Page 88 - Fluid Catalytic Cracking Handbook

FCC Feed Characterization 85

hydrogen per barrel of fresh feed. The typical H 2/CH 4 mole ratio for
a gas oil having less than 0.5 ppm nickel is between 0.25 to 0.35,
The equivalent H 2 make is between 30 and 40 scf/bbl of feed.
It is usually more accurate to back-calculate the feed metals from
the equilibrium catalyst data than to analyze the feed regularly. If
nickel will be a regular component of the feed, passivators are avail-
able. If nickel affects operation and margins, it is often beneficial to
use antimony to passivate the nickel. This can be attractive if the
nickel on the equilibrium catalyst is greater than 1,000 ppm.,

Vanadium
Vanadium also promotes dehydrogenation reactions, but less than nickel.
Vanadium's contribution to hydrogen yield is 20% to 50% of nickel's
contribution, but vanadium is a more severe poison. Unlike nickel,
vanadium does not stay on the surface of the catalyst. Instead, it migrates
to the inner (zeolite) part of the catalyst and destroys the zeolite crystal
structure. Catalyst surface area and activity are permanently lost.
Vanadium occurs as part of organo-metallic molecules of high
molecular weight. When these heavy molecules are cracked, coke
residue containing vanadium is left on the catalyst. During regenera-
tion, the coke is burned off and vanadium is converted to vanadium
oxides such as vanadium pentoxide (V 2O 5). V 2O 5 melts at 1,274°F
(690°C), which allows it to destroy zeolite under typical regenerator
temperature conditions. V 2O 5 is highly mobile and can go from one
particle to another.
There are several theories about the chemistry of vanadium poison-
ing. The most prominent involves conversion of V 2O 5 to vanadic acid
(H 3VO 4) under regenerator conditions. Vanadic acid, through hydrolysis,
extracts the tetrahedral alumina in the zeolite crystal structure, causing
it to collapse.
The severity of vanadium poisoning depends on the following factors:
1. Vanadium Concentration
In general, vanadium concentrations above 2,000 ppm on the
E-Cat can justify passivation.
2. Regenerator Temperature
Higher regenerator temperatures (>1,250°F or 677°C) exceed the
melting point of vanadium oxides, increasing their mobility. This