Chapter 21
Chapter 21 The industrial production of sulfuric acid Factors affecting the production, including rate and equilibrium position, catalysts, temperature, pressure Waste management including generation, treatment and waste reduction Health and safety Uses of Sulfuric acid Name the raw materials used in the production of sulfuric acid Describe the reaction steps by which sulfuric acid is manufactured Explain how the principles of equilibrium and reaction rates play a significant role in determing reaction conditions Explain the reasons for choice of reaction conditions, such as pressure, temprrature and catalysts. Describe waste management procedures Describe health and safety issues involved in the production of sulfuric acid Describe how the production of sulfuric acid through the contact process realates to the principles of green chemistry Recall the major uses of sulfuric acid Absorption tower Contact process Converter Dehydrating agent Diprotic acid Double absorption oleum Sulfuric acid is produced in greater quantities than any other chemical in both Australia and the world. Annual worldwide production is estimated at about 170 million tonnes and Australian production at 4 million tonnes. In future years it is anticipated that Australia will become a major exporter of the chemical. Transport and storage of sulfuric acid are hazardous. A high proportion of the acid is used close to the site of manufacture. 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. It is also used in the manufacturing of paper, household detergents, pigments, dyes and drugs. It is the electrolyte in car batteries. Many Australian soils are phosphorous deficient and it must be added to the land. During superphosphate manufacture, insoluble calcium phosphate contained in rock phosphate is converted to a soluble form that plants can absorb. This reaction takes several weeks to occur: Ca3(PO4)2(s) + 2H2SO4(l) + 4H2O(l) → Ca(H2PO4)2(s) + 2CaSO4.H2O(l) superphosphate The final mixture is superphosphate. It is crushed into a powder and bagged for easy distribution. Although Queensland has deposits of phosphate rock, it is not really used as a reactant. Instead we use rock phosphate that comes from North Africa as it is cheap and readily available. Pure sulfuric acid is a viscous liquid that reacts with water in two steps. H2SO4(l) + H2O(l) → HSO4 - (aq) + H3O +(aq) Ka = 109 mol L-1 HSO4 - (aq) + H2O(l) <==> SO4 2- (aq) + H3O +(aq) Sulfuric acid is a diprotic acid. The first step proceeds virtually to completion. The second step has a much smaller Ka value. H2SO4(l) + H2O(l) → HSO4 - (aq) + H3O +(aq) Ka = 109 mol L-1 HSO4 - (aq) + H2O(l) <==> SO4 2- (aq) + H3O +(aq) Ka = 1.0 x 10-2 mol L-1 It is used as a strong acid in the ‘pickling’ of iron and steel. This is where the iron(III) oxide is removed from the surface of the iron. 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. Concentrated sulfuric acid is a powerful dehydrating agent. Sugar is dehydrated: C12H22O11(s) H2SO4(l) 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 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 following reactions can occur when zinc is added to sulfuric acid: Zn(s) + 2H2SO4(aq) → ZnSO4(aq) + 2H2O(l) + SO2(g) 3Zn(s) + 4H2SO4(aq) → 3ZnSO4(aq) + 4H2O(l) + S(s) 4Zn(s) + 5H2SO4(aq) → 4ZnSO4(aq) + 4H2O(l) + H2S(g) Like other strong acids, dilute sulfuric acid reacts with zinc to produce hydrogen gas: Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) What is the oxidant?? H+ (g) Sulfuric Acid is manufactured in stages from sulfur dioxide. These involve oxidation of sulfur dioxide to sulfur trioxide. Followed by conversion to the acid. The process can be summarised: SO2(from various sources) → SO3 → H2SO4 The sulfur dioxide used to produce sulfuric acid is obtained from two principal sources Combustion of sulfur recovered from natural gas and crude oil Sulfur dioxide formed during the smelting of sulfide ores of copper, zinc or lead. A third process can be used from mining of the underground deposits of elemental sulfur but this is not used in Australia due to the first two being in high abundance. 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 it 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. S(l) + O2(g) → SO2(g); ∆H = -297 kJ mol-1 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. 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. 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. 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? 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. 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) 2. Oleum obtained from the absorption tower is then carefully mixed with water to produce sulfuric acid: H2S2O7(l) + H2O(l) → H2SO4(l) 1. 1. 2. 3. 4. Oxidation of S to SO2 Catalytic oxidation of SO2 to SO3 The absorption of the SO3 by previously prepared sulfuric acid to produce oleum, H2S2O7 The dilution of the oleum with water to make sulfuric acid. 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% 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. 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. 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. Sulfuric acid is highly corrosive and can burn skin and eyes severely. It can cause blindness and third degree burns on contact. Exposure to sulfuric acid mist can cause other health problems. Workers in sulfuric acid plants can also be exposed to the acid through breathing air contaminated with emissions containing oxides of sulfur Strict safety procedures including adequate methods to trap the fumes are required to minimise the risks to workers and the environment in the case of accidental release Work areas must be well ventilated and employees wear protective clothing. Acid spills are contained using materials such as earth, clay or sand and then slowly diluted with water before being neutralised with a base such as limestone or sodium carbonate
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