Dynamics of Transversal Hot Zones in a Shallow Packed Bed Reactor

Feb 7, 2007 - Plenge, F.; Rodin, P.; Scholl, E.; Krischer, K. Breathing current domains in globally coupled electrochemical systems; a comparison with...
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Ind. Eng. Chem. Res. 2007, 46, 1485-1491

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Dynamics of Transversal Hot Zones in a Shallow Packed Bed Reactor during Oxidation of Mixtures of C3H6 and CO Sandhya Sundarram and Dan Luss* Department of Chemical and Biomolecular Engineering, UniVersity of Houston, Houston, Texas 77204

The evolution and motions of transversal hot zones on the top of a shallow packed bed reactor, in which the atmospheric oxidation of either propylene or its mixture with CO took place, were studied using infrared thermography. The hot regions were separated by a sharp temperature front from the adjacent colder region (∆T ∼ 50 °C). The period of the oscillations of mixtures of propylene and carbon monoxide was about 20 times shorter than those of CO and about 2 times shorter than those of propylene. This indicates that the frequency of the hot zone motions is affected mainly by the kinetics of the catalytic reaction and the strength of adsorption of the organic reactants and not by the properties of the bed and/or the flow through it. The mixture of the two reactants led to the formation of a moving hot spot over a much wider range than that of either reaction, and under operating conditions for which neither one of the two reactions led to formation of hot regions. The experiments seem to confirm the prediction by Viswanathan and Luss (Viswanathan, G. A.; Luss, D. AIChE. J. 2006, 52, 705-717), that transversal hot zones are likely to form in a shallow packed bed reactor for reactions whose rates may exhibit oscillatory behavior. Introduction and Background Local hot zones have been observed in both laboratory and industrial packed bed reactors. Among others, Boreskov et al.2 observed several hot regions in the bottom (exit) of a packed bed reactor during the partial oxidation of isobutyl alcohol. Barkelew and Gambhir3 reported clinker formationssmall lumps of molten catalystsduring hydrodesulfurization in trickle bed reactors. Jaffe4 reported a hot zone formation in a hydrogenation reactor. Moving hot zones were observed on Pt-Rh gauze during HCN synthesis and ammonia oxidation. Wicke and Onken5 reported that a nonuniform temperature existed in a cross section of a laboratory packed bed reactor during CO oxidation. We observed hot zone formation in a shallow laboratory packed bed reactor during the oxidation of CO.6,7 Similar hot zones were observed by Sheintuch’s research group on a catalytic glass fiber cloth.8 A hot zone present next to the reactor walls may lead to a safety hazard as it can decrease the mechanical strength of the wall and generate a crack. The subsequent release of reactants may lead to an explosion. It is very difficult to detect small hot spots in a large-diameter industrial reactor. Hence it is important to be able to predict what may lead to formation of these hot zones in order to be able to devise control and operation procedures that circumvent their formation. We have previously observed various types of hot spots on top of a shallow packed bed reactor during CO oxidation.6,7,9,10 The hot spots exhibited several qualitatively different dynamic motions, such as rotation, antiphase motion, and breathing (expansion and contraction) as well as a complex juxtaposition of these motions. Sundarram et al.11 noted that global coupling between the effluents on the top of the reactor and the catalyst affected the dynamic features of the hot zones. The motions observed during CO oxidation were surprisingly slow, with a rotation lasting several hours in a 10 cm i.d. reactor. The characteristic time of this rotation was much longer than either that of the reaction or that of the heat transfer in the reactor. * To whom correspondence should be addressed. E-mail: dluss@ uh.edu.

