Design and Fabrication of a High-Throughput Microreactor and Its

Article pubs.acs.org/IECR

Design and Fabrication of a High-Throughput Microreactor and Its Evaluation for Highly Exothermic Reactions Nageswara Rao Peela,† Ivan C. Lee,‡ and Dionisios G. Vlachos*,† †

Department of Chemical and Biomolecular Engineering, Center for Catalytic Science and Technology, and Catalysis Center for Energy Innovation, University of Delaware, 150 Academy Street, Newark, Delaware 19716, United States ‡ Sensors and Electron Devices Directorate, US Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, United States ABSTRACT: The effect of distributor and channel positions on flow uniformity in a high-throughput reactor is studied using a COMSOL MULTIPHYSICS, CFD package. The best design is subsequently fabricated and evaluated for propane total oxidation reaction at low conversions. Comparison of our design with a literature design is also presented. The results indicate that the conical distributor performs significantly better in terms of flow uniformity as compared to the commonly used disc shaped distributor. The flow uniformity is improved when the central channel is removed from the design, indicating that the channel arrangement also affects the flow uniformity. The experimental results on flow uniformity are in good agreement with simulation. The deviation in conversion of propane from channel to channel is within experimental error, indicating that the reactor can reliably screen catalysts and deliver kinetics. A half a dozen catalysts were tested for complete propane oxidation using the developed technology.

1. INTRODUCTION High-throughput experimentation (HTE) techniques were introduced several years ago to reduce catalyst development times for both batch and continuous processes.1−6 The major phases of HTE operation, particularly involving solid catalysts, include design of experiments, catalyst synthesis and characterization, design and fabrication of a suitable multireactor system, and design of an analytical system. Several studies have been reported in the literature for successful development of catalyst libraries using tools such as genetic algorithms,7,8 artificial neural networks,9 and combinations of them.10,11 Several highthroughput catalyst synthesis methods have also been developed.12 A common analysis technique is gas chromatography. However, this technique is slow, so depending on the reaction system, mass spectroscopy and FTIR are often used.13−15 A comprehensive list of analysis techniques has been presented in the review by Radislav Potyrailo et al.16 Turner et al.17 have given an overview of the synthesis and screening technologies at Symyx Technologies Inc. for rapid and efficient heterogeneous catalyst discovery and optimization. A crucial point in developing a continuous HTE is the flow uniformity with a constant space time of reactants in all the channels. Hoffmann et al.18 developed a 49-channel reactor system and tested it for methane oxidation. The authors showed that the system performance was reasonable with a relative error in conversion of ±6.5%. The main set-back in their design was that the deviation in flow rate was nearly ±10%. The authors overcame this problem using a mass flow controller just before the GC. The same group has used a similar HTE system for screening Au based catalysts for CO oxidation19 and Cu based catalysts for high pressure methanol synthesis.20 Szijjarto et al.21 have used the 16-channel reactor19 for developing non-noble metal based multicomponent catalysts for steam reforming of ethanol, with MgAl2O4 as © 2012 American Chemical Society

support. The authors demonstrated that a four-component system containing Ni, Co, Ce, and Mo exhibited the highest hydrogen yield. Perez-Ramirez et al.22 developed a six-reactor system where the pressure and space velocity of each reactor were controlled simultaneously using separate mass flow controllers at the inlet and back pressure regulators at the outlet of each reactor. The flow could be controlled accurately at the expense of capital cost. It is clear that it is essential to ensure proper flow distribution while reducing the capital cost. Several studies have recently addressed this issue for microchannel devices possessing rectangular parallel channels23−25 and proposed different configurations of inlet and outlet chambers to achieve this. Jensen and co-workers26,27 have developed a micropacked bed reactor in cross-flow configuration. The authors achieved the uniform flow along the width of the catalyst bed using 256 shallow pressure drop channels at the outlet of the catalyst bed. The inlet manifold was designed such that the length from reactor inlet to the bed is same at all widths. The flow distribution in multiphase flows has also been studied28−30 with emphasis on the effect of manifold geometry and inlet pipe length on flow distribution. In this study, we have designed a continuous high-throughput microreactor (HTM) for flow uniformity using the COMSOL MULTIPHYSICS package. Subsequently, we fabricated such a system and tested it experimentally for propane total oxidation reaction at low conversions. Comparison of our design with a literature design is also presented. The smallest dimension used in this study was 1 mm (channel) diameter. It is common to refer to reactors Received: Revised: Accepted: Published: 16270

