Unit 12. Fluidised Catalytic Cracking...Cracking of relatively long-chain paraffins and olefins can go up to 95% completion at cracking conditions. Certain hydrogen transfer reactions

2016

Assistant teacher

Belinskaya Nataliya Segeevna

PROFESSIONAL COURSE IN ENGLISH

“FUNDAMENTALS OF PETROLEUM REFINING”

Unit 12. Fluidised Catalytic Cracking




Introduction

Catalytic cracking is the process in which

heavy low-value petroleum stream such as

vacuum gas oil is upgraded into higher value

products:

2

FCC products:

gasoline

olefins

LPG

Alkylation

unit

Ultra clean

gasoline

(C7–C8

alkylates)

Catalytic cracking processes

Fluidised catalytic cracking (FCC)

Petro-FCC

Residue FCC (RFCC)

Deep catalytic cracking (DCC)

3




Role of FCC in the Refinery

4

Figure 1. Role of FCC in refining

operation




Feedstock

The main feedstock used in a FCC unit is the

gas oil boiling between 316 ºC and 566 ºC.

Some possible

feedstocks are:

atmospheric distillates

coking distillates

visbreaking distillates

vacuum gasoil

atmospheric residue

vacuum residue

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Products

6

Table 1. FCC products




FCC Reactions

The main reaction in the FCC is the catalytic

cracking of paraffin, olefins, naphthenes and

side chains in aromatics.

7 Figure 2. FCC reactions network




Primary Reactions

Primary cracking occurs by the carbenium

ion intermediates in the following steps:

(a) Olefin is formed first by the mild

thermal cracking of paraffin:

nC8H18 → CH4 + CH3 – (CH2)4 – CH = CH2

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Primary Reactions

(b) Proton shift:

(c) Beta scission:

CH3 – (CH2)4 – CH+ – CH3 → CH3 – CH = CH2

+ CH2+ – CH2 – CH2 – CH3

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Primary reactions

The newly formed carbenium ion reacts with

another paraffin molecule and further

propagates the reaction.

The chain reaction is terminated when

(a) the carbenium ion losses a proton to the

catalyst and is converted to an olefin; or

(b) the carbenium ion picks up a hydride ion

from a donor (e.g. coke) and is converted to

paraffin.

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FCC Reactions

Other primary reactions

Olefins – smaller olefins

CH3 – CH = CH – CH2 – CH2 – CH2 – CH3 →

CH3 – CH = CH – CH3 + CH3 – CH = CH2

Alkylaromatics – Dealkylation

Alkylaromatics – Side chain cracking

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FCC Reactions

Hydrogen transfer plays a key role in the

gas oil cracking process.

It reduces the amount of olefins in the

product, contributes to coke formation, and

thereby influences the molecular weight

distribution of the product.

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FCC Reactions

Through intermolecular (bimolecular)

hydrogen transfer, highly reactive olefins

are converted to more stable paraffins and

aromatics as in the following reaction:

3CnH2n + CmH2m → 3CnH2n+2 + CmH2m–6

Olefin Naphthene Paraffin Aromatic

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FCC Reactions

Further loss of hydrogen to olefins by

aromatics or other hydrogen-deficient

products results in more paraffins and coke

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FCC Secondary Reactions

Gasoline formed from primary cracking can

undergo further secondary cracking, which

is generally caused by hydrogen transfer

mechanisms such as isomerisation,

cyclisation and coke formation.

15





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Isomerisation

The final product is the transformation of

paraffins and olefins to isoparaffins.


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Cyclisation

The final result would be the cyclisation of

olefins to naphthenes and possibly further

cyclisation to coke.

FCC Reactions

The main reactions in the FCC reactor can be

summarised as follows:

Paraffins Thermal catalytic cracking

Paraffin cracking → Paraffins + Olefins

Olefins Olefin cracking → LPG olefins

Olefin cyclisation → Naphthenes

Olefin isomerisation → Branched olefins +

Branched paraffins

Olefin H-transfer → Paraffins

Olefin cyclisation → Coke

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FCC Reactions

Naphthenes

Naphthene cracking → Olefins

Naphthene dehydrogenation → Aromatics

Naphthene isomerisation → Restructured naphthenes

Aromatics

Aromatics (side chain) → Aromatics + Olefins

Aromatic transalkylation → Alkylaromatics

Aromatic dehydrogenation → Polyaromatics → Coke

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Thermodynamics of FCC Reactions

The key reaction in cracking is β-scission, which is not equilibrium

limited.

Cracking of relatively long-chain paraffins and olefins can go up to

95% completion at cracking conditions.

Certain hydrogen transfer reactions act in the same way.

Isomerisation, transalkylation, dealkylation and dehydrogenation

reactions are intermediate in attaining equilibrium.

Condensation reactions, such as olefin polymerization and paraffin

alkylation, are less favourable at higher temperatures.

The occurrence of both exothermic and endothermic reactions

contributes to the overall heat balance.

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Thermodynamics of FCC Reactions

The high volume of products caused by the

cracking of larger molecules requires low

operating pressure (1–5 bars).

The high endothermic nature of cracking

reactions requires that the reactor operates

at high temperatures 480–550 ºC.

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FCC Catalyst

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Zeolite type catalyst

It is in a powder form

with an average particle

size of 75 μmmicrometre and

an average surface area

of 800 m2/g.

It has a crystalline structure of

aluminosilicates.

A matrix is added to the zeolite which acts as a

binder and filler.

Zeolite (Y-Zeolite)

The main active component in the catalyst is the

Y-Zeolite. It is a crystalline structure of

aluminosilicates which has the Y-faujasite

structure.

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Figure 3. Structure of Y-faujasite

The highest pore size in the

Y-faujasite structure is 8 ºAangstrom,

which is called the super cage.

It can allow some C18–C25 mono-,

di- and tri-nuclear aromatics

present in the vacuum gas oil to

pass.

Zeolite (ZSM-5)

In the cracking of long chain paraffins, another type

of high silica zeolite is added.

This zeolite is called ZSM-5 and is used to improve

octane number and it is composed of zig-zag

channel systems.

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Figure 4. Schematic representation of shape-selective cracking with ZSM-5 zeolite

Hexane will easily enter the catalyst pore. Then hexane cracks into propane, while iso-hexane

and benzene do not enter the pores. Thus the unreacted stream is enriched with iso-paraffins

and aromatics, which contribute to an increase in the octane number.

Matrix

The matrix is added to the zeolite

to increase the body of the catalyst

to add some improved properties

Three types of substances constitute the matrix:

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is added as a glue

provides cohesion for zeolite particles

make up the body of the catalyst

it is usually a clay (Kaoline)

a small amount (ppm) of metal

metallic oxides

addition of 5% ZSM-5 zeolite

Binder

Filler

provide physical integrity

(density and attrition

resistance)

promotes the combustion of

CO to CO2 in the regenerator

fix SOx on the catalyst

leads to an increase of RON

Additives

Documents

Unit 12. Fluidised Catalytic Cracking...Cracking of relatively long-chain paraffins and olefins can go up to 95% completion at cracking conditions. Certain hydrogen transfer reactions