Study On Catalytic Cracking Of Paraffin For Light Olefin Production

Compared with the conventional steam cracking process, which was dominated by thermal cracking reactions, catalytic cracking of naphtha is advantangeous in lower energy consumption and carbon dioxide emission, higher selectivity to high-valued-compound products. Consequently, it is a competitive alternative for light olefin production. Paraffin, as main constituent of straight-run naphtha, is a perfect feedstock for steam cracking process of light olefin production. However, it is difficult to crack under mild conditions, posing a difficulty for the catalytic cracking process. This thesis focuses on difficulty confronted by catalytic cracking of paraffin; n-heptane was selected as model compound and series of related research work were performed.Protolytic cracking route, based on formation of penta-coordinated carbonium, played an important role in the initiation step during catalytic cracking of n-heptane over HZSM-5 catalyst. It caused that C3/C4 molar ratio was higher than 1 and the ratio of 1-heptene cracking with similar conversion. Over fresh catalyst, high conversion was achieved, accompanied by low light olefin selectivity, due to occurrences of hydrogen transfer reactions. After hydrothermal treatment, surface acidity decreased, especially the strong Bronsted acid sites, thus conversion of n-heptane declined significantly, light olefin selectivity was improved.The paper proposed to introduce reducible metal oxide containing lattice oxygen into HZSM-5 equlibrium catalyst, to improve reactivity of paraffin. This method can overcome the contradiction between conversion and light olefin selectivity brought by adjusting catalyst acidity. V₂O₅/Al₂O₃ was selected as catalyst for providing lattice oxygen; conversion of n-heptane, relative selectivity of propylene plus butylene to ethylene, and content of i-butylene in butylenes could be improved by V₂O₅/Al₂O₃; in fixed bed reactor, conversion of n-heptane could be improved by 30%, and propylene yield was about 4 percentages higher; in circulating fluidized bed unit, conversion could be improved by about 90%, and propylene yield was about 8 percentages higher.Influences of V₂O₅/Al₂O₃ on initiation reaction of n-heptane were investigated in a micro fixed bed reactor chromatography unit by pulse injection. The results demonstrated that V₂O₅/Al₂O₃ introduction improved reaction rate of n-heptane over equilibrium HZSM-5 catalyst and changed initial product selectivities, its presence provided another route for initiation reaction of n-heptane cracking.In fixed bed reactor, active sites of V₂O₅/Al₂O₃ and possible reaction mechanism were studied by designing different reacton modes and catalyst characterization. During the reaction, part of the V⁵⁺ on the surface of V₂O₅/Al₂O₃ was reduced, and lattice oxygen participated into the reaction, leading to formation of CO and H2O product. In continuous reaction, surface lattice oxygen was consumed and the promotion effects disappeared progressively, its participation behaviors were directly responsible for the influences brought by V₂O₅/Al₂O₃; role of lattice oxygen was further confirmed by recovery of V₂O₅/Al₂O₃ activity after oxidation. In the catalyst system composed of HZSM-5 catalyst and V₂O₅/Al₂O₃, the former remained primary active sites for n-heptane cracking; V₂O₅/Al₂O₃ functioned as lattice oxygen supplier, it promoted initial reaction rate and improved reactivity of paraffin. Under coactions of the two catalysts, it was favorable for n-heptane to interact with V₂O₅/Al₂O₃ and generate some active intermediate species before cracking over acidic sites in HZSM-5 catalyst; the cracked products could further promote cracking of other n-heptane molecules by chain transfer reactions.