3242
Ind. Eng. Chem. Res. 1997, 36, 3242-3247
Hydrocracking of n-Alkane Mixtures on Pt/H-Y Zeolite: Chain Length Dependence of the Adsorption and the Kinetic Constants Joeri F. Denayer and Gino V. Baron*,† Department of Chemical Engineering, Vrije Universiteit Brussels, Pleinlaan 2, B-1050 Brussels, Belgium
Wim Souverijns, Johan A. Martens, and Pierre A. Jacobs Center for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium
The conversion of a mixture of n-hexane, n-heptane, n-octane and n-nonane, in the presence of hydrogen over zeolite Pt/H-Y is modeled using a multicomponent adsorption-reaction scheme. Adsorption parameters for the relevant hydrocarbon molecules are determined using the chromatographic technique. Intrinsic reaction parameters are obtained by the fitting of the model to reaction rate data from catalytic experiments, using the independently determined adsorption parameters. Preferential conversion of the longer n-alkanes in mixtures is caused by competitive adsorption favoring the heavy compounds. Apparent reaction rates in mixtures are controlled by adsorption, increasing with chain length of the n-alkanes, rather than by the individual intrinsic reaction rates of the different n-alkanes, which are similar. Introduction
Table 1. Reaction Conditions
Isomerization and hydrocracking of alkanes on zeolite catalysts has already been studied extensively (Flinn et al., 1960; Froment, 1987), and the reaction mechanisms are well-understood (Coonradt and Garwood, 1960; Martens et al., 1986a,b). Most of the investigations on the laboratory scale have been carried out with pure components, despite the fact that very complex feedstocks are used in industrial processes. Modeling of hydrocarbon conversions in shape selective zeolites with multicomponent feeds has typically been conducted toward the diffusion/reaction approach (Haag et al., 1982; Wei, 1982; Beschmann and Riekert, 1993), but even for the conversion of single components on large pore zeolites, Steijns and Froment (1981) have demonstrated that it is necessary to include physisorption in the reaction models. Studies of the conversion of binary alkane mixtures have shown that the reactivities of individual compounds in mixtures may deviate strongly from their behavior as single components. For example, in the conversion of an equimolar mixture of n-heptane and n-decane on Pt/US-Y (Steijns and Froment, 1981) or an equimolar mixture of n-decane and n-dodecane on zeolite Pd/La-Y (Dauns and Weitkamp, 1986), the rate of conversion of the heavy n-alkane in the mixture was not affected by the presence of the lighter compound. The conversion rate of the light n-alkane was drastically reduced compared to a pure feed under the same reaction conditions. These phenomena have been ascribed to competitive physisorption in the zeolite micropores (Steijns and Froment, 1981). In the work presented here, the conversion of a quaternary mixture of n-alkanes (n-hexane, n-heptane, n-octane, n-nonane) on zeolite Pt/H-Y is analyzed using a semilumped kinetic model including a multicomponent adsorption equilibrium for all components of the feed and product stream. In this approach, kinetic and adsorption parameters are determined independently. This allows a much more accurate determination of all parameters, in comparison with the conventional ap†
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temp (°C) total pressure (kPa) H2 pressure (kPa) PH2/Phydrocarbons
233 450 418 13.1
pn-hexane (kPa) pn-heptane (kPa) pn-octane (kPa) pn-nonane (kPa)
3.7 10 11 7
proach, where adsorption is implicitly described and a large number of kinetic and adsorption parameters have to be fitted simultaneously to experimental data from catalytic experiments. Experimental Section Catalytic Experiments. Zeolite Na-Y was exchanged for 30% of the exchange capacity with NH4+, saturated with water, and heated in a covered crucible in a muffle furnace, preheated at 700 °C. After this selfsteaming treatment, the framework Si/Al ratio (determined with 29Si MAS NMR) was increased from 2.5 to 2.8. The obtained deep bed steamed Y was exchanged with excess NH4Cl under reflux conditions and loaded with 0.5 wt % platinum by cation exchange with Pt(NH3)4Cl2 in aqueous solution. The catalyst was activated in the reactor in flowing oxygen, where the temperature was increased from 25 to 400 °C at 6 °C/ min, and kept subsequently at 400 °C for 1 h. After a purge with nitrogen, the platinum was reduced in a flow of hydrogen for 1 h at 400 °C. A reactor tube with an internal diameter of 1 cm was filled with 3 g of Pt/H-Y pellets of 300-500 µm. A mixture of n-hexane, n-heptane, n-octane, and n-nonane vapor was diluted with hydrogen and contacted at various space times with the catalyst bed under the reaction conditions of Table 1. Reaction product analysis was carried out on-line with a HP 5890 gas chromatograph. No catalyst deactivation was observed. The conversion xi of component i is calculated as
xi )
pi,in - pi,out pi,in
The weighed conversion xj is defined as © 1997 American Chemical Society
(1)
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3243
Figure 2. Bifunctional conversion scheme.
