Steady-State Multiplicity and Superadiabatic Extinction Waves in the

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Ind. Eng. Chem. Res. 2003, 42, 37-45

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Steady-State Multiplicity and Superadiabatic Extinction Waves in the Oxidation of CO/H2 Mixtures over a Pt/Al2O3-Coated Monolith Mingyong Sun, Eric B. Croiset, Robert R. Hudgins,* and Peter L. Silveston Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Michael Menzinger Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6

Steady-state multiplicity was observed in the catalytic oxidation of mixtures of H2 and CO over a cordierite monolith wash-coated with Pt/Al2O3. Increasing the CO concentration or decreasing the H2 concentration across the hysteresis limit results in extinction, whereas decreasing the CO concentration or increasing the H2 concentration across the other hysteresis limit causes ignition. CO inhibits the ignition of H2, whereas H2 assists the ignition of CO. Extinction of the reaction due to a step increase of CO or a step decrease of H2 occurred in the form of a downstream-moving wave accompanied by a sharply localized superadiabatic temperature excursion. Axial temperatures were recorded by five equally spaced thermocouples. The superadiabatic rise in temperature and the speed of the extinction wave are related in a complex way to the size of the CO jump into the hysteresis zone. The greater the H2 jump downward in concentration, the lower the maximum temperature and the greater the speed of the reaction front. Introduction Oxidation of carbon monoxide continues to receive attention in the literature because of its role in air pollution control. Multiple steady states exist in the catalytic oxidation of carbon monoxide over supported noble catalysts in packed-bed reactors.1-5 Sharma and Hughes observed a transient temperature increase, termed “wrong-way behavior”, when they decreased the feed temperature.3,4 Recent experimental and modeling investigations have shown that a sudden increase in the concentration of carbon monoxide could also cause similar high-temperature waves in the oxidation of carbon monoxide over a Pt/Al2O3 catalyst in a packedbed reactor.5 Compared to catalytic packed-bed reactors, less attention has been given to catalytic monolith reactors, which are used in the abatement of automotive exhaust gases. Catalytic monoliths differ from packed-bed reactors in the configuration of the catalyst and, thus, in the mechanisms of heat and mass transfer. Whereas the flow in packed-bed reactors is turbulent, in monoliths, it is typically laminar. Because of its large void fraction, a monolith reactor has a much smaller thermal inertia than a packed-bed reactor. It is expected that the temperature responses of monolith reactors to transient operations, such as sudden changes in feed compositions, will be more pronounced than those observed in packed-bed reactors. Oh and Cavendish simulated the temperature responses of monolith catalysts to step increases and decreases in the feed temperature.6 They found that a step decrease in the feed stream temperature could lead to an unexpected transient temperature rise in the solid phase (catalytic wall) above its initial temperature, similar to wrong-way behavior in packed-bed reactors. * Author for correspondence. E-mail: [email protected].

Catalytic converters are complicated systems, through which CO, hydrocarbons, and NOx are to be converted to CO2, H2O, N2, and O2. They are habitually operated under transient conditions. In addition to the cold-start period, when a cold monolith catalyst is suddenly exposed to hot exhaust gas, the flow rate, temperature, and composition of the exhaust gas change with the operating mode of engine. Thus, it is of practical interest to study how transient temperature excursions occur during dynamic operations of monolith catalysts. In the present experimental work, we investigate the multiplicity of steady states relative to CO and H2 concentrations and the occurrence and propagation of hightemperature, superadiabatic extinction waves. Depending on how an automobile is operated, its exhaust gas contains various amounts of hydrogen, from 0.1 to 2%.7 The presence of H2 aids the light-off of CO oxidation6 and leads to a solid temperature that exceeds the adiabatic temperature.6,8-10 The ease with which hydrogen causes light-off has been considered a means of heating the catalyst during cold starts.11,12 Therefore, it is of practical importance to understand how the oxidation of H2 interacts with the oxidation of CO over the catalytic monolith. In the present contribution, we also examine the influence of H2 on CO oxidation behavior, apparently for the first time. Experimental Section Reactor and Monolith Catalyst. The reactor was a quartz tube (1 cm in diameter) with a vacuum jacket (10-6 Torr) to minimize the heat loss (Figure 1). The vacuum jacket was wrapped in a layer of insulating material (Kaowool) to further reduce heat loss. A heating tape was used to preheat the feed mixture and control the inlet gas temperature. The first thermocouple (TC1) was located 5 mm upstream from the entrance of the catalyst to monitor the feed gas temperature.

10.1021/ie020310l CCC: $25.00 © 2003 American Chemical Society Published on Web 11/19/2002

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Figure 1. Schematic of the monolith reactor. Thermocouple placement: TC1 ) -5 mm, TC2 ) 2 mm, TC3 ) 22 mm, TC4 ) 42 mm, TC5 ) 62 mm, TC6 ) 82 mm.

