Liquid Fuels Lectures (GIKI)

Liquid Fuel
Origin, Composition and
Classification of Petroleum

Petroleum
• Latin word Petra – rock, oleum – oil
• Natural occurring brown to black
• Mainly comprising of hydrocarbon
• Found under the crust of earth (on shore / off
shore)

Origin of Petroleum
• Carbide theory:
– Hydrocarbons present in petroleum are formed by the
action of water and inorganic carbide.
– Carbides, in return are formed by reaction of metal
and carbon under high pressure and temperature
under earth crust.
– The lower hydrocarbons formed, undergo
hydrogenation and polymerization to give various
types of other hydrocarbon.
– This theory fails to explain:
• Presence of nitrogen and sulphur in crude oil.
• Presence of chlorophyll and haemin.
• Presence of optically active compound.

Formation of inorganic carbide.
Ca + 2C  CaC2
4Al + 3C  Al4C3
Reaction of inorganic carbide with water.
CaC2 + H2O  Ca(OH)2 + C2H2
Al4C3 + 12H2O  4Al(OH)3 + 3CH3
Hydrogenation
C2H2 + H2
 C2H4
C2H4 + H2
 C2H6
Polymerization
3C2H2
 C6H6
3C2H4
 C6H12

• Engler (1900) theory:
– Petroleum is of animal origin.
– Formed by the decay and decomposition of marine animals
under high pressure and temperature.
– Sulphur dioxide gas given out by the volcanoes beside the
sea-side kills fishes and other sea animals which go on
pilling beside the volcano.
– After hundred of years, these animals started decomposing
under the influence of high temperature and pressure to
form petroleum.
– The theory is supported by following facts:
• Experimental decomposition of fish oil and other animals fats under
high temperature and pressure gives a product similar to
petroleum.
• Presence of brine or sea water together with petroleum.
• Presence of nitrogen and sulphur compounds.
• Presence of optically active compound.
• Presence of fossils in petroleum area.
– The theory fails to account for the presence of chlorophyll in
petroleum plus it does not explain the presence of coal
deposits in vicinity of oil fields.

• Modren theory:
– Petroleum is of animal as well as plant origin.
– Petroleum is believed to be formed by the decay and
decomposition of marine animals as well as plants of pre-historic
forest.
– It is thought that due to some upheavals or earthquakes,
these pre-historic forest and marine animals got buried
under the crust of earth.
– Then due high pressure and temperature under the earth
crust for long period of time these biological matters
decomposed into petroleum.
– The modren theory explains the presence of:
• Brine and coal in the vicinity of petroleum.
• It also explains the presence of nitrogen and sulphur compounds.
• Chlorophyll and optically active compounds.

Composition of Petroleum
• The principal component of petroleum are hydrocarbons,
small amount of sulphur, nitrogen and oxygen compound as
impurities.
• Hydrocarbon found in petroleum are paraffins, naphthenes,
aromatics, olefins etc.
• Sulphur compound found in petroleum are hydrogen
sulphide, thiophene, mercaptans etc.
• Oxygen occur in combined form in phenols and alcohols etc
• Nitrogen compounds includes pyridines, quinolines etc.
• Typical composition by weight percent of petroleum is
given bellow:
– Carbon: 84 – 87%
– Hydrogen: 11 – 15%
– Sulphur: 0.1 – 3%
– Nitrogen: 0.1 – 1.5%
– Oxygen: 0.3 – 1.8%

Detection of Petroleum Deposit
• Geophysical method includes the measurement
of density, elasticity, magnetic and electric
property of rocks in the crust of earth.
• Geological method includes the measurements
of age and nature of rocks inside the earth’s crust
which gives an idea of oil deposit.
• Drilling in the earth crust is the final test when
petroleum deposits have been proved either by
above two method or both.

Geological studies indicates that deposits of crude petroleum were
formed near seashores, but the action of the surrounding water
gradually shifted the location of oil pool. After period of many centuries,
the oil was forced through layers of porous rock until it became trapped
under a dome capped by hard rock. This makes impossible for gas and
oil to escape and water keeps the pool of water under pressure.

