Fractional Distillation of Crude Oil

Fractional distillation separates crude oil into useful groups of substances by heating it and using their different boiling ranges. Crude oil is a mixture mainly of hydrocarbons, and each separated group, called a fraction, contains substances with similar boiling ranges rather than a single pure compound.

The principle of separation

Hydrocarbons differ in volatility, meaning how readily they vaporise. Fractional distillation exploits these differences without deliberately changing their molecular structure.

• More volatile hydrocarbons have lower boiling points and remain as vapour at lower temperatures.
• Less volatile hydrocarbons have higher boiling points and condense at higher temperatures.
• Petroleum fractions have boiling ranges because crude oil contains many different compounds.

How the fractionating column works

Crude oil is heated in a furnace, and the resulting mixture of vapour and liquid enters a fractionating column. The column is hotter near the bottom and progressively cooler towards the top.

• As vapours rise and cool, different fractions condense at different levels according to their boiling ranges.
• Higher-boiling fractions are withdrawn from lower levels, while lower-boiling fractions travel farther upwards before condensing.
• Trays or packing bring rising vapour into contact with descending liquid, improving separation through repeated vaporisation and condensation.
• Some condensed liquid is returned as reflux, which helps sharpen the separation between fractions.

Major fractions and their uses

The broad order from the top towards the bottom is gases, petrol-range material, kerosene, diesel-range material and heavy residue. Exact product boundaries depend on the crude oil and refinery operation.

• Refinery gases include hydrocarbons that can be recovered as liquefied petroleum gas (LPG) for fuel.
• Petrol or gasoline-range fractions provide components for motor fuel; naphtha also serves as a petrochemical feedstock.
• Kerosene fractions are used for lighting and cooking and, after suitable processing, for aviation turbine fuel.
• Diesel or gas-oil fractions provide components for diesel fuel and feedstocks for further refining.
• Heavy residue can undergo further processing to obtain additional fuels, lubricating-oil feedstocks and bitumen.

Why further refining is needed

Distillation is a separation step, not the complete manufacture of finished fuels. Vacuum distillation separates heavy residue at reduced pressure, allowing it to boil at lower temperatures and limiting thermal decomposition.

• Cracking converts larger hydrocarbon molecules into smaller ones, unlike distillation.
• Treatment and blending help remove impurities and bring products to the required fuel specifications.