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Effect of Inlet Flow Distributor for Reagent Equalization on Autothermal Reforming of Ethanol in a Microreformer Hongqing Chen, Hao Yu,* Jichao Li, Feng Peng,* and Hongjuan Wang School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Two types of flow distributors, that is, a jet-flow-splitter and a simplified constructal distributor, were designed to equalize the gaseous reagents in the inlet of a microreformer packed with catalyst supported on ceramic foam. The effects of type and geometry of distributors on flow, temperature distributions, and reaction performance over the catalyst during the autothermal reforming of ethanol were investigated experimentally and computationally. It was found that the jet flow splitter in the shape of cone or hemisphere can effectively equalize the flow distribution on the lower surface of catalyst, thereby improving the temperature distribution and performance of the microreformer. A microreformer with a hemisphere jet-flow-splitter in optimal geometry can convert 91% of ethanol with a selectivity to hydrogen of 74%, equivalent to yielding 3.3 mol of hydrogen per mol ethanol. Such a distribution device can be used to fabricate an efficient microreformer with simple structure for the hydrogen production orienting the portable fuel cell application.

1. INTRODUCTION The portable and compact microreformer fed by light alcohols for onboard hydrogen production has recently attracted much attention owing to the growing fuel cell technique and related industries.1−3 Among the alcohols, biomass derived ethanol is nontoxic and renewable and can be blended with gasoline for vehicle fuel. By cofeeding ethanol with appropriately stoichiometric oxygen and steam, hydrogen can be generated by the autothermal reforming of ethanol (ATRE):

structured 50 kW ethanol fuel processor for hydrogen supply, based on a multichannel plate reformer.8 In regard to the construct of the microreformer based on structured catalysts, reactants are required to be distributed uniformly in the inlet of the reactor and the front surface of the catalyst monolith. The maldistribution of reactants may cause hot spots and thereby blockage and degradation of reactor performance.19−25 For the ATRE reaction, the feed and temperature maldistribution, which deeply depends on the former, could reduce the efficiency for hydrogen production and the catalyst stability by generating hot/cold spots. Many efforts have been devoted to optimize the inlet, channel, and outlet geometry to obtain uniform mass flow and temperature distributions by computational and experimental methods. The theory for the manifold problem has recently been reviewed by Wang JY.26 With a three-dimensional structured catalyst, popularly consisting of monolith or foam, the design of the distributor is particularly important, since the geometry and structure of catalyst are usually constrained by the commercial availability. In a recent review paper, Rebrov EV et al.27 summarized various designs for inlet distributors equalizing the single-phase-flow in microreactors. A properly designed inlet of a microreactor can be achieved by a combination of head and perforated distributor or a distributor with bifurcation/ multiscale network/tree-like structure.23,28−32 The latter provides minimal global resistance and uniform flow distribution. However, it is challenging to manufacture the complex bifurcation structure with multiple generations of split channels or tubes, especially in a compact milli-/microreactor. In the former case, thin-wall or thick-wall screens are usually adopted to form even flow at the front surface of catalyst, which has

C2H5OH + (3 − 2δ)H 2O + δO2 → 2 CO2 + (6 − 2δ)H 2

(δ ≥ 0.61, ΔΗ ≤ 0)

The temperature required for the reaction is sustained by heat released from the reaction, thus no extra heat supply is needed. In addition, the near-equilibrium product of the reaction can be achieved within contact time at the millisecond level over selected catalysts.4 Our previous work demonstrated that the ATRE reformer is able to respond to the feeding rapidly within a start-up time of 1−2 min.5 These features make a compact hydrogen generation system based on the ATRE reaction possible. To this end, structured catalysts, including plates with catalytic microchannels,6−10 monoliths,11−14 and foams,15−17 are desired for configuring the compact microreformer, since they improve heat and mass transfer and reduce pressure drop along the flow direction. Al2O3 monolith-supported Rh-CeO2 catalysts have been reported as an efficient and stable catalyst in the ATRE reaction.4 Our recent works showed that the relatively low-cost Ir/La2O3 catalyst can be used as an effective ATRE catalyst with good resistance to the sintering and coking.18 By coating the Ir/La2O3 on ZrO2 foam support, excellent ATRE performance was achieved in a 700 W scale microreformer.5 Recently, the scientists at the Institute for Mikrotechnik Mainz, Germany, have constructed a micro© 2012 American Chemical Society

Received: Revised: Accepted: Published: 10132

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Figure 1. Schematic diagram of the microreformer with (a) a jet-flow-splitter and (b) a simplifed constructal distributor. Four types of flow splitters were investigated, (A) ball, (B) hemisphere, (C) cone, and (D) inverse cone cap. The lower right inset shows the arrangement of thermowells for measuring temperature distribution of the catalyst.

