Regeneration of Fixed-Bed Catalytic Reactors Deactivated by Coke

Ind. Eng. Chem. Res. 1996, 35, 1813-1823

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Regeneration of Fixed-Bed Catalytic Reactors Deactivated by Coke: Influence of Operating Conditions and of Different Pretreatments of the Coke Deposits C. Royo, J. M. Perdices, A. Monzo´ n, and J. Santamarıá* Department of Chemical and Environmental Engineering, University of Zaragoza, 50 009 Zaragoza, Spain

The regeneration of coked fixed-bed catalytic reactors has been studied experimentally. Different types of coke distribution have been artificially created in order to study their influence upon the temperatures reached during regeneration. The effect of the oxygen concentration in the reactor feed and the total gas feedrate have also been studied. In addition, a study has been carried out of different coke pretreatments with the aim of eliminating part of the hydrogenrich coke fractions prior to regeneration, thereby reducing temperature increases during the subsequent regeneration process. A new parameter, the Thermal Aging Index, has been proposed, as a means to quantify the impact that different regeneration procedures may have on a catalyst. Introduction Coke deposition is a common cause of catalyst deactivation in the chemical and petrochemical industries. Coke deposition decreases the number of active sites available for the reaction by direct (site coverage) and/ or indirect (pore blockage) processes. Unlike other causes of catalyst deactivation such as irreversible poisoning or sintering, the deactivation caused by catalyst coking can usually be reversed using relatively mild treatments. These involve the removal of the coke deposits by gasification agents such as air, hydrogen, steam, or carbon monoxide, alone or in combination. Of these methods, oxidative regeneration is the most commonly used, involving the combustion of the coke deposits with oxygen-containing streams. The main concern during oxidative regeneration is the possibility of large temperature rises caused by the exothermic combustion of the coke deposits (around 108 kcal/mol of coke if a composition of CH0.5 is taken as representative and combustion to CO2 and H2O is assumed). When the coke-forming reaction is carried out in a fixed-bed reactor, an in situ regeneration gives rise to a high-temperature front that moves along the bed as the coke deposits are burnt off (e.g., Byrne et al., 1986; Brito et al., 1993; Royo et al., 1994). The maximum temperature within this high-temperature front must be limited in order to avoid catalyst sintering, damage to the reaction equipment, and unsafe operation conditions. This can be achieved by means of a careful control of the amount of oxygen fed to the reactor. When carried out industrially, the regeneration of fixed-bed reactors starts with a low oxygen concentration that is progressively stepped up (Fulton, 1988). Often, auctioneering control is used, in which the temperature is continuously monitored at a number of measuring points in the bed, and the maximum temperature reading is used to control the amount of oxygen in the reactor feed. It must also be taken into account that hydrogen-rich fractions of coke deposits significantly contribute to the heat evolution during catalyst regeneration and, in particular, are presumed responsible for the rapid temperature rise that is often observed at the onset of * Author to whom correspondence is addressed. Fax: (+34) 76 761159. E-mail: [email protected].

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the regeneration process (e.g., Massoth and Menon, 1969). This opens up another possibility to develop suitable regeneration strategies, based on the partial elimination of the hydrogen-rich fractions of the coke prior to regeneration, as a means to reduce the maximum temperatures reached. This partial elimination could be obtained by aging the coke deposits under an inert atmosphere, as shown, for instance, by Royo et al. (1994) and Ibarra et al. (1995), but it can probably be achieved more efficiently by pretreatment of the coke deposits with gasifying agents such as steam, carbon dioxide, or oxygen. The necessity of a sufficiently fast regeneration process while at the same time avoiding excessive temperature rises has prompted a number of modeling works aimed at the simulation of the regeneration process (e.g., Byrne et al., 1985a, 1989; Acharya et al., 1990, 1992) and at the optimization of the operationregeneration cycles (Dumez and Froment, 1976; Borio et al., 1992; Borio and Schbib, 1995). In contrast, there are relatively few experimental studies on catalyst regeneration. There are two main difficulties in the experimental studies of catalyst regeneration that explain the scarcity of research works in this area. The first is related to the fact that coke is an ill-defined species, whose characteristics change with time on stream (Pieck et al., 1989, 1992; Royo et al., 1994). The aging of the coke deposits leads to lower hydrogen contents and reactivities (Royo et al., 1994). However, the high rate of evolution often present on freshly deposited coke (Gayubo et al., 1993) quickly decreases, which means that the coke deposits can be stabilized in a relatively short aging period. This has led to regeneration studies on evolved coke deposits, also termed “hard coke”. The second difficulty in regeneration studies stems from the nonuniformity of coke deposition along the reactor. Since the amount of heat released during regeneration is proportional to the amount of coke present at a given position, the pattern of coke deposition has a direct influence on the temperatures reached locally in the regeneration process. Therefore, information on the amount of coke deposited at different positions in the reactor is needed before an experimental investigation of catalyst regeneration can be carried out. The coke profiles in the bed can be predicted if the © 1996 American Chemical Society

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Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996

mechanism of coke deposition is known. Thus, it has been recognized for a long time (Froment and Bischoff, 1961) that a parallel-type of coke deposition would give rise to decreasing coke concentrations along the bed, while the opposite would be true for a series-type of coking. In practice, however, detailed kinetics of coke deposition are usually not available. Also, the reaction network responsible for coke formation often includes several species, reactants and/or products, as coke precursors, which strongly deviates from the simple scheme depicted above, in which only reactants or products are responsible for coke deposition. The theoretical prediction of coke profiles is further complicated by the existence of temperature nonuniformities in the bed at the start and during the development of the coking process, which can be very significant (Borio and Schbib, 1995). The pattern of coke deposition could be determined experimentally by sampling at different reactor positions prior to regeneration. However, this disturbs the bed, it is a time-consuming technique, and the results are often questionable in terms of reproducibility. A different approach was followed by Byrne et al. (1985b), who used neutron attenuation techniques to determine coke concentrations at different bed positions. While this procedure is very useful as a noninvasive technique, a systematic study of catalyst regeneration requires not only the knowledge of the coke deposition patterns but also the capability of accurately reproducing a given coke profile in order to study the influence of different variables upon regeneration. In this work, the influence of different experimental variables upon the temperatures reached during regeneration has been studied. To this end, uniform and nonuniform coke profiles have been produced by loading a fixed-bed reactor with coked catalyst particles of known coke content. In this way, the regeneration process can be studied using a known coke distribution that can be reproduced accurately. Also, different pretreatments of the coke deposits have been tested, as a means of eliminating the most reactive, hydrogen-rich coke fractions before the bulk of the regeneration process takes place. Experimental Section The coking experiments were carried out in a 5 cm i.d., 79 cm long, stainless steel fixed-bed reactor, comprising preheating and cooling sections packed with ceramic rings, and a 29 cm long catalyst bed. The reactor was heated using a three-stage electrical furnace. Butene dehydrogenation was used as a cokeforming reaction, passing a mass flow-controlled mixture containing 25% butene in nitrogen over a Cr2O3/ Al2O3 catalyst, at temperatures ranging from 470 to 600 °C. This produced a nonuniform coke deposition along the bed. Preliminary experiments showed that, in spite of the three-stage electrical furnace used to attain a more uniform temperature profile, the reproducibility of the coke distribution along the bed could not be guaranteed. Therefore, as mentioned above, it was decided to create known coke distributions in the bed, using previously coked catalyst particles. It is, however, recognized that the nature of the coke deposits depends not only on the coking molecule but also on the time and temperature of the coking process (Royo et al., 1994). Therefore, the regeneration of a reactor made up with catalyst particles coked under different conditions could be different from that of a reactor with the

