Hydrocracking Reaction

Figure 5.1 Major reactions in catalytic reforming illustrated with specific examples (a) dehydrogenation of cyclohexanes to aromatic hydrocarbons (b) dehydroisomerization of alkylcyclopentanes to aromatic hydrocarbons (c) dehydrocyclization of alkanes to aromatic hydrocarbons (d) isomerization of n -alkanes to branched alkanes (e) fragmentation reactions (hydrocracking and hydrogenolysis) yielding low carbon number alkanes.

Of the main reactions, aromatization takes place most readily and proceeds ca 7 times as fast as the dehydroisomerization reaction and ca 20 times as fast as the dehydrocyclization. Hence, feeds richest in cycloparaftins are most easily reformed. Hydrocracking to yield paraffins having a lower boiling point than feedstock proceeds at about the same rate as dehydrocyclization. 

Magnitudes of /cg, /cp, /c, and indicate the importance of direct reactions with coal, where and are for hydrocracking reactions in the conversion process. Data for and from the experiments with HPO indicate that oil production from coal is increased by the use of a good hydrogen donor solvent. 

All of the above reactions are reversible, with the exception of hydrocracking, so that thermodynamic equilibrium limitations are important considerations. To the extent possible, therefore, operating conditions are selected which will minimize equilibrium restrictions on conversion to aromatics. This conversion is favored at higher temperatures and lower operating pressures. 

Catalytic conversion processes include naphtha catalytic reforming, catalytic cracking, hydrocracking, hydrodealkylation, isomerization, alkylation, and polymerization. In these processes, one or more catalyst is used. A common factor among these processes is that most of the reactions are initiated hy an acid-type catalyst that promotes carhonium ion formation. 

Bond breaking can occur at any position along the hydrocarbon chain. Because the aromatization reactions mentioned earlier produce hydrogen and are favored at high temperatures, some hydrocracking occurs also under these conditions. However, hydrocracking long-chain molecules can produce Ce, C7, and Cg hydrocarbons that are suitable for hydrode-cyclization to aromatics. 

Hydrodealkylation. Hydrodealkylation is a cracking reaction of an aromatic side chain in presence of hydrogen. Like hydrocracking, the... 

As in hydrocracking, this reaction increases the gas yield and changes the relative equilihrium distrihution of the aromatics in favor of benzene. Table 3-7 shows the properties of feed and products from Chevron Rheiniforming process. 

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. 

Other reactions that occur during hydrocracking are the fragmentation followed by hydrogenation (hydrogenolysis) of the complex asphaltenes and heterocyclic compounds normally present in the feeds. 

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