496
The Chemistry and Technology of Petroleum
conversion processes on heavier crude oil. One common misconception of delayed coking is that the
product coke is a disadvantage. Although coke is a low-valued by-product, compared with transportation fuels, there is a significant worldwide trade and demand even for high-sulfur petroleum coke
from delayed cokers as coke is a very economical fuel. However, for coke burning power plants, this
has required the installation of flue gas scrubbers and, in some of these plants, a circulating bed of
limestone captures the sulfur.
Low-pressure coking is a process designed for a once-through, low-pressure operation. The process is similar to delayed coking except that recycling is not usually practiced and the coke chamber
operating conditions are 435°C (815°F), 25 psi. Excessive coking is inhibited by the addition of
water to the feedstock in order to quench and restrict the reactions of the reactive intermediates.
High-temperature coking is a semi-continuous thermal conversion process designed for highmelting asphaltic residua that yields coke and gas oil as the primary products. The coke may be
treated to remove sulfur to produce a low-sulfur coke (5%), even though the feedstock contained as
much as 5% w/w sulfur.
Thus, the feedstock is transported to the pitch accumulator, then to the heater (370°C, 700°F,
30 psi), and finally to the coke oven, where temperatures may be as high as 980°C–1095°C
(1800°F–2000°F). Volatile materials are fractionated, and after the cycle is complete, coke is collected for sulfur removal and quenching before storage.
18.3.2.2 Fluid Coking
Fluid coking is a continuous process that uses the fluidized solids technique to convert residua,
including vacuum residua and cracked residua, to more valuable products (Roundtree, 1997). The
process is useful for processing of residua (Table 18.5). The yield of distillates from coking can be
improved by reducing the residence time of the cracked vapors. In order to simplify handling of the
coke product, and enhance product yields, fluidized-bed coking, or fluid coking, was developed in
the mid-1950s.
The residuum feedstock is decomposed by being sprayed into a fluidized bed of hot, fine
coke particles, which permits the coking reactions to be conducted at higher temperatures and
shorter contact times than can be employed in delayed coking. Moreover, these conditions result
in decreased yields of coke; greater quantities of more valuable liquid product are recovered in the
fluid coking process.
Fluid coking uses two vessels, a reactor and a burner; coke particles are circulated between these
to transfer heat (generated by burning a portion of the coke) to the reactor (Figure 18.5; Blaser,
1992). The reactor holds a bed of fluidized coke particles, and steam is introduced at the bottom of
the reactor to fluidize the bed. The feed coming from the bottom of a vacuum tower at, for example,
260°C –370°C (500°F–700°F) is injected directly into the reactor. The temperature in the coking
vessel ranges from 480°C to 565°C (900°F–1050°F), and the pressure is substantially atmospheric,
so the incoming feed is partly vaporized and partly deposited on the fluidized coke particles. The
material on the particle surface then cracks and vaporizes, leaving a residue that dries to form coke.
The vapor products pass through cyclones that remove most of the entrained coke.
The vapor is discharged into the bottom of a scrubber, where the products are cooled to condense
a heavy tar containing the remaining coke dust, which is recycled to the coking reactor. The upper
part of the scrubber tower is a fractionating zone from which coker gas oil is withdrawn and then
fed to a catalytic cracking unit; naphtha and gas are taken overhead to condensers.
In the reactor, the coke particles flow down through the vessel into a stripping zone at the bottom.
Steam displaces the product vapors between the particles, and the coke then flows into a riser that
leads to the burner. Steam is added to the riser to reduce the solids loading and to induce upward
flow. The average bed temperature in the burner is 590°C–650°C (1095°F–1200°F), and air is added
as needed to maintain the temperature by burning part of the product coke. The pressure in the
burner may range from 5 to 25 psi. Flue gases from the burner bed pass through cyclones and discharge to the stack. Hot coke from the bed is returned to the reactor through a second riser assembly.