Production of petroleum
• Petroleum always occurs along with natural gas.
• After drilling in the crust of earth both natural gas and
oil flow up through pipes initially.
• When the oil pressure decreases, then the residual
either sucked by creating vacuum or pressure is
created by injecting compressed gas.
• When the oil well contain oil and gas it is called wet
well and if it contain only gas then it is called dry well.
• For digging an oil well rotary drilling system is used.
• The main boring is of 20 – 30cm in diameter and its
depth may varies from 1.5 to 4.5 km.
• The entire well is provided with a steel casing to
prevent boring wall to collapse.

The drilling rig consist of
four main parts:
•Derrick - tall steel structure
equipped with gear for raising
and lowering the drill
equipment.
•Drill column or stem – consist
of main section of steel pipes to
lower end of which the drilling
bit is attached.
•Rotating mechanism – at the
upper end of drill column.
•Mud circulation system –
drilling fluid.

Pre-treatment of crude oil
• Oil and gas when they come out of oil field are separated
(Phase separator).
• The natural gas is compressed to liquid (LPG) which is used
for heating domestic and industrial ovens.
• Crude oil is made free from:
– water (up to25%), requires extra heat for distillation.
– salts (MgCl2, CaCl2, NaCl etc. up to 2 – 5 g/lit) scaling, corrosion
and reduces heat transfer coefficient.
– sediments (1 – 1.5%) cause erosion and scaling.
• It is then made free of some dissolved gases into it by the
process called stabilization.
• Crude oil is subjected to centrifuging, filtration and settling
after heating it to 120 to 160 degree C at 6 – 8 atm to
remove these impurities and dissolved gases.

Classification of petroleum
• Depending upon nature of hydrocarbon present in petroleum is
classified into:
– Paraffinic:
Saturated hydrocarbons with general formula CnH2n+2 where n = 1 to
35. When n = 1 to 5 then it is gases (methane, ethane, butane etc)
when n = 5 to 15 then it is liquid (gasoline) and when n = 16 and above
then it is semi-solid (paraffin wax)
– Naphthenic:
Saturated ring hydrocarbon having general formula CnH2n (cyclo-hexane).
– Asphaltic:
Aromatic hydrocarbon contains 6 hydrocarbon in form of hexagon ring
and are unsaturated compound having general formula C6H2n-6
where n = 1 to 6.
– Mixed:
Crude containing all paraffinic, naphthenic and asphaltic constituents.

Uses of petroleum products
• Liquified petroleum gas (LPG) – Domestic and industrial fuel. Also called
refinery gas. Mixture of propane and butane.
• Gasoline (Petrol) – Fuel for spark ignition internal combustion engine.
• Naphtha – Used as a solvent, paint thinner, most important use is in the
production of H2 by its steam reforming.
• Jet fuel – Fuel for jet planes and turbine engines.
• Kerosene – Used for domestic illuminant.
• Diesel – Fuel for compression ignition internal combustion engine.
• Gas oil – It is gasified for fuel gas production.
• Lubricating oil – Used for lubrication in machines and engines.
• Petrolatum – base material for grease which is semi-solid lubricant.
• Light fuel oil – Used as a fuel in industrial furnaces.
• Heavy fuel oil – After blending it with light fuel oil or naphtha, it is used as
a furnace fuel
• Bitumen or tar – Used as a binder and moisture proof coating for roads.
• Wax – Used for making candles and other wax stuffs.
• Residue Pitch – Used for making roads and electrodes.

Typical product yield in crude
distillation (volume percentage)
• Petrol = 18%
• Naphtha = 6%
• Kerosene = 10%
• Diesel = 8%
• Gas oil = 12%
• Lubricating oil = 12%
• Wax = 9%
• Residue pitch = 25%

• Major oil field in Pakistan?
• Crude oil production?
• Nature of Pakistani crude?
• Oil refineries in Pakistan?
• Their capacities?

