Thermal cracking is a refinery process that breaks larger hydrocarbon molecules into smaller molecules like gasoline. The presentation discusses various aspects of thermal cracking including: 1. The necessity of cracking to produce more gasoline from heavier crude oil fractions. 2. The main types of cracking - thermal cracking and catalytic cracking. Thermal cracking uses high temperatures without a catalyst. 3. Key thermal cracking processes like Dubbs, pyrolysis, visbreaking, and coking which use different temperatures and pressures to produce different product yields. 4. The thermal cracking reactions of decomposition, hydrogenation, polymerization, and cyclization that alter the hydrocarbon molecules. 5. Commercial thermal cracking units and how they operate to continuously

2. Presentation Topic
Thermal Cracking
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3. Presentation By
Shabeel Khalid 21
M.Bila 06
Fahad Muzaffar 03
Ateeq Ahmad 31
Nayyar Iqbal 08

4. Contents:
• Cracking
• Necessity for cracking or why we need
cracking
• Types of cracking
• Thermal cracking
• Thermal cracking processes
• Thermal cracking reactions
• Commercial thermal cracking processes

5. Terminologies
There is a wide variety of types of crude oil based on its color, smell, and it's
properties. Some include paraffin based, olefins, and aromatic hydrocarbons.
Paraffin based crudes (a waxy residue)
•
a flammable, whitish, translucent, waxy solid consisting of a mixture of saturated
hydrocarbons, obtained by distillation from petroleum or shale and used in
candles, cosmetics, polishes, and sealing and waterproofing compounds.
• Olefins:
Olefin is a hydrocarbon with two or three covalent bonds in between carbon
atoms. It is formed at a refinery conversion process and used for the manufacture of
petrochemical products.
Aromatic hydrocarbon:
These carbocyclic compounds contain at least one benzene ring.

6. petroleum refining :
Basic refinery processes
 Functions of Refinery Units:
•
(1) separating the many types of Hydrocarbon present in crude oils into
fractions of more closely related properties
•
(2) chemically converting the separated hydrocarbons into more
desirable reaction products, and
•
(3) purifying the products of unwanted elements and compounds.
 Types of Distillation:
•
•
•
•
•
Fractional Distillation
Vacuum Distillation
Atmospheric Distillation
Thermal Cracking
Catalytic Cracking

7. Cracking:
Cracking means heating of higher boiling petroleum fraction like heavy fuel oil at
high temperature and pressure to produce lower boiling lighter fraction.
 It is an endothermic reaction .
The main application of cracking is for production of gasoline from gas oil.
It is also done to produce olefins(for petrochemicals production) from gas oil and
naphtha ,lower viscosity furnaces oil (visbreaking) and coke(coking).
Important chemical reactions in thermal cracking which carried out at
comparatively
• Higher pressure (1-70atm)
• Higher temperature (450-7500C)

8. Necessity for cracking

An average crude oil on distillation yields:
• 20-30% Petrol
• 30-45% intermediate oils (kerosene, naphtha, diesel, lube oil etc)
• 25-50% residual fuel oils.
 Why we need cracking…?
• Increasing demand for high quality (octane number) petrol exceed the
availability of petrol by distillation alone.
• Hence, cracking is heavier fuel oils( obtained by distillation of crude) is
done to produced mainly petrol.
• Cracking is also done to reduce the viscosity of fuel oil (by
visbreaking), to produce coke and to produce olefins which is feed for
petrochemical industry.

9. Types of Cracking
 There are two types of cracking
• Thermal Cracking
• Catalytic Cracking
 When cracking (i.e. breaking larger molecules into smaller ones by heating)
is done without any catalyst(which increases the rate of reaction) then it is
called thermal cracking.
 Cracking in presence of a catalyst is called catalytic cracking.

10.  Thermal cracking
 The fuel which may be gas oil, fuel oil(heavy or light), atmospheric or vacuum
residue is heated up to 450-7500C at pressure ranging from 1-70 atmosphere to
produce gas, petrol, diesel, cracked residue(coke) etc.
 Octane number of petrol obtained may be up to 75.
 The yield and quality of products will depend upon the type of
feed, temperature and pressure.
 Thermal cracking processes
 Depending upon the pressure and temperature employed for the cracking
and the characteristics of feed, there are various thermal cracking processes in
which the product yields and characteristics are different. Some of them are
given below
1.
2.
3.
4.
Low temperature and high pressure thermal cracking process
High temperature and high pressure process
Low pressure and high temperature process
High temperature and low pressure process

