Periodicity and chaos in a catalytic packed bed reactor for CO oxidation

Chemrcal EngrnwNng Science,Vol. Printed

in Great

000%2509/88 $3.00+0.00 Pergamon Pressplc

43, No. 8. pp. 2289-2294,1988

Britain.

PERIODICITY

AND

Inst.

CHAOS

IN

A

CATALYTIC

E.

Wicke

PACKED

and

fiir Pllysikalische Scllloanplatz 4,

H.U.

BED

Onken

REACTOR

FOR

CO

OXIDATION

“1

Chemie, Universitgt 4400 Miinstcr, F.1Z.G.

Miinstcr

The contribution deals with the question: What Ilappens in a packed bed catalytic reactor in the case of an exothermic reaction and under adiabatic conditions when the reaction rate at single catalyst pellets begins to oscillate? pellets, and Such oscillations are presented with the CO oxidation on Pt/Al 0 the mechanism of their generation is discussed. Concerning a pacced bed reac2? tor it is shown that and why a profound influence of those oscillations on t11c dynamics of the reaction is restricted to the region of tIlerma bifurcation, i.e. the transition from the monostable to the bistable regime. In the pertinent ranqe of parameter values reaction rate oscillations at pellets in the entrance section of the packed bed initiate the growth of reaction waves that propagate throuyh the catalyst packing and give rise to large fluctuations of temperature and conversion in the exit section. Under certain conditions these fluctuations develop to deterministic chaos as it is characteristic of open systems with non-linear dynamics in bifurcation regions. 1.

llltrocluctiorr

Oscillations of reaction rates at single pellets of supported catalysts have first been reported on ISCRE 1, 1970, at Washington D.C., observed during tile of CO on Pt containing pellets (Beuscll et al., 1972). Since oxidation of H2 and then a large number of periodic pllenomena of reaction rate in hetcrogencous catalysis, predominantly in oxidation reactions, have been observed, and have been summarized in several review articles (most recently Raz/on and Schmitz, 1986). A number of reasons have been suggested to explain the rate oscillations_ Most likely on noble metals as active components are oxidation-reduction mecltanisms of the metal surface (Wicke et al., 1980: Sales et al., 19801, two-dimcnsional phase transitions in the chemisorbed state of reaction components (Wicke Bijcker and Wicke, 1985). et al., 1981, and/or structural cilanges of the metal surface - reconstruction or even disintegration - as have been observed by Ertl (Imand his group under UHV conditions especially on Pt single crystal planes bihl et al., 1986, Eiswirth and Ertl, 1986). The aim of this contribution is to show what happens when in a packed bed catalytic reactor the reaction rate begins to oscillate at sinqle catalyst pellets. This question was investigated by means of tile oxidation of CO 0,~ a supported catalyst (Onken 1987). In the next section we deal with tile oscll Pt'AIZoZ a ory bei1aviour of the reaction rate at single catalyst pellets and in section 3 wiLh the tlrcrmal bifurcation in an adiabatic packed bed reactor for a non-oscillating reaction (etharie oxidation on a Pd/Al203 catalyst). In section 4 the dynamics of the CO oxidation along the reactor tube is demonstrated, within the parameter range of the thermal bifurcation, affected by rate osciland the mechanism involved is discussed. lations of single pellets, 2.

Reaction

rate

oscillations

of

the

CO

oxidation

at

single

catalyst

pellets

(internal surface 150 m'/y) of cylindrical shape (3.3 x 3.3 ::;':::t:TnL;'a"~ wt. used for the reaction. % of Pt. were An arrangement of qlnss tu4 pellets, 8 mm distant from one ano'cher, in a frame of thin-walled bing as shown in Fig. 1 to the left, was embedded in the uppermost layer of a packing of inactive alumina pellets in the adiabatic reactor described below. The active pellets were held in position by 0.1 mm wires of Ni/CrNi thermocouples the junctions of which were located in the midst of 0.2 mm channels drilled through the pellets. The reaction conditions are specified in Fig. 1: v = linear gas flow rate in the empty tube at STP; To = temperature of pellet b *)

On leave

at

Dept.

