Ind. Eng. Chem. Res. 2005, 44, 4267-4271
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Effects of Microreactor Geometry on Performance: Differences between Posted Reactors and Channel Reactors Zheng Ni, E. G. Seebauer, and Richard I. Masel* Department of Chemical and Biomolecular Engineering, University of Illinois, 600 S Mathews, Urbana, Illinois 61801
Microreactors are useful for a variety of different purposes, but guidelines regarding optimal reactor geometry in microreactors remain poorly delineated in the literature. The present work uses the decomposition of ammonia to hydrogen to compare the conversions obtained in posted and channel microreactors. The experiments include measurements using a differential recycle reactor to obtain an empirical expression for the reaction rate, conversion measurements for comparison to idealized reactors, and flow visualization to characterize flow patterns. The conversion characteristics of the two geometries differ greatly. In the range of conditions studied, posted reactors yield conversion behavior close to those expected for ideal mixed flow, while channel reactor behavior resembles ideal plug flow. Flow visualization showed only steady laminar-like flow in the channel reactor. In contrast, the flow pattern in the posted reactor showed significant mixing. Introduction Microreactors have been proposed as useful devices for microanalysis, the production of fine chemicals, hydrogen production for portable electronics, catalyst testing, and other applications.1-3 The flow in microreactors is often less turbulent than that in conventional reactors, and the small length scales permit diffusion to play a larger role. Hence, the design guidelines appropriate for conventional reactors do not always apply to analogous microreactors. Guidelines regarding optimal reactor geometry in microreactors remain poorly delineated in the literature. Some computationally based literature exists that attempts to relate reactor geometry to conversion in gas-phase systems.4-13 The present work seeks to determine whether the results of such computations accord with experiment by comparing the conversion behavior of posted and straightchannel microreactors. Figure 1 shows the layout of these reactor types. Both reactor types offer significantly lower pressure drops and better heat transfer than conventional packed-bed configurations.14,15 In a previous paper, Vlachos et al.5 suggested that posted reactors should exhibit nearly ideal plug flow behavior. The present experiments instead show mixed flow behavior. Experimental Section The experiments measured conversion as a function of geometry and residence time for a simple test reaction:
2NH3 w N2 + 3H2
(1)
Some experiments also measured reaction kinetics in situ using a differential reactor with a micro air pump in the recycling loop. * To whom correspondence should be addressed. E-mail:
[email protected].
Figure 1. Photos of the (a) posted and (b) channel reactors used in this work.
The procedures for reactor fabrication followed those described in Ganley et al.14-16 Reactors were fabricated from 1100 aluminum (99+% Al) stock. Posted reactors were made using electrical discharge machining (EDM) to cut an array of 385 square posts 0.3 mm wide and 3 mm high on 0.6 mm centers, as shown in Figure 1a. Channel reactors (Figure 1b) had 14 straight channels with dimensions of 0.3 mm × 3 mm × 15 mm. The structures were degreased in acetone and anodized potentiostatically at 45 V in 0.40 M oxalic acid at room temperature for 1 day. This procedure formed a conformal porous alumina layer on the interior surfaces. The alumina was then impregnated with a saturated solution of ruthenium(III) chloride in water. This catalyst
10.1021/ie048956w CCC: $30.25 © 2005 American Chemical Society Published on Web 05/10/2005
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Figure 2. The conversion of a 0.301 cm3 posted reactor as a function of (i) the flow rate of 50% ammonia in argon fed to the reactor at 562 °C and (ii) the recycle rate of products from the exit of the reactor into the reactor inlet.
