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Ind. Eng. Chem. Res. 1996, 35, 1506-1516
Mechanistic Modeling of n-Heptane Cracking on HZSM-5 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.sConn., Columbia, Maryland 21044
A mechanistic model for the catalytic cracking of n-heptane was developed using a novel mechanism-based lumping scheme that exploits the chemical similarities within reaction families. The formal application of 13 reaction family matrices, which correspond to the 11 reaction families in the model, to the matrix representations of the reactants and derived products generated 70 species, 235 elementary steps and 70 ordinary differential equations. The reaction family concept was further exploited to constrain the kinetics within each reaction family to follow a quantitative structure/reactivity Polanyi relationship. Ultimately, four Polanyi relationship parameters and one catalyst specific parameter were optimized using experimental data obtained from the cracking of n-heptane at 500 °C over HZSM-5 with a Si/Al ratio of 21.25. The model correlations were excellent, as were the a priori predictions of experimental results at 450 and 550 °C with an HZSM-5 Si/Al ratio of 21.25 and at 500 °C with HZSM-5 Si/Al ratios of 35.25 and 63.5. The thus validated model was then used to probe the controlling elementary steps of n-heptane cracking. Carbonium ion cracking, β-scission, and hydride transfer were the kinetically significant reactions. Introduction Fluid catalytic cracking (FCC) is the primary refining process for producing gasoline. Recent environmental legislation and issues of product performance have directed the focus of much FCC research to the molecular composition of the feedstock and product spectrum. This focus has generated an associated interest in modeling FCC chemistry at the molecular level. The immediate modeling challenge is that the feedstock and product slates comprise thousands of molecules that react through numerous pathways with at least 104 associated rate constants. Prior investigations of the kinetics of catalytic cracking reactions have focused on both the reactions of complex petroleum feedstocks (John and Wojciechowski, 1975; Coopmans et al., 1992), such as gas oils, and the reactions of pure components (Greensfelder and Voge, 1945; Good et al., 1947; Nace, 1969; Abbot and Wojciechowski, 1985; Corma et al., 1992; Groten and Wojciechowski, 1993). Early studies using complex gas oil feedstocks led to empirical models which predict the interconversion of lumped pseudocomponents (paraffins, olefins, naphthenes, and aromatics) or global reactant classes (boiling point cuts) (Weekman and Nace, 1970; Jacob et al., 1976). These models are typically feedstock specific, and the underlying molecular information is obscured due to the multicomponent nature of each lump. Recent modeling efforts (Liguras and Allen, 1989a,b; Liguras et al., 1992) have been more molecularly explicit, which introduces the need for an extensive database of reaction pathways and kinetics. The literature provides excellent guidance regarding the likely pathways and elementary steps, but available quantitative kinetic information is still less than that required for each reaction in the cracking of a gas oil. * To whom correspondence should be addressed. FAX: (302) 831-1810. E-mail:
[email protected].
In principle, such a database could be generated through pure component and simple mixture experiments. The many studies that have been completed (Greensfelder and Voge, 1945; Good et al., 1947; Nace, 1969; Abbot and Wojciechowski, 1985; Corma et al., 1992; Groten and Wojciechowski, 1993) still fall short of the demand of a molecular model for a gas oil. It is clear that the goal of amassing rate constants experimentally for each reactant and intermediate in gas oil cracking is impractical and unrealistic. Some level and type of lumping are required. Classical physical-organic chemistry provides the basis for a new lumping approach. This use of linear free energy relationships (LFERs) in kinetic lumping is suggested by the observation that even the most complex petroleum feedstocks contain only a few compound classes (paraffins, olefins, naphthenes, and aromatics), which in turn react through a limited number of reaction families. The implied lumping approach would organize a complex reacting mixture into reactions of sets of many similar compounds where, within a given set of compounds, differences occur only in the substituents. Subsequently, differences in reactivity (rate constants) can be attributed to these substituents. 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 reaction. As a result, a small number of reaction matrices (of order 10) can be used to generate thousands of reactions. The physical-organic chemistry base also provides a concise parameter estimation strategy. Quantitatively, the substituent effects on reactions within a reaction family can be handled through the use of 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.
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Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996 1507
An LFER is a semiempirical correlation of kinetic data with reacting species’ properties. The early work of Hammett (Hammett, 1937), Brønsted (Lowry and Richardson, 1987), and Evans and Polanyi (Evans and Polanyi, 1938) provide the classic formalisms. 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). The reactivity index can be a property of one of the molecules involved in the reaction, such as a heat of formation, or a property of the reaction itself, such as the enthalpy change of reaction. Ultimately, LFER-like structure/reactivity correlations for each reaction family will take the form of eq 1:
log kj ) a + bRIj
(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 where 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 thermal cracking (LaMarca, 1992) and hydrocracking (Neurock, 1992; Korre, 1994). Earlier work using a similar approach to describe the catalytic cracking of alkylbenzenes (Mochida and Yoneda, 1967) and isobutane (Dumesic et al., 1993) further demonstrates the validity of the approach to catalytic cracking chemistry. The objective of the present work was to determine LFER kinetic parameters for the catalytic cracking of paraffins by using the reaction family approach at the mechanistic level. These LFER parameters would be, at this stage, catalyst specific but, crucially, feedstock invariant, and would thus ultimately be used in gas oil reaction models. The present work focused on experimental studies of the catalytic cracking of n-heptane, which provided the basis for the ultimate development of a mechanistic model. Future publications will address the catalytic cracking of longer chain paraffins as well as alkylaromatic and alkylnaphthenic compounds. A similar approach will be used to determine the LFER parameters for each compound class. Experimental Section The activity and selectivity measurements reported in this work were taken at W.R. Grace & Co.sConn. with an isothermal fixed bed reactor. The reactor vessel was a quartz tube approximately 50 cm in length and 2 cm in diameter. In each study, 0.10 g of HZSM-5 zeolite was diluted in 4 cm3 of alundum (a low surface area alumina), to maintain a constant thermal mass, and was positioned in the center of the reactor. The quartz tube was heated with a three-zone furnace. Temperature measurements by a thermocouple placed in the center of the bed showed that the bed temperature decreased by less than 5 °C during the measurement. Alundum by itself converted less than 1% of the n-heptane at all temperatures and weight hourly space velocities (WHSVs) reported in this work. Different conversions were measured at constant temperature by changing the feed flow rate. n-Heptane (Aldrich, 99% + purity) was introduced into the reactor with a nitrogen bubbler, which carried the n-heptane to the reactor at its room temperature saturation
Table 1. Physical Properties of the HZSM-5 Catalysts Si/Al ratio BET surface area, m2/g sodium content, wt % mean particle size, µm pore volume, cm3/g
21.25 423 0.043 6.0 0.256
35.25 400 0.025 5.7 0.253
63.5 419 0.027 4.1 0.221
pressure. The n-heptane feed rate was varied between 0.325 and 3.25 g/h by varying the nitrogen flow rate with a calibrated Brookfield mass flow controller. Data were taken at 1 min time on stream and were analyzed with an on-line gas chromatograph. Six consecutive measurements taken from the same zeolite sample showed no decrease in conversion with time, which indicates that these HZSM-5 samples did not deactivate by coking on this time scale. Three HZSM-5 samples with different Si/Al ratios were prepared by a standard synthesis recipe (Shiralkar and Clearfield, 1989). The Si/Al ratios of the materials were varied by changing the ratio of Si/Al in the synthesis gel. The samples had good physical properties, with calcined surface areas exceeding 400 m2/g by nitrogen porosimetry and low sodium contents (
