Catalytic Combustion of Very Lean Mixtures in a Reverse Flow

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Ind. Eng. Chem. Res. 1997, 36, 4198-4206

Catalytic Combustion of Very Lean Mixtures in a Reverse Flow Reactor Using an Internal Electrical Heater F. Cunill,† L. van de Beld, and K. R. Westerterp* Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

An experimental study of the reverse flow reactor, equipped with an internal electrical heater, for the autothermal combustion of very dilute organic compounds, in particular ethene, propane, and their mixtures, has been carried out. The influence of several operating parameters like electrical heater power, cycle period, chemical character, and concentration of the pollutants on the maximum temperature and on the shape of temperature profiles in the stationary state is discussed. Experimental results show that an internal electrical heater can be successfully used to oxidize completely very lean mixtures which would not be able to maintain an autothermal process only by themselves. The predictions with a heterogeneous one-dimensional model without using fit parameters show a good agreement with experiments except for critical situations. Introduction The applicability of a reverse flow reactor (RFR) for the purification of waste air has been demonstrated by several authors by using both modeling and experimental studies (Matros et al., 1988, 1993b; Eigenberger and Nieken, 1988; Chumachenko and Matros, 1990; Sapundzhiev et al., 1991; Nieken et al., 1994a,b; van de Beld et al., 1994; Purwono et al., 1994; van de Beld and Westerterp, 1994, 1996). These works are devoted mainly to analyzing and discussing the influence of the various reaction and reactor parameters, such as the rate constant, activation energy, cycle period, stability and control, gas velocity and properties, adiabatic behavior, methods of cooling or heating part of the bed, ratio of inert solids to catalyst, and other operational considerations. However, literature that specifically deals with design problems is more scarce (Haynes et al., 1995). Although the concept of the RFR usually is attributed to Boreskov and Matrossthey in fact have put a first coordinated effort in its mathematical description, experimental testing, and commercial introductionsthe invention as such is older. Cottrell (1938) patented the RFR already in 1938 in the USA and fully described its operation as in modern plants. A RFR, as any autothermal reactor, has to be run in the ignited steady state, which implicates a danger of overheating in periods of rich pollutant concentrations causing thermal aging of the catalyst, whereas in the case of a lean feed the temperature may decrease below the ignition temperature, quenching the reactor. Therefore, control actions should be taken to avoid these limit situations. In the case of very rich feeds some options have been proposed in the literature to prevent overheating, namely, dilution of the feed with additional clean air, prolongation of the switching period, withdrawal of a portion of the hot gas from the center of the catalyst bed, use of an internal heat exchange, injection of cold gas, and use of a structured fixed bed with different effective axial heat conductivities (Eigenberger and Nieken, 1988; Purwono et al., 1994; Nieken et al., 1994a,b; van de Beld and Westerterp, 1995). Options to prevent extinction due to low pollutant * To whom correspondence should be addressed. † Present address: Departament d’Enginyeria Quı ´mica, Universitat de Barcelona, Martı´ i Franque`s, 1, 08028 Barcelona, Spain. S0888-5885(96)00658-6 CCC: $14.00

concentrations are more scarce (Nieken et al., 1994a,b). It is evident that a minimum amount of combustible components should be present in the feed to supply the necessary energy to sustain the autothermal process. This limit concentration is usually specified in terms of the minimum adiabatic temperature rise, ∆Tad,min, given by the expression

∆Tad,min )

0 H1C1,min

(Fcp)0g

(1)

Below this value the RFR is extinguished, no reaction occurs, and the maximum temperature of the reactor equals the feed temperature. This minimum adiabatic temperature rise depends on the efficiency of the heat exchange and on the ignition temperature, that is on the reactive components and catalyst. Thus, it is a characteristic value for each system and, in general, for a well-designed RFR, ranges from 10 to 30 °C. A simple strategy to warrant the minimum adiabatic temperature rise to keep the reactor ignited during times of lean feed consists of adding an additional gaseous or liquid fuel to the polluted air. This option has already been used successfully on an industrial scale to clean ventilation gases from painting or drying chambers in which the adiabatic temperature rise for the organic solvents like acetone, toluene, and xylene of say between 3 and 15 °C does not ensure the autothermicity of the process (Matros et al., 1993a). Two additional examples of similar industrial applications are mentioned by Chumachenko and Matros (1990). An interesting fact to be noted is when two or more combustibles with different ignition temperatures are treated. When a mixture of pollutants in air with an adiabatic temperature rise higher than the minimum value is treated in a RFR, the maximum temperature reached depends on both the composition and concentration of the pollutants. Several steady states are possible, and complete conversion of the component with the higher ignition temperature cannot be ensured particularly when the component with the lower ignition temperature is in excess (Nieken et al., 1994a,b; van de Beld et al., 1994). Consequently, the fuel to be added to a lean feed should have an ignition temperature similar to that of the pollutant oxidized at the highest ignition temperature. This is why it is difficult to use small amounts of natural gas as an extra fuel, since the © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4199