Processing of Crude Oil
Fractional distillation

Distillation
• Separation of a mixture on the
basis of boiling their boiling
point.
• By applying heat.
• The vapours of a boiling
mixture enrich in the
components that have lower
boiling points.
• Vapours formed are
condensed contain more of
more volatile component.
• The original mixture contain
less of more volatile
component.
• Distillation column are
designed to achieve this
separation efficiently.

Main component of distillation column

Basic terminologies
• The liquid mixture that is to be processed is known as the feed.
• Feed is introduced usually somewhere near the middle of the
column to a tray known as the feed tray.
• The feed tray divides the column into a top
enriching or rectification section and a bottom stripping section.
• The feed with high boiling point flows down the column where it is
collected at the bottom in the reboiler where it is heated again to
produced the vapours and re-introduced into the column.
• The liquid product obtained from reboiler is called residue or
bottom product.
• The vapour moves up the column, and as it exits the top of the unit,
it is cooled by a condenser.
• The condensed liquid is stored in a holding vessel known as the
reflux drum.
• Some of this liquid is recycled back to the top of the column and
this is called the reflux.
• The condensed liquid that is removed from the system is known as
the distillate or top product.

• The crude oil is pumped from storage tanks to the primary column
after passing it through preheat exchangers and preheat furnace.
– Preheat exchangers – out streams from the columns are at high
temperature transferred their heat to incoming crude oil.
– Pre heat furnace – bulk of heat to the crude oil is given by refinery gas
burnt in a tube-still heater.
• The material separated in column according to their boiling point.
• The column operates under pressure to retain and dissolve
substances like butane which improve anti knocking value of
gasoline.
• Gasoline and gases rise to the top are condensed and flow into an
accumulator / reflux drum.
• In order to provide sufficient liquid flow down the column, a part of
condensed liquid is refluxed from refluxed drum. Greater the reflux
greater will be the purity of product.
• The gasoline from primary tower contains certain very low boiling
point compounds which are removed by the process of stabilization
i.e. heating upto 150 degree C under 6 – 8 atmospheric pressure.
• Desired product may be withdrawn as a side stream at appropriate
points on the column.
• The side stream are further fractionated in a small column called
stripper where steam is used to strip out less volatile component
from more volatile one.

Two stage distillation unit: used for separating crude into six - ten narrow cuts

Three stage distillation unit: used to separate heat
sensitive or high boiling point fraction of crude oil by
lowering their boiling point in a vacuum column.

Hydrocarbon cracking
• Cracking: It is the process of breaking long chained
hydrocarbon into shorter one i.e. high boiling point
petroleum fraction into low boiling point lighter fractions.
• Necessity: Increasing demand of gasoline exceed its
availability by distillation alone, hence cracking of heavier
fraction of petroleum is done mainly to produced gasoline.
• Types: There are two types of cracking processes:
– Thermal cracking (450 – 750 C, 1 – 70 atm)
– Catalytic cracking (350 – 650 C, 1 – 15 atm)
• The yield of process and quality of product depend on type
of feed, temperature and pressure.
• Higher temperature gives higher yield of gasoline
• Higher pressure reduces the octane number of gasoline

Thermal cracking processes
• Low temperature and high pressure: (500 C and 20 atm )
Feed is mainly residue from atmospheric distillation unit and
products are mainly fuel oil and gasoline.
The process is also called visbreaking because heavy fuel oil is
thermally cracked to reduced its viscosity so that it can properly
atomised through burners.
• High temperature and high pressure: (530 C and 50 – 70 atm)
Feed is mainly gas oil and product is mainly gas and gasoline.
When the feed is naphtha then it is called thermal reforming.
• Low temperature and low pressure: (>550 C and 2 – 5 atm)
Feed is mainly residue of atmospheric distillation unit and
product mainly are gasoline and gas rich in unsaturated
hydrocarbon.
This is also called pyrolysis when temperature provided is 700 C
at 1 atm to produce product mainly gas rich in aromatic and
unsaturated hydrocarbons.