11. 1. Low temperature and high pressure thermal cracking
process
• Feed- is residue from atmospheric distillation unit
• Products- are mainly gas, fuel oil, and gasoline.
• Temperature and pressure employed are 5000C and 20 atm. Respectively.
• This process is also called visbreaking when heavy fuel oil is thermally
cracked to residue its viscosity so that it can be properly atomised through the
burners.
• This process is employed for heavy oil cracking.
2. High temperature and high pressure process
•Feed- gas oil.
•Products-gas and gasoline mainly.
•T and P-5300C and 50-70 atm. Respectively.
•This is also called light oil thermal cracking process.
•When feed is naphtha, it is called thermal reforming

12. 3. Low pressure and high temperature process
•Feed –residue from atmospheric distillation unit.
•T and P – >5500C and 2-5 atm. Respectively.
•Products –gasoline and gas rich in unsaturated hydrocarbons(a feed for
petrochemical)
4. High temperature and low pressure process
•This is also called pyrolysis.
•T and P – 7000C and 1 atm. Respectively.
•Products –is mainly gas rich in aromatic and unsaturated hydrocarbons.

13.  Coking
• Feed –is vacuum residue.
• T and P – 400-4500C and 2 atm. Respectively.
• Products –are gas, coker gasoline, coke for electrode manufacture(main
product ) and coke residue.
 Thermal cracking reactions
Few important thermal cracking reactions are:
1.
2.
3.
4.
Decomposition and destructive condensation
Hydrogenation and dehydrogenation
Polymerisation
Cyclisation
 Octane number increases in the order
Paraffin
•
•
Olefin
Naphthen
Iso-paraffin
Aromatic
That means aromatics have the highest octane number and paraffins have
the lowest.
Hence, cracking reactions should aim for producing hydrocarbons with higher
octane number , if production of gasoline is the main product.

14.  Decomposition and destructive condensation
• Decomposition and destructive condensation of olefins to produce high
octane number aromatics is shown:
 Hydrogenation
• Hydrogenation (hydro-cracking) of higher boiling paraffin to lower boiling
paraffin shown:
• Since, lower boiling paraffins have higher octane number, hydrogenation
(hydro-cracking) improves octane rating i.e., quality of gasoline(petrol).

15. Dehydrogenation
• Dehydrogenation of naphthenes to aromatics also helps in increasing the octane
number of cracking products.
 polymerisation
• Converts C3 and C4 olefins to higher diolefins

16.  Cyclisation
•Reaction converts n-paraffins to aromatics which have higher octane number.
•This reaction is also called dehydrocyclisation because hydrogen is a co-product.

17.  Commercial Thermal Cracking processes
Mainly there are four commercial processes employed for thermal
cracking in oil refineries. They are:
1. Dubbs thermal cracking process(old process In old refineries)
2. Pyrolysis
3. Visbreaking
4. Coking

18.  Dubbs thermal cracking process
• It is an old thermal cracking process used in oil refineries up to about 1939 on gas oil
having fuel oil stocks.
•
Heavy oil or topped crude is pumped into a bubble cap fractionators where it is
mixed with heavy cracked oil from the process.
•The mixed feed at pressure up to 70kg/cm2 is passed through a tube furnace where it is
heated maximum up to 5400C before passing to the soaking chamber. This permits a
time dwell during which a controlled amount of cracking can take place.
•Pressure is reduced before passing into a flash chamber where heavy cracked fuel oil
separates and then to the bubble cap fractionators from which the cracked gas and
gasoline passes out at the top and furnace distillate from side. Its flow sheet is given in
fig.

19. cooler
Cracked gas
Topped crude
Or
Heavy oil
Cracked gasoline
Octane no.66
5400C
Bubble cap
fractionators
Heater
Furnace distillate
70kg/cm2
Heavy cracked oil
Flash drum
Dubbs thermal cracking process
Socking
chamber

20. Pyrolysis
Pyrolysis (mild thermal cracking) is done mainly for the production of lighter products
mainly unsaturated like olefin(ethylene) and naphthene polymers, diolefin, benzenes and
toluene's etc.
 It is carried out at high temperature(650-7000C ) and low pressure.
Initially this process was used for the production of toluene.
The feed to Pyrolysis is raw crude oil, kerosene and natural gasoline.
 A higher yield of gas, benzene and toluene are obtained by this process.
 The feed is heated in special furnaces.
 The products of the Pyrolysis are cooled and delivered to the fractionating column.
 The Pyrolysis gas is drawn from the top of the fractionators.
The aromatics are taken from middle section from its bottom of the column.
 The residue which is resinous in nature is discharged from its bottom.

