Microreactor Technology and Process Intensification - American

Chapter 7

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: August 9, 2005 | doi: 10.1021/bk-2005-0914.ch007

Fuel Reformation: Catalyst Requirements in Microchannel Architectures D. L. King, K. Brooks, C. Fischer, L. Pederson, G. Rawlings, V. S. Stenkamp, W. TeGrotenhuis, R. Wegeng, and G. Whyatt PacificNorthwestNationalLaboratory,P.O.Box999,Richland,WA99352

Microchannel reactors have unique capabilities for onboard hydrocarbon fuel processing, due to their ability to provide process intensification through high heat and mass transfer, leading to smaller and more efficient reactors. The catalyst requirements in microchannel devices are demanding, requiring high activity, very low deactivation rates, and strong adherence to engineered substrate. Each unit operation benefits from microchannel architecture: the steam reforming reactor removes heat transfer limitations, allowing the catalyst to operate at elevated temperatures at the kinetic limit; the water gas shift reactor uses unique temperature control to reduce catalyst volume requirements; the P R O X reactor provides high C O conversion and minimizes H oxidation through effective control of reactor temperature. 2

© 2005 American Chemical Society

In Microreactor Technology and Process Intensification; Wang, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

119

120 Introduction: The Case for MicroChannel Reactors in Onboard Fuel Processing There are a number of reasons for the implementation of microchannel reactors for onboard hydrocarbon fuel processing via steam reforming. These include: •

Compactness. MicroChannel reactors have high surface to volume ratios, reducing or removing the limitations arising from heat or mass transfer. This allows the catalyst to operate at its kinetic limit. In the case of steam reforming, the kinetic activity of steam reforming catalysts, especially precious metal catalysts, is substantial. For example, turnover numbers for methane steam reforming exceed 10 sec" at 873K for supported platinum [1]. Conventional wisdom that steam reforming is an inherently slow process is incorrect and is based on the assumption that, for larger scale systems, it is difficult to get heat into the catalyst at a rapid enough rate to sustain the endothermic reaction. This is not a limitation with microchannel reactors. A s a result of the high activity that can be realized, reactor volume can be substantially reduced.


1

•

Efficiency. Improved thermal management provides high heat transfer effectiveness in heat exchangers and reactors for maximum heat utilization and high fuel efficiency in microchannel systems. Laminar flow results in very low pressure drop. In typical operation of the P E M fuel cell, hydrogen-containing anode tail gas is produced as consumption of hydrogen is incomplete. A benefit of steam reforming is that the anode tail gas can be burned, providing heat to sustain the endothermic steam reforming reaction. This increases the efficiency in comparison with autothermal reforming or catalytic partial oxidation, where die nitrogen-diluted tail has minimal utility. Moreover, efficiency is gained by producing higher H2 partial pressure in the reformate with no N2 dilution, in comparison with autothermal or partial oxidation approaches.

•

Temperature control. Close and responsive temperature is possible with microchannel devices, providing rapid response to load demands. Temperature control provides benefits in onboard fuel processing, for example for the water gas shift and P R O X reactions. For example, it is possible to control or minimize reaction exotherms, allowing close to isothermal operation or, in some instances, control of the reaction temperature along the axis of flow. This is important for the P R O X reaction. For water gas shift, where there is a significant tradeoff between kinetics and thermodynamics of C O conversion, the ability to decrease the temperature along the flow path in a controllable fashion can lead to higher C O conversion than is available through adiabatic or isothermal operation for the same reactor volume [2].


121


Catalyst Challenges Operation of microchannel reactors is demanding in terms of the performance of the catalyst. Because of the limited volume available in a microchannel reactor, high catalyst activity is required. This requires control of both the chemical formulation of the catalyst and its textural properties on the engineered substrate or support. The operation of the catalyst at or near the kinetic limit implies that there is no excess catalyst for the process to fall back on i f deactivation occurs, thus highly stable catalysts are required. The design of microchannel reactors may require sealing the catalyst in hardware, so that it is difficult to remove and replace the catalyst i f deactivation does occur. Therefore, the catalyst must be long-lived or easily regenerable in situ. Operation in microchannels typically leads to short residence times and high flow rates. Strong adherence of the catalyst to the engineered substrate under high flow conditions is necessary, as catalyst spalling could lead to both loss of activity and possible channel plugging. In our work to date, catalysts have typically been coated onto felts, foams, or monoliths that are then inserted into the microchannel reactor, therefore means to assure good adhesion of the catalyst to the substrate is required. A n alternative method of employing the catalyst is to coat it onto the walls of the reactor. This provides even better heat transfer to the catalyst, however the methods to assure adequate and uniform coating of the catalyst to the reactor walls, especially after the primary assembly of the device, is challenging and has not been pursued thus far in our studies.

