Fractional Distillation of Crude Oil (Edexcel GCSE Chemistry)
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Fractional Distillation of Crude Oil
Crude oil as a mixture is not a very useful substance but the different hydrocarbons that make up the mixture, called fractions, are enormously valuable, with each fraction having many different applications
Each fraction consists of groups of hydrocarbons of similar chain lengths
The fractions in petroleum are separated from each other in a process called fractional distillation
The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain
The size and length of each hydrocarbon molecule determines in which fraction it will be separated into
The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains
Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them
Diagram showing the process of fractional distillation to separate crude oil in a fractionating column
Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top
Crude oil enters the fractionating column and is heated so vapours rise
Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column
Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off
The different fractions condense at different heights according to their boiling points and are tapped off as liquids
The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases
The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column
Examiner Tips and Tricks
As you move up a fractionating column the temperature decreases, so the compounds with higher boiling points come off lower down the column.
The Main Fractions
Uses of the different fractions obtained from petroleum (crude oil)
The petrochemical industry is hugely important for modern society and development
The fuels that are used in most modern methods of transport (cars, trains, airplanes etc.) are all based on oil products
Polymers, lubricants, solvents, detergents and adhesives are all products that are obtained from crude oil
The array of fractions in crude oil and the huge range of compounds we can produce from them all stem from carbon’s ability to form multiple strong covalent bonds with itself leading a huge number of organic compounds
The main fractions and their uses are described in the following table:
Uses of Crude Oil Fractions
Examiner Tips and Tricks
You need to learn the names and uses of the main fractions obtained from crude oil: refinery gases(also known as liquid petroleum gases), gasoline, kerosene, diesel, fuel oil and bitumen. Gasoline and petrol are the same thing; gasoline is the term used in the USA.
Trends in Properties
Properties of the main fractions of crude oil
Some properties of hydrocarbons depend on the size of their molecules, including boiling point, viscosity and flammability
These properties influence how hydrocarbons are used as fuels
Number of Carbon & Hydrogen Atoms
The size and length of each hydrocarbon molecule determines in which fraction it will be separated into
The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains
Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them
Decane is an alkane, C10H22 and is a component in some fuels
Boiling point
As the molecules get larger, the intermolecular forces of attraction between the molecules becomes greater as there is more surface area contact between them
This means that more heat is needed to separate the molecules, hence with increasing molecular size there is an increase in boiling point
Viscosity
Viscosity refers to the ease of flow of a liquid
High viscosity liquids are thick and flow less easily
Viscosity also increases with increasing chain length
This is also due to the increased intermolecular forces of attraction as molecular size increases
Increased viscosity means that higher alkanes are useful as lubricants in machinery as they are less likely to burn and function to reduce friction between moving parts
Trends in the Main Fractions
Ease of Ignition
Molecular size again influences the ease of ignition or flammability of hydrocarbons
Smaller hydrocarbon molecules are more flammable and are easier to ignite than larger molecules
This makes them very useful as fuels, releasing large amounts of energy when they burn
Examiner Tips and Tricks
Makes sure you know the trends in the properties of crude oil fractions.
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