Ind. Eng. Chem. Res. 2007, 46, 9053-9060
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Development of a Novel Metal Monolith Catalyst for Natural Gas Steam Reforming Pradeepkumar O. Sharma,† Martin A. Abraham,† and Sudipta Chattopadhyay*,‡ Department of Chemical and EnVironmental Engineering, The UniVersity of Toledo, Toledo, Ohio 43606, and Catacel Corporation, GarrettsVille, Ohio 44231
Production of hydrogen from natural gas is the most cost-effective and simplest technology for commercial hydrogen generation. Natural gas is also a likely source of hydrogen for residential fuel cell systems, due to its wide availability and ease of conversion via steam methane reforming. Although catalyst technology is available for generating hydrogen from natural gas, the design of new catalysts and new catalytic supports to overcome the limitations associated with ceramic catalyst provides an opportunity to make the conversion of natural gas to hydrogen more cost-effective. In the present study, the performance of Ni-Rh/Al2O3-CeO2ZrO2, Ni-Rh/γ-Al2O3, and Ni-Pd/γ-Al2O3 catalysts were quantitatively evaluated in terms of activity and stability using appropriate kinetic models. Catalysts were tested as powders and supported on metal foil. Results proved rhodium to be a better active agent than palladium, and catalyst activity was found to increase with the increase in rhodium loading at both atmospheric and elevated pressures. γ-Al2O3 supported rhodium catalysts exhibited a better performance at a much lower rhodium loading than the Al2O3-CeO2-ZrO2 supported catalysts. Mass and heat transfer advantages of the metallic support over the powder form were also established by studying the performance of this catalyst in both forms to demonstrate the potential for the use of metal structured support to achieve commercially relevant hydrogen production targets at lower residence times. Introduction Electricity, gasoline, diesel fuel, solar radiation, biofuel, and natural gas serve as energy carriers in the present energy supply system. In the past few decades, there has been a great deal of effort to promote hydrogen as an energy carrier because hydrogen has a higher heating value (HHV), i.e., 140 MJ/kg, than other common fuels such as gasoline, crude oil, bituminous coal, etc. having HHVs in the range of 15-54 MJ/kg. Also, since hydrogen contains no carbon, it cannot produce CO2. Hydrogen also serves as the primary fuel source for fuel cells, desired for use in cars for transportation or for power generation in stationary applications. However, hydrogen is an energy carrier and not an energy source, as it must be produced from other primary energy sources, or through electrolysis of water. Currently, more than 500 billion m3 of hydrogen is produced each year for a wide variety of uses such as production of ammonia and methanol, or for the hydrogenation of fats and oils. The production of hydrogen from hydrocarbon fuels can be performed in three ways: steam reforming, partial oxidation, and autothermal reforming.1 Steam reforming of natural gas on solid ceramic catalysts is currently the most highly developed and costeffective method for generating hydrogen and is also the most efficient, giving methane conversion values of 70-80% on a large scale. Today, 95% of the hydrogen produced in the United States is produced through steam methane reforming (SMR).2 However, a large amount of energy is required because SMR is highly endothermic. SMR is typically carried out at high temperatures (>750 °C) and pressures (≈2 MPa) and produces the highest hydrogen yield per carbon from the possible reaction pathways. * To whom correspondence should be addressed. Tel.: (330) 5270731. Fax: (330) 527-0761. E-mail:
[email protected]. † The University of Toledo. ‡ Catacel Corporation
Although alumina and ceria are the most common supports used for commercial reforming catalysts, academic researchers have extensively studied the effect of support on the reforming catalyst. For example, nickel on zirconia was found to be very effective for steam reforming at 500 °C, because the surface hydrophilicity allows accumulation of OH- resulting in higher H2 and CO2 production.3 Russian scientists also reported4 that, in the case of Ni/zirconia catalysts, prepared by plasma deposition of respective oxides on the metal substrate, zirconia was very effective against carbon deposition. Al2O3-supported nickel catalysts have been found to deactivate easily due to coking and sintering of Ni metal. Additionally, Ni catalysts supported on Al2O3 were also found to undergo severe deactivation due to the formation of spinel and inactive NiAl2O4 phase.5 Promoters including the alkali metal potassium and alkaline earth metals calcium and magnesium serve as carbon deposition suppressants on the Ni catalyst, but also lead to the formation of a NiAl2O4 phase and inhibit the reduction of NiO.6,7 Mixed metal oxide supported catalysts have attracted tremendous interest in recent years.8-12 Ceria addition has been of particular interest due to its oxygen storage capacity. For example, ceria addition enhanced the catalytic activity of Ni/ ZrO2 by improving metal dispersion and Rh/Al2O3 through oxidative stabilization.13,14 Ni/Ce-ZrO2, Ni/La-ZrO2, and Ni/ Si-ZrO2 catalysts were tested for steam reforming of methane, with Ni/Ce-ZrO2 exhibiting the highest activity and stability, even though it did not show any significant increase in terms of BET surface area, Ni surface area, or H2 uptake. The strong metal-support interaction and high oxygen storage capacity of Ce-ZrO2 was found to play a very important role in catalyst activity improvement.15 Ni-loaded Ce-ZrO2 catalysts have also been found to exhibit improved catalytic activity and stability compared to several conventional catalysts.12,13,16,17 While nickel remains the most widely used catalyst for natural gas steam reforming,18 noble metals such as ruthenium,19
10.1021/ie070373+ CCC: $37.00 © 2007 American Chemical Society Published on Web 11/23/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007
Figure 1. High temperature steel support and monolith catalyst strip.
