1. Introduction
In
modern industrial practice, a variety of highly exothermic reactions are
known to be promoted by contacting of the reaction mixture in the
gaseous or vapor phase with a heterogeneous catalyst [1, 2]. In some
cases, these exothermic reactions are carried out in catalyst-containing
structures or vessels where external cooling must be supplied, in part,
because of the inability to obtain sufficient heat transfer and the need
to control the reaction within certain temperature constraints. In these
cases, it is not considered practical to use a monolithic catalyst
structure, where the unreacted portion of the reaction mixture supplies
the cooling for the catalytic reaction, because existing catalyst
structures do not provide an environment whereby the desired reaction
can be optimized while removing the heat of reaction through heat
exchange with unreacted reaction mixture under conditions where
undesired reactions and catalyst overheating are avoided [3, 4].
Thus, the applicability of monolithic catalysts structures to many
catalyzed exothermic reactions could clearly be enhanced if monolithic
catalyst structures could be developed wherein the reaction zone
environment and heat exchange between reacted and unreacted portions of
the reaction mixture are improved.
Highly exothermic catalytic reactors with internal cooling have made
rapid progress [5, 6]. While they have varying applications, the
reactors are typified by exothermic reactions within the catalytic
portion of the reactor and a cooling means to control the temperature
within the catalytic portion to avoid a material failure, either of the
substrate or the catalyst [7, 8]. Cooling in these reactors can be
accomplished by a number of means, including placing the catalyst in a
backside-cooled relationship with the cooling agent [9, 10]. A
backside-cooling arrangement is particularly suitable for catalytic
reactions that are both rapid and highly exothermic, for instance,
catalytic combustion [11, 12]. In this arrangement, the catalyst
substrate, typically a metal foil, is coated with an oxidation catalyst
on only one side, the opposite side or backside remaining free of
oxidation catalyst [13, 14]. The substrate is shaped and assembled,
before or after catalyst coating, to create separate channels for
exothermic reaction, in the channels coated with oxidation catalyst, or
for cooling, in the channels free of oxidation catalyst [15, 16].
Fluid passing through the cooling channels removes a portion of the heat
generated in the exothermic reaction channels.
In a typical catalytic combustor, active catalysts being supported on
various substrates provide an effective means of initiating and
stabilizing the combustion process when they are used with suitable
mixtures of fuel and air [17, 18]. These combustion catalysts have
several desirable characteristics: they are capable of minimizing
nitrogen oxides emission and improving the pattern factor [19, 20].
However, one of their limitations is that their maximum operating
temperature tends to be only marginally acceptable as a catalytic
combustor inlet temperature [21, 22]. This limitation is inherent in
the way the typical catalytic combustor operates. Catalysts initiate the
combustion reaction at their surfaces and at temperatures lower than
normal ignition temperature. However, once the reaction is initiated, it
continues in the gas stream and persists beyond the catalyst in the form
of afterburning [23, 24]. Simultaneously, the catalyst substrate
temperature increases, resulting in an accelerated reaction which moves
the reaction zone further upstream in the catalyst. The result may be
damage of the catalyst and catalyst substrate if the fuel-air ratio is
such as to give an excessive catalyst outlet temperature [25, 26].
There is an example of a backside-cooled catalytic reactor for use in a
catalytic combustion system, with the basic method of splitting a given
fuel-air mixture flow into catalytic and non-catalytic passages. This
example is the use of a ceramic substrate with multiple parallel
channels, generally of the same shape and size, in which the walls which
border and define each catalytic channel are coated with an oxidation
catalyst on the sides facing the catalytic channel, but are not coated
with an oxidation catalyst on the sides facing adjacent non-catalytic
channels. By this method, the percentage of total reactants catalyzed in
the reactor is no greater than the percentage of catalytic channels
[27, 28]. The average temperature rise through the reactor is thus
limited [29, 30]. In addition, the wall temperatures of catalytic
channels bordering adjacent non-catalytic channels are controlled
through the use of backside cooling.
