Recovery of oil from ethanol processing byproducts offers some very promising opportunities for dry-mill plants to expand products and markets. DDGS has become a very valuable live­stock feed in recent years but possesses some characteristics that limit its use in some appli­cations. DDGS contains about 10.5% oil, which is about three times the level found in most corn. This relatively high oil content limits its use as a feed supplement particularly with respect to swine (Plain 2006). Additionally, the high oil content reduces DDGS longevity with respect to storage and shipping because the oil can become rancid, rendering the DDGS unsuitable for use as feed.

A 56-lb bushel of corn produces 2.72gallons of ethanol and approximately 171b of dis­tillers ’ grains in various forms (Renewable Fuels Association 20081. It is estimated that approximately 2.6 billion bushels of corn will be used to produce ethanol per year by 2010 (Baker and Zahniser 2006). About 2/3 of that corn will be used in dry-milling operations (Murthy et al. 2004). Consequently, an anticipated 29 billion pounds of DDGS will be produced from dry — mill plants. The DDGS contains about 10.5% oil, and the oil weighs about 7.6 lb per gallon, so about 400 million gallons of oil is potentially available for alternative uses if it could be separated from the DDGS (about 0.085 gallons of oil for every gallon of ethanol produced). For comparison purposes, a single dry-mill ethanol plant that produces 100 million gallons of ethanol annually would have about 8.5 million gallons of oil available for separation and/or extraction.

The oil can be removed from fractionated germ prior to fermentation, in which case it is suitable for human consumption. Alternatively, the oil can be removed from DDGS after fermentation. In this case, the oil would be unfit for human consumption because of its rela­tively high levels of free fatty acids (about 8%-9%). However, the oil could be used as animal feed or as feedstock in the production of biodiesel (methyl ester). Given that biodiesel pro­duction was 495 million gallons in 2007, the oil contained in DDGS offers considerable potential as a feedstock for biodiesel production without committing additional acreage to biofuel crops (ICM 2009).

Corn kernels are composed of four primary parts. The endosperm is the largest component (about 82%) of the kernel and is made up primarily of starch and protein. This starch fraction is the portion that is fermented into ethanol. The germ is the next largest fraction at 12% of the kernel and is the primary source of corn oil. The pericarp is the seed hull and composed 5% of the kernel, while the tip cap is where the seed was attached to the cob and makes up about 1% of the kernel. Wet-milling processes can efficiently fractionate the kernels and remove the oil-containing germ from the fermentable fraction prior to producing ethanol, thus allowing for the extraction of food — grade oil from the germ. Dry — mill plants can also be retrofitted to remove the germ prior to fermentation. This is accomplished by either (1) quick germ (and quick germ quick fiber) methods or (2) enzymatic milling. In the quick germ process, the whole corn is soaked in water for 3-12 hours at 60°C. Soaking the ground corn in water with enzymes increases specific gravity such that the germ and fiber float prior to fermentation. After soaking, the corn, a conventional Bauer mill, is used for degermination, similar to the methods used in the wet-milling process. The germ is recovered using germ hydroclones, and the rest of the corn is ground and processed through the dry-mill process (Singh and Eckhoff 1997).

In enzymatic milling, soaking is followed by incubating with protease and starch-degrading enzymes for 2-4 hours. After incubation, quick germ processes are used to recover germ and pericarp fibers. The remaining slurry is screened on a 200-mesh sieve to recover endosperm fibers. In either case, separation of the germ allows for the recovery of high-value food-grade corn oil from the germ (Singh et al. 2005). This can be accomplished through the use of an expeller and/or solvent extraction. Removal of endosperm fibers will further increase fermen­tation capacity and reduce fibers in the DDGS and increase the protein content of the DDGS, making the DDGS more suitable for a wider variety of livestock applications. Also, these modified milling processes can produce additional ethanol per batch because non-fermenta — bles (germ, pericarp fibers, and endosperm fibers) are removed. These non-fermentables can be replaced by a more fermentable substrate. Plants performing these modified milling pro­cesses can potentially increase the amount of corn processed and therefore produce more ethanol per batch compared with conventional dry-mill process (Singh et al. 2005).

Retrofitting an existing dry-mill plant to remove the germ prior to fermentation using either the quick germ or enzymatic milling processes is a relatively involved and costly endeavor requiring significant modifications and capital investment in equipment. However, the improvements associated with ethanol production, along with the high value of coproducts such as the food-grade corn oil produced and the improved quality of the DDGS, can justify the investment. Consequently, ethanol production technology providers such as Mercer Energy, FWS, FCStone Carbon, and ICM now offer fractionation technology to their clients with dry-mill plants.

