Monoliths parallel channel

Another important constraint comes from the pressure drop across the catalytic bed, which must be kept to a minimum to avoid a loss in engine power and performance. This requirement is satisfied by a very shallow pellet bed of no more than ten pellets deep, a monolithic structure with many open parallel channels, or a pile of metallic screens and saddles. 

A wealth of structures exists and can be found in the literature [1-3]. Figure 9.1 shows examples of monoliths and arrayed catalysts. MonoHths (Figure 9.1a) consist of parallel channels, whereas arrayed catalysts are built from structural elements that are similar to monolithic structures but containing twisted (zig-zag or skewed) passages and/or interconnected passages (Figure 9.1b,c) or arrays of packets of conventional catalyst particles located in the reaction zone in a structured way, whereby the position of particles inside the packets is random (Figure 9.1d). The latter are mainly used for catalytic distillation and are not discussed further in this chapter. 

Figure 9.1 Monoliths and arrayed structures, (a) Ceramic monoliths with parallel channels. (Reprinted from [4].)...

While conventional monoliths contain parallel channels, in practice, systems are often made from alternate layers that allow lighter structures with better mass transfer characteristics in gas-phase applications, see Figure 9.6 showing interconnected flow paths. They are usually made from metal, mostly Fecralloy , Kanthal , or stainless steel, and widely used in autocatalysts and in environmental... 

At the heart of an automotive catalytic converter is a catalyzed monolith which consists of a large number of parallel channels in the flow direction whose walls are coated with a thin layer of catalyzed washcoat. The monolith catalyst brick is wrapped with mat, steel shell and insulation to minimize exhaust gas bypassing and heat loss to the surroundings. 

Plate type packing to separate the phases is discussed by Carlsson et al. (1983) and by Hatziantoniu etal. (1986). De Vos et al. (1982,1986) describe use of a monolithic porous catalyst with vertical and horizontal channels. The liquid phase flows downward through an array of parallel channels in the monolith, while gas moves in cross flow through a separate set of channels. Another approach treats the catalyst to make part of the surface hydrophobic or lyophobic (Berruti et aL, 1984). The gas phase has direct access to the surface on these unwetted portions of the surface, resulting in partial, spatial segregation of the phases. 

Inclusion of photocatalysts in monolithic structures, namely solid structures with bored parallel channels, enables the pressure drop caused by the passage of the gas through the catalyst to be reduced by several orders of magnitude and improves both chemical and photon contact surfaces... 

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. 

The cordierite extruded monoliths, having 400 square cellsAn, were similar to those used in automobile catalytic converters. However, instead of using an alumina washcoat as in the catalytic converter, these catalyst supports were loaded directly with 12 to 14 wt.% Pt in the same manner as the foam monoliths. Because these extruded monoliths consist of several straight, parallel channels, the flow in these monoliths is laminar (with entrance effects) at the flow rates studied. 

Ceramic honeycomb monoliths are porous macro-structured supports consisting of parallel channels. On the walls a thin layer of active material can be applied (Figure 1). Honeycomb catalyst supports were originally developed for use in automotive... 

In many situations, the monolith reactor can be represented by a single channel. This assumption is correct for the isothermal or adiabatic reactor with uniform inlet flow distribution. If the actual conditions in the reactor are significantly different, more parallel channels with heat exchange have to be simulated (cf., e.g. Chen et al., 1988 Jahn et al., 1997, 2001 Tischer and Deutschmann, 2005 Wanker et al., 2000 Young and Finlayson, 1976). In this section we will further discuss effective single channel models. 

Fro. 37. Two successive 2-D xz images of two-phase flow through the parallel channels of a ceramic monolith rated at 400 cpsi, for a gas flow rate of 200 cm min (a) 74ms after excitation, (b) 220 ms after r.f. excitation. In-plane image resolution is 393 pm (x) x 783 pm (z). Reprinted from reference (84) with permission of Springer Science and Business Media. 

Metal monoliths can be shaped rather freely. A good example is given in Figure 4 (9), where it can be seen that in these parallel-channel systems the structure of the channels is such that the turbulence increases. The reasoning behind that is the wish to counteract the low mass transfer rates associated with laminar flow in the thin channels of the monolith. 

