Fuels additional 406528

*Visit www.gyandharaonline.com

for more notes n updates

Liquid fuels

• All liquid fuels are generally products obtained

from petroleum refining

• Crude Petroleum,

• gasoline,

• Diesel,

• Biodiesel are examples

*

Cracking of Heavy Oils

• Cracking is a process of converting heavy oil

with higher molecular weight hydrocarbons to

the oil with lower molecular weight hydrocarbon

which is known as gasoline. (C5-C9)

• The lower molecular weight hydrocarbons have

low boiling points

• Heavy oil is heated at a high temperature under

pressure or in the presence of catalyst to obtain

gasoline.Visit www.gyandharaonline.com


*

Why Cracking is done?

• Higher hydrocarbons with high boiling points are

decomposed into a variety of lower hydrocarbons that

are more volatile (low boiling)

• The Quality n quantity (20%) of the important fraction -

straight run gasoline/ petrol is not good from fractional

distillation of crude petroleum.

• The middle and heavy surplus fractions are further

cracked to get petrol. This petrol is better for IC engines

.

Visit www.gyandharaonline.com


*

The heavy oil is heated at high temperature

under pressure or in the presence of catalyst

C10H22 C5H12 (n-pentane) + C5H10

C16H34 C8H18 (n-octane) + C8H16

• Thus, n-pentane or octane along with other

lower hydrocarbon is used as gasoline.

• Generally, on cracking a mixture of hydrocarbon

is obtained which is allowed to undergo

fractional condensation to separate gasoline.



*

Types of Cracking

There are two methods for cracking.

i) Thermal cracking which may be

a) In liquid phase or

b) In vapour phase

ii) Catalytic cracking which may be

a) Fixed bed

b) Moving bed



*

Summary of All cracking processes



*

Sr.

No

.

Characteristic Liquid Phase Thermal

Cracking

Vapour Phase

Thermal

Cracking

1. Cracking

temperature

Moderate i.e. 420C –

550C

600C – 650C

2. Pressure High 100 kg/cm2 Low 10 – 20

kg/cm2

3. Yield percentages 50 – 60% Higher

4. Octane rating 65 – 70 > 70 (75 – 80)

5. Pre-requirement

for process

Any type of heavy oil

can be used

Oil has to be

vaporised readily

6. Time required Comparatively more Comparatively

less

Comparison of Liquid n Vapour phase Cracking



*

Sr. No Characteristic Fixed Bed

Catalytic

Cracking

Moving Bed or Fluidised

Bed Catalytic Cracking

1. Cracking

temperature

i.e. 425C –

450C

Around 500C

2. Pressure 1.5 kg/cm2 Around 1 kg/cm2

3. Octane value 80 – 85 85 – 90

Comparison of Fixed Bed n Moving Bed Catalytic cracking



*

Advantages of catalytic process over Thermal

process

i) The cracking reaction can be carried out at lower temperature

and pressure.

ii) Yield is high

iii) No external fuel is required. Heat required for cracking is

derived from the coal embedded in the catalyst.

iv) Better control of the reaction so better choice of product.

v) The cracking is specific in nature and can give proper quality of

gasoline.

vi) The octane value of gasoline is higher by catalytic process,

hence better for petrol engine.





*

Reforming• Reforming is the process of bringing about structural

modifications in the components of straight run gasoline

with the primary objective of improving its anti knock

characteristics.

• Reforming or aromatisation involves the conversion of open

chain (aliphatic) hydrocarbons and/or cycloalkanes in the

presence of a catalyst, into aromatic hydrocarbons (arenes)

containing the same number of carbon atoms.

• It is carried thermally (temp 500-6000C, pressure 85 atm) or

catalytically [Pt (0.75%) supported over Alumina (temp 460-

5300C, pressure 30-35 atm)].

*

-3H2

cyclohexane Benzene

Dehydrogenation

Dehydrocyclasition

n Heptane methyl cyclohexane Toluene

Isomerisation



*

Knocking in IC Engines

• Knocking related to the internal combustion

engine working on petrol.

