Performance of Auto-Cyclic Reactor in Catalytic Combustion of Lean

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Performance of Auto-Cyclic Reactor in Catalytic Combustion of Lean Fuel Mixtures Danilo Klvana,* Jamal Chaouki, Christophe Guy, Jitka Kirchnerova, and Massimiliano Zanoletti Department of Chemical Engineering, Ecole Polytechnique, P.O. Box 6079, Station Centre-ville, Montreal, Quebec, H3C 3A7, Canada

This work examines the experimental assessment of the conditions required for sustainable autothermal catalytic combustion of mixtures of lean fuels (methyl ethyl ketone (MEK), acetone, propane, and methane) in a small nonadiabatic laboratory auto-cyclic reactor (ACR) loaded with a combination of laboratory-prepared monoliths and commercial palladium catalyst pellets. Despite the non-optimized physical parameters of this reactor, the experiments demonstrated that, for a given fuel, the domain of autothermal operation is dependent primarily on fuel/catalyst reactivity that, in turn, dictates the minimum heat output (power) requirement of the air/fuel mixture and, to a lesser degree, flow rate. In correlation with the reactivity of individual fuels, the power requirement for a flow rate of 64 L/min (ambient) increased, from 375 W for MEK and acetone to ∼480 W for propane and 613 W for methane. For propane and methane combusted under the limiting conditions, oscillatory behavior was observed with the periods that correlated with the power of the fuel/air mixture. When the methane/air feed mixture was heated to 400 °C before entering the ACR, sustained combustion was assured for 0.6% methane flowing at a rate of 97.2 L/min. 1. Introduction Catalytic combustion is recognized as one of the efficient methods for eliminating combustible gases (fuels), such as volatile organic compounds (VOCs), from a variety of industrial effluents.1-3 Catalytic combustion is also considered to be the preferred means of exploiting natural gas for heat and energy generation.4-6 One of the main problems of catalytic combustion technologies, which are typically operated in a fixed-bed reactor mode, is the creeping of the reaction zone along the reactor axes.7 Furthermore, for low-calorific-value effluents, the question arises as to how much additional power must be supplied to sustain the combustion of lean mixtures. Indeed, for applications involving very lean fuel mixtures, reactor design ensuring an efficient and autothermal operation presents special challenges. Reverse-flow reactors using regenerative heat exchange are known to alleviate greatly these problems;3,8-12 however, they suffer from several technical inconveniencies, such as valve operation and control. Another option for autothermal operation are countercurrent reactors, using recuperative heat exchange, as discussed by Kolios et al.11 A new auto-cyclic reactor (ACR) that has been conceived in our laboratories offers an attractive alternative to the typical designs of recuperative type of reactors.13 Even though this new reactor closely resembles the reactor that has been described as operating with internal recirculation by Ben-Tullilah et al.,14 there are special features, explained below, that make it distinctive and justify the notation “auto-cyclic”. The unique valveless design of the ACR, which consists of two separate concentric catalytic beds, allows the recuperation of the combustion heat specifically at * To whom correspondence should be addressed. E-mail: [email protected].

the outlet to heat the inlet gas mixture. Thus, the exiting of the moving combustion front can be prevented. The feed mixture entering the ACR flows first across the outer annular catalytic bed to the extremity of the reactor, where it enters the inner bed and flows in the counter-current direction toward the exit. The outlet region of the inner reactor tube is equipped with outer fins to improve heat transfer toward the annulus inlet. This particular physical coupling of the heat transferred between the outlet (inner tube) and entrance (annulus) sections of the ACR provides continuous heat recuperation to improve the autothermality; if the front moves up to the exit of the inner reactor compartment, its heat may re-ignite the incoming combustion mixture in the annulus and a new cycle begins. Primarily, it is this feature, in addition to particular arrangements of the catalytic bed to suit specific applications (which are detailed further in the next section), that makes the ACR unique. In contrast, the similarly designed inner recirculation reactor14 featuring also two concentric tubes is not equipped with heat recuperation fins (in the inlet/outlet) section and its axially symmetrical bed packing consists of a catalytic zone sandwiched between two inert zones. Numerous articles that involve theoretical and mathematical evaluation of autothermal reactors have been published over the years. In comparison, studies concerning the experimental assessment of such reactors have been appearing in the literature more recently but are still relatively few.8-10,13,14 This article concerns experimental results obtained using a small nonadiabatic laboratory ACR that has been loaded with a combination of highly active PdO catalyst supported on alumina wash-coated monoliths prepared in our laboratory and commercial palladium/ alumina pellets. The experiments involved catalytic combustion of several mixtures of lean fuels (methyl