Figure 1. Diffusion, adsorption, and reaction of molecules in the bimodal catalyst pore structure. 4
xj )
f i xi ∑ i)1 4
(2)
fj ∑ j)1 Adsorption Experiments. A detailed description of the adsorption experiments can be found elsewhere (Denayer et al., 1996). The Henry constants were determined with the tracer chromatographic method. Langmuir constants were obtained from single component isotherms recorded by means of perturbation chromatography. Three zeolite samples were investigated: zeolite NaY, H-Y, and Pt/H-Y, all with a Si/Al ratio of 2.7 (Zeocat). Little difference in adsorption constants between Na-Y, H-Y and Pt/Na-Y zeolites was noticed. This justifies the use of the adsorption constants of Na-Y in the modeling. Results and Discussion Adsorption-Reaction Scheme. Isomerization and hydrocracking of hydrocarbons are performed on a bifunctional catalyst, which is commonly an acidic zeolite containing a dispersed noble metal (Maxwell and Stork, 1991). The first event in the conversion is the diffusion of the hydrocarbon molecule from the bulk gas phase to the zeolite pellet and subsequently through the stagnant film layer (Figure 1). Once the molecule has reached the pellet surface, it has to diffuse in the pores of the pellets toward the individual zeolite crystals, which contain the active sites. This macropore diffusion is governed by intermolecular collisions (molecular diffusion) or by collisions with the pore wall (Knudsen diffusion), when the mean free path of the molecules approaches the mesopore diameter. Adsorption, diffusion, and reaction in the micropores of the crystals occur simultaneously. Although the diffusion mechanisms in zeolites are still not fully understood, and several possible mechanisms (for example: creep diffusion (Derouane et al., 1987), configurational diffusion via gas translation or solid vibration (Xiao Wei, 1992)) are under debate, it can be assumed that the molecules do not escape from the force field exhibited by the zeolite micropore walls. Consequently, the micropore diffusion properties are very strongly related to the adsorption properties, as is
reflected for instance, in the Darken equation (Darken, 1948). It has already been shown for the present n-alkane/Y zeolite system, that the slowest step in mass transfer is macropore diffusion (Denayer and Baron, 1996). This macropore diffusion is not limiting as the Weisz modulus is of the order 10-3. Moreover, the more slowly diffusing species are converted more rapidly than the smaller, faster diffusing components, which is in clear contrast with diffusion control. Reaction is assumed to take place after a molecule is adsorbed in the micropore system. For a multicomponent system, the concentration in the micropores (referred to as internal concentration) of the several species which are present in the feed is determined by the multicomponent adsorption equilibrium. Therefore, if one type of feed molecule is adsorbed preferentially, its concentration in the zeolite pore system will be higher than that of the other molecules, and hence this component can be converted more efficiently or prevent other molecules from being transformed. It is obvious that the competition of the different classes of molecules for adsorption in the micropore system, where the catalytic activity is situated, determines the local concentrations and hence the actual reaction conditions. On the bifunctional catalyst investigated, adsorbed alkanes are dehydrogenated on the platinum metal clusters. The alkenes formed are protonated on the Brønsted acid sites and transformed into alkylcarbenium ions (Figure 2). The alkylcarbenium ions are