Viswanathan and Luss1 have shown that transversal hot zones are likely to form in a shallow packed bed reactor for reactions having an oscillatory rate. That analysis predicts that the reaction kinetics and not the transport rate processes in the reactor have a dominant impact on whether hot spots form and on their dynamics. Experimental data verifying this prediction do not yet exist. One goal of our study was to check this prediction by determining the dynamics of the hot zones formed when different reactions are conducted in the same reactor with the same catalyst under the same time-averaged, surface-average temperature. In this way, the impact of the reaction kinetics becomes evident. Various studies showed that the oxidation of olefins on Pt can lead to oscillatory behavior.12,13 Thus, we studied the formation and dynamic features of the hot zones in a shallow packed bed reactor during the oxidation of either CO or propylene or mixtures of CO and propylene. Both theoretical14-16 and experimental17,18 studies revealed that the multiplicity and dynamic features of reaction mixtures are much more intricate than those of the individual reactants. We report here the evolution and dynamic features of hot zones in a shallow packed bed reactor in which two reactions are conducted simultaneously. Experimental System and Procedure We used the shallow packed bed reactor previously used to investigate the evolution and dynamics of hot zones during CO oxidation.6,7 The reactor was a cylindrical stainless steel vessel (SS 316, 125 mm o.d., 5 mm wall thickness, 286 mm long). The shallow packed bed consisted of a single layer of spherical catalyst pellets (0.5 wt % Pd deposited as a thin exterior shell on alumina). The pellets were uniformly arranged on a stainless steel wire mesh that was supported on three stainless steel pins (placed 120° apart around the inner wall, 51 cm from the top of the reactor). A schematic of the experimental system is shown in Figure 1. To minimize the heat loss, an annular ring of alumina/silica wool was placed between the catalyst and the reactor wall. Experiments showed that the ignited state was uniformly hot, indicating that the heat loss at the walls was rather small. Previous experiments in this reactor have shown that operation without the insulating layer leads to premature

10.1021/ie060597c CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007

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Experimental Results

Figure 1. Schematic of the experimental setup used to investigate formation and dynamics of hot zones.

extinction of the bed. The vessel was thermally insulated and heated using electrical heaters. The outer wall temperature was measured using thermocouples (J type) and regulated using a temperature controller (PID, Omega CN2041). Three additional thermocouples were placed inside the reactor. One below the wire mesh support measured the inlet gas temperature, one above the catalyst bed measured the effluent temperature, and one was attached to a single pellet. The reactive mixture was fed through five inlet ports (one at the bottom center and four from the sides of the bottom of the vessel) and exited through four outlet ports (at the top, staggered at 45° with respect to the bottom inlet ports). The bottom of the vessel was packed with glass beads topped with three inert ceramic monolith disks (4 in. o.d. × 0.875 in., 15 ppi, AL 99, Vesuvius Hi-Tech Ceramics) to improve the uniformity of the feed distribution. The feed during the propylene oxidation experiments consisted of 1.5% CP propylene (purity, 99%), 70% extra-dry oxygen (purity 99.6%), and 28.5% prepurified nitrogen (purity, 99.998%). The feed composition during the experiments with a mixture of two reactants was 1.5% propylene, 6% CP carbon monoxide (purity, 99.5%), 70% oxygen, and 22.5% nitrogen. The feed reactants were mixed in a bed of glass beads, purified, and dried by activated charcoal purifiers (Linde) before entering the vessel. Nupro 7-µm particulate filters were attached in the flow line immediately after the pressurized supply cylinder to remove any particulates in the stream. All tubing was stainless steel (0.25 in. o.d.;1/3500 in. wall thickness). The top of the stainless steel vessel was an infrared transparent quartz window. The temperature distribution on the top surface of the pellet layer was measured by an IR camera (Amber Raytheon PM). The radiation from the top was reflected by a gold-plated mirror placed at 45° (above the vessel) to the camera. The camera has a 256 × 256 indium antimonide detector array sensitive to 3-5 µm radiation. The measured temperatures were recorded on a PC using ImageDesk II (software, Amber Raytheon). Calibration was done to correlate the intensity recorded by the camera (counts) to the actual temperature. The effluent concentration was measured by a mass spectrometer (OMNISTAR, GSD 300O) at a sampling frequency of 0.1 Hz. After each temperature change, the system was allowed to remain at the specified temperature for a couple of hours to attain a steady state. Due to the slow motion of the hot zones the IR images were recorded at a frequency of 1-4 min-1.

