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.

•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.

The drilling rig consist offour main parts:

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.

Phase separator

Pre treatment of crude oil

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.

Characteristic of crude oil

Constituents of petroleum

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.

Basic operation

• 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.

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• 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.

Reaction in reforming process

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g • 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 plumbiteto 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 plumbitesolution (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/TWhere 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 Sayboltviscometer Furol viscometer is used for high viscosity oils.

Engler viscometer

S.I units:Pascal-seconds (Pa.s) or milli Pascal-seconds (m pa.s)Units Conversion:1Pa.s = 10 Poise1m Pa.s = 1cp

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 100where 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 vapoursto form an inflammable mixture with air and catches fir momentarily when flame is applied.

• Crude oil = -10 to 0 oC, Gasoline= - 40 to 30oCKerosene = 28 – 55oC, Diesel = > 55oCLube 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.

Aniline point apparatus

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 cetanenumber 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 — naphthenedefins — n-paraffins where as the octane number decreases in the same order.