total hydrocarbon concentration in the inlet and outlet of the reactor is detected on-line by FID analyzers of JUM Engineering. The installation has been automated completely, and all process parameters can be changed via the control computer. The critical parameters like maximum temperature, pressure, and combustibles concentration are checked against alarm values, and if necessary, the installation shuts down automatically and safely. All measured data are collected on a data acquisition and control unit of Hewlett Packard. Mathematical Model Figure 1. Scheme of the reactor. Lengths: recuperator, 0.175 m; catalyst bed, 0.65 m; electrical heater, 0.05 m.

ignition temperature of methane is high and about 600 °C on the catalysts usually applied in catalytic combustion. In this case an alternative option would be to burn the natural gas in a separated burner and to introduce the resulting hot gas in the middle of the RFR (Nieken et al., 1994a,b). In this paper, the use of an internal electrical heater in the middle of the bed is proposed to add the extra energy to treat lean feeds as was proposed by van de Beld and Westerterp (1995). This alternative results in a simpler process that does not need extra fuel. The behavior of the RFR equipped with an internal electrical heater to purify air polluted by ethene and propane will be presented and discussed from an experimental and simulation point of view. The installation of an electric heater directly in the center of the catalyst bed of a RFR was first suggested by Wojciechowski (1985) in a European patent.

In literature different models have been proposed to describe the behavior of a reverse flow reactor. In this paper the one-dimensional heterogeneous model proposed by van de Beld and Westerterp (1995) is used after modification to include the presence of the electrical heater. The model considers the nonadiabaticity of the reactor, and heat transport terms are included to account for heat transport in the radial direction. The heat balance for the reactor wall has also been included because its heat capacity amounts to about 20% of the whole system. The energy released by the electrical heater is included in the model with an additional heat generation term in the solid phase. The following modified dimensionless equation is obtained for the heating zone:

F*s

∂θs ) NTUh(θg - θs) + Da∆θad ∂τ

(2) where P*el is the dimensionless power input defined by

Experimental Setup Basically, the apparatus and procedures are the same as those described in detail elsewhere by van de Beld et al. (1994) and will be summarized briefly. The reactor consists of a tube, 1 m in length and 145 mm in diameter. A scheme of the reactor is shown in Figure 1. To avoid heat losses to the surroundings, the reactor was equipped with an evacuated jacket and four radiation shields. The pressure in the jacket, ranging from 0.03 to 0.05 mbar, was monitored with a vacuum gauge, the pirani-11 of Edwards. The front and end parts of the reactor were filled with inert material. The remainder was filled with cylindrical catalyst pellets of Pd on γ-Al2O3. The pellets have a height and diameter of 4.5 mm. Twenty thermocouples are placed in the reactor to measure the gas temperature, and the solid phase temperature is measured at 12 positions. In the middle of the catalyst bed a standard electrical heater coil has been placed. The heating zone is 8 cm in diameter and about 5 cm in length. The power released by the heater can be varied between 0 and 1000 W and was measured by an ac kWh meter. The main air flow is supplied with a compressor of Hydrovane and measured and controlled by a Bronkhorst thermal mass flow controller. The air velocity in the experiments was about 0.4 m s-1 and the pressure 1.45 bar. Before entering into the reactor, the air was dried to a dewpoint of about 8 °C and filtered to remove all other unwanted components. Brooks 5850 mass flow monitors/controllers have been used to ensure a constant flow of ethene and propane. The flow direction is switched via four ball valves equipped with pneumatic actuators and placed in the feed and exit lines. The

