Introduction to Petroleum Chemistry, Chemistry tutorial


Petroleum as well known as the crude oil is a viscous liquid mixture of organic compounds deposited in the strata of the earth. Crude oil is found in Saudi Arabia, Kuwait, Nigeria, U.S.A., Iraq and so on. It differs broadly in composition however comprises mainly of alkanes (C1 to C40 per molecule), cycloalkanes and aromatic hydrocarbons. It might as well include a few proportions of sulphur, nitrogen, oxygen and so on.

Crude oil is assumed to have been made or prepared by the bacterial decomposition, under pressure, of animal and plant remains. Petroleum is an enormously significant raw material. It is needed both to give fuels for energy generation and as the source of many organic and inorganic chemicals.

Petroleum is extracted from the earth crust basically through drilling. This is then followed through a difficult procedure of separating the individual component via the refining method. The procedure of petroleum refining is principally that of converting crude oil to a range of high quality economically significant products.

The composite mixtures of hydrocarbons present in the crude exist in gaseous, liquid and solid forms and as well present with the crude are a number of inorganic impurities which are detrimental (that is, hazardous) to the refining process; and solid impurities like sand. Petroleum is at times found related with the Natural Gas.

Crude Oil Reserves:

Around 60% of the world's oil reserves are found in the Asia, with the Middle East (such as Saudi Arabia, Iran, Iraq, Kuwait, Oman, Bahrain) having the lion share. China and Indonesia are other Asian countries having large oil reserves. Russia as a country consists of the biggest reserve in the world. Large deposits of crude oil are as well found in United States, Canada, Venezuala, Australia and in several West African countries.

The Refining Process:

There are two main phases, desalting and distillation methods. The desalting is simply a method of removing salt compounds; and is more of a mechanical procedure than chemical.

Fractional Distillation:

This method facilitates the separation of the petroleum to 'fractions' or 'cuts' therefore the name fractional distillation that is the principal process for the purification of petroleum. Crude oil is heated to around 400°C by passing it via coils of pipes in a gas-heated pipe-still furnace.

The resultant hot oil, which is at around 400°C is a mixture of vapors and liquids, is passed to a tall cylinder termed as a fractionating tower or column, The column is sub-divided into a number of compartments by means of trays that encompass holes covered through 'bubble-cap' and 'overflow pipes'.

66_Fractional distillation of crude oil.jpg

Fig:  Fractional distillation of crude oil

The temperature of the fractionating column ranges from around 400°C at the bottom to around 40°C at the top, and each tray is at a controlled temperature. As the mixture passes to the column, the liquids fall to the bottom and at the same time the vapors pass up via the trays. As each and every tray is at a different temperature, different mixtures of vapor condense to give various petroleum fractions in each and every tray.

The major fractions collected and their uses are tabulated in the table shown below:


Boiling range oC

Approx carbon chain


Gas fraction

< 40

1 - 5

Gaseous fuel for cooking and heating; source of alkanes

Petroleum ether


Petrol or gasoline


40 - 60

60 - 80

70 - 180

100 - 200



5 - 10




Solvent; source of alkanes and alkenes

Kerosene or Paraffin oil

200 - 300

11 - 14

Fuel oil lanterns, tractor and jet engines

Light gas - oil or fuel oil

250 - 400

13 - 17

Fuel oil for diesel engines

Heavy gas oil - light lubricating oil

300 - 400

18 - 25

Fuel, lubricants, paraffin wax, medicinal paraffin


> 400

> 25

Bitumen for surfacing road and roofing materials

Quality of petrol - octane number:

Most of the gasoline fraction obtained from the column, needs further treatment as it knocks easily if employed directly. This signifies that the petrol-air mixture in the cylinders, if it is being employed as fuel in engines, explodes prematurely and partly causing a metallic rattle termed as knocking.

The knock properties of a petrol is computed by its octane number, 2, 2, 4-Trimethylpentane (iso-octane) cause almost no knocking (that is, octane number 100) and heptane that knocks very readily (octane number  0), have been selected as standards against which the performance of any petrol can be computed. The octane number of any fuel is the percentage of iso-octane in the mixture of iso-octane and heptane which will knock to the similar extent as the fuel under the similar conditions. The difference in the grades of petrol is the difference in their octane number.

The octane rating of a fuel can be enhanced by adding anti-knock agents like tetraethyl lead. As well, straight-chain alkanes (example: heptane) cause knocking far more readily than branched-chain alkanes (example: iso-octane). Alkenes and aromatic hydrocarbons have better known qualities than the straight-chain alkanes. Therefore delicate conversion methods have been developed that balances all such characteristics to change the composition of the original gasoline fraction.

The Conversion Processes:

The distillation method just separates the crude oil into different fractions however it can't change the proportion of the constituent hydrocarbons in the fractions. Such proportions differ with the source of crude oil and in most cases don't meet up the standard for their desired use. Both the yield and quality of the fractions can be enhanced on by different conversion methods.


In the cracking method larger hydrocarbons in fuel oil fractions are broken down to smaller hydrocarbons required in petrol. This same method as well transforms some of the straight-chain alkanes (that knock easily) into branched-chain alkanes. The cracking can be accomplished at high temperature and pressure (thermal) however cracking using catalyst is more common. Alkanes (that is, saturated) with lower relative molecular mass are produced, altogether having a mixture of gaseous alkenes example: C2H4, C3H6 and C4H8.


Fig: Cracking


In this method, straight-chain Acmes are transformed into branched-chain isomers at around 100°C and under pressure; having AlC13 as catalyst


Fig: Isomerisation


This method comprises the conversion of straight-chain alkanes to aromatic hydrocarbon. The process comprises a simultaneous cyclisation (making ring compound), catalyst by Al2O3; and dehydrogenation, catalyst through platinum.


Fig: Reforming

In these conversion methods, more useful alkanes and alkenes are formed and straight-chain alkanes are transformed to the branched alkanes and aromatic hydrocarbon; all of which enhances the octane number or quality of petrol.


The word petrochemicals is employed to explain the chemicals obtained from petroleum that are helpful raw materials for the chemical industries.

The gas and naphtha fractions from the fractional distillation of petroleum, altogether having gases obtained from the cracking method, and natural gas, provide the main starting materials. They serve up as a source of these petrochemicals - methane, ethane, propane, butane, ethene, propene, butenes and aromatic hydrocarbons like benzene, toluene and xylenes; that are the starting materials for a very broad range of significant products, example: plastics, cosmetics, pharmaceutics, detergents and synthetic fibres like nylon. The petrochemicals are often termed to as the value-added products.

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