521
Catalytic Cracking
naphtha and atmospheric residua. The catalyst is transported from the bottom of the unit to the top
in a gas lift employing compressed flue gas and steam.
The reactor feed and catalyst pass concurrently through the reactor zone to a disengager section, in which vapors are separated and directed to a conventional fractionation system. The spent
catalyst, which has been steam purged of residual oil, flows to the kiln for regeneration, after which
steam and flue gas are used to transport the catalyst to the reactor.
19.3.3.4 Suspensoid Catalytic Cracking
Suspensoid cracking was developed from the thermal cracking process carried out in tube and tank
units (Chapter 18). Small amounts of powdered catalyst or a mixture with the feedstock are pumped
through a cracking coil furnace. Cracking temperatures are 550°C–610°C (1025°F–1130°F), with
pressures of 200–500 psi. After leaving the furnace, the cracked material enters a separator where
the catalyst and high-boiling entrained material (tar) are left behind. The cracked vapors enter a
bubble tower where they are separated into two parts, gas oil and pressure distillate. The latter is
separated into gasoline and gases. The spent catalyst is filtered from the organic carry-over, which
is used as a heavy industrial fuel oil.
The process is actually a compromise between catalytic and thermal cracking. The main effect
of the catalyst is to allow a higher cracking temperature and to assist mechanically in keeping coke
from accumulating on the walls of the tubes. The normal catalyst employed is spent clay (2–10 lb
per barrel of feed) that obtained from the contact filtration of lubricating oils (Chapter 24).
19.4 OPTIONS FOR HEAVY FEEDSTOCKS
The processes described in the following sections are the evolutionary offspring of the fluid catalytic cracking and the residuum catalytic cracking processes. Some of these newer processes use
catalysts with different silica/alumina ratios as acid support of metals such as Mo, Co, Ni, and W.
In general, the first catalyst used to remove metals from oils was the conventional HDS catalyst.
Diverse natural minerals are also used as raw material for elaborating catalysts addressed to
the upgrading of heavy fractions. Among these minerals are clays; manganese nodules; bauxite
activated with vanadium (V), nickel (Ni), chromium (Cr), iron (Fe), and cobalt (Co), as well as
iron laterites, sepiolites; and mineral nickel and transition metal sulfides supported on silica and
alumina. Other kinds of catalysts, such as vanadium sulfide, are generated in situ, possibly in
colloidal states.
In the past decades, in spite of the difficulty of handling heavy feedstocks, residuum fluidized
catalytic cracking (RFCC) has evolved to become a well-established approach for converting a
significant portion of the heavier fractions of the crude barrel into a high-octane gasoline blending component. RFCC, which is an extension of conventional fluid catalytic cracking technology
for applications involving the conversion of highly contaminated residua, has been commercially
proven on feedstocks ranging from gas oil-residuum blends to atmospheric residua, as well as blends
of atmospheric and vacuum residua blends. In addition to high gasoline yields, the RFCC unit also
produces gaseous, distillate and fuel oil-range products.
The product quality from the residuum fluidized catalytic cracker is directly affected by its feedstock quality. In particular, and unlike hydrotreating, the RFCC redistributes sulfur among the
various products, but does not remove sulfur from the products unless, of course, one discount the
sulfur that is retained by ay coke formed on the catalyst. Consequently, tightening product specifications have forced refiners to hydrotreat some, or all, of the products from the resid cat cracking unit.
Similarly, in the future, the emissions of sulfur oxides (SO x ) from a resid cat cracker may become
more of an obstacle for residue conversion projects. For these reasons, a point can be reached where
the economic operability of the unit can be sufficient to justify hydrotreating the feedstock to the
cat cracker.
Catalytic Cracking
naphtha and atmospheric residua. The catalyst is transported from the bottom of the unit to the top
in a gas lift employing compressed flue gas and steam.
The reactor feed and catalyst pass concurrently through the reactor zone to a disengager section, in which vapors are separated and directed to a conventional fractionation system. The spent
catalyst, which has been steam purged of residual oil, flows to the kiln for regeneration, after which
steam and flue gas are used to transport the catalyst to the reactor.
19.3.3.4 Suspensoid Catalytic Cracking
Suspensoid cracking was developed from the thermal cracking process carried out in tube and tank
units (Chapter 18). Small amounts of powdered catalyst or a mixture with the feedstock are pumped
through a cracking coil furnace. Cracking temperatures are 550°C–610°C (1025°F–1130°F), with
pressures of 200–500 psi. After leaving the furnace, the cracked material enters a separator where
the catalyst and high-boiling entrained material (tar) are left behind. The cracked vapors enter a
bubble tower where they are separated into two parts, gas oil and pressure distillate. The latter is
separated into gasoline and gases. The spent catalyst is filtered from the organic carry-over, which
is used as a heavy industrial fuel oil.
The process is actually a compromise between catalytic and thermal cracking. The main effect
of the catalyst is to allow a higher cracking temperature and to assist mechanically in keeping coke
from accumulating on the walls of the tubes. The normal catalyst employed is spent clay (2–10 lb
per barrel of feed) that obtained from the contact filtration of lubricating oils (Chapter 24).
19.4 OPTIONS FOR HEAVY FEEDSTOCKS
The processes described in the following sections are the evolutionary offspring of the fluid catalytic cracking and the residuum catalytic cracking processes. Some of these newer processes use
catalysts with different silica/alumina ratios as acid support of metals such as Mo, Co, Ni, and W.
In general, the first catalyst used to remove metals from oils was the conventional HDS catalyst.
Diverse natural minerals are also used as raw material for elaborating catalysts addressed to
the upgrading of heavy fractions. Among these minerals are clays; manganese nodules; bauxite
activated with vanadium (V), nickel (Ni), chromium (Cr), iron (Fe), and cobalt (Co), as well as
iron laterites, sepiolites; and mineral nickel and transition metal sulfides supported on silica and
alumina. Other kinds of catalysts, such as vanadium sulfide, are generated in situ, possibly in
colloidal states.
In the past decades, in spite of the difficulty of handling heavy feedstocks, residuum fluidized
catalytic cracking (RFCC) has evolved to become a well-established approach for converting a
significant portion of the heavier fractions of the crude barrel into a high-octane gasoline blending component. RFCC, which is an extension of conventional fluid catalytic cracking technology
for applications involving the conversion of highly contaminated residua, has been commercially
proven on feedstocks ranging from gas oil-residuum blends to atmospheric residua, as well as blends
of atmospheric and vacuum residua blends. In addition to high gasoline yields, the RFCC unit also
produces gaseous, distillate and fuel oil-range products.
The product quality from the residuum fluidized catalytic cracker is directly affected by its feedstock quality. In particular, and unlike hydrotreating, the RFCC redistributes sulfur among the
various products, but does not remove sulfur from the products unless, of course, one discount the
sulfur that is retained by ay coke formed on the catalyst. Consequently, tightening product specifications have forced refiners to hydrotreat some, or all, of the products from the resid cat cracking unit.
Similarly, in the future, the emissions of sulfur oxides (SO x ) from a resid cat cracker may become
more of an obstacle for residue conversion projects. For these reasons, a point can be reached where
the economic operability of the unit can be sufficient to justify hydrotreating the feedstock to the
cat cracker.