10.1021/ie050819r CCC: $30.25 © 2005 American Chemical Society Published on Web 10/25/2005

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Figure 1. Schematic of the laboratory auto-cyclic reactor (ACR), showing the positions of the thermocouples.

ethyl ketone (MEK), acetone, propane, and methane). These fuels were selected not only for practical reasons, but because they also represent a wide range of reactivity, which is a parameter of high importance for the design and efficient autothermal operation of catalytic combustors. 2. Experimental Section 2.1. Catalysts. Two types of alumina-supported palladium catalysts were used: commercial 0.2 wt % Pd/Al2O3, 3-mm pellets (Procatalyse, PC263) and PdO/ alumina supported on cordierite monoliths (400 CPSI) prepared in the laboratory as follows. The required pieces (annular cylinders (7.5-4.8 cm) × 7.5 cm in length for the outer reactor compartment and cylinders 3.0 cm in diameter × 7.5 cm in length) were cut from the cordierite cubes (side dimensions of 7.5 cm). Monoliths that were wash-coated with γ-alumina via a 12 wt % suspension were impregnated with palladium by immersing them in a solution of palladium nitrate, using a procedure similar to wash coating. The monoliths were dried under an infrared (IR) lamp in a fume hood and then calcined for 6 h at 650 °C in air. The resulting catalyst contained 8 ( 1.5 wt % alumina washcoat and 0.43 ( 0.06 wt % PdO (2 g Pd/L monolith). 2.2. Catalyst Activity. A steady-state, apparent activity of the catalysts was determined in a U-shaped plug-flow laboratory reactor, as described elsewhere.15 Typically, for the highly active palladium-based catalysts, 0.2 g of crushed catalytic monolith, or crushed pellets, dispersed in 10 mL of precalcined (10 h at 875 °C) inert pumice was used. The temperature of the catalytic bed was monitored by two thermocouples that were touching the bed at the inlet and at the outlet to verify that isothermality was maintained, despite a relatively high methane concentration (3.1%). The reactor was heated in steps of ∼25 °C, and the fuel conversion was determined by gas chromatography (GC) when a constant temperature was attained. For methane, the activity was determined at two different flow rates (200 and 300 mL/min) after in situ catalyst aging at 450 °C of the combustion mixture flowing at a rate of 100 mL/min. For MEK and acetone, the apparent activity was determined for a single set of conditions (0.25 g, 300 mL/min). 2.3. The AC Reactor. The laboratory ACR (Figure 1) was constructed from two 40-cm lengths of standard stainless-steel pipe: the one that served as the outer

annulus had an inside diameter of 9 cm, and the other, which had an inside diameter of 3.2 cm and wall thickness of 0.16 cm, served as the central compartment.16 The two pipes were assembled concentrically, the central pipe being axially displaced by 3 cm, creating a common zone where the flow changed direction. This zone of flow change was filled partially with an alumina fiber mat. The catalytic bed consisted of a combination of pellets (zone of ignition-re-ignition) and monoliths, where the pellets had a significantly higher activity per volume than the monoliths. The space between fins at the ACR entrance was filled with bare precalcined Alcoa CSS100 3/16 alumina pellets, followed by a layer of PC263 catalyst and a 25-cm-long zone of annular monolith pieces stacked together. The central compartment was filled with three monolith pieces (total length of 22.5 cm), followed by a 17-cm layer of PC263 pellets, just before the exit. To minimize bypassing by unreacted fuel, the monolith pieces were held together firmly by a layer (∼0.2 cm) of compacted alumina fiber mat that was impregnated with palladium oxide. Ten K-type thermocouples that were positioned along the reactor (Figure 1) and connected to a computer data acquisition system monitored the temperature profiles during the operation. The complete ACR was wrapped in an insulating fiber mat and enclosed in a protecting box, to minimize the heat loss from the catalytic bed. For the combustion experiment startup, the entrance portion of the ACR was preheated to a given level, typically to ∼350 °C under a low air flow (25 L/min), by a rheostat-controlled ceramic cylindrical heater. In addition, in some of the experiments with the lowest methane concentrations, the inlet gases were continuously heated to ∼400 °C by passage through an independent, computer-controlled preheater. The effluents of the reactor were periodically analyzed using on-line GC. Gases were metered by gas flow meters, whereas flows of liquid MEK and acetone to obtain a given fuel (volatile organic compound, VOC) concentration were controlled by a high-pressure piston pump (for liquidliquid (L-L) chromatography). For all experiments, the heat losses were estimated at 30 W. This estimate was obtained independent of combustion via measurement for several flow rates of air heated to 400 °C, the steadystate temperatures at the inlet and the outlet of the ACR. From the observed difference between the steadystate temperature at the inlet and the outlet, the heat loss was determined using heat balance in the reactor.

