The Contact Process: Sulfuric Acid Production

The Contact Process

• Sulphuric acid is commercially produced using the Contact process.
• Raw materials needed include: sulphur dioxide, air, water, and a catalyst.
• The main stages: (1) production of sulphur dioxide, (2) oxidation of sulphur dioxide to sulphur trioxide, and (3) hydration of sulphur trioxide to sulphuric acid.

Production of Sulphur Dioxide

• Can be produced by: -The burning of sulphur in excess air: S(s) + O2(g) → SO2(g) -Heating of sulphide ores, like pyrite, in excess air: 4FeS2(s) + 11O2(g) → 2Fe2O3(s) + 8SO2(g) -Decomposing calcium sulphate with coke: 2CaSO₄(s) + C(s) → 2CaO(s) + CO2(g) + 2SO2(g)

Oxidation of Sulphur Dioxide to Sulphur Trioxide

• Sulphur dioxide is mixed with excess air and passed through an electrostatic precipitator to remove dust and impurities.
• Purified sulphur dioxide combines with oxygen, using vanadium (V) oxide as a catalyst, to form sulphur trioxide (the anhydride of sulphuric acid).
• Reaction: 2SO2(g) + O2(g) ⇌ 2SO3(g) ΔH = -196 kJ mol⁻¹
• The reaction is reversible, establishing a state of dynamic equilibrium.
• The forward reaction is exothermic and results in a decrease in volume (3 moles of reactants to 2 moles of products).
• Le Chatelier's Principle suggests low temperature and high pressure would favor the forward reaction, increasing sulphur trioxide yield.
• Low temperatures result in a slow reaction rate that is uneconomical.
• High pressures cause sulphur dioxide to liquefy.
• A temperature of 450°C is used with a catalyst, giving a good yield of sulphur trioxide at an acceptable rate.
• Catalysts increase the reaction rate, but have no effect on equilibrium position.
• The catalyst is layered in pellet form in order to increase the surface area available for reaction.
• Impurities are removed from the gaseous mixture, as they reduce the catalyst's surface area and efficiency.
• The Contact process gets its name due to the reaction taking place by contact of the gaseous mixture with a solid catalyst.
• The forward reaction is exothermic, increasing the temperature from 450°C to 600°C as the reaction proceeds.
• At 600°C, the yield reduces to ~70%, thus the gases are cooled down to 450°C between catalyst layers to achieve a yield of around 98%.
• The cooling system is a closed water circuit.
• Heat exchangers recirculate lost heat to heat incoming gases; this reduces the need for external heating.
• Reaction is done close to atmospheric pressure for an approximate yield of 98%.
• Increased oxygen concentration favors the forward reaction (Le Chatelier's Principle), with a 1:1 mixture giving the best sulphur trioxide yield.

Hydrating Sulphur Trioxide to Sulphuric Acid

• Gases are dissolved in water using a counter flow system.
• This method is not used, because the reaction is highly exothermic and creates uncontrollable clouds of sulphuric acid which are difficult to condense.
• Sulphuric acid destroys skin and flesh, and can cause blindness.
• Sulphur trioxide is first dissolved in concentrated sulphuric acid to form oleum: H₂SO₄(l) + SO2(g) → H2S2O,(l)
• Oleum is diluted with water to produce concentrated sulphuric acid: H2S2O,(l) + H2O(l) → 2H₂SO₄(l)

Uses of Compounds of Sulphur

• Most sulphur is used to produce sulphur dioxide to make sulphuric acid.
• Sulphur is used in the vulcanization of natural rubber to improve its elasticity and tensile strength, making rubber less sticky and soluble in organic solvents.
• Applications of sulphur include it being used as a fungicide and fumigant.
• Sulphur dioxide is a reducing agent, used for bleaching wool, straw, and paper, though the effect is temporary; oxygen in the atmosphere can re-oxidize the reduced material.
• Sulphur dioxide is a useful antioxidant in food preservation; used as a preservative in alcoholic drinks and dried fruits.
• Sulphur dioxide prevents mould growth, with its acidity also killing bacteria.
• The main use of sulphur dioxide is in the manufacture of sulphuric acid.
• Sulphuric acid is commonly used in the manufacture of fertilizers, detergents, car batteries, high strength fibres, paints and pigments, petroleum refining, and metallurgy.

Impact of the Sulphuric Acid Industry

• Sulphuric acid production reduces sulphur dioxide emissions, as some sulphur dioxide sources are by-products of metal extraction from sulphide ores and the decomposition of calcium sulphate.
• Sulphur dioxide emission from the Contact process is minimized by recycling unreacted sulphur dioxide and scrubbing the tail gas with a basic solution like ammonium or sodium hydroxide.
• Sulphuric acid is used often as a catalyst in the chemical industry, and it needs to be replaced with fresh concentrated acid when diluted or contaminated.
• Spent sulphuric acid from the chemical industry is regenerated and recycled.
• Thermal decomposition of spent sulphuric acid gives sulphur dioxide, which is regenerated into clean sulphuric acid and is reused in the process.
• Spent catalyst from the Contact process can either be disposed of in a licensed landfill or its vanadium content can be recovered and recycled.
• Regeneration and recycling prevent spent acid and catalysts from becoming waste.
• Sulphuric acid production is a net producer of energy.
• Evolved heat in the process turns water into steam, which generates electricity.
• No carbon dioxide is formed because fossil fuel is not used.
• During production, handling, and storage of sulphuric acid, accidental leaks could impact the environment.
• Precautions need to be taken to reduce the risk of these possibilities.