Experimental studies of transient thermal effects during catalytic oxidation in a packed-bed reactor

9 1997 Elsevier Science B.V. All rights reserved. Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis G.F. Froment and K.C. Waugh, editors

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Experimental studies of transient thermal effects during catalytic oxidation in a packed-bed reactor S. Marengo, P. Comotti. S. Scappatura and M. Vasconi Stazione Sperimentale per i Combustibili Viale A. De Gasperi 3, 20097 San Donato Milanese, MI. Italy

The dynamic phenomena associated with the rhodium-catalyzed oxidation of carbon monoxide, methane and propane have been studied by in-situ infrared thermography. Highresolution temperature maps of the reacting catalyst revealed the mobility of the reaction front during ignition and extinction of the CO oxidation, and the development of thermokinetic oscillations. The catalytic oxidation of methane and propane produced weaker dynamics. Chemisorption and kinetic experiments suggest that the competitive adsorption of the reactants and the occurrence of self-inhibition, represent key factors in the development of the observed transient effects.

1. INTRODUCTION The dynamics of exothermic catalytic reactions has been the object of investigation for almost half a century comprehensive updated studies have been recently reported [1, 2]. Whereas various models have been proposed in a large number of papers, describing mathematical and theoretical approaches, experimental data concerning these phenomena are relatively scarce, due to the difficulty of performing accurate measurements under reaction conditions. Novel in-situ techniques capable of providing reliable spatial information on reactor behaviour can thus represent a valuable tool for extending our knowledge in this field and improving the efficiency and the safety of a number of catalytic processes which are carried out in the ignited state [3, 4]. More efficient experimental methods may also favour the development of new techniques of reactor operation, based on a periodic change of the process parameters [5]. Infrared imaging was utilized in several studies of spatial effects in exothermic catalytic reactions over model catalysts, such as isolated particles, wafers, plates, discs [2]. Our approach has been to characterize the catalysts directly in a packed-bed microreactor, under realistic reaction conditions. In-situ measurements by infrared thermography of the adsorption properties of catalytic materials have been previously reported [6]. In the present study, the catalytic oxidation of compounds having different chemical properties was investigated by the same technique, with the aim of obtaining comparative data useful to better understand the factors governing the complex phenomena associated with catalytic combustion.

430 Experimental data describing the dynamics of ignition and extinction will be reported in detail, and the principal factors determining the behaviour of the different reactants will be analysed.

2. E X P E R I M E N T A L The catalyst was Rh on T-alumina in the form of particles with diameter of 0.1-0.3 mm. In some measurements, supported Pt or Pd were also utilized. The catalyst was packed in a specially designed, 8-mm i.d. tubular reactor, between two layers of inert material. A forced ventilation oven allowed control of reactor temperature. The expressions "reactor inlet" and "reactor outlet" adopted in the text to describe the reaction front motion, are referred to the catalyst bed only, without taking into account the inert layer. High-resolution thermal maps of the surface of the catalyst bed (8-15 mm high) were recorded with an Agema Thermovision 900 apparatus, equipped with optic for close-up view and with a data processing unit for real-time image analysis. An axial multiple thermocouple placed in the catalyst bed allowed the measurement of internal temperature. Catalytic experiments were carried out at 0.1 MPa total pressure, with molar fraction (y) of CO, CH 4 or C3H 8 between 0.01 and 0.04, oxygen slightly above stoichiometric value, and Ar or He as balance gas. Reaction products were monitored by on-line analysis with a gas chromatograph and a quadrupole mass spectrometer. The peculiar aspect of this study is represented by the in-situ measurement of thermal effects produced throughout the reactor under real operating conditions. In this respect, it is important to verify that the thermal map of the surface of the catalyst bed, obtained by thermography, describes reliably the phenomena occurring in the bulk of the bed. For this purpose, the external temperature profile was compared with the profile obtained by an axial multiple thermocouple placed inside the catalyst bed. It was observed that both in the steady and in the transient state, the two profiles have similar shape, although the temperatures are not identical due to axial gradient.

3. RESULTS

3.1. Ignition It is known that the catalytic combustion of hydrocarbons exhibits ignition-extinction phenomena which depend on feed composition [3]. The ignition experiments reported in this study were performed with inlet fuel concentration comprised in the region of surface flammability, in which hysteresis effects occur; typical values were: YCH40.04, YC3H80.02, YCO 0.03. The superficial velocity (u) of the reactant gas was between 1 and 5 cm/s. In order to minimize the effects of external factors on the dynamic behaviour of the reaction system, no external perturbation was introduced during the tests, with the exception of a slow change in the heating rate. Ignition was obtained by gradually raising the reactor heating rate, while keeping feed composition and flow rate constant. In the CO-O2 reaction, light-off was revealed by a fast temperature rise near the reactor outlet; in the same time, CO conversion jumped to about 100 %. Figure 1 shows the

431 temperature map of the surface of the catalyst bed and the temperature profile along a vertical line on this surface.

