INTRODUCTION
Fluid catalytic cracking (FCC) technology is a technology with more than 60 years of commercial operating experience. The process is used to convert higher-molecular-weight hydrocarbons to lighter, more valuable products through contact with a powdered catalyst at appropriate conditions. Historically, the primary purpose of the FCC process has been to produce gasoline, distillate, and C3/C4 olefins from low-value excess refinery gas oils and heavier refinery streams. FCC is often the heart of a modern refinery because of its adaptability to changing feedstocks and product demands and because of high margins that
exist between the FCC feedstocks and converted FCC products. As oil refining has evolved
over the last 60 years, the FCC process has evolved with it, meeting the challenges of cracking heavier, more contaminated feedstocks, increasing operating flexibility, accommodating environmental legislation, and maximizing reliability.
The FCC unit continuously circulates a fluidized zeolite catalyst that allows rapid
cracking reactions to occur in the vapor phase. The KBR Orthoflow FCC unit (Fig. 3.1.1)
consists of a stacked disengager-regenerator system that minimizes plot space requirements. The cracking reactions are carried out in an up-flowing vertical reactor-riser in
which a liquid oil stream contacts hot powdered catalyst. The oil vaporizes and cracks to
lighter products as it moves up the riser and carries the catalyst along with it. The reactions
are rapid, requiring only a few seconds of contact time. Simultaneously with the desired
reactions, coke, a material having a low ratio of hydrogen to carbon, deposits on the catalyst
and renders it less catalytically active. Catalyst and product vapors separate in a disengaging vessel with the catalyst continuing first through a stripping stage and second
through a regeneration stage where coke is combusted to rejuvenate the catalyst and provide heat for operation of the process. The regenerated catalyst then passes to the bottom of the reactor-riser, where the cycle starts again. Hydrocarbon product vapors flow downstream for separation into individual products.
KBR, through its ancestry in The M.W. Kellogg Company, has been a leader in FCC
technology developments since the inception of the process. In recent years, KBR has
worked with its FCC partner, ExxonMobil, to create and refine FCC technology features
that have led the industry. To date, KBR has licensed more than 120 grassroots FCC
units throughout the world, including 13 grassroots units and more than 120 revamps
since just 1990.
FEEDSTOCKS
The modern FCC unit can accept a broad range of feedstocks, a fact which contributes to
FCC’s reputation as one of the most flexible refining processes in use today. Examples of
common feedstocks for conventional distillate feed FCC units are
● Atmospheric gas oils
● Vacuum gas oils
● Coker gas oils
● Thermally cracked gas oils
● Solvent deasphalted oils
● Lube extracts
● Hydrocracker bottoms
Residual FCCU (RFCCU) processes Conradson carbon residue and metals-contaminated
feedstocks such as atmospheric residues or mixtures of vacuum residue and gas oils.
Depending on the level of carbon residue and metallic contaminants (nickel and vanadium),
these feedstocks may be hydrotreated or deasphalted before being fed to an RFCCU.
Feed hydrotreating or deasphalting reduces the carbon residue and metals levels of the
feed, reducing both the coke-making tendency of the feed and catalyst deactivation.
PRODUCTS
Products from the FCC and RFCC processes are typically as follows:
● Fuel gas (ethane and lighter hydrocarbons)
● C3 and C4 liquefied petroleum gas (LPG)
● Gasoline
● Light cycle oil (LCO)
● Fractionator bottoms (slurry oil)
● Coke (combusted in regenerator)
● Hydrogen Sulfide (from amine regeneration)
Although gasoline is typically the most desired product from an FCCU or RFCCU, design
and operating variables can be adjusted to maximize other products. The three principal
modes of FCC operation are (1) maximum gasoline production, (2) maximum light cycle
oil production, and (3) maximum light olefin production, often referred to as maximum
LPG operation. These modes of operation are discussed below:
Maximum Gasoline The maximum gasoline mode is characterized by use of an intermediate cracking temperature (510 to 540°C), high catalyst activity, and a high catalyst/oil ratio. Recycle is normally not used since the conversion after a single pass through the riser is already high. Maximization of gasoline yield requires the use of an effective feed injection system, a short-contact-time vertical riser, and efficient riser effluent separation to maximize the cracking selectivity to gasoline in the riser and to prevent secondary reactions from degrading the gasoline after it exits the riser.
Maximum Middle Distillate The maximum middle distillate mode of operation is a low-cracking-severity operation in which the first pass conversion is held to a low level to restrict recracking of light cycle oil formed during initial cracking. Severity is lowered by reducing the riser outlet temperature (below 510°C) and by reducing the catalyst/oil ratio. The lower catalyst/oil ratio is often achieved by the use of a fired feed heater which significantly increases feed temperature. Additionally catalyst activity is sometimes lowered by reducing the fresh catalyst makeup rate or reducing fresh catalyst activity. Since during low-severity operation a substantial portion of the feed remains unconverted in a single pass through the riser, recycle of heavy cycle oil to the riser is used to reduce the yield of lower-value, heavy streams such as slurry product. When middle distillate production is maximized, upstream crude distillation units are operated to minimize middle distillate components in the FCCU feedstock, since these components either degrade in quality or convert to gasoline and lighter products in the FCCU. In addition, while maximizing middle distillate production, the FCCU gasoline endpoint would typically be minimized within middle distillate flash point constraints, shifting gasoline product into LCO.
If it is desirable to increase gasoline octane or increase LPG yield while also maximizing
LCO production, ZSM-5 containing catalyst additives can be used. ZSM-5 selectively
cracks gasoline boiling-range linear molecules and has the effect of increasing gasoline
research and motor octane ratings, decreasing gasoline yield, and increasing C3 and C4
LPG yield. Light cycle oil yield is also reduced slightly.