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Table 1. Conditions for Evaluating the Catalytic Combustion of Different Fuels in the Auto-cyclic Reactor (ACR) fuel

-∆Hr (kJ/mol)

MEK acetone propane methane

2268 1659 2043 803

concentration (vol %)

Q (L/min)a

powerb (W)

∆Tad (°C)

∆Pc (kPa)

0.29-0.77 0.40-0.53 0.43, 0.53 0.6-1.74

35-97 63-91 77, 63 56-97

335-461 374-413 460, 375 613 (290d)

228-603 229-304 303, 374 166-481

20-70 36-62 35-50 23-70

a Under ambient conditions. b Heat output of a given fuel/air mixture assuming a complete conversion. c Back pressure of the overall system. d For preheated reaction mixtures (400 °C).

Table 2. Activity of Used Catalysts in the Catalytic Combustion of Different Fuels in Terms of Pseudo-First-Order Kinetic Parameters catalyst monoliths monoliths monoliths commercial commercial a

fuel MEK acetone methane methane propane

T50a (°C)

Eapp (kJ/mol)

ln A (mol kg-1 h-1 kPa-1)

ln k625c (mol kg-1 h-1 kPa-1)

235 237 480 440 340

119b

30.2 27.8 16.6 18.8 26.4

7.29 7.28 0.43 1.09 3.69

117 84 92 118

Temperature of 50% conversion. b For temperatures of >245 °C. c Pseudo-first-order rate constant for 352 °C (625 K).

2.4. ACR Operation Conditions. To determine the limits of the autothermal operation and to illustrate the possible cyclical behavior of the system, a relatively narrow range of low fuel concentrations and flow rates of 50 L/min). This procedure, which lasted only a few minutes, assured ignition. Thereafter, conditions were usually changed only when apparently stable combustion was reached. It is remarkable that reactor behavior under different conditions was highly reproducible, as demonstrated by temperature evolution and reaction front movement. After several hours of operation, similar temperature profiles were registered for comparable conditions, regardless of the level of the bed preheating. Thus, we could recognize three zones (regions) of operational conditions: (a) conditions that ensure steady-state (stable) combustion, i.e., stable temperature profiles; (b) conditions that lead to quasi-stable combustion that is characterized by an oscillatory movement of the reaction front and apparently is limited to a fairly narrow range of conditions; and (c) conditions of unstable combustion, under which the reaction front exits and/or combustion simply is extinguished. The velocity of the propagation of reaction front (w) in packed-bed reactors is usually determined by an energy balance around the migration zone of combustion:7

w)

[1 - (∆Tad/∆Tmax )]b(FCp)f [1 - (∆Tad/∆Tmax )]b(FCp)f + (1 - b)(FCp)s

u (1)

where

∆Tad )

( ) - ∆Hr (FCp)f

y0in

(2)

This relationship shows that the reaction front movement is dependent primarily on the three variables: the superficial velocity u, ∆Tmax, and ∆Tad. This equation can equally be described by transforming these three variables to volumetric flow rate, power (heat output) and a term representing the reactivity of the fuelcatalyst system. Indeed, the experimental domains of sustainable catalytic combustion correlate well, for a given flow rate, with the catalyst activity and the heat output (power) of a fuel mixture. Specifically, for the two easily combustible fuels (MEK and acetone), sustainable catalytic combustion was obtained at a power

Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9679 Table 3. Limiting Operational Conditions Required for Sustained Catalytic Combustion in the ACR Value

a

parameter

MEK

acetone

propane

methane

relative reactivity, ln k625 (mol kg-1 h-1 kPa-1) minimum power requirement at 64 L/min (W) range of acceptable flow rates at the MPRa (L/min) maximum flow rate for 461 W (L/min)

7.3 377 55-75.3 97

7.3 374

3.0 480

97

63

0.4 613 55-61 >65b

MPR ) Minimum power requirement. b When the reaction mixture is continuously preheated to 400 °C.

