Manufacture of Ethanol (Edexcel IGCSE Chemistry)
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Industrial Synthesis of Ethanol
- 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: 78oC
- Ethene: -103oC
- Water: 100oC
- The ethanol and water are separated afterwards by fractional distillation
A water molecule adds across the C=C in the hydration of ethene to produce ethanol
Fermentation
- Ethanol (C2H5OH) is one of the most important alcohols
- It is used as fuel (for vehicles in some countries) and as a solvent
- It is the type of alcohol found in alcoholic drinks such as wine and beer
- It can 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 + enzymes → 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
Exam Tip
Fermentation is an anaerobic process. Oxygen is not required for ethanol to be produced by fermentation.
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