Wall-coated monolith channels

For the design of a single-channel wall-coated monoliths (and other similar structured reactors), the control of the residence... 

Scientists from Politecnico di Milano and Ineos Vinyls UK developed a tubular fixed-bed reactor comprising a metallic monolith [30]. The walls were coated with catalytically active material and the monolith pieces were loaded lengthwise. Corning, the world leader in ceramic structured supports, developed metallic supports with straight channels, zig-zag channels, and wall-flow channels. They were produced by extrusion of metal powders, for example, copper, fin, zinc, aluminum, iron, silver, nickel, and mixtures and alloys [31]. An alternative method is extrusion of softened bulk metal feed, for example, aluminum, copper, and their alloys. The metal surface can be covered with carbon, carbides, and alumina, using a CVD technique [32]. For metal monoliths, it is to be expected that the main resistance lies at the interface between reactor wall and monolith. Corning... 

We can extend the hyperbolic model to cases in which the solute diffuses in more than one phase. A common case is that of a monolith channel in which the flow is laminar and the walls are coated with a washcoat layer into which the solute can diffuse (Fig. 4). The complete model for a non-reacting solute here is described by the convection-diffusion equation for the fluid phase coupled with the unsteady-state diffusion equation in the solid phase with continuity of concentration and flux at the fluid-solid interface. Transverse averaging of such a model gives the following hyperbolic model for the cup-mixing concentration in the fluid phase ... 

Gas and wall temperature profiles and methane conversions obtained by integrating Eqs. (1), (2), and (5) after substituting Eq. (7) in the heat balance Eq. (5) are shown in Fig. 6. As for fully coated monoliths, the washcoat temperature is the highest at the inlet and decreases slightly thereafter, asymptotically merging with the gas temperature. Washcoat temperatures can be calculated from Eq. (8), which is derived by substituting Eqs. (4) and (7) in Eq. (5) and letting kr oo. The half-factor reflects the fact that the gas temperatures in the catalytic and noncatalytic channels are the same. 

In addition to the preparation of packed beds and monoliths, wall coating is an alternative method forthe introductionofcatalysts into continuous flow systems, due to the short diffusion distances obtained within micro reaction channels. An early example of this was demonstrated by Yeung and co-workers [59]. who employed a stainless-steel micro reactor [channel dimensions = 300 pm (width) x 600 pm (depth) x 2.5 cm (length)] coated with an NaA zeolite membrane, followed by a layer of... 

A second catalyst support type is commonly called a monolith which is a thin walled multi channeled honeycomb. The ceramic walls between the channels are the base support surfaces for the catalyst. (FIGURE 3) Although they are porous, they are not the direct surface for the precious metal. An intermediate alumina coating called "washcoat provides an ultra high surface for the catalyst sights (FIGURE 4 is an illustration of the washcoat precious metal relationship). The catalyzed monolith is likewise assembled into the metal container using a compressible interface material. 

If, however, the selectivity of a reaction is influenced by the presence of a thick support layer on the channel walls, the use of a different type of monolith support than the classical carbon-coated monolith is recommended. This was shown in the ruthenium-catalyzed reaction. EPMA analyses showed that ruthenium species are present both in the pores of the outer carbon washcoat layer and on the surface of the carbon inclusions located inside the monolith walls. No ruthenium was found in the empty macropores of the cordierite, since (1) there is no strong interaction between the precursor and the cordierite, and (2) after impregnation and washing, the macropores are emptied first upon drying, due to the capillary forces. The presence of active phase in the walls of the monolithic substrate is undesired, since it makes the diffusion path of the reactants to the active ruthenium sites longer. To prevent deposition of ruthenium in the waU,... 

For successful application of carbon-coated monolithic catalysts, the deposition of active phase in the walls of a monolithic substrate should be prevented. To prevent deposition of ruthenium in the wall (1) substrates with nonporous walls can be used, or (2) the cordierite monolithic substrates can be modified with a-AEOs, blocking the macroporosity of the cordierite and rounding the channel cross section to enable a more uniform thickness of the carbon coating layer. Alternatively, ACM monoliths or integral carbon monoliths with very thin walls having a characteristic diffusion length similar to the activated carbon slurry catalysts can be employed. 