iridium, rhodium,20 and platinum21,22 have been found to improve the catalyst performance for SMR.19,23-26 Noble metals exhibited higher reforming activity and less coking tendency compared to Ni catalyst.27 Rh was the most effective component in terms of the promotion in catalyst activity and inhibition of carbon deposition even when the additive amount of Rh was as low as 0.035%.23 Doping by highly dispersed Ru facilitated the reduction of Ni oxides, which eliminated the prereduction step and also greatly suppressed carbon growth on Ni/Al2O3 during steam reforming of CH4.19 The reason for substantial suppression of carbon deposition was found to be the smaller dissolution of the carbon into these noble metal solutions.28 Tests for steam reforming of methane at 450-950 °C and atmospheric pressure revealed the following order of activities: 10% Ni > 0.3% Ru ≈ 0.3% Rh ≈ 10% Co > 0.3% Pt > 0.3% Pd > 10% Fe. This suggested a higher activity of noble metals for the steam reforming reaction.25 However, the use of noble metals (Rh, Ru, and Pt) on a large scale has not been practiced due to their relatively high cost. Commercially, a steam-to-carbon (S/C) ratio of 3 is used to avoid carbon deposition on the catalyst. Since water vaporization and steam consumption have been important factors in determining the economics of this process,30 the economy of natural gas steam reforming might be improved through lower S/C ratios. When operating at lower S/C, less heat recovery will be required. As a result, the increased cost of highly stable and coke-resistant noble metal catalysts may be offset through reduced operating costs if stable performance can be achieved at low S/C ratios. An additional limitation of commercial steam reforming is the use of ceramic catalyst media, which has a coefficient of thermal expansion different from that of the reactor tubes, and thus the ceramic catalyst is crushed during thermal cycling of the reactor. The ceramic support also gives relatively poor heat transfer across the catalyst bed, which adversely affects the performance of highly endothermic methane steam reforming reaction. To overcome the above-mentioned limitations of the ceramic catalyst media, Catacel Corporation has designed the structured stackable reactor (SSR), a catalytic support material made from high temperature steel with internal metal fins that are coated with active reforming catalyst. Matching the heat transfer coefficient of the steel and reactor tubes, SSR provides very high heat transfer through highly conductive steel, and eliminates the catalyst attrition issue associated with the use of ceramic pellets. However, replacement of ceramic catalyst media with SSR decreases the reactor volume available for gas flow, thereby decreasing the residence time available for the reaction. Hence, this technological replacement requires development of
a highly active and stable catalyst that can be coated onto metal foil in order to compete successfully with ceramic catalyst technology. Experimental Section Active catalyst slurry was obtained from commercial vendors ETI and OMG and used as received. Vendors provided Ni catalyst on bimetallic support where the exact composition was unknown. Additional catalyst was prepared in the laboratory according to the following procedure. Known amounts of metal precursors were added to excess volume of the solvent (e.g., methanol, THF), and the solution was stirred to ensure complete dissolution of the precursor. This solution was then added to γ-Al2O3 received from Grace Davison. The solution was again stirred for 5-15 min. Nitric acid was then added with continuous stirring to adjust the pH of the solution. This mixture was kept on a magnetic stirrer, and solvent was allowed to evaporate slowly for 3-4 h. Finally, the catalyst powder was dried in an oven overnight at a temperature of about 50-60 °C. The prepared powder was then milled for about 30-45 min with a specific ratio of nitric acid, binder (γ-Al2O3, obtained from Grace Davison), and deionized water to control the particle size and the adhesion property in preparation for coating onto the foil. Rhodium metal (Rh(NO3)3, obtained from SigmaAldrich), if absent in the catalyst slurry obtained from the vendor, was added as salt solution to the slurry phase, and the whole mixture was agitated for 4-5 min to achieve uniform mixing, resulting in a reforming catalyst ready to be coated on the metal foil. The 1 in. × 36 in. Fe-Cr high temperature alloy strip was initially cleaned and then oxidized at elevated temperature. Finally, catalyst was spray coated to produce a catalyst foil containing 30 mg/in.2 catalyst loading. The pictorial view of the high temperature steel substrate and final catalyst strip used for carrying out the reaction is shown in Figure 1. Table 1 gives the detailed composition of all the catalysts along with the vendor details. The process flow diagram of the experimental system is shown in Figure 2. The reactor system was designed to carry out the steam reforming reactions at high pressure (300 psig) but could also be used for atmospheric runs. Argon was added in the product stream as an internal standard to calculate the product gas flow rate. The inlet flow rate of the gaseous streams, namely natural gas and argon, was controlled through the mass flow controllers (Allicat Scientific) represented as MFC (MC1SLPM-D) in Figure 2. For the high pressure runs a different mass controller (OMEGA Engineering Co. FMA-767A with OMEGA Engineering Co. DP-F64 display) was used to control the flow of natural gas. The liquid water was pumped at a desired flow rate using an ISO 1000 isocratic pump (Chrom
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9055 Table 1. Classification and Composition of Catalysts Studied noble metal loading (%) catalyst series and source
physical form
catalyst no.
