Page 232 - Fluid Catalytic Cracking Handbook

200 Fluid Catalytic Cracking Handbook

BMC! is a function of gravity and midpoint temperature. To achieve
a BMCI of 120, the DO's °API gravity should not exceed 2.0. °API
gravity is a rough indication of aromaticity; the lower the gravity, the
higher the aromaticity.
The ash content of the DO is affected by the reactor cyclone's
performance and catalyst physical properties. To meet the CBFS' ash
requirement (maximum of 0.05 wt%), DO product may need to be
filtered for the removal of the catalyst fines.
Coke

In a cat cracker, a portion of the feed, mostly from secondary
cracking and polymerization reactions, is deposited on the catalyst as
coke. Coke formation is a necessary byproduct of the FCC operation;
the heat released from burning coke in the regenerator supplies the
heat for the reaction.
The coke structure and the chemistry of its formation are difficult
to define. However, coke in FCC comes from at least four sources:

• Catalytic coke is a byproduct of the cracking of FCC feed to
lighter products. Its yield is a function of conversion, catalyst
type, and hydrocarbon/catalyst residence time in the reactor.
• Contaminant coke is produced by catalytic activity of metals such
as nickel, vanadium, and by deactivation of the catalyst caused
by organic nitrogen.
« Feed residue coke is the small portion of the (non-residue) feed
that is directly deposited on the catalyst. This coke comes from
the very heavy fraction of the feed and its yield is predicted by
the Conradson or Ramsbottom carbon tests.
• Catalyst circulation coke is a "hydrogen-rich" coke from the
reactor-stripper. Efficiency of catalyst stripping and catalyst pore
size distribution affect the amount of hydrocarbons carried over
into the regenerator.
A proposed equation [1] to express coke yield is:

n n s (ABc/RTrx)
Coke yield, wt% = g(Z,, . . . Z N) x (C/O) x (WHSV) ~ x [ e ]
Where:
g(Z,, . , .) = function of feed quality, hydrocarbon partial pressure, catalyst
type, , etc.