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Hot zones formation in packed bed reactors
Chemical Engineering Science 58 (2003) 733 – 738
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Hot zones formation in packed bed reactors B. Marwaha, D. Luss∗ Department of Chemical Engineering, University of Houston, Houston, TX 77204-4792, USA
Abstract Stationary and complex moving hot zones were observed during atmospheric oxidation of carbon monoxide on the top surface of a shallow packed bed, consisting of several layers of spherical catalytic pellets (Pd=Al2 O3 ). The test reaction was atmospheric oxidation of carbon monoxide. The reactor was run under conditions for which steady-state multiplicity and hot zone existed for some feed temperatures. The size of the hot zones was much larger than that of individual particles. IR imaging revealed that the hot and cold regions (temperature di4erence of the order of 100◦ C) were separated by a sharp (about 3 mm wide) temperature front. A very intricate periodic motion in which the hot zone repeatedly split and coalesced was observed in the shallow packed bed reactor. The transition from the branch of uniformly ignited to the states with a hot region was usually supercritical. It is not yet clear which rate processes generate the transversal hot zones in uniform packed bed reactors. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Hot zone; Spatiotemporal patterns; Packed bed; Split; Coalesce; CO oxidation
1. Introduction The axial temperature pro7le of a cooled packed bed reactor, in which an exothermic reaction occurs, usually attains a local maximum, which is referred to as a “hot spot”. When the amplitude of this hot spot exceeds some critical value it may lead to reactor runaway caused by an increase in the rate of the desired or of an undesired reaction, which has a negligible rate under normal operating conditions. Extensive literature exists on this subject, reviewed recently by Varma, Morbidelli, and Wu (1999). In an adiabatic reactor the transversal (normal to the >ow direction) temperature is usually uniform. However, transversal hot zones have been reported to exist in adiabatic packed bed reactors. For example, Boreskov, Matros, Klenov, Lugovskoi, and Lakhmostov (1981) observed several hot zones in the bottom of a packed bed reactor used to carry out the partial oxidation of isobutyl alcohol. Moving hot zones have been reported to form on Pt–Rh gauze used in the synthesis of HCN and ammonia oxidation. It has been shown that transversal hot zone may form due to non-uniform activity or packing in the bed. It may also be induced by natural convection when the velocity of the reacting mixture is low (Nguyen & Balakotaiah, 1994; Benneker, Kronberg, & Westerterp, 1998). However, ∗
Tel.: +1-713-743-4305; fax: + 1-713-743-4323. E-mail address: [email protected] (D. Luss).
at present it is not known what causes formation of transversal hot zones in a uniformly active packed bed reactor under the common situation that forced convection dominates the transport processes. Balakotaiah, Christtoforatou, and West (1999) showed that such hot zones may form under the conditions that the transversal dispersion of the reactant is faster than that of the temperature. However, Yakhnin and Menzinger (2001) pointed out that in practice the heat dispersion largely exceeds that of the species. The formation of hot zones may have a deleterious impact on the yield of the desired product(s), and it may deactivate the catalyst. When a hot zone exists next to the walls of a reactor it may decrease the mechanical strength of the wall. This may generate to a leak that may lead to an explosion. Thus, a hot zone may lead to severe safety problems. The formation of these hot zones is of both practical importance and of intrinsic academic interest. This has motivated us to conduct an experimental study of the evolution and dynamics of hot zone formation in shallow packed bed reactors. We report here preliminary results from an on-going study of hot zone evolution and dynamics in packed bed reactors (Marwaha & Luss, 2002; Marwaha, Annamalai, & Luss, 2001). The experiments consisted of IR imaging of the spatiotemporal temperature pro7les at the top of a 125 mm diameter shallow packed bed reactor consisting of several layers of 0:3 wt% Pd on alumina (3 mm diameter) catalytic pellets. The reactor is kept within an insulated oven and its
0009-2509/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0009-2509(02)00602-4
734
B. Marwaha, D. Luss / Chemical Engineering Science 58 (2003) 733 – 738
hot gold plated mirror cold quartz window
insulation effluent
infra-red image
Pellets (Pd/Al 2O 3)
alumina wool
glass beads feed
wire mesh
Fig. 1. Schematic of the system used to measure the spatiotemporal temperature on the top of a shallow packed bed.