same coke distribution but coked in situ. However, the system employed in this work guarantees an accurate reproducibility of the initial coke profiles, which is extremely important in regeneration studies. In addition, using this procedure different types of coke deposition (i.e., uniform, increasing, or decreasing along the bed) can be created, for any given total amount of coke in the reactor, which enables the study of the influence of different types of coke distribution at the same total level of coke deposition. To this end, different batches (450 g) of Cr2O3/Al2O3 catalyst (4 mm pellets) were coked in the abovedescribed reactor, termed reactor 1. After coking was completed, the reactor was carefully unloaded by means of a vacuum line. Using this procedure, the bed was partitioned into several sections, each with a relatively uniform coke content. The different sections were homogenized and stored separately, and their coke content was determined using a thermobalance (C. I. Electronics). The scatter in duplicated analysis of the coke in the different sections was lower than 5% of the average value. The above procedure was repeated using different coking times and temperatures to obtain different coke loadings, until enough stock of catalyst with different coke contents was built up. This allowed the creation of different coke distributions in the bed to be used in the subsequent regeneration experiments. Most of the regeneration experiments were carried out in the above-described fixed-bed reactor. In order to measure the temperatures attained during regeneration, seven thermocouples (one in the preheating section, and six in the catalytic bed) were located axially, at 5 cm intervals. The thermocouples were connected to a computer-controlled data logging system, which read and stored the temperatures every 2 s. The reactor was brought up to the reaction temperature under a nitrogen stream until a relatively uniform temperature was achieved along the bed, by adjusting the current intensities delivered to the three zones of the electrical furnace. The reactor feed was then switched to a mass flow-controlled nitrogen/oxygen mixture, and the regeneration started. During regeneration, the same heat input to each of the three furnace zones was maintained, with the aim of simulating operation near adiabatic conditions. The axial movement of the high-temperature front produced in the combustion of the coke deposits was followed using the thermocouple readings at the six positions within the bed. In addition, continuous analysis of oxygen, CO, and CO2 in the product gases was carried out in some of the experiments to establish more precisely the end of the regeneration process. Some coking and regeneration experiments were carried out using a different experimental system. In this case, a 38 cm long, 3 cm i.d., stainless steel fixedbed reactor containing the uncoked catalyst pellets (reactor 2) was immersed into a sand fluidized-bed reactor at the reaction temperature. This ensured an isothermal temperature along the external reactor wall and gave a very good reproducibility of the coke distribution along the bed. The catalyst loading in this reactor was 240 g, corresponding to approximately 30 cm of the bed length, preceded by a 8 cm long gas preheating section. As in the previous case, six thermocouples were placed axially inside the fixed bed and connected to a data logging system. It must be noted, however, that in this case the regeneration was carried out with heat loss to an isothermal coolant. The

Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996 1815 Table 1. Conditions Used in the Different Pretreatments of the Coke Deposits pretreatment

pretreatment agent

temperature (°C)

total flowrate (L/min (STP))

concentration (%)

duration (min)

1 2 3 4 5 6 7 8 9

none steam steam steam oxygen oxygen oxygen oxygen carbon dioxide

650 650 700 460 460 460 460 700

2.33 2.33 2.33 5 10 10 10 3

36.5 36.5 36.5 1 1 0.5 0.5 100

30 60 60 30 30 30 10 30

fluidized-bed temperatures were set at 590 °C during coking and at 470 °C during regeneration. Pretreatment of the Coke Deposits. In order to establish the influence of different coke pretreatments upon the temperatures reached during regeneration, a set of pretreatment/regeneration experiments was carried out using reactor 1, in which a 30 cm long catalyst bed was placed, containing approximately 148 g of coked catalyst. The same initial coke distribution was used in all the experiments: a uniform coke distribution with a 3.2% coke content, created using the above-described procedures. The reactor was preheated to the desired temperature under a nitrogen flow of 1.5 L/min (STP), using the three independent heating sections to achieve an approximately uniform temperature at the start of the pretreatment and regeneration processes. Once a stable temperature was achieved, the coke deposits were pretreated with different gasifying agents (Table 1): Steam: A dosifyng pump supplied 0.67 g/min of deionized water to an electrically heated pipe in which it was evaporated and the resulting steam heated to about 300 °C, after which it entered the reactor at a higher temperature. Preliminary experiments showed that, under the conditions investigated, the minimum temperature to have a significant gasifying effect of the steam on the hard coke deposits was 650 °C. Carbon dioxide: Pure carbon dioxide was used as a pretreating agent, at 700 °C, since lower temperatures did not have any significant gasifying effect. Oxygen: It was decided to use relatively large flowrates and low oxygen concentrations, in order to have wide pretreatment zones and low temperature increases. After the different pretreatments, the reactor temperature was stabilized at ca. 460 °C, and then a gas stream containing 6.3% oxygen, with a total flowrate of 2.9 L/min (STP), was passed through the reactor in order to achieve complete catalyst regeneration. Results and Discussion Influence of the Type of Coke Distribution in the Reactor. Figure 1 shows two examples of the coke distributions employed in this work. Figure 1A shows a decreasing pattern, comprising four different sections with coke concentrations ranging from 6.2 to 1.5% by weight. The positions of the six thermocouples placed inside the catalytic bed (T2 to T7) are also indicated. Figure 1B is symmetrical respect to Figure 1A, with an increasing coke profile. The same total amount of coke was present in both cases, and thus the differences observed in the temperature evolution during regeneration can be ascribed to the different pattern of coke distribution. The reactors with the coke loadings corresponding to parts A and B of Figure 1 were regenerated using a total flowrate of 2.9 L/min (STP), containing 6.3% oxygen in

