Presentation Topic

Thermal Cracking

when you known Better you do Better…

Shabeel Khalid 21M.Bila 06

Fahad Muzaffar 03Ateeq Ahmad 31Nayyar Iqbal 08

Presentation By

Contents:

• Cracking• Necessity for cracking or why we need

cracking• Types of cracking• Thermal cracking• Thermal cracking processes• Thermal cracking reactions• Commercial thermal cracking processes

Terminologies There is a wide variety of types of crude oil based on its color, smell, and it's properties. Some include paraffin based, olefins, and aromatic hydrocarbons.

Paraffin based crudes (a waxy residue)• a flammable, whitish, translucent, waxy solid consisting of a mixture of saturated

hydrocarbons, obtained by distillation from petroleum or shale and used in candles, cosmetics, polishes, and sealing and waterproofing compounds.

• Olefins: Olefin is a hydrocarbon with two or three covalent bonds in between carbon atoms. It is formed at a refinery conversion process and used for the manufacture of petrochemical products.

Aromatic hydrocarbon:These carbocyclic compounds contain at least one benzene ring.

petroleum refining : Basic refinery processes

Functions of Refinery Units:

• (1) separating the many types of Hydrocarbon present in crude oils into fractions of more closely related properties

• (2) chemically converting the separated hydrocarbons into more desirable reaction products, and

• (3) purifying the products of unwanted elements and compounds.

Types of Distillation:

• Fractional Distillation• Vacuum Distillation• Atmospheric Distillation• Thermal Cracking• Catalytic Cracking

Cracking:

Cracking means heating of higher boiling petroleum fraction like heavy fuel oil at high temperature and pressure to produce lower boiling lighter fraction.

It is an endothermic reaction .

The main application of cracking is for production of gasoline from gas oil.It is also done to produce olefins(for petrochemicals production) from gas oil and naphtha ,lower viscosity furnaces oil (visbreaking) and coke(coking).

Important chemical reactions in thermal cracking which carried out at comparatively

• Higher pressure (1-70atm)

• Higher temperature (450-7500C)

Necessity for cracking An average crude oil on distillation yields:

• 20-30% Petrol

• 30-45% intermediate oils (kerosene, naphtha, diesel, lube oil etc)

• 25-50% residual fuel oils.

Why we need cracking…?

• Increasing demand for high quality (octane number) petrol exceed the availability of petrol by distillation alone.

• Hence, cracking is heavier fuel oils( obtained by distillation of crude) is done to produced mainly petrol.

• Cracking is also done to reduce the viscosity of fuel oil (by visbreaking), to produce coke and to produce olefins which is feed for petrochemical industry.

Types of Cracking

There are two types of cracking

• Thermal Cracking• Catalytic Cracking

When cracking (i.e. breaking larger molecules into smaller ones by heating) is done without any catalyst(which increases the rate of reaction) then it is called thermal cracking.

Cracking in presence of a catalyst is called catalytic cracking.

Thermal cracking

The fuel which may be gas oil, fuel oil(heavy or light), atmospheric or vacuum residue is heated up to 450-7500C at pressure ranging from 1-70 atmosphere to produce gas, petrol, diesel, cracked residue(coke) etc.

Octane number of petrol obtained may be up to 75.

The yield and quality of products will depend upon the type of feed, temperature and pressure.

Thermal cracking processes Depending upon the pressure and temperature employed for the cracking and the characteristics of feed, there are various thermal cracking processes in which the product yields and characteristics are different. Some of them are given below

1. Low temperature and high pressure thermal cracking process2. High temperature and high pressure process3. Low pressure and high temperature process4. High temperature and low pressure process

1. Low temperature and high pressure thermal cracking process

• Feed- is residue from atmospheric distillation unit

• Products- are mainly gas, fuel oil, and gasoline.

• Temperature and pressure employed are 5000C and 20 atm. Respectively.

• This process is also called visbreaking when heavy fuel oil is thermally cracked to residue its viscosity so that it can be properly atomised through the burners.

• This process is employed for heavy oil cracking.

2. High temperature and high pressure process• Feed- gas oil.

• Products-gas and gasoline mainly.

• T and P-5300C and 50-70 atm. Respectively.