Figure 2. Map of ACR performance showing regions of stable ignited and extinguished catalytic combustion separated by a narrow range of quasi-stable combustion.

as low as 377 W (0.44%, 55 L/min, or 0.32%, 75 L/min), whereas at 335 W (0.4%, 55 L/min, or 0.28%, 75 L/min), the combustion was extinguished. In comparison, for propane, the power requirement was >460 W (estimated close to 480 W), and, finally, the methane mixture needed at least 613 W. Table 3 summarizes the limiting operational conditions required for sustained (autothermal) catalytic combustion of the four fuels. As a measure of reactivity, apparent first-order kinetic constant can be used for convenience. To represent the dependence of the observed minimum power requirement for the ignited combustion on the fuel reactivity graphically, we can map the overall performance of the ACR in terms of an apparent rate constant, k625, which represents the fuel reactivity (over a given catalyst) and required power, such as that shown in Figure 2, where the two states (ignited and extinguished) are separated by a fairly narrow region of conditions at which a quasi-stable combustion occurs. Note that, for significantly higher flow rates, the minimum power requirement is expected to increase. Moreover, the limiting power is further bound to a minimum acceptable concentration or, rather, ∆Tad. 3.2.1. Steady-State Combustion. In a regime of steady (stable, ignited) combustion, the temperature profiles in the reactor respond smoothly to changes in either concentration or flow rate by an increase or a decrease in temperature, as well as by movement of the reaction front, depending on the degree of the change. In this regime, the heat required is generated fast enough to maintain a very slow movement of the reaction front. Two examples of stable autothermal combustion are shown in Figure 3 for acetone and Figure 4 for methane. Even though the boundary regions for individual fuels between stable ignited and extinguished states of the ACR were determined only at a single flow rate (63 ( 1 L/min), the overall data permit us to define the range of flow rates that ensures the stable combustion for individual fuels. As our data indicate, when the minimum power requirement and

Figure 3. Example of stable acetone combustion showing the effect of increasing flow rate on temperature profiles and reaction front movement. Region A corresponds to 63 L/min, 0.533% (377 W); in regions B-E, the amount of injected acetone was constant (412 W) while the total flow rate was varied (73 L/min in region B, 78 L/min in region C, 88 L/min in region D, and 91 L/min in region E).

Figure 4. Example of stable methane combustion at 613 W, including the period of ignition. Region A: 55.5 L/min, 2.02%; region B: 61.4 L/min, 1.83%.

minimum ∆Tad (i.e., minimum concentration) are satisfied, stable combustion is independent of flow rate, as illustrated in Figures 3 and 4. In the case of acetone, when operating at powers higher than the minimum, no extinguished state was attained at flow rates up to 91 L/min. In fact, a similar situation was observed for MEK combustion, for which it was possible to operate at a flow rate of 97 L/min while maintaining the minimum concentration (∼0.3%). 3.2.2. Combustion is Not Sustained (Extinguished). When conditions of unstable combustion are reached, the reaction may simply be extinguished without a clear-cut exit from the reactor, such as that shown in Figure 5 for the case of MEK combustion, after an insufficient concentration in the mixture (power) was introduced in the reactor. In this particular example,

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Figure 5. Effect of changing concentration for an intermediate flow rate on catalytic combustion of methyl ethyl ketone (MEK), including extinction, when the concentration is reset below the limiting value. Region A: 50 L/min, 0.543% (419 W); region B: 55 L/min, 0.494% (419 W); region C: 55 L/min, 0.444% (377 W); and region D: 55 L/min, 0.4% f extinction. Figure 7. Comparison of an oscillatory movement of the reaction front during quasi-steady propane catalytic combustion in the ACR; propane (0.53%, total flow rate of 63 L/min, auxiliary heating of 25 W of the ACR entrance) with that of quasi-steady catalytic combustion of methane (1.74%, 64.7 L/min).

