Ind. Eng. Chem. Res. 2005, 44, 9659-9667
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Structured Folded-Plate Reactor for CO Preferential Oxidation Eduardo Lopez,* Grigorios Kolios,† and Gerhart Eigenberger Institute for Chemical Process Engineering (ICVT), University of Stuttgart, Bo¨ blinger Strasse 72/78, 70199 Stuttgart, Germany
An Au/R-Fe2O3-γ-Al2O3 catalyst (0.96% (w/w) of Au and 4.8% of Fe2O3) was tested for activity and selectivity in an isothermal flat-bed reactor. Very good activity and selectivity values were measured for different operation temperatures and compositions. The reaction rate parameters of carbon monoxide and hydrogen oxidation were fitted. Activation energies of 11 and 36.9 kJ/ mol for CO and H2 oxidation, respectively, were obtained. A folded-plate reactor concept with three side ports and one front port for air injection has been proposed to carry out the preferential oxidation (PrOx) reaction. By means of a mathematical model of the reactor, design and optimization studies were conducted. Simulation results indicate a remarkable uniformity of the axial temperature profile in the reactor. For the calculated heat-transfer parameters, a reactor configuration with only one air-side-feed injection port, is the preferred design, combining good operation performance, simplicity of design, and sufficient oxygen conversion. On the basis of the simulation results, a folded-plate reactor for CO-PrOx has been built up and is presently under testing for an experimental proof of concept. 1. Introduction Fuel cell technology has been intensively studied over the past two decades to eventually replace internal combustion engines in vehicles or small-scale decentralized power stations. Among the different types of fuel cells, the proton exchange membrane fuel cell (PEM) appears to be the most promising due to its compact size, high power density and low operating temperature (around 80 °C). Among several propositions, in-situ generation of the required hydrogen for the fuel cell operation from steam reforming of hydrocarbons or alcohols arises as the most technologically viable option nowadays. Typical gas mixtures after the shift stage in reforming systems show CO concentrations of 0.5-2% (v/v). PEM fuel cells require a feed of purified hydrogen (yCO < 50 ppm)1-3 in order to avoid the poisoning of the Pt/Ru-based anode catalyst. Several methods for selective CO removal have been studied, mainly pressure swing adsorption (PSA), methanation, and preferential oxidation (PrOx). Although with the use of palladium diffusion membranes (PSA process) excellent hydrogen selectivity is possible, the operation requires high pressure differences and is inherently expensive. Pressureswing adsorption is an industrially established technology for hydrogen purification in large scale. Methanation involves the consumption of 3 mol of hydrogen/mol of CO converted. PrOx of CO is the method of choice to purify hydrogen streams from small-scale reformers due to its fairly simple implementation, lower operation costs, and minimal loss of hydrogen.4-6 Several catalysts have been proposed to carry out the PrOx reaction, most of them based on noble metals. The PrOx catalyst should exhibit not only good activity but also good selectivity toward the CO oxidation in the H2rich atmosphere. Pt-based catalysts (over alumina or silica) have been used as standard catalysts for CO * To whom correspondence should be addressed. E-mail:
[email protected]. † Current address: Christ A.G., Hau¨ptstrasse 192, 4147 Aesch, Switzerland.
preferential oxidation. The required operating temperature is around 200 °C with selectivities of ∼40%.7,8 This temperature level is intermediate between the operating temperatures of the shift reactor (∼250 °C 8) and the PEM fuel cell (80 °C). This is not very suitable for the design of the complete system with respect to heat integration as it requires an additional cooling step. Ru catalysts have been also introduced as potential catalysts for the PrOx reaction. Slightly lower operating temperatures with a wider operation window than for Pt catalysts have been reported, maintaining the same selectivity levels.2,9-11 Although showing low efficiencies as bulk phase, metal-supported Au nanoparticles have been reported to be highly active for the PrOx reaction, especially combined with a metal oxide.6,12 Typical operating temperatures for gold catalysts are around 80 °C, the same as for PEM fuel cells. This fact points them as suitable candidates since the thermal integration of the PrOx reactor and the PEM fuel cell within the same cooling circuit becomes straightforward. The reported selectivity is also very good, with values