Ind. Eng. Chem. Res. 2003, 42, 1543-1550
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Hydrogasification of Coke in Heptane Reforming over Pt-Re/Al2O3 K. Liu,†,‡,§ S. C. Fung,‡ T. C. Ho,‡ and D. S. Rumschitzki*,† Department of Chemical Engineering, City College of City University of New York, New York, New York 10031, and Corporate Strategic Research Laboratory, ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801
We reported previously that coke can indiscriminately deposit on the metal, acid, and alreadycoked sites on a Pt-Re/Al2O3 reforming catalyst. Here we show that the thus-deposited coke, when still in a freshly formed state, can be indiscriminately gasified by hydrogen at temperatures as low as 477 °C. Once the freshly formed coke transforms into a more refractory form, it cannot be gasified by hydrogen. The kinetics of coke hydrogasification is first order with respect to both the concentration of the freshly formed coke and hydrogen partial pressure, with an apparent activation energy of 9.2 kcal/mol. A random coke deposition model offers a theoretical basis for correlating the decoking data. Introduction It is well-known that hydrogen plays an important role in the coking of reforming catalysts. First, hydrogen inhibits the dehydrogenation reactions. This lowers the concentration of the coke precursors, and so less coke forms. Second, hydrogen hinders the transformation from a reversible to an irreversible coke by an “ensemble effect”.1,2 Third, hydrogen plays a role, possibly related to the first, that has received relatively little attention: it can remove some of the deposited coke from the catalyst. It is the last point that we pursue in this study. We dedicate this paper to Dr. John H. Sinfelt for his immense contributions to heterogeneous catalysis. The majority of the prior work on carbon gasification deals with carbon containing no platinum and requires a temperatures above 650 °C, which is much higher than typical reformer temperatures (450-550 °C). Relatively few studies are concerned with Pt-catalyzed coke gasification.3 Querini and Fung4 have investigated gasification of coke on a Pt-Re reforming catalyst under hydrogen or helium using temperature-programmed technique up to 1000 °C and observed complete coke removal. Work on model systems has helped shed some light on the role played by transition metals in coke gasification.5 Qualitatively, carbon gasification in the presence of transition metals consists mainly of the following steps: hydrogen dissociates on the metal site, then migrates to the coke site, and finally reacts with coke.6-10 The rate-controlling step is temperature dependent.6,11-14 At typical reforming temperatures (450-550 °C), the rate-limiting step for catalytic hydrogasification of coke should be either surface migration or the gasification reaction, with the latter being more likely.7 The hydrogasification rate depends strongly on the amount of deposited coke,15-17 which, as shown later, is not consistent with what we observed in this * Corresponding author. E-mail:
[email protected]. cuny.edu. Tel: 212- 650-5430. † City College of City University of New York. ‡ ExxonMobil Research and Engineering Co. § Current address: UTC FC, Fuel Cell Division of United Technologies Corp., 60 Bidwell Road, South Windsor, CT 06074.
work. As for decoking kinetics, Biswas et al.3 did initial coke deposition (1 h) and removal experiments on the metal site of a Pt-Re/Al2O3 catalyst in a conventional thermogravimetric analysis (TGA) microbalance. Their data are restricted to short time coking. Also, the atmospheric pressure limitation precluded a partial pressure dependence study over a wide range of conditions. Moreover, it is known that conventional TGA microbalances are not accurate for small weight changes because of significant buoyancy and drag effects.18 The present work is a quantitative study of partial decoking of reforming catalysts with hydrogen. Specifically, we developed a hydrogasification kinetic model based on long-term (up to 45 h) data obtained from a vibrational microbalance and a temperature-programmed oxidation (TPO) unit. We conclude that the removable coke is simply the freshly formed, low molecular weight coke, irrespective of whether it resides on metal or on support. This is consistent with the coking mechanism and the random coke deposition model proposed previously.19 Experimental Section Coking and Decoking. The in situ hydrogen-decoking studies employ a vibrational microbalance whose development and operation can be found elsewhere.18,20 To generate coked catalyst samples, we use methylcyclopentane (MCP), an established prodigious