There are a number of ways of controlling the temperature, such as by
dilution with excess air, by controlled oxidation using one or more
catalysts, or by staged combustion using variously lean or rich fuel
mixtures [31, 32]. Combinations of these methods are also known. One
widely attempted method is the use of multistage catalytic combustion
[33, 34]. Most of these processes utilize multi-section catalysts
with metal or metal oxide catalysts on ceramic catalyst carriers [35,
36]. It is, however, difficult to control the temperatures in these
processes. Since the object of each of the processes is to produce a
maximum amount of heat in a form which can be efficiently used in some
later process, the combustion steps are essentially adiabatic.
Consequently, a minor change in any of fuel rate, air rate, or operating
processes in an early stage will cause significant changes in the latter
stage temperatures. Very high temperatures place thermal strain on
downstream catalytic elements [37, 38]. Platinum group metals are
considered useful in catalytic combustion processes [39, 40]. A
metal substrate is used for improved heat conduction to the backside
cooling fluid, and for greater resistance to thermal shock.
Aluminum-containing steels are cited as being preferred. This example is
the use of non-similar shape and size channels, so that the flow split
between catalytic and non-catalytic channels can be varied while
retaining approximately half catalytic channels and half non-catalytic
channels. Despite these changes, the fundamental structure, namely a
multitude of catalytic channels and adjacent non-catalytic channels, is
retained [41, 42]. The structure is refined in which periodic
alterations in channel shape provide different wall heat transfer rates
in the catalytic channels and non-catalytic channels [43, 44].
Again, however, the fundamental structure, namely a multitude of
catalytic channels and adjacent non-catalytic channels, is retained.
Furthermore, while the catalytic and non-catalytic channels have
different shape and tortuosity, the average channel properties over some
finite lengths are not varied in the longitudinal direction, so that the
catalytic reactors taught are effectively one-dimensional or
two-dimensional in terms of channel flow properties such as bulk heat
transfer coefficient, velocity, or average cross-sectional shape or
area.
There is also a clear need to improve the operability of monolithic
catalyst structures in areas where they are currently used or proposed
for use, such as the combustion or partial combustion of fuels or the
catalytic treatment of exhaust emissions from internal combustion
engines, to widen the range of operating, conditions at which the
desired catalytic conversions can be achieved. A catalyst structure made
up of a series of adjacently disposed catalyst-coated and catalyst-free
channels for passage of a flowing reaction mixture can be employed,
wherein the catalytic and non-catalytic channels share a common wall
such that integral heat exchange can be used to dissipate the reaction
heat generated on the catalyst and thereby control or limit the
temperature of the catalyst [45, 46]. That is, the heat produced on
the catalyst in any given catalyst-coated channel flows through the
common wall to the opposite non-catalytic surface to be dissipated into
the flowing reaction mixture in the adjacent catalyst-free channel. The
configuration of the catalytic channels differs from the non-catalytic
channels in one or more critical respects, including the tortuosity of
the flow channel, such that, when applied to catalytic combustion,
catalytic and homogeneous combustion is promoted within the catalytic
channels and not promoted or substantially limited in the non-catalytic
channels while heat exchange is otherwise optimized [47, 48]. These
uniquely configured catalyst structures substantially widen the window
of operating parameters for catalytic combustion and partial combustion
processes.
