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

Fermentation

• 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