Figure 1. Physical representation of the microchannel reactor for the steam reforming process in which tail gas from the fuel cell and hydrogen storage system is used to provide heat needed to reform the feed fuel and regenerate the calcium oxide bed.
Microchannel reactors preferably include microchannels and a plurality of adjacent heat exchange microchannels. The adjacent heat exchange microchannels are illustrated schematically in Figure 2 in which contact time is maintained about constant but the length of each stage increases progressively to accommodate the higher flow rate in each stage. Each stage can have the same length or stages can have different lengths. The plurality of microchannels may contain, for example, 2, 10, 100, 1000 or more channels capable of operating in parallel. In preferred cases, the microchannels are arranged in parallel arrays of planar microchannels, for example, at least 3 arrays of planar microchannels. Performance advantages in the use of this type of reactor architecture for the purposes of the present design include their relatively large heat and mass transfer rates, and the substantial absence of any explosive limits [51, 52]. Pressure drops can be low, allowing high throughput and the catalyst can be fixed in a very accessible form within the channels eliminating the need for separation. In some cases, a reaction microchannel contains a bulk flow path. The term bulk flow path refers to an open path within the reaction chamber. A contiguous bulk flow region allows rapid fluid flow through the reaction chamber without large pressure drops. In devices with multiple manifolds, the design can be characterized by the volume ratio of one manifold to its connecting microchannels, or characterized by the volumetric sum of plural manifolds and their connecting microchannels. However, if connecting channels are connected to a header and footer, then both the header and footer must be included in the calculation of manifold volume [53, 54]. The volume of the submanifold is included in the volume of the manifold. A general methodology to build commercial scale microchannel devices is to form the microchannels in the shims by different methods such as etching and stamping [55, 56]. For example, shims may be stacked together and joined by different methods such as chemical bonding and brazing. After joining, the device may or may not require machining.