How is ethanol made?
Ethanol can be made by a dry mill process or a wet mill process. Most of the fuel ethanol in the U.S. is made using the dry mill method. The major steps in this process are:
The feedstock (corn, wheat, barley, etc.) passes through a hammer mill, which grinds it into a fine powder called meal. The vast majority of ethanol in the U.S. is produced from corn.
The meal is mixed with water and alpha-amylase, then 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.
The mash from the cookers is cooled and the secondary enzyme (gluco-amylase) is added to convert the liquefied starch to fermentable sugars (dextrose).
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 it is fully fermented and leaves the final tank. In a batch process, the mash stays in one fermenter for about 48 hours before the distillation process is started.
The fermented mash, now called beer, contains about 10 percent alcohol, plus all the non-fermentable solids from the corn and yeast cells. The mash is pumped to the continuous flow, multi-column distillation system where the alcohol is removed from the solids and the water. The alcohol leaves the top of the final column at about 96 percent strength and the residual mash, called stillage, is transferred from the base of the column to the co-product processing area.
The alcohol from the top of the column passes 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 ethanol (pure, without water) and is approximately 200 proof.
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.
There are two main co-products created in the production of ethanol: Distillers grains and carbon dioxide. Distillers grains, wet or dry, are valuable livestock feeds. Carbon dioxide is given off in great quantities during fermentation and many ethanol plants collect, compress and sell it for use in other industries.