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Periodic temperature forcing of catalytic reactions
Chemical Engineering Science 59 (2004) 4043 – 4053 www.elsevier.com/locate/ces
Periodic temperature forcing of catalytic reactions Peter L. Silveston∗ , Robert R. Hudgins Department of Chemical Engineering, Reactor Engineering Laboratory, University of Waterloo, Waterloo, Ont., Canada Received 5 January 2004; received in revised form 14 April 2004; accepted 18 May 2004
Abstract The exponential relationship of rate constant to temperature served as a magnet to attract the attention of researchers in the 1960–1970 to periodic temperature forcing. Early theoretical studies and simulation exercises suggested that significant improvements of rate and selectivity could be expected from this type of unsteady state operation. The first experiments in the late 1970s and early 1980 failed to confirm these expectations, so research on periodic temperature forcing languished. The current interest in microreactors with their greatly reduced thermal capacitance has revived activity in temperature modulation. Recent discoveries of the effect of temperature pulses in trickle-bed reactors have further spurred this activity. This review addresses the question of whether temperature modulation of chemical reactors can significantly affect reactor performance. 䉷 2004 Published by Elsevier Ltd. Keywords: Periodic operation; Temperature cycling; Temperature modulation; Catalytic reactors; Trickle beds
1. Introduction Because of the exponential relation of the rate constant to temperature, periodic temperature forcing of catalytic reactions seemed promising to investigators. This mode of forcing was discussed in the earliest studies on periodic operation, such as those of Horn and Lin (1967), Denis and Kabel (1970), Dorawala and Douglas (1971), and Bailey et al. (1971). The reason for this early interest in temperature forcing becomes clear from some simple calculations. If a point in a reactor operating at 500◦ C could be cycled symmetrically with an amplitude of 10◦ C, the time-average rate of reaction at the point would increase by 7.4% over the rate at steady state at 500◦ C for a reaction with a low activation energy of 83.6 kJ/mol. For a large activation energy of 418 kJ/mol, the increase would be 266%. If the amplitude were increased to 20◦ C, rate improvements for the range of activation energies become 30% and 2300%, respectively. Furthermore, Van Neer et al. (1996), in their theoretical study of resonance phenomena under modulation, observed that
∗ Corresponding author. Tel.: +1-519-885-1211; fax: +1-519-746-4979.
E-mail address: [email protected] (P.L. Silveston) 0009-2509/$ - see front matter 䉷 2004 Published by Elsevier Ltd. doi:10.1016/j.ces.2004.05.034
temperature forcing should exhibit much larger effects than those from concentration forcing. Early attempts to carry out temperature-forcing trials foundered because of the thermal inertia of catalyst beds or reactor bodies; as a result, there has been little research activity on temperature forcing for about three decades. The appearance of micro-reactors with relatively low mass in the last few years has reawakened interest in temperature forcing. Further, some remarkable increases in rates of reaction in three-phase trickle beds observed for periodic interruption of liquid phase flow now appear to be caused by temperature variations. Our purpose in this review is to summarize what has been discovered so far about temperature forcing of catalytic reactions and to suggest what experiments should be undertaken. The ultimate goal is to determine whether temperature modulation is a useful practice. 2. Theoretical studies Early studies on periodic temperature forcing, mentioned in the Introduction, were theoretical and are summarized in a review (Bailey, 1974). Two groups of operations are considered: quasi-steady state and relaxed steady state. In
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the former mode, half-cycles are long enough that the steady state is reached, while for the relaxed steady state, switching is so fast that the manipulated system does not change with time. Improved performance for quasi-steady state requires convexity in the equations describing the reactor system. Consider an isothermal CSTR in which a first-order reaction is occurring, the system is simply described as x1 =
1 , 1 + Da exp[−1/x2 ]
(1)
where x1 , x2 are, respectively, the dimensionless concentration and temperature in the reactor. If the temperature is an independent variable, the system exhibits convexity. Indeed, using the numbers given in the introduction, a bang–bang periodic change in reactor temperature (x2 ) can significantly increase conversion. Convexity is also exhibited by a plugflow packed-bed catalytic reactor provided the characteristic time for a change in surface concentration is small measured against the cycle period. The relaxed steady state arises when the variation of the manipulated variable forces a reactor output into a limit cycle that shrinks into approximately a single state as the cycle frequency increases. It will differ from a steady state of the system if the dynamics of the reactor are nonlinear or the relation is nonlinear between an objective function to be optimized, such as conversion, and the state and manipulated variables. In temperature forcing, the manipulated variable is temperature, so that in general the condition for a relaxed steady state that differs from a steady state is met. Whether the relaxed steady-state results in improvement can be established by applying an optimization routine to the system of dynamic and objective function equations. The first application to a reactor system was in the classical paper by Horn and Lin (1967) considering a system of parallel reactions of different order in a CSTR under bang–bang temperature cycling. Applying Pontryagin’s maximum principle, Horn and Lin found a set of temperature states for which the objective function, the concentration of a product from one of the competing reaction, was contained in a small limit cycle located outside of the range of steady states. This situation is shown in Fig. 1. The plot assumes a second-order reaction to the desired product and a firstorder reaction to a waste product. The activation energy for the second reaction is three-quarters of the activation energy for the reaction to the desired product. The cycle contains two temperatures. In the plot, the concentration of the desired product, CB , is plotted against the exit concentration of the reactant CA . The figure shows the variation of CB with CA is concave, so that quasi-steady-state operation cannot improve upon steady state (maximum selectivity to CB at I). Fast cycling of the CSTR temperature leads to at least two relaxed steady-state operations. The operation designated by II shows a modest improvement over the optimal steady state, while the second region just above I is another relaxed steady state but one giving negligible improvement over the best steady-state operation.
Fig. 1. Exit concentrations under steady state and periodic operation with temperature forcing for a parallel reaction (A → B, A → C) in a CSTR. SS = curve of steady-state selectivities, I, II = relaxed steady operations (figure reproduced from Bailey (1974) with permission of the copyright holder, 䉷 2001 Gordon and Breach Science Publishers).
Dorawala and Douglas (1971) considered a consecutive reaction in a CSTR with variable temperature. In this system, the intermediate product was the desired one. They demonstrated using a search technique that there is a relaxed steady state through temperature cycling which provided a selectivity which is superior to the optimal steady-state one. Sterman and Ydstie (1990, 1991) considered both parallel and consecutive reactions in a CSTR with temperature forcing. In the latter paper, they developed an optimization scheme to determine relaxed steady states that improved upon the optimal steady state. An interesting development in the theoretical studies of temperature forcing in a CSTR was the discovery by Sinˇci˙c and Bailey (1977) that forcing can lead to some surprising dynamic behavior. A comprehensive investigation of the dynamic phenomena that may arise is discussed by Kevrekidis et al. (1986). For first-order parallel reactions with different activation energies, Sinˇci´c and Bailey observed a single limit cycle for the reactor, a complex cycle that was 2-periodic, and two stable limit cycles that are -periodic. Several of the cycles exhibited irregular temperature trajectories. One such is illustrated in Fig. 2. The dimensionless temperature x2 is the time-average temperature in the reactor. 3. Simulation studies Perhaps the earliest study of temperature forcing was undertaken by Chang and Bankoff (1968), who examined a jacketed packed-bed reactor. These investigators found that conversion was optimal in a quasi-steady-state operation. Denis and Kabel (1970) undertook simulation of temperature forcing in a packed-bed catalytic reactor. The reaction considered was the vapor-phase dehydrogenation of ethanol over a cation exchange resin. They demonstrated improved conversion over steady-state operation. An interesting aspect of their work was the use of the response to stepchange variations in the reactor jacket temperature to obtain
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Dimensionless concn. X1
1.0
0.8
0.6
0.4
0.2
10
11
12
13
14
Dimensionless temp. X2 Fig. 2. Concentration-temperature trajectory for the A → B, A → C reaction system in a CSTR that is forced by modulation of the feed temperature. Points in the figure represent steady operating states. The two loops indicate 2-periodicity (figure reproduced from Sincic and Bailey (1977) with permission of the copyright holder, 䉷 2001 Pergamon Press).
parameters for their dynamic packed-bed model. This model allowed for adsorption/desorption from the catalyst surface and considered variations of velocity in the reactor. Cyclic temperature variations in the above type of system were considered in a simulation study by Kim and Hulburt (1972). These authors also examined simultaneous modulation of concentration and feed rate along with temperature. They observed improvements that they claimed were due primarily to temperature cycling. Temperature forcing of a CSTR was simulated by Dorawala and Douglas (1971) assuming temperature modulation by changing the coolant flow rate in the CSTR jacket. This caused a variation in reactor temperature. The same authors also examined forcing of the feed temperature to the reactor. Generally, these simulation studies found that temperature cycling with the proper frequency and cycle split improved reaction selectivity. Many of these studies are summarized in Bailey’s masterly review (Bailey, 1977). In 1980, Lee and Bailey (1980) considered consecutiveparallel reactions in a CSTR under forcing and demonstrated a significant increase in selectivity to the desired intermediate product in the reaction chain. The optimism of this and earlier simulations and theoretical studies has not stood up to the rigor of experiment.
