Ind. Eng. Chem. Res. 2005, 44, 5159-5169
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Acid-Metal Balance of a Hydrocracking Catalyst: Ideal versus Nonideal Behavior Joris W. Thybaut,† C. S. Laxmi Narasimhan,† Joeri F. Denayer,‡ Gino V. Baron,‡ Pierre A. Jacobs,§ J. A. Martens,§ and Guy B. Marin*,† Ghent University, Laboratorium voor Petrochemische Techniek, Krijgslaan 281 - S5, B-9000 Ghent, Belgium, Vrije Universiteit Brussel, Chemie Ingenieurstechniek, Pleinlaan 2, B-1050 Brussels, Belgium, and Katholieke Universiteit Leuven, Centrum voor Oppervlakchemie en Katalyse, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium
n-Alkane hydrocracking has been previously performed in a perfectly mixed flow reactor at temperatures from 473 to 553 K, pressures from 0.5 to 10 MPa, and molar hydrogen-tohydrocarbon inlet ratios from 50 to 300 on a USY zeolite loaded with 0.5 wt % Pt1. For a given ratio of metal to acid sites, four important causes affecting the ideal character (i.e., the quasi equilibration of the (de)-hydrogenation reactions) were identified. In the investigated range of operating conditions, (1) a decreasing total pressure, (2) an increasing temperature, (3) an increasing molar hydrogen-to-hydrocarbon inlet ratio, and (4) a higher reactant carbon number lead to nonideality. A kinetic model for nonideal hydrocracking has been derived on the basis of a lumped reaction scheme. With literature values for the kinetic parameters, this allows us to rationalize the deviations from ideality in hydrocracking observed as a function of the operating conditions and to describe the high isomerization yield in ideal hydrocracking and the high cracking yield in nonideal hydrocracking. The results obtained with this lumped kinetic model will serve as a basis for the future development of single-event microkinetics (SEMK) for nonideal hydrocracking. 1. Introduction Hydrocracking is an important industrial process for the production of diesels, aviation fuels, and lubricating oils.2,3 Among others, one of the attractive features of the hydrocracking process is its capability of transforming n-alkanes into iso-alkanes to a high extent before the cracking reactions become important. Processes focusing on this isomerization rather than on cracking are referred to as hydroisomerization processes. Isodewaxing is an example of such a process and has the advantage that the waxy molecules are isomerized into the desired lube oil molecules instead of being cracked into lighter products.4 Hydrocracking is a bifunctional process requiring metal as well as acid sites.5 Saturated hydrocarbons are dehydrogenated on the metal sites; subsequently, the unsaturated species migrate to the acid sites where they are protonated, yielding carbenium ions. Such carbenium ions undergo further isomerization and cracking reactions. The product carbenium ions desorb as alkenes that are hydrogenated into the observable saturated species. Prior to these chemical steps, physisorption occurs in the micropores of the catalyst.6,7 The balance between the number and the activity or strength of the metal and the acid sites plays a key role in the product selectivities observed in hydrocracking.8-11 Compared to acid catalysts used in catalytic cracking, the presence of a metal phase on hydrocracking cata* To whom correspondence should be addressed. Tel: + 32 9 264 45 17. Fax: + 32 9 264 49 99. E-mail: Guy.Marin@ UGent.be. † Ghent University. ‡ Vrije Universiteit Brussel. § Katholieke Universiteit Leuven.
