Absolute alcohol is named to 100% concentrated ethanol with chemical formula C2H5OH. Well a rectified spirit is a mixture with composition of ethanol 95% and water with 5%. The process of fermentation mostly makes rectified spirit in an alcohol industry. However, to attain pure form of ethanol requires one more separation step so that the water content in rectified spirit is removed completely. Based on the chemical nature of ethanol and water, their concentration in the liquid mixture do not alter when heated to its boiling point. Whatever the composition present in liquid mixture is equal to the concentrations of the component in the vapour obtained from the ethanol and water liquid mixture when heat to its boiling condition.
This situation of solution is called as Azeotropic mixture and the boiling point of ethanol and water mixture is 78.13 oC, which near to the boiling point of pure ethanol 78.3 oC. Distillation operation is not good enough to separate water from rectified spirit. Two selective processes are operational to get the job done. One was using quick lime (CaO) to absorb water content and finally contacting the solution with anhydrous copper sulphate removes the left over traces of water. The second method uses a third component like benzene, toluene, carbon tetrachloride and diethyl ether in azeotropic distillation operation. These dehydrating agents alter the boiling point and partial pressure of ethanol and water so that when heat supplied to the solution, one of the components boiling point range is changed and so we can separate them using distillation technique. In recent, another method called anhydrous salt method is preferred from the above two.
Absolute alcohol product, which obtained from azeotropic distillation method, is somewhat complicated when compared to others. Its process description involves distillation column, separators and a decanter as unit operations. Even dehydrating agents selection make the operation differ in it parameters. For instants when a dehydrating agent having low boiling point condition when compared to rectified spirit. Than that particular dehydrating agent which is mainly C6CH6, C6H5CH3, C2H5OC2H5 and CCL4 make the partial pressure of alcohol to decrease than water at same condition. When a distillation operation is performed than the lowered partial pressure, alcohol will rise up to the top of the column and water is drawn out at the bottom of the tower along with the dehydrating agent. If a dehydrating agents like ethylene glycol or glycerol which have high boiling point are selected than it reverse the situation by lowering the partial pressure of water to lesser value and the operation is same for separation as before. Most ethanol processing industries select this method.
Azeotrope distillation using benzene:
By using benzene as a third component to break the azeotrope, a process for absolute alcohol can be described as follows. For conversion of rectified spirit, required amount of benzene is added continuously from the makeup and recycle to the feed line. The mixture is fed to ethanol distillation column. A ternary mixture is formed inside the column. At 64.8 oC benzene 74.2%, water 7.4% and alcohol with 18.4% concentration will exist in equilibrium. The vapors from the column are sent to condenser. Part of the condensed liquid used as reflux and the other part fed to decanter. In decanter, two layers are separated. One with benzene – alcohol as top layer and the other with water- alcohol as bottom. The top layer is recycled to ethanol distillation column. The bottom is sent to aqueous alcohol distillation column to separate the trace of benzene and recycle it. The bottom of aqueous alcohol distillation column is at 78.3 oC and is fed to another distillation column that removes excess water and sends the rectified sprite as recycle into main feed stream of ethanol distillation column.
Anhydrous Salt Method to Produce Absolute Ethyl Alcohol from Rectified Spirit:
Glycerine well known as dehydrator, it is used to remove the water content in rectified spirit turning it into an absolute alcohol. In small or medium scale industries, anhydrous salt method is selected as a choice of operation. By using simple scrubbing technique, the rectified spirit vapours are contacted counter currently so that glycerine vapours absorb the water present in the vapours of rectified spirit. Absolute ethanol vapours collected at the top of the scrubbing column. They are condensed to liquid. The bottom stream of the scrubber contains a mixture of glycerine, water and traces of alcohol. This bottom mixture sent to a distillation column to obtain dehydrated glycerine and recycle it to the scrubber.