Figure 3. Effect of wall thermal conductivity on the average Nusselt
number in the exothermic process of the combined parallel plate heat
exchanger-reactor.
The effect of wall thermal conductivity on the average Nusselt number
are illustrated in Figure 4 in the endothermic process of the combined
parallel plate heat exchanger-reactor. The process may be carried out in
a single autothermal reactor without the need to provide multiple
sequential catalyst zones in the reactor. The catalytic partial
oxidation reaction is exothermic in nature and the heat generated
thereby is used to carry out the steam reforming reaction which is
endothermic in nature. By having the catalytic partial oxidation layers
in intimate contact with the steam reforming catalyst layers, the
process heat can be more effectively managed in an adiabatic reactor,
namely an autothermal reactor. By having the two catalyst layers in
contact with one another, heat loss which otherwise occurs from the use
of multiple autothermal reactors or an autothermal reactor containing
multiple catalyst zones is significantly minimized. The process results
in savings in reactor volume and monolith substrate costs as well as
less pressure drop throughout the catalytic partial oxidation and steam
reforming reactions. The process thereby provides more efficient
utilization and uniform usage of the heat generated by the exothermic
catalytic partial oxidation reaction, thus allowing the endothermic
steam reforming reaction to be carried out at a somewhat higher
temperature due to lower heat loss and concomitant higher reaction rate
and under adiabatic conditions. The result is that the catalytic partial
oxidation reaction temperature is somewhat lowered, estimated to be by
about 60 degrees and concomitantly, the steam reforming reaction
temperature is estimated to be raised by about 60 degrees, thereby
improving catalyst life and resulting in higher steam reforming reaction
rates. Moreover, by utilizing the catalytic partial oxidation and steam
reforming catalysts as layers in contact with one another, adverse
reactions such as the reaction of oxygen with rhodium and the reaction
of oxygen with platinum, may be avoided. The overall process consists of
first preheating the reactants to the required temperature. It ensures
good thermal management for the products leaving the reactor to be used
to preheat the incoming reactants to a temperature close to the reaction
temperature. The methanol, oxygen, and associated nitrogen flow through
the inlets of the inlet manifold and into the reaction channels of the
flow path. Heterogeneous oxidation occurs in the catalyst attached to
the wall. As the stream flows down through the flow path, the conversion
increases until the stream passes through the outlets. In the adjacent
flow paths, preheated methanol and steam enter the second discrete set
of channels through inlets, contact the catalyst coated onto the wall,
and reaction occurs. The heat for the reaction is supplied directly
through the wall from the oxidation channels occurring on the opposing
side of the dividing wall. As the heat transfer characteristics are
highly independent of the bulk reactants velocity, a velocity can be
chosen to ensure that the reactants exiting the reactor has attained the
desired level of conversion or indeed reached any equilibrium. It is
interesting to note that in such an arrangement it is desirable to
operate the reactants in a co-current flow arrangement. This ensures
that the area with the greatest heat generation is adjacent to the area
with the greatest heat requirement. However, cases may exist where a
countercurrent flow arrangement is desirable.