PETROLEUM AND NATURAL GAS
Petroleum or rock oil (petra-rock and oleum-oil) usually floats on a layer of salt water, deep below the rocks and has a layer of natural gas upon it. Natural gas contains about 80% methane and 10% ethane, the remaining 10% being a mixture of higher gaseous hydrocarbons. Compressed Natural Gas (CNG) is natural gas filled in cylinders under high pressure. Most of it is used as an industrial fuel, without purification. The propane-butane fraction is separated from the more volatile components by liquifaction: it is compressed into cylinders and sold as bottled gas or LPG (liquified petroleum gas) which is used as domestic fuel for cooking.
Mining of oil is done by drilling holes in the earth's crust and sinking pipes up to the oil bearing porous rock. As the pipe reaches the oil, it rushes up through the pipe due to the pressure of the gases inside. As the gas pressure gets low, the oil is pumped out by applying air pressure. The crude oil is obtained as a viscous, dark coloured liquid, often with a distinct odour. It consists of a mixture of gaseous, liquid and solid hydrocarbons. Besides hydrocarbons, small amounts of organic compounds containing oxygen, nitrogen and sulphur are also present. The hydrocarbons present in crude oil are paraffins (C₁ to C40 alkanes), naphthenes (cycloalkanes), olefins and aromatic compounds in varying percentages. It is separated into various fractions by the process known as refining of petroleum. According to the nature of the main constituents present, the crudes are classified into three groups:
(a) Paraffin-base petroleum mainly consists of hydrocarbons of the paraffin series. On distillation, solid paraffin wax is left behind.
(b) Asphalt-base petroleum is rich in nonparaffinic hydrocarbons, like aromatic and naphthenic compounds, and on distillation asphalt or bitumen is left behind.
(c) Mixed base has a composition between the above two types.
◆ The history of petroleum in India began with its accidental discovery in Digboi, Assam in 1889.90. After independence (1947), extensive oil exploration began which changed the picture of the petroleum industry in India. In 1956, Oil and Natural Gas Commission (ONGC) was set up. It has been very successful in locating new oil fields and mining the crude. The chief oil producing areas are Assam, Gujarat and Bombay High (offshore).
Refining of Petroleum The refining of crude oil involves separation into fractions with different boiling points. The distillation of petroleum is carried in tubular furnace with a tall steel fractionating column. The fractionating column consists of a vertical steel cylinder divided by horizontal plates into a number of sections. These plates are called trays. The trays have a large number of perforations, through which vapours pass in the upward direction. Each perforation is mounted by a short pipe and a cap, called the bubble cap. The system is such that the vapours have to pass through the liquid collected on one tray so that reasonably good contact between the two is accomplished.
Uses of Various Fractions
● Gaseous Hydrocarbons :- This fraction is mainly used as industrial or domestic fuel.
● Petroleum Ether :- It is used as solvent in perfumery and also for drycleaning clothes.
● Gasoline :- This fraction, when obtained directly from refining process is called straight run gasoline, commonly known as petrol. Crude petroleum contains only a small percentage of this low-boiling fraction. Its yield is there fore, increased by cracking of high boiling fractions, eg kerosene or diesel oil..
● Kerosene :- Kerosene was the most important petroleum product for almost fifty years since crude oil was first processed. Although it has now lost its leading place to gasoline, it is still in considerable demand. It is used in lamps, burners (stoves), and turbojet aircraft. It is also subjected to the process of cracking to meet the increas ing demand for gasoline.
● Diesel oil or Gas Oil :- This fraction is used as fuel for diesel engines, for industrial heating (hence also called furnace oil) and as a feed stock for cracking processes where it is converted to high grade gasoline.
● Lubricating oils and Greases :- As the name indicates,they are chiefly used for lubrication. Lubricating greases are semi-solid substances made by thickening lubricating oils with soaps, clays, silica gel or other thickening agents.
● Paraffin Wax :- It is used in making candles, shoe polish, floor polish, waxed paper, etc. White petroleum jelly (vaseline) is a paraffin wax with little oil content and is a major constituent of many ointments and cosmetics.
● Asphalt and Coke :- These are dark semisolid or solid residues of the distillation process. They are used for roofing, road building, making underground cables, bat tery boxes and electrodes, etc.
Other Fractions
Other fractions are less useful and are converted by various processes into useful chemicals, e.g. plastics, detergents, solvents, rubber, etc. All chemicals derived from petroleum fractions are called petrochemicals. Another way of converting the high-boiling fractions into useful gasoline is by cracking when smaller hydrocarbons (C₁ - C₂) are formed. The quality of petrol for use in car engines is denoted by their anti-knock properties. The anti-knock property of petrol samples are measured by the values of their octane numbers. The higher the octane number of a petrol sample, the more anti-knocking property it has and more useful it is. The highest octane number is 100. To increase octane number, tetra ethyl lead (TEL) is added to petrol.