Catalyst Preparation Methods

We have used engineered catalyst supports, based on either metal felts or foams. Examples are shown in Figure 1. The majority of the testing in the single channel reactor employed FeCrAlY metallic felts (Technetics; 24% dense). In our single channel test reactor (Figure 2), the strips were cut from sheets of the FeCrAl felt, with dimensions typically lx8x.025 cm. Calcination of the felts at 890°C in air for 6-10 hours facilitates migration of the aluminum to the external surface of the felt, providing a surface of polycrystalline aluminum oxide to which the catalyst readily adheres, as seen in the S E M photographs provided in Figure 3. As-received catalysts were ball milled to facilitate producing a high solids content suspension in isopropanol solvent and a more uniform particle size



122

Figure 1. Engineered Catalyst Supports

Figure 2. Single Channel Test Reactor

Figure 3. SEM of Aluminum Oxide Surface Layer on FeCrAlY Felts at Different Magnification.



123 distribution. Ball milling was for a period of 10 minutes and achieved a submicron particle size. The catalyst was added to the support without a binder by several dip-and-dry cycles in the stirred suspension, with drying provided with flowing nitrogen to remove any catalyst particles that were not strongly adhering to the support. The final catalyst loading varied between 40-250 mg per felt strip. A cross section of the felt coated with catalyst at high and low loadings is shown in Figure 4. The lighter regions are the metallic felt filaments, the darker regions contain the catalyst. Electrophoretic deposition of the catalyst onto the support was also evaluated as a means to replace the tedious dip-and dry approach. However, we found evidence of catalyst spalling with the electrophoretic method, and did not pursue methods to optimize this latter approach. Catalysts Catalysts were predominantly obtained from vendors, in order to assure that any high performing catalysts could be obtained at significant scale. Since many of the catalysts obtained were "developmental", a non-analysis agreement was typically required, so that characterization of catalyst post reaction was only possible by working with the vendor. This is the tradeoff under which we chose to operate. In most cases, we obtained catalyst powders from the vendor, allowing us to control the amount of catalyst deposited on the support. In other cases the vendor carried out the coating onto our felt supports, somewhat limiting flexibility. For the steam reforming reaction, a proprietary precious metal catalyst from Battelle was utilized. For the water gas shift reaction, we evaluated both copperbased and precious metal-based catalysts provided by Slid Chemie Inc. For our purposes, the superior activity of the precious metal catalysts was determined to be critical to the operation of our unit, in order to minimize catalyst requirements. For the P R O X reaction, we employed a combination of nonprecious metal catalyst from Sud Chemie and precious metal catalyst from Engelhard Corporation. MicroChannel Reactor Testing Many of the details of the microchannel hardware employed for fuel reforming are provided in a paper by K . P . Brooks, et. a l , included in this monograph [3],



124

Figure 4. Cross Section of Ball Milled Catalyst on FeCrAlY Felt at Two Different Loadings.

Steam Reformation The steam reforming reaction stoichiometry for alkane feedstock can be summarized: C„H

2n+2

+ n H 0 = nCO + (2n+l)H 2

2

The reaction is endothermic, and heat is provided by flowing fuel through a combustor and then into a separate set of interleaved channels (no catalyst within the channels). The efficient microchannel heat exchange provides heat necessary to support the steam reforming reaction. A n important goal is to minimize hydrocarbon slip (unconverted hydrocarbon). Table 1 shows the operating temperature, space velocity (on catalyst), and calculated reactor core volume necessary to provide reformate for a 50kWe fuel cell with no more than 0.1% hydrocarbon slip. These data were obtained using benchmark fuel as feedstock (74% isooctane, 20% xylenes, 5% methyl cyclohexane, 1% 1-pentene) and a S/C ratio of 3. High temperature operation is clearly beneficial in reducing reformer volume.


125 Table 1; Microchannel Steam Reformer Performance With Precious Metal Catalyst Temperature Gas Hourly Space Velocity, h f Reformer volume, Kter

U

650 C 83,000 12.7

1

U

850 C 257,000 4.1


The water gas shift reaction also occurs over this catalyst, CO + H 0 = C 0 + H , 2

2

2

(1)

and generally reaches equilibrium (CO concentration in the range of 10-14 vol%) under the conditions of operation. Water Gas Shift In this unit operation, C O is converted to less than 1 v o l % (H 0-free basis) according to reaction (1). The reaction is mildly exothermic, ~9.5 kcal/mole. Typically water gas shift is carried out in two stages [4]. The first stage operates at 370-400°C with iron-chrome catalysts, producing C O in the range of 2-4 vol %. The second stage, based on copper-zinc catalyst formulations, reduces C O to as low as 0.1-0.3 vol % operating at or below 250°C. In an onboard fuel processor, this combination of catalysts results in large catalyst beds. Moreover, copper-based catalysts are sensitive to elevated temperatures, unstable in the presence of liquid water (which may collect during shutdown) and pyrophoric (air leakage into the catalyst bed may also occur during frequent startup and shutdown). For this reason, we chose to employ precious metal water gas shift catalysts. These catalysts operate over a wide temperature range, and have acceptable activity for a compact fuel processor. Potential concerns regarding these catalysts are their cost and their long term stability in the microchannel environment under frequent startup and shutdown. We are continuing to investigate the latter issue. 2

