Industrial production of ethanol from sugar cane

Fermentation

  • Ethanol is produced from molasses, a product of sugar cane.
  • A concentration of typically 60% w/w molasses is fed to the fermenter at a pre-set flow rate so residual sugars are maintained at a very low concentration (<0.2% w/w).
  • Immediate conversion of sugars to ethanol occurs
  • Alcohol concentration is maintained constant at about 4-5% ww by ethanol removal.
  • This permits fermentation of very concentrated substrates and eliminates ethanol inhibition of the yeast activity.
  • Fermenter aeration for at least 3 days and nutrients such as phosphoric acid and urea ensure adequate yeast growth.
  • 1200 kJ heat generated per kg of ethanol – excess heat removed by continuous circulation of the broth through an external plate heat exchanger
    • C6H12O6(aq) => 2CO2(g) + 2C2H5OH(l) + 134 kJ
  • Fermenter vent gases contain residual air/CO2/ethanol/volatiles scrubbed with dilution water to recover entrained ethanol – overall alcohol yield can be increased by about 1%
  • Fermentation takes 28-72hr to produce ethanol concentration of 8-10%
Fig3OverviewofDryGrindEthanolProcesscopycopy.gif
F4SimplifiedFlowDiagram.gif