Fractional distillation
Definition
Fractional distillation
A method of separating a mixture of liquids by vaporising it and condensing the parts at different temperatures according to their boiling points.
Definition
Fraction
A mixture of hydrocarbons with similar chain lengths and boiling points that is collected at one height of a fractionating column.
- Petroleum is separated into useful parts called fractions by fractional distillation.
- The separation works because the hydrocarbons in petroleum have different boiling points.
- The petroleum is heated in a furnace to about 400 ∘C400\ ^\circ\text{C}400 ∘C, so that most of it boils and turns into vapour.
- The hot vapour is passed into the bottom of a tall fractionating column.
- The column has a temperature gradient, so it is hottest at the bottom and coolest at the top.
- The vapours rise up the column through holes in a series of horizontal trays, and they cool as they rise.
- Each hydrocarbon condenses to a liquid when it reaches a height where the temperature is below its boiling point.
- Hydrocarbons with high boiling points condense low down in the column, where it is still hot.
- Hydrocarbons with low boiling points stay as vapour for longer and condense higher up, where it is cooler.
- The liquid that collects on the trays at each height is piped off as a separate fraction.
- The hydrocarbons with the lowest boiling points never condense, so they leave the top of the column as refinery gas.
- The largest molecules do not boil at all, so they collect at the bottom of the column as a thick residue called bitumen.
- Each fraction is still a mixture of hydrocarbons, but the hydrocarbons in it have similar chain lengths and similar boiling points.
- Fractional distillation is a physical process, because the molecules are separated without any of their bonds being broken.
Trends up the column
Definition
Volatility
How easily a liquid evaporates to form a vapour.
Definition
Viscosity
The resistance of a liquid to flowing, so a more viscous liquid is thicker and flows more slowly.
- The properties of the fractions change in a regular way from the bottom to the top of the column.
- Chain length
- The hydrocarbon chains get shorter towards the top, so the molecules are smaller.
- Boiling point
- The boiling points get lower towards the top.
- Smaller molecules have weaker intermolecular forces, so less energy is needed to separate them and the boiling point is lower.
- Volatility
- The fractions become more volatile towards the top, so they evaporate more easily.
- A fraction with a low boiling point turns into a vapour easily, which is why the most volatile fractions are collected highest up.
- Viscosity
- The fractions become less viscous towards the top, so they are runnier and flow more easily.
- Long chains tangle together and attract each other more strongly, which is why the fractions near the bottom are thick and sticky.
- All four trends come from the same cause, which is the decrease in chain length from the bottom to the top of the column.
Example
Comparing petrol and bitumen
- The petrol fraction is collected near the top, and its molecules typically contain about 5–10 carbon atoms.
- The bitumen fraction is collected at the bottom, and its molecules typically contain more than 50 carbon atoms.
- Petrol has the shorter chains, so it has the lower boiling point, the higher volatility and the lower viscosity.
- These properties explain the uses, because a runny, volatile liquid turns into vapour easily in a car engine while a thick, sticky solid binds stones together on a road.
Uses of the fractions
- The eight fractions are listed below in order from the top of the column to the bottom.
- Refinery gas
- Refinery gas is used as gas for heating and cooking, often sold as bottled gas.
- Gasoline, also called petrol
- Gasoline is used as fuel for cars.
- Naphtha
- Naphtha is used as a chemical feedstock, which is the raw material for making other chemicals such as plastics.
- Kerosene, also called paraffin
- Kerosene is used as jet fuel for aircraft.
- Diesel oil, also called gas oil
- Diesel oil is used as fuel for diesel engines, such as those in lorries, buses and trains.
- Fuel oil
- Fuel oil is used as fuel for ships and for home heating systems.
- Lubricating oil
- Lubricating oil is used to make lubricants, waxes and polishes.
- Bitumen
- Bitumen is used for making roads, because it is a thick, black material that holds the road surface together.
Practical
Fractional distillation of synthetic crude oil
- Aim: to separate a sample of crude oil substitute into fractions and compare how runny and how flammable each fraction is.
- Apparatus: boiling tube with a side arm, −10-10−10 to 400 ∘C400\ ^\circ\text{C}400 ∘C thermometer in a bung, ceramic wool, synthetic crude oil, four small test tubes, beaker of cold water, test-tube rack, Bunsen burner, clamp stand, watch glasses, wooden splints, heat-resistant mat and eye protection.
- Method:
- Soak a plug of ceramic wool with about 2 cm32\ \text{cm}^32 cm3 of synthetic crude oil and push it to the bottom of the side-arm boiling tube.
- Fit the thermometer so that its bulb sits level with the side arm, because that is where the vapour leaves the tube.
- Clamp the tube and place the end of the side arm inside a small test tube standing in a beaker of cold water.
- Heat the bottom of the boiling tube gently and collect the liquid that condenses up to 100 ∘C100\ ^\circ\text{C}100 ∘C as the first fraction.
- Swap to a clean test tube each time the thermometer reaches 150 ∘C150\ ^\circ\text{C}150 ∘C, 200 ∘C200\ ^\circ\text{C}200 ∘C and 250 ∘C250\ ^\circ\text{C}250 ∘C, so you collect four fractions.
- Pour a few drops of each fraction onto a separate watch glass, tilt it to judge the runniness, and touch it with a lighted splint.
- Results:
- The first fraction is pale yellow, very runny, catches fire at once and burns with a clean yellow flame.
- Each later fraction is darker, thicker, harder to light and burns with a smokier flame.
- A dark, sticky residue is left in the ceramic wool at the end.
- Conclusion: fractions that boil at lower temperatures have shorter chains, so they are less viscous, more volatile and more flammable, which matches the trend up a real fractionating column.
- Safety: wear eye protection, use synthetic crude oil because real crude oil contains toxic compounds, work in a well-ventilated room or fume cupboard, keep the collected fractions away from the flame, and lift the side arm out of the collecting tube before you stop heating.
Common Mistake
- Do not reverse the trends, because chain length, boiling point and viscosity decrease up the column while volatility increases.
- Do not say that the column separates single pure compounds, because each fraction is a mixture of hydrocarbons with similar boiling points.
- Do not say that the hydrocarbons are separated by their melting points or their density, because they are separated by their boiling points.
Exam technique
Describing fractional distillation
- Mention the temperature gradient, saying that the column is hotter at the bottom and cooler at the top.
- State that the fractions condense at different heights because they have different boiling points.
- Give each use in the exact form, such as fuel for cars for gasoline and jet fuel for kerosene, because "fuel" on its own is too vague.
- Call naphtha a chemical feedstock, not a fuel.
- When asked about a trend, name the direction and the property, such as "volatility increases from the bottom to the top".
Self review
- Why can petroleum be separated by fractional distillation?
- Where in the column is the temperature highest?
- How do chain length, boiling point, volatility and viscosity change from the bottom to the top of the column?
- Which fraction is used as a chemical feedstock?
- State the uses of the kerosene, fuel oil and bitumen fractions.