Preferential C O Oxidation (PROX) The preferential C O oxidation reaction, 0 0 + 72 02 = 002,

(2)

is employed to convert the C O exiting the water gas shift reactor to a concentration of less than ~10ppm. Low C O concentration is required as C O


126 poisons the anode of the P E M fuel c e l l The reaction is exothermic, ΔΗ = -67.8 kcal/mole. The challenge is to reduce C O to these low levels without oxidizing the H , present in the reformate at much higher concentrations, to H 0 . Reaction exothermicity exacerbates the challenge, as with increasing temperature the P R O X catalyst becomes much less selective toward C O . For this reason, the P R O X is typically a series of staged adiabatic beds with interstage cooling. MicroChannel hardware with excellent heat transfer has the potential to operate the first stage reactor at near isothermal conditions, leading to low C O exit concentration and minimizing the load on the second stage reactor. The C O output from the second stage reactor is sensitive to temperature and again can benefit from the temperature control provided by the microchannel architecture. Our testing in the single channel reactor identified both precious metal and nonprecious metal catalysts that successfully converted C O to C 0 to below 0.1 vol% with approximately 50% selectivity (50% selectivity means as many moles of H were oxidized as were C O , a tolerable amount when initial C O concentration is low). The conditions for the test were: 190-200°C; 0 : C O = 1; gas hourly space velocity (GHSV) = 100,000, see Figure 5. In order to achieve the final 10 ppm C O , the second stage P R O X reactor operated with a precious metal catalyst at a lower space velocity of 50,000 hr" and lower temperature of 120°C and required slightly higher 0 : C O ratio of 1.4 to achieve the necessary level of C O conversion, as shown in Figure 6. Further details are provided in the paper by Brooks et. al. in this same monograph [3].


2

2

2

2

2

1

2

Summary and Conclusions MicroChannel reactors have unique capabilities for onboard hydrocarbon fuel processing, due to their ability to provide process intensification through high heat and mass transfer, leading to smaller and more efficient reactors. The catalyst requirements in microchannel devices are demanding, requiring high activity, very low deactivation rates, and strong adherence to engineered substrate. Each unit operation benefits from microchannel architecture. The steam reforming reactor achieves high catalyst activity realized by removing heat transfer limitations, allowing the catalyst to operate at elevated temperatures at the kinetic limit. The water gas shift reactor takes advantage of the high activity of the precious metal water gas shift catalyst and unique temperature control to reduces catalyst volume requirements by trading off kinetics and thermodynamics. The two stage P R O X reactor provides high C O conversion and minimizes H oxidation through effective control of reactor temperature. 2


127 Wet GH8V • 100,000, S/G » 0.3,0 /CO =» 1.0


2

40

-I

,

,

,

,

170

180

190

200

210

.

1 220

Temperature, °C Figure 5. First Stage PROX Reaction With Non-precious Metal Catalyst

0.1%CQ 120°C,S/G = 0.3

50%

40%")

03

,

r

0.6

0.7

,

OA

,

0Λ

,

,

•

,

1

1.1

12

13

r-

14

1 1Λ

CVCO Ratio

Figure 6. Second Stage PROX Reaction With Precious Metal CatalystEffect ofOi/CO

Ratio


128 Acknowledgments Support from the U.S. Department of Energy. Office of FreedomCAR & Vehicle Technologies Program, with direction from Dr. Nancy Garland, is gratefully acknowledged.


References [1] J. Wei, and E . Iglesia. Mechanism and Site Requirement for Activation and Chemical Conversion o f Methane on Supported Pt clusters and Turnover Rate Comparisons Among Noble Metals. J. Phys, Chem. Β 2004, 108, 4094-4103. [2] W . E . TeGrotenhuis, K . P . Brooks, R . A . Dagle, B . F . Davis, J . M . Davis, J. Holladay, M . J . Kapadia, D . L . King, L . R . Pederson, V . S . Stenkamp, R.S. Wegeng. Microchannel Reformate Cleanup: Water Gas Shift and Preferential Oxidation. 2004 DOE Hydrogen, Fuel Cells & Infrastructure Technologies Program Review, Poster Presentation, May 2004. [3] K . P . Brooks, J . M . Davis, C.M. Fischer, D . L . King, L . R . Pederson, G . C . Rawlings, V . S . Stenkamp, W . Tegrotenhuis, R.S. Wegeng, G . A . Whyatt. Fuel Reformation: Microchannel Reactor Design. Microreaction Technology and Process Intensification, A C S National Meeting, New York, September 8, 2003. [4] L . Lloyd, D . E . Ridler, and M . V . Twigg. The Water Gas Shift Reaction. Catalyst Handbook, 2 ed., Chapter 6. M.V. Twigg, ed. nd


Microreactor Technology and Process Intensification - American

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