petroleum refining

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2. CHAPTER 5
C R A C K I N G

3. CHAPTER 5 OUTLINE
• Cracking
• Fluid catalytic cracking
I. Principles
II. Recent developments
III. Feedstock
IV. Product yields and qualities
V. Catalyst and operating parameters
• Hydrocracking
I. Principles
II. Process requirements
III. Product yields and qualities
IV. Residue cracking

4. OIL REFINING
• Method by which crude oil converted to
petroleum products
– (I think that a barrel (42 gal—produces 44 gal of
petroleum products)
• Distillation (fractionation)
– At high temperature the lightest fractions rise to
the top of a tower, heavier fractions condense at
bottom

5. OIL REFINING
Typical Oil
– Gasoline C4 to C10 27%
– Kerosene C11 to C13 13%
– Diesel C14 to C18 12%
– Heavy gas oil C19 to C25 10%
– Lubricating oil C26-C40 20%
– Residue >C40 18%

6. OIL REFINING
• Thermal Cracking
• Catalytic Cracking

7. OIL REFINING
• What we get out of oil now with modern
refineries:
– 50% gas
– 30% fuel oil
– 7.5% jet fuel
HOW??

9. CRACKING
• Crude oil contains many large molecules. If these are to
be used as fuels or feedstock for the chemical industry
then they have to be cracked into smaller molecules.
• When hydrocarbons burn they are reacting with oxygen
in the air. In general, the smaller the molecule the better
it will mix and then react with the air.
Fuel gas
Naphtha
Diesel
Petrol
Kerosine
Fuel Oil and bitumen

10. CRACKING
Involves the breaking of C-C bonds in alkanes
Converts heavy fractions into higher value products
• THERMAL
proceeds via a free radical mechanism
• CATALYTIC
proceeds via a carbocation (carbonium ion) mechanism

11. • High Pressure ... 7000 kPa
• High Temperature ... 400°C to 900°C
• Free Radical Mechanism
• Homolytic fission
• Produces mostly alkenes ...e.g. ethene for
making polymers and ethanol
• Produces Hydrogen ... used in the Haber
Process and in margarine manufacture
• Bonds can be broken anywhere in the molecule
by C-C bond fission or C-H bond fission
THERMAL CRACKING

12. • Slight pressure
• High Temperature
• Use catalyst to
Catalysts include
speed up the cracking reaction.
zeolite, aluminium hydrosilicate,
bauxite and silica alumina.
• Carbocation Mechanism
• Heterolytic fission
• Produces branched and cyclic alkanes, naromatic
hydrocarbons used for motor fuels
**ZEOLITES are crystalline aluminosilicates;
clay like substances
CATALYTIC CRACKING

13. •Catalytic cracking is similar to thermal cracking except that catalysts
facilitate the conversion of the heavier molecules into lighter products.
•Use of a catalyst (a material that assists a chemical reaction but does
not take part in it) in the cracking reaction increases the yield of
improved-quality products under much less severe operating conditions
than in thermal cracking.
•Typical temperatures are from 450°-510° C at much lower pressures of
10-20 psi.
•The catalysts used in refinery cracking units are typically solid materials
(zeolite, aluminum hydrosilicate, treated bentonite clay, fuller's earth,
bauxite, and silica-alumina) that come in the form of powders, beads,
pellets or shaped materials called extrudates.
CATALYTIC CRACKING

14. There are three basic functions in the catalytic
cracking process:
I. Reaction: Feedstock reacts with catalyst and
cracks into different hydrocarbons;
II. Regeneration: Catalyst is reactivated by
burning off coke; and
III. Fractionation: Cracked hydrocarbon stream is
separated into various products.
BASIC FUNCTIONS IN
CATALYTIC CRACKING

15. molecules• Large hydrocarbons are broken into smaller
using heat and a catalyst.
• This process is known as catalytic cracking.
• The small molecules produced are then separated by
distillation.
CATALYTIC CRACKING PROCESS
Catalytic
crackerHeat to
vaporise
Distillation
tower
pressure
Big Molecules
Molecules break
up

16. In the catalytic cracker long chain molecules are
‘cracked’. An example of such a reaction is:
C8H1
8
 C6H14 +C2H4
C C
H
H
H
H
+
ethene
H H H H H H H H
H C C C C C C C C H
H H H H H H H H
Octane
H
H H H H H H C
C C C C C H H
H H H H
H
hexane
Ethene
is used
to make
plastics
Heat
pressure
catalyst
Used as a
fuel
CATALYTIC CRACKING REACTION

