Coker naphtha to an FCCU reactor riser is bascially uncrackable.
We would expect no more than 5% of the total amount of this stream to crack
and that is if it is added neat to the Wye/J-bend lift. If it is added as a blend
component of the feed through the feed injectors very little (<1%) is expected
to crack.
The most useful reason to employ a material such as Coker naphtha is to replace lift steam
at the base of the riser. It provides the lift for making the transition from the regen cat
standpipe thru the wye (or J-bend) into the reactor riser without having the negative thermal
degradation effects on the catalyst that steam imparts. It also provides some Nickel (Ni)
passivation by sulphiding the Ni. However, the coker naphtha does consume additional
capacity/loading within the Vapor Recovery Unit (VRU. aka the GRU Gas Recovery Unit,
mostly in the de-butanizer and naphtha splitter, if one is present).
If ZSM-5 is employed, a fair percentage of the C7, C8, and C9 olefins present in the
coker naphtha, will crack to LPG. The amount is dependent on the amount/concentration of the
ZSM-5 additive present in the inventory.
Almost all of the sulphur present in the coker naphtha will pass through into the FCC
stream, which is another good reason not to put it into the FCC.