3.1.6 Alkanes - Fractional distillation of crude oil

Specification

Students should:
  • know that alkanes are saturated hydrocarbons
  • know that petroleum is a mixture consisting mainly of alkane hydrocarbons
  • understand that different components (fractions) of this mixture can be drawn off at different levels in a fractionating column because of the temperature gradient

Alkanes

These are saturated hydrocarbons, i.e. they have only single bonds between atoms.

methane structure ethane structure propane structure butane structure
methane ethane propane butane

Hydrocarbons are chemical compounds composed of only carbon atoms and hydrogen atoms. Each hydrocarbon has a different boiling point. This is handy because we use the different boiling points to separate them.


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Physical properties of hydrocarbons

The lower members of the alkanes are hydrocarbons with low boiling points. It is not until C30H62 that they become solids at room temperature.

The boiling points of organic compounds can give important clues to other physical properties. A liquid boils when its vapor pressure is equal to the atmospheric pressure. Vapor pressure is determined by the kinetic energy of molecules. Kinetic energy is related to temperature and the mass and velocity of the molecules. When the temperature reaches the boiling point, the average kinetic energy of the liquid particles is sufficient to overcome the forces of attraction that hold molecules in the liquid state. Then these molecules break away from the liquid forming the gas state.

graph of the boiling points of the alkanes against their relative mass.

Vapour pressure is caused by an equilibrium between molecules in the gaseous state and molecules in the liquid state. When molecules in the liquid state have sufficient kinetic energy, they may escape from the surface and turn into a gas. Molecules with the most independence in individual motions achieve sufficient kinetic energy (velocities) to escape at lower temperatures. The vapor pressure will be higher and therefore the compound will boil at a lower temperature.


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Fractional distillation

Molecules which strongly interact or bond with each other through a variety of intermolecular forces cannot move easily or rapidly and therefore, do not achieve the kinetic energy necessary to escape the liquid state. Therefore, molecules with strong intermolecular forces will have higher boiling points. This is a consequence of the increased kinetic energy needed to break the intermolecular bonds, so that individual molecules can escape the liquid as gases.

The boiling point is a measure of the amount of energy needed to separate molecules in a liquid from one another

Petroleum refining is the process of separating the many compounds present in crude petroleum. The principle which is used is that the longer the carbon chain, the higher the temperature at which the compounds will boil. The crude petroleum is heated and changed into a gas. The gases are passed through a distillation column which becomes cooler as the height increases. When a compound in the gaseous state cools below its boiling point, it condenses into a liquid. The liquids may be drawn off the distilling column at various heights.

Fractional distillation

Although all fractions of petroleum find uses, the greatest demand is for gasoline. One barrel of crude petroleum contains only 30-40% gasoline. Transportation demands require that over 50% of the crude oil be converted into gasoline. To meet this demand some petroleum fractions must be converted to gasoline.

This may be done by "cracking" - breaking down large molecules of heavy heating oil; "reforming" - changing molecular structures of low quality gasoline molecules; or "polymerization" - forming longer molecules from smaller ones.

Cracking in the laboratory

For example if pentane is heated to about 500º C the covalent carbon-carbon bonds begin to break during the cracking process. Many kinds of compounds including alkenes are made during the cracking process. Alkenes are formed because there are not enough hydrogens to saturate all bonding positions after the carbon-carbon bonds are broken.


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Petroleum

The petrol that we put in the car is actually a mixture of many substances, designed to burn smoothly with the characteristics required by the engine. The main compound is the branched alkane 2,2,4-trimethylpentane, C8H18, an isomer of octane.

2,2,4-trimethylpentane structure
2,2,4-trimethylpentane

All petrols are given an octane 'rating', based on how easily that fuel tends to self-ignite causing uneven burning in the pistons. This can give rise to an effect known as 'knocking' where the pistons impact on the piston walls making a knocking sound and leading to engine damage over time.

The fuel is compared in a test engine with mixtures of 2,2,4-trimethylpentane (called iso-octane in the industry) and heptane. The mixture that burns with the same characteristics as the petrol under test defines its octane rating.

For example, petrol with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90. A rating of 90 does not mean that the petrol contains just iso-octane and heptane in these proportions, but that it has the same ignition and burning characteristics.


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quick test