| Contents for this page | Related topics |  |
|---|---|---|
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Introduction Ammonia Nitric acid Sulphuric acid Additional questions |
The petroleum industry The chloralkali and Solvay processes The polymer industry (1) The polymer industry (2) The fertiliser industry Electrochemical cells |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will be familiar with the chemical principles underlying the industrial production of a) ammonia, b) nitric acid, and c) sulphuric acid. | ||
South Africa has the most highly developed chemical industry on the African continent, producing much of its internal needs for so-called "heavy chemicals" (that is, chemicals produced in very large quantities), and exporting important quantities to other countries. For this, it relies on well trained chemical engineers. Several South African universities offer degree courses in Chemical Engineering, thus providing opportunities to take up rewarding and challenging careers. In this section, we deal with three important chemicals which are not only the mainstay of the fertiliser industry, but also of great importance in other industrial sectors.
Ammonia is produced industrially by the Haber-Bosch process which involves the catalytic reduction of nitrogen by hydrogen at temperatures of 450-500 ºC and pressures of 35-40 MPa.
The process provides an excellent illustration of Le Chatelier's Principle (
). If we examine the above equation, we see that 4 volumes of the reacting gases form 2 volumes of ammonia. Le Chatelier's Principle predicts that the forward reaction will be favoured by applying a stress on the system such as to reduce the overall volume. This is achieved by increasing the pressure on the reacting system. In practice, the pressure that is used is around 40 MPa (about 400 times normal atmospheric pressure).
We note also that heat is evolved in the process (DH is negative - the reaction is exothermic). The forward reaction will therefore be promoted by a reduction in temperature, thus improving the yield of ammonia at equilibrium. However, at low temperatures the rate of attaining the equilibrium is so slow as to make the process impracticable. So, in order to carry out the process, a temperature of about 500 ºC is used, and even so, it has to be speeded up by the use of an iron oxide catalyst. The equilibrium mixture contains about 25-30% ammonia. This gas is easily liquefied, thus enabling its separation from the unreacted nitrogen and hydrogen, which are recycled.
Nitric acid is produced industrially by the Ostwald Process, which involves three steps:
Sulphuric acid, H2SO4, is one of the most important industrial chemicals. It is an oily liquid having a boiling point of 335 ºC, which evolves much heat on dilution with water. Millions of tons of sulphuric acid are made every year by the CONTACT PROCESS, which converts raw sulphur, oxygen and water to sulphuric acid.
Step 1: Melted sulphur is burned in a furnace, using air, producing sulphur dioxide, SO2.
Step 2: The SO2 gas is passed through a tower called a precipitator in order to remove dust and other impurities which might interfere with the catalyst.
Step 3: The SO2 is then washed with water, in a scrubbing tower.
Step 4: The SO2 is then dried in a drying tower.
Step 5: After passing through a heating chamber, the SO2, which is still mixed with air, is passed through a reactor. There, using vanadium pentoxide, V2O5, as catalyst, the SO2 is converted to sulphur trioxide, SO3.
Step 6: Finally, the SO3 is absorbed in concentrated sulphuric acid, giving the so-called oleum or pyrosulphuric acid. This is the diluted with water to give about 98% pure H2SO4.
