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benium ions rearrangement via protonated cyclopropane rings and cracking by /3 scission are the only observed catalytic reactions in the hydroisomerization and hydrocracking of isooctane on Pt/US-Y zeolites besides the usual proton and methyl shifts. The formation of dibranched, monobranched, and linear octanes proceeds over a consecutive mechanism. Cracking of isomers following energetically favored cleavage routes is superimposed upon the isomerization and as a consequence equilibrium is not established, even within octane fractions with the same degree of branching, in contrast with what is observed with linear alkanes. This is emphasized by the increasing relative concentrations of the cup-isomers (higher than their equilibrium values), viz. 2,2,3- and 2,3,3-trimethylpentanes and 2,2-, 3,3-dimethylhexanes, all of which have to form sec-carbenium ions before rearranging or cracking. With feedstocks having a high degree of branching and therefore forming the more stable tert-carbenium ions, high H2to hydrocarbon ratios and pressures of hydrogen above 10 bars have to be used to prevent deactivation if low process temperatures are desired.
Acknowledgment This work was undertaken thanks to a "Center for Excellence" Grant awarded by the Belgian Ministry of Scientific Affairs within the framework of the "Action Concertge Interuniversitaire Catalyse". Registry No. n-Octane, 111-65-9; isooctane, 540-84-1; isobutane, 75-28-5;2,2-dimethylhexane,590-73-8;2,3-dimethylhexane,
584-94-1; 2,4-dimethylhexane, 589-43-5; 2,5-dimethylhexane, 592-13-2; 3,3-dimethylhexane, 563-16-6; 3,4-dimethylhexane, 583-48-2. Literature Cited API Research Project N 44, "Selected Values of Properties of Hydrocarbons and Related Compounds", 1974. Billon, A.; Franck, J. P.; Perles, J. P. Hydrocarbon Process. Sept 1975, 139. Bolton, A. P. ACS Monogr. 1978, 171, 714. Brouwer, D. M.; Hogeveen, H. R e d . Trav. Chim. Pays-Bas 1970 89, 211. Ciapetta, F. G.; Hunter, J. B. Ind. Eng. Chem. 1953, 4 5 , 147. Kelley, A. E.; Peralta, B.; Reeg, C. P. Chem. Eng. Monogr. 1979, 13, 34. Kerr, G. T. J. C a b / . 1989, 15, 200. Jacobs, P. A.; Uytterhoeven, J. 6.; Steljns, M.; Froment, G. F.; Weitkamp, J. "Proceedings, 5th International Conference on Zeolites"; Rees, L. V. C.. Ed.; Heyden: London, 1980; p 607. Jacobs, P. A,; Martens, J. A,; Weitkamp, J.: Beyer, H. K. Faraday Discuss. Chem. SOC. 1981, 72, 353. McDaniel, C. V.; Maher, P. K. 1st International Congress on Molecular Sieves, London, 1967. Steijns, M.; Froment, G. F.; Jacobs, P. A,; Uytterhoeven, J.; Weitkamp, J. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 654. Thinh, T. P.; Duran, J. L.; Ramalho, R. S. Kaliaguine, S. Hydrocarbon Process Jan 1981, 98. Vaughan, D. E. W. Symposium on Properties and Applications of Zeolites, London, Aprll 18-20, 1979. Ward, J. W. Hydrocarbon Process. Sept 1975, 101. Weltkamp, J.; Farag, H. Acta Phys. Chem. 1978, 2 4 , 327. Weitkamp, J.; Jacobs, P. A. Award Symposium on Fundamentals of Catalysis and Thermal Reactions, 181st National Meeting of the American Chemical Society, Atlanta, GA, March 29-April 3, 1981. Weltkamp, J. 7th International Congress on Catalysis, Tokyo, June 30-July 4, 1980, Comm. C7. Weltkamp, J. "Catalysis by Zeolites"; Imelik, B.;Naccache, C.; Taarit, Y. B.; Vedrine, J. C.; Coudurier, G.; Praliaud, H., Ed.; Elsevier: Amsterdam, 1980; p 65.
