In coke formation

On the one hand, high-throughput techniques can be used to achieve more specific catalysts, while the use of conditions favoring a reduction in coke formation during the reaction (i.e., the use supercritical conditions) could also be of crucial importance in the rapid incorporation of these catalysts into industrial processes. 

The results described above suggest that the methylation of naphthalene over MFI- metallosilicates occurs inside crystalline pores by a restricted transition-state mechanism, not with unordered sites at or near external sites. Weaker acid sites preferentially form 2,6-DMN and favor a decease in coke formation. 

A problem may occur when higher-molecular-weight materials are employed as feedstocks and result in coke formation and deposition. When an alkali-promoted catalyst is employed, corrosion and fouling problems in the reformer (or even in equipment downstream of the reformer because of the tendency of the alkali to migrate) may occur with some frequency. However, coke formation can be eliminated by the use of a proprietary alkali-free catalyst that has an extremely high activity and resistance to poisoning. 

Figure 9 summarizes some of the intermolecular reaction pathways deemed important in catalytic cracking. For example, hydrogen transfer between paraffin and olefin and between olefin and naphthenes can occur to form energetically more stable reaction products (37,38). Transalkylation, i.e., scrambling of short chain alkyl groups on aromatics, is also prevalent. Condensation reactions have been implicated in coke formation pathways. 

Obviously the acidity and the pore structure of the zeolite catalysts play a significant role in coke formation. These parameters influence both the reactions involved in the formation of coke molecules and their retention. Thus the stronger the acid sites the faster the reactions and the slower the diffusion of basic intermediates hence the faster the coke formation. The density of the acid sites has also a positive effect on coke formation, which can be related to the intervention of... 

The zeolite was pelletized, sieved, and the fraction between 0.5 and 0.71 mm diameter was retained. Scanning electron microscopy did not reveal any modification of the zeolite crystals by the pelletization. Three experiments were performed using different particle sizes, to ensure that the pelletization did not introduce transport limitations. No difference in coke formation, conversion, or selectivities was observed. 

The sequence of events in coke formation was studies in the model reaction [70] of H-Y zeolite with propene at 723 K. Under these drastic conditions the soluble white coke formed rapidly within 20 min and was converted into insoluble coke within 6h under inert gas without loosing carbon atoms in the deposit. Due to the larger pores in the Y-zcolites compared to the ZSM type zeolites used in the other studies mentioned so far, the structure of the aromatic molecules is somewhat different. The soluble coke in this system consisted of alkyl cyclopentapyrenes (C H2 -26. Type A) as the hydrogen-rich primary product and of alkyl benzoperylenes (C H2n- 2. Type B) and alkyl coro-nenes (CrtH2 -36. Type C) as matured components. The temporal evolution of the various products is presented in Fig. 14. It clearly emerges that the soluble coke fractions are precursors for the insoluble coke and that within the soluble coke fraction the final steps of dehydrogenation-polymerization are very slow compared to the initial formation of smaller aromatic molecules from propene. The sequential formation of precursors with decreasing C H ratio follows from the shift of the maximum in the abundance of each fraction on the time axis. 

Table 1. Classification of phenomena occurring in coke formation at various levels. ...

J.G. McCarty, P.Y. Hou, D. Sheridan, and H. Wise, Reactivity of Surface Carbon on Nickel Catalysts Temperature-Programmed Surface Reaction with Hydrogen and Water, in Coke Formation on Metal Surfaces, eds. L.G. Albright and R.T.K. Baker, American Chemical Society, Washington D.C., 1982, p. 253. 

The effect of quinoline and phenanthrene additions to a n-hexadecane feedstock has been determined for a model four-component FCC catalyst by means of a MAT reactor with analysis of all products and characterisation of the coke produced. Both additions lead to an overall loss in conversion quinoline is considered to act as a poison while phenanthrene participates strongly in coke formation and the resultant coke becomes more aromatic in nature. The cracking propensity and associated coke formation have been measured for a series of FCC catalysts with differing compositions. Increasing amounts of zeolite in a matrix lead to increasing extents of conversion but with little effect on the extent of coke production. However, a pure zeolite gave a very high coke content. 

A wide variety of reactions occur during the thermal treatment of crude residue, as was shown earlier. However, not all these reactions lead to the formation of coke. In this section, we will look at the reactions that are involved in coke formation for both the catalytic treatment process and the purely thermal treatment process. 

In contrast to catalytic treatment, coke formation during thermal treatment leads only to pipe blocking and poor thermal conductivity of the reactor walls. The most important difference between coke formation in catalytic and thermal treatment is that a free radical mechanism in coke formation is not possible in the case of thermal processing. 

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