∑r*jH*j + P*el

P*el )

PelL Vhz(FCp)goTougo

(3)

The mathematical model consists of three heat balances for the solid, the wall, and the gas and as many mass balances for the solid and gas phases as there are components. To solve the model, a finite difference technique is applied using 300 grid points. It is to be noted that the comparison between the experiments and the calculations will be done without using fit procedures; the radial heat transport has been determined in the same setup without chemical reaction and the kinetic parameters in a separate installation. Axial heat- and mass-transport coefficients are estimated from literature relations. For details of the model, the kinetics, the physical properties, and the transport relations used and details on the numerical method, we refer to van de Beld and Westerterp (1994, 1996). Experimental Results and Discussion The influence of several operating parameters, such as the adiabatic temperature rise, the power of the electrical heater, the cycle period, and the mixtures composition, on the behavior of the reverse flow reactor will be discussed on the basis of the axial temperature profiles, the maximum temperature, and the overall conversion. Unless stated otherwise the operational parameters given in Table 1 were used in all subsequent experiments. A standard experiment is carried out regularly to check whether the catalyst activity was constant or not. The greater temperature difference observed in the maximum temperatures between re-

4200 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997

Figure 2. Experimental axial temperature profiles for two different concentrations. Table 1. Experimental Operating Conditions pressure total number of cycles feed temperature

1.4 bar 195 30 °C

cycle period gas velocity (at STP) contaminants

400 s 0.4 m/s ethene, propane

peated experiments was less than 5 °C. The temperature profiles remained the same. (i) Influence of the Internal Electrical Heater. Before going into details on the influence of the electrical heater, we will give a brief description of the temperature profiles obtained in a RFR without an internal electrical heater. In our installation, it takes the temperature profiles 10-20 h to become completely constant over a cycle, i.e., to reach the pseudo-steadystate (PSS) (van de Beld et al., 1994). For simplicity only the profiles in one flow direction are given, so in the PSS the temperature profile for the reverse direction is a mirror image of the previous one. Figure 2 shows the temperature profiles for two different adiabatic temperature rises of ethene. As can be seen, the shapes of the profiles are completely different. The ethene conversion is very high for the higher adiabatic temperature rise whereas the combustion is not complete for the lower value, which is very near to the minimum adiabatic temperature rise, as will be seen later. The

upper curve of Figure 2 is the standard profile found in a RFR for ∆Tad > ∆Tad,min. Now the top of the temperature has been shifted from the center to the inlet and outlet parts of the reactor. If the reactor were truly adiabatic, the shape of the temperature profile would be plateau-like. However, radial heat losses inevitably occur, and as a result, a shallow valley appears in the middle of the reactor. The reaction is completed just before the maximum temperature is reached and in the middle part no reaction occurs anymore, so here heat losses dominate. Figure 3 shows experimental temperature profiles for different power inputs by the internal electrical heater for Cethene ) 0.0244 vol % after 195 cycles. The adiabatic temperature rise for this concentration is 11 °C, a value well below the critical one of about 21 °C (van de Beld et al., 1994). Thus, without the extra energy input the reactor is extinguished. Examination of Figure 3 shows several notable characteristics. First, the shape of the profiles resembles the one of Figure 2 for the low adiabatic temperature rise. There is no plateau, and the maximum temperature is always in the middle of the reactor where the heater is located. This can also be observed for other ethene concentrations, for propane, and for their mixtures as long as the adiabatic temperature rise is lower than the minimum one. In this situation, the heat released by the reaction has little influence on the stationary temperature profiles. Second, there is a critical electrical heater power for the ethene concentration used; for lower values the reactor will extinguish; in Figure 3 for 40 and 65 W the temperatures are decreasing and ultimately will reach the PSS, where the bed temperature equals the feed temperature. Under adiabatic conditions the heat released by the electrical heater produces a temperature rise given by

∆Tel )

4Pel πuo(FCp)goD2t

(4)

Adding ∆Tel to the temperature rise due to reaction ∆Tr, we have the total adiabatic temperature rise for the system, ∆Tt:

Figure 3. Influence of the electrical power input on the axial temperature profiles for Cethene ) 0.0244 vol % or ∆Tr ) 11 °C.


