Figure 3. Effect of reactor length on the methanol and hydrogen mass fraction profiles along the length of the micro-structured heat-exchanger reactor for hydrogen production by steam methanol reforming.
The effect of reactor length on the methanol and hydrogen mole fraction profiles is illustrated in Figure 4 along the length of the micro-structured heat-exchanger reactor for hydrogen production by steam methanol reforming. A microchannel reactor is preferably designed to achieve a temperature trajectory down the length of the reaction chamber that approaches a predetermined temperature trajectory. Typically, this predetermined temperature trajectory is substantially different from the temperature trajectory that would occur if the reaction were allowed to proceed adiabatically or isothermally. In preferred forms, this predetermined temperature trajectory approaches a theoretically determined optimum temperature trajectory based on the reaction rate and design parameters specific to the particular application. Many catalytic reactions begin with gas phase reactants, for example steam reforming, partial oxidation, water gas shift and others. However, equipment, specifically reactor volume is generally large because of mass and heat transfer limitations. Microchannel reactors offer less resistance to heat and mass transfer thus creating the opportunity for dramatic reductions in process hardware volume. Further, the micro-combustor or micro-reformer can be part of an efficient integrated system, which can reform lower alcohols and even higher alcohols that require higher processing temperatures. Carbon dioxide selectivity over carbon monoxide, a poison to fuel cells, of the steam reforming process is high, so that it is possible to avoid or reduce requirements for removing carbon monoxide after reforming and before supplying the gas to the fuel cell, thereby greatly simplifying the overall system. Since catalytic combustion is used, stable low temperature performance is easily attained for the combustor to provide uninterrupted operational heat for vaporizers and steam reformer units so they may operate in a steady optimum manner. The excess air should not be too much, since the extra air removes heat from the steam reformer. Air and methanol flows are adjusted until the steam reformer is at the desired temperature. The reformer fuel mixture flow is initiated at this point. Combustor flows are adjusted as necessary to maintain desired temperatures.