Design and testing of a microchannel reactor for the PROX reaction

Design and testing of a microchannel reactor for the PROX reaction S. Cruz1, O. Sanz1, R. Poyato1, O.H. Laguna1*, F. J. Echave2, L.C. Almeida2, M.A. Centeno1, G. Arzamendi3, L.M. Gandia3, E.F. Souza-Aguiar4, M. Montes2, J.A. Odriozola1 1 Inst. of Material Sciences of Seville, CSIC – Universidad de Sevilla. Avda. Américo Vespuccio 49, 41092 Seville, Spain 2 Dept. Appl. Chemistry, Universitiy of the Basque Country, Paseo Manuel Lardizábal 3, 20018, San Sebastián, Spain 3Public University of Navarre, Campus de Arrosadía, s/n, E-31006 Pamplona, Spain 4Petrobras, Rio de Janeiro, Brazil *Corresponding author: Oscar Hernando Laguna Espitia e-mail address: [email protected] Telephone: +34 954489501 Ext. 9221 Fax: +34 954460665 Abstract The different steps for manufacturing a microchannel reactor for the PROX reaction are discussed. Transient Liquid Phase bonding (TLP) using a Ni-B-Si amorphous melt spun is used for joining micromilled Al-alloyed ferritic stainless steel plates followed by recrystallization at 1200ºC for 5 hours. A CuOx-CeO2 catalyst synthesized by the coprecipitation method was washcoated on the microchannel block resulting in a homogenous 20-30 µm thick layer. The catalytic activity for CO-PROX reaction is similar in both the powder catalyst and the microchannel coated reactor but the selectivity is higher in the microchannel reactor. Keywords 1) Microchannel reactor 2) CO-PROX 3) CuOx-CeO2 4) Transient Liquid Phase bonding 5) Al-alloyed ferritic stainless steel 1. Introduction The widespread use of portable electric and electronic devices increases the need for efficient autonomous power supplies that replace the currently predominant battery technology. The use of common fuels/chemicals (hydrocarbons or alcohols) as energy sources is a promising alternative when combined with the recent developments in microchannel reactor technology. The high power density, rapid start-up time, and low-temperature operation of PEMFC make these devices as the most promising for powering up electric or electronic devices. However, an essential requirement for the reformate-fed PEMFC is the removal of CO from the H2 stream after the reforming and water– gas shift (WGS) reactions. After the WGS units the CO content of the hydrogen streams may vary between ca. 0.5 and 2 vol.% being, therefore, mandatory clean-up units for the removal of CO to trace level. Preferential CO oxidation (CO-PROX) is among the preferred technologies for small-scale fuel processor applications [1]. CO-PROX reactions allow reducing CO contents from 1 % in the feed to the ppm level. When using PEMFCs for portable or automotive applications packed-bed reactors have several drawbacks such as pressure drop within catalyst layer, temperature gradients, and hot spot due to the high exothermicity of the oxidations of CO and H2. Microreactors have the advantages of fast response time, easy integration, and small footprint, which are ideal for portable power systems. In addition, enhanced mass and heat transport properties are also widely recognized as advantages of microreactors [2-6]. In addition to this, recent studies of simulated CO-PROX reactions have shown that the reverse water-gas-shift side reaction is favoured in the case of packed bed reactors with respect to thin catalytic films deposited on microchannel walls [4]. Therefore, CO-PROX units based on microchannel reactors have been reported as part of integrated fuel processors in a wide power range [6-9]. Different catalytic systems and reactor designs have been proposed for the COPROX reaction. In a recent study Bion et al. [10] review the performances of noble metals (including Pt, Ru, Rh, Pd and Au) and transition metal oxides catalysts and compare the advantages and drawbacks for each type of catalysts in terms of activity and selectivity including the influence of the presence of CO2 and H2O in the reactants flow. They conclude that CuO–CeO2 catalysts are very attractive for industrial applications since their excellent performances, stability and low cost compared to noble metal-based catalysts. Dudfield et al. [11, 12] operated a compact fin heat-exchanger reactor containing 2.5% Pt-Ru catalyst in the CO-PROX reaction. Using a two-stage configuration this unit allowed reductions in the CO concentrations down to 15 ppm. Silicon wafers [13] or austenitic stainless steel plates [14, 15] were used for building microchannel reactors for the PROX reaction. Hwang et al. [16] studied the reaction over a silicon-based microreactor coated with a Pt/Al2O3 catalyst, yielding 99.4% CO conversion. Kim et al. [15, 17] obtained CO outlet concentration below 50 ppm at O2/CO ratio of 2.5 by using a 13-layered microchannel reactor built by stacking chemically etched stainless steel plates coated with a Pt–Co/Al2O3 catalyst. Despite most of the work on CO-PROX in microreactors has been carried out using noble metal based catalysts a few insights on copper based catalysts have been reported. Snytnikov et al. [18] compared a 5 wt.