coke precursor,18 MCP in toluene, and n-heptane to precoke a bimetallic Pt-Re/Al2O3 catalyst. In all of the coking experiments, we load the fresh catalyst into the microbalance cell and heat the catalyst from room temperature to 789 K at 3 K/min overnight with the same hydrogen flow rate (88 cm3/min at ambient conditions) and maintain this temperature for 8 h to reduce the catalyst in situ. A gradual 3-h cooling of the reactor to 643 K precedes the introduction of the reforming feed. A subsequent slow 3-h heating of the reactor brings the reactor to the desired run temperature.18 The total pressure for all of the experiments is 207 kPa, and the weight hourly space velocity (WHSV) is 40-50 g/(g of catalyst)‚h. Coking experiments are carried out at varying partial pressures of hydrogen and hydrocarbon and temperatures to generate samples with various coke levels. Once the coking experiment
10.1021/ie020410r CCC: $25.00 © 2003 American Chemical Society Published on Web 11/27/2002
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is complete, we stop both the hydrocarbon and hydrogen feeds and turn on the helium flow to strip the physically adsorbed hydrocarbon from the catalyst surface. We then adjust the hydrogen and helium flow rates according to the desired hydrogen partial pressure and collect decoking data for about 30 h under flowing hydrogen (or hydrogen + helium) and at the same temperature as that used in coking in most cases. Because of the highly sensitive nature of the vibrational microbalance, when a reactant is added or removed from the carrier gas, the change in the gas density produces a mass change in the void space of the balance. We developed an adequate procedure to account for this.18,20 In this study the coking and decoking curves are presented in one of two formats: either corrected or uncorrected for gas density changes due to feed switching. Because we collect both coking and decoking rate data with the same catalyst loading, we do not need to load or discharge the catalyst or to heat up and cool the system. However, as the data below show, the coking conditions do not seem to strongly affect the measurement of the hydrogendecoking kinetics, which is different from previous observations.15-17 Coke Analyses. At the end of each run, we cool the catalyst bed under hydrogen or helium flow and discharge the catalyst. To ascertain what type of coke has been removed by hydrogen, we analyze the H2-decoked catalyst sample and the nondecoked sample with a highly sensitive TPO technique (modified Altamira temperature-programmed unit, model AMI-1). A Ru catalyst converts the CO2 produced during coke oxidation to methane that a flame ionization detector continuously monitors. TPO analyses use a 1% O2 in He mixture flowing at 60 cm3/min (room temperature) through the sample while increasing the temperature at a rate of 13 K/min from room temperature to 1043 K. Sample weights are about 20 mg. Fung and Querini21 have described the details of the TPO analysis. Materials. The gases used are cylinder hydrogen of electrolytic grade (99.95%), cylinder helium (99.90%), and cylinder nitrogen (99.95%). MCP, toluene, and n-heptane are all analytical pure grade (99 mol %). The reforming catalysts used are 0.3 wt % Pt/Al2O3, 0.6 wt % Pt/Al2O3, and 0.3 wt % Pt-0.3 wt % Re/Al2O3. We neither presulfide the catalyst nor add sulfur to the feed during the run. The catalyst comes in the form of 1/16 in. diameter extrudates. We crushed and sieved the particles and retained the 60-80 mesh (177-250 µm) fraction for the microbalance studies. Results and Discussion Decoking Behavior. With the flow-through vibrational microbalance and the TPO unit, we can pinpoint the type of coke that is removed during hydrogen decoking. The former allows us to determine what impact, if any, intermittent H2-decoking treatments may have on the coking rate and the amount of coke deposited. An early study18 indicated that n-heptane produces a total of ∼4 wt % five-membered-ring naphthenes at the microbalance exit. Accordingly, our microbalance run starts with precoking with a feed containing 4 wt % MCP and 96 wt % toluene at the same conditions (750 K, 207 kPa, 50 WHSV, and a hydrogen/hydrocarbon molar ratio of 3:1), followed by two intermittent decoking treatments. Referring to Figure 1, we account for the effect of gas density change when the feed is introduced at the start of the experiment. That is, the
Figure 1. Coking and hydrogen decoking of Pt-Re/Al2O3 catalyst (750 K, 207 kPa, 50 WHSV; coking of a toluene-MCP feed mixture). Abrupt weight changes occurring at the 10th, 20th, 56th, and 66th h are due to gas density changes.