fabricated by stainless steel. A quartz liner was fixed between reformer and catalytic foam to reduce radial heat loses. A circular electrical heater was placed around the reformer to provide heat during reduction of the catalyst. The reactor was thermally insulated with quartz wool. Two distribution devices were designed and tested. In the first case (Figure 1a), the gasified EtOH−H2O mixture and air were fed and mixed in the inlet of the reformer, and were subsequently introduced to a catalytic foam with 40 mm in diameter and 20 mm in length. A jet-flow-splitter made of stainless steel was fixed at the entrance of reactor to split the gaseous flow at the inlet as highlighted by the circle in Figure 1a. Four types of splitters were designed in this work, ball, hemisphere, cone, and inverse cone cap. They were denoted as ball(R), hemisphere(R), cone(H−r1), and cone cap(H-r2), respectively, where R represents the radius of ball or hemisphere, r1 and r2 represent the radius of cone and inverse cone cap, and H is the height of the cone or cone cap. All the units are in mm. The distribution devices are oppositely facing the inlet of reactant with a distance of 1 mm. Eight Φ3 mm stainless steel thermowells were installed to permit monitoring of the temperature distribution on the lower surface of catalyst. The thermowells were arranged around the outlet collector at a radius of 4, 7, and 10 mm, as shown in the lower right inset of Figure 1. The thermowells were numbered from 0 to 8, as shown in Figure 1. Thermocouples 0−7 were connected to a multiple-channel thermocouple input USB module (Advantech, USB-4718) to monitor the reaction temperature distribution. Thermocouple 8 was for temperature control during the catalyst reduction. The product gas was collected from the tube between thermocouples 7 and 8. We also tested a simplified constructal distributor, as shown in Figure 1b. Two stainless steel conduits were used to introduce an ethanol−steam mixture and air, respectively. The separate introduction of reactants was employed to minimize the early mixing and thereby rapid gas-phase combustion/ oxidation. Both of the conduits were equally branched into four gas lines, and each line was connected to a spray-head with four perpendicular Φ0.5 mm side apertures. The spray-heads were

been employed to improve the performances of microstructured reactors for the oxidation of organic compounds33 and high-throughput screening of catalysts for ammoxidation of ethylene to acetonitrile.34−36 However, the screen structure, as well as the tree-like constructal distributor, may pose a risk of blockage in the narrow slots or holes or channels as applied in the microreformer for the ATRE reaction, because this reaction is easy to coke at high temperatures up to 800 °C. In addition, it is undesirable that the large wall surface may induce noncatalytic decomposition and oxidation reactions in the distributor section. Actually, we have found that even bare oxide supports will contribute to the conversion of ethanol under ATRE conditions.37 These drawbacks require a special design for the reactant equalizer at the inlet of the ATRE microreformer. Hitherto, few papers concentrated on the experimental investigation to the effect and optimization of feed distribution in an ATRE microreformer for hydrogen production. We consider that a microreformer should be designed and optimized to guarantee (i) compactness, (ii) low overall pressure drop; (iii) low risk of blockage caused by coking. In this paper, we present the effort to fabricate an ATRE microreformer equipped with a flow equalizer with a simple structure, such as hemisphere, cone, etc., which was designed as a jet-flow-splitter in the flow direction of reactant gas to disturb the penetration. Ceramic ZrO2 foam-supported Ir/La2O3 was used as structured catalyst. The effect of distribution device on flow equalization in ATRE reaction was investigated and optimized by experimental approach. The improved ATRE performance was achieved with a hemisphere internal (radius = 7 mm) due to the relatively uniform distribution of feed in the reactor. A microreformer that can power a fuel cell on 1100 W scale was fabricated with the optimal distribution device.

2. EXPERIMENTAL SECTION 2.1. Fabrication of Microreformer. A homemade microreformer, as shown in Figure 1, was used to evaluate the effect of flow equalization on ATRE reaction. The reformer was 10133

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In this work, we employed the temperature distribution to indicate the reactant distribution, because (i) the reaction is exothermic, and the reaction temperature is sustained by the heat released, and (ii) the ATRE reaction is very fast (usually within a contact time of ∼10 ms),4 thus the diffusion rate of heat and mass is relatively slow. Figure 2 shows a typical reactor

