Monoliths catalytic combustion

Fig. 1.4 Illustration of the chemically reacting boundary-layer flow in a single channel of a catalytic-combustion monolith.

There are many chemically reacting flow situations in which a reactive stream flows interior to a channel or duct. Two such examples are illustrated in Figs. 1.4 and 1.6, which consider flow in a catalytic-combustion monolith [28,156,168,259,322] and in the channels of a solid-oxide fuel cell. Other examples include the catalytic converters in automobiles. Certainly there are many industrial chemical processes that involve reactive flow tubular reactors. Innovative new short-contact-time processes use flow in catalytic monoliths to convert raw hydrocarbons to higher-value chemical feedstocks [37,99,100,173,184,436, 447]. Certain types of chemical-vapor-deposition reactors use a channel to direct flow over a wafer where a thin film is grown or deposited [219]. Flow reactors used in the laboratory to study gas-phase chemical kinetics usually strive to achieve plug-flow conditions and to minimize wall-chemistry effects. Nevertheless, boundary-layer simulations can be used to verify the flow condition or to account for non-ideal behavior [147]. 

Catalysts are often implemented in the walls of monolith channels, with overall performance depending on a balance between surface reactivity and flow conditions. Consider a situation that represents a catalytic-combustion monolith such as in a gas-turbine system (e.g., Fig. 17.17), where an individual channel diameter of d = 2 mm and a length of L = 5 cm. The channel walls may be assumed to be isothermal at Tw = 800°C. A CH4-air mixture enters the monolith with a equivalence ratio of(p = 0.3, inlet temperature of Tm — 400°C, and pressure of p = 1 atm. 

L.L. Raja, R J. Kee, O. Deutschmann, J. Wamatz, and LD. Schmidt. A Critical Evaluation of Navier-Stokes, Boundary-Layer, and Plug-Flow Models of the Flow and Chemistry in a Catalytic-Combustion Monolith. Catalysis Today, 59 47-60,2000. 

Raja LL, Kee RJ, Deutschmann O, Warnatz J, Schmidt LD. A critical evaluation of Navier-Stokes, boundary-layer, and plug-flow models of the flow and chemistry in a catalytic-combustion monolith. Catalysis Today 2000 59 47-60. 

Tischer S, Correa C, Deutschmann O (2001) Transient three-dimensional simulation of a catalytic combustion monolith using detailed models for heterogeneous and homogeneous reactions and transport phenomena. Catal Today 69 57-62... 

Catalyst monoliths may laos be employed as catalytic combustion chambers preceding aircraft and stationary gas turbines. As shown diagramatically in Fig. 16, a catalytic combustor comprises a preheat region, a catalyst monolith unit and a thermal region. In the preheat region, a small fuel-rich flame burner is employed to preheat the fuel-air mixture before the hot gases reach the monolith unit. Additional fuel is then injected into the hot gas stream prior to entry to the monolith where... 

Catalytic combustion in a monolith channel provides an illustration of boundary-layer flow in a channel [322], Figure 17.18 shows a typical monolith structure and the particular single-channel geometry used in this example. Since every channel within the monolith structure behaves essentially alike, only one channel needs to be analyzed. Also a cylindrical channel is used to approximate the actual shape of the channels. 

Comprehensive reviews on catalytic combustion, including the use of monoliths for automotive converters, have been published in the last decades [1-5]. Reviews on mono-... 

Because of the outstanding prospects for catalytic combustions, a lot of R D work has been earned out in recent decades on this subject. For the above-mentioned reasons, a considerable proportion of the research is dedicated to the development of novel materials for monoliths. Pilot and demonstration plants for monolithic combustion are in operation. 

Catalytic combustion is another area where monolithic catalysts will find their place. After all material problems (refractoriness and life of the catalyst) and engineering... 

