Liquid fuels

By
Mr. Chandra Sekhar Devarapu M.Sc., M.Phil., (Ph.D)
Assistant Professor
Department of Engineering Chemistry,
Sagi Rama Krishnam Raju Enineering College,
Bhimavaram, Andhra Pradesh, India
Liquid fuels – Petroleum :
 Petroleum is made from the remains of plants and animals buried
millions of years ago.
 It is a non-renewable resource.
Composition
 Petroleum is a dark, greenish brown, viscous liquid that is found
underground. It comprises hydrocarbons such as:
 Straight paraffins or cycloparaffins such as methane, ethane,
propane,
butane, isobutane, pentane, hexane.
 Olefins such as ethylene, butene, isobutene and acetylene,
butadienes.
 Aromatics such as benzene, naphthalene, cyclohexane, methyl
cyclopentane.
 Some organic compounds containing nitrogen, oxygen and
sulphur.
 The average composition of crude oil is C =80 - 87%, H
=11-15%, S = 0.1 -3.5%, (N +O) =0.1- 0.5%.
Classification of petroleum
 Petroleum is classified into three types based on variation of
chemical nature of crude oil found in the earth.

 Paraffinic-base type crude oil: It contains saturated hydrocarbons

from CH4 to C35H72 and little amount of naphthalenes and
aromatics.

 Asphaltic-base type crude oil: It contains mainly cycloparaffins or

naphthalenes with smaller amount of paraffins and aromatic
hydrocarbons.

 Mixed-base type crude oil : It contains both paraffinic and

asphaltic hydrocarbons and are generally in the form of semi-
solid waxes.
Refining of crude oil:
 The petroleum obtained by mining is viscous and dark colored liquid. Due to the
presence of sulphur, it has an unpleasant smell. It also contains impurities of
sand, brine or sea water. Hence it is called crude oil. The process of refining
involves the following steps.

 Separation of water (Cottrell's process):

The crude oil from the oil well is an extremely stable emulsion 0 f oil and salt
water. The crude oil is allowed to flow between two highly charged electrodes,
where colloidal water droplets coalesce to form large drops, which is then
separated out from the oil.

 Removal of harmful impurities:

a) The presence of NaCI and MgCI in the crude oil can corrode the refining
equipment, hence these salts are removed by electrical desalting and
dehydration methods.
b) The sulphur compounds present in the crude oil is removed by treating oil
with copper oxide, which results in the formation of copper sulphide (solid),
which is then removed by filtration.
Fractional distillation :
• The crude oil is then heated to about 400°C in an
iron retort, whereby all volatile substances (except
asphalt or coke) are evaporated. The hot vapors are
then passed up a fractionating column, which 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 vapors of the oil go up in the

fractionating column, they become gradually
cooler and get condensed at different heights of
column. The fractions having higher boiling points
condense at lower trays whereas the fractions
having lower boiling points condense at higher
trays.
Various fractions of crude oil and their composition and uses :

Composition
of
Boiling
S.No Name of the fractions Hydrocarbo Uses
range (OC)
ns

As domestic and
1. Uncondensed gases Below 30°C C1 to C4 industrial fuel under
the name LPG

2. Petroleum ether 30 70 °C C5 to C7 As a solvent.

Gasoline (or) petrol. 40 - 120 °C As motor fuel, solvent and in

3. C5 to C9 dry cleaning.

Naphtha ( or}solvent spirit As solvent and in dry

4. 120. - 180 °C C9 to C10
cleaning.
As fuel for jet engines
5. Kerosene oil. 180 - 250 °C C10 to CI6
and an illuminant.

6. Diesel oil (or) gas oil 250 320 °C C10 to CI8 As Diesel engine fuel.

Production of gasoline by
7. Heavy oil. 320 - 400 °C C17 to C30 cracking
process.
Various fractions recovered from Heavy Oil :

Boiling Composition
S.no Name of the fractions Uses
range (OC) of Hydrocarbons

Used as lubricants
1. Lubricating oil - C17 to C20

Used in medicines and

2. Petroleum jelly (Vaseline) - - cosmetics

3. Grease - C20 to C28 Used as lubricants.

Used in candles, wax

4. Paraffin wax - - paper, boot polish,
etc.
Used for making roads,
5. Asphalt or bitumen Above 400°C C30 and above water proof roofing,
etc
Cracking:
 The decomposition of bigger hydrocarbon molecules into simpler, low boiling
hydrocarbons of lower molecular weight is called cracking.

 The gasoline obtained from the fractional distillation of petroleum, has the
highest demand as a motor fuel, but the yield of this fractions is only 20-30%
(Crude oil) and also quality as straight-run gasoline which is not good and
hence is used only after proper blending. To overcome these difficulties, the
higher boiling fractions (e.g. fuel oil and gas oil) are converted into lower
boiling fractions gasoline (petrol) by cracking process.

 The cracked gasoline gives better engine performance i.e., they are suitable for
spark ignition engines of automobiles. In cracking process, higher saturated
hydrocarbon molecules are converted into simpler molecules such as
paraffinic and olefinic hydrocarbons.