21. Pyrolysis
gas
pyrolysis
Feed
heated
fractionator
aromatics
Pyrolysis
residue

22. Visbreaking
• Visbreaking, a mild form of thermal cracking, significantly lowers the viscosity of heavy
crude-oil residue without affecting the boiling point range.
• Residual from the atmospheric distillation tower is heated (800 -950 F) at atmospheric
pressure and mildly cracked in a heater.
• It is then quenched with cool gas oil to control over cracking, and flashed in a
distillation tower.
• Visbreaking is used to reduce the pour point of waxy residues and reduce the viscosity
of residues used for blending with lighter fuel oils.
• Middle distillates may also be produced, depending on product demand.
• The thermally cracked residue tar, which accumulates in the bottom of the fractionation
tower, is vacuum-flashed in a stripper and the distillate recycled.
 Visbreaking process:
Feedstock
From
Process
Residual
Atmospheric
Decompose
tower
&
Vacuum tower
Typical products - to - unit
•Gasoline or distillate To Hydrotreating
•Vapor To Hydrotreater
•Residue To Stripper or recycle
•Gases To Gas plant

24. Coke Formation
 Thermal cracking reactions are highly endothermic and
require heat which is either provided by heating
furnaces or generated by burning some of the
produced coke.
 The formation of coke from alkanes, alkenes and
cycloalkanes is generally endothermic, while it is
exothermic for aromatics.

25. Coking Processes
Coking is a severe method of thermal cracking used to upgrade heavy residuals into
lighter products or distillates. Coking produces straight-run gasoline (coker naphtha) and
various middle-distillate fractions used as catalytic cracking feedstock. The process so
completely reduces hydrogen that the residue is a form of carbon called "coke." The two
most common processes are delayed coking and continuous (contact or fluid) coking.
Three typical types of coke are obtained (sponge coke, honeycomb coke, and needle
coke) depending upon the reaction mechanism, time, temperature, and the crude
feedstock.
Delayed Coking
•In delayed coking the heated charge (typically residuum from atmospheric distillation
towers) is transferred to large coke drums which provide the long residence time needed
to allow the cracking reactions to proceed to completion. Initially the heavy feedstock is
fed to a furnace, which heats the residuum to high temperatures (900 -950 F) at low
pressures (25-30 psi) and is designed and controlled to prevent premature coking in the
heater tubes. The mixture is passed from the heater to one or more coker drums where
the hot material is held approximately 24 hours (delayed) at pressures of 25-75 psi, until
it cracks into lighter products. Vapors from the drums are returned to a fractionator
where gas, naphtha, and gas oils are separated out. The heavier hydrocarbons
produced in the fractionator are recycled through the furnace.

26. After the coke reaches a predetermined level in one drum, the flow is diverted to another
drum to maintain continuous operation. The full drum is steamed to strip out uncracked
hydrocarbons, cooled by water injection, and decoked by mechanical or hydraulic
methods. The coke is mechanically removed by an auger rising from the bottom of the
drum. Hydraulic decoking consists of fracturing the coke bed with high-pressure water
ejected from a rotating cutter.
Continuous Coking
Continuous (contact or fluid) coking is a moving-bed process that operates at
temperatures higher than delayed coking. In continuous coking, thermal cracking occurs
by using heat transferred from hot, recycled coke particles to feedstock in a radial mixer,
called a reactor, at a pressure of 50 psi. Gases and vapors are taken from the reactor,
quenched to stop any further reaction, and fractionated. The reacted coke enters a
surge drum and is lifted to a feeder and classifier where the larger coke particles are
removed as product. The remaining coke is dropped into the preheater for recycling with
feedstock. Coking occurs both in the reactor and in the surge drum. The process is
automatic in that there is a continuous flow of coke and feedstock.

27. Coking Processes
Feedstock
From
Process
Typical products - to - unit
Naphtha,
gasoline
Distillation
column,
blending
Decomposition
•Gasoline or distillate To
Hydrotreating
•Vapor To Hydrotreater
•Residue To Stripper or recycle
•Gases To Gas plant
Clarified oil
Tars
Catalytic
cracker
Various units
Wastewater,
(sour) Gases
Treatment
Gas plant
Coke to Shipping, recycle
Gas oil to Catalytic cracking

29. Ref:
http://www.setlaboratories.com/therm/tabid/107/default.aspx
http://en.wikipedia.org/wiki/Cracking_(chemistry)
Book:
Title:
Elements of Fuel, Furnaces and Refractories
Author:
O.P. Gupta

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