4. Experimental studies Experimental investigations also began shortly after the Bailey review. One of these dealt with small amplitude, lowfrequency perturbations intended to test dynamic, packedbed reactor models (Hansen and Jorgensen, 1974). These experimenters used pilot-scale equipment (reactor dimensions:
Fig. 3. Temperature modulation input, temperature variation at the reactor wall used by Abdul-Kareem et al. (1980) in their investigation of CO oxidation in a packed-bed reactor (figure reproduced from Abdul-Kareem et al. (1980) with permission of the authors).
diameter=9.6 cm, wall thickness=0.1 mm, length=50 cm) and considered the oxidation of hydrogen on a Pt/Al2 O3 catalyst. Oxidation went to completion in the reactor, so the authors were unable to gain information about the effect of temperature modulation on the rate. They did observe wrong-way behavior on a step-input and amplification as great as 20 db in the reactor interior when a periodic, squarewave disturbance of small amplitude was introduced. The Hansen experiments have been recently extended and we will discuss their new data briefly at the end of this section. The question of improvement through temperature modulation in a packed-bed reactor was attacked by Abdul-Kareem et al. (1980). These researchers examined temperature forcing of CO oxidation over a vanadia catalyst in a packed bed, 6.35 mm in diameter. This reactor was immersed in a fluidized sand bath, so that the reactor wall was essentially at the temperature maintained in the sand bath. A high-gas flow rate and fine particles (50–60 mesh) minimized radial temperature gradients in the reactor. This also made the reactor function as a differential reactor. Forcing was produced by switching the temperature in the sand bath using high-output electrical heaters as well as by adjusting the air flow rate. Temperature in the catalyst bed was measured by a steel-sheathed chromel–alumel thermocouple. Because of the thermal inertia, temperature in the bed could not be cycled as a square wave. The wave form obtained is illustrated in Fig. 3. As the cycle period increased, 1 > 1 and a square-wave input was approached. Steady-state rate measurements over the forcing range plotted linearly against the reciprocal absolute temperature gave an activation energy of 103 ± 5 kJ/mol, in good agreement with published values. Cycle periods from 1 to 8 h were used. The lower value was chosen because about 10 min were needed to switch temperatures between the two levels chosen. Thus, with a 60-min cycle, the bed resided at the desired temperature for just 20 min in each partial cycle.
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Fig. 4. Comparison of time-average CO oxidation rate with the quasi-steady-state oxidation as a function of cycle period and input amplitude. Inlet CO = 16% by volume (figure reproduced from Abdul-Kareem et al. (1980) with permission of the authors).
Abdul-Kareem et al. observed that the time-average rate of CO oxidation under temperature modulation increased with increasing cycle period for both amplitudes (10◦ C and 20◦ C). Above 6 h, the quasi-steady-state rate was obtained. Low-frequency modulation, thus, improves the time-average reaction rate, but the quasi-steady-state limit is not exceeded. As expected, the quasi-steady-state rate is greater than the steady-state rate at any temperature. The oxidation rate versus temperature curve is convex. What is surprising about the measurements is that the time-average rate lies below the quasi-steady-state rate at cycle periods from 2 to 6 h. Abdul-Kareem et al. point out that the sand bath reached a constant temperature in 10 min after a change, while the catalyst particles moved across the temperature difference in about 1 s and heat transport through the small diameter packed bed has a characteristic time of perhaps 60 s (Fig. 4). Thus, particle temperature follows the input temperature closely. The remarkably long period of 6 h needed to reach the quasi-steady state indicates that changing temperature on the catalyst surface probably retards rather than enhances the rate of the catalytic reaction. Experimental results with cycle periods less than 2 h and a 10◦ amplitude only reached the steady-state rate at the time-average temperature. This rate is well below the quasi-steady-state rate, again indicating that the transient temperature experience by the particles does not enhance the rate of oxidation. Consequently, for the CO oxidation system studied, temperature cycling offered no advantage. The best strategy to increase the rate of reaction is to operate at the highest permissible temperature.
Fig. 5. Schematic of the jacketed CSTR used by Lee et al. (1980) in their experiments on temperature forcing of the saponification of diethyl adipate (figure reproduced from Lee et al. (1980) with permission of the authors).
Bang–bang temperature cycling of the heating fluid in the jacket of a CSTR reactor was undertaken by Lee et al. (1980) in order to verify their earlier predictions of improvement obtained by simulation. The small jacketed CSTR used by Lee et al. (1980) is shown in Fig. 5. Saponification of diethyl adipate with sodium hydroxide was studied. This reaction has a consecutive-parallel structure: A + B → C + D, C + B → E + D. Temperature cycling was performed by switching the coolant in the CSTR jacket from a stream at 90◦ C to one at 15◦ C. Experiments were carried out using diethyl adipate dissolved in an equivolume mixture of isopropanol and water. Results for temperature forcing are shown in Fig. 6 for a cycle period of 5 min and cycle splits from 0.1 to 0.9 based on the duration of the high-temperature coolant in the jacket. Experimental measurements fell on the steady-state line, but are not shown because of uncertainty in the measurements of adipate concentrations. The curve marked “periodic” in the figure is a simulation based on careful kinetic measurements and a model for heat transfer in the jacketed CSTR that closely reproduced the experimental temperature change in the CSTR in response to a step-change in jacket temperature. The figure suggests an improvement in the intermediate yield ranging from only 3% at the highest conversion used to 2% at the lowest conversion. This is quite small and suggests that temperature forcing in CSTRs is ineffective, contrary to earlier simulation results. Dynamic measurements
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Fig. 6. Time-average yield of sodium monoethyl adipate salt as a function of diethyl adipate conversion for cooling jacket temperature cycling at an amplitude of 37.5◦ C with a cycle period of 5 min and various cycle splits. Steady-state yield at 52.5◦ C shown for comparison (figure reproduced from Lee et al. (1980) with permission of the authors).
Fig. 7. Hand-size microreactor designed for temperature cycling showing insertable heating cartridge and insertion point for TC monitor (figure reproduced from Brandner et al. (2001) with permission of the copyright holder, 䉷 2001 Microreactor Technology).