• Thermal cracking reactions:
– Decomposition and destructive condensation of olefins to
produce high octane number aromatic.
– Hydrogenation (hydrocracking) and dehydrogenation

– Polymerisatization and cyclisation
Octane number increase in order of paraffin  Olefin 
Naphthene  iso – paraffins  Aromatics. Hence cracking
reaction are aim for producing higher hydrocarbon with
higher octane number, if production of gasoline is main
objective.

Catalytic cracking
• Cracking of heavy hydrocarbon in presence of catalyst.
• It gives higher yield and higher octane number of gasoline than
thermal cracking.
• Moreover the process requirement for temperature and pressure
are 450 – 550 C and 1.5 to 2.5 atm respectively which is lower than
thermal cracking.
• Feed mainly is light gas oil to reduced crude.
• The more naphthenic the feed, the higher is the yield of gasoline.
• High boiling point feed sometime contain appreciable amount of
Conradson carbon which gets deposited on the catalyst which are
discarded and replaced by fresh one.
• Primary catalyst used were natural or artificial clay e.g. bentonite,
metallic catalyst being used presently are platinum, chromium, iron
and nickel. Synthetic catalyst being used include silica-alumina and
silica-magnesia.
• The size of catalyst in moving bed catalytic process is 3 – 5 mm
while in fluidized bed catalytic process is about 20 – 40 micron.

Properties of catalyst
• High reactivity
• Good selectivity
• Sufficient strength
• Easy regenerability
• High surface area
• High porosity
Type of processes
• Fixed bed
• Moving bed
• Fluidized bed

Catalytic cracking reactions
• Cracking reactions are endothermic whereas the regeneration of
catalyst is exothermic.
• The reaction occurring during thermal cracking are accelerated in
catalytic cracking.
• Paraffins crack faster to lower paraffins and olefins.
• n-paraffins crack faster than corresponding thermal process.
• iso-paraffins crack faster than n-paraffins.
• Dehydro-cyclisation reaction takes place resulting in the formation
of aromatics from paraffins.
• Oliefins being more reactive cracks thousands times faster than
thermal process.
• Oliefins undergos isomerization to give iso-olefins and coke.
• Oliefins undergos polymerization.
• Aromatic are not responsive to cracking catalytic conditions.

Factors effecting on catalytic cracking
• Temperature:
– Rate of reaction increases with increase in temperature.
– For every 40oC the decomposition rate doubles.
– For a given conversion gasoline yield is reduced at higher temperature.
• Pressure:
– For high pressure more residence time is required which result in
formation more unsaturated hydrocarbon as a consequence less
octane number gasoline is achieved.
– At given conversion, increase in pressure increases the production of
coke.
• Catalyst to oil ratio:
– Conversion increases with increase in catalyst to oil ratio as it shorten
the time required to pass through reactor and hence reduces the
extent of its deactivation from coke production.
• Coke concentration on catalyst:
– Lesser the concentration of coke on catalyst, better be the effective
activity of catalyst.
– Lower coke concentration is obtained by increasing catalyst to oil ratio.
• Catalyst activity:
– With decrease in catalyst activity, the conversion declines making less
product yield.
– Fresh catalyst are being replaced by old catalyst to increase catalytic
activity.

Fluidized bed catalytic cracking
• Fluidized bed catalytic cracking is the most widely used process in
oil refineries.
• The plant consisting of three main column:
– Reactor
– Regenerator
– Fractionator
• The catalyst is in powdered form and is circulated between rector
and regenerator.
• Fluidized bed of catalyst is obtained by forcing the vapors through
bed until minimum fluidized velocity is reached at which the
particles of catalyst arranged a loose packing in vapours.
• A slight increase in the velocity cause the vapours to flow through a
zone of low solid concentration in which high mass transfer rates
between catalyst surface and vapours enables high reaction rates.
• A catalyst stand pipe with control valve projects from the bottom of
each vessel, the outlet of which are connected by pipeline to the
other vessels.

