higher conversion levels.
Conclusions Combined analysis by supercritical fluid extraction (SFE) and spectroscopic techniques (XPS, NMR, MS) provides a good picture of FCC coke compositions and structure. Our study confirmed that nitrogen compounds, particularly the basic nitrogen and amides, are among the major precursors of FCC coke. XPS revealed two different N species in FCC coke; one is in a more polarized environment than the other as indicated by
Acknowledgment. The authors acknowledge Drs. K. Rajagopalan, R. Gatte, G. Hatfield, and J. Onuferko for valuable comments and suggestions on the paper. Mr. M. Jones helped to conduct a part of the SFE experiments and should be acknowledged. We also thank W.R. Grace Co-Conn. and Grace Davison for permission to publish this work. EF960204U
