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Non-Uniform Sintering in Oxyregeneration of Fixed Bed Catalytic Reactors
C.H. Bartholomew and J.B. Butt (Editors1, Catalyst Deactivation 1991 01991 Elsevier Science Publishers B.V., Amsterdam
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NON-UNIFORM SlNTERlNG IN OXYREGENERATION OF FIXED BED CATALYTIC REACTORS V.BLASCO1, C.ROYO1, M.MENENDEZ1, J.SANTAMARIA1 AND J.L.G. FIERROZ 1 Department of Chemical Engineering, University of Zaragoza, 50009 Zaragoza, Spain. Zlnstituto de Catalisis y Petroleoquimica, CSIC,28006 Madrid, Spain.
SUMMARY An experimental study of the possibility of non-uniform sintering during oxidative regeneration of fixed bed catalytic reactors has been carried out. Both kinetic measurements and XPS results show that a different degree of sintering can be expected in catalyst particles sampled at different reactor positions. INTRODUCTION Coke formation is a common cause of catalyst deactivation. Regeneration of coked catalyst can generally be achieved by gasification of the coke deposits with oxygen, carbon dioxide, steam or hydrogen. The first of these methods gives rise to oxidative regeneration processes with which this work is concerned. The main problem in oxidative regeneration is the possibility of high temperature rises caused by the exothermic oxidation of the coke, which can lead to sintering of the catalyst and/or damage to the reaction equipment. Thus, considerable effort has been devoted to the development of theoretical models capable of predicting the temperature rises attained during regeneration of catalytic reactors. A closely related research effort is concerned with studying the effect of the high temperatures reached during catalyst regeneration on the sintering of supported metal catalysts. Thus, many authors have investigated the loss of catalytic activity and the modification of the catalyst surface after one or more regenerations (e.9. refs 1-6). In general, oxidative regeneration of fixed bed catalytic reactors gives rise to the formation of a high-temperature front which travels along the bed, as it burns off the coke deposited on the catalyst. This is well documented in the literature, and it is a common industrial practice to use
630
the oxygen concentration in the feed as a means of controlling the maximum temperature reached during the course of the regeneration process. However, coke deposition does not usually occur uniformly in the reactor, which means that the amount of heat evolved in the combustion of the coke deposits may vary locally whithin the bed. The dynamics of propagation of the hot fronts generated also contribute to the heterogeneity of the thermal history of the catalyst particles in bed, and thus, those located near the exit region are usually exposed to comparatively higher temperatures (refs 7-9). It is therefore reasonable to expect a different activity loss by thermal degradation in catalyst particles located at different positions in the bed. In the present work, an experimental study has been carried out of the loss of catalytic activity at different positions in a fixed bed reactor, in order to assess the extent of the thermal degradation of the catalyst. EXPERIMENTAL The deactivation/regeneration experiments were carried out in a 78 cm long 4 cm i.d. fixed bed reactor provided with three separate electric heating systems with independent power inputs, in order to achieve a more uniform control of the temperature along the bed at the start of the regeneration. Temperature measurements during regeneration were made at regular intervals by means of 6 thermocouples positioned axially along the bed, using a computer-controlled data acquisition system. Coking was accomplished by passing a 50% butenehitrogen mixture over a commercial Cr203/A1203 catalyst (Harshaw) at 580 "C.After coking, the catalyst was regenerated using 02/N2 mixtures. The reactant flows to the reactor during the cokinglregeneration cycles were controlled automatically. At the end of a given number of cokinglregeneration cycles, catalyst pellets were sampled at selected reactor locations. The sampling points were always in the vicinity of a thermocouple, in order to know the thermal history of the particles sampled. These particles were then subjected to kinetic tests in order to determine their activity, and to XPS measurements, from which the catalyst surface area and the atomic ratios of the different elements on the catalyst surface were obtained. X-ray photoelectron spectra were recorded on a Leybold LHS 10 spectrometer equipped with a hemispherical electron analyzer and a M g K a (1253.6 eV) X-ray radiation source. The samples were mounted on a sample rod, placed in an introduction chamber and turbo-pumped at ca. 10-5 torr
before they were moved into the ion-pumped analysis chamber. The residual pressure during data acquisition was maintained below 7'1 0-9 torr. Each spectral region was signal-averaged for a number of scans to obtain good signal-to-noise ratios. Accurate binding energies (BE) were determined by charge referencing with the Si 2p peak at 103,4 eV.