• In IC engine - gasoline vapours + air = fuel

• Burning of a fuel is initiated by a spark in the

cylinder.

• Due to combustion, gases are formed which

moves the piston down the cylinder.

• The movement of the piston must be

even/uniform without any vibration.



*

• But sometimes, the rate of combustion/ oxidation

becomes so great that the fuel on ignition,

instantaneously produces sudden increase in

the gaseous volume which causes uneven

movement of the piston with rattling noise in the

engine.

• It is called as knocking of the engine.

• The knocking results in the loss of efficiency of

I.C. engine & damage to the piston and cylinder



*

• The petrol is a mixture of various lower

hydrocarbons and the knocking of the engine

depends upon the structure of hydrocarbons,

present in the petrol.

• Straight chain saturated hydrocarbons have

more knocking tendency than the straight chain

unsaturated hydrocarbons, as it burns with a

little difficulty.



*

• Further, the cyclic compounds have less

tendency to knock than the straight chain

compounds. The presence of double bond and

aromaticity is also important.

• The aromatic hydrocarbons burn uniformly, thus

it has a little tendency towards knocking.

• Thus, tendency of gasoline to knock is in the

following order. Straight chain paraffins >

Branch chain paraffins > Olefins > Cyclo

paraffins > Aromatics.



*

Octane number

• The knocking property of the fuel is expressed as

Octane number. It has been understood that the

phenomenon of knocking is related to the chemical

composition of the fuel.

• The fuel containing large number of straight chain

hydrocarbons possesses more knocking tendency,

thereby proving itself to be worst.

• N-heptane knocks very badly, so its anti-knock value is

arbitrarily zero. Isooctane gives very little knocking, so its

anti-knock value is 100.



*

The octane number of a fuel may be defined as, the

percentage of iso-octane in a mixture of iso-octane and n-

heptane that matches the fuel under test in its knocking

character.



Aromatic hydrocarbons have octane numbers higher than 100,

while the straight-run gasoline has the octane number of 55.

Thus, the aromatic hydrocarbons prove better

*

Sr.

No.

Parts of

Iso-Octane

Parts of n-

heptane

Knocking Octane

Number

1. 100 Zero Minimum 100

2. Zero 100 Maximum Zero

3. 78 22 Moderate 78

4. 85 15 Less 85

Antiknocking Agents/Compounds :

Antiknocking agents are the compounds which help to

increase the octane number of the fuel or decreasing the

knocking. By the addition of these agents, the gasoline/

petrol can be improved in its quality. Some of the

compounds normally used are TEL, i.e. Tetra Ethyl Lead

[(C2 H5)4 Pb].



*

Diesel Engine

• In diesel engine fuel is exploded not by spark, but by the

application of heat n pressure.

• Diesel engine fuels should easily ignite below the

compression temperature n there should be as short an

induction lag as possible.

• So essentially the hydrocarbons molecules in diesel

engine fuels should be as far as possible the straight

chain ones, with minimum admixture of aromatic n side

chain hydrocarbons.



*

Centane Number

• In the diesel engines, the fuel with straight chain paraffin

is only used. The diesel fuels generally consists of the

fractions boiling between kerosene and heavier

lubricating oils.

• The term centane number is introduced as the mixture

that was first studied was cetane (n-hexa-decane) and

-methyl naphthalene.

• Centane has high ignition while 2-methyl naphthalene

has very low ignition quality.



*

Sr.

No

Parts of

Cetane C16H34

Parts of -

methyl

Naphthalene

Ignition Delay Cetane Number

1. 100 Zero Minimum 100

2. Zero 100 Maximum Zero





*

• Centane number is defined as the percentage by

volume of cetane in a mixture of cetane and -methyl

naphthalene which exactly matches in its ignition

characteristics with the diesel under test.

• The cetane value of the diesel can be improved by

adding substances (Dopes) like ethyl nitrite, iso-amyl

nitrite and acetone peroxide. The dopes are added upto

2%.



Fuels additional 406528

Engineering

Transcript of Fuels additional 406528