Fluidized bed catalytic cracking process

• Sequence of operation in fluidized catalytic bed cracking process
are:
– The feed is heated and vaporized by a stream of hot catalyst passing
into the reactor at about 450o C.
– The catalyst separates from the vapours and sinks to the base of
reactor from which it flows with air into the regenerator.
– In regenerator the temperature rises to about 680o C due to
combustion of carbon on the catalyst.
– The regenerated catalyst flows down the stand pipe into the fresh oil
feed.
– The cracked vapours pass from top of the reactor to the fractionating
column from which the heavier fractions are recycled through the
reactor and the products are drawn off.
• The catalyst oil ratio is 5 – 15 and the reactor space velocity is 1 – 3
kg oil/hr/kg catalyst
• Gasoline yield = 53 – 59 % Heavy gas oil = 2 – 3 %
light gas oil = 18 – 25 % coke = 5 – 8 %
• Octane number of gasoline = 92 – 94

Reforming
• Reforming means rearrangement of molecules without much
affecting the average molecular weight of feed which is generally
naphtha of gasoline boiling range.
• Reforming carried out to produce high quality octane number
gasoline.
• Like cracking reforming can also be done thermally and by using
catalyst.
• Low octane thermally cracked gasoline is also subjected to
reforming to improve its octane number.
• Gasoline produced by reforming is called reformed gasoline or
reformate.
• Thermal reforming has been almost completely replaced by
catalytic reforming.
• Besides the main product i.e. reformate or reformed gasoline,
reforming also produces lighter hydrocarbons, hydrogen and traces
of high boiling materials.

Factors effecting on catalytic reforming
• Temperature:
– Temperature range in catalytic reforming is 450 – 550oC.
– Temperature less than 450oC making catalyst inactive while temperature
exceeding 550oC cracking predominates.
– cyclization recation occur at 470 – 490oC, isomerization at 490 – 500oC and
dehydrogenation occur at 500 – 550oC.
• Pressure:
– Low pressure and high temperature are the desirable condition for catalytic
reforming which also favor the formation of coke which deactivate catalyst.
– Deposit coke on catalyst can be reduced by operating in atmosphere of
hydrogen.
– Hence catalytic reforming is carried under substantial pressure with hydrogen
product recycled to reaction zone.
• Catalyst particle size:
– With very active catalyst, the interior portion (incase of porous material) is
ineffective because the reaction occurs before the reactant penetrate to the
core. In this case the activity of catalyst increase as the particle size decrease.
– With platinum catalyst the effect is substantial.
• Coke concentration on catalyst:
– High sulphur in the feed inhibits dehydrogenation and promotes
hydrocracking over platinum catalyst thereby reducing the yield of liquid
hydrocarbons.
– Presence of nitrogen inhibts in dehydrocyclization of paraffins .
– Halides promotes activity of platinum-alumina catalyst.
– Metals such as lead and arsenic poison the catalyst

Fluidized bed hydroforming process
• In this process the powdered catalyst containing 10% of molybdena
(molybdenum atoms sandwiched between sheets of sulphur atoms)
and 90 % alumina is circulated between a reactor and a regenerator
by fluidized technique.
• Fluid hydroreformers operates at 480o C and 14 – 20 kg/cm2.
• To maintain this reaction temperature while supplying the
endothermic heat of reaction, the recycle gas is heated to 650o C.
• The feed temperature is kept below 540o C to minimize thermal
cracking.
• Fluidization enables the temperature to be controlled in the reactor,
which prevent the excessive reforming which produces excessive
reformed gas.
• Regeneration temperature is more closely controlled by cooling fins
in regeneration unit.
• Catalyst circulation rate is about 1 kg catalyst per kg naphtha.
• Gasoline yield is about 80% with octane number of 95.

Fluidized bed hydroforming Unit

Purification of petroleum products
• Sweetening process:
– Removal of sulphur and its compounds like hydrogen sulphide,
mercaptans etc from petroleum products.
– Sulphur compounds darken gasoline, give it an offensive odor
and increase toxic sulphur dioxide engine emissions.
– Presence of these compound make the oil sour.
– Petroleum products are termed as sweet if they pass doctor’s
test in which oil sample is shaken with equal volume of sodium
plumbite solution and a small amount of sulphur.
– The mercaptans present in oil first reacts with sodium plumbite
to form oil soluble lead mercaptides which on further reaction
with elemental sulphur to form sweet smelling disulphide and
give a black precipitate of lead sulphide.
– Hydrogen sulphides also give black precipitate with the plumbite
solution (sodium plumbite)and sulphur.

