Znd. Eng. Chem. Res. 1994,33,2563-2570
2563
Regeneration of Coked Catalysts: The Effect of Aging upon the Characteristics of the Coke Deposits Carlos Royo, Jose V. Ibarra,?Antonio Monz6n, and Jestis Santamaria' Departarnento de Ingenieria Quirnica y Tecnologias del Medio Arnbiente, Universidad de Zaragoza, 50009 Zaragoza, Spain
The effect of aging in nitrogen upon the regeneration characteristics of the coke deposits on chromia-alumina catalysts has been investigated. To this end, the coked catalysts have been subjected to various treatments in nitrogen, and the chemical composition and reactivity of the deposits have been investigated. The results show that the process of aging in nitrogen gives rise to significant changes in both the composition and reactivity of the coke deposits, due to the stripping of the coke fractions with a higher volatility. This obviously has important consequences upon the subsequent regeneration, which are also discussed and tested in regeneration experiments using coked catalyst of different ages.
Introduction Coke deposition may cause the decrease of catalytic activity by site coverage, pore blockage, or both. Catalyst deactivation by coke can usually be reversed by removing the coke deposited on the catalyst. This involves the gasification of the coke deposits using one or more reactants, although oxidative regeneration (i.e., the combustion of the coke deposits) is the preferred method. Since the combustion reaction is highly exothermic, when oxidative regeneration is used the main concern is the possibility of an excessive temperature rise which could damage the reactor and cause irreversible deactivation of the catalyst by sintering. The avoidance of high-temperature excursions is especially important in fixed bed reactors, where the combustion gives rise to a high-temperature reaction front that moves along the bed as the coke deposits are depleted. Thus for instance, Blasco et al. (1992) have shown that large-temperature increments during regeneration may cause irreversible loss of catalytic activity by sintering in chromia-alumina catalysts. In this case, the extent of the deactivation was related to the time-temperature history in a series of operation-regeneration cycles. High-temperature excursions can be avoided if a careful monitoring of the temperature along the bed is carried out during regeneration and the amount of oxygen fed to the reactor is adjusted to keep the temperature within acceptable limits. A 2.3% oxygen concentration in the feed is usually low enough to start regeneration, and this level can be progressively stepped up as the regeneration progresses (Fulton, 1988). Given the importance of avoiding excessive temperature rises, a great deal of effort has been spent on the simulation of catalyst regeneration in fxed bed reactors, with the aim of studying the influence of operating conditions upon the dynamics of propagation and upon the magnitude of the high-temperature fronts generated within the bed. A recent example of this kind of studies is the work of Acharya et al. (1992). Another interesting problem is the optimization of the operation of catalytic reactors working in production/ deactivation-regeneration cycles. One cycle consists of a production stage during which the catalyst is deacti-
* Author to whom correspondence should be addressed. E-Mail: QTMIGUEUXC.UNIZAR.ES. + Permanent address: Instituto de Carboquimica, CSIC, Plaza Paraiso, 50080 Zaragoza, Spain. 0888-5885/94/2633-2563$04.50/0
vated by coke, followed by purge and evacuation steps to eliminate the reactants and volatile hydrocarbons, and then by a regeneration stage during which the coke deposits are burnt off. The optimization of the operation of a chromia-alumina catalytic reactor coked during 1-butene dehydrogenation has been carried out by Dumez and Froment (1976) and by Borio et al. (1992). Other authors have also simulated the production/ deactivation stage (Acharya and Hughes, 1990) and the regeneration stage (Acharya et al., 1992)for this system, although the optimization of the cycle was not attempted in this case. None of the previous works on catalyst regeneration has studied the effect that the purge and evacuation steps may have on the coke deposits and on the subsequent regeneration. In spite of this, there is experimental evidence which shows that, depending on the operating conditions, an aging period may have a very significant effect upon the characteristics of the coke deposits. For instance, four decades ago, Plank and Nace (1955) found a large variation in the C/H ratio of the coke deposits obtained from cumene cracking, with larger C/H ratios corresponding to higher values of coking time. Furthermore, these authors found that, if the coke deposits were subjected to a purge in nitrogen for a certain period of time, part of the coke could be displaced, with a substantial recovery in the cracking activity. Appleby and co-workers (1962) proposed a coke-forming mechanism that included dehydrogenation, cracking, and condensation reactions, which would also lead to a lower hydrogen content of the coke with increased time. Haldeman and Botty (1959) also found that, depending on operating conditions, a considerable amount of hydrogen and low molecular weight hydrocarbons could be evolved during nitrogen aging of the coke deposits on cracking catalysts. This led to an increase in the C/H ratio from about 0.5 to 2-2.6. Recently, Gayubo et al. (1993a) have reported a very strong devolatilization of the coke deposits during stripping with helium under a temperature ramp, with more than half of the coke weight being removed prior to the start of the combustion stage. Although no coke analysis was performed, it can be presumed that a substantial modification of the composition of the coke deposits also took place in this case. Among other reasons, the variation in the C/H ratio of the coke is important because it is likely to have a strong influence upon catalyst regeneration. For in@ 1994 American Chemical Society
2564 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
stance, Haldeman and Botty (1959), Massoth (19671, Acharya et al. (19871, and Piek et al. (1992) have reported that the hydrogen-rich fractions of the coke are burnt before the rest of the coke deposits. In particular, Massoth (1967) reported a very rapid initial temperature rise upon introduction of the combustion gas, which could be ascribed to the adsorption of oxygen but also t o the faster combustion of the hydrogen-rich coke that he reported. There is a wide variation among the reported C/H ratios in the literature, from about 0.5 t o 2.8 (Wolf and Alfani, 1982; Hughes, 1984). Part of the scatter in these values is, most likely, the result of the different temperatures and reaction times employed, with higher temperatures or longer reaction times leading to higher C/H ratios. In addition, it has been reported in the literature (e.g., Byrne et al., 1986) that the C/H ratio of the coke deposits may change somewhat with the position inside a fured bed reactor. In this case, the regions with a higher hydrogen content would dehydrogenate at a different rate (presumably faster) than those with a high C/H ratio. The aim of this work is to study the effect of the aging of the coke deposits upon the subsequent regeneration. To study aging as independently from coking as possible, the aging step will be carried out in nitrogen, after the interruption of the coking process. However, as stated above, it must be taken into account that during coking the aging of the coke deposits also takes place, to an extent that it increases with coking time and/or temperature.
Experimental Section Catalyst coking was achieved in the dehydrogenation of butene to butadiene over a CrzOdAlzO3 catalyst, at temperatures ranging from 753 to 873 K. The coked catalyst was then subjected to an aging treatment in nitrogen, either in a thermobalance or in a fixed bed reactor. The aging process was carried out at the same temperature used for coking, using 99.999% pure nitrogen (Air Liquide). Although the industrial process usually involves the stripping of the coke with steam, it was decided to use nitrogen, in order to study the aging process independently from other reactions, such as those that would take place if steam (a well-known coke gasifying agent) was used. Thus, in what follows both terms stripping and aging will be used to describe the processes that take place when the coke deposits are subjected t o contact with a stream of inert gas, at a given temperature. When the coking and aging processes were carried out in the thermobalance, the weight loss due to devolatilization in nitrogen could be followed. The thermobalance employed was a C. I. Instruments MK2, fitted with automatic controls of flow (Brooks mass flow controllers) and temperature. The weight data were acquired a t regular intervals using a computer-controlled data logging system. The accuracy of the weight readings was g. During the reactiodcoking stage, the exit gases from the thermobalance were directed to a gas chromatograph for analysis of butene, butadiene, and hydrogen. The thermobalance was also used to compare the reactivity of cokes with different degrees of aging. This was carried out by means of temperature-programmed oxidation experiments using 3% of 0 2 in nitrogen, with a heating rate of 3 Wmin. The heat evolved in the combustion of coke deposits with varying degrees of aging was measured in a different set of temperature-programmed experiments, using a Set-
aram TG.DSC92 differential scanning calorimeter (DSC), with a heating rate of 5 Wmin in air. When a fured bed reactor was used to perform the coking and aging steps, the higher mass of coke present in the system provided enough sensitivity to carry out elemental analysis of the aged coke. The reactor is described below. After the coking and aging stages, the C/H ratio of the coke was determined by in situcontrolled combustion of the coke deposits. The exit gases from the combustion were directed to a bed of quantitative water absorbent (drierite) and then to a gas storage system. All the gases produced in the experiment were collected and analyzed for CO and COz, which yielded the total amount of carbon in the exit gases. Taking into account the weight increase of the drierite bed, the composition of the coke could be calculated. Repeated runs showed a reproducibility within 4% of the measured C/H value. FTIR spectra of coked catalyst were run on KBr pellets (120 mg, 1wt %I. Spectra were recorded by coadding 120 scans at a resolution of 2 cm-l in a Nicolet 550 spectrometer. Software facilities were used for baseline corrections of spectra which were scaled to 1 mg of sample/cm2. For quantitative measurements of spectra, duplicate