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Ind. Eng. Chem. Res. 2003, 42, 1662-1673
Hysteresis and Extinction Waves in Catalytic CO Oxidation Caused by Reactant Concentration Perturbations in a Packed-Bed Reactor Attasak Jaree Laboratory of Chemical Reactor Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Robert R. Hudgins,* Hector M. Budman, and Peter L. Silveston Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Vladimir Z. Yakhnin and Michael Menzinger Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6
The occurrence of large-temperature excursions in response to periodic changes in the inlet reactant concentration was demonstrated in a packed-bed reactor for CO oxidation over Pt/ Al2O3, a reaction known to be reactant-inhibited. Hysteresis or rate multiplicity was observed by changing the inlet CO concentration. A sufficiently large, sudden, and persistent increase of CO introduced into the reactor resulted in a transient temperature spike that exceeded the adiabatic temperature and was followed by complete extinction. Periodic perturbations of the inlet CO concentration near the hysteresis limit also resulted in high-temperature responses, the amplitude of which near the hot spot depended on the input frequency. A pseudohomogeneous model with modified Langmuir-Hinshelwood reaction kinetics was used to simulate concentration hysteresis effects and extinction waves. Sigmoid functions were used to relate the sticking probabilities of CO and O2 to the CO surface coverage on Pt. With these functions, the observed response to input disturbances could be adequately reproduced. Introduction The packed-bed reactor (PBR) is widely used in the chemical and petrochemical industries in applications such as hydrogenation of paraffins, partial oxidation of methylcyclohexane, oxidation of carbon monoxide, etc. With exothermic reactions, PBRs can exhibit complex dynamic features that are beyond the power of steadystate models to predict. The difficulty of analyzing these systems is increased by the fact that they are highly nonlinear. Multiplicity and stability are well-studied examples of steady-state problems encountered with exothermic reactions. Interesting dynamic phenomena exist, such as wrong-way behavior,1 differential-flow instability (DIFI),2 various types of travelling waves,3,4 bifurcation behavior,5 etc. An understanding of such phenomena and the ability to model them are essential for the development of reliable reactor control systems. There have been some theoretical investigations of the sensitivity of an exothermic reaction operating in a PBR to periodic or random inlet disturbances. DIFI results, on the one hand, from the positive feedback due to the released heat and, on the other hand, from the difference in the propagation velocities between heat and matter through the bed.6 Concentration waves travel with the mean speed of the flow stream, whereas thermal waves lag behind because of the large thermal inertia of the catalyst bed. When a wave of reactant concentration overtakes a thermal wave, the resulting increased rate of reaction can cause temperature excursions. DIFI, a so-called convective instability,2 plays a key role in most of the above-mentioned dynamic phenomena. * To whom correspondence should be addressed. E-mail:
[email protected].
As a model system for DIFI, we have chosen the oxidation of carbon monoxide. This reaction, catalyzed by transition-metal surfaces, has received much attention because of its role in the abatement of environmental pollution. An example is automobile catalytic combustion of exhaust gas. It is known that the feed composition and temperature of catalytic converters can oscillate, displaying several dominant frequencies superimposed on white noise.7 Monolithic catalysts are often employed, and these exhibit unexplained temperature excursions that can result in thermal degradation or even melting of the monolithic converters.8 The reaction is convenient for experimentation because only one reaction occurs if the O2 partial pressure is high enough. Our objective in this contribution is to illustrate the behavior of extinction waves and hysteresis caused by changing the CO inlet concentration during CO oxidation over Pt/Al2O3 in a PBR. Extinction waves are a special case of DIFI. CO oxidation on noble metal catalysts involves self-inhibition when the CO concentration is relatively high because of the adsorption characteristics of CO and O2. Thus, the use of high CO feed concentrations leads to extinction, accompanied by a high temperature rise during transients. Experimental data demonstrating extinction waves, steady-state multiplicity, and hysteresis phenomena will be presented. A mathematical model will be set up and shown to reproduce the phenomena observed in the experiments. Experimental Setup A schematic representation of a tubular PBR used to investigate extinction waves and hysteresis is shown in Figure 1. The reactor was constructed of two concentric
10.1021/ie020500w CCC: $25.00 © 2003 American Chemical Society Published on Web 03/22/2003
Ind. Eng. Chem. Res., Vol. 42, No. 8, 2003 1663
Figure 1. Schematic of the experimental apparatus, giving the positions of the thermocouples.
stainless steel tubes of different diameters. The inner tube with an external diameter of 2.54 cm contained a mixed bed of catalyst and inert particles, while the outer tube with a diameter of 7.6 cm formed an evacuated, heat-insulating annulus. Both tubes were 102 cm long. The inner tube had a wall thickness of 0.0508 cm. Two 2.54 cm holes along the side of the outer tube provided access to the outer annulus. One, located 2.54 cm below the top flange, was connected to a vacuum gauge. The second, 2.54 cm above the bottom flange, was connected to a liquid nitrogen trap and a 2 in. oil diffusion pump. To reduce end effects, the catalyst bed was placed 25.4 cm away from both the top and bottom of the reactor. Above a section of the Al2O3 support, the catalyst section was a homogeneous mixture of 0.2 wt % Pt/Al2O3 and Al2O3, the overall concentration of which was 0.02 wt % Pt/Al2O3. To monitor the time-dependent temperature profile resulting from the inlet concentration perturbations, nine thermocouples were arranged axially through the reactor about the centerline. The first two thermocouples were located in the empty space, 6.35 cm above the catalyst bed. One was the temperature sensor used to control the heating tape; next to it, thermocouple TC1 recorded the temperature of the feed gas to the reactor. The next thermocouple (TC2) was located at the boundary of the first layer of particles in the bed. TC2 was the first of seven thermocouples (TC2-TC8), located 7.6 cm apart along the bed axis, that provided measurements of bed temperatures. The final thermocouple (TC8) was situated 5.1 cm above the bottom of the catalyst bed. Individual thermocouples were bundled together to form an imperfect cone of diameter 0.95 cm at TC2, tapering to 0.08 cm, the diameter of a single thermocouple, at the location of TC8. The upper portion of the bundle was caulked with silicone into a tube fitted with a Swagelok coupling that allowed it to pass through a cap on the 2.54-cm-diameter inner stainless steel tube. Thermocouples were insulated with fiberglass sheathing. At the set-point temperature, the reactant gas mixture (N2, O2, and CO) flowed through
Table 1. Properties of the Gases Used in This Study composition
minimum purity (%) total hydrocarbons (ppm) oxygen (ppm) moisture
oxygen (zero gas)
nitrogen (prepurified)
carbon monoxide (CP)
>99.6 99.998
>99.5