support
nickel loading (%)
100, ETI/Catacel
metal foil
101
Al2O3-CeO2-ZrO2; Al2O3 ) 15-17%, CeO2 ) 22-23%, ZrO2 ) 9-10%
8-15% (NiO)
0
200, OM group/Catacel
metal foil
102 201
Al2O3-CeO2-ZrO2; Al2O3 ) 36-37%, CeO2 ) 22-23%, ZrO2 ) 9-10%
32% (NiO)
0-1 1.25
300, Catacel
metal foil
202 203 301
32% (NiO)
400, UT Laboratory
powder
Al2O3-ZrO2; Al2O3 ) 36-37%, ZrO2 ) 63-64% γ-Al2O3
1.5 2.4 1
10%
0.5
500, UT/Catacel
metal foil powder
γ-Al2O3a γ-Al2O3a
10% 10%
0.5 0.5
a
401 402 501 501A
Rh
Pd
3
With additives like cellulose, 2%; CuO, 5%; ZnO, 2.5%; and binder used in the wash-coating technology.
Figure 2. Flow sheet of reactor system used to carry out the natural gas steam reforming reaction.
Tech, Inc.). A rupture disk (Zook Enterprises) was connected in the gaseous feed line, as a safety precaution. The reactor consisted of a stainless steel tube (SS316) provided with compression couplings and proper connections for the reactant inlet and product outlet lines at the ends. Two different reactor tubes (High Pressure Equipment Company (HIP) 20-LM16-22), based on their maximum temperature and pressure ratings, were used for atmospheric and high pressure runs. Atmospheric runs used a reactor tube with an outside diameter of 0.0254 m, an inside diameter of 0.0175 m, and a length of 0.56 m, placed in a 0.46 m long, single-zone furnace (Applied Test Systems). High pressure runs used a thicker tube with the same outside diameter of 0.0254 m but an inside diameter of 0.0142 m and a length of 0.56 m. A stainless steel static mixer of about 0.15 m was also placed in the feed line prior to the reactor to ensure good mixing of the reactants at the catalyst. The product gases coming out of the reactor passed though insulated stainless steel lines to a metal condenser which was a stainless steel tube having an outside diameter of 0.0254 m, an inside diameter of 0.0175 m, and a length of 0.56 m. The same tube was used for atmospheric and high pressure runs since the
pressure ratings of the metal condenser tube were in the acceptable limits at the temperatures of interest. The gases coming out of the metal condenser passed through a safetyrelief/pressure-relief valve (Swagelok SS-4R3A) as a safety precaution. A back-pressure regulator (Tescom Co.) was used to maintain the pressure of the system at the desired value. After the back-pressure regulator the product gases passed through a glass condenser to ensure complete condensation of trace liquid products. The outlet from the glass condenser passed through a flow meter (Allicat Scientific M-2SLPM-D) to an OMNISTAR Mass Spectrometer (Pfeiffer vacuum GSS 300, GSD 301 C2), and the product gases were analyzed on-line using a Faraday detector. Quadstar-32 bit software was used for data acquisition and analysis. Condensate collected was analyzed using a total organic carbon analyzer (Shimadzu TOC VCPN) for the presence of carbon. Results and Discussion The effect of rhodium loading on catalyst activity obtained in natural gas steam reforming is shown in Figure 3. Catalyst activity was expressed in terms of methane conversion (XCH4)
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Figure 3. Evaluation of catalyst activity for commercially available washcoats: methane conversion for CAT-100 and -200 series Ni-Rh/ Al2O3-CeO2-ZrO2 catalysts (T ) 800 °C, P ) 1 atm, S/C ) 3, τ ) 250 ms).
