Ethanol Science

How is Ethanol Made?

Ethanol can be made by a dry mill process or a wet mill process. All of the new fuel ethanol plants in the U.S. use the dry mill method. The major steps in this process are:

Milling: The feedstock (corn, wheat, barley, etc.) passes through a hammer mill which grinds it into course flour called meal. The vast majority of ethanol in the U.S. is produced from corn.

Liquefaction: The meal is mixed with water and alpha-amylase enzyme to create a "mash" which is passed through cookers where the starch is liquefied. Heat is applied at this stage to enable liquefaction. Cookers with a high temperature stage (120-150 degrees Celsius) and a lower temperature holding period (95 degrees Celsius) are used. High temperatures reduce bacteria levels in the mash.

Saccharification. The mash from the cookers is cooled and the secondary enzyme (gluco-amylase) is added to convert the liquefied starch to fermentable sugars (dextrose).

Fermentation: Yeast is added to the mash to ferment the sugars to ethanol and carbon dioxide. Using a continuous process, the fermenting mash is allowed to flow through several fermenters until is it fully fermented and leaves the final tank. In a batch process, the mash stays in one fermenter for about 48 hours before it flows to the distillation process.

Distillation: The fermented mash, now called beer, contains about 15% alcohol plus all the non-fermentable solids from the corn and yeast cells. The beer is pumped to the continuous flow, multi-column distillation system where, using heat, the alcohol is removed from the solids and the water. In the final distillation column, the alcohol leaves the top of the final column at about 96% ethanol. The residue mash from the distillation process, called stillage, is transferred from the base of the first column to the co-product processing area.

Dehydration: The alcohol from the top of the last column passes through a dehydration system where the remaining water is 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 ethanol (pure, without water) and is approximately 200 proof.

Denaturing: Ethanol that will be used for fuel is denatured, or made unfit for human consumption, with a small amount (2-5%) of gasoline at the facility which produces the ethanol.

Co-Products: There are two main co-products created in the production of ethanol: distillers grains and carbon dioxide. Distillers grains in either wet or dry form, is a valuable animal feed. Carbon dioxide is given off during fermentation and many ethanol plants collect, compress, and sell it for use in carbonated beverages, food processing and other industries.

Source: BBI International HYPERLINK "" and American Coalition for Ethanol

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