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Single-Event Rate Parameters for the Hydrocracking of Cycloalkanes on Pt/US-Y Zeolites Gert G. Martens, Joris W. Thybaut, and Guy B. Marin* Laboratorium voor Petrochemische Techniek, Universiteit Gent, Krijgslaan 281, B-9000 Belgium
A single-event kinetic model is applied to the hydrocracking of cycloalkane model components on two bifunctional Pt/US-Y zeolites over a wide range of experimental conditions (T ) 493573 K, P ) 10-50 bar, molar H2-to-hydrocarbon ratio ) 50-300). Values for the standard activation entropies of the elementary steps were obtained from transition state theory, while independently determined Henry coefficients were used to describe the physisorption. Values for the composite activation energies, i.e., the sums of the protonation enthalpies of the alkene intermediates and the activation energies of the elementary carbenium ion transformations, were obtained from a regression of the experimental data. The composite activation energies for intra-ring alkyl shifts vary from 21 to 25 kJ/mol, which is higher than those for the corresponding acyclic alkyl shifts, which vary from 10 to 16 kJ/mol. The composite activation energies for cyclic protonated cyclopropane branching reactions range between 27 and 40 kJ/ mol and are comparable to those of the corresponding acyclic reactions. The same holds for exoand acyclic β-scission, with values from 22 to 76 kJ/mol. Endocyclic β-scissions, in general, have lower composite activation energies, varying from 31 to 56 kJ/mol, than acyclic β-scissions. However, because of a much lower preexponential factor, they proceed at a lower rate than acyclic β-scissions. Introduction Because of the large number of compounds present in typical feeds for oil refining processes such as catalytic cracking, hydrocracking, and catalytic reforming and because of the rather poor knowledge concerning the chemical composition of these mixtures caused by limitations in the chemical analysis, most kinetic models drastically simplify the kinetics of the conversion of these complex mixtures by grouping molecules into a discrete number of lumps. A straightforward lumping strategy is to choose the lumps on the basis of the boiling point range of typical products, i.e., LPG, gasoline, diesel, etc., and/or on the component types present in the feed mixture.1-3 The predictive capabilities of these models improve as the number of lumps increases. In practice, this increase is limited, as the number of reactions, and hence the number of parameters to be determined, increases more than proportionally with the number of lumps defined, while their values often depend on the composition of the feedstock used. A model comprising a manageable set of fundamental rate parameters requires a judicious translation of the reaction chemistry, in the present case carbenium ion chemistry, into appropriate rate expressions. There are relatively few families of elementary reactions involved in hydrocracking. Alkyl shifts, protonated cyclopropane isomerizations, and β-scissions are the most important. The number of kinetic parameters can be reduced by introducing assumptions concerning the effects of the types and structures of the carbenium ions involved in these families of elementary reactions. For the singleevent model used in the present paper, Baltanas et al.4 neglected a possible effect of variations in the skeletal * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +32 9 2644999.
structure. Only the type of the carbenium ion intermediates, i.e., secondary or tertiary, is considered to be relevant, while the effect of changes in the global symmetry of the reactant and transition state on the reaction rate is also taken into account by describing the elementary steps in terms of single events. The number of single events is defined as the ratio of the global symmetry numbers of the reactant and activated complex. The application of the single-event model to hydrocracking has been restricted to alkanes until now. From ample kinetic studies4-7 using various model components, it has emerged as an excellent model that is able to describe, in detail, the product composition over a broad range of process conditions. Recently, the carbon number independence of this model was verified, and its applicability was extended to a series of (US)-Y zeolites instead of one single USY zeolite.8 Moreover, the number of kinetic parameters to be estimated by regression of experimental hydrocracking data was decreased via the calculation of the preexponential factors.8 The single-event model has not yet been applied to the hydrocracking of cycloalkanes, however. Elaborate studies with model cycloalkanes9-13 have been performed. Although most of the reactions involving the ring, e.g., by altering the position and number of alkyl substituents on the ring, are mechanistically identical to the corresponding elementary reactions in aliphatic hydrocarbons, the more rigid structure of the ring is known to have a certain impact on the corresponding reaction rates. Compared to β-scission in an aliphatic chain (acyclic β-scission), cleavage of a C-C bond that is part of a ring (endocyclic β-scission) is known to proceed at a much lower rate.14 Another type of β-scission influenced by the presence of a ring is exocyclic cracking, cleavage of the bond between a substituent on the ring and the ring itself. A typical
