Industrial alcohol is called as ordinary rectified spirit. Ethylene is a product obtained from petroleum industry used to produce industrial ethyl alcohol also known as ethanol by esterification and hydrolysis technology. Ethylene with a chemical formula C2H4 it is derived mainly by cracking propane and ethane.
Industrial alcohol production technology:
- Direct hydration
- Indirect hydration
Process description of industrial alcohol production by indirect hydration:
Ethanol manufacturing involves two unit processes absorption and hydrolysis. Absorption is carried out by absorbing ethylene with sulphuric acid. The chemical changes take place are:
CH2=CH2 + H2SO4 → CH3CH2OSO3H (ethylhydrogen sulphate)
2CH2=CH2 + H2SO4 → (CH3CH2)2SO4 (diethyl sulphate)
An absorption tower operating at 150 to 200 psi pressure conducts this operation. Ethylene of concentration 35 – 95 % is allowed to flow from the bottom of the absorption tower. Sulphuric acid perfectly sprayed at the top section of the column by acid proof spray nozzles. Its concentration 90-95% should be maintained throughout the operation. Excess flow of sulphuric acid will absorb ethylene effectively at the temperature ranging from 50 to 80 oC. The chemical reaction occurs in the equipment is exothermic in nature. Heat is evolved in the column and so increase in temperature will decrease the absorption rate. Feed rates are controlled to maintain the temperature in allowed range. Packed absorption tower filled with catalysts of iron oxide or copper oxide will improve the performance of absorption process. The bottom product of ethylene absorption column is a mixture of diethyl sulphate and ethyl hydrogen sulphate. Off gases are used as fuel and based on ethylene concentration off-gases from top of the column are recycled in its feed stream.
The bottom stream now enters into the second unit process of hydrolysis. Here water is added to the reacting stream and allowed to react to form ethanol and release sulphuric acid as per the reactions.
CH3CH2OSO3H + (CH3CH2)2SO4 + 3 H2O → 3 CH3CH2OH + 2 H2SO4
CH3CH2OSO3H + H2O → CH3CH2OH + H2SO4
(CH3CH2)2SO4 + 2H2O → 2CH3CH2OH + H2SO4
The mixture is allowed for 4 hours space-time to complete the reaction at 70 oC. So formed esters can be converted to ethanol with less diethyl ether formation. In any case, 5-10% of diethyl ether is formed by the side reaction.
2C2H5OH → C2H5OC2H5 + H2O
After hydrolysis, the mixture with ethanol, sulphuric acid, unreacted reactants and diethyl ether are fed to stripping operation section. A stripping column separates sulphuric acid as bottom stream and rest of the mixture as a top stream. Sulphuric acid is concentrated and recycled to absorption tower as absorbent. The vapour mixture is treated with sodium hydroxide to neutralize the traces of sulphuric acid. Diethyl Ether is separated from ethanol in ether separation column. Finally, a fractionating column produces industrial alcohol with 95% ethanol and 5% water that is also called as rectified spirit.
Process flow diagram of indirect hydration of ethylene for industrial alcohol production: