The FCC unit process consists of those seven steps:

① The raw material molecules diffuse to the catalyst in the independent gas flow;

② The raw material molecules close to the catalyst diffuse to the inner surface of the micropore;

③ The raw material molecules close to the catalytic surface are adsorbed by the catalyst;

④ The adsorbed molecules undergo chemical reactions under the action of catalysts;

⑤ The generated product molecules are desorbed from the catalyst;

⑥ The desorbed product molecules diffuse outward from the micropores;

⑦ Product molecules diffuse from the outer surface of the catalyst into the main gas flow. Then leave the reactor.