Fractional Distillation & Fractions
Crude oil, or petroleum, is a complex mixture of thousands of hydrocarbon compounds formed over millions of years from the remains of ancient marine organisms subjected to intense heat and pressure underground. Because it contains hydrocarbons ranging from tiny molecules with just one or two carbon atoms to enormous chains with more than 70, crude oil in its raw form has very limited direct usefulness. Fractional distillation is the industrial process used at oil refineries to separate this mixture into groups of hydrocarbons called fractions, each with a characteristic boiling-point range and therefore a distinct set of physical and chemical properties. The separation relies on the fact that intermolecular forces — primarily London dispersion (van der Waals) forces — increase in strength as hydrocarbon chain length increases. Longer chains have larger electron clouds and greater surface area, producing stronger temporary dipole interactions. This means longer-chain hydrocarbons require more energy to vaporise, giving them higher boiling points. When crude oil is heated and the vapour rises through a fractionating column that is hottest at the bottom and coolest at the top, each fraction condenses at a different height and is drawn off separately. The major fractions collected, from lightest to heaviest, are: refinery gas (C₁–C₄), gasoline/petrol (C₅–C₁₀), naphtha (C₅–C₁₂), kerosene/paraffin (C₁₁–C₁₅), diesel oil/gas oil (C₁₅–C₁₉), fuel oil (C₂₀–C₃₀), lubricating oil (C₂₀–C₅₀), and bitumen (> C₇₀). As chain length increases across these fractions, boiling point, viscosity, and density all rise, while volatility and flammability decrease. Understanding these trends is central to matching petroleum fractions to their correct industrial and domestic applications.
- Heating the Crude Oil
- The Temperature Gradient
- Condensation and Collection
- Fraction Profiles and Their Uses
- Trends Across the Fractions
- Why Supply and Demand Create a Problem
Heating the Crude Oil
Crude oil is pumped into a furnace and heated to approximately 350–400 °C, converting most of it into a mixture of vapour and liquid. This hot mixture is then fed into the base of the fractionating column, a tall steel tower that can reach 60 metres in height.
The Temperature Gradient
The column is designed so that the temperature decreases continuously from roughly 350 °C at the bottom to about 20–25 °C at the top. This gradient is maintained by the continuous flow of hot vapour rising upward and cooler liquid trickling downward over a series of horizontal trays (bubble-cap trays) punched at intervals along the column's height.
Condensation and Collection
As vapour rises, it cools. When a vapour mixture reaches a tray whose temperature equals or falls below the boiling point of a particular fraction, those hydrocarbon molecules condense back into liquid and are collected through a side pipe. Very short-chain molecules (C₁–C₄) remain gaseous all the way to the top and are collected as refinery gases. Residue that does not vaporise at all accumulates at the bottom as a thick black sludge — bitumen.
Fraction Profiles and Their Uses
Refinery gas (C₁–C₄, bp < 25 °C): methane, ethane, propane, butane; used as LPG fuel and chemical feedstock. Gasoline/petrol (C₅–C₁₀, bp 25–75 °C): used as motor fuel. Naphtha (C₅–C₁₂, bp 75–120 °C): feedstock for the petrochemical industry. Kerosene (C₁₁–C₁₅, bp 150–250 °C): jet fuel, heating fuel. Diesel/gas oil (C₁₅–C₁₉, bp 220–350 °C): road vehicle fuel, central heating. Fuel oil (C₂₀–C₃₀): marine fuel, power stations. Lubricating oil (C₂₀–C₅₀): engine lubricants, waxes. Bitumen (> C₇₀): road surfacing, roofing.
Trends Across the Fractions
Moving from refinery gas to bitumen, chain length increases → boiling point increases (stronger London forces) → viscosity increases (molecules harder to slide past each other) → density increases → colour deepens from colourless to black → volatility decreases (fewer molecules escape to vapour at room temperature) → flammability decreases (harder to vaporise means harder to ignite). These trends are entirely explained by the change in intermolecular London dispersion forces with molecular size.
Why Supply and Demand Create a Problem
The fraction that distillation naturally produces in the largest quantity is heavy fuel oil and residue, but the fraction in highest commercial demand is gasoline. This mismatch is resolved by cracking — a further refining process in which large hydrocarbon molecules are broken into smaller, more useful ones, often using high temperature, pressure, or a catalyst (catalytic cracking, or 'cat cracking'). Cracking is the subject of a related concept article.
The Key Equation
- \ce{C_{16}H_{34}} — Hexadecane (—): A straight-chain alkane with 16 carbon atoms, found in the diesel/gas oil fraction; general formula CₙH₂ₙ₊₂.
- \ce{C_8H_{18}} — Octane (—): An 8-carbon alkane in the gasoline fraction; its isomer 2,2,4-trimethylpentane (iso-octane) defines the 100-point on the octane rating scale.
- \ce{C_4H_8} — Butene (—): A 4-carbon alkene (general formula CₙH₂ₙ); the presence of a C=C double bond makes alkenes more reactive than alkanes and valuable as chemical feedstocks.
Worked Example
Problem. A student is given four unlabelled samples of petroleum fractions and told they are: refinery gas (C₁–C₄), kerosene (C₁₁–C₁₅), diesel (C₁₅–C₁₉), and bitumen (> C₇₀). Using only observations at room temperature (approximately 20 °C), describe how the student could identify all four samples, and explain the chemical reasoning behind each observation.
Answer. Refinery gas: identified as the gaseous sample. Kerosene: pale, low-viscosity liquid, lowest flash point of the liquids. Diesel: darker, more viscous liquid, higher flash point. Bitumen: black semi-solid, essentially non-flammable at room temperature.
Origins · 1 minJames Young and how this was discovered
James Young is often called the 'Father of the Petroleum Industry'. In 1850 he patented a process for distilling 'paraffin oil' (kerosene) and solid paraffin wax from shale oil and coal, establishing the world's first commercial oil refinery at Bathgate, Scotland. His systematic separation of crude mineral oil into fractions of different boiling-point ranges laid the technical and commercial foundation for the petroleum refining industry that would later be applied to crude oil on a global scale. Young's process was so successful that the Scottish shale oil industry he started kept operating for over a century — the last shale oil mine in Scotland closed only in 1962. His fortune funded explorer David Livingstone's expeditions to Africa; the two had been close friends since studying chemistry together in Glasgow.
Related Concepts
- Cracking of Hydrocarbons — Cracking is the industrial follow-up to fractional distillation: it breaks large-chain fractions that are in excess supply into smaller, more commercially valuable molecules such as petrol and ethene.
- Alkanes — Structure and Properties — The molecules within each petroleum fraction are predominantly alkanes. Understanding their general formula (CₙH₂ₙ₊₂), saturated bonding, and how chain length affects physical properties underpins all trends in fractional distillation.
- Intermolecular Forces — London Dispersion Forces — The entire basis for why petroleum fractions have different boiling points is the variation in London dispersion forces with molecular size. This concept is essential for explaining all physical trends across the fractions.
- Combustion of Hydrocarbons — The primary use of most petroleum fractions (petrol, diesel, kerosene, fuel oil) is as fuels. Complete and incomplete combustion reactions, energy output, and pollutant formation are directly connected to petroleum chemistry.
- Alkenes and Addition Reactions — Cracking of petroleum fractions produces alkenes such as ethene and propene alongside alkanes. These alkenes are vital feedstocks for making polymers and other chemicals, linking petroleum chemistry to organic synthesis.