Frontal area, monoliths

The consideration of the pressure drop over the monoliths containing a variety of CPSI (cells per in ) for the modeling of honeycomb reactor may be required, since Ap of the reactor strongly depends on CPSI of monolith. Eqn. (7) for the pressure drop of the honeycomb was employed to develop the reactor model describing the performance of the honeycomb fabricated in the present work [8]. and Ke indicate contraction and expansion loss coefficient at the honeycomb inlet and outlet, respectively and o is the ratio of free flow area to frontal area. 

The SCR catalysts are used in the form of honeycomb monoliths or plates to guarantee low pressure drops in view oflarge frontal area with parallel channels, high external surface area per unit volume of catalyst, high attrition resistance and low tendency for fly ash plugging. The SCR monoliths and plates are assembled into standard modules and inserted into the reactor to form catalyst layers. 

Figure 64. Influence of the design of the converter inlet cones on the distribution of the exhaust gas flow velocity over the radius of a monolith at the outlet frontal area (above) converter with long inlet cones (below) converter with short inlet cones.

Figure 65. Effect of the gas to washcoat heat transfer rate on the distribution of solids temperature over the radius of a ceramic monolith at its outlet frontal area, for a washcoat with a high heat transfer rate (C2) and for a washcoat with a low heat transfer rate (Cl). Reprinted with permission from ref [34], 1991 Society of Automotive Engineers, Inc.

Although the early honeycombs were ceramic, recently metallic monolith structures have been used in certain niche markets for exhaust control because they can be made with thinner walls and have open frontal areas close to 90%,... 

Due to its monolithic structure, the catalyst can be represented by a single monolith channel (Fig. 22.1) if a uniform gas flow distribution over the whole catalyst frontal area is assumed [15-18]. This is nearly the case for the major part of operation, justifying a simplihed ID consideration instead of a 2D model. 

Monolith type (ceU density/ waU thickness) Open frontal area (OFA) Hydraulic diameter (mm) Geometric specific area (m /m ) Bulk density (kg/m )... 

Ratio of free flow area in monolith to frontal area... 

As a final illustration of reactors in parallel, flip ahead to Chapter 9 and have a look at Figure 9-5. This figure shows a form of catalyst known as a honeycomb or monolith. These catalysts consist of several hundred parallel chaimels per square inch of frontal area. Each channel is an independentreactor (although not necessarily an ideal PFR) since there is no flow of fluid between channels. One of the challenges of using this form of catalyst is to ensure that the value of W/Fxo is the same, or as close as possible, from channel to chaimel. 

A variety of monoliths are offered to the market which differ in the size of the channels and in the thickness of the walls. Together, these parameters fix what is called the cell density, the number of cells per unit frontal surface area. With ceramic monoliths, these two parameters can be varied to some extent independently. As of today, the most commonly used supports have about 62 channels per square centimeter, which corresponds to a channel width of about 1 mm and a wall thickness of about 0.15 mm. The bulk density of such a support is about 420kgm . The supports currently used have, within the production variance, homogeneous channel... 

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