When fermentation products are more volatile than water, recovery by distillation is often the used technology. Distillation means, that the bioethanol is separated from water in the liquid mixture. In virgin bioethanol, the content of water is normally higher than 80 %. In order to concentrate the ethanol to 95.6 %, larger quantities of energy are required (azeotrope mixture of ethanol with water). Maximum concentration of bioethanol, viable to microorganisms is about 10 wt% at 303 K, but this maximum decreases with increasing temperature.
Firstly, a recovering process of bioethanol is done (in a distillation or beer column), where most of the water with solid parts remain. The bioethanol (about 37 %) is concentrated in a rectifying column. This is done to a concentration just below the azeotrope (95 %). The residual bottom product is fed to the stripping column for the elimination of additional water. The bioethanol distillate is thereby recombined with the feed to the rectifier. In order to reduce bioethanol losses, the recovery of bioethanol in the distillation columns in the plant is ficed to be 99.6 %.
Secondly, a separation step is done with the solids (using a centrifuge) and afterwards, solids are dried in a rotary dryer. About 25 % of the centrifuge effluent is recycled to fermentation and the remaining effluent is evaporated a second and third time. The most part of the evaporator condensate is returned to the process as nearly clean condensate. The concentrated syrup contains about 15 – 20 % by weight total solids.