lysts enhances isomer formation. The higher the (de)hydrogenation activity of the metal compared to the acid strength of the catalyst, the higher the isomer yield.12 The higher the (de)-hydrogenation activity, the more likely it is that the unsaturated products are hydrogenated rather than being subject to another acidcatalyzed skeleton rearrangement or cracking reaction. From a certain level of (de)-hydrogenation activity on (i.e., when the (de)-hydrogenation activity is sufficiently high to establish quasi equilibrium), the isomer yield does not increase further.8,9 Because of the high isomer yields obtained using hydrocracking catalysts with a high (de)-hydrogenation activity, the term ideal hydrocracking has been introduced.12,13 In the current terminology, ideal hydrocracking is equivalent to the quasi equilibration of the (de)-hydrogenation reactions.1,9 In ideal hydrocracking experiments, considerable insight into the isomerization and cracking mechanism can be obtained.14 The occurrence of ideal hydrocracking is not only catalyst-dependent but also depends on the operating conditions (i.e., a catalyst exhibiting ideal hydrocracking under one set of operating conditions may exhibit nonideal hydrocracking under other set of operating conditions). The effect of the operating conditions on the ideality of the hydrocracking behvior on a given catalyst is illustrated in Figure 1.1 Under ideal hydrocracking conditions, an apparently unique relationship between product yields and total conversion is obtained on a NiMo/Al2O3 15 and on a Pt/ USY zeolite1,16-18 (vide Figure 2). Hence, it can be concluded that the effects of operating conditions on the isomerization and cracking rates are identical or at least similar. In particular for the temperature effect on the
10.1021/ie049375+ CCC: $30.25 © 2005 American Chemical Society Published on Web 12/24/2004
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Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 Table 2 (a) Chemisorption Enthalpies and Activation Energies Used in the Simulations of n-Octane Hydrocracking metal sites acid sites
∆Hchema
EA,deha
-68 -85
60
EA,isoa
EA,cra
120
118
(b) Preexponential Factors Used in the Simulations of n-Octane Hydrocracking Achemb metal sites acid sites a
Figure 1. Simulated isomerization conversion of n-alkane on Pt/ US-Y as a function of the total conversion of n-alkane under ideal and nonideal hydrocracking conditions: at 520 K (diamonds), 540 K (circles), 560 K (triangles), and 580 K (squares) and at 0.1 MPa (open symbols), 0.35 MPa (light shaded symbols), 1 MPa (dark shaded symbols), and 10 MPa (closed symbols).
Figure 2. Simulated isomerization conversion of n-alkane on Pt/ US-Y as a function of the total conversion of n-alkane under ideal hydrocracking conditions: at 480 K (squares), 500 K (triangles), and 520 K (diamonds) and at 0.1 MPa (open symbols), 1 MPa (shaded symbols), and 10 MPa (closed symbols). Table 1. Experimental Operating Conditions and Space Times T/K n-octane n-decane n-dodecane n-tetradecane n-hexadecane
493-553 493-533 493-533 493-533 483-533
0 pt/MPa (H2/HC)0 W/Fn-P (kg s mol-1)
0.5-5 0.5-5 0.5-5 0.5-2 1-2
50-200 50-200 50-300 50-300 160-400
180-720 140-720 110-720 110-360 360-720
reaction rates, this lead to the conclusion that the activation energies of the acid-catalyzed steps are similar.17 This paper reports on how hydrocracking rate equations can reflect the transition from ideal to nonideal hydrocracking and vice versa. In contrast to the work of Degnan and Kennedy,9 attention is focused on the effect of the operating conditions rather than that of the catalyst properties. 2. Procedures 2.1. Equipment and Operating Conditions. Experiments have been performed with n-alkanes in the range of octane to hexadecane at the operating conditions mentioned in Table 1.1 This allowed us to investigate the effect of the total pressure, temperature, inlet molar hydrogen-to-hydrocarbon ratio, and carbon num-
10-4
3.3 × 3.0 × 10-5
Adehc 1.3 ×
1016
Aisc
Acrc
1.0 × 1013
1.0 × 1013
kJ mol-1. b kgcat mol-1. c s-1.
