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.