Figure 4. Evaluation of catalyst activity for laboratory developed catalysts: methane conversion for CAT-300, -400, and -500 series (T ) 800 °C, P ) 1 and 20 atm, S/C ) 3, τP)1atm ) 250 ms, τP)20atm ) 4 s).
and compared to thermodynamic equilibrium methane conversion at the reaction temperature (800 °C) and pressures (1 and 20 atm). CAT-101 was a nickel-only catalyst without any rhodium loading, whereas other catalysts had both nickel and rhodium supported on an Al2O3-CeO2-ZrO2 matrix. A substantially higher conversion was obtained for CAT-102 compared to CAT-101, demonstrating the advantage of adding rhodium to the Ni catalyst. Researchers previously identified a similar promoting effect due to noble metal addition on the Ni catalyst for oxidative steam reforming of methane and have also reported higher activities for noble metal catalysts compared to Ni catalyst.23,29 The conversion for CAT-102 (0-1 wt % Rh) gave a methane conversion that was still below the thermodynamic equilibrium value at the reaction temperature and pressure. In order to improve the conversion, the rhodium loading was increased in further testing. CAT-201, CAT-202, and CAT-203 with 1.25, 1.5, and 2.4 wt % rhodium loading, respectively, were studied to evaluate the effect of rhodium loading on catalyst activity. As expected, results show an increase in methane conversion with an increase in rhodium loading. Methane conversion obtained for CAT-202 with 1.5 wt % rhodium almost matched the equilibrium conversion, and the methane conversion remained constant with further increase in rhodium content. CAT200 series materials were then tested at elevated pressure, and a similar increase in activity was observed as the rhodium loading increased. However, even CAT-203, containing the highest rhodium content in our sample matrix, gave a methane conversion significantly less than the equilibrium conversion calculated at 20 atm. Since commercially procured washcoats (Ni-Rh/Al2O3CeO2-ZrO2) designated as CAT-200 series of catalysts could not attain thermodynamic equilibrium conversion at high pressure even with a very high rhodium content of 2.4 wt %, different supports were investigated for improving the catalytic performance at high pressure. In addition, palladium at relatively higher loading was considered to be an active metal. These catalysts were designated as 300-500 series of catalysts in the sample matrix, as indicated in Table 1. Figure 4 shows the catalyst activity expressed in terms of methane conversion obtained during the methane steam reforming reaction for 300-500 series of catalysts. The CAT-400 series was tested in powdered form before they were coated on the
metal foil. Experiments at atmospheric pressure indicated that CAT-401 with 0.5 wt % Rh gave better performance than CAT402 with 3 wt % Pd, indicating that rhodium was more active than palladium. Methane conversion of CAT-401 fairly matched with the equilibrium methane conversion; hence CAT-401 was then coated on metal foil for its high pressure testing and was designated as CAT-501. At elevated pressure, CAT-501 (NiRh/γ-Al2O3) containing 0.5 wt % Rh gave a better performance than CAT-301 (Ni-Rh/Al2O3-ZrO2) containing 1 wt % Rh, indicating both the importance of support materials and the rhodium impregnation method onto the catalyst. CAT-501 was found to be superior in terms of catalytic performance among several catalysts prepared in our laboratory, and CAT-203 was identified as the best performing catalyst among those supplied by vendors. Thus, these materials were selected for further study. Besides demonstrating high activity in terms of methane conversion, an SMR catalyst also needs to be stable by exhibiting constant methane conversion for an extended period of time on stream. Hence, deactivation studies were performed to evaluate and quantify the catalyst stability. Each catalyst was tested for a period of 50-100 h on stream. Figure 5 shows results of one such test performed on CAT-102 wherein methane conversion (XCH4) and CO2 selectivity (SCO2 ) FCO2/(FCO + FCO2), Fi ) molar flow rate of species i in the product stream) are plotted as a function of time on stream and compared against thermodynamic equilibrium. The plot clearly indicates the deactivation of the catalyst illustrated by a decrease in methane conversion of about 6% and a smaller but still perceptible decrease in CO2 selectivity in approximately 100 h of operation. Methane conversion data as a function of time on stream can be used to estimate a reaction rate constant and a catalyst deactivation constant, assuming a reasonable model for the reaction kinetics and deactivation mechanism. Starting from first-order kinetics and first-order deactivation, allowing for gas expansion () and assuming plug flow behavior within the reactor, provides a rate expression as follows:
(
ln (1 + ) ln
(
)
)
1 - XA ) ln(kτ′) - kdt 1 - XA
(1)
where τ′ ) WCA,0/FA,0 is weight-time expressed in g of catalyst‚min/cm3. Thus, plotting ln((1 + ) ln[1/(1 - XA)] XA) versus time on stream (t) should yield a straight line, where
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9057
Figure 5. Performance of CAT-102 as a function of time on stream (T ) 800 °C, P ) 1 atm, S/C ) 3, τ ) 250 ms). Table 2. Kinetic Results Indicating Estimated Values of First-Order Rate Constants and Deactivation Constants from Trial Catalysts; All Experiments at 800 °C catalyst
P (atm)
k0 (cm3/g of catalyst min)
kd × 105 (h-1)
CAT-101 CAT-102 CAT-201 CAT-201 CAT-202 CAT-202 CAT-203 CAT-203 CAT-301 CAT-401 CAT-402 CAT-501
1 1 1 20 1 20 1 20 20 1 1 20
1604.7 ( 139.2 7436.6 ( 276.9 8367.9 ( 258.8 43.737 ( 1.464 11007 ( 91 54.957 ( 2.443 10999 ( 64, 9611.5 ( 936.9a 83.634 ( 0.416, 627.58 ( 25.53a 160.78 ( 3.77 13416 ( 160 9148.4 ( 224.9 338.08 ( 0.34, 798.28 ( 26.25a
-532.1 ( 132.5 360.6 ( 57.82 164.3 ( 41.94 150.9 ( 48.23 116.1 ( 28.11 834.5 ( 159.8 83.36 ( 20.69 61.37 ( 31.94 409.5 ( 134.2 -86.17 ( 45.31 1102 ( 103.3 4.705 ( 1.535
a These estimates for first-order rate constants were obtained from independent experiments in which W/FA,0 was varied.
