Figure 5. Effect of temperature on the ratio of the methanol mole fraction along the fluid centerline to that on the catalyst surface along the length of the process microchannel of the microchannel reactor.
The effect of temperature on the effective factor of the microchannel reactor is illustrated in Figure 6 with different shapes of the cross section of the process microchannel. Although effective for relatively short periods of use, low temperature shift catalysts also introduce several difficulties to practical long-term use of these catalysts as methanol steam reforming catalysts in commercial products. For example, the copper-zinc low temperature shift catalysts described above are pyrophoric, which means that these catalysts will spontaneously combust in the presence of air. The heat produced by this spontaneous oxidation of the catalyst may damage the catalyst and other portions of the reformer, as well as being a safety hazard. Therefore, steam reformers using a low temperature shift catalyst as a reforming catalyst generally include sufficient seals, guards or related mechanisms to minimize or prevent air from contacting the catalyst. Another disadvantage of the low temperature shift catalysts, as used as methanol steam reforming catalysts, is that the copper oxide component of the catalyst is easily reduced to elemental copper and then sintered at the temperatures in which methanol steam reforming is conducted. The speed at which the low temperature shift catalyst is reduced and then sintered increases as the temperature at which the low temperature shift catalyst is used as a methanol steam reforming catalyst. For example, the low temperature shift catalysts tend to be sintered and deactivated within approximately 200 hours or less when used at temperatures at or above 300 °C. This active life decreases even further when used at more preferred methanol steam reforming temperatures of at least 300 °C. The sintered catalyst is deactivated and therefore decreases the ability of the steam reformer to produce hydrogen gas from the feed stream. The present study is directed to methanol steam reforming catalysts that do not exhibit one or both of the above-described disadvantages of copper-zinc low temperature shift catalysts while still providing a comparable, or even greater, conversion of the feed stream into hydrogen gas. The methanol steam reforming catalyst also should not be reduced from an oxidized state and deactivated during use as a steam reforming catalyst in the temperature range of 200-300 °C. As such, the steam reforming catalyst will have a much longer useful life than low temperature shift catalysts when used as a methanol steam reforming catalyst. A methanol steam reforming catalyst is additionally or alternatively not pyrophoric. A benefit of such a catalyst is that the reforming catalyst beds do not need to be shielded or otherwise isolated from contact with air to prevent spontaneous oxidation of the catalyst, as is typically required for low temperature shift catalysts. Therefore, the reforming catalyst beds may be air permeable or otherwise exposed to air.