% Cu/CeO2 catalyst in both fixed bed and microchannel reactors with the latter exhibiting higher conversion and selectivity. This catalyst allowed the reduction of the CO concentration from 1 vol.% to 10 ppm. Kim et al. [15] compared a CuO/CeO2 coated microreactor with a Pt-Co coated microreactor showing better selectivities in the CO-PROX reaction for the copper-based catalyst. In the present work, we provide a description of the different steps required for manufacturing a microchannel reactor for the CO-PROX reaction. The reactor is tested using conventional CuOx/CeO2 catalysts and the results compared with those obtained for powdered catalysts. 2. Materials and methods 2.1. Catalyst preparation The CuOx-CeO2 catalyst was prepared by the coprecipitation method. The amount necessary for preparing a 0,5 M solution of Cu(NO3)2·3H2O and Ce(NO3)3·6H2O were mixed, under vigorous stirring, to get a 9:1 Ce(OH)3:Cu(OH)2 weight ratio. After homogenizing the system, a NaOH solution (2 M) was added dropwise until a stable pH of 9. The precipitate was filtered and washed with distilled water in order to remove the Na+ and NO3ions. Finally, the obtained solid was dried overnight at 60°C and finally, calcined 2h at 300 ºC. 2.2. Catalysts characterization BET specific surface areas were measured by nitrogen adsorption at liquid nitrogen temperature in a Micromeritics ASAP 2000 apparatus. Before analysis, the samples were degassed 2 h at 150ºC in vacuum. The cerium and copper contents of the samples were determined by X-ray fluorescence spectrometry (XRF) in a Panalytical AXIOS PW4400 sequential spectrophotometer with a rhodium tube as the source of radiation. X-ray diffraction (XRD) analysis was performed on a Siemens D 500 diffractometer. Diffraction patterns were recorded with Cu Ka radiation (40 mA, 40 kV) over a 10-80º 2θ range using a position-sensitive detector with a step size of 0.01º and a step time of 7 s. The Raman spectra were recorded in a dispersive Horiva Jobin Yvon LabRam HR800 microscope, with a 20 mWHe-Ne green laser (532,14 nm), without filter, and with a 600 g·mm-1 grating. The microscope used a 50x objective and a confocal pinhole of 100 μm. The Raman spectrometer is calibrated using a silicon wafer. The reducibility studies were carried out on a pilot plant built by PID Eng&Tech in TPR mode equipped with a VINCI thermal conductivity detector. The experiments were done by a thermo-programmed reduction (TPR) of 50 mg of catalyst, in a H2 flow of 5 % diluted in Ar (total flow = 50 mL/min), using a temperature ramp of 10 °C/min from room temperature to 900 °C. The Zeta Potential was measured by using a MALVERN Zetasizer 2000 instrument. The solids were dispersed in an aqueous solution of 0.003 M NaCl. The pHs of the solutions were adjusted with HNO3 or NaOH solutions. Rheological properties of the slurries were measured in a rotational viscosimeter HAAKE, model VT 500, geometry NV. The adherence of the catalytic layer deposited onto the substrates was evaluated using an ultrasonic technique. The weight loss caused by the exposure of the sample to ultrasound is measured. The structured supports immersed in petroleum ether were submitted to an ultrasonic treatment for 30 min at room temperature. After that, the samples were dried and calcined. The weight loss was determined by the difference in the mass of the samples before and after the ultrasonic test. The results are presented in terms of the retained amount of coating on the monolith, expressed as percentage. 2.3. Catalytic activity measurements The CO-PROX reaction was carried out at atmospheric pressure in a PID Eng&Tech Microactivity set-up, employing a stainless steel tubular reactor with internal diameter of 9 mm and a constant feed stream flow rate of 100 cm3·min-1 (STP). The catalyst (100 mg, particle size in the 100–200 mm range) was diluted with crushed glass particles in the same particle size range forming a bed of about 5 mm in length. The experimental runs were carried out in a flow containing 50 vol.