wall and feed temperature are the same. The catalytic oxidation of carbon monoxide was run under conditions for which steady state multiplicity existed for some feed temperatures. Hopefully, the understanding of this behavior will enable development of predictive mathematical models. These models will, in turn, be used to determine operation procedures, which avoid the hot zone formation, or at least minimize their impact. It is not clear what is, if any, the relation between the formation of hot zones on an individual catalytic pellet to the formation of a hot zone by an ensemble of pellets in a packed bed reactor. We shall report on the in>uence of the number of catalyst layers on the evolution and dynamic features of the hot zones. 2. Experimental set-up and procedure The formation and evolution of hot zones in a shallow packed bed reactor were studied during the atmospheric oxidation of carbon monoxide. The reactor was a cylindrical stainless steel (SS 316) vessel (125 mm o.d., 5 mm wall thickness, 286 mm long) and the shallow packed bed consisted of several layers of spherical catalyst pellets (3–4 mm). The spherical catalyst contained 0:3 wt% palladium on alumina deposited in a thin surface shell (Oxy-Catalyst, Inc.). The catalyst was placed on top of two layers of glass beads (3 mm), used to enhance the feed distribution. They were placed on a thin wire mesh, supported on three pins, 51 mm from the top of the vessel (Fig. 1). Preliminary experiments have shown that heat loss from the vessel wall can distort the temperature pro7les and cause premature extinction. To minimize this heat loss, we 7lled a 3 mm annular space between the catalyst layers and the vessel walls with alumina wool. This vessel was thermally insulated and electrically heated from the outside. The vessel wall temperature was measured by thermocouples
and regulated by a temperature controller (PID, Omega CN2041). The vessel eKuent (CO) concentration was measured by an infrared gas analyzer (AR-411; Anarad) at a sampling frequency of 0:1 Hz. The reactive mixture was fed into the reactor through 7ve, 6 mm, inlet ports at the vessel bottom (4 on the periphery and 1 in the center) and exited the vessel through four, 6 mm, outlet ports at the top. The bottom of the SS vessel was packed to a height of 127 mm with glass beads to enhance the feed distribution. The reactive feed consisted of extra dry grade oxygen (purity 99.6%), carbon monoxide (Aeriform) and puri7ed grade nitrogen (purity 99.998%). The carbon monoxide was puri7ed by passing through a carbonyl trap of a molecular sieve adsorbent (5A zeolite; Linde) kept at 240◦ C. The feed gases were mixed in a bed of glass beads, puri7ed, and dried by activated charcoal puri7ers (Linde) before entering the vessel. The gas >ow rates were regulated by thermal mass >ow controllers (FC-280, FC-261 and FC-260; Tylan; accuracy ±1%). The top cover of the SS vessel was a infra-red transparent quartz disc. The temperature distribution on the top layer of catalyst pellets was monitored by an IR camera (Amber Radiance PM). The radiation from this layer was re>ected by a gold-plated mirror, placed at 45◦ above the vessel, to the infrared camera. It had a 256 × 256 indium-antimonide detector array, sensitive to 3–5 m radiation. The 7eld of view of the 50 mm lens was 11◦ ×11◦ and the spatial resolution was 0:4 mm2 . The measured temperatures were recorded on a computer using Imagedesk II (software; Raytheon Amber). During an experiment the vessel temperature was step-wise increased from room temperature until the whole packed bed (top layer) was ignited and attained a uniform high temperature. After attaining a fully ignited state, the vessel temperature was stepwise decreased until the reactor extinguished. At each vessel temperature we waited till a stationary state was reached (1.5 –2 h) before recording any infrared images. A typical run took 36 h, with 75% of the total data taken close to the extinction point, as patterned state were found only in that region. IR images were recorded at the rate of 1– 4 image per minute to capture e4ectively the slow hot spot motion. The feed composition in all the 7gures shown hence was CO—6 vol%; O2 —70 vol% and N2 —24 vol%: 3. Experimental results Slow cooling of the reactor led to the formation of a hot zone from a fully ignited state, i.e. a uniform high temperature that existed across the top of the bed. This evolution of a non-uniform state exhibiting hot zone from a fully ignited state (TV = 160◦ C) is depicted by a sequence of snapshots of the temperature of the top of a four layers of catalyst bed (Fig. 2). The total >ow rate was 1200 cm3 =min. The uniform surface temperature of the top of the bed decreased as the vessel temperature was reduced from 160◦ C to 90◦ C.
B. Marwaha, D. Luss / Chemical Engineering Science 58 (2003) 733 – 738
Tv = 160 oC
90
130
100
85
79
Fig. 2. Evolution of spatiotemporal patterns on the top of a four catalyst layers bed. 80
c b a
4 layers 1 layer
40 2 layers
Exit CO conversion [%]
60
20
0
60
90
120
Vessel temperature
150
180
[oC]
Fig. 3. E4ect of number of layers on bifurcation plot. Total >ow rate 1200 cm3 =min.
However, no cold zones formed on the top layer. Upon cooling below 90◦ C an annular cold zone formed and completely engulfed a hot zone in its center (Fig. 2, TV =85◦ C). The hot zone shrunk on further cooling and eventually extinguished. A similar non-uniform state with a hot zone always formed close to the extinction temperature. Throughout its evolution (Fig. 2) the hot zone remained a single entity. However, in some cases the hot zone split and underwent complex motion. The evolution of patterns did not change qualitatively upon changing the number of catalyst pellet layers, but the temperature range for non-uniform states changed. The bifurcation diagrams for three cases with di4erent number of catalyst layers are compared in Fig. 3. As expected, for a particular vessel temperature, the conversion increased with the number of layers. In addition, the extinction temperature decreased as the number of layers increased. Thus, the range of vessel temperatures over which non-uniform states existed depended on the number of layers.
735
Fig. 4. Instantaneous snapshots of topmost layer for same operating conditions as in Fig. 3.
