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Energy & Fuels 2007, 21, 3548–3554
Effect of Relative Bed Lengths on a Platinum/Nickel Stratified Dual Bed Catalyst for the Catalytic Partial Oxidation of Methane at Millisecond Contact Times Christa J. Bell and Corey A. Leclerc*,† Department of Chemical Engineering, McGill UniVersity, 3610 UniVersity St., Montreal, Quebec H3A 2B2, Canada ReceiVed July 11, 2007. ReVised Manuscript ReceiVed August 30, 2007
Stratified, dual bed catalysts composed of platinum followed by nickel have shown reactant conversions and product selectivities similar to rhodium, which has shown the highest activity for the millisecond catalytic partial oxidation reaction. The goal of the stratified catalyst is to carry out the catalytic partial oxidation in a two-step process: combustion catalyzed by platinum followed by steam and carbon dioxide reforming catalyzed by nickel. Previous studies have shown that sequential platinum and nickel beds with the same space velocity have high activity and long-term stability. In this work, the relative bed length of the platinum and nickel catalysts is investigated. Since combustion occurs much faster than reforming, the platinum bed can be much shorter than the nickel bed. Experiments show that the relative bed length of platinum can be much smaller than that of nickel in the dual bed catalyst. Reducing the amount of platinum increases the methane conversion and hydrogen selectivity while reducing the carbon monoxide selectivity. Degradation studies under harsh operating conditions show that the catalyst is stable. The number of potential experiments required to optimize this system leads one to believe a simulation-based approach will be more efficient.
Introduction Catalytic partial oxidation (CPO) is a viable fuel conversion process to produce hydrogen from a wide variety of fuels. 1 CH4 + O2 f CO + 2H2 2
∆H ) –36 kJ/mol
(1)
For methane (1) and other fuels, CPO occurs at millisecond residence times and is still able to convert 95% of the fuel fed to the reactor with a selectivity to hydrogen around 95%. CPO has shown excellent activity for the conversion of methane,1 isooctane,2,3 hexadecane,4 ethanol,5 glycerol,6 and many other fuels .7,8 The process can start in less than 5 s due to the small * Corresponding author: Ph (514) 398-8308; Fax (514) 398-6678; e-mail
[email protected]. † Present address: Department of Chemical Engineering, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801. Ph (575) 835-5293; Fax (575) 835-5210; e-mail
[email protected]. (1) Hickman, D. A.; Schmidt, L. D. Production of syngas by direct catalytic oxidation of methane. Science 1993, 259, 343. (2) O’Connor, R. P.; Klein, E. J.; Schmidt, L. D. High yields of synthesis gas by millisecond partial oxidation of higher hydrocarbons. Catal. Lett. 2000, 70, 99. (3) Schmidt, L. D.; Klein, E. J.; Leclerc, C. A.; Krummenacher, J. J.; West, K. N. Syngas in millisecond reactors: higher alkanes and fast lightoff. Chem. Eng. Sci. 2003, 58, 1037. (4) Krummenacher, J. J.; West, K. N.; Schmidt, L. D. Catalytic partial oxidation of higher hydrocarbons at millisecond contact times: decane, hexadecane, and diesel fuel. J. Catal. 2003, 215, 332. (5) Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E. Renewable hydrogen from ethanol by autothermal reforming. Science 2004, 303, 993. (6) Dauenhauer, P. J.; Salge, J. R.; Schmidt, L. D. Renewable hydrogen by autothermal steam reforming of volatile carbohydrates. J. Catal. 2006, 244, 238. (7) Wanat, E. C.; Suman, B.; Schmidt, L. D. Partial oxidation of alcohols to produce hydrogen and chemicals in millisecond-contact time reactors. J. Catal. 2005, 235, 18.
