The economics of ethanol production from dilute solutions may be improved by adding a concentrated sugar feed The production of 94% ethanol from a raw material containing 3% of fermentable sugars, carried out in a cascade of three continuous fermenters with 75% cell recycle and in a single distillation column was taken as a reference case. The parameter df variation was the ratio of the concentrated feed flow containing 50% of sugar to the dilute feed flow. The effects of the additional sugarfeed an the fermentation, centrifugation and distillation capacities of the plant were analysed and the retrofit of the fermentation unit was optimized using the fermenter volume and cell recycle as optimization parameters. Approximate economic evaluations of retrofit were made by comparing some essential costs in the retrofitted system to these costs in the above reference system and in a conventional fermentation plant, respectively.