Figure 1. Two examples of artificially created coke distributions with the same total coke content. The positions of the thermocouples are also indicated: (A) decreasing coke distribution; (B) increasing coke distribution.

nitrogen. The temperature evolution during regeneration at each of the six axial positions in the bed is shown in parts A and B of Figure 2, respectively. In both cases the displacement of the high-temperature front corresponding to the moving combustion zone can be followed neatly. The velocity of the high-temperature front increases or decreases, depending on the amount of coke that it finds along the bed, which is a function of the coke deposition profile. Thus, for instance, Figure 2A (decreasing coke distribution) shows that the center of the moving combustion zone is located 7 cm inside the catalyst bed (thermocouple T3), after approximately 86 min of regeneration time, while when an increasing coke distribution is used (Figure 2B), the movement of the combustion zone is faster initially, reaching 7 cm inside the bed before 30 min of regeneration time. This behavior is caused by the different amounts of coke present in the entrance region of the bed. With Cr2O3/Al2O3 catalysts at the temperatures employed, the combustion zone moves along the bed, leaving behind completely regenerated catalyst particles, as could be assessed in partial regeneration experiments (Acharya et al., 1987). As the coke content is depleted in a given region of the bed, the combustion front moves further into the reactor (Figure 2). At a given time, the hightemperature combustion zone is localized in a relatively

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Figure 2. Evolution of the temperatures at the different thermocouple positions during regeneration. Total flowrate, 2.9 L/min (STP), 6.3% O2. T2-T7 denote the curves corresponding to each of the six thermocouples inside the catalyst bed. Parts A and B correspond to the regeneration of coked catalyst beds with the coke distributions given in Parts A and B of Figure 1, respectively.

narrow region of the bed, in which practically all the oxygen is consumed. This can be seen in Figure 2A, where, when thermocouple T2 reaches its maximum temperature (i.e., the center of the high-temperature zone is around the position of T2), thermocouples T3 and T4 (5 and 10 cm from T2, respectively) show only very small temperature increases, while the remaining thermocouples are approximately at their initial temperatures. The same behavior, with initially narrower temperature peaks can be observed in Figure 2B. The width of the temperature-time peaks is related to the velocity of the combustion zone, which, for a fixed set of inlet conditions, is a function of the amount of coke present in a given region of the bed. Thus, in Figure 2A (decreasing coke profile), the peaks are wider at the beginning of the bed where the coke concentration is high (slowly moving combustion zone) and narrower toward the end (rapidly moving combustion zone), while the opposite behavior can be observed in Figure 2B. It is important to note that the width of the temperature peaks at a given reactor position affects the subsequent deactivation by sintering. In this respect, not only are the temperatures reached important but also the time spent at high temperatures. Thus, wide peaks would give rise to a greater extent of sintering than narrow peaks of the same maximum temperature. Acharya and co-workers (1987) carried out partial regeneration experiments in which the regeneration was interrupted before completion. From their neutron scattering measurements, these authors found that the coke content of the catalyst downstream of the moving combustion zone was lower than the initial value, and elementary analysis of these deposits also showed a lower hydrogen content. To explain these results,

Acharya et al. (1987) postulated a dual combustion mechanism: A high-temperature, fast combustion of the coke deposits within the moving combustion zone, where most of the oxygen would be consumed, and a lowtemperature, slow combustion of the coke deposits downstream from the moving front, caused by small amounts of oxygen escaping from the high-temperature zone. This low-temperature combustion would preferentially affect the hydrogen-rich fractions of the coke. The hot exit gases from the high-temperature combustion zone heat up the downstream regions of the reactor, and as a result, when these regions are reached by the moving combustion front, the main combustion of the coke deposits starts at a higher temperature. Therefore, in an adiabatic fixed-bed reactor with a uniform coke distribution, the temperatures reached by each of the thermocouples would be expected to increase monotonically along the bed. Obviously, a coke distribution of the increasing type should enhance this effect. However, this does not take place in the results shown in Figure 2B. While the maximum temperature reached by thermocouple T3 is 57 °C higher than that of T2 and the temperature reached by T4 is 29 °C higher than that of T3, thermocouple T5 shows a maximum temperature which is 3 °C lower than T4. This result was typical of many experiments carried out, in which the increasing temperature pattern was always discontinued at an intermediate position. To explain the above behavior, two factors must be taken into account. First, the reactor is not truly adiabatic: In the system used, net heat losses are unavoidable when the temperature increases above the initial value. Second, the coke characteristics change with reaction time, as shown by the above-discussed findings of Acharya et al. (1987) and also by the recent results of Royo et al. (1994), in which it was shown that the aging of the coke deposits on Cr2O3/Al2O3 catalysts gives rise to a very significant decrease in their hydrogen content. Coke deposits with higher hydrogen contents release a higher amount of heat when burnt. Thus, a coke deposit with an empirical composition of CH1.5 burnt to CO2 and H2O would release around 27% more heat than a coke deposit with a composition of CH0.5. Whether through the low-temperature combustion mechanism or through the aging of the coke deposits or both, when the high-temperature combustion zone reaches regions located further inside the bed, it will encounter coke deposits with a lower hydrogen content and, hence, with a lower heat of combustion. In Figure 2B, the high-temperature front rapidly leaves behind thermocouple T2; thermocouple T3, away from the cold inlet gases, reaches a higher maximum temperature. Thermocouple T4 is in a region with a higher coke concentration, and this is sufficient to offset the lower hydrogen content of the coke deposits after approximately 1 h on stream, giving an increase in the maximum temperature. However, by the time thermocouple T5 reaches its maximum temperature, the regeneration time approaches 2 h, and the depletion of the coke deposits and of their hydrogen content is enough to cause the experimentally observed decrease in the maximum temperature reached. This probably represents the largest effect in terms of hydrogen depletion, since by this time the combination of the aging and low-temperature combustion processes would have eliminated most of the hydrogen-rich fractions of the coke. Therefore, from this point onward the pattern

Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996 1817

of increasing maximum temperatures restarts, as shown in Figure 2B. When a decreasing coke distribution is used (Figure 2A), both the hydrogen content of the coke and the amount of coke encountered by the moving combustion front decrease along the bed, which causes a pattern of decreasing temperatures. It is worth noting that the evolution of the temperature in the reactor cannot be the only parameter used to determine the end of the regeneration process. Thus, in the experiment presented in Figure 2B, an on-line measurement of the amount of oxygen present in the reactor exit stream was also carried out. At a reaction time of 268 min, when thermocouple T7 reached its maximum temperature, the exit oxygen concentration was 0.2%. At this point, the temperature of thermocouple T7 started decreasing, and the exit oxygen concentration slowly increased. The oxygen concentration regained its inlet value (6.3%) only 20 min after the temperature reading of thermocouple T7 returned to its initial value. This explains the apparent inconsistency shown by the regeneration times in parts A and B of Figure 2: In Figure 2A (decreasing coke distribution), the maximum of the high-temperature front (i.e., approximately the center of the high-temperature reaction zone) reaches thermocouple T7 at about 287 min. When an increasing coke distribution was used (Figure 2B), the same position is reached much earlier (268 min), giving an apparently faster regeneration. However, from the above results it is clear that, by the time the high-temperature front reaches the last thermocouple, the regeneration is not complete and, given the different amounts of coke in the last section of the reactor (6.2% vs 1.5%), the time required to complete regeneration in Figure 2B is considerably higher. In fact, the total regeneration times were very similar in both cases, since during most of the regeneration process the oxygen feed is totally consumed within the catalyst bed. Influence of the Oxygen Concentration in the Reactor Feed. The intrinsic regeneration kinetics for this catalyst (e.g., Royo et al., 1991), are approximately first order in oxygen. Thus, the reaction rate directly increases with increasing oxygen concentration. With a Cr2O3/Al2O3 catalyst, at the regeneration temperatures used in this work, the regeneration process is diffusion controlled rather than kinetically controlled, but, even in this case, the reaction rate is proportional to the oxygen concentration. Therefore, an increase in the oxygen concentration directly increases the reaction rate and hence the rate of heat release and of temperature increase at a given reactor position, thus justifying the industrial practice of using the oxygen concentration in the feed as a temperature-limiting parameter. A compromise must, therefore, be achieved between the need to have a fast regeneration and the requirement of sufficiently low maximum temperatures. Parts A and B of Figure 3 show two regeneration experiments carried out in reactor 2 (i.e., heat loss to an external fluidized bed), using two different oxygen concentrations. In both cases the catalyst was coked in situ, with the same coke distribution in the bed (corresponding to an average 3.3% coke concentration, with a smoothly increasing coke profile), and regenerated without unloading the reactor. This allowed one to carry out experiments in which the coke deposits were aged for only 25 min before regeneration, which gave younger, less evolved coke deposits with a higher hydrogen content, compared to the experiments carried

Figure 3. Evolution of the temperatures at the different thermocouple positions during regeneration. Total flowrate, 4.3 L/min (STP), reactor 2, “in situ” coking. (A) 4.2% oxygen concentration; (B) 6.3% oxygen concentration.

out in reactor 1, in which relatively long cooling and preheating periods were necessary. As could be expected, a higher oxygen concentration gives rise to a faster regeneration (thermocouple T7 reaches its maximum temperature at 87 min when a feed with 6.3% oxygen is used, compared to 122 min for a 4.2% feed) and also to considerably higher temperatures. The temperature peaks obtained at the higher oxygen concentration were sharper, and the maximum temperatures exceeded 660 °C for most of the reactor, in spite of the good heat-transfer characteristics of the external fluidized bed, which was kept at 450 °C throughout the experiment. The main difference in the experiments carried out in reactor 2 concerns the relative magnitude of the maximum temperatures reached by the different thermocouples. Thus, in both experiments represented in parts A and B of Figure 3, the first three thermocouples show a pattern of decreasing maximum temperatures, which changes to an approximately constant plateau in the remainder of the reactor. In particular, the first thermocouple showed a much higher maximum temperature than the rest. This is due to the lower age of the coke deposits in the experiments carried out in reactor 2. A less evolved coke has a higher hydrogen content and a higher reactivity (Haldemann and Botty, 1959, Takahashi and Watanabe, 1978). The rapid reaction of the hydrogen-rich coke causes a fast temperature increase (Massoth and Menon, 1969), that in our case exceeds 150 °C/min (Figure 3B). To this temperature increase contribute not only the hydrogenrich coke deposited directly on the catalyst but also an undetermined amount of hydrocarbons (often termed green oils), that can also form on the reactor walls (Royo et al., 1994) from pyrolysis, polymerization, and condensation processes. As the high-temperature reaction

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Figure 5. Temperature evolution at the different thermocouple positions during regeneration. Total flowrate, 2.9 L/min (STP), 6.3% O2, reactor 2, “in situ” coking.

Figure 4. Regeneration of coked reactors with a stepup in the oxygen concentration of the reactor feed. Reactor 1, increasing pattern of coke deposition (Figure 1B). (A) 3% oxygen concentration in the feed for 75 min, 6.3% thereafter; (B) 3% oxygen concentration in the feed for 289 min, 6.3% thereafter.

front moves along the bed, it encounters less of the green oil hydrocarbons and also coke with a lower hydrogen content, which gives rise to a decrease in the maximum temperatures. The decreasing pattern of temperatures disappears only after about 1 h of reaction time has elapsed. At this time, most of the green oils and the hydrogen-rich fractions of the coke on the catalyst would have been eliminated, giving a stabilized coke deposit that yields the expected pattern of maximum temperatures. Finally, two experiments were carried out in which the regeneration was started with a relatively low oxygen concentration in the feed (3%), which was stepped up to a higher value (6.3%), after a certain reaction time. In the first of these experiments the oxygen stepup takes place in the earlier stages of regeneration, while in the second it was carried out after more than half of the regeneration was completed, with the aim of investigating the effect of an increase of the oxygen concentration with different levels of coke remaining in the reactor. Both experiments were performed using reactor 1, with a total flowrate of 2.9 L/min (STP), and the same ascending coke profile shown in Figure 1B. It can be seen that the stepup in the oxygen concentration is instantaneously followed by a sharp increase in the slope of the temperature curve of the “hot zone” thermocouple, i.e., the thermocouple corresponding to the region of the bed where the temperature is increasing (T4 in Figure 4A and T6 in Figure 4B). Thus, for instance, the heating rate shown by thermocouple T6 in Figure 4B is approximately 8 times faster after the increase in oxygen concentration. Also, a temperature hike is clearly noticeable in the temperature registered at the previous thermocouple position, in which the temperature is decreasing, while