% H2, 2 vol. % CO and 1 vol. % O2 concentrations using N2 as balance. The reaction temperature was increased from 50–60 to 190–250 ºC in steps of 10 ºC. For each step the temperature was stabilized and data were recorded at steady-state conditions. Fresh catalyst was loaded into the reactor after each complete run. Some experiments carried out at the same space-time but at varying gas linear velocities confirmed the absence of external mass transfer effects. On-line analyses of the feed and products streams were performed on an Agilent 7890 gas chromatograph equipped with a Porapak Q, two Molecular Sieve 5A and two Hayesep Q columns and two TCD detectors and a FID detector. The same computerized PID Eng&Tech Microactivity set-up was used to study the reaction in the microchannel reactor, replacing the tubular stainless steel reactor for the built microchannel reactor the same compositions were chosen but 300 cm3·min-1 (STP) feed stream flow rate since 300 mg catalyst was loaded in the microchannel block. To study the effect of CO2 and H2O, a series of experiments keeping constant the H2 content of the feed stream set at 50 vol.%, and both the CO and O2 concentrations fixed at 1 vol.% were carried out. The CO2 and H2O concentrations in the feed were varied within the 2–10 vol.% and 0–20 vol.% ranges, respectively, using N2 as balance. The microactivity reference hot box controlled the reaction temperature. For each step the temperature was stabilized and data were recorded at steady-state conditions. 3. Microchannel reactor The metallic microchannel reactor was manufactured using Al-alloyed ferritic stainless steel (for instance, Fecralloy®), since ferritic alloys containing 3–5% of aluminium produce by thermal treatment an Al2O3 layer that favours the interaction with the catalytic coating [19-22]. The Fecralloy consisted of Cr 22%, Al 4.8%, Si 0.3%, Y 0.3%, and Fe balance [21]. Characterization of the joined steel plates were carried out by optical and electron microscopy. Specimens were extracted from the samples, grinded with SiC paper of #240, #400, #600 and #1200 grain size and mirror polished with 0,3 and 0,1 µm Al2O3 powder. Etching with Vilella’s reagent developed the microstructure. Sample observation in the as polished an etched conditions was done in a Leica-DM-IRM optical microscope equipped with a digital camera (Leica DC300). In order to identify metallic and non-metallic inclusions as well as to determine local compositions, the as-polished samples were also observed by SEM using a JEOL 5400 system equipped with secondary and backscattered electron and x-ray detectors. Microchannels were fabricated by micro-milling 1 mm thick ferritic stainless steel plates. Each plate has 10 square channels of 750 µm separated between them 300 µm machined in a 20x20 mm2 plate. This process results in channels with 56 mm2 geometric surface and 700 µm hydraulic radius. In order to minimize the high pressures and processing times required for solid state bonding the ferritic stainless steel plates were joined together using the transient liquid phase (TLP) bonding process. The TLP bonding process uses interlayers, which either contain melting point depressants (e.g. B, Si or P) or form an eutectic with the parent metal being bonded. The joint is held at the bonding temperature until the melting point depressants are lost from the liquid interlayer by diffusion and the liquid interlayer solidifies isothermally due to the change in composition of the bond. This technique has been previously used to join different alloy systems including duplex stainless steels [23-29]. Figure 1 is a schematic picture of the steel plates-metallic glass assembly used in this work. For TLP bonding, a nickel-based interlayer with a composition of Ni-14B-7Si (wt %, Goodfellow) was used. This interlayer was an amorphous melt spun foil with a thickness of 25 µm. The bonding process was performed in vacuum using a test machine developed by Microtest that allows the control of the applied force, temperature and time allowing rapid heating up to the bonding temperature. Various trials were conducted to establish the optimum bonding parameters, varying the temperature, applied force and time. A bonding temperature of 850°C with an applied force of 2,8 kN (~7 MPa) was selected. Low temperature and light pressure applied is required for keeping the mechanical integrity of the machined plate since the low creep strength of the selected alloy [30]. Figure 2 shows the macroscopic aspects of joined plates