thermal mass of the catalyst.9,10 The ability to convert liquid fuels, start up quickly, and achieve high yields in millisecond residence times make CPO ideal for small, mobile, hydrogenfueled applications. One drawback of CPO is that rhodium gives the highest hydrogen yields and best long-term stability of any other single metal catalyst.11 Unfortunately, rhodium is expensive with prices over U.S.$4300/oz in the past 5 years.12 An alternative catalyst composed of two sequential beds of a combustion catalyst followed by a reforming catalyst achieves yields of hydrogen that are similar to those for rhodium. This stratified bed catalyst has been demonstrated with platinum followed by nickel13 as well as copper, chromium, or manganese oxides followed by nickel.14 The overlying concept is that the partial oxidation of methane can occur sequentially in two steps. The first step is the total oxidation (2) in which all of the oxygen reacts with a fraction of the methane to form combustion products. CH4 + 2O2 f CO2 + 2H2O
∆H ) –801 kJ/mol (2)
(8) Nguyen, B. N. T.; Leclerc, C. A. Int. J. Hydrogen Energy 2007, accepted for publication. (9) Leclerc, C. A.; Redenius, J. M.; Schmidt, L. D. Fast lightoff of millisecond reactors. Catal. Lett. 2002, 79, 39. (10) Williams, K. A.; Leclerc, C. A.; Schmidt, L. D. Rapid lightoff of syngas production from methane: A transient product analysis. AIChE J. 2005, 51, 247. (11) Torniainen, P. M.; Chu, X.; Schmidt, L. D. Comparison of monolithsupported metals for the direct oxidation of methane to syngas. J. Catal. 1994, 146, 1. (12) United States Geological Survey, Mineral Commodities Summaries 2007; United States Department of the Interior, 2007. (13) Tong, G. C. M.; Flynn, J.; Leclerc, C. A. A dual catalyst bed for the autothermal partial oxidation of methane to synthesis gas. Catal. Lett. 2005, 102, 131. (14) Nguyen, B. N. T.; Leclerc, C. A. Metal oxides as combustion catalysts for a stratified, dual bed partial oxidation catalyst. J. Power Sources 2007, 163, 623.
10.1021/ef700398f CCC: $37.00 2007 American Chemical Society Published on Web 10/06/2007
RelatiVe Bed Lengths in Pt/Ni Partial Oxidation Catalysts
Energy & Fuels, Vol. 21, No. 6, 2007 3549
The second step is the steam (3) and dry (4) reforming of the remaining methane by the combustion products. CH4 + H2O T CO + 3H2
∆H ) 206 kJ/mol
(3)
CH4 + CO2 T CO + 2H2
∆H ) 247 kJ/mol
(4)
Reactions 2–4 when combined linearly with the proper coefficients lead to reaction 1. Because of the high gas velocities, the heat generated in the upstream, exothermic catalyst is quickly convected to the second catalyst to drive the endothermic reforming reactions of the remaining methane with the products formed in the first bed. This catalyst concept has produced yields and stabilities similar to the rhodium catalyst at a fraction of the cost. The only metal of comparable price to rhodium in the stratified bed catalyst is platinum, which is used in smaller amounts. The stratified bed catalysts have been investigated with equal length beds of combustion catalysts followed by reforming catalysts. Since combustion occurs at a faster rate than steam reforming, it is expected that the combustion section of the catalyst can be much smaller than the reforming section. Indeed, experiments have shown that for a rhodium catalyst all of the oxygen is consumed within millimeters of the catalyst entrance.15,16 The combustion section should not be much longer, since combustion will not occur after the oxygen is converted. By using a platinum bed that is shorter than the nickel bed, theory predicts that the hydrogen yield should increase, and economics predict that the cost of the catalyst will decrease. When the use of natural gas (or other fuel) reformers becomes widespread, even small increases in hydrogen yield will turn into large financial savings. In this work, we have investigated the effect of changing the relative bed length of a platinum combustion catalyst to a nickel reforming