Ethanol Process
Take a step-by-step interactive tour of an ethanol plant.

In dry milling, which is the process used in all five of Michigan's ethanol plants, the entire corn kernel or other starchy grain is first ground into flour, which is referred to in the industry as "meal." The meal is then processed without separating out the various component parts of the grain and slurried with water to form a "mash." Enzymes are added to the mash to convert the starch to dextrose, a simple sugar. Ammonia is added for pH control and as a nutrient to the yeast.

The mash is processed in a high-temperature cooker to reduce bacteria levels ahead of fermentation. The mash is cooled and transferred to fermenters where yeast is added and the conversion of sugar to ethanol and carbon dioxide (CO2) begins.

The fermentation process generally takes about 40 to 50 hours. During this part of the process, the mash is agitated and kept cool to facilitate the activity of the yeast. After fermentation, the resulting "beer" is transferred to distillation columns where the ethanol is separated from the remaining "stillage." The ethanol is concentrated using conventional distillation and then is dehydrated in a molecular sieve system to increase the alcohol content.

The anhydrous ethanol is then blended with about 5% denaturant (such as natural gasoline) to render it undrinkable. It is then ready for shipment to gasoline terminals or retailers.

The stillage is sent through a centrifuge that separates the coarse grain from the solubles. The solubles are then concentrated to about 30% solids by evaporation, resulting in Condensed Distillers Solubles (CDS) or "syrup." The coarse grain and the syrup are then dried together to produce dried distillers grains with solubles (DDGS), a high quality, nutritious livestock feed. The CO2 released during fermentation is captured and sold for use in carbonating soft drinks and beverages and the manufacture of dry ice.


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