Petrochemical
Industry

Composition of crude oil and hydrocarbon structure

Crude oil is primarily composed of hydrocarbons-molecules made up of hydrogen and carbon atoms. These hydrocarbons vary in size, structure and length. The smallest, methane, contains only one carbon atom per molecule and exists as a gas under standard conditions. As the number of carbon atoms increases, hydrocarbons form liquids and, at even higher molecular weights, solids. This variation in molecular structure is what enables crude oil to be transformed into such a wide range of products, including butane, jet fuel, petrol, diesel, lubricants and many types of plastics. Crude oil is the raw material for a wide range of petrochemicals, including fuels, solvents, lubricants and polymers. These are derived through fractional distillation and advanced refining processes such as cracking, reforming and hydrocracking.

Distillation of crude oil into component fractions

The length of each hydrocarbon molecule determines its boiling point, making it possible to separate crude oil into fractions through distillation. In this process, crude oil is vaporised by mixing it with superheated steam. The vapour then enters a tall distillation column, which is approximately 600°C (1112°F) at the base and gradually cools to 20°C (68°F) at the top. As the vapour rises through the column, it encounters a series of trays. Each hydrocarbon fraction condenses at a height corresponding to its boiling point and is collected. The fractions recovered include gases, petrol, naphtha, kerosene, diesel, fuel oil and various residues.

Further processing and conversion of oil fractions

Not all distillation fractions have equal value, so additional processing is often used to convert less valuable fractions into more desirable products. This is achieved through methods such as cracking, unification and alteration. Cracking is the most common of these techniques, involving the breakdown of larger hydrocarbon molecules into smaller, more volatile and combustible ones. These lighter molecules are particularly useful as fuels due to their improved flow and ignition characteristics. Unification combines small molecules to form larger ones, while alteration changes the molecular structure without changing the number of atoms.

Types of cracking used in oil refining

Cracking can be achieved through several methods, each adapted to specific industrial goals. Thermal cracking employs high temperatures, and in some cases high pressure, to break molecular bonds. Steam cracking involves diluting hydrocarbons with steam and briefly exposing them to extremely high temperatures to induce breakdown. Catalytic cracking uses a catalyst in a fluidised bed reactor at lower temperatures to stimulate the chemical reactions needed to split the hydrocarbons. When hydrogen is introduced into the catalytic process, it becomes hydrocracking- a technique that improves both product yield and quality. The products of cracking often undergo further distillation to separate the resulting components into useful fractions.