Monoliths forms

The carbons are broadly comparable in terms of their maximum concentration and implied energy efficiency but the two monolithic forms offer the advantage of smaller pressure vessel sizes and improved heat transfer. 

Most of the NO reducing catalysts in pellet or monolithic form begin to lose their activity at 2000 miles and fail to be effective at 4000 miles. This lack of durability may well be connected to the usage of the NO bed for oxidation purposes during the cold start, which exposes the NOx catalysts to repeated oxidation-reduction cycles. Better catalyst durability can be anticipated in the single bed redox catalyst with a tightly controlled air-to-fuel ratio, since this oxidation-reduction cycle would not take place. Recent data indicates that the all metal catalysts of Questor and Gould may be able to last 25,000 miles. 

These materials are produced in monolithic form or as coatings. The coatings are generally applied by CVD on ceramic substrates and by sputtering, electron-beam evaporation, or ion-beam assisted deposition on steel substrates (see Appendix). 

As with the automotive exhaust converter, the SCR catalyst is designed to handle large flows of gas (e.g. 300 N s for a 300 MW power plant) without causing a significant pressure drop. Figure 10.12 shows a reactor arrangement with about 250 m of catalyst in monolithic form, sufficient for a 300 MW power plant. 

What are monoliths While packed beds are made from individual particles, monoliths form a continuous porous structure throughout the... 

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. 

Because of the close similarity of ionic radii, transition metal ions (M = Mn, Cu, Fe, Cr, Co, Ni) can be partially substituted for A1 ions. These transition metal ions can provide significant activity in combustion reaction.5 At low metal concentrations, the structural and morphological properties are not significantly affected by substitution, so that high thermal stability is maintained. Substituted hexaaluminates can be directly shaped in the monolith form required by the combustor, providing bulk active catalysts without need of ceramic supports. 

Because of its effectiveness and flexibility, this method was scaled up to the preparation of catalysts in the monolith form to be used in pilot and full-scale combustors.17... 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

The incorporation of fly ash lowers the initial heat evolution during setting, thereby reducing the incidence of cracking and spalling. It is desirable to maintain the modified cement in monolithic form for optimum leach resistance. 

The traditional or conventional ceramics are generally in monolithic form. These include bricks, pottery, tiles and a variety of art objects. The advanced or high-performance monolithic ceramic materials represent a new and improved class of ceramic materials where, frequently, some sophisticated chemical processing route is used to obtain them. Generally, their characteristics are based on the high quality and purity of the raw materials used. Examples of these high-performance ceramics include oxides, nitrides, carbides of silicon, aluminium, titanium and zirconium, alumina, etc. 

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. 

Fig. 11. a SEM image of an ordered macroporous monolith, formed by assembling PS spheres coated with five silicalite nanoparticle/polyelectrolyte layer pairs into a close-packed arrangement, and subsequently calcining, b TEM image of the silicalite framework in a. c Corresponding diffraction pattern. (Adapted from [42], by permission of the American Chemical Society)... 

CIM disk (convective interaction media) BIA, Ljubljana, Slovenia Disk/monolith formed by copolymer 19-21... 

The kinetics of chemical corrosion for inorganic membranes can be quite different from that for the same materials in dense monolithic form. This is particularly true with porous inorganic membranes where the porous network resembles interconnected finely divided particles. Compared to dense bulk bodies, fine particles normally are dissolved more rapidly in corrosive chemicals. 

There is the possibility to make substrates in various materials Alumina is an obvious possibility, but monoliths formed from alumina are particularly susceptible to thermal shock problems, and they readily crack during rapid temperature excursions. Silicon carbide and boron nitride are other possible materials having good properties, but they are expensive. 

Crynes et al. [35] have developed what they call a monolith froth reactor. They introduce the gas through a porous glass frit just below the monolith, forming a froth that is fed into the reactor. Oxidation of phenol was studied in a 5-cm-wide and 33-cm-long... 

The Amoco PX 21 carbon has the highest BET surface area of aU carbons listed in Table 22.3. Yet, the isothermal deliverable CH4 capacity for this carbon is about lOOm /m [18]. This is due to its low bulk density (high external void). If this carbon can be produced in a monolith form so that the external void in the ANG tank is negligible, then the isothermal deliverable capacity ( 210m /m ) approaches that of the CNG [18]. Recendy, a carbon monolith was fabricated by Lozano-CasteUo and coworkers, but it produced an isothermal deliverable capacity of 126m /m only [40]. 

This method requires less experimental expertise and less equipment than other techniques however, up to now it is limited to the development of monolithic forms, and complete disintegration cannot be detected. 

One of the causes of deactivation of the exhaust gas catalyst, in both pellet and monolith form, is clogging by lead, phosphorus and zinc traces contained in exhaust gases. 

Deactivation Mechanisms- To provide a perspective on the nature of catalyst deactivation in air emissions control systems, consider first the general characteristics of the catalyst itself. The catalyst can be employed generally in the form of pellets or in a monolithic form often called a... 

The copolymer (PhMeSi) (Mc2Si) is also used as a silicon carbide precursor, especially for monolithic formed objects. 

Because the application of carbon-based monoliths forms an emerging field in (bio)chemical engineering, new applications and process developments can be expected in the future. One might think of applications of monoUth-CNF combinations in water-air filter systems [66], carbon precnrsors with tailored porons texture for production of integral/coated monoliths, ultrathin coatings on ACM monoliths for higher mass-transfer rates, and carbon monoliths in adsorption processes or as high-surface-area electrodes. 

Two catalyst formulations were used in this study. The first, a Pd catalyst having a loading of 50 g/ft.3 (0.29 wt.%), was obtained in monolith form (cell density 400 cells/in ). This washcoat formulation is considered to represent a state-of-the-art Pd catalyst for use in a rapid HC lightoff close-coupled (within 10-15" of the exliaust manifold) converter, and has been processed with several additives which serve to promote the activity of the Pd and to thermally stabilize the support and supported metal. The Pt-Rh monolith catalyst was also obtained in monolith fonn, having a cell density of 400 cells/in and a loading of 23.5 g/ft3 Pt, 1.68 g/ft3 Rli (0.13 wt.% Pt, 0.0093 wt.% Rh). This washcoat is designed for use in converters which will be exposed to exhaust temperatures consistent with an underfloor location (at least 25-30" from the exhaust manifold), which are significantly lower than those encountered in a close-coupled location. 

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