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80 DISTILLING OIL When plants and animals die, they normally decay, helped along by fungi and bacteria in the environment. Once decomposed, L&)ch provide nutrients for living organisms, and the respiration of fungi and bacteria causing decay releases carbon dioxide into the atmosphere. Over a period of several hundred million years, however, comparatively small amounts of organic material have remained under layers of silt, soil or volcanic rock and, as there is no oxygen, have not fully decomposed. Instead, they have formed deposits of coal, natural gas and oil, often located far below the land surface or the sea-bed. Oil is usually found in porous rock under a layer of hard rock which prevents it from escaping. It can, then, only be reached by drilling. The initial rush of oil out of a drill pipe is caused by the pressure of the gas compressed immediately above the oil deposits. In time, this pressure decreases and the oil has to be' pumped to the surface. Raising oil from below the sea-bed is an immensely difficult and dangerous operation. Although drilling engineers are exposed to high winds and heavy seas, they have to make test bores to see if it is worth exploring further. After they are satisfied that they have found an oifield, they set up a platform. The quality of the oil which is piped up to the surface varies, but it all has to be brought ashore. This is done either by pumping it along pipelines or carrying it in tankers. In the North Sea, as they are constantly threatened by the weather, the big oil companies have, on the whole, preferred pipelines. In other locations, where they are favoured by better weather conditions, they often use tankers. The crude oil raised directly from wells is not yet ready for use. It has to be refined. The first stage in this process is fractional distillation in a fractionating column. Those fractions, such as petrol and kerosene, which are lighter and more volatile, move towards the top of the column before condensing. The heavy residual fuel at the base of the column is extremely impure. The fractional distillation of crude oil results in the production of several useful substances, all of them normally liquids except the gas from the top of the column and the solid residue at the base. Straight petrol, which vaporizes between 300 and 2000 Centigrade, is used (when mixed with petrol produced from kerosene and heavy gas oil) as fuel for motor cars. The gas, which boils between 200 and 1 640C, also has a use - many households rely on it for heating and cooking.Kerosene has, of course, become invaluable as the fuel consumed by jet planes. It boils between 2000 and 3000C, whereas heavy gas oil and fuel oil vaporize within the range 3000C. The former is used to produce diesel fuel for lorries, buses and some cars, and the latter is redistilled to produce other fractions. The heavier fractions, such as petroleum jelly and paraffin, The former with a boiling point over 3500C and the latter with a melting point between 520 and 570C, have a variety of uses1 Petroleum jelly is a useful lubricant and is used on the skin, and paraffin is the main component of wax candles. The pitch and tar at the bottom of the column, which boil at over 4300C, are used to make asphalt. So, there is little wastage. But distillation does not produce enough high grade petrol to meet today's high demand. The petrol offered for sale to motorists is a mixture of straight petrol and distilled petrol produced by chemical modification from certain other distillates.
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