Instantaneous temperature distribution over the topmost layer at three di4erent vessel temperatures are shown in Fig. 4 for three di4erent packing con7gurations (one, two and four layers) and three vessel temperatures 110◦ C; 102◦ C and 93◦ C that are marked on the bifurcation plots by vertical lines a, b and c. The snapshots show that for two cases (one and two layers), the reactor exhibits a non-uniform state in which a hot zone is engulfed by a cold zone for a vessel temperature of 102◦ C. For the four-layers bed the vessel cooling merely reduced the surface temperature of the topmost layer and did not a4ect the conversion as seen in Fig. 3. At 93◦ C the reactor is already extinguished for the one-layer case. The snapshots in Fig. 4 show that the temperature on the top of the one and two-layers bed was higher than that on the top of the four-layers bed, even though the conversion of the four-layers bed was higher, as shown in Fig. 3. The reason for this is that heat losses to the surrounding caused the four-layers bed to act as a cooled packed bed, the maximum temperature of which is attained at some intermediate position ahead of its downstream section. Thus, the local temperature at this section is not indicative of the local level of conversion. When the reactor attained a non-uniform state, the hot zone exhibited several kinds of motion, ranging from simple lateral translation to a complex motion in which the hot zone repeatedly split and coalesced. Figs. 5(a) and (b) shows two kinds of motion for the single-layer case. In the 7rst (Fig. 5(a)), the hot zone moved from left to right upon cooling of the vessel from 110◦ C to 105◦ C. While the above translation motion shown in Fig. 5(a) was induced by cooling, the intricate motion shown in Fig. 5(b) occurred at a constant vessel temperature of 105◦ C. The hot region that was initially centered at the 2 o’clock position stretched to the 7 o’clock position (60 and 120 min). After 180 min two hot regions existed, one at the 2 o’clock and the second at the 7 o’clock position, i.e., the hot region split. In the remainder of the sequence the two hot regions coalesced into one hot region at the original 2 o’clock location (300 min).
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B. Marwaha, D. Luss / Chemical Engineering Science 58 (2003) 733 – 738
Fig. 5. Several motions of hot zone for (a) single-layer case when the vessel was cooled from 110◦ C to 107◦ C, (b) single-layer case at vessel temperature of 105◦ C and (c) for two-layer case at vessel temperature of 93◦ C. The total >ow rate was 1200 cm3 =min.
The splitting and merging of the hot region was more obvious in the two-layers bed as illustrated in Fig. 5(c). Initially two hot regions existed at the 2 o’clock and the 7 o’clock locations. After 12 min the hot zone at 7 o’clock started to merge with the other hot zone. At 20 min only one hot zone existed (at the 2 o’clock position). Then this hot zone started to expand. At 360 min the large hot zone started to split into two, located at the 2 o’clock and 7 o’clock positions (600 min). Subsequently the two hot zones again coalesced. In contrast to the merging that took place initially (12 min), this time (700 min) the hot zone at 2 o’clock position merged into the one at the 7 o’clock position. Moreover, in the 7rst merging the hot zone moved through the center of the top pellet layer, whereas at 700 min it moved along the periphery. After 720 min the single hot zone at the 7 o’clock position expanded and split into two
hot zones at the 7 o’clock and 2 o’clock positions (847 min). The hot zone moved along the periphery of the pellet layer in both these splitting occasions (600 and 847 min). Due to the long period of this motion (order of 10 h) we were unable to determine conclusively if the motion was strictly periodic. In addition to the hot zone motion described above, some hot zones exhibited bursting and breathing motions. Two examples of these motions are shown in Figs. 6(a) and (b) along two lines on the top of a four-layer bed. A repeated bursting of a cold zone into the hot zone is shown in Fig. 6(a) The space–time in Fig. 6(b) shows a more complex motion, in which two hot regions separated by a cold zone repeatedly contracted and expanded (breathing).The amplitude and frequency of this motion increased on cooling the vessel.
B. Marwaha, D. Luss / Chemical Engineering Science 58 (2003) 733 – 738
0
737
3
0
length [cm]
Tv=82oC
a)
2
80 oC
150
Tv=78oC
a) 0
3
time [hr]
Fig. 6. Breathing motion on 2 lines on top of the four-layer bed at 82◦ C (a) 78◦ C (b). Total >ow rate of 1200 cm3 =min.
Tv=155oC, one layer exit CO concentration[%]
Tv=93oC, two layer
a)
b)
Fig. 7. Temporal oscillations in eKuent CO concentration were (a) chaotic close to the bifurcation of the ignited state (3200 cm3 =min) and (b) bursting spikes during the non-uniform state (1200 cm3 =min).
4. Discussion The experiments revealed that the dynamics of the hot zones on the top of the bed may be rather intricate. In addition to breathing and translation on the surface, the hot zones may split, coalesce, move around and re-split (Fig. 5).
3.4
% Exit CO (t + τ)
The eKuent CO concentration exhibited oscillatory behavior including chaotic oscillations (Fig. 7(a)) and bursting/spiking (Fig. 7(b)). The chaotic motion of the single-layer reactor, shown in the time series Fig. 7(a), occurred when the reactor was close to bifurcation from the uniform ignited state. The amplitude of the oscillation was small compared to that in the time series shown in Fig. 7(b) when the hot zone that existed in the two-layer reactor underwent repeated splitting and merging. Long relaxation periods are also noticed in this case (Fig. 7(b)). The time series of the eKuent CO concentration shown in Fig. 7(a) was used to construct a pseudo-phase-plane (Fig. 8). A time delay of 1=3 s was used in the construction. Fig. 8 indicates that the motion was of a strange attractor. We did not determine the dimension of that attractor.
τ=1/3 sec
3
2.6 2.6
3
3.4
% Exit CO (t)
Fig. 8. A two-dimensional pseudo-space-plane of the motion of the eKuent CO concentration shown in Fig. 7(a).