Figure 4. Maximum temperature as a function of the temperature rise for ethene with Cethene ) 0.0244 vol % and ∆Tr ) 11 °C with and without electrical heater.

∆Tt ) ∆Tr + ∆Tel

(5)

In Figure 4 the maximum temperature is given as a function of the temperature rise calculated by eq 5. As can be seen in our experimental setup, an autothermal process with high conversion cannot be guaranteed below ∆Tad,min ≈ 22 °C. Above this critical value a high conversion is obtained and the temperature rise in the reactor reaches values several times the total adiabatic temperature rise. At Pel ) 85 W, very near to the critical point, the conversion is about 96% and increases sharply on an increase in the electrical input, becoming 100% at Pel ) 177 W. Figure 4 also shows a comparison with the case of no electrical supply (see van de Beld et al., 1994), with all the other parameters being unchanged. There is an excellent agreement between the critical values with and without electrical power supply. This result allows us to suppose that for a single compound and for a given installation the ∆Tad, min is a constant for the ignited state. This conclusion enables us to estimate the minimum electrical power to be supplied to the RFR when lean mixtures are treated. Further there is a significant difference in how the maximum temperature increases with increasing ∆Tt; it is higher with than without electrical power supply for the same ∆Tt. This result can be explained if we take into account that the electrical energy, unlike the energy of reaction, is always added at the center of the reactor. Experiments performed with propane showed that the shape and relative position of the maximum temperature using propane are very similar to those of Figure 4 and, as a result, we can draw the same general conclusions as for ethene; but now ∆Tad,min ≈ 27 °C and the maximum temperatures are over 390 °C. The effect of the component can be observed in Figure 5 by comparing the maximum temperature and the adiabatic temperature rise in the critical zone for ethene and propane in the case of an electrical power supply. The critical maximum temperature for propane is about 100 °C higher than that for ethene; this is in good agreement with literature data because propane has lower reaction rates and a higher ignition temperature than ethene. An important result also is that above the minimum adiabatic temperature rise for propane both lines coincide practically. It seems that for this range the reactor behavior is independent of the component being burned. Consequently, the slope of these lines should be a function of the reactor characteristics including the electrical heater.

Figure 5. Maximum temperatures in the case of electrical power supply as a function of the adiabatic temperature rises for ethene with Cethene ) 0.0244 vol % and ∆Tr ) 11 °C and for propane with Cpropane ) 0.0195 vol % and ∆Tr ) 13 °C.

(ii) Influence of the Cycle Period. For different cycle periods the axial temperature profiles are given in Figure 6 just before the flow is reversed. The other operational parameters are given in Table 1. The power supply is 177 W or ∆Tel ) 22.5 °C, and the ethene concentration is about 0.0244 vol %. It can be seen that for a too high cycle period, too much heat is removed from the reactorsthe outlet temperature increases for longer periodssand the reactor is extinguished. This is in good agreement with literature data. Further the maximum temperature decreases for increasing cycle periods. This apparently does not agree with experimental data in literature by Nieken et al. (1994a) and van de Beld et al. (1994). These authors found that the maximum temperature in the PSS is not affected by the increase of the cycle period. However, they observed that the width of the plateau becomes smaller. In our case with ∆Tr < ∆Tad,min the self-adaptative behavior of the RFR reduces the maximum temperature in order to compensate for the heat removed by convection when the cycle period is increased on the other hand, simulations by Sapundzhiev et al. (1991) and by Eigenberger and Nieken (1988) agree with the experimental ones of Figure 6. The conversion was 100% up to a cycle period of 800 s, and it reduces to 95% for a period of 1200 s. Finally we can conclude that the presence of the electrical heater will make it more difficult to find the optimum cycle period. (iii) Behavior of Mixtures. In industrial practice different organic components or mixtures of components can be present in contaminated air. In order to study the effect of an internal electrical power supply in a RFR, experiments were performed for a mixture of ethene and propane. Different compositions were employed such that the total adiabatic temperature rise n of the mixture ∆Tad,r ) Σi)1 Tad,iswhere ∆Tad,i holds for complete oxidation of the ith component and n equals the number of components, in our case two componentsswas below and above the minimum adiabatic temperature to reach the ignited state. Figure 7 shows the periodic steady-state temperature profiles for a mixture with Cethene ) 0.0244 vol % and Cpropane ) 0.0195 vol %, which gives ∆Tad,r ) ∆Tad,ethene + ∆Tad,propane ) 11.3 + 13.5 ≈ 25 °C. This mixture has a total adiabatic temperature rise slightly lower than the minimum one since the reactor cools down without the heat being supplied by the heater, but is very near to this critical value because just a ∆Tel off about 5-8 °C was necessary to have an ignited state. The shapes


Figure 6. Influence of the cycle period on the axial temperature profiles with using ethene as the contaminant with Cethene ) 0.0244 vol % and Pel ) 177 W.