ber of the hydrocarbon feed on the ideality of hydrocracking. A Berty reactorsa gas-phase reactor with complete internal mixingswas used. The equipment used was essentially the same as that used by Steijns et al.17 The quality of the hydrogen used (99.99%, L’Air Liquide, H2O + O2 content 10 and hence (γ + 1) ≈ γ), all terms between brackets in the numerator exhibit a linear dependence on γ, and all terms in the denominator exhibit a square dependence on γ. As a result, no difference in the relative importance of the terms in the numerator and the denominator is observed. A similar conclusion is drawn when the rate equations are constructed on the basis of other possible (de)-hydrogenation reaction mechanisms (vide Appendix A). A possible explanation for the effect of the molar hydrogen-tohydrocarbon ratio is related to the relative hydrocarbon concentrations on the metal and the acid sites. Substituting the full expression for the composite dehydrogenation rate coefficient (i.e., eq 7) in the expression for the first term in the numerator results in
(
γpt
M kdeh,isoKM iso Ct
∑
Kdeh,iso 1 + Csat
KM i KL,iyi pt/(1 + γ) +
i
∑ j
1+
∑K
L,kyk
pt/(1 + γ) +
k
)
(20)
KM j KL,jyj(1/γ)
∑K
L,lyl(1/γ)
l
Performing the same substitution in the first term of the denominator leads to γ2pt2
(
M M 2 kdeh,nkdeh,isoKM deh Kiso(Ct )
∑
Kdeh,nKdeh,iso 1 + Csat
KM i KL,iyi pt/(1 + γ) +
i
1+
∑ j
∑K
L,kyk
k
pt/(1 + γ) +
)
KM j KL,jyj(1/γ)
∑K
L,lyl(1/γ)
l
(21)
2
When the hydrocarbon concentrations on the metal sites M are high (i.e., Csat(∑i KM i KL,iyi pt/(1 + γ) + ∑j Kj KL,jyj1/γ) . 1) and the physisorbed concentrations are not too high (i.e., ∑k KL,kyk pt/(1 + γ) + ∑l KL,lyl(1/γ) , 1), the molar hydrogen-to-hydrocarbon ratio has an extra enhancing effect on the value of the expressions in eqs 20 and 21 apart from the linear, respectively, with the squared effect already present in the numerator of these expressions. A similar dependence on the molar hydrogen-tohydrocarbon ratio is found for the composite isomerization or cracking coefficients. A transition from ideal to nonideal hydrocracking may occur when the carbenium ion concentration relative to the total acid site concentration is significantly higher than the sum of the alkane and alkene concentrations relative to the total metal-site concentration. In the extreme case, the alkane and alkene concentrations on the metal sites are
Table 5. Ratios between the Number of (De)Hydrogenations, Alkyl Shifts, PCP Branchings, and β-Scissions in the n-Alkane Hydrocracking Network and in the n-Heptane Hydrocracking Network carbon number
(de)hydrogenation
alkyl shift
PCP branching
βscission
8 9 10 11 12
2.4 5.5 12.1 24.3 46.1
3.5 9.5 22.7 47.6 90.5
2.9 6.7 14.7 27.9 50.2
3.4 9.9 24.7 54.2 108.5
negligible, and the carbenium ion concentrations are close to the total acid site concentration. In this case, the term in the denominator of eq 13 corresponding to ideal hydrocracking has a square dependence on the molar hydrogen-to-hydrocarbon ratio, and the term corresponding to nonideal hydrocracking has a fourth power dependence on the molar hydrogen-to-hydrocarbon ratio. In view of a carbenium ion concentration on the acid sites under typical operating conditions of about 15% of the total acid site concentration,22 the alkane and alkene concentrations on the metal sites are expected to be negligible compared to the total metal site concentration. Analogous to the effect of temperature on nonideal hydrocracking (vide Figure 4), a shift in the maximum conversion as a function of the total pressure to higher total pressures is expected with increasing molar hydrogen-to-hydrocarbon ratio. However, in the range of operating conditions investigated, the effect is not pronounced enough to be visualized on the same figure. 5.4. Carbon Number Effect. The fourth factor identified to affect the nonideal hydrocracking behavior is the feedstock’s carbon number. The carbon number effect on the relative importance between the terms in the numerator and denominator of eq 13 is twofold. The number of elementary acid-catalyzed steps increases faster with the carbon number than the number of (de)hydrogenation reactions (vide Table 5), especially for carbon numbers lower than 10. Hence, the lumped rate coefficients for acid-catalyzed reactions are also expected to increase faster with the carbon number than the lumped rate coefficients for the (de)-hydrogenation reactions. As a result, the relative importance of the last with respect to the first terms in both the numerator and the denominator will increase with the carbon number, and nonideal hydrocracking will be favored for higher feedstock carbon numbers. This first carbon number effect is reinforced by a second effect related to the hydrocarbon concentration on the metal sites. Higher hydrocarbons have more negative chemisorption enthalpies and hence higher concentrations on the metal surface. Higher metal surface concentrations lead to decreases in the values of the terms in eq 13 corresponding to ideal hydrocracking and hence to a transition from ideal to nonideal behavior with increasing carbon number of the feed molecule under identical hydrocracking conditions. 6. Reaction Pathway Analysis 6.1. Ideal versus Nonideal Hydrocracking Reaction Pathway. Ideal or nonideal hydrocracking behavior is strongly reflected in the isomer yield. Under ideal hydrocracking conditions, the isomer yield is maximized. The first term in the numerator of eq 13 (i.e.,
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Figure 5. Lumped hydrocracking reaction scheme accounting for several isomer and cracked product lumps. The composite activation energies (i.e., sum of the protonation enthalpy and the real activation energy) of the acid-catalyzed elementary steps between the lumps are indicated (kJ/mol).
