Metal-supported catalyst beds with improved thermal and flow characteristics for promoting endothermic chemical reactions are undergoing development. In the proposed original application, catalyst beds of this type would be used on Mars to produce hydrogen from an endothermic reaction between carbon dioxide and methane. The empirical equation for this reaction is:
CO2 + CH4 --> 2CO + 2H2
(The subsequent reaction of carbon monoxide with water will generate additional hydrogen as shown in CO + H2O --> CO2 + H2.) Hydrogen is then available as a propellant for launching payloads from the Martian surface or as a fuel for fuel cells generating power for surface mobility or stationary electricity.
The basic principles of operation of these catalyst beds are also applicable to steam reformers and other catalytic reactors used on Earth to produce fuels and some specialty chemicals. With respect to endothermic reactions that one seeks to catalyze, the developmental catalyst beds offer advantages over traditional packed catalyst beds, as explained below.
In a typical traditional case, the heat needed for an endothermic reaction is generated outside a reactor and transferred through the reactor wall to a bed of packed catalyst particles inside the reactor. The energy efficiency of the reactor is diminished because (1) the thermal resistance of the wall and (2) the parasitic loss of heat from the external heater to the environment.
A catalyst bed of the type now undergoing development consists of a metal support coated with a material that consists of, or contains, one or more catalyst(s). The center mandrel and the outside wall of the metal support are connected to a source of dc electric power. In operation, the metal support is heated by passing an electric current through it. Efficiency is increased because the losses associated with the thermal resistance of the reactor wall and parasitic transfer of heat from an external heater to the environment are eliminated. In addition, because heat is supplied to the catalyst more directly, it is possible to exert better control over the endothermic reaction.
The metal support is corrugated (see figures) to make the reactants flow turbulently; this feature enhances (relative to the packed-bed case) both the mixing of the reactants and the transfer of heat from the support to the reactant mixture. As a result of the combination of internal electrical heating and turbulence, the overall conversion efficiency for a reactor of a given size is increased or, equivalently, the size of the reactor needed to achieve a given conversion efficiency is reduced. Moreover, the degree of utilization of catalyst applied to the surface of the internally heated support is greater than the degree of utilization of catalyst in or on conventional catalyst rings, cylinders, and pellets, so that less of the catalyst is needed to drive a reaction with a given conversion efficiency.
This work was done by Gerald Voecks of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.