positioned on a ring-shaped plane (i.d. = 20 mm, o.d.= 36 mm) with 45° angle with the neighbor. 2.2. Catalyst Preparation. A washcoating method was used to prepare the Ir−La/ZrO2 ceramic foam structured catalyst, as described in detail elsewhere.5 Ceramic foams (Feite material Co., Ltd. Foshan) with 25 pores per inch were employed. Prior to coating, the foam was cleaned by distilled water and acetone to remove salts and organic contaminants. Then, the clean foam was immersed into a La(NO3)3 aqueous solution and dried at 110 °C overnight. The resulting foam was calcined at 500 °C for 2 h to decompose the nitrate species. The loading of lanthanum was selected at 8 wt %. The La2O3coated foam was then impregnated in a 5 wt % aqueous solution of iridium chloride, followed by drying and calcining at 500 °C for 2 h. The loading of Ir was 4.8 wt %. The resulting catalyst was denoted as Ir(4.8)−La(8)/ZrO2. 2.3. Catalytic Reaction. In a typical ATRE reaction, the catalyst was activated by hydrogen of 50 mL/min at 500 °C for 80 min prior to the ATRE reaction. A mixture of ethanol and water was then fed into a preheater at 250 °C by a syringe pump at a flow rate of 5−20 mL/min. Air was introduced at room temperature. The molar ratio of ethanol/oxygen/water was 1:0.83:2. The gas hourly space velocity (GHSV) was in the range of (0.3−1.5) × 105 h−1. The reformate was analyzed by an Agilent 6820 GC equipped with Haysep (for the analysis of CO2, C2H4, ethanol, acetaldehyde, water, and acetone), 13× molecular sieve (for H2, N2, CO, CH4,) packed columns and a thermal conductivity detector (TCD). N2 was used as the inert standard to calculate the mass balances. Because Ar was used as a carrier gas in GC for accurately measuring hydrogen, the small difference between the thermoconductivities of Ar, N2, and carboncontaining products may result in a relatively lower accuracy for these products. The carbon balances of GC analysis were within 100 ± 8%. The ethanol conversion and product selectivities were calculated by Xethanol =

SH 2 =

Si =

Figure 2. Temperatures at thermowells 0−7 in a course from reaction startup to shutdown of the ATRE reaction. The inset shows the magnified temperature profiles. Conditions: cone(4-3) distributor, GHSV = 6.0 × 104 1/h.

temperature evolution at the lower surface of a reduced catalyst in the course from start-up to shutdown of ATRE. The reactor was started up at about 360 °C. Once the reactant was introduced, the temperatures dropped due to the cooling effect of the reagents. After about 20 s, the exothermic oxidative reaction elevated the temperatures to 480−540 °C within about 50 s, meanwhile the hydrogen-rich gas was produced. Subsequently, the temperatures ascended slowly, because longer time was needed to achieve thermal-equilibrium due to the low thermoconductivity of the ceramic. The steady local temperatures varied in a range from 480 to 540 °C with a descending order of the thermowells, 1 > 7 > 4 > 0 > 3 > 5 > 2 > 6, since the different flow rate of reactant resulted in different reaction heat released. The higher temperature indicated higher flow rate, producing higher volumetric heat flux. The linkage between temperature and flow rate can also explain the cooling behavior during the introduction of feed as shown in the inset of Figure 2. The temperature minimums and the time to reach them varied with the thrmowell positions in an inverse ascending order, 1 < 7 < 4 < 0 < 3 < 5 < 2 < 6, since the higher flow rate would cool down the catalyst faster. The above results suggested that the flow distribution during the ATRE reaction can be indicated by local temperature distribution. At present, although it is difficult to establish the quantitative relationship between the temperature and reagent distributions, it is rational to conclude that a uniform temperature distribution can be a reflection of uniform flow distribution. 3.2. Effect of Distributor Geometry on Reagent Distribution. The single-phase flow entering the reactor from the inlet leads to a jet flow penetrating the catalyst, which results in short contact time in the center of catalyst and low reactant concentration in the majority of the catalyst. To overcome this, we introduced an internal device in the opposite direction of flow to split the jet flow. Prior to the experimental study, a CFD analysis was conducted to assist the design of the distribution device geometry. Commercial computational fluid dynamics (CFD) code Fluent 6.0 was used to simulate the fluid

Fethanol,in − Fethanol,out Fethanol,in FH2out

(6 − 2δ)Fethanol,inXethanol

mFi,out 2Fethanol,inXethanol

where F is the normal flow rate, and m is the number of carbon atoms in a product molecular. δ is the stoichiometric number of O2 in ATRE reaction. We performed all the experiments with one catalyst sample to exclude the possible differences in ceramic foam structure and metal loading. Our previous work5 showed that the catalyst is stable either in steady operation or dynamic start-up and shut-down operation in a microreformer. To ensure the reproducibility, after a set of experiments with a distributor design, the catalyst sample was tested again under the reaction condition first investigated (e.g., GHSV). The examination gave reproducible results, indicating neligible catalyst deactivation.

