Figure 2. Nitrogen and steam mole fraction contour maps in the micro-structured heat-exchanger reactor for hydrogen production by steam methanol reforming.
The effect of reactor length on the methanol and hydrogen mass fraction profiles is illustrated in Figure 3 along the length of the micro-structured heat-exchanger reactor for hydrogen production by steam methanol reforming. Channels having a dimension between one millimeter and one centimeter are sometimes referred to as meso-channels, with the term microchannels used for those less than one millimeter. However, for the purposes of the present study, a microchannel or a microchamber has at least one dimension, typically the depth, less than about one millimeter, and still more often less than about 0.8 millimeter. The width of a microchannel may be any magnitude, but typically will be constrained by the desire to control manufacturing processes or by the desire to control fluid distribution in a reactor or heat exchanger that has multiple microchannels. Length is unlimited, but as a practical matter for the overall purpose of miniaturization, the length is typically on the order of centimeters to tens of centimeters. Where the depth is the micro-dimension, microchannels will typically, though not essentially, have a large ratio of length to width, for example greater than about 8. The present design is a microchannel chemical reactor having a reaction flow path in thermal contact with a heat exchange channel. The heat exchange channel may also be a reaction channel. Either the reaction flow path or the heat transfer channel, or both, include microchannels where the smallest dimension of the microchannel is generally parallel to the direction of heat flux, which would be in a vertical direction. Reactants flow through the reaction flow path from an inlet to an outlet. Between the inlet and outlet is a reaction chamber defined by the presence of a reaction catalyst in the flow path, which can span some or substantially all of the length of the flow path. Heat exchange fluid flows through the heat exchange channel from a fluid inlet to a fluid outlet. Typically, though not essentially, at least one solid wall separates the heat exchange channel from the reaction chamber to prevent mass transport between the fluids. An optional heater is also provided adjacent the inlet end of the reaction chamber. The heater at one end of the device can be used to help maintain a temperature gradient down the length of the reaction chamber. A cooler could be used at the other end of the reaction chamber in place of or in addition to the heater. When the reaction in the reaction chamber is a reversible exothermic reaction, heat is generated in the reaction chamber and transferred to the heat exchange fluid to cool the reactants as they proceed through the reaction chamber. Conversely, when the reaction in the reaction chamber is a reversible endothermic reaction, heat is transferred from a heating fluid in the heat exchange channel to the reacting fluid as the reactants proceed through the reaction chamber. When the heat transfer channel is also a reaction channel, heat is transferred from one reaction channel to the other, as the reactants proceed through their respective flow paths.