the temperatures read at the rest of the measuring points were not affected. This confirms the narrowness of the high-temperature combustion zone (i.e., between 5 and 10 cm in this case). The temperature hike displayed by thermocouples T3 in Figure 4A and T5 in Figure 4B indicates that there is still carbon to be burnt at these positions, thereby confirming that a decreasing temperature cannot be taken as a signal of complete regeneration at a given position in the bed. The maximum temperature readings of thermocouples T6 and T7 were 680 and 684 °C in Figure 4A and 684 and 687 °C in Figure 4B; i.e., approximately the same maximum values were obtained in both experiments. Thus, the maximum temperature was determined by the oxygen concentration (since the coke concentration was the same in both cases) and not by the time at which the oxygen concentration was stepped up. This is so in spite of the fact that, at the time of the oxygen concentration step in Figure 4A, a considerably higher amount of coke remains in the reactor compared to the experiment in Figure 4B. Influence of the Total Gas Flowrate. An increase in the total gas flowrate would be expected to lower the external mass-transfer resistance, thus increasing the reaction rate. However, under the conditions used in this work, the process is controlled by the intraparticle diffusion resistance (Byrne et al., 1986), which makes this effect negligible. It must also be taken into account that, for a given oxygen concentration in the feed, an increase in the total gas flowrate would increase the amount of oxygen fed to the reactor (and, therefore, the amount of oxygen reacted on the whole of the catalyst bed). While this would increase the rate of heat release in the reactor, it would at the same time provide a greater mass of gas capable of acting as a heat sink. Figure 5 shows the temperature evolution for an experiment carried out in the same experimental system (reactor 2) and with the same initial coke profile as in parts A and B of Figure 3. The comparison between the experiments in Figure 5 and in Figure 3B shows the effect of changing the total flowrate while keeping the same inlet oxygen concentration. As expected, the regeneration with the higher flowrate (Figure 3B) is faster, since the oxygen input to the reactor (at the same oxygen concentration) has increased by a factor of 1.5. It is interesting to note that the maximum temperatures reached at each of the thermocouple positions in both experiments (Figures 3B and 5) are very similar. This means that the greater rate of heat release caused by


the faster regeneration of Figure 3B is compensated by the increase in the total gas flowrate. The experiment presented in Figure 5 (2.9 L/min, 6.3% O2) can also be compared to that of Figure 3A (4.3 L/min, 4.2% O2). This is relevant because both experiments were carried out with different total flowrates and oxygen concentrations but with the same rate of oxygen input to the reactor (8.15 mmol/min). Since during most of the regeneration all the oxygen is consumed within the reactor, this enables the comparison of the temperature evolution in the reactor at approximately the same overall rate of reaction. It can be seen that the temperatures reached in this experiment are 50-60 deg lower in thermocouples T3 to T7, and about 100 deg lower in thermocouple T2, compared to the results in the experiments carried out with a 6.3% oxygen concentration in the feed. It may therefore be concluded that, under the conditions employed, the feed oxygen concentration rather than the oxygen input to the reactor is the main factor governing the temperature increase at a given position. The comparison between the experiments carried out with the same rate oxygen input (Figures 3A and 5) gives some clues to the interpretation of the results obtained. There are at least three significant factors contributing to the decrease in the maximum temperatures observed in the experiment with the higher total gas flowrate: (i) The greater mass flowrate to the reactor provides the above-mentioned heat sink effect. (ii) A wider reaction zone is obtained, therefore distributing the heat released in the reaction along an extended area in the reactor. (iii) Related to the previous point, the reaction rate at a given position in the bed depends on the local oxygen concentration. A higher concentration gives rise to a fast reaction and hence to high rates of heat release that cannot be dissipated rapidly enough. If oxygen is supplied at a sufficient rate, the resulting temperature increases further accelerate the reaction and the rate of temperature rise, resulting in a local runaway such as that observed for position T2 in Figures 3 and 5. This leads to a fast catalyst deactivation by sintering (Blasco et al., 1992) and may also cause damage to the reactor itself. Pretreatment of the Coke Deposits. Estimation of the Degree of Thermal Aging. The coked reactor was subjected to pretreatments using steam, carbon dioxide, or oxygen, as described above. The evolution of the temperature profiles during the subsequent regeneration was obviously affected by the type of pretreatment carried out. Our main concern here is to assess the effectiveness of the different pretreatments to avoid an excessive loss of activity due to the thermal aging of the catalyst during regeneration. While it is useful to describe the regeneration process in terms of the regeneration time and the maximum temperatures reached at different reactor positions as done in the above discussion, this does not measure the extent of thermal aging of the catalyst. The degree to which a catalyst is permanently deactivated by sintering depends on the temperature-time history of the catalyst and on the reaction atmosphere surrounding it. The relationship describing catalyst deactivation as a function of the above variables may be complex and is not usually available. However, in a previous work Blasco et al. (1992) reported such a relationship for the Cr2O3/ Al2O3 catalyst. In this investigation, the activity loss of a given catalyst sample was found to be approximately independent of the reaction atmosphere used in

a series of coking-regeneration cycles, and the deactivation by sintering could be described according to

-da/dt ) 0.147e-73600/RTa2.2

(1)

In general, if the deactivation by sintering can be accounted for by an equation of the type

-da/dt ) k0e-Ea/RTad

(2)

the integration of eq 2 yields the variation of catalytic activity after a certain time on stream:

∫0te-E /RT dt

a01-d - at1-d ) (1 - d)k0

a

(3)

where the integral term depends on the temperaturetime history of a given catalyst sample. In a fixed-bed reactor, the temperature-time history of the different regions can be considerably different. Thus, depending on the coke distribution pattern and the regeneration conditions used, certain regions of the catalyst bed may undergo a fast regeneration at relatively low temperatures, while others may be subjected to prolonged periods at high temperatures (see, for instance, Figure 2B). It would be useful to define a single parameter capable of characterizing the loss of activity due to the thermal load of the regeneration process. Based on the above deactivation kinetics (eq 2), we propose the definition of the Thermal Aging Index (TAI), according to n

