Ethanol as a Fuel (Oxford AQA IGCSE Chemistry)

The Production of Ethanol

• Ethanol is an alcohol with many important applications such as:

  • As a fuel (for vehicles in some countries)

  • As a solvent

  • In alcoholic drinks such as wine and beer

• It can be produced by:

  • The reaction of ethene with steam 

  • Fermentation 

Ethene & Steam

• The reaction of ethene and steam to produce ethanol is known as hydration

• It uses 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.

  • The ethanol and water are separated afterwards by fractional distillation

• This method of making ethanol is very efficient as it is a continuous process

• However, it uses ethene derived from crude oil which is a non-renewable resource 

The reaction between ethene and steam

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

Fermentation

• Ethanol can be produced by fermentation whereby sugar is dissolved in water and yeast is added

• The mixture is then fermented between 20 and 35°C with the absence of oxygen for a few days

• Yeast acts as the catalyst in this reaction

• The yeast respires anaerobically using the glucose to form ethanol and carbon dioxide:

C₆H₁₂O₆  → 2CO₂ + 2C₂H₅OH

• 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 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

• Unlike ethene, sugar is a renewable resource

Comparing the methods of making ethanol

Biofuels

• Biofuels are fuels produced from plant materials

• Examples include:

  • Biodiesel

  • Ethanol

• They can be used as alternatives to hydrocarbon fuels

Biofuels

Advantages of Biofuels

• Biofuel is a renewable resource

• Biofuel is considered to be carbon neutral

  • This means there is no overall net gain or loss of carbon dioxide in the atmosphere

  • This helps reduce our carbon footprint (the total amount of greenhouse gases released)

• No sulphur dioxide is produced

  • Therefore, biofuels do not contribute to acid rain

Disadvantages of Biofuels

• Growing the crops takes a lot of land, which could be used for food production

• Crops of biofuel producing plants must be grown which takes time

• Burning biofuels releases carbon dioxide into the atmosphere

  • Carbon dioxide is a greenhouse gas that contributes to climate change

Ethanol as Biofuel

• The ethanol used as a biofuel is produced from a dilute solution of ethanol

• This dilute solution of ethanol is produced by fermentation of plant materials

• Fermentation is carried out between 20 and 35 ^oC