Manufacture of Ethanol (Edexcel IGCSE Chemistry)

Hydration of ethene

• Ethanol can be synthesised by the hydration of ethene
• Ethene is a by-product of the cracking of hydrocarbons and is a valuable feedstock for making many chemicals
• The hydration reaction is very important industrially for the production of alcohols and it occurs using the following conditions:
  • Temperature of around 300ºC
  • Pressure of 60 – 70 atm
  • Concentrated phosphoric acid catalyst

• When the reaction is complete, the reaction chamber holds unreacted ethene, ethanol and water
• The contents are transferred to a condenser where ethene is separated easily as it has a much lower boiling point than ethanol and water:
  • Ethanol: 78^oC
  • Ethene: -103^oC
  • Water: 100^oC

• The ethanol and water are separated afterwards by fractional distillation

Hydration of ethene

A water molecule adds across the C=C in the hydration of ethene to produce ethanol

• Ethanol can also be produced by fermentation where sugar or starch is dissolved in water and yeast is added
• The mixture is then fermented between 25 and 35°C (the optimum temperature is 30 °C) with the absence of oxygen for a few days
• Yeast contains enzymes that break down sugar to alcohol
• If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured
• The yeast respires anaerobically using the glucose to form ethanol and carbon dioxide:

C₆H₁₂O₆ → 2CO₂ + 2C₂H₅OH

• The yeast is killed off once the concentration of alcohol reaches around 15%, hence the reaction vessel is emptied and the process is started again
• This is the reason that ethanol production by fermentation is a batch process
• At the end, there is a mixture of ethanol and water which is separated by fractional distillation