RESULTS A typical coke distribution along the catalyst bed is shown in Figure 1. This was obtained by dividing the catalyst bed into 7 sections which were then individually mixed and analyzed separately using a themobalance. It can be seen that the coke profile in the bed is clearly non-uniform. Even greater differences among coking levels at different positions could be obtained by increasing the reaction time and/or temperature.
REACTOR LENGTH (cm) Figure 1. Coke distribution in the catalyst bed. Coking was accomplished by passing a 1:3 Butene/N2 mixture at 475" C over the catalyst for 5 hours. In addition to the coke distribution in the reactor the main factor influencing the temperature rises attained at a given position during regeneration is the oxygen input to the reactor. Thus, figure 2 shows the maximum temperatures reached at each axial position in the bed for two different operating conditions (series A and B respectively). It can be seen that considerable differences can be expected for different regenerations and also for different positions in the bed during the course of a regeneration. However,the extent of thermal deactivation is also likely to be influenced not only by the maxirnun temperature reached, but also by the
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REACTOR LENGTH (cm) Figure 2. Maximum temperatures reached at different axial positions in the bed under different experimental conditions. Series A: Experiments carried out with a total gas flow rate of 2 Vmin and 6% oxygen concentration in the feed. Series B: 3 I/min, 10% oxygen concentracion. The catalyst bed used in series A was slightly longer. time-temperature history of the catalyst. Thus, in figure 3 the temperatures recorded at different axial positions in the reactor are represented as a function of time for a series of cokinghegeneration cycles carried out under the experimental conditions corresponding to series A. Different temperature zones can be observed in these diagrams corresponding to the coking and regeneration stages, with their associated preheating and cooling periods. The comparison of the time-temperature plots for different reactor positions during the same coking/regeneration series shows that the temperature peaks corresponding to the different regenerations occur at later times as we move towars the reactor exit. This is due to the time interval required for the displacement of the hightemperature reaction zone from one position of the reactor to another. It can also be seen that under mild regeneration conditions such as those of series A only small temperature differences were obtained near the reactor inlet, while considerable temperature rises were reached at other positions in the bed. Under more severe regeneration conditions (series B ) the temperature rises near the reactor inlet increased notably. Given the different thermal history of different regions of the catalyst bed, varying degrees of thermal deactivation could also be expected at different locations. To measure the extent of thermal deactivation, kinetic tests were perfomed under the same experimental conditions with fresh
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Figure 3. Temperature-time history of particles sampled at different positions in the bed. 7 coking/regeneration cycles. The regenerations were carried out with 6% oxygen concentration in the ‘ ~ x dgas and a total gas flow rate of 2 I/rnin. catalyst and with deactivated catalyst from different positions in the bed and/or regeneration runs. The results are shown in figure 4. It can be seen that the fresh catalyst loses activity rapidly as it becomes coked. After 7 regenerations with a 6% oxygen concentration in the feed to the reactor (figure 4A) the catalyst particles sampled at different reactor positions show clearly different initial activities . Their deactivation rate is also lower than that of the fresh catalyst. The results in figure 4B, obtained under more severe reaction conditions (series B), show considerably lower initial reaction rates for particles sampled at a given reactor position compared to the corresponding values in figure 4A. Also, the
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Figure 4. Results of the kinetics tests performed on fresh catalyst and on catalyst samples subjected to different regeneration treatments. Series A: Experiments carried out with a total gas flow rate of 2 I/rnin and 6% oxygen concentration in the feed gas. Series B: 3 I/rnin, 10% oxygen concentracion. Key: + : Fresh catalyst. u : Catalyst from position 2 (reactor entrance). I Catalyst from position 7 (reactor exit). difference in the initial reaction rate between particles sampled at the entrance and at the exit regions of the reactor is considerably larger than in the previous case.