on the jacketed CSTR indicate a characteristic time of about 2.5 min. This suggests the cycle period used was too short to benefit from the amplitude imposed on the coolant. Thus, damping through heat exchange may explain the small improvement compared to steady state. Although the early experimental results are not encouraging, they do not rule out the use of temperature cycling. After a lapse of some 20 years, interest in periodic temperature forcing has re-surfaced driven by the availability of microreactors with fast heating and relatively low-thermal inertia. Fig. 7 is a photograph of a typical unit that has been examined for fast temperature cycling. The Swage-
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lock fittings for 41 -in tubing gives a good indication of the size of the unit. Layered construction of the unit is given in Fig. 8a. The wafer layers separate reacting fluid, usually a gas, and the cooling fluid, usually a liquid. Manifolds on either side of the stack introduce the feed and coolant. On the opposite side-reaction products and the heated coolant are withdrawn. The manifolds are arranged to give cross-flow. Metallic wafers are used for rapid heat transfer, so that just a single layer is adequate for heat input. These wafers are 44 mm × 33 mm × 2 mm. Tracks are machined into each wafer to form the flow paths for each fluid. The path structure is shown in the bottom of the figure as Fig. 8b. In the reacting fluid layer, the channel is 380 m wide and 150 m deep. Its length is 710 mm. Catalyst can be deposited in the channel in several ways. The usual first step is to roughen the surface chemically and then lay down a porous washcoat. This wash coat is then impregnated with a decomposable salt of the catalyst. Drying and calcinations complete the process. Details are given by Rouge and Renken (2001a). Alépée et al. (2000) describe an earlier version. Rouge and Renken (2001a) employed heating and cooling fluids, so a somewhat different design was needed. They used a 20 mm × 20 mm stack consisting of nine 14-mmthick ceramic wafers (MACOR) and nine steel plates for flow control. Stack structure and flow routing are shown in Fig. 9a and the machined wafers are given in Fig. 9b. Parallel channels are machined into the wafers as flow channels. There were 34 channels in each wafer. Each channel was 300 m wide, 240 m deep and 20 mm in length. Use of a ceramic wafer simplifies application of the catalyst to the channel walls. A wash coat can be applied and then impregnated or in some case the wafer alone can act as the catalyst support alone. Brandner et al. (2001) were concerned with microreactor response and so did not demonstrate forced temperature modulation for a reacting system. They employed N2 as the flowing fluid and used water as the coolant. High-frequency on–off water flow was required and this was accomplished by switching water flow between the micro reactor and a bypass. Early tests showed some water hold up in the cooling channels that interfered with the heating half-cycle. This was solved by employing a 10-ms air blast to strip water from the channels. Electrical heating power as high as 1675 W could be used. At this heat input about 93% was transferred to the flowing gas. The investigators demonstrated an amplitude of about 90◦ C within the reactor, while the exit gas temperature had an amplitude of about 33◦ C at a water flow rate of 15 kg/h, a power input of 450 W and a cycle period of 60 s. Increasing power input and water flow rate increased both amplitudes. If the cycle period was reduced to 20 s, amplitudes were sharply reduced and the temperature oscillations were no longer symmetrical. Brandner et al. claimed amplitudes of about 50◦ C in the reactor walls can be achieved for 4-s periods. On the gas side, mixing was sufficient to cause the temperature variation at the outlet to disappear. Further modifications were made to increase the heating and
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Fig. 8. Construction of the micro reactor-heat transfer stack (a) and flow path layout for reacting and cooling fluids (b) (figure reproduced from Brandner et al. (2001) with permission of the copyright holder, 䉷 2001 Microreactor Technology).
Fig. 9. Schematic of directional masks for reacting fluids and heat transfer fluids (a) for a micro reactor-heat transfer stack for forced temperature modulation (b). Stacking of the machined out wafers showing the flow channels (c) (figure reproduced from Rouge and Renken (2001a) with permission of the copyright holder, 䉷 2001 Microreactor Technology).
cooling rates (Brandner et al., 2002). The researchers claimed that wall temperature amplitudes of 50◦ C could be reached with cycle periods as short as 1 s.
Rouge and Renken (2001b) investigated the dehydration of isopropanol over alumina using step-changes in temperature as well as modulation employing a 20-s cycle and a
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Fig. 10. Response of isopropanol and propene concentrations after step-changes in microreactor coolant temperature. Operating conditions: P = 1.5 bar, Q0 = 0.66 cm3 /(STP), (CIP)0 = 0.92 mol/m3 (STP) (figure reproduced from Rouge and Renken (2001a,b) with permission of the copyright holder, 䉷 2001 Microreactor Technology).
10◦ C amplitude. Dehydration is both product- and reactantinhibited. Adjacent sites are needed for the surface reaction. Even at low isopropanol concentrations, isopropanol and the water product on the surface severely reduce the reaction rate. Both species, however, are reversibly adsorbed, so the reaction rate can be increased several orders of magnitude by flushing the catalyst surface with a non-adsorbing carrier gas and then restoring the isopropanol flow (Moravek, 1992; Silveston, 1998). Fig. 10 shows that an abrupt increase in temperature also increases the dehydration rate. Data shown in the figure were collected in the unit illustrated schematically in Fig. 9. The delay in the concentration peaks after the step-up from 190 to 210◦ C results partly from heat transfer lag and partly from the endothermic desorption step. Rouge and Renken found a 5–10 s lag in wall temperature after a coolant temperature change. Reaching the upper temperature is also slowed by consumption of heat for desorption. Nevertheless, desorption from the surface due to higher temperature and competition from water adsorption causes the isopropanol concentration to peak. It declines then as the alcohol is consumed at the higher reaction temperature. The propene concentration peaks earlier as desorption from the surface rapidly increases the dehydration rate. The small dip seen in the figure arises from increasing inhibition from adsorbed water. This is counteracted after 20 s by increased availability of alcohol on the catalyst surface. With the opposite change in coolant temperature the concave peak in isopropanol concentration about 10 s after the coolant temperature change arises from an increase in adsorption because of the lower temperature. The lag is just 10 s because of heat release on adsorption. Dynamic effects in Fig. 10 suggest that temperature modulation should be undertaken in a range of cycle periods between 20 and 60 s. Rouge and Renken (2001a,b) report
modulation measurements at = 20 s for the system and conditions given in Fig. 10. Their data are reproduced in Fig. 11. Noticeable is the 5 s phase shift in the isopropanol and propene concentrations. This is less than the lag in Fig. 10 on the temperature step-up, but has undoubtedly the same explanation. Propene and isopropanol peaks and troughs under modulation in Fig. 11 are less and greater than the maxima or minima seen for the step-changes in temperature in Fig. 10. These are attributable to thermal lags in the microreactor. Comparison of the steady-state portions of the time trace in Fig. 10 with the time-average concentrations in Fig. 11 indicate, at best, a small improvement, perhaps 1–2%, in the dehydration rate due to modulation. This is not conclusive and further measurements are needed. There are many candidate systems for microreactor forcing experiments. Dehydration reactions of amines and alcohols, such as isopropanol discussed above, are reactant and product inhibited. They often exhibit the “stop” effect. Periodic desorption should improve reaction rates. Thus, periodic temperature pulses providing desorption should stimulate reaction. The Rouge and Renken work on isopropanol needs to be extended to include a much wider range of cycle periods, amplitudes and cycle splits. Dehydration reactions also have side reactions, mainly parallel, so that effects on selectivity could be explored by operating at higher timeaverage temperatures and feed concentrations. CO oxidation over Pt and Pd catalyst experiences educt inhibition, so that it, too, is a candidate for microreactor studies. The saponification reaction, discussed above, is a homogeneous reaction that would also be a candidate for selectivity studies. Halogenation and nitration are other homogeneous reactions that are in the consecutive-parallel reaction class. Partial oxidation of hydrocarbons over metal oxide catalysts also fall into the same class. Experiments should use a wide range of cycle periods, splits, and amplitudes.
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Fig. 11. Response of isopropanol and propene concentrations to coolant temperature modulation in a microreactor. Operating conditions as in Fig. 4 (figure reproduced from Rouge and Renken (2001a,b) with permission of the copyright holder, 䉷 2001 Microreactor Technology).
teraction of concentration and temperature waves moving through the packed-bed reactor at different velocities. Faster moving concentration waves overtake the slower temperature waves causing bursts of energy generation. This is the same phenomenon associated with wrong-way behavior. Reaction goes to completion in the experimental studies of Jaree et al. (2003a,b) and Na-Ranong et al. (2003), just as was the case for the Hansen and Jorgensen experiments. This is evident in the steady-state profile at t = 0 in Fig. 12 which shows a gentle maximum with position in the reactor. Consequently, whether inlet temperature modulation increases the oxidation rate could not be determined. Further experiments with the Jaree system seem warranted. Increasing the flow rate or reducing catalyst activity so that the reaction does not reach completion should provide an answer to the rate improvement question.
5. Temperature modulation of trickle beds
Fig. 12. Amplification of a triangular temperature oscillation in the feed stream during transit through a catalytic packed bed in which CO oxidation takes place (figure reproduced from Jaree et al. (2003a) with permission of the copyright holder, 䉷 2003 by Elsevier Science Publishers).