3. RESULTS AND DISCUSSION 3.1. In-Situ Monitoring Flow Distribution via Temperature Distribution. During the ATRE reaction, the flow rate distribution is difficult to measure in a compact microreformer. 10134

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flow distribution along a microreformer equipped with the distribution devices as shown in Figure 1a (see Supporting Information for details). We computed the flow fields in the model illustrated in Figure S1. The gas velocity profile at the outlet of the model was characterized by a flow nonuniformity factor δv defined as δν(%) =

100 v̅

The steady temperature distributions of the lower surface of catalyst foam of the reformer equipped with a cone(4-3) distributor are presented in Figure 3 as a function of GHSV. The temperature values were taken by averaging a time series of temperatures for 3 min in the steady state. Two hypotheses were adopted to plot the two-dimensional pseudocolor temperature distribution: (i) the temperature distribution was centrosymmetric about the center of the catalytic foam; and (ii) the reformer wall temperature was equal to the lowest temperature on the catalytic foam periphery. As shown in Figure 3, the central temperature increased with space velocity from 516 to 561 °C, with an exception at 9.0 × 104 1/h. Owing to the obstruction of the distributor, the temperature at the catalyst center was not the highest one. The highest temperature was detected at a radius of about 9 mm. The maximal temperature difference measured was in the range from 45 °C at 4.8 × 104 1/h to 57 °C at 1.2 × 105 1/h. It is interesting that the temperature distributions are rotationally asymmetric, which may be caused by manufacture tolerance. Another reason responsible for the asymmetry may be the way the splitters were installed. They were fixed by welding them with two 1 mm stainless steel wires to prevent suspension. Such an installation violates strictly cylindrical symmetry of the inlet structure, partially contributing to the asymmetry. At a GHSV of 1.2 × 105 1/h, two relatively high temperature regions located at a radius of about 9 mm indicated the strong flow segregation caused by the splitting effect of the distribution cone. Previous analysis from CFD indicated that hemisphere 7 has lower flow nonuniformity factor compared with cone(4-3). The effect of hemisphere(7) on the reagent equalization at different GHSVs is presented in Figure 4. The uniformity of temperature distribution was significantly improved by applying the hemisphere distribution device. The maximal temperature differences were in the range from 16 °C at 4.8 × 104 1/h to 21 °C at 1.2 × 105 1/h, only 1/4−1/3 of those in the case of the cone(4-3) distributor. At GHSVs of (4.8−6.0) × 104 1/h, the highest temperature was reached at the center of catalyst and a symmetric distribution can be achieved. However, further increasing space velocity resulted in hot spots near the wall (at r = ∼18 mm, θ = 60 or 240°) and an asymmetric overall distribution pattern, as shown in Figure 4c,d. It is indicative of the undesired flow segregation caused by the splitting effect of the distributor. 3.3. Effect of Simplified Constructal Distributor on Temperature Distribution. The effects of cone(4-3) and hemisphere(7) distributors were compared with a simplified constructal distributor with two generations, as shown in Figure 1b. In this design, air and the mixture of EtOH and steam were separately introduced into the first splitting generation to avoid their combustion in conduit and resultant risk of blockage by soot. Remarkable temperature nonuniformity across the surface of catalytic foam was observed with the simplified constructal distributor. A maximum temperature difference of 161 °C (from 439 to 600 °C) was observed at a GHSV of 6.0 × 104 1/ h. As the GHSV increased to 9.0 × 104 1/h, the local temperatures on the catalyst surface were elevated by 10−30 °C, while the maximum temperature difference increased to 169 °C, suggesting the negative effect of high space velocity on the uniformity of reagent distribution. As the constructal distributor employed, the temperature profile exhibited two high temperature regions with radii between 5 and 12 mm (see Supporting Information, Figure S2), implying that the ATRE

n

1 ∑ (vi − v ̅ )2 n − 1 i=1

where v ̅ is the average velocity in the outlet, vi is the mesh unit velocity on the outlet surface, and the number of mesh (n) is 476. The computation was carried out at Reynolds number of 762.6. As shown in Table 1, all the flow splitters significantly decrease the flow nonuniformity compared with that without Table 1. Effect of Size and Geometry of Distribution Device on the Computed Flow Nonuniformitya distribution device

δv (%)

without distributor hemisphere(3) hemisphere(5) hemisphere(7) hemisphere(8) hemisphere(9) ball(3) cone(4-2) cone(4-3) cone(4-4) cone(4-5) inverse cone cap(4-3)

49.55 14.77 11.39 9.27 7.55 5.54 17.48 15.42 12.47 8.18 5.15 6.68

notes

segregated flow near the wall segregated flow near the wall

segregated flow near the wall segregated flow near the wall segregated flow near the wall

a Steam under 600 K was selected as the working fluid. The inlet gas velocity was 10 m/s, correspondig to Re = 762.6.

