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Ind. Eng. Chem. Res. 2007, 46, 8710-8721
Aggregation State Effects in Shape-Selective Hydroconversion C. S. Laxmi Narasimhan,†,‡ J. W. Thybaut,*,† J. F. Denayer,§ G. V. Baron,§ P. A. Jacobs,| J. A. Martens,| and G. B. Marin† Laboratory for Chemical Technology, Ghent UniVersity, Krijgslaan 281 S-5, B-9000 Ghent, Belgium, Chemie Ingenieurstechniek, Vrije UniVersiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium, and Centrum Voor OpperVlakchemie en Katalyse, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg 23, B-3001 HeVerlee, Belgium
A unified model for the description of vapor- and liquid-phase hydroconversion has been developed. The high isomer yield, which is typical for the shape selectivity exhibited by Pt/H-ZSM-22 at vapor-phase conditions, is still observed at liquid-phase conditions. However, the molar cracking yield is enhanced by at least 10 mol % when operating at liquid-phase conditions. The latter effect is mainly due to an increased physisorption competitiveness of lighter alkanes, which also results in enhanced cracking of lighter alkanes. The effect amounts to approximately 3 orders of magnitude for a difference of 2 in carbon number. These aggregation state effects are accounted for in the model through (i) liquid phase thermodynamic nonideality, (ii) destabilization of the physisorbed state by compression of the adsorbate by the bulk fluid, and (iii) carbenium ion stabilization by sorbent solvation by the bulk fluid. F-values of the order of 103 indicate the model’s adequacy in describing the observed phenomena. 1. Introduction Vapor-phase hydroconversion on shape-selective catalysts such as Pt/H-ZSM-22 has revealed the role of multiple types of sites on the reaction network and the resulting product distribution.1,2 Pore mouths, owing to strong physisorption, selectively contribute to branching isomerization. Sites at the bridges between the pore mouths, where physisorption is weak, contribute only to nonbranching isomerization reactions. Inside the micropores, only n-alkane cracking reactions occur. The kinetics have been quanititatively described by a so-called single-event microkinetic (SEMK) model through elementary reaction networks occurring on the different types of sites accounting for steric effects and physisorption.1-3 Hydroconversion on Pt/H-zeolites leads to clearly distinct results in the liquid phase compared to the vapor phase.4-8 Comparing the effects on Pt/H-Beta, Pt/H-USY, Pt/H-MCM22 and Pt/H-Y, the differences are relatively more pronounced for Pt/H-Beta and Pt/H-Y, which have smaller pores than USY or the wide-pore MCM-22. For a non-shape-selective wide-pore zeolite, e.g., Pt/H-USY, hydroconversion of n-C7-n-C9 showed that the preferential conversion of the heaviest component, c.q., n-C9, is more pronounced at vapor-phase than at liquid-phase conditions.4-6 The n-C7 hydroconversion rate relative to n-C9 was enhanced by more than a factor of 2 in the liquid phase, indicating that n-C7 is “more competitive” with n-C9 in the liquid phase. These effects have been attributed to enhanced sorbatesorbate lateral interaction effects in liquid-phase physisorption compared to that in the vapor phase.4-6 In a quantitative description, these differences in vapor- and liquid-phase kinetics have been described by accounting for (i) the thermodynamic nonideality of the liquid phase, (ii) the destabilization of the physisorbed phase, and (iii) the carbenium ion stabilization at dense-phase conditions rather than at ideal gas-phase conditions.7 * Corresponding author. E-mail:
[email protected]. Tel.: +32(0)9 264.45.19. Fax: +32(0)9 264.49.99. † Ghent University. ‡ Present address: Shell Technology India, Bangalore. § Vrije Universiteit Brussel. | Katholieke Universiteit Leuven.
