507
Thermal Cracking
In the process, the separated bitumen is (1) distilled—atmospheric and vacuum; (2) the residuum
is deasphalted—deasphaltened to remove the asphaltene fraction; (3) the deasphalted oil is sent to
a thermal cracking unit; and (4) the cracked products are recycled into the feedstock to the atmospheric distillation, so as to recover the distillates and separate the asphaltene component produced
by the thermal treatment.
The process produces distillates and a heavy residue (asphaltene fraction), which is used as feedstock for the gasification plant that produces syngas to generate energy and the steam needed for
extraction of the bitumen via SAGD (steam assisted gravity drainage) (Chapters 6 and 7), as well as
the hydrogen needed for further upgrading of the products.
18.4.10 seleCtIve CrACkIng
Selective cracking is a thermal conversion process that utilizes different conditions depending on
feedstock composition. For example, a heavy oil may be cracked at 494°C–515°C (920°F–960°F)
and 300–500 psi; a lighter gas oil may be cracked at 510°C–530°C (950°F–990°F) and 500–700 psi
(Moschopedis et al., 1998; Speight and Ozum, 2002).
Each feedstock has its own particular characteristics that dictate the optimum conditions of temperature and pressure for maximum yields of the products. These factors are utilized in selective
combination of cracking units in which the more refractory feedstocks are cracked for longer periods of time or at higher temperatures than the less stable feedstocks, which are cracked at lower
temperatures.
The process eliminates the accumulation of stable low-boiling material in the recycle stock and
also minimizes coke formation from high-temperature cracking of the higher boiling material. The
end result is the production of fairly high yields of gasoline, middle distillates, and olefin gases.
18.4.11 sHell tHermAl CrACkIng
The Shell thermal distillate cracking unit is based on the principle of converting high boiling feedstocks to lower boiling products (Table 18.12; Figure 18.8). Thermal cracking of heavy gas oil takes
place in the liquid phase in a furnace, at elevated pressure and temperature and the products are
residuum and distillate products. The majority of the heavy gas oil is converted to light gas oil.
In the process (Figure 18.9), for example, heavy gas oil from the atmospheric distillation unit
or vacuum gas oil from the vacuum distillation unit is sent to a surge drum. Liquid from this drum
is pumped to the distillate heater, which typically operates at a pressure of approximately 490°C
(915°C) and 290 psi. Under these conditions, the cracking reactions take place in the liquid phase.
Fluid from the distillate heater is then routed to the combi tower where separation is achieved
between residue, gas oil, and lighter products. In addition, a heavy gas oil fraction is taken from
the combi tower, returned to the surge drum, and then recycled through the distillate heater. The
bottom product of the combi tower is routed to a vacuum flasher where heavy gas oil is recovered
from the residuum stream and routed back to the distillate heater. The vacuum flashed residuum
TABLE 18.12
Typical Product Yields from the SHELL
Thermal Distillate Cracking Process
Product
Yield, % w/w
Gas
9
Naphtha
17
Endpoint 165°C (330°F)
Gas oil
54
Endpoint 350°C (660°F)
Residuum
20
Thermal Cracking
In the process, the separated bitumen is (1) distilled—atmospheric and vacuum; (2) the residuum
is deasphalted—deasphaltened to remove the asphaltene fraction; (3) the deasphalted oil is sent to
a thermal cracking unit; and (4) the cracked products are recycled into the feedstock to the atmospheric distillation, so as to recover the distillates and separate the asphaltene component produced
by the thermal treatment.
The process produces distillates and a heavy residue (asphaltene fraction), which is used as feedstock for the gasification plant that produces syngas to generate energy and the steam needed for
extraction of the bitumen via SAGD (steam assisted gravity drainage) (Chapters 6 and 7), as well as
the hydrogen needed for further upgrading of the products.
18.4.10 seleCtIve CrACkIng
Selective cracking is a thermal conversion process that utilizes different conditions depending on
feedstock composition. For example, a heavy oil may be cracked at 494°C–515°C (920°F–960°F)
and 300–500 psi; a lighter gas oil may be cracked at 510°C–530°C (950°F–990°F) and 500–700 psi
(Moschopedis et al., 1998; Speight and Ozum, 2002).
Each feedstock has its own particular characteristics that dictate the optimum conditions of temperature and pressure for maximum yields of the products. These factors are utilized in selective
combination of cracking units in which the more refractory feedstocks are cracked for longer periods of time or at higher temperatures than the less stable feedstocks, which are cracked at lower
temperatures.
The process eliminates the accumulation of stable low-boiling material in the recycle stock and
also minimizes coke formation from high-temperature cracking of the higher boiling material. The
end result is the production of fairly high yields of gasoline, middle distillates, and olefin gases.
18.4.11 sHell tHermAl CrACkIng
The Shell thermal distillate cracking unit is based on the principle of converting high boiling feedstocks to lower boiling products (Table 18.12; Figure 18.8). Thermal cracking of heavy gas oil takes
place in the liquid phase in a furnace, at elevated pressure and temperature and the products are
residuum and distillate products. The majority of the heavy gas oil is converted to light gas oil.
In the process (Figure 18.9), for example, heavy gas oil from the atmospheric distillation unit
or vacuum gas oil from the vacuum distillation unit is sent to a surge drum. Liquid from this drum
is pumped to the distillate heater, which typically operates at a pressure of approximately 490°C
(915°C) and 290 psi. Under these conditions, the cracking reactions take place in the liquid phase.
Fluid from the distillate heater is then routed to the combi tower where separation is achieved
between residue, gas oil, and lighter products. In addition, a heavy gas oil fraction is taken from
the combi tower, returned to the surge drum, and then recycled through the distillate heater. The
bottom product of the combi tower is routed to a vacuum flasher where heavy gas oil is recovered
from the residuum stream and routed back to the distillate heater. The vacuum flashed residuum
TABLE 18.12
Typical Product Yields from the SHELL
Thermal Distillate Cracking Process
Product
Yield, % w/w
Gas
9
Naphtha
17
Endpoint 165°C (330°F)
Gas oil
54
Endpoint 350°C (660°F)
Residuum
20