Article pubs.acs.org/EF
Development of a Comprehensive Naphtha Catalytic Cracking Kinetic Model Daniel C. Longstaff* R&DC, Saudi Aramco, P.O. Box 62, Dhahran 31311, Saudi Arabia S Supporting Information *
ABSTRACT: A comprehensive naphtha cracking kinetic model was developed and regressed to predict the cracking of FCC (olefinic) and saturated naphthas with blends of Y-based and ZSM-5 based catalysts. A comprehensive model was required because hydrogen transfer reactions occurring on the Y-zeolite portion of the catalyst affect the apparent activity for naphtha conversion and the selectivity of olefins even when the olefins are formed in the ZSM-5 portion of the catalyst blend. The model accounted for 13 reaction classes comprising 360 separate reactions applied to 37 lumps consisting of paraffins, olefins, naphthenes, and aromatics broken down based on carbon number. Because the regression was overdetermined, it was simplified by regressing compensation effects for individual reaction classes plotted on W − Ea variable space where W = ln Ao. The model was validated by predicting the effect of changing temperature and catalyst composition on the conversion and product selectivity. Butene-to-butane and C3-to-C4 ratios for 50/50 blends of Y-based and ZSM-5 based catalysts were accurately predicted based on “training” the model with the end members, i.e., 100% Y and 100% ZSM-5 catalysts.
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BACKGROUND Worldwide demand for propylene is increasing at a faster rate than ethylene. Consequently, there is increasing demand for on-purpose propylene production by fluidized catalyst cracking (FCC) of vacuum gas oils with catalysts containing ZSM-5 additives. A recent innovation in FCC cracking is the use of FCC downflow reactors (downers) in place of the conventional FCC risers. A downer FCC allows increased severity through a combination of higher temperatures and higher catalyst-to-oil ratios (C/O ratio) resulting from higher catalyst circulation rates that are possible in downflow. The increased severity permits conversion of saturated (olefin-free) naphthas, which are less reactive than vacuum gas oils or olefinic naphthas. FCC cracking of saturated naphthas is economically driven by the decreasing demand for gasoline due to dieselization of various fuel markets, such as Europe. Conversion of naphtha to propylene is a means for refineries to increase propylene production at the expense of lower value gasoline. Development of downer FCC technology to convert unconventional feeds, such as saturated naphtha, distillate, or condensate, in downer FCC units requires an extensive process simulation in specialized simulation software to determine economics, operating conditions, and practicality. The fundamental core of process simulation software is the kinetic model, which predicts the effect of feed composition, temperature, composition of the catalyst blend (fraction of Yzeolite catalyst vs ZSM-5 catalyst), residence time, and C/O ratio on feed conversion and product yields. A stand-alone downer FCC unit or a conventional riser FCC unit that has been retrofitted with an auxiliary downer will operate with a blend of Y- and ZSM-5-based catalysts. The ZSM-5 content of the catalyst blend can be changed to adjust the product yields and selectivity. Kinetic models for naphtha cracking should therefore be able to predict the effect of changing ZSM-5 catalyst content of the catalyst blend. © 2012 American Chemical Society
Different naphtha feedstocks, residence times, C/O ratios, operating temperatures, and composition of the catalyst blend (Y- vs ZSM-5-based catalyst) all affect the yield of propylene, and other petrochemicals, such as ethylene, butenes, and aromatics. It is therefore desirable to develop a naphtha cracking model to predict yield as a function of feedstock, catalyst composition (Y vs ZSM-5 content) in the catalyst blend and operating conditions. A simple lumping kinetic model, which includes lumps, such as light and heavy aromatics, naphthenes, and paraffins, is suitable for predicting yields and conversion at a single temperature, feed, and catalyst composition; however, such a model is not fundamental enough to account for changes in the important variables of interest. Detailed naphtha conversion models have been developed for dehydrocyclization on noble metal catalysts1 and for thermal/catalytic cracking.2 Lumps are defined based on carbon number and compound types. Compound types include