• This is also called light oil thermal cracking process.

• When feed is naphtha, it is called thermal reforming

3. Low pressure and high temperature process

•Feed –residue from atmospheric distillation unit.

•T and P – >5500C and 2-5 atm. Respectively.

•Products –gasoline and gas rich in unsaturated hydrocarbons(a feed for petrochemical)

4. High temperature and low pressure process

•This is also called pyrolysis.

•T and P – 7000C and 1 atm. Respectively.

•Products –is mainly gas rich in aromatic and unsaturated hydrocarbons.

Coking

• Feed –is vacuum residue.

• T and P – 400-4500C and 2 atm. Respectively.

• Products –are gas, coker gasoline, coke for electrode manufacture(main product ) and coke residue.

Thermal cracking reactionsFew important thermal cracking reactions are:

1. Decomposition and destructive condensation2. Hydrogenation and dehydrogenation3. Polymerisation4. Cyclisation

Octane number increases in the order Paraffin Olefin Naphthen Iso-paraffin

Aromatic• That means aromatics have the highest octane number and paraffins have

the lowest.• Hence, cracking reactions should aim for producing hydrocarbons with higher

octane number , if production of gasoline is the main product.

Decomposition and destructive condensation

• Decomposition and destructive condensation of olefins to produce high octane number aromatics is shown:

2C2H4 C4H6 + H2

C4H6 + C2H4 C6H6 + H2

Hydrogenation

• Hydrogenation (hydro-cracking) of higher boiling paraffin to lower boiling paraffin shown:

On white board

• Since, lower boiling paraffins have higher octane number, hydrogenation (hydro-cracking) improves octane rating i.e., quality of gasoline(petrol).

Dehydrogenation

• Dehydrogenation of naphthenes to aromatics also helps in increasing the octane number of cracking products.

On white board

polymerisation

• Converts C3 and C4 olefins to higher diolefins

On white board

Cyclisation

•Reaction converts n-paraffins to aromatics which have higher octane number.

•This reaction is also called dehydrocyclisation because hydrogen is a co-product.

On white board

Commercial Thermal Cracking processes

Mainly there are four commercial processes employed for thermal cracking in oil refineries. They are:

1. Dubbs thermal cracking process(old process In old refineries)

2. Pyrolysis

3. Visbreaking

4. Coking

Dubbs thermal cracking process

• It is an old thermal cracking process used in oil refineries up to about 1939 on gas oil having fuel oil stocks.

• Heavy oil or topped crude is pumped into a bubble cap fractionators where it is mixed with heavy cracked oil from the process.

•The mixed feed at pressure up to 70kg/cm2 is passed through a tube furnace where it is heated maximum up to 5400C before passing to the soaking chamber. This permits a time dwell during which a controlled amount of cracking can take place.

•Pressure is reduced before passing into a flash chamber where heavy cracked fuel oil separates and then to the bubble cap fractionators from which the cracked gas and gasoline passes out at the top and furnace distillate from side. Its flow sheet is given in fig.

Bubble cap fractionatorsH

eater

Socking chamber

Flash drum

Topped crude

OrHeavy oil

Cracked gas

Cracked gasolineOctane no.66

Furnace distillate

cooler

Heavy cracked oil

Dubbs thermal cracking process

70kg/cm2

5400C

Pyrolysis

Pyrolysis (mild thermal cracking) is done mainly for the production of lighter products mainly unsaturated like olefin(ethylene) and naphthene polymers, diolefin, benzenes and toluene's etc.

It is carried out at high temperature(650-7000C ) and low pressure.

Initially this process was used for the production of toluene.

The feed to Pyrolysis is raw crude oil, kerosene and natural gasoline.

A higher yield of gas, benzene and toluene are obtained by this process.

The feed is heated in special furnaces.

The products of the Pyrolysis are cooled and delivered to the fractionating column.

The Pyrolysis gas is drawn from the top of the fractionators.

The aromatics are taken from middle section from its bottom of the column.

