Making Ethanol

Dry mill, wet mill, and the cellulosic process

The production of ethanol, or ethyl alcohol, from starch or sugar-based feedstocks is among man's earliest ventures into value-added processing. While the basic steps remain the same, the process has been considerably refined in recent years, leading to a very efficient process. There are two production processes: wet milling and dry milling. The main difference between the two is in the initial treatment of the grain.

Dry Milling
In dry milling, the entire corn kernel or other starchy grain is first ground into flour, which is referred to in the industry as “meal,” and processed without separating out the various component parts of the grain. The meal is 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 to 190 proof using conventional distillation and then is dehydrated to approximately 200 proof in a molecular sieve system.

The anhydrous ethanol is then blended with about five percent denaturant (such as natural gasoline) to render it undrinkable and thus not subject to beverage alcohol tax. 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 percent 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.

Wet Milling
In wet milling, the grain is soaked or “steeped” in water and diluted sulfurous acid for 24 to 48 hours. This steeping facilitates the separation of the grain into its many component parts.

After steeping, the corn slurry is processed through a series of grinders to separate the corn germ. The corn oil from the germ is either extracted on site or sold to crushers who extract the corn oil. The remaining fiber, gluten and starch components are further segregated using centrifugal, screen and hydroclonic separators.

The steeping liquor is concentrated in an evaporator. This concentrated product, heavy steep water, is co-dried with the fiber component and is then sold as corn gluten feed to the livestock industry. Heavy steep water is also sold by itself as a feed ingredient and is used as a component in Ice Ban, an environmentally friendly alternative to salt for removing ice from roads.

The gluten component (protein) is filtered and dried to produce the corn gluten meal co-product. This product is highly sought after as a feed ingredient in poultry broiler operations.

The starch and any remaining water from the mash can then be processed in one of three ways: fermented into ethanol, dried and sold as dried or modified corn starch, or processed into corn syrup. The fermentation process for ethanol is very similar to the dry mill process described above.

Cellulosic Biomass Opens New Opportunities
While ethanol is typically produced from the starch contained in grains such as corn and grain sorghum, it can also be produced from cellulose. Cellulose is the main component of plant cell walls and is the most common organic compound on earth. It is more difficult to break down cellulose to convert it into usable sugars for ethanol production. Yet, making ethanol from cellulose dramatically expands the types and amount of available material for ethanol production. This includes many materials now regarded as wastes requiring disposal, as well as corn stalks, rice straw and wood chips or “energy crops” of fast-growing trees and grasses.

There are a variety of options for pretreatment and other steps in the process.Several technologies combine two or all three of the hydrolysis and fermentation steps within the shaded box. (Chart courtesy of the National Renewable Energy Lab)

Producing ethanol from cellulose promises to greatly increase the volume of fuel ethanol that can be produced in the U.S. and abroad. A recent report found the land resources in the U.S. are capable of producing a sustainable supply of 1.3 billion tons per year of biomass, and that one billion tons of biomass would be sufficient to displace 30 percent or more of the country's present petroleum consumption.

Importantly, it offers tremendous opportunities for new jobs and economic growth outside the traditional “grain belt,” with production across the country from locally available resources. Cellulose ethanol production will also provide additional greenhouse gas emissions reductions. Several existing ethanol plants in the U.S. are engaged in research and demonstration projects with the U.S. Department of Energy utilizing the existing fiber in their facility that typically goes into the livestock feed co-product. With continued advancements in pretreatment technology, fermentation and collection and storage logistics, the commercial production of cellulose ethanol becomes more economically feasible.

ONLINE EXCLUSIVE Take a video tour of an ethanol plant and learn about the production process from start to finish!

The Biorefinery
The concept of a biorefinery is modeled after petrochemical refineries, with production of multiple products at a single facility. Existing biorefineries include wet-mill corn processing and pulp and paper mills. As with petrochemical refineries, the vision is that the biorefinery would integrate several conversion processes to produce both transportation fuel (ethanol and biodiesel) and high-value chemicals or products, including ones that would otherwise be made from petroleum. Industrial biorefineries have been identified as the most promising route to the creation of a new domestic biobased industry.
This article was provided by the Renewable Fuels Association, www.ethanolrfa.org.

   Current Issue   |   Past Issues   |   Contact Us   |   Send Files    |   Advertising   |   www.pei.org

The PEI Journal • Second Quarter 2007 • Volume 1, No. 2 • Contents are Copyright © Data Key Communications, Inc.
All rights reserved. • Nothing may be reproduced in whole or part without written permission of the publisher.