pellets were used. The aromatic C=C stretching zone (1630-1500 cm-') and the aliphatic C-H bending region (1450-1300 cm-l) of the FTIR spectra were studied by curve-fitting analysis using commercial data processing software. Aliphatic structures were studied in the region near 1400 cm-l rather than in the 2900 cm-l zone, due to the strong absorption of the OH structures at 3450 cm-l. Finally, regeneration experiments were carried out using fixed bed reactors which had been coked under the same conditions but subjected to different degrees of coke aging. These experiments were carried out in a 30 mm internal diameter, 380 mm long stainless steel fured bed reactor. In order to have a good control over the coking and regeneration conditions, the reactor was immersed in a sand-fluidizedbed to attain isothermality of the heat transfer medium during the course of the experiment. The reactor consisted of a gas preheating section followed by the fixed bed itself, which comprised approximately 244 g of catalyst. The flow rates of the different gases to the reactor (nitrogedbutene during coking, nitrogen during aging, nitrogedoxygen during regeneration) were mass flow controlled (Brooks). The temperature variations in the reactor during the different stages were measured at six axial positions (at bed depths of 2, 7, 12, 22 and 27 cm), at regular time intervals using 6 K-type thermocouples connected t o a computer-controlled data acquisition system.
Results and Discussion Figure 1 shows the result of a coking/aging experiment carried out in two stages using a thermobalance. In this case, the catalyst was coked using a 50% nitrogenhutene mixture at 853 K for about 70 min until a coke level of approximately 7% was achieved. The coke deposits were then aged in nitrogen for 110 min a t the same temperature. After aging, coking was resumed for a further period of 30 min, and then the coke deposits were aged again for about 18 min. The experiment was ended by the combustion of the coke deposits, which approximately restored the initial weight of the sample as well as the catalytic activity (not shown). In Figure 1, it can be observed that, during the aging period, only a small decrease in the weight of
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2666
120
90
150
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210
v.v
time (min) Figure 1. Evolution of the weight of coke deposited on the catalyst and of the hydrogen production during butene dehydrogenation at 853 K.
coke takes place, due to the stripping of the most volatile fractions of the coke. The 110 min aging period of Figure 1 seems to have little or no effect upon the rate of coke formation. After the butenehitrogen feed is restarted at about 180 min the curve of coke formation continues in such a way that it would smoothly continue the weight increase of the first coking period if the aging stage was eliminated from the figure. As for the effect of the coking and aging stages on the reaction rate, Figure 1 also shows the variation of the hydrogen production with time during the first and second coking periods. It can be seen that the rate of hydrogen production decreases with the deposition of coke on the catalyst to give exit concentrations of less than 0.2% after about 40 min on stream, which corresponds to a coke content of approximately 5.8%. Also, after about 50 min on stream the activity of the catalyst for the main reaction remains approximately constant at a residual level, in spite of significant increases in the weight of coke. This was interpreted in an earlier work as evidence of coke formation in multiple layers (Pefia et al., 1993). The first aging of the coke deposits removes approximately 0.15 mg of coke (slightly over 1%of the coke deposited on the catalyst). Upon restarting the reaction, a very small increase on the hydrogen production can be observed in Figure 1, which is quickly depleted as new coke is deposited on the catalyst. This was typically found in the different coking/aging experiments carried out, but the activity increases were fairly modest in all cases. Therefore, in this work, both the devolatilization of the coke deposits on the catalyst and the recovery of catalytic activity during aging were of a small magnitude. This is in contrast to the results of other authors such as Gayubo et al. (1993a), who reported a considerable removal of coke (and an important recuperation of catalytic activity) during stripping with nitrogen. It must, however, be noted that these authors coked their samples during the isomerization of cis-butene on silica-alumina, which took place at 493 K, while the stripping was carried out under a temperature ramp from 493 K to the regeneration temperature of 798 K. This means that the coke was deposited at a low temperature and therefore it was little evolved, which was confirmed by the high solubility of the coke in
0.0 0
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time (min) Figure 2. Variation of the C/H ratio of the coke deposits with aging time. Curve 1 shows only coke deposited directly on the catalyst. The coking and aging temperature was 853 K, and the combustion was carried out at 743 K. Curve 2: total coke content. Coking, aging and regeneration were carried out at 803 K. Curve 3: total coke content. The coking and aging temperature was 853 K, and the combustion was carried out at 743 K.