Apart from the higher competitiveness of the lighter hydrocarbons at liquid-phase conditions, an enhanced cracking on Pt-/ H-ZSM-22 is comparable to that observed on Pt/H-USY.8,9 For example, liquid-phase hydroconversion of a mixture of n-C9-n-C14 alkanes on Pt/H-ZSM-22 resulted in a 20% yield of cracked products at 45% conversion of the n-C9-n-C14 in the mixture, which is comparable with that obtained on Pt/HUSY under identical conditions.8 In contrast, the vapor-phase hydroconversion of an even more reactive molecule such as n-C18, on ZSM-22, resulted in barely 5% cracked products at 60% conversion, as a result of pronounced isomerization compared to cracking.2,10,11 In the description of vapor-phase hydroconversion, data on each of the elementary steps obtained from independent experiments were used. In particular, physisorption data obtained at vapor-phase conditions were used to determine the physisorption equilibrium coefficient. The same approach was followed for the liquid-phase kinetics.7,12 At low pressure, a gas phase has a low density and can be considered to be close to the ideal gas state. Physisorption at these conditions typically falls in the Henry regime.13 At these circumstances, the surface enrichment, defined as the density difference between the physisorbed phase and the bulk phase, is high14 and varies with zeolite type and composition. With an increase in fluid-phase density, the competitive effects of the sorbates affect the physisorption and, hence, deviations from the Henry regime occur. The surface enrichment reduces with the increase in the bulk fluid density.13-22 Aranovich and Donohue21,22 comprehensively analyzed the effect of bulk density on physisorption and postulated a destabilization effect on the physisorbed phase due to the interaction of physisorbed species with the dense bulk-phase molecules and termed the effect as “adsorption compression effects”. At these conditions, adsorbates become so compressed that they start repelling each other and interact strongly with the bulk-phase molecules, resulting in lowering of surface enrichment.13-22 The extent of this destabilization varies with the density of the bulk fluid phase and the sorbent. Protonation is also affected by the bulk-phase density.7,12,19,20 The higher bulk-phase density leads to a more pronounced carbenium ion stabilization compared to that in a low-density bulk phase.7,12
10.1021/ie070788v CCC: $37.00 © 2007 American Chemical Society Published on Web 11/07/2007
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The beneficial features of a catalyst such as ZSM-22 used under liquid-phase conditions are twofold. The first benefit is the removal of normal alkanes from relatively heavy fractions by isomerization or cracking. This is important for selective dewaxing of lubricating oils in the range of n-C22-n-C33, aviation turbine fuels in the range of n-C9-n-C12, and diesel feedstock in the range of n-C14-n-C19, where only the undesired high carbon number n-alkanes are to be selectively converted to isomers and cracked products yielding desired product properties such as low pour point. The other feature is the enhanced selective production of liquefied petroleum gas (LPG) containing large quantities of n-C3-n-C4, which provides strategic advantages for potential emerging markets in fastdeveloping countries. Selective n-alkane cracking results in a maximum at n-C3-n-C4. The effect of the liquid phase on non-shape-selective hydroconversion, e.g., on Pt/H-USY, was the subject of our earlier work7,12 and led to the introduction of excess parameters for physisorption and protonation. In the present paper, the effect of liquid-phase conditions on hydroconversion on a shapeselective zeolite, i.e., Pt/H-ZSM-22, is quantitatively described using the same methodology. The enhanced cracking observed in the liquid-phase hydroconversion on Pt/H-ZSM-22 compared to that in the vapor phase is quantitatively described using a single-event microkinetic model accounting for aggregation state effects at the various types of active sites on ZSM-22, i.e., pore mouth, bridge, and micropore sites. An industrial application making use of the shape-selective properties of ZSM-22 is also discussed to illustrate the importance of fundamental understanding for appropriate scale-up. An adequate kinetic model that accounts for aggregation state effects as well as shape selectivity exhibited by the catalyst is a useful tool in further catalyst, reactor, and process development.23 2. Procedures Previously generated data from physisorption and kinetic experiments at liquid-phase conditions were used for the purpose of kinetic modeling.5,6,8 Liquid-phase kinetic data for ZSM-22 were generated at the Laboratory for Chemical Technology, Ghent University,8 while physisorption and additional kinetic data were obtained at CHIS, Vrije Universiteit, Brussel.5,6 The ZSM-22 catalyst was synthesized by COK, Katholieke Universiteit Leuven. 