In the past, the principles described have been implicitly recognized in several attempts to convert monolithic catalysts into catalytic heat exchangers. While the use of millimeter dimensions and nanoporous ceramic supports meets the primary criteria already mentioned, the parallel channel structure of monoliths is not ideally tailored for heat exchanger applications, and complex header structures are required to uniformly distribute and collect reaction medium and coolant to and from the individual channels (Figure 9). The unsatisfactory interface between the milli- and macroscale has been a major weakness of such concepts. 

Figure 2 shows the shape and size of the Monolith alumina supports. These are in the form of cylindrical segments of about 2.54 cm in length and about 1.0 cm in diameter. These have longitudinal and parallel channels along their length. The size, shape and thickness of the walls of the channels are also shown in Figure 2. The Monolith structure has about 60 to 80 percent of its cross-sectional area open. Therefore, a bed of regularly stacked Monoliths would offer significantly less pressure drop than that encountered in conventional packed beds. This has been observed by Satterfield and Ozel (1) for a water-air system.

The above requirements are to some extent contradictory, which has led to the proposition of a large number of different catalyst shapes and arrangements. However, only a few of these have proved really effective in practical operation. Suitable catalyst forms and arrangements include random packings of spheres, solid cylinders, and hollow cylinders, as well as uniformly structured catalyst packings in the form of monoliths with parallel channels, parallel stacked plates, and crossed, corrugated-plate packets (Fig. 3). 

With nonadiabatic reaction control, heat must be transported through the fixed bed to the integrated heat exchange surfaces. At the usual mass flow rates of G > lkgm-2s-1, this heat transport takes place mainly by convection, i.e. the fixed bed must allow for a cross flow transverse to the main flow direction. Monolith structures with straight parallel channels are thus unsuitable for nonadiabatic reaction control. 

Monolith forms can have very high specific surfaces combined with a very low pressure loss. Monoliths with straight, parallel channels, such as used for automobile exhaust control have only very poor radial heat transport properties. Crossed corrugated structures are considerably more favorable for isothermal reaction control. They have a very high radial thermal conductivity which is almost independent of the specific surface area the latter can be varied over a wide range by means of the channel dimensions. 

Note that, despite the typically high operating temperatures of fuel cells, radiative heat transfer was neglected. Lee and Aris (16) have discussed such effects in parallel-channel monoliths. The importance of radiative transport depends on the emissivity of the surface for the low (about 0.1) emissivity of Pt-coated catalyst-electrodes, their analysis suggests that radiative effects can be neglected. 

Figure 2 shows examples of monoliths, the most popular structured reactors (2). They consist of large numbers of parallel channels. Figure 3... 

Monoliths, with their parallel channels, possibly internally finned, were shown to be advantageous for these reactions. Film flow is possible, both for cocurrent and for countercurrent operation, because of the low degree of interaction between the two phases (23). 

All the monolith composites were prepared at a 1 1 ratio between the magnesium silicate clay binder and the AC or alumina. After premixing of the dry powders by careful addition of water a dough was formed. This dough was extruded as honeycomb monolithic structures with parallel channels of square section at a cell density of 8 cells cm and a wall thickness of 0.9 mm using a Bonnot single screw extruder. 

A generalization of this concept of a monolithic multi-channel honeycomb structure is described in a patent by Hoover and Roberts [1978]. An integral support of porous ceramic material has a multiplicity of parallel passageways (or open channels). These passageways are substantially uniformly spaced. On the surface of these channels are coated with a permselective membrane layer. The feed stream flows inside the channels. The membrane, being the first layer in direct contact with the process stream, is selective to one or more species in the stream. In the normal cases of properly weued membrane pores, the permeate under a driving force will uansport through the membrane, any... 

The main feature of monolithic catalysts is a high ratio of geometric surface area to volume and a low pressure drop with distnbution of gas flow through a large number of parallel channels. The most significant application for structured catalyst units of this type is in the control of exhaust emissions from cars this area is discussed here under five subheadings. Nonautomotive applications are discussed later, in Section IV.B. 

The distribution of gas over all parallel channels in the monolith is not necessarily uniform [2,5,6]. It may be caused by a nonuniform inlet velocity over the cross-sectional area of the monolith, due to bows in the inlet tube or due to gradual or sudden changes of the tube diameter. Such effects become important, because the pressure loss over the monolith itself is small. Also, a nonuniformity of channel diameters could be a cause at the operative low Reynolds numbers, as was reported for packed beds [7]. A number of devices were proposed to ensure a uniform inlet velocity [5,8], which indeed increases the total pressure drop. 

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