k/deh,iso/Kdeh,isoγ pt), which dominates its value in ideal hydrocracking, is found again in the initial net production rate of the isomer lump. In the product of rate coefficients in the numerator of the initial net production rate of the isomer lump (i.e., k/iso k/deh,iso/Kdeh,iso k/deh,n, vide eq 14), the reaction pathway from the normal alkane to the isomer lump can be identified: (1) dehydrogenation of the normal alkane to the normal alkenes, k/deh,n, (2) isomerization of the normal alkenes to the isomer alkenes, k/iso, and (3) hydrogenation of the iso-alkenes to the iso-alkanes, k/deh,iso/Kdeh,iso. The second term in the numerator of eq 13 (i.e., (1 + γ)k/cr) corresponds to the formation of cracked products directly from the normal alkane (i.e., via the formation of iso-alkenes that are cracked prior to hydrogenation) and is present in the expression for the initial net production rate of cracked products (vide eq 15). The product of the three rate coefficients, k/deh,nk/isok/cr, in this expression again reflects the reaction pathway: (1) dehydrogenation of the normal alkane to normal alkenes, k/deh,n, (2) isomerization of the normal alkenes to iso-alkenes, k/iso, and (3) cracking of the iso-alkenes to cracked products, k/cr. 6.2. Uniqueness of the Isomer Yield in Ideal Hydrocracking. One of the peculiarities of ideal hydrocracking is the unique relationship between product yields and conversion.1,6,9,15 By substitution of eqs 16 and 17 into eqs B.4 to B.6, it is clear that under ideal hydrocracking conditions and under ideal hydrocracking conditions only, the effects of the total pressure and the molar inlet hydrogen-to-hydrocarbon ratio on the isomerization and cracking rate are identical. The temperature effect, however, depends on the sum of the protonation enthalpies and the real activation energies for branching isomerization via protonated cyclopropane (PCP) isomerization and cracking via β-scission. These so-called composite activation energies are not necessarily the same for the various reaction types and within the reaction type can also be assumed to depend on the type of carbenium ions involved as reactant and product (i.e., secondary or tertiary23). From previous work, differences in composite activation energies of more than 45 kJ mol-1 have been observed22 (vide Figure 5). Two important observations can be made: (i) The composite activation energies along the main n-alkane hydrocracking reaction pathway indicated by the gray arrow act,/ act,/ act,/ in Figure 5 (i.e., Eact,/ PCP (s; s), EPCP (s; t), EPCP (t; s), EPCP (t; act,/ t), and Eβ (t; t)) span a range of only 15 kJ mol-1. (ii)
The values of the composite activation energies not (s; belonging to the main reaction pathway (i.e., Eact,/ β act,/ (s; t), and E (t; s)) are higher than the s), Eact,/ β β highest composite activation energy in the main reaction pathway, which is Eact,/ PCP (s; s). As a result, in the main reaction pathway the rate of the isomerization reactions increases more strongly with temperature than the rate of the cracking reactions. However, the rate of the cracking reactions not belonging to the main reaction pathway increases even more strongly with the temperature than the rate of the isomerization reactions in the main reaction patwhay. Hence, the increase of the overall isomerization and cracking rates with temperature is expected to be rather similar, resulting in an apparently unique relationship between product yields and total conversion. Different product yield patterns as a function of the total conversion can be expected in ideal hydrocracking when the reaction scheme such as the one presented in Figure 5 changes, for example, by the use of a shapeselective catalyst such as ZSM-22. On ZSM-22, tertiary carbenium ions are sterically hindered, and hence, the product yield pattern as a function of the total conversion is solely determined by the composite activation energies of reactions