Monolith boundary

At high flowrate, jet flow prevails in the centre of the monolith. The fluid velocity can be more than twice the mean velocity. Conversely, fluid velocity at the monolith boundary can be much smaller. This velocity distribution depends on the mean flowrate and on the presence of flow deflectors in the divergent section. 

As expected, a slightly earlier light-off occurs when there are channels with high gas velocity. Conversely, steady state conversions decrease when the velocity is not uniform. For non uniform velocity distributions, at steady state the maximum temperature difference between adjacent channels is about 25 K. Given that the model ignores heat conduction in the solid, the actual temperature difference is probably much smaller except at monolith boundary. 

The ceramic oxide carrier is bonded to the monolith by both chemical and physical means. The bonding differs for a ceramic monolith and a metallic monolith. Attrition is a physical loss of the carrier from the monolith from the surface shear effects caused by the exhaust gas, a sudden start-up or shutdown causing a thermal shock as a result of different coefficients of thermal expansion at the boundary between the carrier and the monolith, physical vibration of the cataly2ed honeycomb, or abrasion from particulates in the exhaust air (21) (see Fig. 6d). 

There is a general trend toward structured packings and monoliths, particularly in demanding applications such as automotive catalytic converters. In principle, the steady-state performance of such reactors can be modeled using Equations (9.1) and (9.3). However, the parameter estimates in Figures 9.1 and 9.2 and Equations (9.6)-(9.7) were developed for random packings, and even the boundary condition of Equation (9.4) may be inappropriate for monoliths or structured packings. Also, at least for automotive catalytic converters. 

Open-channel monoliths are better defined. The Sherwood (and Nusselt) number varies mainly in the axial direction due to the formation ofa hydrodynamic boundary layer and a concentration (temperature) boundary layer. Owing to the chemical reactions and heat formation on the surface, the local Sherwood (and Nusselt) numbers depend on the local reaction rate and the reaction rate upstream. A complicating factor is that the traditional Sherwood numbers are usually defined for constant concentration or constant flux on the surface, while, in reahty, the catalytic reaction on the surface exhibits different behavior. 

This is explained by a possible higher activity of pure rhodium than supported metal catalysts. However, two other reasons are also taken into account to explain the superior performance of the micro reactor boundary-layer mass transfer limitations, which exist for the laboratory-scale monoliths with larger internal dimensions, are less significant for the micro reactor with order-of-magnitude smaller dimensions, and the use of the thermally highly conductive rhodium as construction material facilitates heat transfer from the oxidation to the reforming zone. 

The strong intra-phase diffusion limitations are accounted for by the following equations for diffusion-reaction of the reactants in the catalytic monolith wall (Equation 13.21) with the appropriate boundary conditions (Equation 13.22) ... 

Fig. 1.4 Illustration of the chemically reacting boundary-layer flow in a single channel of a catalytic-combustion monolith.

There are many chemically reacting flow situations in which a reactive stream flows interior to a channel or duct. Two such examples are illustrated in Figs. 1.4 and 1.6, which consider flow in a catalytic-combustion monolith [28,156,168,259,322] and in the channels of a solid-oxide fuel cell. Other examples include the catalytic converters in automobiles. Certainly there are many industrial chemical processes that involve reactive flow tubular reactors. Innovative new short-contact-time processes use flow in catalytic monoliths to convert raw hydrocarbons to higher-value chemical feedstocks [37,99,100,173,184,436, 447]. Certain types of chemical-vapor-deposition reactors use a channel to direct flow over a wafer where a thin film is grown or deposited [219]. Flow reactors used in the laboratory to study gas-phase chemical kinetics usually strive to achieve plug-flow conditions and to minimize wall-chemistry effects. Nevertheless, boundary-layer simulations can be used to verify the flow condition or to account for non-ideal behavior [147]. 

Catalytic combustion in a monolith channel provides an illustration of boundary-layer flow in a channel [322], Figure 17.18 shows a typical monolith structure and the particular single-channel geometry used in this example. Since every channel within the monolith structure behaves essentially alike, only one channel needs to be analyzed. Also a cylindrical channel is used to approximate the actual shape of the channels. 

L.L. Raja, R J. Kee, O. Deutschmann, J. Wamatz, and LD. Schmidt. A Critical Evaluation of Navier-Stokes, Boundary-Layer, and Plug-Flow Models of the Flow and Chemistry in a Catalytic-Combustion Monolith. Catalysis Today, 59 47-60,2000. 

Schematics illustrating (a) laminated composite, consisting of the strong layer and the weak layer, and (b) fibrous monolithic ceramic, consisting of the strong cell and the weak cell boundary. 

Fibrous monolithic ceramics consist of dense cells separated by a continuous cell boundary, in which the cells provide most of the strength of the FM and... 

In the last 10 years, significant advances in fibrous monolithic ceramics have been achieved. A variety of materials in the form of either oxide or nonoxide ceramic for cell and cell boundary have been investigated [1], As a result of these efforts, FMs are now commercially available from the ACR company [28], These FMs are fabricated by a coextrusion process. In addition, the green fiber composite can then be wound, woven, or braided into the shape of the desired component. The applications of these FMs involve solid hot gas containment tubes, rocket nozzles, body armor plates, and so forth. Such commercialization of FMs itself proves that these ceramic composites are the most promising structural components at elevated temperatures. 

---