Page 28 - Curriculum Visions Dynamic Book

Manufacturing sulphuric acid
More sulphuric acid is made than almost any other substance.
This is because the acid is useful in making so many other products.
Sulphuric acid is made by reacting sulphur dioxide with oxygen
and water. Sulphur dioxide is obtained either from hydrogen sulphide in natural gas, or as pure sulphur is burned in air. It may also be collected from power station smokestacks.
Sulphur dioxide reacts only slowly with oxygen, so the process is made quicker by reacting the substances at a high temperature (500°C) and in the presence of a catalyst. The catalyst used in this reaction (called the Contact process) is a material called vanadium pentoxide.
Sulphur dioxide and air are forced through a tower containing the catalyst in the form of pellets. The sulphur trioxide gas then flows into a tower containing quartz bathed in a circulating bath of sulphuric acid. As the gas dissolves in the sulphuric acid, the acid becomes more concentrated and some is drawn off and diluted.
The gas is not added directly to water because this would give off too much heat energy and make the water evaporate.
The Contact process
Sulphur dioxide and oxygen are brought together in a converter containing vanadium pentoxide. Usually more than one stage of conversion is needed. The reaction produces heat. If the reacting gases get too hot they
stop reacting, so the gases are led from one converter, cooled and then fed into another. Up to four stages of this conversion are needed. By this time nearly all of the sulphur dioxide has been converted to sulphur trioxide.
The absorber is a tank of sulphuric acid
that absorbs sulphur trioxide, concentrating the sulphuric acid even further. Some of this liquid can be drawn off and diluted as required. Sulphur trioxide is not added to water because it releases a great deal of heat and produces a fine mist that is difficult to handle.
 A bank of Contact process converters.
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