Figure 2. Schematic cross section of the monolith catalyst. Table 1. Physical Properties of Monolith Catalyst Pt content, g/L washcoat loading, g/L thickness of wash coat, mm density of monolith, g/cm3 density of washcoat, g/cm3 bare substrate channel width, mm wall thickness, mm

1 117 0.034 0.41 1.5 1.105 0.165

The Pt/Al2O3 washcoated cordierite monolith was provided by DCL, Concord, Ontario, Canada. Its main physical properties are listed in Table 1. The washcoat was evenly distributed to a thickness of about 0.034 mm. The monolith catalyst was sculptured so that 20 channels symmetrically surrounded a central channel, as illustrated in Figure 2. As shown in Figure 1, five thermocouples (TC2-TC6) were evenly spaced axially within the central monolith channel, TC2 being located 2 mm downstream from the entrance, and the remainder being spaced 20 mm apart from each other. The diameter of a single thermocouple was 0.23 mm. The five thermocouples were bundled together and inserted from the outlet end of the monolith. The inlet end of the central channel was blocked with high-temperature cement to prevent gas from flowing through this channel; thus, the thermocouples were in a dead-ended tube that was open at the downstream end. Furthermore, the thermocouples were slightly bent to make the measuring tips touch the channel wall and, hence, to measure the monolith temperature. Experimental Procedure. H2 and CO were supplied from high-pressure cylinders. Air was obtained from pressurized supply lines, passed through 4A-type molecular sieves and then through a bed of anhydrous calcium sulfate (Drierite) to remove water and other impurities. Mass flow controllers regulated the flows of each of the above gases, so as to produce a desired

Figure 3. Multiplicity resulting from step changes in the carbon monoxide concentration. The CO concentration is on the right coordinate, the balance being dry air; the total flow rate is 0.214 mol/min.

composition and total flow rate of the gas mixture to the reactor. At the beginning of each experiment, the air alone was heated and passed through the reactor at the desired flow rate. Once the temperature profile of the reactor was constant, either carbon monoxide or H2 was added to the air at a certain flow rate to obtain a desired concentration, while the total flow rate remained unchanged. Then, the reaction was ignited, and the heat generated by the two exothermic reactions raised the temperature of the catalyst. The temperature rise along the monolith catalyst was recorded by the five thermocouples that were connected to an interface residing in a computer and read at a frequency of 0.1 Hz. Before any further changes were made, the system was allowed to reach a steady state. For reference, the adiabatic temperature rise from the combustion of CO in air is 100 °C for every 1% CO; for the combustion of H2 in air, it is 75 °C for every 1% H2. Results and Discussion Step Changes in CO Concentration. Figure 3 illustrates the hysteresis phenomenon that arises when stepwise changes are made in the concentration of CO in air. The feed temperature was set at 150 °C, and the total flow rate was 0.214 mol/min, corresponding to a mass average velocity of 6.2 m/s at 150 °C. At time 0 ks, an extinguished steady state is obtained at a CO concentration of 1.0% because of the strong inhibition effect of carbon monoxide.13 The temperature distribution decreases monotonically, at a rate of about 2 °C/ cm along the reactor axis, because of heat loss. This temperature decrease is much smaller than the temperature rise due to steady-state or transient reaction. Therefore, we shall tend to neglect heat losses in what follows. As the CO concentration is then decreased stepwise from 1.0% to 0.5, 0.4, and 0.3%, the reaction ignites when the concentration reaches 0.3%. TC2 displays the highest temperature and a rise of about 34 °C (from 153 °C, the feed gas temperature, to 187 °C), which is just short of the adiabatic temperature rise expected from the complete combustion of 0.3% CO. This suggests that CO is completely oxidized just ahead of TC2, which is within a very narrow region near the entrance of the monolith. No reactant remains downstream from the reaction front. In the downstream region, the monolith is heated by the hot gas through convective heat

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Figure 4. Temperature at TC6 as a function of inlet CO concentration (feed temperature ) 150 °C, flow rate ) 0.214 mol/ min).

transfer. The reaction remains ignited when the CO concentration is further decreased to 0.2 and 0.1%, and the temperatures decrease to correspondingly lower levels because the exothermicity falls in proportion to the lower concentrations of reactant. Then, in Figure 3, the CO concentration is stepwise increased to 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7% in turn. At each concentration, the system is allowed to reach steady state before the next step increase occurs. The temperatures of the thermocouples increase proportionately, and the highest temperature is still observed at TC2 as the CO concentration is stepped up to 0.4%, again meaning that complete CO oxidation occurs near the entrance of the reactor. When the CO concentration is stepped to 0.5%, the temperature recorded at TC2 drops, following a transient increase, and TC3 registers the highest temperature at steady state under this concentration, indicating that the reaction front has shifted downstream. A further increase in the CO concentration to 0.6% causes a further decrease in the temperature at TC2 and a drop in the temperature at TC3 after an initial increase. TC4 records the highest temperature under this condition. TC3 now displays a temperature of 173 °C, indicating that a CO conversion of roughly 35% has been achieved within the first 2 cm of the monolith. For a 0.6% CO feed, the steady temperature is 207 °C at TC4, close to the adiabatic value for complete conversion. The reaction front is inferred to lie between TC3 and TC4 (see below). Increasing the CO concentration further to 0.7% results in a rapid decrease of the temperatures recorded at TC2 and TC3 to the original feed gas temperature, i.e., complete extinction within the entrance region of the monolith. The CO concentration wave travels through the reactor with the mean speed of the flow (>6 m/s) and takes less than 0.01 s to pass from TC2 to TC5. The temperature wave, on the other hand, travels much more slowly because of the thermal inertia of the monolith.14 As the stream containing the excess CO impinges on the still hot, slow-moving reaction front, the temperature spikes to provide a superadiabatic temperature, a temperature higher than that obtainable under steady-state adiabatic operation. This mechanism is closely related to that of the well-known wrong-way behavior.14 In Figure 4, the temperature of TC6, an indicator of the ignited or extinguished state of the reactor, is

