With conventional distillation at atmospheric pressure, the maximum achievable ethanol concentration is 90-95%, because in the system ethanol-water there is an azeotrope at 95.6% (w/w) ethanol, boiling at a temperature of 78.2°C. For the production of anhydrous ethanol further dehydration of the concentrated ethanol is required. This can be achieved by employing azeotropic distillation, extractive distillation, liquid-liquid extraction, adsorption, membrane separation or molecular sieves (Hatti-Kaul, 2010; Huang et al., 2008).

Separation of ethanol from water is an energy-intensive process. The energy required for production of concentrated ethanol by distillation also depends very much on the feed concentration (Zacchi & Axelsson, 1989). The search for solutions for the reduction of the energy required is a field of intensive research. Membrane separation processes need much less energy for ethanol separation but are not in operation on an industrial scale. First results from a pilot plant using the SiftekTM membrane technology show a reduction of the energy required for dehydration of about 50% (Cote et al., 2010). Process and heat integration techniques also play an important role in energy saving in the bioethanol process (Alzate & Toro, 2006; Wingren et al., 2008). Maximum energy saving in the distillation of about 40% is possible by applying mechanical vapour recompression (Xiao-Ping et al., 2008). Solar distillation of ethanol is under investigation for distillation of bioethanol in smaller plants (Vorayos et al., 2006). The production of solid biofuel or biogas for thermal energy supply also reduces the net energy requirement of bioethanol production (Eriksson & Kjellstrom, 2010; Santek et al., 2010).