4_7 The Contact Process: How Sulphuric Acid Is Manufactured

More sulphuric acid is made than any other chemical. Production is so large that it was once used as a measure of a nation's industrial strength. Almost all of it is made by the contact process, named after the step where the gases come into contact with a solid catalyst. The process is a classic case study in using reaction rates and equilibrium together to choose economical conditions.

The Three Chemical Steps

1. Burn sulphur (or roast sulphide ores) to make sulphur dioxide.
2. Oxidize sulphur dioxide to sulphur trioxide over a vanadium(V) oxide catalyst.
3. Absorb the sulphur trioxide to make sulphuric acid.

Step 1: Making and Cleaning Sulphur Dioxide

Molten sulphur is sprayed into a furnace and burned in excess dry air:

\mathrm{S + O_2 \rightarrow SO_2}

. Where sulphide ores are used instead, roasting iron pyrites gives the same gas:

\mathrm{4FeS_2 + 11O_2 \rightarrow 2Fe_2O_3 + 8SO_2}

. Gas from ores carries dust, moisture and arsenic compounds. These would poison the catalyst, coating its surface and destroying its activity. So the gas passes through a purification train:

• Dust removal: steam or electrostatic precipitators remove fine particles.
• Cooling and washing: the gas is cooled and scrubbed with water to remove soluble impurities.
• Drying tower: concentrated sulphuric acid trickling over packing absorbs water vapour.
• Arsenic removal: passing the gas over freshly made iron(III) hydroxide removes arsenic oxide.
• Testing: a beam of light through the gas shows whether any particles remain (the Tyndall effect). If they do, the gas goes back for more cleaning.

Step 2: Catalytic Oxidation and the Choice of Conditions

The heart of the process is a reversible, exothermic reaction:

O=S=O.O=S=O.O=O>>O=S(=O)=O.O=S(=O)=O

\mathrm{2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)} \qquad \Delta H = -197\ \text{kJ/mol}

How each condition is chosen

Condition	What equilibrium favours	What is actually used	Why
Temperature	Low (exothermic forward reaction)	About 400–450 °C	Below about 400 °C the catalyst is too slow; this is a compromise between yield and rate
Pressure	High (3 moles of gas become 2)	Only 1–2 atm	Conversion is already high, and high pressure equipment is expensive
Oxygen	Excess shifts the equilibrium right	Excess air	Cheap way to push the conversion up
Catalyst	No effect on position	V₂O₅ on silica	Speeds up reaching equilibrium; cheaper and harder to poison than platinum

The converter holds several layers of catalyst. Because the reaction releases heat, the gas is cooled between layers to keep the temperature where the equilibrium is still favourable, and that heat is recovered to raise steam. No outside heating is needed once the plant is running. In modern plants, the gas is passed through an absorber partway, then sent back over more catalyst. This double absorption brings total conversion above 99.5%, which keeps sulphur dioxide emissions very low.

Step 3: Absorption as Oleum

You might expect sulphur trioxide simply to be dissolved in water, but that reaction is so exothermic that it forms a dense mist of tiny acid droplets that is almost impossible to condense. Instead, the

\mathrm{SO_3}

is absorbed in 98% sulphuric acid, where it dissolves smoothly to form oleum, also called fuming sulphuric acid or disulphuric acid:

OS(=O)(=O)O.O=S(=O)=O>>OS(=O)(=O)OS(=O)(=O)O

Oleum is then carefully diluted with water to give sulphuric acid of whatever strength is needed:

\mathrm{H_2S_2O_7 + H_2O \rightarrow 2H_2SO_4}

. In practice, water is added to the circulating acid continuously, so the absorber always contains about 98% acid.

Energy and the Environment

The contact process is a net exporter of energy. Burning sulphur, oxidizing

\mathrm{SO_2}

and absorbing

\mathrm{SO_3}

are all strongly exothermic, and the heat from each stage raises high-pressure steam. That steam drives the plant's air blowers and often generates electricity for nearby industry. A large plant can supply more power than it uses.

Sulphur dioxide that escapes from the stack would form acid rain, which is why plants use double absorption and sometimes a final alkaline scrubber on the tail gas. Much of the sulphur fed to modern plants is itself a by-product of removing sulphur compounds from natural gas and crude oil. That cleaning step means less

\mathrm{SO_2}

is released when those fuels are burned, so one environmental clean-up supplies the raw material for another industry.

Tips for remembering

• Three S-steps: S → SO₂ → SO₃ → H₂SO₄: burn sulphur, oxidize the SO₂ over a catalyst, then absorb the SO₃.
• 450 °C is a compromise: a lower temperature gives a better yield of this exothermic reaction, but the catalyst works too slowly below about 400 °C.
• Never add SO₃ straight to water: it forms an acid mist that will not condense, so it is absorbed in 98% acid to make oleum, which is then diluted.