ZIMSEC O Level Combined Science Notes: Ethanol as a source of energy

  • Ethanol is also known as alcohol
  • It is made by fermenting plants with a high sugar content
  • Sugar cane is commonly used to make ethanol in Zimbabwe
  • The plant is grown in Zimbabwe’s Lowveld
  • Water from Mutirikwe and Bangala Dams is used to irrigate the crop
  • Biomas is a crop that is grown as an energy source
  • The mass of sugar cane in a field is its biomass
  • For example if 2 tonnes of sugar cane are reaped per every 1 hactre then the biomass of sugar cane would be 2 tonnes per hactre
  • Ethanol produced from plants Triangle and other places in the Lowveld is used to blend with Petrol
  • Such fuels as E10 (90% Petrol and 10% ethanol) are then produced
  • This reduces Zimbabwe’s fuel import bill and allows limited petrol supplies to go further

How ethanol is made

  • The sugarcane is harvested and sent to the crushing mill
  • In the sugarcane crushing mill the cane is crushed and the juice extracted from it
  • Bagasse-is the fibre that remains
  • The Bagasse is used as fuel for boilers to generate steam
  • The juice is filtered and evaporated to produce sugar crystals
  • These crystals are separated from the molasses
  • The molasses is then taken to the ethanol plant
  • Where it is diluted with water and mixed with a yeast culture
  • The pH is lowered by adding dilute sulphuric acid
  • The yeast will survive at this pH but bacteria will not
  • Air is bubbled through the tank and the yeast multiplies
  • When the yeast population has reached the required level the culture is removed to the main fermentation vats
  • Air is not admitted at this stage
  • The fermentation is anaerobic
  • Under these conditions the yeast cannot multiply
  • The pH is adjusted and the mixture is allowed to brew for 40 hours
  • The enzyme in the yeast cause the breakdown of the sugars into smaller glucose molecules
  • The glucose is then converted into ethanol by the action of the zymase in the yeast
  • glucose\xrightarrow{zymase} \text{ethanol+carbon dioxide}
  • Once the alcohol content reaches 15% the yeast becomes inactive
  • The dilute alcoholic solution is then pumped into the middle of a distillation column
  • As the ethanol vaporizes it rises up into the column
  • 1% Benzol from coking plants at Hwange is added to the pure ethanol to make it unfit for human consumptionAny combustible material removed from the ethanol during the distillation process are added back to the final product
  • The carbon dioxide bubbles released during the fermentation process are piped to the carbon dioxide plant
  • There they are liquefied and solidified into dry ice
  • The dry ice which has very low temperatures is used as a coolant
  • Some of the carbon dioxide is also used to make carbonated drinks (fizzy drinks)
  • Methylated spirits is ethanol to which a poison has been added
  • It is used as a fuel
  • It burns with a pale blue flame and leaves no residue
  • It is less polluting than hydro-carbonate fuels such as Petrol

Uses of ethanol

  • Ethanol has several uses including:
  • It is blended with Petrol to form Blend Petrol for example E10 and E85
  • To make flavourants and essance in the food industry
  • In the manufacture of drugs and medicines
  • As an antiseptic in hospitals and clinics
  • To preserve biological specimens
  • To make varnishings and perfumes
  • As a disinfectant in hospitals and clinics
  • To make hand sanitisers
  • In thermometers
  • As a solvent

To access more topics go to the Combined Science Notes page.

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