transformed through numerous reaction steps including alkyl and hydride shifts, skeletal branching reactions, and β-scission, which can be reduced to a limited number of single kinetic events (Svoboda et al., 1995). After transformation, the branched or cracked alkylcarbenium ions are deprotonated to yield rearranged alkenes, which are in turn hydrogenated to isoalkanes and cracked alkanes occurring as final products. Mono-, di-, and multibranched and cracked alkanes are formed in consecutive reaction steps (Martens et al., 1987). The following reaction scheme for the conversion of n-hexane, nheptane, n-octane, and n-nonane is adopted (Froment, 1987): kMB
kMTB
kCR
n-alkane 9 8 MB 9 8 MTB 98 CR r r
(3)
where MB stands for monobranched, MTB for multibranched, and CR for cracked alkanes. The thermodynamic equilibrium constants between monobranched and linear alkanes on one hand and multibranched and monobranched alkanes on the other hand are given by KMB and KMTB. This lumped model is without doubt oversimplified but is nevertheless sufficient for a first study of the influence of adsorption on the relative reaction rates. Reaction Rate Equations. The rate equations are written in terms of internal concentrations. These internal concentrations are related to the measurable,
3244 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 2. Henry and Langmuir Adsorption Constants and Saturation Capacities of Alkanes on Zeolite Na-Y at 233 °C Obtained by Fitting of the Experimental Isotherms K′n-alkane ((mol/kg)/Pa) K′MB ((mol/kg)/Pa) K′CR ((mol/kg)/Pa) Ln-alkane (1/Pa) LMB (1/Pa) LMTB (1/Pa) LCR (1/Pa) ns,n-alkane (mol/kg)
nonane
octane
heptane
hexane
148 × 10-5 154 × 10-5 180 × 10-5 143 × 10-5 150 × 10-5 173 × 10-5 10.8 × 10-5 1.03
56.4 × 10-5 58.1 × 10-5 66.0 × 10-5 52.8 × 10-5 54.3 × 10-5 61.7 × 10-5 4.1 × 10-5 1.07
21.5 × 10-5 22.2 × 10-5 24.1 × 10-5 17.6 × 10-5 18.2 × 10-5 19.8 × 10-5 3.03 × 10-5 1.22
7.88 × 10-5 8.08 × 10-5 8.27 × 10-5 6.1 × 10-5 6.26 × 10-5 6.41 × 10-5 0.3 × 10-5 1.29
external partial pressure through the adsorption equilibrium. The multicomponent equilibrium is described by an extended Langmuir isotherm.
K′i pi
qi )
1+
(4)
∑j Lj pj
This multicomponent isotherm accounts for adsorption of the linear alkanes of the feed, but also for the adsorption of the products and the intermediates, such as the mono- and multibranched alkanes, and the cracked alkanes. The rate limiting steps in the reaction are the isomerization and cracking of the alkylcarbeniumions on the Brønsted acid sites (Coonradt and Garwood, 1964). Adsorption, desorption, and (de)hydrogenation are assumed to be in quasi-equilibrium. This leads to the following rate equations:
ralkane, i ) -
dpalkane,i pHCd(W/FHC)
)
(
kMB,i qalkane,i -
)
qMB,i /p (5) KMB,i H2
dpMB,i ) pHCd(W/FHC) kMB,i qMB,i kMTB,i qMTB,i qalkane,i qMB,i (6) p H2 KMB,i pH2 KMTB,i
rMB,i )
rMTB,i )
(
)
(
)
dpMTB,i ) pHCd(W/FHC) kMTB,i qMTB,i kCR,iqMTB,i qMB,i (7) pH2 KMTB,i pH2
(
)
kCR,iqMTB,i ) rCR,i ) pH2 pHCd(W/FHC) dpCR,i
(8)
It was shown by Froment (1987) that a similar form for the rate equations applies in various reaction conditions. A much more detailed reaction network, based on elementary reaction steps, was developed later on by Froment and co-workers (Baltanas et al., 1989). This model gives a much better approximation of the real kinetics and allows easier extrapolation to other hydrocarbons and different reaction conditions. However, it was our major concern to investigate the influence of the adsorption properties on the reaction kinetics of the hydrocracking of a mixture of alkanes. For this, we have only used the more simple kinetic model.
Figure 3. Adsorption isotherms of n-hexane, n-heptane, n-octane, and n-nonane on Na-Y at 233 °C (symbols, experimental data; curves, fitted isotherms).
Chain Length Dependence of the Adsorption and Kinetic Constants. The Henry constants of the linear alkanes increase exponentially with chain length, with about a factor of 2.6 per extra carbon group. The Henry constants of the isomers are slightly higher than those of the linear chains (Denayer and Baron, 1996) (Table 2). The adsorption isotherms of n-hexane, nheptane, n-octane, and n-nonane on zeolite Na-Y are shown in Figure 3. Langmuir constants, obtained from fitting of the experimental isotherms with the Langmuir model, are given in Table 2. The saturation capacities, calculated from
ns ) K′/L
(9)
decrease with increasing molecular weight. The experimentally determined conversion curves for the mixture of n-hexane, n-heptane, n-octane, and n-nonane on zeolite Pt/H-Y versus space time and weighed conversion are shown in Figure 4. n-Nonane is rapidly converted, followed in order by n-octane, n-heptane, and n-hexane. Since the adsorption constants are known, only the intrinsic kinetic parameters and the equilibrium constants had to be fitted with the experimental data using eqs 4-8. The rate equations were implemented in a FORTRAN based program and integrated with a fourthorder Runge-Kutta algorithm. The fitting was performed with subroutine VA05A of the Harwell Subroutine Library (Harwell, 1973), which is a compromise between the Newton-Raphson, the steepest descent, and the Marquardt algorithm. Good starting values of
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3245 Table 3. Kinetic and Equilibrium Parameters for the Conversion of Alkanes on Zeolite Pt/H-Y at 233 °C kMB (Pa/s) kMTB (Pa/s) kCR (Pa/s) KMB KMTB
Figure 4. Conversion of mixture of n-alkanes on zeolite Pt/H-Y at 233 °C as a function of space time and as a function of the weighed conversion (symbols, experimental data; lines, theoretical curves).
the kinetic constants were obtained by a simulation in Microsoft Excel. A complication in the fitting procedure arises from the formation of six carbon products from nonane hydrocracking (Martens et al., 1986b). To obtain a good fit of the curves for the conversion of hexane, the formation of monobranched hexane isomers from the cracking of nonane was included in the model. It was assumed that 12% of the cracked nonane consists of monobranched hexane chains (Martens et al., 1986b). The cracking of monobranched and linear alkanes is negligible. Due to experimental difficulties in determining the cracking products of n-hexane, the rate constants for the cracking of n-hexane could not be determined accurately. However, sensitivity analysis showed that these constants do not strongly influence the fitting of the other curves. The conversion of the mixture is very well represented by the model (Figure 4). In Figure 5, the evolution of the composition of the different carbon number fractions with weighed conversion is shown. Correlation factors r2 ranging from 0.992 to 0.999 were found between the experimental and the fitted data for the different product lumps. Linear alkanes are first transformed into monobranched alkanes. For nonane, the maximum concentration of monobranched chains is reached at a weighed conversion of 30%. After this maximum, the concentration of monobranched chains decreases due to secondary isomerization, leading to multibranched alkanes. For the shorter chains, this maximum is approached at
nonane
octane
heptane
hexane
217 189 80.0 7.2 1.9
196 139 68 5.0 1.2
167 94.7 56.3 3.5 0.7
106 85.6 19.1 2 0.03