The dynamic features of hot zone formation during CO oxidation have been already studied previously by us.6,7,9,10 We initially conducted experiments to determine the dynamic features of hot zones formed during propylene oxidation. We selected this test reaction because its ignition temperature is lower than 300 °C, which is the maximum temperature attainable in the reactor, and it ignited for a feed concentration lower than the explosive limit. Following the single reactant oxidation experiments, we conducted those involving the oxidation of propylene and CO mixtures. Propylene Oxidation. The reactant mixture during the propylene oxidation experiments contained 1.5 vol % C3H6, 28.5 vol % N2, and 70 vol % O2. The total flow rate was between 1000 and 1600 cm3/min. Following slow heating of the reactor vessel from room temperature, ignition occurred at 275 °C. The catalyst layer temperature of the ignited state was uniform, and the conversion was high (93%). As the temperature of the reactor vessel was slowly decreased, the bed initially cooled uniformly. The effluent concentration was constant when the reactor attained either a uniform ignited state or a stationary hot zone. Changes in the size or motion of a hot zone led to variations in the effluent concentrations. Hot zone motion was observed for propylene oxidation for two (total) flow rates of 1200 and 1000 cm3/min. Propylene oxidation at the high flow rate (1200 cm3/min) led to the formation of a hot zone motion that we refer to as a pulse, which was not encountered during the CO oxidation experiments.6,7 It consisted of a hot zone that periodically formed from a uniformly extinguished state, grew in size, and then contracted until the bed was again uniformly quenched. In a given experiment the periodic hot zone always emerged at the same position. However, the hot zone often emerged at another location following a change in the reactor operating conditions (such as vessel temperature or feed concentration) and a return to the original operating conditions. Pulse behavior occurred for vessel temperatures between 220 and 175 °C. Two qualitatively different types of pulses were observed. During a breathing pulse, a hot zone evolved from a uniformly cold bed, expanded, reached a maximum size, and then contracted and vanished. Snapshots of a breathing pulse, observed at a reactor temperature of 200 °C, are shown in Figure 2b. The typical period for this behavior was 90 min. As the vessel temperature was lowered to 175 °C, a new type of motion, a rotating pulse, formed. Here, a hot zone emerged in one part of the bed, rotated (less than 360°) around the bed, and then extinguished. Snapshots of a rotating-pulse motion are shown in Figure 2c. A typical period of this behavior was 60 min. An interesting long transient rotating hot zone was observed at the same flow rate following rapid cooling (order of minutes) of the reactor vessel from 250 to 200 °C. The hot zone evolved from a uniform ignited state and rotated around the bed at an almost constant angular velocity of about 9.8 deg/min. This angular velocity was almost an order of magnitude faster than the 1.72 deg/min observed during CO oxidation.6,7 Snapshots of this rotating hot zone are shown in Figure 2a. The hot zone changed its direction of rotation after every three to four cycles. After about 16 complete rotations, the motion became a breathing pulse (Figure 2b), described in the previous paragraph. At the lower flow rate of 1000 cm3/min breathing patterns, similar to those formed during CO oxidation,6,7 were observed. As the reactor temperature was reduced, the frequency of the breathing motion decreased. The plots of angular position with time at a fixed radial position on the catalyst bed of such a

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Figure 4. Range of vessel temperatures for which nonuniform dynamic states are observed as a function of total flow rate. (a) CO, C3H6 oxidation; (b) oxidation of mixtures.

Figure 2. Snapshots of temperature distribution on top of the bed. Flow rate 1200 cm3/min; 1.5% C3H6. (a) Rotating hot zone; (b) breathing pulse, Tv ) 200 °C; (c) rotating-pulse motion, Tv ) 175 °C.

Figure 3. Plot of angular position vs time, depicting the decrease in the frequency of the oscillations as the reactor vessel is cooled during C3H6 oxidation.

case are shown in Figure 3. Further cooling of the reactor vessel led to extinction at a reactor temperature of 170 °C. Stationary and moving hot zones existed between 220 and 175 °C for a flow rate of 1200 cm3/min and between 220 and 170 °C for a flow rate of 1000 cm3/min (Figure 4a). The decrease in the total feed flow rate (1200 f 1000 cm3/min) increased the range of vessel temperatures for which hot zones were observed. This impact of the velocity is the opposite of that observed during CO oxidation. In that case, decreasing the total flow rate decreased the range of temperatures for which oscillations occurred (Figure 4a). The same impact of velocity was observed (during CO oxidation) as the number of catalytic pellet layers was increased from one to three.

