Ind. Eng. Chem. Res. 2007, 46, 5503-5509
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Evolution of Light Hydrocarbons from a Coked Ferrierite Catalyst during Temperature-Programmed Gasification Tuomo J. Keskitalo,*,† Satu T. Korhonen,† Kyo1 sti J. T. Lipia1 inen,‡ and A. Outi I. Krause† Laboratory of Industrial Chemistry, Helsinki UniVersity of Technology, P.O. Box 6100, FI-02015 TKK, Finland, and Neste Oil Corporation, Technology Centre, P.O. Box 310, FI-06101 PorVoo, Finland
The evolution of light hydrocarbons during temperature-programmed gasification with helium (TPHe) of a coked ferrierite catalyst was studied to gain information on partial coke removal by heat treatment under inert flow. The catalyst had been deactivated in the skeletal isomerization of alkenes. Characterization by DRIFTS, GC-MS, and 13C CPMAS NMR revealed that the coke contained two- and three-ring aromatic hydrocarbons, as well as polyaromatics. Evolution predominantly of C2-C5 hydrocarbons was observed during TPHe between 200 and 500 °C. The main components were propene and ethene. The carbon content of the catalyst dropped from 4.3 to 3.5 wt % during TPHe. Kinetic models suitable for the purpose of designing a coke removal unit were derived for the evolution of carbon fractions between C2 and C5 from the ferrierite zeolite. The results of the kinetic modeling suggest that one or several bimolecular reactions are relevant in the formation of light hydrocarbons from coke. 1. Introduction Formation of coke is the main mode of catalyst deactivation in industrial processes such as fluid catalytic cracking (FCC), continuous catalytic reforming, and skeletal isomerization.1 Formation of coke in these processes is an unavoidable side reaction and mostly unwanted. Coke deactivates the catalyst by blocking the active sites and the pores of the catalyst, preventing the transport of reactants between the bulk phase and the active sites. Coke formation can occur quickly, in FCC on a time scale of seconds,2 which makes regeneration of the catalyst of critical importance. The catalyst can be regenerated by oxidation of the coke with air. Inert gases such as nitrogen or steam can also be applied to achieve at least partial reactivation by gasification of the coke. Hydrocarbon processes in which coke formation plays an essential role could feasibly be designed with separate units for the main reaction and the regeneration.3 In this design the deactivated catalyst is continuously fed to the regenerator, from which the regenerated catalyst is routed back to the main reactor. The coked catalyst passes through a dip leg, in which the particles remain as a bed until they are fed to the regenerator. Inert gas or steam is fed to the dip leg, countercurrent to the catalyst flow, to remove light hydrocarbons and part of the coke from the catalyst. Thus, partial catalyst decoking may be performed already in the dip leg. We studied the removal of coke in a nonoxidative gas stream to gain information on the overall reactions taking place in the dip leg. The study is also relevant for catalyst regeneration by coke oxidation since treatment of the catalyst in the dip leg affects the nature and amount of coke on the catalyst fed to the regenerator. The removal of coke from heterogeneous catalysts under inert atmosphere has been studied by Finelli et al.4 using a ferrierite catalyst coked in the skeletal isomerization of 1-butene. The group reported that the part of the coke that can be removed by * To whom correspondence should be addressed. Tel.: +3589-451 2522. Fax: +358-9-451 2622. E-mail:
[email protected]. † Helsinki University of Technology. ‡ Neste Oil Corporation.
