Figure 3. Effect of reactor length on the methanol and hydrogen mass
fraction profiles along the length of the micro-structured
heat-exchanger reactor for hydrogen production by steam methanol
reforming.
The effect of reactor length on the methanol and hydrogen mole fraction
profiles is illustrated in Figure 4 along the length of the
micro-structured heat-exchanger reactor for hydrogen production by steam
methanol reforming. A microchannel reactor is preferably designed to
achieve a temperature trajectory down the length of the reaction chamber
that approaches a predetermined temperature trajectory. Typically, this
predetermined temperature trajectory is substantially different from the
temperature trajectory that would occur if the reaction were allowed to
proceed adiabatically or isothermally. In preferred forms, this
predetermined temperature trajectory approaches a theoretically
determined optimum temperature trajectory based on the reaction rate and
design parameters specific to the particular application. Many catalytic
reactions begin with gas phase reactants, for example steam reforming,
partial oxidation, water gas shift and others. However, equipment,
specifically reactor volume is generally large because of mass and heat
transfer limitations. Microchannel reactors offer less resistance to
heat and mass transfer thus creating the opportunity for dramatic
reductions in process hardware volume. Further, the micro-combustor or
micro-reformer can be part of an efficient integrated system, which can
reform lower alcohols and even higher alcohols that require higher
processing temperatures. Carbon dioxide selectivity over carbon
monoxide, a poison to fuel cells, of the steam reforming process is
high, so that it is possible to avoid or reduce requirements for
removing carbon monoxide after reforming and before supplying the gas to
the fuel cell, thereby greatly simplifying the overall system. Since
catalytic combustion is used, stable low temperature performance is
easily attained for the combustor to provide uninterrupted operational
heat for vaporizers and steam reformer units so they may operate in a
steady optimum manner. The excess air should not be too much, since the
extra air removes heat from the steam reformer. Air and methanol flows
are adjusted until the steam reformer is at the desired temperature. The
reformer fuel mixture flow is initiated at this point. Combustor flows
are adjusted as necessary to maintain desired temperatures.