between secondary carbenium ions.24 As a result, the compensation effect between global isomerization and cracking rates on USY-zeolites as described above cannot occur on ZSM-22, leading to maximum isomerization yields dependent on the operating conditions.25 7. Conclusions Apart from the catalyst’s acid-metal balance, the operating conditions also strongly affect the ideality of the hydrocracking behavior. Increasing total pressures, decreasing temperatures, and decreasing molar hydrogen-to-hydrocarbon ratios were found to favor ideal hydrocracking, whereas a higher reactant carbon number was found to cause nonideal hydrocracking. The total pressure dependence of the reaction rate and the isomerization conversion as a function of the total conversion provide two useful means of monitoring the ideality of the hydrocracking behavior. Rate equations based on a lumped reaction scheme accounting for potentially rate-determining metal- and acid-catalyzed surface reactions allow us to rationalize the experimentally observed trends. The analysis of the rate equations is independent from the exact reaction mechanism for the (de)-hydrogenation reactions. The changes in selectivity according to the deviation from ideal hydrocracking are reflected in the model equations. Acknowledgment This research has been carried out in the framework of the Interuniversity Attraction Poles Program funded by the Belgian Science Policy. J.F.D. is grateful to the F.W.O.sVlaanderen for a fellowship as a postdoctoral researcher. Notation Roman Symbols C ) concentration (mol kgcat-1)
Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 5167 Eact ) activation energy (J mol-1) F ) flow rate (mol s-1) H ) enthalpy (J mol-1) K ) equilibrium coefficient (-) K(A1; A2) ) equilibrium coefficient between A1 and A2 (-) KL ) Langmuir physiorption coefficient (Pa-1) KA ) chemisorption () protonation) coefficient on acid sites (kgcat mol-1) KM ) chemisorption coefficient on metal sites (kgcat mol-1) k ) rate coefficient (s-1) O ) alkene P ) alkane p ) pressure (Pa) R ) net production rate (mol kgcat-1 s-1) R+ ) carbenium ion T ) temperature (K) W ) catalyst weight (kg) y ) mole fraction (-)
to the replacement in eq 4 of the hydrogen partial pressure by the hydrogen surface concentration:
Rf-P )
(
Rf-P ) M (1 + KH p + 2 H2
Appendix A. Effect of the (De)-hydrogenation Mechanism on the Qualitative Interpretation of the Rate Equation For simplicity of the rate equations it was assumed that the (de)-hydrogenation reaction mechanism occurs via an Eley-Rideal mechanism. It is demonstrated in this Appendix that other possible assumptions on the (de)-hydrogenation reaction mechanism, such as surface reaction, molecular or dissociative hydrogen chemisorption, molecular or atomic hydrogen addition, first or second hydrogen addition as rate-determining step,...26 lead to analogous qualitative interpretations of the rate equations. A.1. Molecular Hydrogen Chemisorption and Surface Reaction as Rate-Determining Steps. The assumption that hydrogen passes through a chemisorbed state in the (de)-hydrogenation mechanism leads
)
(A.1)
)
Cf-OpH2 Kdeh,f
M M Cg-P + ∑Kh-O Ch-O)2 ∑g Kg-P h
(A.2)
with M Kdeh,f ) Kdeh,f
M Kf-P M M Kf-O KH 2
(A.3)
The composite dehydrogenation rate coefficient is defined in this case as follows: 2 M (CM kdeh,f Kf-P t )
Subscripts * ) free active site chem ) chemisorption cr ) cracking/cracked products deh ) dehydrogenation f ) lump f g ) lump g h ) lump h i ) component index iso ) isomerization/isomers l ) pool of hydrocarbons in the reaction mixture n ) normal phys ) physisorption prot ) protonation sat ) saturation t ) total
M Kdeh,f
2 M (CM -kdeh,f Kf-P t ) Cf-P -
Superscripts * ) composite rate coefficient 0 ) inlet conditions 0 )standard conditions A )acid site M ) metal site
-
M M Cf-O CH 2
Accounting for the chemisorption on the metal surface, this equation can be rewritten using the concentrations of physisorbed lumps in the pores and the hydrogen partial pressure
