Thermal monolithic cordierite

Two types of catalytic converters are currently being used for meeting the passenger car emission standards in the U.S. three-way converters and dualbed converters. Both converters contain three-way catalysts, but with the dual-bed converter the three-way catalyst is followed by an air injection/ oxidation catalyst system. As for the earlier oxidation catalysts two forms of catalyst support are used pellets (thermally stable transitional alumina) and monoliths (cordierite honeycombs coated with a thin alumina washcoat). Figure 7 shows four catalytic converters currently being used by General Motors. 

Although common catalyst support materials such as alumina, silica, and carbon are also used for the production of monoliths, the most common material for commercial monolith production is cordierite, because of its high thermal stability and low coefficient of expansion. 

In general, both cordierite and metallic monoliths are unsuitable as catalytic supports. To process a monolith into an active monolithic catalyst, a layer of porous catalytic support must be deposited on the walls between channels. y-Alumina appeared to be the most effective support for automotive catalysts. The alumina layer is deposited by sol-gel technique (so called washcoating). Adherence of 7-alumina to cordierite is relatively strong. However, to form the stable 7-alumina layer on a metallic surface, we need to use an appropriate alloy that is appropriately processed before the layer is deposited. Stainless steel containing chromium, aluminum, and yttrium subjected to thermal treatment under oxidizing conditions meets requirements of automotive converters. Aluminum in the steel is oxidized to form 7-alumina needles (whiskers) protruding above the metal... 

In the earliest catalysts, two basic support configurations were used. The first was thermally stable alumina in the form of cylindrical pellets or spheres, typically 3 mm in diameter, that had for several decades been used in the chemical processes industry, such as petroleum refining. The second type of catalyst support was the so-called monolith, made from metal, as described in this chapter, or a ceramic material such as cordierite (2MgO 2Al203 5Si02) the subject of the previous chapter. The monolith has strong thin... 

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. 

Design of the catalysts Is critical [42]. The volume flow rate of exhaust gas can be high, and pressure drop must be avoided. As a result, it is more common to use a monolithic structure as the base of the catalyst. However, pellets can and have been used. This base material Is usually fabricated from cordierite or from a metal alloy [42], these materials being chosen for their stability and thermal expansion characteristics. 

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). 

A key property of cordierite monolith is the ability to resist fracture from thermally induced stresses. 

Cordierite (2Mg0-2Al20y5Si02) - Cordierite is, together with alumina, the most common substrate for monolith extrusion. This material has one of the lowest thermal expansions among the proposed materials. This parameter... 

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. 

Karatzas et al. [34] performed autothermal reforming of tet-radecane, low sulfur, and Fischer-Tropsch diesel in a monolithic reformer over rhodium/ceria/lanthana catalyst. The reformer had a thermal power output of 14 kW. It was composed of an inert zirconia-coated alumina foam for feed distribution at the reactor inlet and two 400 cpsi cordierite monoliths coated with the catalyst switched in series. At an O/C ratio of 0.45, a S/C ratio of 2.5 and temperatures exceeding 740°C, full conversion of the low sulfur feed was achieved, while the formation of the byproduct ethylene was between 100 and 200 ppm. As shown in Figure 14.7, an increasing S/C ratio suppresses ethylene formation. The catalyst showed stable performance for 40 h duration. Karatzas et al. [44] determined experimentally as shown in Figure 14.8 that the efficiency of their ATR increased with increasing fuel inlet temperature and O/C ratio. 

Catalyzed ceramic foam as a catalytic diesel particulate filter is an application different from other tems discussed in this paper. Here high temperattoe resistance and low bulk density are important, but tortuous flow paths together with open porosity are key features. As with conventional monolithic catdysts, cordierite foams have been used because of the very low thermal coefficient of e q)ansion [40-45]... 

BET data for some prepared modified cordierites presented in Table 1 indicate that all samples have low specific surface area (0.02-4.9 m /g). It was found that increase in calcination time and temperature as well as in MnOx contents results in lower specific surface area and internal pore volume that usually leads to better thermal shock resistance and high durability of monoliths. The prepared cordierite-like materials have low open porosity and the mean pore diameter does not exceed 1 pm. 

For tigheat transfer mechanism in the channel, enhancing the upstream propagation of the reaction front. A numerical experiment was conducted to quantify the effect of radiation on the apparent thermal conductivity of the solid wall, by recomputing Case 15 and assuming an artificial material (cordierite ) for the microreactor wall, which had essentially the same properties with cordierite in Table 8.1 except for a higher thermal conductivity the value of was increased at 0.5 W/mK steps, until fig and fit assumed values close to the ones of Case 16. For = 5.5 W/mK, ignition and steady-state times for a microreactor without surface radiation, became essentially identical to the characteristic times of a microreactor with — 2.0 W/mK and a surface emissivity of 8 = 0.6 (see Case 17 in Table 8.2). This apparent increase in the thermal conductivity of the solid, however, should not be confused with numerical models employing effective wall thermal conductivity for catalytic monoliths [23]. 

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