between 40 and 60% and even higher.12 Many literature studies have presented the performance of Au catalysts using idealized reformate gas, without CO2 and water. In other papers however it was shown that these components play an important role in the reaction and should be included in the study of the catalyst performance.13 In addition to the choice of the catalytic system, the design of the CO-PrOx reactor requires special attention. CO and H2 oxidations are highly exothermic reactions, the hydrogen oxidation being favored by higher temperatures. Therefore, an efficient control of temperature is required for selectivity reasons (minimizing undesired hydrogen combustion). In addition, compactness and low pressure drop are also necessary. Several design configurations have been proposed to carry out this process. The classic fixed-bed reactor with a single oxygen injection in the reactor inlet seems not too adequate for this application since the temperature evolution is difficult to control and the generation of a
10.1021/ie050282h CCC: $30.25 © 2005 American Chemical Society Published on Web 08/25/2005
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hot-spot in the inlet region is almost inevitable. If, however, very good heat-transfer conditions can be guaranteed, it still can be an attractive alternative due to the simplicity in design and operation. In contrast, multistaged reactors, with intermediate oxygen addition, were proposed in the literature.14,15 Although these designs have the disadvantage of requiring equipment for the dosification of oxygen (air) in each intermediate stage of the reactor, they are able to handle this highly exothermic reaction system with acceptable selectivity. In this contribution the folded-plate reactor (FPR)16-19 will be proposed for the PrOx reaction. It is based on the design of plate heat exchangers and combines multifunctionality with the efficient heat-transfer characteristics of wall reactors. The FPR concept offers an attractive solution for the CO/H2 oxidation since in this design the chambers are open to the side and oxygen (air) can be distributed along the entire length of the channels according to the process requirements. Effective cooling is also assured due to the excellent heattransfer properties achieved by this concept. A gold-based catalyst for the PrOx reaction will be used. Its preparation procedure is briefly described and its activity and selectivity tested in a kinetic setup, consisting of a flat-bed reactor and its periphery. Using the data collected in the kinetic experiments, the parameters of the rate equations representing the CO and H2 oxidations were fitted. A spatially one-dimensional mathematical model of the PrOx reactor has been used for design and optimization studies of the FPR for CO preferential oxidation. On the basis of the simulation results, a folded-plate reactor for CO-PrOx has been built up and is presently under test for an experimental proof of concept. Some details about this experimental setup are also presented. 2. Catalyst 2.1. Preparation and Characterization. In the frame of a cooperation project (Fuel Cell State Cooperation Project Baden-Wu¨rttemberg, 2003-2005) an Au/ R-Fe2O3-γ-Al2O3 catalyst was prepared by the Center for Solar Energy and Hydrogen Research (ZSW, Ulm, Germany) for use in the CO-PrOx reaction. The 1.6 mm diameter, 1.6 mm height cylindrical pellets (SBET ∼ 165 m2/g) were impregnated by a coprecipitation-deposition method with a HAuCl4 solution at pH ∼ 7 onto a “Fe(OH)3”-γ-Al2O3 substrate. The latter was prepared by soaking the γ-Al2O3 raw pellets with concentrated Fe(NO3)3 solution and subsequent neutralization with concentrated soda up to pH 9. Final contents of 0.96% (w/w) of gold and 4.8% of Fe2O3 were achieved. The average Au and Fe2O3 crystallite sizes of the calcined (2 h at 350 °C) catalyst from difference X-ray diffraction (XRD) scans are ∼3 and ∼15 nm, respectively. 2.2. Kinetic Setup. An aluminum flat-bed reactor (Figure 1) was used in order to test the performance of the above-described catalyst and to make kinetic measurements under practical conditions (same temperature and pressure ranges, same catalyst, and similar flow conditions as in the PrOx folded-plate reactor). It consists of two aluminum bars in the middle of which the catalyst channel is machined. Both bars are heated by temperature-controlled heating coils to compensate for heat losses and to ensure a uniform wall temperature of the catalyst channel. The catalyst is placed in three sections in the rectangular reactor channel (30 mm width, 1.6 mm height). The sections are separated
Figure 1. Schematic representation of the aluminum flat-bed reactor.