August 4, 2012 November 21, 2012 November 27, 2012 November 27, 2012 dx.doi.org/10.1021/ie302093u | Ind. Eng. Chem. Res. 2012, 51, 16270−16277

Industrial & Engineering Chemistry Research

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regions and the Brinkman equations31 in the porous catalyst bed (the violet colored region in Figure 1). The Navier−Stokes equation in the free flow region is

with a diameter less or equal to 1 mm as microreactors. Therefore, the reactor is named a high-thoughtput microreactor. Since the results of the simulation section were used for fabricating the HTM, we used the following paper structure. The model details were presented first followed by simulation results, then the details of catalyst synthesis and reactor design were summarized, followed by the experimental results and discussion and finally the conclusions.

ρ(u·∇)u = ∇·[−pI + μ(∇u + (∇u)T )]

ρ(∇·u) = 0 The Brinkman equation in the porous catalyst bed is ⎡ ⎞ μ ρ⎛ ⎜⎜(u·∇) u ⎟⎟ = ∇·⎢ −pI + (∇u + (∇u)T ) εp ⎝ εp ⎠ εp ⎢⎣

2. MODELING The schematics of various designs considered in this study are shown in Figure 1. As shown in Figure 1b, the HTM system

−

⎛μ ⎞ ⎤ 2μ (∇·u)I − ⎜ ⎟u⎥ 3εp ⎝ k br ⎠ ⎥⎦

ρ(∇·u) = 0 Here ρ is the density of fluid, kg m−3; u is the velocity, m s−1; p is the pressure, Pa; εp is the porosity of the catalyst bed; μ is the viscosity of the fluid, Pa·s; and Kbr is the permeability of the bed. The boundary conditions were constant velocity at the inlet, constant pressure at the outlet, and no slip condition at all the walls. The normal inflow velocity at the inlet was set to 6 m/s. At the outlet, the pressure was specified to be 0 Pa and the viscous stress was assumed to be zero. For comparison with literature design,18 we kept everything else the same except for the distributor. Ten channels were used in this comparison, including a central channel. After analyzing the results obtained in the above design, the central channel was removed in a modified design and tested for flow uniformity. The HTM was simulated in 3D using the COMSOL MULTIPHYSICS package (a commercial software), which uses the finite element method. A free tetrahedral mesh was used in our simulations. To strike a balance between computational time and accuracy, the mesh size varied at different parts of the computational domain, depending on the dimension. For example, the diameter of the reactor and capillary connected to the bottom of each reactor was 5 and 1 mm, respectively, and the maximum mesh size in each subpart was 1 and 0.2 mm, respectively. Further details on the mesh size in different sections are presented in Table 1.

Figure 1. Computational domain of different designs: (a) Literature design with disc shaped distributor and 10 channels (D1). (b) Our design with conical distributor and 10 channels (D2). The modified design with conical distributor and 9 channels (D3) is similar to (b) without a central channel. The violet colored regions indicate the catalyst beds where Brinkman’s equation was used. The remaining regions are free flow where the Navier−Stokes equations were used.

Table 1. Characteristic Scale in Various Sections of the Computational Domain

consists of six major parts: flow inlet tube, flow distributor, diffuser capillaries, reactors, catalyst beds, and reactor endcapillaries. The fluid is passed through a distributor (disc shaped or conical) and is then distributed over all the channels through a diffuser plate to avoid any back mixing. The reactors are filled with a porous catalyst bed with an assumed porosity of 0.4 and a permeability of 10−9 m2. The length of the bed is 10 mm. The end-capillaries are useful to equalize the pressure in all the channels and thereby the flow. To assess the effect of end-capillaries, simulations were carried out by varying their length from zero (no end-capillaries) to long enough that asymptotic behavior was achieved. The model assumptions were steady state, negligible gravitational forces, and incompressible laminar flow (maximum NRe = 600). The underlying model consists of the Navier−Stokes equations in the free-flow