• Dewaxing:
– Removal of wax from petroleum product, mainly lubricating oil.
– Petroleum waxes are solid hydrocarbons which are soluble in
petroleum and its products and cannot be separated by
distillation as their boiling points (35 – 95oC) overlap with other
petroleum products.
– At low temperatures, waxes crystallize and precipitate out from
diesel and lubricating oil, hence to maintain their fluidity they
must be dewaxed.
• Deasphalting:
– Removal of asphalts from heavy stocks.
– Asphalts are high molecular weight compounds present in
petroleum and have high boiling point in range of lube oil and
heavy gas oil.
– Asphalts are undesirable in catalytic cracking because they form
coke.
– Asphalts are readily oxidize and form carbonaceous sludge and
hence must be removed.
– Deasphalting is done by distillation (atmospheric or vacuum),
acid treating and by solvent (propane) extraction.

Properties of petroleum products

Specific gravity
• Knowledge of specific gravity predicts the quality of product. It
determines the energy per unit weight or per unit volume.
• Higher the specific gravity – heavier the hydrocarbon – higher
the carbon to hydrogen ratio
• Lower the specific gravity – lighter the hydrocarbon – lower the
carbon to hydrogen ratio.
• Lighter hydrocarbon have high calorific value per unit weight
than heavier hydrocarbon.
• However high calorific value of normal paraffins can not be
fully utilized because of their low anti knocking rating where as
aromatics produce more carbon deposits than paraffins these
consideration favour the use of iso-paraffins as a fuel.
• Lighter the crude oil, the lager is the content of lighter
constituents like gasoline and kerosene.

• Hydrometer is used for determination of specific gravity upto 0.001.
• For heavy liquid specific gravity bottles are used.
• For semi-solid masses like tar specific gravity is determined by
mixing it with an equal volume of kerosene and using the relation
Sp.GrTar=2Sp.GrMix- Sp.Gkerosene.
• Specific gravity can be used to find the gross calorific value of
petroleum products using formula of US Bureau of mines i.e.
G.C.V = 12400 – 2100 Sp.Gr.2
where specific gravity is measured at 60oF or 15.5oC.
• Another measure of specific gravity is oAPI given by American
Petroleum Institute which is:
[(141.5/Sp.gr) – 131.5]
where specific gravity is measure at 60oF or 15.5oC.
• Typical specific gravity of petroleum and its products are given
below:

Molecular weight
• Molecular weight of petroleum and its products are given by:
M = (44.29 x Sp.Gr.) / (1.03 – Sp.Gr.)
Where Sp.Gr. again is at 60oF or 15.5oC.
• Another way of measuring molecular weight is:
M = a + bT + cT2
where T is the average boiling point in degree Rankine and
constants a, b and c depends upon the characterization factor K
determined by:
K = t1/3/Sp.Gr
• Molecular weight of crude and some of its products are given
below:

Vapour pressure
• Vapour pressure is important because:
– With decreased atmospheric pressure at high altitudes the
vapor comes out of the Liquid fuel causing vapor-locking and
difficulty in starting of spark ignition engines.
– If the vapor pressure of fuel is too low, starting of engine and
lubricating oil dilution becomes difficult.
– If the vapor pressure is high, vapor occurs in fuel tanks and
pipelines causing pumping and metering difficulties.
– If the vapor pressure is high that means flash point is low
resulting in danger of fire and explosion. Besides, breathing loss
is high.
– Lubricants having high vapor pressure will be lost quickly
particularly in case of thin film boundary Lubrication.
• Vapour pressure of a liquid fuel is measured by Reid Bomb
and reported as Reid vapour pressure in psi or mm of Hg at
100oF.

