Figure 7. Oxidation channel and wall centerline temperature profiles along the length of the micro-structured heat-exchanger reactor for hydrogen production by steam methanol reforming.
The reforming channel and wall centerline temperature profiles are presented in Figure 8 along the length of the micro-structured heat-exchanger reactor for hydrogen production by steam methanol reforming. Advantages can be realized by using one or more reactors with controlled temperature trajectories as compared to two adiabatic reactors with intercooling, which is the typical approach used in fuel steam reforming processes [59, 60]. In the case of adiabatic reactors with intercooling, reactor productivity is maximized for a given total conversion by optimizing the two inlet temperatures and the amount of conversion in the first reactor [61, 62]. When comparing this three-component configuration to the optimum temperature trajectory for the steam reformate stream and using the same kinetic rate expression, approximately double or more catalyst is required for 80 percent conversion in the optimized two-stage adiabatic reactor system than is required if the optimized temperature trajectory is achieved [63, 64]. Of course, the actual size of a single reactor operating with the temperature trajectory would likely be larger than any single component of the three-component system [65, 66]. However, if the entire system of two reactors plus the intervening heat exchanger is considered, the overall size and mass will likely be smaller with the optimum temperature profile [67, 68]. The optimum temperature system is also simplified by combining three components into one. In addition, the steam reforming catalyst may be an important cost element, so improving catalyst productivity may be sufficient alone for pursuing an optimized profile. Microchannel reactors offer the advantage of exceptional heat exchange integration and can be utilized for approaching optimum temperature trajectories for exothermic, reversible reactions. Catalytic monoliths are located at the center of each of an array of reaction flow channels such that reactants flow by both sides of the catalyst structures. Reactants from the reaction flow channel diffuse into pores in the catalyst structure to react, generating heat. Reaction products then diffuse out of the steam reforming catalyst structure and into the bulk reactant flow path. Diffusion into and out of the steam reforming catalyst is in a direction generally transverse to the bulk flow direction. The reaction flow channel arrays are interleaved with heat exchange channels, and a heat exchange fluid flowing co-current or counter-current to the reaction flow removes the heat of reaction and cools the gas, thereby establishing a desired temperature trajectory for the reaction. The choice of coolant, the temperature and flow of the coolant, and the geometry and relative orientation of the flow channels are among the design variables that can be modified for achieving a desired temperature profile for a given reaction and catalyst. In a preferred form, design variables are selected to substantially maximize catalyst productivity.