any distribution device. Taking the radius of ball, hemisphere, cone, and inverse cone cap as characteristic size, which determines the frontal area of the internal device, we compared the effect of shape on the distribution at the identical characteristic size of 3 mm. The inverse cone cap(4-3) has the lowest δv at Re of 762.6, indicating the highest uniformity of gas velocity. However, serious flow segregation near the wall was observed with the inverse cone cap(4-3). The high flux of reactant near the wall will result in high flux of heat loss from the reactor walls, which is detrimental to the ATRE reaction because of the importance of heat insulation on the reaction performance.38,39 The lower δv of 12.5% was also obtained with cone(4-3) distributor, showing that it will perform well in equalizing reagent. The nonuniformity reduced with increasing cone radius considerably and achieved 5.2% with the cone(4-5). Taking into account the flow segregation, the computation suggested that the cone radius should not exceed 3 mm. Similar analysis was carried out for ball and hemisphere type distributors. The nonunifomity factor δv indicates that the most uniform gas velocity distribution without significant flow segaragation will be achieved with the hemisphere(7) distributor. According to the computational results, we fabricated two microreformers equipped with cone(4-3) and hemisphere(7), respectively, for the experimental investigation of the flow distribution in the ATRE reaction. 10135

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Figure 3. Steady temperature distributions on the lower surface of catalyst of the reformer equipped with a cone(4-3) distributor. GHSV: (a) 4.8 × 104 1/h; (b) 6.0 × 104 1/h; (c) 9.0 × 104 1/h; (d) 1.2 × 105 1/h.

Figure 4. Steady temperature distributions on the lower surface of catalyst of the reformer equipped with a hemisphere(7) distributor. GHSV: (a) 4.8 × 104 1/h; (b) 6.0 × 104 1/h; (c) 9.0 × 104 1/h; (d) 1.2 × 105 1/h.

10136

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Figure 5. Effect of the type of distributors on the ATRE performance of the microreformer: (a) without distributor, with (b) cone(4-3), (c) hemisphere(7), and (d) simplified constructal distributor as shown in Figure 2. Legends: (■) conversion, (□) H2, (●) CO, (▲) CO2, (○) CH4, (Δ) C2H4O.

and high flow uniformity, allowing for a robust and compact ATRE microreformer. 3.4. Effect of Reagent Distribution on ATRE Perfomances. The ATRE performance of the microreformer without distributor is shown in Figure 5a as a base case. The ATRE reaction in the microreformer produced H2, CO2, CO, and CH4 as main gaseous products. At GHSV of 3.0 × 104 1/h, the microreformer converted more than 90% of ethanol fed with a hydrogen selectivity of 62%. However, the conversion reduced with space velocity. As the GHSV was elevated to 1.2 × 105 1/h, only about 80% ethanol conversion was achieved. Meanwhile, considerable acetaldehyde was produced, indicating the insufficient reforming reactivity for acetaldehyde reforming and carbon−carbon bond cleavage caused by short contact time with the catalyst. The selectivities of hydrogen and CO increased with GHSV, while the CH4 selectivity decreased with GHSV. This may be caused by the increased reaction temperature with space velocity under autothermal operation, which suppressed the exothermic water−gas shift (WGS) reaction and favored the reforming of CH4. Figure 5b shows the ATRE performance of a microreformer with cone(4-3) distributor as a jet flow splitter. About 100% conversion of ethanol, which was much higher than that in the case without distributor, was obtained with a selectivity to hydrogen of 64% at 4.8 × 104 1/h. This result confirmed that the proper flow equalization can effectively improve the reactivity, since all the reaction conditions remained the same except for the installation of the distributor. As the GHSV rose, the ethanol conversion gradually decreased to 85% at 1.2 × 105 1/h, suggesting that the flow segregation at high space velocity as shown in Figure 3 should be avoided. Even more superior performance was achieved in the microreformer with the hemisphere(7) distributor, in which the conversion of ethanol was almost unchanged (89−93%) with GHSV, as shown in Figure 5c. Meanwhile, the hydrogen selectivity achieved 70− 74% in the range of the GHSV investigated. This can be attributed to the excellent reagent distribution of the hemisphere(7) distributor, therefore the catalyst efficiency was improved. With the design of cone(4-3) and hemisphere(7), the selectivity to hydrogen slightly increased with GHSV, due to the increase of reactor temperature with GHSV. The

reaction mainly took place there. Combined with the fact of a large temperature difference, it can be expected that the overall conversion efficiency of the reformer with the constructal distributor will be low. We defined a temperature nonuniformity factor δT to evaluate the temperature distribution on the lower catalyst surface: δT(%) =