replotted from the steady-state data in Figure 3 as a function of inlet CO concentration. The result is a “clockwise” ignition-extinction hysteresis with the CO concentration as the control parameter. The feed gas temperature affects the CO concentration at the hysteresis limits (also termed bifurcation points or ignition/extinction limits). Generally, the hysteresis limits decrease as the feed gas temperature is lowered. For example, when the feed gas temperature decreases from 150 to 125 °C, the ignition CO concentration drops from 0.3 to 0.1%, and the extinction CO concentration drops from 0.7 to 0.3%. The extinction/ignition phenomenon can be understood qualitatively as follows. At a high CO concentration in the gas phase, the adsorbed CO species dominates the catalytically active surface as a result of the strong affinity of CO for adsorption on Pt (i.e., the large CO adsorption coefficient). Thus, CO inhibits the adsorption of oxygen on the active surface,6,13 and therefore, the oxidation proceeds too slowly to ignite the reaction. At low CO concentrations, on the other hand, a certain fraction of the catalytically active surface remains uncovered by CO, permitting O2 to adsorb and to increase the CO oxidation rate. Increasing the temperature decreases the CO coverage and thus increases the relative proportion of adsorbed oxygen as well as the reaction rate constant. Therefore, a higher temperature is needed to ignite feeds that contain higher CO concentrations. Effect of H2 on CO Oxidation. When H2 is added, several new phenomena occur. When 1% H2 is mixed with 0.7% of CO in air, the mixture cannot be ignited at 150 °C, even though H2 alone in air is spontaneously ignited over the catalyst at room temperature. For the combustion of a mixture of CO and H2, increasing the CO concentration can cause extinction as it does in the combustion of CO alone. Clockwise hysteresis cycles exist for CO concentration similar to those observed in the absence of hydrogen, as shown in Figure 4, except that the presence of H2 shifts the bifurcation points of extinction and ignition to higher CO concentrations. At a feed gas temperature of 150 °C, the extinction concentration of CO is 0.7% without H2 and 0.9% with H2; the ignition concentration of CO is 0.3% without H2 and 0.4% with H2. This behavior suggests that CO and H2 compete for the same active sites on the platinum catalyst. CO blocks almost all surface sites, and the adsorption of H2 is thus inhibited because CO has a much greater affinity for platinum than does H2;6,15 consequently, the catalytic rate of H2 is also inhibited. On the other hand, H2 assists the ignition of CO oxidation. Figure 3 shows that, when the CO concentration is at 0.4% or higher, the reaction cannot be ignited at 150 °C; only when the concentration of CO decreases to 0.3% does ignition occur. Although not shown in the figure, in the presence of 0.2% of H2, the reaction can be ignited at a CO concentration of 0.4%. This means that, at a given temperature, with the assistance of H2, the oxidation of CO can be ignited at higher CO concentrations. The catalytic oxidation of H2 can occur readily, even at a low coverage on the catalyst surface, because the activation energy of the reaction between the adsorbed H2 and oxygen species is much lower than that between adsorbed CO and oxygen.15 A small amount of H2 consumption leads to a slight temperature increase,

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Figure 5. Hydrogen aids the ignition of CO combustion.

Figure 6. Hysteresis with hydrogen concentration in the combustion of mixtures of CO and hydrogen.

which shifts the adsorption-desorption equilibrium of CO toward desorption. With a decrease in the coverage of CO, the surface coverages of H2 and O2 increase, and the resulting H2 oxidation further raises the temperature and leads to a greater fraction of uncovered surface. Finally, the reaction of CO lights-off when the number of uncovered surface sites is large enough to allow a sufficient amount of oxygen to adsorb. At higher CO concentrations, it takes more H2 to provide higher surface temperatures to achieve the necessary fraction of uncovered surface to light-off the CO oxidation. Therefore, the greater the CO concentration in the feed, the higher the H2 concentration needed to ignite the reaction. For example, at 150 °C, when the CO concentration is 0.7%, the H2 concentration should be 1.1% to ignite the reaction, whereas the H2 concentration has to be as high as 1.3% to ignite the reaction when the CO concentration is 1.0% (see Figure 5). At a constant CO concentration, when the hysteresis is plotted with H2 as the control parameter, it is counterclockwise, as shown in Figure 6. A higher CO concentration requires a higher H2 concentration to ignite the reaction and to keep the ignited reaction from collapsing to the extinguished state. The relationship between the CO and H2 concentrations needed for ignition is not linear. When the CO concentration is higher than 1%, ignition is more difficult. The H2 concentration has to be as high as 2.5% to ignite the reaction at 150 °C when the CO concentration is 1.1%. Presumably, the number of active sites

