hydrocracker but go a long way to achieving the heavy oil cracking objective of
converting low-quality material into more valuable oil products. The process family
of thermal crackers has three members, which are:
• Thermal crackers
• Visbreakers
• Cokers
The term thermal cracking is given to those processes that convert heavy oil
(usually fuel oil or residues) into lighter product stock such as LPG, naphtha, and
middle distillates by applying only heat to the feed over a prescribed element of
time. The term thermal cracker when applied to a specific process usually refers to
the processing of atmospheric residues (long residues) to give the lighter products.
The term visbreaking refers to the processing of vacuum residues (short residues) to
reduce the viscosity of the oil only and thus to meet the requirements of a more
valuable fuel oil stock. Coking refers to the most severe process in the thermal
cracking family. Either long or short residues can be fed to this process whose
objective is to produce the lighter products and petroleum coke. The coker process
is extinctive – that is, it converts all the feed. In the other two processes, there is
usually some unconverted feed although the thermal cracker can be designed to be
“extinctive” by recycling the unconverted oil. The three thermal cracking processes
have the same basic process configuration. This consists of a cracking furnace, a
“soaking” vessel or coil, and a product recovery fractionator(s). The feed is first
preheated by heat exchange with hot product streams before entering the cracking
furnace or heater. The cracking furnace raises the temperature of the oil to its
predetermined cracking temperature. This is always in excess of 920
F and, by
careful design of the heater coils, the oil is retained in the furnace at a prescribed
cracking temperature for a predetermined period of time (the residence time). In
some cases an additional coil section is added to the heater to allow the oil to “soak”
at the fixed temperature for a longer period of time. In other cases the oil leaves the
furnace to enter a drum which retains the oil at its cracking temperature for a little
time. In the coker process, the oil leaves the furnace to enter one of a series of coker
drums in which the oil is retained for a longer period of time at its coking
temperature for the production of coke.
The cracked oil is quenched by a cold heavy oil product stream on leaving the
soaking section to a temperature below its cracking temperature. It then enters a
fractionator where the distillate products are separated and taken off in a manner
similar to the crude distillation unit. In the case of the cokers, the coke is removed
from the drums by high-velocity water jets on a regular batch basis. The coking
process summarized here refers to the more simple “delayed coking” process. There
are other coking processes which are more complicated such as the fluid coker and
the proprietary Flexicoker.
Further details on thermal cracking are provided in the chapter “▶ Upgrading the
Bottom of the Barrel.” This chapter includes also the treating of residues by
hydrocracking and fluid catalytic cracking.
Introduction to Crude Oil and Petroleum Processing
39
converting low-quality material into more valuable oil products. The process family
of thermal crackers has three members, which are:
• Thermal crackers
• Visbreakers
• Cokers
The term thermal cracking is given to those processes that convert heavy oil
(usually fuel oil or residues) into lighter product stock such as LPG, naphtha, and
middle distillates by applying only heat to the feed over a prescribed element of
time. The term thermal cracker when applied to a specific process usually refers to
the processing of atmospheric residues (long residues) to give the lighter products.
The term visbreaking refers to the processing of vacuum residues (short residues) to
reduce the viscosity of the oil only and thus to meet the requirements of a more
valuable fuel oil stock. Coking refers to the most severe process in the thermal
cracking family. Either long or short residues can be fed to this process whose
objective is to produce the lighter products and petroleum coke. The coker process
is extinctive – that is, it converts all the feed. In the other two processes, there is
usually some unconverted feed although the thermal cracker can be designed to be
“extinctive” by recycling the unconverted oil. The three thermal cracking processes
have the same basic process configuration. This consists of a cracking furnace, a
“soaking” vessel or coil, and a product recovery fractionator(s). The feed is first
preheated by heat exchange with hot product streams before entering the cracking
furnace or heater. The cracking furnace raises the temperature of the oil to its
predetermined cracking temperature. This is always in excess of 920
F and, by
careful design of the heater coils, the oil is retained in the furnace at a prescribed
cracking temperature for a predetermined period of time (the residence time). In
some cases an additional coil section is added to the heater to allow the oil to “soak”
at the fixed temperature for a longer period of time. In other cases the oil leaves the
furnace to enter a drum which retains the oil at its cracking temperature for a little
time. In the coker process, the oil leaves the furnace to enter one of a series of coker
drums in which the oil is retained for a longer period of time at its coking
temperature for the production of coke.
The cracked oil is quenched by a cold heavy oil product stream on leaving the
soaking section to a temperature below its cracking temperature. It then enters a
fractionator where the distillate products are separated and taken off in a manner
similar to the crude distillation unit. In the case of the cokers, the coke is removed
from the drums by high-velocity water jets on a regular batch basis. The coking
process summarized here refers to the more simple “delayed coking” process. There
are other coking processes which are more complicated such as the fluid coker and
the proprietary Flexicoker.
Further details on thermal cracking are provided in the chapter “▶ Upgrading the
Bottom of the Barrel.” This chapter includes also the treating of residues by
hydrocracking and fluid catalytic cracking.
Introduction to Crude Oil and Petroleum Processing
39