temperature-programmed oxidation (TPO) between about 300 and 500 °C can also be removed by temperature-programmed gasification with helium (TPHe), and on that basis the coke was suggested to be rich in hydrogen. In a study of coke and its removal from Y-zeolite, mordenite, and L-zeolite deactivated in isobutane alkylation, Querini and Roa5 found that the amount and nature of the coke is specific for each catalyst, as is the proportion of coke that can be removed from a catalyst in helium flow. The elimination of coke from lanthanum-exchanged H-Y zeolite resulted in the evolution of hydrocarbons ranging from C1 to C8 and even higher. Concerning the cracking of oligomers from zeolites, Gricus Kofke and Gorte6 reported that oligomerization products of propene, ethene, and 2-methyl-2-propanol on H-ZSM-5 catalysts evolve mainly as C2-C7 hydrocarbons. Thomazeau et al.7 studied the deactivation of a 5A zeolite, a small-pore catalyst, under propene feed, and found that the reversible part of the coke was evolved as propene under vacuum. However, hydrogen transfer and cyclization reactions transformed the coke toward aromatics, which could not be eliminated from the small pores of the catalyst by thermal vacuum treatment. Clearly, the transformation and evolution of hydrocarbons from zeolites is case specific. Temperature-programmed gasification is highly similar to temperature-programmed desorption (TPD), which is a wellknown thermal analysis technique8,9 commonly used in the study of heterogeneous catalysts to characterize adsorption and desorption. In addition to the commonly applied qualitative characterization, the results of properly designed TPD experiments10 can be utilized to characterize adsorption, desorption, and surface reactions through kinetic modeling. Temperatureprogrammed experiments are particularly interesting for kinetic modeling, because information on reactions is produced over a wide temperature range and because chemical reactions are usually highly dependent on temperature. In short, as this paper will demonstrate, kinetic analysis of the results from temperature-programmed experiments adds to the information that can be obtained with these thermal techniques. We focus here on the evolution of light hydrocarbons during TPHe of a deactivated ferrierite zeolite, to study the partial decoking of the catalyst taking place in the dip leg of the skeletal
10.1021/ie070063r CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007
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isomerization reactor under an inert gas stream. In earlier work we studied the same ferrierite catalyst by TPO.11,12 This paper presents information on the evolved hydrocarbons, quantified by gas chromatography, and reports results of the analysis of the coke on the catalyst. The reactions that produce the evolved hydrocarbons are characterized through kinetic modeling. The derived models describe the evolution from the coked catalyst of hydrocarbon fractions from C2 to C5, and the models are suitable for the purpose of designing a decoking unit for this catalyst operating under inert gas flow. 2. Experimental Section 2.1. Catalyst Characterization. The catalyst was a ferrierite zeolite, deactivated in skeletal isomerization of alkenes ranging from C5 to C7.3 The reaction was carried out in a pilot-scale reactor operating at 285 °C for 21 h. The catalyst was retrieved from the pilot reactor after flushing with nitrogen at 285 °C overnight. In this study, coke refers to any hydrocarbons present in the deactivated catalyst after the flushing. Characterization of the coke on the ferrierite was carried out by in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), gas chromatography combined with mass spectrometry (GCMS), and 13C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy. Additionally, the total carbon and hydrogen content of the catalyst was measured with a Model CHN-2000 elemental analyzer (LECO). The in situ DRIFTS equipment consisted of a Nicole Nexus Fourier transform infrared spectrometer and a Spectra Tech hightemperature and high-pressure reaction chamber equipped with ZnSe windows. Gasification experiments were carried out by heating the coked catalyst samples under constant nitrogen flow of 50 cm3/min. The samples were examined both as received and after dilution with KBr (5 wt % catalyst, 95 wt % KBr). KBr dilution was used to reveal the bands originating from the coke in the low wavenumber region (2000-1300 cm-1), which otherwise were covered by strong vibrations of the zeolite framework. The spectrum measured with an aluminum mirror (4 cm-1, 200 scans) was used as a background in all experiments. The DRIFT spectra were recorded every 100 °C (4 cm-1, 100 scans) during the heat treatment, after which the cell was flushed with nitrogen for 30 min at 500 °C before the samples were cooled to room temperature. During the cooling stage the spectra were recorded every 100 °C (4 cm-1, 100 scans). For the GC-MS analysis of coke, the catalyst sample was demineralized by a treatment with HF, adapted from Snape et al.13 The sample was first flushed in nitrogen flow at 280 °C to desorb any light hydrocarbons that might still be adsorbed on the catalyst. Five grams of sample was then stirred in a decanter at 60 °C overnight with 50 cm3 of 2 M HCl. After that, the sample was washed with water, and demineralized by stirring with 100 cm3 of 40% HF at room temperature for 2 h. The sample was filtered, washed with water, and dried in an oven at 100 °C. Finally, the remaining coke sample was dissolved in dichloromethane for the GC-MS analysis. The gas chromatograph (Agilent 5890) was equipped with a DB-1 column. The NMR analysis was carried out with a Chemagnetics model CMX270MHz Infinity instrument. 2.2. Temperature-Programmed Experiments. Temperatureprogrammed experiments were carried out in a packed-bed microreactor system11 operating at atmospheric pressure. The equipment consisted of a glass-tube reactor furnished with a fast, small Au-film furnace (Fortum), temperature controller (KS40, Phillips), mass-flow controllers (Brooks), and six-way valves (Valco). The flow from the reactor was separated into
Figure 1. DRIFT spectra recorded during heat treatment of coked ferrierite. The spectra on the left were recorded using an undiluted ferrierite sample, while those on the right were recorded with a KBr-diluted sample. The spectra of a fresh catalyst recorded at 25 °C are presented for comparison.