Figure 2. Data (overlapping open triangles) taken from Figure 1 by removal of the decoking time period. The solid curve is calculated from the random coke deposition model19 at the same conditions.
rapid weight gain shown in Figure 1 at time t ) 0 is solely due to the hydrocarbon adsorption and coking of the most active sites. At t ) 10 h, we stop the hydrocarbon feed and let pure hydrogen flow through the coked catalyst for 10 h at 750 K to decoke the catalyst. After that, we feed hydrocarbon for an additional 36 h, followed by a second 10 h H2-decoking treatment, and finally followed by a third coking period that lasts until the end of the run. Note that the abrupt weight changes (Figure 1) caused by intermittent coking-decoking feed switchings (t > 0) are due to gas density change. Here we want to show the raw data so no density-change corrections are made. It should be instructive to replot the data in Figure 1 by excluding the decoking time period and removing the data points due to gas density changes. The thick solid curve shown in Figure 2 is such a plot, which matches reasonably well the thin solid curve calculated from our random coke deposition model.19 This says that the hydrodecoking process removes the same amount of coke as the coke being generated by the recovered active sites; i.e., the coking process appears to resume as if it had never been decoked. It thus appears that hydrogen decoking has little, if any, influence on the coking rate of the catalyst. Previously, we have shown that the catalyst coking kinetics model is quite robust,19,22 which predicts the rate at which the coking reaction deactivates itself as being exponential in the coke amount, consistent with a Poisson process in probability theory. To gain a further understanding of the H2-decoking process, we perform several microbalance runs by stopping the MCP-toluene feed at different coking
Ind. Eng. Chem. Res., Vol. 42, No. 8, 2003 1545
Figure 3. TPO spectra for a 1-h nC7-coked Pt-Re/Al2O3 catalyst before and after hydrogen decoking at 750 K and 207 kPa; 30-h decoking.
Figure 4. TPO spectra for a 2.5-h nC7-coked Pt-Re/Al2O3 catalyst before (thick solid curve) and after (dotted curve) hydrogen decoking at 750 K and 207 kPa. The thin solid curve is obtained by scaling the dotted curve to the same height as that for the coked catalyst; 30-h decoking.
times. These runs provide us with coked catalyst samples corresponding to 1, 2.5, and 100 h of precoking. Specifically, the experimental procedure is as follows. After stopping the MCP-toluene and hydrogen feeds, one cools the catalyst under helium flow, picks out some of the coked catalyst for TPO analysis, and reloads the rest of the catalyst for hydrogen decoking of up to ∼30 h. One then performs the TPO analysis on the coked catalyst samples with and without hydrogen decoking. Figures 3-5 show the TPO spectra for these catalyst samples (0.3 wt % Pt-0.3 wt % Re/Al2O3). It is generally accepted that the low-temperature peak, 330 °C, corresponds to coke deposited on, or in contact with, the metal sites and the high-temperature peak, 550 °C, corresponds to the coke on the acid sites, i.e., on the alumina support and away from the metal sites.20,21,23-25 As Figures 3-5 show, the TPO profiles of the coked catalyst before and after hydrogen decoking, except for their sizes, show a similar shape. In fact, they become almost indistinguishable from each other when the TPO profile for the H2-decoked catalyst is scaled to the same height as that for the coked catalyst; see Figures 4 and 5. This implies that H2 decoking essentially results in a proportional removal of coke from metal, acid, and already-coked sites. The foregoing assertion is at variance with the result of Figoli et al.,25-27 who reported that hydrogen removes
Figure 5. TPO spectra for a 100-h nC7-coked Pt-Re/Al2O3 catalyst before (thick solid curve) and after (dotted curve) hydrogen decoking at 750 K and 207 kPa. The thin solid curve is obtained by scaling the dotted curve to the same height as that for the coked catalyst; 30-h decoking.