Oxidation of Mixtures of CO and C3H6. The oxidation of mixtures of CO and C3H6 were conducted in the shallow packed bed to determine whether the interaction between the two reactions would introduce new behavioral features different from those exhibited by a moving hot zone when only one reaction was carried out. During these experiments, initially a single reactant was fed to the reactor, either 6 vol % CO or 1.5 vol % C3H6. The reactor temperature was then slowly increased until ignition occurred (at 160 °C for CO or 275 °C for C3H6). After a uniform high temperature state was attained, the reactor vessel temperature was slowly decreased and the second reactant was introduced into the feed. The feed composition of the mixture was 6% CO, 1.5% C3H6, 22.5% N2, and 70% O2 (by volume) in all these experiments. Three qualitatively different types of hot zone motions, similar to those observed during CO oxidation,6,7 were observed during the oxidation of the feed containing both CO and C3H6. In none of these experiments the dynamic behavior was similar to the pulse motion observed during the C3H6 oxidation (Figure 2b,c). At vessel temperatures below 75 °C the hot zone attained a breathing motion (periodic contraction and expansion). During this motion, the hot zone length expands and contracts as its breadth contracts and expands. This breathing motion is more complex than the one previously reported with only one front moving back and forth. Snapshots of this behavior are shown in Figure 5a. The second type of motion is antiphase, in which the hot zone oscillated between two diametrically opposite locations on the bed. The two opposite states were symmetric with respect to a rotation by π. This antiphase motion is similar to a classical standing wave motion.19 The period of this motion was much shorter than that observed during CO oxidation. For example, at a vessel temperature of 120 °C, the 35 min period was about 0.25 of that observed during CO oxidation.6,7 Some snapshots of this motion are shown in Figure 5b. The third motion was rotation. Snapshots of a rotation motion that emerged from a nonuniform stationary state when 1.75% C3H6 was added to the feed containing CO, O2, and N2 at a flow rate of 1200 cm3/min and vessel temperature of 120 °C are shown in Figure 5c. Both clockwise and counterclockwise rotations were observed at this temperature. A rotating, breathing hot zone was the primary motion observed at a flow rate of 1400 cm3/min at all vessel temperatures between 110 and 80 °C. The three dynamic behaviors observed during the oxidation

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Figure 5. Snapshots of hot zone motions observed during oxidation of a mixture containing CO and C3H6. (a) Breathing, (b) antiphase, (c) rotation; total flow rates 1200-1000 cm3/min.

of the mixture remained unchanged for long periods of time (up to 16 h). The surface temperature has a large impact on the period of the dynamics of the oscillations. Thus, we determined the hot zone dynamics in the same reactor for feed containing different reactants under conditions of equal time-averaged, surfaceaverage temperature 〈T〉 defined as

1

〈T〉 ) tpA

∫t ∫A T da dt

(1)

p

where tp is the period of the motion and A is the surface area. Figure 6 describes experiments conducted at a 〈T〉 of 130 °C. When the feed contained a mixture of both CO and propylene, the period of the spatiotemporal motions was much shorter than when it contained only one of these reactants. The angular positions vs time (at r ) 0.8 R, where R is the bed radius) of the rotation motion during the oxidation of CO and a mixture of CO and C3H6 are shown in parts a and c, respectively, of Figure 6. During the oxidation of CO, the hot zone after completing a rotation around the bed remained stationary for a while at a particular location before starting the next rotation (Figure 6a). The period of this cyclic motion was about 9 h. The corresponding temporal surface-average temperature is shown in Figure 6b. When using as a feed a mixture of 6 vol % CO and 1.5 vol % C3H6, a back-and-forth rotation motion was observed at the same 〈T〉 with a time period of only 40 min (Figure 6b). The hot zone did not remain stationary at any point of the bed. The figure clearly shows a large difference in the frequency and features of the oscillations of the surfaceaverage temperature of these two reactant feeds. The dynamic features (frequency and amplitude) of the temporal conversion were similar to those of the surface-average temperature. Additional experiments (that have not been reported here) indicated that, during the oxidation of mixtures, an increase in propylene and decrease in CO while maintaining the operating conditions, as well as 〈T〉 constant, decreased the time period of observed dynamic motions. The 〈T〉 ranges for which steady-state multiplicity and hot zones formed during the oxidation of CO, propylene, or a mixture of CO and propylene are shown in Figure 7. The arrow tips denote the ignition and extinction points. The solid lines

