Figure 6. Effect of temperature on the effective factor of the microchannel methanol steam reformer reactor with different shapes of the cross section of the process microchannel.
The methanol mole fraction contour maps are illustrated in Figure 7 for the microchannel reactor that comprises a plurality of process and heat exchange microchannels. The acceptable maximum concentrations presented herein are illustrative examples, and concentrations other than those presented herein may be used and are within the scope of the present study. For example, particular users or manufacturers may require minimum or maximum concentration levels or ranges that are different than those identified herein. Similarly, when steam reformers are used with a fuel cell stack that is more tolerant of these impurities, then the product hydrogen stream may contain larger amounts of these gases [61, 62]. Similarly, when the steam reformers are used to produce product hydrogen streams that are used for applications other than as a fuel stream for a fuel cell stack, it may be desirable to remove other components from the product hydrogen stream and it may not be necessary to utilize a separation process [63, 64]. For example, when the product hydrogen stream is intended for use in a proton-exchange membrane fuel cell stack or other device that will be damaged if the stream contains more than determined concentrations of carbon monoxide or carbon dioxide, it may be desirable to include a methanation catalyst [65, 66]. Polishing regions may also include a steam reforming catalyst to convert any unreacted feedstock into hydrogen gas [67, 68]. Since the steam reforming process is highly endothermic, external heating sources are required. Burners installed within the furnace housing combust natural gas or some other fuel to support endothermic reactions within catalyst filled tubes. Heat released from oxidation reactions is transferred by radiation and convection to tubular reactor outer wall, then by conduction from the outer wall to the inner wall, and then by conduction and convection to the reaction mixture in the tubular reactor interior. A portion of the heat absorbed by the tubular reactor is utilized to bring natural gas and steam feeds from their feed temperature to reaction temperature in a range of from about 200 °C to about 300 °C to achieve desired methanol conversion. Improving heat flux from tubular reactor outer environment to inner environment is a critical step to increase reactor efficiency. The catalytic endothermic reforming reactions occur on the catalyst particle exterior surface, as well as within the pores accessible to the reactants. The heat absorbed by the tubular reactor, conducts through the tube wall into the interior to support endothermic reactions. Both convective and conductive heat transfer mechanisms are in play inside the reactor tube. The lower thermal conductivity of catalyst particles affects heat available for endothermic reactions in the reactor interior. The upper limits on gas velocity to minimize pressure drop and prevent catalyst particle fluidization affect the heat transfer rate from the tube wall to the catalyst particles, on whose surface the endothermic catalytic reactions occur and where heat is needed. The tube wall temperature typically ranges from about 200 °C to about 300 °C. Such higher temperatures cause considerable expansion of tubes. Since the catalyst particles have a lower coefficient of thermal expansion, the potential exists for considerable slumping of the catalyst particles upon reactor heat up. This may cause suboptimal reactor performance due to inadequate heat transfer, higher pressure-drop, and increased diffusional resistances. The potential also exists for catalyst attrition due to crushing forces when the tube contracts. Attempts can be made to address these problems by providing support structures to hold the catalyst particles in position or dividing the catalyst bed within the tube into multiple beds with support structures in between, however these add complexities and can lead to undesirable higher pressure drop.