Figure 7. Experimental axial temperature profiles for a mixture of ethene and propane with a total adiabatic temperature rise of 25 °C, slightly lower than the critical one for different electrical power inputs.

of the curves are similar to those of Figure 3, but they seem to show some trend to a plateau profile, particularly at small electrical power. The slope of the variation of the maximum temperature with the total adiabatic temperature rise is very similar to that of Figure 5. The total conversions in the ignited states were very high. For 65 W they were about 98% and went up to 100% for a power of 85 W. The unburned hydrocarbon was propane because ethene is oxidized easier than propane, the difference in the ignition temperatures being near 100 °C. For 40 W the reactor behavior is critical. The quenching rate depended on the initial preheat temperature. At T0 ) 400 °C the decrease in maximum temperature and total conversion was extremely slow, whereas at T0 ) 310 °C it was fast. From these results we can conclude that for a mixture of ethene and propanesin which an incomplete combustion of the compound with the higher ignition temperature occurssthe addition of a small amount of energy

of a few watts can complete the combustion of all contaminants. Other experiments were performed using a mixture of ethene and propane with a total adiabatic temperature rise higher than the critical one. In Figure 8 the axial temperature profiles for different compositions and electrical power inputs are shown. The initial temperature profile is established with propane only. For the other experiments the composition was changed to ∆Tad ) 36 °C for ethene and ∆Tad ) 15 °C for propane and kept constant at ∆Tr ) 51 °C. From the first to the second experiment, the temperature decreased about 15 °C and the width of the plateau increased as expected (see van de Beld et al. (1994) and Nieken et al. (1994b)), whereas the total conversion was maintained constant between 99 and 100%. In the following experiments the effect of the electrical power supply on the plateau temperature can be noticed. The slope of the temperature profiles is almost insensitive to the power supply,


Figure 8. Experimental axial temperature profiles for a mixture of ethene and propane with an adiabatic temperature rise of 48 °C, greater than the critical one. Effect of the electrical power supply on the plateau temperature.

Figure 9. Start-up using the internal electrical heater and axial temperature profiles for a mixture of ethene and propane with an adiabatic temperature rise slightly lower than the critical one.

whereas the maximum temperature increased quickly with increasing electrical power. It is important to note that, also in this case, the slope of the variation of the maximum temperature with the total adiabatic temperature rise is very similar to that of Figure 5, backing up the hypothesis that the increase of the maximum temperature or plateau temperature with an increase in the electrical power supply is independent of the contaminants and composition. An experiment was performed with a feed of ethene corresponding to ∆Tad ) 26 °C, just 4 °C higher than the critical adiabatic temperature rise. In the PSS a plateau temperature of 332 °C and an ethene conversion of 96-97% were obtained, as expected from previous results. After that an amount of propane with Cpropane ) 0.032 vol % and ∆Tad ) 22 °C was added to the feed so that the total adiabatic temperature rise became 48 °C. The response of the reactor was unexpected since the maximum temperature 332 °C was too low to oxidize propane. Not only an increase of the plateau temperature to 374 °C but also an increase in the conversion

to 99% was produced. Thus, some small propane conversion already takes place at 332 °C which subsequently increases Tmax and hence the ethene and propane conversion. (iv) Start-Up and Development of Temperature Profiles. For start-up the RFR has to be preheated in a separate first step to the temperature level for the reaction. This is about 290 °C for ethene and 390 °C for propane in our installation, if we use hot air from an external heater. An experiment has been performed to find out whether the start-up may be carried out using the internal electrical heater only. Figure 9 shows the experimental development of the temperature profiles for a mixture of ethene with Cethene ) 0.024 vol % and propane with Cpropane ) 0.0195 vol % and an electrical power of 223 W. The initial temperature was about 60 °C. It can be seen that the start-up using only the internal heater is feasible. Moreover, for this power level it took less time to reach the minimum temperature for the ethene reaction than using the external heater. Therefore, a RFR equipped with an internal


Figure 10. Comparison of experimental and calculated temperature profiles for the oxidation of ethene. Cethene ) 0.0244 vol %; ∆Tad ) 11 °C; Pel ) 265 W.