The size of the hot zones observed on the top of the shallow bed were much larger than those of typical catalytic pellets used in commercial reactors. Commercial packed bed reactors have a much larger diameter than ours. Hence several distinct hot regions may co-exist and interact. It is of both academic and industrial importance to predict the interaction between the dynamics of these hot zones, their motion,
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B. Marwaha, D. Luss / Chemical Engineering Science 58 (2003) 733 – 738
the distances among the di4erent zones and how their dynamics a4ects the overall reaction rate. The overall reaction rate was strongly a4ected by the hot zone dynamics in our experiments (Figs. 7(a) and (b)). The experiments in the shallow packed bed reactor revealed some similarity with the dynamics of hot regions on a radial >ow reactor (Marwaha & Luss, 2001). In both cases the temperature patterns formed close to the extinction of the uniform states, and the amplitude and frequency of the breathing motion increased upon cooling of the vessel. This behavior follows the prediction of Hagberg and Meron (1994) that the temperature front stability decreases upon an increase in the ratio between the time constants of the inhibitor and the activator. A decrease of the temperature increased 1=k(T ), the time constant of the inhibitor (limiting reactant concentration). In addition, in both cases, the velocity of the fronts was rather low, of the order of mm/min. Transversal hot zone evolution and motion in packed bed are a4ected by the interaction among di4usion processes, reaction rate, global coupling and catalyst non-uniformity. In addition other factors a4ecting transversal hot zones are reactant convection, changes in the reactants concentrations and temperature in the >ow direction, transport of heat and species in the radial direction and the impact of the reaction on the >uid physical properties. While the dynamics of temperature patterns on single catalytic pellets is rather well understood, it is not yet established what are the rate processes and interactions leading to transversal hot zone formation in uniformly active packed bed reactors. Numerical studies by Yakhnin, Menzinger, Delmon, and Froment (1999) show that non-uniform catalyst distribution may generate transient temperature excursions exceeding those in reactors with uniform catalytic activity. These predictions were recently veri7ed experimentally (Jaree et al., 2001). Patterns may evolve in di4usion-reaction systems when the di4usivity of the inhibitor exceeds that of the activator (Turing, 1952; Segel & Jackson, 1972). Similarly, Zeldovich, Barenblatt, Librovich, and Makhviladze (1985) predicted that a laminar >ame front may be destabilized when the diffusivity of the reacting species exceeded that of the thermal di4usivity. We conjecture that transversal hot zones may be predictable by a two-phase model, which accounts for the difference in the local states of the >uid and the solid phases. This model can describe ignition due to the transport resistances between the >uid and solid, a phenomenon, which is not accounted for by the pseudo-homogeneous model. Accounting for this ignition may enable the prediction of the transversal hot zones under practical operating conditions. The prediction and analysis of the dynamic features of hot
zones by a three-dimensional, two-phase model of a packed bed reactor will be rather intricate and tedious. However, it may be needed to explain the formation of the transversal hot zones. Availability of a reliable model of transversal hot spots will enable a rational design and control, which avoid their undesirable formation. Acknowledgements We are thankful for the support of this research from National Science Foundation and the Welch Foundation. References Balakotaiah, V., Christoforatou, E. L., & West, D. H. (1999). Transverse concentration and temperature nonuniformities in adiabatic packed-bed catalytic reactors. Chemical Engineering Science, 54, 1725–1734. Benneker, A. H., Kronberg, A. E., & Westerterp, K. R. (1998). In>uence of buoyancy forces on the >ow of gases through packed beds at elevated pressures. A.I.Ch.E. Journal, 44, 263–270. Boreskov, G. K, Matros, Yu. Sh. Klenov, O.P. Lugovskoi, V. I., & Lakhmostov, V. S. (1981). Local nonuniformities in a catalyst bed. Doklardy Akademii Nauk SSSR, 258, 1418. Hagberg, A., & Meron, E. (1994). Pattern formation in non-gradient reaction-di4usion systems: the e4ects of front bifurcations. Nonlinearity, 7, 805. Jaree, A., Budman, H. M., Hudgins, R. R., Silveston, P. L., Yakhnin, V., & Menzinger, M. (2001). Temperature excursions in reactors packed with segregated layers of catalyst and inert solids. Chemical Engineering Science, 56, 5719–5726. Marwaha, B., Annamalai, J., & Luss, D. (2001). Hot zone formation during carbon monoxide oxidation in a radial >ow reactor. Chemical Engineering Science, 56, 89–96. Marwaha, B., & Luss, D. (2002). Formation and dynamics of a hot zone in radial >ow reactor. A.I.Ch.E. Journal, 48, 617–624. Nguyen, D., & Balakotaiah, V. (1994). Flow mal-distributions and hot spots in down->ow packed-bed reactors. Chemical Engineering Science, 49, 5489–5505. Segel, L. A., & Jackson, J. L. (1972). Dissipative structure: An explanation and an ecological example. Journal of Theoretical Biology, 37, 545–559. Turing, A. M. (1952). The chemical basis of morphogenesis. Philosophical Transactions of the Royal Society of London, Series B, 237, 37–72. Varma, A., Morbidelli, M., & Wu, H. (1999). Parametric sensitivity in chemical systems. Cambridge: Cambridge University Press. Yakhnin, V., & Menzinger, M. (2001). On transverse patterns in packed-bed catalytic reactors. Chemical Engineering Science, 56, 2233–2236. Yakhnin, V., Menzinger, M., Delmon, B., & Froment, G. F. (1999). Studies in surface science and catalysis, Vol. 126 (p. 291). Amsterdam: Elsevier. Zeldovich, Ya. B., Barenblatt, G. I., Librovich, V. B., & Makhviladze, G. M. (1985). The mathematical theory of combustion and explosives. New York: Consultants Bureau.
