fluidised catalytic cracking for student
TRANSCRIPT
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Chemistry and technology ofpetroleum
By Dr. Dang Saebea
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Fluidised Catalytic Cracking
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Introduction The fluidised catalytic cracking (FCC) unit is
the heart of the refinery and is where heavy
low-value petroleum streamsuch as vacuum
gas oil (VGO) is upgraded into higher valueproducts, mainly gasoline and C3/C4 olefins,
which can be used in the alkylation unit for
production of gasoline (C7C8 alkylates).
Major developments have occurred in two
areas: new catalysts and new reactor and
regenerator designs.
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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 producesolefins (C5 =and C4 =)
and LPG.
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FeedstockThe 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, feed pre-treatment by hydrotreating
is required in order to remove contaminants andimprove crackingcharacteristics and yields.
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Nitrogen tends to poison the catalyst by
neutralising its acid sites. However, the FCC
process is unaffectedif the nitrogen content
level is controlled below 0.2%.
Some possible feedstocks atmospheric
distillates, coking distillates, visbreaking
distillates, VGO, atmospheric residue
(desulphurised) and vacuum residue
(desulphurised, deasphalted).
Feedstock
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FCC products
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Fluidisation When a fluid flowsupward through a packed
bed of catalyst particles
at low velocity, theparticles remain
stationary
As the fluid velocity
increases, the pressuredrop increases. Then the
particles begin to move.
This is the minimum
fluidisation velocity. As the velocity increases,
the bed expands and bed
porosity increases while
the pressure drop
remains practicallyModes of fluidisation
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FCC Reactions
The VGO undergoes the desired primary cracking into
gasoline and LCO.
A secondary reaction also occurs, which must be limited,such as a hydrogen transfer reaction which lowers the
gasoline yield and causes the cycloaddition reaction.
The latter could lead to coke formation (needed to provide
heat for catalyst regeneration).
The main
reaction in the
FCC is thecatalytic
cracking of
paraffin, olefins,
naphthenesand side chains
in aromatics.
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Primary Reactions
Primary cracking occurs by the carbenium ionintermediates in the following steps:
(a) Olefin is formed first by the mild thermal
cracking of paraffin:
(b) Proton shift:
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(c) Beta scission:
Carboncarbon scission takes place at the
carbon in the position beta to the carbenium ions
and olefins.
The newly formed carbenium ion reacts with
another paraffin molecule and further propagatesthe reaction.
Primary Reactions
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Primary Reactions
1. Olefinssmaller olefins
2. AlkylaromaticsDealkylation
3. AlkylaromaticsSide chain cracking
Beside paraffins:
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Secondary Reactions
Isomerisation
The final product is the transformation of paraffins and olefinsto isoparaffins.
The final result would be the cyclisation of olefins to
naphthenes and
possibly further cyclisation to coke.
Cyclisation
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Process Description
riserSteam and VGO heated are fed to the bottom of the riser.
The regenerated hot catalyst at is also fed to the bottom of the
riser.
316427 C649760 C
-endothermic reactions
- residence time =210 s
P D i ti
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Process Description
-Steam is injected into the stripper section, and the oil is removedfrom the catalyst with the help of some baffles installed in the
stripper.
- The oil is stripped in this way from the catalyst and the spent
catalyst is sent to the regenerator.
-The coke in the spent catalyst is burned off in the regenerator by
CO, CO2
, H2
O, Air
482538 C
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Process Description
-Hydrocyclones are installed to catch any solid particles carriedout in the overheated stream.
-The product gases from the reactor are sent to the fractionator.
-The light gases are sent to the gas concentration unit.
-The filtered decant can be used as an aromatic solvent or
recycled back to the riser with the HCO.
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The End
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