Crude petroleum is a complex, dark, and viscous mixture of hundreds of different hydrocarbons, ranging from light gases to heavy solids. To make it industrially useful, it must be separated into different portions called petroleum fractions.
The Principle of Fractional Distillation
This separation is achieved through fractional distillation, a physical process based on the differences in boiling points of the constituent hydrocarbons.
- The Process: Crude oil is heated to approximately 400°C in a furnace, converting it into vapor. This vapor is fed into the base of a tall fractionating column (distillation tower).
- Thermal Gradient: The column possesses a temperature gradient, being hottest at the bottom and progressively cooler toward the top.
- Condensation: As the vapors rise through the tower, different hydrocarbons condense back into liquids at the specific heights where the column temperature drops below their respective boiling points.
- Separation Mechanics: Hydrocarbons with smaller carbon chains (C1 to C4) have low boiling points and rise to the very top as gases. Conversely, heavy hydrocarbons with long carbon chains (>C20) have high boiling points, condensing rapidly near the bottom of the column.
Detailed Profile of Major Petroleum Fractions
The following table outlines the systematic distribution of petroleum fractions from the top of the fractionating column to the bottom residue, along with their chemical characteristics and primary applications:
| Fraction Name | Boiling Point Range (°C) | Hydrocarbon Composition | Primary Uses and Significance |
| Refinery / Petroleum Gases | < 20 | C1 to C4 | Liquefied under pressure to form LPG (Propane and Butane); raw material for petrochemical synthesis. |
| Gasoline (Petrol) | 40 – 170 | C5 to C10 | High-volatility fuel for light internal combustion engines (automobiles); industrial solvent. |
| Naphtha | 120 – 180 | C7 to C11 | Primary feedstock for the petrochemical industry; cracked to produce plastics, synthetic rubber, and chemicals. |
| Kerosene (Paraffin Oil) | 170 – 250 | C10 to C16 | Illuminant, domestic heating fuel, and highly purified straight-run fuel for aviation jet engines. |
| Diesel Oil (Gas Oil) | 250 – 350 | C15 to C18 | Fuel for compression-ignition heavy engines (buses, trucks, trains, and agricultural tractors). |
| Fuel Oil (Heavy Gas Oil) | 350 – 450 | C17 to C25 | Fuel for marine vessels, industrial boilers, central heating systems, and thermal power stations. |
| Lubricating Oil and Greases | > 400 (under vacuum) | C20 to C30 | Viscous fluid used to minimize mechanical friction and wear in heavy machinery and automotive engines. |
| Paraffin Wax | Solidifies at room temp | C20 to C30 | Used in making candles, wax paper, waterproof coatings, polishes, and ointments. |
| Bitumen / Asphalt | Non-volatile Residue | > C30 | Viscous black residue used for surfacing roads, waterproofing roofs, and anti-corrosive coatings. |
Upgrading Fractions: Cracking and Reforming
The natural yield of direct straight-run fractions from fractional distillation often does not match commercial market demand. To resolve this imbalance, refineries use secondary chemical processes to alter the molecular structures.
Cracking (Thermal and Catalytic)
- Definition: The chemical process of breaking down heavier, long-chain, high-boiling-point hydrocarbons into lighter, short-chain, low-boiling-point hydrocarbons.
- Example: Heavy Gas Oil (C18H38) can be thermally or catalytically cracked into Petrol (C8H18) and gases like Ethene (C2H4).
- Catalytic Cracking: Uses catalysts like Zeolites (synthetic aluminosilicates) at moderate temperatures and pressures, yielding high-quality petrol with less waste.
Catalytic Reforming (Isomerization)
- Definition: The chemical rearrangement of straight-chain alkanes into branched-chain alkanes, cycloalkanes, or aromatic hydrocarbons.
- Purpose: This process does not change the carbon number but significantly improves the Octane Number of the petrol, enhancing its anti-knock properties in spark-ignition internal combustion engines.
Key Fact-Sheet for UPSC Prelims
- Vacuum Distillation: The heavy residue left at the bottom of an atmospheric distillation column contains high-boiling compounds that would decompose (crack) if heated further at normal pressure. To avoid decomposition, they are separated in a separate column under reduced pressure (vacuum), which lowers their boiling points. This process yields heavy fuel oils, lubricating oils, and bitumen.
- Sweet vs. Sour Crude: Crude oil containing less than 0.5% sulfur is classified as “sweet,” whereas oil with higher sulfur content is classified as “sour.” Sour crude requires extensive processing to remove sulfur, preventing the release of hazardous Sulfur Dioxide (SO2) emissions during combustion.
- Petrochemical Feedstock: Naphtha and refinery gases are the dual pillars of the modern petrochemical industry. Through steam cracking, they yield olefins (ethylene, propylene) and aromatics (benzene, toluene, xylene), which are converted into daily commodities like plastics, synthetic fibers (polyester, nylon), fertilizers, and pharmaceuticals.