Manufacture of sulfuric acid

Sulfuric acid usually manufactured by the contact process, which is the three-step process illustrated in figure 1

Figure 1 The contact process for the production of sulfuric acid. The first step involved the oxidation of sulfur to sulfur dioxide. This is achieved by

burning sulfur in air.

S(s) + O2(g) ---> SO2(g)

The second in the process, the oxidation of sulfur dioxide to sulfur trioxide, is reversible reaction and is carried out under carefully controlled conditions to maximize the yield of SO3 produced.

2SO2(g) + O2(g) ---> 2SO3(g) + heat

As with many industrial chemical process, the conditions used are a compromise between rate, equilibrium yield and economic considerations. The equilibrium yield of SO3 is favouredby:

• Low temperatures-the reaction is exothermic.
• High pressures-the reaction leads to a decrease in the concentration of gaseous molecules.

The reaction rate is favoured by:

• High temperat ures
• High pressure
• The use of catalyst

The actual process is carried out at a temperature oa about 600oC with a V2O5 catalyst and at pressure close to one atmosphere. At this temperature using a catalyst the reaction occurs rapidly but does not occur to completion. The yield is increased by cooling the gas mixture to about 400oC and passing it over additional amounts of a catalyst. This result in almost complete conversion of SO2 to SO3. Because high yield can be obtained at atmospheric

pressure it is not necessary to use expensive high pressure equipment.

In the third step sulfur trioxide is dissolved in concentrated sulfuric acid to form oleum, H2S2O7.

SO3(g) + H2SO4(l) ---> H2S2O7(l)

Water is then added to  H2S2O7 to produce sulfuric acid.

 H2S2O7(l) + H2O(l) --->  H2SO4(l)

The SO3 is not directly absorbed in water because this reaction forms a fog of H2SO4 droplets with which it is difficult to work. The sulfuric acid produced is up 98% H2SO4 and has a concentration of about 18 mol L-1 .Both the oxidation of sulfur and sulfur dioxide are exothermic process. The large quantities of heat released in these reactions are used to generate steam and electricity and help to improve the economics of the process.

Sulfuric acid is a very important chemical in industry and is produced in greater

quantities than any other manufactured chemical. It is used in many major industries and the annual production is sometimes used as an index of a nation’s industrial activity. In Australia the annual production is about one-sixth of a tone per head of population. The major use of sulfuric acid in Australia is in the manufacture of fertilizers, but it is also used in many other industries,