two flows. With one flow the total content of hydrocarbons was analyzed continuously with a flame ionization detector (FID), and with the second flow the exact composition of samples was quantified with a gas chromatograph (Agilent 5890). In the experiments the deactivated catalyst sample (about 10 mg, mean particle diameter 120 µm) was loaded in the middle of the glass reactor tube over a quartz wool layer and kept in helium stream for 12 h at 200 °C to remove water adsorbed from air. TPHe was carried out from 200 to 500 °C at a constant heating rate (5, 10, or 15 °C/min) in helium stream (flow rate 30 cm3/min, NTP). The FID response was calibrated with a gas containing 0.99 vol % CO and 1.99 vol % CO2 (AGA), which was transformed completely to methane by hydrogenation over a Ni/γ-Al2O3 catalyst. 3. Results 3.1. Characterization of Coke. The DRIFT spectra of the coked ferrierite were compared with those measured for a fresh sample. Figure 1 presents the spectra recorded during the heat treatment in nitrogen. The spectra in the high wavenumber region (left) were obtained with an undiluted sample, while those presented in the low wavenumber region were obtained with a sample diluted with KBr. The dilution had no effect on the frequency of the bands (not shown for the sake of brevity) in the high wavenumber region but significantly lowered their intensity. For the fresh sample the Si-OH species were observed at 3747 cm-1 and the Brønsted acid species (Si(OH)Al in Figure 1) were observed at 3598 cm-1.14,15 Both bands were lower in intensity for the coked sample. At room temperature, the main bands on the coked sample were the broad feature between 3700 and 3000 cm-1 and the intense band at 1624 cm-1. These bands were assigned to adsorbed water, which desorbed below 200 °C. Bands originating from carbon-containing adsorbed species were observed at 2964, 2926, 2878, 1588, 1558, 1518, and 1447 cm-1.
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Figure 2. DRIFT spectra of coked ferrierite, measured at 25 °C after heat treatment. The spectra on the left were recorded with an undiluted sample, and those on the right were recorded with a KBr-diluted sample. The spectra of the fresh catalyst are presented for comparison.
Figure 3. Evolution of hydrocarbons as detected by FID from the coked ferrierite catalyst during TPHe carried out at the heating rates of 5 (continuous line), 10 (dashed line), and 15 °C/min (dotted-dashed line).
