Year 12 Chemistry: Chapter 21:~ Production of Sulfuric Acid
Annual worldwide production is estimated at about 170 million tonnes and Australian production at 4 million tonnes. In future years it is anticipated that Australian will become a major exporter of the chemical.
As transport and storage are both hazardous, a high proportion of the acid is used close to the site of manufacture. Sulfuric acid plants are located near smelting and refining industries that produce waste sulfur dioxide, which is the raw material for the production of sulfuric acid.
21.1 Uses of Sulfuric acid
Most of its uses involve making other chemicals. 75% is used to make fertilisers. Sulfuric acid is also used in the manufacture of paper, household detergents, pigments, dyes and drugs. It is the electrolyte in car batteries.
Superphosphate
An adequate supply of phosphorus is vital for plant growth. Superphosphate is manufactured by adding sulfuric acid to finely powdered rock phosphate.
Ca3(PO4)2 (s) + 2H2SO4 (l) + 4H2O (l) ® Ca(H2PO4)2 (s) + 2CaSO4.2H2O (s)
Superphosphate
Although Queensland has deposits of phosphate rock, these have not yet been exploited to any major extent. Companies using rock phosphate that comes from North Africa, where it is cheap and readily available.
Other Uses
As a strong acid
Pure sulfuric acid is a viscous liquid that reacts with water in two steps:
H2SO4 (l) + H2O (l) ® HSO4- (aq) + H3O+ (aq) (large K)
HSO4- (aq) + H2O (l) SO42-(aq) + H3O+ (aq) (small K)
Sulfuric acid acts as a diprotic acid. A large amount of heat is evolved during this process, therefore when preparing a sulfuric acid solution you must always take care to add acid to water slowly with continuous stirring. If water is added to pure acid, steam or mist will form and can cause the acid to splatter violently.
‘Pickling’ of iron and steel. The surface layer of iron (III) oxide must be removed, treatment with sulfuric acid converts the oxide into water and soluble iron (III) sulfate.
As a dehydrating agent
Concentrated sulfuric acid is a powerful dehydrating agent.
H2SO4(l)
C12H22O11(s) ® 12C(s) + 11H2O(l)
Concentrated sulfuric acid dehydrates (evaporates water) hydrated copper sulfate (blue) to the anhydrous salt (white).
H2SO4(l)
CuSO4.5H2O(s) ® CuSO4(s) + 5H2O(l)
(Blue) (White)
The dehydrating ability of sulfuric acid is often utilised in laboratories to dry gas mixtures that are being prepared or analysed. It is suitable to dry gases, such as air, carbon dioxide and nitrogen. It is not suitable for bases as they will react with the acid.
As an oxidant
Concentrated sulfuric acid is a strong oxidant, especially when hot. 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)
In this reaction it is the H+(aq) ion that acts as the oxidant
21.2 The Contact Process
SO2 à SO3 à H2SO4
Raw materials for making Sulfuric acid
The contact process was developed in 1831 by Peregrine Phillips. The sulfur dioxide used to produced sulfuric acid is obtained from two principal sources:
J Combustion of sulfur recovered from natural gas and crude oil
J Sulfur dioxide formed during the smelting of sulfide ores of copper, zinc or lead.
A third source is from the mining of underground deposits of elemental sulfur by a method known as the frasch process, but this is rare used in Australia.
The sources of sulfur dioxide used in Australia are attractive from an environmental viewpoint as they use a by-product of other industries and limit the amount of sulfur dioxide emitted into the atmosphere. Several smelters and refineries in Australia act as sources of sulfur dioxide or sulfur for conversion to sulfuric acid.
Production of Sulfuric Acid: The Contact Process
1. Sulfur Burning:
Involves spraying molten sulfur under pressure into a furnace, where it burns in air to produce sulfur dioxide gas. The high surface area of the sulfur allows combustion to be rapid.
Temperatures up to 1000°C may be reached. The sulfur dioxide gas is then cooled for the next step in the process.
2. Catalytic Oxidation of Sulfur dioxide:
Sulfur dioxide gas is oxidised to sulfur trioxide gas by oxygen, using vanadium (V) oxide as a catalyst:
This step is performed in a reaction vessel called a converter. Sulfur dioxide is mixed with air and passed several times through trays containing loosely packed porous pellets of catalyst (catalyst beds). The reaction is exothermic, therefore it is necessary to cool the gas mixture as it passes from one tray of catalyst to another to maintain the desired reaction temperature. The temperature in the converter is maintained between 400°C and 500°C and the gas pressure is close to 1 atmosphere. Nearly complete conversion of sulfur dioxide to sulfur trioxide is achieved.