Viscosity
• Viscosity is an important property because:
– high viscosity fuel oil cannot be properly atomized resulting in loss of fuel.
– highly viscous oil has to be preheated to reduce the viscosity to minimize the pressure drop in
pumping to different refineries from the oil field.
– high viscosity lubricating oil reduces its fluidity besides causing undue friction.
– It affects the amount of fuel that can be drawn through a wick to a flame by capillary action.
• Viscosity is determined by three makes of commercial viscometers:
– The Redwood viscometer (used in commonwealth countries) – time for 50 cc
– saybolt viscometer (used in U.S.A.) – time for 60 cc
– Engler viscometer (used in Europe) – time for 200 cc
• A fixed volume of a liquid at fixed temp is allowed to flow through a standard
capillary tube & the time of flow is noted.
• The results are expressed in terms of time by taken oil to flow through a particular
instrument e.g. Viscosity=100 Redwood sec at 20°C.
• Kinematic viscosity measured by this method is expressed in stokes or centistokes.
Kinematic viscosity of liquid fuel is given by
ɳ = AT-B/T
Where T = Time of flow of oil (at a fixed temp) through the viscometer. A and B are
viscometer constants and depend upon the dimensions of the viscometer capillary
through which oil flows.
• Redwood Viscometer I is used for low viscosity oils whereas Redwood Viscometer
II is used for highly viscous oil.
• Saybolt universal viscometer is used for low viscosity of oils where as Saybolt
viscometer Furol viscometer is used for high viscosity oils.

S.I units:
Pascal-seconds (Pa.s) or milli Pascal-seconds
(m pa.s)
Units Conversion:
1Pa.s = 10 Poise
1m Pa.s = 1cp
Engler viscometer

Redwood Viscometer
Saybolt Viscometer

Viscosity index
• Viscosity of liquid decreases with increases in temperature.
• The change in viscosity with change in temperature is sometime
express in term of viscosity index.
• It is an empirical number indicating the rate of change of viscosity
of an oil from 100oF to 210oF.
• Low viscosity index means a large change in viscosity with change in
temperature while high viscosity index shows a relatively small
change in viscosity with temperature.
• Paraffins oils which have very high viscosity index taking equals 100
while naphthenic oil which have very low viscosity index taking
equals 0 as a standard.
• Viscosity index – V.I = (L-U)/(L-H) x 100
where U = viscosity of oil sample at 100oF, L = viscosity of standard
oil of V.I = 0 at 210oF and H = viscosity of standard oil of V.I = 100 at
210 oF.
• Lubricating oil should have high viscosity index.
• Viscosity index can be improved by adding polybutene.

Flash & Fire point
• It is the minimum temperature at which an oil gives out sufficient vapours
to form an inflammable mixture with air and catches fir momentarily
when flame is applied.
• Crude oil = -10 to 0 oC, Gasoline= - 40 to 30oC
Kerosene = 28 – 55oC, Diesel = > 55oC
Lube oil = 130 – 300oC, Fuel oil = > 66oC
• Flash point less then 23oC is dangerous and highly inflammable. The safe
value is greater than 60oC.
• Pensky – Martin’s apparatus used for flash point above 50oC.
• Cleveland open cup tester used for volatile oils having flash point > 50oC.
• Flash point give the idea about the volatility of fuel and the explosion
hazard.
• Fire point is the lowest temperature at which vapours given off by oil,
ignite and continue to burn for at-least five seconds, when flame is
applied.
• Fire point is 5 – 40oC higher than flash point and is measured in same
apparatus used for determination of flash point.

Penske Martin Apparatus
Cleveland open cup tester

Cloud, Pour and Freezing Point
• When oil is cooled at a specified rate, the temperature at
which it become hazy or cloudy is called cloud point of oil.
The haziness can be due to the presence of wax or increase
in viscosity at low temperature.
• The temperature at which the oil just ceased to flow for five
second when it is kept horizontal is called pour point. It
determine temperature at which an oil cannot be used as a
lubricant.
• The temperature at which the oil freezes completely and
cannot flow at all is called freezing point. Important in
aviation fuels because at high altitude the temperature is
low enough that the fuel can freeze and chock the pipeline.
• The apparatus used for determination of these values is
called cloud and pour point apparatus.
• The freezing mixture used is ice+CaCl2.

