Honeycomb elements

Prior to spin coating, both ends of a pre-fabricated porous support, either in the form of a tube or a monolithic honeycomb element, are filled with the slip and temporarily sealed with gaskets. Slips of Ni, A1 and AI2O3 have been spin coated at a rotational speed of up to about 1300 rpm to form membrane layers [Sumitomo Electric Ind., 1981 NGK Insulators, 1986]. The development of a coating layer is expected to be faster with the in coating process than the dip coating process due to the centrifugal force. 

From this catalyst performance test the lifetime of SCR catalysts may be predicted, thus providing a replacement strategy for SCR elements. Honeycomb elements or plate-type elements are placed in the oven of a bench-scale setup. With this method the integral activity of an element is obtained. 

The bench-scale setup is shown in Fig. 19. Flue gas is generated by a natural gas boiler, in which ammonia is burnt for NO.v production. Flue gas amounts of 100 to 200m h are passed to complete honeycomb elements. Optionally, NH3, SO2, CO, HCl, HF, and fly ash can be added to the flue gas, simulating real flue gas. 

Figure 20 shows the results of an activity test of a honeycomb element and the analysis of unreacted ammonia. 

In a full-scale SCR unit two or three layers of honeycomb elements are used. In these layers several elements are placed for ex situ testing at regular time intervals [129]. Testing these samples in bench-scale equipment produces overall rate constants which depend on the location in the SCR reactor. Moreover, information is obtained when catalyst layers have to be exchanged. 

Meijer and Janssen [130, 131] studied SCR monolithic catalysts by means of depth profiling (SEM/EDS). Depth profiles of W, Ti, Si, and S were obtained near the entrance and at the exit of a honeycomb element after 29 000 h of operation in a coal-fired power plant. The profiles, shown in Fig. 24, shows the behavior of the... 

In the field of technical ceramics, these requirements are usually satisfied. Extrusion is practiced with well-prepared, homogeneous compounds conducive to constant, uniform production. On the other hand, extrusion involves very high pressures, often in excess of 200 bar, and, frequently, body that is difficult to extrude. This calls for extrusion tools - inch everything from the extruder itself to the die exit - of hydraulically optimized design. Moreover, the extrudates and honeycomb elements for technical applications must display high levels of exactitude. For example, the engineering of a honeycomb die for thin-walled honeycombs, i.e., ceramic monoliths, with web thicknesses of 0.15 mm and less demands a degree of precision that can only be achieved with state-of-the-art machine tools. 

The increasing use of sihconized coatings for weather durabiUty caused severe masking problems for the all-metal, filter mesh-like catalyst elements available in the 1970s. Interest in catalytic afterburners increased when dispersed-phase precious metal—alumin a-on-ceramic honeycomb catalysts offered economically attractive results. 

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. 

The catalyst, used in the form of a ceramic honeycomb monolith, is constituted, as in mobile applications, by a noble metal and an absorber element, such as potassium, deposited on a Y-AI2O3 wash-coat layer. In the oxidation and absorption cycle, the SCON Ox catalyst works by simultaneously oxidizing CO and UHCs to CO2 and H2O, while NOj, are captured on the adsorber compound. Catalyst regeneration is accomplished by passing a controlled mixture of regeneration gases across the surface of the catalyst in the absence of oxygen. 

The chemical reactions by which these pollutants are converted to harmless emissions take place as exhaust gases pass through two chambers. Each chamber contains a catalyst that has been deposited on large numbers of very small ceramic beads or on the surfaces of a honeycomb-shaped hlter. In the hrst chamber, unburned hydrocarbons and water from exhaust gases react to form elemental hydrogen (H2). The most common catalyst in this chamber of the converter is hnely divided rhodium metal. 

Biology offers innumerable examples where the increase of complexity is attended by the onset of sophisticated emergent properties. In the previous chapter the beehive-like structure of silicon fluids was mentioned if we now consider a real honeycomb, each bee appears to behave as an independent element, acting apparently on its own account, but the whole population of bees gives a highly sophisticated collective emergent structure. The same can be said for an anthill or... 

There was no microparticle evidence of increased oil-combustion emissions in the high-[SO -] air mass. Oil fly ash is generally platy or honeycombed carbon which is enriched in V and Ni (19.22,30). Two particles yielded very small V x-ray peaks but these peaks were not associated with a specific particle type or elemental combination. 

Trabecular (cancellous) bone lies between cortical bone surfaces and consists of a network of honeycombed interstices containing haematopoietic elements and bony trabeculae. The trabeculae are predominantly oriented perpendicular to external forces to provide structural support. Trabecular bone continually undergoes remodelling on the internal endosteal surfaces. 

Figure 5.1 Photograph of alumina membrane elements in tubular and monolithic honeycomb shapes [Courtesy of U.S. Filters]...

The porosity at both ends of a tubular or monolithic honeycomb membrane element can be a potential source of leakage. These extremities need to be made impervious to both permeates and retentates so that the two streams do not remix. Typically the end surfaces and the outer surfaces near the ends of a commercial membrane element are coated with some impervious enamel or ceramic materials. 

Figure 11.2 Schematic diagram of a honeycomb ceramic membrane element with rectangular channels [Dolccek, 1995]...

Membranes that arc catalytically active or impregnated with catalyst do not suffer from any potential catalyst loss or attrition as much as other membrane reactor configurations. This and the above advantage have the implication that the former requires a lower catalyst concentration per unit volume than the latter. It should be mentioned that the catalyst concentration per unit volume can be further increased by selecting a high "packing density" (surface area per unit volume) membrane element such as a honeycomb monolith or hollow fiber shape. 

Current commercial inorganic membranes come in a limited number of shapes disk, tube and monolithic honeycomb. Compared to other shapes such as spiral-wound and hollow-fiber that are available to commercial organic membranes, these types of membrane elements have lower packing densities and, therefore, lower throughput per unit volume of membrane element or system. 

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