Figure 6. Comparison of time period of spatiotemporal hot zone motion at an average catalyst surface temperature of 130 °C. (a) Angular position vs time plot depicting complete rotation during CO oxidation and (b) corresponding average surface temperature. (c) Angular position vs time plot depicting back-and-forth rotation during CO + C3H6 oxidation and (d) corresponding average surface temperature. Angular position vs time plots were drawn at a radial position ) 0.8 R.

Figure 7. Region of steady-state multiplicity and of hot zone formation dependence on the time-averaged, surface-average temperature 〈T〉, during oxidation of CO (6%), propylene (1.5%), or mixtures of CO (1%) and propylene (1%).

denote uniform temperature states, while the dotted lines denote states with hot zones. For the oxidation of 6 vol % CO ignition occurred at a vessel temperature of about 160 °C with a 〈T〉 ) 203 °C, while extinction occurred at 〈T〉 ) 120 °C. The state with hot zones existed for 〈T〉 between 120 and 154 °C. During

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the oxidation of propylene, ignition occurred at 〈T〉 of about 300 °C and nonuniform states existed for 〈T〉 between 180 and 257 °C. While CO had a uniform ignited state for 〈T〉 > 203 °C, the propylene oxidation led to spatiotemporal hot zone motion under some of these temperatures. During the oxidation of a mixture of CO and propylene, moving hot zones existed for 〈T〉 between 88 and 203 °C. The data show that the range of 〈T〉 for which moving hot zones existed was much larger than those for either one of the single reactant. The interaction between the two reactants enabled a hot zone to exist for 〈T〉 values in the range of 154-180 °C for which a nonuniform temperature did not exist for either of the two reactions. An increase in the total feed flow rate decreased the range of the temperatures for which periodic hot zone motion was observed. It was 55-185 °C for a total flow rate of 1200 cm3/ min and only 70-85 °C for 1600 cm3/min. This effect, shown in Figure 4b, is similar to that observed for C3H6 oxidation. In contrast, during CO oxidation the range of vessel temperatures for which hot zone motion existed increased from 115-120 °C to 80-120 °C as the total flow rate was increased from 1200 to 1600 cm3/min. Discussion of Results The experiments show that similar to CO oxidation6,7,9,10 the oxidation of propylene can lead to moving hot zones on a shallow packed bed reactor. The breathing pulse observed by us is similar to breathing motion observed during CO oxidation on a catalytic glass cloth by Nekhamkina et al.20 Their qualitative model to describe the observed motions included an oscillatory rate expression and accounted for global coupling between the catalyst and effluents. Sheintuch and Luss21 reported that the oxidation of propylene could lead to steady-state multiplicity. Oscillatory and chaotic overall reaction rates were observed during the oxidation of propylene in air on a thin Pt ribbon, the average temperature of which was kept at a preset value.22,23 In CO oxidation under atmospheric pressure, the oscillations are caused by the formation and removal of subsurface oxygen. The slow motion of these oscillations is attributed to the slow removal of subsurface oxygen by CO. Propylene combustion probably proceeds through dissociation and hydrogen oxidation. The mechanism that leads to rate oscillations during propylene oxidation has not yet been established, but it is probably different from that of CO oxidation. Figure 7 shows that the two reactions have steady-state multiplicity over different ranges of temperatures. Moreover, as Figure 7 shows, the range of average surface temperature over which hot spots form during CO oxidation does not overlap that in which they form during propylene oxidation. The oxidation of a mixture of CO and propylene leads to hot zone formation over a much wider range than that of either reaction and even for some in which neither reaction leads to hot zone formation. A prediction of the evolution and dynamic features of the hot zones requires information on the detailed surface reactions that lead to the overall reaction. This information is also essential for any attempt to predict the impact of the interaction between the two reactions. Unfortunately, this information is not available at present. The finding of moving hot zones during the oxidation of CO, C3H6, or their mixtures in a shallow packed bed reactor confirms the prediction by Viswanathan and Luss1 that hot zones are likely to form when the rate of the catalytic reaction may oscillate under constant ambient conditions. The period of the oscillations in the reaction rate due to the spatiotemporal motion of the hot zones during C3H6 oxidation was 60-90 min, which is much shorter than that observed

