Jan 2022_CPB 30503_Refining Process-Thermal Cracking

Petrochemical & Petroleum
Refining Technology

Conversion Process (Thermal Cracking)

Refining Concept

1. Pre-treatment
2. Separation
3. Conversion

 Decomposition (cracking)
[thermal & catalytic cracking, coking and visbreaking]

 Unification (combining)
[alkylation & polymerization]

 Alteration (rearranging)
[isomerization & catalytic reforming]

4. Treatment
5. Blending



Introduction

1. Thermal cracking is the cracking of heavy residues under severe
thermal conditions.

2. The liquid products of this process are highly olefinic, aromatic and
have high sulphur content.

3. Coking is the process of carbon rejection from the heavy residues
producing lighter components lower in sulphur, since most of the
sulphur is retained in the coke.

4. The thermal treatment of hydrocarbons follows a free radical
mechanism where cracking reactions take place in the initiation step.

5. The reactions in the final step result in the formation of heavy
fractions and products like coke.

6. Reaction pathways of different fractions are expressed below:

Thermal Cracking Mechanism

Introduction

7. There are three classes of industrial thermal cracking processes. The
first is mild cracking (as in visbreaking) in which mild heating is
applied to crack the residue just enough to lower its viscosity and also
to produce some light products.

8. The second process is delayed coking in which moderate thermal
cracking converts the residue into lighter products, leaving coke
behind.

9. The third process involves severe thermal cracking: part of the coke is
burned and used to heat the feed in the cracking reactor, as in fluid
coking.

Visbreaking

1. Visbreaking is a mild thermal cracking of vacuum or atmospheric
residues to produce light products and 75–85% cracked material of
lower viscosity that can be used as fuel oil.

2. The feed to visbreaker can be either
 Atmospheric residue (AR)
 Vacuum residue (VR)

3. Vacuum residue is the heaviest distillation product and it contains two
fractions: heavy hydrocarbons and very heavy molecular weight
molecules, such as asphaltene and resins.

Visbreaking Reaction

1. The main reaction in visbreaking is thermal cracking of heavy
hydrocarbons, since resins are holding asphaltene and keep them
attached to the oil.

2. The cracking of resin will result in precipitation of asphaltene forming
deposits in the furnace and will also produce unstable fuel oil.

3. The possible reactions in visbreaking are:
 Paraffinic side chain breaking which will also lower the pour point;
 Cracking of naphthens rings at temperature above 482C (900F);
 Coke formation by polymerization, condensation,
dehydrogenation and dealkylation;
 Further cracking will be the result of asphaltene and coke leaving
the liquid phase (delayed coking).

Process Description

1. There are two types of visbreakers: coil visbreaking (thermal cracking
occurs in the coil of the furnace) and the soak visbreaker, (cracking
occurs in a soak drum).

A. Coil type visbreaker; B. Soaker type visbreaker

Coil Visbreaker

1. Vacuum or atmospheric residue feedstock is heated and then mildly
cracked in the visbreaker furnace.

2. Reaction temperatures range from 850 to 900F (450 to 480C), and
operating pressures vary from as low as 3 bar to as high as 10 bar.

3. The coil furnace visbreaking is used and the visbroken products are
immediately quenched to stop the cracking reaction.

4. The quenching step is essential to prevent coking in the fractionation
tower.

5. The gas oil and the visbreaker residue are most commonly used as
quenching streams.

Coil Visbreaker

6. After quenching, the effluent is directed to the lower section of the
fractionator where it is flashed.

7. The fractionator separates the products into gas, gasoline, gas oil and
visbreaker tar (residue).

8. The gas oil withdrawn from the fractionator is steam-stripped to
remove volatile components and then blended with the visbreaker
bottoms or routed for further processing, such as hydrotreating,
catalytic cracking or hydrocracking.

9. The unstabilized naphtha and fuel gas, recovered as overhead
products, are treated and then used as feedstock for catalytic
reforming, blended into finished products or sent to the fuel system.

10. The visbreaker bottoms are withdrawn from the fractionator, heat
exchanged with the visbreaker feedstock, mixed with stripped gas oil
(optional) and routed to storage.

Soaker Visbreaker

1. Some visbreakers employ a soaker between the visbreaker furnace
and the quenching step, similar to the conventional thermal cracking
processes.

2. This type of operation is termed soaker cracking as shown in.
3. According to the fundamentals of thermal cracking technology, the

conversion is mainly a function of two operating parameters,
temperature and residence time.
4. Coil cracking is described as a high temperature, short residence time
route whereas soaker cracking is a low temperature, long residence
time route.
5. The yields achieved by both options are in principle the same, as are
also the properties of the products.

Soaker Visbreaker

6. Both process configurations have their advantages and applications.
Coil cracking yields a slightly more stable visbreaker products, which
are important for some feedstocks and applications.

7. It is generally more flexible and allows the production of heavy cuts,
boiling in the vacuum gas oil range.

8. Soaker cracking usually requires less capital investment, consumes
less fuel and has longer on-stream times.

Delayed Coking

1. Delayed coking is a type of thermal cracking in which the heat
required to complete the coking reactions is supplied by a furnace,
while coking itself takes place in drums operating continuously on a
24 h filling and 24 h emptying cycles.

2. The process minimizes residence time in the furnace, while sufficient
time is allowed in the drums where coking takes place.

3. Coke is rejected in the drums, thus increasing the H/C ratio in the rest
of the products. However, these products are still unstable and
unsaturated, and require further hydrogenation.

