Catalyst monolith reactors

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 

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Firstly, there are technical reasons concerning catalyst and reactor requirements. In the chemical industry, catalyst performance is critical. Compared to conventional catalysts, they are relatively expensive and catalyst production and standardization lag behind. In practice, a robust, proven catalyst is needed. For a specific application, an extended catalyst and washcoat development program is unavoidable, and in particular, for the fine chemistry in-house development is a burden. For coated systems, catalyst loading is low, making them unsuited for reactions occurring in the kinetic regime, which is particularly important for bulk chemistry and refineries. In that case, incorporated monolithic catalysts are the logical choice. Catalyst stability is crucial. It determines the amount of catalyst required for a batch process, the number of times the catalyst can be reused, and for a continuous process, the run time. 

G., and Forzatti, P. (1998) Monolithic catalysts for selective reduction of NOx with NH3 from stationary sources, in Structured Catalysts and Reactors, 1st edn. Chapter 5 (eds A. Cybulski and J.A. Moulijn), Marcel Dekker, New York, p. 121 (b) Nova, 1., Beretta,... 

Irandoust, S., Cybulski, A., and Moulijn, J.A. (1998) The use of monolithic reactors for three-phase reactions, in Structured Catalysts and Reactors,... 

Static mixing catalysts Operation Monolithic reactors Microreactors Heat exchange reactors Supersonic gas/liquid reactor Jet-impingement reactor Rotating packed-bed reactor... 

Concerning the reaction pathway, two routes have been proposed the sequence of total oxidation of methane, followed by reforming of the unconverted methane with CO2 and H2O (designated as indirect scheme), and the direct partial oxidation of methane to synthesis gas without the experience of CO2 and H2O as reaction intermediates. The results obtained by Schmidt and his co-workers [4, 5] indicate that the direct reaction scheme may be followed in a monolith reactor when an extremely short contact time is employed at temperatures in the neighborhood of 1000°C. However, the majority of previous studies over numerous types of catalysts show that the partial oxidation of methane follows the indirect reaction scheme, which is supported by the observation that a sharp temperature spike occurs near the entrance of the catalyst bed, and that essentially zero CO and H2 selectivity is obtained at low methane conversions (<25%) where oxygen is not fully consumed [2, 3]. A major problem encountered... 

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

Tronconi et al. [46] developed a fully transient two-phase 1D + 1D mathematical model of an SCR honeycomb monolith reactor, where the intrinsic kinetics determined over the powdered SCR catalyst were incorporated, and which also accounts for intra-porous diffusion within the catalyst substrate. Accordingly, the model is able to simulate both coated and bulk extruded catalysts. The model was validated successfully against laboratory data obtained over SCR monolith catalyst samples during transients associated with start-up (ammonia injection), shut-down (ammonia... 

This study uses novel monolith structured catalysts in aiming to improve process productivity and selectivity. Apart from the advanced characteristics of low pressure drop, less backmixing, convenient change out of catalysts, the monolith reactor structure reduces the mass transfer limitation and potentially improves the selectivity. 

Monolith reactor This type of reactor is used extensively for the abatement of automobiles exhaust emissions. The gas flows continuously through the reactor, whereas the catalyst is a continuous phase consisting of a ceramic support and the active phase, which is dispersed onto the support. The support is structured in many channels and shapes that achieve large catalytic surface at small volume. A typical application of monolith reactors is the exhaust gas cleaning. 

With a monolith reactor, diffusion from the channels to the catalyst coated on the channel walls is the sole means by which reactants are able to reach the catalyst (Section III). It seems reasonable that a similar diffusion process occurs in a coated filter. 

The catalyst and particulate filter models were developed individually with different university partners. They are described in the following sections. A key issue for all models is robustness and scalability as the applications range from passenger cars to heavy-duty commercial vehicles. The models are physical and chemically based, consisting of a transport model for heat and mass transfer phenomena in the monolith in gas and solid phases, cf. Fig. 6. The monolith reactor modeling is discussed in more detail in Section III. 

The second contribution comes from a major catalyst manufacturer and illustrates how insight in the reaction paths involved in three-way conversion leads to a fundamental, i.e. based on first principles, model. The emphasis in this contribution is on the chemistry rather than on the reactor model, i.e. on the description of the physical phenomena occurring in the monolith reactor. In this sense, this contribution is the bridge from the first to the third contribution. 

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