at different applied forces clearly shown that for applied pressures above 15 MPa (6,0 kN) and 900ºC creep results in a strong deformation of the machined microchannels. The TLP bonded samples were finally treated at 1200ºC for 5 h. After the joining procedure the two ferritic steel plates are separated by a metallic alloy ca. 25 µm thick whose composition mainly corresponds to that of the Ni-based interlayer. The microstructure of the joined area is characterized by the existence of two ferritic layers in which the precipitation of chromium carbides is evident separated by the Ni-based interlayer. A strong chromium carbide precipitation occurs at the stainless steel-interlayer interfaces, figure 3A. Upon post-processing at 1200ºC a single phase recrystallizes. EDX analysis across the joined area show that the Fe, Cr, Al and Al line profiles are almost flat indicating interdiffusion of the Ni-based interlayer and the ferritic steel, except for some chromium maximum and iron minimum corresponding to the presence of chromium carbide precipitates. Using nickel as a marker, it should be noted that this element is absent in the ferritic steel this diffusion is clearly seen. Figure 3D shows the Gaussian-fitted Ni line profile across the joined area, the FWHM of the Gaussian distribution, ca. 400 µm, is considerable higher than the thickness of the used interlayer demonstrating the formation of a single phase upon the designed joining procedure. Micromachining and joining of the steel plates resulted in a microblock that was housed in Al-alloyed ferritic steels cases designed using CFD algorithms for ensuring homogeneous flow through all the channels, the housing was join together using graphite seals, figure 4. Finally, the microchannel block was fitted with thermocouples monitoring inlet and outlet temperatures as well as temperature gradients within the microblock. Manifolding and instrumentation of the microreactor was implemented in a computerized Microactivity Reference Catalytic Reactor from PID Eng&Tech that controls the reaction temperature in the reactor through the temperature control of the hot box. Prior to the assembly of the microreactor the microchannel block was coated with the catalyst. For improving the interaction between the washcoated layer and the metallic support, the surface of the microchannel block was modified generating an oxide scale that enhances adhesion of the catalytic layer both mechanically, through the generated roughness, and chemically through the interaction between this oxide scale and the catalytic material. Modified surfaces are obtained upon heating in air at elevated temperatures being the resulting oxide scale an excellent substrate to adhere catalysts [1922]. The optimal treatment parameters remain usually undisclosed. Upon heating at 900 ºC for 22 h in air the microchannel block a homogeneous surface layer of α-Al2O3 is formed as confirmed by DRX. SEM micrographs, figure 5, clearly show the needle-like structure of the formed whiskers having thicknesses ranging between 3 and 6 µm. Washcoating was selected for coating the microchannels with the CuOx/CeO2 catalyst. The first step for washcoating a metallic substrate is to prepare stable slurries of the catalyst to be deposited. Particle size, solid content of the suspension and pH of the catalyst slurry are parameters that influence the slurry stability. The particle size of our catalyst, d90=0.5μm, is well below 10µm the upper recommend limit for preparing stable slurries [31] therefore the usual ball milling process for reducing particle size was avoided. The isoelectric point (IEP) of the catalyst is ca. 7 and therefore the pH was fixed at 4 for ensuring high values of zeta potential and then high repulsions between the particles, favouring the stability of the slurries [32]. The use of additives for the slurry formulation attempts to improve the catalyst adherence and the washcoating drying process. The addition of colloidal alumina, that presents a narrow particle size distribution, improves the catalyst adherence [31, 33] according to the model previously proposed by Nijhuis et al. [34] a bimodal particle size distribution increase adherence since the smaller particles are located between the bigger ones. The use of polyvinyl alcohol (PVOH) helps in preventing crack formation during the drying process improving the wetting properties of the catalytic layer. After several trials of slurry formulation for washcoating the following proportions of catalyst and additives was selected: 76% catalyst