Greek Symbols β ) beta scission γ ) molar hydrogen-to-hydrocarbon ratio ∆ ) difference
(
M -kdeh,f Cf-P CM /
/ kdeh,f
) M (1 + KH p + 2 H2
M M Cg-P + ∑Kh-O Ch-O)2 ∑g Kg-P h
(A.4)
Compared to eq 7 an extra term appears in the denominator accounting for the hydrogen chemisorption and the denominator is squared because two active sites are involved in the rate-determining step. If the hydrogen surface concentration is low, leading to a negligible chemisorption term for hydrogen, the same chemisorption term remains in the denominator as in the original expression, eq 7, however, because of the squared denominator the effects related to nonnegligible hydrocarbon concentrations on the metal sites will already occur at lower total pressures. For higher hydrogen surface concentrations, the / , excomposite dehydrogenation rate coefficient, kdeh,f hibits an inverse squared dependence on the total pressure and, hence, the linear dependence on the total pressure of the term which dominates the value of the numerator of eq 13 in the case of ideal hydrocracking, γptk/deh,iso/Kdeh,iso, changes to an inverse dependence. Because this is in contradiction with the experimental observations, the assumption of high hydrogen surface coverage can be rejected. A.2. Dissociative Hydrogen Chemisorption: First H Addition as RDS. Assuming dissociative hydrogen chemisorption and the first hydrogen atom addition as the rate-determining step leads to the replacement of the hydrogen partial pressure in eq 4 by the atomic hydrogen surface concentration and the replacement of the alkane surface concentration by the surface concen-
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Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005
tration of the species from which one hydrogen atom has been abstracted:
(
M Cf-O CM H
M Rf-P ) -kdeh,f Cf-P CM / -H
M Kdeh,f (f-P-H;f-O)
)
value of the numerator of eq 13 under ideal hydrocracking conditions is replaced by a square root dependence of the total pressure. Hence, compared to the rate equations developed under the simplifying assumptions, higher total pressures are expected to be required for ideal hydrocracking to be established under the assumptions discussed in this paragraph. For higher hydrogen surface concentrations, the linear dependence on the total pressure of the term which dominates the numerator of eq 13 in the case of ideal hydrocracking changes to an inverse square root pressure dependence. Because this is in contradiction with the experimental observation that higher total pressures favor ideal hydrocracking, the assumption of high hydrogen surface coverage can be rejected in this case. Analogous equations can be derived and similar conclusions can be drawn in the case of dissociative hydrogen chemisorption with the second H-atom addition as the rate-determining step or without assuming any of the H-atom additions as being rate determining.
(A.5)
Via the equilibrium between the alkane and the species with one hydrogen atom abstracted, the alkane surface concentration can be introduced:
Rf-P ) -kdeh,f
(
M M 2 KM deh(f-P;f-P-H)Cf-P(C/ )
-
CM H
M Cf-O CM H
KM deh(f-P-H;f-O)
)
(A.6)
Accounting for the chemisorption on the metal sites, the hydrocarbon concentrations physisorbed in the pores and the hydrogen partial pressure can be introduced
Rf-P )
(
M 2 (CM -kdeh,fKdeh(f-P;f-P-H)Kf-P t ) Cf-P -
xK
M H2pH2(1
+
xK
M H2pH2
+
)
Cf-OpH2 Kdeh,f
Appendix B. Solution of the Linear Set of Equations 9 and 11 The two eqs 9 and 11 constitute a linear set of algebraic equations of the form
M M Cg-P + ∑Kh-O Ch-O)2 ∑g Kg-P h
[ ][
(A.7) with M Kdeh,f ) KM deh(f-P;f-P-H)Kdeh(f-P-H;f-O)
(A.8)
a)
/ kdeh,n p + k/iso Kdeh,n H2
and the expression for the composite dehydrogenation rate coefficient becomes
b)-
/ kdeh,f )
c ) -k/iso 2 M (CM kdeh,fKdeh(f-P;f-P-H)Kf-P t )
x
+
x
M KH p 2 H2
+
∑g
M Kg-P Cg-P
+
∑h