The Chemistry and Technology of Petroleum
conversion processes on heavier crude oil. One common misconception of delayed coking is that the
product coke is a disadvantage. Although coke is a low-valued by-product, compared with transportation fuels, there is a significant worldwide trade and demand even for high-sulfur petroleum coke
from delayed cokers as coke is a very economical fuel. However, for coke burning power plants, this
has required the installation of flue gas scrubbers and, in some of these plants, a circulating bed of
limestone captures the sulfur.
Low-pressure coking is a process designed for a once-through, low-pressure operation. The process is similar to delayed coking except that recycling is not usually practiced and the coke chamber
operating conditions are 435°C (815°F), 25 psi. Excessive coking is inhibited by the addition of
water to the feedstock in order to quench and restrict the reactions of the reactive intermediates.
High-temperature coking is a semi-continuous thermal conversion process designed for highmelting asphaltic residua that yields coke and gas oil as the primary products. The coke may be
treated to remove sulfur to produce a low-sulfur coke (5%), even though the feedstock contained as
much as 5% w/w sulfur.
Thus, the feedstock is transported to the pitch accumulator, then to the heater (370°C, 700°F,
30 psi), and finally to the coke oven, where temperatures may be as high as 980°C–1095°C
(1800°F–2000°F). Volatile materials are fractionated, and after the cycle is complete, coke is collected for sulfur removal and quenching before storage.
18.3.2.2 Fluid Coking
Fluid coking is a continuous process that uses the fluidized solids technique to convert residua,
including vacuum residua and cracked residua, to more valuable products (Roundtree, 1997). The
process is useful for processing of residua (Table 18.5). The yield of distillates from coking can be
improved by reducing the residence time of the cracked vapors. In order to simplify handling of the
coke product, and enhance product yields, fluidized-bed coking, or fluid coking, was developed in
the mid-1950s.
The residuum feedstock is decomposed by being sprayed into a fluidized bed of hot, fine
coke particles, which permits the coking reactions to be conducted at higher temperatures and
shorter contact times than can be employed in delayed coking. Moreover, these conditions result
in decreased yields of coke; greater quantities of more valuable liquid product are recovered in the
fluid coking process.
Fluid coking uses two vessels, a reactor and a burner; coke particles are circulated between these
to transfer heat (generated by burning a portion of the coke) to the reactor (Figure 18.5; Blaser,
1992). The reactor holds a bed of fluidized coke particles, and steam is introduced at the bottom of
the reactor to fluidize the bed. The feed coming from the bottom of a vacuum tower at, for example,
260°C –370°C (500°F–700°F) is injected directly into the reactor. The temperature in the coking
vessel ranges from 480°C to 565°C (900°F–1050°F), and the pressure is substantially atmospheric,
so the incoming feed is partly vaporized and partly deposited on the fluidized coke particles. The
material on the particle surface then cracks and vaporizes, leaving a residue that dries to form coke.
The vapor products pass through cyclones that remove most of the entrained coke.
The vapor is discharged into the bottom of a scrubber, where the products are cooled to condense
a heavy tar containing the remaining coke dust, which is recycled to the coking reactor. The upper
part of the scrubber tower is a fractionating zone from which coker gas oil is withdrawn and then
fed to a catalytic cracking unit; naphtha and gas are taken overhead to condensers.
In the reactor, the coke particles flow down through the vessel into a stripping zone at the bottom.
Steam displaces the product vapors between the particles, and the coke then flows into a riser that
leads to the burner. Steam is added to the riser to reduce the solids loading and to induce upward
flow. The average bed temperature in the burner is 590°C–650°C (1095°F–1200°F), and air is added
as needed to maintain the temperature by burning part of the product coke. The pressure in the
burner may range from 5 to 25 psi. Flue gases from the burner bed pass through cyclones and discharge to the stack. Hot coke from the bed is returned to the reactor through a second riser assembly.