In cases where the integral heat exchange structure is used to carry out
catalytic partial combustion of a fuel followed by complete combustion
after the catalyst, the catalyst must burn a portion of the fuel and
produce an outlet gas sufficiently hot to induce homogeneous combustion
after the catalyst [49, 50]. In addition, it is desirable that the
catalyst not become too hot since this would shorten the life of the
catalyst and limit the advantages to be gained from this approach. As
the operating condition of the catalyst is changed, it is noted with the
integral heat exchange structures that operating window of such
catalysts are limited. That is, that the gas velocity or mass flow rate
must be within a certain range to prevent catalyst overheating. In
general, the backside-cooled catalytic reactors include a multitude of
catalytic channels, where each individual catalytic channel is in
essence a separate catalytic reactor [51, 52]. As a result,
variations in fuel-air ratio from channel to channel, due to imperfect
premixing, for instance, can lead to different degrees of combustion and
heat release in different channels. Likewise, variations in inlet
temperature from channel to channel can also lead to variations in
combustion behavior in different channels. Rate of reaction, catalyst
light-off length, and maximum gas or surface temperature can all be
affected by the temperature and fuel-air ratio at a channel inlet [53,
54]. In addition, manufacturing tolerances may result in unequal
physical properties of different channels. Properties which may vary
include channel size, wall thickness, catalyst or washcoat thickness,
and catalyst loading; each of these may affect combustion behavior
[55, 56]. In essence, multiple catalytic channels can produce widely
varying degrees of catalytic combustion.
Because there is no mixing between separate catalytic channels in the
backside-cooled reactors, the reactors suffer the above-mentioned
disadvantages of sensitivity to premixing, for instance, the fuel-air
ratio, and sensitivity to inlet temperature uniformity [57, 58].
Given that all real systems have some level of gas-stream
non-uniformity, these sensitivities translate to a narrowed operating
range. Structures and methods that provide an un-partitioned exothermic
catalytic reaction channel and multiple cooling channels offer superior
performance [59, 60]. The un-partitioned exothermic catalytic
reaction channel allows for continual mixing of the fuel-oxidant stream
within the channel leading to a more uniform combustion and a wider
operating range. A catalytic reactor may employ an exothermic catalytic
reaction channel cooled by numerous cooling channels, where the cooling
fluid is a portion of the ultimate fuel-oxidant-product mixture [61,
62]. The structure of the reactor is more flexible, facilitating
cross-stream area changes in the streamwise or longitudinal direction,
since there is no constraint that walls contact each other to form
multiple catalytic channels [63, 64]. Thus, the reactor can be used
to vary the bulk fluid properties in the streamwise or longitudinal
direction via cross-stream area changes. In particular, it may be
desirable to reduce the velocity of the fuel-air mixture after it has
entered the exothermic catalytic reaction channel, to provide greater
residence time for reaction within the reactor, while maintaining
sufficient velocity at the reactor inlet to prevent flashback to the
fuel-oxidant mixture upstream of the reactor [65, 66]. Therefore, it
is clear that a need exists for improved catalytic structures employing
integral heat exchange which will substantially widen the window or
range of operating conditions under which such catalytic structures can
be employed in highly exothermic processes like catalytic combustion or
partial combustion. It is necessary to capitalize on certain critical
differences in the configuration of the catalytic and non-catalytic
passageways or channels in an integral heat exchange structure to
materially widen the operating window for such catalysts.
The present study relates to a catalyst structure employing integral
heat exchange in an array of longitudinally disposed, adjacent reaction
passageways or channels which are either catalyst-coated or
catalyst-free, as well as a method for using the catalyst structure in
highly exothermic processes, such as combustion or partial combustion
processes. More particularly, the present study is directed to such a
catalyst structure employing integral heat exchange wherein the
catalytic and non-catalytic channels differ from each other in certain
critical respects whereby the exothermic reaction in the catalytic
channels and heat exchange between the catalytic and non-catalytic
channels are optimized while undesired exothermic reaction in the
non-catalytic channels is suppressed. The present study is focused
primarily upon the integral heat exchange in an array of longitudinally
disposed adjacent reaction passage-ways or channels, which are either
catalyst-coated or catalyst-free, wherein the configuration of the
catalyst-coated channels differs from the non-catalyst channels such
that, when applied in exothermic reaction processes, such as catalytic
combustion, the desired reaction is promoted in the catalytic channels
and substantially limited in the non-catalyst channels. 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, preferably the improved
catalyst structures, 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.