location

maximum size (mm)

entrance of the HTM distributor diffusor channels catalyst bed end-capillaries

1.0 2.0 0.2 0.7 0.2

3. SIMULATION RESULTS To compare with literature design (D1) of a disc shaped distributor, reported in ref 18, a similar design (D2) was considered but with a conical distributor. To find out the effect of a distributor on flow uniformity, the simulations of both designs were run without any capillary tube at the outlet of the reactor. The average flow rates were estimated using COMSOL and are presented in Figure 2b. As shown in Figure 2a, channel 4 is the central channel, channels 1−3 are the middle channels, 16271

dx.doi.org/10.1021/ie302093u | Ind. Eng. Chem. Res. 2012, 51, 16270−16277


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Figure 2. (a) Schematic of the top view of HTM with the reactors numbered. (b) Flow distribution as a function of channel number and distributor type.

capillary required to reduce the deviation in flow to a particular value was always higher in the disc distributor (design D1). Packed beds usually provide fluidic resistance, and this effect is much more pronounced in the case of industrial packed bed reactors where the packing is much longer. However, in the case of laboratory scale reactors, the bed lengths are much shorter (of the order of 1 cm), and therefore, the entrance effects play an important role when it comes to the flow distribution in parallel packed bed channels, such as the case here. That really depends on the relative resistance that exists in the flow path. This short bed issue is essential in high throughput screening used in laboratory reactors. On comparing the conical and disc shaped distributors, one can physically visualize that the distance traveled by a particle is relatively different in a disc shaped distributor as compared to that in a conical distributor. This is the reason for the large difference in flow in the central channel in the case of a disc

and channels 5−10 are the outer channels. The maximum deviation in the flow with the conical distributor is about 8%, whereas that with the disc shaped distributor is nearly 25%. Moreover, the flow rate in the middle channel was very high as compared to outer channels in D1. This clearly indicates that the conical distributor achieves better distribution of flow among the channels. In Figure 3a and b, a 2D slice of the HTM is shown with conical and disc shaped distributors, respectively. During flow through the conical distributor, the gas distributes toward all the channels in D2, whereas, in D1, the gas is not uniformly distributed even in the vicinity of channels. To find out how the capillary at the end of each reactor modifies the flow distribution, simulations were run with different lengths of capillaries. The average deviation in flow among the channels obtained in each design is plotted in Figure 4. The percentage difference decreased with an increase in capillary length in both designs and leveled off at higher capillary lengths. The length of 16272



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compared to that with a central channel (0.5%), for the same end-capillary length. This indicates that the arrangement of channels also affects the flow uniformity. For comparison, one simulation was also carried out using the same arrangement of channels (D3) with a disc shaped distributor and with a 30 mm end-capillary. The deviation in flow was found to be similar to that with a conical distributor. The 3D pressure profile with design D3 is shown in Figure 5a. Most of the pressure drop occurs in the capillary tube at the

Figure 3. (a) Velocity profile in an xy-plane at z = 0 with a conical distributor. (b) Velocity profile, at the same plane with a disc shaped distributor. The velocity is in m/s.

Figure 5. Simulation results from design D3: (a) 3D pressure profile; (b) pressure profile in a yz-plane at x = 20 mm; (c) velocity profile, at the same plane. Pressure is in Pa, and velocity is in m/s.

end of each reactor. As a result, the pressure difference between the channels at any axial position is negligible. For example, the difference in pressure at a particular y−z plane among the channels (Figure 5b) was ∼3 Pa, which is negligible compared to the total pressure drop in the system (500 Pa). The velocity profile was also highly uniform at any cross section (Figure 5c).

4. CATALYST SYNTHESIS AND REACTOR DESIGN The catalysts were synthesized using incipient wetness impregnation of commercial alumina powder with an aqueous precursor solution of suitable salt (Table 2). The synthesized catalysts were dried at 120 °C for 10 h and then calcined at 290 Figure 4. Average deviation in flow as a function of length of the endcapillary.