only metal-site coke, which is only a small fraction of the total coke. This, if true, would rejuvenate metal’s protocoke-forming activity, which, upon restarting the coking experiment, would produce more coke than if one had never decoked the catalyst. Put differently, when there is a preferential removal of coke from metal sites without removing a proportional amount of coke on acid sites and already-coked sites, the decoked metal sites, paradoxically, would increase the total coke deposit: coke deposit on metal sites, acid sites, and already-coked sites.19 Our microbalance data, however, indicate that hydrogen decoking has no effect on the total coke content and are entirely consistent with our random coke deposition model.19 Let us explain. Formation of Removable Coke. The model posits the following sequence of events. The first is an equilibrium conversion of five-membered-ring naphthenes (e.g., MCP) to coke precursors (e.g., dehydrogenated MCP) on metal sites. The precursors adsorbed on metal sites quickly equilibrate with the same precursors adsorbed on acid sites, via either readsorption or surface spillover. These equilibrated precursors slowly form protocoke, which quickly and randomly deposits on all surface sites to form soft, mobile coke, whether they are metal, acid, or coked sites. So, the coking rate depends on the density of both metal and acid sites. The formation and disposition rates of the protocoke quickly become comparable, so the protocoke concentration is in a pseudo steady state. We further posit that protocoke is the predecessor of soft coke. Then H2 decoking essentially is a process for gasifying soft coke from sites previously occupied by protocoke. Once these sites are exposed to hydrocarbon again, they collect soft coke at the same rate as that before H2 decoking. As a result, the overall coke buildup and coking rate show virtually no effect of intermittent H2 decoking. It is important to note that here we loosely define H2-gasifiable coke as soft coke. Under reforming conditions, soft coke continuously converts and polymerizes to harder, more refractory coke whose removal can best be achieved through combustion rather than hydrogasification. Once one stops the hydrocarbon flow and starts H2 decoking, it is reasonable to expect that the rate of soft-to-hard coke transformation should become much slower because of the overabundance of hydrogen and the absence of coke precursors.
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Ind. Eng. Chem. Res., Vol. 42, No. 8, 2003 Table 1. Amount of Coke before and after H2 Decoking coke amount of percent of coking before or amount coke removed decoking time after (wt %) (%) figure (h) H2 decoking (wt %) 3
1
4
2.5
5
Figure 6. Coking of 0.3 wt % Pt/Al2O3, 0.6 wt % Pt/Al2O3, and Al2O3 (corrected for gas density change; MCP feed, 207 kPa, 750 K, 50 WHSV).
Figure 7. TPO spectra for the coked catalysts of different metal loadings (MCP feed; 207 kPa, 750 K, 50 WHSV).
It bears emphasizing that, although H2 decoking is site indifferent, it is the small amount of metal sites that produces coke precursors. Figure 6 shows the coking rates of catalysts with different metal loadings with the MCP feed and otherwise reforming conditions. Figure 7 displays the corresponding TPO spectra for the coked catalysts discharged at the end of the microbalance runs. As these figures show, Al2O3 alone produces little coke (∼1.0 wt %) over 60 h, but the addition of a small amount of Pt (0.3 wt %) results in a dramatic increase in the coke produced (∼6.0 wt %) over the same time period. However, doubling the metal loading only marginally increases the coke level. The reasons are that the concentration of metal sites is much smaller than that of acid sites and the coking rate (protocoke formation) depends on both metal and acid sites. The acid sites collect the majority of the coke. Extent of Decoking. Figure 1 indicates that at 477 °C hydrogen removes most of the 10-h coke, but a far smaller fraction of the latter coke as the harder, less removable coke is continuously formed. Moreover, for the same decoking period of 10 h, the absolute amounts of removable coke at coking times of 10 and 46 h are almost the same, about 0.15 wt %. A similar behavior can also be seen from Figures 3-5 for a decoking period of 30 h. The catalyst was coked at different conditions than those used for Figure 1. This is summarized in Table 1, which shows the coke levels before and after decoking for three different precoking times. As can be seen, H2 removes most (76%) of the coke on the 1-hcoked sample but only a small fraction (11%) of the longterm coke on the 100-h-coked sample. Moreover, after 2.5 h of coking time, the absolute amount of soft coke stays approximately constant (0.65 vs 0.61 wt % coke
100
before after before after before after
1.13 0.27 1.98 1.33 5.47 4.85
0.86
76.20
0.65
32.88
0.61
11.24
on catalyst) over a wide range of coking times. This suggests that, after an initial period, the conversion of soft to hard coke proceeds at a rate comparable to that of soft coke formation. The initial period is for the soft coke to establish a pseudo steady state. At the first hour of coking, there are more uncoked sites and hence a quicker buildup of soft coke (0.86 wt %). Beyond this period, the total amount of soft coke decays slowly with coking time (more on this later). We should point out that the foregoing results, at first glance, appear to contradict Querini and Fung’s findings.4 Their temperature-programmed hydrogen or helium (TPH or TPHe) decoking shows complete removal of all coke from a coked Pt-Re reforming catalyst at temperatures up to 1000 °C. They concluded that coke removal at such a high temperature results from the reaction between the hydroxyl groups on the alumina support and the coke deposit. The maximum removable amount of coke is limited by the number of surface hydroxyl groups, and this limit is 5-6 wt % for high surface area alumina. Hydrogen actually inhibits this reaction at high temperatures because CO and H2 are the products of the decoking reaction. At temperatures below 500 °C, the amount of coke removed via TPH is very small ( r4 . r1, implying that, essentially, protocoke and soft coke are kinetically indistinguishable. r1
r2
r4
coke precursor 98 protocoke 98 soft coke 98 CH4 (2) The key message of the foregoing argument is that the coking-decoking data suggest that for practical purposes the behavior of the soft coke may be qualitatively inferred from that of the protocoke. It is therefore helpful to briefly discuss the behavior of the protocoke based on the treatment of Liu et al.19 Let Ck be the concentration of deposited coke and C/k be that of protocoke. Suppose that the rates of formation and consumption of C/k are r and rd(S0,C/k), respectively. The balances for Ck and C/k can be written as
Figure 8. Hydrogen-decoking data obtained at 788 K and 207 kPa (corrected for gas density change). The solid curve is the model prediction.