Received for review March 7, 1983 Accepted June 20, 1983
Hydroisomerization and Hydrocracking. 5. Kinetic Analysis of Rate Data for n-Octane Mlguel A. Baltanas, Herman Vanslna, and Gllbert F. Froment Laboratorium voor Petrochemische Techniek, Rijksuniversiteit-Gent, Krogslaan 28 1, 8-9000 Gent, Belgium
The kinetics of the hydroisomerization and hydrocracking of n-octane were studied in a CSTR (Berty type) reactor at T = 180-240 OC and P = 5-100 bar using 0.5 wt % R/US-Y zeolite. The same reaction network and lumping structure used for higher boiling point n-alkanes was found to be applicable to n-C,. Physical adsorption of both octanes and cracked products determining the concentrations at the active sites were accounted for in the finally retained kinetic models. The latter also indicate low concentration of carbenium ions with respect to the total nupber of active sites.
Introduction Hydroisomerization and hydrocracking of n-alkanes has been shown to proceed in consecutive steps (Steijns et al., 1978; 1981; Weitkamp and Jacobs, 1981). In a previous paper it was reported that the primary products of the reaction of n-octane are the monobranched isomers, with further formation of multibranched isooctanes that may crack according to energetically favored routes (part 4, Vansina et al., 1983). The kinetics of these reactions is the subject of the present study. The modeling of the multicomponent reaction networks found in hydrotreating has evolved from the pseudofirst-order kinetics approach (Flock et al., 1976; Bhinde, 1979) to rate expressions accounting for competitive adsorption on the active sites, with Langmuir-Hinshel0196-432 1/83/1222-053l$Ol.50/0
wood-Hougen-Watson (LHHW) kinetic models being the most practical (Lo, 1981). The inclusion of physical adsorption of the hydrocarbons in the pores of zeolite catalysts has been the lattest refinement (Steijns and Froment, 1981). A higher effective concentration of hydrocarbon in the vicinity of a site results from the strong adsorption in the zeolite cages. This work extends previous studies on n-decane and n-dodecane. n-Octane was isomerized and cracked in a Berty type reactor at T = 180-240 "C and P = 5-100 bar. A 0.5 wt % Pt/US-Y zeolite was used, as detailed elsewhere (part 4, Vansina et al., 1983). Whenever possible, former approaches have been followed, in order to get a homogeneous set of results that may lead to an eventual prediction of the behavior of homologues in n-alkane hy0 1983 American Chemical Society
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90 150
___________
10 60
*P
0
m
30
-A*#--
i
05
'n-Ce
_
(bar)
Figure 2. Rate of n-octane hydroisomerization vs. n-Cs partial pressure at various total pressure levels (2' = 217-225 "C). Scheme I c
200
100
3 00
ux)
c 8 (g.h/mol)
W/F:-
Figure 1. Conversion of n-octane vs. W/Fonc8around 220 "C.
droisomerization and hydrocracking. Parameter Estimation and Model Discrimination Procedure. The data analysis was performed as outlined by Froment (1975). Parameter estimates were obtained by minimizing the objective function m m
n
s = c c wJ'i=c1(Yi; - 9,) ( Y j l - 9iJ '
;=11=1
where m is the number of responses, n the number of experiments, and wjl the G,l) elements of the inverse of the covariance matrix of the experimental errors on the resposes y (Himmelblau et al., 1967). A matrix was determined from an unweighted preliminary parameter estimation and this matrix was used to come to a second one which was then employed in the final regression. S was minimized by means of a multiresponse Marquardt-Levenberg algorithm. To reduce the computational difficulties arising from the strong correlation between frequency factors and the activation energy, all rate constants were reparameterized as
where ! i is'the average temperature of all the experiments (Kitrell, 1970). The significance of the overall regression was tested by means of the ratio of the mean regression sum of squares to the mean residual sum of squares (Draper and Smith, 1966). The significance of the individual estimates for the model parameters was expressed by their t values. Initial parameter estimates were obtained from the modeling of isothermal and nonisothermal data at low conversions, or from simplified models containing one parameter less. The adequacy of the models was tested by analyzing their residuals. Discrimination among rival models was based on this statistical testing and on their compatibility with the physicochemical constraints.