Table 2. Summary of Catalysts Used in This Study

shaped distributor as compared to that with a conical distributor. The case might be different in industrial packed bed reactors where the resistance to flow (because of the larger length of the bed) is much larger as compared to that due to entrance effects. Figure 2b shows that the flow in the central channel (channel 4 in Figure 2a) was highest as compared to that in other channels. Therefore, in our modified design D3, the central channel was removed and the simulation was run with the remaining nine channels, with an end-capillary length of 30 mm. The results show that the average deviation in the flow rate is less than 0.35%. The deviation in this design is less

catalyst 1% Pt/ Al2O3 1% Pd/ Al2O3 1% Rh/ Al2O3 5% Ni/ Al2O3 1% Co/ Al2O3 1% Cu/ Al2O3 16273

nominal composition (wt %)

weight of catalyst (mg)

tetraammineplatinum(II) nitrate palladium(II) nitrate

1.0

2.0

1.0

2.0

rhodium(III) chloride

1.0

4.0

nickel(II) nitrate

5.0

10.0

cobalt(II) nitrate

1.0

10.0

copper(II) nitrate

1.0

10.0

precursor salt



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°C for 2 h. The catalysts were then made into pellets, crushed, and sieved to a final size of 105−150 μm. The optimum HTM design (D3) obtained in the simulations was fabricated in-house. Photographs of various parts and of the assembled HTM are shown in Figure 6. The HTM consists of

the selector valve were heated at 160 °C to avoid any condensation of water that formed during the reaction. The water in the effluent passing to the GC was condensed, using an ice trap, after the selector and prior to the GC. Product analysis was carried out with a four-channel micro-GC (Agilent Technologies, USA; model G3581A) with three different capillary columns, molecular sieve 5A, porapak-U, and AlOx. Each column was connected to an individual TCD detector. To achieve temperature uniformity, the reactors were heated with three heating cartridges, each of 170 W capacity. The heating rate and temperature of the HTM were controlled by a PID controller. The exit temperatures of all nine reactors were measured using 1/16” K-type thermocouples. The system was first tested, for reproducibility, for propane oxidation with 1% Pt/Al2O3 catalyst in eight channels and the remaining channel (one of three inside channels) was filled with SiC only. The reaction conditions used in this study were as follows: weight of catalyst = 2 mg (weighing of catalysts was carried out using a microbalance with an accuracy of 0.001 mg); catalyst dilution ratio = 1:200; propane flow = 1.98 SCCM (partial pressure of propane 0.5 kPa); oxygen flow = 98.7 SCCM (partial pressure of O2 = 25 kPa); He flow = 299 SCCM (partial pressure of He = 74.5 kPa); total inlet flow rate = 400 SCCM; temperature = 400−480 °C. The diluent used was SiC with the same particle size as that of the catalyst. Six different catalysts have been tested in propane oxidation activity. The catalysts used and their nominal composition are summarized in Table 2. On the basis of preliminary experiments, the catalyst weight (Table 2) was adjusted to obtain detectable conversion with all the catalysts and to maintain conversion below 40%, to avoid light-off and also to minimize the local volume change. The mass balance in all the experiments and channels closes within 3%.

Figure 6. Photographs of the high throughput experimental system: (a) expanded view; (b) assembled view.

5. EXPERIMENTAL RESULTS Oxidation reactions operated under light-off conditions could result in ignition/extinction bistability, due to thermal crosstalk with the neighboring wells, e.g., ref 18. Light-off is undesirable for kinetics studies. Moreover, when dealing with highly exothermic or endothermic reactions, the heat produced or consumed in one reactor might affect the performance of the catalyst in the reactor adjacent to it. In the present study, to avoid light-off and to avoid thermal effects, the catalyst bed has been diluted 200 times with the same size SiC particles. Another important aspect that could influence the flow distribution is the local volume change. To overcome this issue, reactants were used with a high degree of dilution. With the operating conditions used in this study, the maximum increase in flow rate (at 100% conversion) would be 2.0 SCCM (out of 400 SCCM), which is 0.5% of the total flow used in this study. Moreover, the conversion level was restricted to 40% to further reduce the deviation to 0.2%. To assess the effect of dispersion on the kinetics, we have calculated the Peclet number at different sections of the HTM. The Peclet number is found to be much larger than 1 at all the sections, indicating that the axial dispersion can be neglected. The flow was measured in empty reactors and with a catalyst bed (at both room temperature and reaction temperature). The results showed that the maximum deviation in the flow without the catalyst bed and with a catalyst bed at room temperature was less than ±0.25%. For comparison, one simulation was also carried out with the same length of the end-capillary as in the experiment. The average deviation in flow among channels was

