Manufacture of Sulfuric Acid (H2SO4)
Most of the sulfuric acid manufactured is produced using the Contact Process.
(combustion of sulfur)
(conversion of sulfur dioxide)
(sulfur trioxide absorbed
into the sulfuric acid mist
||Hydration of Oleum|
to produce sulfuric acid
The Contact Process is a process involving the catalytic oxidation of sulfur dioxide, SO2, to sulfur trioxide, SO3.
- Solid sulfur, S(s), is burned in air to form sulfur dioxide gas, SO2
S(s) + O2(g) → SO2(g)
- The gases are mixed with more air then cleaned by electrostatic precipitation to remove any particulate matter
- The mixture of sulfur dioxide and air is heated to 450oC and subjected to a pressure of 101.3 - 202.6 kPa (1 -2 atmospheres) in the presence of a vanadium catalyst (vanadium (V) oxide) to produce sulfur trioxide, SO3(g), with a yield of 98%.
2SO2(g) + O2(g) → 2SO3(g)
Any unreacted gases from the above reaction are recylced back into the above reaction
- Sulfur trioxide, SO3(g) is dissolved in 98% (18 M) sulfuric acid, H2SO4, to produce disulfuric acid or pyrosulfuric acid, also known as fuming sulfuric acid or oleum, H2S2O7.
SO3(g) + H2SO4 → H2S2O7
This is done because when water is added directly to sulfur trioxide to produce sulfuric acid
SO3(g) + H2O(l) → H2SO4(l)
the reaction is slow and tends to form a mist in which the particles refuse to coalesce.
- Water is added to the disulfuric acid, H2S2O7, to produce sulfuric acid, H2SO4
H2S2O7(l) + H2O(l) → 2H2SO4(l)
The oxidation of sulfur dioxide to sulfur trioxide in step III above is an exothermic reaction (energy is released), so by Le Chatelier's Principle, higher temperatures will force the equilibrium position to shift to the left hand side of the equation favouring the production of sulfur dioxide.
Lower temperatures would favour the production of the product sulfur trioxide and result in a higher yield.
However, the rate of reaching equilibrium at the lower temperatures is extremely low.
A higher temperature means equilibrium is established more rapidly but the yield of sulfur trioxide is lower.
A temperature of 450oC is a compromise whereby a faster reaction rate results in a slightly lower yield.
Similarly, at higher pressures, the equilibrium position shifts to the side of the equation in which there are the least numbers of gaseous molecules.
2SO2(g) + O2(g) → 2SO3
On the left hand side of the reaction there are 3 moles of gaseous reactants, and the right hand side there are 2 moles of gaseous products, so higher pressure favours the right hand side, by Le Chatelier's Principle.
Higher pressure results in a higher yield of sulfur trioxide.
A vanadium catalyst (vanadium(V) oxide) is also used in this reaction in order to speed up the rate of the reaction.