Fuels additional 406528

Liquid fuels
• All liquid fuels are generally products obtained
from petroleum refining
• Crude Petroleum,
• gasoline,
• Diesel,
• Biodiesel are examples
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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.
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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
.

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The heavy oil is heated at high temperature
under pressure or in the presence of catalyst
CH CH(n-pentane) + CH1022
512 510
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.

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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

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Summary of All cracking processes

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Comparison of Liquid n Vapour phase Cracking
Sr.
No
.
Characteristic Liquid Phase Thermal
Cracking
Vapour Phase
Thermal
Cracking
1. Cracking
temperature
Moderate i.e. 420C –
550C
600C – 650C
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

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Comparison of Fixed Bed n Moving Bed Catalytic cracking
Sr. No Characteristic Fixed Bed
Catalytic
Cracking
Moving Bed or Fluidised
Bed Catalytic Cracking
1. Cracking
temperature
i.e. 425C –
450C
Around 500C
2. Pressure 1.5 kg/cm2 Around 1 kg/cm2
3. Octane value 80 – 85 85 – 90

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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.

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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)].

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-3H2
Dehydrogenation
cyclohexane Benzene
Dehydrocyclasition
n Heptane methyl cyclohexane Toluene
Isomerisation

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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
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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

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• 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.

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• 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.

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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.

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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

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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].

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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.

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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.

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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

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Fuels additional 406528