 There are two methods of cracking in use

1. Thermal cracking 2. Catalytic cracking
 Catalytic cracking:
 In this process, cracking is carried out in presence of a catalyst at lower temperature (300° C to
450° C) and pressures (l to 5 kg/cm2). The catalyst like aluminium silicate [Al2(SiO3)] or
alumina [Al2O3] used in cracking gives higher yield and better quality of gasoline. There are two
types of catalytic cracking in use. .

 Fixed-bed catalytic cracking

The heavy oil is passed through the heater, where the oil is vapourised and heated to 400 to
500°C and then forced through a catalytic champers containing the catalyst of silica alumina gel
(SiO2, Al2O3) or bauxite, is mixed with clay and zirconium oxide maintained at 400 to 500 °C
and 1.5 kg/cm2 pressure. During their passage through the tower, cracking takes place about 30-
40% of the charge is converted into gasoline and about 2- 4% carbon is formed which gets
deposited on the catalytic bed.

The vapours produced are then passed through a fractionating column, where heavy oil fractions
condensed. The vapours are then admitted into a cooler, where some of the gaseous products are
condensed along with gasoline and uncondensed gases move on. The gasoline containing some
dissolved gases is then sent to a stabilizer, where the dissolved gases are removed and pure
gasoline is obtained.
Fixed-bed catalytic cracking
Fluid (Moving)-bed catalytic cracking
 In this process, solid catalyst is finely powdered, so that it behaves almost as a
fluid, which can be circulated in gas stream.
 The vapours of cracking stock (gas oil, heavy oil, etc.,) mixed with fluidized
catalyst is forced up into a large reactor bed in which cracking of the heavier
molecules into lighter molecules occurs at a temperature of 530 °C and
pressure of about 3 to 5 kg/cm2.
 The top of the reactor, there is a centrifugal separator, which, allows the low
boiling lighter molecules move up to the top of the reactor and enter into the
fractionating column but retains all the catalyst powder in the reactor itself.
 The carbon deposited on the catalyst powder are burnt off in the regenerator
and the temperature rises to about 590°C or more.
 The cracked gases and gasoline are removed from the top of the fractionating
column and sent to a cooler, where gasoline is condensed.
 It is then sent to a stabilizer to recover pure gasoline. The product contains a
higher proportion of aromatics and iso-paraffins.
Moving-bed catalytic cracking
 Synthesis of Gasoline:
 The gasoline obtained from the fractional distillation of crude
petroleum oil is not enough to meet the requirement of the
present community due to vast increase of automobiles. Hence
an alternate source need of finding out to manufacture synthetic
petrol.

 Synthetic petrol can be manufactured by the process of

hydrogenation of coal. The preparation of liquid fuels from solid
coal is called hydrogenation of coal.
Gasoline is synthesized by the following methods.
Fischer- Tropsch process.
Bergius process.
Fischer- Tropsch process :

 In this process, water gas (CO+H2) is produced by the action of

steam over red hot coke. The gas is purified by passing through
Fe2O3 to remove sulphur.
 It is mixed with hydrogen and the mixture is compressed to 5-25
atmospheres. The compressed gases are then led through a
converter which is maintained at a temperature of 200-300°C.
 The converter is provided with a suitable catalyst consisting of a
mixture of 100 parts cobalt, 5 parts thoria, 8 parts magnesia and
200 parts kieselguhr. A mixture of saturated and unsaturated
hydrocarbons occurs as a result of polymerisation.
 n CO + 2 n H2 CnH2n + n H2O

n CO + (2 n + 1) H2 CnH2n+2 + n H2O

Fischer-Tropsch Process
Bergius process:

 This method was developed by Bergius in Germany during the First World
War. The low ash coal is finely powdered and made into a paste with heavy oil
and then a catalyst (composed of tin or nickel oleate) is incorporated.
 The whole is heated with hydrogen at 450°C and under a pressure 200-250 atm
for about 1.5 hours, during which hydrogen combines with coal to form
saturated hydrocarbons, which decompose at prevailing high temperature and
pressure to yield low-boiling liquid hydrocarbons.
 The gases from the reaction vessel are led to condenser, where a liquid
resembling crude oil is obtained, which is then fractionated to get: (i) gasoline,
(ii) middle oil, and (iii) heavy oil.
 The heavy oil latter is used again for making paste with fresh coal dust. The
middle oil is hydrogenated in vapour-phase in presence of a solid catalyst to
yields more gasoline. The yields of gasoline in about 60% of the coal dust
used.
Bergius Process
Knocking:
 Knocking is a kind of explosion due to rapid pressure rise occurring in
an IC engine.
 In a petrol engine, a mixture of gasoline vapour and air at 1: 17 ratio is
used as fuel. This mixture is compressed and ignited by an electric
spark. The products of oxidation reaction (combustion) increases the
pressure and pushes the piston down the cylinder.
 If the combustion proceeds in a regular way, there is no problem in
knocking. But in some cases, the rate of combustion (oxidation) will
not be uniform due to unwanted chemical constituents of gasoline.
 The rate of ignition of the fuel gradually increases and the final portion
of the fuel-air mixture gets ignited instantaneously producing an
explosive sound known as "Knocking". Knocking property of the fuel
reduces the efficiency of engine. So a good gasoline should resist
knocking.
• The efficiency of an IC engine increases with the compression ratio, which is
dependent on the nature of the constituents present in the gasoline used.
The ratio gaseous volume in the cylinder at the end of suction stoke to the
volume at the end of compression stroke of the piston, is known as compression ratio.

