Undergraduate · 20 min

The contact process: making sulfuric acid

Follow the industrial chain S → SO₂ → SO₃ → oleum → H₂SO₄ and understand why each unit is needed.

Goal

Trace the mass flow and explain the roles of purification, catalysis and oleum absorption in the contact process.

Apparatus and reagents

Virtual contact plant: sulfur feed, air supply, V₂O₅ catalyst bed, oleum absorption tower, water for dilution.

Procedure

Set the flow slider slow and name each of the five units as the gas passes: $\ce{S}$ , $\ce{SO2}$ , $\ce{SO3}$ , oleum, $\ce{H2SO4}$ .
Write the three key reactions: $\ce{S + O2 -> SO2}$ , $\ce{2SO2 + O2 <=>[V2O5] 2SO3}$ , $\ce{H2SO4 + SO3 -> H2S2O7}$ then dilution.
Increase the flow rate and discuss why higher throughput needs larger heat exchange and why purity of the feed protects the catalyst.
Explain why $\ce{SO3}$ is absorbed into concentrated acid (oleum) instead of water directly.

What to observe

The exothermic converter step $\ce{2SO2 + O2 <=> 2SO3}$ runs best near 450 °C over V₂O₅ — a compromise between rate and equilibrium.
Direct absorption of $\ce{SO3}$ in water would form an uncontrollable acid mist; oleum dissolves it smoothly.

Explanation

The contact process is the backbone of the chemical industry — sulfuric acid is made in greater tonnage than any other chemical. Purification removes catalyst poisons, the V₂O₅ bed speeds the oxidation to SO₃, and staged absorption into oleum tames the violent hydration of SO₃. Each unit exists because thermodynamics, kinetics and safety each impose a constraint.

Chemists behind it

Wilhelm Ostwald