dC/k dCk / ) rd(S0,Ck); ) r(PH,PMCP,Ck) - rd(S0,C/k) dt dt (3) where
rd(S0,C/k) ) kdS0C/k (4)
r ) r0(PH,PMCP) φ(Ck)
(
r0(PH,PMCP) ) κ1 -
)
κ2 PMCP ; φ(Ck) ) e-RCk (5) PH PH
where the deactivation rate constants κ1 and κ2 are functions of the initial acid and metal site densities, adsorption equilibrium constants, and polymerization initiation rate constants.19 Note also that φ(Ck) is the fraction of surface sites that are active, which the random deposition model predicts to be φ(Ck) ) exp(-RCk), with R a function only of the catalyst preparation and metal loading. Assuming that protocoke is a reactive intermediate allows one to set dC/k/dt ≈ 0 after a short coking time. This approximation, coupled with the observation that PH and PMCP are essentially time independent in microbalance experiments, leads to an expression for Ck as a function of coking time.
Ck )
∫0tr0(t′) dt′] ≈ R1 ln(1 + Rr0t)
1 ln[1 + R R
(6)
Equation 6 indicates that the coke buildup is initially linear and then logarithmic in t. The fraction φ of uncoked sites decays slowly with time. Thus, some uncoked sites remain even at very long coking times. It also follows that
C/k )
r0
∫0 r0(t′) dt′]
kdS0[1 + R
t
≈
r0 kdS0(1 + Rr0t)
(7)
Because of the quasi-steady-state assumption, eq 7 is valid only after a short coking time. The amount of protocoke decreases slowly for long coking times with a time constant of 1/Rr0. Because the total coke level increases logarithmically with time, the fraction of the removable coke decreases with time, consistent with the H2-decoking experiments. Hydrodecoking Kinetics. To develop a hydrodecoking kinetics model, we measure the decoking rates in the vibrational microbalance at different hydrogen
Figure 9. Hydrogen-decoking data obtained at 773 K and 207 kPa (corrected for gas density change). The solid curve is the model prediction. Table 2. Precoking and Decoking Conditions, 477 °C PH PMCP (kPa) (kPa) WHSV 155 155 103 155
103 20.6 4.5 52
50 40 40 50
coking coke coke time deposited decoking decoking removed (h) (wt %) PH (kPa) time (h) (wt %) 58 160 35 62
6.61 3.82 3.63 5.95
207 103 158 207
46 31 28 27
0.56 0.20 0.26 0.39
partial pressures and temperatures. Figures 8 and 9 show two typical coke removal curves for up to ∼20 h. We have data for a large number of experiments, such as those in Figures 8 and 9, but under different conditions (not shown). All of these runs precoked the catalyst by feeding MCP-toluene at 477 °C for the various hydrogen and MCP partial pressures, WHSVs, and times listed in Table 2. After the MCP and hydrogen feeds are stopped, helium introduction strips all of the physically adsorbed hydrocarbons. Following this, the helium flow stops and a hydrogen flow at a rate of 75 cm3/min (ambient conditions) begins for 15-45 h. The total amount of coke removed as a weight percent of catalyst appears in Table 2. Even after 17 h (Figure 8), the decoking curve has not yet leveled off, but the hydrodecoking rate does decrease as the level of coke on the catalyst decreases. The amount of the hydrogen-removable coke is only 0.2-0.6 wt %, compared with a total of 3.6-6.6 wt % coke on the catalyst. This indicates that hydrodecoking is a relatively slow process and only a small fraction (∼5-8.5%) of the total coke is removed by hydrogen within a reasonable time range.