These bands were assigned to the ν(CH2) and ν(CH3), ν(Cd C), and δ(CH2 or CH3) vibrations of aromatic species.14,15 During the heat treatment, the ν(CH2) and ν(CH3) vibrations at 2964, 2926, and 2878 cm-1 decreased in intensity, while the ν(CH) vibrations of aromatic species above 3000 cm-1 16 increased in intensity, indicating that the amount of hydrogen in the carbonaceous surface species was decreased through condensation reactions. The decrease in the amount of hydrogen was also observed in the low wavenumber region as increasing intensity of the ν(CdC) vibrations of aromatic rings at 1575 and 1514 cm-1.14 Figure 2 presents the spectra recorded at room temperature after the heat treatment. The multiplicity of bands indicated the presence of a heterogeneous group of alkylated aromatic species. The bands were assigned to the ν(CH) vibrations of aromatic species (3089, 3067, and 3067 cm-1), the ν(CH2 or CH3) vibrations of aliphatic groups (2974, 2949, 2924, and 2866 cm-1), the ν(CdC) vibrations of aromatic ring structures (1620, 1606, 1584, and 1520 cm-1), and δ(CH2 or CH3) vibrations (1475, 1446, 1422, 1394, and 1349 cm-1). The broad feature centered at about 3400 cm-1 most likely originates from OH groups hydrogen bonded to the coke.16 According to Paze´ et al.,14 the bands observed at 1620, 1606, 1584, 1520, and 1422 cm-1 could originate from xylenes, toluene, or polycyclic molecules. The 13C MAS NMR and GC-MS analyses gave similar results. The NMR spectra of the coked ferrierite showed bands around 20, 55, 130, and 205 ppm. The band at 20 ppm corresponds to aliphatic carbons, the band at 130 ppm corresponds to aromatic carbon structures, and the bands at 55 and 205 ppm result from spinning sidebands. The NMR analysis thus suggests that the coke is composed of aromatic compounds and that methyl groups are attached to the aromatic skeleton of the coke molecule. These findings are in accordance with the DRIFTS results. Concerning the GC-MS analysis, all of the demineralized sample dissolved in dichloromethane. The analysis revealed a number of mainly polycyclic aromatic hydrocarbons, the main coke components containing two- or three-ring aromatics. In conclusion, the results from all characterization methods suggest that the coke was composed of aromatic hydrocarbons with short-chain alkyl groups. The aromaticity of the coke was shown by DRIFTS to increase during heat treatment. 3.2. Temperature-Programmed Experiments. TPHe of the coked catalyst sample resulted in evolution of light hydrocarbons. Figure 3 shows the measured FID response at the three
heating rates tested. Each TPHe spectrum shows a single peak spanning the temperature range from about 300 to 500 °C with the peak maximum around 380-400 °C, depending on the heating rate. As expected, the peak maximum shifts to higher temperature as the heating rate increases. The fact that the baseline is not reached by 500 °C (Figure 3) suggests that a second peak starts to form around 450 °C, but it does not emerge because the temperature program was terminated at 500 °C. The kinetic modeling, described below, focused on the description of the first peak and therefore utilized experimental data only up to 450 °C. FID responses were measured in TPHe experiments carried out at the heating rates of 5, 10, and 15 °C/min. Two experiments were carried out at each heating rate. The response of the FID was assumed to be linearly proportional to the number of carbon atoms in the molecule being detected.17 Integration of the TPHe peak areas, spanning the whole range from 200 to 500 °C, gave the total evolved amount of carbon as 0.58 and 0.56 mmolC/gcat for the two experiments carried out at the heating rate of 5 °C/min, 0.70 and 0.69 mmolC/gcat for those carried out at 10 °C/min, and 0.66 and 0.61 mmolC/ gcat for those carried out at 15 °C/min. The total amount of carbon desorbed during TPHe was on average 0.63 ( 0.06 mmolC/gcat, equal to 0.8 wt %. According to the elemental analysis, the carbon content of the coked catalyst was 4.3 wt %. On average, then, the carbon content was decreased from 4.3 to 3.5 wt % during TPHe. Clearly, inert gas treatment at 500 °C removes only part of the coke from the catalyst. The reactor outflow was sampled during TPHe experiments corresponding to reactor temperatures close to 300, 350, 400, 450, and 480 °C and analyzed with the gas chromatograph. As an example, Table 1 lists some of the GC results obtained from the TPHe experiment carried out at the heating rate of 10 °C/ min. In general, the C3 fraction dominated, followed by C2, C4, and C5. Most of the C3 fraction was composed of propene, and most of the C2 fraction was composed of ethene. The C4 fraction included butenes. Hydrocarbons containing more than five carbon atoms were not identified in the experiments. The fraction of methane and especially the fraction of ethene in C2 increased with temperature, and the fractions of C3 and C4 decreased. 