Cordierite ceramic monoliths

A ceramic monolith catalyst support, cordierite, consisting of silica, alumina and magnesium oxide. The purpose of this is to provide support, strength and stability over a wide temperature range. 

Composite ceramic monolith catalysts are also included among the commercial SCR systems. They are manufactured by depositing a layer of catalytic ingredients onto strong, thin-walled ceramic honeycomb supports (usually cordierite). They may suffer from erosion problems in the presence of dust, and their use may be preferably limited to a clean environment. 

Ceramic monoliths can be manufactured either by extrusion [6-31 ] or by corrugation [32-44], the former being the technique mainly used. By extrusion, ceramic monoliths of various materials can be produced, though cordierite or mullite monoliths are most used, especially as catalyst carriers in exhaust gas treatment [6-31]. 

The monolithic substrate can be made of either ceramic or steel. Ceramic monoliths are extruded in the form with cordierite being the main material used. Metallic monoliths are made by corrugation, followed by rolling or folding the metal into monoliths of the desired shape and size. 

The production procedure for ceramic monoliths has been described in the literature. Basically, they are made by extrusion of a paste that contains the cordierite precursors together with processing aids, followed by drying and reactive calcination [24]. 

The trapping component was formulated into a washcoat and supported on a ceramic monolith with 400 cells per square inch (cpsi). The trap material was chosen for NOx adsorption, regenerability, thermal stability and rate of adsorption/desorption. Platinum is incorporated within the trap to oxidize the NO and the injected hydrocarbon. The lean NOx catalyst was Pt (60 gft- ) deposited on y-Al203 on a 400 cpsi cordierite monolith. 

The preparation and use of active catalysts coated on a structure packing was further studied as an attractive replacement for conventional catalysfs in randomly packed beds or slurry reactions. A method was developed in which catalytically active and selective BEA coatings could be prepared onto ceramic monoliths constituted either of pure silica or cordierite (Figure 4.7a) and mefallic wire gauze packings (Figure 4.7b). ... 

Two basic catalyst structures were used, distinguished by the configuration of the catalyst support. The two support types are alumina pellets and alumina coated ceramic monoliths (Figure 2). The pellets are approximately 1/8th inch in diameter and are composed of thermally stable transitional alumina. The monoliths are made of a ceramic material such as cordierite (2Mg,2Al203,5Si02). 

The resistance of the ceramic monolith to melting has been the object of research since it was first developed. The melting temperature of cordierite monolith (1460°C) is well in excess of that reached in... 

The substrate consisted of ceramic monolith made of cordierite (2 MgO 5 Si02 2 AI2O3) manufactured by Coming with a cell density of 62 cells/cm. The... 

A ceramic monolith can be coated with an agarose gel and activated using the same procedure used to activate a bed of agarose beads. It is possible to increase or decrease the coating load in order to have a thicker or thinner coat, and it is possible to make a monolith bed as large as any commercially available monolith. There is no indication that any of the chemicals present in cordierite interfere with our separation process or that they even come in contact with the enzyme solutions l. 

Washcoat technology was initially developed for the automobile catalytic converter, consisting of a ceramic monolith of many small parallel channels, in the mid-1970s. Monoliths have since then developed into a variety of different materials and configurations, essentially cordierite based [3]. 

Honeycomb monoliths are obtained by extruding a paste made by catalytic material, whereas plate catalysts are made by depositing the catalytic material onto a stainless steel net or a perforated metal plate. Composite ceramic monolith catalysts, consisting of a monolith matrix made of cordierite coated with metal oxide SCR material, are also offered. However, they may suffer adhesion problems in the presence of dust and their use may be preferably limited to clean environment. Coated metal monoliths are constructed of thin metal foil and are characterized by large cell densities. In view of this, they are used exclusively in dust-free applications. 

Heat management in monoUth reactors via external heating or cooling is not as effective as in PBRs due to lack of convective heat transport in the radial direction. At this point, the material of construction of the monolithic structure affects the overall performance. Monolith reactors can be made of metals or ceramics. In case of nonadiabatic reactions, metallic monoliths are preferred due to their higher thermal conductivity which partially eliminates the lacking convective contribution. Ceramic monoliths, on the other hand, have very low thermal conductivities (e.g., 3 W/m.K for cordierite [11]) and are suitable for use in adiabatic operations. 

Owing to the low surface area of the monoliths, a catalyst carrier such as alumina or ceria is deposited onto them usually by wash coating [11]. The ceramic construction material is well compatible with these carriers, does not migrate into the catalyst coatings, and the precious active metal species of the catalysts do not migrate into the monoUth bodies [1]. The most widely used construction material for ceramic monolith carriers is cordierite [9] with alumina being an alternative. 

A parametric study on the effects of axial heat conduction in the solid matrix has shown that i) such effects are negligible in ceramic monoliths (cordierite, kj = 1.4 w/m/K) but expectedly significant in metallic monoliths (Fecralloy, k i = 35 W/m/K) when a constant heat flux is imposed at the external matrix wall ii) however, the influence of axial conduction in metallic monoliths is much less apparent if a constant wall temperature condition is applied, since the monolith tends to an isothermal behavior. Metallic matrices exhibit very flat axial and radial temperature profiles, which seems promising for their use as catalyst supports in non-adiabatic chemical reactors. 

Figure 3 - Effect of cell density and of channel size on heat transfer efficiency of a monolith with constant external wall temperature = 500 K. Case of ceramic monolith (Cordierite). Gas=air.

Materials that are routinely coated with catalyst wash-coats are ceramics such as cordierite, which is the construction material of ceramic monoliths, metals such as Fecralloy, the construction material of metallic monoliths (see Section 6.2) and stainless steel [57]. The amount of catalyst material that can be coated onto a monolith ranges between 20 and 40 g m , while plate heat-exchangers may even take up more catalyst when coated prior to the sealing procedure, because the access to the channels is better. 

Pellets and ceramic monolithic substrate structures were initially involved in three-way catalytic converters for washcoat deposition, while metal foil monolithic substrates were also introduced since the late 1970s. TWCs manufactures were soon concentrated on cordierite (2Mg0-2Al203-5Si02) ceramic monoliths or on Fe-based alloys foil monoliths (iron-chromiimi—alimiimmi ferritic steels). Both options are used nowadays, although ceramic monoliths are preferably used, despite the several advantages of the latter [2]. 

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