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Table 3. Hydrogasification Rate Constants Measured at Different Hydrogen Partial Pressures at 477 °C coking H2/MCP
coking time (h)
Ck0 [g/(g of catalyst)]
decoking PH (kPa)
1/khPH
kh × 103 (1/h‚kPa)
C∞ [g/(g of catalyst)]
(%)
1.5 3.0 3.33 3.33 7.5
58 60 55 24 160
0.0661 0.0592 0.0426 0.0363 0.0382
207 207 189 158 103
12.45 12.76 13.27 16.69 24.46
0.389 0.379 0.398 0.380 0.395
0.0605 0.0553 0.0399 0.0343 0.0362
8.46 7.08 6.36 5.59 5.18
Table 3 shows that the amount of coke removed correlates with the decoking hydrogen partial pressure and the H2/MCP ratio used in the coking experiment: the higher the decoking hydrogen partial pressure, 189 kPa vs 158 kPa, the more coke it removes during decoking for a fixed time, 6.36% vs 5.59% of total coke, and the coking time is less important when it exceeds 20 h. Also the higher the H2/MCP ratio, the lower r0(PH,PMCP) and hence the lower the amount of the removable coke due to a low C/k. Finally, methane is the only detectable off-gas product in the microbalance runs because the overabundance of hydrogen shifts the reactions toward cracking. Accordingly, the process has the following overall stoichiometry for the conversion of soft coke (CHx) to CH4:
(
CHx + 2 -
kh x H2 98 CH4 2
)
(8)
Figure 10. Hydrogen-decoking rate constants measured in different tests at 477 °C.
where typically 0.5 < x < 1 and the total coke Ck ) Csoft + Chard. Within the time frame of the decoking process, the conversion of soft to hard coke is negligibly slow, so Chard is essentially time independent and the final coke after H2 decoking for an infinite amount of time, or simply Chard ) C∞. To minimize the number of adjustable parameters, we assume that the kinetics is first order with respect to the driving force Ck - C∞ and PH
d(Ck - C∞) ) -khPH(Ck - C∞) dt
(9)
where t is the decoking time and kh the rate constant. The initial condition at t ) t0 is Ck ) Ck0, where Ck0 is a function of the precoking conditions (temperature, coking time, partial pressures of hydrocarbon and hydrogen, etc.). The decoking environment in the vibrational microbalance is such that PH ≈ constant throughout the experiment. Integration of eq 8 yields
Ck(t) ) C∞ + (Ck0 - C∞) exp[-khPH(t - t0)] (10) Equation 10 is a two-parameter (kh and C∞) model, with Ck f C∞ as t f ∞. It is convenient to define ≡ 100(Ck0 - C∞)/Ck0 as the percent of coke removal. We shall test eq 10 against the data collected at different conditions such as those shown in Figures 8 and 9 and in Table 3 by regressing Ck vs t data from time t0 onward. If eq 9 holds, not only should the curves fit the data but also kh should be independent of PH; i.e., it should only be a function of temperature. As Figures 8 and 9 show, the model (solid lines) predicts the experimental data. Table 3 summarizes the best values of kh and C∞ for five different runs and the corresponding values. The fit is very good, with χ2 on the order of 10-9-10-10 (e.g., at 103 kPa, 1/[khPH] ) 24.46 ( 0.94, C∞ ) 0.0362 ( 5.02 × 10-5). One sees that kh is nearly constant over a wide range of hydrogen partial pressures. To provide the best kh value for all runs, we plot the product khPH from each data fit versus the PH of that
Figure 11. Arrhenius plot of kh at 207 kPa. Table 4. Rate Constant Measured at Different Temperatures and Ck0 (Coking and Decoking Are at the Same Temperature) temp (°C)
Ck0 (wt % coke)
1/khPH
kh × 103 (1/h‚kPa)
(%)
477 500 515 520
5.92 7.33 1.38 10.33
12.76 10.97 9.47 9.05
0.383 0.441 0.511 0.534
6.6 6.0 29.0 2.9
fit (Figure 10). The intercept is very nearly zero, and the best-fit value for kh is 0.383 × 10-3 h-1 kPa-1 at 477 °C. Moreover, kh is independent of the initial coke level Ck0. The activation energy of the hydrodecoking reaction is obtained by conducting experiments at 477, 500, 515, and 520 °C while keeping the hydrogen partial pressure fixed at 207 kPa. The data obtained at different decoking temperatures such as those in Figures 8 and 9 yield the kh and C∞ (through ) values shown in Table 4. Figure 11 shows the corresponding Arrhenius plot, which yields an overall apparent activation energy of 9.2 kcal/mol. This low value, suggestive of a diffusionlimited process, rules out the direct, noncatalytic reaction of gas-phase hydrogen with the soft coke to generate