Maximum Light Olefin Yield
The yields of propylene and butylenes may be increased above that of the maximum gasoline operation by increasing the riser temperature above 540°C and by use of ZSM-5 containing catalyst additives. The FCC unit may also be designed specifically to allow
maximization of propylene as well as ethylene production by incorporation of MAXOFIN
FCC technology, as described more fully in the next section. While traditional FCC operations typically produce less than 6 wt % propylene, the MAXOFIN FCC process can produce as much as 20 wt % or more propylene from traditional FCC feedstocks. The process increases propylene yield relative to that produced by conventional FCC units by combining the effects of MAXOFIN-3 catalyst additive and proprietary hardware, including a second high-severity riser designed to crack surplus naphtha and C4’s into incremental light
olefins. Table 3.1.1 shows the yield flexibility of the MAXOFIN FCC process that can
alternate between maximum propylene and traditional FCC operations.
PROCESS DESCRIPTION
The FCC process may be divided into several major sections, including the converter section, flue gas section, main fractionator section, and vapor recovery units (VRUs). The
number of product streams, the degree of product fractionation, flue gas handling steps,
and several other aspects of the process will vary from unit to unit, depending on the
requirements of the application. The following sections provide more detailed descriptions
of the converter, flue gas train, main fractionator, and VRU.
Converter
The KBR Orthoflow FCCU converter shown in Fig. 3.1.2 consists of regenerator, stripper,
and disengager vessels, with continuous closed-loop catalyst circulation between the
regenerator and disengager/stripper. The term Orthoflow derives from the in-line stacked
arrangement of the disengager and stripper over the regenerator. This arrangement has the
following operational and cost advantages:
● Essentially all-vertical flow of catalyst in standpipes and risers
● Short regenerated and spent catalyst standpipes allowing robust catalyst circulation
● Uniform distribution of spent catalyst in the stripper and regenerator
● Low overall converter height
● Minimum structural steel and plot area requirements
Preheated fresh feedstock, plus any recycle feed, is charged to the base of the riser reactor.
Upon contact with hot regenerated catalyst, the feedstock is vaporized and converted to lower boiling fractions (light cycle oil, gasoline, C3 and C4 LPG, and dry gas). Product vapors are separated from spent catalyst in the disengager cyclones and flow via the disengager
overhead line to the main fractionator and vapor recovery unit for quenching and fractionation.
Coke formed during the cracking reactions is deposited on the catalyst, thereby reducing
its activity. The coked catalyst, which is separated from the reactor products in the
disengager cyclones, flows via the stripper and spent catalyst standpipe to the regenerator.
The discharge rate from the standpipe is controlled by the spent catalyst plug valve.
In the regenerator, coke is removed from the spent catalyst by combustion with air. Air
is supplied to the regenerator air distributors from an air blower. Flue gas from the combustion of coke exits the regenerator through two-stage cyclones which remove all but a
trace of catalyst from the flue gas. Flue gas is collected in an external plenum chamber and
flows to the flue gas train. Regenerated catalyst, with its activity restored, is returned to the
riser via the regenerated catalyst plug valve, completing the cycle.
ATOMAX Feed Injection System
The Orthoflow FCC design employs a regenerated catalyst standpipe, a catalyst plug
valve, and a short inclined lateral to transport regenerated catalyst from the regenerator to
the riser. The catalyst then enters a feed injection cone surrounded by multiple, flat-spray, atomizing feed injection nozzles, as shown in Fig. 3.1.3. The flat, fan-shaped sprays provide
uniform coverage and maximum penetration of feedstock into catalyst, and prevent catalyst
from bypassing feed in the injection zone. Proprietary feed injection nozzles, known
as ATOMAX nozzles, are used to achieve the desired feed atomization and spray pattern
while minimizing feed pressure requirements. The hot regenerated catalyst vaporizes the
oil feed, raises it to reaction temperature, and supplies the necessary heat for cracking.
The cracking reaction proceeds as the catalyst and vapor mixture flow up the riser. The
riser outlet temperature is controlled by the amount of catalyst admitted to the riser by the
catalyst plug valve.
Riser Quench
The riser quench system consists of a series of nozzles uniformly spaced around the upper
section of riser. A portion of the feed or a recycle stream from the main fractionator is
injected through the nozzles into the riser to rapidly reduce the temperature of the riser
contents. The heat required to vaporize the quench is supplied by increased fresh feed preheat or by increased catalyst circulation. This effectively increases the temperature in the
lower section of the riser above that which would be achieved in a nonquenched operation,
thereby increasing the vaporization of heavy feeds, increasing gasoline yield, olefin production, and gasoline octane.
Riser Termination
At the top of the riser, all the selective cracking reactions have been completed. It is important to minimize product vapor residence time in the disengager to prevent unwanted thermal or catalytic cracking reactions which produce dry gas and coke from more valuable
products. Figure 3.1.4 shows the strong effect of temperature on thermal recracking of
gasoline and distillate to produce predominantly dry gas.
Closed cyclone technology is used to separate product vapors from catalyst with minimum
vapor residence time in the disengager. This system (Fig. 3.1.5) consists of riser
cyclones directly coupled to secondary cyclones housed in the disengager vessel. The riser
cyclones effect a quick separation of the spent catalyst and product vapors exiting the
riser. The vapors flow directly from the outlet of the riser cyclones into the inlets of the
secondary cyclones and then to the main fractionator for rapid quenching. Closed cyclones
almost completely eliminate postriser thermal cracking with its associated dry gas and
butadiene production. Closed cyclone technology is particularly important in operation at
high riser temperatures (say, 538°C or higher), typical of maximum gasoline or maximum
light olefin operations.