2.1. Catalyst. ZSM-22 has tubular micropores with a relatively uniform cross section delineated by a framework of 10 rings of framework oxygen atoms. Acid sites are present inside the micropores, at the pore mouths of these micropores, and at the bridges between the pore mouths.2,24,25 ZSM-22 was obtained by hydrothermal synthesis in stainless steel autoclaves according to a recipe described by Ernst et al.26 The Si/AlF ratio in the framework as determined by quantitative27Al MAS NMR amounted to 30. Inductively coupled plasmaatomic emission spectroscopy (ICP-AES) measurements lead to the same Si/Al ratio, indicating that no extraframework aluminum was present. The corresponding number of Bro¨nsted acid sites, which corresponds to the total concentration of acid sites in the micropores, amounted to 0.54 mol kgcat-1. A pore mouth concentration of 0.34 mmol kgcat-1 was estimated.24 The Pt loading amounted to 0.5 wt %.27 With a Pt dispersion of 30%, this corresponds to a Pt particle diameter of 3 nm.28 2.2. Liquid-Phase Physisorption and Kinetics. Binary physisorption isotherms for n-C8-n-C6 and n-C6-n-C10 mixtures were measured previously to investigate the effect of carbon number on liquid-phase physisorption on ZSM-22. In addition, binary isotherms for n-C8-2MeC7 and n-C6-2MeC5 were also generated to assess the effect of monobranching.5,29
Liquid-phase hydroconversion kinetic data were obtained in a Robinson Mahoney reactor.8,9 A hydrocarbon feed mixture containing n-C9-n-C14 hydrocarbons denoted as “parapur” was used. The mixture contained 0.3 mol % n-C9, 9.4 mol % n-C10, 26 mol % n-C11, 44.7 mol % n-C12, 19.2 mol % n-C13, and 0.4 mol % n-C14. A full range of typical operating conditions was covered in the experimental program. The temperatures ranged from 503 to 533 K. The total pressure ranged between 6 and 9 MPa. The molar inlet hydrogen-to-hydrocarbon ratio was always in the range of 4.5-5. Within this range, the catalyst stability against coking was ensured without excessive liquid vaporization. Space times from 504 to 900 kgcat s molHC-1 have been applied. 2.3. Separation Factor. Liquid-phase physisorption characteristics are typically expressed in terms of separation factors or partition coefficients.5 The separation factor at every point of an experimental binary physisorption isotherm is defined as
Ri-j )
yi/(1 - yi) xi/(1 - xi)
(1)
yi and xi are the mole fractions of component i in the physisorbed phase and in the bulk fluid phase, respectively. Consequently, the composition of the physisorbed phase can be obtained from
yi )
exp Ri-j xi exp (1 + Ri-j xi - xi)
(2)
exp if Ri-j is assumed to be constant over the entire range of fluidphase composition xi. 2.4. Conversion and Yields. The conversion of a feedstock component i is calculated by
Xi )
Foi - Fi Foi
(3)
The total feedstock conversion is calculated as an average of the individual feedstock component’s conversions weighted according to the feedstock composition. nfeed
Xtot )
yiXi ∑ i)1
(4)
The behavior of a prominent feedstock component with the highest carbon number, which has no possibility to be formed from cracking of heavier components in the feedstock, is taken as a basis for discussion. For the parapur mixture, n-C13 is taken as the reference component. Isomer and cracked-product yields are expressed on a molar basis as niso
yiso )
∑ i)1 ncr
ycr )
∑ j)1
Fi Foi
(5)
Fj Foj
(6)
with niso and ncr being the number of isomers and cracked products, respectively. 2.5. Parameter Estimation. Parameter estimations were performed using the Levenberg-Marquardt method for minimization of the following objective function, S(b), considering the full error variance-covariance matrix calculated from observed and calculated responses:
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Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 nob nresp nresp
S(b) )
∑ ∑ ∑ σi,j[yi,q - yˆ i,q][yj,q - yˆ j,q] f min
(7)
q)1 i)1 j)1
where σi,j corresponds to an element in the inverse of the error variance-covariance matrix, yi,q is the observed value for the response i in experiment q, and yˆ i,q is the corresponding modelcalculated value. Liquid-phase mole fractions were used as responses when modeling physisorption, whereas molar outlet flow rates were used when performing kinetic modeling. Equation 2 is used to calculate the liquid-phase mole fractions. The calculation of the separation factors and the required physisorption model are detailed in Sections 3.1 and 2.6.1, respectively. In the former case, only the excess physisorption coefficient was estimated, while in the latter case, the only adjustable parameter was the excess standard protonation enthalpy detailed in Section 3. The calculation of the molar outlet flow rates is explained in Section 2.6.2. The ODRPACK-package version 2.01 was used for regression.30,31 Initial parameter estimates were obtained from a qualitative interpretation of the experimental data and some preliminary simulations on a trial-and-error basis. The elements of σi,j of the inverse of the error covariance matrix were estimated by an iterative procedure using successive substitution starting from a diagonal error variance-covariance matrix. These diagonal elements were calculated for the individual responses according to
Figure 1. Liquid-phase physisorption on ZSM-22: binary isotherm of n-C6-n-C8 and n-C6-n-C10 at 277 K. Comparison of experimental data with the calculated values using the model described by eq 2 with the value of separation factor calculated from eq 21 using excess free enthalpy parameter cE estimated through regression of experimental binary isotherms as 3.68 kJ mol-1. Henry coefficients as reported by Narasimhan et al. Fugacity coefficients calculated using Peng-Robinson equation of state. Symbols, experimental: triangles, n-C6-n-C10; squares. n-C6-n-C8. Lines, model: thick solid line, n-C6-n-C10 isotherm; thin solid line, n-C6-n-C8 isotherm.
