fuels part iii

7/31/2019 Fuels Part III

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PETROLEUM

Petroleum or crude oil is naturally

occuring liquid fuel. It is a dark brown orblack coloured viscous oil found deep in

earths crust.

The oil is usually floating over a

brine solution and above the oil, natural

gas is present. Crude oil is a mixture of

paraffinic, olefinic and aromatic

hydrocarbons with small amounts of

organic compounds like N, O and S.


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The average composition of crude oil is as follows

Constituents

Percentage (%)

C 80 87

H 11 15

S 0.1 3.5

N + O 0.1 0.5

Classification of Petroleum1.Paraffinic-Base type crude oil

2.Naphthenic (or) Asphaltic Base type

crude oil

3.Mixed Base type crude oil


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REFINING OF PETROLEUM CRUDE OIL

The crude oil obtained from the earth is amixture of oil, water and unwanted impurities. After

the removal of water and other impurities, the crude

oil is subjected to fractional distillation. During

fractional distillation, the crude oil is separatedinto various fractions.

Step : 1

Separation of water (Cottrells process)

Step : 2

Removal of harmful sulphur compound

Step : 3

Fractional distillation


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Step-1Separation of water (Cottrells Process)

The crude oil from oil well is an extremely stable

emulsion of oil and salt water. The crude oil is allowedto flow between two highly charged electrodes, wherecolloidal water droplets combine to form large drops,which is then separated out from the oil.

Step 2: Removal of harmful sulphurcompounds

Sulphur compounds are removed by treating the crudeoil with copper oxide. The copper sulphide formed is

separated out by filtration.Petrolium oil + CuO/Cu2O CuS + Sulphur free oil


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Step 3: Fractional distillation

The purified crude oil is then heated to about 400C in an

iron retort, where the oil gets vapourised. The hot vapours

are then passed into the bottom of a fractionatingcolumn.

The fractionating column is a tall cylindrical tower

containing a number of horizontal stainless steel trays at

short distances. Each tray is provided with small chimney

covered with a loose cap.

When the vapours of the oil go up in the fractionating

column, they become cooler and get condensed at

different trays. The fractions having higher boiling points

condense at lower trays whereas the fractions having

lower boiling points condense at higher trays.


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The gasoline obtained by this fractional distillation is called straight-rungasoline. Various fractions obtained at different trays are given in table

Various fractions, compositions and their uses


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Heavy oils on Refractionation gives


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CRACKING

Definition

Cracking is defined as the decomposition of highboiling hydrocarbons of high molecular weight intosimpler, low boiling hydrocarbons of low molecularweight.

C10H 22 C5H12 + C5H10

Decane n-Pentane Pentane

B.Pt : 174OC B.Pt : 36OC


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Necessity for Cracking The crude oil on fractional distillation yields only about 15

- 20% gasoline. This is known as straight rungasoline.

The quality of straight run gasoline is not so good.

It contains mainly straight chain paraffins, which ignite

readily and more rapidly than any other hydrocarbonsand hence it produces knocking (unwanted

sound) in IC engines.

Inorder to overcome these difficulties and also to improve the

quality and quantity of gasoline, high boiling fractions are

cracked into more valuable low boiling fractions suitable for SI

engines. Thus gasoline obtained by cracking is called Cracked

Gasoline.


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Cracking leads

Straight chain alkanes are converted to

branched chain hydrocarbons. (ii) Saturated higher hydrocarbons are

converted to mixture of saturated and

unsaturated lower hydrocarbons.

(iii) Aliphatic alkanes may also be converted tocyclic compounds.

(iv) All the hydrocarbons obtained by cracking

have lower boiling point than the parent

hydrocarbons.


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Types of Cracking

There are two types of cracking.

1. Thermal Cracking.

2. Catalytic Cracking.

Thermal Cracking

If the cracking is carried out at higher temperature and pressure

without any catalyst, it is called thermal cracking. There are two typesof thermal cracking.

Liquid Phase Thermal Cracking

In this method, the heavy oil is cracked at a temperature of 475 530C

under high pressure of 100 kg/cm

2

to keep the reaction product inliquid state. The cracked products are then separated into various

fractions in a fractionating column.

The yield of gasoline is about 50 - 60% and the octane number is 65 - 70.


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Vapour phase thermal cracking

In this method, the heavy oil is first vapourised and then cracked at

a temperature of 600 650C under a lower pressure of 10 20

kg/cm2. The yield of gasoline is about 70%. This process is

suitable only for those oils which are readily vapourised.

Catalytic Crakcing

If the cracking is carried out at lower temperature and pressure

in the presence of suitable catalyst, it is called catalytic cracking.

The catalyst used are aluminium silicate or alumina. There are

two types of catalytic cracking.Fixed Bed Catalytic Cracking

The heavy oil vapour is heated to 420 450C in a preheater (Fig

4.3). The hot vapours are then passed through a catalytic chamber,

maintained at 425 450C and 1.5 kg/cm2 pressure, where

catalysts (artificial clay mixed with zirconium oxide), are kept in

fixed beds.

During this passage, through the catalytic chamber, about

40% of the heavy oil is converted into gasoline and about 2-4 %

carbon is formed.