10.1021/ie000799n CCC: $20.00 © 2001 American Chemical Society Published on Web 03/23/2001
Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001 1833 Table 1. Specifications of the Pt/US-Y Zeolites catalyst
MC-301
MC-389
Al2O3 (wt %) Pt content (wt %) R activity Na content (ppm)
0 0.5 53 -
35 0.64 Exo(s,s) The predominance of the paring reaction is reflected in the high rate of the (t,t) exocyclic β-scission mode, the final step of this reaction pathway. Conclusions The ability of the single-event kinetic model to describe the hydrocracking of cycloalkanes has been demonstrated. Reliable kinetic parameters have been determined for the reaction families typical for cycloalkane hydrocracking, i.e., intra-ring alkyl shift, cyclic PCp branching, and exo- and endocyclic β-scission. The composite activation energies associated with these reaction steps were the only parameters that were estimated by a regression of experimental hydrocracking data on model cyclkoalkanes. All other parameters were fixed at calculated or previously determined values. The preexponential factors were calculated from first principles, while the kinetic parameters for secondary isomerization and cracking were fixed at values obtained from previous regressions of alkane hydrocracking data. Literature physisorption data of alkanes on similar USY zeolites were used. The use of two different USY zeolites does not interfere with the determination of the kinetic parameters as the latter are USY-zeoliteindependent. With a minimal number of fundamental kinetic parameters and a limited set of USY-dependent parameters, the presented model is able to describe the hydrocracking of both alkanes and cycloalkanes. It can also be used for the design and simulation of industrial hydrocracking units with complex feedstocks. Acknowledgment This research has been performed as a part of the program “Interuniversitaire attractiepolen, funded by the Belgian government, Diensten van de Eerste Minister-Federale diensten voor wetenschappelijke, technische en culturele aangelegenheden”. Nomenclature Roman Symbols A ) preexponential factor, kgcat (mol s)-1 Ci ) surface concentration of i, mol kgcat-1 Csat,i ) saturation concentration of a hydrocarbon i, mol kgcat-1 Ct ) total concentration of acid active sites, mol kgcat-1 E ) activation energy, kJ mol-1 Fi,j ) experimental molar flow rate of the jth response of the ith experiment, mol s-1
Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001 1843 F ˜ i,j ) calculated molar flow rate of the jth response of the ith experiment, mol s-1 h ) Planck’s constant , 6.626 × 10-34 J s HL,i ) Henry coefficient for physisorption of hydrocarbon i, mol (kgcat Pa)-1 k(m1;m2) ) rate coefficient of a reaction step that converts a carbenium of type m1 into one of type m2, s-1 k˜ (m1;m2) ) single-event rate coefficient of a reaction step that converts a carbenium of type m1 into one of type m2, s-1 kB ) Boltzmann constant , 1.38 × 10-23J K-1 kL(g;h) ) lumped rate coefficient for conversion of lump g into lump h, Pa s-1 K ) equilibrium coefficient K ˜ ) single-event equilibrium coefficient KL,i ) Langmuir coefficient for physisorption of hydrocarbon i, Pa-1 (LC)isom(m1;m2)(g;h) ) lumping coefficient for isomerization of carbenium ions of type m1 of lump g into carbenium ions of type m2 of lump h, Pa mik, mlo ) type of carbenium ion k (o) corresponding to alkane i (l), s, or t ne ) number of single events pg ) (partial) pressure, Pa Rw j ) net rate of formation of component j via reaction pathway w, mol (kgcat s)-1 S ) entropy, J (mol K)-1 SSQ ) sum of squares, (mol/s)2 T ) temperature, K Vm,i ) saturated molar volume of hydrocarbon i, m3 mol-1 Vp ) pore volume of the catalyst, m3 kgcat-1 wj ) weighing factor of jth response Wi ) mass of catalyst used in ith experiment, kgcat X ) conversion, % y ) mole fraction Greek Symbols ∆H ) enthalpy difference, kJ mol-1 ∆S ) entropy difference, J (mol K)-1 σ ) global symmetry number γ ) hydrogen-to-hydrocarbon ratio, mol mol-1 Subscripts and Superscripts 0 ) inlet 0 ) standard ads ) adsorption AS ) alkyl shift comp ) composite COuv ) cycloalkene with index v resulting from cycloalkane with index u Cr ) acyclic β-scission CR+ik ) cyclic carbenium ion with index k resulting from cycloalkane with index i cycPCP ) cyclic PCP branching DH,ij ) dehydrogenation of alkane i into alkene j endo ) endocyclic β-scission, ring opening exo ) exocyclic β-scission, dealkylation irAS ) intra-ring alkyl shifts isom ) isomerization Ni ) cycloalkane with index i Oij ) alkene with index j resulting from alkane with index i OR+ik ) alkene carbenium ion with index k resulting from alkane with index i Pi ) alkane with index i PCP ) branching involving protonated cyclopropyl carbonium ions phys ) physisorption prot ) protonation reac ) reaction
R+ik ) carbenium ion with index k resulting from alkane with index i RO ) ring opening, endocyclic β-scission rot ) rotational t ) total trans ) translational
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Received for review September 5, 2000 Revised manuscript received January 18, 2001 Accepted January 23, 2001 IE000799N
