Figure 5. Velocity contour plots of flow 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 gas-phase reaction rate profiles along the fluid centerline of the
catalytic reactor are presented in Figure 6 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 method takes advantage of a sequence of fuel
combustion zones, and improves the system by conducting at least a
portion of the thermal combustion in a gas expansion zone of the
catalytic reactor. Part of the thermal combustion may occur in both of
these types of expansion areas. As a result, a continuous reheating
effect occurs within the catalytic reactor and this system provides for
the highly efficient use of the fuel, greater than if no combustion
occurs in the expansion zone of the catalytic reactor, without the
formation of undesirable amounts of nitrogen oxides during such
combustion and the development of excessively high temperatures. Part of
the catalytically supported thermal combustion of the fuel-air mixture
is conducted upstream of the catalytic reactor by combustion of the
mixture while passing through an insufficient amount of catalyst to
effect complete combustion of the fuel prior to passing through the
expansion zone. The partially combusted effluent from the catalyst, with
or without substantial intermediate but incomplete further combustion,
is introduced into a gas expansion zone of the catalytic reactor so that
the partially combusted effluent is further thermally combusted while
undergoing expansion in the catalytic reactor. 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. At least a portion of the fuel is combusted in the
catalytic reactor under essentially adiabatic conditions. 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 adiabatic flame temperature is determined at catalyst
inlet conditions. The resulting effluent is characterized by high
thermal energy useful for generating power and by low amounts of
atmospheric pollutants. Where desired, combustible fuel components, for
instance, un-combusted fuel or intermediate combustion products
contained in the effluent from the catalytic zone, or fuel-air admixture
which has not passed through a catalytic zone, may be combusted in a
thermal zone following the catalytic zone. Sustained
catalytically-supported, thermal combustion occurs at a substantially
lower temperature than in conventional adiabatic thermal combustion and
therefore it is possible to operate without formation of significant
amounts of oxides of nitrogen. Combustion in the catalytic reactor is no
longer limited by mass transfer as in the case of conventional catalytic
combustion, and at the specified operating temperatures the reaction
rate is substantially increased beyond the mass transfer limitation, for
instance, at least about 8 times greater than the mass transfer limited
rate. In the catalytic reactor, reaction rates of up to about 80 or more
times the mass transfer limited rate may be attainable. Such high
reaction rates permit high fuel space velocities which normally are not
obtainable in catalytic reactions. One can employ, for instance, at
least an amount of fuel equivalent in heating value to about 200 pounds
of ethane per hour per cubic foot of catalyst, and this amount may be at
least several times greater, for instance, an amount of fuel equivalent
in heating value to at least about 800 pounds of ethane per hour per
cubic foot of catalyst. There is, moreover, no necessity of maintaining
fuel-to-air ratios in the flammable range, and consequently loss of
combustion due to variations in the fuel-to-air ratio is not the problem
it is in conventional combustors [71, 72]. When the fuel-air ratio
in the cooling channels is within the limits of flammability, it is
preferred that the cross-stream area for flow through the cooling
channels is not increased after the cooling stream enters the conduits
[73, 74]. The cross-stream area of the cooling passages may in fact
be decreased to increase the cooling flow velocity for greater
resistance to flashback or pre-ignition in the cooling portion of the
reactor. The minimum residence time for pre-ignition to occur in the
conduits is dependent upon fuel type, fuel-air ratio, temperature, and
pressure, and can be measured experimentally or calculated on the basis
of elementary chemical reaction rates, if known. Gas temperatures in the
cooling portion of the reactor may rise to near the material limit of
the catalyst and substrate, resulting in very short ignition delay
times. To prevent pre-ignition in the cooling portion of the reactor,
the gas residence time in the cooling portion of the reactor should be
less than the ignition delay time. Increasing the cooling flow velocity
within the cooling passages reduces the residence time of the cooling
fluid within the reactor, and reduces the cooling fluid’s propensity for
pre-ignition. Similarly, decreasing the velocity of the fluid in the
exothermic catalytic reaction channel provides increased residence time
for reaction in a given length reactor, while the cooling fluid’s
residence time within the same length reactor remains at a smaller
value, allowing a reduced propensity for pre-ignition of the cooling
fluid. When applied to the catalysis of highly exothermic reactions, the
catalyst structures are typically monolithic-type structures 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
portion of the channels are coated on at least a part of their interior
surface with a catalyst for the reaction mixture, catalyst-coated
channels, and the remaining channels are not coated with catalyst on
their interior surface, catalyst-free channels, such that the interior
surface of the catalyst-coated channels are in heat exchange
relationship with the interior surface of adjacent catalyst-free
channels and wherein the catalyst-coated channels differ in
configuration from the catalyst-free channels such that the desired
reaction is promoted in the catalytic channels and suppressed in the
non-catalytic channels of the catalytic reactor [75, 76]. In cases
where the catalyst structure is employed in a catalytic combustion or
partial combustion process, the critical difference in the design of the
catalytic versus non-catalytic channels will insure more complete
combustion of the fuel in the catalytic channels and minimum combustion
in the non-catalytic channels over a wider range of linear velocity,
inlet gas temperature and pressure [77, 78]. The highly exothermic
process can be carried out in a single catalytic reaction zone employing
the catalyst structure or in multiple catalytic reaction zones using
catalyst structures designed specifically for each catalytic stage. In
most cases the catalytic reaction zone will be followed by a homogeneous
combustion zone in which the gas exiting from the earlier catalytic
combustion zone is combusted under non-catalytic, non-flame conditions
to afford the higher gas temperature. The homogeneous combustion zone is
sized to achieve substantially complete combustion and to reduce the
carbon monoxide level to the desired concentration.