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Figure 1. Temperature map and profile of the catalyst bed upon ignition of the CO-O2 reaction. Downward flow, u=4.8 cm/s; Yco =0.03.

Figure 2. Temperature map and profile in the ignited state for the CO-O2 reaction.

Figure 3. Temperature map and profile during extinction of the CO-O2 reaction.

432 Ignition was followed by upstream creeping of the reaction front, with a linear velocity comprised between 0.8 and 1.2 mm/s. In the ignited state, the reaction front was stabilized near the inlet (Figure 2), where a sharp temperature profile and significant radial gradient (internal catalyst temperature was 20 ~ higher than at the surface) were measured. When external heating was lowered, the maximum temperature in the bed decreased, the reaction front broadened and moved downstream, eventually reaching the end of the bed, where extinction occurred (Figure 3). Such a behaviour seems to be a characteristic of the CO-O2 reaction, since it was observed in our study with different active metals (Rh, Pt, Pd), different loadings, various catalyst shapes and with different superficial velocities and flow directions (upward and downward). Similar dynamics has been reported for the CO oxidation on Pt/alumina in a larger tubular reactor [7]. Varying inlet CO concentration did not alter substantially this dynamics; however, for Yco below 0.02, the reaction front after ignition moved only up to the middle of the bed, were it was stabilized or moved back again, depending on heating rate. Between ignition and extinction, temperature oscillations were observed in the active portion of the catalyst bed. The oscillating patterns were complex and multiform, depending to a large extent on the catalyst composition and reaction conditions. When, starting from the ignited state, external heating was lowered, the amplitude of oscillations increased and the period decreased, until extinction took place. Real-time analysis of the temperature map revealed synchronization of oscillations in a portion of the reaction front (Figure 4, points 1, 2). In the downstream zone (point 3), oscillations in opposite phase were detected, revealing influence of mass transfer on the dynamic phenomena. In the same time, CO2 and O2 concentration in the effluent gas oscillated in opposite phase, with the same period as the slow temperature fluctuations.

Figure 4. Local temperature oscillations during CO oxidation over 3% Rh/AI203. Yco =0.03, u=4.8 cm/s. Points 1, 2, 3 are on a vertical line at 1-mm distance.

433 In the combustion of methane, a different transition to the ignited state was observed. An incipient reaction front formed near the reactor inlet, due to release of the heat of reaction. Upon ignition, the reaction front developed slowly, holding its position and showing no significant dynamics. Propane exhibited a behaviour similar to methane. However, the initial reaction front formed slightly downstream of the reactor inlet, and shifted upstream to the very initial part of the bed (Figure 5). The radial gradient in the active zone was remarkable (40 ~ between axial and external position). With both methane and propane, lowering heating caused extinction near the inlet, without any significant shift of the reaction front.

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Axial position (mm) Figure 5. Temperature profiles during the ignition of the C3H8-O 2 reaction over 3% Rh/AI203. YC3H8=0.02, Yo2 =0.12, u= 3.3 cm/s.

In propane oxidation, only weak kinetic oscillations were revealed by the spectrometer signal of CO2 at reactor outlet. The combustion of methane under similar operating conditions showed no oscillating behaviour. 3.2. Adsorption and kinetic measurements

To investigate the factors controlling the catalytic combustion in the reaction systems investigated, adsorption measurements of the different reactants were performed at room temperature on freshly reduced Rh/A1203 (Rh 1%). A series of 3-gmol pulses were injected into an Ar stream flowing through the reactor with u=l cm/s. The pulses of CO produced a significant thermal effect in definite portions of the catalyst bed (Figure 6). The rise of the catalyst temperature was about 2 ~ after each pulse. Interestingly, a peak at mass 2 was observed by the quadrupole analyser during CO adsorption, indicating displacement of H2 by CO. Pulses of oxygen on the reduced catalyst in the same conditions, produced much stronger thermal effects than CO (Figure 7). The temperature rise after each pulse was about 8 ~ On

434 the other hand, the portion of catalyst bed involved in the adsorption of one pulse of 02 was larger than with CO, as revealed by the position of the temperature peak.

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Figure 6. Thermal effect produced by injection of a 3-gmol pulse of CO onto reduced RMA1203.

Figure 7. Thermal effect produced by injection of a 3-gmol pulse of 0 2 o n t o reduced Rh/AI203.