As evident from the square root of the hydrogen partial pressure in the denominator of the expression for the composite dehydrogenation rate coefficient, the rate of (de)-hydrogenation reactions will be less dependent on the total pressure than in the original development. For low hydrogen surface concentrations the linear total pressure dependence of the term dominating the
1+ Rn-P )
∑i KL,ipi
(
-
k/iso
/ kdeh,iso
Kdeh,iso
/ / kdeh,n kdeh,iso
Kdeh,n Kdeh,iso
d)
k/iso Kiso
/ kdeh,iso k/iso pH2 + + k/cr Kdeh,iso Kiso
/ e ) kdeh,n Cn-P / f ) kdeh,iso Ciso-P
(B.2)
which has as solution
M Kh-O Ch-O)2
(A.9)
CsatKL,n-P
(B.1)
with
M Kf-P M M Kf-O KH 2
M KH p (1 2 H2
] []
a b Cn-O e ) c d Ciso-O f
(
( (
de - bf ad - bc
Ciso-O )
af - ce ad - bc
(B.3)
In eq 5 for normal, isomer and cracked alkanes the alkene concentrations can be substituted by expressions [B.2] and [B.3] which leads to the following net rates of formation of the alkane lumps:
/ kdeh,n pH2 pn-P-
p H 22 +
Cn-O )
/ kdeh,n k/iso
Kdeh,n Kiso
)
KL,iso-Ppiso-P KisoKL,n-P
)
+ k/cr +
/ kdeh,iso
Kdeh,iso
/ - k/iso k/cr kdeh,n pn-P
)
k/iso pH2 + k/iso k/cr
)
(B.4)
Csat 1+ Riso-P )
(( k/iso
Kdeh,n
Kdeh,n Kdeh,iso
Rcr-P )
Csat
∑i KL,ipi
(
/ kdeh,n KL,n-PpH2 pn-P -
/ kdeh,n
-
/ / kdeh,n kdeh,iso
2 1+
/ kdeh,iso
Kdeh,iso
∑i KL,ipi
(
pH22 +
/ / kdeh,n kdeh,iso
Kdeh,n Kdeh,iso
(
KisoKL,n-P
) )
/ / k/cr kdeh,iso pH2 + k/iso k/cr kdeh,iso KL,iso-Ppiso-P
( (
/ kdeh,n k/iso
Kdeh,n Kiso
/ kdeh,n
Kdeh,n
pH22 +
)
KL,iso-Ppiso-P
)
+ k/cr +
/ kdeh,iso
Kdeh,iso
)
/ / k/cr kdeh,iso pH2 + k/iso k/cr kdeh,iso KL,iso-Ppiso-P
( (
/ kdeh,n k/iso
Kdeh,n Kiso
Literature Cited (1) Ward, J. W. Hydrocracking processes and catalysts. Fuel Proc. Technol. 1993, 35, 55. (2) Scherzer, J.; Gruia, A. J. Hydrocracking Science and Technology; Marcel Dekker: New York, 1996. (3) Maxwell, I. E.; Minderhoud, J. K.; Stork, W. H. J.; van Veen, J. A. R. Hydrocracking and Catalytic Dewaxing In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; VCH: New York, 1997; pp 2017-2038. (4) Weisz, P. B. Polyfunctional heterogeneous catalysis Adv. Catal. 1962, 13, 137. (5) Steijns, M.; Froment, G. F. Hydroisomerization and Hydrocracking. 3. Kinetic Analysis of Rate Data for n-Decane and n-Dodecane. Ind. Chem. Product Res. Dev. 1981, 20, 660. (6) Denayer, J. F.; Baron, G. V.; Souverijns, W.; Martens, J. A.; Jacobs, P. A. Hydrocracking of n-Alkane Mixtures on Pt/H-Y Zeolite: Chain Length Dependence of the Adsorption and the Kinetic Constants. Ind. Eng. Chem. Res. 1997, 36, 3242. (7) Giannetto, G. E.; Perot, G. R.; Guisnet, M. R. Hydroisomerization and Hydrocracking of n-Alkanes. 1. Ideal Hydroisomerization PtHY Catalysts. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 481. (8) Degnan, T. F.; Kennedy, C. R. Impact of catalyst acid/metal balance in the hydroisomerization of normal paraffins. AIChE J. 1993, 39, 607. (9) Alvarez, F.; Ribeiro, F. R.; Perot, G.; Thomazeau, C.; Guisnet, M. Hydroisomerization and hydrocracking of alkanes: 7. Influence of the balance between acid and hydrogenating functions on the transformation of n-decane on PtHy catalysts. J. Catal. 1996, 162, 179. (10) Girgis, M. J.; Tsao, Y. P. Catalytic Hydroprocessing of Simulated Heavy Coal Liquids. 2. Reaction Network of Aromatic Hydrocarbons and Sulfur and Oxygen Heterocyclic Compounds. Ind. Eng. Chem. Res. 1994, 33, 2301. (11) Schulz, H. F.; Weitkamp, J. H. Zeolite Catalysts. Hydrocracking and Hydroisomerization of n-Dodecane. Ind. Eng. Chem. Prod. Res. Dev. 1972, 11, 46. (12) Pichler, V. H.; Schulz, H.; Reitemeyer, H. O.; Weitkamp, J. Uber das Hydrocracken gesatterigter Kohlenwasserstoffe. Erdo¨ l Kohle, Erdgas, Petrochem. 1972, 25, 494. (13) Debrabandere, B.; Froment, G. F. Influence of the hydrocarbon length on the kinetics of the hydroisomerization and hydrocracking of n-paraffins. Stud. Surf. Sci. Catal. 1997, 106, 379. (14) Weitkamp, J. Influence of Chain Length in Hydrocracking and Hydroisomerisation of n-Alkanes. ACS Symp. Ser. 1975, 20, 1.
