Petroleum

Petroleum is an essential source of hydrocarbons and fuels. It has petrol, kerosene, diesel and other important fuels in addition to many useful solvents, lubrication oil, paraffin wax and many other useful compounds.

• Source of Petroleum

Crude petroleum is dark yellow, brown or green coloured oily liquid lighter than water. It is found inside the impervious rocks and thus named as petroleum(petra-rocks; oleum-oil). It is also known as crude oil or mineral oil.

• Composition of Petroleum:

petroleum is a complex oily liquid mixture of hydrocarbons. In addition to alkanes it has cyclo alkanes, aromatic hydrocarbons and compounds of O, S and N. The composition of petroleum depends upon its source from where it is obtained.

• Refining & Mining of Petroleum

Imprevious rocks are drilled by machines and the petroleum comes out of these holes due to the pressure developed by gases inside the rocks. The final extraction is made by passing air at high pressure inside the rocks.

The crude petroleum is refined in refinery. Petroleum is mainly found in Assam, Gujrat, Maharashtra (Bombay high).

Petroleum is refined by fractional distillation. Further fractional distillation of its products gives a no. of refined products given below.

Refer to the following  video for refining of petroleum

• Knocking

In an internal combustion engine, the fuel mixture of gasoline vapours and air burns in cylinder. The mixture is fired by means of a spark in the compressed state. For better and maximum efficiency, the fuel mixture is highly compressed before firing in cylinder. A good fuel on sparking burns steadily, completely in time at a definite compression ratio. However, if compression ratio exceeds a limit, the sparking to a fuel gives rise to knocking i.e., all the fuel does not burn smoothly and a part of fuel burns suddenly with explosive nature giving rise to metallic rattle or knocking. This results in loss of power. The knocking however falls with nature of product fuel.

Knocking order falls with branching, unsaturation, aromatization etc. The knocking order is:

Straight chain paraffins > branched chain paraffins > olefins > naphthenes > Arenes.

The knocking may be prevented by adding compounds like TEL or tetra ethyl lead. The gasoline from petrol pump which we get contain anti knock compounds (TEL).

Octane no. of gasoline is the no. for rating anti knock qualities of gasoline. The higher the octane no., lesser the tendency for knocking and better being the fuel. Octane no. is derived by assuming 100 arbitrary value for isooctane and zero for n-heptane. Thus no. is the % of isooctane in a mixture of isooctane and n-heptane whose knocking resembles with the knocking of fuel used.

The octane no. of fuel is increased by two to three units by adding 2 to 4 ml of TEL in one Gallon petrol. Tetra ethyl lead is supposed to dissociate as

(C₂H₅)₄Pb  →   4C₂H₅^.  +Pb

                     Ethyl free radical

The ethyl free radical converts some of the straight chain hydrocarbons into branched chain hydrocarbons and thus increases the octane number. In addition to TEL, a little ethylene dibromide is also added which converts Pb to PbBr₂, which comes out through exhaust and prevents its deposition in engine due to decomposition of TEL.

 Pb + CH₂BrCH₂Br → PbBr₂ + CH₂=CH₂

                                Volatile

A gasoline fuel with octane no. 30 or above is good fuel. The efficiency of fuel increases as the octane no increases.

Addition of TEL in petro, no doubt improves the octane number of fuel but it also causes lead pollution in atmosphere. The attempts are therefore been made to improve octane number of fuel by some other means. Unleaded or lead free gasoline marketed today is the gasoline obtained by isomerisation and alkylation blended with BTX (Benzene, toluene, xylene) or methyl-t. butyl ether. Its octane number is 90. Thus following processes are used for getting lead free gasoline with better octane number.

(i) Isomerisation (Reforming) : By passing gasoline vapours over aluminium chloride (AICI₃) at 200^oC.

(ii) Alkylation : Isobutylene formed during cracking on alkylation with isobutene forms iso-octane in presence of concentrated H₂SO₄.

(iii) Aromatisation : A mixture of benzene, toluene and xylenes known as BTX is obtained when crude naptha vapours are passed over a catalyst 
(Pt + AI₂O₃) at 500^oC.

Note :

1.Octane number may be less than zero (e.g. n-nonane has octane number = -45) as well as it may be greater than 100 (e.g. Triptane or 2, 3, 3-trimethyl butane has octane no. = 124)           

2.To avoid lead pollution, a new anti knock agent AK-33-X i.e. cyclopentadienyl manganese carbonyl        is used in developed countries in unleaded petrol.

3.Commercial TEL used as antiknock has following mixture. TEL = 63%; Ethylene bromide = 25%; Ethylene chloride = 9%, methylene blue dye (2%)

4.Octane number of gasoline can be increased by addition of BTX (benzene, toluene and xylene).

5.The fraction obtained during fractional distillation with offensive odour due to the presence of sulphur compounds as well as dark coloured due to the presence of aromatic compound is called sour gasoline.

6.The process of purification of sour gasoline is called Doctors sweetening process.

Synthetic Petrol : With the advantage of civilization, the consumption of gasoline has increased much and demand still on increase led chemists to prepare gasoline. Following method were given during the short supply of petrol in II world war by German chemists.

1. Bergius method : Coal is a mixture of solid hydrocarbons and may be hydrogenated to produce low boiling hydrocarbons.

Coal tar or Coal dust + heavy oil → make a paste

Paste + H₂     Crude oil

Crude oil       1. Gasoline (Petrol) , 2. Middle oil , 3. Heavy oil

2.Fischer Tropsch Process :

Pass steam to red hot coke → Water gas (CO + H₂)

• Cetane Number

Cetane number is used for grading the diesel oils. Hexadecane (cetane) has been assigned cetane number 100 while α-methyl naphthalene is assigned zero cetane number arbitrarily.

The cetane number of a diesel oil is the percentage of cetane (hexadecane) by volume in a mixture of cetane and α-methyl naphthalene which has the same ignition property as the fuel oil in question under similar experimental conditions. The diesel oil having cetane number 75 has same ignition property as a mixture of 75% cetane and 25% α-methyl naphthalene.

• Flash Point

Volatile nature of a liquid hydrocarbon determines its explosive nature. The lowest temperature at which an oil gives sufficient vapours to form an explosive mixture with air is referred to as flash point of the oil.

The flash point fixed for a particular oil varies from country to country, depends on its climate and controls the percentage of highly volatile hydrocarbons in the oil. The flash point is high for a hot country and low for a cold country. The flash point in India is fixed at 44^oC, in France it is fixed at 35^oC and in England at 22.8^oC.

You might be interested in referring IIT JEE Organic Chemistry Syllabus, Books and Revision Notes on Organic Chemistry.