Of

Chemical

Engng.,

Univ.

2289

of

Notre

Dame,

Indiana,

U.S.A.

2290

E. WICKIZ

without

CO

sur~~lv

to

the

qas

flow.

and H. U.

H9

ONKEN

shown in Fig. 1 to the riqht the temperature of the pellets jumfis up and dbwn f.53 by AT = 2 to 4 K with periods of 5 to :51 30 min, i.e. the reaction rate jumps indicating an inactive hack and forth, and an active state of the catalyst. In active state the reaction rate L57 the displays a fine structure of short-time breaks-in that increase steadily until L53 finally the rate falls down to its value in the inactive state. Here a process of reactivation proceeds gradually that becomes perceptible by the small . but distinct temperature rise In these Fig. 1: A 4 pellet arrangement on an regions in Fig. 1. Tile amplitude and inert packing the frequency of tile temperature jumps are not uniform among the pellets of a especially the amplitude increases with increasing Pt content catalyst charge, of the individual pellet, visible by the darkness of its grey colour. The four pellets shown in Fig. 1 flave therefore been selected wit11 regard to a rather strong and uniform grey, in order to get a fairly uniform oscillating behaviour. Patterns of square-shaped fluctuations that are surprisingly similar to those 1 have been observed in the group of Ertl with the CO oxidation shown in Fig. on Pt (110) single crystal planes at ca. 70 K higher temperatures under lligh vacuum conditions (Eiswirth and Ertl, 1986). LEED investigations (Ladas et al., 1988) revealed that the (110) plane becomes unstable during the reaction. It gradually changes its flat shape to a rather rough structure, displaying many the numerous edges and steps of this new crystal facets of Iligh-indexed planes; "active" state of the catalyst (high sticking facetted structure constitute the The jump back to the inactive state is attributed to coefficient for oxygen). surface that predominates at the higher tempcthermal reordering of tile (110) ratures during reaction in the active state. It seems probable that the jumps of reaction rate observed on the catalyst pel1 may be traced back also to structural changes - reconstruction lets in Fig. Pt crystalljtes in the planes at the surface of the or facetting - of crystal The jumps of reaction rate at those planes qivc rise to temcatalyst pellets. and the LIeat conductivity of the alumina pcrakure jumps of 'ihe Pt crystallites, tion of the relaxation jumps among the 1013 to support provides for synchroniza 10 14 Pt crystallites (average size 5 nm, Keil and Wicke, 1980) in the porous the uniform collective bellaviour of interior of a catalyst pellet. In this way the reaction rate of a pellet as shown in Fig. 1 can be understood. 3.

Thermal

bifurcation

in

a

catalyst

As

packed

bed

under

adiabatic

conditions

1945) that in the case of an exottlermic It is known for a long time (Wagner, catalytic gas reaction the heat balance conditions predicl; - within certain - three steady states of the catalyst: a quenranges of the parameter values ched state with low and an ignited state with high reaction rate alld ternThe heat balance diagram perature, and between these an unstable steady state. based on the ideas of Wagner is recalled in Fig. 2. The chemical heat producincreases like an s-shaped curve with intion, K Ij'reaction temperature T, representing creasin the Arrhenius law at low temperatures and the levelling off to the limit of external mass transfer at high the value of this limit being proportemperatures; tional to the concentration of the minority component in the reaction mixture. The flux of heat removal, bypassing gas flow is represented by a Q # by the s E raight line, that rises from the abscissa at the gas flow temperature (at A, B, or C in Fig. 2) with to the heat transfer coefficient a slope proportional at the external surface of the catalyst pellet, thus the slope increases with increasing gas flow rate. At BC AT A one high flow rates, line C in Fig. 2, there is on1 f; 2: 1Icat balance diain>ersection, i.e. one temperature, where Q = ._ I Fiq. steadyrstatg. representing one single (and stable) qram of an e:totllcrr.~ic At smaller flow rates catalytic reaction (and reduced gas temperature) curve that crosses the 4 line A is obtained for Q steady sta!?es mentioned sbove.rThe three times, corresponding to the three transitions back and forth between the lower and the upper stable state - ignifor instance by changing the fuel content in the gas flow tion and quenching, The limiting case between the monostable and are connected with a hysteresis. line B that touclres the heat production the bistahle reyime is displayed by the