paraffins, isoparaffins, cyclopentanes, cyclohexanes, and aromatics.1,2 The general dehydrocyclization reactions of importance, such as isomerization, cyclohexane dehydrogenation, and hydrogenolysis, were modeled with kinetic rate expressions.1 Ho Lee et al.2 estimated kinetic parameters for thermal cracking gas-phase reactions using thermodynamics.2 Catalytic reactions were too complicated to be estimated because of surface adsorption and diffusion. In addition, neither model1,2 was designed to predict a continuous change in catalyst composition (Y vs ZSM-5). Activation energies for reaction families were assumed to be constant irrespective of their carbon number. This assumption was required because the large number of kinetic parameters cause the regression to be Received: October 9, 2011 Revised: December 28, 2011 Published: January 31, 2012 801
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overdetermined. A technique for reducing the number of values to be regressed while still maintaining the required detail was required to develop this model. The complexity is too great, however, for the problem to be amenable to a typical nonlinear regression in which discrete kinetic parameters for individual reactions are regressed. Kinetic parameters must also be scientifically meaningful. For example, higher olefins should be more reactive than lower olefins because higher carbon number olefins have more skeletal configurations that can form tertiary carbenium ion transition states. Hydrogen transfer reactions for ethylene and propylene are expected to be lower than for higher olefins because tertiary carbenium ions hydrogen transfer more than 10 times faster than secondary carbenium ions.3
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DATA Two main sources of naphtha cracking data were available for regressing the model. One source consisted of literature data from Wang et al.4 for cracking FCC (olefinic) naphtha in an isothermal riser pilot plant. The second data set consisted of results from cracking a saturated (olefin-free) naphtha in a microactivity testing (MAT) reactor under conditions similar to the ASTM D3907 test method. The microactivity test is used to make relative comparisons between FCC cracking catalysts. In this test, a 1.33-g sample of hydrocarbon feed is injected into a nitrogen stream, which passes over 4 g of hot catalyst. The hydrocarbon products are collected and analyzed. The catalyst is also analyzed to determine the coke yield. In the MAT testing in this work, the normal temperatures, flow rates, and feed specified in the ASTM D3907 test were varied for the purpose of studying the effect of temperature and C/O ratio on the yields and selectivity of naphtha cracking. Although the objective of the model was to predict the conversion of saturated naphthas, the FCC pilot plant results4 with the olefinic feed were useful because the mechanisms of cracking saturated naphtha are dominated by the reactions of olefins. The FCC pilot plant data4 were collected as part of a study to consider recycling FCC (olefinic) naphtha from the primary riser of an FCC unit cracking a conventional vacuum gas oil to a secondary riser, which would be operated solely to further convert olefinic naphtha to C2−C4 olefins. The catalyst had been removed from an operating FCC unit so it represented equilibrium catalyst. In contrast, the MAT testing was conducted with the intent to investigate the potential of cracking saturated (olefin-free) naphthas under the highseverity conditions of downer FCC units. In the MAT testing, the gas product was analyzed, but the naphtha product was not analyzed by the ASTM D-2887 method as was done with the FCC pilot plant data.4 The catalyst used in the MAT study was taken as fresh from the manufacturer and was steamed prior to use. The compositions of the FCC naphtha from the pilot plant study4 and from the MAT testing are presented in Figure 1a and b. The FCC pilot plant data were collected with a catalyst blend containing 5% ZSM-5-based catalysts in 95% Y-based catalyst. Three catalyst blends were used in the MAT data: 100% Ybased, 100% ZSM-5-based, and a 50/50 blend of ZSM-5- and Y-based catalysts. The FCC naphtha cracking data4 were gathered by varying three process conditions: temperature, C/O ratio, and residence time, τ. The test conditions of FCC naphtha cracking (temperature, C/O ratio, and residence time) are presented as
Figure 1. (a) Composition of FCC naphtha feed in pilot plant data.4 (b) Composition of saturated naphtha feed in MAT testing.