during CO oxidation (10-12 h). Addition of C3H6 to a catalyst bed in which CO oxidation was conducted decreased the period of the hot zone motion from 10-12 h to 30-45 min, while maintaining the qualitative features (breathing, antiphase, and rotation) observed during the CO oxidation. The adsorption strength of CO on a Pd(111) surface is much stronger than that of oxygen. Thus, CO displaces oxygen from a surface initially saturated with oxygen. However, oxygen does not displace CO from a saturated surface,24 but oxygen can displace C3H6 from a saturated Pd surface.25 The faster oscillation period during the oxidation of C3H6 and its mixtures with CO is most probably due to the difference in the reaction mechanisms. The observed period of oscillations during the simultaneous oxidation of CO and C3H6 is similar to the 15 min observed when the reaction was conducted on cylindrical Pt/alumina pellets held in an isothermal CSTR.26 An elementary step reaction model which included the nonequilibrium adsorption and desorption processes, was used to explain the cause of those oscillations. Shinjoh et al.25 reported that the rate of C3H6 oxidation over Pd catalyst can be described by an nth-order rate expression (n ) 0.6) with respect to oxygen and is not always dependent on the hydrocarbon partial pressure. On the other hand, the rate of CO oxidation on Pd satisfies a Langmuir-Hinshelwood kinetic expression and has the features of a negative order reaction at high CO concentration. Moreover, the reaction rate exhibits oscillatory behavior due to CO-driven oxidation-reduction reactions.27 The large difference in the frequencies of the motions in the same packed bed reactor suggest that they are caused by the differences in the kinetics mechanisms and not by the thermal properties and/or flow through the shallow catalyst bed. The qualitative features of the three spatiotemporal patterns (breathing, antiphase, and rotation) observed during the oxidation of CO + C3H6 mixtures are strikingly similar to those observed in electrochemical systems.28-31 Those studies concluded that the evolution of these patterns was caused by the presence of (negative) global coupling. Global coupling effects are generated in our experiments by interaction between the effluents in the top of the vessel and the catalytic bed. The interaction of the effluents with the catalytic layer have been shown to affect the stability and formation of spatiotemporal patterns during CO oxidation.11 An increase of the flow rate decreased the range of temperatures over which stable spatiotemporal temperature patterns existed during the oxidation of the propylene and that of the two-reactant mixture (Figure 4). This effect is due to the decrease in the strength of the global coupling caused by the decreased residence time of the effluents on top of the reactor as the flow rate is increased. The strength of global coupling may also change the qualitative features of the patterns. Lee at al.31 reported that a change from standing waves and pulses to homogeneous oscillations occurred upon a change in the strength of the global coupling in their electrochemical system. Similarly, we observed that during propylene oxidation only pulse motion existed at a high flow rate, whereas only breathing motion existed for a lower flow rate. The ignition temperature of C3H6 oxidation was 275 °C, while in the mixture of CO and C3H6 the ignition temperature was 135 °C (Figure 7a). Harold18 observed a similar result as the addition of CO to a feed stream containing C2H6, O2, and N2 decreased the ignition temperature on a single pellet from 300 to 200 °C. This effect is due to the increase in the heat generated by the CO oxidation. During CO and C3H6 oxidation, moving hot zones existed for a narrow range of vessel temperatures close

