Institute for Space Systems Operations * 2001 Annual Report * 98-101

Advanced Catalysts and Reactors for Mars Exploration Sabatier Processors

James T. Richardson, Ph.D., Professor, Department of Chemical Engineering, Cullen College of Engineering

In-situ resource utilization (ISRU) is one of five areas with the highest cost leverage for manned missions to Mars.¹ Specifically, manufacturing the propellant for the return journey from Martian resources reduces earth-to-orbit mass by 20-45 percent, thereby increasing the cost-effectiveness of the mission. NASA plans to establish chemical plants on Mars prior to the arrival of the first astronauts. These chemical plants will process carbon dioxide from the Martian atmosphere to make methane, using the Sabatier reaction,

with H₂ shipped from Earth or produced from indigenous water. The principal product, CH₄, will be used as fuel for the return journey. In addition, H₂O produced in Reaction (1) can be converted via electrolysis to generate more H₂ (recycled to the process) and O₂ (for use as an oxidant and for life support). System studies show that the cost of Earth launch is reduced by one half, since a unit of CH₄ generates more thrust than the H₂ used to make it, and less cryogenic storage is required.

Reaction (1) occurs in a tubular fixed-bed of catalyst pellets comprising Ru dispersed on g-Al₂O₃. The thermodynamics of the reaction indicate high exothermicity and equilibrium limitation. Heat liberated must be removed to prevent temperature increases in the bed that could destroy the catalyst and the reactor. Usually, long, narrow reactor tubes with high surface-to-volume ratios transfer the heat of reaction outside to some heat transfer medium. A problem is the resulting high-pressure drop, usually overcome by using bigger catalyst pellets. This, in turn, leads to mass diffusion restrictions, so that the effectiveness of the catalyst decreases and larger reactor volumes are needed. Since the main objective of ISRU is mass reduction, increasing heat transfer from the tubes will remove these problems and give a more compact, efficient and safer reactor system.

In this ISSO project, we have explored the feasibility of using catalyst beds made of ceramic foam to accomplish better heat removal from NASA's Sabatier reactor.

Technical Plan and Equipment
Ceramic foams are preformed retriculated structures that are positive images of plastic foams.^2,3 They exhibit extremely high porosities (85 to 90 percent), formed by megapores 0.04 to 1.5 mm in diameter, and spherical-like cells connected through windows. Figure 1 demonstrates these features with a micrograph of an a-Al₂O₃ foam with a pore density of 30 pore per inch (PPI). The pore structure has a high degree of interconnectivity and is characterized by a mean pore diameter d_p. High bed porosity is important, since it gives a much lower pressure drop in a reactor filled with a foam "cartridge" rather than packed particles, thus reducing compression requirements.³ Furthermore, the extensive pore tortuosity enhances turbulence, mixing, and transport. These features result in significant advantages for catalytic processes limited by mass or heat transfer.

Figure 1. Microstructure of 30 PPI a-Al₂O₃ ceramic foam.

We have measured pressure drop over a wide range of foam samples, and the results confirm a pressure drop advantage of over a factor of ten for foam beds compared to an equivalent bed of packed particles.⁴ Similarly, heat transfer measurements showed that radial heat transfer out of the tubes increases by a factor of five to seven (Fig. 2).⁵

Figure 2. Comparison of radial heat transfer with foam and pellet catalyst beds.

In this project, we have investigated the use of ceramic foam structures in Sabatier reactors planned for Mars missions. The principal objectives were (1) to demonstrate that ceramic foams could be successfully loaded with catalyst formulations identical to the Sabatier pellets, (2) to measure kinetic rate equations for the foam-Sabatier reactor and compare them to the pellets, and (3) to construct a simulation model for the reactor based on experimentally-determined parameters.

Based on previous experience, the following advantages were expected:

Higher volume activity. The external surface of ceramic foam is equivalent to a bed of small particles, much smaller than could be used in a reactor. Mass transfer and pore diffusional resistances to reaction are reduced, and the bed should exhibit much higher catalytic activity per unit volume, resulting in smaller reactors, i.e. reduced mass.

Greatly enhanced heat transfer. Figure 2 demonstrates the effect of increasing the wall heat transfer coefficients, with a corresponding temperature decrease in the reactor temperature, resulting in improved overall heat control. Operation of the reactor is more stable, and catalyst deactivation, due to sintering at high temperatures, greatly reduced. In addition, reaction heat is more easily recovered for use in other parts of the plant.

Lower pressure drop. Pressure drop is reduced by about a factor of ten, thereby saving mass since smaller pumps will suffice. Energy consumption is also be lowered.

Results and Discussion
Objective No. 1. Loading the foam with the catalyst. Sabatier catalyst particles are 2-3 mm diameter pellets of g-Al₂O₃ loaded with 0.5-5.0wt% Ru. The g-Al₂O₃ has a BET surface area of about 100 m²g^-1 and the Ru is present as crystallites 5-20 nm in size. The first task was to reproduce these properties in the a-Al₂O₃-foam, which has a low surface area (1-2 m²g^-1). A washcoat of g-Al₂O₃ was added to increase the area for Ru deposition. Pellets of the foam (1-cm diameter, 2.5 cm in length) were dipped into a 15 percent slurry of high purity boehmite containing 15 percent aluminum nitrate and 0.5 percent glycerol. After one hour, the pellets were removed, drained by shaking and air blowing, and then dried in a microwave oven for three to five hours. This was followed by calcining in an oven for two hours at 550ºC. The active component (Ru) was added by impregnation with a solution of RuCl₃·×H₂O, 99.9 percent in ethanol with a concentration calculated to yield a desired 1.0 wt% Ru on the finished catalyst. Enough solution was used to just fill the estimated pore volume of the washcoat. The wet pellets were dried in the microwave oven at 105°C for one hour and then calcined at 550ºC for four hours to convert the salt to the oxide.