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The results from catalyst characterization measurements are given in Table I . It can be seen that the atomic surface ratio of Cr and Al, which can be used as an indicator of the Chromium dispersion, after a few regenerations is considerably diferent from that of the fresh catalyst. This is especially true of the series B catalyst, subjected to more severe regeneration conditions. In the series B samples the Cr/AI ratios measured are different for catalyst samples from the entrance and from the exit regions of the reactor. The Na/AI surface ratio also changes, as can be seen from the results of series A, in which there is a considerable difference between the Na/AI ratios at different positions in the bed, as well as with respect to those of the fresh catalyst.
TABLE 1. CATALYST CHARACTERIZATION RESULTS1 : XPS ANALYSIS. Atomic ratio SAMPLE Fresh A-7-2 A-7-7 8-5-2 8-5-7 8-3-7
O/Al
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2.080
Na/AI 0.1 01 0.036 0.01 8
Cr/AI 0.167 0.106 0.105 0.083 0.074 0.073
1 The notation i-ii-iii indicates: i) the type of cokinghegeneration series perfomed (A= 6 % oxygen concentration, 2 Umin; B=lO% oxygen concentration, 3 Vmin), ii) the number of cokinghegeneration cycles and iii) the reactor position (l=reactor entrance, 7= reactor exit )
CONCLUSIONS During regeneration of coked fixed bed catalytic reactors different regions of the bed are subjected to widely different time-temperature environments. This is partly a consecuence of the dynamics of propagation of the high-temperature fronts generated during coke combustion, although the effect is enhanced by the existence of varying conditions (such as coke distribution) along the bed. As a consequence of the different thermal history, the catalyst particles sampled from different reactor locations exhibit various degrees of thermal deactivation. Non-uniform deactivation was confirmed from kinetic measurements, and XPS data from series B experiments also
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indicate different degrees of chromium dispersion in the catalyst bed. The results of this study suggest that, when the catalyst of a reactor is discarded because a high degree of deactivation has been reached on average for the whole reactor (which is usually quantified by a given drop in conversion at the reactor exit), large regions of the bed may still retain considerable catalytic activity . ACKNOWLEDGEMENTS Financial support from DGICYT, Spain (Project PB87-631) for this work is gratefully acknowledged. REFERENCES 1 Z.M. George, P. Mohammed and R. Tower, Regeneration of a spent hydroprocessing catalyst, Proc. 9th Int. Congress on Catalysis, Calgary, Canada, 1988. 2 H.W. Pennline and S.S. Pollack, Deactivation and regeneration of a promoted transition-metal zeolite catalyst, Ind. Eng. Chem. Prod. Res. Dev. 25,(1986),11-14. 3 J.M. Bogdanor and H.F. Rase, Characteristics of a commercially aged NiM o l A 1 2 0 3 hydrotreating catalyst: Component distribution, coke characteristics and effects of regeneration, Ind. Eng. Chem. Prod. Res. Dev. 25, (1986), 220-230. 4 A. Arteaga, J.L.G. Fierro, P. Grange and B. Delmon, CoMo HDS catalysts: simulated deactivation and regeneration. Role of various regeneration parameters, in B. Delmon and G.F. Froment (Eds.), Catalyst Deactivation 1987, Elsevier Amsterdam, 1987,pp 59-80. 5 A.Arteaga, J.L.G. Fierro, P. Grange and B. Delrnon, Simulated regeneration of an industrial CoMoly-Al2Og catalyst. Influence of the regeneration temperature. Applied Catalysis 34, (1987), 89-1 07. 6 A. Arteaga, J.L.G. Fierro, F. Delannay and B. Delmon, Simulated deactivation and regeneration of an industrial CoMoly-A120 3 hydrodesulphurization catalyst, Applied Catalysis 26, (1 986), 227249. 7 A. Byrne, R. Hughes, J. Santamaria, The influence of the initial coke profile and hydrogen content of the coke on the regeneration of fixed beds of catalyst, Chem.Eng.Sci., 40,(1985),1507-16. 8 A. Byrne, R. Hughes, J. Santamaria, Effect of deposited coke profiles on transient temperatures during regeneration of a fixed bed catalytic reactor, C hem.Eng. Sci.44,(1986),153-62 9 J. Santamaria, A. Monzon, M. Berbegal, R. Hughes, Regeneration strategies for coked fixed bed reactors, Chem. Eng. Sci. in press
629
NON-UNIFORM SlNTERlNG IN OXYREGENERATION OF FIXED BED CATALYTIC REACTORS V.BLASCO1, C.ROYO1, M.MENENDEZ1, J.SANTAMARIA1 AND J.L.G. FIERROZ 1 Department of Chemical Engineering, University of Zaragoza, 50009 Zaragoza, Spain. Zlnstituto de Catalisis y Petroleoquimica, CSIC,28006 Madrid, Spain.