a three-dimensional scatter plot in Figure 2. The C/O ratio and residence time both represent changing conditions of time
Figure 2. Scatter plot of operating conditions.4.
dimension, so they may potentially be mathematically equivalent in terms of defining the kinetics. If, however, thermal cracking were occurring, then increasing the residence time would result in higher total conversion than increasing the C/O ratio alone. This is because residence time acts with the catalyst to increase catalytic conversion, but it could also act in the gas phase to increase thermal conversion. The effects of increasing the C/O ratio and increasing residence time are presented in Figure 3. The product of residence time and C/O ratio, τ·(C/O), was designated effective time, t*. The effectiveness of using effective time as a substitute for either or both C/O ratio and residence time was determined by comparing the C5+ feed remaining with changing C/O ratios, residence times, and effective times. 802
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The 37 ODEs were formulated based on individual reactions and rate constants. Thirteen reaction classes were considered in developing the reaction network involved in naphtha cracking. These reaction classes are presented in Table 1. ODEs were developed by summing kinetic terms that quantify the conversion and production of each reactant or product in each reaction for each reaction class. All reactions were assumed to be first order. Bimolecular reactions were first order with respect to both reactants. It was assumed that the rate-limiting step for olefin−naphthene hydrogen transfer was the initial reaction between a naphthene and an olefin to form a cyclohexene, which rapidly hydrogen transferred to produce an aromatic so the reaction was first order with respect to both the naphthene and the initiating olefin. Carbonium ion reactions (i.e., paraffin dehydrogenation, demethylation, and deethylation) were assumed to be inhibited by the adsorption of C4− C11 olefins on pristine Bronsted acid sites. ODEs were constructed for each of the lumps by summing the creation and destruction terms for the various reactions which formed or consumed the lump of interest. The system of 37 ODEs formed contained about 1370 terms, which included 360 rate constants. The various ODEs contained from 10 to 80 terms and are not listed here for brevity. Example kinetic terms for each reaction class are presented in Table 2 as an illustration of the general forms of the kinetic terms in the ODEs. Kinetic Linearization. Because of the complexity of the model (360 pre-exponential factors, 360 activation energies, present in about 1370 terms distributed through 37 ODEs) conventional nonlinear regression of kinetic parameters was not possible. A new kinetic approach was required. The approach consisted of linearizing kinetic parameters into a kinetic map. This was based on linearizing the relationship between Arrhenius parameters Ea and Ao to a semilog coordinate system where ln Ao was correlated with Ea. A compensation effect is known to occur in catalysis such that there is a linear correlation between the observed parameters of the Arrhenius equation Ea and ln Ao for a series of catalysts7−9 used in a single reaction on a series of catalysts.
Figure 3. Effect of t* on C5+ feed in the products at 600 °C.
Increasing t* by increasing the C/O ratio or by increasing the residence time resulted in the same decrease in C5+ remaining in the products. It was therefore concluded that the C/O ratio and the residence time were mathematically equivalent and could both be combined into effective time, t*.