Chemical structure and knocking

• The knocking tendency of fuel hydrocarbons mainly depends on their
chemical structures. The knocking tendency decreases in the following order.
• Straight chain paraffins> Branched chain paraffins> Cycloparaffins > Olefins
> Aromatics.
Octane rating is the measure of fuel’s ability to resist knocking.
Introduced by Edger in 1872, found that n-heptane knocks badly and its anti
knock value given zero. On the other hand, isooctane gives very little
knocking and its anti knock value given 100.
Octane number of a gasoline is the percentage of isooctane present in the
mixture of isooctane and n-heptane, which matches the fuel under test.
Improvement of antiknock characteristics
• The octane number of fuel can be improved by
i. Blending petrol of high octane number with petrol of low octane number, so that
the octane number of the latter can be improved.
ii. The addition of anti-knock agents like Tetra-Ethyl Lead (TEL).

• The anti-knock properties of a gasoline can be improved by the addition of suitable

additives. Tetraethyl lead (TEL) or (C2H5)4 Pb is an important additive added to
petrol. Thus the petrol containing tetra ethyl lead is called leaded petrol.
• TEL reduces the knocking tendency of hydrocarbon. Knocking follows a free radical
mechanism, leading to a chain growth which results in an explosion. If the chains are
terminated before their growth, knocking will cease.
Disadvantages of using TEL
• When the leaded petrol is used as a fuel, the TEL is converted to lead oxide and
metallic lead. This lead deposits on the spark plug and on cylinder walls which is
harmful to engine life.
• To avoid this, small amount of ethylene dibromide is added along with TEL. This
ethylene dibromide reacts with Pb and PbO to give volatile lead bromide, which goes
out along with exhaust gases. But this creates atmospheric pollution.
• Nowadays aromatic phosphates, methyl tertiary butyl ether, benzen, etc., are used as
antiknock agent because it avoids lead pollution.
Cetane Number:

 Cetane number is introduced to express the knocking characteristics of diesel. Cetane

has a very short ignition lag and hence its cetane number is taken as 100. On the other
hand 2-methyl naphthalene has a long ignition lag and hence its cetane number is taken
as zero. CH3

CH3 – (CH2)14 – CH3

n-cetane (hexa decane) = 100
2-methyl naphthalene (cetane number = 0)

 Thus the cetane number is defined as "the percentage of hexa decane present in a
mixture of hexa decane and 2-methyl napthalene, which has the same ignition lag as the
fuel under test".
 The cetane number decreases in the following order.
n-alkanes > Cycloalkanes > alkenes >branched alkanes >aromatics
 The cetane number of a diesel oil can be increased by adding additives called dopes.
Important dopes: Ethyl nitrate, Iso-amyl nitrate.
Liquefied Petroleum Gas (LPG):
 It is also called bottled gas or refinery gas.
 Its calorific value 27,800 kcal/m3.
 LPG consists of hydrocarbons of such volatility that they can exist
as gas under atmospheric pressure, but can be readily liquefied
under pressure.
 The largest use of LPG is as domestic fuel and industrial fuel.
 High efficiency and heating rate. It is cheaper than gasoline, it
gives better manifold distribution and mixes easily with air.
 It is highly knock resistant, residue and oil contamination is small
as it is burn cleanly.
Compressed natural gas (CNG):
 Natural Gas is obtained from wells dug in the oil bearing regions. When natural gas occurs
along with petroleum in oil wells, it is called as ‘wet gas’ and contains gaseous hydro
carbons from C1 to C4. The wet gas is then suitably treated to remove propane, propene,
butane and butane, which is used as LPG.When the natural gas is compressed, it is called
Compressed Natural Gas (CNG). The primary component present in CNG is methane. It is
mainly derived from natural gas.
 The natural gas can either be stored in a tank of a vehicle as compressed natural gas
(CNG) at 3,000 or 3,600 psi or as liquified natural gas (LNG) at typically 20-150 psi. A
suitably designed natural gas engine may have a higher output compared with a petrol
engine because the octane number of natural gas is higher than that of petrol.
 Compressed natural gas vehicles require a greater amount of space for fuel storage than
convention gasoline power vehicles. This makes it difficult to design smaller vehicles that
look and operate like the vehicles that people are accustomed to.

 Properties
l. CNG is; the cheapest, cleanest and least environmentally impacting alternative fuel.
2. Vehicles powered by CNG produce less carbon monoxide and hydrocarbon (HC)
emission.
3. It is less expensive than petrol and diesel.
4. The ignition temperature of CNG is about 550°C. CNG requires more air for ignition.
Thank you
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