Page 147 - Fluid Catalytic Cracking Handbook

122 Fluid Catalytic Cracking Handbook

Metal Passivation

As discussed in Chapter 2, nickel, vanadium, and sodium are the
metal compounds usually present in the FCC feedstock. These metals
deposit on the catalyst, thus poisoning the catalyst active sites. Some
of the options available to refiners for reducing the effect of metals
on catalyst activity are as follows:
• Increasing the fresh catalyst makeup rate
• Using outside E-cat
• Employing metal passivators
• Incorporating metal trap into the FCC catalyst
• Using demetalizing technology to remove the metals from the catalyst
• The MagnaCat separation process (demetalizing technology),
which allows discarding the "older" catalyst particles containing
higher metal levels

Metal passivation in general, and antimony in particular, are discussed
in the following section.
In recent years, several methods have been patented for chemical
passivation of nickel and vanadium. Only some of the tin compounds
have had limited commercial success in passivating vanadium. Although
tin has been used by some refiners, it has not been proven or as widely
accepted as antimony. In the case of nickel, antimony-based com-
pounds have been most effective in reducing the detrimental effects
of nickel poisoning. It should be noted that, although the existing
antimony-based technology is the most effective method of reducing
the deleterious effects of nickel, the antimony is fugitive and can be
considered hazardous. In this case, a bismuth-based passivator may be
a better choice.

Antimony

Antimony-based passivation was introduced by Phillips Petroleum
in 1976 to passivate nickel compounds in the FCC feed. Antimony is
injected into the fresh feed, usually with the help of a carrier such as
light cycle oil. If there are feed preheaters in the unit, antimony should
be injected downstream of the preheater to avoid thermal decomposi-
tion of the antimony solution in the heater tubes.
The effects of antimony passivation are usually immediate. By
forming an alloy with nickel, the dehydrogenation reactions that are