78
around the stream also serves as a scrubbing medium to wash down catalyst fines
entrained in the vapors.
The bottom product of the fractionation is called slurry oil or clarified oil (CLO)
or decant oil (DO). This product was used as a cutter stock to make fuel oil, but that
is being discontinued. CLO contains the entrained catalyst particles (expressed as
ash content wt%), which had escaped the reactor. Hence, CLO is stored in a settler,
and a slip stream is recycled back to the riser. The HCO is a product obtained from
a side draw above the CLO. Heavy cycle oil (HCO) is generally used as a pump
around for the main fractionator or recycled to the riser. Light cycle oil (LCO) is a
side draw, which is actually used as a product. It is added to the diesel pool post
hydrotreatment or can be used as a pump around the stream. The heavy naphtha side
draw is used as an extraction medium in the gas concentration section or blended
into the gasoline pool. The top gaseous product is processed in the gas concentration
unit after it being pressurized in a wet gas compressor. In the gas concentration unit,
the gaseous product is separated into dry gas, C3, C4, and light stabilized naphtha
(Fig. 10). The C3 hydrocarbons can be later separated to obtain olefins, and C4
hydrocarbons can be separated to obtain isobutene [33].
The most important product from the conventional FCC unit is gasoline, whose
yield ranges between 46 and 51 wt% for standard feedstock and can increase to
about 60 wt% for hydrotreated feedstock. The second commercial product is
LPG. LPG yield is about 12–15 wt%. Other yields dry gas (∼5 wt%), LCO
(∼15 wt%), and HCO (∼8 wt%) are not subjected to quality standards, but it is felt
that the lower the aromatic content in liquid products, the better the quality. About
4–6 wt% of the original feedstock converts to coke, which deposits on the catalyst
surface, blocking the pores and, consequently, decreasing catalyst activity. All these
yields are subject to change, depending on the type of feedstock (from heavy to
hydrotreated) and the production objectives of the catalyst [32, 33].
4.5 Modified Catalytic Cracking Processes
Different high-severity FCC units are developed for a variety of feedstocks, starting
from C4 hydrocarbons to all the way up to residue. A few common features of the
above processes are similar, except that some of them have a downer in place of a
riser or double risers [35]. Table 3 summarizes all the processes.
Importantly, all of them require very high reaction severity, e.g., riser temperature of 550–675 °C, the catalyst to oil ratio of 15–30, reactor pressure of 0.5 kg/cm
2
(g) to 1.5 kg/cm
2
(g), and using mixed catalyst composition consisting of USY
zeolite-based FCC catalyst and ZSM-5 zeolite-based additive in different ratios
[35]. The innovative features of all the processes are mainly (1) hardware configuration and (2) catalyst formulation depending upon feedstock chosen for processing.
All of the above processes, except ACO and Superflex, are employed for the conversion of mainly hydrotreated VGO, or heavy feedstock.
A. R. Khande et al.
around the stream also serves as a scrubbing medium to wash down catalyst fines
entrained in the vapors.
The bottom product of the fractionation is called slurry oil or clarified oil (CLO)
or decant oil (DO). This product was used as a cutter stock to make fuel oil, but that
is being discontinued. CLO contains the entrained catalyst particles (expressed as
ash content wt%), which had escaped the reactor. Hence, CLO is stored in a settler,
and a slip stream is recycled back to the riser. The HCO is a product obtained from
a side draw above the CLO. Heavy cycle oil (HCO) is generally used as a pump
around for the main fractionator or recycled to the riser. Light cycle oil (LCO) is a
side draw, which is actually used as a product. It is added to the diesel pool post
hydrotreatment or can be used as a pump around the stream. The heavy naphtha side
draw is used as an extraction medium in the gas concentration section or blended
into the gasoline pool. The top gaseous product is processed in the gas concentration
unit after it being pressurized in a wet gas compressor. In the gas concentration unit,
the gaseous product is separated into dry gas, C3, C4, and light stabilized naphtha
(Fig. 10). The C3 hydrocarbons can be later separated to obtain olefins, and C4
hydrocarbons can be separated to obtain isobutene [33].
The most important product from the conventional FCC unit is gasoline, whose
yield ranges between 46 and 51 wt% for standard feedstock and can increase to
about 60 wt% for hydrotreated feedstock. The second commercial product is
LPG. LPG yield is about 12–15 wt%. Other yields dry gas (∼5 wt%), LCO
(∼15 wt%), and HCO (∼8 wt%) are not subjected to quality standards, but it is felt
that the lower the aromatic content in liquid products, the better the quality. About
4–6 wt% of the original feedstock converts to coke, which deposits on the catalyst
surface, blocking the pores and, consequently, decreasing catalyst activity. All these
yields are subject to change, depending on the type of feedstock (from heavy to
hydrotreated) and the production objectives of the catalyst [32, 33].
4.5 Modified Catalytic Cracking Processes
Different high-severity FCC units are developed for a variety of feedstocks, starting
from C4 hydrocarbons to all the way up to residue. A few common features of the
above processes are similar, except that some of them have a downer in place of a
riser or double risers [35]. Table 3 summarizes all the processes.
Importantly, all of them require very high reaction severity, e.g., riser temperature of 550–675 °C, the catalyst to oil ratio of 15–30, reactor pressure of 0.5 kg/cm
2
(g) to 1.5 kg/cm
2
(g), and using mixed catalyst composition consisting of USY
zeolite-based FCC catalyst and ZSM-5 zeolite-based additive in different ratios
[35]. The innovative features of all the processes are mainly (1) hardware configuration and (2) catalyst formulation depending upon feedstock chosen for processing.
All of the above processes, except ACO and Superflex, are employed for the conversion of mainly hydrotreated VGO, or heavy feedstock.
A. R. Khande et al.