Resumen del autor, extraído del trabajo. Since 1993 the RUHR OEL GmbH has been operating at Gelsenkirchen Horst (Germany) a UOP type side by side FCC unit. In former times the driving force for optimizing the unit was mainly driven by motor gasoline production. But regarding both the market and the more stringent specifications for transportation fuels in Europe, refiners are forced to look for new directions and are forced to be more flexible as well. Due to the increasing propylene demand at Gelsenkirchen, the strategy operating the FCCU has changed towards that direction. The FCCU has become one of the key units regarding the integration within the Refinery/Petrochemical-complex. With the given presentation the author will focus on experiences with both the addition of ZSM-5 catalyst and recycling naphtha to the bottom of the riser in order to extend the propylene production of the unit. The results will be reflected against traditional FCC operation and will be discussed in some detail. Benchmarking the different modes of operation, differences in yield slates and additional catalyst costs have to be taken into account, as well as the flexibility referring to naphtha recycling.