Figure 4. Ethane mole fraction contour plots in 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 velocity contour plots of flow in the catalytic reactor are
illustrated in Figure 5 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. A catalyst
structure comprising a heat resistant support material composed of a
plurality of common walls which form a multitude of adjacently disposed
longitudinal channels for passage of a gaseous reaction mixture wherein
at least a part of the interior surface of at least a portion of the
channels is coated with a catalyst and the interior surface of the
remaining channels is not coated with any catalyst such that the
interior surface of the catalyst-coated channels are in heat exchange
relationship with the interior surface of adjacent catalyst-free
channels. The preferred catalyst structure can be readily modified to
increase the number and tortuosity of the catalytic channels by
inserting additional corrugated sheets having a herringbone corrugation
pattern between the two flat sheets with sharp peaked corrugations. If
additional corrugated sheets are inserted in the repeat unit, they can
be coated on one side of the other or remain uncoated depending on the
catalyst structure desired. Catalytic channels and non-catalytic
channels are formed by selectively coating one side of the two flat
sheets and one side of one of the corrugated sheets with a catalyst. The
reactor can incorporate streamwise variation of the fluid velocities.
Specifically, the velocity of the fluid in the exothermic catalytic
reaction channel can be decreased after the fluid enters the channel by
streamwise geometric changes in the reactor wherein the channel's
entrance flow area is less than the channel's cross-stream flow area at
some streamwise location where the catalyst is deposited. The channel's
entrance flow area is defined as the channel's cross-stream flow area
immediately downstream of the most downstream conduit inlet. If the
conduit wall thicknesses are approximately constant, streamwise changes
in flow velocity can be produced by providing, for instance, either
contracted sections of the conduits, expanded sections of the casing, or
a combination of both. In a reactor employing expanded-end conduits, the
cross-stream area of the conduits decreases just after the cooling flow
enters the cooling passages, where the expanded conduit sections taper
down to the nominal conduit size in the central portion of the reactor.
As a result, the cooling flow velocity is increased to a value greater
than its entrance velocity. Conversely, because the casing is of
constant cross-sectional size, the cross-stream flow area of the
exothermic catalytic reaction channel increases just after the
fuel-oxidant mixture enters. As a result, the flow velocity in the
exothermic catalytic reaction channel is decreased to a value less than
its entrance velocity. The gas flow velocity entering the exothermic
catalytic reaction channel should exceed the minimum required to prevent
flashback into the fuel-oxidant stream upstream of the reactor if the
fuel-oxidant mixture entering the exothermic catalytic reaction channel
is within the limits of flammability. The laminar flame propagation
velocity is typically less than 0.8 meters per second for hydrocarbon
fuels in air, but the turbulent flame propagation velocity may exceed 8
meters per second and may approach 28 meters per second for highly
turbulent flow. To prevent flashback, the gas flow velocity should
exceed 8 to 28 meters per second at gas turbine engine conditions, or
more if a safety margin is allowed. Because catalyst light-off becomes
increasingly difficult with increasing velocity, it is desirable to
reduce the velocity of the fuel-air stream once it has entered the
exothermic catalytic reaction channel, by a streamwise variation of
cross-stream area. The flow velocity of the fuel-air stream over the
exothermic reaction surface in the exothermic catalytic reaction channel
is nominally 28 meters per second or less. This reduction in velocity is
achieved by a streamwise increase in the cross-stream area for flow over
the exothermic reaction surface. Streamwise changes in cross-stream area
are fixed by the geometry of the reactor, and do not change in time. The
velocity of the cooling stream should exceed the maximum flame
propagation velocity at the exit of the cooling conduits, if the cooling
fluid exiting the cooling passages is within the limits of flammability,
to prevent flashback from a downstream combustion chamber. If expanded
conduit outlets are employed, it is also very important that the
downstream increase in cross-stream area of the cooling conduits is
sufficiently gradual that recirculation of the cooling flow does not
occur, so that there is no possibility of flashback or flame holding in
the downstream expanded conduit section. Typically, the cone angle for
an axis-symmetric diffuser section should not exceed approximately 6 to
8 degrees for good pressure recovery and minimal recirculation. If
continued backside cooling is a consideration, the angle should be
especially shallow to ensure that there is no local separation of the
cooling flow and concurrent loss of cooling effectiveness.