The feed modulation experiments mentioned at the beginning of this section have been recently extended in order to investigate the amplification phenomena first reported by Padberg and Wicke (1967). CO oxidation over Pt (Jaree et al., 2001, 2003a,b) and over a Cu catalyst (Na-Ranong et al., 2003) carried out in a 2.54-cm diameter bed of catalyst surrounded by an annular chamber under high vacuum exhibited significant amplification of an inlet temperature, triangular wave, periodic disturbance. This is illustrated in Fig. 12. Amplification exhibits a frequency resonance and saturation as the amplitude of the inlet disturbance increases. The figure shows that the periodicity of the inlet modulation is preserved. The gain shown in Fig. 12 is about 6–7 db. This is well below the gain measured by Hansen and Jorgensen (1974). Yakhnin et al. (1995a,b) have studied amplification in packed-bed reactors. Their simulations indicate gains as great as 70 db. Explanation of the phenomenon is the in-
Haure et al. (1989) observed that periodic interruption of the liquid flow to a trickle bed dramatically increases the reaction rate in the bed. The authors demonstrated that part of the rate increase arises from a temperature increase in the trickle bed after flow is stopped. Liquid flow is the major carrier of heat from a trickle bed in which an exothermic reaction takes place. Stop flow has been used by Haure and co-workers to generate temperature pulses as is illustrated in Fig. 13. The system is the catalytic hydrogenation of -methylstyrene in a cocurrent down-flow trickle bed. Exothermicity of this reaction coupled with the cessation of heat removal through the flowing liquid causes the temperature rise. Periodically increasing temperature results in a large increase in the rate of reaction. This increase is illustrated in Fig. 14. At the shortest periods used, there is about a four-fold increase in the rate of hydrogenation. This remarkable rate increase arises from parallel gas- and liquid-hydrogenation pathways with the gas-phase pathway as much as an order of magnitude faster. Periodically increasing temperature vaporizes -methyl styrene. The reaction is not conducted in the gas phase because of the high heat generation that can lead to runaway. Consequently, periodic temperature pulsing provides a means of avoiding runaway while taking at least partial advantage of the rapid gas-phase reaction. Temperature pulses can also be created by periodically altering the feed composition to the reactor. This is illustrated in Fig. 15. Of course with a three-phase reactor, feed temperature modulation can be undertaken very simply without resorting to flow interruption or feed composition switching. With respect to the effect of feed temperature modulation on trickle-bed performance, the positive result that the Gabarain experiments suggest is clouded by interference of flow interruption that profoundly changes reactor operation. We believe the question needs further experimental study. Experiments need to be undertaken using feed
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Fig. 13. Periodic temperature excursion in a trickle bed generated by flow interruption. Liquid flow runs for 2 min in every 6-min cycle. Reaction = hydrogenation of -methylstyrene (figure reproduced from Gabarain et al. (1997) with permission of the copyright holder; 䉷 2003 by the AIChE).
Fig. 14. Normalized time-average rate of cumene formation as a function of cycle period for different cycle splits. Data for -methyl styrene hydrogenation over a Pd/Al2 O3 catalyst at feed conditions of 41◦ C, 1 bar. Time-averaged rate data normalized by the steady-state rate at 41◦ C (figure reproduced from Gabarain et al. (1997) with permission of the copyright holder; 䉷 2003 by the AIChE).
temperature modulation directly. Both the effect on rate of reaction and on selectivity deserve attention. A suitable candidate system for this study would be the hydrogenation of crotonaldehyde. In this reaction there are parallel pathways: hydrogenation of the double bond to produce butylaldehyde and hydrogenation of the aldehyde group to give crotyl alcohol. Stradiotto et al. (1999) used this reaction in a recent study of flow modulation in trickle beds.
6. Overview The question posed in this review is whether temperature modulation of chemical reactors significantly influ-
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Fig. 15. Temperature variation initiated by cyclohexene concentration changes through dilution with cyclohexane. Curves show the effect of lag in a temperature controller used to begin the composition switch. Experimental conditions: initial bed temperature and feed inlet temperature = 50◦ C, gas flow rate = 15 l/h, liquid flow rate = 340 ml/h and the temperature set point = 70◦ C for both a concentration step-up and step-down (figure reproduced from Hanika et al. (1990) with permission, 䉷 1990 Elsevier/Sequoia Science Publishers).
ences reactor performance in terms of conversion or yield. Theoretical studies and early simulations indicated that, for fluid–catalyst systems, significant improvements could be expected. To date, limited numbers of direct experiments have not confirmed such expectations. Early simulations and theoretical work failed to allow for the effect of the thermal capacity of the reactor containment and the heat supply. If this influence could be eliminated or reduced, would temperature modulation be effective? It now seems possible to answer this question through the use of microreactors. Although any commercial applications of temperature modulation on a microreactor scale are not obvious, an answer to this question should be sought for the sake of completeness. For three-phase reactors or, at least for trickle beds, indirect experiments using flow interruption show an important effect of temperature modulation on the rate of reaction. This improvement is caused by a phase change. However, the experimental investigations introduced other phenomena, so further studies are certainly needed. Potential commercial applications would justify the research effort. Additional studies should be directed towards selectivity, rather than improving rate or conversion. The amplification investigations mentioned above suggest that temperature modulation may influence reaction rate. The work needs to be repeated under conditions in which the reaction does not continue to completion in order to measure an effect on reaction rate.
7. Conclusions The conflicting simulation, theoretical work and experiments, make it impossible to state confidently that temperature modulation is a useful tool for reaction engineering. Further experimental studies along the lines advocated in the previous section would be worthwhile.
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Notations Ci CSTR Da STP s x1 x2
concentration of species i, mol/m3 continuous stirred tank reactor, fully backmixed reactor Damköhler number standard temperature and pressure cycle split dimensionless educt concentration dimensionless temperature, bed temperature K/feed temperature K
Greek letters
dimensionless activation energy dimensionless time cycle period, r, s, min
Subscripts 1 2 A B P 0
concentration temperature reactant product product initial
Acknowledgements Preparation of this manuscript was supported by research grants to the authors by the Natural Sciences and Engineering Research Council of Canada.
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Microreaction Technology, March 10–14, 2002, New Orleans (Paper No. 105a). Chang, K.S., Bankoff, S.G., 1968. Oscillatory operation of jacketed tubular reactors. Industrial & Engineering Chemistry Fundamentals 7, 633–639. Denis, G.H., Kabel, R.L., 1970. The effect of temperature change on tubular heterogeneous catalytic reactors. Chemical Engineering Science 25, 1057–1071. Dorawala, T.G., Douglas, J.M., 1971. Complex reactions in oscillating reactors. A.I.Ch.E. Journal 17, 974–981. Gabarain, L., Castellari, A.T., Cechini, J., Tobolski, A., Haure, P.M., 1997. Analysis of rate enhancement in a periodically operated trickle-bed reactor. A.I.Ch.E. Journal 43, 166–172. Hanika, J., Lange, R., Turek, F., 1990. Computer-aided control of a laboratory trickle bed reactor. Chemical Engineering Processing 28, 23–27. Hansen, K.W., Jorgensen, S.B., 1974. Experimental investigations of the dynamics of a catalytic fixed bed reactor. Advances in Chemistry Series 133, 505–518. Haure, P.M., Hudgins, R.R., Silveston, P.L., 1989. Periodic operation of a trickle-bed reactor. A.I.Ch.E. Journal 35, 1437–1444. Horn, F.J.M., Lin, R.C., 1967. Periodic processes—a variational approach. Industrial & Engineering Chemistry Process Development and Design 6, 21–30. Jaree, A., Budman, H., Hudgins, R.R., Silveston, P.L., Yakhnin, V., Menzinger, M., 2001. Temperature excursions in reactors packed with segregated layers of catalysts. Catalysis Today 69, 137–146. Jaree, A., Hudgins, R.R., Budman, H., Silveston, P.L., Yakhnin, V., Menzinger, M., 2003a. Hysteresis and extinction waves in catalytic CO oxidation caused by reactant concentration perturbations in a packedbed reactor. Industrial & Engineering Chemistry Research, in press. Jaree, A., Hudgins, R.R., Budman, H., Silveston, P.L., Yakhnin, V., Menzinger, M., 2003b. Amplification of inlet temperature disturbances in a packed-bed reactor for CO oxidation over Pt/Al2 O3 . Chemical Engineering Science 58, 833–839. Kevrekidis, I.G., Schmidt, L.D., Aris, R., 1986. Some common features of periodically forced reacting systems. Chemical Engineering Science 41, 1263–1276. Kim, D.K., Hulburt, H.M., 1972. Preprint. Sixty-fourth Annual A.I.Ch.E. Meeting, San Francisco. Lee, C.K., Bailey, J.E., 1980. Modification of consecutive-competitive reaction selectivity by periodic operation. Industrial & Engineering Chemistry Process Development and Design 19, 160–166. Lee, C.K., Yeung, S.Y.S., Bailey, J.E., 1980. Experimental studies of a consecutive-competitive reaction in steady-state and forced periodic CSTRs. Canadian Journal of Chemical Engineering 58, 212–218. Moravek, V., 1992. Steady-state and transient kinetics of displacement adsorption and educt inhibition in dehydration of alcohols on alumina. Journal of Catalysis 133, 170–178. Na-Ranong, D., Jaree, A., Hudgins, R.R., Silveston, P.L., Yakhnin, V., Menzinger, M., 2003. Amplification of inlet temperature disturbances in a packed-bed reactor for CO oxidation over CuO–Al2 O3 catalyst. Canadian Journal of Chemical Engineering, in press. Padberg, G., Wicke, E., 1967. Stabiles und instabiles verhalten eines adiabatischen rohrreaktors am beispiel der katalytischen CO-oxydation. Chemical Engineering Science 22, 1035–1051. Rouge, A., Renken, A., 2001a. Forced periodic temperature oscillations in microchannel reactors. In: Proceedings of the Fifth International Conference on Microreaction Technology, May 27–30, 2001.Strasbourg, France. Rouge, A., Renken, A., 2001b. Microchannel reactors for fast periodic operation: The catalytic dehydration of isopropanol. Chemical Engineering Science 56, 1419–1427. Silveston, P.L., 1998. Composition Modulation of Catalytic Reactors. Gordon and Breach Science Publishers, London. Sincic, D., Bailey, J.E., 1977. Pathological dynamic behavior of forced periodic chemical processes. Chemical Engineering Science 32, 281–286.