by metallic net structures located in the regions of side withdrawals to allow analysis after each section. The flat-bed reactor design provides well-controlled temperature conditions due to a good contact between the reactor body and the catalyst sample and the possibility of individual temperature control in each one of its eight heating elements. Uniform flow conditions are achieved by a proper geometry of the channels and the usage of mixing elements. Side-flow withdrawals permit the measurement of axial concentration profiles and also to check the temperature in the reaction channel. Thermal mass-flow controllers (El-Flow, Bronckhorst) are used for the dosage of the gaseous components. The liquid water, dosed using a liquid pump (L-6200, Hitachi), is continuously evaporated (electric evaporator, ICVT) and, after mixing with the gaseous components, fed to the flat-bed reactor via heated pipes. Temperatures are measured with thermocouples (Type K) by a multichannel-temperature controller (HT-60, Horst) and by a data acquisition board (PCI-6071E, National Instruments). The system pressure is controlled by a back-pressure regulator (El-Press, Bronckhorst). All components of the plant are connected to the PC by the data acquisition board, a digital IO board (DIO-96, National Instruments), and serial ports (RS 232). The plant is controlled by the instrumentation software LabView (National Instruments). The gas probes, extracted from each of the reactor side-flow withdrawals through thermostatized pipes, are analyzed by online chromatography (Hewlett-Packard, HP 5890) using two different capillary columns (CP PoraPlotQ/CP Molsieve 5 A, Chrompack) and a TCD detector.20 All the components present in the reaction mixture are measured by GC to ensure a correct close of the mass balance. 2.3. Activity Measurements and Kinetic Fitting. Kinetic expressions for the CO and H2 oxidation over the Au catalyst are required by the mathematical model of the reactor described in a later section. The kinetic setup described in the previous section was used to obtain the necessary experimental data. After a first activation of the catalyst under reaction conditions and T ) 60 °C for 10 h, 17 sets of axial CO and O2 concentration profiles for different temperatures, λ ratios (amount of oxygen in feed divided by the amount of oxygen needed to only oxidize the CO present completely; see eq 1) and flows were measured. Several measurements were repeated frequently to ensure catalyst stability. Selectivity values were calculated by eq 2. No methane could be detected at the reactor outlet showing that the methanation reaction can be neglected in these studies for the temperature range under evaluation. In addition, in experiments conducted with a CO2/O2-free mixture (2% CO, 10% water) only negli-
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Figure 2. CO selectivity (empty symbols) and conversion (full symbols) over temperature for different λ ratios. Flow rate, 0.91 slpm; yinlet CO ) 1.3% (see text for other components).
gible quantities of CO2 were detected as product from the water-gas-shift (WGS) reaction and minimal quantities of CO (around 15 ppm) could be observed for the reverse-WGS reaction (with a CO-free atmosphere, also 10% water), concluding that these side reactions are also neglectable in the tested system. It should be mentioned, however, that the reverse-WGS reaction may be important at the low CO levels tried to reach in these COPrOx reactors.
Figure 3. Axial mole fraction and selectivity profiles for CO over bed length for different temperatures at λ ) 2. Table 1. Fitted Kinetic Parameters of CO and H2 Oxidation Rates (Equations 3 and 4) parameter
value
k∞,CO ECO k∞,H2 EH2 nCO nH2 nO2,rCO ) nO2,rH
6.5 × 10-2 (mol/m2)/s 10 990 J/mol 7.01 (mol/m2)/s 36 900 J/mol 0.79 1.08 0.56
2
λ)
S)
inlet 2CO 2
Cinlet CO
∆n˘ O2fCO2 ∆n˘ O2fCO2 + ∆n˘ O2fH2O
(1)
(2)
The explored temperature range for the operation of the Au catalyst was between 60 and 100 °C (isothermal axial profiles). The flow rates were set to values which correspond to residence times of approximately half of those assumed for the folded-plate reactor, to achieve a wider spectrum of CO conversions, mainly after the middle of the reactor. A variation in (30% of the base flow used in the experiments (∼0.9 slpm) was used to explore the reactor behavior to changes in residence time. Different CO and O2 inlet concentrations corresponding to λ ratios between 0.5 and 4 were also explored. Mixtures with a realistic proportion of H2, CO2, and H2O (∼52%, 16%, and 9%, respectively) were used in all cases, resembling the outlet from a methanol reformer.19 Figure 2 shows outlet CO conversions and selectivities (eq 2) for different operating temperatures (isothermal axial profiles), with different λ ratios as the parameter. For the case with λ ) 4, nearly total CO conversion was achieved in the temperature range of 60-80 °C. At higher temperatures the decrease in selectivity produces a deficiency in oxygen and hence a decrease in CO conversion. Operation with λ rations lower than the unity was also tested since such conditions will be present in the first reactor section if distributed oxygen feed is used. In such cases, where the oxygen amount is not sufficient to totally oxidize the CO content in the feed, very good CO selectivities were measured, with a maximum of 75% for T ) 60 °C but low conversion. In the opposite situation, i.e., the operation with a large excess of oxygen (λ ) 4), selectivity values between 25 and 35% at almost total CO conversion were observed.
