Page 86 - Fluid Catalytic Cracking Handbook

FCC Feed Characterization S3

which is the formation of "chicken wire" aromatic molecules. Hydrogen
and coke yields are increased and gasoline yields are reduced. Metals
reduce the catalyst's ability to produce the desired products.
These metals permanently poison the FCC catalyst by lowering the
catalyst activity, thereby reducing its ability to produce the desired
products. Virtually all the rnetals in the FCC feed are deposited on
the cracking catalyst. Paraffinic feeds tend to contain more nickel than
vanadium. Each metal has negative effects.

Nickel (Ni)

As discussed in Chapter 3, an FCC catalyst has two parts:
« The non-framework structure called matrix
• The crystalline structure called zeolite
In contact with the catalyst, nickel deposits on the matrix. Nickel
promotes dehydrogenation reactions, removing hydrogen from stable
compounds and making unstable olefins, which can polymerize to
heavy hydrocarbons. These reactions result in high hydrogen and coke
yields. The higher coke causes higher regenerator temperatures. This
lowers the catalyst-to-oil ratio and lowers conversion.
High nickel levels are normally encountered when processing heavy
feed. Neither excess hydrogen nor excess regenerator temperature is
desirable. Excess hydrogen lowers the molecular weight of the wet gas;
since the compressor is usually centrifugal, this limits the discharge
pressure. Lower pressure means less capacity and this can force a
reduction in charge or operation at lower conversion.
A number of indices relate metal activity to hydrogen and coke
production. (These indices predate the use of metal passivation in the
FCC process but are still reliable). The most commonly used index is
4 x Nickel + Vanadium. This indicates that nickel is four times as
active as vanadium in producing hydrogen. Other indices [9] used are:

Jersey Nickel Equivalent Index = 1,000 x (Ni + 0.2 x V + 0.1 x Fe)

Shell Contamination Index = 1,000 x (14 x Ni + 14 x Cu + 4 x V + Fe)
V
Davison Index = Ni + Cu H—
4
V
Mobil = Ni + —
4