Refining of Petroleum: Fractional Distillation
Crude oil extracted from the Earth is a highly complex mixture of various hydrocarbons, water, and impurities. In its raw form, it cannot be used directly as a fuel or raw material. It must be separated into its useful, individual components.
The process of separating crude oil into various useful products (fractions) is called refining, and the industrial facilities where this is done are called oil refineries. The primary method used in these refineries is a physical process known as Fractional Distillation.
The Basic Principle
Fractional distillation works on a very simple principle: different hydrocarbons have different boiling points.
- Lighter molecules (with fewer carbon atoms) have lower boiling points and turn into gas easily.
- Heavier molecules (with more carbon atoms) have higher boiling points and require much more heat to vaporize.
The Step-by-Step Process
Step 1: Heating the Crude Oil
The raw crude oil is pumped into a large furnace and heated to a very high temperature, usually around 400°C. At this intense heat, most of the crude oil boils and turns into a mixture of hot vapours.
Step 2: The Fractionating Column
These hot vapors are then fed into the bottom of a tall, cylindrical tower known as a fractionating column. This tower is designed to have a temperature gradient: it is very hot at the bottom and gradually becomes cooler toward the top.
Step 3: Rising and Cooling
As the hot vapors rise up the column, they gradually cool down.
Step 4: Condensation and Collection
When a specific vapor reaches a height in the tower where the temperature drops below its boiling point, it condenses back into a liquid. Trays are positioned at various heights inside the column to collect these condensed liquids.
- The heaviest fractions with the highest boiling points condense almost immediately at the hot bottom of the tower.
- The lightest fractions with the lowest boiling points travel all the way to the cooler top of the tower before condensing or exiting as a gas.
Major Fractions Obtained from Petroleum
As the crude oil is processed in the fractionating column, several essential products are collected at different levels. Here is a breakdown of the major fractions from top to bottom:
Fraction (Product) | Approximate Boiling Range | Primary Uses in the Economy |
Refinery Gases | Below 20°C (Collected at the very top) | Bottled as Liquefied Petroleum Gas (LPG) for domestic cooking and industrial heating. |
Petrol (Gasoline) | 20°C to 70°C | Used as a primary fuel for light motor vehicles like cars and motorcycles. |
Naphtha | 70°C to 120°C | An essential raw material (feedstock) for the petrochemical industry to manufacture plastics, synthetic fibers, and fertilizers. |
Kerosene | 120°C to 250°C | Used as Aviation Turbine Fuel (ATF) for jet aircraft, and as a domestic fuel in rural areas. |
Diesel Oil | 250°C to 350°C | Used as a heavy-duty fuel for trucks, buses, tractors, trains, and electric generators. |
Lubricating Oil | 350°C to 400°C | Used to reduce friction in machinery and vehicle engines (motor oil). |
Fuel Oil | Above 400°C | Used as a heavy fuel for industrial furnaces, boilers, and large ships. |
Residue (Bitumen/Asphalt) | Does not boil; collected at the bottom | A thick, sticky black substance used extensively for road surfacing (macadamizing) and waterproofing roofs. |