TAI )

∑1 ∫0 e-E /RT dt tf

a

(4)

where n is the number of thermocouples in the reactor, Ea is the apparent activation energy for deactivation by sintering (73 600 kJ/kmol for the Cr2O3/Al2O3 catalyst, Blasco et al., 1992), and tf is the duration of the experiment, in seconds, taken in this work as the time at which the temperature registered by the last thermocouple (T7), decreased below 525 °C. The integral term of eq 4 is calculated at each of the temperaturemeasuring positions and, because it takes into account both time and temperature, it gives a realistic indication of the degree of thermal aging. The TAI value is calculated by adding the contributions corresponding to each of the thermocouple positions, and thus, given a sufficient number of thermocouples, the TAI value provides a reliable indication of the average degree of thermal aging for the reactor. In this way, for a reactor with a given number of thermocouples, the TAI values corresponding to regeneration under different operating conditions can be calculated and compared. A high TAI value means that the severity of a given process (in terms of time-temperature) is such that a large drop in catalytic activity is to be expected. Below, the TAI values will be used to assess the potential benefits of coke pretreatments, by comparing the total TAI values (i.e., pretreatment plus regeneration), for catalyst regenerations carried out after different coke pretreatments and for regenerations without pretreatment of the coke deposits. In order to have a standard for comparison, an experiment was carried out without any pretreatment of the coke deposits. This experiment will serve as a reference for the experiments with different pretreatments. The evolution of temperatures in the reference experiment is shown in Figure 6A. Thermocouple T2 was located axially 2 cm inside

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Figure 6. Regeneration without pretreatment of the coke deposits (uniform coke profile, 3.2% coke content). (A) Temperature evolution at each of the different thermocouple positions. (B) Temperature profiles along the bed at the times (indicated in minutes in the figure), when each of the thermocouples registered its maximum temperature.

the catalyst bed, and the rest of the thermocouples were placed at 5 cm intervals, with the last one (T7) 3 cm before the end of the catalyst bed. Thermocouple T1 (not shown) read the temperature of the preheating zone. Figure 6B shows the axial temperature increase profiles in the bed at the times when each of the thermocouples reached its maximum temperature. The time (in minutes) at which each maximum temperature was reached is also shown in the figure. The temperature increments were calculated by subtracting the maximum temperatures and the initial regeneration temperatures at each of the thermocouple positions. The evolution of temperatures shown in parts A and B of Figure 6 is consistent with the above discussion on the effect of the operating conditions: A very rapid temperature increase takes place at thermocouple position T2, due to the high hydrogen content (and therefore reactivity), of the relatively fresh coke deposits at this position, and the high concentration of oxygen in the entrance region of the reactor. T3 and T4 show smoother temperature increases and lower maximum temperatures, due to the decrease in the hydrogen content of the coke caused by the gases exiting the hot reaction zone. Finally, the last three thermocouples show a pattern of increasing maximum temperatures, corresponding to the usual behavior of pseudoadiabatic reactors with coke deposits of an approximately stable composition. The TAI value for the reference experiment, calculated according to eq 4, was 0.81, corresponding only to the regeneration stage, as no pretreatment was carried out in this case. The maximum temperature was 674 °C (reached at T2), and the

duration of the regeneration, tf, according to the criterium established above was 104 min. Pretreatments with Steam. Steam is often used in practice before regeneration (Fulton, 1988; Duprez et al., 1991) in order to remove adsorbed hydrocarbons and to eliminate the most reactive fractions of the coke. Given the appropriate conditions, steam can also react with the coke deposits, which are partly gasified. In this work, three different pretreatments with steam are reported, with varying degrees of severity according to the different temperatures and/or durations used (Table 1). After each treatment, the regeneration was carried out as indicated above. The results (not shown) indicate that the pretreatment with steam is not appropriate as a means to reduction of the maximum temperatures reached during regeneration. Thus, pretreatment 2 gave rise to only very small reductions in the maximum temperatures reached, and even after pretreatment 3 (1 h at 650 °C), the average decrease in the maximum temperatures compared to the reference experiment was less than 15 °C. To obtain a noticeable maximum temperature reduction throughout the reactor, the coke deposits required pretreatment with steam for 1 h at a 700 °C (pretreatment 4). This gave maximum temperatures which were on average about 35 °C lower than those of the reference experiment and also a considerably shorter regeneration stage (82.6 vs 96 min). The above results show that increasing the duration of the pretreatments with steam or the temperature at which they are carried out can indeed give rise to shorter regenerations and lower temperature maxima. However, these reductions are obtained at the expense of pretreatments of considerable severity, which also contribute to the thermal aging of the catalyst. In order to assess the thermal load on the catalyst, the overall TAI values (i.e., pretreatment plus regeneration) were calculated for pretreatments 2-4, giving values of 1.39, 2.02, and 2.96, respectively. These values can be compared to the no pretreatment or reference case, in which the TAI value was 0.81. Therefore, in spite of the benefits observed during the regeneration stage in terms of reduction of regeneration time and temperature, when the whole process is considered, these benefits are outweighed by the impact of the pretreatment stage, with overall TAI values up to 3.6 times that of the regeneration of unpretreated catalysts. Pretreatments with Oxygen. Four different pretreatments with oxygen were assayed (pretreatments 5-8 in Table 1), using low oxygen concentrations (0.5 or 1%), at ca. 460 °C. In this case, increases over the nominal temperature could be observed during pretreatment, as a consequence of the combustion of the coke deposits that takes place. However, the combination of a low oxygen concentration and a relatively large gas flowrate resulted in a wide combustion zone, and the temperature increases were moderate (e.g., Figure 7A,B). The subsequent regeneration was carried out under the conditions already described. An example of the temperature evolution observed is given in Figure 8A, corresponding to the regeneration carried out after pretreatment 6. The most significant change in Figure 8A when compared to the reference experiment (Figure 6A) is the much lower temperature maxima at the entrance region of the reactor (e.g., 577 vs 674 °C in position T2). This indicates the effectiveness of the treatment with low oxygen concentrations in preferen-


Figure 7. Temperature evolution at each of the different thermocouple positions during pretreatment of the coke deposits with oxygen. (A) Pretreatment 5. (B) Pretreatment 7.

