Page 27 - Curriculum Visions Dynamic Book

What a petrochemical plant does
A refinery is a collection of tall towers, called distillation towers, connected by a maze of pipes. The crude oil is heated in the towers so that the lighter parts of the mixture boil off,
or vapourize. The compounds can be separated out because petroleum is a mixture. This means that the separate fractions can be obtained by making use of the various boiling points of the substances in the mixture.
The gases then condense in the upper, cooler parts of the towers and form into liquids. By using a series of towers, the various chemicals can be separated out. The aim is to produce such different products as liquified petroleum gas (LPG), gasoline (vehicle fuel), kerosene (aircraft fuel), heating oil, diesel fuel, and asphalt (for road surfaces).
A simple process of boiling separates only about one-tenth of the crude oil. The remainder has to be processed again, using a procedure called “cracking”, which involves splitting the large, heavy hydrocarbon molecules into smaller ones. A variety
of methods are used, including heat and pressure, vacuum,
and chemicals such as hydrogen. In this way the products of the refinery can be tailored to market needs. So, for example,
if the demand is for an increase in lubricating oils, then the cracking process is adjusted to produce these products; if the oil demand falls and the gasoline demand rises, the type of cracking is changed to give the new balance of products.
cracking: breaking down complex molecules into simpler components. It is a term particularly used in oil refining.
crude oil: a chemical mixture of petroleum liquids. Crude oil forms the raw material for an oil refinery.
fraction: a group of similar components of a mixture. In the petroleum industry the light fractions of crude oil are those with the smallest molecules, while the medium and heavy fractions have larger molecules.
petroleum: a natural mixture of a range of gases, liquids
and solids derived from the decomposed remains of plants and animals.
 A diagrammatic representation of
the cracking process.
Cracking, the breaking up of the oil fraction into smaller components, takes place in
Waste gases are this fluid bed unit, at 500°C. cleaned and burned.
The cracked mixture passes to the fractionating column.
110°C
Catalyst is passed to the fluid bed unit.
The spent and contaminated catalyst of alumina–silica gel is regenerated in this chamber, which is heated to 600°C.
Air
Spent catalyst passes to the regenerator.
Oil fractions to be cracked come from fractionating tower and may be mixed with some crude oil
400°C
Products such as ethene, propene and heptane are separated from fractionating tower.
Steam
27
27