different solvents (Gayubo et al., 1993a). When the temperature was raised, a considerable evolution took place, which was accompanied by the elimination of volatile coke fractions. However, in our case the coke was deposited at a much higher temperature, which means that most of the devolatilization took place during the coking stage itself. As a result, a considerably evolved coke was produced, which was much more stable and showed little solubility in dichloromethane or pyridine. Analysis of the Coke Deposits. As explained in the introduction, the aging process is expected to change the chemical composition of the coke deposits. Figure 2 shows the evolution of the carbon to hydrogen (CEO, ratio of the coke deposits with aging time. Curves 1 and 3 show the results of experiments in which the coke was deposited and aged at 853 K and regenerated at 743 K. In this case, coke compositions are reported for aging times ranging from 15 to 180 min. Aging times below 15 min could not be used, since this was the minimum time required to cool the external fluidized bed, which contains about 7 kg of sand, from the cokingl
2566 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
aging temperature to the controlled combustion temperature. Therefore, the 15 min old coke corresponds to an aging period during which the temperature changed from 853 to 743 K. In these experiments, the coking and aging steps were carried out in a 14 mm diameter stainless steel reactor which contained the catalyst bed. Obviously, the coke can be deposited on the reactor walls as well as on the catalyst, and therefore both types of deposits contributed to the overall C/H ratio obtained from the combustion process. In addition, condensed hydrocarbons could often be observed at the entrance and exit regions of the reactor when it was unloaded in coking experiments without regeneration. These partly dehydrogenated hydrocarbons, often referred to as “green oils” in industrial practice, are also present when the regeneration step is carried out and contribute to the total temperature rise. Thus, it is important to distinguish between the coke deposited directly on the catalyst and the total amount of combustible material, which will be referred hereinafter as total coke (coke deposits on the catalyst, plus coke on the reactor walls and “green oils”). Some experiments were carried out in order to discriminate between the contributions of both deposits. In these experiments, after coking the reactor was unloaded, cleaned thoroughly, and then repacked with the coked catalyst. In this way, the C/H ratio determined by combustion of the coke corresponded exclusively to the coke deposits on the catalyst. Thus, curve 3 in Figure 2 shows the variation of the overall composition of the coke deposits (i.e., the total coke), whereas curve 1 corresponds t o the coke deposited on the catalyst. It can be seen that the coke deposited on the catalyst has a higher C/H ratio than the total coke present in the reactor, which means that the coke deposits on the reactor walls have a much higher hydrogen content. Figure 2 shows a significant increase of the C/H ratio with aging time for both types of deposits. The increase is greater for the total coke, whose composition approaches that of the coke deposits on the catalyst after approximately 3 h on stream. Therefore, the total coke is more easily stripped of hydrogen-rich fractions than the coke deposited directly on the catalyst. The extrapolation of the observed behavior of the C/H ratio to an industrial reactor is uncertain. On the one hand, green oils would also be present in industrial operation. On the other, the effect of the coke deposited on the reactor walls would be less important, given the large dimensions of the industrial reactors, which would result in a much lower ratio of wall surface to reactor volume. To assess this, a different experiment was carried out in a 30 mm diameter reactor which contained 7 times more catalyst than the previously described 14 mm reactor. In addition, the coking, aging, and regeneration steps were all carried out at the same temperature of 803 K, which enabled the regeneration to be carried out almost immediately after the coking step was concluded. The results are shown in curve 2 of Figure 2 and correspond to total coke (catalyst plus reactor). In this case, in spite of the lower coking temperature, the initial C/H ratio is higher than that of curve 3, due t o the more important contribution of the coke deposited on the catalyst. As time increases, the contribution of the coke deposited on the reactor wall becomes less important, and eventually the total C/H ratio of the coke deposited at 853 K (curve 3 ) becomes higher than that of the coke deposited at 803 K. In the process, a considerable
1700
1600 Wavenumbers (cm-1)
.02 T
1500
I400 _ _ _ _
1350
1400
Wavenumbers (cm-1)
