Industrial preparation of ethanol

Ethanol can be manufactured by the fermentation of:
  • Molasses
  • Starch.

Slow decomposition of organic compounds is called fermentation. This is the principle behind souring of milk, batter, putrefaction of meat, and preparation of wine and vinegar. Fermentation was the earliest method used for preparing alcohol in industries. This is still used for the manufacture of alcohol and alcoholic drinks like beer, wine, brandy, etc,.

Raw materials

Cheap starchy materials like potatoes, maize, barley, rice etc.

OR Molasses, a byproduct of sugar industry.

From Molasses

The syrup left after the separation of cane sugar or beet sugar crystals from the concentrated sugar cane juice is called molasses. It is a dark coloured syrupy mass and contains about 30% of uncrystallizable sucrose and about 32% of invert sugar (a mixture of glucose and fructose). The different steps in the manufacture of ethanol by fermentation of molasses are:

Dilution

The molasses is diluted with water until a concentration of 8-10% sugar is obtained in solution. To discourage bacterial growth, this is acidified with a little sulphuric acid. If sufficient yeast (food for the ferment) is not present, a nutritive solution of ammonium salts is added.

Alcoholic Fermentation

Molasses fermentation

Highly Magnified Yeast Cells

The dilute solution obtained as above is taken in big fermentation tanks and some yeast is added (5% by volume). The temperature is maintained at 330K and the mixture is allowed to stand for a few days. Fermentation sets in and the enzyme (organic catalyst) invertase present in yeast, converts sucrose into glucose and fructose. Zymase, another enzyme present in yeast converts glucose and fructose into ethanol and carbon dioxide.

The carbon dioxide formed is allowed to escape but air is not allowed to enter. In presence of air ethanol formed would be oxidised to acetic acid.

The fermentation is complete in 3 days. The carbon dioxide obtained as byproduct is recovered and can be sold.

Distillation

The fermented liquor contains 9-10% of ethanol and is called wash or wort. It is distilled in a Coffey still (Distillation of wash in a Coffey still) to remove water and other impurities present in wash. The Coffey still consists of two tall fractionating columns with perforated plates. These columns are called the analyser and the rectifier. This works on the counter-current principle as the steam and alcohol travel in opposite directions through the still.

Steam passes up the analyzer and takes away the alcohol vapors from the dilute alcohol that is coming down. The mixture leaves the analyzer at the top. It then enters the rectifier at the base. The mixture heats the wort flowing through the pipes on its way to the analyzer. The steam condenses and the alcohol vapors escaping near the top are condensed in the condenser. The distillate contains about90% alcohol and the residue left in the still is used as cattle feed.

Rectification

The alcohol obtained contains other impurities besides water. These impurities are further removed by fractional distillation. Low boiling impurities like acetaldehyde distil over as first fraction. The middle fraction contains about 93-95% alcohol and is called rectified spirit. Often, distillation and rectification is carried out in the same operation.

From Starch

Starchy raw materials

Wheat, barley, rice, maize and potatoes.

Conversion of starch into maltose

Conversion of starch into maltose or saccharification is carried out as follows:

Malting

Moist barley is allowed to germinate in dark at 290K. Germinated barley is called Malt and this is heated to 330K (to stop further germination). It is then crushed and extracted with water. This Malt extractcontains the enzyme diastase.

Mashing

To break the cell walls, starch is reacted with superheated steam. This exposes the starch inside that forms a paste like mass called Mash.

Hydrolysis

Mash and malt extract are treated together at 320-330K. In about half an hour, hydrolysis is complete and maltose is formed.

Alcoholic Fermentation

Maltose obtained from starch is fermented in the presence of yeast. Maltase present in yeast converts maltose into glucose. Another enzyme zymase present in yeast, then converts glucose into ethanol and carbon dioxide.

Subsequent distillation and rectification yields rectified spirit.

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