available for H2 adsorption is then so small that a very high H2 concentration is needed to make the number of adsorbed H2 species large enough to ignite the reaction. At CO concentrations higher than 1.2%, the catalyst active sites are so heavily covered by CO that very little H2 or O2 is adsorbed. Under this condition, increasing the H2 concentration to as high as 4% cannot ignite the reaction; instead, increasing the feed gas temperature will lead to ignition because of CO desorption and a relative increase in the surface coverages of H2 and O2. Extinction Waves Due to Increasing CO Concentration. For the combustion of CO and H2 in air on the platinum catalyst, an increase in the CO concentration or a decrease in the H2 concentration across the bifurcation points results in extinction, accompanied by a superadiabatic transient or extinction wave. Figure 7 shows a series of extinction waves initiated by increasing the CO concentration across the bifurcation point in steps of different sizes for CO oxidation in the presence of 1% H2 in the feed gas. The exothermicity of 1% H2 would raise the catalyst temperature by 75 °C. For a feed gas temperature of 150 °C, the temperature at the reactor entrance should be 225 °C before the introduction of CO. This means that the reaction should not be able to be extinguished by increasing the CO concentration; thus, no extinction waves should be observed. In previous experiments for a feed gas temperature of 125 °C, it was observed that increasing the CO concentration from 0.2 to 0.3% would cause extinction of the reaction. For this series of runs, the feed gas temperature was set at 50 °C; then, with 1% H2 in the feed and no CO, the temperature at the reactor entrance was observed to be 125 °C because of the catalytic oxidation of the hydrogen. As Figure 7 shows, from an initial 0.2% of CO, we stepped up the CO concentration to different levels, namely, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9% in turn, to study the effect of the step increases in CO concentration on the extinction behavior. The series of high-temperature extinction waves created by increasing the CO concentration from 0.2% to different levels is discussed in detail below. At the beginning of the experiment, only air flows through the reactor at a flow rate of 0.214 mol/min. At about 3 ks, 0.002 14 mol/min of H2 (1% of the total flow rate) is added. The total flow rate of the gas mixture is held constant at 0.214 mol/min by an appropriate decrease in the air flow rate. Once H2 enters the monolith, the reaction immediately ignites, as indicated by the sharp increase in the temperatures from TC2 to TC6. TC2 records the highest temperature, which means that the reaction front is located close to the entrance of the monolith. At about 3.8 ks, CO is introduced to the 1% H2/air mixture at 0.2% of the total flow. The temperature readings increase because of the added exothermicity of the CO oxidation reaction. After the temperature profiles become stable at 5.3 ks, the CO concentration is further increased from 0.2 to 0.3%, that is, the concentration enters the region that causes the reaction to be extinguished. Figure 7a-c (of which parts b and c are enlargements of two sections of part a) prompts many interesting observations. For purposes of describing various thermal phenomena, let us label four distinct parts of the temperature response to a concentration step increase at a given thermocouple, using that of TC6 shown in Figure 7b as an example. Zone i refers to the initial response of the temperature; zone ii applies to the

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Figure 7. Pre-extinction waves due to increases in CO concentration (H2 concentration ) 1%, total flow rate ) 0.214 mol/min, feed gas temperature ) 50 °C): (a) all experiments, (b) CO step increase from 0.2 to 0.3%, (c) CO step increase from 0.2 to 0.7%, (d) CO step increase from 0.2 to 0.8%, (e) CO step increase from 0.2 to 0.9%.

following section in which the temperature rises less steeply than in zone i or even plateaus; zone iii pertains to another rapid rise in the temperature following zone ii; and zone iv applies to the extinction of the temperature peak. First, following a step increase in CO, regardless of whether the hysteresis limit is crossed or not, all thermocouples respond very rapidly in zone i on the time scale of Figure 7a. The thermocouples rise rapidly either to a constant temperature with perturbations that do not cross the hysteresis limit (i.e., step changes to CO concentrations that equal or fall below 0.2%) or to a temperature shoulder followed by a more slowly rising ramp in the cases of supercritical perturbations in zone ii (i.e., step changes to CO concentrations that equal or exceed 0.3%). The steady-state temperature increase of thermocouple TC2 for 0.2% CO corresponds closely to the adiabatic temperature rise (ATR), whereas the thermocouples further downstream show smaller increments because of heat losses, as explained above. For the sequence of experiments with increasing CO perturbations into the extinction zone, the temperature rise in zone i increases in proportion to the size of the CO perturbation, whereas the superadiabatic temperature rise (SATR) spikes (in zone iii) remain roughly constant. Following zone ii, the duration of which increases linearly in the downstream direction from TC2 to TC6, the thermocouples experience a second delayed temperature rise, this time above the ATR, which we call a superadiabatic temperature rise or SATR (in zone iii). The peak temperature of the SATR is nearly constant