Results and Discussion Modeling of the Hydroisomerization of LI -Octane. A t low conversions n-octane reacts to form monomethyl
I I1
n-alkane (g) n-alkane (ads) 111.1 n-alkene (adsj + H+ 111.2 R + 111.3 i-R' IV isoalkene (ads) + H, V isoalkane (ads)
=+ n-alkane (ads)
=+ n-alkene (ads) + H, (metal) R+ (acid + i-R+ =+ isoalkene fads) + H.. =+isoalkane (adsj (metal) t.isoalkane ( 9 )
*
.
I
I
.
isomers only (part 4, Vansina et al., 1983). Therefore, the hydroisomerization rates were considered as a starting point for a single response discrimination procedure (Froment and Bischoff, 1979), using sets of data a t 180, 200, and 220 "C for which the conversion was 1 1 2 % and consecutive reactions such as hydrocracking were still negligible. The rate of n-octane hydroisomerization follows the trends already shown for n-Clo and n-Clz (Steijns and Froment, 1981). Some differences are observed; e.g., the rate is nearly inversely proportional to the total pressure (Figure 1)but varies with the partial pressure of n-octane in a nearly hyperbolic way (Figure 2). A nearly zero-order dependence with respect to the hydrocarbon partial pressure was observed by Steijns and Froment for n-decane and n-dodecane; this led them to consider physical adsorption in the zeolite pores to arrive at the concentration of the hydrocarbons inside the solid catalyst. Several expressions for physical adsorption isotherms relating the hydrocarbon concentration inside the zeolite to the partial pressure in the gas phase were considered by these authors. Their arguments also hold for lighter hydrocarbons: at 200 "C, the concentration of n-C7 in Na faujasite pores is about 400 kg/m3 of pore volume (Breck, 1974; Gates et al., 1979). Obviously, a lighter (physisorbed) hydrocarbon will be farther from its saturation pressure than a heavier one: at a given reaction temperature the last one may be ucondensedn,but not necessarily the lighter. A modified version of the classical bifunctional mechanism (Hosten and Froment, 1971) accounting for the (physically) adsorbed hydrocarbons can be used to model the hydroisomerization. See Scheme I. Both the physical adsorption and hydrogenationldehydrogenation steps are assumed to be in equilibrium, since it is known that above 0.1% Pt the hydrogenation-dehydrogenation function is not a rate-determining step (Sinfelt, 1964; Starnes, 1959). Taking the first step of stage 111-the formation of the carbenium ion-as RDS, the following rate expression can be derived, assuming a Langmuir-Hinshelwood mechanism for chemisorption
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 533
where kl = forward rate coefficient of step 111.1;(C = total concentration of active sites; c A , ~ = concentration of physically adsorbed n-alkane; P H = hydrogen partial pressure; K’iA = chemisorption equilibrium constant of step 111.3; Ki = equilibrium constant of step 111.2; KDH = dehydrogenation equilibrium of A; Km, KHj = hydrogenation equilibrium of iA and other species j (i # A, iA); Cj(Kj’/KHj)cjP= chemisorption terms for products or other species j (i # A, iA); c ~= concentration ~ , ~ of physically adsorbed isoalkane. Using a Langmuir-type isotherm to express the hydrocarbon concentrations (physically adsorbed inside the zeolites) in terms of observables in the gas phase leads to
mechanism, since both bifunctional or monofunctional schemes give the same rate expressions when the carbenium ion rearrangement on the surface is the rate determining step. The models based upon the rearrangement as being the RDS yield the best fit of the rate data (Steijns et al., 1981) and have a sound basis: the rearrangement of carbenium ions that leads to additional branching is slower than the rearrangement that preserves the degree of branching (part 4, Vansina et al., 1983). It should also be noted that the use of a Langmuir-type physical adsorption isotherm allows the inclusion of several physisorbed species condensing simultaneously on the zeolite pores and therefore is the most realistic and adequate at high surface coverages (Santacesaria et al., 1982). For the sake of completeness other models were considered. The different rate expressions that are obtained for physical adsorption using the Langmuir, Freundlich, and Drachsel isotherms and for the three possible steps of stage I11 in the bifunctional Scheme I, together with appropriate expressions derived from rival models without physical adsorption, are presented in Table I. Simplified versions of involved expressions were also included, allowing for equal values of adsorption constants or neglect of carbenium ion coverage fractions on the catalyst surface (CR+