HY

Catalytic packed

bed reactor

for CO

oxidation

curve in the inflection point and represents the state of (thermal) bithe inflection point symbolized by the insertion in Fig. 2. About furcation, the line B coincides narrowly with the heat production curve, the state of reSmall perturbations of the action is therefore not firmly fixed in this range. system from external or from internal sources become strongly amplified and give rise to sometimes large fluctuations of conversion and temperature. Such fluctuations have been measured already in an early investigation of the CO (Fieguth and Wicke, 1971). They represent a oxidation in a fixed bed reactor special case of those fluctuations that occur generally in bifurcation regions of open systems with non-linear dynamics far from equilibrium (Glansdorff and Priqogine, 1971). For a new and more thorough investigation of the fluctuating CO oxidation it was necessary to eliminate the possibility that the fluctuations might be initiated by random perturbations affecting the system from the outside. To this (silver-plaend an adiabatic reactor with a well insulated double mantle tube 3 (Onken and Wicke, 1986: ted and evacuated) was constructed as shown in Fig. The packed beds of catalvst nellets of different heiahts were DOOnken, 1987). -sitioned between layers of inert support pellets that provide for an established radial flow distribution and for adiabatic conditions in vertical direction. The parameters of the feed - flow rate, composition, temperature - were adjusted and kept constant by means of electronic control devices as precisely as possible A number of thermocouples, the junctions of which where placed in the centres of catalyst pellets as described above positioned along the (Fig. l), where axis of the packing. A thermocouple No.6 was located near the tube wall in order to observe inhomogeneities in radial direction. From the effluent flow a small part could be separated for concontent, tinuous recording of the CO i.e. the degree of converse .z n, by IR analysis_ The equipment was tested for perturbance-free operation by means of a nonoscillating reaction, namely the oxidation of ethane with air on a Pd/Al 0 catalyst. The 3 mm pellets were pa?k$d in a bed of 110 mm height between layers of inert alumina pellets similar to the arrangement shown in Fig.3. Fig. 4 preFig. 3: Adiabatic fixed-bed reactor sents the results of measurements about content in the effluent is olotted as a the bifurcation region. Here the CO2 function of the feed temperature at different fixed values of T f?ow rate and ethane content in With a flow rate of the feed. v = 30 cm/s the monostable regime was verified (Fig. 4 to the right) with v = 20 cm the bistable regime with hysteresis (to the left). In order to follow the hysteresis loop the feed temperature was first increased for "ignition", and then decreased again for "quenching" at the exit of the catalyst bed. The curve with the steepest slope in the midst of Fig. 4 represents the bifurcation point. The agreement with the schematic concept in Fig. 1 is obvious Fig. 4: Transition from monoto bistability and the proof of a disturbancethrough the bifurcation region. Ethane oxifree operation of the equipment satisfactory, because no irrequdation on Pd/A1203 larity from external perturbations could be detected despite the high sensitivity of the system in this bifurcation region (Wicke and Onken, 1986).

2291

2292

E.

4.1

The dynamics Travelling

For

these

4.

of the reaction

measurements

CO oxidation waves the

and H. U.

WICKE

in

the

H9

ONKEN

bifurcation

region

arrangement

shown in Fig. 3 with a 90 mm packing of represents the variations with time of the f~~~~~~~ur~~1~"'~0"~" YZZ"of ZZ*CO content in the effluent, obtained with the in the entrance cross rather low fee a tempzrature of 431 l?. The temperature T section remains constant within Ts 0.1 K. The tempera T2, 22 mm down& ure however, streams, shows humps of up to 5 K occurring in quasi-periodic sequence every 10 to 15 min. These humps travel along ttle catalyst bed with an averagc rate of 0.2 mm/s as indicated by the dotted lirles Some of tllenr grow up to give rise to maxima in the (T ) and steep peaks of up to 100 K in tlre exit 5 above). (Fiy. Some of the frumps, however, remain small or even effluent CO fade away: z hese, obviously, don't reach the threshold value necessary to trigger the ignition of a reaction wave. 5

c P i q . 5: CO bed reactor. in air, T 0

Oxidation v = 21 = 431 K.