Figure 3. FTIR spectra of the fresh and coked catalyst in the aromatic carbon stretching zone (A, top) and in the aliphatic C-H bending zone (B, bottom). Key: (a) Fresh catalyst, (b) catalyst coked at 753 K and aged for 15 min, (c) catalyst coked at 803 K and aged for 15 min, (d) catalyst coked at 853 K and aged for 15 min, (e) catalyst coked at 853 K and aged for 180 min. Table 1. Evolution of the Area of the Main Curve-Fitted Bands of the IR Spectrum in the 1750-1385 ern-’ Zone”
sample fresh catalyst catalyst coked a t 753 K catalyst coked at 803 K catalyst coked a t 853 K catalyst coked at 853 K, and aged for 180 min
15701520
13851400
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0.28 0.48 0.45
0.09 0.33 0.24 0.34 0.33
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17501650
1633 1590
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0.86 0.84 1.21 0.82 0.80
a Values expressed as integrated absorbance area in arbitrary units. All the samples were coked for 90 min and then aged in nitrogen for 15 min except the last one.
amount of coke has been devolatilized, to the point that the analysis of the combustion gases showed that the total coke present in the reactor after 3 h of aging at 803 K is about 30% less than after only 3 min of aging. Coke deposits of different age were also characterized using FTIR analysis. Infrared spectroscopy has often been used to characterize the nature of the coke deposits, and many authors have discriminated the contribution of the different species present in the coke to the observed absorbance bands (e.g, Eberly et al., 1966; Eisenbach and Gallei, 1979; Blackmond et al., 1982a,b; Bilbao et al., 1985; Piek et al., 1992; Gayubo et al., 199313; Datka et al., 1994). The general views of the 1750-1480 and 1500-1350 cm-l regions of the infrared spectrum are shown in Figures 3A and 3B, respectively, and the areas corresponding to each of the main absorbance bands established by curve fitting are given in Table 1. Four curves appear in Figure 3A while there are five in Figure 3B. The reason is that both curves correspondingto 853 K (15 and 180 min of aging) in Figure 3A are practically coincident, and therefore only one has been drawn.
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2567 The fresh catalyst presents a strong peak over 1633 cm-l, attributed t o OH groups. The presence of oxygencontaining structures was also made manifest by several small bands in the 1750-1650 cm-l region. These bands disappear with increasing temperature of reaction, as shown in Table 1. The band a t 1633-34 cm-l has also been assigned to stretch vibrations of the double bonds in olefinic chains (Gayubo et al., 1993). At a coking temperature of 753 K, which is 100 K lower than the usual temperature for the reaction considered, there is enough coke deposited on the catalyst t o give rise to the appearance of a band in the 1570-1520 cm-l zone, which remains at approximately the same value of area as the coking temperature is increased. However, the amount of coke deposited on the catalyst a t 753 K is so low (approximately 0.3 wt %) that the contribution of the coke a t 1633 cm-l is not distinguishable from the absorbance due to the catalyst at the same frequency. At 803 K the curve-fitting analysis shows a marked increase of the 1633 cm-l band (Table 11, which could be attributed t o the deposition on the catalyst of polyolefinic structures (Gayubo et al., 1993). Likewise, a new band around 1590 cm-l can also be observed at 803 K. This is the “coke peak, which many authors (e.g., Eisenbach and Gallei, 1979; Blackmond et al., 1982a; Gayubo et al., 1993a,b) have considered to be representative of coke formation, and can be assigned to C=C bands in aromatic and highly conjugated structures. Again, it cannot be observed in the sample coked a t 753 K due to its low coke content. This peak continues to grow as the coking temperature is increased t o 853 K and does not seem to be affected by aging at this temperature. It is interesting to note that, as the coking temperature is increased from 803 to 853 K, the band at 1633 cm-l decreases, and the band at 1590 cm-l increases, as shown by the shift of the main peak in Figure 3. This can be interpreted as an increase in the aromaticity of the coke deposits with reaction temperature. The increase in aromaticity is not observed in the samples aged for longer times a t 853 K. However, a significant loss of hydrogen was caused by aging, as evidenced from the results in Figure 2. The dehydrogenation of the samples can be followed by Figure 3b by the bands at 1400 and 1385 cm-l (shoulder) of the spectrum, which corresponds to symmetric deformation of methyl groups (Wang et al., 1985; Bilbao et al., 1985; Gayubo et al., 1993a,b). It can be seen that even with the low coke content of the sample a t 753 K an important increase in the absorbance area is noticeable. The