These pulse experiments reveal a higher heat of adsorption of 02 compared to CO. Instead, the sticking coefficient of CO on rhodium appears higher than that of 02, as can be deduced from the volume of catalyst saturated by one pulse of each reagent under the same conditions. This result is in qualitative agreement with data reported for the competitive adsorption of the same species on Pt [8]. The dynamics of adsorption of alkanes is totally different. Both methane and propane were detected at reactor outlet already after the first pulse, revealing a very low sticking probability compared to CO and 02. Methane produced no thermal effect on the catalyst, whereas propane caused a weak temperature rise (about 0.2 ~ Activity measurements were carried out on 3% RMA1203 under isothermal conditions; the rate dependence on the fuel partial pressure was determined by gradually raising the concentration of one reactant, while keeping other parameters constant. In the CO oxidation, the effect of partial pressure of CO is described by a typical "volcano" curve, as reported in the literature for Pt group metals [9-11 ]. At low partial pressure, CO exhibits a strong positive effect on the reaction rate; for C0/02 ratio higher than about 0.3, a self-inhibiting effect takes place (Figure 8). The oxygen dependence is 1.2, revealing relatively weak adsorption in the presence of CO. In methane oxidation, a positive effect of methane partial pressure was measured in a wide range of feed composition; the reaction order is 0.7 in CH4 and 0.1 in 02. In propane oxidation, the rise of fuel partial pressure promotes the reaction only at low reactant/oxygen ratio; at higher values, surface saturation seems to take place (Figure 9). The oxygen dependence is 0.8, suggesting stronger competition by propane for surface sites, compared to methane.

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PC3H8 (kPa) Figure 9. Propane dependence of CO2 formation rate on 3% Rh/A1203 . T=200 ~ P 02 =10 kPa.

4. DISCUSSION During the oxidation of CO, CH 4 and C3H8, the ignited state is characterized by a reaction front stabilized in a thin portion of the bed near the reactor inlet. This condition, corresponding to a diffusion-controlled reaction, is predicted by the known models of exothermic catalytic reactions [4]. The chemical factors determining this dynamics are the heat of reaction and the activation energy. For all of the reactants considered in this study, a similar behaviour in the ignited state is observed. Marked differences between the reactants are exhibited in the transition from the low activity state to the ignited state. The dynamic phenomena occurring in the catalytic oxidation of CO can be explained by assuming that the main factors governing ignition are the inhibiting effect of the reactant and the strong dependence of the desorption rate on temperature. In the pre-ignition state, the surface is predominantly covered by CO, as suggested by the high sticking coefficient of the latter and by literature data [12]; the rate of reaction is therefore controlled by the adsorption of oxygen. Initial CO conversion takes place along the catalyst bed rather than at reactor inlet, where surface CO coverage is presumably slightly higher. Heat is transferred towards the reactor outlet, where the concentration of CO is lower due to partial conversion, and the desorption rate from the surface is higher [13]. As a consequence, the surface CO coverage continues to drop in this region, eventually reaching the level corresponding to the sharp transition to the high activity state. Ignition near the outlet is followed by an upstream front motion, according to the dynamics described in numerous studies [1, 14, 15]. The downstream front motion accompanying extinction of the CO-O2 reaction, is controlled by the same factors determining ignition.

436 In the oxidation of C H 4 and C3H 8, no inhibition by the reactant is operative, and initial conversion is higher near the reactor inlet, where favourable conditions for ignition are stabilized. This relationship between dynamic behaviour and adsortion-desorption properties, finds support in the literature. Site competition between hydrocarbons and oxygen has been recognized as a key point in the mechanism of catalytic oxidation [3]; in the same study, a substantially different behaviour in catalytic combustion was reported for alkanes and ethylene, and this was related to the strong adsorption properties of the latter. The mechanism of self-sustained oscillations is not completely understood at present, despite the remarkable number of studies reported in the literature [ 11, 16]. It has been shown that in the CO-O2 reaction over Pt catalysts, oscillations occur in the transition region between the high and low activity state, corresponding to predominantly CO- or O- covered surface [ 11 ]. The existence of such a region of sharp transition appears a basic requisite for oscillating behaviour to take place. This statement applies also to the Rh/A1203 catalyst of this study, for which a similar transition region in the CO oxidation has been revealed by kinetic experiments. The weaker oscillations observed with propane and the absence of any significant dynamics in methane oxidation can similarly be related to the adsorption properties of the alkanes. This conclusion is supported by the fact that oscillations have been rarely reported for hydrocarbon oxidation, and in the case of methane, their occurrence has been excluded for a Pt/A1203 catalyst [ 17].

5. CONCLUSIONS High-resolution infrared imaging of the working catalyst provided information on the dynamics of combustion reactions with unprecedented detail. The capability of exhibiting competitive adsorption in the presence of 02, together with self-inhibiting effect, represents a key factor in the development of dynamic phenomena. This assumption is able to explain the different behaviour of CO during catalytic oxidation, with respect to light hydrocarbons. The same chemical factors that determine the mobility of the reaction front, seem also to play a role in the mechanism of oscillations. These results can help define general criteria for predicting unstable behaviour in processes carried out in the ignited state.

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