Values of selectivity of around 45% were calculated for the intermediate case of λ ) 2. The fitting procedure of kinetic expressions for CO and H2 oxidation was based upon a steady-state, 1-D, isothermal, heterogeneous mathematical model of the flat-bed reactor and the measured data in the flat-bed reactor (136 experimental values). The parameters of the power-law expressions for the CO and H2 oxidation, represented by eqs 3 and 4, have been adjusted and are shown in Table 1. The calculated activation energies for CO and H2 oxidations are 11 and 36.9 kJ/mol, respectively. These values agree well with others published in the literature for the CO-PrOx reaction over Aubased catalysts and with the ones measured by AOK (Institute for Catalysis and Surfaces, University of Ulm, Germany) for the same catalyst as used here but in powder form and with use of a differential reactor. The adjusted reaction orders are also in the range of previously published results.12 CO nO2,rCO pO2 rCO ) k∞,CO e-ECO/(RT)pnCO
(3)
nH2 O2,rH rH2 ) k∞,H2 e-EH2/(RT)pH pO 2 2
(4)
n
2
In Figure 3, experimental and calculated (using the fitted kinetics) axial profiles of CO molar fractions can be compared for runs with different operating temperatures. Experimental and calculated values of the selectivities are also shown in Figure 3 (right y-axis). Figure 4 presents the corresponding axial profiles of oxygen molar fraction. All the data presented in both figures were obtained using λ ) 2. As can be seen in Figure 3, higher conversions are achieved for higher temperatures as long as O2 is present in the reaction mixture. After this point, and especially near the reactor end, the tendency changes: more CO conversions are achieved at lower temperatures due to a more selective
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Figure 4. Axial mole fraction for O2 over bed length for different temperatures at λ ) 2.
Figure 7. Yield over temperature for feed conditions, yinlet CO ) inlet ) 63% (solid line), and for the desired exit 1.7% and yH 2 exit conditions, yexit CO ) 50 ppm and yH2 ) 53% (dashed line).
Figure 5. Axial mole fraction and selectivity profiles of CO for different λ values at T ) 80 °C.
Figure 8. Scheme of two adjacent channels of the folded-plate reactor modeled.
Figure 6. Axial mole fraction of O2 for different λ values at T ) 80 °C.
use of the available oxygen for CO oxidation (see selectivities in Figure 3 and oxygen profiles in Figure 4). Figures 5 and 6 present axial molar fraction profiles of CO and O2, respectively, with the lambda ratio as parameter. Experimental and calculated selectivities are also reported in Figure 5. The operation temperature for these sets of data was 80 °C. As seen, selectivities tend to descend as the reaction proceeds due to the reduction in CO molar fraction. Lower selectivities but high conversions are achieved for higher λ ratios. In the operation with λ ) 4 almost complete CO conversion was achieved, although the residence time is about half of that proposed for the folded-plate reactor.
It can be seen in Figures 3-6 that the simulations (lines) using the fitted kinetic parameters agree very well with the experimentally measured values (points). A measure for the intrinsic yield φ of the reaction can be obtained from the quotient of the two rates:
φ)
k∞,CO p0.79 CO 25910/(RT) e k∞,H2 p1.08
(5)
H2
It shows that the CO yield (or selectivity) is not influenced by the oxygen partial pressure but only by temperature. Figure 7 shows curves of yield over teminlet perature for feed conditions (yinlet CO ) 1.7%, yH2 ) 63%) exit exit and for the desired exit conditions (yCO ) 50 ppm, yH 2 ) 53%). A decrease in the yield of 2 orders of magnitude is observed for the entire range of temperatures when the CO concentration drops from inlet to the desired exit values. For high selectivity the temperature should be as low as possible, a practical lower limit being the onset of condensation of the feed mixture at about 50 °C.
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Figure 9. Simulation results of the folded-plate reactor performance: (a) CO, O2, and H2 molar fraction axial profiles; (b) catalyst, gas, and water axial temperature profiles; (c) heat generation profile; (d) CO conversion and selectivity axial profiles. Total flow ) 17 slpm; inlet inlet lateral flows of air ) 0.6, 1.3, and 0.7 slpm; yinlet ) Twater ) 80 °C. CO ) 1.7% (dry basis); Tg Table 2. Transport Coefficients and Other Necessary Parameters
Table 3. Operating Conditions and Design Specifications for the Simulations of the FPR
parameter
value
parameter
value
Deff Rg-w β λw λeff,g av,g-w
5 × 10-4 m2/s 200 (W/m2)/K 0.096 m/s 21 (W/m)/K ∼ 0.5 (W/m)/K 210 m2/m3 0.32 (experimentally measured)
total flow yinlet CO yexit CO (desired exit concn) gas inlet temp (also for side feeds) coolant (water) inlet temp coolant flow reactor length
17 slpm (τ ∼ 0.5 s) 1.7%
