Hydrocracking of n-heptane

In Figure 14 hydrocracking of n-heptane and n-octane have been selected to demonstrate the influence of the reaction temperature and, hence, of the degree of cracking conversion. Again, a classification of the carbon number fractions is useful ... 

Figure 14. Hydrocracking of n-heptane and n-octane. Influence of reaction temperature and degree of cracking conversion on content of branched isomers.

Table 1 Hydrocracking of n-Heptane, 17.8 conversion Product Distribution, Mol2 ...

Figure 4. Carbon number distribution in products of hydrocracking of n-heptane. 3 he same experiment as Figure 3 (i h lime on stream).

Hydroisomerization and hydrocracking of n-heptane on PtH zeolites. Effect of the porosity and of the distribution of metallic and acid sites Catalysis Today 1, 415-433. 

Thermodynamics. The reaction of hydrocracking of n-heptane, for example, is the following ... 

Various relatively crude lumped reaction schemes were presented in the literature. Vazquez et al (1988) described the hydrocracking of n-heptane in terms of a set of parallel reactions. 

Vazquez M.I., Escardino A. and Ancejo A. (1988) Hydrocracking of n-Heptane with a NiO-MoOj/HY Ultrastable Zeolite as Catalyst. The Network of the Reaction , Ind. Eng. Chem. Res. 27, 2039-2043. 

Beltramini and Trimm (67) utilized Pt-, Sn- and Pt-Sn- supported on y-alumina for the conversion of n-heptane at 500°C and 5 bar. They observed that during six hours less coke per mole of heptane converted was deposited on the Pt-Sn-alumina catalyst than on Pt-alumina however, the total amount of coke formed during six hours was much greater on Pt-Sn-alumina than on Pt-alumina. The addition of tin increased the selectivity of dehydrocyclization. Since hydrocracking and isomerization activity of a Sn-alumina catalyst remained high in spite of coke formation, the authors concluded that there was little support for the suggestion that tin poisons most of the acid sites on the catalyst. These authors (68) also measured activity, selectivity and coking over a number of alumina supported catalysts Pt, Pt-Re, Pt-Ir, Pt-Sn and Pt-... 

When the same Pt-Re-S/Al202 catalyst was coked in laboratory, the fraction of coke on the metallic function was higher than when the catalyst was coked in the commercial unit, as shown in Figure 2, curves C and A. For this reason, the metal function is deactivated more extensively in the case of laboratory coking the residual activity of the reactions controlled by the metal function is smaller and the initial drop of activity in n-pentane hydrocracking and n-heptane dehydrocyclization is greater. 

Dehydrocyclization is highly desirable but, unfortunately, is the least favorable kinetically (relative activity = 1), and thermodynamically favored at high temperatures. Because of their low relative activities, hydrocracking and dehydrocyclization do not operate near equilibrium. The equilibrium of dehydrocyclization is illustrated by the stability diagram in Figure 6.9.3 for the example of n-heptane conversion into toluene (+4H2)- The equilibrium constant is given by ... 

For the hydroconversion of -heptane and n-decane, the group of Jacobs [96,165-169] reported a hnear relation between the rates of hydroisomerization normalized to the concentration of tetrahedral A1 and the concentration of pentacoordinated and tetrahedrally distorted A1 (analyzed by Al MAS NMR) [166]. This emphasizes that the interaction of extraframework aluminum species with Bronsted acid sites creates more active sites. The work by Blomsma et al. [96,165,167] shows that hydroconversion of n-heptane over Pd/H-Beta zeolites is a combination of classic bifimctional hydrocracking and cracking of dimerized C7 species. 

The first suggestions concerning the mechanism of catalytic reforming were based on studies with hydrocarbon mixtures that permitted only observation of composition changes.91,98,120 It was observed, for example, that about 30% of the Ci—C4 product consisted of methane and ethane. These, however, are not common products in catalytic cracking processes. In fact, when n-heptane was hydrocracked, less methane and ethane were formed than expected, according to the stoichiometry of Eqs. (2.15) and (2.16). Therefore, C5 and C6 hydrocarbons were not considered... 

In view of the complicated reaction kinetics of multicomponent systems, it was not clear whether or not the diffusional effects would also affect the relative rate of conversion of feed molecules in a mixture. To answer this question we studied the hydrocracking of three multicomponent systems. The first was a C5-C8 mixture, a C5 360° C boiling range midcontinent reformate which contained 12.5 wt % n-paraffins including 4.2% n-pentane, 4.3% n-hexane, 2.9% n-heptane, l.l%n-octane, and <1% C9+ n-paraffins, with the remainder isoparaffins and aromatics. The reaction was carried out at 400 psig, 2 H2/HC, 2 LHSV, and 800°F. Secondly, a Cg-Cie mixture... 

Figure 5 shows the relative catalytic activities for n-heptane dehydrocyclization to toluene and for dehydrogenation of cyclohexane. On this catalyst, dehydrocyclization of paraffins can be produced simultaneously by a monofunctional metallic mechanism and a bifunctional one controlled by the acid function (refs. 16-18). The deactivation produced by a small coke deposition should correspond to the deactivation of the contribution of the metallic mechanism to dehydrocyclization. This linear deactivation for the rest of the coke deposition should correspond to the deactivation of the contribution of the bifunctional acid controlled mechanism. The decrease in dehydrocyclization observed during the lineout period is smaller than the decrease in gas formation (by hydrocracking-hydrogenoly5is)i therefore, the selectivity to aromatic hydrocarbons increases during this period. 

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