the slope is equivalent to the deactivation constant kd, and the intercept can be related to the assumed first-order rate constant, k. A statistical analysis was performed using the ANOVA regression tool in Microsoft Excel to estimate deactivation constants, along with their 95% confidence limits; the results are provided in Table 2. A positive value for the deactivation constant implies deactivation with time, whereas a negative value implies that catalyst activity increased with time on stream. Of those catalysts prepared from slurries provided by commercial vendors, CAT-203, with the highest rhodium content, gave the smallest deactivation constant of all catalysts in the 200 series, suggesting that it was the most stable catalyst. CAT201 and CAT-203 showed only a very minor effect of pressure on deactivation, whereas CAT-202 exhibited a relatively higher rate of deactivation at high pressure compared to atmospheric pressure. Among the in-house formulations, Ni-Rh/γ-Al2O3 (CAT-501) catalyst exhibited the lowest deactivation constant value, suggesting a stable performance of this catalyst. While CAT-401 (Ni-Rh/γ-Al2O3) gave a slightly negative deactivation value, CAT-402 (Ni-Pd/γ-Al2O3) exhibited a very high deactivation constant, indicating a very high rate of deactivation for the palladium catalyst. Palladium sintering is a very common phenomenon reported in the oxidizing steam environment and is known to cause deactivation of palladium catalysts.30 Deactivation studies suggested that Ni-Rh/γ-Al2O3 catalyst with 0.5 wt % rhodium was the best performing catalyst in the sample matrix. The estimated deactivation rate constant can be
Figure 6. Effect of W/FA,0 on CAT-203 and CAT-501 at different pressures (T ) 800 °C, S/C ) 3).
used to estimate the catalyst activity after extended time on stream. For example, the methane conversion obtained from CAT-501 after 8000 h on stream at 20 atm is predicted to decrease by about 10-11%. Extrapolation from short time data to long times on stream is always suspect, and thus the indicated estimate should be considered with due caution. As one of the goals of the current work was to demonstrate enhanced catalyst performance at short residence times, the performance of CAT-203 and CAT-501 were studied as a function of W/FA,0 at two different pressures (1 and 20 atm). At both pressures, W/FA,0 was varied over the range of 10400 g of catalyst‚min/mol, corresponding to a residence time of 0.5-16 s in the catalyst bed at high reaction pressures. The reactant gas flow rates were kept constant for 3-5 h at each W/FA,0, and conversion was measured after a steady state had been achieved. Figure 6 shows the influence of W/FA,0 on methane conversion. Results for CAT-203 at atmospheric pressure and CAT-501 at 20 atm showed the expected increase in methane conversion with an increase in W/FA,0, with an asymptotic approach to equilibrium conversion at higher values of W/FA,0. However, CAT-203 approached equilibrium conversion at a relatively low value of W/FA,0 but then the conversion decreased at higher W/FA,0 values. Although it is not clear why this should occur, these results were reproducible and may be explained by competing reactions that become more significant at longer residence times. Further insight can be obtained from Figure 7, which provides composition information as a function of W/FA,0. Note that the selectivity to CO (relative to CO2) increases at higher W/FA,0, and the ratio of H2/CO decreases. The water gas shift reaction
CO + H2O S CO2 + H2 competes with steam reforming, particularly at higher W/FA,0. If the reverse reaction (as written) is favored at higher W/FA,0, then the product selectivity would vary as indicated in Figure 7. The performance equation shown previously as eq 1 can be rearranged for a nondeactivated catalyst as
(1 + ) ln
(
)
1 - XA 1 - XA W )k CA,0 FA,0
(2)
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Figure 7. Effect of W/FA,0 on methane conversion, CO selectivity, and H2/CO ratio for CAT-203 (T ) 800 °C, P ) 20 atm, S/C ) 3).