absorbance continues to increase in the sample coked at 803 K (1.7 wt % of coke) and then decreases for the sample at 853 K, due to the more dehydrogenated coke obtained at this higher temperature. The decrease in the area of the peaks at 1400 and 1385 cm-l is produced in spite of the higher coke content of the sample (4.1%). When the sample is further maintained at this temperature for 3 h an important decrease in the area of the peak can be observed. This suggest that aliphatic CH3 structures are present in the coke deposits, possibly attached to aromatic structures, and they ‘ tend to disappear during the dehydrogenation process which takes place at long reaction times. Catalyst Regeneration. In addition to the observed effect on the chemical composition of the coke deposits, the aging process also affects their reactivity toward combustion. This is illustrated in Figure 4A, where the results of temperature-programmed oxidation of coke
deposits of 15 and 180 min of nominal age are shown. The term “nominal”means that the particles were aged for the above stated times in nitrogen, but the aging period was followed by approximately 20 min more of cooling time until the reactor could be unloaded and the particles sampled and placed on a thermobalance for the temperature-programmed oxidation experiment. Therefore, the differences in reactivity between the two original samples are likely to be reduced by the postaging cooling period. Figure 4A shows that the reactivity of the coke deposits decreases significantly with aging: The derivative of the weight curve for the youngest coke reaches a peak approximately 12 K lower than that corresponding to the coke aged for 180 min. This behavior was typical of several experiments carried out, and even greater differences in reactivity could be observed when the experiments were carried out with aging periods shorter than 15 min. The heat of combustion of the coke is a function of its composition,and therefore the variation of the C/Hratio can have a large effect upon the temperature rises attained during regeneration. Figure 4B shows the results of a temperature-programmed experiment carried out in a DSC apparatus with cokes of 15 and 180 min of age. In this case, the heat flow curve indicates that approximately 10% more energy is released from the combustion of the younger coke. Obviously, the aging-devolatilization process reduces the amount of coke present of the sample. However, as shown in Figure 1, when the only coke involved is the coke deposited on the catalyst as in Figure 4B, the devolatilization during aging from 15 to 180 min gives rise to a weight loss of only about 1%of the sample weight. This is unlikely t o cause important effects on the total heat released when a given mass of catalyst is regenerated, and therefore the reason of the lower heat flow for longer aging times lies in the increase of the C/H ratio, which gives a lower heat of combustion per unit mass of coke. As shown above, the aging process gives rise to a less reactive coke, which also has a lower heat of combustion. This has important implications for the regeneration of a coked catalytic bed. To test these, four experiments were carried out in which the catalytic reactor was coked, aged for different lengths of time, and then regenerated. Again, in this case the same temperature (803 K) was used for coking and regeneration, in order to be able to proceed with the regeneration step without the time delay necessary to cool the reactor. The results are shown in Figures 5A-C, where the variation of temperature with time at six fured axial positions in the bed is represented. It can be observed that the aging process has a very strong effect upon the temperature reached in the entrance region of the reactor. Thus, the first thermocouple showed temperatures in excess of 1045 K for the cokes aged for 3 and 15 min (not shown), about 950 K for the coke aged for 1 h, and only about 865 K when the aging time was extended up to 180 min. It must be taken into account that the high temperatures in the first thermocouple of Figure 5A are attained in spite of the fact that it is located near the beginning of the fixed bed, and therefore the catalyst in that area is exposed to comparatively colder gases than the rest of the reactor. In addition to the effects observed with regard to the maximum temperature reached, the shape of the temperature peak varied strongly as the aging time was increased. Since the oxygen feed rate to the reactor
2568 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
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time (s) Figure 4. Temperature-programmed oxidation of the coke deposits on the catalyst after aging for 15 and 180 min at 853 K. (A, Top) Experiment carried out in a thermobalance, with 75 mg of catalyst. (B, Bottom) Experiment carried out in a DSC,with 18 mg of catalyst.