100 T

n

1 ∑ (Ti − T̅ )2 n − 1 i=1

where T̅ is the average temperature obtained by T̅ = 1/n × Σni = 1Ti; the number of measuring points n is 8. δT shows the deviation of the temperatures from the average one. The lower δT means more homogeneous temperature distribution, thereby more uniform flow distribution. The δT values varied markedly with the type of distribution device, while changed little with GHSV (see Figure S3). Taking the case at GHSV of 9.0 × 104 1/h as an example, the δT values of constructal distributor, cone(4-3) and hemisphere distributor were 11.7%, 3.2% and 0.7%, respectively, indicating the significant improvement of temperature and flow distributions by properly designed flow distribution device. The lowest δT was obtained on hemisphere 7 distributor, which is either consistent with the analysis of maximum temperature difference, or the previous computational results in Table 1. The δT value slightly increased to 1.05% at higher GHSV of 1.2 × 105 1/h, indicating that the flow segregation, as illustrated in Figure 4d, is unfavorable to the flow equalization. It is unexceptional that the worst equalization performance was observed with the simplified constructal distributor. Although more generations of tube splitting may increase the flow uniformity, it will occupy more space at the expense of compactness. Another possible reason responsible for the low performance is the separate introduction of air and EtOH−steam mixture, which hinders the intermixing of reagents. However, such a design helped to prevent the combustion reaction and the formation of soot in the inlet conduit. To summarize the performances of the distribution devices used in this work, the hemisphere(7) and cone(4-3) distributors are superior to the constructal distributor, because of their advantages on simple structure 10137

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Technology Project (No. 2010B050200003), and the Fundamental Research Funds for the Central Universities of China (No. 2009zm0246, 2012ZZ0039).

microformer with hemisphere(7) can convert 91% of ethanol with a selectivity to hydrogen of 74% at 1.2 × 105 1/h, equivalent to yielding 3.3 mol of hydrogen per mol ethanol, with a hydrogen flow rate of ∼0.58 m3/h. As predicted in Section 3.3, the ATRE performance of microreformer with the simplified constructal distributor was much lower than those in the cases of cone(4-3) and hemisphere(7), because of the serious flow and temperature nonuniformity. As shown in Figure 5d, very low ethanol conversion and hydrogen selectivity were obtained in the GHSV range employed here. At 1.2 × 105 1/h, the reaction converted 73% of ethanol with a H2 selectivity of 64%, even lower than that without a distributor (81% and 66%, respectively), indicating the negative effect of flow nonuniformity caused by the annular flow. Combined with the results of flow uniformity, it can be concluded that the reactor performance is strongly linked with the flow uniformity across the catalyst, and the distribution device providing homogeneous flow distribution can be one of the most important design factors of a microreformer for ATRE. By comparing the case studies in this work, a properly designed distributor may contribute more than 10% additional yield of hydrogen. The results presented here indicate that the hemisphere(7) distributor may be used to fabricate an efficient microreformer with simple structure for the hydrogen production orientating the portable fuel cell application.