Figure 11. Comparison of experimental and calculated temperature profiles for the oxidation of ethene at these cycle periods. Cethene ) 0.0244 vol %; ∆Tad ) 11 °C; Pel ) 177 W.

electrical heater can be started up with a high power input but not so high that high local temperatures can damage the catalyst. After that, when the minimum temperature for reaction is reached, the power must be adjusted to just oxidize completely the contaminants in the feed. Simulation Results In the figures discussed in the following some experimentally measured profiles will be compared to the calculated temperature profiles with the model. As stated before, we did not attempt to improve the comparisons by adjusting the input values of the parameters. On the basis of these comparisons, the validity of the model can be discussed and also the RFR behavior examined under conditions where our experimental unit could not be operated. Figure 10 shows the comparison of the calculated temperature profile with the experimental temperatures of Figure 3 for a power of 265 W using ethene as a contaminant. The model predictions are quite good, both for the shape of the profiles and for the position of the temperature maximum. The reaction heat is released in a narrow zone between 0.4 and 0.45 m from the inlet, according to the simulation. This heat input (∆Tad ) 11 °C) is not seen in the figure because the released heat is masked by the heat produced by the heater (∆Tad ) 33 °C). On the other hand, the model can reproduce the lines of Figure 5, but the predicted values of the maximum temperature are somewhat higher than the experimental ones. In Figure 11 the influence of the cycle period on the temperature profiles and the maximum temperature is shown. For the two periods the experimentally determined maximum temperature is predicted quite well by the model. The discrepancies are smaller than 5%. Also the shape of the profiles are well described, but some quantitative discrepancies can be observed. Generally such differences increase on approaching the critical values of parameters of either the total adiabatic temperature rise or the maximum cycle period. The deviations are probably caused by inaccurate heattransfer coefficients and the way in which the internal electrical heater has been modeled, assuming a homogeneous heat distribution throughout the heating zone

Figure 12. Comparison of experimental and calculated temperature profiles for the oxidation of a lean mixture of ethene and propane with ∆Tr ) 25 °C and an electrical power input of 131 W.

and considering that the heater material behaves as if it were catalyst. A two-dimensional model accounting for radial temperature gradients and the actual position of the heater and its own transport properties would improve the predictions. Only a 15% reduction in the value of the radial heat-transfer coefficient improves the theoretical predictions of the model significantly. In other words, the model is extremely sensitive toward that coefficient, particularly near the critical conditions. Moreover, in the experimental setup the temperatures are measured on the center line, whereas the onedimensional model predicts a temperature averaged over the radius. It is worthwhile to mentioned the results of Matros et al. (1993a); they found using a twodimensional heterogeneous model that heat losses through the wall may strongly affect the reverse process for gases with a low adiabatic temperature rise. Other sources for the deviations can be the kinetic data and experimental error. In Figures 12 and 13 the experimental and calculated temperature profiles for lean mixtures of ethene and propane are shown for ∆Tr ) 25 and 51 °C, respectively, and an electrical power input of 131 W or ∆Tel ) 17 °C. For the lean mixture the predictions of the model are


Figure 13. Comparison of experimental and calculated temperature profiles for the combustion of a rich mixture of ethene and propane with ∆T ) 51 °C and an electrical power input of 131 W.

