Figure 7. Oxidation channel and wall centerline temperature profiles
along the length of the micro-structured heat-exchanger reactor for
hydrogen production by steam methanol reforming.
The reforming channel and wall centerline temperature profiles are
presented in Figure 8 along the length of the micro-structured
heat-exchanger reactor for hydrogen production by steam methanol
reforming. Advantages can be realized by using one or more reactors with
controlled temperature trajectories as compared to two adiabatic
reactors with intercooling, which is the typical approach used in fuel
steam reforming processes [59, 60]. In the case of adiabatic
reactors with intercooling, reactor productivity is maximized for a
given total conversion by optimizing the two inlet temperatures and the
amount of conversion in the first reactor [61, 62]. When comparing
this three-component configuration to the optimum temperature trajectory
for the steam reformate stream and using the same kinetic rate
expression, approximately double or more catalyst is required for 80
percent conversion in the optimized two-stage adiabatic reactor system
than is required if the optimized temperature trajectory is achieved
[63, 64]. Of course, the actual size of a single reactor operating
with the temperature trajectory would likely be larger than any single
component of the three-component system [65, 66]. However, if the
entire system of two reactors plus the intervening heat exchanger is
considered, the overall size and mass will likely be smaller with the
optimum temperature profile [67, 68]. The optimum temperature system
is also simplified by combining three components into one. In addition,
the steam reforming catalyst may be an important cost element, so
improving catalyst productivity may be sufficient alone for pursuing an
optimized profile. Microchannel reactors offer the advantage of
exceptional heat exchange integration and can be utilized for
approaching optimum temperature trajectories for exothermic, reversible
reactions. Catalytic monoliths are located at the center of each of an
array of reaction flow channels such that reactants flow by both sides
of the catalyst structures. Reactants from the reaction flow channel
diffuse into pores in the catalyst structure to react, generating heat.
Reaction products then diffuse out of the steam reforming catalyst
structure and into the bulk reactant flow path. Diffusion into and out
of the steam reforming catalyst is in a direction generally transverse
to the bulk flow direction. The reaction flow channel arrays are
interleaved with heat exchange channels, and a heat exchange fluid
flowing co-current or counter-current to the reaction flow removes the
heat of reaction and cools the gas, thereby establishing a desired
temperature trajectory for the reaction. The choice of coolant, the
temperature and flow of the coolant, and the geometry and relative
orientation of the flow channels are among the design variables that can
be modified for achieving a desired temperature profile for a given
reaction and catalyst. In a preferred form, design variables are
selected to substantially maximize catalyst productivity.