Figure 3. Effect of catalyst layer thickness on the enthalpy of reaction in the oxidation and reforming processes of the thermally coupled reactor for conducting simultaneous endothermic and exothermic reactions.
The effect of catalyst layer thickness on the methanol conversion and hydrogen yield in the oxidation and reforming processes is illustrated in Figure 4 in the thermally coupled reactor for conducting simultaneous endothermic and exothermic reactions. In addition to the reaction channels, additional features such as microchannel or non-microchannel heat exchangers may be present. Microchannel heat exchangers are preferred. Adjacent heat transfer microchannels enable temperature in the reaction channel to be controlled precisely to promote steam reforming and minimize unselective reactions in the gas phase. The thickness of a wall between adjacent process channels and heat exchange channels is preferably 0.80 mm or less. Each of the process or heat exchange channels may be further subdivided with parallel subchannels. The heat exchange fluids can be gases or liquids and may include steam, liquid metals, or any other known heat exchange fluids. Especially preferred heat exchangers include combustors in which a fuel is oxidized to produce heat for the steam reforming reaction. The incorporation of a simultaneous exothermic reaction to provide an improved heat source can provide a typical heat flux of roughly an order of magnitude above the convective cooling heat flux. The flow of hot fluid through a heat exchanger may be cross flow, counter-flow or co-flow. For all of the above conditions, the approach to equilibrium conversion is the ratio of measured hydrocarbon conversion to equilibrium hydrocarbon conversion. The equilibrium composition or moles hydrocarbon out at equilibrium is based upon the measured average pressure of the inlet and outlet of the reactor zone and the inlet molar composition. The equilibrium distribution or composition for a given temperature, pressure, and inlet mole fraction distribution can be calculated using Gibbs free energies with programs. The catalyst requires catalytically active surface sites that reduce the kinetic barrier to the steam reforming reaction. The catalyst comprises one or more of the following catalytically active materials: ruthenium, rhodium, iridium, nickel, palladium, platinum, and carbide of group VIB. Rhodium is particularly preferred. The catalytically active materials are typically quite expensive. Therefore, it is desirable to minimize the amount used to accomplish the desired performance. The catalyst also contains an alumina support for the catalytically active materials. An alumina support contains aluminum atoms bonded to oxygen atoms, and additional elements can be present. Preferably, the alumina support comprises stabilizing element or elements that improve the stability of the catalyst in hydrothermal conditions. Stabilizing elements typically are large, highly charged cations. Preferably, the catalytically active materials are present in the form of small particles on the surface of the stabilized alumina support. The catalytically active layer is preferably disposed on a porous substrate. Preferably, the catalyst contains an alumina layer disposed on a thermally conductive surface. The surface could be, for example, a porous substrate or reaction chamber walls.