Principle of catalytic cracking reaction

The principle of catalytic cracking is different from that of thermal cracking. The thermal cracking of hydrocarbons is carried out according to the free group principle, while the catalytic cracking is carried out according to the carbon ion reaction principle. C-C bond homolysis occurs during thermal cracking:

                                                        C:C-→C· +C·   (1) 

In catalytic cracking, the C-C bond is heterocracked under the action of a catalyst to generate ions:

                                                        C:C-→C++C:-   (2) 

The raw oil used in catalytic cracking is composed of alkanes, alkenes and aromatics, so the main reaction is:

Alkane cracking:

                                                      C„H2n+2→CmH2m+CpH2p+2   (3) 

Among them, n=m+p.

Olefin cracking:

                                                     C„H2n→CmH2m+CpH2p  (4) 

Aromatics cracking:

                                                    ArC„H₂n+1 -→ ArH + CnH2n   (5) 

In the process of catalytic cracking, side reactions such as isomerization, hydrogen transfer, aromatization, alkylation, superposition and polycondensation also occur obviously. The last three side reactions will cause coking of catalyst and promote premature deactivation of catalyst.

The product composition of catalytic cracking is about 40%~50% gasoline, 20%~40% diesel, and generates 15%~30% gas hydrocarbon (mainly composed of C3~C4, of which propylene, butene and isobutane account for more than half).