catalyst in order to minimize the amount of platinum used in the stratified bed reactor, while maintaining a high activity with no evidence of degradation over a 10 h period of time. Experimental Section Apparatus. The reactor, shown in Figure 1, consists of a quartz tube 40 cm in length and 1.8 cm in inner diameter. Methane and air are fed to the reactor through Tylan mass flow controllers. The gases pass over the catalyst assembly which consists of upstream and downstream 45 ppi R-alumina foam monoliths which act as radiation heat shields and four catalytic monoliths that are coated with either platinum or nickel. A K-type thermocouple passes through the downstream heat shield to contact the back face of the catalyst bed. The product gases are exhausted, or they can be sampled using a gas-tight syringe. Sampled gases are run in an Agilent 6890 gas chromatograph which uses a Poraplot Q column to separate carbon dioxide from the sample and a molsieve 5A to separate the remaining gases. A thermal conductivity detector is used to determine the concentrations of all species except hydrogen and water with nitrogen as an internal standard and helium as the carrier gas. Water is measured by closing the atomic oxygen balance, and hydrogen is measured by subsequently closing the atomic hydrogen balance. For all data shown, the atomic carbon balance closed to within 4%. Catalyst Preparation. The catalysts are supported on 3 mm, 80 pores per inch (ppi) R-alumina spongelike foam monoliths. The (15) Klein, E. J.; Tummala, S.; Schmidt, L. D. Catalytic partial oxidation of methane to syngas: staged and stratified reactors with steam addition. 6th Natural Gas Conversion Symposium, 2001. (16) Horn, R.; Degenstein, N. J.; Williams, K. A.; Schmidt, L. D. Spatial and temporal profiles in millisecond partial oxidation processes. Catal. Lett. 2006, 110, 169.
Figure 1. Schematic of the catalyst assembly. Four catalytic monoliths are placed in sequence between two blank monoliths, which act as heat shields. The gases flow from top to bottom. The first catalyst is always coated with platinum, and the subsequent monoliths are coated with either platinum or nickel.
blank monoliths are first coated with R-alumina to add surface area and decrease the pore size. They are then dried, followed by the addition of either hexachloroplatinic acid or nickelous nitrate solutions. The monoliths are allowed to dry, and more solution is added to obtain the desired loading. Once all of the solution has been added and the monoliths have dried, the catalysts are calcined for 6 h at 510 or 600 °C for platinum and nickel coatings, respectively. All monoliths had washcoat loadings of 1.6–2 wt % and either 1.8–2.2 wt % platinum or 6.6–6.9 wt % nickel. Reactor Performance. The activity of the catalyst was determined by calculating the conversion of reactants and selectivity to form each product. The conversion of species i is defined as the amount of moles of i fed (Fi,in [mol/min]) minus the moles of i exiting (Fi,out [mol/min]) all divided by the moles of i fed: Xi )
Fi,in - Fi,out Fi,in
The selectivity of atom i to form product j is defined as the stoichiometric amount (ν) of atom i in species j divided by the total amount of atom i in all of the products, k: Si,j )
vi,jFj
∑v
i,kFk
k
To simplify the notation, we only report the selectivity for a species without specifying the atom, i. The selectivities of carbon monoxide and carbon dioxide are with respect to the carbon atom and the hydrogen, and water selectivities are with respect to hydrogen atoms. These were the only products detected by the gas chromatograph. The effects of changing the methane to oxygen ratio and the gas hourly space velocity (GHSV) were investigated. Methane to oxygen ratios of 1.6–2.4 were investigated. These ratios bracket the stoichiometric methane to oxygen ratio for partial oxidation of 2. They are significantly higher than the methane to oxygen ratio for combustion, which is 0.5. The GHSVs range from 70 000 to 360 000 h-1. The GHSV is calculated on the basis of the entrance gas flow rate at standard temperature and pressure: GHSV )
