ZIMSEC O Level Combined Science Notes: The Haber process and the Manufacture of Ammonia

  • Sable Chemicals produces ammonia at their Kwekwe plant
  • Ammonia is mainly used to make nitrogen fertilizers such as ammonium nitrate which are used in the agriculture industry
  • Ammonia is made using Hydrogen and Nitrogen
  • Hydrogen is obtained from electrolysis of water and
  • Nitrogen is obtained from the liquification of air
  • The Sable Chemicals plant is located close to these raw materials
  • It is close to the company’s own electrolysis and liquification plant
  • ZISCO which is close to the company also gets its oxygen from Sable Chemicals

The Haber Process

The haber process

The haber process

  • Ammonia is produced using the Haber Process
  • At the plant hydrogen and nitrogen are mixed in the ration 3:1 by volume
  • The gas pressure is raised to 200 atm (atmospheres)
  • The temperature is also raised to 500°C
  • The mixture is then passed over an iron catalyst
  • About 17% of it is then converted to ammonia
  • The ammonia produced here is then separated by condensation in a cooler
  • The unconverted gases are mixed with more nitrogen and oxygen and passed over the catalyst again
  • This process can be shown using the following word equation:
  • \text{nitrogen+hydrogen}\xrightleftharpoons[\text{iron catalyst}]{500 \degree C, 200atm}\text{ammonia+heat}
  • The reaction between nitrogen and hydrogen is reversible and exothermic
  • As much hydrogen and nitrogen must be converted to ammonia to make the process economic
  • This is done by increasing the pressure of the mixture and moderating the temperature
  • As ammonia is produced the pressure of the gases drops
  • If pressure is increased more ammonia is produced
  • A good yield is obtained at lower temperatures since this reaction is exothermic
  • The temperature of the reaction is kept as low as possible (around 450°C)
  • Reducing the temperature below this results in the reaction slowing down and becoming uneconomic
  • To speed the reaction up at these low temperatures an iron catalyst is used
  • The catalyst merely speeds up the rate of the reaction but not the amount of ammonia produced
  • The catalyst itself is not used up in the reaction and is thus used over and over again
  • The exothermic reaction produces heat in the converter which might reach temperatures of about 560°C
  • This heat is used to heat up steam which is used in other parts of the factory

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