P.L. Silveston, R.R. Hudgins / Chemical Engineering Science 59 (2004) 4043 – 4053 Sterman, L.E., Ydstie, B.E., 1990. The steady-state process with periodic perturbations. Chemical Engineering Science 45, 721–736. Sterman, L.E., Ydstie, B.E., 1991. Periodic forcing of the CSTR: An application of generalized Pi-criterion. A.I.Ch.E. Journal 37, 986–996. Stradiotto, D.A., Hudgins, R.R., Silveston, P.L., 1999. Hydrogenation of crotonaldehyde under periodic flow interruption in a trickle bed. Chemical Engineering Science 54, 2561–2568. Van Neer, F.J., Kodde, R.A.J., den Uil, H., Bliek, A., 1996. Understanding of resonance phenomena on a catalyst under forced concentration and temperature oscillations. Canadian Journal of Chemical Engineering 74, 664–673.
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Yakhnin, V.Z., Rovinsky, A.B., Menzinger, M., 1995a. Convective instability induced by differential transport in a tubular packed-bed reactor. Chemical Engineering Science 50, 2853–2859. Yakhnin, V.Z., Rovinsky, A.B., Menzinger, M., 1995b. Absolute instability of a tubular packed-bed reactor with recycling. Chemical Engineering Science 50, 1591–1593.
Periodic temperature forcing of catalytic reactions Peter L. Silveston∗ , Robert R. Hudgins Department of Chemical Engineering, Reactor Engineering Laboratory, University of Waterloo, Waterloo, Ont., Canada Received 5 January 2004; received in revised form 14 April 2004; accepted 18 May 2004
Abstract The exponential relationship of rate constant to temperature served as a magnet to attract the attention of researchers in the 1960–1970 to periodic temperature forcing. Early theoretical studies and simulation exercises suggested that significant improvements of rate and selectivity could be expected from this type of unsteady state operation. The first experiments in the late 1970s and early 1980 failed to confirm these expectations, so research on periodic temperature forcing languished. The current interest in microreactors with their greatly reduced thermal capacitance has revived activity in temperature modulation. Recent discoveries of the effect of temperature pulses in trickle-bed reactors have further spurred this activity. This review addresses the question of whether temperature modulation of chemical reactors can significantly affect reactor performance. 䉷 2004 Published by Elsevier Ltd. Keywords: Periodic operation; Temperature cycling; Temperature modulation; Catalytic reactors; Trickle beds
1. Introduction Because of the exponential relation of the rate constant to temperature, periodic temperature forcing of catalytic reactions seemed promising to investigators. This mode of forcing was discussed in the earliest studies on periodic operation, such as those of Horn and Lin (1967), Denis and Kabel (1970), Dorawala and Douglas (1971), and Bailey et al. (1971). The reason for this early interest in temperature forcing becomes clear from some simple calculations. If a point in a reactor operating at 500◦ C could be cycled symmetrically with an amplitude of 10◦ C, the time-average rate of reaction at the point would increase by 7.4% over the rate at steady state at 500◦ C for a reaction with a low activation energy of 83.6 kJ/mol. For a large activation energy of 418 kJ/mol, the increase would be 266%. If the amplitude were increased to 20◦ C, rate improvements for the range of activation energies become 30% and 2300%, respectively. Furthermore, Van Neer et al. (1996), in their theoretical study of resonance phenomena under modulation, observed that
∗ Corresponding author. Tel.: +1-519-885-1211; fax: +1-519-746-4979.
E-mail address: [email protected] (P.L. Silveston) 0009-2509/$ - see front matter 䉷 2004 Published by Elsevier Ltd. doi:10.1016/j.ces.2004.05.034
temperature forcing should exhibit much larger effects than those from concentration forcing. Early attempts to carry out temperature-forcing trials foundered because of the thermal inertia of catalyst beds or reactor bodies; as a result, there has been little research activity on temperature forcing for about three decades. The appearance of micro-reactors with relatively low mass in the last few years has reawakened interest in temperature forcing. Further, some remarkable increases in rates of reaction in three-phase trickle beds observed for periodic interruption of liquid phase flow now appear to be caused by temperature variations. Our purpose in this review is to summarize what has been discovered so far about temperature forcing of catalytic reactions and to suggest what experiments should be undertaken. The ultimate goal is to determine whether temperature modulation is a useful practice. 2. Theoretical studies Early studies on periodic temperature forcing, mentioned in the Introduction, were theoretical and are summarized in a review (Bailey, 1974). Two groups of operations are considered: quasi-steady state and relaxed steady state. In
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the former mode, half-cycles are long enough that the steady state is reached, while for the relaxed steady state, switching is so fast that the manipulated system does not change with time. Improved performance for quasi-steady state requires convexity in the equations describing the reactor system. Consider an isothermal CSTR in which a first-order reaction is occurring, the system is simply described as x1 =
1 , 1 + Da exp[−1/x2 ]
(1)
where x1 , x2 are, respectively, the dimensionless concentration and temperature in the reactor. If the temperature is an independent variable, the system exhibits convexity. Indeed, using the numbers given in the introduction, a bang–bang periodic change in reactor temperature (x2 ) can significantly increase conversion. Convexity is also exhibited by a plugflow packed-bed catalytic reactor provided the characteristic time for a change in surface concentration is small measured against the cycle period. The relaxed steady state arises when the variation of the manipulated variable forces a reactor output into a limit cycle that shrinks into approximately a single state as the cycle frequency increases. It will differ from a steady state of the system if the dynamics of the reactor are nonlinear or the relation is nonlinear between an objective function to be optimized, such as conversion, and the state and manipulated variables. In temperature forcing, the manipulated variable is temperature, so that in general the condition for a relaxed steady state that differs from a steady state is met. Whether the relaxed steady-state results in improvement can be established by applying an optimization routine to the system of dynamic and objective function equations. The first application to a reactor system was in the classical paper by Horn and Lin (1967) considering a system of parallel reactions of different order in a CSTR under bang–bang temperature cycling. Applying Pontryagin’s maximum principle, Horn and Lin found a set of temperature states for which the objective function, the concentration of a product from one of the competing reaction, was contained in a small limit cycle located outside of the range of steady states. This situation is shown in Fig. 1. The plot assumes a second-order reaction to the desired product and a firstorder reaction to a waste product. The activation energy for the second reaction is three-quarters of the activation energy for the reaction to the desired product. The cycle contains two temperatures. In the plot, the concentration of the desired product, CB , is plotted against the exit concentration of the reactant CA . The figure shows the variation of CB with CA is concave, so that quasi-steady-state operation cannot improve upon steady state (maximum selectivity to CB at I). Fast cycling of the CSTR temperature leads to at least two relaxed steady-state operations. The operation designated by II shows a modest improvement over the optimal steady state, while the second region just above I is another relaxed steady state but one giving negligible improvement over the best steady-state operation.
