The contact process is used to manufacture sulphuric acid through three main stages. In stage one, sulphur is burned in air to produce sulphur dioxide. In stage two, sulphur dioxide and oxygen are passed over a vanadium oxide catalyst at 450°C to produce sulphur trioxide. In stage three, the sulphur trioxide first reacts with concentrated sulphuric acid to form oleum, which is then diluted with water to produce concentrated sulphuric acid in large quantities. The use of oleum prevents the too violent reaction that would occur if sulphur trioxide reacted directly with water.

1. USES OF
SULPHURIC ACID

5. 4. Manufacture of paint

9. CONTACT PROCESS
Manufacture of sulphuric acid

10. The Manufacture of H2SO4, through the Contact Process

11. Contact Process: Stage 1
In the furnace, molten
Sulphur is burnt in dry air to
produce Sulphur dioxide,
SO2. The gas produced is
purified and cooled.

12. Contact Process: Stage 2
In the converter, SO2 and
excess oxygen gas, O2 are
passed over a few plates of
vanadium (V) oxide,
V2O5 catalyst at 450oC to
produce Sulphur trioxide,
SO3
The reaction is slow and the
V2O5 catalyst speeds up the
rate of reaction, at a
moderate temperature of
about 450oC.

13. Contact Process: Stage 2
About 99.5% of the sulphur
dioxide, SO2 is converted into
sulphur trioxide, SO3 through
this reversible reaction.

14. Contact Process: Stage 3
In the absorber, the sulphur
trioxide, SO3 is first reacted
with concentrated sulphuric
acid, H2SO4 to form a
product called oleum,
H2S2O7.

15. Contact Process: Stage 3
The oleum, H2S2O7 is then
diluted with water to
produce concentrated
sulphuric acid, H2SO4 in large
quantities.

16. FAQ 1
• The two reaction in the third stage are equivalent to
adding Sulphur trioxide, SO3, directly to water
• Then why can’t we just skipped concentrated
sulphuric acid step [thus not forming oleum]?

17. Answer
• This is not done in the industry, because
Sulphur trioxide, SO3 reacts too violently with
water. This produces a lot of heat and a large
cloud of sulphuric acid, H2SO4 mist.
• The mist is corrosive, pollutes the air and is
difficult to condense
X

18. Refresh again…
Stage 1, in furnace
Stage 2, in converter
Stage 3, in absorber
Catalyst: Vanadium (V) oxide
Temperature: 450oC
**O2 = burning
**O2 = passed over catalyst