Fin,total επRcat2Lcat
3550 Energy & Fuels, Vol. 21, No. 6, 2007
Bell and Leclerc
In this equation, ε is the void volume (equal to 0.83 for an 80 ppi monolith17) of the catalytic monolith, Rcat is the radius of the catalytic monolith, and Lcat is the axial length of the catalyst. Experiments. The catalyst bed was composed of four catalytic monoliths, which are all 3 mm in length. This gives a total catalyst bed length of 12 mm. The composition of the catalysts was varied to determine the effect of changing the relative GHSV. Experiments were performed with 1 platinum catalyst followed by 3 nickel catalyst denoted as Pt3/Ni9 to indicate the length of each metal as well as 2 platinum with 2 nickel (Pt6/Ni6) and 4 platinum catalysts (Pt12). A catalyst bed composed of four nickel catalysts was not investigated due to the previously known problems with deactivation of nickel.11
Uncertainty Analysis An uncertainty analysis was performed by using the general equation for the propagation of uncertainty as presented in Holman.18 For a function R with the dependent variables x1, x2, ..., xn, the absolute uncertainty in R is given by wR: wR )
(
∂R w ) ∑ ( ∂x n
i
i
xi
)
2
1⁄2
Based on a 2% error in inlet mass flow, 1% error in area measurement from the gas chromatogram, and the certified uncertainty from the calibration standards, the uncertainty in conversion and selectivity can be calculated using the following equations: Fout,i ) FN2RFi
Ai AN2
Ai is the area for species i as measured from the gas chromatogram, RFi is the response factor of species i with respect to nitrogen, and Fout,i is the outlet flow of species i in mol/min at standard conditions. The flow of nitrogen is not specified as outlet or inlet, since it is assumed that it does not react in the reactor and remains unchanged. This equation holds for all species except water and hydrogen whose selectivities are obtained by closing molecular oxygen and hydrogen balances. The equations for these two species are based on the flow rates of other species and corresponding stoichiometric coefficients. Fout,H2O ) 2Fin,O2 - 2Fout,O2 - Fout,CO - 2Fout,CO2 Fout,H2 ) 2Fin,CH4 - 2Fout,CH4 - Fout,H2O Based on these equations, the conversion and selectivities can be calculated as well as the uncertainties in these values. Results Varying Feed Ratio. Experiments were carried out at fixed flow rates on all three catalyst configurations with five different methane to oxygen feed ratios varying from 1.6 to 2.4. Figures 2 and 3 show the (a) conversion of methane, (b) selectivity to hydrogen, and (c) selectivity to carbon monoxide for a gas hourly space velocities of 75 000 and 340 000 h-1, respectively. Low Flow Rate Experiments. The low flow rate experiments (Figure 2) show that the conversion of methane is the same for each catalyst assembly. The conversion decreases almost linearly from 0.95 to 0.65 over the range of methane to oxygen ratios (17) Twigg, M. V.; Richardson, J. T. Theory and application of ceramic foam catalysts. Trans. Inst. Chem. Eng. 2002, 80, 183. (18) Holman, J. P. Experimental Methods for Engineers, 7th ed.; McGraw-Hill: New York, 2001; p 52.
Figure 2. Methane conversion (a), hydrogen selectivity (b), and carbon monoxide selectivity (c) for the catalytic partial oxidation of methane over a Pt3/Ni9 (9), a Pt6/Ni6 (2), and a Pt12 (() catalyst in air for varying methane to oxygen feed ratios at a GHSV of ∼75 000 h-1. Lines are added to aid the visual presentation of the plot.
RelatiVe Bed Lengths in Pt/Ni Partial Oxidation Catalysts
Figure 3. Methane conversion (a), hydrogen selectivity (b), and carbon monoxide selectivity (c) for the catalytic partial oxidation of methane over a Pt3/Ni9 (9), a Pt6/Ni6 (2), and a Pt12 (() catalyst in air for varying methane to oxygen feed ratios at a GHSV of ∼350 000 h-1. Lines are added to aid the visual presentation of the plot.
