Fuels - Part IV

Petroleum : Crude oil or petroleum is a dark viscous liquid which has a strong smell. Petroleum is a mixture of several compounds of carbon and is found deep within the earth’s crust. The name petroleum is derived from Latin : petra means rock and oleum means oil. Petroleum is formed due to decomposition of microorganisms which got buried under the sea over millions of years ago. Dead marine micro organisms got buried under sand and the effects of pressure, heat and bacteria got them converted into oily liquids. The decomposition took place in the absence of air or oxygen. Petroleum wells are generally found under the sea surface. Occasionally there are wells on landmasses also, but these are thought to be landmasses formed over the sea surface due to the motion of earth’s crust.

Petroleum wells occur between layers on non-porous, impervious rocks. The schematics of such a well is shown below.  

Oil deposits are found with water, dust particles, rocks, salt and sand. Wherever there is such a well, natural gas is also found accumulated in pockets of spaces within rocks.

Petroleum can be extracted from oil wells by drilling. Wherever natural gas is found, a petroleum well close by is a possibility. The first oil well was found in Pennysilvania in the USA in the year 1859. In India the first oil well was found in 1867 in Makum in Assam. These days oil wells are generally found under the sea and therefore off shore oil wells have to be dug by building platforms at sea. In India the Oil and Natural Gas Commission (ONGC) and Oil India Limited (OIL) are the two companies that survey, drill and extract oil within India and in its territorial waters.

Refining petroleum : Petroleum is an oily, viscous liquid which has many impurities like water, salt, sand. It is also a complex mixture of many hydrocarbon compounds. The process whereby the petroleum is separated into its components along with the separation of impurities is called refining of petroleum. The refining is done by fractional distillation. Each hydrocarbon component with its own boiling point separates out neatly when the petroleum is heated.

Figure below shows how the fractional distillation of petroleum is done.  

The crude oil obtained from the oil wells is heated to 400°C. The volatile components from the oil start evaporating and they are made to pass through a fractionalizing column.  As the mixtures of vapours rise, they cool. As they cool, different components condense and are then separated out by collecting funnels. The gradual cooling of the vapours along the tower height means that vapours with higher boiling points condense first.  The vapours having lower boiling points are collected at higher levels in the tower.  Gases do not liquefy at such high temperatures and are removed at the top of the tower. Petroleum is distilled into many fractions, and there may be five to six fractions. The residue from the petroleum is removed from the bottom of the furnace and is subjected to further fractional distillation by raising the temperature above 400°C in another fractional distillation tower. The fractions obtained from distillation of crude oil are : fuel oil, diesel oil, gasoline or petrol, petroleum gas. Other fractions obtained after fractional distillation above 400°C include lubricating oil, paraffin wax and asphalt.

All the fractions of crude oil or petroleum are not single compounds but they are mixtures of hydrocarbons. But these mixtures are less complex when compared to the original crude oil. The various fractions, along with their uses and boiling points are given below.

• Fuel oil : These hydrocarbons have 15 to 18 carbon atoms. Their boiling points are between 350°C and 400°C. These compounds are used in industries for furnaces and boilers. Fuel oil is better than coal, as it does not leave any residue.

• Diesel oil : These hydrocarbons have 13 to 15 carbon atoms. Their boiling points are between 250°C and 350°C. Diesel is used to run buses, trucks, railways, ships, etc. Diesel is also used to run electric generators.

• Kerosene oil : These hydrocarbons have 10 to 12 carbon atoms. Their boiling points are between 170°C and 250°C. Kerosene is used in homes for cooking, lighting lamps. High quality kerosene is also used as aviation fuel.

• Gasoline or petrol : These hydrocarbons have 5 to 10 carbon atoms. Their boiling points are between  40°C and 170°C. Petrol is used in cars and other vehicles as fuels. Petrol is also used as a solvent in industries.

• Petroleum gas : These hydrocarbons have 1 to 4 carbon atoms. Their boiling point is  below  40°C.

• Lubricating oil : These hydrocarbons have 10 to 20 carbon atoms. Their boiling point is more than 400°C. As the name suggests, this type of oil is used for lubricating various mechanical and machine parts. The oil is not used as a fuel.

• Paraffin wax : These hydrocarbons have 20 to 30 carbon atoms. Their boiling point is above 400°C. Paraffin wax is used to make candles, wax paper, ointments, greese and many toiletry goods. Paraffin wax is not used as a fuel.

• Asphalt : These hydrocarbons have 30 to 50 carbon atoms. It is a black sticky, viscous substance and does not vapourise easily. It is used for making road surfaces. Asphalt is not used as a fuel.

Other byproducts of petroleum industries are called petrochemicals. Hydrocarbons obtained from the gaseous components on fractional distillation of crude oil are used further for production of other organic compounds. These organic compounds are therefore called petrochemicals.  Methyl alcohol, ethyl alcohol, ethylene, benzene, toluene, D.D.T are some important petrochemicals. Many processes such as cracking, cyclisation, oxidation, hologenation, etc. of the initial gas are used for the manufacture petrochemicals.  Also the petroleum oil is used for manufacture of many products. These products are synthetic fibers, synthetic rubber, perfumes, fertilizers, explosives, dyes, etc.

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