The residue which is resinous in nature is discharged from its bottom.

fractionatorFeed heated

aromatics

residue

Pyrolysis

gas

Pyrolysis

pyrolysis

Visbreaking• Visbreaking, a mild form of thermal cracking, significantly lowers the viscosity of heavy crude-oil residue without affecting the boiling point range.• Residual from the atmospheric distillation tower is heated (800°-950° F) at atmospheric pressure and mildly cracked in a heater.• It is then quenched with cool gas oil to control over cracking, and flashed in a distillation tower.• Visbreaking is used to reduce the pour point of waxy residues and reduce the viscosity of residues used for blending with lighter fuel oils.• Middle distillates may also be produced, depending on product demand.• The thermally cracked residue tar, which accumulates in the bottom of the fractionation tower, is vacuum-flashed in a stripper and the distillate recycled.

Visbreaking process:

Feedstock From Process Typical products - to - unit

Residual Atmospheric tower & Vacuum tower

Decompose •Gasoline or distillate To Hydrotreating•Vapor To Hydrotreater•Residue To Stripper or recycle•Gases To Gas plant

Coke Formation Thermal cracking reactions are highly endothermic and

require heat which is either provided by heating furnaces or generated by burning some of the produced coke.

The formation of coke from alkanes, alkenes and cycloalkanes is generally endothermic, while it is exothermic for aromatics.

Coking ProcessesCoking is a severe method of thermal cracking used to upgrade heavy residuals into lighter products or distillates. Coking produces straight-run gasoline (coker naphtha) and various middle-distillate fractions used as catalytic cracking feedstock. The process so completely reduces hydrogen that the residue is a form of carbon called "coke." The two most common processes are delayed coking and continuous (contact or fluid) coking. Three typical types of coke are obtained (sponge coke, honeycomb coke, and needle coke) depending upon the reaction mechanism, time, temperature, and the crude feedstock.

Delayed Coking •In delayed coking the heated charge (typically residuum from atmospheric distillation towers) is transferred to large coke drums which provide the long residence time needed to allow the cracking reactions to proceed to completion. Initially the heavy feedstock is fed to a furnace, which heats the residuum to high temperatures (900°-950° F) at low pressures (25-30 psi) and is designed and controlled to prevent premature coking in the heater tubes. The mixture is passed from the heater to one or more coker drums where the hot material is held approximately 24 hours (delayed) at pressures of 25-75 psi, until it cracks into lighter products. Vapors from the drums are returned to a fractionator where gas, naphtha, and gas oils are separated out. The heavier hydrocarbons produced in the fractionator are recycled through the furnace.

After the coke reaches a predetermined level in one drum, the flow is diverted to another drum to maintain continuous operation. The full drum is steamed to strip out uncracked hydrocarbons, cooled by water injection, and decoked by mechanical or hydraulic methods. The coke is mechanically removed by an auger rising from the bottom of the drum. Hydraulic decoking consists of fracturing the coke bed with high-pressure water ejected from a rotating cutter.

Continuous Coking  Continuous (contact or fluid) coking is a moving-bed process that operates at temperatures higher than delayed coking. In continuous coking, thermal cracking occurs by using heat transferred from hot, recycled coke particles to feedstock in a radial mixer, called a reactor, at a pressure of 50 psi. Gases and vapors are taken from the reactor, quenched to stop any further reaction, and fractionated. The reacted coke enters a surge drum and is lifted to a feeder and classifier where the larger coke particles are removed as product. The remaining coke is dropped into the preheater for recycling with feedstock. Coking occurs both in the reactor and in the surge drum. The process is automatic in that there is a continuous flow of coke and feedstock.

Feedstock From Process Typical products - to - unit

Naphtha, gasoline

Distillation column, blending

Decomposition •Gasoline or distillate To Hydrotreating•Vapor To Hydrotreater•Residue To Stripper or recycle•Gases To Gas plant

Clarified oilTars

Catalytic crackerVarious units

  Coke to Shipping, recycleGas oil to Catalytic cracking

Wastewater, (sour) Gases

TreatmentGas plant

   

 Coking Processes

Ref:http://www.setlaboratories.com/therm/tabid/107/default.aspxhttp://en.wikipedia.org/wiki/Cracking_(chemistry)

Book:Title:Elements of Fuel, Furnaces and Refractories Author:O.P. Gupta

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