SUMMARY An experimental study of the possibility of non-uniform sintering during oxidative regeneration of fixed bed catalytic reactors has been carried out. Both kinetic measurements and XPS results show that a different degree of sintering can be expected in catalyst particles sampled at different reactor positions. INTRODUCTION Coke formation is a common cause of catalyst deactivation. Regeneration of coked catalyst can generally be achieved by gasification of the coke deposits with oxygen, carbon dioxide, steam or hydrogen. The first of these methods gives rise to oxidative regeneration processes with which this work is concerned. The main problem in oxidative regeneration is the possibility of high temperature rises caused by the exothermic oxidation of the coke, which can lead to sintering of the catalyst and/or damage to the reaction equipment. Thus, considerable effort has been devoted to the development of theoretical models capable of predicting the temperature rises attained during regeneration of catalytic reactors. A closely related research effort is concerned with studying the effect of the high temperatures reached during catalyst regeneration on the sintering of supported metal catalysts. Thus, many authors have investigated the loss of catalytic activity and the modification of the catalyst surface after one or more regenerations (e.9. refs 1-6). In general, oxidative regeneration of fixed bed catalytic reactors gives rise to the formation of a high-temperature front which travels along the bed, as it burns off the coke deposited on the catalyst. This is well documented in the literature, and it is a common industrial practice to use
630
the oxygen concentration in the feed as a means of controlling the maximum temperature reached during the course of the regeneration process. However, coke deposition does not usually occur uniformly in the reactor, which means that the amount of heat evolved in the combustion of the coke deposits may vary locally whithin the bed. The dynamics of propagation of the hot fronts generated also contribute to the heterogeneity of the thermal history of the catalyst particles in bed, and thus, those located near the exit region are usually exposed to comparatively higher temperatures (refs 7-9). It is therefore reasonable to expect a different activity loss by thermal degradation in catalyst particles located at different positions in the bed. In the present work, an experimental study has been carried out of the loss of catalytic activity at different positions in a fixed bed reactor, in order to assess the extent of the thermal degradation of the catalyst. EXPERIMENTAL The deactivation/regeneration experiments were carried out in a 78 cm long 4 cm i.d. fixed bed reactor provided with three separate electric heating systems with independent power inputs, in order to achieve a more uniform control of the temperature along the bed at the start of the regeneration. Temperature measurements during regeneration were made at regular intervals by means of 6 thermocouples positioned axially along the bed, using a computer-controlled data acquisition system. Coking was accomplished by passing a 50% butenehitrogen mixture over a commercial Cr203/A1203 catalyst (Harshaw) at 580 "C.After coking, the catalyst was regenerated using 02/N2 mixtures. The reactant flows to the reactor during the cokinglregeneration cycles were controlled automatically. At the end of a given number of cokinglregeneration cycles, catalyst pellets were sampled at selected reactor locations. The sampling points were always in the vicinity of a thermocouple, in order to know the thermal history of the particles sampled. These particles were then subjected to kinetic tests in order to determine their activity, and to XPS measurements, from which the catalyst surface area and the atomic ratios of the different elements on the catalyst surface were obtained. X-ray photoelectron spectra were recorded on a Leybold LHS 10 spectrometer equipped with a hemispherical electron analyzer and a M g K a (1253.6 eV) X-ray radiation source. The samples were mounted on a sample rod, placed in an introduction chamber and turbo-pumped at ca. 10-5 torr
before they were moved into the ion-pumped analysis chamber. The residual pressure during data acquisition was maintained below 7'1 0-9 torr. Each spectral region was signal-averaged for a number of scans to obtain good signal-to-noise ratios. Accurate binding energies (BE) were determined by charge referencing with the Si 2p peak at 103,4 eV.
