Two of the most important and widely produced industrial chemicals are sulfuric acid and phosphoric acid. Both acids are essential to the agriculture industry as fertilizer. Sulfuric acid has a vast range of specific uses in most industries such as petroleum and explosives, whilst phosphoric acid is added to soft drinks. However, both acids are used in the manufacturing of detergents and batteries. In terms of physical properties both acids are colourless and have a strong affinity to water. Due to their affinity with water, they are ideal dehydrating agents, although the reaction between phosphoric acid and water is considerably safer.
Sulfuric acid is usually manufactured by the contact process, a three-stage process, which sees the catalytic oxidation of sulfur dioxide to sulfur trioxide, whilst phosphoric acid is, in this case, manufactured by a completely different process which sees the conversion of phosphate rock to phosphoric acid through several steps.
In the production of phosphoric acid the raw material is the phosphoric rock, which is heated to extreme temperatures with SiO2 and graphite in an electric furnace (1.1). No furnace is used in the production of sulfuric acid, as in the first stage molten sulfur is sprayed into a container and simply burnt in the air to produce SO2 in an oxidation reaction (1.2).
1.1- 2Ca3(PO4)2(s) + 6SiO2(s) + 10C(s) 6CaSiO3(l) + 10CO(g) + P4(g)
1.2- S(s) + O2(g) SO2(g)
The next stage of the process for both sulfuric and phosphoric acid involves the oxidation of SO2 to SO3 (2.1), and P4 to P4O10 (2.2) respectively. To maximise the yield of SO3 produced, this stage is carried out carefully under controlled conditions. To improve the economics of the industrial plants the large quantities of heat from both these reactions are recycled to generate steam turbines and produce electricity to help power the manufacturing.
2.1- 2SO2(g) + O2(g) 2SO3(g) + 198kJ
2.2- P4(g) + 5O2(g) P4O10(g) + 3053kJ
As with the chemical manufacturing of many other acids and industrial chemicals, the conditions that the system is subjected to are a compromise between economic considerations, equilibrium yield and reaction rates. Firstly, for the production of sulfuric acid the equilibrium yield of SO3 is maximised at lower temperatures, due to the exothermic nature of the reaction, and higher pressures, to increase the concentration of gaseous molecules. The reaction rate is increased by higher pressures and the use of the catalyst V2O5. However, unlike for the equilibrium yield, the reaction rate is increased with higher temperatures.
A comprise is made so that the process is carried out at a temperature of 600C and a slightly higher pressure than that of 1 atmosphere. Whilst the reaction does occur rapidly, it does not occur to completion. The gaseous mixture is then cooled to 400C, and passed over additional layers of catalysts which results in the near complete conversion of SO2 to SO3.
Finally, phosphoric acid is formed by passing P4O10 through a spray of water in a tower (3.1). In the final stage the sulfur trioxide is dissolved in concentrated sulfuric acid to produce oleum (3.2), H2S2O7, which is then, like P4O10, added to water to produce the final chemical, an increased amount of 18M sulfuric acid (3.3).
3.1- P4O10 + 6H2O(l) 4H3PO4(l)
3.2- H2SO4(l) + SO3(g) H2S2O7(l)
3.3- H2S2O7(l) + H2O(l) H2SO4(l)