Aromatics, hydrocracking reactions

Catalytic reformers are normally designed to have a series of catalyst beds (typically three beds). The first bed usually contains less catalyst than the other beds. This arrangement is important because the dehydrogenation of naphthenes to aromatics can reach equilibrium faster than the other reforming reactions. Dehydrocyclization is a slower reaction and may only reach equilibrium at the exit of the third reactor. Isomerization and hydrocracking reactions are slow. They have low equilibrium constants and may not reach equilibrium before exiting the reactor. 

Much progress has been made in understanding the catalytic activity of zeolites for several type of reactions. The number of reactions catalyzed by zeolites has been extended, and new multi-component polyfunctional catalysts with specific properties have been developed. In addition to cracking and hydrocracking, reactions such as n-alkane isomerization, low temperature isomerization of aromatic C8 hydrocarbons, and disproportionation of toluene are industrially performed over zeolite-containing catalysts. Moreover, introduction of various compounds (C02, HCl) into reaction mixtures allows one to control the intensity and selectivity of the reactions. There are many reviews on the catalytic behavior of zeolites and even more original papers and patents. This review emphasizes the results, achievements, and trends which we consider to be most important. 

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... 

The IFP hydrocracking process features a dual catalyst system the first catalyst is a promoted nickel-molybdenum amorphous catalyst. It acts to remove sulfur and nitrogen and hydrogenate aromatic rings. The second catalyst is a zeolite that finishes the hydrogenation and promotes the hydrocracking reaction. 

Pressure. Pressure affects dehydrogenation and hydrocracking reactions. Increasing the pressure will increase hydrocracking but adversely affect equilibrium aromatic... 

Naphtha reforming catalysts are mostly based on metals (Pt, Pt-Re, Pt-lr, Pt-Sn, Pt-Re-Sn) supported on chlorinated-ALOs or on a KL zeolite. Non-acidic KL zeolite in combination with Pt has been applied in a new reforming process. The non-acidic zeolite support inhibits undcsircd isomerization and hydrocracking reactions leading to enhanced aromalization selectivities [69]. Besides the absence of acidity, the presence of highly dispersed Pt clusters inside the zeolite channels and the shape-selective effects imposed by the monodirectional channel structure (0.71 nm diameter) of the zeolite may also contribute to the excellent aromatization performance of Pt/KL catalysts. 

Perovskites such as SrTi03 have been proposed as selective hydrogenating materials in the hydrocracking reactions of polynuclear aromatics... 

When dehydrogenation was carried out in a stream of hydrogen at 325°C, the yield of aromatic hydrocarbons formed in the dehydrogenation of cyclohexane and cyclohexene derivatives was 87—99%. As a result of side-reactions an insignificant amount of dealkylation products (benzene, toluene) was also formed. Decalin was most difficult to dehydrogenate, and in this instance, together with naphthalene, tetralin was also formed. Under these conditions isomerization of cyclopentane derivatives into cyclohexane hydrocarbons did not take place, and aromatic hydrocarbons were not formed from cyclopentane hydrocarbons. The process, however, was complicated by hydrocracking reactions. 

Effect on Gasoline Properties. The hydrocracking reaction is the C—C bond breaking catalyzed by the acid function. In this reaction, either a paraffin molecule is broken into two molecules of lower molecular weight, or a naphthene ring is opened. The aromatics are very difficult to be hydrocracked under normal reforming conditions. 

A significant amount of heat is released in the hydrogenation of aromatics/ naphtheno-aromatics and in the hydrocracking reactions. The liquid phase temperature profile inside the reactor beds is obtained from... 

Figure 15. Hydrocracking reaction chain for poly aromatic and naphtheno-aromatic compounds, per Filimononv, et al. The relative rates of Reactions 1 to 12 are shown in Table 6.

It is known that, when heavy feedstocks such as heavy vacuum gas oil are processed simultaneously with all the hydrocracking reactions, heavy polynuclear aromatics are formed. These compounds are concentrated in the heaviest fraction of the effluent they have a detrimental effect on the activity of the catalyst and on its life. For an operation with recycling for complete conversion, these heavy polynuclear aromatics accumulate in the liquid recycle stream causing fouling and rapid deactivation of the catalyst. A solution often practised is to accept a slight reduction in conversion by purging part of ftie liquid recycle stream catalyst activity and stability are thereby improved. 

As reaction temperature (WAIT) increases, the yield of aromatic components increases significantly. However, at higher temperatures (greater than 520 °C), the H2HC ratio is not sufficient to prevent undesired hydrocracking reactions. 

Ring dealkylation reaction 2 Ring open reaction 3 Aromatic satjration 4. Paraffin hydrocracking reaction P paraffins N naphthenes A aromatics AN ring compound which has naphthene and aromatics... 

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