487
Thermal Cracking
Refiners will be faced with the choice of purchasing light sweet crude oils at a premium price, or adding
bottom of the barrel upgrading capability, through additional new investment, to reduce the production
of high-sulfur residual fuel oil and increase the production of low-sulfur distillate transportation fuels.
There are a number of different processing technologies available to meet the refiners differing needs.
In this chapter, we examine the various options available and the variables that influence the choice of
one technology over another.
There are several pressures that are expected to push refiners toward increased bottom of the
barrel upgrading: (1) increasing low-sulfur distillate fuel demand that will require increased refining capacity, (2) heavier crude oil slates that will result in a greater high-sulfur residual fuel oil
production, if conversion is not added, and (3) environmental restrictions that will result in reduced
demand for high-sulfur residual fuel oil. One answer to the need for conversion of the heavier feedstocks is the installation and use of one or more thermal cracking process.
Thermal cracking processes offer attractive methods of conversion of heavy feedstocks because
they enable low operating pressure, while involving high operating temperature, without requiring
expensive catalysts. Currently, the widest operated residuum conversion processes are visbreaking
(q.v.) and delayed coking (q.v.). And, these are still attractive processes for refineries from an economic point of view (Bland and Davidson, 1967; Dickenson et al., 1997).
The majority of regular thermal cracking processes use temperatures of 455°C–540°C
(850°F–1005°F) and pressures of 100–1000 psi. The Dubbs process, to which reduced crude is
charged, may be taken as a typical application of conventional thermal cracking. The feedstock
(reduced crude) is preheated by direct exchange with the cracked products in the fractionating
columns. Cracked gasoline and middle distillate fractions are removed from the upper section of
the column. Light and heavy distillate fractions are removed from the lower section and are pumped
to separate heaters. Higher temperatures are used to crack the more refractory fraction. The streams
from the heaters are combined and sent to a soaking chamber, where additional time is provided to
complete the cracking reactions. The cracked products are then separated in a low-pressure flash
chamber where a heavy fuel oil is removed as bottoms. The remaining cracked products are sent to
the fractionating columns.
Low pressures (<100 psi) and temperatures in excess of 500°C (930°F) tend to produce lower
molecular weight hydrocarbons than those produced at higher pressures (400–1000 psi) and at temperatures below 500°C (930°F). The reaction time is also important; light feeds (gas oils) and recycle
oils require longer reaction times than the readily cracked heavy residues. Mild cracking conditions
(defined here as a low conversion per cycle) favor a high yield of gasoline components with low gas
production and decreased coke production, but the gasoline quality is not high, whereas more severe
conditions give increased gas production and increased coke production and reduced gasoline yield
(but of higher quality). With limited conversion per cycle, the heavier residues must be recycled.
However, the recycled oils become increasingly refractory upon repeated cracking, and if they are
not required as a fuel oil stock they may be subjected to a coking operation to increase gasoline yield
or refined by means of a hydrogen process.
Although new thermal cracking units are now under development for heavy feedstocks, processes that can be regarded as having evolved from the original concept of thermal cracking are
visbreaking and the various coking processes (Table 18.1).
It is the purpose of this chapter to present these processes in the light of their use in modern
refineries and the information that should be borne in mind when considering and deciding upon the
potential utility of any process presented throughout this and subsequent chapters.
18.3.1 vIsBreAkIng
Visbreaking (viscosity reduction, viscosity breaking) is a relatively mild thermal cracking operation
used to reduce the viscosity of residua to produce fuel oil that meets specifications (Ballard et al.,
1992; Dominici and Sieli, 1997). Residua are sometimes blended with lighter heating oils to produce
Thermal Cracking
Refiners will be faced with the choice of purchasing light sweet crude oils at a premium price, or adding
bottom of the barrel upgrading capability, through additional new investment, to reduce the production
of high-sulfur residual fuel oil and increase the production of low-sulfur distillate transportation fuels.
There are a number of different processing technologies available to meet the refiners differing needs.
In this chapter, we examine the various options available and the variables that influence the choice of
one technology over another.
There are several pressures that are expected to push refiners toward increased bottom of the
barrel upgrading: (1) increasing low-sulfur distillate fuel demand that will require increased refining capacity, (2) heavier crude oil slates that will result in a greater high-sulfur residual fuel oil
production, if conversion is not added, and (3) environmental restrictions that will result in reduced
demand for high-sulfur residual fuel oil. One answer to the need for conversion of the heavier feedstocks is the installation and use of one or more thermal cracking process.
Thermal cracking processes offer attractive methods of conversion of heavy feedstocks because
they enable low operating pressure, while involving high operating temperature, without requiring
expensive catalysts. Currently, the widest operated residuum conversion processes are visbreaking
(q.v.) and delayed coking (q.v.). And, these are still attractive processes for refineries from an economic point of view (Bland and Davidson, 1967; Dickenson et al., 1997).
The majority of regular thermal cracking processes use temperatures of 455°C–540°C
(850°F–1005°F) and pressures of 100–1000 psi. The Dubbs process, to which reduced crude is
charged, may be taken as a typical application of conventional thermal cracking. The feedstock
(reduced crude) is preheated by direct exchange with the cracked products in the fractionating
columns. Cracked gasoline and middle distillate fractions are removed from the upper section of
the column. Light and heavy distillate fractions are removed from the lower section and are pumped
to separate heaters. Higher temperatures are used to crack the more refractory fraction. The streams
from the heaters are combined and sent to a soaking chamber, where additional time is provided to
complete the cracking reactions. The cracked products are then separated in a low-pressure flash
chamber where a heavy fuel oil is removed as bottoms. The remaining cracked products are sent to
the fractionating columns.
Low pressures (<100 psi) and temperatures in excess of 500°C (930°F) tend to produce lower
molecular weight hydrocarbons than those produced at higher pressures (400–1000 psi) and at temperatures below 500°C (930°F). The reaction time is also important; light feeds (gas oils) and recycle
oils require longer reaction times than the readily cracked heavy residues. Mild cracking conditions
(defined here as a low conversion per cycle) favor a high yield of gasoline components with low gas
production and decreased coke production, but the gasoline quality is not high, whereas more severe
conditions give increased gas production and increased coke production and reduced gasoline yield
(but of higher quality). With limited conversion per cycle, the heavier residues must be recycled.
However, the recycled oils become increasingly refractory upon repeated cracking, and if they are
not required as a fuel oil stock they may be subjected to a coking operation to increase gasoline yield
or refined by means of a hydrogen process.
Although new thermal cracking units are now under development for heavy feedstocks, processes that can be regarded as having evolved from the original concept of thermal cracking are
visbreaking and the various coking processes (Table 18.1).
It is the purpose of this chapter to present these processes in the light of their use in modern
refineries and the information that should be borne in mind when considering and deciding upon the
potential utility of any process presented throughout this and subsequent chapters.
18.3.1 vIsBreAkIng
Visbreaking (viscosity reduction, viscosity breaking) is a relatively mild thermal cracking operation
used to reduce the viscosity of residua to produce fuel oil that meets specifications (Ballard et al.,
1992; Dominici and Sieli, 1997). Residua are sometimes blended with lighter heating oils to produce