Thread Content
Section 7 Dealkylation of Aromatic Hydrocarbons The dealkylation reaction of toluene is difficult to carry out on acidic centers, and is generally considered to be carried out on metal centers (hydrogenolysis reaction). For alkylbenzenes with three or more carbon atoms in the alkyl side chain, dealkylation (hydrocracking) can also occur at the acidic center. In short, hydrogenolysis and hydrocracking are moderately exothermic reactions, and their thermal effects are approximately -50kJ/mol. Because its equilibrium constant is very large, it can be regarded as an irreversible reaction. In the catalytic reforming process, such reactions will lead to a decrease in liquid product yield and consumption of hydrogen, and are therefore undesirable side reactions. The reaction rate of hydrocracking (γ1) is relatively small. However, in the early stages of operation of the catalytic reforming unit, the activity of the catalyst is relatively high, and hydrogenolysis and hydrocracking reactions tend to occur more easily. Section 8 Coke Reaction During the catalytic reforming reaction, deep dehydrogenation of hydrocarbons will produce olefins, diolefins and condensed ring aromatic hydrocarbons, which will be firmly adsorbed on the catalyst surface and further dehydrogenated and condensed into coke, deactivating the catalyst. In addition to propylbenzene, five-membered cycloalkanes are also a class of substances that can easily generate coke. It is generally believed that cyclopentane and its dehydrogenation products cyclopentene and cyclopentadiene are important precursors of coke. In production, appropriate operating conditions must be selected to inhibit the coking reaction and extend the use time of the catalyst. To sum up, it can be seen that the dehydrogenation of six-membered cycloalkanes is the most basic reaction in catalytic reforming. It has the largest equilibrium constant and the largest reaction rate.: Although the equilibrium constant for the isomerization of five-membered cycloalkanes into six-membered cycloalkanes is very small and the reaction rate is also small, a very high conversion rate can still be achieved because six-membered cycloalkanes are easily dehydrogenated into aromatics once formed. ； Although the equilibrium constant of the alkane dehydrocyclization reaction is large, its reaction rate is very small, so its actual conversion rate is not very high. In terms of the thermal effects of the reaction, dehydrogenation of six-membered cycloalkanes and dehydrogenation of alkanes are both strongly endothermic reactions, isomerization is a mild exothermic reaction, and hydrocracking is a moderately exothermic reaction. In summary, catalytic reforming is a strongly endothermic reaction.