Industrial method of preparation of alcohol

Industrial method of preparation of alcohol

There are three methods to get alcohols, industrially. all these methods utilise raw materials which can be obtained from petroleum, natural gas, coal and biomass. The methods are outlined below.

1. By hydration of alkenes.
2. By oxo process.
3. By fermentation of carbohydrates.

1. By hydration of alkenes.

Alkenes can be obtained by the cracking of petroleum can be easily converted into alcohols by hydration in the following two ways.

a. By the direct addition of water.

Alkenes can be converted into alcohols by direct addition of water in the presence of the phosphoric acid which acts as the catalyst.

b. By the indirect method of hydration.

Alkenes are directly hydrated at high temperature and pressure to form alcohols. This reaction is carried out in presence of acid like H3PO4.

$$\underbrace{C=C}_{Alkene}+H_2O\xrightarrow{Pressure\,H_3PO_4}\underbrace{CH_2OH}_{Alcohol}$$

example.

$$\underbrace{CH_2=CH_2}_{ethene}+H_2O\xrightarrow{Pressure\,H_3PO_4}\underbrace{CH_3-CH_2-OH}_{Ethanol}$$

$$\underbrace{CH_3-CH=CH_2}_{Propene}+H_2O\xrightarrow{Pressure\,H_3PO_4}\underbrace{CH_3-CH-OH-CH_3}_{2-propanol}$$

Hydration of unsymmetrical alkenes followMarkovinkov's rule. Alkenes can also be hydrated indirectly i presence of Conc. H₂SO₄.

$$\underbrace{CH_2=CH_2}_{ethene}+H_2SO_4→\underbrace{CH_3-CH_2-HSO_4}_{Ethyl\,hydrogen\,sulphate}\xrightarrow{H_2O}+\underbrace{CH_3-CH_2-OH}_{ethanol}+H_2SO_4$$

2. Oxo process.

Alkenes react with water gas at high temperature and pressure in a present of Cobalt carbonyl [Co(Co)₄]₂ as the catalyst to form aldehydes. These aldehydes yields on reduction furnish alcohols. This reaction is called oxo process.

example.

$$\underbrace{CH_2=CH_2}_{Enene}+CO+H_2\xrightarrow{[CO(CO)_4]_2}\underbrace{CH_3-CH_2-CHO}_{Propanal}\xrightarrow{H_2\,Ni}\underbrace{CH_3-CH_2CH_2OH}_{1-propanol}$$

3. Fermentation.

Fermentation is defined as the slow process of decomposition of large and complex organic molecules into simpler ones. by the action of different enzymes. It is a complex biochemical process which is regarded as one of the oldest methods of manufacture of alcohol.

By this method ethyl, alcohol can be manufactured using two types of raw material molasses and starch.

a. From Molasses.

Molasses is a dark coloured mother liquor obtained after crystallisation of sugar. It contains carbohydrates like sucrose, glucose, fructose etc. in significant quantity. Molasses is first hydrolyzed with water to form the dilute solution which is then mixed with yeast )a unicellular fungus containing enzymes like zymase, invertase, diastase etc). The different steps involved in this process are.

$$\underbrace{C_{12}H_{22}O_{11}}_{Sucrose}+H_2O\xrightarrow{invertase}\underbrace{C_6H_{12}O_6}_{Glucose}+\underbrace{C_6H_{12}O_6}_{Fructose}$$

$$\underbrace{C_6H_{12}O_6}_{Glucose}\xrightarrow{Zymase}\underbrace{2C_2H_5OH}_{Ethanol}+2CO_2$$

The alcohol obtained from molasses is called "wash" which contains the low percentage of ethyl alcohol (15-30%) . When impure ethyl alcohol is subjected to fractional distillation, pure alcohol is obtained containing the purity of about 90-97%. This alcohol is called 'rectified spirit'.

b. from starch.

Ethyl alcohol also is manufactured from starch containing food grains like millet, rice, wheat, maize, buckwheat etc. In this methods, food grains are cooked at first to release starch granules. The solution is diluted with water and the resulting solution is called 'mash'.

'Mash' is mixed with malt which is geominated barely containing a inicellular plant, yeast. Yeast contains different enzymes required for fermentation.

$$\underbrace{2(C_6H_{10}O_5)_n}_{Starch}+nH_2O\xrightarrow{diastase}\underbrace{nC_{12}H_{22}O_{11}}_{Maltose}$$

$$\underbrace{C_{12}H_{22}O_{11}}_{Maltose}+H_2O\xrightarrow{Maltose}\underbrace{2C_6H_{12}O_6}_{Glucose}$$

$$\underbrace{C_6H_{12}O_6}_{Glucose}\xrightarrow{Zymase}\underbrace{2C_2H_5OH}_{Ethanol}+2CO_2$$

The fermented liquor obtained by this method is called 'wash' which is subjected to fractional distillation to obtained pure alcohol having the purity of about 90-97% and is called 'rectified spirit'.

Reference.

Bahl, B S, Bahl, and Arun. Advanced Organic chemistry. S. Chand and company Ltd., n.d.

Sthapit, M K, R R Pradhananga, and K B Bajracharya. Foundations of chemistry. Taleju Prakashan, n.d.

Tewari, K S, S N Mehrotra, and N K Vishnoi. A textbook of organic chemistry. Vikash publishing House Pvt. ltd., n.d.

Verma, N K and S K Khanna. Compressive chemistry. 8th edition. Laxmi publications P. Ltd., 1999.