Low-temperature coke

Schwel-kohle,/. a kind of lignite rich in volatile matter (and hence suitable for carbonization). -koks, m. low-temperature coke. 

In this way, the conjunct polymers serve as a reservoir of hydride ions. Under some conditions, the polymers are a source of hydride ions, but they accept these ions under other conditions. Substantial amounts of the saturated products are supposedly formed via this route with sulfuric acid. In zeolites, species similar to conjunct polymers also form. The heavy hydrocarbon molecules, which deactivate the catalyst by pore blocking or by site blocking, are generally termed soft coke or low-temperature coke , because of the absence of aromatic species. 

Previous work hod shown that low temperature coke is formed from cools hooted to between 450° and 500° C. by a process of nudeation and growth of spherical bodies in the plastic vitrinite. An essentially similar process has now been found to occur with coke-oven and petroleum pitches, with polyvinyl chloride, and with some polynuclear hydrocarbons, all of which yield carbons which grophitize readily at high temperatures. The process is probably general for the initial stages of formation of such carbons from the liquid phase. Some control of the solidification process has been achieved on the laboratory scale, and the physical and chemical structure of the spherulites has been investigated. 

The condensation products are mainly or wholly insoluble in organic solvents but do not differ markedly in C/H ratio from the high-boiling pitch compounds from which they have separated. The average molecular weight of the pitch shortly before conversion to spheres was about 450 and that of the material of the newly formed spheres was about 1700. Presumably further condensation reactions occur within the ordered structure of the spheres at higher temperatures after complete conversion to low-temperature coke. 

Ralph J. Gray. Dr. Taylor states that the transformation of plastic vitrinite or coal-tar pitch into low temperature coke has some of the characteristics of a crystallization process. The authors place much emphasis on the development of spherical bodies in lx>th vitrinite and pitch. My question concerns the type of pitch, the method of its production, and the method of pitch coke production. 

Fig. 12. IINS spectra of three different types of coke 46). (b) High-temperature coke collected from the surface of a Pt/A Os catalyst and low-temperature cokes from deactivated Pd/Si02 catalysts (a) after six months of operation, and (c) after two years of operation.

Coal, which contains a high fraction of volatile components, has to be preoxidized and then carbonized at low temperatures, whereas anthracite can in principle be activated directly. In practice, however, pulverization, briquetting with the aid of a binder, briquette communition and low temperature carbonization are generally carried out. This makes subsequent activation much easier. Lignite low temperature coke can be directly activated. 

Studies of catalysts deactivation by coke are abundant in the literature most of them are usually conducted at high temperatures (around 500°C) using metal catalysts supported on oxides with low surface area such as silica, aluminas or silica-alumina [2 and references therein]. The deactivation by coke of zeolite catalysts has also been studied and such studies have mostly been done for high temperature reactions such as the conversion of n-hexane or the isomerization of xylenes [2,4]. However, low temperature coke formation (20-25°C) combining the effect of high acidity and size specificity for a high coking component such as nickel, has not yet been considered from the point of view of the presence of compounded effects of crystalline structure and location of metal particles. 

Effects of crystalline structure and acidity differentiate the catalytic behavior of ZSM5, USY and mordenite zeolites. Compounded with the nature of the support, the location of nickel particles leads to very peculiar behaviors in the formation of low-temperature coke during the hydrogenation of phenylacetylene. The principal differences in the high temperature deactivation are determined by the size specificity of the zeolitic supports, and by the high acidity available to the reactant molecules, especially for the USY support. The contribution of nickel to coke formation at low temperatures occurs mainly at the internal surface of Ni/mordenite and Ni/USY and at the external surface of Ni/ZSM-5. This conclusion is supported by the TPR patterns as well as by the relative values of low, intermediate and high temperamre coke for each individual support. 

Coke formation on H-Mordenite during olefins reaction was studied both under static and under on-stream conditions . Under static conditions, it was found that the carbonization of ethylene and propylene could be separated into two processes, depending upon the temperature. Below 227°C, the radicals of a low-temperature coke are formed and subsequently annihilated. These radicals do not appear to be precursors of the high-temperature coke. Above 227°C, highly unsaturated radicals of high-temperature coke are formed. The presence... 

UV-VIS Spectroscopy was used for specifying the chemical identity of the components of coke formed during the transformation of methanol or olefins over acid zeolites [34,35]. On ZSM5 catalysts, low temperature Coke exhibited predominantly bands around 325 and 360-385 nm attributable to alkenyl and polyenylic carbocations, high-... 

At low temperature coking occurs rapidly from olefins and from polyaromatics and very slowly from monoaromatics. This is also true from alkylaromatics such as cumene since their cracking into olefins is slow [47]. However a rapid formation of coke occurred during benzene hydrogenation at 80 C on PtUSHY and PtHMOR catalysts. All the coke components resulted from condensation reactions through... 

Like at low temperature coking occurs rapidly from olefins and from polyaromatics. However at high temperatures, alkylbenzenics (e.g. cumene) and branched alkanes crack rapidly, leading to olefins which are rapidly transformed into coke. Coking is slow only from the monoaromatics such as benzene or toluene and from the linear alkanes which crack slowly [10]. 

Feed materials for briquettes include coals of different properties pretreated coals, low-temperature coke, high-temperature coke breeze, or mixtures of these. Various binders are used in manufacturing the briquettes. The binder may be obtained partly during the pretieatment of the coal or during the carbonization of the briquette. The thermal posttreatment of briquettes at different temperatures, with or without the influence of oxygen in the air, leads to briquettes that (depending on the feed material, the treatment method, and the treatment temperature) have properties more or less similar to those of coke. 

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