tially attacking the hydrogen-rich fractions of the coke, which give rise to the sharp initial temperature rise observed in Figure 6A. Also, as could be expected, the regeneration is faster after the pretreatment with oxygen (86 min). The temperature maxima reached during regeneration after each of the 4 different pretreatments with oxygen are compared in Figure 8B, where the maximum temperature profile of the reference experiment is also represented for comparison. It can be seen that the main decrease of the temperature maxima takes place near the entrance region of the reactor in all the experiments. The smallest decrease corresponds to the regeneration carried out after the mildest pretreatment (pretreatment 8, carried out for 10 min with 0.5% O2 concentration). Furthermore, in the experiment corresponding to the pretreatment of the highest severity (pretreatment 6), a very significant reduction of the maximum temperatures can be observed not only at the reactor entrance but also at any bed position. It would be interesting to estimate the extent of combustible material removed from the different bed sections by each of the pretreatments performed. While a rigorous assessment would involve a destructive combustion analysis of the coke deposits before and after pretreatment, an approximate indication can be obtained by examining the velocity of the hot combustion zone. The set of thermocouples located axially in the reactor is useful for this purpose: The time elapsed between two consecutive temperature maxima roughly indicates the time necessary to consume the coke deposits between two thermocouples. The catalyst loaded initially onto the reactor had a uniform coke content of 3.2%. Since the amount of catalyst between any two thermocouples is known (24.6 g), the theoretical amount of coke burnt when the combustion front moves from one position to the next can be calculated, which

Figure 8. (A) Temperature evolution at each of the different thermocouple positions during regeneration of a fixed-bed reactor after pretreatment of the coke deposits with oxygen (pretreatment 6). (B) Maximum temperature profiles along the bed during regeneration of fixed bed reactors. Pretreatments 1 (reference experiment) and 5-8.

Figure 9. Apparent combustion velocities along the bed. Pretreatments 1 and 5-8.

gives the apparent combustion velocity for each of the different bed sections. Figure 9 shows the apparent combustion velocities for each of the regenerations carried out after the pretreatments with oxygen and also for the reference experiment. For a uniform coke profile, with a constant rate of oxygen input to the reactor, a roughly constant apparent combustion rate would be expected. However, Figure 9 shows that the apparent combustion velocity is higher at the beginning of the bed, decreasing thereafter. This means that the removal of combustible material by the oxygen treatments takes place preferentially at the bed entrance, thus giving a faster displacement of the hot combustion zone during regeneration.

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In spite of the temperature increases observed during pretreatment, low overall TAI values were obtained for the processes involving pretreatments with oxygen: 0.81, 0.67, 0.73, and 0.83, corresponding respectively to pretreatments 5-8 and the subsequent regenerations. These values are clearly below those corresponding to pretreatments with steam and, in two cases (pretreatments 6 and 7), even considerably lower than those of the process without pretreatment, which shows the usefulness of the pretreatments with oxygen in order to reduce the thermal load on the catalyst. Pretreatments with Carbon Dioxide. In spite of preliminary experiments carried out on a thermobalance that seemed to indicate enough reactivity of the coke deposits when treated with carbon dioxide, CO2 pretreatments were not useful for the purpose of reducing the temperatures reached during regeneration. The maximum temperature profiles were approximately the same with and without pretreatment, even under pretreatment conditions that represented a severe thermal load to the catalyst, as indicated by the comparison (not shown) between the results obtained after pretreatment 9 (30 min, 700 °C) and those of the reference experiment, in which virtually the same maximum temperatures were obtained. Conclusions In the preceding pages, we have reported on the influence of the operating conditions and of different pretreatments of the coke deposits on the regeneration of a Cr2O3/Al2O3 catalyst. While we believe that most of the trends depicted are of a general nature, it should be noted that all the experiments have been conducted with the same catalytic system, and the regeneration of other catalysts (e.g., supported metals) may present deviations from the above-discussed behavior. From the results obtained in the study of different operating variables, it can be concluded that in the system studied the type of coke deposition does not have a direct influence on the duration of the regeneration, but it does significantly affect the maximum temperatures reached in the process. The most critical situation arises when adiabatic reactors with an increasing pattern of coke deposition are regenerated. In this case, the usual pattern of increasing maximum temperatures corresponding to adiabatic reactors is reinforced by the increasing coke profile, while the opposite is true in coked reactors with decreasing coke profiles. An increase in the oxygen concentration of the feed gases has an immediate effect upon the rate of regeneration and upon the temperatures reached in the reactor. A high oxygen concentration gives a fast regeneration, but also sharp temperature peaks, leading to excessive temperatures that accelerate the permanent deactivation of the catalyst by sintering. The maximum temperatures in the reactor are directly related to the oxygen concentration in the feed, while the rate of regeneration depends on the amount of oxygen fed to the reactor. Increasing the dilution of the feed gases provides lower local reaction rates, a wider reaction zone, and a significant heat sink effect, resulting in a decrease of the maximum temperatures reached. Therefore, for a given coke concentration in the bed the oxygen supply to the reactor can be set (within certain limits) from the desired regeneration rate, while the oxygen concentration (i.e., the dilution of the feed with inert gases or with steam) depends on the maximum temperature limit. The design of an optimum regeneration

Figure 10. Calculated TAI values (pretreatment, regeneration, and overall) for each of the different pretreatments reported in this work.

algorithm requires the knowledge of the type of coke deposition, the reaction-diffusion kinetics, and the heat-transfer characteristics of the system concerned. Pretreatment of the coke deposits should also be considered when planning regeneration strategies. To this end, the Thermal Aging Index (TAI), defined in this work, is a useful parameter to compare the extent of thermal damage that can be expected on a catalyst after a given treatment. Figure 10 summarizes the TAI values calculated for each of the pretreatments and subsequent regenerations, as well as the overall (pretreatment plus regeneration) TAI value. It can be seen that any of the pretreatments (except number 8) is able to reduce the regeneration TAI value. However, some of the pretreatments themselves present high TAI values. This is particularly true of the pretreatments carried out with steam (2-4) and carbon dioxide. In these cases, the TAI value of the pretreatment more than offsets the benefits achieved during the regeneration stage, giving high values of the overall TAI value. Clearly, the most effective pretreatments are those carried out with diluted, oxygen-containing streams. All of the four pretreatments (5-8) display small values of the thermal aging index, in spite of which significant reductions in the regeneration TAI value can be obtained. The most effective of the four pretreatments carried out was pretreatment 6, carried out for 30 min, with a 1% oxygen concentration and a relatively high flowrate. This produced an overall TAI value of 0.67, or 17% lower than the nonpretreated process. It is also interesting to note that the mildest pretreatment with oxygen (pretreatment 8) does not eliminate enough of the coke deposits to achieve a significant reduction of the regeneration TAI value, and as a consequence the overall TAI value is slightly higher than the reference experiment. In summary, by choosing the appropriate pretreatment of the coke deposits (pretreatment agent, temperature, duration), the thermal aging of the catalyst can be reduced considerably. Pretreatments with oxygen were found to be most effective, but they only achieve significant reductions in TAI values if the pretreatment conditions are carefully selected. The optimum pretreatment conditions will depend on the type of catalyst and the amount and nature of the coke deposited, but in general it can be concluded that procedures based on pretreatments of the coke deposits offer ample scope for improving the current industrial practice, in terms of attaining lower temperatures and faster regeneration processes.


