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.