Fractional Distillation of Crude Oil
Quick Notes
- Alkanes are saturated hydrocarbons, meaning they only contain single bonds between carbon atoms.
- Petroleum (crude oil) is a mixture made up of mainly alkane hydrocarbons.
- Fractional distillation separates crude oil into fractions based on boiling points of compounds in the mixture.
Full Notes
What Are Alkanes?
Alkanes are saturated hydrocarbons, meaning they contain only single C–C and C–H bonds.
General formula: CnH2n+2
They are non-polar and insoluble in water.
What Is Crude Oil?
Crude oil (petroleum) is a naturally occurring mixture made up of mostly alkane hydrocarbons.
It is a finite resource and needs refining to be useful.
It contains molecules of different chain lengths, which have boiling points and properties, meaning they all have different uses (especially as types of fuel).
Fractional Distillation Process
Fractional distillation is used to separate crude oil into fractions based on boiling points.
Step-by-step process:
Crude oil is heated in a furnace until it evaporates.
The vapour enters a fractionating column, which is hot at the bottom and cooler at the top.
Hydrocarbons with different boiling points condense at different levels, forming fractions that contain hydrocarbons with similar boiling points.
Large molecules (high boiling points) condense at the bottom.
Small molecules (low boiling points) condense at the top.
Hydrocarbons with similar boiling points condense at a similar height in the fractionating column and are collected together, making fractions. Different fractions are used for different purposes.
Fractions from Fractional Distillation
| Fraction | Approx. C range | Typical boiling range | Main uses |
|---|---|---|---|
| Gases | C1–C4 | below ~40 °C | LPG, bottled gas |
| Gasoline (Petrol) | C5–C10 | ~40–200 °C | Car fuel |
| Naphtha | C6–C11 | ~70–200 °C | Petrochemical feedstock |
| Kerosene | C10–C16 | ~180–250 °C | Jet fuel / heating |
| Diesel (Gas oil) | C15–C20 | ~250–350 °C | Diesel engines |
| Fuel oil / Lubricating oil | C20–C40 | ~300–400 °C | Ships, heating, lubricants |
| Bitumen | > C35 | > 350 °C | Road surfacing, roofing |
Summary Table: Fractional Distillation
| Key point | Why it happens |
|---|---|
| Fractions are separated by boiling point | Different chain lengths have different strength intermolecular forces and hence different boiling points |
| Bottom of column is hotter than the top | Creates a temperature gradient so vapours condense where their boiling point is reached |
| Larger molecules condense lower down | They have higher boiling points due to stronger intermolecular forces |
| Smaller molecules condense higher up | They have lower boiling points and remain gaseous until reaching cooler regions |