The hydrocarbon vapours rise up the fractionating column. The column has a temperature gradient, being hotter at the bottom and cooler at the top. Different hydrocarbons (fractions) have different boiling points. When a hydrocarbon vapour reaches a level in the column that is cool enough (at or below its boiling point), it condenses back into a liquid and is collected on a tray. Kerosene is a fraction with a medium-range boiling point, so it condenses and is collected in the middle part of the column.
Fractional distillation is a process used to separate crude oil, which is a complex mixture of hydrocarbons, into simpler, more useful mixtures called fractions.
1. **Heating:** The crude oil is heated to a high temperature (around 350°C), causing most of the hydrocarbons to evaporate and turn into a gas (vapour).
2. **Fractionating Column:** This mixture of vapours is pumped into the bottom of a tall fractionating column. The column has a temperature gradient – it is very hot at the bottom and gradually gets cooler towards the top.
3. **Separation:** The hot vapours rise up the column. As they rise, they cool down. Hydrocarbons have different chain lengths and therefore different boiling points.
- Hydrocarbons with long chains have high boiling points. They condense back into a liquid at lower levels in the column where it is still hot.
- Hydrocarbons with short chains have low boiling points. They continue to rise further up the column to where it is cooler before they condense.
4. **Collection:** Kerosene consists of hydrocarbons with medium chain lengths and medium boiling points. Therefore, the kerosene vapours will rise part-way up the column and condense into liquid on trays in the middle section, where the temperature matches their boiling point range. They are then piped off.
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