Cracking-Thermal and

Catalytic
Introduction (Cracking ) :
➢ The process in which heavy hydrocarbon or petroleum fractions
having high boiling points are decomposed at elevated temperatures
(above decomposition temperatures) and pressure into variety of lower
mol. wt. hydrcarbons, which are more volatile is called cracking. Any
fraction of the crude oil from naphtha to residue – can be processed
thermally.

➢ It is an endothermic reaction.

➢ The main application of cracking is for production of gasoline from gas oil.
Production of olefins from gas oil and naphtha, lower viscosity furnace oil
and coke.
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➢ Important chemical reactions in thermal cracking(non catalytic) which is
carried out at comparatively higher pressure (1-70 atm) and
temperature (450-750 C) by heating the feed are decomposition,
dehydrogenation, isomerization and polymerization.

➢ First the higher paraffins decompose to a lower paraffin and an olefin

n-decane →n-hexane + butene

➢ The olefin then undergoes isomerization, dehydrogenation and

polymerization reaction.

Butene → iso-octene
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➢ Apart from liquid fuels, gases and solids (coke) are also produced
during cracking.

➢ Catalytic cracking produces less coke, less gas but more liquid
products.

➢ Thermal cracking is mainly used for production of olefins, coking and

visbreaking.
 Types of cracking
Two common types are-
1. Thermal cracking: cracking i.e breaking larger molecules into the smaller ones by heating, in
the absence of a catalyst
➢Fuel which may be gas oil, fuel oil, atmospheric or vacuum residue is heated upto 450-750C
degrees at P (1-70atm) to produce gas, petrol, diesel cracked residue (coke) etc.

2. Catalytic cracking: cracking in the presence of a catalyst

➢Here the feed (which may be kerosene, fuel oil, gas oil, lubricating oil etc.) is heated in the
presence of catalyst (Pt, Ni, Fe, Cr, silica-alumina, aluminium chloride, sulfuric acid, phosphoric
acid etc.) at 350-650C and 1-15 atm to produce gas, petrol, diesel and residue (coke).
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A typical yield pattern in cracking is given as

Feed-residue of vacuum distillation of crude

• Petrol 55%
• Diesel 20%
• Fuel oil 15%
• Coke 10%

Difference between simple and Vacuum distillation ?

Advantages of catalytic over thermal cracking

• Lower T and P requirement

• High yield and octane no. of the petrol product
• Low Sulfur content of the products
Thermal cracking processes
• Depending upon the characteristics of feed, T and P for the cracking,
there are various thermal cracking processes in which products yields
and characteristics are different.

1. Low T and high P thermal cracking

2. High T and high P process
3. High T and low P
Thermal Cracking Reactions
➢ Few important thermal cracking reactions are:
1. Decomposition and destruction condensation
2. Hydrogenation and dehydrogenation
3. Polymerization
4. Cyclisation

➢ Octane number increases in order of paraffins < Olefins < Naphthenes < iso
paraffins < Aromatics
➢ Hence cracking reactions should aim for producing hydrocarbons with
higher octane number if the production of gasoline is the main product.
Decomposition and destructive condensation

Decomposition and destructive condensation of olefins to produce high octane

number aromatics is shown below.

Butadiene
Hydrogenation

Hydrogenation ( i.e. hydrocracking ) of higher boiling paraffins to lower boiling

paraffins ( since lower boiling paraffins have higher octane number) is shown
below.

So hydrocracking improves the octane rating of gasoline.

Dehydrogenation
Dehydrogenation of naphthenes to aromatics also helps in increasing the
octane number of cracking products.
Polymerization
Converts C3 and C4 olefins to higher diolefins.
Cyclisation
Cyclisation reaction converts n- paraffins to aromatics which have higher
octane number.