4. The feed to coker is usually vacuum residue which is high on
asphaltenes, resins, aromatics, sulphur and metals.

5. The deposited coke contains most of the asphaltenes, sulphur, and
metals present in the feed, and the products are unsaturated gases
(olefins) and highly aromatic liquids.

Role of Delayed Coker

1. The feed to the delayed coker can be any undesirable heavy stream
containing high metal content.

2. A common feed is vacuum residue but it can also accept fluid catalytic
cracking slurry and visbreaking tar (residues).

3. The products from the coker are unsaturated gases (C1–C4), olefins
(C2-C4) and iC4. The olefins are very desirable feedstocks to the
petrochemical industry.

4. Isobutane and olefins can be sent to alkaylation units and the C3/C4
gases are sent to the LPG plant.

5. The highly aromatic naphtha does not need reforming and is sent to
the gasoline pool.

6. Light gas oil (LCO) is hydrotreated and sent to the kerosene pool.
Heavy coker gas oil is sent to the FCC for further cracking.

7. The role of the delayed coker is to handle very heavy undesirable
streams and to produce desirable refinery products.

Role of delayed coker in refinery

Process Description

1. The process includes a furnace, two coke drums, fractionator and
stripping section.

2. Vacuum residue enters the bottom of the flash zone in the distillation
column or just below the gas oil tray.

3. Fractions lighter than heavy gas oil are flashed off and the remaining
oil are fed to the coking furnace.

4. Steam is injected in the furnace to prevent premature coking. The
feed to the coker drums is heated to just above 482C (900F).

5. The liquid–vapour mixture leaving the furnace passes to one of the
coking drum.

6. Coke is deposited in this drum for 24 h period while the other drum is
being decoked and cleaned.

Process Description

7. Hot vapours from the coke drum are quenched by the liquid feed,
thus preventing any significant amount of coke formation in the
fractionator and simultaneously condensing a portion of the heavy
ends which are then recycled.

8. Vapours from the top of the coke drum are returned to the bottom of
the fractionator. These vapours consist of steam and the products of
the thermal cracking reaction (gas, naphtha and gas oils).

9. The vapours flow up through the quench trays of the fractionator.

10. Above the fresh feed entry in the fractionator, there are usually two
or three additional trays below the gas oil drawoff tray.

11. These trays are refluxed with partially cooled gas oil to provide fine
trim control of the gas oil end point and to minimize entrainment of
any fresh feed liquid or recycle liquid into the gas oil product.

Process Description

12. Steam and vaporized light ends are returned from the top of the gas
oil stripper to the fractionator, one or two trays above the draw tray.

13. A pump around reflux system is provided at the draw tray to recover
heat at a high temperature level and minimize the low-temperature
level heat removed by the overhead condenser.

14. This low-temperature level heat cannot normally be recovered by
heat exchange and is rejected to the atmosphere through a water
cooling tower or aerial coolers.

15. Eight to ten trays are generally used between the gas oil draw and the
naphtha draw or column top.

16. If a naphtha side draw is employed, additional trays are required
above the naphtha draw tray.

Delayed coker unit

Types of Coke & Properties

1. Coke amount can be up to 30 wt% in delayed coking. It is produced as
green coke which requires calcination to remove the volatiles as fuel
product.

2. Green coke can also be used as fuel. The most common types of coke
are:
 Sponge coke
Sponge coke is named for its sponge-like appearance. It is
produced from feeds having low to moderate asphaltene content.

 Needle coke
This coke has a needle-like structure and is made from feed having
no asphaltene contents such as decant oils from FCC. It is used to
make expensive graphite electrodes for the steel industry.

Types of Coke & Properties

 Shot coke
This coke is an undesirable product and is produced when
feedstock asphaltene content is high and the drum temperature is
too high. Discrete mini-balls of 2–5 cm in diameter are produced.

Coke charaterization and used from delayed coking

Fluid Coking

1. Fluid coking is a thermal cracking process consisting of a fluidized bed
reactor and a fluidized bed burner.

2. Vacuum residue is heated to 260C (500F) and is fed into the
scrubber which is located above the reactor for coke fine particle
recovery, and it operates at 370C (700F).

3. The heavy hydrocarbons in the feed are recycled with the fine
particles to the reactor as slurry recycle.

4. The reactor operating temperature is 510–566C (950–1050F).
5. The heavy vacuum residue feed is injected through nozzles to a

fluidized bed of coke particles.
6. The feed is cracked to vapour and lighter gases which pass through

the scrubber to the distillation column.

Fluid Coking

7. Coke produced in the reactor is laid down on the coke bed particles in
a layering manner.

8. Steam is introduced at the bottom of the reactor, where a scrubber is
also added to scrub any heavy hydrocarbons from the surface of the
coke particles.

9. This steam is also used to fluidize the bed. Part of the coke flows into
the burner where 15–30% is combusted by the injection of air into
the burner.

10. The rest of the hot coke is recycled back to the reactor to provide the
required heat.

11. The operating temperature of the burner is in the range of 593–677C
(1100–1250F).

Block diagram of fluid coking

Fluid coking process

Yield and end uses of fluid coker process

Flexicoking

1. The flexicoking process is a development of the fluid coking process
where only 2 wt% of coke is produced, thus most of the coke is used
to heat the feed.

2. A fluidized bed is added to the process which acts as a gasifier in
which steam and air are injected to produce synthesis gas called Low
Btu Gas (LBG).

3. The gasifier which operates at 816–982C (1500–1800F) produces
hot coke which remains after combustion.

4. This coke flows into the middle vessel, which acts as a heat exchanger
to heat cold coke coming from the reactor. It operates at 593C
(1100F).

5. The operation of the reactor is the same as fluid coker.

Flexicoking process

Block diagram for flexicoking