Production of Ethanol using Dry Milling .

Ethanol is produced from biomass, primarily by fermenting the glucose derived from sugars obtained from sugar cane, sugar beet, and molasses, or starches obtained from corn, wheat, grains or using cellulose as raw materials. Industrial production of ethanol is majorly carried on using either the wet mill or dry mill process. Wet milling process involves separating the grain kernel into its constituents of germ, fiber, protein, and starch, before fermentation. Whereas in the dry mill process, the entire grain kernel is directly ground into flour. The starch in the flour is converted to ethanol during the fermentation process, creating carbon dioxide and distillers’ grain. In the USA, around 67% of ethanol is produced by the ethanol industry using the dry-grinding process. The various procedure involved in the ‘dry-milling’ process is discussed below.

Mash Formation

Screening is used to remove debris from the grains mass. These filtered kernels are then ground and mixed with water to form a slurry called ‘mash’.

Cooking

The mash is cooked, followed by the addition of enzyme which converts the starch into sugar. Yeast is then added to ferment the sugar which results in a mixture of ethanol and solids. The ethanol is extracted using the distillation & dehydration process. The solid which remains is dried to find its application as a distiller’s dried grain soluble or DDGS. DDGS is popularly used as a high-protein supplement in cattle, swine, poultry, and fish diets. Starch usually comprises of 25-30% amylose and rest is amylopectin. In order to metabolize this starch, yeast is used break it down into glucose, prior to fermentation.

Liquefaction

The slurry is then pumped through a pressurized jet cooker at high temperatures and held for a few minutes. The mixture is then cooled using a condenser. After the condensation cooling, the mixture is held for a few hours at a predefined temperature to give the alpha-amylase enzyme time to break down the starch. The slurry is then heated to reduce viscosity and to provide mechanical shearing to rupture starch molecule, especially of high molecular weight. The mash is further liquefied for at least 30 min to reduce the size of the starch polymer. This dextrinized mash is further cooked to facilitate the addition of glucoamylase to convert liquefied starch into glucose. To accomplish the saccharification of starch to glucose, glucoamylase is added in enough quantity.

Saccharification

Once inside the fermentation tanks, the dextrins are broken down to form simple sugars. Yeast is then added to convert the sugar to ethanol and carbon dioxide. The mash is afterward allowed to ferment for 48–72 hours, resulting in a mixture that contains about 10% ethanol as well as the solids from the grain and added yeast.

Fermentation 

After saccharification, cooling is done and mash is transferred to fermenter and yeast is added. The whole process requires 48-72 hours and can concentrate up to 10-12% of ethanol. CO2 released during this process can be captured and sold for the use in carbonated soft drinks, dry ice, and some beverages industries.

Distillation and Dehydration

Distillation is the process of separating ethanol from the solids and water in the mash. The fermented mash is pumped into the distillation columns where additional heat is added. The columns utilize the differences in the boiling points of ethanol and water as a milestone to boil off and separate the ethanol. By the time the product stream is ready to leave the distillation columns, it contains about 95% ethanol by volume. To carry out the separation of the remaining 5% water from ethanol, It is passed through a molecular sieve. This step produces 100% pure ethanol.