Production of Sulfuric Acid

Sulfuric Acid Sulfuric acid is produced in greater quantities than any other chemical in the world. Most sulfuric acid plants are located near smelting and refining industries that produce waste sulfur dioxide, a raw material for the production of sulfuric acid.

Uses of sulfuric acid It is also used in the manufacturing of paper, household detergents, pigments, dyes and drugs. It is the electrolyte in car batteries.

Used as a strong acid Pure sulfuric acid is a viscous liquid that reacts with water in two steps. Write them down? What type of acid is sulfuric acid? The first step has proceeds virtually to completion. The second step has a much smaller Ka value.

Strong Acid A large amount of heat is evolved during this process. For this reason when preparing sulfuric acid, you ALWAYS add the acid to water slowly with continuous stirring. Never add water to acid as this can cause the water to boil and the acid to splatter.

As a dehydrating agent Concentrated sulfuric acid is a powerful dehydrating agent. Sugar is dehydrated: C12H22O11(s) 12C(s) + 11H2O(l) The dehydrating ability of sulfuric acid is often utilised in laboratories to dry gas mixtures that are being prepared or analysed. It is not suitable for bases as they will react with the acid H2SO4(l)

As an oxidant Concentrated sulfuric acid is a strong oxidant, especially when hot. Sulfuric acid can be reduced to sulfur dioxide (SO2), sulfur (S) or hydrogen sulfide (H2S), depending on the temperature, the strength of the reductant involved and the mole ratio of the reactants.

The contact process The converter contains pellets of a catalyst, vanadium (V) oxide (V2O5). The sulphur dioxide reacts with more air to form sulphur trioxide. This reaction is reversible and reaches an equilibrium. It is also an exothermic reaction and the temperature will rise to over 600oC.

The contact process – raw materials The sulfur dioxide used to produce sulfuric acid is obtained from two principal sources Combustion of sulfur recovered from natural gas and crude oil According to the Le Chatelier's principle, a lower temperature should be used to shift the chemical equilibrium towards the right, hence increasing the percentage yield. However too low of a temperature will lower the formation rate to an uneconomical level.  2 SO2(g) + O2(g) ⇌ 2 SO3(g) : ΔH = -197 kJ·mol−1

Step 1: Burning Sulfur S(l) + O2(g) → SO2(g); ∆H = -297 kJ mol-1 If sulfur is used as a raw material, the first step is to spray molten sulfur under pressure into a furnace up to 1000°C. Here is burns in air to produce sulfur dioxide gas. The sulfur dioxide gas is then cooled for the next step The high surface area of the sulfur spray allows combustion to be rapid. Figure 2  Vanadium(v) oxide catalyst used for the manufacture of sulfuric acid.  The gas inlet duct can be seen in the middle of the picture. S(l) + O2(g) → SO2(g); ∆H = -297 kJ mol-1

Step 2: Catalytic oxidation of sulfur dioxide Sulfur dioxide gas is oxidised to sulfur trioxide gas by oxygen, using Vanadium oxide as a catalyst 2SO2(g) + O2(g) 2SO3(g); ∆H = -197 kJ mol-1 This step is performed in a reaction vessel called a converter. Sulfur dioxide is mixed with air and passed through trays containing loosely packed porous pellets of catalysts.

Step 2: Catalytic oxidation of sulfur dioxide The converter contains several catalyst beds and the gas mixture passes over each in succession. Because the reaction is exothermic it is necessary to cool the gas mixture as it passes from one tray to another to maintain the desired reaction temperature. The temperature in the converter is maintained between 400°C and 500°C and the pressure is close to 1 atm. Nearly complete conversion of sulfur dioxide to sulfur trioxide is achieved.

Stage 2: Equilibrium yield Using Le Chatelier’s principal, the equilibrium yield of sulfur trioxide will increase: As temperature decrease. Since the reaction is exothermic a decrease in temperature will favour the forward reaction. As pressure increases. Since there are more gas particles on the reactants the forward reaction will result in a decreased pressure. If excess reactants are added.

Stage 2: Rate of reaction The rate of reaction will be faster: As temperature increases As pressure increases If a catalyst is employed What compromises have been made to get the fastest reaction with the best yields?

Stage 3: Absorption of sulfur trioxide Sulfur trioxide reacts with water to form sulfuric acid: SO3(g) + H2O(l) → H2SO4(aq); ∆H = -130 kJ mol-1 However direct reaction with water is not used, because so much heat evolves when sulfur trioxide is added to water that a fine mist of acid is produced which is difficult to collect.

Stage 3: Absorption of sulfur trioxide Instead, sulfur trioxide gas is passed into concentrated sulfuric acid in an absorption tower. This reaction occurs in two steps The sulfur trioxide gas dissolves almost totally in the acid to form a liquid known as oleum SO3(g) + H2SO4(l) → H2S2O7(l) Oleum obtained from the absorption tower is then carefully mixed with water to produce sulfuric acid: H2S2O7(l) + H2O(l) → H2SO4(l)

Waste Management Sulfuric acid plants use sulfur or sulfur dioxide that is a by-product from other industries. To maximise their conversion of sulfur dioxide to sulfur trioxide most plants now use a double absorption process. Any unreacted gas from the absorption tower is passed over the catalytic beds again and re passed through the absorption tower. This improves the percentage of sulfur dioxide converted from 98% to better than 99.6%

Waste Management Emissions from the plant have to be continuously monitored for sulfur dioxide as this can cause acid rain. The amount of sulfuric acid mist emitted from the process is minimised by controlling the operating temperature of the absorber, gas flow rates and concentrations.

Waste Management Improvements in conversion have also been made by adding small amounts of caesium to the vanadium oxide catalyst to increase its efficiency and allow it to operate at lower temperatures Caesium-doped catalysts are about 3x more expensive than the usual vanadium oxide catalyst.

Waste Management There is relatively little solid waste produced from sulfuric acid manufacturing. The catalyst is dumped in landfill after recovering the mildly toxic vanadium. The cooling water is recycled. All three processes are exothermic, meaning energy is produced. This energy is used to generate its electricity or as a source to produce other chemicals.