during regeneration was kept constant for the four experiments, the width of a given temperature peak, corresponding to one of the six thermocouples within the catalytic bed, provides a rough indication of how much oxygen is consumed in that particular region of the bed. If the temperature peaks corresponding t o the first thermocouple in the bed are compared for the experiments in Figures 5A-C, it can be observed that considerably less oxygen is consumed as the aging time is increased. This means that either there is less coke to be consumed or there is a low-hydrogen coke (which would consume less oxygen) or both. As we have seen, probably both causes make their contribution to the observed behavior: On the one hand, although it has been shown that the total amount of coke on the catalyst does not change much when aged in nitrogen, it seems likely that the coke deposited on the reactor walls is more easily stripped during aging. Also, it seems very probable that the “green oils”in the entrance region are partly volatilized under a nitrogen stream. Both mechanisms would give less coke in the reactor after longer aging times. On the other, it has already been shown
that the coke deposited directly on the catalyst loses as much as 50%of its hydrogen content during a 3 h aging period. It is also interesting to compare the evolution of the temperature peaks of the different thermocouples in the same experiment. Thus, in Figure 5A, the first peak is wider than the second, and this is wider than the third. This is consistent with the above discussion on the effect of coke aging: As the coked catalyst in the entrance region of the bed is regenerated, the hot gases from the high-temperature moving combustion zone would cause the aging of the coke deposits downstream of it, therefore increasing its C/H ratio. In addition, any nonconsumed oxygen from the high-temperature reaction zone would preferentially attack hydrogen-rich fractions of the coke. Therefore, as the reaction zone moves along the bed it encounters less combustible material and older coke deposits, with a lower hydrogen content. As a result the different aging periods before regeneration would be expected to have less influence upon the temperatures reached in deeper regions of the reactor. This can be observed in Figure 5, where, as the aging
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2569
E
reached during regeneration using cokes with a different degree of aging. These effects will probably be more important in less evolved cokes, which are likely to be more affected by aging processes. It has been shown that, when the regeneration of coked catalysts was carried out in fixed bed reactors after prolonged aging with an inert gas, lower maximum temperatures and a faster regeneration resulted. This means that a better use can be made of the purge period by carrying it out under optimal conditions and duration. These would be different for every catalytic system but in general will involve longer purge periods than those used in the current industrial practice.
1000-
Acknowledgment This work was carried out with financial support from DGICYT (Project No. PB-91-0697).
Literature Cited
0
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50
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90 1 0 0 110
time (min)
Figure 5. Evolution of temperature during regeneration at six axial positions in the reactor. The coking, aging and regeneration temperature was 803 K. Key: (A) coked catalyst aged for 3 min; (B) coked catalyst aged for 60 min; (C) coked catalyst aged for 180 min.
time is increased, the first peak becomes narrower and reaches a lower temperature than the rest of the bed, while the temperatures reached from the third peak onward are of comparable width and magnitude. Finally, Figure 5 also shows that the regeneration is faster as the aging time is increased. Thus, with a very short purge period (3 min), the regeneration requires about 110 min. The regeneration time reduces to about 78 min when the coke deposits are aged for long times. This is consistent with the reduction of the total coke content of the reactor (about 30%), which was mentioned above when the coke deposited at 803 K was aged for 180 min.
Conclusions The aging of coked catalysts has important effects upon the regeneration characteristics of the coke deposits. Although under the conditions investigated the aging in nitrogen does not cause a significant volatilization of the coke deposited directly on the catalyst particles, as the coke deposits age their chemical composition shifts to higher C/H values, leading to more aromatic and dehydrogenated structures, as shown by FTIR analysis. This results in a lower heat of combustion and in a somewhat lower reactivity of the coke deposits. Thus, the chances of a runaway reaction at the start of the regeneration process are correspondingly lower. The results presented in this paper have been obtained with coke deposited and aged at a relatively high temperature, which is sometimes known as hard coke. Even with this type of highly evolved coke, significant differences have been obtained upon the temperatures
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Abstract published in Advance ACS Abstracts, October 1, 1994. @