content, 7% PVOH (w/w), 17% colloidal alumina (w/w) and pH of the suspension adjusted to 4 with diluted HNO3. Assuming the Einstein model for the diluents dispersion of hard spheres, the viscosity of ideal water slurries only depends on the solid content, being higher as the solid content increases [35]. In the case of the CuOx/CeO2 catalyst the viscosity ranges from 20 cps for 15% solid content to 200 cps for a dispersion containing 25% solids. The viscosity of the catalytic slurry is the key parameter in controlling the coating process; a detailed description of the different factors influencing the washcoating process is given elsewhere [31, 33]. Slurry with 18% solid concentration provides an excellent compromise between the slurry wetting properties and catalyst loading. Figure 6 presents the amount of loaded catalyst over the substrates as a function of the number of coatings. The specific load increases almost linearly with the number of coatings. The washcoating method gives additive and homogenous results. Low viscosities allow to obtain highly adherent and homogeneous coatings but with low specific loads. Thus for obtaining the target loading numerous coating are required. On the contrary, high viscosity will allow high specific load per coating although with lower homogeneities (i.e. accumulations, channel blocking) resulting in less adherent coatings [33]. The slurry prepared in these conditions was stable for 96 hours. Once the microchannels were immersed in the slurry the elimination of the excess was done by air blowing for microchannels (2L·min-1). Finally, the microchannel block was dried at 120ºC for 30 minutes between coatings and after the last coating procedure the microchannel block was calcined at 300ºC for 3 hours (1ºC/min). This procedure resulted in a microchannel block loading of 5,46 mg·cm-2 catalyst after eight washcoating processes. 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Textural properties of the catalysts Table 2. Hydrogen consumption during the TPR analysis for the CuOx-CeO2 solid Table 3. Parameters chosen for welding the microchannel plates Figure 1: Schematic picture of the steel plates-metallic glass assembly in crossflow configuration. Figure 2. Joined Al-alloyed ferritic stainless steel plates at 900ºC as a function of the applied uniaxial pressure. A) 10 MPa and B) 15 MPa Figure 3. Microstructures of the joined area. A) SEM micrigraph before the recrystallization process; B) Optical micrograph after recrystallizing the microreactor for 5h at 1200ºC; C) EDX line profile across the joined are after recrystallization; D) Gaussian fit of the Ni EDX line profile across the joined are after recrystallization. The profile is taken along the line marked in pannel B. Figure 4. Microchannel block and microchannel reactor for the CO-PROX reaction. Figure 5. SEM micrographs of the α-Al2O3 formed upon heating at 900 ºC for 22 h in air the microchannel block Figure 6. Evolution of the catalyst loading after the drying process as a function of the number of coatings performed. The amount loaded upon calcination is indicated by an arrow and the specific loading is indicated after 4, 6 or 8 coatings Figure 7. XRD pattern of the CuOx-CeO2 solid Figure 8. Raman spectra of the CuOx-CeO2 solid at different temperatures Figure 9. TPR profile for the prepared CuOx-CeO2 solid Figure 10. CO conversion (A) and O2 selectivity to CO2 (B) for the CO-PROX reaction over CuOx-CeO2 catalysts. Open circles synthesized catalyst; Full circles after leaching in HNO3 at pH=2 Figure 11. CO conversion (A) and O2 selectivity to CO2 (B) for the CO-PROX reaction over CuOx-CeO2 catalysts in a atmosphere with CO:O2:H2:N2 ratios of 2:1:50:47. F(powder) = 100 Ncm3·min-1; F(microchannel) = 300 Ncm3·min-1; W(powder) = 100 mg; W(microchannel) = 300 mg Figure 12. CO conversion for the CO-PROX reaction over CuOx-CeO2 catalysts in the microchannel block as a function of the CO2 and H2O concentrations in a atmosphere with CO:O2:H2:N2 ratios of 2:1:50:balance. F = 300 Ncm3·min-1; W(microchannel) = 300 mg Table 1 Catalyst BET Surface Area m2/g Pore Volume cm3/g Average pore diameter Å CuOx-CeO2 75.9 0.136 74.1 CuOx-CeO2 Slurry 91.6 0.200 100.0 Table 2 Reducton Zone H2 consumption (mol) Chemical composition (mol) A 1.21x10-4 Cu Ce B 3.57x10-5 1.00x10-4 2.63x10-4 Table 3 Joining Post-processing Applied force (kN) 2.8 --- Temperature (ºC) 850 1200 Heating rate (ºC·min-1) 100 Time (minutes) 40 300 Atmosphere Vacuum Inert gas Figure 1 Figure 2 Figure 3 Figure 10 Figure 11 Figure 12