Coking reaction routes

The criterion of evaluated commercial catalyst takes in account a large rage of characterization. On of the most important is the microactivity catal34ic test, that presents good sensibility to detect metal, acid sites and coke deactivation. The great number of reactionals routes makes the specific characterization more difficult. In this way cyclohexane presents ease improvement and can characterize simultaneously metallic and acid sites. 

Manufacture. An extensive technology was developed initially ia the 1930s for isolation of ammonium thiocyanate from coke-oven gases, but this technology is no longer practiced ia the United States (372). However, such thiocyanate recovery processes are used iadustrially ia Europe. Likewise, the direct sulfurization of cyanides to thiocyanates is not practiced commercially ia the United States. The principal route used ia the United States is the reaction of carbon disulfide with aqueous ammonia, which proceeds by way of ammonium dithiocarbamate [513-74-6]. Upon heatiag, the ammonium dithiocarbamate decomposes to ammonium thiocyanate and hydrogen sulfide. 

Reaction of coke with calcium oxide gives calcium carbide, which on treatment with water produces acetylene. This was for many years an important starting point for the production of acrylonitrile, vinyl chloride, vinyl acetate and other vinyl monomers. Furthermore, during World War II, Reppe developed routes for many other monomers although these were not viable under normal economic conditions. 

Phenol was originally recovered during the coking of coal, essentially being a by-product. Eventually, commercial routes were developed based on benzene (from coal or petroleum) for example, sulfonation of benzene to ben-zenesulfonic acid followed by reaction with water to phenol plus regenerated sulfuric acid. Phenol is used to make plastics (phenol-formaldehyde and epoxy resins) and textile fibers (nylon). Phenol is also used in solution as a general disinfectant for cleaning toilets, stables, floors, drains, etc. and is used both internally and externally as a disinfectant for animals. 

Decomposition of the adsorbed carbenium ions is the main reaction charmel. However, isomerization (aromatization) and oligomerization reactions also proceed, and are the route to coke formation. 

The F / Cl exchange in chloroalkanes is a route to HFCs. For example, different routes can be possible for the synthesis of CF3CH2F [1,2 ]. Our focus is on its preparation from CF3CH2CI and HF with chromium (HI) oxide as a catalyst. This fluorination is accompanied by a dehydrofluorination which produces chloroalkenes (mainly CF2=CHC1) resulting in a deactivation of the catalyst Indeed this haloalkene could polymerise and thus lead to coke formation. The reactions involved are ... 

Figure 4.1 summarizes the different routes that can potentially lead to carbon deposition during FTS (a) CO dissociation occurs on cobalt to form an adsorbed atomic carbon, which is also referred to as surface carbide, which can further react to produce the FT intermediates and products. The adsorbed atomic carbon may also form bulk carbide or a polymeric type of carbon. Carbon deposition may also result (b) from the Boudouard reaction and (c) due to further reaction and dehydrogenation of the FTS product (what is commonly called coke), a reaction that should be limited at typical FT reaction conditions. Carbon formed on the surface of cobalt can also spill over or migrate to the support. This is reported to readily occur on Co/A1203 catalysts.43 The chemical nature of the carbonaceous deposits during FTS will depend on the conditions of temperature and pressure, the age of the catalyst, the chemical nature of the feed, and the products formed. 

Pure titanium is obtained commercially from rutile (Ti02) by an indirect route in which Ti02 reacts with Cl2 gas and coke to yield liquid TiCl4 (bp 136°C), which is purified by fractional distillation. Subsequent reduction to Ti metal is then carried out by reaction with molten magnesium at 900°C, and further purification is effected by melting the titanium in an electric arc under an atmosphere of argon. 

The traditional synthesis route involves the direct reaction of silicon with nitrogen at temperatures above 1,300 °C, or by heating silica with carbon (coke) in a stream of nitrogen and hydrogen at 1,500 °C.41 However, as in the case of silicon carbide, the high processing and fabrication temperatures focused attention on the need for alternative access routes based on preceramic polymers. 

A comprehensive study on coke deposition in trickle-bed reactors during severe hydroprocessing of vacuum gas oil has been carried out. On the basis of results obtained with different catalysts on the one hand, and analytical and catalytic characterisation of the coke deposits on the other, it is argued that coke is formed via two parallel routes, viz. (i) thermal condensation reactions of aromatic moieties and (ii) catalytic dehydrogenation reactions. The catalyst composition has a large impact on the amount of catalytic coke whilst physical effects (vapour-liquid equilibria, VLE) predominate in determining the extent of thermal coke formation. The effect of VLE is related to the concentration of heavy coke precursors in the liquid phase under conditions which promote oil evaporation such as elevated temperatures. A quantitative model which describes inter alinea the distinct maximum of coke deposited as a function of temperature is presented. 

In order to explain the effects of the catalyst composition we postulate that two major routes to coke exist, viz. (i) radical reactions giving rise to condensation of the aromatic structures which ultimately lead to thermal coke and (ii) dehydrogenation reactions which... 

However there are several major hurdles. The most common catalysts are based on acid catalysis with Bronsted or Lewis acid sites these sites promote the formation of propylene rather than ethylene as is witnessed by conventional FCC operations. Ethylene is promoted by free radical processes. Catalysis of free radical reactions is rare, but not unknown . One route is to take a conventional acid catalysis and to neutraUse the acid sites with alkaline metals (magnesium, calcium) or phosphorus or a mixture of such. This can generate a further problem, in that the catalyst promotes the formation of carbon (coke) and hydrogen which are thermodynamically favoured at the reaction temperatures. 

In spite of the above, there are some aspects that are not clear at present such as the influence of the preparation method on the SMSI state, and the effect of SMSI phenomena on secondary reactions such as coking. In a previous work we examined partial aspects of the catalytic performance of Ni0-Al203-Ti02 mixed oxides, prepared by the sol-gel route, in the hydrogenation of acetylene and phenylacetylene (13). 

Petroleum chemicals fulfill two functions. They provide alternative and more economic routes to existing chemicals already made from other raw materials, and they lead to new industrial chemicals. The reactions of and outlets for chemicals more economically synthesized from petroleum have already been worked out, although perhaps not completely, in connection with the older routes. Those countries not favored with petroleum as an economic raw material have had to make use of alternative sources for these chemicals. In surveying the literature, it is, therefore, necessary to take account of the history of those petroleum chemicals which have been made from alternative sources. The reactions of methane are the same whether it is obtained from natural gas or as a by-product of the hydrogenation of coal or as a fraction in the liquefaction of coke oven gas depending on the source, the economics may be quite different. 

Analysis of Table 8.3 leads to an important conclusion, that deactivation can be treated using the general framework of the theory of complex reactions, simply considering deactivation as an independent route leading to coke on the catalyst surface. 

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