89
5.4.4 Metal Passivators
The various mechanisms by which the FCC catalyst loses its activity were discussed
earlier. Deactivation of FCC catalyst over a period of time during operation affects
the overall activity and selectivity toward the desired products. Deactivation of FCC
catalysts can be classified as reversible and irreversible. Reversible deactivation is
contributed by the organic nitrogen compounds in the hydrocarbon feed, which
reduce the acidity of the catalyst and, also because of heavy polycyclic aromatic
compounds, get adsorbed on the catalyst and are unable to escape the lattice and
eventually end up as coke [59, 60]. Irreversible deactivation is caused by three
mechanisms. The turbulent hydrothermal conditions of the FCC regenerator contribute to attrition of the catalyst causing loss of material in the form of fines, which
escapes the unit as part of flue gas. In addition to this, the loss of porosity and surface area due to steam also affect the catalyst activity. Zeolites, in particular, are the
most susceptible to such hydrothermal deactivation.
Metallic compounds (particularly Ni and V) better known as catalyst poisons
present in the feed get adsorbed on to the FCC catalyst’s pores as nonstrippable and
nonregenerable complexes. Vanadium contaminants destroy the zeolite structure of
the catalyst, and both nickel and vanadium contaminants promote the undesirable
hydrogenation reactions in the FCC unit, which increases the yield of dry gas. In the
oxygen-rich environment of the FCC regenerator, the vanadium compounds
adsorbed on the catalyst (typically in an oxidation state of +3) get oxidized. This
oxidizes vanadium to +5 state and reacts to form vanadic acid (H 3 VO 4 ); at this state,
vanadium has high intraparticle mobility and can easily penetrate the catalyst particle and destroy the zeolite structures. This leads to a permanent reduction of surface area and pore clogging of the catalyst. A fixed amount of fresh catalyst addition
rate is maintained continuously or at intervals to account for this loss of activity
coupled with off-loading a portion of equilibrium catalyst. Unfortunately, the metal
level on FCC catalysts is seldom in equilibrium, and catalyst deactivation by vanadium does not take place in isolation, but combined with and influenced by hydrothermal deactivation [61–64]. The vanadium species at its elevated oxidation state
hence cannot be completely phased out without the use of dedicated metal passivators [65].
The incorporation of metal passivators in the feed has become the norm due to
trends such as high regenerator temperature and low API hydrocarbon feed of resid
nature [66]. Phillips Petroleum Company discovered and developed the antimony
metal passivation process in the early 1970s and successfully applied the process at
its Borger, Texas heavy oil cracker (HOC) in 1976 [67]. Hydrogen and coke yields
were significantly lower, and gasoline yield increased. FCC units with limited gas
handling capacity (wet gas compressor and main air blower) will benefit immensely
due to the reduction in gas formation and less coking. Other benefits include higher
conversion and throughput, as well as improved resid upgradation (Fig. 17).
Incorporation of metal passivators in any FCC unit is unique and tailor-made
with respect to hydrocarbon feed and operating parameters. Models and laboratory
testing are often used to predict yields that are often used to optimize passivation
Recent Developments in FCC Process and Catalysts
5.4.4 Metal Passivators
The various mechanisms by which the FCC catalyst loses its activity were discussed
earlier. Deactivation of FCC catalyst over a period of time during operation affects
the overall activity and selectivity toward the desired products. Deactivation of FCC
catalysts can be classified as reversible and irreversible. Reversible deactivation is
contributed by the organic nitrogen compounds in the hydrocarbon feed, which
reduce the acidity of the catalyst and, also because of heavy polycyclic aromatic
compounds, get adsorbed on the catalyst and are unable to escape the lattice and
eventually end up as coke [59, 60]. Irreversible deactivation is caused by three
mechanisms. The turbulent hydrothermal conditions of the FCC regenerator contribute to attrition of the catalyst causing loss of material in the form of fines, which
escapes the unit as part of flue gas. In addition to this, the loss of porosity and surface area due to steam also affect the catalyst activity. Zeolites, in particular, are the
most susceptible to such hydrothermal deactivation.
Metallic compounds (particularly Ni and V) better known as catalyst poisons
present in the feed get adsorbed on to the FCC catalyst’s pores as nonstrippable and
nonregenerable complexes. Vanadium contaminants destroy the zeolite structure of
the catalyst, and both nickel and vanadium contaminants promote the undesirable
hydrogenation reactions in the FCC unit, which increases the yield of dry gas. In the
oxygen-rich environment of the FCC regenerator, the vanadium compounds
adsorbed on the catalyst (typically in an oxidation state of +3) get oxidized. This
oxidizes vanadium to +5 state and reacts to form vanadic acid (H 3 VO 4 ); at this state,
vanadium has high intraparticle mobility and can easily penetrate the catalyst particle and destroy the zeolite structures. This leads to a permanent reduction of surface area and pore clogging of the catalyst. A fixed amount of fresh catalyst addition
rate is maintained continuously or at intervals to account for this loss of activity
coupled with off-loading a portion of equilibrium catalyst. Unfortunately, the metal
level on FCC catalysts is seldom in equilibrium, and catalyst deactivation by vanadium does not take place in isolation, but combined with and influenced by hydrothermal deactivation [61–64]. The vanadium species at its elevated oxidation state
hence cannot be completely phased out without the use of dedicated metal passivators [65].
The incorporation of metal passivators in the feed has become the norm due to
trends such as high regenerator temperature and low API hydrocarbon feed of resid
nature [66]. Phillips Petroleum Company discovered and developed the antimony
metal passivation process in the early 1970s and successfully applied the process at
its Borger, Texas heavy oil cracker (HOC) in 1976 [67]. Hydrogen and coke yields
were significantly lower, and gasoline yield increased. FCC units with limited gas
handling capacity (wet gas compressor and main air blower) will benefit immensely
due to the reduction in gas formation and less coking. Other benefits include higher
conversion and throughput, as well as improved resid upgradation (Fig. 17).
Incorporation of metal passivators in any FCC unit is unique and tailor-made
with respect to hydrocarbon feed and operating parameters. Models and laboratory
testing are often used to predict yields that are often used to optimize passivation
Recent Developments in FCC Process and Catalysts