Coke formation catalysts

Certain catalyst properties appear to increase coke formation. Catalysts with high rare earth content tend to promote hydrogen transfer reactions. Hydrogen transfer reactions are bimolecular reactions that can produce multi-ring aromatics. 

On the other hand, there is the question of the close relationship between coke formation (catalyst stability) and aromatics cyclization (catalyst selectivity) over the acid sites present in the zeolites. On naphtha reforming, coke formation is a bifunctional reaction requiring the dehydrogenation capacity of the metallic function and the condensation capacity of the acidic function. Therefore, it is interesting to... 

Froment, G. F. The modeling of catalyst deactivation by coke formation. Catalyst Deactivation 1991 C. H. Bartholomew J. B. Butt (Eds.) Elsevier Sciences Amsterdam, 1991,53. 

Early catalysts were produced from calcined ferric oxide, potassium carbonate, a binder when required, and usually chromium oxide. Subsequently a wide range of other oxides replaced the chromium oxide typical compositions are shown in Table 7.5. The paste was extruded or granulated to produce a suitable shape and then calcined at a high temperature in the range 900°-950°C. Solid solutions of a-hematite and chromium oxide (the active catalyst precursors) were formed and these also contained potassium carbonate to inhibit coke formation. Catalyst surface area and pore volume were controlled by calcination conditions. It has been confirmed by X-ray diffraction studies that a-hematite is reduced to magnetite and that there is some combination of potash and the chromium oxide stabilizer. There is little change in the physical properties of the catalyst during reduction and subsequent operation. 

Nickel catalysts are also used for steam methane reforming. Moreover, nickel catalysts containing potassium to inhibit coke formation from feedstocks such as LPG and naphtha have received wide appHcation. 

Zeolite-Based All lation. Zeohtes have the obvious advantages of being noncorrosive and environmentally benign. They have been extensively researched as catalysts for ethylbenzene synthesis. Eadier efforts were unsuccessful because the catalysts did not have sufficient selectivity and activity and were susceptible to rapid coke formation and deactivation. 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

Engelhard Corporation, The Chemistry of FCC Coke Formation, The Catalyst Report, Vol. 7, Issue 2. 

In a cat cracker, a portion of the feed, mostly from secondary cracking and polymerization reactions, is deposited on the catalyst as coke. Coke formation is a necessary byproduct of the FCC operation the heat released from burning coke in the regenerator supplies the heat for the reaction. 

Example 11.15 Coke formation is a major cause of catalyst deactivation. Decoking is accomplished by periodic oxidations in air. Consider a micro-porous catalyst that has its internal surface covered with a uniform layer of coke. Suppose that the decoking reaction is stopped short of completion. What is the distribution of residual coke under the following circumstances ... 

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 ... 

Coke formation on these catalysts occurs mainly via methane decomposition. Deactivation as a function of coke content (see Fig. 3 for Pt/ y-AljO,) seems to involve two processes, i e, a slow initial one caused by coke formed from methane on Pt that is non reactive towards CO2 (see Table 3) In parallel, carbon also accumulates on the support and given the ratio between the support surface and metal surface area at a certain level begins to physically block Pt deactivating the catalyst rapidly. The coke deposited on the support very close to the Pt- support interface could be playing an important role in this process. 

If the catalyst degrades (e.g. as a result of coke formation on the surface), then a fixed-bed device will have to be taken off-line to regenerate the catalyst. This can either mean shutting down the plant or using a standby reactor. If a standby reactor is to be used, two reactors are periodically switched, keeping one online while the other is taken offline to regenerate the catalyst. Several reactors might be used in this way to maintain an overall operation that is... 

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