Modeling of catalytic combustors has been the subject of a number of studies. The models used varied in degree of complexity and could therefore answer various types of questions. General issues of modeling monolith catalytic reactors are discussed in Chapter 8 of this book and in the reviews of Irandoust and Andersson [57] and Cybulski and Moulijn [58]. Hence, only topics that are specific to the modeling of catalytic combustion in monolith catalysts are considered here. A description of some important aspects of different types of models are as follows. 

Both studies show that at relatively low temperatures, i.e., during ignition of the catalyst, the rate-limiting step shifts from chemical kinetics to diffusion in the washcoat. This is clear from Fig. 7, computed using a one-dimensional model by Nakhjavan et al. [54]. Figure 7A shows the Thiele modulus and Fig. 7B an external diffusion limiting factor F versus dimensionless axial position in the reactor at various times on-stream for the catalytic combustion of propene in monolith reactors. The time is defined as the time after injection of the fuel in a preheated air flow. 

Materials such as aluminum titanate and silicon carbide appear to be promising for high-temperature catalytic combustion. However, problems such as extrudability, the application of washcoats, and reaction with deposited washcoats are not solved yet. For instance, when hexa-aluminate, presented in the introduction to this section, was applied to silicon carbide monoliths, solid-state reactions occurred at 1200-1400 C [76], causing exfoliation of the coating and the formation of new phases. The application of an intermediate mullite layer was suggested as an approach to hinder these solid-state reactions. 

B. Kucharczyk, W. Tylus, L. Kepinski, Pd-based monolithic catalysts on metal supports for catalytic combustion of methane, Appl. Catal. B-Environ. 49 (2004) 27. 

The introduction of solid catalysts into a traditionally non-catalytic free-radical process like combustion occurred in recent years under the influence of two pressures, the energy crisis and the increased awareness of atmospheric emissions. The major applications of catalytic combustion are twofold at low temperatures to eliminate VOC s and at high temperatures (>1000 C) to reduce NOx emission from gas turbines, jet motors, etc. Both these applications are briefly reviewed here. Some recent developments in high-temperature catalytic combustion are trend-setters in catalysis and hence of particular interest. For instance, novel materials are being developed for catalytic applications above 1000 C for sustained operation for over one year. Where material/catalyst developments are still inadequate, systems engineering is coming to the rescue by developing multiple-monolith catalyst systems and the so-called hybrid reactors. 

Fig. 7. Three systems engineering solutions for high-temperature catalytic combustion. A multiple monolith catalyst design B partial catalytic combustion C hybrid (catalytic + thermal) combustion LGC Lean gas-phase combustion.

The first innovative systems engineering approach is of Osaka Gas Company in Japan. They developed a multiple monolith catalyst design for the gas-turbine combustion of methane. In this design, different materials are used to fulfil different functions within the catalyst. A palladium catalyst is placed at the entrance of the catalyst system to ignite the catalytic combustion reaction and to raise the temperature. This temperature is then enough... 

Most of the work cited above has dealt with treating the soot in some way before doing the combustion experiments. We wish to report experiments conducted on soot from a diesel vehicle which has been deposited onto catalytic monolithic substrates. This sooted substrate is then placed in a laboratory apparatus where a synthetic gas mixture flows over the sample, and the soot combustion is monitored as a function of temperature. The laboratory set up simulates regeneration conditions on a vehicle. Using this technique we have been able to obtain kinetic information about the oxidation of soot and gaseous products. Comparisons of base metal and noble metal catalysts were also conducted and are reported. It is intended that this work will help elucidate the mechanism involved in the catalytic combustion of soot which should help in developing improved catalytic materials. 

Catalytic combustion of SO2, toluene and 1,2-dichloroethane were conducted at atmospheric pressure in a tubular flow reactor with an inner diameter of 18 mm. Catalyst extrudates or monoliths were packed into the reactor with glass wool plugs at each end. The reactor was placed in a furnace equipped with a temperature control to maintain a constant reactor temperature, and two thermocouples to measure the inlet and outlet reactor temperatures. Gas compositions and flow rates were set by mass flow controllers. The following reaction conditions were used to test the cataljTic activity of the 0.2 wt. % Pt supported samples ... 

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