3.3. Kinetic Modeling. The experimental data applied in the kinetic modeling were obtained as described in the following. Each of the six TPHe spectra, quantified as reported above, was divided into carbon fractions on the basis of the composition obtained from the GC analyses. Linear interpolation was applied
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ri ) kicdi,
i ) C2, ..., C5
(2)
Equation 2 describes the rate of formation (r) of each carbon fraction as a function of a reaction rate constant (k), the surface concentration of reversible coke carbon (c), expressed as mol/ gcat, and an order parameter (d). The difference between the two models is in the order parameter. Model 1 assumes that all of the reversible coke is accessible to the reactions at all times and therefore the value of the order parameter is fixed at 1.0.18 Model 2 allows the order parameter to vary freely during optimization. The amount of reversible coke carbon is described by the equation
Figure 4. Composition of the outflow in TPHe experiments carried out at the heating rate of 10 °C/min. The black line represents the response by FID, and lines with symbols represent the molar fraction at each temperature. (crosses) C2; (circles) C3; (diamonds) C4; (squares) C5. Table 1. Composition of Outflow at 300, 395, and 480 °C from a TPHe Experiment Carried out at Heating Rate of 10 °C/min, Analyzed with a Gas Chromatograph compound (mol %) methane ethane ethene propane propene 2-methylpropene 1-butene trans-2-butene cis-2-butene 1-pentene trans-2-pentene other compounds
300 °C
395 °C
480 °C
1 1 14 9 43 4 2 8 5 1 5 7
2 2 30 10 37 3 2 6 4 0 1 3
6 2 60 2 22 2 1 2 2 0 0 1
i ) C2, ..., C5
(3)
The total amount of reversible coke carbon on the catalyst (c0) was a parameter in models 1 and 2. Model 3 assumes that all of the coke on the catalyst is initially reversible and, in principle, can evolve as light hydrocarbons. However, in addition to the formation of C2-C5 fractions, reversible coke is also assumed to react into irreversible coke that is inactive and remains on the catalyst surface. The reactions of the reversible coke carbon (C) in model 3 are thereby
iC f Ci(g),
i ) 2, ..., 5
(4)
and
(5)
2C f Cirrev
between the GC data points so that approximately 50 temperature points were obtained for each TPHe spectrum. Since the composition at a certain temperature could be expected to differ with the heating rate, the division of the TPHe spectra was made on the basis of experiments made with the same heating rate. Figure 4 shows the results of experiments carried out at the heating rate of 10 °C/min as an example. The model for the microreactor system consisted of a reactor model and a kinetic model for the formation reactions of the carbon fractions C2-C5. The reactor was modeled as gradientless, and the system was assumed to be free of diffusional limitations, for reasons discussed elsewhere.11 No readsorption was assumed for the desorbed species. The molar amount of gaseous carbon fraction in the microreactor (ni) was described by the equation
Vni dni ) rim - , dt V
dc ) (-2rC2 - 3rC3 - 4rC4 - 5rC5)m dt
(1)
in which r is the formation rate, expressed as mol/(gcat s), m denotes the catalyst mass, V is the volumetric flow rate, and V denotes the volume of the reactor gas phase. Three kinetic models were evaluated. All models assume that the coke is homogeneous on average. It is also assumed that the part of the coke that does not result in evolution of hydrocarbons (irreversible coke) does not affect the modeled reactions. Models 1 and 2 are power-law models that deal only with the part of the coke that evolves as the observed hydrocarbons during TPHe (reversible coke). The models postulate that all of the reversible coke can produce any carbon fraction between C2 and C5. These assumptions lead to a kinetic power-law model
The formation of C2-C5 fractions (eq 4) is described with eq 2, in which the order parameter is fixed at 1.0, as in model 1. The formation of irreversible coke (eq 5) is described by a second-order rate law with respect to carbon concentration according to