in the fixedcm/s, 1 vol.%

Fig. at v

CO

6: Same as Fig. = 36.7 cm/s, To

5 =

440.5

K.

flow rate give rise to A slight increase of the feed temperature and of the gas a much more irregular pattern of reaction waves, Fig. 6, showing a faster seLittle correspondence only can be observed here quence and higher amplitudes. It seems that the maxima at the exit. between the temperature peaks and the CO but that regions cross section, maxima do no more extend uniformly o 3 er the co2 of different temperature and reaction rate alternate locally and temporally the variation wit11 time of the temperaalso in radial direction. Accordingly, (drawn in Fig. 5 below) shows little corof ttle eccentric thermocouple ture T respon 8 ence with the signals of the thermocouples near- tire axis. Since these fluctuations are not initiated by external disturbances - the temno perturbations also in this case - tile fluctuations obperature Tl shows they are initiated by tile reaction viously originate in the system itself, i.e. rate jumps of single catalyst pellets. 4.2

Generation

of

the

reaction

7: Shift Fig. heat removal by temperature of a catalyst

waves

of the line jumps pellet.

by

rate fluctuations at Single pellets Imaqine in the entrance section of the packed bed a catalyst pellet displaying quasi-periodic temperature jumps similar to those presented in Fig. 1. Each upward jump gives rise to a local temperature step in the bypassing gas flow, and line of the pelthereby shifts the Q lets in the downstre& wake to a higher temperature, i.e. towards P" in Fig. 7. It depends on the height and the duration of this temperature step if it is successful in initiating a reaction wave, or if it is insufficient and fades away as happens frequently in the case of

H9

Catalytic packed

bed reactor for CO

2293

oxidation

6) shifts the position of the feed temperature (Fig. 5+Fig. Fig. 5. A raise line nearer to the ignition point: then smaller local temperature of the Q humps ari$ sufficient for initiation of a reaction wave, and therefore more osThe faster sequence leads to the sicillation peaks of pellets are successful. tuation that the following reacting wave propagates into the still non-uniform temperature field left behind by the foregoing one. This causes irregularities of temperature and reaction rate over the cross section and finally to chaos as exhibited by the measurements in Fig. 6. 4.3

Chaotic

OLI

behaviour

TO

5.

=

fine

structure

’

Fig. co v 2

and

,,,I

8: Long-time registration of the fluctuations in the effluent. 0.5 vol.% CO in air, 36.7 cm/s, 440 K.

of

the

reaction

waves

In order to test the behaviour of the system during longer times several longtime series of measurements - over 3 content in the hours each - of the CO effluent were carried sut. Part of such a long-time measurement (performed with 0.5 % CO in the feed, but otherwise the same conditions as in Fig. 6) is pre8 below: above is shown sented in Fig. the first part of this series with tenfold extended time scale. One perceives here a detailed fine structure of many high-frequency, low-amplitude fluctuasuperimposed to the large variations, content caused by the tions of the CO reaction waves grriving at the exit. The series' of the large variations have been evaluated to check for deterministic chaos; the method of the largest Lyapunov exponent (Wolf et al., 1985) was applied and the procedure of the correlation dimension (Grassberger and Procaccia 1983). It may be sufficient here to state that both methods led to the result that the large fluctuations indeed represent deterministic chaos (Onken, 1987; Onken and Wicke, 19881, but that the fine structure seems to be stochastic. This fine structure obviously originates from rate oscillations of single pellets and pellet groups that did not initiate reaction waves. Similar high-frequency fluctuations have been observed in the monostable regime beyond the bifurcation region.