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DERIVATION OF MATHEMATICAL MODEL
The objective of the model was to predict the yields of the following lumps: • H2, LCO and coke • C1−C11 paraffins • C2−C11 olefins • C5−C11 naphthenes • C6−C11 aromatics All told, there were 37 total lumps. Each lump had a defined hydrogen and carbon content. The hydrogen to carbon ratios of LCO and coke were assumed to be 1.25 and 0.2, respectively. Fixed hydrogen and carbon contents were required for the model to close the hydrogen and carbon mass balances. Incomplete carbon and hydrogen mass balances were used as a tool to ensure that the ordinary differential equations (ODEs) contained all the relevant terms. Table 1. Reaction Classes and Sample Reactions reaction type
sample reaction
dehydrogenation
C5Paraffin → C5Olefin + H2
demethylation
C8Paraffin → C7Olefin + CH 4
de-ethylation
C9Paraffin → C7Olefin + C2H6
olefin−paraffin hydrogen transfer (forward and reverse)
C5Olefin + C10Paraffin ⇔ C5Paraffin + C10Olefin
olefin−naphthene hydrogen transfer
3C4 Olefin + C10Naphthene ⇔ 3C4Paraffin + C10Aromatic
olefin cracking
C10Olefin → C3Olefin + C7Olefin
aromatic cracking LCO formation
C9Aromatic → C7Aromatic + C2Olefin
a
C5H10 + C9H12 → 14CH1.25 + 2.25H2
coke formationa
C10Aromatic →
10 2 C1H 0.2 + C10 Naphthene 3 3
naphthene cracking
C11Naphthene → C8Naphthene + C3Olefin
olefin cyclization
C7Olefin → C7Naphthene
naphthene dehydrogenation
C10Naphthene → C10Aromatic + 3H2
olefin−olefin metathesis (one carbon only)5,6
C3Olefin + C5Olefin → 2C4 Olefin
a
LCO and coke are lumps that reflect a large number of individual components. For derivation of ODEs and closure of the hydrogen and carbon balances, the chemical formulas for LCO and coke were assumed to be CH1.25 and CH0.2, respectively. 803
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Table 2. Sample Kinetic Terms for Reaction Classesa reaction type
sample reaction
dehydrogenation
dyC 8P
=−
dt * demethylation
=−
dt *
=−
dt *
dt * olefin cracking
dt *
dyC 9A dyLCO dyCoke dyC11N dt *
olefin cyclization
dyC 7O dyC10N dt *
olefin−olefin metathesis (one carbon only)
dyC 5O dt *
= − k1037OCyC10O + other terms
= k1138NCyC11N + other terms
= − k 7OCyC 7O + other terms
dt * naphthene dehydrogenation
yC 8P + other terms
= kcokeyC10A + other terms
dt * naphthene cracking
1 + Kol ∑11 4 yC ′
= 1.5kC 9LCOyC 9A + other terms
dt * coke formation
kC 8DE
= − k 927ACyC 9A + other terms
dt * LCO formation
yC 8P + other terms
= − 3k410NX yC 4O yC10N + other terms
dyC10O
aromatic cracking
1 + Kol ∑11 4 yC ′
= − k510FPX yC 5O yC10P + k510RPX yC 5P yC10O + other terms
dt *
dyC 4
kC 8DM
i
dyC10P
olefin−naphthene hydrogen transfer
yC 8P + other terms
i
dyC 8P
olefin−paraffin hydrogen transfer (forward and reverse)
1 + Kol ∑11 4 yC ′ i
dyC 8P
de-ethylation
kC 8DH
= − k10NDHyC10N + other terms = − k35OMt yC 3O yC 5O + 2k44OMtRyC 4O yC 4O + other terms
a Kol is the olefin adsorption equilibrium constant, assumed to be equal for all C4−C11 olefins. kAxy is the rate constant (the subscripts, Axy, are defined in Table 1 in the Supporting Information). A refers to the lump, xy refers to the reaction class. yCnX is the yield of lump A where the nomenclature refers to the carbon number of the lump and the compound class, P − paraffins, N − naphthene, O − olefin, and A − aromatics, as well as hydrogen, LCO, and coke. yCnX is expressed in units of mol/mol feed.