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+ k/cr +
/ kdeh,iso
Kdeh,iso
(B.5)
k/iso pH2 + k/iso k/cr
/ k/iso k/cr kdeh,n KL,n-Ppn-P
+
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Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 5169
)
)
(B.6)
k/iso pH2 + k/iso k/cr
(15) Goldfarb, Yu. Ya.; Katsobashvili, Ya. R.; Rozenthal, A. L. Kinetics of n-decane on an aluminium-nickel-molybdenum catalyst. Kinet. Catal. 1977, 18, 364. (16) Weitkamp, J. Hydrocracken, cracken und isomerisieren von kohlenwasserstoffen. Erdo¨ l Kohle, Erdgas, Petrochem. 1978, 31, 13. (17) Steijns, M.; Froment, G. F.; Jacobs, P.; Uytterhoeven, J.; Weitkamp, J. Hydroisomerization and Hydrocracking. 2. Product Distributions from n-Decane and n-Dodecane. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 654. (18) Denayer, J. F.; Baron, G. V.; Vanbutsele, G.; Jacobs, P. A.; Martens, J. A. Evidence for Alkylcarbenium Ion Reaction Intermediates from Intrinsic Reaction kinetics of C6-C9 n-Alkane Hydroisomerization and Hydrocracking on Pt/H-Y and Pt/USY Zeolites. J. Catal. 2000, 190, 469. (19) http://www.netlib.org. (20) Svoboda, G.; Vynckier, E.; Debrabandere, B.; Froment, G. F. Application of a Single-Event Kinetic Model to Octane Hydrocracking on a Pt/US-Y Zeolite. Ind. Eng. Chem. Res. 1995, 34, 3793. (21) Thybaut, J. W.; Marin, G. B.; Baron, G. V.; Jacobs, P. A.; Martens, J. A. Alkene Protonation Enthalpy Determination from Fundamental Kinetic Modeling of Alkane Hydroconversion on Pt/ H-(US)Y-Zeolite. J. Catal. 2001, 202, 324. (22) Thybaut, J. W.; Laxminarasimhan, C. S.; Marin, G. B.; Denayer, J. F. M.; Baron, G. V.; Jacobs, P. A.; Martens, J. A. Alkylcarbenium Ion Concentrations in Zeolite Pores during Octane Hydrocracking on Pt/H-USY Zeolite. Catal. Lett. 2004, 94, 81. (23) Vynckier, E.; Froment, G. F. In Kinetic and Thermodynamic Lumping of Multicomponent Mixtures; Astarita, G., Sandler, S. I., Eds.; Elsevier: Amsterdam, 1991. (24) Laxmi Narasimhan, C. S.; Thybaut, J. W.; Marin, G. B.; Jacobs, P. A.; Martens, J. A.; Denayer, J. F.; Baron, G. V. Kinetic modelling of pore mouth catalysis in the hydroconversion of n-octane on Pt/H-ZSM-22. J. Catal. 2003, 220, 399. (25) Martens, J. A.; Parton R.; Uytterhoeven L.; Jacobs P. A.; Froment G. F. Selective Conversion of Decane into branched Isomers. A Comparison of Pt/ZSM-22, Pt/ZSM-5 and Pt/USY zeolite catalysts. Appl. Catal. 1991, 76, 95. (26) Dumez, F. J.; Froment, G. F. Dehydrogenation of 1-Butene into Butadiene. Kinetics, Catalyst Coking, and Reactor Design. Ind. Eng. Chem. Proc. Des. Dev. 1976, 15, 291.
Received for review July 16, 2004 Revised manuscript received September 15, 2004 Accepted September 30, 2004 IE049375+