Energy & Fuels, Vol. 21, No. 6, 2007 3551
investigated. None of the three assemblies are distinguishable from each other. The hydrogen selectivity increases by 0.01 to 0.02 as the methane to oxygen ratio increases. The 3Pt/9Ni catalyst shows higher selectivity than the others by 0.02. The carbon monoxide selectivities do not change by more than 0.01 over the range of methane to oxygen ratios investigated. As the relative bed length of platinum increases, the carbon monoxide selectivity also increases. High Flow Rate Experiments. For the high flow rate experiments (Figure 3), the methane conversion is 0.97 for all three catalyst assemblies at the lowest methane to oxygen ratio. The rate at which the methane conversion decreases as the methane to oxygen ratio increases is higher for catalysts containing more platinum. The 3Pt/9Ni declines to a conversion of 0.65 at the highest methane to oxygen ratio whereas the 12Pt catalyst decreases to 0.62. The hydrogen selectivies are similar for all three catalyst assemblies at the lowest methane to oxygen feed ratio. As the feed ratio increases, the 3Pt/9Ni catalyst shows a constant increase from 0.92 to 0.96 over the range of feed ratios. As the length of the platinum bed increases, the hydrogen selectivity decreases at the higher ratios. The carbon monoxide selectivies of all catalyst assemblies vary by only 0.02 over the range of feed ratios. The selecitivity decreases as the amount of platinum in the catalyst decreases. Varying Flow Rate. Experiments were carried out at fixed methane to oxygen feed ratios on all four catalyst configurations with five GHSVs varying from 70 900 to 360 000 h-1. Figures 4 and 5 show the (a) conversion of methane, (b) selectivity to hydrogen, and (c) selectivity to carbon monoxide for a methane to oxygen ratios of 1.6 and 2.4, respectively. Low Methane to Oxygen Ratio Experiments. The low methane to oxygen ratio experiments (Figure 4) show that the methane conversion is nearly the same for all three catalyst assemblies and nearly constant over the range of GHSVs investigated. The conversion is above 0.94 in all cases. The conversion increases slightly as the amount of platinum in the catalyst decreases. The hydrogen selectivity is nearly constant for all three catalyst assemblies over the range investigated. The catalysts that contain nickel reach selectivities of 0.93 whereas the catalyst without nickel has a selectivity of 0.90. The carbon monoxide selectivities increase over the range the range of GHSVs. For each catalyst assembly, the selectivity increases by 0.03–0.04. The catalysts that contain nickel achieve lower selectivities than the catalyst without nickel. High Methane to Oxygen Ratio Experiments. The high methane to oxygen ratio experiments (Figure 5) show that the methane conversion decreases slightly over the range of GHSVs. Except at the lowest GHSV, the conversion increases as the amount of platinum in the assembly decreases. The hydrogen selectivity is nearly constant over the range of GHSVs investigated. The Pt3/Ni9 catalyst obtains a selectivity over 0.95 over the entire range. The Pt12 catalyst achieves a selectivity of 0.91 over the same range. The carbon monoxide selectivity increases over the range of GHSVs investigated with the Pt12 showing the highest selectivity over the entire range. The Pt12 catalyst’s selectivity ranges from 0.84 up to 0.88. The Pt3/Ni9 catalyst has a selectivity that ranges from 0.81 to 0.84. The Pt6/Ni6 catalyst’s selectivities lie in between the other two catalysts. Catalyst Degradation. Experiments were carried out over a 3Pt/9Ni catalyst at methane to oxygen ratios of 1.6 and 2.4.
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The experiments were carried out at constant air flow rates, which correspond to GSHVs of 320 000 and 360 000 h-1 for the two methane to oxygen ratios used. Figure 6 shows the conversion of methane, selectivity to hydrogen, selectivity to carbon monoxide, and backface temperature for (a) the carbon to oxygen ratio of 1.6 and (b) a ratio of 2.4. In the case of the 1.6 feed ratio, only slight changes (
