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The following points highlight the chief methods used to distil crude oil. The methods are: 1. Fractional Distillation 2. Thermal Cracking 3. Catalytic Cracking 4. Polymerization.
Method # 1. Fractional Distillation:
This is the basic method of oil distillation that splits the crude oil into its various ‘fractions’. The process is based on the fact that each of the fractions has a different boiling point and thus a different condensation point. The lighter fractions such as petrol (38°C or 100°F) and paraffin (49°C or 300°F) have fairly low boiling points but the heaviest fractions boil at about 427°C (800°F).
The crude oil from the storage tanks is pumped through a furnace where it is heated sufficiently for all the fractions to vaporize. The vapour then enters a cylindrical fractionating tower. This structure may be about 30 metres (100 ft) high and 2.5-8 metres (8-25 ft) in diameter and the interior is divided into several compartments by a series of horizontal trays.
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Each tray is perforated to allow vapours to rise in the tower and each tray has an outlet by which the condensed fractions can be led out of the tower. When the vaporized oil enters the tower, the vapour rises and as it rises it cools. The various fractions condense into liquids at different levels depending on the temperature.
The lighter fractions such as petrol and paraffin have the lowest condensing temperatures and thus liquefy at the top of the tower, collecting in the uppermost trays; the medium fractions, such as diesel, gas oil and some fuel oil, condense at various levels in the middle of the tower; the heavy fractions, such as fuel oils and asphaltic residues, condense at the bottom at high temperatures, and are drained from the base of the tower.
The disadvantage of the fractionating process is that only a relatively small proportion of the distilled oil is of the lighter, most valuable fractions. Several improved systems have been devised which can increase the yield of lighter oils.
Method # 2. Thermal Cracking:
The thermal cracking process increased the yield of petrol from 26.1 per cent in 1920 to about 50 per cent today. This is done by cracking the heavier oils in tanks at more than 538°C (1,000° F), and at a pressure of 700 N/cm2 * (1,000 lb per sq.inch).
The combination of high temperature and high pressure causes the larger hydrocarbon molecules to crack or split into lighter fractions which have a smaller number of carbon atoms. These lighter fractions are then allowed to condense in the fractionating tower.
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Method # 3. Catalytic Cracking:
The cracking method was greatly improved in the 1930s, increasing the yield of petrol and also improving its quality by the development of catalytic cracking. A catalyst is added to the medium fractions which helps to break down the oil molecules, without, however, affecting the oil’s chemical composition.
The catalyst is in the form of a fine powder either of platinum or of clay. The catalyst and the hot oil-vapour are brought into contact in a large cracking chamber called a reactor. The vaporized oil is broken down into lighter fractions such as petrol, and subsequently distilled, leaving its carbon atoms and impurities with the clay.
The catalayst, coated with carbon, then flows out of the reactor to a regenerator where the carbon is burnt off in a stream of air. The catalytic cracking process is thus a chemical treatment of the oil which splits hydrocarbon molecules and also removes impurities including the sulphur compounds- sulphuretted hydrogen, smelly ‘mercaptans’, and ‘aromatics’.
Method # 4. Polymerization:
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The cracking processes improve the petrol yield by splitting the heavier oils but polymerization increases petrol yield from the very lightest fractions-the gases, which might otherwise be burnt off or otherwise wasted. These gases can now be made to combine into larger molecular compounds and thus increase the yield of petrol, and its octane content.
After distillation has separated the fractions to produce the various commercial grades of oil, further processes are applied to purify the oils and blend them together to produce the most desirable types of fuel with a varied octane content. A variety of processes, such as alkylation, is also applied to improve the anti-knock quality of the petrol when used in cars.
In addition polymerization is used to produce petrochemical by-products such as plastics, detergents, synthetic rubber and synthetic fibres. Only a small proportion of the total refinery products is in the form of heavier oils but this proportion is increasing, because of the increasing demand for fuel oils as factories change over from coal to oil-based power plants, especially in Europe. The types and proportions of the various oil fractions produced in refineries can be altered readily to make use of different types of crude oil and to take advantage of market trends.