Dry Milling Corn Ethanol Process

In comparison to the wet milling ethanol process where the corn kernel has to be separated into its components of germ, fiber, gluten, and starch prior to the fermentation step, the dry milling ethanol process first grinds the entire corn kernel into coarse flour form and then ferments the starch in the flour directly into ethanol. The dry milling corn ethanol process by ICM, Inc. is outlined below [21].

1. In the first step, corn receiving and storage, the corn grain is delivered by truck or rail to the corn ethanol fermentation plant. Grains are loaded in storage bins (silos) designed to hold sufficient amounts of grain to supply the plant operation continuously for 7-12 days.

2. The second step, milling, is where the grain is inspected and screened to remove debris (including corn cobs, stalks, finer materials, stones, and foreign objects) and ground into coarse flour. The screening is usually done using a blower and screen. Coarse grinding is typically
performed using a hammer mill. The feed rate from the milling step to the next stage of hot slurrying is typically controlled by the use of weighing tanks.

3. During the so-called cooking process, also called hot slurry, primary liquefaction, and secondary liquefaction, the starch in the flour is physically prepared and chemically modified for fermentation.

a. In the hot slurry method the coarsely ground grain is soaked in hot process water, the pH of the solution is adjusted to about 5.8, and an alpha-amylase enzyme is added. The slurry is heated to 180-190°F (82-88°C) for 30-60 minutes to reduce its viscosity. Agitation needs to be provided.

b. Primary liquefaction follows, where the slurry is then pumped through a pressurized jet cooker at 221°F (105°C) and held for 5 minutes. The jet cooker is also known as a steam injection heater. The mixture is then cooled by an atmospheric or vacuum flash condenser. The jet cooker is a critical component as steam helps to evenly hydrolyze and rapidly heat the slurry. The fluid dynamic relationship between the jet cooker’s steam injector and condens­ing tube produces a pressure drop to help maximize shear action to improve starch conversion [22].

c. The third process, secondary liquefaction, occurs after the flash condensation cooling. The mixture is held for 1-2 hours at 180- 190°F (82-88°C) to give the alpha-amylase enzyme sufficient time to break down the starch into short-chain, low-molecular- weight dextrins. This chemical conversion is called gelatinization. Generally speaking, during the gelatinization step, there is a sharp increase in the slurry viscosity that is rapidly decreased as the a-amylase hydrolyzes the starch into lower molecular weight dextrins. Dextrins are a group of low-molecular-weight carbo­hydrates produced by the hydrolysis of starch and are mixtures of polymers of D-glucose units linked by a-(1^4) or a-(1^6) gly — cosidic bonds. After pH and temperature adjustment, a second enzyme, glucoamylase, is added as the mixture is pumped into the fermentation tanks. Glucoamylase is an amylase enzyme that cleaves the last alpha-1,4-glycosidic linkages at the nonreducing end of amylase and amylopectin to yield glucose. In other words, glucoamylase is an enzyme that cleaves the chemical bonds near the ends of long-chain starches (carbohydrates) and releases maltose and free glucose. Maltose, or malt sugar, is a disaccha­ride that is formed from two units of glucose joined with an a(1^4)bond.

4. The fourth step is called simultaneous saccharification fermen­tation. Once inside the fermentation tanks, the mixture is now referred to as mash, because it is an end product of mashing (which involves mixing of the milled kernel and water followed by mixture heating). The glucoamylase enzyme breaks down the dextrins, oligosaccharides, to form simple sugars, that is, monosac­charides. Yeast is added at this stage to convert the sugar to ethanol and carbon dioxide via an alcohol fermentation reaction. The mash is then allowed to ferment for 50-60 hours, resulting in a mixture that contains about 15% ethanol as well as the solids from the grain and added yeast [21, 23].