RESULTS A typical coke distribution along the catalyst bed is shown in Figure 1. This was obtained by dividing the catalyst bed into 7 sections which were then individually mixed and analyzed separately using a themobalance. It can be seen that the coke profile in the bed is clearly non-uniform. Even greater differences among coking levels at different positions could be obtained by increasing the reaction time and/or temperature.
REACTOR LENGTH (cm) Figure 1. Coke distribution in the catalyst bed. Coking was accomplished by passing a 1:3 Butene/N2 mixture at 475" C over the catalyst for 5 hours. In addition to the coke distribution in the reactor the main factor influencing the temperature rises attained at a given position during regeneration is the oxygen input to the reactor. Thus, figure 2 shows the maximum temperatures reached at each axial position in the bed for two different operating conditions (series A and B respectively). It can be seen that considerable differences can be expected for different regenerations and also for different positions in the bed during the course of a regeneration. However,the extent of thermal deactivation is also likely to be influenced not only by the maxirnun temperature reached, but also by the
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REACTOR LENGTH (cm) Figure 2. Maximum temperatures reached at different axial positions in the bed under different experimental conditions. Series A: Experiments carried out with a total gas flow rate of 2 Vmin and 6% oxygen concentration in the feed. Series B: 3 I/min, 10% oxygen concentracion. The catalyst bed used in series A was slightly longer. time-temperature history of the catalyst. Thus, in figure 3 the temperatures recorded at different axial positions in the reactor are represented as a function of time for a series of cokinghegeneration cycles carried out under the experimental conditions corresponding to series A. Different temperature zones can be observed in these diagrams corresponding to the coking and regeneration stages, with their associated preheating and cooling periods. The comparison of the time-temperature plots for different reactor positions during the same coking/regeneration series shows that the temperature peaks corresponding to the different regenerations occur at later times as we move towars the reactor exit. This is due to the time interval required for the displacement of the hightemperature reaction zone from one position of the reactor to another. It can also be seen that under mild regeneration conditions such as those of series A only small temperature differences were obtained near the reactor inlet, while considerable temperature rises were reached at other positions in the bed. Under more severe regeneration conditions (series B ) the temperature rises near the reactor inlet increased notably. Given the different thermal history of different regions of the catalyst bed, varying degrees of thermal deactivation could also be expected at different locations. To measure the extent of thermal deactivation, kinetic tests were perfomed under the same experimental conditions with fresh
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Figure 3. Temperature-time history of particles sampled at different positions in the bed. 7 coking/regeneration cycles. The regenerations were carried out with 6% oxygen concentration in the ‘ ~ x dgas and a total gas flow rate of 2 I/rnin. catalyst and with deactivated catalyst from different positions in the bed and/or regeneration runs. The results are shown in figure 4. It can be seen that the fresh catalyst loses activity rapidly as it becomes coked. After 7 regenerations with a 6% oxygen concentration in the feed to the reactor (figure 4A) the catalyst particles sampled at different reactor positions show clearly different initial activities . Their deactivation rate is also lower than that of the fresh catalyst. The results in figure 4B, obtained under more severe reaction conditions (series B), show considerably lower initial reaction rates for particles sampled at a given reactor position compared to the corresponding values in figure 4A. Also, the
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Figure 4. Results of the kinetics tests performed on fresh catalyst and on catalyst samples subjected to different regeneration treatments. Series A: Experiments carried out with a total gas flow rate of 2 I/rnin and 6% oxygen concentration in the feed gas. Series B: 3 I/rnin, 10% oxygen concentracion. Key: + : Fresh catalyst. u : Catalyst from position 2 (reactor entrance). I Catalyst from position 7 (reactor exit). difference in the initial reaction rate between particles sampled at the entrance and at the exit regions of the reactor is considerably larger than in the previous case.