www.elsevier.com/locate/ces
Hot zones formation in packed bed reactors B. Marwaha, D. Luss∗ Department of Chemical Engineering, University of Houston, Houston, TX 77204-4792, USA
Abstract Stationary and complex moving hot zones were observed during atmospheric oxidation of carbon monoxide on the top surface of a shallow packed bed, consisting of several layers of spherical catalytic pellets (Pd=Al2 O3 ). The test reaction was atmospheric oxidation of carbon monoxide. The reactor was run under conditions for which steady-state multiplicity and hot zone existed for some feed temperatures. The size of the hot zones was much larger than that of individual particles. IR imaging revealed that the hot and cold regions (temperature di4erence of the order of 100◦ C) were separated by a sharp (about 3 mm wide) temperature front. A very intricate periodic motion in which the hot zone repeatedly split and coalesced was observed in the shallow packed bed reactor. The transition from the branch of uniformly ignited to the states with a hot region was usually supercritical. It is not yet clear which rate processes generate the transversal hot zones in uniform packed bed reactors. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Hot zone; Spatiotemporal patterns; Packed bed; Split; Coalesce; CO oxidation
1. Introduction The axial temperature pro7le of a cooled packed bed reactor, in which an exothermic reaction occurs, usually attains a local maximum, which is referred to as a “hot spot”. When the amplitude of this hot spot exceeds some critical value it may lead to reactor runaway caused by an increase in the rate of the desired or of an undesired reaction, which has a negligible rate under normal operating conditions. Extensive literature exists on this subject, reviewed recently by Varma, Morbidelli, and Wu (1999). In an adiabatic reactor the transversal (normal to the >ow direction) temperature is usually uniform. However, transversal hot zones have been reported to exist in adiabatic packed bed reactors. For example, Boreskov, Matros, Klenov, Lugovskoi, and Lakhmostov (1981) observed several hot zones in the bottom of a packed bed reactor used to carry out the partial oxidation of isobutyl alcohol. Moving hot zones have been reported to form on Pt–Rh gauze used in the synthesis of HCN and ammonia oxidation. It has been shown that transversal hot zone may form due to non-uniform activity or packing in the bed. It may also be induced by natural convection when the velocity of the reacting mixture is low (Nguyen & Balakotaiah, 1994; Benneker, Kronberg, & Westerterp, 1998). However, ∗
Tel.: +1-713-743-4305; fax: + 1-713-743-4323. E-mail address: [email protected] (D. Luss).
at present it is not known what causes formation of transversal hot zones in a uniformly active packed bed reactor under the common situation that forced convection dominates the transport processes. Balakotaiah, Christtoforatou, and West (1999) showed that such hot zones may form under the conditions that the transversal dispersion of the reactant is faster than that of the temperature. However, Yakhnin and Menzinger (2001) pointed out that in practice the heat dispersion largely exceeds that of the species. The formation of hot zones may have a deleterious impact on the yield of the desired product(s), and it may deactivate the catalyst. When a hot zone exists next to the walls of a reactor it may decrease the mechanical strength of the wall. This may generate to a leak that may lead to an explosion. Thus, a hot zone may lead to severe safety problems. The formation of these hot zones is of both practical importance and of intrinsic academic interest. This has motivated us to conduct an experimental study of the evolution and dynamics of hot zone formation in shallow packed bed reactors. We report here preliminary results from an on-going study of hot zone evolution and dynamics in packed bed reactors (Marwaha & Luss, 2002; Marwaha, Annamalai, & Luss, 2001). The experiments consisted of IR imaging of the spatiotemporal temperature pro7les at the top of a 125 mm diameter shallow packed bed reactor consisting of several layers of 0:3 wt% Pd on alumina (3 mm diameter) catalytic pellets. The reactor is kept within an insulated oven and its
0009-2509/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0009-2509(02)00602-4
734
B. Marwaha, D. Luss / Chemical Engineering Science 58 (2003) 733 – 738
hot gold plated mirror cold quartz window
insulation effluent
infra-red image
Pellets (Pd/Al 2O 3)
alumina wool
glass beads feed
wire mesh
Fig. 1. Schematic of the system used to measure the spatiotemporal temperature on the top of a shallow packed bed.
