Hybrid operation of the bio-ethanol fermentation

• Bio-ethanol, as a clean and renewable fuel, is gaining increasing attention.
• Fermentation and subsequent distillation are energy-intensive operations.
• The process of fermentation and subsequent distillation is examined in detail.
• Pervaporation is experimentally and economically assessed.
• A hybrid fermentation/distillation seems.

Bio-ethanol is a clean and renewable fuel of increasing importance. It is biochemically produced by fermentation of different feedstock in an aqueous broth. Its purification is generally achieved by distillation (till the water/ethanol azeotropic mixture of ∼95 wt% ethanol is reached), followed by molecular sieve or membrane dehydration to obtain anhydrous ethanol. Drawbacks of the current production process are the high energy consumption of the distillation and dehydration, and the potential inhibition of the fermentation at high ethanol concentrations. To reduce the impact of both drawbacks, the use of pervaporation in the fermenter broth offers a significant potential. The present paper assesses this hybrid operation mode and its potential in large-scale applications. The experimental results demonstrate a high permeate flux and a good membrane selectivity. At a feed temperature of ∼70 °C, a membrane unit of ∼900 m2 can reduce the steam requirements, whilst also lowering the effective ethanol concentration in the fermenter to below the inhibition threshold. Overall operating costs are reduced by nearly 20 €/ton bio-ethanol.