Ind. Eng. Chem. Res., Vol. 42, No. 8, 2003 1549
Figure 12. Percent of coke removal ≡ 100(Ck0 - C∞)/Ck0 vs initial coke level at PH ) 207 kPa. The solid curve is the model prediction based on the random coke deposition model (eq 13).19
CH4. Because of alumina’s relatively large pore size (around 50 Å) and the low coke level (about 1-7 wt %) on the catalyst, neither pore plugging nor pore diffusion can be important. To rationalize the low activation energy, two possible mechanisms come to mind. Once hydrogen is dissociatively adsorbed on the metal sites, the hydrogen atoms may migrate to acid and already-coked sites for the gasification to occur. Alternatively, the soft coke on the acid and coked sites may migrate to metal sites to be gasified by hydrogen catalyzed by the metal. Migration of atomic hydrogen should be faster than that of soft coke and should not be limited by diffusion. However, its generation may be limited by too low a surface coverage on clean metal sites at this high temperature. On this basis, migration of the soft coke to metal sites appears more likely to be the rate-controlling step. This is consistent with the low hydrogasification rate and low apparent activation energy. Quantitative Protocoke-Soft Coke Relationship. From Tables 2 and 4, we obtain Figure 12, which indicates that decreases precipitously with increasing Ck0. Note that the data in Table 3 show the effect of hydrogen partial pressure and, hence, are excluded in Figure 12. As was alluded to earlier, our theory posits that the removable (or soft) coke in hydrogasification is related to protocoke in coking. This begs the question, to what extent can the random coke deposition model19 predict the data shown in Figure 12? It is this question which we address here. For a given coking time t ) τ, and Ck0 may be roughly estimated from eqs 6 and 7 as follows:
C/k Rr0 ∼ 100 ) Ck kdS0(1 + Rr0τ) ln(1 + Rr0τ)
(11)
Ck0 ∼ ln(1 + Rr0τ)/R
(12)
Let A ≡ r0/kdS0 and Y ≡ ln(1 + Rr0τ)/R; then
∼
A Y exp(RY)
(13)
which serves as a quantitative link between coking (Y) and decoking () and indicates that is an exceedingly strong decreasing function of Y. Because R ) 0.568 wt %,19 we use eq 13 as a one-parameter model for
correlating vs Ck0 data. The best fit, with A ) 143.2, is shown as the solid curve in Figure 12. The agreement is not bad given that the theory is at best asymptotic and that A is not constant because of the diverse conditions (r0 and kd vary) used in the experiments. The important point here is that the random coke deposition theory, developed solely based on coking data, can at least provide a theoretical basis for correlating the overall decoking data. A much better fit (not shown) can be obtained if eq 13 is used as a two-parameter model for estimating in practical situations. Figure 12 shows that decoking with hydrogen is rather inefficient. For one thing, to achieve a high degree of coke removal requires very frequent regeneration with hydrogen. Moreover, even with frequent regeneration, the removal efficiency decreases because of the conversion of soft to hard coke. Oxidation vs Hydrogasification. We now attempt to contrast the two coke removal processes, hydrogasification vs oxidation. Our proposition is that during hydrogasification, as coke freshly deposited on metal sites is hydrogasified, coke freshly deposited on the acid sites and already-coked sites can migrate to the metal sites, resulting in the removal of all soft coke. Why then does not the TPO profile of the coked catalyst show more coke being burned at the metal location? One may suggest that the migration of the soft coke from the acid sites to the metal sites is a slow process, as indicated by the 10-h decoking time. Because the TPO experiment spends only a short duration (