Using Le Chatelier’s principle, the equilibrium yield of sulfur trioxide will increase:
J as temperature ______. Since the reaction is ______, the equilibrium system will compensate for the loss of energy associated with the temperature decrease by favouring the direction that releases energy. The ______reaction.
J as pressure ______. The equilibrium system reacts to high pressure by favouring the direction that will results in a decrease in pressure. There are more gas particles on the left hand side than the right hand side, therefore a ______reaction will result in a ______reaction.
J if excess reactants (oxygen or sulfur) are added. This will result in a ______reaction.
The rate of reaction will be faster:
J as temperature ______
J as pressure ______
J if a ______is employed.
A conflict exists because a high equilibrium yield is favoured by low temperatures, whereas a fast rate of production of sulfur trioxide is favoured by high temperatures. By using a catalyst it is possible to use lower temperatures in the converter and still achieve an acceptable reaction rate.
The most effective catalyst is not necessarily used, since cost and the chance of the catalyst being ‘poisoned’ must also be considered. Vanadium (V) oxide (V2O5) is chosen, and dust particles that could poison the catalyst are removed by passing air and sulfur dioxide through electrostatic precipitators before they enter the converter.
The equilibrium yield of sulfur trioxide is also improved by using an excess of the cheaper reactant, oxygen, in the form of air.
3. Absorption of Sulfur trioxide:
The sulfur trioxide reacts with water to form sulfuric acid:
Direct reaction with water is not used, because so much heat evolves that a fine mist of acid forms which is difficult to collect.
Sulfur trioxide gas is passed into concentrated sulfuric acid (not water – mist) in an absorption tower. The reaction happens in two steps:
1. The sulfur trioxide gas dissolves almost totally in the acid to form a liquid known as oleum (H2S2O7).
2. Oleum obtained from the absorption tower is then carefully mixed with water to produce sulfuric acid.
In practice, many plants trickle in the additional of water, together with the concentrated sulfuric acid, to meet the incoming sulfur trioxide. This results in two moles of sulfuric acid being produced for every one introduced into the absorption tower.
Questions: 2, 5, 6, 7, 8, 9.
21.3 Waste Management
From an environmental perspective, an attractive feature of sulfuric acid plants is that they use sulfur or sulfur dioxide that is a by-product from other industries such as metal smelters and petroleum refining.
Manufacturers of sulfuric acid must maximise the conversion of sulfur dioxide to sulfur dioxide to sulfur trioxide because strict limits are set for sulfur dioxide emissions into the atmosphere to minimise the formation of acid rain. Most plants built after 1970 now use a double absorption process. After the gas mixture is passed through the absorption tower the unreacted gases are then recycled to the converter for one or two more passes over the catalyst before being returned for absorption. In this way the percentage of sulfur dioxide converted can be increased from 98% or better than 99.6%.
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 possible by adding small amounts of caesium to the vanadium (V) oxide catalyst to increase its efficiency and allow it to operate at lower temperatures. Unfortunately it is three times more expensive.
Sulfuric acid has a high boiling temperature of 290°C, it has a low vapour pressure therefore no appreciable air pollution problem with storage, handling and transport.
J Relatively no solid waste produced.
J After recovery of the mildly toxic vanadium from spent catalyst, the catalyst is disposed of in landfill sites.
J Cooling water is usually recycled.
All three major steps: burning sulfur, catalytic conversion to sulfur trioxide and absorption, are exothermic. As a consequence, manufacturers have a ready supply of energy for other operations in the plant. Some sulfuric acid plants use the energy to generate electricity. Since no carbon dioxide is produced. The energy can be described as ‘green’ compared to other forms of energy production.
Green Chemistry and Sulfuric acid Production
The contact process is being modified to enable it to make use of the very low concentrations of sulfur dioxide emissions that are emitted from some industries, and already some plants use dilute waste sulfuric acid and hydrogen sulfide gas as their feedstock.
Questions: 3, 4, 13.
21.4 Health and safety
One of the most significant risks associated with the industry is in the transportation of sulfuric acid.
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 may also cause other health problems, including a build-up of fluid in the lungs (pulmonary oedema).
Both sulfur dioxide and sulfur trioxide are respiratory irritants, damage plants, and contribute to a major extent to acid rain. Oleum is a highly corrosive oily liquid that produces sulfur trioxide fumes.
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 (CaCO3) or sodium carbonate.
Questions: 1, 18.
Amount of acid manufactured mainly depends upon the demand for fertilisers and the size of the market held by local fertiliser producers.