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to the extinction temperatures (Figure 4a). At a flow rate of 1200 cm3/min, a hot zone motion was observed between 120 °C and the extinction temperature (Text) of 115 °C for CO oxidation and between 225 and 180 °C for C3H6. However, during the oxidation of the mixture of the two reactants, moving hot zones were observed for vessel temperatures exceeding by more than 55 °C the extinction temperature. Moving hot zones during the oxidation of the mixtures existed for intermediate temperatures, but none were observed using either reactant by itself. For example, at a vessel temperature of 155 °C, the reactor was uniformly ignited during the oxidation of CO and completely extinguished during that of C3H6. However, a breathing motion was observed when the mixture of both reactants was fed to the vessel at the same temperature. The exothermicity of the CO oxidation reaction enabled the propylene oxidation to be sustained at this vessel temperature. Similar behavior was observed during single pellet experiments.18 Oxidation of a mixture containing 4.2 vol % C2H6 led to multiple steady states in the range of 190-192 °C, while that of a mixture containing 4.4 vol % CO led to multiple steady states in the range of 180200 °C. However, for a feed containing both reactants multiple steady states existed in the much wider temperature range of 80-200 °C.18 Concluding Remarks The large difference in the period of the motion of the hot zones formed during the oxidation of CO, propylene, and the mixture of CO and propylene indicate that the hot zone formation is affected mainly by the kinetics of the reactions and the strength of reactant adsorption and not by the thermal or flow properties through the reactor. It also indicates that the very slow hot zone motion encountered during CO oxidation is a feature of that catalytic reaction and not one to be expected in general. The formation of hot zones during the oxidation of either CO, propylene, or both seems to confirm the prediction by Viswanathan and Luss1 that transversal spatiotemporal patterns are likely to form in a shallow adiabatic packed bed reactor in which the rate of the catalytic reaction may oscillate under constant ambient conditions. It would be of interest to check if hot zones may also form for reactions for which the catalytic reaction does not lead to isothermal oscillations. The hot zones formed during the oxidation of propylene exhibited two types of qualitatively different spatiotemporal behavior such as breathing and pulse motions. When a mixture of CO and C3H6 was oxidized, three types of qualitatively different hot zone motions were observed, i.e., rotation, breathing, and antiphase motions, which were qualitatively similar to those observed during CO oxidation. The frequency of spatiotemporal hot zone motion was highest for the oxidation of mixtures followed by those of C3H6 and CO. Hot zones may form in a packed bed reactor due to nonuniform catalytic activity, nonuniform flow, spontaneous symmetry breaking, or global coupling. It is difficult to conclusively differentiate among the impacts of these phenomena on the observed hot zones. While the impact of nonuniform properties of the bed and of global coupling can be readily understood, there is still a need to gain a better understanding if and when spontaneous symmetry breaking may generate such hot zones, and their magnitude and impact. Further understanding of the above mechanisms is necessary to establish clear operating procedures that circumvent the occurrence of stationary and moving hot zones.