wall and feed temperature are the same. The catalytic oxidation of carbon monoxide was run under conditions for which steady state multiplicity existed for some feed temperatures. Hopefully, the understanding of this behavior will enable development of predictive mathematical models. These models will, in turn, be used to determine operation procedures, which avoid the hot zone formation, or at least minimize their impact. It is not clear what is, if any, the relation between the formation of hot zones on an individual catalytic pellet to the formation of a hot zone by an ensemble of pellets in a packed bed reactor. We shall report on the in>uence of the number of catalyst layers on the evolution and dynamic features of the hot zones. 2. Experimental set-up and procedure The formation and evolution of hot zones in a shallow packed bed reactor were studied during the atmospheric oxidation of carbon monoxide. The reactor was a cylindrical stainless steel (SS 316) vessel (125 mm o.d., 5 mm wall thickness, 286 mm long) and the shallow packed bed consisted of several layers of spherical catalyst pellets (3–4 mm). The spherical catalyst contained 0:3 wt% palladium on alumina deposited in a thin surface shell (Oxy-Catalyst, Inc.). The catalyst was placed on top of two layers of glass beads (3 mm), used to enhance the feed distribution. They were placed on a thin wire mesh, supported on three pins, 51 mm from the top of the vessel (Fig. 1). Preliminary experiments have shown that heat loss from the vessel wall can distort the temperature pro7les and cause premature extinction. To minimize this heat loss, we 7lled a 3 mm annular space between the catalyst layers and the vessel walls with alumina wool. This vessel was thermally insulated and electrically heated from the outside. The vessel wall temperature was measured by thermocouples
and regulated by a temperature controller (PID, Omega CN2041). The vessel eKuent (CO) concentration was measured by an infrared gas analyzer (AR-411; Anarad) at a sampling frequency of 0:1 Hz. The reactive mixture was fed into the reactor through 7ve, 6 mm, inlet ports at the vessel bottom (4 on the periphery and 1 in the center) and exited the vessel through four, 6 mm, outlet ports at the top. The bottom of the SS vessel was packed to a height of 127 mm with glass beads to enhance the feed distribution. The reactive feed consisted of extra dry grade oxygen (purity 99.6%), carbon monoxide (Aeriform) and puri7ed grade nitrogen (purity 99.998%). The carbon monoxide was puri7ed by passing through a carbonyl trap of a molecular sieve adsorbent (5A zeolite; Linde) kept at 240◦ C. The feed gases were mixed in a bed of glass beads, puri7ed, and dried by activated charcoal puri7ers (Linde) before entering the vessel. The gas >ow rates were regulated by thermal mass >ow controllers (FC-280, FC-261 and FC-260; Tylan; accuracy ±1%). The top cover of the SS vessel was a infra-red transparent quartz disc. The temperature distribution on the top layer of catalyst pellets was monitored by an IR camera (Amber Radiance PM). The radiation from this layer was re>ected by a gold-plated mirror, placed at 45◦ above the vessel, to the infrared camera. It had a 256 × 256 indium-antimonide detector array, sensitive to 3–5 m radiation. The 7eld of view of the 50 mm lens was 11◦ ×11◦ and the spatial resolution was 0:4 mm2 . The measured temperatures were recorded on a computer using Imagedesk II (software; Raytheon Amber). During an experiment the vessel temperature was step-wise increased from room temperature until the whole packed bed (top layer) was ignited and attained a uniform high temperature. After attaining a fully ignited state, the vessel temperature was stepwise decreased until the reactor extinguished. At each vessel temperature we waited till a stationary state was reached (1.5 –2 h) before recording any infrared images. A typical run took 36 h, with 75% of the total data taken close to the extinction point, as patterned state were found only in that region. IR images were recorded at the rate of 1– 4 image per minute to capture e4ectively the slow hot spot motion. The feed composition in all the 7gures shown hence was CO—6 vol%; O2 —70 vol% and N2 —24 vol%: 3. Experimental results Slow cooling of the reactor led to the formation of a hot zone from a fully ignited state, i.e. a uniform high temperature that existed across the top of the bed. This evolution of a non-uniform state exhibiting hot zone from a fully ignited state (TV = 160◦ C) is depicted by a sequence of snapshots of the temperature of the top of a four layers of catalyst bed (Fig. 2). The total >ow rate was 1200 cm3 =min. The uniform surface temperature of the top of the bed decreased as the vessel temperature was reduced from 160◦ C to 90◦ C.
B. Marwaha, D. Luss / Chemical Engineering Science 58 (2003) 733 – 738
Tv = 160 oC
90
130
100
85
79
Fig. 2. Evolution of spatiotemporal patterns on the top of a four catalyst layers bed. 80
c b a
4 layers 1 layer
40 2 layers
Exit CO conversion [%]
60
20
0
60
90
120
Vessel temperature
150
180
[oC]
Fig. 3. E4ect of number of layers on bifurcation plot. Total >ow rate 1200 cm3 =min.
However, no cold zones formed on the top layer. Upon cooling below 90◦ C an annular cold zone formed and completely engulfed a hot zone in its center (Fig. 2, TV =85◦ C). The hot zone shrunk on further cooling and eventually extinguished. A similar non-uniform state with a hot zone always formed close to the extinction temperature. Throughout its evolution (Fig. 2) the hot zone remained a single entity. However, in some cases the hot zone split and underwent complex motion. The evolution of patterns did not change qualitatively upon changing the number of catalyst pellet layers, but the temperature range for non-uniform states changed. The bifurcation diagrams for three cases with di4erent number of catalyst layers are compared in Fig. 3. As expected, for a particular vessel temperature, the conversion increased with the number of layers. In addition, the extinction temperature decreased as the number of layers increased. Thus, the range of vessel temperatures over which non-uniform states existed depended on the number of layers.
735
Fig. 4. Instantaneous snapshots of topmost layer for same operating conditions as in Fig. 3.