nob
( σi,i )
nresp
yi,q)-n ∑ q)1 nob
(8)
(∑ ypj,q)-n ∑ j)1 q)1
After completion of every parameter estimation, the elements of the inverse error covariance matrix σi,j were recalculated and used in the next iteration for parameter estimation. This procedure was continued till convergence of σi,j was obtained. Typically, this was found to take 3-4 iterations. The statistical significance of the regression was expressed by means of the F-test, comparing the mean sum of squares of the calculated response values with the mean residual sum of squares. The individual significance of the parameters on the 95% probability level is tested using Students’ t-value and the corresponding confidence intervals. 2.6. Models. 2.6.1. Physisorption. 2.6.1.1. Phenomenological Description. Typically, the hydrocarbon physisorption properties are determined from vapor-phase experiments.3-5 These properties are then used to describe the physisorption from a more dense phase. Physisorption from a dense phase can be linked to vapor-phase physisorption at the given operating conditions through a Born-Haber cycle; see Figure 27,12,15, o,V o,E L ∆Go,L phys ) -RT ln φ + ∆Gphys + ∆Gphys
(9)
where -RT ln φL accounts for the thermodynamic nonideality of the bulk phase with respect to the ideal gas state with φL being the dense-phase fugacity coefficient. ∆Go,V phys is the standard free enthalpy of physisorption determined from ideal gasphase physisorption, and ∆Go,E phys is the excess free enthalpy for physisorption, which is further discussed in this section. The thermodynamics of the physisorbed state have been shown to depend on the density of the bulk fluid,7 i.e., setting ∆Go,E phys ) 0 did not allow one to describe the observed separation factors. This was illustrated by Narasimhan et al.7 for alkane physisorption on a USY-zeolite and will be explained
Figure 2. Born-Haber cycle for vapor- and liquid-phase alkane physisorption.
in Section 3.1 for ZSM-22. The terminology “excess” is drawn from the analogy to liquid-phase thermodynamic nonideality. ∆Go,E phys varies with the bulk-phase density and becomes significant for dense bulk phases.7,12 The intermolecular distances in a dense bulk phase are of molecular order, and entry or exit of a molecule from the dense bulk phase to the sorbent is felt by the neighboring molecules both in the dense bulk phase and in the entire physisorbed phase.13-22 As a result, sorbate molecules are being “compressed” and repulsive interactions between these sorbate molecules become more important and destabilize the physisorbed state.21,22 Because of the analogy of this effect to the bulk-phase nonideality, a proportionality between the standard excess free enthalpy for physisorption and the logarithm of the fugacity coefficient for the species involved was put forward:7,12 E L ∆Go,E phys ) -c ln φ
(10)
The corresponding proportionality factor, cE, can be determined from physisorption data. It has been observed experimentally that the extent of this compression of adsorbates depends on the sorbent used.5,29 This suggests that the sorbent itself may be affected by changes in the density of the bulk fluid phase, which can be considered as a solVation. Variations in the bulk-phase density cause the sorbent framework to adapt its structure to establish the minimum free enthalpy configuration resulting in small changes of the pore geometry. Because zeolite pore sizes are of molecular order, small changes in pore size can have important effects on the physisorption properties. The extent of sorbent framework adaptation to the bulk-phase density depends, among other things, on the framework topology. Hence, the proportionality factor cE depends on the sorbent considered and can be denoted as a catalyst descriptor. Its value can be obtained by regression of liquid-phase binary physisorption isotherms for a given zeolite.
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There exist two important factors during physisorption on strong sorbents in a dense bulk fluid. First, the vapor-phase standard free enthalpy of physisorption acts in the direction of increasing surface enrichment. This corresponds to the second term in eq 9. The first term in eq 9, related to the dense-phase fugacity coefficient, which is typically
