Figure 2. Nitrogen and steam mole fraction contour maps in the
micro-structured heat-exchanger reactor for hydrogen production by steam
methanol reforming.
The effect of reactor length on the methanol and hydrogen mass fraction
profiles is illustrated in Figure 3 along the length of the
micro-structured heat-exchanger reactor for hydrogen production by steam
methanol reforming. Channels having a dimension between one millimeter
and one centimeter are sometimes referred to as meso-channels, with the
term microchannels used for those less than one millimeter. However, for
the purposes of the present study, a microchannel or a microchamber has
at least one dimension, typically the depth, less than about one
millimeter, and still more often less than about 0.8 millimeter. The
width of a microchannel may be any magnitude, but typically will be
constrained by the desire to control manufacturing processes or by the
desire to control fluid distribution in a reactor or heat exchanger that
has multiple microchannels. Length is unlimited, but as a practical
matter for the overall purpose of miniaturization, the length is
typically on the order of centimeters to tens of centimeters. Where the
depth is the micro-dimension, microchannels will typically, though not
essentially, have a large ratio of length to width, for example greater
than about 8. The present design is a microchannel chemical reactor
having a reaction flow path in thermal contact with a heat exchange
channel. The heat exchange channel may also be a reaction channel.
Either the reaction flow path or the heat transfer channel, or both,
include microchannels where the smallest dimension of the microchannel
is generally parallel to the direction of heat flux, which would be in a
vertical direction. Reactants flow through the reaction flow path from
an inlet to an outlet. Between the inlet and outlet is a reaction
chamber defined by the presence of a reaction catalyst in the flow path,
which can span some or substantially all of the length of the flow path.
Heat exchange fluid flows through the heat exchange channel from a fluid
inlet to a fluid outlet. Typically, though not essentially, at least one
solid wall separates the heat exchange channel from the reaction chamber
to prevent mass transport between the fluids. An optional heater is also
provided adjacent the inlet end of the reaction chamber. The heater at
one end of the device can be used to help maintain a temperature
gradient down the length of the reaction chamber. A cooler could be used
at the other end of the reaction chamber in place of or in addition to
the heater. When the reaction in the reaction chamber is a reversible
exothermic reaction, heat is generated in the reaction chamber and
transferred to the heat exchange fluid to cool the reactants as they
proceed through the reaction chamber. Conversely, when the reaction in
the reaction chamber is a reversible endothermic reaction, heat is
transferred from a heating fluid in the heat exchange channel to the
reacting fluid as the reactants proceed through the reaction chamber.
When the heat transfer channel is also a reaction channel, heat is
transferred from one reaction channel to the other, as the reactants
proceed through their respective flow paths.