How is ethanol manufactured from sugarcane ?

Explanation

Ethanol Manufacture :
(i) Ethanol is manufactured in industries by the fermentation of molasses, which is a by-product obtained during the manufacture of sugar from sugarcane.
(ii) Molasses is a dark coloured syrupy liquid left after the crystallization of sugar from the concentrated sugarcane juice .
(iii) Molasses contain about 30% of sucrose , which cannot be separated by crystallization . It is converted into ethanol by the following steps :
a) Dilution of molasses : Molasses is first diluted with water to bring down the concentration of sugar to about 8 to 10% .
b) Addition of Nitrogen source :
(i) Molasses usually contains enough nitrogeneous matter to act as food for yeast during the fermentations process.
(ii) If the nitrogen content of the molasses is pooer , it may be fortified by the addition of ammonium sulphate or ammonium phosphate .
(c) Addition of Yeast : The solution obtained in step . Is collected in large ' fermentation tanks' and yeast is added to it .
The mixture is kept at about 303 K for a few days . During this period , the enzymes invertase and zymase present in yeast , bring about the conversion of sucrose into ethanol .
`underset("sugar")(C_(12)H_(22)O_(11))+H_2O overset("invertase")to underset("glucose")(C_6H_(12)O_6)+ underset("fructose")(C_6H_(12)O_6)`
` underset("glucose or fructose ")(C_6H_(12)O_6) overset("zymase")to underset("ethanol")(2C_2H_5OH)+2CO_2 `
The fermented liquid is technically called wash .
d) Distillation of 'Wash' :
(i) The fermented liquid (i.e. wash) , containing 15 to 18 percent alcohol , is now subjected to fractional distillation.
(ii) The main fraction drawn in an aqueous solution of ethanol which contain 95.5% of ethanol and 4.5% of water . This is called rectified spirit . (iv) On distillation of this mixture , pure alcohol (100%) is obtained . This is called absolute alcohol .