Figure 14. Effect of the width (parameter of the curves in centimeters) of the electrical heater on the temperature profiles for the combustion of ethene with Cethene ) 0.0244 vol % and Pel ) 265 W.

quite similar to those for ethene only except that now the maximum temperature is 7% higher than the experimental one. The model shows that the ethene and propane oxidations are completed before the maximum temperature is reached. In Figure 13 we can see that the effect of the electrical heater on the plateau temperature profile is well predicted by the model. Now the reaction is completed as soon as the temperature is high enough, which occurs according to the model at about 0.3 m on the axial position. Once having demonstrated that the model predictions are acceptable, we can study the effect of the width of the electrical heater by simulation. Figures 14 and 15 show calculations for the combustion of a lean ethene feed, varying the width of the heater from the approximated value of 5 cm in the experimental setup to a maximum of 65 cm corresponding to the full catalyst bed, at the same conditions. From the temperature profiles for Pel ) 265 W in Figure 14 we can conclude that the ignited state is always obtained and that the maximum temperature decreases with an increase in the width of the heater. Complete ethene reaction is

Figure 15. Effect of the width (parameter of the curves in centimeters) of the electrical heater on the temperature profiles for the combustion of ethene with Cethene ) 0.0244 vol % and Pel ) 177 W.

also achieved. Simulations for Pel ) 177 W in Figure 15 show a different behavior. The maximum temperature decreases and so does the ethene conversion. In the range of 5-20 cm the ethene conversion is 100%. It reduces to 97% for a width of 40 cm and to 0% for a width of 65 cm. This indicates the heater width is important when the reactor works near the minimum temperature for ignition. Therefore, for a reactor with an internal electrical heater the heater width should be as short as possible but not so short that a hot spot can develop. Moreover, a short width allows us to use a short catalytic bed since the reaction generally occurs near the heater. Experimental data in Figures 3 and 7 back up this conclusion. We should realize that this discussion only holds when the cycle period is so short that the electrical heat supplied is kept in the bed and not removed by the cold feed. Further the electrical power supply can be interrupted as soon as combustion is complete as, for example, determined in on-line instruments. Conclusions The catalytic combustion of a lean feed in a reverse flow reactor using an internal electrical heater has been studied experimentally. The reactor has been operated successfully, and high conversions of the contaminants have been obtained using the internal electrical heater for lean mixtures, which required an additional heat supply for the ignited state. The total minimum adiabatic temperature rise was 22 °C for ethene and 26 °C for propane in the ignited state in our equipment. The effect of the cycle period is similar to when there is no electrical heat supply, but it influences the maximum temperature. It should be taken into account since the minimum temperature for reaction can be affected if the cycle is too high and the reactor may be blown out. All the same, the optimum cycle period should not be far from the optimum value obtained for no internal heater. The behavior of mixtures of combustible components is difficult to predict because the minimum adiabatic temperature rise is not a simple function of those of the components, at least for lean mixtures. The use of an internal electrical heater will be very useful because the maximum temperature is a function of the electrical


power and independent of the mixture components and composition. Thus, a mixture of contaminants with very different ignition temperatures can be burned completely, provided that the electrical power is high enough. The minimum power for complete reaction may be difficult to estimate due to possible interaction effects between the components of the mixture. In the case of rich mixtures an internal electrical heater is not necessary to maintain the ignited state. Nevertheless, it can be used to start up the reactor and to ensure the complete reaction of mixtures. Simulation results show that for lean mixtures and a given electrical power supply the width of the electrical heater reduces the maximum temperatures. Problems of quenching may arise with an increase in the heater width if the reactor is near the minimum temperature for the ignited state. So, the narrower the width of the electrical heater, the better. Furthermore, a narrow width of the internal heater allows us to use shorter catalyst beds because the reaction usually takes place near the heater. Acknowledgment These investigations are supported by The Netherlands’ Foundation for Chemical Research (SON) with financial aid from The Netherlands’ Technology Foundation and DSM and by the Direccion General de Investigacion Cientifica y Tecnica (DGICYT) of the Spanish Government. The authors thank G. H. Banis, P. Flanagan, F. ter Borg, and A. H. Pleiter for technical assistance. Nomenclature Cp ) heat capacity, kJ/(kg K) C ) concentration, mol/m3 Dt ) reactor diameter, m Fs* ) Cps/Cops H ) heat of reaction, kJ/mol of HC L ) reactor length, m Pel ) power input, kW P*el ) dimensionless power input (eq 3) r* ) dimensionless reaction rate T ) temperature, K T0 ) preheat temperature, K ∆Tel ) “electrical” temperature rise, K ∆Tr ) adiabatic temperature rise due to reaction, K ∆Tt ) total adiabatic temperature rise defined by eq 5, K ∆Tad,min ) minimum temperature rise for an autothermal process at reference conditions, K t ) time tc ) cycle period u ) superficial gas velocity, m/s Uoverall ) overall radial heat-transfer coefficient, kW/(m2 K) Vhz ) heating zone volume, m3 Greek Letters F ) density, kg/m3 θ ) T/T0, dimensionless temperature