Thus an appropriate plot of the left-hand side vs W/FA,0 should yield a straight line, the slope of which is the value of the assumed first-order rate constant. Rate constants and 95% confidence limits were determined by performing statistical analysis using the ANOVA regression tool in Microsoft Excel, with the resulting values reported in Table 2. Comparing the first-order kinetic rate constant values obtained for CAT-203 and CAT-501 by the above method against the one determined from the intercept of the deactivation plot generated for reaction carried out at constant W/FA,0 shows that values at 1 atm are relatively close, whereas there is a substantial difference in the values at 20 atm. A possible reason for this difference is that the reaction at constant W/FA,0 was carried out at very high W/FA,0, where mass transfer limitations adversely affected the kinetic rate constant values. As a result, the mass transfer limited values obtained at 20 atm are smaller relative to the kinetic values obtained from the lower W/FA,0 region. However, the mass transfer limitation is absent for the atmospheric runs because of the higher linear velocities across the catalyst bed. Estimation of the Thiele modulus and effectiveness factor shows a value close to 1 for atmospheric pressure runs, confirming the absence of mass transfer limitations for these cases. Thus the rate constants obtained from these two methods are similar, within the experimental limitation. In order to evaluate the effect of temperature on the steam reforming reaction, methane conversion over CAT-203 and CAT-501 was evaluated in the range of 500-900 °C, maintaining a steam-to-carbon (S/C) ratio of 3, with the results as shown in Figure 8. The reaction temperature and reactant gas flow rates were kept constant for 3-5 h at each temperature before the temperature set point was changed. Equilibrium conversion was obtained for CAT-203 at nearly all conditions, except at high temperature and 20 atm. On the other hand, CAT-501 promoted equilibrium conversion over the whole range of temperatures, confirming the fact that CAT-501 was a better catalyst for natural gas steam reforming at high temperatures and high pressures. These results were used to determine the activation energies for these catalysts, which are reported in Table 3. The activation energy of CAT-501 is slightly lower than that of CAT-203, once again indicating the advantage of CAT-501 relative to commercially available materials for use at elevated pressure and high temperature. Table 3 also provides a direct comparison of the activation energies obtained in this work with those available in the literature on Ni and Ni/Rh catalysts on
Figure 8. Effect of temperature on activity of CAT-203 and CAT-501 at different pressures (P ) 1 atm, S/C ) 3, τ ) 250 ms; and P ) 20 atm, S/C ) 3, τ ) 8.38 s). Table 3. Activation Energies Calculated for Selected Catalysts activation energies (kJ/mol) (E ( 95% confidence limits) catalyst CAT-203 CAT-501 sulfided nickel on γ-Al2O331 Ni/R-Al2O332 Ni-Rh/γ-Al2O333
P ) 1 atm 28.11 ( 2.18
P ) 20 atm 30.73 ( 2.26 27.71 ( 1.90
211.5 109.4 54.3
alumina supports.31,32 Interestingly, the value obtained here is substantially lower than those obtained previously, most likely a result from the inclusion of rhodium in the current formulation. One additional estimate was recently reported, for Ni/Rh on γ-alumina supported on metal substrate,33 which provides a value much closer to that obtained in the current work. The use of metal-supported catalyst for the SSR application provides an advantage in terms of lower pressure drop, reduction in internal mass transfer limitations, and better heat transfer (a particular advantage for a highly endothermic reaction such as natural gas steam reforming (∆Hr ) 205 kJ/mol)). In order to verify the performance of the supported catalyst relative to the powder, temperature ramp reactions were carried out. A 0.29 g sample of catalyst was supported on a 4 in. metal foil strip and compared with an equal weight of catalyst used in powder form. The amount of total catalyst powder required to maintain the same W/FA,0 and same residence time of the reactants in the catalyst bed between the two reactions was obtained by diluting 0.29 g of active catalyst powder with inactive γ-Al2O3 support. This powdered catalyst was referred to as CAT-501A. To confirm the inactivity of γ-Al2O3 for natural gas steam reforming reaction, a reaction with blank γ-Al2O3 support was carried out at atmospheric pressure and 800 °C. Very low methane conversions in the range of 2-5% were obtained. The activity of these two catalysts was studied over a range of temperature from 650 to 900 °C maintaining a constant residence time at each temperature, with the results shown in Figure 9. The catalyst supported on metal foil gave a higher activity than the powder catalyst over the entire range of temperature, and close to equilibrium conversion. Researchers have previously observed a similar enhancement in catalyst performance using microchannel technology, suggesting that the metal foil support might provide benefits similar to those obtained using microchannel reactors.27
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9059 Table 4. Kinetic Results Indicating Estimated Values of First-Order Rate Constants and Deactivation Constants from CAT-501 at Different S/C Ratios; All Experiments at 800 °C and 20 atm
Figure 9. Effect of catalyst form on the activity of CAT-501 (P ) 20 atm, S/C ) 3, τ ) 4 s).