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(1) Holladay, J. D.; Hu, J.; King, D. L.; Wang, Y. An overview of hydrogen production technologies. Catal. Today 2009, 139 (4), 244− 260. (2) Holladay, J. D.; Wang, Y.; Jones, E. Review of developments in portable hydrogen production using microreactor technology. Chem. Rev. 2004, 104 (10), 4767−4789. (3) Mitsos, A.; Chachuat, B.; Barton, P. I. Methodology for the design of man-portable power generation devices. Ind. Eng. Chem. Res. 2007, 46 (22), 7164−7176. (4) Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E. Renewable hydrogen from ethanol by autothermal reforming. Science 2004, 303 (5660), 993−997. (5) Chen, H. Q.; Yu, H.; Yang, G. X.; Peng, F.; Wang, H. J.; Yang, J. Auto-thermal ethanol micro-reformer with a structural Ir/La2O3/ZrO2 catalyst for hydrogen production. Chem. Eng. J. 2011, 167 (1), 322− 327. (6) Wanat, E. C.; Venkataraman, K.; Schmidt, L. D. Steam reforming and water-gas shift of ethanol on Rh and Rh−Ce catalysts in a catalytic wall reactor. Appl. Catal. a 2004, 276 (1−2), 155−162. (7) Casanovas, A.; Saint-Gerons, M.; Griffon, F.; Llorca, J. Autothermal generation of hydrogen from ethanol in a microreactor. Int. J. Hydrogen. Energy. 2008, 33 (7), 1827−1833. (8) Kolb, G.; Men, Y.; Schelhaas, K. P.; Tiemann, D.; Zapf, R.; Wilhelm, J. Development Work on a Microstructured 50 kW Ethanol Fuel Processor for a Small-Scale Stationary Hydrogen Supply System. Ind. Eng. Chem. Res. 2011, 50 (5), 2554−2561. (9) Kolb, G.; Baier, T.; Schurer, J.; Tiemann, D.; Ziogas, A.; Ehwald, H.; Alphonse, P. A micro-structured 5 kW complete fuel processor for iso-octane as hydrogen supply system for mobile auxiliary power units - Part I. Development of autothermal reforming catalyst and reactor. Chem. Eng. J. 2008, 137 (3), 653−663. (10) Kolb, G.; Baier, T.; Schurer, J.; Tiemann, D.; Ziogas, A.; Specchia, S.; Galletti, C.; Germani, G.; Schuurman, Y. A microstructured 5 kW complete fuel processor for iso-octane as hydrogen supply system for mobile auxiliary power unitsPart II. Development of water−gas shift and preferential oxidation catalysts reactors and assembly of the fuel processor. Chem. Eng. J. 2008, 138 (1−3), 474− 489. (11) Dominguez, M.; Taboada, E.; Molins, E.; Llorca, J. Co−SiO2 aerogel-coated catalytic walls for the generation of hydrogen. Catal. Today 2008, 138 (3−4), 193−197. (12) Casanovas, A.; Dominguez, M.; Ledesma, C.; Lopez, E.; Llorca, J. Catalytic walls and micro-devices for generating hydrogen by low temperature steam reforming of ethanol. Catal. Today 2009, 143 (1− 2), 32−37. (13) Casanovas, A.; de Leitenburg, C.; Trovarelli, A.; Llorca, J. Catalytic monoliths for ethanol steam reforming. Catal. Today 2008, 138 (3−4), 187−192. (14) Llorca, J.; Casanovas, A.; Trifonov, T.; Rodriguez, A.; Alcubilla, R. First use of macroporous silicon loaded with catalyst film for a chemical reaction: A microreformer for producing hydrogen from ethanol steam reforming. J. Catal. 2008, 255 (2), 228−233. (15) Dauenhauer, P. J.; Dreyer, B. J.; Degenstein, N. J.; Schmidt, L. D. Millisecond reforming of solid biomass for sustainable fuels. Angew. Chem. Int. Ed. 2007, 46 (31), 5864−5867. (16) Horn, R.; Williams, K. A.; Degenstein, N. J.; Bitsch-Larsen, A.; Nogare, D. D.; Tupy, S. A.; Schmidt, L. D. Methane catalytic partial oxidation on autothermal Rh and Pt foam catalysts: Oxidation and reforming zones, transport effects, and approach to thermodynamic equilibrium. J. Catal. 2007, 249 (2), 380−393. (17) Williams, K. A.; Horn, R.; Schmidt, L. D. Performance of mechanisms and reactor models for methane oxidation on Rh. Aiche J 2007, 53 (8), 2097−2113.

4. CONCLUSIONS In summary, the effectiveness of flow equalization on the performance of ATRE microreformer was experimentally demonstrated in a reformer system packed with Ir/La2O3 catalyst supported on ZrO2 ceramic foam. The flow equalization was realized by a properly designed flow splitter installed at the inlet of reactor. The geometry of the flow splitter was optimized with computational fluid dynamics. Among the shapes investigated, hemisphere displayed the best ATRE performance, since it provided the most homogeneous flow and temperature distributions. We also investigated the simplified constructal distributor. Compared with flow splitter, the constructal distributor performed worse due to its serious flow nonuniformity. Thus, the flow splitter type distributor is superior to the constructal design in this study. The design presented in this paper also takes the advantages of simple structure, low pressure drop and easy maintenance.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures as described in the tex. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86 20 8711 4916. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program of China (863 Program, No.2009AA05Z102), the Natural Science Foundation of China (No. 20176094), the Guangdong Provincial Science and 10138