Fig. 1. Exit concentrations under steady state and periodic operation with temperature forcing for a parallel reaction (A → B, A → C) in a CSTR. SS = curve of steady-state selectivities, I, II = relaxed steady operations (figure reproduced from Bailey (1974) with permission of the copyright holder, 䉷 2001 Gordon and Breach Science Publishers).
Dorawala and Douglas (1971) considered a consecutive reaction in a CSTR with variable temperature. In this system, the intermediate product was the desired one. They demonstrated using a search technique that there is a relaxed steady state through temperature cycling which provided a selectivity which is superior to the optimal steady-state one. Sterman and Ydstie (1990, 1991) considered both parallel and consecutive reactions in a CSTR with temperature forcing. In the latter paper, they developed an optimization scheme to determine relaxed steady states that improved upon the optimal steady state. An interesting development in the theoretical studies of temperature forcing in a CSTR was the discovery by Sinˇci˙c and Bailey (1977) that forcing can lead to some surprising dynamic behavior. A comprehensive investigation of the dynamic phenomena that may arise is discussed by Kevrekidis et al. (1986). For first-order parallel reactions with different activation energies, Sinˇci´c and Bailey observed a single limit cycle for the reactor, a complex cycle that was 2-periodic, and two stable limit cycles that are -periodic. Several of the cycles exhibited irregular temperature trajectories. One such is illustrated in Fig. 2. The dimensionless temperature x2 is the time-average temperature in the reactor. 3. Simulation studies Perhaps the earliest study of temperature forcing was undertaken by Chang and Bankoff (1968), who examined a jacketed packed-bed reactor. These investigators found that conversion was optimal in a quasi-steady-state operation. Denis and Kabel (1970) undertook simulation of temperature forcing in a packed-bed catalytic reactor. The reaction considered was the vapor-phase dehydrogenation of ethanol over a cation exchange resin. They demonstrated improved conversion over steady-state operation. An interesting aspect of their work was the use of the response to stepchange variations in the reactor jacket temperature to obtain
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Dimensionless concn. X1
1.0
0.8
0.6
0.4
0.2
10
11
12
13
14
Dimensionless temp. X2 Fig. 2. Concentration-temperature trajectory for the A → B, A → C reaction system in a CSTR that is forced by modulation of the feed temperature. Points in the figure represent steady operating states. The two loops indicate 2-periodicity (figure reproduced from Sincic and Bailey (1977) with permission of the copyright holder, 䉷 2001 Pergamon Press).
parameters for their dynamic packed-bed model. This model allowed for adsorption/desorption from the catalyst surface and considered variations of velocity in the reactor. Cyclic temperature variations in the above type of system were considered in a simulation study by Kim and Hulburt (1972). These authors also examined simultaneous modulation of concentration and feed rate along with temperature. They observed improvements that they claimed were due primarily to temperature cycling. Temperature forcing of a CSTR was simulated by Dorawala and Douglas (1971) assuming temperature modulation by changing the coolant flow rate in the CSTR jacket. This caused a variation in reactor temperature. The same authors also examined forcing of the feed temperature to the reactor. Generally, these simulation studies found that temperature cycling with the proper frequency and cycle split improved reaction selectivity. Many of these studies are summarized in Bailey’s masterly review (Bailey, 1977). In 1980, Lee and Bailey (1980) considered consecutiveparallel reactions in a CSTR under forcing and demonstrated a significant increase in selectivity to the desired intermediate product in the reaction chain. The optimism of this and earlier simulations and theoretical studies has not stood up to the rigor of experiment.
4. Experimental studies Experimental investigations also began shortly after the Bailey review. One of these dealt with small amplitude, lowfrequency perturbations intended to test dynamic, packedbed reactor models (Hansen and Jorgensen, 1974). These experimenters used pilot-scale equipment (reactor dimensions:
Fig. 3. Temperature modulation input, temperature variation at the reactor wall used by Abdul-Kareem et al. (1980) in their investigation of CO oxidation in a packed-bed reactor (figure reproduced from Abdul-Kareem et al. (1980) with permission of the authors).
diameter=9.6 cm, wall thickness=0.1 mm, length=50 cm) and considered the oxidation of hydrogen on a Pt/Al2 O3 catalyst. Oxidation went to completion in the reactor, so the authors were unable to gain information about the effect of temperature modulation on the rate. They did observe wrong-way behavior on a step-input and amplification as great as 20 db in the reactor interior when a periodic, squarewave disturbance of small amplitude was introduced. The Hansen experiments have been recently extended and we will discuss their new data briefly at the end of this section. The question of improvement through temperature modulation in a packed-bed reactor was attacked by Abdul-Kareem et al. (1980). These researchers examined temperature forcing of CO oxidation over a vanadia catalyst in a packed bed, 6.35 mm in diameter. This reactor was immersed in a fluidized sand bath, so that the reactor wall was essentially at the temperature maintained in the sand bath. A high-gas flow rate and fine particles (50–60 mesh) minimized radial temperature gradients in the reactor. This also made the reactor function as a differential reactor. Forcing was produced by switching the temperature in the sand bath using high-output electrical heaters as well as by adjusting the air flow rate. Temperature in the catalyst bed was measured by a steel-sheathed chromel–alumel thermocouple. Because of the thermal inertia, temperature in the bed could not be cycled as a square wave. The wave form obtained is illustrated in Fig. 3. As the cycle period increased, 1 > 1 and a square-wave input was approached. Steady-state rate measurements over the forcing range plotted linearly against the reciprocal absolute temperature gave an activation energy of 103 ± 5 kJ/mol, in good agreement with published values. Cycle periods from 1 to 8 h were used. The lower value was chosen because about 10 min were needed to switch temperatures between the two levels chosen. Thus, with a 60-min cycle, the bed resided at the desired temperature for just 20 min in each partial cycle.
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Fig. 4. Comparison of time-average CO oxidation rate with the quasi-steady-state oxidation as a function of cycle period and input amplitude. Inlet CO = 16% by volume (figure reproduced from Abdul-Kareem et al. (1980) with permission of the authors).
Abdul-Kareem et al. observed that the time-average rate of CO oxidation under temperature modulation increased with increasing cycle period for both amplitudes (10◦ C and 20◦ C). Above 6 h, the quasi-steady-state rate was obtained. Low-frequency modulation, thus, improves the time-average reaction rate, but the quasi-steady-state limit is not exceeded. As expected, the quasi-steady-state rate is greater than the steady-state rate at any temperature. The oxidation rate versus temperature curve is convex. What is surprising about the measurements is that the time-average rate lies below the quasi-steady-state rate at cycle periods from 2 to 6 h. Abdul-Kareem et al. point out that the sand bath reached a constant temperature in 10 min after a change, while the catalyst particles moved across the temperature difference in about 1 s and heat transport through the small diameter packed bed has a characteristic time of perhaps 60 s (Fig. 4). Thus, particle temperature follows the input temperature closely. The remarkably long period of 6 h needed to reach the quasi-steady state indicates that changing temperature on the catalyst surface probably retards rather than enhances the rate of the catalytic reaction. Experimental results with cycle periods less than 2 h and a 10◦ amplitude only reached the steady-state rate at the time-average temperature. This rate is well below the quasi-steady-state rate, again indicating that the transient temperature experience by the particles does not enhance the rate of oxidation. Consequently, for the CO oxidation system studied, temperature cycling offered no advantage. The best strategy to increase the rate of reaction is to operate at the highest permissible temperature.
Fig. 5. Schematic of the jacketed CSTR used by Lee et al. (1980) in their experiments on temperature forcing of the saponification of diethyl adipate (figure reproduced from Lee et al. (1980) with permission of the authors).