Figure 6. Example of unstable propane combustion showing the reaction front exiting the reactor. Region A: 63 L/min, 0.53%; region B: 76.7 L/min, 0.43%.

when at a flow rate of 55 L/min, the concentration was reduced from 0.444% (region C, corresponding to 377 W) to 0.4% (region D, 336 W) the combustion extinguished. A similar situation was observed for higher flow rates, specifically when at a flow rate of 75.4 L/min, the concentration was reduced from 0.32% (370 W) to 0.29% (336 W). Alternatively, a situation may develop where the combustion was ignited, but the conditions are on the verge of sustainability and the reaction front exits as illustrated in Figure 6 for the case of propane combustion. It is worthwhile to mention that almost the same temperature evolution and movement of the reaction front also was observed for unstable methane combustion. Nevertheless, in the case of methane, the time period between the two sharp peaks of the reaction front just before exiting was slightly longer, i.e., 25 min in the case of propane, versus 45 min in the case of methane. 3.2.3. Quasi-stable Combustion. The most interesting situation arises when the conditions are just on the border of sustainability, i.e., the regime of quasi-steady combustion is attained. In this particular case, an oscillatory movement of the reaction front develops, as the system tends toward stable combustion, or toward an exit of the front for that matter. Such a situation was observed for propane and methane. For both fuels, oscillations were observed for as long as 12 h. In the

case of propane combustion evaluated in two independent experiments only, we have encountered difficulty to pinpointing the conditions of sustained combustion, even when using a mixture that corresponded to 460 W and a relatively high concentration (0.53%, 63 L/min). Thus, we have decided to maintain a weak heating of the entrance portion of the catalytic bed, which is estimated at 25 W, to ensure the ignited state. On the other hand, a quasi-stable oscillatory (cycling) regime without additional heating was reached for a methane concentration of 1.74% flowing at a rate of 64.7 L/min. The oscillatory behavior of the reaction front for the two fuels is compared in Figure 7, representing only a small segment of the data. Inasmuch as the oscillating regimes in Figure 7 resemble each other, as well as those observed by Lauschke and Gilles19 during ethene catalytic combustion in a packed-bed loop reactor, some obvious differences are evident. These are mainly in the period of oscillations, temperature amplitude at different positions, and finally in the position of the reaction front and its movement along the reactor axes. Because the flow rates in the two experiments were almost identical, the differences in the oscillatory regime of the fuels reflect mainly the different fuel reactivity and ∆Tad value of the reaction mixture (i.e., 303 and 481 K for propane and methane, respectively). Interestingly, note that the two observed periodss95 and 130 min for propane and methane, respectivelysseem to be in the same ratio as that between the minimum required power for the two fuels (∼480 and 613 W). Although prediction of the period of oscillation (τ) is a fairly difficult task, as a first simple approximation, it may be estimated from the velocity of the reaction front, calculated using eq 1 and the reactor geometry, according to

τ)

li

∑i w

(3)

