Coke formation olefins

These interrelations are consistent with the above model of high temperature deactivation by coke formation through a reaction of coke growth with methanol. However, this mechanism needs coke seeds provided as "olefin coke" on external acidic centers. Development of ZSM5-catalysts for high temperature application with long life time thus concerns minimizing of acid sites on crystallite surfaces. 

Although the reaction classes discussed earlier are sufficient to describe the hydrocarbon conversion kinetics, an understanding of the elementary reaction sequence is needed to describe catalyst deactivation. Several of the overall reactions require formation of olefinic intermediates in their elementary reaction sequence. Ultimately, these olefinic intermediates lead to coke formation and subsequent catalyst deactivation. For example, the ring closure reaction... 

For example, in the ring isomerization reaction, methylcyclopentane forms a methylcyclopentene intermediate in its reaction sequence to cyclohexane. The intermediate can also further dehydrogenate to form methylcyclo-pentadiene, a coke precursor. Bakulen et al. (4) states that methylcyclo-pentadiene can undergo a Diels-Alder reaction to form large polynuclear aromatic coke species. Once any olefinic intermediate is formed, it can either go to desired product or dehydrogenate further and polymerize to coke precursors. This results in a selectivity relationship between the desired products and coke formation as shown on the next page. 

In our mechanism, coke formation is due to the presence of olefins, which occur as intermediate species during the reforming reactions. As discussed in Section II, these olefins can go either to products or to coke precursors. The deactivation caused by feed poison, catalyst sintering during regeneration, or improper regeneration techniques is not considered in this development. 

The effect of rare-earth HY zeolites is somewhat similar, except that it produces less olefin as a result of enhanced hydrogen transfer reactions. Decreased hydrogen transfer is the main feature of USY zeolites, yielding a product significantly richer in olefins. It is slightly richer in aromatics because of the retardation of their secondary transformations (condensation, coke formation). 

Nor did catalytic cracking escape the probing attention of Paul Emmett. At Johns Hopkins his students used labeled molecules extensively to examine the nature of secondary reactions in the cracking of cetane over amorphous silica-alumina and crystalline zeolites. They demonstrated that small olefins (e.g., propylene) are incorporated extensively into higher-molecular-weight molecules, especially aromatics, and are the primary source of coke formation on these catalysts. 

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. 

A key question related to the design of HC catalysts is the importance of the proximity, or as it is often termed, the intimacy of the hydrogenation function to the acidic sites. Studies based on model HC catalyst systems (25) led to the development of intimacy criteria that need to be satisfied for sufficient rates of diffusion of olefinic intermediates between acidic and hydrogenation functions to achieve equilibria. Similarly, intimacy rules will apply to aromatic intermediates in order to minimise coke formation. Literature data on this aspect are rather scarce, however. 

Fig. 6 shows that the DME conversion increased slightly with coke formation up to about 5 wt% coke. Coke obviously has a promoting effect besides a deactivating effect for the formation of olefins. A set of experiments with DTO and MTO on SAPO-34 that had been pre-coked with propene was performed at 698 K to... 

A lthough coke formation is always of importance during pyrolysis processes that are used for production of ethylene and other valuable olefins, diolefins, aromatics, etc., relatively little is known about the factors affecting such coke formation. It has been found that operating conditions, feedstock, pyrolysis equipment, and materials of construction and pretreatments of the inner walls of the pyrolysis tubes all affect the production of coke. General rules that have been devised empirically at one plant for minimizing coke formation are sometimes different than those for another plant. It can be concluded that there is relatively little understanding of, or at least little application of, fundamentals to commercial units. 

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