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Programme 11: Industrial Chemistry Background Information
Ammonia Plants and Fertilisers To ensure healthy growth plants need a number of key elements, such as nitrogen, phosphorus and potassium. Nitrogen is particularly important for the construction of protein. In nature, these elements are continually recycled by systems such as the nitrogen cycle. When farmers repeatedly grow crops on the same land and remove the plants for food the soil eventually becomes short of nutrients such as nitrogen and these must be replaced by using fertilisers. Traditionally, natural sources of nitrogen such as animal manure were used, but by the late nineteenth century such sources proved inadequate for the world’s growing population and it became necessary to make artificial fertilisers. These fertilisers need to be rich in nutrients and soluble in water so they can be absorbed through plant roots. The most common nitrogen-based fertiliser used in Britain is ammonium nitrate. The starting materials for ammonium nitrate are ammonia and nitric acid. As nitric acid is also manufactured from ammonia, ammonia is the key compound in the making of fertilisers. Making Ammonia Approximately 2.5 million tonnes of ammonia are manufactured annually in the UK using the Haber process. Nitrogen is supplied by the fractional distillation of air. The steam reforming of methane produces the hydrogen. In this process methane is passed over a nickel-chromium catalyst with excess steam at a pressure of 30 atmospheres and a temperature of 900°C. 
The reaction between hydrogen and nitrogen is slow and the % yield is quite low. To make the industrial process economical careful choice of reaction conditions must be made. 
The reaction is reversible and sets up equilibrium. The forward reaction is exothermic. If the temperature is raised the equilibrium will shift to the left and the yield of ammonia will be reduced. This means that low temperatures are useful. However, low temperatures will slow down the reaction and so a compromise temperature of about 450°C is used. There are four molecules on the left of the equation and two on the right. This means that as ammonia is made the volume of gas is reduced and the forward reaction will be favoured by an increase in pressure. High-pressure chemistry is expensive and a compromise pressure of around 250 atmospheres is adopted. The mixture of one part nitrogen to three parts hydrogen is passed over a finely divided iron catalyst to speed up the reaction and about 10 percent of the gas is converted to ammonia. The ammonia is cooled and turned to a liquid. The remaining gas is then recycled. Nitric Acid About 5 percent of all ammonia is converted into nitric acid. The liquid ammonia is allowed to vaporise and is then mixed with air at a pressure of about 10 atmospheres. The mixture is then passed over a platinum rhodium catalyst at about 1000°C. At this point about 98 percent of the ammonia is converted to nitrogen monoxide. 
The mixture of gases is cooled and further air is added converting the nitrogen monoxide to nitrogen dioxide. Finally, this mixture of gases is passed through one or more absorption towers where they meet a stream of water and more oxygen. At this point the mixture contains about 60 percent nitric acid. 
Ammonium Nitrate The nitric acid and ammonia are mixed and the neutralisation results in a solution of ammonium nitrate. The water is evaporated to leave solid fertiliser. The solid is then melted and sprayed into the top of a tall tower called a prilling tower. Here the solid meets an upward stream of air and forms into small droplets that are easy for the farmer to spread. 
Polymers Hydrocarbons Crude oil is a complex mixture of hydrocarbon molecules. Hydrocarbons are compounds of hydrogen and carbon only. The alkanes are a family of hydrocarbons with the general formula CnH2n+2. When alkanes are heated in the presence of a catalyst they break into smaller hydrocarbons and form members of another hydrocarbon family called the alkenes. Alkenes have the general formula CnH2n. Alkanes are reactive because they contain carbon-carbon double bonds. Because of the presence of a double bond, alkenes such as ethene can undergo polymerisation. This is the linking together of small molecules called monomers into large molecules called polymers. 
Ethene is heated at a pressure of 100 atmospheres and a temperature of 180°C with a small amount of oxygen to start the reaction. The final polymer, polyethene may contain many hundreds of combined ethene molecules. This method of polymerisation is called addition polymerisation. Polymers are widely used because of their useful combination of properties. They have low density, do not corrode and can be easily coloured and shaped. Disposal can be a problem as they do not decompose and they produce toxic gases such as hydrogen cyanide when burnt. Increasing amounts are recycled and some are burnt in special incinerators to produce heat energy. Sulphuric Acid and Manufacture Approximately two million tonnes of sulphuric acid are manufactured annually in the UK. It is used in the manufacture of a wide variety of products including polymers, fertilisers and steel. The main ingredient is sulphur, which may be from elemental sulphur or sulphide minerals such as zinc sulphide. The sulphur is burned in air to form sulphur dioxide. 
Next the sulphur dioxide is oxidised to sulphur trioxide by more oxygen. Similar considerations are applied here as those applied to the Haber process and a temperature of 450°C and atmospheric pressure are used. Higher pressure would increase the yield but using only a pressure of one atmosphere, 98 percent conversion is achieved. The mixture of sulphur dioxide and oxygen is passed over a vanadium oxide catalyst. 
In the final step, the sulphur trioxide should be mixed with water but the reaction is so exothermic it produces a mist which is dangerous. To get round this problem the sulphur trioxide is dissolved in concentrated sulphuric acid to produce a super concentrated acid called oleum. The oleum is then diluted to produce the required concentration of sulphuric acid. 
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