Raw Materials

Background Information

Oil

Crude oil contains a mixture of compounds, mostly hydrocarbons, with different boiling points. The boiling point of a hydrocarbon depends on its chain length: the longer the chain, the higher the boiling point. The degree of branching of the hydrocarbon chain is another factor which affects the boiling point. For example, the straight-chain form of butane has a higher boiling point than its branched-chain isomer.

Distillation

The variation in the boiling points of the hydrocarbons in crude oil allows them to be separated by distillation. The separation of liquids with similar boiling points can be achieved in the laboratory with the help of a fractionating column. There is a temperature gradient in the column which allows separation of the components or fractions in the mixture according to their boiling points. Fractions with high boiling points condense near the bottom of the column while those with low boiling points condense near the top.

This is the basis of the fractional distillation method, which is used in the petroleum industry for the distillation of crude oil. Crude oil is vaporised in a furnace and passed through a fractionating column in which there is a series of trays. Condensation occurs on these trays, and fractions are separated according to their boiling points, as in the laboratory apparatus. Each fraction contains a mixture of hydrocarbons within a narrow range of boiling points.

Some Properties of Crude Oil Fractions

Name	Carbon Chain Length	Boiling Range (oC)
petroleum gases	1 - 4	<5
gasoline	5 - 10	20 - 60
naphtha	8 - 12	60 - 180
kerosine	10 - 16	180 - 260
gas oil (diesel oil)	14 - 20	260 - 340
lubricating oil	20 - 50	370 - 600
fuel oil	20 - 70	>330
residue (bitumen)	>70	(non-distillate)

Air

Air is an important resource for the chemical industry. The gases in air are important raw materials for many chemical processes. The combustion of sulphur in oxygen to make sulphur (IV) oxide (sulphur dioxide) is the first stage in the production of sulphuric acid by the Contact Process. Fertiliser production involves the separation of nitrogen from the air and its conversion to ammonia by the Haber Process.

Liquid oxygen boils at -183^oC and liquid nitrogen at -196^oC. They can be separated from liquefied air by the same process of fractional distillation as is used to obtain useful products from crude oil.

Reducing Metal Oxides

The principles of metal oxide reduction are of great economic importance in the metals industry. The method used to extract a metal from its ore depends on the position of the metal in the reactivity series. A very unreactive metal such as gold can be found in the Earth's crust in its native state. Copper, a slightly more reactive metal, is easily obtained by roasting the ore in air and then purifying the crude metal electrolytically.

More reactive metals such as iron came later in the history of humanity because of the difficulty of reducing the ores of these metals. To make iron, iron ore must be heated with coke and limestone to a very high temperature (about 1,500^oC) in a blast furnace. Carbon monoxide reduces iron (III) oxide in the furnace, while siliceous impurities in the ore are removed as a slag by limestone.