Figure 7. Oxidation reaction rate profiles along the length of the thermally coupled reactor for conducting simultaneous endothermic and exothermic reactions.
The reforming reaction rate profiles are presented in Figure 8 along the length of the thermally coupled reactor for conducting simultaneous endothermic and exothermic reactions. This steam reforming reaction is carried out using, as raw materials, methanol and steam fed into the ceramic tubular reactors so as to yield a reformed gas containing hydrogen and carbon dioxide. In such a steam reforming reaction, methanol is reacted with steam while the reforming material containing the methanol and steam is heated with part of combustion heat so as to yield a reformed gas containing hydrogen and carbon dioxide. In a low-temperature region where the reactivity is low, a steam reforming reaction is carried out by the catalysis of a reforming catalyst. The term catalyzed hardware is used for a catalyst system where a layer of catalyst is fixed on a surface of another material, for example, metallic surfaces. The other material serves as the supporting structure giving strength to the system. This allows to design catalyst shapes which would not have sufficient mechanical strength in itself. The steam reforming technology makes use of reforming catalyst in the form of pellets of various sizes and shapes. The catalyst pellets are placed in fixed bed reactors. The reforming reaction is endothermic. In conventional reformers, the necessary heat for the reaction is supplied from the environment outside the tubes usually by a combination of radiation and convection to the outer side of the reformer tube. The heat is transferred to the inner side of the tube by heat conduction through the tube wall and is transferred to the gas phase by convection. Finally, the heat is transferred from the gas phase to the catalyst pellet by convection. The catalyst temperature can be more than 80 °C lower than the inner tube wall temperature at the same axial position of the reformer tube. Heat transport is more efficient when catalyzed hardware is used in the steam reforming process. The heat transport to the catalyst occurs by conduction from the inner tube wall. This is a much more efficient transport mechanism than the transport by convection via the gas phase. The result is that the temperatures of the inner tube wall and the catalyst are almost identical. Furthermore, the tube thickness can be reduced, which makes the temperature difference between the inner and outer side of the reformer tube smaller. It is hence possible to have both a higher catalyst temperature and a lower tube temperature, all other conditions being the same when replacing the conventional reformer tubes with catalyzed hardware tubes. A low outer tube wall temperature is desirable since it prolongs the lifetime of the tube. A high catalyst temperature is advantageous since the reaction rate increases with temperature and since the equilibrium of reaction is shifted to the right-hand side resulting in a better utilization of the feed. Finally, the catalyst amount is reduced when using catalyzed hardware reformer tubes compared to the conventional reformer with a fixed bed of reforming catalyst. The critical steam to carbon ratio decreases when the operating pressure is increased. The operating pressure in the thermally coupled reactor is the critical parameter for suppressing soot formation. By increasing the operating pressure, it is possible to operate advantageously at a lower steam to carbon ratio. The actual critical pressure will depend on the burner design used in the thermally coupled reactor.