Bang–bang temperature cycling of the heating fluid in the jacket of a CSTR reactor was undertaken by Lee et al. (1980) in order to verify their earlier predictions of improvement obtained by simulation. The small jacketed CSTR used by Lee et al. (1980) is shown in Fig. 5. Saponification of diethyl adipate with sodium hydroxide was studied. This reaction has a consecutive-parallel structure: A + B → C + D, C + B → E + D. Temperature cycling was performed by switching the coolant in the CSTR jacket from a stream at 90◦ C to one at 15◦ C. Experiments were carried out using diethyl adipate dissolved in an equivolume mixture of isopropanol and water. Results for temperature forcing are shown in Fig. 6 for a cycle period of 5 min and cycle splits from 0.1 to 0.9 based on the duration of the high-temperature coolant in the jacket. Experimental measurements fell on the steady-state line, but are not shown because of uncertainty in the measurements of adipate concentrations. The curve marked “periodic” in the figure is a simulation based on careful kinetic measurements and a model for heat transfer in the jacketed CSTR that closely reproduced the experimental temperature change in the CSTR in response to a step-change in jacket temperature. The figure suggests an improvement in the intermediate yield ranging from only 3% at the highest conversion used to 2% at the lowest conversion. This is quite small and suggests that temperature forcing in CSTRs is ineffective, contrary to earlier simulation results. Dynamic measurements
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Fig. 6. Time-average yield of sodium monoethyl adipate salt as a function of diethyl adipate conversion for cooling jacket temperature cycling at an amplitude of 37.5◦ C with a cycle period of 5 min and various cycle splits. Steady-state yield at 52.5◦ C shown for comparison (figure reproduced from Lee et al. (1980) with permission of the authors).
Fig. 7. Hand-size microreactor designed for temperature cycling showing insertable heating cartridge and insertion point for TC monitor (figure reproduced from Brandner et al. (2001) with permission of the copyright holder, 䉷 2001 Microreactor Technology).
on the jacketed CSTR indicate a characteristic time of about 2.5 min. This suggests the cycle period used was too short to benefit from the amplitude imposed on the coolant. Thus, damping through heat exchange may explain the small improvement compared to steady state. Although the early experimental results are not encouraging, they do not rule out the use of temperature cycling. After a lapse of some 20 years, interest in periodic temperature forcing has re-surfaced driven by the availability of microreactors with fast heating and relatively low-thermal inertia. Fig. 7 is a photograph of a typical unit that has been examined for fast temperature cycling. The Swage-
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lock fittings for 41 -in tubing gives a good indication of the size of the unit. Layered construction of the unit is given in Fig. 8a. The wafer layers separate reacting fluid, usually a gas, and the cooling fluid, usually a liquid. Manifolds on either side of the stack introduce the feed and coolant. On the opposite side-reaction products and the heated coolant are withdrawn. The manifolds are arranged to give cross-flow. Metallic wafers are used for rapid heat transfer, so that just a single layer is adequate for heat input. These wafers are 44 mm × 33 mm × 2 mm. Tracks are machined into each wafer to form the flow paths for each fluid. The path structure is shown in the bottom of the figure as Fig. 8b. In the reacting fluid layer, the channel is 380 m wide and 150 m deep. Its length is 710 mm. Catalyst can be deposited in the channel in several ways. The usual first step is to roughen the surface chemically and then lay down a porous washcoat. This wash coat is then impregnated with a decomposable salt of the catalyst. Drying and calcinations complete the process. Details are given by Rouge and Renken (2001a). Alépée et al. (2000) describe an earlier version. Rouge and Renken (2001a) employed heating and cooling fluids, so a somewhat different design was needed. They used a 20 mm × 20 mm stack consisting of nine 14-mmthick ceramic wafers (MACOR) and nine steel plates for flow control. Stack structure and flow routing are shown in Fig. 9a and the machined wafers are given in Fig. 9b. Parallel channels are machined into the wafers as flow channels. There were 34 channels in each wafer. Each channel was 300 m wide, 240 m deep and 20 mm in length. Use of a ceramic wafer simplifies application of the catalyst to the channel walls. A wash coat can be applied and then impregnated or in some case the wafer alone can act as the catalyst support alone. Brandner et al. (2001) were concerned with microreactor response and so did not demonstrate forced temperature modulation for a reacting system. They employed N2 as the flowing fluid and used water as the coolant. High-frequency on–off water flow was required and this was accomplished by switching water flow between the micro reactor and a bypass. Early tests showed some water hold up in the cooling channels that interfered with the heating half-cycle. This was solved by employing a 10-ms air blast to strip water from the channels. Electrical heating power as high as 1675 W could be used. At this heat input about 93% was transferred to the flowing gas. The investigators demonstrated an amplitude of about 90◦ C within the reactor, while the exit gas temperature had an amplitude of about 33◦ C at a water flow rate of 15 kg/h, a power input of 450 W and a cycle period of 60 s. Increasing power input and water flow rate increased both amplitudes. If the cycle period was reduced to 20 s, amplitudes were sharply reduced and the temperature oscillations were no longer symmetrical. Brandner et al. claimed amplitudes of about 50◦ C in the reactor walls can be achieved for 4-s periods. On the gas side, mixing was sufficient to cause the temperature variation at the outlet to disappear. Further modifications were made to increase the heating and
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Fig. 8. Construction of the micro reactor-heat transfer stack (a) and flow path layout for reacting and cooling fluids (b) (figure reproduced from Brandner et al. (2001) with permission of the copyright holder, 䉷 2001 Microreactor Technology).
Fig. 9. Schematic of directional masks for reacting fluids and heat transfer fluids (a) for a micro reactor-heat transfer stack for forced temperature modulation (b). Stacking of the machined out wafers showing the flow channels (c) (figure reproduced from Rouge and Renken (2001a) with permission of the copyright holder, 䉷 2001 Microreactor Technology).
cooling rates (Brandner et al., 2002). The researchers claimed that wall temperature amplitudes of 50◦ C could be reached with cycle periods as short as 1 s.
Rouge and Renken (2001b) investigated the dehydration of isopropanol over alumina using step-changes in temperature as well as modulation employing a 20-s cycle and a
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Fig. 10. Response of isopropanol and propene concentrations after step-changes in microreactor coolant temperature. Operating conditions: P = 1.5 bar, Q0 = 0.66 cm3 /(STP), (CIP)0 = 0.92 mol/m3 (STP) (figure reproduced from Rouge and Renken (2001a,b) with permission of the copyright holder, 䉷 2001 Microreactor Technology).
10◦ C amplitude. Dehydration is both product- and reactantinhibited. Adjacent sites are needed for the surface reaction. Even at low isopropanol concentrations, isopropanol and the water product on the surface severely reduce the reaction rate. Both species, however, are reversibly adsorbed, so the reaction rate can be increased several orders of magnitude by flushing the catalyst surface with a non-adsorbing carrier gas and then restoring the isopropanol flow (Moravek, 1992; Silveston, 1998). Fig. 10 shows that an abrupt increase in temperature also increases the dehydration rate. Data shown in the figure were collected in the unit illustrated schematically in Fig. 9. The delay in the concentration peaks after the step-up from 190 to 210◦ C results partly from heat transfer lag and partly from the endothermic desorption step. Rouge and Renken found a 5–10 s lag in wall temperature after a coolant temperature change. Reaching the upper temperature is also slowed by consumption of heat for desorption. Nevertheless, desorption from the surface due to higher temperature and competition from water adsorption causes the isopropanol concentration to peak. It declines then as the alcohol is consumed at the higher reaction temperature. The propene concentration peaks earlier as desorption from the surface rapidly increases the dehydration rate. The small dip seen in the figure arises from increasing inhibition from adsorbed water. This is counteracted after 20 s by increased availability of alcohol on the catalyst surface. With the opposite change in coolant temperature the concave peak in isopropanol concentration about 10 s after the coolant temperature change arises from an increase in adsorption because of the lower temperature. The lag is just 10 s because of heat release on adsorption. Dynamic effects in Fig. 10 suggest that temperature modulation should be undertaken in a range of cycle periods between 20 and 60 s. Rouge and Renken (2001a,b) report
modulation measurements at = 20 s for the system and conditions given in Fig. 10. Their data are reproduced in Fig. 11. Noticeable is the 5 s phase shift in the isopropanol and propene concentrations. This is less than the lag in Fig. 10 on the temperature step-up, but has undoubtedly the same explanation. Propene and isopropanol peaks and troughs under modulation in Fig. 11 are less and greater than the maxima or minima seen for the step-changes in temperature in Fig. 10. These are attributable to thermal lags in the microreactor. Comparison of the steady-state portions of the time trace in Fig. 10 with the time-average concentrations in Fig. 11 indicate, at best, a small improvement, perhaps 1–2%, in the dehydration rate due to modulation. This is not conclusive and further measurements are needed. There are many candidate systems for microreactor forcing experiments. Dehydration reactions of amines and alcohols, such as isopropanol discussed above, are reactant and product inhibited. They often exhibit the “stop” effect. Periodic desorption should improve reaction rates. Thus, periodic temperature pulses providing desorption should stimulate reaction. The Rouge and Renken work on isopropanol needs to be extended to include a much wider range of cycle periods, amplitudes and cycle splits. Dehydration reactions also have side reactions, mainly parallel, so that effects on selectivity could be explored by operating at higher timeaverage temperatures and feed concentrations. CO oxidation over Pt and Pd catalyst experiences educt inhibition, so that it, too, is a candidate for microreactor studies. The saponification reaction, discussed above, is a homogeneous reaction that would also be a candidate for selectivity studies. Halogenation and nitration are other homogeneous reactions that are in the consecutive-parallel reaction class. Partial oxidation of hydrocarbons over metal oxide catalysts also fall into the same class. Experiments should use a wide range of cycle periods, splits, and amplitudes.
