Monolithic honeycomb design

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. 

The concept of monolithic module design is associated with Argonne National Laboratories [111, 112]. Power density of about 8 kW/kg or 4kW/1 and fuel efficiency over 50% are expected to be achieved with monolithic SOFCs. The monolithic structure started with a co-flow version where the cell consists of a honeycomb-like array of adjacent fuel and oxidant channels that look like corrugated paperboard, as shown in Fig. 42. Multilayer laminates of the active cell components (anode-elec-ttolyte-cathode) are appropriately corrugated and stacked alternatively between flat multilayer laminates in the following sequence anode- interconnection material-cathode. Tape casting [111] and hot-roll calendering [113, 114] are used to fabricate the monolithic structure. A cross-flow version where oxidant and fuel channels are... 

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. 

Conversion efficiency is definitely affected by the large void fraction, which is apparent in the results from changes in the total throughput, or space velocity (0.56 versus 1.11 sec ), shown in Fig. 7. In this comparison, the concentration of unconverted hexane increased tenfold when the flow rate was doubled. The impact of improvements in conductive heat transfer, combined with the mass transfer limitations associated with the cell size and honeycomb design, and a catalyst loading that was nearly one-half Chat of commercial pellet catalysts (average, 11.5% versus 19.2%) suggested that both carbon formation and steam/hydrocarbon reactions were better controlled with monolithic supports under the conditions employed. This comparison was made where the extent of the endothermic reaction is equal between the pellet bed and the hybrid cordierite/metal monolith bed. 

The monolithic support for this reactor was of a honeycomb design, which required that the flow velocity be high enough that the exothermic reaction rate between the fuel and air that was catalyzed with a noble metal would be mediated by axial heat transfer through the bed and by the steam diluent. Under these conditions, heal was radiated... 

Effect of Monolith Geometry. Technical constraints are often imposed on the design of the monolith honeycomb geometry by the extrusion process, as well as by the mechanical properties of the extrudate the specific SCR application (eg, high dust vs low dust) is also crucial for the definition of the catalyst geometric features. Here, attention is paid to the influence that the monolith parameters (channel shape, wall thickness, and channel size) exert on both the DeNO reaction and the SO2 oxidation to advance guidelines for optimization of the catalyst geometry. 

A comprehensive description of PMG honeycomb monolithic catalysts of the type shown in figure 13-6 is given by Heck et al. (1988). They point out that the honeycomb design provides the following advantages compared to packed beds of granules ... 

The kinetic parameters estimated by the experimental data obtained frmn the honeycomb reactor along with the packed bed flow reactor as listed in Table 1 reveal that all the kinetic parameters estimated from both reactors are similar to each other. This indicates that the honeycomb reactor model developed in the present study can directly employ intrinsic kinetic parameters estimated from the kinetic study over the packed-bed flow reactor. It will significantly reduce the efibrt for predicting the performance of monolith and estimating the parameters for the design of the commercial SCR reactor along with the reaction kinetics. 

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

A billion cars and coimting, himdreds of millions of them with catalytic converters—this application is a landmark success of catalytic science and technology. Automobile catalytic converters are mostly monoliths— like ceramic honeycombs with porous catalyst layers on their inner wall surfaces. These monoliths are the most widely used structured reactors, the topic addressed by Moulijn, Kreutzer, Nijhuis, and Kapteijn. In contrast to the classical reactors containing discrete particles of catalyst and characterized by random and chaotic behavior, structured reactors are characterized by regular structures and predictable laminar flow. Structured reactors can be designed in full detail up to the local surroimdings of the... 

III. DESIGN OF HONEYCOMB MONOLITH COMBUSTION CATALYSTS A. Pros and Cons of Catalysis at High Temperatures... 

These results became the basis for investigating another modification to improve the performance of a monolith catalyst bed for steam reforming, i.e, use of a metal monolith support that, although made up of parallel cells, was not honeycomb in design and had a multipath flow [10] (see Chapter 14) This modification would be expected to... 

They are basically of the cellular design of ceramic monoliths but the honeycomb is formed normally by spirally wound alternate sheets of flat and corrugated metal. 

In the field of technical ceramics, these requirements are usually satisfied. Extrusion is practiced with well-prepared, homogeneous compounds conducive to constant, uniform production. On the other hand, extrusion involves very high pressures, often in excess of 200 bar, and, frequently, body that is difficult to extrude. This calls for extrusion tools - inch everything from the extruder itself to the die exit - of hydraulically optimized design. Moreover, the extrudates and honeycomb elements for technical applications must display high levels of exactitude. For example, the engineering of a honeycomb die for thin-walled honeycombs, i.e., ceramic monoliths, with web thicknesses of 0.15 mm and less demands a degree of precision that can only be achieved with state-of-the-art machine tools. 

The dominant catalyst support for the auto exhaust catalyst is a monolith or honeycomb structure. (For some early history on the use of bead catalyst, see Reference (6).) The monolith can be thought of as a series of parallel tubes, with a cell density ranging from 300 to 1200 cpsi (cells per square inch). Advances in monolith technology, catalyst-mounting methods, flexibility in reactor design, low pressure drop, and high heat transfer and mass transfer rates are the main reasons the monolithic support dominates the entire market as the preferred catalyst support. 

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