was calcined for 6 h in air at 550 °C, followed by reduction in hydrogen for 2 h at the same temperature. Conversion was measured in a titanium reactor housing heated by a Lindberg Mini-Mite tube furnace. Temperature was measured by inserting a 0.15 mm o.d. thermocouples (Omega) into a hole drilled into the reactor base, and subsequently back-filled with tin solder to ensure intimate thermal contact. A 14 mm i.d. tube filled with blank aluminum pellets (Alfa Aesar) was placed upstream to serve as a reactant preheater. Effluent concentrations were monitored by a differentially pumped quadrupole mass spectrometer system (Balzers QMG-112). The base chamber pressure was 10-10 Torr, and pressure was increased to around 10-7 Torr for measurement. The mass spectrometer was calibrated by passing an equimolar ratio of ammonia and argon through the reactor bypass. Control experiments throughout the temperature range of interest showed that the reactor housing induced no conversion in the absence of catalyst. The flow of technical grade (99.99%) anhydrous ammonia was controlled using calibrated mass flowmeters. Argon was used as the reference gas. All experiments were carried out with the reactor vented to the atmosphere. Results Figure 2 shows the variation of conversion with flow rate for the posted reactor at 562 °C. The flow rate in the plot is the feed flow rate of the ammonia/argon mixture into the reactor and excludes the extra flow that arises because of the recycle. Not surprisingly, the conversion decreased as the feed rate increased. Figure 2 also shows results of experiments in which products were recycled back to the reactor inlet at rates of 3000 and 4000 sccm. The conversion was largely independent of recycle rate above 2000 sccm. Interestingly, at any feed flow rate, the conversion increases when the products of the reaction are recycled to the inlet. Figure 3 shows the rate as a function of the ammonia pressure calculated from the recycle data in Figure 2, assuming that, at the high recycle ratios, the reactor itself produced differential conversion. Figure 3 also shows the rate calculated via the empirical equation
mol rNH3 ) -45.8 (PNH3)1.34 lit - min - atm1.34
(2)
Figure 3. (9) The rate of the reaction as a function of the ammonia pressure calculated from the data in Figure 2. (-) The rate calculated via eq 2.
Figure 4. A comparison of the no recycle data in Figure 2 (9) to that expected for a CSTR and a PFR following the kinetics in eq 2.
Equation 2 fits the data quite well over the entire pressure range. Other experiments with hydrogen addition to the feed showed no significant effect on the rate. One can use eq 2 to calculate the expected behavior using idealized design equations for a plug flow reactor (PFR) or perfectly mixed continuous stirred tank reactor (CSTR). Figure 4 compares the results to the actual conversion of the posted reactor. Notice that the conversion in the posted reactor is much closer to that expected for a CSTR than for a PFR, although the measured conversion is always slightly below that expected for a CSTR. Figure 5 shows corresponding results for channel reactors. The channel reactors behave much differently; the conversion is always much higher in the absence of recycle. The empirical expression derived from the recycle data is
mol rNH3 ) -271 (PNH3)1.43 lit - min - atm1.43
(3)
Generally, rates are higher in the channel reactor than in the posted reactor. Experimentally, when posted reactors and channel reactors15 are anodized under identical conditions, the alumina that forms is different because the local electric field is different in the two cases. Generally, the alumina in the posted reactor has a lower pore volume and surface area than the alumina in the channel reactors. This leads to a lower catalyst
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Figure 8. An image of the flow pattern emerging from the channel reactor. In this experiment a line of smoke was injected from the left and emerges from the reactor on the right.
Figure 5. The conversion of a 0.189 cm3 channel reactor as a function of the flow rate of 50% ammonia in argon feed to the reactor at 492 °C and the recycle rate of products from the exit of the reactor into the reactor inlet.
Figure 9. An image of the flow pattern emerging from the posted reactor. In this experiment a line of smoke was injected from the left and emerges from the reactor on the right. The spots in the photo are scratches on the reactor housing.
Figure 6. A comparison of (b) the data in Figure 5 to that expected for a CSTR and a PFR following the kinetics in eq 3.
Figure 7. A replot of the data in Figure 6 on an expanded scale.
loading and dispersion.15 The differences result in a lower rate in the posted reactors than in the channel reactors, even though the catalysts were prepared identically in both cases. Figure 6 and Figure 7 compare the data in Figure 5 to that expected for a CSTR and PFR calculated via eq 3. The expected conversions always lie close to, but slightly below those, of the PFR. In fact, the conversion is always 97 ( 0.6% of the PFR conversion, independent of the flow rate. Flow Visualization Flow visualization experiments were conducted by injecting a single thin stream of smoke at the entrance of each type of reactor, along with a flow of 200 sccm of
