Cracked products, branching in the

The degree of branching in the cracked products has been discussed, so far, for a medium degree of cracking conversion. 

Figure 12. Hydrocracking of n-alkanes (Xr = 50%). Influence of chain length of the feed on content of branched isomers in the cracked products.

Stress-corrosion cracks tend to branch along the metal surfaces. Typically, evidence of corrosion, such as accumulations of corrosion products, is not observed, although stains in the cracked region may be apparent. Stress-corrosion cracks tend to originate at physical discontinuities, such as pits, notches, and corners. Areas that may possess high-residual stresses, such as welds or arc strikes, are also susceptible. 

The relative reactivity of hexane and 3-methylpentane (about 1000) in the isomerization mode was shown to be the same as found for isomerization in HF-SbF5.102 In the cracking mode, however, the ratio is about 10, resulting from the dramatic acceleration of the reaction of hexane compared to that of 3-methylpentane. Further characteristics of the cracking mode are a large excess of branched isomers in the C4—C5 fractions, the absence of unsaturated cracking products, and formation of... 

Hydrogenolysis has been concluded in a preceeding section to be mainly responsible for the formation of + Cm 1 and Cg + Cm 2 Branching in the fractions p = m-1 and p = m-2, then, may be due to hydroisomerization of either the feed or the cracked products according to the reaction sequences ... 

One of the significant features of hydrocarbon free radicals is their resistance to isomerization, for example, migration of an alkyl group and, as a result, thermal cracking does not produce any degree of branching in the products other than that already present in the feedstock. 

Relative hydrogen transfer activity can be determined using an HTI test (11), where the index is a measure of the degree of saturation in the reaction product. The test determines the product ratio of 3-methylpentenes to 3-methylpentane derived from a 1-hexene feed. While the branched products come mainly from oligomerization followed by cracking, the results should be relevant here as well. The higher the index, the lower the relative hydrogen transfer activity. 

The liquid products of the pyrolysis of PP contain primarily olefins that resemble the molecular skeleton of PP (i.e. branched hydrocarbons). A distinguishing feature of PP pyrolysis is the predominant formation of a particular C9 olefin in the pyrolysis product. The level of this C9 compound identified as 2,4-dimethylhept-l-ene can be as high as 25%. Also present are C5 olefin, Cs olefin, several C15 olefins and some C21 olefins [2]. The tertiary carbon sites in PP allows for the facile chain cleavage and rearrangements according to the Rice-Kossiakoff cracking mechanism shown in Figure 15.2. The noncondensable gas from PP pyrolysis contains elevated levels of propylene, isobutylene and n-pentane. 

Many attempts have been made to interpret the origin of the cracked products (propene and pentenes) formed during the reaction of n-butenes and to relate the formation of isobutylene, propene, and pentenes to the acidic and structural properties of the molecular sieve catalyst. In the introduction it was noted that the key reaction intermediates are (i) methylcyclo-propyl cations (formed in the monomolecular path) and (ii) di- (or tri-) branched methyl Ce (or C5) cations (formed in the bimolecular path) and that high selectivity to isobutylene can be achieved only when the monomolecular path predominates. 

Figure 6.23a shows the conversion of the n-alkane as a function of temperature. In the hydrocracking reaction, the cracked products, which are formed by consecutive reactions of the isoalkane, are of interest. The maxima in the selectivity curves correspond to temperatures where cracking becomes dominant. Hydrocarbons with more than six carbon atoms can give doubly branched isomers. These molecules have an enhanced cracking rate, because yS-CH scission can occur via tertiary carbenium ions (Section 3.2.3). This shifts the selectivity for cracking of longer hydrocarbons to lower temperatures. 

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