Figure 6. Mesh and methanol mole fraction contour maps in the
micro-structured heat-exchanger reactor for hydrogen production by steam
methanol reforming.
The oxidation channel and wall centerline temperature profiles are
presented in Figure 7 along the length of the micro-structured
heat-exchanger reactor for hydrogen production by steam methanol
reforming. The reaction rate for a single reaction with a given catalyst
is a function of the composition and the temperature. The temperature
corresponding to the maximum reaction rate at a given composition is
determined by setting the partial derivative of the reaction rate with
respect to temperature equal to zero. Assuming an ideal plug flow
reactor, a theoretical optimum temperature trajectory is determined from
the mass balance equation. Integrating this equation gives the minimum
reactor length required to achieve a given level of conversion. The
appropriate catalyst loading is also calculated from the reaction rate
equation. An exemplary reaction useful in the present design is the
water-gas-shift reaction. The water-gas-shift reaction is employed in
fuel processors that reform liquid fuels to produce hydrogen for fuel
cells. The shift reaction increases hydrogen yield while reducing carbon
monoxide, which is a poison for the proton-exchange membrane fuel cell
anode. The water-gas-shift reaction is exothermic and reversible. The
initial composition is representative of a reformate stream generated
from steam reforming of isooctane at a 3:1 steam to carbon ratio and
contains. The initial maximum reaction rate occurs at a temperature of
about 280 °C. As the reaction proceeds, the peak reaction rate rapidly
drops. The temperature at which the peak rate occurs also drops with
increasing carbon monoxide conversion. The size of a reactor to
accomplish high conversion and the amount of catalyst required is
dependent on the temperature trajectory through the reactor. For a
reactor operating with this temperature trajectory, most of the
conversion would occur in the first third of the reactor, and the
remaining two-thirds of the reactor would be required for the remaining
percent of conversion, a direct result of much lower activity as the
temperature decreases. For a variety of reasons, however, it may not be
practical or desirable to follow a theoretically optimum temperature
profile during the entire length of the reactor. For example, concerns
over methane formation, coking, or catalyst sintering may place
constraints on the inlet temperature to the reactor or the maximum
temperature in the reactor. Likewise, cost constraints can become
manifest if following the ideal temperature trajectory would require
that the reactor system be manufactured in more expensive materials than
would otherwise be practical. An alternative temperature trajectory is
to enter the reactor at a temperature near an upper limit temperature
and operate substantially isothermally through the initial stage of the
reactor. Once the reaction has proceeded to a point where the optimum
temperature drops below an upper constraint, then the theoretically
optimum temperature profile can be followed.