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Energy & Fuels 1997, 11, 354-363
Articles Mechanistic Modeling of n-Hexadecane Cracking on Rare Earth Y Beth A. Watson and Michael T. Klein* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Robert H. Harding W. R. Grace & Co.-Conn., Columbia, Maryland 21044 Received May 28, 1996. Revised Manuscript Received December 4, 1996X
The cracking reaction pathways and mechanisms of n-hexadecane with a rare earth Y (REY) catalyst were studied. Experiments at 500 °C indicated that the dominant reactions were isomerization and cracking to smaller paraffins and olefins. These results were described in terms of a kinetic model that was based on a novel mechanism-oriented lumping scheme that exploits the chemical similarities within reaction families of elementary steps. Thus, 13 reaction family matrices were able to describe all of the elementary steps. Formal application of these reaction matrices to the matrix representations of the reactants and derived products generated the model. The reaction family concept was further exploited to constrain the kinetics within each reaction family to follow a quantitative structure/reactivity Polanyi relationship. Ultimately, three Polanyi relationship parameters, one catalyst-specific parameter and two coking/deactivation parameters, were determined by optimizing the model fit to the experimental data. The resulting model correlations were excellent, which suggests the optimized parameters contain fundamental structure/reactivity information.
Introduction The significance of solid acid catalyzed reaction chemistry can be measured by the process chemistry it underlies: fluid catalytic cracking (FCC), hydrocracking, alkylation, re-forming, and isomerization are major refinery units based on acid chemistry. The 1990s have also witnessed increased attention to the detailed molecular kinetic modeling of these processes. The molecular nature of these models provides an opportunity to produce kinetic parameters of fundamental significance that has hitherto been prevented by globally lumped models. This motivated the present program of model compound kinetic studies aimed at producing kinetic information that would be common to both model compounds and the components of a gas oil. Gas oils contain paraffinic, aromatic, and naphthenic compound classes. We have previously reported on catalytic cracking experiments and mechanistic modeling of the short-chain paraffin heptane.1 The present study extends that work and focuses on the cracking of a longer chain paraffin that is more typical of molecules found in gas oil feedstocks to FCC units. The * Author to whom correspondence should be addressed [fax (302) 831-1810; e-mail
[email protected]]. X Abstract published in Advance ACS Abstracts, February 1, 1997. (1) Watson, B. A.; Klein, M. T.; Harding, R. H. Mechanistic Modeling of n-Heptane Cracking on HZSM-5. Ind. Eng. Chem. Res. 1996, 35, 1506-1516.
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literature2-5 also indicates that paraffin reaction rates and the complexity of the associated reaction pathways increase with chain length. The primary pathways are cracking and isomerization to smaller paraffins and olefins, and these are accompanied by a large number of secondary reaction pathways. Thus, for not only gas oils but even a simple model compound, some type of organizational scheme is needed to keep the number of reaction paths and required rate constants from placing an insurmountable burden on the model. The organizational scheme we have been developing1,6,7 derives from classical physical-organic chemistry. This lumping approach organizes even the most complex petroleum feedstocks into a set of only a few compound classes (paraffins, olefins, naphthenes, and aromatics), which, in turn, react through a limited number of reaction families. Intrafamily reactivity differences are due to the electronic effects of substit(2) Greensfelder, B. S.; Voge, H. H. Catalytic Cracking of Pure Hydrocarbons. Ind. Eng. Chem. 1945, 37, 514-520. (3) Abbot, J.; Wojciechowski, B. W. Kinetics of Catalytic Cracking of n-Paraffins on HY Zeolite. J. Catal. 1987, 104, 80-85. (4) Abbot, J. Catalytic Cracking of Long-Chain Paraffins and Olefins on HY Zeolite. J. Catal. 1990, 124, 548-552. (5) Corma, A.; Miguel, P. J.; Orchilles, A. V. Kinetics of the Catalytic Cracking of Paraffins at Very Short Times on Stream. J. Catal. 1994, 145, 58-64. (6) Neurock, M.; Klein, M. T. When you can’t measuresmodel. CHEMTECH 1993, 23 (9), 26-32. (7) Korre, S. C. Quantitative Structure/Reactivity Correlations as a Reaction Engineering Tool: Applications to Hydrocracking of Polynuclear Aromatics. Ph.D. Dissertation, University of Delaware, 1994.