Conclusions

Oscillations of reaction rate at single catalyst pellets affect the dynamics of an exothermic reaction in a fixed-bed reactor severely in the region of thermal bifurcation only. Here reaction waves can be initiated that grow up during propagation through the catalyst packing and may result in chaotic fluctuations of temperature and conversion. Beyond the bifurcation region the oscillations produce small irregularities only of temperature and conversion. Acknowledgements: schung des Landes mie are gratefully

Financial support by the Minister NRW, the Max-Buchner-Forschungsstiftung acknowledged.

fiir Wissenschaft and the

Fond

und Forder Che-

References Beusch, H., Fieguth, P., and Wicke, E., 1972, Unstable Behavior of Chemical Reaction at Single Catalyst Particles. Xiv. Chem, Ser. 109, 615-21. Thermisch und kinetisch verursachte Instabilititen im Reaktionsverhalten einzelner Katalysatork&ner. Chem.-Ing.-Techn. 44, 445-51. &cker, D., and Wicke, E., 1985, In-situ IR Study during Oscillations of the Catalytic CO Oxidation. Ber. Bunsenges. Phys. Chezn. 89, 629-33. Eiswirth, M., and Ml, G., 1986, Kinetic Oscillations in the Catalytic CO Oxidation on a Pt(ll0) surface. Surf. Sci. 177, 90-100. van Zi.ind/L&ch-Verhalten zu stabilen ReaktionszuFieguth, P., and Wicke, E., 1971, Der ubergang &&den bei einem adiabatischen Rohrreaktor. Qlem.-Ing.-Techn. 43, 604-08. Glansdorff, P., and Prigcgine, I., 1971, "Structure, Stability and Fluctuations", Wiley, New York. Grassberger, P., and Prccaccia, I., 1983, Measuring the Strangeness of Strange Attractors. Physica z, 189-209.

2294

E. WICKE and H. U.

ONKEN

Imbihl, R., Cox, M.B., Ertl, G., 1986, Kinetic Oscillations in the Catalytic CO Oxidation on J. Chem. Phys. 84, 3519-34. Ft(ll0) : Experiments. Keil, W., and Wicke, E., 1980, fiber die kinetischen InstabilitZten bei der CD-Oxidation an PtKatalysatoren. Ber. Eunscnges. Phys. Chem. 84, 377-83. Ladas, S., Imbihl, R., and Ertl, G., 1988, Microfacetting of a Pt(ll0) Surface during Catalytic CO Oxidativn. Surf. Sci., 197, 153-82. Onken, H-U., 1987, Der iiberyang "on pericdischen Oszillationen zum Chaos bei der Oxidation van Kohlenrwnoxid an einem Platin-Tr;igerkatalysator. Dissertation Univ. MCinster. Onken, H-U.. and Wicke, E., 1986, Statistical Fluctuations of Temperature and Conversion at the Catalytic CO Oxidation in an Adiabatic Packed Bed Reactor. ~r.Bur~scn~es.PI~ys.Clr~. 90, 976-81. Onkcn, H.U., and Wicke, E., 1988, Identification of Chaos durinq tile Oxidation of CU on Pt/A1203 in an Adiabatic Fixed-bed Reactor. IMACS wrld Congress, Paris. Razon, L-F., and Schmitz, R-A., 1986, Intrinsically Unstable Behavior during the Oxidation of CO on Pt. Cat. Rev. Sci. Ehgng. 28, 89-164. J.E., Sales, B.C., and Maple, M-R., 1981, Oscillatory Oxidation of CO over a Pt CataTurner, lyst. Surf. Sci. 103, 54-74. den Umsatz an Hijchstleistungskatalysatoren. Chemische Technik 18, l-7; Wagner, C., 1945, ij= iiber die Temperoturcinstellung an Hiichstleistungskatalysatoren. ibid. 28-34. Behavior in Wicke, E., Kwmlann, P., Kcil, W., and Schiefler, J., 1980, Unst,able and Oscillatory Heterogeneous Catalysis. Bcr. Bunsenges. PIlys. Chem. 84, 315-23. Fluctuations of Conversion and Temperature in an Wicke, E., and Onken, H-U., 1986, Statistical Adiabatic Fixed-bed Reactor for CO Oxidation. Chem. Ehgng. Sci. 41, 1681-87. Lyapunov Expnents Il.L., and Vastano, J.A., 1985, Determining Wolf, A., Swift, J-B., Swinney, from a Time Series. Fhysica m, 285-317.

H9