5. In the distillation step the fermented mash is pumped into a multicol­umn distillation system. The distillation columns utilize the boiling point difference between ethanol and water to distill and separate the ethanol from the solution. By the time the product stream is ready to leave the distillation columns, it contains about 95% ethanol by volume (which is 190-proof). This point is just immediately below the azeotropic concentration of the ethanol-water binary system, as explained in Section To overcome this azeotropic limitation of maximum achievable ethanol concentration via straight distilla­tion, several optional methods are being used, including jumping over the azeotropic point or bypassing the distillation. The residue from this process, called stillage, contains nonfermentable solids and water and is pumped out from the bottom of the distillation col­umns into the centrifuges.

6. The sixth step is that of dehydration. The 190-proof ethanol still con­tains about 5 vol.% water. This near-azeotropic binary mixture is passed through a molecular sieve to physically separate the remain­ing water from the ethanol based on the size difference between the two molecules [21]. This dehydration step produces 200-proof anhy­drous (waterless) ethanol, that is, near 100% ethanol.

7. Product ethanol storage is the seventh step. Before the purified eth­anol is sent to storage tanks, a small amount of denaturant chemi­cal is added, making it unsuitable for human consumption. There are so many different kinds of denaturants available on the market for diverse purposes other than fuel ethanol. However, only cer­tain gasoline-compatible blendstocks are suitable as denaturants for fuel ethanol. Some ethanol refineries also sell their denaturants for other ethanol industries. The ASTM D4806-11a specification covers nominally anhydrous denatured fuel ethanol intended for blending with unleaded or leaded gasolines for use as a spark-igni­tion automotive engine fuel. According to this specification, the only denaturants used for fuel ethanol shall be natural gasoline, (also known as natural gas liquid [NGL]), gasoline components,

or unleaded gasoline at the minimum concentration prescribed. Methanol, pyrroles, turpentine, ketones, and tars are explicitly listed as prohibited denaturants for fuel ethanol meant to be used as gasoline blendstock [24]. Most ethanol plants’ storage tanks are sized to allow storage of 7-12 days’ production capacity.

8. During the ethanol production process, two valuable coproducts are created: carbon dioxide and distillers grains. Their recover­able values are very important to the overall process economics and this is why they are called coproducts rather than simply by-products.

During yeast fermentation, a large amount of carbon dioxide gas is gen­erated. Because CO2 is a major greenhouse chemical, its release into the atmosphere is not desirable. The carbon dioxide generated by fermentation is of high concentration and its purification is relatively straightforward. Therefore, carbon dioxide from ethanol fermentation is commonly captured and purified with a scrubber so it can be marketed to the food processing industry for use in carbonated beverages and flash-freezing applications. Dry ice is a common coproduct of the ethanol refineries.

The stillage from the bottom of the distillation columns contains solids derived from the grain and added yeast as well as liquid from the water added during the process. The stillage is sent to centrifuges for separation into thin stillage (a liquid with 5-10% solids) and wet distillers grains [21].

Some of the thin stillage is recycled back to the cook/slurry tanks as makeup water, reducing the amount of fresh water required by the cook­ing (hot slurry) process. The rest is sent through a multiple-effect evapo­ration system where it is concentrated into a syrup containing 25-50% solids. This syrup, which is high in protein and fat content, is then mixed back in with the wet distillers grains [21]. This is a step intended to recover most of the nutritive components from the stillage. With the added syrup, the WDG still contains most of the nutritive value of the original feed­stock plus the added yeast and as such it makes excellent cattle feed. After the addition of the syrup, it is conveyed to a wet cake pad, where it is loaded for transport.

Many ethanol refinery facilities do not have enough nearby cattle farms or established markets to utilize all of their WDG products. However, WDG must be used soon after it is produced, because it gets spoiled rather easily. Therefore, WDG is often sent through an energy-efficient drying system to remove moisture and extend its shelf life. Dried distillers grains are com­monly used as a high-protein ingredient in cattle, swine, poultry, and fish diets. Modified forms of DDGs are also being researched for human con­sumption due to the outstanding nutritive values. In more practical senses, DDG is better known as a corn ethanol coproduct than WDG.


A schematic of ICM’s dry milling corn ethanol process [21] is shown in Figure 3.7.