Structured monolithic honeycombs

The activated coating layer must possess two additional properties. It must adhere tenaciously to the monolithic honeycomb surface under conditions of rapid thermal changes, high flow, and moisture condensation, evaporation, or freezing. It must have an open porous structure to permit easy gas passage into the coating layer and back into the main exhaust stream. It must maintain this porous structure even after exposure to temperatures exceeding 900°C. 

Table 2 Results of Computer Search for Publications on Monolithic/Honeycomb Structures...

The monolith honeycomb structure is widely used as a catalyst support for gas treatment applications such as the cleaning of automotive exhaust gases and industrial off-gases [1,2]. In these applications, in which large volumetric flows must be handled, monoliths offer certain advantages, such as low pressure drop and high mechanical strength. 

M.F.M. Zwinkels, S.G. Jaras, and P.G. Menon, "Catalytic Fuel Combustion in Monolithic Honeycomb Reactors", Chapter in A. Cybulski and J.A. Moulijn (Eds), Structured Calaly.st.s and Reactors, Marcel Dekker, New York, 1997. 

Hayes, RE, Rojas, A, Mmbaga, J. The effective thermal conductivity of monolith honeycomb structures. Catal. Today 2009 147S S113-S119. 

A desiccant wheel assembly consists of i) a core that contains the desiccant materials, ii) a core/wheel support structure, iii) a drive system to rotate the wheel at a very low speed, and iv) a set of air seals to separate the process air stream from the regeneration air stream. The core has a monolith honeycomb type structure. Both sinusoidal and hexagonal channels are used in commercial equipment. Various types of materials have been used for support matrix that includes ceramic, glass fibers, and corrugated aluminum sheets. The desiccant materials may be washcoated, impregnated, or formed in situ on the support matrix. 

The structural elements of commercial inorganic membranes exist in three major geometries disk, tube or tube bundle, and multichannel or honeycomb monolith. The disks are primarily used in laboratories where small-scale separation or purification needs arise and the membrane filtration is often performed in the flow-through mode. The majority of industrial applications require large filtration areas (20 to over 200m ) and, therefore, the tube/tube bundle and the multichannel monolithic forms, particularly the latter, predominate. They are almost exclusively operated in the cross-flow mode. 

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... 

A variety of photocatalyst supports has been examined experimentally. Dip-coated glass slides or plates have been used in many experimental systems as a simple lab-scale supported photocatalyst system. Coated glass offers many of tlte important features of a supported photocatalyst while still offering relatively simple preparation. Honeycomb monoliths, widely used as commercial catalyst supports for a variety of gas-phase applications, have also been examined as photocatalyst supports (Fig. 3). Although these monoliths offer good stability and excellent throughput, providing illumination for the photocatalyst inside the monolith channels can be problematic [41,42]. Randomly structured support materials, like fiber-based filters, reticulated foams, and similar materials, have been used... 

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... 

Figure 4 is a picture of another reactor with widely varying scales—a catalytic monolith. Like the tubular reactor, the monolith itself has two intrinsic length scales, the radius and the length, which are typically 10-20 cm and between 30 and 50 cm, respectively. The monolith cross-section has a honeycomb structure... 

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... 

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. 

Many commercial ceramic membranes nowadays come in the form of a monolith consisting of multiple, straight channels parallel to the axis of the cylindrical structure (Figure 3.6). The surfaces of the open channels are deposited with permselective membranes and possibly one or more intermediate support layers. The porous suppon of these multi-channel structures are produced by extrusion of ceramic pastes described above with a channel diameter of a few millimeters. Their lengths are somewhat limited by the size of the furnaces used to dry, calcine and sinter them and also by such practical considerations as the total compact weights to be supported during heat ueatment and the risk of distortion in the middle section. It should be noted that this type of honeycomb... 

The porous mass of the monolithic structure has a low pressure drop. If further reduction of the pressure drop in the bulk support is desired, there arc a few possibilities [Atomic Energy Commission (France), 1971]. Additional grooves can be machined or swaged with dowels which will produce channels, or a network of additional channels perpendicular to the honeycomb feed channels is provided by using materials such as carbon that can be burned off. 

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 general advantages reported for monoliths in SCR applications still hold for reverse-flow operation Furthermore, it has been claimed that the use of honeycomb structures in reverse-NOjj processes allows for autothennal treatment of gas stream with even lower concentrations with respect to packed beds. On the basis of a general two-phase model, it has been evaluated that an adiabatic temperature rise of less than 10°C (NOx < 500 ppm) permits a stable penodic regime with monoliths, whereas at least 15 C is required with packed beds Reverse-flow SCR on honeycomb structures has been... 

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