ABSTRACT
Because of the carbenium ion mechanism, several features are observed in the reactions in FCC. The fractions of methane, ethane, and ethylene are small in the products. For the formation of these products, the reactions should go through primary cabenium ions, which are energetically unstable. Ethylene is produced by thermal cracking in the current industry. Hydrocarbons containing less than six carbon atoms hardly undergo cracking, which would form primary carbenium ions. Linear-chained alkanes do not crack directly but isomerize first to branched hydrocarbons and crack, which avoids going through primary carbenium ion intermediates. Among the reactions occurring simultaneously with the b-scission, hydrogen transfer is an important reaction that exerts a strong influence on the product distribution and coke formation. Examples of hydrogen transfer are as follows: 4 CnH2n 3 CnH2n + 2 + CnH2n – 6 5 butene 2 xylene + 2 butane benzene + 3 butene 6 C (coke) + 3 butaneThese hydrogen transfer reactions are composed of repeated occurrences of hydride transfer and proton transfer in alternation. Proton transfers from carbenium ion either to alkenes to form carbenium ion or to catalyst surface to restore acidic site. For butene to p-xylene and butane, hydride transfer and proton transfer occur three times after dimerization to isooctane, cyclization and ring expansion being also included as shown below. As a result of hydrogen transfer in general, alkenes decreases and alkanes, aromatics and coke increase.
