Ethanol (C2H5OH) is a light alcohol and is a volatile, colourless, flammable liquid with a characteristic odour. It is also known as ethyl alcohol and often abbreviated as EtOH. The most common way of production is the fermentation of sugar or starch from agricultural crops by yeasts or bacteria.
Ethanol can either be used as high blend (E85) in dedicated flex-fuel vehicles, or as low blend in most current vehicles without modifications. The blending rate is usually up to 10%.
The feedstock for ethanol production can be any material containing appreciable amounts of sugar or substances that can be converted to sugar. Conventional production uses sugar (from sugar cane and sugar beet), starch (from corn, wheat or potatoes) or other polysaccharides. The production process of second generation ethanol, also called cellulosic alcohol, uses cellulosic feedstock (e.g. from agricultural residues) which require further pretreatment.
In a fermentation process sugar (glucose, fructose or other monosaccharides) is converted to ethanol by microbes (mostly varieties of the yeast Saccharomyces cerevisiae), which are inoculated to the feedstock. The monosachharides originate either directly from disaccharides, which are broken up via invertase enzymes, or from starch which is hydrolysed with amylase enzymes. In addition to ethanol, water and carbon dioxide are prdouced also.
The glucose-to-ethanol reaction is represented by the equation below:
C6H12O6 + 2 ADP + 2 Pi → 2 C2H5OH + 2 CO2 + 2 H2O + 2 ATP
Common processes produce a fermentation broth with concentration of 5% - 10% ethanol per volume, as ethanol itself is toxic to the microorganisms. More advanced facilities are able to increase the concentration up to 20% due to the use of adapted and specialized yeasts.
The upgrading includes the recovery and concentration of ethanol from the fermentation broth. Therefore the following technological steps are applied:
- Evaporation of ethanol: in this step the first evaporation of ethanol is performed in order to obtain “crude” ethanol with concentration about 45% per volume.
- Rectification: in the rectification step the ethanol concentration is increased to 96% per volume
- Dehydration: by dehydration the remaining azeotropic water is removed in order to obtain a required concentration of 98.7% per mass and water content below 0.3% per mass.
The potential and challenges of drop-in biofuels, IEA Bioenergy Task 39, 2014