AS/A-level Chemistry - Fractional distillation

• It is a continuous process that separates the different lengths of hydrocarbons in crude oil, using their differing volatility.

• It separates crude oil with different sizes, weights and boiling temperatures.

• The crude oil is vaporised and travels up the column and is separated into fractions that each has a specific boiling point range.

When it comes to AS/A-level Chemistry, do you remember the process of fractional distillation?

• Heat the mixture of two or more substances (liquids) with different boiling points to a high temperature.
• Heating is usually done with high pressure steam to temperatures of about 1112 degrees Fahrenheit / 600 degrees Celsius.
• The mixture boils, forming vapor (gases); most substances go into the vapor phase.
• The vapor enters the bottom of a long column (fractional distillation column) that is filled with trays or plates.
• The trays have many holes or bubble caps in them to allow the vapor to pass through. They increase the contact time between the vapor and the liquids in the column and help to collect liquids that form at various heights in the column.
• There is a temperature difference across the column (hot at the bottom, cool at the top).
• The vapor rises through the trays in the column and it cools.
• When a substance in the vapor reaches a height where the temperature of the column is equal to that substance's boiling point, it will condense to form a liquid.
• The substance with the lowest boiling point will condense at the highest point in the column; substances with higher boiling points will condense lower in the column.
• The trays collect the various liquid fractions.
• The collected liquid fractions may pass to condensers, which cool them further, and then go to storage tanks, or they may go to other areas for further chemical processing.

• Hydrocarbons with high boiling points condense first, low down the tower.
• Hydrocarbons with low boiling points can get all the way to the top before they condense.
• Some hydrocarbons have very low boiling points and so they are gases. They don't condense but are collected as 'fuel gases'.
• Separating the mixture of substances in crude oil into fractions by evaporation and condensation is fractional distillation.

References:

1. https://chembam.com/online-resources/gcse-resources/fractional-distillation/

That's the end of the topic!

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Drafted by Bonnie (Chemistry)