(85c) Impact of Recycling Light Olefins to the FCC

Recycling light olefin product streams back to a fluid catalytic cracking (FCC) unit has potential applications in US refineries due to the growing level of Shale Oils in the crude slate as well as in maximum propylene units such as many built in Asia.  In refineries that process high levels of Shale Oil, light naphtha streams may be a potential target for recycle to obtain more valuable C4 olefins or possibly heavier gasoline range compounds.  The light naphtha products from the FCC such as C5 compounds have relatively low value due to the abundance of these hydrocarbons in the crude and RVP requirements for gasoline.  In the case of max propylene units, C4 olefins have been proposed for recycle to further maximize the production of more desirable propylene through either oligomerization and re-cracking or shifting the light olefin equilibrium distribution.  The goal of this work is to determine how product yields shift when light olefins are recycled back to the FCC under different operating conditions.  To this end, propylene, 1-butene, and 1-pentene were each co-fed with a Mid-Continent vacuum gasoil into a circulating FCC pilot unit to study the impact on the process and the resulting product distribution.  The experiments were performed using a commercial high matrix FCC catalyst alone and with varying amounts of ZSM-5 additive to simulate a max propylene operation.   In addition, the reactor outlet temperature was changed during the course of the study to evaluate the effect of operating conditions.  Incremental olefin yields as a function of olefin type, reactor temperature and ZSM-5 content are reported.  These results help to understand the potential impact on the FCC process and resulting products from different olefin recycling schemes.