air. Figure 8 shows how the line of smoke moved through the channel reactor. The smoke all stays within the channel and emerges in the same thin-line shape. Movies taken at 8 frames/s showed that when the feed smoke is moved to another channel, the corresponding exit line moves instantaneously at the exit. That is, there is no tapering off of the original trail in time as would be expected with significant back mixing. In contrast, very different behavior was seen with the posted reactor as shown in Figure 9. When smoke was injected into one side of the reactor, a broad and diffuse stream emerged that was too pale to photograph adequately. We always observed a uniform, well-mixed smoke plume at the exit, independent of where we injected the smoke. There were also concentrated regions of smoke near the walls of the reactor housing, indicating bypassing. Thus, it is clear that there is significant radial- and or back mixing in the posted reactor, even though the Reynolds number based on the channel width is only about 10. Other experiments employing reactors with fewer posts showed significant smoke channeling. Discussion The results in this paper show some agreement with theory, but also some important differences. The channel reactor behaved as expected. In the absence of recycle. the reactor operates at Reynolds numbers between 1 and 25. Under these conditions, the flow should be laminar consistent with the visualization in Figure 8. The Peclet number based on length (PeL) was above 10 in all experiments, so back mixing and axial dispersion should have been negligible. The Peclet number based on the channel radius (Per) was below 0.1 in all of the experiments, so radial diffusion was rapid. The standard Taylor dispersion model indicates
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that one would expect the reactor to behave like a plug flow reactor (PFR) under these conditions. We observe PFR behavior in Figure 6 as expected, although there was a small difference at high conversions as shown in Figure 7. We believe the small difference is due to bypassing since our design left a small gap above the reactor housing to make insertion of the reactor easier. We have also observed bypassing in our flow visualization. Note that the channel reactor was apparently operating near a “sweet spot” in experimental conditions where radial mixing is nearly complete by diffusion but there is little axial mixing. Since the channel aspect ratio (i.e., length over width) is only about 50, changing the flow conditions would likely move the reactor off this sweet spot into regions of higher back mixing. With faster flow, flow would dominate diffusion, and the parabolic velocity profile would give a spread-out residence time distribution not characteristic of plug flow. With slower flow, axial diffusional back mixing would show up. The flow visualization experiments cannot capture these nuances because the smoke particles diffuse more slowly than gas molecules. Thus, the PFR behavior is probably limited to a specific set of conditions. However, the benefits for high conversion make such conditions desirable as a goal for other kinds of channel-like microreactors. The kinetics measurements also reflected expectations. If the decomposition of ammonia followed the classic Temkin-Pyzhev rate law,17 the rate of ammonia decomposition should be proportional to the ammonia pressure to the 1.5 power. Other investigators have reported that the rate is proportional to the ammonia pressure to the 0.8 to 1.7 power.18-20 Our kinetic results are within the range of these reports. We do not see evidence for significant hydrogen inhibition of the rate, however, presumably because our pressures are more than an order of magnitude lower than those used previously. Also, most of the recycle measurements were done at conditions where there is significant hydrogen in the reactor due to conversion of ammonia. The results using the posted reactor in Figure 4 did not come out quite as expected, however. Previous calculations of Vlachos et al.5 suggested that, in the absence of recycle, the conversion from the channel should also follow the PFR equation but Figure 4 shows that the conversion in the posted reactor is much closer to that expected from a CSTR than a PFR. Experimentally, we find that the conversion of the channel reactor decreases markedly when we recycle products back from the inlet stream. In contrast, the conversion of the posted reactor increases slightly. Clearly, the posted reactor is not behaving like a PFR in the absence of recycle. The flow visualization also came out somewhat unexpected. Our flow visualization showed that the exit of the reactor is always well-mixed under the conditions used during our experiments. We do not observe discrete streams of gas, as was suggested by the previous calculations. There is some bypassing along the wall of the reactor housing consistent with our observation that the conversion increases when we recycle but, otherwise, the flow visualization was consistent with what one would expect for a CSTR, not a PFR. We have not measured residence time distributions yet, so we do not know for sure that the posted reactor behaves completely like a CSTR. Still, both the flow