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(18) Chen, H. Q.; Yu, H.; Peng, F.; Wang, H. J.; Yang, J.; Pan, M. Q. Efficient and stable oxidative steam reforming of ethanol for hydrogen production: Effect of in situ dispersion of Ir over Ir/La2O3. J. Catal. 2010, 269 (2), 281−290. (19) Kim, Y. J.; Joshi, Y. K.; Fedorov, A. G.; Lee, Y. J.; Lim, S. K., Thermal Characterization of Interlayer Microfluidic Cooling of ThreeDimensional Integrated Circuits with Nonuniform Heat Flux. J Heat Trans.-T ASME 2010, 132 (4). (20) Heibel, A. K.; Scheenen, T. W. J.; Heiszwolf, J. J.; Van As, H.; Kapteijn, F.; Moulijn, J. A. Gas and liquid phase distribution and their effect on reactor performance in the monolith film flow reactor. Chem. Eng. Sci. 2001, 56 (21−22), 5935−5944. (21) Heibel, A. K.; Vergeldt, F. J.; van As, H. Gas and liquid distribution in the monolith film flow reactor. AIChE J. 2003, 49 (12), 3007−3017. (22) Delsman, E. R.; de Croon, M. H. J. M.; Elzinga, G. D.; Cobden, P. D.; Kramer, G. J.; Schouten, J. C. The influence of differences between microchannels on microreactor performance. Chem. Eng. Technol. 2005, 28 (3), 367−375. (23) Saber, M.; Commenge, J. M.; Falk, L. Microreactor numberingup in multi-scale networks for industrial-scale applications: Impact of flow maldistribution on the reactor performances. Chem. Eng. Sci. 2010, 65 (1), 372−379. (24) Regatte, V. R.; Kaisare, N. S. Propane combustion in nonadiabatic microreactors: 2. Flow configuration in posted microreactors. Chem. Eng. Sci. 2011, 66 (17), 3732−3741. (25) Arzamendi, G.; Dieguez, P. M.; Montes, M.; Centeno, M. A.; Odriozola, J. A.; Gandia, L. M. Integration of methanol steam reforming and combustion in a microchannel reactor for H2 production: A CFD simulation study. Catal. Today 2009, 143 (1− 2), 25−31. (26) Wang, J. Y. Theory of flow distribution in manifolds. Chem. Eng. J. 2011, 168 (3), 1331−1345. (27) Rebrov, E. V.; Schouten, J. C.; de Croon, M. H. J. M. Singlephase fluid flow distribution and heat transfer in microstructured reactors. Chem. Eng. Sci. 2011, 66 (7), 1374−1393. (28) Rebrov, E. V.; Ismagilov, R. Z.; Ekatpure, R. P.; de Croon, M.; Schouten, J. C. Header design for flow equalization in microstructured reactors. AIChE J. 2007, 53 (1), 28−38. (29) Fan, Z. W.; Zhou, X. G.; Luo, L. G.; Yuan, W. K. Experimental investigation of the flow distribution of a 2-dimensional constructal distributor. Exp. Therm. Fluid Sci. 2008, 33 (1), 77−83. (30) Yue, J.; Boichot, R.; Luo, L. G.; Gonthier, Y.; Chen, G. W.; Yuan, Q. Flow Distribution and Mass Transfer in a Parallel Microchannel Contactor Integrated with Constructal Distributors. AIChE J. 2010, 56 (2), 298−317. (31) 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 (1−3), 379−390. (32) Saber, M.; Commenge, J. M.; Falk, L. Rapid design of channel multi-scale networks with minimum flow maldistribution. Chem. Eng. Process. 2009, 48 (3), 723−733. (33) Ismagilov, I. Z.; Michurin, E. M.; Sukhova, O. B.; Tsykoza, L. T.; Matus, E.; Kerzhentsev, M. A.; Ismagilov, Z. R.; Zagoruiko, A. N.; Rebrov, E. V.; De Croon, M.; Schouten, J. C. Oxidation of organic compounds in a microstructured catalytic reactor. Chem. Eng. J. 2008, 135, S57−S65. (34) Mies, M. J. M.; Rebrov, E.; Schiepers, C.; de Croon, M.; Schouten, J. C. High-throughput screening of Co-BEA and Co-ZSM-5 coatings in the ammoxidation of ethylene to acetonitrile in a microstructured reactor. Chem. Eng. Sci. 2007, 62 (18−20), 5097− 5101. (35) Mies, M. J. M.; Rebrov, E. V.; de Croon, M. H. J. M.; Schouten, J. C. Design of a molybdenum high throughput microreactor for high temperature screening of catalytic coatings. Chem. Eng. J. 2004, 101 (1−3), 225−235. (36) Mies, M. J. M.; Rebrov, E. V.; Deutz, L.; Kleijn, C. R.; de Croon, M.; Schouten, J. C. Experimental validation of the performance of a

microreactor for the high-throughput screening of catalytic coatings. Ind. Eng. Chem. Res. 2007, 46 (12), 3922−3931. (37) Chen, H. Q.; Yu, H.; Tang, Y.; Pan, M. Q.; Yang, G. X.; Peng, F.; Wang, H. J.; Yang, J. Hydrogen production via autothermal reforming of ethanol over noble metal catalysts supported on oxides. J. Nat. Gas Chem. 2009, 18 (2), 191−198. (38) Moreno, A. M.; Wilhite, B. A. Autothermal hydrogen generation from methanol in a ceramic microchannel network. J. Power Sources 2010, 195 (7), 1964−1970. (39) Park, H. G.; Malen, J. A.; Piggott, W. T.; Morse, J. D.; Greif, R.; Grigoropoulos, C. P.; Havstad, M. A.; Upadhye, R. Methanol steam reformer on a silicon wafer. J. Microelectromech. Syst. 2006, 15 (4), 976−985.

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