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The Chemistry and Technology of Petroleum
and industrial fuel oils or as diesel fuels when blended with straight-run gas oils. The gas oils
produced by cracking were also a further important source of gasoline. In a once-through cracking operation, all of the cracked material is separated into products and may be used as such.
However, the gas oils produced by cracking (cracked gas oils) are more resistant to cracking (more
refractory) than gas oils produced by distillation (straight-run gas oils) but could still be cracked to
produce more gasoline. This was achieved using a later innovation (post-1940) involving a recycle
operation in which the cracked gas oil was combined with fresh feed for another trip through the
cracking unit. The extent to which recycling was carried out affected the yield of gasoline from
the process.
The majority of the thermal cracking processes use temperatures of 455°C–540°C
(850°F–1005°F) and pressures of 100–1000 psi; the Dubbs process may be taken as a typical
application of an early thermal cracking operation. The feedstock (reduced crude) is preheated by
direct exchange with the cracking products in the fractionating columns. Cracked gasoline and
heating oil 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 light distillate 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.
Mild cracking conditions, with a low conversion per cycle, favor a high yield of gasoline components, with low gas and coke production, but the gasoline quality is not high, whereas more
severe conditions give increased gas and coke production and reduced gasoline yield (but of
higher quality). With limited conversion per cycle, the heavier residues must be recycled, but
these recycle oils become increasingly refractory upon repeated cracking, and if they are not
required as a fuel oil stock they may be coked to increase gasoline yield or refined by means of
a hydrogen process.
The thermal cracking of higher-boiling petroleum fractions to produce gasoline is now virtually
obsolete. The antiknock requirements of modern automobile engines together with the different
nature of crude oils (compared to those of 50 or more years ago) has reduced the ability of the
thermal cracking process to produce gasoline on an economic basis. Very few new units have been
installed since the 1960s, and some refineries may still operate the older cracking units.
15.5.2.2 Visbreaking
Visbreaking (viscosity breaking) is essentially a process of the post-1940 era and was initially introduced as a mild thermal cracking operation that could be used to reduce the viscosity of residua
to allow the products to meet fuel oil specifications. Alternatively, the visbroken residua could be
blended with lighter product oils to produce fuel oils of acceptable viscosity. By reducing the viscosity of the residuum, visbreaking reduces the amount of light heating oil that is required for blending
to meet the fuel oil specifications. In addition to the major product, fuel oil, material in the gas oil
and gasoline boiling range is produced. The gas oil may be used as additional feed for catalytic
cracking units, or as heating oil.
In a typical visbreaking operation (Figure 15.5), a crude oil residuum is passed through a
furnace where it is heated to a temperature of 480°C (895°F) under an outlet pressure of about
100 psi. The heating coils in the furnace are arranged to provide a soaking section of low
heat density, where the charge remains until the visbreaking reactions are completed and the
cracked products are then passed into a flash-distillation chamber. The overhead material from
this chamber is then fractionated to produce a low-quality gasoline as an overhead product and
light gas oil as bottom. The liquid products from the flash chamber are cooled with a gas oil flux
and then sent to a vacuum fractionator. This yields a heavy gas oil distillate and a residual tar
of reduced viscosity.
The Chemistry and Technology of Petroleum
and industrial fuel oils or as diesel fuels when blended with straight-run gas oils. The gas oils
produced by cracking were also a further important source of gasoline. In a once-through cracking operation, all of the cracked material is separated into products and may be used as such.
However, the gas oils produced by cracking (cracked gas oils) are more resistant to cracking (more
refractory) than gas oils produced by distillation (straight-run gas oils) but could still be cracked to
produce more gasoline. This was achieved using a later innovation (post-1940) involving a recycle
operation in which the cracked gas oil was combined with fresh feed for another trip through the
cracking unit. The extent to which recycling was carried out affected the yield of gasoline from
the process.
The majority of the thermal cracking processes use temperatures of 455°C–540°C
(850°F–1005°F) and pressures of 100–1000 psi; the Dubbs process may be taken as a typical
application of an early thermal cracking operation. The feedstock (reduced crude) is preheated by
direct exchange with the cracking products in the fractionating columns. Cracked gasoline and
heating oil 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 light distillate 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.
Mild cracking conditions, with a low conversion per cycle, favor a high yield of gasoline components, with low gas and coke production, but the gasoline quality is not high, whereas more
severe conditions give increased gas and coke production and reduced gasoline yield (but of
higher quality). With limited conversion per cycle, the heavier residues must be recycled, but
these recycle oils become increasingly refractory upon repeated cracking, and if they are not
required as a fuel oil stock they may be coked to increase gasoline yield or refined by means of
a hydrogen process.
The thermal cracking of higher-boiling petroleum fractions to produce gasoline is now virtually
obsolete. The antiknock requirements of modern automobile engines together with the different
nature of crude oils (compared to those of 50 or more years ago) has reduced the ability of the
thermal cracking process to produce gasoline on an economic basis. Very few new units have been
installed since the 1960s, and some refineries may still operate the older cracking units.
15.5.2.2 Visbreaking
Visbreaking (viscosity breaking) is essentially a process of the post-1940 era and was initially introduced as a mild thermal cracking operation that could be used to reduce the viscosity of residua
to allow the products to meet fuel oil specifications. Alternatively, the visbroken residua could be
blended with lighter product oils to produce fuel oils of acceptable viscosity. By reducing the viscosity of the residuum, visbreaking reduces the amount of light heating oil that is required for blending
to meet the fuel oil specifications. In addition to the major product, fuel oil, material in the gas oil
and gasoline boiling range is produced. The gas oil may be used as additional feed for catalytic
cracking units, or as heating oil.
In a typical visbreaking operation (Figure 15.5), a crude oil residuum is passed through a
furnace where it is heated to a temperature of 480°C (895°F) under an outlet pressure of about
100 psi. The heating coils in the furnace are arranged to provide a soaking section of low
heat density, where the charge remains until the visbreaking reactions are completed and the
cracked products are then passed into a flash-distillation chamber. The overhead material from
this chamber is then fractionated to produce a low-quality gasoline as an overhead product and
light gas oil as bottom. The liquid products from the flash chamber are cooled with a gas oil flux
and then sent to a vacuum fractionator. This yields a heavy gas oil distillate and a residual tar
of reduced viscosity.