Petroleum Refining — Fractional Distillation

Crude oil (petroleum) is a complex mixture of hundreds of hydrocarbons (compounds of carbon and hydrogen only) with different chain lengths. It is found trapped in sedimentary rock layers.

"Petroleum" = "petra" (rock) + "oleum" (oil) — literally rock oil.

Crude oil is separated by fractional distillation using the difference in boiling points of its components.

Process:

1. Crude oil is heated to ~400°C in a furnace → becomes a vapour mixture
2. Vapour enters a tall fractionating column (hotter at bottom, cooler at top)
3. Hydrocarbons with higher boiling points condense lower in the column
4. Hydrocarbons with lower boiling points rise higher before condensing
5. Each fraction is collected at a different height — called a tray

Fractions of Petroleum (bottom to top)

• Shorter carbon chains = lower boiling point = more volatile = collected at top
• Longer carbon chains = higher boiling point = collected at bottom
• Distillation is a physical process (no chemical reaction) — fractions have the same molecules as in crude oil
• After distillation, further cracking (breaking long chains) produces more petrol from heavy fractions

Thermal or catalytic cracking breaks large hydrocarbon molecules (like fuel oil) into smaller, more useful ones (petrol):

C₁₀H₂₂ → C₅H₁₂ + C₅H₁₀ (example)

This is a chemical process — new, smaller molecules are formed.

• Petroleum = mixture of hydrocarbons (not compound) — can be separated physically
• Fractional distillation separates by boiling point differences
• Lower in column = higher boiling point = more C atoms = heavier fraction
• LPG = Liquefied Petroleum Gas (propane + butane, C3–C4) — cooking use
• Kerosene = "mitti ka tel" — C8–C12, used in stoves and jet engines
• Cracking = chemical change; distillation = physical change

1. Why does crude oil need to be refined?
2. Where in the fractionating column is diesel collected — top or bottom? Why?
3. What is the difference between distillation and cracking?
4. Name two uses of kerosene.
5. Stretch: Why do larger hydrocarbon molecules have higher boiling points?