rirrev ) kirrevc2
(6)
because a first-order dependence proved only as good as model 1 (see below). The balance equation for the reversible coke carbon in model 3 is
dc ) (-2rirrev - 2rC2 - 3rC3 - 4rC4 - 5rC5)m dt
(7)
and for the irreversible coke carbon it is
dcirrev ) rirrevm dt
(8)
In model 3, the total amount of reversible carbon on the catalyst (c0) was fixed at the value obtained in the elemental analysis (3.4 mmol/gcat). The reaction rate constants in all models are dependent on temperature according to the reparametrized Arrhenius equation
ki ) kref,i exp
( (
-Ei 1 1 R T Tref
))
(9)
in which Tref is the reference temperature (673 K), kref is the rate constant at the reference temperature, E is the activation energy, and R is the universal gas constant. Nonlinear regression was applied to estimate the parameters of the kinetic models. Parameter optimization was aimed at minimizing the residual root-mean-square error (RRMS) be-
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Figure 5. Results and the fit of models 1-3 in a TPHe experiment carried out at the heating rate of 10 °C/min. Experimental (symbols) and simulated (continuous lines) concentrations in outflow for each carbon fraction. (crosses) C2; (circles) C3; (diamonds) C4; (squares) C5. Table 2. Estimated Parameter Values, 95% Confidence Intervals, and Residual Root-Mean-Square (RRMS) Values for the Kinetic Models
a
parameter
model 1
model 2
model 3
kref,C2 (1/s) EC2 (kJ/mol) dC2 kref,C3 (1/s) EC3 (kJ/mol) dC3 kref,C4 (1/s) EC4 (kJ/mol) dC4 kref,C5 (1/s) EC5 (kJ/mol) dC5 kref,irrev (gcat/(mol s)) Eirrev (kJ/mol) c0 (mmol/gcat) RRMS (nmol/s)
(5.7 ( 0.2) × 10-4 159 ( 4 fixed at 1.0 (8.3 ( 0.3) × 10-4 101 ( 3 fixed at 1.0 (2.8 ( 0.2) × 10-4 95 ( 5 fixed at 1.0 (7.8 ( 1.1) × 10-5 70 ( 12 fixed at 1.0
(6.5 ( 3.5) × 10-3 167 ( 5 1.35 ( 0.08 (5.8 ( 2.9) × 10-1 147 ( 3 1.90 ( 0.08 (2.2 ( 1.5) × 10-1 137 ( 6 1.91 ( 0.10 9.9a 118 ( 13 2.63 ( 0.32
1.38 ( 0.03 0.25
1.44 ( 0.02 0.13
(3.3 ( 0.2) × 10-4 190 ( 4 fixed at 1.0 (4.6 ( 0.2) × 10-4 125 ( 3 fixed at 1.0 (1.5 ( 0.1) × 10-4 113 ( 4 fixed at 1.0 (3.8 ( 0.1) × 10-5 70 ( 9 fixed at 1.0 2.1 ( 0.2 180 ( 6 fixed at 3.4 0.18
Large confidence interval.
tween N experimental (exp) and calculated (calc) flow rates (F) of all carbon fractions C2-C5 out of the reactor, expressed as nmol/s, as described by the objective function:
RRMS )
x∑ 1
N
N j)1
(Fj,exp - Fj,calc)2
(10)
All the calculations were carried out with Matlab 7 (The MathWorks, Inc.). The optimization of eq 10 was carried out by the polytope search algorithm of Nelder and Mead,19 as implemented in Matlab. The stiff ordinary differential equation solver ode15s of Matlab was employed. The absolute error tolerance in the calculations was 10-6 nmol/s, and the relative error tolerance was 10-8 nmol/s. The estimated parameters for the models are presented in Table 2. At least two initial values were tested to check for convergence toward the values reported in Table 2. Models 1 and 3 exhibited no strong correlations between parameters, but in model 2 the correlation coefficients between the rate constant and order parameter for each carbon fraction were over 0.95. Figure 5 depicts the fits of all models for an experiment carried out at the heating rate of 10 °C/min as an example. 4. Discussion 4.1. Characterization of Coke. Characterization by DRIFTS showed the coke on the ferrierite to contain structures of xylenes, toluene, and polycyclic aromatics. Similar results were obtained in the NMR analysis, which suggested that aromatics are present on the coked catalyst and methyl groups are attached to aromatic compounds. The GC-MS results showed the coke to contain two- and three-ring aromatics. In summary, the results of the three characterization methods applied indicate that the coke is composed of aromatic hydrocarbons with short-chain alkyl