These procedures resulted in 30-PPI foam pellets containing 15wt% washcoat as a 50-mm porous layer over the struts of the foam, with an overall surface area of 13.3 m²g^-1. After reduction in H₂ at 550ºC, X-ray diffraction measurements showed that the washcoat contained 0.85wt% Ru metal in the form of crystallites 19.4 nm in size.

Samples of 1.8-mm spherical pellets of g-Al₂O₃ were treated in a similar manner to produce regular pelleted catalysts for comparison. Typical results are shown in Fig. 3. The spheres in the upper left are black in color and measurements showed that Ru was concentrated in the outer layers of the pellet. Since this was undesirable, it was remedied by adding HCl to the impregnation solution, resulting in the uniformly loaded pellets in the upper right. A sample of the Ru-loaded ceramic foam is shown in the center. The foam pellets is loaded uniformly throughout the structure.

Figure 3. Sabatier catalyst pellets and foams. Upper left: 1.8 mm g-Al₂O₃ spheres loaded with about 1 wt% Ru, metal on the exterior. Upper right: 1.8 mm g-Al₂O₃ spheres loaded with about 1 wt% Ru, uniform metal loading. Center: 30-PPI foam pellet loaded with about 1 wt% Ru.

Objective No. 2. Measurement of catalytic kinetics. An existing kinetic apparatus, consisting of a gas metering manifold, a fixed bed reactor, temperature control, and a chromatograph-based analytical system was adapted for studies on Reaction (1). Differential (low conversion) rate measurements were made on the foam, the 1.8-mm pellets and 60-100 mesh (200 mm) powder made by crushing the pellets. Typical results for the foam at 200ºC (Fig. 3) show the dependence of the rate on the partial pressure of CO₂ and H₂, the other held constant. This is consistent with a rate equation of the form

(2)

based on an Eley-Rideal mechanism for the reaction. The apparent activation energy is 52 kJ mol^-1. Similar rate equations were determined for the pellets and powder. The results of these kinetic studies are summarized in Table 1.

Table 1. Summary of kinetic differences between powder, pellet and foam catalysts. Temperature = 225ºC, = 0.100 atm, = 0.500 atm.

These results show that the pellet has an effectiveness factor of 0.19 when compared to the powder and that the foam is very nearly equivalent to the powder. The critical factor, however, is the difference in the rate per unit volume of the reactor between the pellet and the foam. Table 1 shows an increase by over a factor of two. Possibly, the loading of the foam could be increased to give an even better comparison.

These results imply that the volume of the reactor could be reduced to half the original size with an equivalent saving in space and mass. Alternatively, heat could be removed from the reactor more efficiently, giving better heat recovery, more stable reactor operation, and greater catalyst lifetime.

Objective No 3. Model simulation of the Sabatier reactor. Development of a 2-dimensional model reactor for the Sabatier reactor is in progress. This model generates both axial and radial temperature profiles and is superior to the 1-D model in Fig. 2 that uses only average bed temperatures. The kinetic rate equations measured in this project, together with 2-D heat transfer correlations already found in this laboratory, will be used in this model to accurately predict temperature profiles under a wide range of conditions. Parametric data for process optimization, detailed designs, and system analyses will possibly use this model.

The final step in this project will be testing a prototype reactor under Martian conditions. Although the simulation model will predict many of the results, only pilot testing can validate performance, operability, and long-term stability or reliability.

This project has demonstrated that the NASA Sabatier reactor is a candidate for improved heat removal using ceramic foam as the catalyst bed. The feasibility of loading the foam with a washcoat containing the Sabatier catalyst has been demonstrated. Kinetic rate equations have been measured and compared with a conventional pelleted catalyst. A kinetic advantage of over a factor of two was demonstrated and this could possibly be increased at least a factor of two. This improvement, together with previously proven advantages for enhanced heat transfer, could lead to smaller process equipment (mass savings) and to safer process control and longevity.

Figure 4. Typical kinetic data with the ceramic foam catalyst in UH experiments

References
¹"Workshop on Research for Space Exploration: Physical Sciences and Process technology," NASA/CP-1998-207431.
²L. J. Gibson and M. F. Ashby. Cellular Solids, Structures and Properties. Oxford: Pergamon Press, 1988.
³M. V. Twigg and J. T. Richardson. Proc., 6th Int'l Symp. on the Scientific Bases for the Preparation of Heterogeneous Catalysts, Elsevier Science B. V., Amsterdam, Netherlands, (1994): 345-59
⁴J. T. Richardson, Y. Peng, and D. Remue. "Properties of Ceramic Foams as Catalyst Supports: Pressure Drop," Appl. Catal. A: Gen. 204 (2000): 19.
⁵J. T. Richardson, J.-K. Hung, and D. Remue. "Properties of Ceramic Foams as Catalyst Supports: Reactor Mass and Heat Transfer," Appl. Catal. A: Gen. (Submitted for publication.)

Publications
Twigg, M. V. and J. T. Richardson. "Theory and Application of Ceramic Foam Catalysts," Trans. I Chem E 80 (2002): 183.

Theses or student reports
Peng, Y. "Transport Properties of Ceramic Foam for Catalyst Support Applications," Ph.D. Dissertation, Department of Chemical Engineering, Aug. 2001.
Brown, S. A. "The Kinetics of Exothermic Reactions on Ceramic Foam Catalysts," Ph.D. Dissertation, Department of Chemical Engineering, Aug. 2001

Funding and proposals
"Improved Catalytic Combustion System." Texas Coordinating Board of Higher Education ATP Program, 2002-2003, $155,000.

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Institute for Space Systems Operations - 2001 Annual Report
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