visualization measurements and the conversion measurements are consistent with the idea that our posted reactor is behaving like a CSTR with bypassing. We offer the following speculation about why these experiments differ from the calculations of Vlachos et al.5 Those calculations were performed in two dimensions not three dimensions. This in effect assumes infinite boundary conditions in the vertical direction. In contrast, there exist walls bounding the height of our reactor. It is possible that fluid impinging on the walls spreads both forward and against the net axial flow, thereby creating back mixing. The boundary conditions of the calculations could not have reproduced such effects. Conclusions In summary, this work has shown that, under some conditions, a channel microreactor can behave like a PFR, while the posted reactors tend to resemble a CSTR. Clearly, reactor geometry plays a key role in determining reactor conversionssometimes in unexpected ways. Acknowledgment This work was supported by the Department of Defense Multidisciplinary University Research Initiative (MURI) program administered by the Army Research Office under contract DAAD19-01-1-0582. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Department of Defense or the Army Research Office. The authors thank Craig Miesse for assistance in taking the photographs in Figure 8 and Figure 9. Literature Cited (1) Masel, R. I.; Gold, S.; Ni, Z. Microreactors and Microreaction Engineering. Encyclopedia of Chemical Processes, to appear. (2) Jensen, K. F. Microreaction engineeringsis small better? Chem. Eng. Sci. 2001, 56, 293-303. (3) Ehrfeld, W.; Hessel, V.; Lo¨we, H. Microreactors: New technology for modern chemistry; Wiley-VCH: New York, 2001. (4) Deshmukh, S. R.; Mhadeshwar, A. B.; Vlachos, D. G.; Lebedeva, M. I., Multiscale modeling of microchemical devices: Application to hydrogen production for portable fuel cells. Int. J. Multiscale Comput. Eng., to appear. (5) Deshmukh, S. R.; Mhadeshwar, A. B.; Vlachos, D. G. Microreactor modeling for hydrogen production from ammonia decomposition on ruthenium. Ind. Eng. Chem. Res. 2004, 43, 2986-2999. (6) Steinfeldt, N.; Dropka, N.; Wolf, A.; Baerns, M. Application of multichannel microreactors for studying heterogeneous catalysed gas-phase reactions. Chem. Eng. Res. Des. 2003, 81, 735-743. (7) Ibashi, W.; Groppi, G.; Forzatti, P. Kinetic measurements of CH4 combustion over a 10% PdO/ZrO2 catalyst using an annular flow microreactor. Catal. Today 2003, 83, 115-129. (8) Amador, C.; Gavriilidis, A.; Angeli, P. Flow distribution in different microreactor scale-out geometries and the effect of manufacturing tolerances and channel blockage. Chem. Eng. J. 2004, 101, 379-390. (9) Delsman, E. R.; de Croon, M. H. J. M.; Kramer, G. J.; Cobden, P. D.; Hofmann, Ch.; Cominos, V.; Schouten, J. C. Experiments and modelling of an integrated preferential oxidationheat exchanger microdevice. Chem. Eng. J. 2004, 101, 123-131. (10) Hsing, I. M.; Srinivasan R.; Harold, M. P.; Jensen, K. F.; Schmidt, M. A. Simulation of micromachined chemical reactors for heterogeneous partial oxidation reactions. Chem. Eng. Sci. 2000, 55, 3-13. (11) Quiram, D. J.; Hsing, I. M.; Franz, A. J.; Jensen K. F.; Schmidt, M. A. Design issues for membrane-based, gas-phase microchemical systems. Chem. Eng. Sci. 2000, 55, 3065-3075.
Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4271 (12) Rebrov, E. V.; Duinkerke, S. A.; de Croon, M. H. J. M.; Schouten, J. C. Optimization of heat transfer characteristics, flow distribution, and reaction processing for a microstructured reactor/ heat-exchanger for optimal performance in platinum catalyzed ammonia oxidation. Chem. Eng. J. 2003, 93, 201-216. (13) Keoschkerjan, R.; Richter, M.; Boskovic, D.; Schnurer, F.; Lobbecke, S. Novel multifunctional microreaction unit for chemical engineering. Chem. Eng. J. 2004, 101, 469-475. (14) Ganley, J. C.; Riechmann, K. L.; Seebauer, E. G.; Masel R. I. Porous anodic alumina optimized as a catalyst support for microreactors. J. Catal. 2004, 227, 26-32. (15) Ganley, J. C.; Seebauer, E. G.; Masel, R. I. Porous anodic alumina microreactors for production of hydrogen from ammonia. AIChE J. 2004, 50, 829-834. (16) Ganley, J. C.; Thomas, F. S.; Seebauer, E. G.; Masel, R. I. A Priori Catalytic Activity Correlations: The Difficult Case of Hydrogen Production from Ammonia. Catal. Lett. 2004, 96, 117122.
(17) Temkin, M. I.; Pyzhev, V. Kinetics of ammonia synthesis on promoted iron catalysts. Acta Physicochim. URSS 1940, 12, 327-356. (18) Kiperman, S. Kinetics of the ammonia synthesis on ruthenium. J. Phys. Chem. (U.S.S.R.) 1947, 21, 1435-48. (19) Choudhary T. V.; Sivadinarayana C.; Goodman D. W. Production of COx-free hydrogen for fuel cells via stepwise hydrocarbon reforming and catalytic dehydrogenation of ammonia. Chem. Eng. J. 2003, 93, 69-80. (20) Bradford, M. C. J.; Fanning, P. E.; Vannice, M. A. Kinetics Of NH3 Decomposition Over Well Dispersed Ru. J. Catal. 1997, 172, 479-484.
Received for review October 27, 2004 Revised manuscript received March 14, 2005 Accepted March 31, 2005 IE048956W