Acknowledgment Financial support from DGICYT, Spain (Project PB940568), is gratefully acknowledged. Literature Cited Acharya, D. R.; Brito-Alayoń, A.; Hughes, R.; Santamarıá-Ramiro, J. Partial regeneration of coked catalyst in fixed bed reactors. In Recent Trends in Chemical Reaction Engineering; Kulkarni, B. D., Mashelkar, R. A., Sharma, M. M., Eds.; Wiley Eastern: New Delhi, India, 1987; pp 138-148. Acharya, D. R.; Hughes, R.; Kennard, M. A.; Liu, Y. P. Regeneration of fixed beds of coked chromia-alumina catalyst. Chem. Eng. Sci. 1992, 47, 1687-93. Blasco, V.; Royo, C.; Monzoń, A.; Santamarıá, J. Non-uniform sintering in fixed bed reactors. Deactivation rate and thermal history. AIChE J. 1992, 38, 237-46. Borio, D. O.; Schbib, N. S. Simulation and optimization of a set of catalytic reactors used for dehydrogenation of butene into butadiene. Comput. Chem. Eng. 1995, 1, 345-350. Borio, D. O.; Meneńdez, M.; Santamarıá, J. Simulation and optimization of a fixed bed reactor operating in cokingregeneration cycles. Ind. Eng. Chem. Res. 1992, 31, 2699-2707. Brito, A.; Borges, M. E., Arvelo, R.; Garcıá, T. Catalyst characterization. Variation of several parameters with the deactivation/regeneration cycles. Appl. Catal. 1993, 103, 17-21. Byrne, A.; Hughes, R.; Santamarıá-Ramiro, J. The influence of initial coke profile and hydrogen content of the coke regeneration of fixed beds of catalyst. Chem. Eng. Sci. 1985a, 40, 15071516. Byrne, A.; Dakessian, V.; Hughes, R.; Santamarıá-Ramiro, J.; Wright, J. Determination of coke profiles in fixed-bed catalytic reactors by a neutron attenuation technique. J. Catal. 1985b, 96, 146-153. Byrne, A.; Hughes, R.; Santamarıá-Ramiro, J. Effect of deposited coke profiles on transient temperatures during regeneration of a fixed bed catalytic reactor. Chem. Eng. Sci. 1986, 41, 773779. Byrne, A.; Hughes, R.; Santamarıá, J. The influence of operating and coke-related variables on the regeneration on fixed beds of catalyst. Chem. Eng. Sci. 1989, 44, 2197-2206. Dumez, F. J.; Froment, G. F. Dehydrogenation of 1-butene into butadiene. Kinetics, catalyst coking and reactor design. Ind. Eng. Chem. Proc. Des. Dev. 1976, 15, 291-301. Duprez, D.; Hadj-Aissa, M.; Barbier, J. Effect of steam on the coking and on the regeneration of metal catalysts: A compara-

tive study of alumina-supported Platinum, Rhenium, Iridium and Rhodium catalysts. In Catalyst Deactivation. Bartholomew, C. H., Butt, J. B., Eds; Amsterdam, The Netherlands, 1991; pp 111-118. Froment, G. F., Bischoff, K. B. Non-steady state behaviour of fixed bed catalytic reactors due to catalyst fouling. Chem. Eng. Sci. 1961, 16, 189-201. Fulton, J. W. Regenerating Spent Catalyst. Chem. Eng. 1988, 95, 111-14. Gayubo, A. G.; Arandes, J. M.; Aguayo, A.; Ola´zar, M.; Bilbao, J. Reaction-regeneration cycles in the isomerization of cis-butene and calculation of the reactivation kinetics of a silica-alumina catalyst. Chem. Eng. Sci. 1993, 48, 2741-52. Haldeman, R. G.; Botty, M. C. On the nature of the carbon deposit of cracking catalysts. J. Phys. Chem. 1959, 63, 489-95. Ibarra, J. V., Royo, C.; Monzoń, A.; Santamarıá, J. Fourier transform infrared spectroscopy study of coke deposits on a Cr2O3/Al2O3 catalyst. Vib. Spectrosc. 1995, 9, 191-96. Massoth, F. E.; Menon, P. G. Active species on coked silicaalumina catalyst. Ind. Eng. Chem. Prod. Res. Dev. 1969, 8, 383385. Pieck, C. L.; Jablonski, E. L.; Verderone, R. J.; Grau, J. M.; Parera, J. M. The burning of coke on Pt-Re/Al2O3. Catal. Today 1989, 5, 463-72. Pieck, C. L.; Jablonski, E. L.; Parera, J. M.; Frety, R.; Lefebvre, F. Characterization of residual coke during burning. Ind. Eng. Chem. Res. 1992, 31, 1017-21. Royo, C.; Meneńdez, M.; Santamarıá, J. Kinetics of catalyst regeneration by coke combustion. I. Increased reaction rate due to the presence of chromium. React. Kinet. Catal. Lett. 1991, 44, 445-50. Royo, C.; Ibarra, J. V.; Monzoń, A.; Santamarıá, J. Regeneration of coked catalysts: The effect of aging upon the characteristics of the coke deposits. Ind. Eng. Chem. Res. 1994, 33, 2563-2570. Takahashi, T.; Watanabe, K. The role of hydrogen-carbon ratio of coke in cracking of aromatic hydrocarbons on silica-alumina catalyst. J. Jpn. Pet. Inst. 1978, 21, 85-88.

Received for review October 19, 1995 Accepted March 11, 1996X IE950639P

X Abstract published in Advance ACS Abstracts, May 1, 1996.

Regeneration of Fixed-Bed Catalytic Reactors Deactivated by Coke

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