Inside Ethanol

Farm Journal, Nov 2, 2004 by Jeanne Bernick

Ethanol Production Process (Dry Mill)

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Ethanol-blended gasoline lit lamps in the early 1800s. Henry Ford's Model T ran on it. Hitler's armies were powered by it. Americans have traveled more than two trillion miles--

80 million trips around the world--on it since 1978.

You may already know ethanol is made by fermenting corn (or some other starchy crop, such as sorghum or barley), but how does 1 bu. of corn turn into 2.5 gal. of ethanol fuel?

Even teetotalers can swallow that the process is similar to that of making liquor. Starch from corn is broken down into simple sugars, which are then fermented to produce alcohol (ethanol). In fact, commercial ethanol plants are just slightly modified distilleries. Most ethanol plants in the U.S. use a dry milling process to convert corn into ethanol, which means the corn is not separated into basic components, such as corn oil or germ meal.

Current ethanol production offers a net energy gain of 21,105 British thermal units--or 34% more energy than what is used to grow, harvest and distill grain into ethanol, reports the USDA.

Dry milling ethanol plants use seven major steps to convert corn to fuel.

Step 1. Milling: Corn that is delivered to the ethanol plant is first cleaned, after which it passes through hammer mills that grind it into a fine powder referred to as meal.

Step 2. Liquefaction: The meal next moves into cookers, where the starch is liquefied and turned into a soupy mash. To do this, the meal is mixed with water and alpha-amylase, an enzyme that functions as a catalyst to speed up chemical changes. The mash passes through cookers at a high temperature of between 248degrees and 300degrees to kill off any bacteria. From the cookers, the material passes into a lower-temperature cooker (203degrees) for a holding period.

Step 3. Saccharification: During this process, the mash from the cookers is cooled and a secondary enzyme called gluco-amylase is added to the mash to convert the liquefied starch to fermentable sugars, or dextrose.

Step 4. Fermentation: The mash now goes into a fermenting tank (which can range from the size of a coffee can to thousands of gallons) where yeast is added to convert the sugars into alcohol (ethanol) and carbon dioxide. Yeast is a single-celled fungi that feeds on sugars.

In most dry mill plants, the fermenting process occurs in one tank and is called batch fermentation. Some plants, however, use continuous flow fermentation, where the fermenting mash flows, or cascades, through several large, cylindrical steel fermenters. The reason for using several fermenters is to increase production.

In a typical batch fermentation process, the mash will stay in each fermenter for about 48 hours before distillation. Carbon dioxide is also given off during fermentation, and many plants collect the gas, clean it of any residual alcohol, compress it and sell it for use in carbonated beverages or in flash-freezing of meat.

Step 5. Distillation: The fermented mash, now called beer, will contain alcohol as well as water and nonfermentable solids from the corn and yeast cells. The beer is pumped into a continuous flow, multi-column distillation system where the alcohol is removed from the solids and water.

Ethanol distillation works by continually heating the beer and letting its vapors--which include alcohol--rise to the top of the column. The vapors bubble up until their temperatures drop to the point that they liquefy into a pure form of alcohol.

When alcohol leaves the top of the column, it is about 96% strength. The leftover mash at the base of the last column, now called stillage, is transferred to a co-product processing area, where it is dried to create distillers' dried grains, a high-protein livestock feed.

Step 6. Dehydration: The alcohol from the top of the column passes through a dehydration system where all remaining water is removed. The alcohol product at this stage is called anhydrous ethanol, meaning pure or without water, and is about 200 proof.

Step 7: Denaturing: Ethanol to be used for fuel is denatured with a small amount of gasoline (about 2% to 5%) to make it unfit for human consumption while it is held in storage. Before it goes to the pump, ethanol is further diluted with more gasoline to create fuel blends like E-85.

Editor's note: This article was prepared with the technical advice of the National Corn to Ethanol Research Center, National Corn Growers Association, BBI International and the American Coalition for Ethanol.