- Biological process in which sugars (glucose, fructose, sucrose) are converted into cellular energy.
- Ethanol is produced in the result of this process.
- It is colorless, volatile or flammable liquid.
- It is widely used as biofuel as well as an alcoholic beverage is increasingly being consumed globally.
- This is mainly because of the cheap raw materials available.
- Ethanol can be derived from either sugar, starchy materials or lignocelluloses.
- The main feedstock for ethanol production includes sugarcane, sugar beet, corn, wheat.
- Sugarcane (molasses & juice)
- Cane sugar (clarified concentrated syrup)
- Sugar beet
- Sweet sorghum
- Sugarcane bagasse
- Corn stover
- Cereal straws
Feedstock conditioning and Pretreatment
- Molasses must be diluted to below to 25 °Bx (Brix) as yeast start to ferment quickly at this concentration.
- To prevent any incrustation in the pipelines or distillation towers due to ash content in molasses greater than 10%.
- The special chelating agent can also be used to remove the incrustation.
Addition of org & inorganic compounds:
- Done to offset the negative effect of salt which in turn increases the osmotic pressure.
- Yeast strains resistant to salts are also developed.
- To remove the impurities that stick to the surface of the biocatalyst when immobilize cells are used.
- Zymomonas mobilis
- Clostridium acetobutylicum
- Saccharomyces cerevisiae
- Saccharomyces uvarum
- Candida utilis
- Kluyveromyces fragilis
Features of Microbes:
- Due to the small size, having a high surface ratio.
- Due to having a resistant cell wall, producing high concentration substances leads to a faster fermentation rate.
- The intense metabolism permits the development of a continuous fermentation process.
- Cells growth rate offsets at which cells are removed from the bioreactor.
- Have the ability to “predigest” the available food source and release both products and the intermediate metabolites.
- Using immobilized cells of microbes by treating with Calcium alginate to adsorbed on the surface of materials.
- Using genetically modified microbes to enhance the fermentation process.
- The ideal pH is around 4.0-4.5.
- The initial temperature is kept between 21-26 ºC.
- Ethanol gets evaporated at 27 ºC.
- Aeration is initially required for the growth of microbes.
- Later, anaerobic condition are created by withdrawing oxygen coupled with the production of carbon dioxide.
- Urea is the most suitable source.
- Gaseous ammonium increases the pH of the medium.
- Ammonium sulfate can lead to incrustation.
- Diammonium phosphate used as a source.
- They can also be added to convert biopolymers and non-fermentable substances in the molasses to monosaccharides or amino acids.
Classical fermentation can be achieved in three steps:
- During the first phase (22-24 h), yeast cells multiply aerobically by consuming oxygen present in the mash.
- In the middle phase (24-48 h), alcohol production occurs with post saccharification of sugars and multiplication of yeast falls off.
- The decrease in alcohol formation along with insignificant yeast growth at the final stage (48-72 h).
Image Source: https://doi.org/10.1016/j.biortech.2018.02.125
There are following steps in ethanol production:
- The feedstock is passed through a hammer mill which grinds it into a fine powder called a meal.
- The meal is mixed with water and alpha-amylase.
- Then passed through cookers where the starch is liquified and heat is applied here to enable liquefaction.
- Cookers with the high-temperature stage (120-150˚C) and lower temperature holding period (95˚C) are used.
- High temperatures reduce bacteria levels in the mash.
- The mash from the cookers is cooled.
- And secondary enzyme glucoamylase is added.
- This converts the liquified starch into the fermentable sugars.
- Yeast is added to ferment the sugars to ethanol and carbon dioxide.
- In a continuous process, the fermenting mash can flow through several fermenters until it is fully fermented and leaves the final tank.
- In a batch process, the mash stays in one fermenter for about 48 hours before distillation starts.
´Yeast reuse results in a decrease in new growth with no more sugar available for ethanol production and an increase in the yield from 2 to 7%.
´Traditional yield 1-3g/L.
- To ensure system homogeneity and reduce the concentration gradient in the culture broth, CSTRs are employed.
- Reduce construction costs of bioreactors
- Lower requirements of maintenance and operation
- Better control of the process
- Higher productivities
- Cultivation of yeast under anaerobic conditions for a long time diminish their ability to produce ethanol.
- Aeration is important which can enhance cell concentration.
Extraction of the final product:
- Mash is pumped to continuous flow.
- Multicolumn distillation system where the alcohol is removed from solid and water.
- The alcohol leaves the top of the final column at about 96% strength.
- The residue mash is called stillage which is transferred from the base of the column to the co-product processing area.
- The alcohol from the top of the column is passed through a dehydration system where the remaining water will be removed.
- Most ethanol plants use a molecular sieve to capture the last bit of water in the ethanol.
- The alcohol product at this stage is called anhydrous alcohol.
Dried distiller grains with soluble (DDGS):
- The form is available to the feed industry.
- The liquid is separated from mash during the distillation process.
- It is partially dehydrated into syrup.
- Then added back on to the dried distiller’s grain to create DDGS.
- Used to carbonate the beverages.
- Manufacture dry ice.
- Used to flash freeze meat.
- Used by paper mills and food industries.