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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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