© 1997 American Chemical Society
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uents. The resulting implication is that many chemical reactions will involve only a small number of irreducibly different atomic rearrangements in which the substituents affect the rate but do not otherwise alter the connectivity change of the reaction. As a result, a small number of reaction matrices (of order 10) can be used to generate thousands of reactions. This approach also provides a concise parameter estimation strategy. Quantitatively, the substituent effects on reactions within a reaction family can be handled through the use of linear free energy relationships (LFERs) to describe the rate constants. The reduction of complexity is realized as a small number of LFER parameters (of order 10) instead of independent rate constants (of order 104) as the ultimate gas oil model parameters. A LFER is a semiempirical correlation of kinetic data with reacting species’ properties. In the present application, each sterically similar reaction family is defined by a single Arrhenius A factor, and its LFER relates the change in activation energy to a reactivity index (RI). Ultimately, LFER-like structure/reactivity correlations for each reaction family will take the form of eq 1:
log kj ) a + b(RIj)
(1)
The constants a and b are determined from a basis set of pure component and simple mixture experimental data. This correlation can then be used to estimate rate constants for other molecules for whichwhere no data exist. The utility of using the LFER lumping approach for the modeling of complex mixtures, such as petroleum feedstocks, is clear. The use of these correlations will greatly reduce the amount of experimental data needed and the total number of kinetic parameters required. The approach has been used successfully for n-heptane and phenyloctane catalytic cracking,1,8 thermal cracking,9 and hydrocracking.6,7,10 Earlier work using a similar approach to describe the catalytic cracking of alkylbenzenes11 and isobutane12 further demonstrates the validity of the approach to catalytic cracking chemistry. The objective of the present work was twofold: (1) to study reaction pathways and mechanisms for n-hexadecane cracking and (2) to determine LFER kinetic parameters (a and b in eq 1) for the catalytic cracking of paraffins by using the reaction family approach at the mechanistic level. An experimental study of nhexadecane cracking over rare earth Y (REY) was used to identify reaction products and their molar selectivities, which in turn specified the operative reaction pathways and mechanisms. This provided the basis for the development of a mechanistic model to describe the catalytic cracking of hexadecane. Optimizing the model (8) Watson, B. A.; Klein, M. T.; Harding, R. H. Catalytic Cracking of Alkylbenzenes: Modeling the Reaction Pathways and Mechanisms Appl. Catal. 1996, submitted for publication. (9) LaMarca, C. Kinetic Coupling in Multicomponent Pyrolysis Systems. Ph.D. Dissertation, University of Delaware, 1992. (10) Neurock, M. A Computational Chemical Reaction Engineering Analysis of Complex Heavy Hydrocarbon Reaction Systems. Ph.D. Dissertation, University of Delaware, 1992. (11) Mochida, I.; Yoneda, Y. Linear Free Relationships in Heterogeneous Catalysis 1. Dealkylation of Alkylbenzenes on Cracking Catalysts. J. Catal. 1967, 7, 386-392. (12) Dumesic, J. A.; Rudd, D. F.; Aparicio, L. M.; Rekoske, J. E.; Trevino, A. A. The Microkinetics of Heterogeneous Catalysis; American Chemical Society: Washington, DC, 1993.
predictions to the data gave LFER parameters for paraffin cracking that are catalyst specific but feedstock invariant and could ultimately be used in gas oil reaction models. Experimental Section The reactant, n-hexadecane (Aldrich, 99%+ purity), was obtained commercially and used as received. The activity and selectivity measurements were taken at W. R. Grace & Co.Conn. with an isothermal fixed bed reactor. The reactor vessel was a quartz tube approximately 50 cm in length and 2 cm in diameter. The catalyst was pressed to 40/80 mesh size, positioned in the center of the reaction tube, and preheated for 30 min at 500 °C under 10 cm3/min nitrogen flow. The quartz tube was heated with a three-zone furnace, and the actual catalyst temperature was measured with a type-K thermocouple in the center of the bed. The reactant was pumped into the reaction chamber with a syringe infusion pump at the rate of 0.6 g/min. Nitrogen was cofed with the reactant at the rate of 10 cm3/min (at STP) set by a mass flow controller. The cracking reactions were run for 3 min time on stream at 500 °C, 0-1 psig. The hexadecane partial pressure was 87.8 kPa. The weight hourly space velocity (WHSV) was varied by changing the amount of catalyst in the reactor tube. The space velocities were in the range of 721440 h-1. To maintain a constant thermal mass, the catalyst was diluted with alundum (a low surface area alumina) to a constant bed volume of 4 cm3. Alundum by itself converted