nine channels. The reactant gases (propane, oxygen) and the inert (He) gas flows were controlled with mass flow controllers (model RS-485, MKS Instruments, USA). The flow rate in each channel was measured using a mass flow meter (model ADM2000, Agilent Technologies, USA). The gases were then mixed and passed to the conical distributor, where the gas was evenly distributed to each channel. A diffusor plate, with holes of 1.5 mm diameter and 6 mm height, was used between the distributor and reactors to avoid any back-mixing of reactor products with incoming reactants. The length of each reactor was about 75 mm, and the catalyst bed length was approximately 10 mm. The channel-to-channel distance was approximately 10 mm. The three partsdistributor, diffusor plate, and reactor systemwere combined using nut bolts and graphite seals to avoid any leaks. On the basis of the CFD results discussed above, the end of each reactor channel was connected to a 20 cm long 1/16” tube with an inside diameter of approximately 0.4 mm to ensure that the pressure drop in the tube after the reactor was higher than the pressure drop in the catalyst bed to even out any small differences arising from preparing the catalyst bed. This also ensured that the pressure drop in the effluent passed to the GC sampling valve is equal to that in the effluents connected to the vent. One of the nine reactor effluents was selected at a time for analysis using a 10port selector (VICI instruments, USA) and the remaining eight effluents were vented. The tubes after the reactor channels and 16274



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±0.15%, in reasonable agreement with the experiment. The average percentage deviation with a catalyst bed in place at typical reaction temperatures (∼400 °C) was nearly ±1.0%. This deviation is on the higher side compared to that in simulations. This could be because of the higher pressure drop at high temperature in the catalyst bed. This deviation could be reduced using an even longer capillary at the end of each reactor at the expense of higher pressure drop. Therefore, there is a compromise between the pressure drop and the percentage deviation in flow. Moreover, this deviation was far less than the value reported by Hoffmann et al.18 Therefore, we have used a 20 cm length end-capillary for all the experiments. The conversion of propane on 1% Pt/Al2O3 is presented in Figure 7. The deviation in conversion is small and nearly the

Table 4. p Values from 1-Way ANOVA with Reactor Number as a Factor temperature (°C)

p values

400 420 440 460 480

0.0036 0.0007 0.0718 0.055 0.1258

screening of catalysts at an enhanced speed and for obtaining kinetics. The conversion of propane obtained as a function of temperature with six catalysts is shown in Figure 8. The

Figure 8. Variation of propane conversion with temperature on different catalysts.

conversion is highest with 1% Pd/Al2O3 catalyst, which is expected32,33 as the reactant mixture is very O2 rich. The O2 rich conditions oxidize Pd to PdO, which is known to be active for oxidation reactions.34−36 Pt is known to be the best catalyst for oxidation of paraffins with a carbon number more than 2;37,38 Pt is the active phase, and PtO2 may act as an inhibitor.39−41 In this study, because the O2 to hydrocarbon ratio is very high, Pt may be converting to PtO2. Detailed characterization will be necessary to understand the lower activity of Pt as compared to Pd. Uneven phase changes (i.e., due to different conversions in different channels) and significant accumulations, such as of coke, could influence the flow distribution. Therefore, one has to be cautious when using high-throughput technology. However, neither phase changes nor accumulation of coke are expected at our operating conditions. Catalyst particle shape and morphology may also affect the flow distribution, particularly for multiphase flows. Though this topic is beyond the scope of our study, one has to be cautious. As compared to the high-throughput experimentation system developed in ref 18, our HTM is slower because of the lower number of reactors. However, the data obtained in our HTM is proved to be highly reliable as compared to that from ref 18. As compared to conversional single packed bed reactors, our HTM is much faster. It takes about 10 h for cleaning, weighing, loading, and unloading of the catalysts. For pretreatment (such as reduction) and catalyst activity testing (at five different temperatures), the typical time required is about 13 h. The total man hours required for testing nine catalysts is 23 h. It is estimated that 120 h (including cleaning, loading, unloading, pretreatment, and testing) total man hours are required for testing nine catalysts with a conventional fixed reactor. The man hours are reduced 5-fold with the use of HTM developed as compared to a conventional single packed bed reactor we run in our lab. The HTM is even more beneficial when used for

