**6. Distillation and purification**

A distillation process is necessary for separation of ethanol from mixture and purification of ethanol after fermentation process. Process is performed simply with boiling ethanol-water mixture. Because of boiling point of water (100°C) is higher than boiling point of ethanol (78 °C), ethanol vaporized before water [110]. However, due to being an azeotrop mixture, high amount of energy is used for distillation [42]. In order to separate azeotrop mixtures, an agent which changes the azeotrop structure must be added to the mixture. Added substance changes the volatility of mixture by effecting the molecular attractions in the mixture. Various separa‐ tion agents such as benzene, pentane, cyclohexane, hexane, acetone, and diethyl ether can be used in this process [111]. Distillation column which has two streams as top and bottom, separates most of the bioethanol from the mixture. While top stream is rich in bioethanol, bottom stream is rich in water. 37% bioethanol then concentrated in rectifying column to approach concentration of 95% [78]. Product which is remained in the bottom is fed to stripping column in order to remove excess water [112]. Mostly in plants, recovery of bioethanol in distillation columns is fixed to be 99.6% due to decrease bioethanol loss [54].
