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.