Catalyst regeneration problems

In addition, some processes create considerable amounts of particulate matter and other emissions from catalyst regeneration or decoking processes. Volatile chemicals and hydrocarbons are also released from equipment leaks, storage tanks, and wastewaters. Other cleaning units, such as the installation of filters, electrostatic precipitators, and cyclones, can mitigate part of the problem. 

The tendency in the past decades has been to replace them with solid acids (Figure 13.1). These solid acids could present important advantages, decreasing reactor and plant corrosion problems (with simpler and safer maintenance), and favoring catalyst regeneration and environmentally safe disposal. This is the case of the use of zeolites, amorphous sihco-aluminas, or more recently, the so-called superacid solids, that is, sulfated metal oxides, heteropolyoxometalates, or nation (Figure 13.1). It is clear that the well-known carbocation chemistry that occurs in liquid-acid processes also occurs on the sohd-acid catalysts (similar mechanisms have been proposed in both catalyst types) and the same process variables that control liquid-acid reactions also affect the solid catalyst processes. 

The regeneration problem when to stop a run and either discard or regenerate the catalyst. This problem is easy to treat once the first problem has been solved for a range of run times and final catalyst activities. Note each pair of values for time and final activity yields the corresponding mean conversion.)... 

Commercial Development of Fixed-Bed Process. From the above process considerations it became obvious that the capacity of a commercial unit, and its economic value, were closely related to its ability to burn coke. Indeed, most of the design problems associated with catalytic cracking have been centered around the question of catalyst regeneration. To obtain the most favorable economic return from a 10,000-barrel-per-day unit, it was designed to burn approximately 6000 pounds per hour of coke. This coke yield represents approximately 5% by weight of the charge. 

The recovery, regeneration, and repeated reuse of the active catalyst are of prime importance in substantially reducing the overall cost of coal liquefaction. The used catalysts usually remain in the bottoms products, which consist of nondistillable asphaltenes, preasphaltenes, unreacted coal, and minerals. The asphaltenes and preasphaltenes can be recycled with the catalyst in bottoms recycle processes. However, unreacted coal and minerals, if present in the recycle, dilute the catalyst and limit the amount of allowable bottoms recycle because they unnecessarily increase the slurry viscosity and corrosion problems. Hence, these useless components should be removed or at least reduced in concentration. If the catalyst is deactivated, reactivation becomes necessary before reuse. Thus, the design of means for catalyst regeneration and recycle is necessary for an effective coal liquefaction process. Several approaches to achieving these goals are discussed below. 

A second problem of catalyst regeneration is often the modification of the dispersion of the active component. Several studies [24, 230] clarify that carbon deposition originating from hydrocarbons not only covers an active particle but may remove it from its support. This mode of carbonization occurs effectively with metals catalyzing the formation of carbon filaments (see above). Figure 35 summarizes this effect. A metal... 

As outlined above, supramolecular binding offers new possibilities in this regard. Solids functionalized with a single acceptor motif can be used in more than one application, and the effective cost of the synthesis of the support is reduced. After (partial) catalyst decomposition, the catalyst can be removed easily, and the support can be reused and the catalyst regenerated. Leaching of immobilized catalysts remains the key problem, even without decomposition the leached catalyst can be handled by applying reverse-flow techniques in an "oversized bed. However, no applications of this approach have been reported, but it can be improved. 

Fixed-bed reactors arc suitable for lower temperature ranges, but because of slow heat transfer, their temperature control problem can only partly be overcome by high gas recycling. For catalyst regeneration the process must be interrupted. 

Until recently only a few papers were available on moving beds in cross flow [11-18]. This type of reactor is sometimes a favorable process solution for a selective catalytic process with a moderate catalyst rcsidence time and with a short gas residence time, especially when the process is accompanied by a continuous catalyst regeneration. The use of conventional short-contact-time reactors like fluidized-bed reactors, risers, and fixed-bed reactors does not always yield satisfactory results. This may be explained by problems connected with gas back-mixing, channeling of gas, low catalyst holdup, attrition of the solid catalyst, or difficulties in temperature control. 

Supported metal catalysis are employed in a variety of commercially important hydrocarbon conversion processes. Such catalysts consist, in general, of small metal crystallites (0.S to 5 nm diameter) dispersed on non-metallic oxide supports. One of the major ways in which a catalyst becomes deactivated is due to accumulation of carbonaceous deposits on its surface. Catalyst regeneration, or decoking, is normally achieved by gasification of the deposit in air at about 500°C. However, during this process a further problem is frequently encountered, which contributes to catalyst deactivation, namely particle sintering. Other factors which can contribute to catalyst deactivation include the influence of poisons such as sulfur, phosphorus, arsenic and... 

In the present paper consideration is given to the solution of all three problems by use of mechanochemical and barothermal methods applied to catalyst regeneration. 

Carbon monoxide is a potential emission problem. Fortunately, it is a valuable industrial fuel and may be burned in a CO boiler to recover energy as steam and discharge carbon dioxide. However, it can also arise from the catalyst regenerators of some recent models of catalytic cracking, units which ran at temperatures too low to obtain complete oxidation of carbon to carbon dioxide. The most recent designs of regenerators operate at higher temperatures to achieve complete conversion of carbon to carbon dioxide. 

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