4. Conclusions
The highly exothermic process characteristics of catalytic reactors are
investigated with integral heat exchange structures. Ethane mole
fraction and gas-phase reaction rate profiles in catalytic reactors are
presented for a catalytic reactor with an integral heat exchange
structure, and ethane mole fraction, flow velocity, gas-phase reaction
rate, and temperature contour plots are illustrated for catalytically
supported thermal combustion systems. The present study aims to provide
an improved reaction system and process for combustion of a fuel wherein
catalytic combustion using a catalyst structure employing integral heat
exchange affords a partially-combusted, gaseous product which is passed
to a homogeneous combustion zone where complete combustion is promoted
by means of a flame holder. Particular emphasis is placed upon the
catalytic reactor configuration that allows the oxidation catalyst to be
backside cooled by any fluid passing through the cooling conduits. The
major conclusions are summarized as follows:
- The percentage of reaction completed in the exothermic catalytic
reaction channel depends both upon the flow rate of the fuel-oxidant
mixture through the exothermic catalytic reaction channel and upon the
physical characteristics of the catalytic reactor. The critical
difference in the design of the catalytic versus non-catalytic
channels for the catalytic structure, in its most basic terms, is that
the catalytic channels are designed so that the reaction mixture flow
passages defined by the catalytic channels possess a higher or
increased tortuosity over the corresponding flow passages formed by
the non-catalytic channels.
- The tortuosity of the catalytic channels is increased by changing
their cross-sectional area at a multiplicity of points along their
longitudinal axes. One preferred way of accomplishing this change in
tortuosity for the catalytic channels involves the use of a stacked
arrangement of non-nesting corrugated sheets of catalyst support
material which are corrugated in a herringbone pattern with at least a
portion of one side of a given corrugated sheet facing and stacked
against another corrugated sheet being coated with catalyst such that
the stacked sheets in question form a plurality of catalytic channels.
- 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 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.
- Catalytically-supported thermal combustion in the catalytic reactor is
achieved by contacting at least a portion of the carbonaceous fuel
intimately admixed with air with a solid oxidation catalyst having an
operating temperature substantially above the instantaneous
auto-ignition temperature of the fuel-air admixture. Combustion in the
catalytic reactor is characterized by the use of a fuel-air admixture
having an adiabatic flame temperature substantially above the
instantaneous auto-ignition temperature of the admixture but below a
temperature that would result in any substantial formation of oxides
of nitrogen.
- 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
- The temperature of the catalyst zone is controlled by controlling the
composition and initial temperature of the fuel-air admixture as well
as the uniformity of the mixture. The total residence time in the
combustion system should be sufficient to provide essentially complete
combustion of the fuel, but not so long as to result in the formation
of oxides of nitrogen. Any hot surface acts as a catalyst to some
degree, hence even the non-catalyzed passages may tend to provide some
surface combustion. This effect will be minimized by selecting a
ceramic base material with minimal catalytic properties. It may also
be possible to control the boundary layer, decrease the surface area
and the residence time, and perhaps even provide a chain breaking or
ignition delaying surface.