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Fig. 11. Response of isopropanol and propene concentrations to coolant temperature modulation in a microreactor. Operating conditions as in Fig. 4 (figure reproduced from Rouge and Renken (2001a,b) with permission of the copyright holder, 䉷 2001 Microreactor Technology).
teraction of concentration and temperature waves moving through the packed-bed reactor at different velocities. Faster moving concentration waves overtake the slower temperature waves causing bursts of energy generation. This is the same phenomenon associated with wrong-way behavior. Reaction goes to completion in the experimental studies of Jaree et al. (2003a,b) and Na-Ranong et al. (2003), just as was the case for the Hansen and Jorgensen experiments. This is evident in the steady-state profile at t = 0 in Fig. 12 which shows a gentle maximum with position in the reactor. Consequently, whether inlet temperature modulation increases the oxidation rate could not be determined. Further experiments with the Jaree system seem warranted. Increasing the flow rate or reducing catalyst activity so that the reaction does not reach completion should provide an answer to the rate improvement question.
5. Temperature modulation of trickle beds
Fig. 12. Amplification of a triangular temperature oscillation in the feed stream during transit through a catalytic packed bed in which CO oxidation takes place (figure reproduced from Jaree et al. (2003a) with permission of the copyright holder, 䉷 2003 by Elsevier Science Publishers).
The feed modulation experiments mentioned at the beginning of this section have been recently extended in order to investigate the amplification phenomena first reported by Padberg and Wicke (1967). CO oxidation over Pt (Jaree et al., 2001, 2003a,b) and over a Cu catalyst (Na-Ranong et al., 2003) carried out in a 2.54-cm diameter bed of catalyst surrounded by an annular chamber under high vacuum exhibited significant amplification of an inlet temperature, triangular wave, periodic disturbance. This is illustrated in Fig. 12. Amplification exhibits a frequency resonance and saturation as the amplitude of the inlet disturbance increases. The figure shows that the periodicity of the inlet modulation is preserved. The gain shown in Fig. 12 is about 6–7 db. This is well below the gain measured by Hansen and Jorgensen (1974). Yakhnin et al. (1995a,b) have studied amplification in packed-bed reactors. Their simulations indicate gains as great as 70 db. Explanation of the phenomenon is the in-
Haure et al. (1989) observed that periodic interruption of the liquid flow to a trickle bed dramatically increases the reaction rate in the bed. The authors demonstrated that part of the rate increase arises from a temperature increase in the trickle bed after flow is stopped. Liquid flow is the major carrier of heat from a trickle bed in which an exothermic reaction takes place. Stop flow has been used by Haure and co-workers to generate temperature pulses as is illustrated in Fig. 13. The system is the catalytic hydrogenation of -methylstyrene in a cocurrent down-flow trickle bed. Exothermicity of this reaction coupled with the cessation of heat removal through the flowing liquid causes the temperature rise. Periodically increasing temperature results in a large increase in the rate of reaction. This increase is illustrated in Fig. 14. At the shortest periods used, there is about a four-fold increase in the rate of hydrogenation. This remarkable rate increase arises from parallel gas- and liquid-hydrogenation pathways with the gas-phase pathway as much as an order of magnitude faster. Periodically increasing temperature vaporizes -methyl styrene. The reaction is not conducted in the gas phase because of the high heat generation that can lead to runaway. Consequently, periodic temperature pulsing provides a means of avoiding runaway while taking at least partial advantage of the rapid gas-phase reaction. Temperature pulses can also be created by periodically altering the feed composition to the reactor. This is illustrated in Fig. 15. Of course with a three-phase reactor, feed temperature modulation can be undertaken very simply without resorting to flow interruption or feed composition switching. With respect to the effect of feed temperature modulation on trickle-bed performance, the positive result that the Gabarain experiments suggest is clouded by interference of flow interruption that profoundly changes reactor operation. We believe the question needs further experimental study. Experiments need to be undertaken using feed
P.L. Silveston, R.R. Hudgins / Chemical Engineering Science 59 (2004) 4043 – 4053
Fig. 13. Periodic temperature excursion in a trickle bed generated by flow interruption. Liquid flow runs for 2 min in every 6-min cycle. Reaction = hydrogenation of -methylstyrene (figure reproduced from Gabarain et al. (1997) with permission of the copyright holder; 䉷 2003 by the AIChE).
Fig. 14. Normalized time-average rate of cumene formation as a function of cycle period for different cycle splits. Data for -methyl styrene hydrogenation over a Pd/Al2 O3 catalyst at feed conditions of 41◦ C, 1 bar. Time-averaged rate data normalized by the steady-state rate at 41◦ C (figure reproduced from Gabarain et al. (1997) with permission of the copyright holder; 䉷 2003 by the AIChE).
temperature modulation directly. Both the effect on rate of reaction and on selectivity deserve attention. A suitable candidate system for this study would be the hydrogenation of crotonaldehyde. In this reaction there are parallel pathways: hydrogenation of the double bond to produce butylaldehyde and hydrogenation of the aldehyde group to give crotyl alcohol. Stradiotto et al. (1999) used this reaction in a recent study of flow modulation in trickle beds.
6. Overview The question posed in this review is whether temperature modulation of chemical reactors significantly influ-
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Fig. 15. Temperature variation initiated by cyclohexene concentration changes through dilution with cyclohexane. Curves show the effect of lag in a temperature controller used to begin the composition switch. Experimental conditions: initial bed temperature and feed inlet temperature = 50◦ C, gas flow rate = 15 l/h, liquid flow rate = 340 ml/h and the temperature set point = 70◦ C for both a concentration step-up and step-down (figure reproduced from Hanika et al. (1990) with permission, 䉷 1990 Elsevier/Sequoia Science Publishers).
ences reactor performance in terms of conversion or yield. Theoretical studies and early simulations indicated that, for fluid–catalyst systems, significant improvements could be expected. To date, limited numbers of direct experiments have not confirmed such expectations. Early simulations and theoretical work failed to allow for the effect of the thermal capacity of the reactor containment and the heat supply. If this influence could be eliminated or reduced, would temperature modulation be effective? It now seems possible to answer this question through the use of microreactors. Although any commercial applications of temperature modulation on a microreactor scale are not obvious, an answer to this question should be sought for the sake of completeness. For three-phase reactors or, at least for trickle beds, indirect experiments using flow interruption show an important effect of temperature modulation on the rate of reaction. This improvement is caused by a phase change. However, the experimental investigations introduced other phenomena, so further studies are certainly needed. Potential commercial applications would justify the research effort. Additional studies should be directed towards selectivity, rather than improving rate or conversion. The amplification investigations mentioned above suggest that temperature modulation may influence reaction rate. The work needs to be repeated under conditions in which the reaction does not continue to completion in order to measure an effect on reaction rate.
7. Conclusions The conflicting simulation, theoretical work and experiments, make it impossible to state confidently that temperature modulation is a useful tool for reaction engineering. Further experimental studies along the lines advocated in the previous section would be worthwhile.
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Notations Ci CSTR Da STP s x1 x2
concentration of species i, mol/m3 continuous stirred tank reactor, fully backmixed reactor Damköhler number standard temperature and pressure cycle split dimensionless educt concentration dimensionless temperature, bed temperature K/feed temperature K
Greek letters
dimensionless activation energy dimensionless time cycle period, r, s, min
Subscripts 1 2 A B P 0
concentration temperature reactant product product initial
Acknowledgements Preparation of this manuscript was supported by research grants to the authors by the Natural Sciences and Engineering Research Council of Canada.
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