Figure 10. Effect of S/C on the performance of CAT-501 (T ) 800 °C, P ) 20 atm, τ ) 4 s).
One of the objectives of the current research was to develop a catalyst that can be used for carrying out natural gas steam reforming on a commercial scale. While a steam-to-carbon ratio (S/C) of 1 is sufficient to carry out the steam reforming reaction, excess steam enhances the water gas shift reaction according to Le Chatelier’s principle. Also, excess steam reduces the deactivation of the catalyst by minimizing carbon formation on the catalyst surface. While a S/C ratio of 3 is normally used in commercial processes, natural gas steam reforming at a low S/C ratio could be advantageous because (a) low CO2 production with high CO/CO2 ratios can be achieved, beneficial for downstream processes such as methanol synthesis and oxo synthesis,27 and (b) reduced energy for vaporizing and heating water is required.34 As a result, the reforming activity of CAT-501 was evaluated over a range of S/C ratios from 1 to 3 at 20 atm and 800 °C, maintaining a residence time of 4 s at each S/C ratio. Figure 10 shows the effect of S/C ratio on the performance of CAT-501 for natural gas steam reforming. Solid curves indicate the methane conversion and CO2 selectivity values at thermodynamic equilibrium over different S/C ratios. Methane conversion increases with S/C ratio due to an increase in the
S/C
k0 (cm3/g of catalyst min)
kd × 105 (h-1)
3 2.5 2 1.5 1
337.73 ( 0.35 285.49 ( 0.08 233.54 ( 0.40 178.01 ( 0.73 155.72 ( 2.66
4.705 ( 1.535 6.055 ( 3.753 132.1 ( 27.08 230.9 ( 64.62 432.9 ( 37.46
extent of water gas shift reaction indicated by the increase in CO2 selectivity. Comparing the values of methane conversion and CO2 selectivity obtained for CAT-501 against the thermodynamic equilibrium values demonstrates that this catalyst exhibits excellent activity at all S/C ratios. The reaction was run for about 15 h at each S/C ratio, and catalyst stability was studied by determining the deactivation constants at each S/C ratio, as reported in Table 4. Deactivation constant values at S/C ratios of 3 and 2.5 were found to be very low, suggesting a stable performance of CAT-501 at these S/C ratios. As the S/C ratio decreased, higher deactivation rate constants were obtained. Deactivation was identified based on a decrease of about 1% methane conversion every 10 h for a S/C ratio of 1. Researchers who previously studied the effect of S/C ratio on the stability of 10% Rh/MgO-Al2O3 reported no noticeable deactivation of the catalyst for a very short time on stream of about 5 h at each S/C ratio,27 perhaps due to their higher rhodium loadings or shorter time on stream (or both). The objective of the current research was to develop a catalyst that could be used commercially for carrying out natural gas steam reforming. Since Ni-Rh/γ-Al2O3 (0.5% Rh), referred to as CAT-501, was the best performing catalyst among the catalysts studied, scale-up calculations were performed using the results obtained for this catalyst and then compared with the performance of an existing pilot plant operation. A commercial application using ceramic pellets exhibits a specific reaction rate of ≈1 mol g of catalyst-1 h-1 at a residence time of about 8.5 s. Replacement of the ceramic catalyst by SSR technology blocks off about half the volume in the center of the reactor tube, thereby reducing the active volume available. A commercial system generates approximately 150 m3/h hydrogen using roughly 3.2 kg of ceramic catalyst. Scale-up results for CAT-501 predict a hydrogen productivity of about 350 m3/h for a pilot plant using the same catalyst weight, an operating temperature of 800 °C, and a pressure of 20 atm, with a residence time of 1.43 s. Thus, it is shown that CAT-501 may be used in the SSR configuration as a replacement for commercial methane steam reforming catalyst supported on ceramic beads. Conclusions Catalyst performance results for Ni-Rh/γ-Al2O3 and NiPd/γ-Al2O3 catalysts revealed a higher activity and stability for steam reforming of natural gas catalyzed by rhodium, indicating that it is the preferred noble metal for improvement of the nickel catalyst. Tests on Ni-Rh/Al2O3-CeO2-ZrO2 catalysts with increased rhodium loading confirmed an increase in catalyst activity and stability, leading to an increase in the extent of natural gas steam reforming reaction. Comparison between the performance of commercially available Ni-Rh/Al2O3-CeO2ZrO2 catalysts and laboratory-made Ni-Rh/γ-Al2O3 catalysts revealed γ-Al2O3 as a better support material at elevated pressure as it exhibited higher activity and stability with much smaller rhodium content. That might be attributed to the absence of any sintering for Ni-Rh/γ-Al2O3 catalysts at high pressure reaction conditions.
