Fermentation and distillation

 

All ENA production involves two steps: Fermentation – the enzymatic transformation of organic compounds (like sugar) by micro-organisms (yeast). Followed by Distillation – the separation process of components with different boiling points and physical properties.

 

Fermentation is very sensitive to temperature and acidity, and requires close control. Industrial fermentation can be a batch process (the entire process is contained and completed in individual tanks) or continuous (the product overflows from one tank to the next). The cycle lasts 20 to 30 hours at a temperature of 30° to 32°C and the resulting product, known as ‘low wine’ or ‘low beer’, will be 12 % alcohol by volume at most (yeast becomes inactive in higher alcohol concentrations). An alcohol yield of 50 kg per 100 kg of sugar is considered as acceptable. Fermentation residue can be used as animal feed.

 

Distillation concentrates and purifies the alcohol. At industrial levels, ethanol is produced through a continuous distillation process where a distillation column brings ascending vapour into contact with the descending ‘low wine’ or ‘low beer’ liquid using devices such as bubble caps, valves or sieve trays. This leads to a mass transfer; the alcohol vapour is concentrated in the top of the column, while the original liquid, stripped of alcohol, is discarded as slop at the bottom of the column. Removal of impurities is achieved by boiling point differences by which ‘heads’ (esters, aldehydes etc.) are withdrawn at the top of the column and ‘tails’ (such as furfural, higher alcohols or fusel oils) at the bottom. Distillation produces a raw alcohol of 95 or 96% strength by volume.