Monolithic honeycombs

Automobile exhaust catalysts have been developed that maximize the catalyst surface area available to the flowing exhaust gas without incurring excessive pressure drop. Two types have been extensively studied the monolithic honeycomb type and the pellet type. 

Use of the peUetted converter, developed and used by General Motors starting in 1975, has declined since 1980. The advantage of the peUetted converter, which consists of a packed bed of small spherical beads about 3 mm in diameter, is that the pellets were less cosdy to manufacture than the monolithic honeycomb. Disadvantages were the peUetted converter had 2 to 3 times more weight and volume, took longer to heat up, and was more susceptible to attrition and loss of catalyst in use. The monolithic honeycomb can be mounted in any orientation, whereas the peUetted converter had to be downflow. AdditionaUy, the pressure drop of the monolithic honeycomb is one-half to one-quarter that of a similar function peUetted converter. 

The activated coating layer must possess two additional properties. It must adhere tenaciously to the monolithic honeycomb surface under conditions of rapid thermal changes, high flow, and moisture condensation, evaporation, or freezing. It must have an open porous stmcture to permit easy gas passage iato the coating layer and back iato the main exhaust stream. It must maintain this porous stmcture even after exposure to temperatures exceeding 900°C. 

Catalytic Support Body Monolithic Honeycomb Unit. The terms substrate and brick are also used to describe the high geometric surface area material upon which the active coating material is placed. Monolithic honeycomb catalytic support material comes in both ceramic and metallic form. Both are used in automobile catalysts and each possesses unique properties. A common property is a high geometric surface area which is inert and does not react with the catalytic layer. 

ActivatedL yer Loss. Loss of the catalytic layer is the third method of deactivation. Attrition, erosion, or loss of adhesion and exfoHation of the active catalytic layer aU. result in loss of catalyst performance. The monolithic honeycomb catalyst is designed to be resistant to aU. of these mechanisms. There is some erosion of the inlet edge of the cells at the entrance to the monolithic honeycomb, but this loss is minor. The peUetted catalyst is more susceptible to attrition losses because the pellets in the catalytic bed mb against each other. Improvements in the design of the peUetted converter, the surface hardness of the peUets, and the depth of the active layer of the peUets also minimise loss of catalyst performance from attrition in that converter. 

Some OCs were of the monolithic honeycomb type, but all General Motors cars used peUetted OCs. For a period in the late 1970s and throughout the 1980s, both TWC and OC were used in a dual-bed catalyst. Oxygen needed for OC performance was provided by an engine driven air pump or a reed... 

Most catalysts consist of active components dispersed as small crystallites on a thermally stable, chemically inactive support such as alumina, ceramics, or metallic wires and screens. The supports are shaped into spheroids, cylinders, monolithic honeycombs, and metallic mesh or saddles. 

Weiland et al observed that a small amount of Pt metal present in the Rh-based catalyst could significantly improve the catalyst activity for ATR of gasoline range fuels. They claimed that the role of Pt is to enhance oxidation activity, whereas Rh provides high SR activity. The Rh-Pt/alumina catalyst used in the study was supported on monolithic honeycombs and had a Rh to Pt ratio of 3-10 by weight. The geometry (metal monolith, ceramic monolith, or ceramic foam) of the support did not affect the product composition. ... 

Kinetic runs over small monolithic honeycomb catalyst samples were performed in two different rigs at Politecnico di Milano and in the Daimler... 

The catalysts in use contain Pt and Rh as major constituents. A converter typically contains 1-2 g Pt and 0.2-0.3 g Rh. The design of the reactor has received a lot of attention. Beads were also applied initially, but nowadays monolithic honeycombs are used almost exclusively. 

Prior to spin coating, both ends of a pre-fabricated porous support, either in the form of a tube or a monolithic honeycomb element, are filled with the slip and temporarily sealed with gaskets. Slips of Ni, A1 and AI2O3 have been spin coated at a rotational speed of up to about 1300 rpm to form membrane layers [Sumitomo Electric Ind., 1981 NGK Insulators, 1986]. The development of a coating layer is expected to be faster with the in coating process than the dip coating process due to the centrifugal force. 

Figure 5.1 Photograph of alumina membrane elements in tubular and monolithic honeycomb shapes [Courtesy of U.S. Filters]...

The porosity at both ends of a tubular or monolithic honeycomb membrane element can be a potential source of leakage. These extremities need to be made impervious to both permeates and retentates so that the two streams do not remix. Typically the end surfaces and the outer surfaces near the ends of a commercial membrane element are coated with some impervious enamel or ceramic materials. 

Current commercial inorganic membranes come in a limited number of shapes disk, tube and monolithic honeycomb. Compared to other shapes such as spiral-wound and hollow-fiber that are available to commercial organic membranes, these types of membrane elements have lower packing densities and, therefore, lower throughput per unit volume of membrane element or system. 

Table 2 Results of Computer Search for Publications on Monolithic/Honeycomb Structures...

J.S. Howiit and T.C. Sekella, Flow effects in monolithic honeycomb automotive catalytic converters, SAE Paper 740244 (1974). 

The monolith honeycomb structure is widely used as a catalyst support for gas treatment applications such as the cleaning of automotive exhaust gases and industrial off-gases [1,2]. In these applications, in which large volumetric flows must be handled, monoliths offer certain advantages, such as low pressure drop and high mechanical strength. 

Monolithic honeycomb have found major application in the selective catalytic reduction process (Fig. 16). The pressure op is about 250-1000 Pa. The SCR reactor asks for a flat velocity profile of the flue gas. Therefore, a layer of dummy honeycombs is installed for flow-straightening purposes. 

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