at ∼215°C for all thermocouples, despite the gradual decrease of the slope in zone ii (see Figure 7b, between 5.3 and 6.1 ks), indicating that the secondary SATR increases in the downstream direction. The extinction wave slows as it moves downstream, and its slope decreases with time (e.g., zones iv in Figure 7b and 7c). The rise time between the initial CO jump and the attainment of the ATR (beginning of zone ii) and the secondary rise time to the SATR peak (i.e., zone iii) both lengthen with distance along the reactor axis (i.e., from TC2 to TC6). As the CO jump across the hysteresis limit increases (from 0.3 to 0.9% CO), the SATRs and the extinction fronts (zone iv) follow each other at shorter time intervals, indicating an acceleration of the extinction waves. For the CO jump to 0.8%, only TC6 records an ATR and SATR, whereas instant extinction is noted on all the other thermocouples. To interpret these phenomena, we note that four types of waves, characterized by different time scales, come into play. (a) The fastest is the flow vflow ≈ 6 m/s, which transports inert molecules from TC2 to TC6 in 1.3 ms. (b) The second is the passive transport of heat with velocity vheat ) vflow/Le ) 0.6 cm/s, where Le is the Lewis number, defined as the ratio of the sum of heat capacities of the solid plus gaseous phases to that of the gaseous phase alone.18 Its value is on the order of Le ) 1000. Hence, vheat ) 0.6 cm/s, and the heat wave takes

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only 13 s to travel the distance of 8 cm from TC2 to TC6. This passive heat wave accounts for the rapid heating of the downstream thermocouples following the initial CO perturbation. (c) The third wave is the combined SATR-extinction front, the speed of which depends on the magnitude of the CO jump, as well as on catalyst inhibition by CO. For small CO jumps (final CO concentration ) 0.30.4%), its speed vextinctn is on the order of vextinctn ≈ 0.010.02 cm/s; for large jumps (final CO concentration ) 0.9%), vextinctn becomes immeasurably fast. The length of zone ii increases with the downstream distance from TC2 for small CO jumps (Figure 7b), as the slow extinction wave lags behind the fast, passive heat wave. (d) The extinction wave is driven downstream by progressive inhibition of the catalyst surface by CO, and its velocity rises with the CO content in the feed stream. The high-temperature SATR spike at thermocouple TCi signals extinction of the reaction just upstream of TCi. As a result of catalyst inhibition and rapid passive cooling of the monolith by convective heat transfer upstream of the slowly moving reaction zone, this moving reaction zone is narrower than the stationary reaction zone at lower values of CO concentration. The unreacted, cool fuel is rapidly swept into this steep reaction zone by the flow, and the reaction heat is released in a very narrow axial section. As a result, the temperature rises high above the adiabatic limit, in a manner similar to that described elsewhere.16,17 The mechanism of the SATR extinction waves is closely related to the well-known wrong-way behavior.18 (e) The fact that the relaxation time τ1 from the temperature just preceding the CO perturbation to the ATR shoulder, as well as the relaxation time τ2 from the shoulder to the SATR peak increase from TC2 to TC6 indicates the action of heat dispersion. (f) The acceleration of the extinction wave with increasing size of the CO jump is linked to the increased steepness of the moving reaction zone because of enhanced catalyst inhibition. Heat dispersion tends then to drive the reaction zone downstream in proportion to the temperature gradient. From a close examination of the evolution of the temperature of individual thermocouples (Figure 7b,c), it is evident that the rate of temperature increase varies with time. When the CO concentration is suddenly increased, the concentration front passes through the reactor within 0.01 s. We have already noted that there are four different waves differing greatly in speed and that the temperature rise upon perturbation has a significant fine structure, termed ATR and SATR. In the process, some CO molecules in the feed initially pass into the downstream part of the reactor unconverted. The monolith temperature through the reactor increases rapidly because of the reaction heat evolved from the increased amount of CO during this initial period. With the increase in catalyst temperature, the conversion of CO in the upstream region also increases. At a certain point, all CO molecules are converted in the reaction front upstream of TC2, and no reaction occurs on the catalyst surfaces further downstream. At the reaction front, the monolith is heated directly by the release of the exothermicity of the reaction at the catalytic surface. The gas is heated in turn by the hot monolith through convective heat transfer from its surface. Downstream from the reaction front, no reaction occurs on the catalyst surface: the monolith is heated by the hot gas