The reaction is also called dehydrocyclisation because hydrogen is a co

product.
Visbreaking
Reactions in visbreaking include cracking of side chains of aromatics.
Commercial thermal cracking processes
Mainly there are four commercial processes employed for
thermal cracking in oil refineries
1. Dubbs thermal cracking in oil refineries
2. Pyrolysis
3. Visbreaking
4. Coking
( point 1 and 3 is your assignment )
Pyrolysis (mild thermal cracking)
➢It is done mainly for the production of lighter products mainly
unsaturates like olefin (ethylene) and naphthene polymers, diolefins,
benzenes, and toluenes.

➢Carried out at high T and low P (650-700C)

➢Feed is raw crude oil, kerosene and natural gasoline(petrol).

➢A higher yield of gas, benzene and toluene are obtained

Coking
➢ Coking is a thermal process for the continuous conversion of residual into
lower-boiling products.

➢ In coking more severe reaction conditions are used than visbreaking so that
the feed is completely converted to lighter products resulting from cracking.

➢ The feedstock can be thermal cracking residual, pitch from the pyrolysis
plant and atmospheric column residue.

➢ The products are gases, naphtha, fuel oil, gas oil, and coke.

➢ When the primary product in coking is coke, then gas and gasoline are
secondary products. If gasoline is primary product, then coke is the
secondary product.
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➢ The gas oil may be the major product of a coking operation, and serves primarily
as a feedstock for catalytic cracking units.

➢ The coke obtained is usually used as fuel, but processing for specialty uses,
such as electrode manufacture, production of chemicals, and metallurgical coke,
is also possible and increases the value of the coke.

➢ For these uses, the coke may require treatment to remove sulfur and metal
impurities.

➢ Coking is severe thermal cracking. The residue feed is heated to about 475 to
520 °C (890 to 970 °F) in a furnace with very low residence time and is
discharged into the bottom of a large vessel called a coke drum for extensive
and controlled cracking.Coking processes generally utilize longer reaction times
than thermal cracking processes.
Decoking from the drums
➢ Decoking is a routine daily occurrence accomplished by a high-
pressure water jet.

➢ First the top and bottom heads of the coke drum are removed.
Next a hole is drilled in the coke from the top to the bottom of the
vessel.

➢ Then a rotating stem is lowered through the hole, spraying a

water jet sideways. The high-pressure jet cuts the coke into
lumps, which fall out the bottom of the drum for subsequent
loading into trucks or railcars for shipment to customers.
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➢ Typically, coke drums operate on 24-hour cycles, filling with coke over one
24-hour period followed by cooling, decoking, and reheating over the next
24 hours.

➢ Cokers produce no liquid residue but yield up to 30 percent coke by weight.

➢ Much of the low-sulfur product is employed to produce electrodes for the

electrolytic smelting of aluminium.
Delayed Coking
➢ Delayed coking is a thermal cracking process in which a hydrocarbon
feedstock, mainly residue is converted to lighter and more valuable
products and coke.

➢ Main advantage of the process is that it can take residual stocks from a
wide variety of process (even the heaviest of residues) unit in a Refinery
Coking Furnace.

➢ Coking provides partial or complete conversion of feed to naphtha and

diesel along with coke formation .

➢ Feed to these units is normally heavy atmospheric residues, although heavy

catalytic cycle oils and cracked tars may also be used.
 Catalytic cracking
➢ Cracking of the feedstock in the presence of a catalyst is called catalytic cracking
which gives greater yield and higher octane number of gasoline as compare to
thermal cracking or reforming.

➢ Catalytic cracking is the most important and widely used refinery process for
converting heavy oils into more valuable gasoline and lighter products.

➢ Originally cracking was accomplished thermally but the catalytic process has
almost completely replaced thermal cracking because more gasoline having a
higher octane number and less heavy fuel oils and light gases are produced.

➢ It reduces severity of operation as T and P is low 450 oC and 0.5 to 1.5 atm.

( Difference between cracking and reforming ???? )

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➢ Gasoline produced by catalytic cracking consists largely of iso-paraffins
and aromatics. The iso -paraffins and aromatic hydrocarbons have high
octane numbers and greater chemical stability than mono-
olefins(alkenes)

➢ The light gases produced by catalytic cracking contain more olefins

than those produced by thermal cracking.