higher weighed conversions. Since longer alkanes have more isomers, the equilibrium between linear and branched chains shifts toward the branched chains with rising chain length. The same trend is observed for the formation of multibranched alkanes. Almost no multibranched alkanes are formed during the n-heptane and the n-hexane conversion, as reflected in the very low equilibrium constants KMTB (see Table 3). Cracking occurs only after the formation of multibranched alkanes. The equilibrium and intrinsic reaction rate parameters for the conversion of the n-alkanes obtained from the fitting of the model with the experimental data are given in Table 3. It has to be mentioned that the intrinsic reaction rate constants are still lumped parameters which contain a term for the dehydrogenation of the alkanes to alkenes. The thermodynamic equilibrium constant for dehydrogenation increases with the carbon number. Apparently, the intrinsic reaction rate constants increase much slower with chain length than the adsorption parameters (Figure 6). The Henry and Langmuir adsorption parameters increase exponentially, whereas the intrinsic kinetic parameters vary in a more linear way. Kissin (1990) determined relative reactivities of alkanes in catalytic cracking on zeolite Y and found that the reactivities increase with a factor of 2 per extra carbon in the molecule. On the basis of the present results, it can be speculated that Kissin’s experiments are another example where the increase of observed reactivities is caused by the dependence of the adsorption constants on the molecular weight, and not by an increase of intrinsic reactivities. Conclusions Microporous catalysts such as zeolites are selective adsorbents, and the internal concentrations of reacting molecules can be very different from that in the bulk fluid phase, even in the absence of diffusion limitations. In this paper, an example of a catalytic conversion in a zeolite is reported where competitive adsorption, rather than diffusion, is the main factor in controlling the reactivity order in multicomponent feedstocks. A Langmuir model of multicomponent adsorption was used to express the reaction kinetics in terms of the actual internal concentrations. Using a relatively simple kinetic model, it was shown that the intrinsic kinetic constants are less dependent on the alkane chain length than the adsorption parameters. In the isomerization and hydrocracking of n-alkanes on zeolite Pt/H-Y, the longer chains are converted preferentially as they are concentrated in the zeolite. Acknowledgment This research is supported by IWT (scholarships to J.F.D. and W.S.) and the Flemish F. W. O. (Research Grant to G.V.B. and P.A.J. (G.B. 9.0231.951) and a Research Position to J.A.M.).
3246 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997
Figure 5. Conversion of a mixture of n-alkanes and formation of branched and cracked alkanes on zeolite Pt/H-Y at 233 °C (nA, linear chain; MB, monobranched; MTB, multibranched; CR, cracked alkanes) (symbols, experimental data; lines, fitted curves).
Figure 6. Intrinsic kinetic constants for mono- and multibranching and Henry and Langmuir constants of n-alkanes at 233 °C as a function of the carbon number.
Nomenclature CR: cracked product fi: mole fraction of component i in hydrocarbon feed FHC: molar feed rate of n-alkanes at reactor inlet (mol/s) kMB, i: rate constant for the conversion of linear into monobrached alkanes (Pa/s) kMTB, i: rate constant for the conversion of monobranched into multibrached alkanes (Pa/s) kCR, i: rate constant for the conversion of multibranched to cracked alkanes (Pa/s)
KMB: equilibrium constant between linear and monobranched alkanes KMTB: equilibrium constant between monobranched and multibranched alkanes K′i: Henry constant of component i [mol/(kg‚Pa)] Li: Langmuir constant of component i (1/Pa) MB: monobranched alkane MTB: multibranched alkane nA: normal alkane ns: adsorption capacity (mol/kg) pH2: partial pressure of hydrogen (Pa) pHC: total hydrocarbon pressure at reactor inlet (Pa) pi: partial pressure of component i (Pa) pi,in: partial pressure of component i at reactor inlet (Pa) pi,out: partial pressure of component i at reactor outlet (Pa) ri: reaction rate of component i [(mol/kg)/s] W: catalyst weight (kg) xi: conversion of component i xj: weighed conversion
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Received for review October 16, 1996 Revised manuscript received March 13, 1997 Accepted April 12, 1997X IE960657M
X Abstract published in Advance ACS Abstracts, July 1, 1997.