through convective heat transfer. Therefore, the downstream thermocouples record a slow increase in temperature that follows the initial rapid increase. The increase in CO concentration from 0.2 to 0.3% just crosses the bifurcation limit for extinction. The increased CO concentration inhibits the reaction at the entrance of the monolith. As the stream containing unconsumed reactants encounters the hot reaction zone, the temperature rises locally well above the adiabatic temperature rise. The reaction cannot remain in the ignited state because of the strong inhibition effect of CO at such a high concentration. The temperature collapses because the reaction is extinguished. Thus, the concentration front (i.e., reaction front) moves downstream, where the catalyst is still hot. Now, the monolith at the new reaction front is heated directly by the oxidation of CO, and the temperature increases more rapidly than by convective heat transfer from the gas phase. Thus, at a given location, the temperature rises rapidly in the monolith when the temperature upstream from it falls to the inlet temperature. Therefore, a temperature spike initiated at the reaction front close to the entrance of the catalyst travels downstream, where it is eventually blown out of the reactor. The average speed of the high-temperature peak between TC2 and TC6 is about 0.09 mm/s. The behavior of these extinction waves has been previously described16,17 and is closely related to the mechanism of the well-known phenomenon of wrong-way behavior.14 Additional experiments were conducted to investigate the influence of the size of the concentration step on the temperature excursion. Removal of CO from the feed gas results in reignition of the H2 reaction. A subsequent return of the CO concentration to 0.2% restores the temperatures previously obtained under the same conditions. This indicates that the steady-state temperature profile at 1% of H2 and 0.2% of CO is reproducible. Starting from the same steady state, at a CO concentration of 0.2% and a H2 concentration of 1.0%, the CO concentration is increased to 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9% in sequence while the H2 concentration is held constant. Figure 7 shows that the maximum excursion temperature increases with increasing step changes in the CO concentration until the CO concentration reaches 0.8%. Increasing the CO concentration from 0.2 to 0.3% results in a temperature overshoot of 67 °C. The overshoots caused by increasing the CO concentration from 0.2% to 0.4, 0.5, and 0.6% are 82, 94, and 106 °C, respectively. Each increase of 0.1% in CO concentration adds about 12-15 °C to the maximum temperature. Let us now examine the development of the SATR as can be seen in Figure 7a. Consider a given axial coordinate, z, in the reactor; z is chosen just downstream of the reaction front so that all of the CO has been converted upstream of z. Thus, the catalyst surface at z is predominantly oxygen-covered and is heated largely by convective heat transport from upstream of z. Then, for some particular CO step increase in the feed, a sudden increase in the CO partial pressure occurs at z. The oxygen-covered catalyst surface at z (and for some distance downstream) becomes progressively covered by CO as the adsorbed oxygen reacts and disappears from the surface. The rate of reaction at z and beyond is a function of the CO partial pressure as well as of the surface oxygen. In addition, the reaction rate increases exponentially with temperature. This increase in reaction rate with local CO partial pressure and tempera-

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Figure 8. Traveling speed of the thermal front as a function of final CO concentration (H2 concentration ) 1%, total flow rate ) 0.214 mol/min, feed gas temperature ) 50 °C).

ture produces the SATR. The higher the CO partial pressure, the greater the reaction rate, and the higher the catalyst surface temperature; thus, a further transient increase will occur in the reaction rate. However, once the surface oxygen species becomes depleted, the catalytic surface becomes mainly CO-covered, and the CO oxidation process is reduced to the point of extinction. For large CO step increases, the surface oxygen species disappear rapidly, and the extinction process is accelerated. This explains the narrower reaction zone (Figure 7a) and faster extinction waves (Figure 8) as the CO concentration step size increases. However, TC2 fails to reach the maximum temperature before the temperature collapses when the CO concentration is increased from 0.2 to 0.7%, and only TC6 rises to the maximum temperature when the CO concentration is increased from 0.2 to 0.8%. Furthermore, when the CO concentration exceeds about 0.9%, the reaction is quickly extinguished without the observation of any high-temperature excursion. As explained above, four types of waves are involved in these extinction phenomena: the concentration wave (fastest), the passive heat wave, the combined SATRextinction front, and the extinction wave. The speed of the concentration wave is, at maximum, determined by the gas flow rate, which transports inert and unconverted CO molecules downstream with a speed of more than 6 m/s. (The actual speed of the reactant concentration wave is reduced by the extent of chemisorption of the reactant on the catalyst.) The passive heat wave speed, determined by the heat capacities of the reacting system, is on the order of 1000-fold lower than that of the concentration wave. The establishment of the SATR is due to the reaction between the entering CO molecules with preadsorbed oxygen on the still hot downstream catalyst surface. The speed of the combined SATR-extinction front is determined by the reaction rate between the unconverted CO and preadsorbed oxygen, which depends on the local CO concentration as well as the local catalyst temperature. After a small step increase in CO concentration occurs, the combined SATR-extinction front moves at 0.1-0.2 mm/s, a speed much less than that of the passive heat wave. Thus, as the SATR is developing at position z, the temperature downstream of z is determined by passive heat transport. This is the situation when the CO concentration is stepped from 0.2 to 0.7% (Figure 7c). After a large CO concentration step (e.g., from 0.2 to 0.8% as in Figure 7d), a large entering CO concentration consumes the preadsorbed oxygen at a high rate. When the