i

To apply eq 3, the overall length of the reactor bed was

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divided into five sections of different packing and/or geometry of the bed. Furthermore, because of a continuous heating of the entrance portion of the bed in the case of propane, we assumed that the temperature of the propane/air feed was higher than that in the case of methane. In this way, we obtained periods of 47 and 68 min, which are values that are largely underestimated, but approximately represent the ratio of the experimental values. Obviously, much more refined analysis of the data is needed to obtain a better prediction. 3.2.4. Catalytic Combustion of Hot Fuel/Air Mixtures. Many industrial or combustion effluents that require treatment of the combustible (unburned) pollutants arrive already hot. Thus, because of this extra energy content, the requirements for stable ignited catalytic combustion are less demanding. In view of a potential development of treatment for low concentrations of unburned methane, few experiments have been conducted in which the feed gas mixture was continuously preheated to 400 °C, increasing the power of the mixture by ∼400 W. This addition would shift the power demand from methane for its stable combustion to T4 > T3 ≈ T7 ≈ T6 ≈ T5 ≈ T2 . T1 (see Figure 1). Reducing the heat output from methane combustion to an equivalent power of 280 W, by decreasing the methane concentration to 0.8% while maintaining the same flow rate, was marked not only by temperature reduction throughout the reactor but also by the movement of the reaction front from position T9 to position T0, which is located at the ACR exit. Under these conditions, the temperature distribution was as follows: T0 ≈ T9 . T7 > T8 > T6 . T4 ) T3 > T5 ) T2 . T1. Alternatively, a preheated mixture of 0.6% methane was successfully combusted at a flow rate of 97 L/min. 3.2.5. ACR Scaleup. The presented laboratory experiments, which were conducted in a small nonadiabatic laboratory ACR reactor, illustrate well that sustained operation of the described device is possible, despite its far-from-optimal dimensions. Furthermore, they also indicate or confirm the key issues involved in scaling-up of the apparatus for sustained and reliable operation for different applications. In fact, technical requirements for operation with relatively high fuel concentrations (i.e., high reaction heat release for heat generation, for example) may largely differ from those for treating low fuel concentrations, such as those observed in the treatment of VOC or methane emissions. For low fuel concentrations, for which the fuel reactivity is very important, the overall ACR design must ensure approximately adiabatic operation. In this case, the reaction front should move by approximately the same velocity in both compartments. In contrast, for applications with higher fuel concentration, heat evacuation from the surface of the external tube by different means must be envisaged. To compensate for a faster heat loss

from the annular compartment, the diameter of the central tube could be smaller to ensure faster flow in this compartment. 4. Conclusions Even though the design of the auto-cyclic reactor (ACR) was not optimized in this study, experiments have shown the domain of autothermal operation, as a function of fuel reactivity, power, and, to some degree, flow rate. For a flow rate of 64 L/min, the power requirement increases from ∼375 W for methyl ethyl ketone (MEK) and acetone to ∼480 W for propane and ∼613 W for methane, in correlation with the reactivity of individual fuels. For propane and methane that have been combusted under the limiting conditions, oscillatory movement of the reaction front was observed, with the periods correlating with the power. When the methane/air feed mixture was heated to 400 °C before entering the ACR, sustained combustion was assured for 0.6% methane flowing at 97.2 L/min (320 W). The results of this study indicate clearly the attractive and versatile characteristics of the ACR, despite the non-optimal reactor dimensions. The ACR is suitable not only for small units of heat generators, but also as a volatile organic compound (VOC) and methane emissions removal system. Although the ACR probably cannot handle fuel concentrations as low as those treatable in the reverse-flow adiabatic reactor, the ACR is ideal for low and intermediate fuel concentrations and definitely presents several advantages. The applicability of the ACR is dependent on the temperature of the feed stream, the fuel concentration, its calorific value, and the total flow rate (i.e., the power equivalent (heat output) and the efficiency of heat recuperation at the inlet ignition zone). These latest experimental results will be compared with simulation studies to optimize the overall reactor design, including the catalyst physical characteristics and its activity for the best required performance (this is a focus of our current research). With improved designs, and especially with more-efficient catalysts that can certainly be prepared, and possibly good insulation, much lower methane concentrations should be treatable. Acknowledgment This work was supported by grants from Natural Sciences and Engineering Research Council of Canada and from Natural Gas Technology Center, Montreal. Nomenclature -∆Hr ) heat of combustion (kJ/mol) Q ) flow rate (L/min) ∆Tad ) adiabatic temperature rise (°C) ∆P ) back pressure (kPa) T50 ) temperature of 50% conversion (°C) Eapp ) apparent activation energy (kJ/mol) A ) pre-exponential factor (mol kg-1 h-1 kPa-1) k625 ) pseudo-first-order kinetic constant for 625 K (mol kg-1 h-1 kPa-1) (Cp)f ) heat capacity of fluid (kJ/kg) (Cp)s ) heat capacity of solid (kJ/kg) li ) length of a reactor segment i (m) u ) gas velocity (m/s) w ) velocity of reaction front movement (m/s) y0in ) initial fuel molar fraction b ) void fraction of the catalytic bed

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(F)f ) density of fluid (kg/L) (F)s ) density of solid (kg/L) τ ) period of oscillation (min)

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Received for review July 12, 2005 Revised manuscript received September 16, 2005 Accepted September 27, 2005 IE050819R