Page 87 - Fluid Catalytic Cracking Handbook

64 Fluid Catalytic Cracking Handbook

In every equation, nickel is the most active. These indices convert
all metals to a common basis, generally either vanadium or nickel.
Metals are most active when they first deposit on the catalyst. With
time, they lose their initial effectiveness through continuous oxidation-
reduction cycles. On average, about one third of the nickel on the
equilibrium catalyst will have the activity to promote dehydrogena-
tion reactions,
A small amount of nickel in the FCC feed has a significant influence
on the unit operation. In a "clean" gas oil operation, the hydrogen yield
is about 40 standard cubic feet (scf) per barrel of feed (0.07 wt%).
This is a manageable rate that most units can handle. If the nickel
level increases to 1.5 ppm, the hydrogen yield increases up to 100 scf
per barrel (0.17 wt%). Note that in a 50,000 barrel/day unit, this
corresponds to a mere 16 pounds per day of nickel. Unless the catalyst
addition rate is increased or the nickel in the feed is passivated (see
Chapter 3), the feed rate or conversion may need to be reduced. The
wet gas will become lean and may limit the pumping capacity of
the wet gas compressor.
In most units, the increase in hydrogen make does not increase coke
yield; the coke yield in a cat cracker is constant (Chapter 5). The coke
yield does not go up because other unit constraints, such as the
regenerator temperature and/or wet gas compressor, force the operator
to reduce charge or severity. High hydrogen yield also affects the
recovery of C 3+ components in the gas plant. Hydrogen works as an
inert and changes the liquid-vapor ratio in the absorbers.
On a wt% basis, the increase in hydrogen is negligible, but the sharp
increase in gas volume impacts unit performance,
Catalyst composition and feed chloride have a noticeable impact on
hydrogen yield. Catalysts with an active alumina matrix tend to
increase the dehydrogenation reactions. Chlorides in the feed reactivate
aged nickel, resulting in high hydrogen yield.
Two common indicators track the effects of nickel on the catalyst.
These are:
* Hydrogen/methane ratio
* Volume of hydrogen per barrel of feed
The H 2/CH 4 ratio is an indicator of dehydrogenation reactions.
However, the ratio is sensitive to the reactor temperature and the type
of catalyst. A better indicator of nickel activity is the volume of