combined SATR-extinction front moves at a speed faster than the passive heat wave, the catalyst has insufficient time to heat; thus, no SATR response can be observed under such conditions. This occurs with a step increase in CO concentration from 0.2 to 0.9% (Figure 7e) or higher. A step increase in CO concentration to 0.8% is presumed to be in a transition region as in Figure 7d. The speed of the combined SATRextinction front slows further downstream. Upstream of TC5, the speed of the combined SATR-extinction front is close to that of the passive heat wave, so the SATR does not have time to develop fully before complete extinction. Further downstream, around TC6, the speed of the combined SATR-extinction front decreases and becomes lower than that of the passive heat wave; therefore, about TC6, the full SATR is developed. The size of the CO concentration step not only influences the magnitude of the temperature excursion but also increases the traveling speed of the temperature spike. When the CO concentration is increased to 0.8%, the inhibition effect at the entrance of the reactor is so strong that the temperature at TC2 collapses without reaching the expected peak temperature. Figure 8 shows the average speed of the thermal front from TC3 to TC6 as a function of final CO concentration. When the CO concentration is 0.2%, the system just ignites, but no temperature excursion occurs; hence, the front speed is taken as 0. The front speed increases monotonically with CO concentration. The data for CO ) 0.9% suggest a practically instantaneous and simultaneous extinction on all thermocouples. Hence, the rate of the extinction front seems to fall outside the range extrapolated in Figure 8. Also, the very rapid cooling at all thermocouples invites comment. Temperatures of the traveling extinction waves are ramped up (in zone iii) by the extinction of the reaction immediately upstream. This can be seen in the two enlarged temperature-time plots of Figure 7b,c. Also, the maximum slope (at the turning points) decreases in the order TC2 > TC3 > TC4 > TC5 > TC6, and the time interval increases, signaling deceleration of the extinction wave. It appears that, after the reaction is extinguished upstream, unreacted CO penetrates more deeply into the bed, and the driving force for reaction (i.e., the CO concentration) is momentarily refreshed. It is also to be noted that the extinction rate, as measured by the rate of temperature drop, falls slightly between TC2 and TC6, possibly as a result of axial dispersion in the monolith channels. At high CO concentrations, the reaction is effectively blocked, unreacted CO passes through the monolith, and the rapid rate of cooling must be passive, advective cooling. Thus, the “apparent” acceleration of the extinction wave appears to arise from a gradual, inhibitioninduced switch from a slow reaction-diffusion-convection wave (mechanism c) to passive, advective heat transport/cooling (mechanism b). The distance between two neighboring thermocouples is 20 mm, and the data recording interval is 10 s; therefore, if the thermal front travels with a speed higher than 2 mm/s, the hightemperature peak cannot be recorded. For example, Figure 8 shows that, when the CO concentration is 0.7%, the speed of the thermal front appears to rise sharply to the highest value observed. At CO concentrations of 0.8 and 0.9%, the thermal front moves even more rapidly, but the exact speed can only be approximated.

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Ind. Eng. Chem. Res., Vol. 42, No. 1, 2003

Figure 9. Pre-extinction waves due to decrease in hydrogen concentration (total flow rate ) 0.214 mol/min, inlet gas temperature ) 50 °C).

Extinction Waves Due to Decreasing H2 Concentration. For the oxidation of CO/H2 mixtures at 150 °C, the reaction can remain ignited when the hydrogen concentration is as low as 0.2% with 1% of CO in the feed gas. Such a narrow concentration range (0.2-0% H2 in the feed) makes it difficult to investigate the effect of a step decrease in H2 concentration on the extinction behavior. On the other hand, lower feed gas temperatures require higher H2 concentrations to maintain ignition. Thus, at 50 °C, with 0.2% CO in the feed, a minimum H2 concentration of 1.1% is needed to keep the reaction ignited. Therefore, starting from 1.1% H2, we have sufficient latitude to study the effect of the step size on the extinction behavior as the H2 concentration is decreased from 1.1 to 0.9%, from 1.1 to 0.7%, and from 1.1 to 0.5%. Figure 9 shows a series of extinction waves caused by decreasing the H2 concentration across the extinction limit in steps of different sizes. At the beginning of the experiments, no H2 or CO is added to the air flow. At 0.35 ks, 1.1% H2 is introduced to ignite the reaction, and then 0.2% CO is added at 0.9 ks. After the steady state is reached, the H2 concentration is suddenly decreased to 0.9%. Immediately, TC2 responds with a high-temperature excursion from 155 to 197 °C, followed by extinction and successively by similar transients of the other thermocouples. The temperature peak travels downstream faster at the beginning and then gradually slows. Its average speed is 0.11 mm/s. Removing CO from the feed stream reignites the reaction, and the same temperature profile as before is obtained when the CO and H2 concentrations are returned to 0.2 and 1.1%, respectively. Then, the H2 concentration is decreased to 0.7%, resulting in a slightly altered extinction wave. The maximum temperature decreases by about 17 °C over the previous run, and the propagation speed of the high-temperature extinction front increases to 0.31 mm/s. This trend is confirmed by a further decrease in the step size of the H2 concentration. When the H2 concentration is decreased from 1.1 to 0.5%, the maximum temperature reaches only 161 °C, about 17 °C lower than that observed in the previous 0.7% H2 run, and the propagation speed of the high-temperature peak increases to 0.62 mm/s. Variations in reactant concentrations within or outside of the multiplicity region do not cause temperature excursions that exceed the adiabatic temperature rise.

