Monolithic products

Although common catalyst support materials such as alumina, silica, and carbon are also used for the production of monoliths, the most common material for commercial monolith production is cordierite, because of its high thermal stability and low coefficient of expansion. 

In extrusion, in addition to the nature and the properties of the materials used to make the moldable mixture, the additives used, the pH, the water content, and the force used in extrusion are also of importance with respect to the properties of the monolith products [31]. The additives applied in extrusion are, e.g., celluloses, CaCl2, ethylene, glycols, diethylene glycols, alcohols, wax, paraffin, acids [13,16,24,30], and heat-resistant inorganic fibers [24]. Besides water, other solvents can also be used, such as ketones, alcohols, and ethers [15,16]. The use of additives may lead to improved properties of the monoliths, such as the production of microcracks that enhance the resistance to thermal shock [4,12], better porosity and adsorbability [15,16], and enhanced mechanical strength or a low thermal expansion [24]. 

Examples of Commercially Available Membrane and Monolithic Products... 

One of the greatest challenges in monolith production is to increase the diameter. Presently, silica monoliths with a diameter of up to 50 mm can be produced. Commercial monolithic silica columns e.g. Chromolith columns of Merck KGaA possess a column permeability equivalent to a column packed with 15 xm particles, but show a column performance of a column packed with 5 xm beads. The major benefit of monolithic columns, however, is their robustness in use, their tailored pore structure and the tunable surface chemistry. 

Figure 28.3 reports, for instance, the results of propane oxidative dehydrogenation tests over the VMgO-washcoated monolith. Product distributions are plotted as a function of propane conversion, which was progressively increased by increasing the temperature of an external heating furnace. 

Refractories may be preformed, ie, shaped, or formed and installed on-site, ie, specialties. Iimovations in placement and vessel constmction has led to a greater emphasis on specialty refractory products. Castables, gunning mixes, and plastic and ramming mixes are used either for repair or for complete new constmction of what is known as monolithic linings. The tendency to use monolithics instead of constmctions using shaped products has been steadily increasing. As of the mid-1990s, monolithic installations are as common as conventional shaped product constmction. 

The sol—gel process can be utilized to yield products within a wide range of appHcations. Some of these appHcations include production of nanocomposites, films, fibers, porous and dense monoliths, and biomaterials. 

In the second procedure, calcium nitrate was replaced by calcium alkoxide (60). Calcium and sificon alkoxides have very different rates of hydrolysis. To avoid the production of inhomogeneities, a slow and controlled hydrolysis of a mixture of sificon, calcium, and phosphorous alkoxide was performed. The resulting materials were highly homogenous, and monolithic pieces could be produced. The bioactivity of the gel-derived materials is equivalent or greater than melt-derived glasses. 

Scale-up in fixed-bed reactors is limited by the maximum size of the matrix that can be manufactured as a monolith. Hence, this system is appHcable for small- to medium-scale production of antibodies and other proteins, usually for the diagnostic market. This system has been described in greater detail ia the Hterature (22). 

Since NO production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore, many new designs incorporate DENOX units that employ catalytic methods to reduce the NO limit. Platinum-containing monolithic catalysts are used (36). Each catalyst performs optimally for a specific temperature range, and most of them work properly around 400°C. 

A recently developed adsorbent version of ORNL s porous carbon fiber-carbon binder eomposite is named carbon fiber composite molecular sieve (CFCMS). The CFCMS monoliths were the product of a collaborative researeh program between ORNL and the University of Kentueky, Center for Applied Energy Researeh (UKCAER) [19-21]. The m.onoliths are manufactured in the manner deseribed in Section 2 from P200 isotropic pitch derived fibers. While development of these materials is in its early stages, a number of potential applieations can be identified. 

Often, the immobilized product has a structural strength sufficient to prevent fracturing over time. Solidification accomplishes the objective by changing a non-solid waste material into a solid, monolithic structure that ideally will not permit liquids to percolate into or leach materials out of the mass. Stabilization, on the other hand, binds the hazardous constituents into an insoluble matrix or changes the hazardous constituent to an insoluble form. Other objectives of solidiflcation/stabilization processes are to improve handling of the waste and pri uce a stable solid (no free liquid) for subsequent use as a construction material or for landfilling. 

Serious research in catalytic reduction of automotive exhaust was begun in 1949 by Eugene Houdry, who developed mufflers for fork lift trucks used in confined spaces such as mines and warehouses (18). One of the supports used was the monolith—porcelain rods covered with films of alumina, on which platinum was deposited. California enacted laws in 1959 and 1960 on air quality and motor vehicle emission standards, which would be operative when at least two devices were developed that could meet the requirements. This gave the impetus for a greater effort in automotive catalysis research (19). Catalyst developments and fleet tests involved the partnership of catalyst manufacturers and muffler manufacturers. Three of these teams were certified by the California Motor Vehicle Pollution Control Board in 1964-65 American Cyanamid and Walker, W. R. Grace and Norris-Thermador, and Universal Oil Products and Arvin. At the same time, Detroit announced that engine modifications by lean carburation and secondary air injection enabled them to meet the California standard without the use of catalysts. This then delayed the use of catalysts in automobiles. 

Chemical vapor deposition (C VD) is a versatile process suitable for the manufacturing of coatings, powders, fibers, and monolithic components. With CVD, it is possible to produce most metals, many nonmetallic elements such as carbon and silicon as well as a large number of compounds including carbides, nitrides, oxides, intermetallics, and many others. This technology is now an essential factor in the manufacture of semiconductors and other electronic components, in the coating of tools, bearings, and other wear-resistant parts and in many optical, optoelectronic and corrosion applications. The market for CVD products in the U.S. and abroad is expected to reach several billions dollars by the end of the century. 

In this book, the CVD applications are classified by product functions such as electrical, opto-electrical, optical, mechanical and chemical. This classification corresponds roughly to the various segments of industry such as the electronic industry, the optical industry, the tool industry, and the chemical industry. CVD applications are also classified by product forms such as coatings, powders, fibers, monoliths, and composites. 

Provide a detailed assessment of the technology of CVD and its relation to the production of coatings, fibers, powders, and monolithic shapes. 

Coatings are by far the largest area of application of CVD at the present but by no means the only CVD process. Other areas of CVD, such as production of powders, fibers, monoliths, and composites, are growing rapidly. 

In this particular example as in many others, a proper analysis of cost is a crucial factor. An example is the production of balls for ball bearings. Coated ball bearings (or monolithic silicon nitride) greatly outperform steel balls but their cost is considerably higher. Steel balls in passenger-automobile applications are satisfactory and normally last the life of the car and the far-longer life of the ceramic balls is not needed. 

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