Cloud and pour point apparatus

Aniline point
• The lowest temperature at which an oil is completely miscible with
equal volume of aniline is called aniline point.
• Since the aromatics dissolve aniline which itself is aromatic
compound more readily than paraffins or iso-paraffins therefore
aniline point is the measure of aromatic content in oil and hence
the characteristic of diesel.
• Higher the aniline point – lower the aromatics – higher the paraffin
content – higher the cetane number – making oil suitable for use in
diesel engine.
• Aniline point of oil also gives an indication of the possible
deterioration of rubber sealing since aromatics have tendency to
dissolve natural and some types of synthetic rubber. Therefore
lubricants should have high aniline point.
• Aniline point of cetane is 95oC and for hexyl benzene is -12oC.

Smoke point
• This is the maximum height of flame in mm without
smoke formation when the kerosene is burned in a
standard lamp under closely controlled conditions.
• Smoking of kerosene is mainly due to presence of
aromatic hydrocarbons.
• Paraffins are desirable in kerosene as it has got highest
smoke point where as aromatics are undesirable as it
has got lowest smoke point.
• Standard smoke point of kerosene is 20-30mm.
• Aromatic should removed from kerosene to improve its
smoke point.

Octane Number
• Octane number is the property of spark ignition engine and expresses its knocking
characteristics.
• Knocking produces when engine is made to operate at high load and slow speed.
• Knocking basically produces by unsteady or uncontrolled combustion in the
combustion chamber of internal combustion engine. This noise is called knocking.
• Knocking is harmful as its parts are set to vibration thereby reducing the life of
engine.
• It has been found that n-heptane knocks very badly so its anti-knock property
taken as standard i.e. zero and iso-octane give very little knocking so its anti-knock
property has been taken as standard i.e. 100.
• Octane number of fuel is defined as percentage by volume of iso-octane in a
mixture of n-heptane and iso-octane with the same knocking tendency as fuel.
• Thus if a gasoline mixture produces as much as knocking as a mixture which is
comprises of 80 parts iso-octane and 20 parts n-heptane then the octane number
of that gasoline will be 80.
• Higher the octane number – better the fuel octane number increases in the order
n-paraffins – olefins – naphthenes – iso-paraffins – aromatic.
• Octane number is determined by burning the fuel in a standard engine and knock
produces is measured by knockmeter.
• Tetra-ethyl lead (TEL) was used as an additive to gasoline to increase its octane
number which is now replaced by methyl tertiary butly ether (MTBE).

Cetane number
• It is a characteristic property of diesel and is used to indicate its quality and
performance in compression ignition engine.
• In case of diesel engine, the fuel should ignite as soon as it is injected into the
cylinder. If it does not ignite instantaneously, (The time lag between fuel injection
and fuel ignition is called ignition delay period) abnormal combustion takes place
resulting in shock waves, due to fluctuating pressure rise in the cylinder.
• With diesel of long ignition delay period, much of the charge is injected into the
cylinder before ignition is initiated, causing violent combustion, sudden increase in
pressure and rough and bumpy running. This is called diesel knock.
• Cetane (C16H34) has a very small ignition delay period hence it is given a cetane
number rating of 100 and α-methyl naphthalene has a very high ignition delay
period, hence it is given a rating of zero.
• Cetane number of diesel oil is the percentage by volume of cetane in a cetane - α-
methyl naphthalene mixture that has the same ignition delay period and
performance in a standard compression ignition engine as that of the fuel.
• Oils for high speed (>1500 rpm), medium speed and low speed should have
cetane number not less than 50, 35 and 20 respectively.
• Cetane number increases in the order aromatics — iso-paraffins — naphthene
defins — n-paraffins where as the octane number decreases in the same order.

Liquid Fuels Lectures (GIKI)

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