Mechanism of coke formation

This experimental work gives data that help us to understand the mechanisms of coke formation during in-situ combustion. 

Equations for the kinetic mechanisms of coke formation with the exponential form of the deactivation function are obtained by integrating eqs. (6)—(8) ... 

Figure L Profiles relevant to simple parallel mechanism of coke formation a - rate of blocking the active sites versus normalized distance from the pellet centre b - fraction of blocked sites versus normalized distance from the pellet centre ...

Jn corroboration of these arguments, the plots on Fig. 4 illustrate profiles of the main reaction rate inside the catalyst grain on assuming different mechanisms of coke formation. Curve 1 corresponds to the rate... 

At this point, it is necessary to distinguish between experiments in which mixtures are introduced as feed for the purpose of accelerated aging and those for other purposes. The early papers of Blue and coworkers [4,5], for example, introduced tetralin and decal in with various butenes, but this was done primarily to find a detailed mechanism of coke formation using isotopic techniques. This is not of interest in this work. In this work, we report on the effect of accelerated coking, brought about by cokegenerating additives of various types and at various levels. 

The mechanism of coke formation is changed in the presence of oxygen, which is strongly adsorbed on coked surface. 

The formation of hydrocarbons from methanol catalyzed by zeolite H-MFI has been investigated extensively 60,61). As with many hydrocarbon conversions, the catalytic activity of the methanol-to-hydrocarbons reaction decreases over time as a result of the buildup of high-molecular-weight carbonaceous deposits (coke). UV Raman spectroscopy was employed to characterize the accumulation and chemical nature of deposited hydrocarbons as a function of time and reaction temperature with both methanol and dimethyl ether as reactants and with zeolite MFI of various Si/Al atomic ratios as catalysts the first account of this work reported results for a zeolite MFI with low acid content (Si/Al = 90) (62). Both polyolefin and a cyclopentadienyl species were observed as intermediates during the formation of polyaromatic retained hydrocarbons. These observations strongly confirm the mechanism of coke formation proposed by Schulz and Wei (63) involving aromatic formation via a five-membered ring... 

Why was iron the predominant metal in filament, ribbon, and needle cokes Further information on the mechanism of coke formation is needed. 

Advancing as well is our understanding of tiie many mechanisms of coke formation, but these are so reaction/catalyst-specific that we shall for the foreseeable future still find it necessary to... 

Thus we will consider coke formation on the surface of the adsorbent/catalyst as the polycondensation of starting organic substances or the products of an earlier reaction of these substances which are capable of further reactions [13]. The process of formation of polycondensation products cannot be presented by only one chemical reaction (Scheme 1 illustrates only the general mechanism of coke formation.) This process can be imagined as a multitude of different polycondensation reaction processes leading to hypermolecular structure of the condensation products. 

The nature of the coke and the kinetics and mechanism of coke formation and removal have been studied in some detail [3-7]. Minimisation of coking requires minimal coke formation and maximal coke removal - by gasification of carbon or of intermediates which can lead to carbon. 

Generally the mechanism of coke formation can by described be polycondensation reactions of asphaltenes or asphaltenes with aromatic or unsaturated compounds. In the simplest form all these reactions can be represented by the following reactions ... 

In just the same way two or more mechanisms of coke formation were observed in the decomposition of the naphthenes, of the paraffin hydrocarbons, olefins, acetylene, carboxylic acids, etc. In all cases the mechanisms of coke and tar formation could be represented by a cyclic sequence of a number of elementary stages, which involve the addition of every new molecule of the coke-forming material. Depending on whether the initial organic substance when the temperature is rising can give one, two, or more kinds of molecules of coke-forming material capable of... 

According to different mechanisms of coke formation, we have observed different products of polycondensation using chromatographic, luminiscent, and UV-spectroscopic methods. For example, in the case of decomposition of benzene on different catalysts only products of the dehydrocondensation of benzene with preservation of nuclei were observed (biphenyl, biphenylbenzenes, triphenylene, products of condensation of more than four benzene nuclei, etc.) and such products as naphthalene, anthracene, and phenanthrene were not observed. In tar and coke formation from ethylene on silica gel and aluminosilicates the formation of naphthalene, chrysene, 1,2-dibenzanthracene, fluorene, its derivatives, and others, takes place and if the process is carried out on alumina at a temperature lower than 500°, mainly anthracene, phenanthrene, pyrene, and coronene are formed, but aliphatic hydrocarbons, etc., do not appear. 

This study looks at the rearrangement of a range of ketoximes and aldoximes over zeolite and alumina based solid acid catalysts with a view to elucidating the factors which determine the reactivity of the various oximes and the selectivity to the amide products. Another factor to be addressed is the mechanism of coke formation, in particular the role of acidic and basic surface sites in this process) 16]. 

The mechanism of coke formation in catalysts and its nature have been widely reported in the literature [1, 2, 8-14]. For the hydroisomerization catalysts, however, the mechanism is not well known. Herein, oin interest is to study in detail the coke formation and its combustion... 

Langner, B.E. and Meyer, S. (1980), The mechanism of coke formation in the reaction of butadiene on calcined NaNH -Y in B. Delmon and G.F. Froment (eds.). Studies in Surface Science and Catalysis, Catalyst Deactivation, Elsevier, Amsterdam, pp. 91-102. 

The mechanisms of coke formation on the acid function involves naphthenes with five carbon atoms ring as one of the most important coke precursors,. Cy-clopentadiene and methylcyclopentadiene can polymerize through a Dials-Alder condensation mechanism to form polynuclear aromatics and then coke (130). Figure 14 shows the reaction sequence involved in the formation of coke from re-paraffins (97). It has been found both on Pt and Pt-Re using re-heptane as feed. 

Fig. 14. Mechanism of coke formation from n.-paraffins. DHC dehydrocyclization, DH dehydrogenation or hydrogen transfer, C condensation.

Siinivasan, N.S. McKnight, C.A. Mechanism of coke formation from hydrocracked Athabasca residuum, F e/1994, 73,1511-1517. 

Fig. 5.2 Mechanism of coke formation from n-paraflins. DHC dehydrocyclization (on metal and acid sites) DH dehydrogenation (on metal sites) or hydrogen transfer (on acid sites) C condensation on acid sites, CP cyclopentane, CPde cyclopenta-diene., MCP methylcyclopentane, MCPde methylcyclopentadene.

For coking on the metal, many reaction mechanisms had been tested, and only the following mechanism of coke formation on the metal was found to be able to describe the experimental observation (Li et al., 2011b) ... 

The description of the mechanism(s) of coke formation remain essentially the same as described before. With increasing experimental information on coking in the literature there appears now to be at least tacit agreement that it is not particularly rewarding to look for a mechanism of coke formation when there are probably as many mechanisms as there are reactions and catalysts. In this sense, simple reaction schemes may provide a starting point for analysis but may also fall far short of the mark in confrontation with experiment. 

Coke formation reactions reported above in VII may be more complex than realized previously. In very recent work on steam reforming of n-hexane, Bett, et al. (94) conclude that carbon formation may result from interaction of unreacted hexane and the products of secondary cracking reactions, or from unstable intermediates in the cracking process. A selection of other thoughts concerning the mechanism of coke formation in reforming on Ni would include the works of Whalley, et al. (95), Presland and Walker (96). and Renshaw, et al. (97). 

Coking kinetics has been the subject of various researches reported in the literature, with special emphasis on the mechanism of coke formation, interconversion of the solubility class components during conversion, role of these components in coke formation, influence of structural properties on coking rate and yields, development of correlations, among others. 

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