3. Results and discussion
The ethane mole fraction profiles along the fluid centerline of the
catalytic reactor are presented in Figure 3 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. Regardless of the specific catalytic reactor
configuration, the reactor should be capable of providing good contact
between the oxidation catalyst and the fuel-oxidant mixture in the
exothermic catalytic reaction channel. For catalytic combustion
applications, it is preferred that the reactor be sized such that the
reaction of the fuel-oxidant mixture in the exothermic catalytic
reaction channel should proceed more than 50 percent of the way to
completion before exiting. For fuel-lean mixtures, this means that more
than 50 percent of the fuel entering the channel should be consumed.
Most preferably, more than 80 percent of the fuel entering the
exothermic catalytic reaction channel should burn before exiting. 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 chemical composition of
the fuel-oxidant mixture may also affect the percentage of reaction
completed, particularly if the rate of chemical reaction is
significantly limiting when compared to the rate of mass transfer to the
catalyst surface. With regard to percentage of reaction completed,
important physical characteristics within the exothermic catalytic
reaction channel include the rate of mass transfer to the oxidation
catalyst surface, the ratio of oxidation catalyst surface area to
reaction channel volume, and the activity of the oxidation catalyst. The
catalyst-coated substrate may be fabricated from any of various high
temperature materials. High temperature metal alloys are preferred,
particularly alloys composed of iron, nickel, and cobalt, in combination
with aluminum, chromium, and other alloying materials. High temperature
nickel alloys are especially preferred. Other materials which may be
used include ceramics, metal oxides, intermetallic materials, carbides,
and nitrides. Metallic substrates are most preferred due to their
excellent thermal conductivity, allowing effective backside cooling of
the catalyst. The support material, preferably metallic or
intermetallic, may be fabricated using conventional techniques to form a
honeycomb structure, spiral rolls or stacked patterns of corrugated
sheet, sometimes inter-layered with sheets which may be flat or of other
configuration, or columnar or other configuration which allow for the
presence of adjacent longitudinal channels which are designed to present
flow channels in accordance with the design criteria set forth above. If
intermetallic or metallic foil or corrugated sheet is employed, the
catalyst will be applied to only one side of the sheet or foil or in
some cases the foil or sheet will remain uncoated depending on the
catalyst structure design chosen. Applying the catalyst to only one side
of the foil or sheet, which is then fabricated into the catalyst
structure, takes advantage of the integral heat exchange concept,
allowing heat produced on the catalyst to flow through the structure
wall into contact with the flowing gas at the opposite non-catalytic
wall thereby facilitating heat removal from the catalyst and maintaining
the catalyst temperature below the temperature for complete adiabatic
reaction. In this regard, the adiabatic combustion temperature is the
temperature of the gas mixture if the reaction mixture reacts completely
and no heat is lost from the gas mixture. 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
concept of tortuosity, as used herein, is defined as the difference
between the length of the path which a given portion of reaction mixture
will travel through the passage formed by the channel as a result of
changes in direction of the channel and changes in channel
cross-sectional area versus the length of the path traveled by a similar
portion of the reaction mixture in a channel of the same overall length
without changes in direction or cross-sectional area, in other words, a
straight channel of unaltered cross-sectional area. The deviations from
a straight or linear path, of course, result in a longer or more
tortuous path and the greater the deviations from a linear path the
longer the traveled path will be. When applied to the catalyst
structures, differences in tortuosity between catalytic and
non-catalytic channels is determined by comparing the average tortuosity
of all of the catalytic channels in the structure to the average
tortuosity of all of the non-catalytic channels in the structures.