Figure 6. Mesh and methanol mole fraction contour maps in the micro-structured heat-exchanger reactor for hydrogen production by steam methanol reforming.
The oxidation channel and wall centerline temperature profiles are presented in Figure 7 along the length of the micro-structured heat-exchanger reactor for hydrogen production by steam methanol reforming. The reaction rate for a single reaction with a given catalyst is a function of the composition and the temperature. The temperature corresponding to the maximum reaction rate at a given composition is determined by setting the partial derivative of the reaction rate with respect to temperature equal to zero. Assuming an ideal plug flow reactor, a theoretical optimum temperature trajectory is determined from the mass balance equation. Integrating this equation gives the minimum reactor length required to achieve a given level of conversion. The appropriate catalyst loading is also calculated from the reaction rate equation. An exemplary reaction useful in the present design is the water-gas-shift reaction. The water-gas-shift reaction is employed in fuel processors that reform liquid fuels to produce hydrogen for fuel cells. The shift reaction increases hydrogen yield while reducing carbon monoxide, which is a poison for the proton-exchange membrane fuel cell anode. The water-gas-shift reaction is exothermic and reversible. The initial composition is representative of a reformate stream generated from steam reforming of isooctane at a 3:1 steam to carbon ratio and contains. The initial maximum reaction rate occurs at a temperature of about 280 °C. As the reaction proceeds, the peak reaction rate rapidly drops. The temperature at which the peak rate occurs also drops with increasing carbon monoxide conversion. The size of a reactor to accomplish high conversion and the amount of catalyst required is dependent on the temperature trajectory through the reactor. For a reactor operating with this temperature trajectory, most of the conversion would occur in the first third of the reactor, and the remaining two-thirds of the reactor would be required for the remaining percent of conversion, a direct result of much lower activity as the temperature decreases. For a variety of reasons, however, it may not be practical or desirable to follow a theoretically optimum temperature profile during the entire length of the reactor. For example, concerns over methane formation, coking, or catalyst sintering may place constraints on the inlet temperature to the reactor or the maximum temperature in the reactor. Likewise, cost constraints can become manifest if following the ideal temperature trajectory would require that the reactor system be manufactured in more expensive materials than would otherwise be practical. An alternative temperature trajectory is to enter the reactor at a temperature near an upper limit temperature and operate substantially isothermally through the initial stage of the reactor. Once the reaction has proceeded to a point where the optimum temperature drops below an upper constraint, then the theoretically optimum temperature profile can be followed.