However, increasing the CO concentration or decreasing the H2 concentration from inside to outside the multiplicity region produces large temperature excursions. The maximum temperature depends on the step size of the concentration changes. Every 0.1% increase in the CO concentration of the feed yields about a 12 °C higher maximum temperature; every 0.2% decrease in the H2 concentration of the feed causes about a 17 °C reduction in maximum temperature. When the feed CO concentration is sufficiently high or the feed H2 concentration is sufficiently low, well beyond the ignition limits in either direction, then the reaction cannot ignite, even in the hottest part of the reactor, and no superadiabatic temperature excursion occurs. The step size also affects the traveling speed of the temperature peak. The further the final concentrations are from the bifurcation points, the faster the hightemperature peaks travel downstream. Increasing CO concentrations and decreasing H2 concentrations both produce higher relative CO concentrations. The relatively higher CO concentration yields a higher rate of CO adsorption on the active sites, thus causing a higher degree of blocking of the catalytic sites. As a result, the rate-limiting mechanism changes from a slow reactiondiffusion-advection extinction front at low relative CO concentration to a fast, passive heat wave at high CO concentration. The rate of coverage of CO on the catalyst surface changes, which leads to a faster movement of the thermal front. Conclusions Under experimental conditions, steady-state multiplicity phenomena exist in a certain range of CO and H2 concentrations for the catalytic oxidation of mixtures of CO and H2 over catalytic monolith wash-coated with Pt/Al2O3. Step increases in the CO concentration or step decreases in the H2 concentration across bifurcation concentrations cause extinction waves accompanied by superadiabatic temperature excursions. The final CO and H2 feed concentrations attained after crossing a hysteresis limit determine the maximum temperatures and propagation speeds of the temperature waves. Thermal waves propagate faster at high CO concentrations and low H2 concentrations. This behavior is consistent with a switch from extinction dominated by a slow reaction-diffusion-advection wave for small CO jumps into the extinction zone to rapid blocking of the catalytic sites and rapidly propagating, passive heat waves for large CO concentration jumps into the extinction zone. Acknowledgment This work was performed as part of the strategic project Packed Bed Reactors and Catalytic Converters with Enhanced Dynamical Stability funded by the Natural Sciences and Engineering Research Council of Canada. We express special thanks to Mr. J. Aleixo and Mr. S. Williams of Diesel Controls Ltd. (DCL) for providing us with samples of the wash-coated cordierite monolith. We also thank Dr. A. Jaree for fruitful discussions. Literature Cited (1) Hegedus, L. L.; Oh, S. H.; Baron, K. Multiple Steady States in an Isothermal Integral Reactor: The Catalytic Oxidation of Carbon Monoxide Over Platinum-Alumina. AIChE J. 1977, 23, 632.

Ind. Eng. Chem. Res., Vol. 42, No. 1, 2003 45 (2) Herskowitz, M.; Kenney, C. N. CO Oxidation on Pt Supported Catalysts: Kinetics and Multiple Steady States. Can. J. Chem. Eng. 1983, 6, 194. (3) Sharma, C. S.; Hughes, R. The Behavior of an Adiabatic Fixed Bed Reactor for the Oxidation of Carbon Monoxide: I. General Parametric Studies. Chem. Eng. Sci. 1979, 34, 613. (4) Sharma, C. S.; Hughes, R. The Behavior of an Adiabatic Fixed Bed Reactor for the Oxidation of Carbon Monoxide: II. Effect of Perturbations. Chem. Eng. Sci. 1979, 34, 625. (5) Jaree, A.; Hudgins, R. R.; Budman, H.; Silveston, P. L.; Yakhnin, V.; Menzinger, M. Modeling of Hysteresis and Extinction Waves Caused by Reactant Concentration Perturbations in a NearAdiabatic Packed-Bed Reactor, to be published. (6) Oh, S. H.; Cavendish, J. C. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 29. (7) Wei, J. Adv. Catal. 1975, 24, 57. (8) Hegedus, L. L. Temperature Excursions in Catalytic Monoliths. AlChE J. 1975, 21, 5. (9) Heck, R. H.; Wei, J.; Katzer, J. R. Mathematical Modeling of Monolithic Catalysts. AIChE J. 1976, 22, 477. (10) Young, L. C.; Finlayson, B. A. Mathematical Models of the Monolith Catalytic Converter: Part II Application to Automobile Exhaust. AIChE J. 1976, 22, 343. (11) Appleby, A. J. U.S. Patent 5,953,908, 1999. (12) Murphy, O. J.; Andrews, C. C. U.S. Patent 6,122,909, 2000.

(13) Behm, R. J.; Thiel, P. A.; Norton, P. R.; Ertl, G. The Interaction of CO and Pt(100) I: Mechanism of Adsorption and Pt Phase Transition. J. Chem. Phys. 1983, 78, 7437. (14) Yakhnin, V.; Menzinger, M. Moving Hot Spots and Resonance in Adiabatic Packed-Bed Reactor. AIChE J. 1998, 44, 1222. (15) Deutschmann, O.; Maier, L. I.; Riedel, U.; Stroemman, A. H.; Dibble, R. W. Hydrogen Assisted Catalytic Combustion of Methane on Platinum. Catal. Today 2000, 59, 141. (16) Jaree, A.; Silveston, P. L.; Hudgins, R. R.; Budman, H.; Menzinger, M. Temperature Response to Reactant Concentration Perturbations in the Near-Adiabatic Packed-Bed Reactor. Can. J. Chem. Eng. 2001, 79, 823. (17) Jaree, A.; Budman, H.; Hudgins, R. R.; Silveston, P. L.; Yakhnin, V.; Menzinger, M. Temperature Excursions in Packedbed Reactors with an Axial Variation of Catalyst Activity, Catal. Today 2001, 69, 137. (18) Jensen, K. F.; Ray, W. H. The Bifurcation Behavior of Tubular Reactors, Chem. Eng. Sci. 1982, 37, 199.

Received for review April 24, 2002 Revised manuscript received October 10, 2002 Accepted October 15, 2002 IE020310L