groups. The evolution of mainly C2 and C3 fractions suggests the same. Our results on coke characterization are similar to those reported by Andy et al.16 The mechanisms relevant in skeletal isomerization20 and catalytic cracking of alkenes21,22 suggest possible reaction routes from coke to the observed light hydrocarbons. The acid sites on zeolites play a key role in these mechanisms. It has been established that alkenes react with the acid sites and form carbenium ions, which are the reactive species in both skeletal isomerization and catalytic cracking. Among the main reactions of carbenium ions21 are fast proton, double bond and alkylgroup shifts, and oligomerization reactions. C-C bond scission reactions of the intermediates lead to the observed cracking products. The paring reaction23 may also be relevant. These catalyzed reactions are possibly responsible for the cracking of coke as well, though thermal cracking may play a role, too. The double bonds in the coke molecules probably react with acid sites of the catalyst, and the formed carbenium ions react through a combination of all of the reactions mentioned above. In view of the literature dealing with skeletal isomerization and catalytic cracking of alkenes,20-22 the light hydrocarbons evolved during TPHe may be a result of coke cracking reactions taking place in the catalyst. 4.2. Temperature-Programmed Experiments. The TPHe profile recorded at 15 °C/min (Figure 3) resembles the helium stripping spectra presented by Finelli et al.,4 detected with FID from a ferrierite catalyst deactivated during isomerization of 1-butene. The evolved products were unfortunately not identified in their study. The conclusion from the oligomer cracking study of Gricus Kofke and Gorte6 was that the precursor of the oligomer (ethene, propene, or 2-methyl-2-propanol) does not markedly influence the ratios of the cracking products. This would explain the similarity of our results, in which the feed was C5-C7 alkenes, to those of Finelli et al. If the alkene feed
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composition does not greatly affect the reactions of coke, our findings reported in this paper are meaningful for other ferrierite deactivation studies, too. The beginning of the formation of a second peak at about 450 °C (Figure 3) indicates that, in this case, there are at least two modes of cracking of coke to light hydrocarbons. We studied the one observed in the temperature range between 300 and 500 °C in detail, but not that detected above 450 °C, because the heating program was terminated at 500 °C and few experimental data were available for it. Furthermore, the helium stripping spectra of Finelli et al.4 of a ferrierite catalyst deactivated during 1-butene isomerization showed that the second peak, in the temperature range from about 450 to 600 °C, is small compared with the peak observed between 300 and 500 °C. Examination of the products detected during TPHe (Table 2) shows that the carbon-to-hydrogen ratio of the coke increases with heating. The DRIFTS measurements confirmed this trend. Evidently the composition of the coke becomes more aromatic as temperature increases. The focus of the product spectrum also changes from C3-C5 hydrocarbons to ethene, ethane, and methane. This may be due to a change in the preferred C-C bond scission mechanism as the temperature rises.21 Another possibility is that the active coke intermediate is transformed as a result of the cracking reactions so that the light C2 and C1 units, mostly ethene, are more easily produced from the coke than the heavier C3-C5 fractions. 4.3. Kinetic Modeling. All three kinetic models describe the experimental data adequately (Figure 5). The residual root-meansquare (RRMS) values in Table 2 show that model 2 performs better than model 1, and model 3 describes the experimental data slightly worse than model 2, but better than model 1. The differences between the experimental and calculated data were seemingly random, except that all models slightly overestimated the formation of C2-C5 for experiments carried out at 5 °C/min. The probable reason for this is that those experiments have the least weight in the objective function (eq 10) because of the low concentrations (see ref 11). The values of the optimized parameters in Table 2 appear to be physically meaningful. The activation energies decrease as a function of carbon fraction for all models. Since the order parameters of models 1 and 2 were different, it is no surprise that the estimated rate constants and activation energies differ, too. The large confidence intervals exhibited by the parameter estimates are partly due to errors in the experimental data, especially the data related to C4 and C5, because of their low concentrations compared to C2 and C3. In the case of model 2, the strong correlations between the rate constant and the order parameter are reflected as large confidence intervals. The estimated total concentration of carbon that is removed during TPHe (reversible coke carbon) is about the same for models 1 and 2. The estimated concentration (about 1.4 mmol/ gcat) is larger than the