Dimensionless Groups

Da ) r01L/ugoC01, Damkohler number F ) (1 - )(FCp)so/(FCp)go, extraction factor for heat NTUh ) Rap(1 - )L/ (FoCp)gugo, number of heat-transfer units ∆θad ) H1C01/(FCp)goT0, dimensionless adiabatic temperature rise τ ) (ugot/(1/L)/F), dimensionless time: time divided by the heat front residence time Literature Cited Chumachenko, V. A.; Matros, Yu. Sh. Catalytic purification of gases from organic substances under unsteady-state conditions; results and predictions. Proceedings of the International Conference, Novosibirsk, U.S.S.R., June 5-8, 1990; VSP BV: Utrecht, The Netherlands, 1990; pp 605-612. Cottrell, F. G. Purifying gases and apparatus therefor. U.S. Patent Office, 2, 121, 733, June 21, 1938. Eigenberger, G.; Nieken, U. Catalytic combustion with periodic flow reversal. Chem. Eng. Sci. 1988, 43, 2109-2015. Haynes, T. N.; Georgakis, C.; Caram, H. The design of reverse flow reactor for catalytic combustion systems. Chem. Eng. Sci. 1995, 50, 401-416. Matros, Yu. Sh.; Noskov, A. S.; Chumachenko, V. A.; Goldman, O. V. Theory and application of unsteady-state catalytic detoxication of effluent gases from dioxide, nitrogen oxides and organic compounds. Chem. Eng. Sci. 1988, 43, 2061-2066. Matros, Yu. Sh.; Bunimovich, G. A.; Noskov, A. S. The decontamination of gases by unsteady-state catalytic method. Theory and practice. Catal. Today 1993a, 17, 261-274. Matros, Yu. Sh.; Noskov, A. S.; Chumachenko, V. A. Progress in reverse-process application to catalytic incineration problems. Chem. Eng. Prog. 1993b, 32, 89-98. Nieken, U.; Kolios, G.; Eigenberger, G. Fixed-bed reactors with periodic flow reversal: experimental results for catalytic combustion. Catal. Today 1994a, 20, 335-350. Nieken, U.; Kolios, G.; Eigenberger, G. Control of the ignited steady state in autothermal fixed-bed reactors for catalytic combustion. Chem. Eng. Sci. 1994b, 49, 5507-5518. Purwono, S.; Budman, H.; Hudgins, R. R.; Silveston, P. L.; Matros, Yu. Sh. Runaway in packed bed reactors operating with periodic flow reversal. Chem. Eng. Sci. 1994, 49, 5473-5487. Sapundzhiev, C.; Grosev, G.; Elenkov, D. Non-steady-state catalytic decontamination of waste gases. Chem. Eng. Technol. 1991, 14, 209-212. van de Beld, L.; Westerterp, K. R. Air purification by catalytic oxidation in a reactor with periodic flow reversal. Chem. Eng. Technol. 1994, 17, 217-226. van de Beld, L.; Westerterp, K. R. Decontamination of polluted air in a reverse flow reactor: Comparison of model simulations and experiments. AIChE J. 1996, 42, 1139-1148. van de Beld, L.; Westerterp, K. R. Can. J. Chem. Eng. 1997, accepted. van de Beld, L.; Borman, R. A.; Derkx, O. R.; Woezik, B. A. A.; Westerterp, K. R. Removal of volatile organic compounds from polluted air in a reverse flow reactor: An experimental study. Ind. Eng. Chem. Res. 1994, 33, 2946-2956. Wojciechowski, J. European Patent Office 0 037 119, January 30, 1995.

Received for review October 15, 1996 Revised manuscript received March 14, 1997 Accepted July 11, 1997X

Subscripts and Superscripts g ) gas j ) component j s ) solid o ) at reference conditions, 298 K and 1 bar * ) dimensionless

IE960658E

Abstract published in Advance ACS Abstracts, September 1, 1997. X

Catalytic Combustion of Very Lean Mixtures in a Reverse Flow

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