Figure 7. Conversion of propane as a function of temperature. The error bars indicate the deviation in conversion among channels.

same at all conversions. The maximum deviation in temperature in our HTM was less than ±4 K. To further investigate the causes for the small deviation in conversion, the experiment was repeated by performing product analysis twice. A 2-way analysis of variance (2-way ANOVA) was performed for the data obtained in the above experiment. The p values obtained from ANOVA at each temperature are given in Table 3. From Table 3. p Values from 2-Way ANOVA with Reactor Number and GC Analysis as Factors temperature (°C)

p values from reactor no.

p value from GC analysis

400 420 440 460 480

0.0003 0.0021 0.0041 0.0005 0.0088

0.0533 0.4447 0.1332 0.1133 0.3510

the p values, it can be concluded that, at a significance level of 5%, the deviation in conversion can be considered to be solely from the randomness in the reactors and not from the GC analysis. To check whether the deviation present in the conversion is statistically significant, we performed 1-way ANOVA on the conversion with reactor number as a factor. The results obtained are summarized in Table 4. The deviation is statistically significant only at low conversions but not significant at high conversions (i.e., for temperatures above 420 °C). This indicates that the deviation in conversion from channel to channel is within the usual experimental error. Therefore, the HTM developed in this study can be used for 16275



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(6) Trapp, O. Gas Chromatographic High-Throughput Screening Techniques in Catalysis. J. Chromatogr., A 2008, 1184, 160. (7) Paul, J. S.; Janssens, R.; Denayer, J. F. M.; Baron, G. V.; Jacobs, P. A. Optimization of MoVSb Oxide Catalyst for Partial Oxidation of Isobutane by Combinatorial Approaches. J. Comb. Chem. 2005, 7, 407. (8) Wolf, A.; Schüth, F. A Systematic Study of the Synthesis Conditions for the Preparation of Highly Active Gold Catalysts. Appl. Catal., A 2002, 226, 1. (9) Tompos, A.; Margitfalvi, J. L.; Tfirst, E.; Végvári, L.; Jaloull, M. A.; Khalfalla, H. A.; Elgarni, M. M. Development of Catalyst Libraries for Total Oxidation of Methane: A Case Study for Combined Application of “Holographic Research Strategy and Artificial Neural Networks” in Catalyst Library Design. Appl. Catal., A 2005, 285, 65. (10) Corma, A.; Serra, J. M.; Serna, P.; Valero, S.; Argente, E.; Botti, V. Optimisation of Olefin Epoxidation Catalysts with the Application of High-Throughput and Genetic Algorithms Assisted by Artificial Neural Networks (Softcomputing Techniques). J. Catal. 2005, 229, 513. (11) Rodemerck, U.; Baerns, M.; Holena, M.; Wolf, D. Application of A Genetic Algorithm and A Neural Network for the Discovery and Optimization of New Solid Catalytic Materials. Appl. Surf. Sci. 2004, 223, 168. (12) Zheng, Z. Q.; Zhou, X. P. High Speed Screening Technologies in Heterogeneous Catalysis. Comb. Chem. High Throughput Screening 2011, 14, 147. (13) Hendershot, R. J.; Fanson, P. T.; Snively, C. M.; Lauterbach, J. A. High-Throughput Catalytic Science: Parallel Analysis of Transients in Catalytic Reactions. Angew. Chem., Int. Ed. 2003, 42, 1152. (14) Snively, C. M.; Oskarsdottir, G.; Lauterbach, J. Parallel Analysis of the Reaction Products from Combinatorial Catalyst Libraries. Angew. Chem., Int. Ed. 2001, 40, 3028. (15) Snively, C. M.; Oskarsdottir, G.; Lauterbach, J. Chemically Sensitive Parallel Analysis of Combinatorial Catalyst Libraries. Catal. Today 2001, 67, 357. (16) Potyrailo, R.; Rajan, K.; Stoewe, K.; Takeuchi, I.; Chisholm, B.; Lam, H. Combinatorial and High-Throughput Screening of Materials Libraries: Review of State of the Art. ACS Comb. Sci. 2011, 13, 579. (17) Turner, H. W.; Volpe, A. F., Jr.; Weinberg, W. H. HighThroughput Heterogeneous Catalyst Research. Surf. Sci. 2009, 603, 1763. (18) Hoffmann, C.; Schmidt, H.