Instantaneous temperature distribution over the topmost layer at three di4erent vessel temperatures are shown in Fig. 4 for three di4erent packing con7gurations (one, two and four layers) and three vessel temperatures 110◦ C; 102◦ C and 93◦ C that are marked on the bifurcation plots by vertical lines a, b and c. The snapshots show that for two cases (one and two layers), the reactor exhibits a non-uniform state in which a hot zone is engulfed by a cold zone for a vessel temperature of 102◦ C. For the four-layers bed the vessel cooling merely reduced the surface temperature of the topmost layer and did not a4ect the conversion as seen in Fig. 3. At 93◦ C the reactor is already extinguished for the one-layer case. The snapshots in Fig. 4 show that the temperature on the top of the one and two-layers bed was higher than that on the top of the four-layers bed, even though the conversion of the four-layers bed was higher, as shown in Fig. 3. The reason for this is that heat losses to the surrounding caused the four-layers bed to act as a cooled packed bed, the maximum temperature of which is attained at some intermediate position ahead of its downstream section. Thus, the local temperature at this section is not indicative of the local level of conversion. When the reactor attained a non-uniform state, the hot zone exhibited several kinds of motion, ranging from simple lateral translation to a complex motion in which the hot zone repeatedly split and coalesced. Figs. 5(a) and (b) shows two kinds of motion for the single-layer case. In the 7rst (Fig. 5(a)), the hot zone moved from left to right upon cooling of the vessel from 110◦ C to 105◦ C. While the above translation motion shown in Fig. 5(a) was induced by cooling, the intricate motion shown in Fig. 5(b) occurred at a constant vessel temperature of 105◦ C. The hot region that was initially centered at the 2 o’clock position stretched to the 7 o’clock position (60 and 120 min). After 180 min two hot regions existed, one at the 2 o’clock and the second at the 7 o’clock position, i.e., the hot region split. In the remainder of the sequence the two hot regions coalesced into one hot region at the original 2 o’clock location (300 min).
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Fig. 5. Several motions of hot zone for (a) single-layer case when the vessel was cooled from 110◦ C to 107◦ C, (b) single-layer case at vessel temperature of 105◦ C and (c) for two-layer case at vessel temperature of 93◦ C. The total >ow rate was 1200 cm3 =min.
The splitting and merging of the hot region was more obvious in the two-layers bed as illustrated in Fig. 5(c). Initially two hot regions existed at the 2 o’clock and the 7 o’clock locations. After 12 min the hot zone at 7 o’clock started to merge with the other hot zone. At 20 min only one hot zone existed (at the 2 o’clock position). Then this hot zone started to expand. At 360 min the large hot zone started to split into two, located at the 2 o’clock and 7 o’clock positions (600 min). Subsequently the two hot zones again coalesced. In contrast to the merging that took place initially (12 min), this time (700 min) the hot zone at 2 o’clock position merged into the one at the 7 o’clock position. Moreover, in the 7rst merging the hot zone moved through the center of the top pellet layer, whereas at 700 min it moved along the periphery. After 720 min the single hot zone at the 7 o’clock position expanded and split into two
hot zones at the 7 o’clock and 2 o’clock positions (847 min). The hot zone moved along the periphery of the pellet layer in both these splitting occasions (600 and 847 min). Due to the long period of this motion (order of 10 h) we were unable to determine conclusively if the motion was strictly periodic. In addition to the hot zone motion described above, some hot zones exhibited bursting and breathing motions. Two examples of these motions are shown in Figs. 6(a) and (b) along two lines on the top of a four-layer bed. A repeated bursting of a cold zone into the hot zone is shown in Fig. 6(a) The space–time in Fig. 6(b) shows a more complex motion, in which two hot regions separated by a cold zone repeatedly contracted and expanded (breathing).The amplitude and frequency of this motion increased on cooling the vessel.
B. Marwaha, D. Luss / Chemical Engineering Science 58 (2003) 733 – 738
0
737
3
0
length [cm]
Tv=82oC
a)
2
80 oC
150
Tv=78oC
a) 0
3
time [hr]
Fig. 6. Breathing motion on 2 lines on top of the four-layer bed at 82◦ C (a) 78◦ C (b). Total >ow rate of 1200 cm3 =min.
Tv=155oC, one layer exit CO concentration[%]
Tv=93oC, two layer
a)
b)
Fig. 7. Temporal oscillations in eKuent CO concentration were (a) chaotic close to the bifurcation of the ignited state (3200 cm3 =min) and (b) bursting spikes during the non-uniform state (1200 cm3 =min).
4. Discussion The experiments revealed that the dynamics of the hot zones on the top of the bed may be rather intricate. In addition to breathing and translation on the surface, the hot zones may split, coalesce, move around and re-split (Fig. 5).
3.4
% Exit CO (t + τ)
The eKuent CO concentration exhibited oscillatory behavior including chaotic oscillations (Fig. 7(a)) and bursting/spiking (Fig. 7(b)). The chaotic motion of the single-layer reactor, shown in the time series Fig. 7(a), occurred when the reactor was close to bifurcation from the uniform ignited state. The amplitude of the oscillation was small compared to that in the time series shown in Fig. 7(b) when the hot zone that existed in the two-layer reactor underwent repeated splitting and merging. Long relaxation periods are also noticed in this case (Fig. 7(b)). The time series of the eKuent CO concentration shown in Fig. 7(a) was used to construct a pseudo-phase-plane (Fig. 8). A time delay of 1=3 s was used in the construction. Fig. 8 indicates that the motion was of a strange attractor. We did not determine the dimension of that attractor.
τ=1/3 sec
3
2.6 2.6
3
3.4
% Exit CO (t)
Fig. 8. A two-dimensional pseudo-space-plane of the motion of the eKuent CO concentration shown in Fig. 7(a).