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The results from catalyst characterization measurements are given in Table I . It can be seen that the atomic surface ratio of Cr and Al, which can be used as an indicator of the Chromium dispersion, after a few regenerations is considerably diferent from that of the fresh catalyst. This is especially true of the series B catalyst, subjected to more severe regeneration conditions. In the series B samples the Cr/AI ratios measured are different for catalyst samples from the entrance and from the exit regions of the reactor. The Na/AI surface ratio also changes, as can be seen from the results of series A, in which there is a considerable difference between the Na/AI ratios at different positions in the bed, as well as with respect to those of the fresh catalyst.
TABLE 1. CATALYST CHARACTERIZATION RESULTS1 : XPS ANALYSIS. Atomic ratio SAMPLE Fresh A-7-2 A-7-7 8-5-2 8-5-7 8-3-7
O/Al
-
2.080
Na/AI 0.1 01 0.036 0.01 8
Cr/AI 0.167 0.106 0.105 0.083 0.074 0.073
1 The notation i-ii-iii indicates: i) the type of cokinghegeneration series perfomed (A= 6 % oxygen concentration, 2 Umin; B=lO% oxygen concentration, 3 Vmin), ii) the number of cokinghegeneration cycles and iii) the reactor position (l=reactor entrance, 7= reactor exit )
CONCLUSIONS During regeneration of coked fixed bed catalytic reactors different regions of the bed are subjected to widely different time-temperature environments. This is partly a consecuence of the dynamics of propagation of the high-temperature fronts generated during coke combustion, although the effect is enhanced by the existence of varying conditions (such as coke distribution) along the bed. As a consequence of the different thermal history, the catalyst particles sampled from different reactor locations exhibit various degrees of thermal deactivation. Non-uniform deactivation was confirmed from kinetic measurements, and XPS data from series B experiments also
636
indicate different degrees of chromium dispersion in the catalyst bed. The results of this study suggest that, when the catalyst of a reactor is discarded because a high degree of deactivation has been reached on average for the whole reactor (which is usually quantified by a given drop in conversion at the reactor exit), large regions of the bed may still retain considerable catalytic activity . ACKNOWLEDGEMENTS Financial support from DGICYT, Spain (Project PB87-631) for this work is gratefully acknowledged. REFERENCES 1 Z.M. George, P. Mohammed and R. Tower, Regeneration of a spent hydroprocessing catalyst, Proc. 9th Int. Congress on Catalysis, Calgary, Canada, 1988. 2 H.W. Pennline and S.S. Pollack, Deactivation and regeneration of a promoted transition-metal zeolite catalyst, Ind. Eng. Chem. Prod. Res. Dev. 25,(1986),11-14. 3 J.M. Bogdanor and H.F. Rase, Characteristics of a commercially aged NiM o l A 1 2 0 3 hydrotreating catalyst: Component distribution, coke characteristics and effects of regeneration, Ind. Eng. Chem. Prod. Res. Dev. 25, (1986), 220-230. 4 A. Arteaga, J.L.G. Fierro, P. Grange and B. Delmon, CoMo HDS catalysts: simulated deactivation and regeneration. Role of various regeneration parameters, in B. Delmon and G.F. Froment (Eds.), Catalyst Deactivation 1987, Elsevier Amsterdam, 1987,pp 59-80. 5 A.Arteaga, J.L.G. Fierro, P. Grange and B. Delrnon, Simulated regeneration of an industrial CoMoly-Al2Og catalyst. Influence of the regeneration temperature. Applied Catalysis 34, (1987), 89-1 07. 6 A. Arteaga, J.L.G. Fierro, F. Delannay and B. Delmon, Simulated deactivation and regeneration of an industrial CoMoly-A120 3 hydrodesulphurization catalyst, Applied Catalysis 26, (1 986), 227249. 7 A. Byrne, R. Hughes, J. Santamaria, The influence of the initial coke profile and hydrogen content of the coke on the regeneration of fixed beds of catalyst, Chem.Eng.Sci., 40,(1985),1507-16. 8 A. Byrne, R. Hughes, J. Santamaria, Effect of deposited coke profiles on transient temperatures during regeneration of a fixed bed catalytic reactor, C hem.Eng. Sci.44,(1986),153-62 9 J. Santamaria, A. Monzon, M. Berbegal, R. Hughes, Regeneration strategies for coked fixed bed reactors, Chem. Eng. Sci. in press