Q.(i) Describe the manufacture of ethanol from molasses

Ethanol is manufactured industrially from molasses by yeast fermentation followed by distillation; 'denatured' alcohol is ethanol deliberately made undrinkable for industrial use; and ethanol heated with concentrated H2SO4 at 443 K undergoes dehydration to ethene.

(i) Manufacture of ethanol from molasses: Molasses, the dark, sugar-rich viscous liquid left over after crystallisation of sugar from sugarcane juice, is diluted with water and treated (e.g., with sulphuric acid) to remove impurities. The diluted molasses solution is then fermented by adding yeast, which supplies two enzymes:

• Invertase, which hydrolyses the sucrose present in molasses into glucose and fructose: C12H22O11 + H2O --invertase--> C6H12O6 (glucose) + C6H12O6 (fructose)
• Zymase, which then converts these simple sugars into ethanol and carbon dioxide: C6H12O6 --zymase--> 2C2H5OH + 2CO2

The resulting fermented liquid ('wash'), containing about 8-10% ethanol, is then subjected to fractional distillation to obtain about 95% ethanol (called rectified spirit). Pure, water-free (100%) 'absolute alcohol' can be obtained by removing the last traces of water via azeotropic distillation (e.g. with benzene).

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