every reaction the following relationship between rate constant, k, and the Arrhenius parameters, Ao and Ea, is as follows:
An alternative compensation effect is when a series of reactions occur on a single catalyst. The linear compensation effect is described mathematically as follows:
ln Ao = bo + b1Ea
⎛ E ⎞ k = Aoexp⎜ − a ⎟ ⎝ RT ⎠
(1)
where bo is the y-intercept and b1 is the slope of the compensation effect plot. If a plot of ln Ao vs Ea results in a steadily progressing trend then this is evidence that there is a compensation effect6,7 between ln Ao vs Ea. In other words, ln Ao and Ea are not independent because there is a correlation between the two. The compensation effect is normally assumed to be linear. The possibility of nonlinear compensation effects was assumed in this work in that ln Ao vs Ea may exhibit a nonlinear correlation with respect to one another. A plot of ln Ao vs Ea for a series of reactions for a single catalyst, or a single reaction on a series of catalysts is referred to as a Constable plot.7 A kinetic map is defined in this work as a plot of ln Ao vs Ea for a series of compensation effects that apply for the reaction classes that comprise the entire reaction network describing the conversion of feeds to products. For simplicity a new variable, W, is defined where W = ln Ao. W is incorporated into reaction kinetics by considering that for
(2)
Substituting Ao = exp(W) and rearranging yields the following:
⎛ E ⎞ k = exp⎜W − a ⎟ ⎝ RT ⎠
(3)
Derivation of a Reference Isokine. An isokine can be defined for a reaction in W − Ea reaction space to serve as a reference for regression of kinetic parameters. A reference isokine is defined such that all reactions lying along the isokine exhibit the same rate constant at the reference temperature, TR, for the isokine. A reference isokine will only be defined for a single reference temperature because the rate constants of reactions with higher activation energies increase with increasing temperature to a greater degree than lower activation energy reactions. Once the temperature changes, the rate constants of the reactions along the original isokine will no longer exhibit the same rate constant. 804
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The equation for a reference isokine is:
W = Wo +
1 Ea RTR
(4)
This is a similar form to the relationship in eq 1, with yintercept b0 = Wo and slope b1 = (1/RTR). The slope of the reference isokine is therefore
m=
1 RTR
(5)
Different values of the isokine intercept, Wo, produce parallel isokines, which are offset from the reference isokine by an increment or W-bias. This relationship is presented graphically in Figure 4.
Figure 5. Discrete kinetic parameters for a hypothetical reaction type plotted in W −Ea space.
The regression was further simplified by regressing W biases relative to the reference isokine rather than separate W values. This eliminated the cross correlation between Ea and W (or Ao) for data collected at TR for the isokine. This also permitted rapid identification of W biases, which were causing instability in the kinetic model by being too high or were inconsequential because they were too low. In the example illustrated in Figure 5, the W bias for the reaction plotted at 20 kcal/mol is zero and is −10 for the reaction plotted at 60 kcal/mol. A reaction with a W bias of 0 occurs e10 ≈ 22 000 times faster at the isokine reference temperature than a reaction with a W bias of −10. Equilibrium Thermodynamics. Consider a hypothetical equilibrium reaction with forward and reverse rate constants and forward and reverse kinetic parameters in W − Ea space: Wf, Wr, Ea,f, and Ea,r. The kinetic parameters for the forward reaction, Wf, and Ea,f in W − Ea are determined by regression. The kinetic parameters for the reverse reaction, Wr and Ea,r are calculated from thermodynamics. Ea,r is determined from the heat of reaction by the following relationship:
Figure 4. Sample isokine for 600 °C.
The solid line represents the reference isokine. The dashed lines are example isokines that are ±10 W points offset from the reference isokine. Point A represents a hypothetical reaction whose kinetic parameters, W and Ea, lie on the reference isokine. Point B represents a different reaction with a higher activation energy, which is 10 W points below the reference isokine. Although the reaction mapped at Point A exhibits a lower W (and lower pre-exponential factor) than the reaction mapped at Point B, the reaction at Point A occurs e10 ≈ 22 000 times faster than the reaction B at the reference temperature of the isokine (600 °C). Compensation effects are generally expected to be linear.7 A semilog plot of olefin cracking rate constant vs carbon number is not linear10 so there is some precedent for nonlinear compensation effects. Given that the apparent activation energies of olefin cracking rate constants are evenly spaced (which would be expected if the apparent activation energies are related to the sum of the intrinsic activation energy and the heat of adsorption of the olefin6), then the kinetic parameters plotted in a Constable plot may exhibit curvature, resulting in a nonlinear compensation effect. A hypothetical nonlinear compensation effect is illustrated in Figure 5. The kinetic curve depicted by the dotted line is biased to an isokine with TR = 600 °C. It was assumed in this work that compensation effect curves exist for each naphtha cracking reaction class and that discrete rate constants for a homologous series of carbon numbers lie along each reaction class curve. The objective of the regression was to regress reaction class curves for each reaction class (similar to Figure 5) and to determine the location of individual reactions along the curve as a function of the carbon number(s) of the reacting species. This greatly reduced the number of parameters to be regressed for a full-scale kinetic model.