There are three possibilities for the implementation of SPE in microfluidic devices, namely, (1) to fill a channel with particles that serve as the extraction material, (2) coat the channel wall with the extraction material, and (3) fill a channel with a polymeric rod (monolithic phase) extraction material. 

Two types of catalyst systems are commonly used (1) packed beds and (2) monolith blocks. The first type is a perforated container containing spherical beads coated with catalytic ingredients it allows the reactants to flow through the catalytic beds. It has high external surface area and high mass transfer efficiency between reactants and catalyst. The second type has the walls of monolithic honeycomb wash coaled with the catalyst ingredients and allows the reactants to flow through the channels in the monolith. 

Both reactor types R3 and R4 use the segmented flow (Taylor) principle. They are divided into two categories R3 has very small channels (<1 mm) and R4 are monolith reactors (honeycomb), well developed on the laboratory scale with at least one example of industrial application. Category R3 includes single-channel and multi-ple-channel reactors [10], etched in silicon [10] or glass [10,11], with wall-coated or immobilized catalysts in the case of gas-liquid-solid additions [12], and capillary microreactors for gas-liquid-liquid systems [13]. 

Recently, the hydrogenation a mixture of toluene, styrene and 1-octene, representing a model feed for hydrotreating in the refining industry, was performed in monolith reactors [37]. One is a y-alumina monolith of diameter 1 cm and 15 or 30 cm long and the other is a more conventional cordierite monolith with a wall-coated layer of y-alumina. In both monoliths, the channels size is 1-2 mm and the catalyst is based on Ni. Substantial alkene conversions of more than 50% were observed in the small-channel reactors, which was attributed to the intensified mass-transfer rate generally measured in monolith reactors [16]. 

For the negligible heat transport limitations in the case of the wall-coated microreactor the equations have to be different due to the geometric changes for derivation of the criteria. Looking at Fig. 1 it becomes dear that two cases have to be distinguished with regard to the microreactor stacking scheme A) the strictly cooled case and B) the monolithic system, where most of the channels with reaction are located next to each other. 

Monoliths are continuous structures consisting of narrow parallel channels, typically with a diameter of 1-3 mm. A ceramic or metallic support is coated with a layer of material in which catalytically active components are dispersed (washcoat). The walls of the channels may be either permeable or impermeable. In the former case, the term membrane reactor (see above) is used. Figure 4.10.78 shows an example of a monolith. The shape of the monolith can be adapted to fit in the reaction chamber. 

Early converters were designed to oxidize CO, H2, and unburnt hydrocarbons (oxidizing converters). More recently, "three-way converters have been developed to oxidize hydrocarbons, H2, and CO, and reduce NO emissions simultaneously. This goal is achieved by using a suitably promoted Platinum-Rhodium catalyst supported on alumina and by carefully controling the air/fuel ratio. The catalyst is a thin porous alumina layer "wash-coated" on the wall of the monolith channels. Typically the wash-coat is about a few tens of a micrometer thick, except at the corners of the channel where it is thicker. 

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. 

With a monolith reactor, diffusion from the channels to the catalyst coated on the channel walls is the sole means by which reactants are able to reach the catalyst (Section III). It seems reasonable that a similar diffusion process occurs in a coated filter. 

Figure 7.15 Monolithic ceramic microfilter. The feed solution passes down the bores of the channels formed in a porous ceramic block. The channel walls are coated with a finely porous ceramic layer...

Since typical monolith catalysts have a thin coating of catalytic ingredients on the channel walls, they can be susceptible to poisoning. 

Channels y are perpendicular to channels x and the two alternate along the z axis of the monolith. The walls of channels y are coated with a suitable metal or conductive metal oxide catalyst which catalyzes the fuel s anodic oxidation. 

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