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The carbon gets adsorbed on the catalyst bed. Thecracked vapours are then passed through thefractionating column, where heavy oil getscondensed at the bottom. The vapours of gasolineare then sent through the cooler where gasolinegets condensed along with some gases.

The gasoline containing some dissolved gasesis then sent to a stabiliser, where the dissolvedgases are removed and pure gasoline is recovered.

After 8-10 hours, the catalyst loses its activity dueto the deposition of carbon. It is reactivated byburning off the deposited carbon.


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Moving bed (or) Fluid bed catalytic cracking

In this process, the solid catalyst is finely powdered, sothat it behaves as a fluid, which can be circulated in oilvapour.

The heavy oil vapour is heated to 420 450C in apreheater and it is mixed with the catalyst powder.

Then this mixture is forced into the reactor, whichis maintained at a temperature of 500C and apressure of 5 kg/cm2, where cracking takes place.

Near the top of the reactor, there is a centrifugalseparator (called cyclone), which allows only thecracked oil vapours to pass on to the fractionatingcolumn leaving behind the catalyst powder in thereactor itself.


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The catalyst powder gradually becomes heavier, due to

coating of carbon and it settles down at the bottom of the

reactor.

Then it is forced into the regenerator maintained at

600C, where carbon is burnt and the regenerated

catalyst is again recirculated along with the heavy oil

vapour. From the reactor the cracked oil vapours are passed into

the fractionating column, where heavy oil settles down

and the vapours are then passed through the cooler

where gasoline condenses along with some gases.

The dissolved gases are separated from gasoline by

passing it through a stabiliser.


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Advantages of Catalytic Cracking Over Thermal Cracking1. The yield of petrol is higher.

2. The quality of petrol produced is better.

3. The production cost is very less, since high temperature andhigh pressure are not required.

4. No external fuel is necessary for cracking. The heat required forcracking is derived by burning the carbon deposited on thecatalyst.

5. The percentage of gum and gum forming compounds is

very low.

6. The products contain less sulphur compounds.

7. The octane number of cracked gasoline is higher whencompared to straight-run gasoline. This is due to thepresence of branched paraffins and aromatic hydrocarbons in cracked gasol

8. The cracking process can be easily controlled, so the

desired products can be maintained


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SYNTHETIC PETROL

The gasoline, obtained from the fractional distillation of crudepetroleum oil, is called straight run petrol.

As the use of gasoline is increased, the amount of straight rungasoline is not enough to meet the requirement of the presentcommunity.

Hence, we are in need of finding out a method of synthesizingpetrol.

Hydrogenation of coal (or) Manufacture of syntheticpetrol

Coal contains about 4.5% hydrogen compared to about 18% inpetroleum.

coal is a hydrogen deficient compound. If coal is heatedwith hydrogen to high temperature under high pressure, it is

converted to gasoline. The preparation of liquid fuels fromsolid coal is called hydrogenation of coal (or) synthetic petrol.


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There are two methods available for the hydrogenation of

coal

1. Bergius process (or direct method).2. Fischer-Tropsch process (or indirect method).

Bergius process (or direct method).

In this process, the finely powdered coal is made into a

paste with heavy oil and a catalyst powder (tin or nickel

oleate) is mixed with it.

The paste is pumped along with hydrogen gas intothe converter, where the paste is heated to 400 450C

under a pressure of 200 250 atm.


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Bergius Process


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During this process hydrogen combines with coal to

form saturated higher hydrocarbons, which undergo

further decomposition at higher temperature to

yield mixture of lower hydrocarbons.

The mixture is led to a condenser, where the

crude oil is obtained.

The crude oil is then fractionated to yield(i) Gasoline (ii) Middle oil (iii) Heavy oil.

The middle oil is further hydrogenated in vapourphase to yield more gasoline. The heavy oil isrecycled for making paste with fresh coal dust. Theyield of gasoline is about 60% of the coal used.


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Fischer-Tropsch process (or indirect method

In this process coal is first converted into coke.Then water gas (CO + H2) is produced by

passing steam over red hot coke.

C + H2O 1200 C CO + H2Water gas

The water gas is mixed with hydrogen and the mixture is purified bypassing through Fe2O3 (to remove H2S) and then into a mixture ofFe2O3 + Na2CO3 (to remove organic sulphur compounds).

The purified gas is compressed to 5 to 25 atm and then ledthrough a converter, which is maintained at a temperature of 200 300C. The converter is provided with a catalyst bed consisting of amixture of 100 parts cobalt, 5 parts thoria, 8 parts magnesia and 200parts keiselghur earth.


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A mixture of saturated and unsaturated hydrocarbonis produced as a result of polymerisation

nCO + 2nH2 CnH2n + nH2OCO + 3H2 CH4 + H2OnCO + (2n + 1) H2 Cn H2n+2 + nH2O

The outcoming gaseous mixture is led to acondenser, where the liquid crude oil is obtained.

The crude oil is fractionated to yield (i) gasoline

and (ii) heavy oil. The heavy oil is used for cracking to get more

gasoline.

fuels part iii

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