Catalytic cracking of heavy atmospheric gas oil to light olefins over ZSM-5 zeolite: Effect of crystal size in the absence/presence of steam

Abstract

The catalytic cracking of heavy atmospheric gas oil (AGO) has been investigated to study the effect of ZSM-5 zeolite crystal size on the selectivity to light olefin in the absence and presence of steam. Dodecane (used as model compound) and heavy AGO feed streams were utilized to study the influence of changing pore shape selectivity when comparing cracking performance by nano and micro ZSM-5 zeolites. The zeolites were characterized using BET, XRD NMR, NH3-TPD and Py-FTIR. In the presence of steam, the short pores length nano ZSM-5 crystals showed enhanced catalytic cracking activity and selectivity to light olefins. The nano ZSM-5 catalyst promoted higher selectivity to light olefins through β-scission while reducing the extent of hydrogenation reactions towards the formation of liquefied petroleum gas components (LPG). In contrast, the long pores length of micro ZSM-5 tended to favor the production of more naphtha, kerosene and LPG through hydrogen transfer routes, resulting in lower selectivity to light olefins. The resulting propylene/ethylene (P/E) ratios were altered by the type of feed used, cracking reaction temperature, influence of steam and catalyst crystal size. Specifically, higher P/E ratio was achieved over nano ZSM-5 zeolite, especially with the cracking of heavy AGO feed in the presence of steam, due to an increase of monomolecular reactions through β-scission cracking mechanism.