When the process objective is maximum propylene production, specific technology features must be added to the fluid catalytic cracking (FCC)/resid FCC (RFCC) unit. The challenge is particularly great when the feedstock contains residue.
Description: ZSM-5 additive is able to crack only C7 to C10 olefins to LPG. Consequently, most of the C5 and C6 cut are not converted by ZSM-5 in the main riser. To convert this cut, it has been published by IFP and others that the optimum catalytic system is a recycle in a separate riser operating under more severe conditions—a PetroRiser.
Indeed, recycling with the feed does not allow converting this light naphtha since the temperature is too low in the main riser. If the naphtha recycle is injected before the feed zone where the catalyst temperature is above 700°C, production of fuel gas is very high due to thermal cracking as well as detrimental side reactions specific to this thermal level. In addition, injecting light naphtha below the main feed alters the riser conditions at the point of injection of the main resid feed resulting in less than optimum performance.
The conclusion of the R&D work is that recycling light naphtha to a separate riser at a temperature higher than the main riser allows cracking C5 and C6 olefins and also enables paraffins to produce more LPG and less C5–70°C naphtha.
An additional feedstock for propylene production is the indirect recycle of C4 olefins. As with light naphtha, the C4 olefins will not crack in the main riser, and a simple recycle to the PetroRiser will result in nonselective conversion of C4 olefins. The easiest and most selective way to recycle crack the C4 olefins into propylene is to use the benefit of a C4 oligomerization unit (Polynaphtha) to produce longer olefins (C8 and C12 olefins). These longer chain olefins will crack very selectively in the PetroRiser, thus producing more propylene as well as good quality gasoline. This integration is called FlexEne and presented in more details in a dedicated paragraph of the handbook.