13. Ethanol

Ethanol is generally referred to as alcohol. All alcoholic liquids and some cough syrups contain ethanol. The molecular formula for ethanol is \(C_2H_5OH\).

 

Ethanol is prepared in industries via the fermentation of molasses obtained through sugar from sugarcane. Molasses is a dark coloured syrupy liquid left after the crystallisation of sugar from the concentrated sugarcane juice. Molasses contain approximately \(30%\) of sucrose, which cannot be separated by using crystallisation.

 

 

Molasses is first diluted with water to decrease the concentration of sugar to approximately \(8-10\%\).

 

  

During the fermentation process, the nitrogenous matter that is present in molasses acts as food for yeast.  Ammonium sulphate or ammonium phosphate is added if the nitrogen content of the molasses is poor.

 

  

The solution extracted from a nitrogen source is collected in large ‘fermentation tanks’, and yeast is mixed. The aggregate is preserved at \(303K\) for some days. During this time, the enzymes invertase and zymase found in yeast result in the conversion of sucrose into ethanol.

 

(i) \(\text{Sugar + Water \(\xrightarrow {invertase}\) Glucose + Fructose}\)

\(C_{12}H_{22}O_{11} + H_2O \xrightarrow{invertase} C_6H_{12}O_6 + C_6H_{12}O_6 \)

(ii) \(\text{Glucose or Fructose \(\xrightarrow {zymase}\) Ethanol + Carbon dioxide}\)

 

\(C_6H_{12}O_6 \xrightarrow{zymase} 2C_2H_5OH + 2CO_2\)

 

The fermented liquid from this process is called 'Wash'.

 

The fermented liquid (i.e. wash), containing \(15%\) to \(18%\) alcohol, is now subjected to fractional distillation. The primary fraction drawn is an aqueous solution of ethanol. \(95.5%\) of ethanol and \(4.5%\) of water proportion is known as rectified spirit.
 
Rectified spirit is then refluxed over quicklime for about \(5\) to \(6\) hours and then allowed settling for \(12\) hours. Pure alcohol (\(100%\)) is obtained on the distillation of the above mixture. This alcohol is known as absolute alcohol.