➢ Sulfur compounds are changed in such a way that the sulfur content of
gasoline produced by catalytic cracking gasoline is lower than the
sulfur content of gasoline produced by thermal cracking.
 Feed for catalytic cracking
Feed ranges from light gas oil to reduced crude. For production of
aviation gasoline(petrol), the feed in the boiling range of 350 – 420 oC
should be used. Whereas for motor gasoline, this boiling range should
be 420 to 500 oC .
Feed stocks are generally
❑Crude oil/ primary products from atmospheric distillation unit
❑Kerosene and gas oil from atmospheric distillation unit
❑Light lubricating oil (its cracking produces aromatics which
decompose to give deposits on catalyst)
❑Deasphalted crude
 Catalytic cracking
➢ In general, catalytic cracking may be regarded as the modern method
for converting high-boiling petroleum fractions, such as gas oil, into
high-quality gasoline and other added value products.

➢ The cracking process produces carbon (coke) which remains on the

catalyst particle and rapidly lowers its activity.

➢ To maintain the catalyst activity at a useful level, it is necessary to

regenerate the catalyst by burning off this coke with air.

➢ As a result, the catalyst is continuously moved from reactor to

regenerator and back to reactor.
 Catalysts used in catalytic cracking
Presently metallic catalysts are used such as Pt, Cr, Fe and Ni. Synthetic catalysts
used are silica-alumina and silica magnesia.
Desirable properties of the catalysts are
➢High reactivity
➢Good selectivity
➢Sufficient hardness and strength
➢Absence of sulfur, nitrogen and metallic constituents
➢Easy regenerability
➢High surface area
➢High porosity
Types of Catalytic cracking
➢ Depending upon the physical conditions of the catalyst bed, there are
three main processes for catalytic cracking namely.
1. Hourdry’s fixed bed catalytic cracking process.
2. T.C.C. (Thermoformer catalytic cracking) moving bed process.
3. Fluidised bed catalytic cracking ( F.C.C.) process
Houdry’s Fixed Bed Process
➢ This catalytic cracking process was first used in 1936 has become
obsolete now by the development of moving bed and fluidised bed
reactor.
Effect of reaction conditions
What happens in a moving bed or fluidised bed catalytic cracking reactor depends
upon the
➢ Temperature
➢ Pressure
➢ Ratio of catalyst to oil passing through the reactor
➢ Catalyst activity
➢ Effect of coke concentration on catalyst

Effect of Temperature

➢ Rate of reaction increases by increasing temperature.

➢ For every 40 oC rise, the decomposition rate doubles.
➢ For a given conversion gasoline yield is reduced at higher temperature.
Effect of Pressure
➢ At a given conversion, increasing the pressure increases the production of
coke.
Effect of catalyst to oil ratio
Conversion increases with increase in catalyst to oil ratio as it shortens the time
required for catalyst to pass through the reactor and thereby reduces the extent
of its deactivation from coke production.
Effect of coke concentration on catalyst
➢ Lesser is the concentration of coke on the catalyst, better is the effective
activity of the catalyst.
➢ Lower coke concentration is obtained by increasing catalyst to oil ratio.

Effect of catalyst activity

➢ With decrease in catalyst activity, the conversion declines.
➢ Product yield is poorer at low catalyst activity hence after several cycles
of regeneration some catalyst is deliberately discarded so that more
fresh catalyst can be added to improve activity and yields.
Hydrocracking
➢Cracking in the presence of hydrogen is called hydrocracking. It is also
called destructive hydrogenation and leads to the formation of
saturated compounds of lower mol. wt. by the cracking of heavy
fractions and residues under pressure of hydrogen.
➢Hydrocracking achieves high conversion into gasoline and produces
high quality diesel fuels.
➢Hydrocracking and other hydrogenation processes in refinery have
been possible mainly because of the availability of surplus hydrogen
rich gases from the catalytic reforming of naphthas.