Study of cracking FCC naphtha in a secondary riser of the FCC unit for maximum propylene production

To satisfy the increasing propylene demand, reprocessing FCC naphtha in a secondary riser of the FCC unit was investigated. To this aim, a full range FCC naphtha was cracked over a mixture of two kinds of commercial equilibrium FCC catalysts, which contained 95 t.% Y zeolite-based catalyst and 5 wt.% ZSM-5 zeolite-based additive. The effects of operating parameters such as reaction temperature (temperature of the riser outlet), catalyst-to-oil ratio and residence time on FCC naphtha cracking were studied in a continuous pilot plant. This work demonstrates that FCC naphtha requires high operating severities to crack, and approximately 12–19 wt.% FCC naphtha can be transformed into propylene. The conversion and yield of propylene showed a rapid increase with increasing reaction temperature, and the increase of catalyst-to-oil ratio also enhanced FCC naphtha cracking, even at high reaction temperature. However, at high catalyst-to-oil reactions, hydrogen-transfer reactions constrain further increases in light olefin yields. At these high operating severities, shortening residence time is an appropriate way to obtain high yields of propylene combined with (i) lower yields of dry gas and (ii) a lower apparent hydrogen-transfer coefficient.