The size of the hot zones observed on the top of the shallow bed were much larger than those of typical catalytic pellets used in commercial reactors. Commercial packed bed reactors have a much larger diameter than ours. Hence several distinct hot regions may co-exist and interact. It is of both academic and industrial importance to predict the interaction between the dynamics of these hot zones, their motion,
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the distances among the di4erent zones and how their dynamics a4ects the overall reaction rate. The overall reaction rate was strongly a4ected by the hot zone dynamics in our experiments (Figs. 7(a) and (b)). The experiments in the shallow packed bed reactor revealed some similarity with the dynamics of hot regions on a radial >ow reactor (Marwaha & Luss, 2001). In both cases the temperature patterns formed close to the extinction of the uniform states, and the amplitude and frequency of the breathing motion increased upon cooling of the vessel. This behavior follows the prediction of Hagberg and Meron (1994) that the temperature front stability decreases upon an increase in the ratio between the time constants of the inhibitor and the activator. A decrease of the temperature increased 1=k(T ), the time constant of the inhibitor (limiting reactant concentration). In addition, in both cases, the velocity of the fronts was rather low, of the order of mm/min. Transversal hot zone evolution and motion in packed bed are a4ected by the interaction among di4usion processes, reaction rate, global coupling and catalyst non-uniformity. In addition other factors a4ecting transversal hot zones are reactant convection, changes in the reactants concentrations and temperature in the >ow direction, transport of heat and species in the radial direction and the impact of the reaction on the >uid physical properties. While the dynamics of temperature patterns on single catalytic pellets is rather well understood, it is not yet established what are the rate processes and interactions leading to transversal hot zone formation in uniformly active packed bed reactors. Numerical studies by Yakhnin, Menzinger, Delmon, and Froment (1999) show that non-uniform catalyst distribution may generate transient temperature excursions exceeding those in reactors with uniform catalytic activity. These predictions were recently veri7ed experimentally (Jaree et al., 2001). Patterns may evolve in di4usion-reaction systems when the di4usivity of the inhibitor exceeds that of the activator (Turing, 1952; Segel & Jackson, 1972). Similarly, Zeldovich, Barenblatt, Librovich, and Makhviladze (1985) predicted that a laminar >ame front may be destabilized when the diffusivity of the reacting species exceeded that of the thermal di4usivity. We conjecture that transversal hot zones may be predictable by a two-phase model, which accounts for the difference in the local states of the >uid and the solid phases. This model can describe ignition due to the transport resistances between the >uid and solid, a phenomenon, which is not accounted for by the pseudo-homogeneous model. Accounting for this ignition may enable the prediction of the transversal hot zones under practical operating conditions. The prediction and analysis of the dynamic features of hot
zones by a three-dimensional, two-phase model of a packed bed reactor will be rather intricate and tedious. However, it may be needed to explain the formation of the transversal hot zones. Availability of a reliable model of transversal hot spots will enable a rational design and control, which avoid their undesirable formation. Acknowledgements We are thankful for the support of this research from National Science Foundation and the Welch Foundation. References Balakotaiah, V., Christoforatou, E. L., & West, D. H. (1999). Transverse concentration and temperature nonuniformities in adiabatic packed-bed catalytic reactors. Chemical Engineering Science, 54, 1725–1734. Benneker, A. H., Kronberg, A. E., & Westerterp, K. R. (1998). In>uence of buoyancy forces on the >ow of gases through packed beds at elevated pressures. A.I.Ch.E. Journal, 44, 263–270. Boreskov, G. K, Matros, Yu. Sh. Klenov, O.P. Lugovskoi, V. I., & Lakhmostov, V. S. (1981). Local nonuniformities in a catalyst bed. Doklardy Akademii Nauk SSSR, 258, 1418. Hagberg, A., & Meron, E. (1994). Pattern formation in non-gradient reaction-di4usion systems: the e4ects of front bifurcations. Nonlinearity, 7, 805. Jaree, A., Budman, H. M., Hudgins, R. R., Silveston, P. L., Yakhnin, V., & Menzinger, M. (2001). Temperature excursions in reactors packed with segregated layers of catalyst and inert solids. Chemical Engineering Science, 56, 5719–5726. Marwaha, B., Annamalai, J., & Luss, D. (2001). Hot zone formation during carbon monoxide oxidation in a radial >ow reactor. Chemical Engineering Science, 56, 89–96. Marwaha, B., & Luss, D. (2002). Formation and dynamics of a hot zone in radial >ow reactor. A.I.Ch.E. Journal, 48, 617–624. Nguyen, D., & Balakotaiah, V. (1994). Flow mal-distributions and hot spots in down->ow packed-bed reactors. Chemical Engineering Science, 49, 5489–5505. Segel, L. A., & Jackson, J. L. (1972). Dissipative structure: An explanation and an ecological example. Journal of Theoretical Biology, 37, 545–559. Turing, A. M. (1952). The chemical basis of morphogenesis. Philosophical Transactions of the Royal Society of London, Series B, 237, 37–72. Varma, A., Morbidelli, M., & Wu, H. (1999). Parametric sensitivity in chemical systems. Cambridge: Cambridge University Press. Yakhnin, V., & Menzinger, M. (2001). On transverse patterns in packed-bed catalytic reactors. Chemical Engineering Science, 56, 2233–2236. Yakhnin, V., Menzinger, M., Delmon, B., & Froment, G. F. (1999). Studies in surface science and catalysis, Vol. 126 (p. 291). Amsterdam: Elsevier. Zeldovich, Ya. B., Barenblatt, G. I., Librovich, V. B., & Makhviladze, G. M. (1985). The mathematical theory of combustion and explosives. New York: Consultants Bureau.