-W.; Schüth, F. A Multipurpose Parallelized 49-Channel Reactor for the Screening of Catalysts: Methane Oxidation as the Example Reaction. J. Catal. 2001, 198, 348. (19) Hoffmann, C.; Wolf, A.; Schuth, F. Parallel Synthesis and Testing of Catalysts Under Nearly Conventional Testing Conditions. Angew. Chem., Int. Ed. 1999, 38, 2800. (20) Kiener, C.; Kurtz, M.; Wilmer, H.; Hoffmann, C.; Schmidt, H. W.; Grunwaldt, J. D.; Muhler, M.; Schuth, F. High-Throughput Screening Under Demanding Conditions: Cu/ZnO Catalysts in High Pressure Methanol Synthesis as an Example. J. Catal. 2003, 216, 110. (21) Szijjártó, G. P.; Tompos, A.; Margitfavi, J. L. High-Throughput and Combinatorial Development of Multicomponent Catalysts for Ethanol Steam Reforming. Appl. Catal., A 2011, 391, 417. (22) Pérez-Ramírez, J.; Berger, R. J.; Mul, G.; Kapteijn, F.; Moulijn, J. A. The Six-Flow Reactor Technology: A Review on Fast Catalyst Screening and Kinetic Studies. Catal. Today 2000, 60, 93. (23) Commenge, J. M.; Falk, L.; Corriou, J. P.; Matlosz, M. Optimal Design for Flow Uniformity in Microchannel Reactors. AIChE J. 2002, 48, 345. (24) Griffini, G.; Gavriilidis, A. Effect of Microchannel Plate Design on Fluid Flow Uniformity at Low Flow Rates. Chem. Eng. Technol. 2007, 30, 395. (25) Liu, H.; Li, P.; Lew, J. V. CFD Study on Flow Distribution Uniformity in Fuel Distributors Having Multiple Structural Bifurcations of Flow Channels. Int. J. Hydrogen Energy 2010, 35, 9186. (26) Ajmera, S. K.; Delattre, C.; Schmidt, M. A.; Jensen, K. F. Microfabricated Cross-Flow Chemical Reactor for Catalyst Testing. Sens. Actuators, B 2002, 82, 297.

kinetics study of catalysts with negligible deactivation, since the time required to reach steady state can be eliminated for eight catalysts. Therefore, the HTM is 8 times faster than the conventional packed bed reactor when a large number of operating conditions is explored.

6. CONCLUSIONS The design and fabrication of a high-throughput millireactor has been presented. From the simulation and experimental results, it can be concluded that the conical distributor significantly improves the flow distribution as compared to a disc shaped distributor used in the literature. The microchannel arrangement also affects the flow uniformity. Simulation results indicated that the flow rates in individual channels deviate by only ±0.15% with an end-capillary length of 20 cm for nine channels. Experimental and simulation results are in reasonable agreement. The results from a test reaction, propane oxidation, indicate that this device can deliver reliable data on catalyst screening without any external devices, such as mass flow controllers to manipulate the flow. This can simplify the design and reduce the capital cost. A half a dozen catalysts were tested for propane oxidation using the developed HTM technology. Extension of this design with other designs, such as that developed by Perez-Ramirez et al.,22 in which there is more flexibility to have different flow rates and total pressures in each channel, to obtain larger throughput and flexibility for kinetic studies is entirely feasible.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 302 831 2830. E-mail: [email protected]. Notes

The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-10-2-0047.

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REFERENCES

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