AZEOTROPIC DISTILLATION

• Azeotropic distillation has been used for decades by chemists for the purification of chemicals from azeotropic mixtures. 
• Azeotropic distillation involves adding a third chemical, called an entrainer, into the system during the distillation process. 
• This third chemical interacts with both of the other chemicals in the system making a ternary azeotrope ( three component azeotrope ) which is stronger than the original binary azeotrope. 
• When separation of ethanol from water is performed by azeotropic distillation, benzene or cyclohexane is commonly used as the entrainer chemical.
• When benzene is fed as the entrainer, three distinct regions are present each with different composition in the distillation column. 
• The bottom of the column contains the first fraction which is composed of almost pure water. 
• The middle of the column contains a near-ternary azeotrope mixture of water, benzene, and alcohol. 
• And the top of the column contains anhydrous ethanol. 
• The trace quantity of water in the ethanol is controlled by the energy input into the column, product removal rate, and specific column design. 
• Downfalls of the azeotropic system are the high energy input required, the complex system of columns required to regenerate the entrainer, and the propensity for ethanol contamination with the entrainer when plant upsets occur. 
• In addition to these issues there are significant logistical and safety concerns with the storage of highly flammable and often carcinogenic (e.g. benzene) chemicals on site. 
• Due to the problems associated with entrainer distillation, an alternate system was developed which uses molecular sieves for the separation of water from ethanol.