Page 233 - Fluid Catalytic Cracking Handbook

Products and Economics 201

n = 0.65
C/O = cat-to-oil ratio
WHS V = weight of hourly space velocity, weight of total feed/hr divided
by weight of catalyst inventory in reaction zone, hr -1
= activation energy ~ 2,500 Btu/lb-mole (5828 J/G - mole)
A EC
R = gas constant, 1.987 Btu/lb-mole-°R (8.314 J/G- mole °K)
= reactor temperature, °R
T RX
The coke yield of a given cat cracker is essentially constant. The
FCC produces enough coke to satisfy the heat balance. However,
a more important term is delta coke. Delta coke is the difference
between the coke on the spent catalyst and the coke on the regenerated
catalyst. At a given reactor temperature and constant CO 2/CO ratio,
delta coke controls the regenerator temperature.
Reducing delta coke will lower the regenerator temperature. Many
benefits are associated with a lower regenerator temperature. The
resulting higher cat/oil ratio improves product selectivity and/or
provides the flexibility to process heavier feeds.
Many factors influence delta coke, including quality of the FCC
feedstock, design of the feed/catalyst injection system, riser design,
operating conditions, and catalyst type. The following is a brief
discussion of these factors:

* Feedstock quality. The quality of the FCC feedstock impacts the
concentration of coke on the catalyst entering the regenerator. A
"heavier" feed containing a higher concentration of coker gas oil
will directionally increase the delta coke as compared with a
"lighter," resid-free feedstock.
* Feed/catalyst injection. A well-designed injection system provides
a rapid and uniform vaporization of the liquid feed. This will
lower delta coke by minimizing non-catalytic coke deposition as
well as reducing the deposits of heavy material on the catalyst.
• Riser design. A properly designed riser will help reduce delta coke
by reducing the back-mixing of already "coked-up" catalyst with
fresh feed. The back-mixing causes unwanted secondary reactions.
• Cat/oil ratio. An increase in the cat/oil ratio reduces delta coke
by spreading out some coke-producing feed components over
more catalyst particles and, thus, lowering the concentration of
coke on each particle.