Bioethanol production process

Pretreatment

• Sugar can be directly fermented to alcohol
• Starch first has to be converted (in a process called "enzymatic hydrolysis") into sugars (monosaccharides) before fermentation 
• During enzymatic hydrolysis first starch containing feedstock is crushed and mashed 
• Then enzymes (amylases) are added

Under the following link you can watch more about ethanol production from corn: click here

Fermentation

• Sugars contained in the mash are fermented by yeast (living bacteria, known from the production wine or beer). This process also results in the formation of CO2: 

• From 1 kg of glucose 0.51 kg ethanol (~0.64 l) and 0.48 kg CO2 are produced
• Fermentation is an exothermic reaction
• Temperature should not increase to much, optimal temperature for fermentation is around 37°C (higher temperatures would deactivate enzymes and yeasts)
• Process has to take place under anaerobic conditions
• Resulting CO2 has to be removed from the reaktor constantly
• Fermentation process takes about 48-60 hours
• Since ethanol is toxic for the yeast ethanol concentration is limited - i.e the process stops at ethanol concentrations above 14%
• Ethanol tolerant yeasts are selected for the process, nevertheless, ethanol must be removed constantly to ensure continuous ethanol production
• Continuous fermentation can take place in one vessel (e.g. flow from the top to bottom) or in multi-step processes with 2-8 fermenters connected in a cascade
• Ethanol concentration after fermentation is typically around 10%-14%

Distillation or rectification

To increase the ethanol concentration of the fermented juice, distillation is carried out:

• water is separated from the ethanol by heating the fermented juice
• the process is taking advantage of the fact that ethanol has a lower boiling point (78°C) than water (100°C)
• the concentration of highly volatile components (e.g. ethanol) is higher in the produced vapour
• after condensation the distillate has a higher ethanol concentration than the initial liquid but still contains water

• by repeated distillation of the concentrate, a higher ethanol concentration can be reached in the condensate
• 100% purity of bioethanol cannot be achieved because hydrogen bonds between water and ethanol are too strong to separate them completely

Under the following link you can watch a video about distillation: click here

Usually industrial ethanol distillation is performed in huge distillation towers or columns, called rectification or fractional distillation (see following figure).

• By repeated, continuous distillation the ethanol concentration can increased up to 94%
• The rectification column typically consists of several trays through which vapour and liquid pass in countercurrent flow (see scheme in the following figure)
• The feed enters the column at the bottom and is heated up
• The vapor produced moves upwards in the column where it cools down and condenses on the surfaces and walls in the column
• Condensed liquid is sent to further stages where the distillation process is repeated
• Part of the vapor at the top is discharged and condensated
• Part of the condensed liquid flows back into the column as reflux and moves downwards as liquid counterphase to cool the ascending vapours and thus to increase the efficiency of the distillation tower

Watch a video about continuous distillation here: click here

• After distillation a liquid waste is remaining, called "stillage", containing 0.002 wt% ethanol
• Stillage contains a lot of substances like proteins, fiber etc. and can be further extracted and used for further applications
• Stillage is separated into coarse grain and solubles
• Stillage can be used as substrate in biogas plants to generate heat or electricity from biogas
• Dried and condensed stillage is used as livestock feed (DDGS = dried distillers grains with solubles)

Dehydration

• Reduces the remaining water content to 0.01 vol.% and increases ethanol content up to 99.5% (anhydrated ethanol)
• Bioethanol that is to be blended with gasoline has to be anhydrated ethanol
• Usually done with pressure swing adsorption (PSA) in molecular sieves, i.e. adsorption with porous materials (e.g. zeolites) taps the water molecules

Take a look how zeolith molecular sieve looks like