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Ni-Rh/γ-Al2O3 catalyst with 0.5 wt % rhodium and 10 wt % nickel was found to be the best performing catalyst of those evaluated, achieving thermodynamic equilibrium methane conversion at atmospheric and elevated pressures. Kinetic studies revealed that this catalyst also exhibited higher rate constant and lower activation energy compared to those catalysts available from commercial vendors. The performance of the Ni-Rh/γ-Al2O3 catalyst when supported on metal foil was shown to be enhanced relative to its performance as a powder, which was attributed to mass and heat transfer advantages. NiRh/γ-Al2O3 catalyst also gave an excellent performance matching the equilibrium performance at lower S/C ratios in the range of 3 and 1. Scale-up calculations suggest that the CAT-501 formulation, supported on metal foil as in the SSR, may be appropriate as a replacement system for commercial methane steam reforming operations. Acknowledgment Funding for this research was provided by the Edison Materials Technology Center through a grant from the U.S. Department of Energy. Literature Cited (1) Ahmed, S.; Krumpelt, M. Hydrogen from hydrocarbon fuels for fuel cells. Int. J. Hydrogen Energy 2001, 26, 291. (2) Hydrogen fact sheet: Hydrogen production-Steam methane reforming (SMR). http://www.getenergysmart.org/Files/HydrogenEducation/6HydrogenProductionSteamMethaneReforming.pdf (accessed Dec 8, 2006), part of www.nyserda.org (accessed Dec 8, 2006). (3) Matsumura, Y.; Nakamori, T. Steam reforming of methane over nickel catalysts at low reaction temperature. Appl. Catal., A 2004, 258 (1), 107. (4) Makunin, A. V.; Serdyukov, S. I.; Safonov, M. S. Methane steam reforming over nickel-zirconium oxide catalysts. Neftekhimiya 1996, 36 (5), 418-442. (5) Gadalla, A. M.; Bower, B. The role of catalyst support on the activity of nickel for reforming methane with carbon dioxide. Chem. Eng. Sci. 1988, 43 (11), 3049. (6) Hou, Z.; Yokota, O.; Tanaka, T.; Yashima, T. Characterization of Ca-promoted Ni/R-Al2O3 catalyst for CH4 reforming with CO2. Appl. Catal., A 2003, 253, 381. (7) Hou, Z.; Yokota, O.; Tanaka, T.; Yashima, T. A novel KCaNi/RAl2O3 catalyst for CH4 reforming with CO2. Catal. Lett. 2003, 87, 37. (8) Fornasiero, P.; Monte, R. D.; Rao, G. R.; Kaspar, J.; Meriani, S.; Trovarelli, A.; Graziani, M. Rh-loaded CeO2-ZrO2 solid-solutions as highly efficient oxygen exchangers: Dependence of the reduction behavior and the oxygen storage capacity on the structural-properties. J. Catal. 1995, 151, 168. (9) Leitenburg, C.; Trovarelli, A.; Llorca, J.; Cavani, F.; Bini, G. The effect of doping CeO2 with zirconium in the oxidation of isobutene. Appl. Catal., A 1996, 139, 161. (10) Hori, C. E.; Permana, H.; Ng, K. Y. S.; Brenner, A.; More, K.; Rahmoeller, K. M.; Belton, D. Thermal stability of oxygen storage properties in a mixed CeO2-ZrO2 system. Appl. Catal., B 1998, 16, 105. (11) Thammachart, M.; Meeyoo, V.; Risksomboon, T.; Osuwan, S. Catalytic activity of CeO2-ZrO2 mixed oxide catalysts prepared via solgel technique: CO oxidation. Catal. Today 2001, 68, 53. (12) Bozo, C.; Gaillard, F.; Guilhaume, N. Characterization of ceriazirconia solid solutions after hydrothermal ageing. Appl. Catal., A 2001, 220, 69. (13) Dong, W.-S.; Roh, H.-S.; Jun, K.-W.; Park, S.-E.; Oh, Y.-S. Methane reforming over Ni/Ce-ZrO2 catalysts: Effect of nickel content. Appl. Catal., A 2002, 226, 63. (14) Kurungot, S.; Yamaguchi, T. Stability improvement of Rh/Al2O3 catalyst layer by ceria doping for steam reforming in an integrated catalytic membrane reactor system. Catal. Lett. 2004, 92 (3-4), 181.
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ReceiVed for reView March 12, 2007 ReVised manuscript receiVed September 11, 2007 Accepted September 20, 2007 IE070373+
