In 2006, more than 5 billion gallons of fuel ethanol were produced in the U.S. A number of grains can be used to produce fuel ethanol (e.g., corn, sorghum, wheat, barley, etc.), but in the U.S., most ethanol (> 90%) is produced from corn, using about 2.15 billion bushels, or 20% of the US corn harvest in 2006. Corn is largely composed of starch which can be broken down into sugars and fermented to ethanol using yeast. U.S. commercial production of corn ethanol uses either a dry-grinding process or a wet milling process, which differ with respect to their complexity and associated capital costs, the numbers and types of co-products produced, and the flexibility to produce different kinds of primary products. Dry-grind is the most prevalent process and much of the current expansion of the industry uses this technology. However, fuel ethanol is also produced using wet milling processes. Corn wet mills are generally larger in size than dry-grind facilities due to higher capital costs. And, in addition to the production of fuel ethanol, most corn wet mills also produce a number of food-grade products such as specialty starches, high-fructose corn syrup (HFCS), corn oil, acidulants (citric acid) and thickeners (xanthan gum).
Producing fuel ethanol using the wet mill process involves preparing the grain, fermentating the sugars, recovering the ethanol, and handling the co-products (figure 3). The principal differences between the ethanol dry-grind process and the ethanol wet mill process are the grain preparation steps and the numbers and types of co-products recovered. Once the starch has been recovered the process of converting it to fuel ethanol and recovering the ethanol is similar in both wet mill and dry-grind facilities.
Steeping, the first step in the wet milling process, is what differentiates this process from dry milling (for cereal processing) and dry-grinding (for ethanol production) processes. In the steeping process, the corn kernels (figure 4) are soaked in an acidic solution to soften the kernels which aids in the separation of the starch from the protein and other components of the grain. The corn kernel is further fractionated by coarsely grinding the grain followed by several steps to recover the oil-rich germ and to separate the hull and the fiber from the starch and protein (i.e., gluten). The recovered fiber is used to produce corn gluten feed. The starch and the protein are next separated and the protein is used to produce corn gluten meal. Corn gluten feed and corn gluten meal are used as livestock feed.
For fuel ethanol production, the recovered starch is fermented to ethanol using processes similar to those used in the ethanol dry-grind process. Ammonia and lime are added to adjust the acidity of the slurry (starch and water mixture) and to serve as a nutrient for the yeast during fermentation. The slurry is heated and an enzyme is added to cut the starch molecules into smaller pieces. Next, sulfuric acid is added to make the slurry more acidic and another enzyme is added to further break down the starch into individual glucose molecules. At this point, the slurry (now called mash) is cooled and transferred to the fermentation tanks.
The fermentation of the starch in a wet mill is similar to the process used in dry-grind facilities with the only major differences being the presence of fewer insoluble solids in the fermentation liquid (due to the removal of the fiber and germ), and the use of steep water.
An industrial yeast (Saccharomyces cerevisiae which is also used for brewing beer and making bread) is used to convert sugars to ethanol. The fermentation process usually occurs in batches with each fermentation tank filled and fermentation completed before being drained and refilled with a new batch. While the exact size of each fermentor varies between plant designs, common fermentor sizes range between 300,000–500,000 gallons each and a typical facility will have several fermentors operating simultaneously (i.e., one or more filling, one or more fermenting, and one or more emptying and resetting for the next batch). Some facilities use continuous fermentation processes in which new fermentation material is continuously added and fermented product continuously removed. Continuous fermentation has greater reactor productivity, but there are fewer opportunities for contamination in batch reactors.
The fermented corn mash (now called beer) contains 8-10% ethanol by weight. Separation and recovery of the ethanol is accomplished through a series of distillation and condensation steps finishing with a pass through a molecular sieve and final condensation of the ethanol vapor, to produce a final product that is at least 99.6% pure ethanol (Kwiatkowski, 2006). The ethanol is mixed with a denaturant (e.g. gasoline) to render it as non-potable fuel ethanol.
In addition to ethanol, germ, carbon dioxide, corn gluten meal, and corn gluten feed are produced in the wet milling process. The germ, which is separated from the other kernel constituents after the coarse grind, contains most of the oil contained in the grain. It can be processed to extract the oil, which can then be further refined into food-grade corn oil. Carbon dioxide produced during fermentation can be captured and used to carbonate beverages or to produce dry ice. Corn gluten meal and corn gluten feed are used as animal feed (Loe, 2006). Approximately 2.5 gallons of ethanol, 16.4 pounds of carbon dioxide, 2.1 pounds of oil, 2.6 pounds (dry mass) of corn gluten meal, and 11.2 pounds (dry mass) of corn gluten feed are produced per bushel of corn using the wet milling process.