Ea , r = Ea , f − ΔHr
(6)
Wr is calculated as follows based on the free energy and entropy of reaction:
Wr = Wf +
ΔG ΔH ΔS − = Wf − RT RT R
(7)
ΔS (8) R The relationships are presented graphically in Figure 6. Wr = Wf −
Figure 6. Graphical presentation of kinetic parameters for a forward and reverse reaction in W − Ea space.
Although Wr > Wf, the reverse reaction is slower than the forward reaction (Keq > 1) to the degree that Wr lies below the isokine. The application of these thermodynamically based relationships in W − Ea space were used to address reversible reactions in the regression. 805
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selectivities derived from regression of the FCC pilot plant data of Wang et al.4 were imposed into the regression of the MAT data due to the incompleteness of the MAT data. The scientific value of the MAT data was that they were collected for blends of USY and ZSM-5 catalysts. Kinetic parameters for 100% USY and 100% ZSM-5 could therefore be used to predict conversions of blends of these catalysts.
The use of compensation effect curves for each reaction class defined three levels of activity and selectivity: 1 Overall activity determined by the reference isokine. If the catalyst is diluted or the reactor type is changed, (e.g., fluidized bed vs MAT reactor) the fundamental reactivity of the system will change. This change will be reflected in an upward or downward shift of the reference isokine. 2 Overall selectivity determined by the relative bias between compensation effect curves and the relative isokine. A change in the zeolite content or type will affect the relative importance of various reaction classes, such as olefin cracking and hydrogen transfer. Selectivity addresses why some reaction classes (e.g., olefin cracking) proceed faster than others (e.g., paraffin cracking). 3 Subselectivity defined by the slope and/or curvature of the individual compensation effect curves. Subselectivity addresses why some carbon numbers convert faster than others in a given reaction class, e.g., C10 olefins crack faster than C5 olefins. Regression. Ordinary differential equations were numerically solved using a Runge−Kutta solver programmed in Visual Basic in MS Excel. The least-squares solver had the objective to minimize the relative errors between predicted and measured values. Step size was optimized to minimize computing time without creating overflow errors. The initial guess was developed by assigning a single W value for all reaction rates for all reaction classes. The initial W value was assigned to lie on an isokine with a reference temperature TR = 600 °C. Data collected for 600 °C4 at varying effective time, t*, were regressed with reference to this reference isokine. Data for varying temperature were only regressed later to obtain activation energies. The y-intercept of the isokine was regressed with a slope calculated from eq 5. The initial W value was then assigned a W bias of zero relative to the reference isokine, which provided the initial guess. The values of the W bias for individual reaction classes were then regressed, which determined the overall reactivity of the naphtha cracking network. The overall selectivities were determined next by regressing separate W biases for each reaction class. The regression results were such that faster reaction classes (e.g., olefin cracking) exhibited positive W biases, and slower reaction classes (e.g., deethylation, aromatic cracking, etc.) exhibited negative W biases. The subselectivities were finally determined by regressing the slope or curvature of the compensation effect for the individual reaction classes. This determined the reactivity difference for individual carbon numbers in each reaction class. During each stage of the regression the predicted and measured data trends were compared to determine if reaction classes should be added or removed or to increase the detail of regression of individual rate constants in a reaction class. Data for varying T was regressed when the regression for varying t* was completed. An additional regression was performed by adapting the model developed for the pilot plant data to the second set of naphtha cracking data consisting of MAT data collected for the cracking of a saturated naphtha at varying C/O ratios. MAT data consisted of gas composition and yield, total yield, and coke yield. Complete naphtha composition data were not available for the products of the MAT testing. Because of the lack of composition data for the C5+ fraction, only the overall activity and overall selectivities were regressed. The sub-
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RESULTS AND DISCUSSION Prediction of FCC Naphtha Cracking. One of the main objectives of this method was to explore regressing kinetic parameters in W − Ea space instead of conventional Ao − Ea space. Following the regression the results were plotted on a kinetic map in W − Ea space, which is presented in Figure 7a and b.