17. CATALYTIC CRACKING REACTION
• Products formed are the result of both primary
and secondary reactions.
• Primary rxns – involve the initial C-C bond
scission and the immediate neutralization of the
carbonium ion.
• Primary rxns as below:
Paraffin paraffin + olefin
Alkyl napthene napthene + olefin
Alkyl aromatic aromatic + olefin

21. CLASSIFICATION OF
CATALYTIC CRACKING
• Catalytic cracking processes can be classified
as either moving-bed (Thermafor catalytic
cracking - TCC) or fluidized-bed units (FCC).
• Very few TCC units in operation today, FCC unit
has taken over the field – where the major
fraction of the cracking reaction occurs.

22. PROCESS FLOW OF
CATALYTIC CRACKING
• Process flows for FCC and TCC are similar.
• The hot oil feed is contacted with catalyst in either the
feed riser or the reactor.
• The catalyst is progressively deactivated by the
formation of coke on the surface of the catalyst.
• Catalyst and hydrocarbon vapors are separated
mechanically, oil remaining on the catalyst is removed
by steam stripping before catalyst enters the
regenerator.
• The oil vapors are taken overhead to a fractionation
tower for separation into streams having the desired
boiling ranges.

23. CATALYST REGENERATED
What happen to the catalyst then?
• It flows into the regenerator and is activated by burning
off the coke deposits with air.
• Regenerator temperatures are carefully controlled to
prevent catalyst deactivation by overheating and to
provide the desired amount of carbon burn-off. – by
manipulating the air flow in the exit flue gas.
• Flue gas & catalyst are separated by cyclone separators
and electrostatic precipitators.
• Important to make sure the catalyst is steam-stripped as
it leaves the generator, to remove the adsorbed oxygen
before it is contacted with the oil feed.

24. FLUIDIZED –BED CATALYTIC CRACKING

25. Introduction - FCC
• The fluidized catalytic cracking (FCC) unit is the
heart of the refinery and is where heavy low-value
petroleum stream such as vacuum gas oil (VGO) is
upgraded into higher value products, mainly
gasoline and C3/C4 olefins, which can be used in
the alkylation unit for production of gasoline (C7–
C8 alkylates).
• Major developments have occurred in areas of new
catalysts and new reactor and regenerator designs.

26. Role of FCC in the Refinery
• The role of the FCC is to
take heavy desulphurised
feedstock and crack it into
lighter, mainly high octane
gasoline.
• The FCC also produces
olefins (C5 = and C4 =) and
LPG.

27. FCC Process Flow Diagram

28. Introduction - FCC
• FCC employs a catalyst in the form of very fine
particles (70 microns), that can behave as a
fluid when aerated with a vapor.
• Two type of FCC units:
I. Side-by –side type, where the reactor and
regenerator are separate vessels adjacent to
each other
II. Orthoflow/stacked type, reactor is mounted
on top of the regenerator.

29. Zeolite as Catalyst in FCC
• Early attempts to increase production of light
olefins from the FCC were based primarily on
process variables.
• Poor selectivity of this approach resulted in
excess production of dry gas and coke.
• By 1970s, researchers found that non-Y
zeolites could also co-produce light olefins
(C2= to C5=), often at the expense of gasoline.

30. Zeolite as Catalyst in FCC
• The
development
chronology of
for
catalyst and additives
the
light
to enhance
production of
olefins in FCCs.

31. Feedstock
• The main feedstock used in a FCC unit is the gas oil ,
which can be considered mixtures of aromatic,
naphthanic and paraffinic molecules.
• There are also varying amounts of contaminants
such as sulphur, nitrogen and metals. To protect the
catalyst,
required
feed pre-treatment by hydrotreating is
in order to remove contaminants
(especially sulphur) and improve cracking
characteristics and yields.

32. • Nitrogen tends to poison the catalyst by neutralising
its acid sites. However, the FCC process is unaffected
if the nitrogen content level is controlled below 0.2%.
• The acidity and unique porous structures of zeolites
play an important role in controlling the activity and
selectivity of many zeolite-based catalysts.
• Some possible feedstocks atmospheric distillates,
coking distillates, visbreaking distillates, VGO,
atmospheric residue (desulphurised) and vacuum
residue (desulphurised, deasphalted).
Feedstock

33. FCC products