Fractional distillation is a physical separation process used to separate a mixture of miscible liquids with different boiling points. In the context of fuels and petrochemicals, it serves as the foundational process in oil refineries to separate crude oil into useful components called fractions.
Vapor Pressure and Boiling Point Mechanics
- Boiling Point Relationship: A liquid boils when its internal vapor pressure equals the surrounding atmospheric pressure. Hydrocarbons with short carbon chains have weak intermolecular forces (Van der Waals forces), low boiling points, and high volatility. Conversely, long-chain hydrocarbons possess strong intermolecular forces, high boiling points, and low volatility.
- Continuous Vaporization and Condensation: Inside a fractionating column, a continuous cycle of vaporization and condensation takes place. As vapor ascends the column, it hits physical barriers (trays or packing). The heavier, higher-boiling-point molecules condense into liquid and flow downward, while the lighter, lower-boiling-point molecules remain as vapor and continue to rise. Each cycle increases the purity of the lighter component in the vapor phase.
Architecture of an Industrial Distillation Column
Industrial refinery distillation columns are tall, vertical steel towers designed to handle thousands of barrels of crude oil per day using a steady temperature gradient.
Structural Components
- The Furnace: Before entering the column, crude oil is heated to approximately 350°C to 400°C, vaporizing most of the hydrocarbons into a hot, multi-phase mixture.
- The Temperature Gradient: The column is engineered to be hottest at the bottom (near the feed inlet) and progressively cooler toward the top.
- Distillation Trays (Bubble Cap / Valve Trays): Horizontal trays are spaced throughout the tower. These trays hold a shallow layer of condensed liquid. Rising vapors are forced to bubble through this liquid layer. This layout maximizes thermal contact, forcing heavier molecules out of the vapor into the liquid phase, while vaporizing lighter components out of the liquid phase.
- Side-Streams: Liquid fractions are continuously drawn off from the trays at various heights where the column temperature matches their specific condensation points.
The Fractionation Profile of Crude Oil
The table below traces the step-by-step separation of crude oil fractions as vapors rise from the hot base to the cool top of an atmospheric distillation column.
| Column Level | Fraction Name | Boiling Point Range (°C) | Carbon Chain Length | Primary Applications |
| Top (Coolest) | Refinery Gases | < 20 | C1 to C4 | Liquefied under pressure to form LPG; petrochemical feedstock. |
| Upper Middle | Gasoline (Petrol) | 40 – 170 | C5 to C10 | Fuel for spark-ignition automobile engines; industrial solvents. |
| Middle | Naphtha | 120 – 180 | C7 to C11 | Primary feedstock for steam crackers to manufacture plastics and polymers. |
| Lower Middle | Kerosene | 170 – 250 | C10 to C16 | Aviation turbine fuel (ATF); domestic cooking and heating. |
| Lower | Diesel Oil | 250 – 350 | C15 to C18 | Fuel for compression-ignition heavy vehicles and generators. |
| Bottom (Hottest) | Atmospheric Residue | > 350 | > C20 | Dispatched to a vacuum distillation column for further separation. |
Vacuum Distillation: Processing the Heavy Residue
The thick, unvaporized residue collected from the bottom of the atmospheric distillation tower contains long-chain hydrocarbons with boiling points exceeding 350°C to 400°C.
The Problem of Thermal Cracking
If these heavy residues are heated to their normal boiling points at standard atmospheric pressure, the thermal energy will break the carbon-carbon bonds. This unwanted thermal cracking produces low-value coke and plugs the refinery equipment.
The Vacuum Solution
To prevent cracking, the residue is transferred to a Vacuum Distillation Unit (VDU).
- The Principle: Reducing the internal pressure of the distillation column lowers the boiling point of the liquids inside.
- The Outcome: Under highly reduced pressure (vacuum), these heavy hydrocarbons vaporize smoothly at much lower temperatures. This allows refineries to safely separate high-boiling fractions into heavy gas oils, lubricating oils, paraffin wax, and asphalt/bitumen.
Key Fact-Sheet for UPSC Prelims
- Azeotropes (Azeotropic Mixtures): A mixture of two or more liquids whose proportions cannot be altered or changed by simple fractional distillation. This occurs because when an azeotrope is boiled, the vapor has the exact same ratio of constituents as the unboiled liquid (e.g., a mixture of 95.6% ethanol and 4.4% water).
- Theoretical Plates: A quantitative measure used to evaluate the efficiency of a fractional distillation column. One theoretical plate corresponds to a single, perfect cycle of vaporization and condensation. Columns with more theoretical plates achieve sharper separation of components with close boiling points.
- Straight-Run Fractions: Petroleum fractions obtained directly from the physical separation of crude oil in a distillation column without undergoing chemical modifications like cracking, reforming, or alkylation.
- Refinery Turnaround: A highly planned, temporary shutdown of a refinery distillation unit to clean out accumulated carbon residues, replace trays, inspect for corrosion, and maximize the efficiency of future fractional distillation cycles.