Figure 7. (a) Kinetic map of FCC naphtha cracking kinetic parameters, Part I. (b) Kinetic map of FCC naphtha cracking kinetic parameters, Part II.
The legend of abbreviations used in Figure 7a and b is presented in Table 3. The activation energies for olefin−paraffin hydrogen transfer were excessively high (100−120 kcal/mol). The rate-limiting step for paraffin cracking is hydride transfer from a paraffin to an adsorbed olefin present as a C3−C4 carbenium ion on an acid site. The activation energy for this step should have been in the order 5−10 kcal/mol.11 Measured and predicted yields for C2−C5 olefins and C6−C11 naphthenes at varying effective times, t*, at 600 °C are presented in Figures 8 and 9, respectively. Measured and predicted yields for C2−C4 olefins produced at varying temperature at constant t* = 36 gcat/gfeed·s are presented in Figure 10. The butene yields passed through a maximum at very low effective time, t*. This suggests that there is a deficiency in the model. Perhaps the model deficiency could be identified and eliminated by incorporating data at lower values of effective time, t*, in future work. 806
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C11 naphthenes were not reported in the pilot plant data.4 The presence of C11 naphthenes in the products was only inferred from olefin cyclization reactions of C11 olefins. The data4 reported that C7 naphthenes were about five times more abundant than either C6 or C8 naphthenes. It was concluded that an artifact in the GC analysis was the most likely explanation for the higher than expected levels of C7 naphthene because other authors12 reported C7 naphthene levels, which were intermediate between C6 and C8 naphthene levels. The model predicted a decline in ethylene yield at higher temperatures. This is an artifact from the regression resulting from the high activation energy for olefin−paraffin hydrogen transfer. The most facile reactions were (1) metathesis reactions involving pairs of olefins with n ≥ 4 carbon numbers (W bias = 0−6), (2) cyclization of C9+ olefins (W bias = 0−2), and (3) olefin cracking (W bias = −6−1). The slowest reactions were (1) metathesis reactions involving ethylene and propylene (W bias < −20), (2) LCO forming reactions for 10−20% Y-zeolite catalyst provided enough hydrogen transfer activity to result in significant conversion of butenes to butanes. The increase in C3/C4 ratio with increasing ZSM-5 content was more linear. ZSM-5 is selective for converting olefins to propylene. Y-zeolite is more selective for producing butenes. As ZSM-5 content was increased, C4 selectivity decreased and C3 selectivity increased by an amount proportional to the content of the respective zeolites. Curvature of the trend was due to differences in hydrogen transfer activation of paraffins on the two different zeolite catalysts and differences in the base olefin cracking activity of ZSM-5 relative to Y-based zeolite catalysts.
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CONCLUSIONS A comprehensive naphtha cracking kinetic model was developed to predict naphtha cracking kinetics. The model predicted the effect of changing temperature, residence time, C/O ratio, catalyst, and feed composition on product yields. The model predicted the yields of 37 separate lumps whose conversion and production were affected by 13 reaction types comprising 360 reactions. Because the regression was overdetermined, compensation effects for reaction classes were regressed rather than kinetic parameters for single reactions. The regression of the kinetic parameters was simplified by regressing compensation effects in W − Ea variable space where W = ln Ao. Olefin cracking reactions, metathesis, and cyclization reactions were faster for higher carbon numbers. Aromatic cracking was slow because the aromatics probably consisted mainly of polymethyl aromatics, which are less reactive than aromatics with longer alkyl chains. 809
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