Acknowledgment We wish to acknowledge the support of this research by grants from the ACS-PRF and the U.S.-Israel Bi-National Science Foundation. We are thankful to G. A. Viswanathan for helpful comments and suggestions. Literature Cited (1) Viswanathan, G. A.; Luss, D. Moving Transversal Hot zones in Adiabatic Shallow Packed-bed Reactors. AIChE. J. 2006, 52, 705-717. (2) Boreskov, G. K.; Matros, Ju. Sˇ .; Klenov, O. P.; Lugovskij, V. I.; Lachmostov, V. S. Local non uniformities in a catalyst bed. Dokl. Akad. Nauk SSSR 1981, 258, 1418-1420. (3) Barkelew, C. H.; Gambhir, B. S. Stability of Trickle-Bed Reactors. ACS Symp. Ser. 1984, 237, 61-81. (4) Jaffe, S. B. Hot Spot Simulation in Commercial Hydrogenation Processes. Ind. Eng. Chem. Process Des. DeV. 1976, 15, 410-416. (5) Wicke, E.; Onken, H. U. Periodicity and Chaos in a Catalytic PackedBed Reactor for CO oxidation. Chem. Eng. Sci. 1988, 43, 2289-2294. (6) Marwaha, B.; Sundarram, S.; Luss, D. Dynamics of transverse hot zones in shallow packed bed reactors. J. Phys. Chem. B 2004, 108, 1447014476. (7) Marwaha, B.; Sundarram, S.; Luss, D. Dynamics of hot zones on top of packed bed reactors. Chem. Eng. Sci. 2004, 59, 5569-5574. (8) Digilov, R.; Nekhamkina, O.; Sheintuch, M. Thermal imaging of breathing patterns during CO oxidation on a Pd/glass cloth. AIChE. J. 2004, 50, 163-172. (9) Marwaha, B.; Luss, D. Formation and Dynamics of a hot zone in Radial Flow Reactor. AIChE. J. 2002, 48, 617-624. (10) Marwaha, B.; Luss, D. Hot zone formation in packed bed reactors. Chem. Eng. Sci. 2003, 58, 733-738. (11) Sundarram, S.; Marwaha, B.; Luss, D. Global-coupling induced temperature patterns on top of packed-bed reactors. Chem. Eng. Sci. 2005, 60, 6803-6805. (12) Sheintuch, M.; Luss, D. Reaction Rate Oscillations during Propylene Oxidation on Platinum. J. Catal. 1981, 68, 245-248. (13) Sheintuch, M.; Schmidt, J.; Rosenberg, S. Kinetic falsification by Symmetry Breaking. Ind. Eng. Chem. Res. 1989, 28, 955-960. (14) Balakotaiah, V.; Luss, D. Analysis of the Multiplicity Patterns of a CSTR. Chem. Eng. Commun. 1982, 19, 185-189. (15) Balakotaiah, V.; Luss, D. Multiplicity Features of Chemically Reacting Systems: Dependence of the Steady States on the Residence Time. Chem. Eng. Sci. 1983, 38, 1709-1721. (16) Balakotaiah, V.; Luss, D. Global Analysis of the Multiplicity Features of Multi-Reaction Lumped-Parameter Systems. Chem. Eng. Sci. 1984, 39, 865-881. (17) Harold, M. P.; Luss, D. An experimental study of steady state multiplicity features of two parallel catalytic reactions. Chem. Eng. Sci. 1985, 40, 39-42. (18) Harold, M. P. An experimental and theoretical study of the steady state multiplicity features of single or two parallel heterogeneous catalytic reactions. Ph.D. Dissertation, University of Houston, 1985. (19) Golubitsky, M.; Stewart, T.; Schaeffer, D.G. Singularities and Groups in Bifurcation Theory; Springer-Verlag: New York, 1984; Vol. II. (20) Nekhamkina, O.; Digilov, R.; Sheintuch, M. Modeling of temporally complex breathing patterns during Pd-catalyzed CO oxidation. J. Chem. Phys. 2003, 119, 2322-2332. (21) Sheintuch, M.; Luss, D. Application of Singularity Theory to Modeling of Steady-State Multiplicity: Propylene Oxidation on Platinum. Ind. Eng. Chem. Fundam. 1983, 22, 209-215. (22) Philippou, G.; Schultz, F.; Luss, D. Spatiotemporal Temperature Patterns on an Electrically Heated Catalytic Ribbon. J. Phys. Chem. 1991, 95, 3224-3229. (23) Philippou, G.; Luss, D. Temperature Patterns on a Catalytic Ribbon Heated by a Constant Electric Current. J. Phys. Chem. 1992, 96, 66516656. (24) Gasser, R. P. H. An introduction to chemisorption and catalysis by metals; Oxford University Press: Oxford, U.K., 1985. (25) Shinjoh, H.; Muraki, H.; Fujitani, Y. Periodic Operation Effects in Propane and Propylene Oxidation over Noble Metal Catalysts. Appl. Catal. 1989, 49,195-204. (26) Cutlip, M. B.; Kenney, C. N.; Morton, W.; Mukesh, D.; Capsaskis, S. C. Transient and Oscillatory Phenomenon in Catalytic Reactors. Inst. Chem. Eng. Symp. Ser. 1984, 87, 135-142. (27) Sales, B. C.; Turner, J. E.; Maple, M. B. Oscillatory Oxidation of CO Over Pt, Pd, and Ir Catalysts: Theory. Surf. Sci. 1982, 14, 381-394.

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ReceiVed for reView May 15, 2006 ReVised manuscript receiVed December 7, 2006 Accepted January 3, 2007 IE060597C