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A well is drilled so that the crude oil and other liquids travel up the bore hole. When it comes to the surface the crude oil has to be moved closer to where it is needed.
Finding oil and gas trapped deep underground and drilling a well is very complicated and expensive. It costs millions of pound to drill a well and only a few are successful. The liquids found underground can be a complicated mixture of water, crude oil and gas. The crude oil and gas need to be separated before they can be transported safely.
How is crude oil separated?
Crude oil is separated by fractional distillation; this is the best method to use as it separates gasses according to their boiling points so therefore the majority of substances are formed. Also vacuum distillation produces products with lower boiling points by reducing pressure.
There are two types of distillation process of fractional distillation which take place, the first being primary distillation which is the separation of crude oil. A further process takes place under reduced pressures which lower the boiling points, this is called vacuum distillation.
Primary distillation - method of fractionation. Distillation is based on the difference in boiling point of the liquids in the mixture to be separated. Successive vaporisation and condensation of crude oil in a fractioning column will separate out the lighter fractions, leaving a residue.
Crude oil is often a dark, sticky liquid that cannot be used without changing it.
Crude oil is a mixture of different chemical called hydrocarbons. As you can see from the diagram, the temperature at the bottom of the column is kept very high about 350°C and cooler towards the top. This is because the first step to refine crude oil is to heat it until it boils. The burning oil turns into a mixture of gases in the column. The gases then rise up the distillation column this separates gases according to its boiling point. The fractions with higher boiling point hydrocarbons are condensed (turned back into a liquid) first and fractions with the lowest boiling point hydrocarbons are condensed last and at the very top. Fractions in the order which they occur with the highest boiling point first, are: bitumen (tar), fuel oil, lubricating oil, diesel oil, paraffin (kerosene), naphtha (this is not shown in diagram this occurs at a boiling point of 100 – 180°C), petrol (gasoline), and refinery gases. Generally in primary distillation only diesel oil, paraffin (kerosene), naphtha, petrol (gasoline), and refinery gases are formed the rest is residue which has a boiling point of over 350°C.
Liquids from refining oil still have to be changed to make them more useful. Sometimes it's to make them clean enough to be used. Sometimes it's to turn some of the unwanted liquids into things people want to buy. The heavier liquids are in less demand from customers so are turned into lighter products that are in demand. One of the processes is called catalytic cracking. It breaks down some of the heavy liquids from the distillation column. The heavy liquids are changed into simple and more useful liquids and gases. Cracking is just one of many chemical changes in an oil refinery.
Vacuum distillation - distillation under reduced pressure (less than atmospheric) which lowers the boiling points of the liquid mixture being distilled. This technique, involving a relatively lower temperature, prevents cracking or decomposition of the very heavy hydrocarbon stocks which normally boil at very high temperatures.
Vacuum distillation deals with residue that is formed at the end of primary distillation. This residue contains the non-volatile hydrocarbons which only vaporise above 350°C but at atmospheric pressure. This residue needs to be distilled further for more fractions to be obtained. But to obtain these fractions they bust be distilled at reduced pressures. This allows mixture to be separated at a temperature below the normal boiling point, and prevents the componenets from decomposing. The fractions obtained from this process in order of boiling pints with the highest first are: bitumen (tar), paraffin waxes, fuel oil, and lubricating oil.
What are the uses of the products formed?