concentration calculated by integration over the TPHe spectra (0.63 mmol/gcat). The estimated concentration is nevertheless reasonable compared with the total carbon content measured by elemental analysis (3.4 mmol/gcat). The kinetic modeling appears to give some insight into the mechanism on coke removal under inert flow. Model 1, in which the order parameters dC2, dC3, dC4, and dC5 were fixed at 1.0, performs worst at the high-temperature range of the TPHe spectra (Figure 5). The order parameters in model 2 are all relatively close to 2.0, and according to RRMS values (Table 2), model 2 fits the experimental data better than model 1. The second-order rate law for the formation of irreversible coke in
model 3 (eq 6) is also necessary to describe the experimental results, since the test with a first-order dependence resulted in a fit only as good as model 1. Since the second-order rate law is indicative of a bimolecular reaction, the results of the kinetic modeling suggest that one or several bimolecular reactions are relevant in the formation of light hydrocarbons from the coke. Model 3 has a possible mechanistic interpretation. The formation of irreversible coke, described by the second-order rate law with respect to coke carbon concentration, may represent the combination of two molecules formed from the coke. This combination reaction may be alkylation, since it is bimolecular and relevant in coke formation.24 The first order for the formation of C2-C5 fractions in model 3 might be explained as carbon-carbon scission or paring23 reactions, which involve one coke molecule. In summary, model 3 appears to be physically realistic, although we have no experimental evidence of the actual limiting reaction steps in the formation of the light hydrocarbons. Because models 2 and 3 describe the experimental data best, they are appropriate for design purposes for processes involving the studied ferrierite catalyst. However, since the parameters of model 3 were well identified, it is more appropriate if extrapolation is attempted. Because of the approximations reported above and the errors in the available experimental data, more detailed kinetic modeling was not carried out since it would likely result in overfitting. Because the possible reaction network in the system is large, the present study does not allow us to specify the reactions taking place on the ferrierite in detail. Nevertheless, the kinetic modeling resulted in new information on the removal of coke under inert flow. First, kinetic models useful for the design of a coke removal unit for the ferrierite catalyst were designed. Second, the parameters of the derived models characterize the observed coke removal reactions and can be used to compare the results of different case studies focused on coke removal. Third, the kinetic modeling indicated mechanistic details of the coke cracking reactions. In this case, the approximately secondorder rate laws with respect to carbon concentration in model 2 suggest that one or more bimolecular reaction steps are relevant in the formation of the light hydrocarbons from coke. The results with model 3 suggest that the formation of irreversible coke may be the second-order reaction, while the formation of C2-C5 is of first order. The kinetic modeling shows that a simplified kinetic model describes the overall reactions, even though the chemistry involved is complex. 5. Conclusions Partial coke removal from a coked ferrierite catalyst taking place under heat treatment in an inert atmosphere was studied by kinetic modeling utilizing the results of temperatureprogrammed gasification experiments. The coke on the catalyst included aromatics and polyaromatics with short alkyl side chains. The evolution of propene, ethene, and other hydrocarbons in the range from C2 to C5 during TPHe between 300 and 500 °C was determined continuously with a flame ionization detector. Quantitative analysis of the products in outflow samples was carried out with a gas chromatograph. Kinetic models derived for the cracking reactions of coke described the results of the TPHe experiments. The results of kinetic modeling suggest that one or more bimolecular reactions are essential in the transformation of coke to light hydrocarbons. The derived models can be applied in the design of a decoking unit for the coked ferrierite catalyst.
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ReceiVed for reView January 11, 2007 ReVised manuscript receiVed June 4, 2007 Accepted June 13, 2007 IE070063R
