Figure 6. Gas-phase reaction rate profiles along the fluid centerline of
the catalytic reactor with an integral heat exchange structure in an
array of longitudinally disposed, adjacent reaction passageways or
channels which are either catalyst-coated or catalyst-free.
The reaction rate contour plots in the gas phase of the catalytic
reactor are illustrated in Figure 7 with an integral heat exchange
structure in an array of longitudinally disposed, adjacent reaction
passageways or channels which are either catalyst-coated or
catalyst-free. In preferred catalyst structures, the channels coated
with catalyst differ from the catalyst-free channels by having an
average hydraulic diameter which is lower than the average hydraulic
diameter of the catalyst-free channels and by having a higher film heat
transfer coefficient than the catalyst-free channels. More preferably,
the catalyst-coated channels have both a lower hydraulic diameter and a
higher film heat transfer coefficient than the catalyst-free channels.
Thus, for the catalyst structures, the average hydraulic diameter can be
determined by first finding the hydraulic diameter for all of the
catalyst-coated channels in the structure by calculating the average
hydraulic diameter for any given channel over its entire length and then
determining the average hydraulic diameter for the catalyst-coated
channels by totaling up all of the calculated hydraulic diameters for
the individual channels, multiplied by a weighing factor representing
the fractional open frontal area for that channel. Following the same
procedure, the average hydraulic diameter for the catalyst-free channels
in the structure can also be determined. The finding that the
catalyst-coated channels most advantageously have a lower average
hydraulic diameter than the catalyst-free channels can be explained, in
part, by the fact that the catalyst-coated channels desirably have a
surface to volume ratio which is higher than that of the catalyst-free
channels, since hydraulic diameter bears an inverse relationship to
surface to volume ratio. Further, in the catalyst structures, the
difference in average hydraulic diameter of the catalyst-coated channels
and catalyst-free channels gives an indication that the catalyst-free
channels, on average, must be more open channeled and therefore, the gas
flow through these channels is less affected by changes in the channel
diameter than the catalyst-coated channels, again, in part, because of
the higher surface to volume ratios in the catalyst-coated channels.
Preferably, the numeric ratio of the average hydraulic diameter of the
catalyst-coated channels to the average hydraulic diameter of the
catalyst-free channels, that is, average hydraulic diameter of
catalyst-coated channels divided by average hydraulic diameter of
catalyst-free channels is between about 0.08 and about 0.88 and, most
preferably, the ratio of average hydraulic diameter of catalyst-coated
channels to catalyst-free channels is between about 0.2 and 0.8. The
film heat transfer coefficient is a dimension-less value, which is
measured experimentally by flowing gas, for instance, air or air-fuel
mixtures, at a given inlet temperature through an appropriate test
structure having the specified channel geometry and temperature and
measuring the outlet gas temperature. Since the gas composition, flow
rates, pressures and temperatures in the catalytic and non-catalytic
channels of the catalyst structure are very similar, the film heat
transfer coefficient provides useful means of characterizing the
different flow geometries provided by the various flow channel
configurations which distinguish the catalyst-coated channels from the
catalyst-free channels of the catalyst structure. Since these different
flow geometries, in turn, are related to the tortuosity of the flow path
formed by the channels, the film heat transfer coefficient provides some
measure of tortuosity as it is employed in the catalyst structures.
While one could conceive of a variety of methods to measure or otherwise
determine the film heat transfer coefficient in the catalyst structures,
one convenient method would involve constructing an experimental test
structure, for instance, a solid thick metal structure, with internal
space machined to simulate the desired channel shape; and then to test
it in environments where the wall temperature is essentially constant
from inlet to outlet or varies from inlet to outlet and is measured at
several points along the channel length in the structure. The catalyst
structure is particularly useful when equipped with appropriate
catalytic materials for use in a combustion or partial combustion
process wherein a fuel, in gaseous or vaporous form, is typically
partially combusted in the catalyst structure followed by complete
homogeneous combustion downstream of the catalyst. With the catalyst
structure, it is possible to obtain more complete combustion of fuel in
the catalytic channels with minimum combustion in the non-catalytic
channels over a wider range of linear velocities, gas inlet temperatures
and pressures than has here-to-for been possible with catalyst
structures of the previous designs, including those employing integral
heat exchange. Accordingly, the design also encompasses an improved
catalyst structure for use in the combustion or partial combustion of a
combustible fuel, as well as a process for combusting a mixture of a
combustible fuel and air or oxygen-containing gas, using the catalyst
structure.