Two primary options, centrifugation and solvent extraction, are available to dry-mill ethanol producers that can facilitate recovery of oil from either individual process streams or the combined DDGS byproduct. The centrifugation method is relatively straightforward, requiring a relatively low capital investment ($500,000 to $1 million to implement for an average ethanol plant). Solvent extraction methods are considerably more involved, require considerable capital investment, and create hazards that require careful consideration prior to implementation.

During the distillation process, solids comprising the grain and added yeast, as well as liquid from the water added during the process, accumulate in the bottom of the distillation tanks (ICM 2009) . The solids are processed through centrifuges for separation into thin stillage (a liquid with 5%-10% solids) and wet distillers’ grain. Some of the thin stillage is routed back upstream in the process for use as makeup water, reducing the amount of fresh water required. The rest is sent through a multiple-effect evaporation system where it is concentrated into syrup containing 25%-50% solids. This syrup, which is also high in protein and oil content, is then mixed back in with the distillers’ grain and further processed to create animal feed. In an effort to recover the valuable oil, some facilities have added a centrifugation step to the syrup prior to mixing it with distillers’ grain (ICM 2009).

The thin stillage syrup contains about 3.5% to 7% oil, depending on moisture content. Centrifugation will remove about 1/3 of the oil contained in the syrup. Therefore, a 60 gallon per minute centrifuge (the size implemented by ethanol plants that produce 50 to 100 million gallons of ethanol per year) could realistically produce about 500,000 to 1 million gallons of oil per year. Implementation of centrifugation to remove oil from the syrup is a relatively inexpensive and nondisruptive process change. However, centrifugation of the thin stillage syrup would only recover about 10% to 15% of the total oil available from the entire plant. The remaining oil is present in the DDGS and requires extraction with a solvent-based process.

Solvent extraction systems are much more efficient methods with respect to removing nearly all of the available oil from either the germ or DDGS. However, they tend to be complex and hazardous to operate because of the flammable solvents used for extraction. With solvent extraction processes, the germ or DDGS is fed into an extractor, where the material forms a uniform shallow bed and is washed with a solvent such as hexane as it is conveyed across the upper, horizontal section of the extractor, counter current to the solvent. The concentrated oil-solvent mixture discharges from the extractor through a hydroclone. The hydroclone “scrubs” the fines from the oil-solvent mix before being pumped further to a distillation system. On a typical extractor, there are seven stages of oil-solvent mixture, ranging from about 2% oil concentration to approximately 25%. From the extractor, the concentrated oil-solvent mixture is sent to a distillation system, where the oil and solvent are separated. The solvent is recycled back into the extraction process and the oil is available for sale or for conversion into other products. De-oiled DDGS would be available for markets that need less oil for the livestock it will be fed to, or for applications that require long-term storage or shipping.

Most conventional large- s cale biodiesel plants use base — catalyzed transesterification to convert triglyceride in the form of vegetable oil to methyl ester (Steinbach 2007).

Base

Triglyceride + Methanol —Catalyst > Methyl esters + Glycerol

This process alone is not suitable for processing oil derived from DDGS because of the relatively high levels of free fatty acids present in the oil. The base catalyst converts the free fatty acids to soap, thus reducing the yield of methyl ester. Consequently, the base-catalyzed process must be supplemented with a pretreatment step with an acid catalyst to convert the fatty acids to methyl ester. Following the pretreatment step, the remaining triglycerides can be converted to methyl ester using the base-catalyzed process.

Acid

Fatty Acid + Methanol —Catalyst > Fatty Acid Methyl Esters + Water

Biodiesel is now well accepted as a petroleum diesel fuel alternative offering multiple advantages such as renewability, energy security, and superior environmental performance. It is anticipated that feedstocks for biodiesel production will be increasingly in short supply in coming years. The number of biodiesel producers increased by more than 400% from 2004 to 2007. The total biodiesel production capacity available in the United States, as of January 2008, was 2.2 billion gallons from 171 plants, of which 137 came on line in 2004 (Steinbach 2007). Less than 1/2 billion gallons (less than 25% capacity) was actually produced in 2007 partly due to issues associated with feedstock availability (particularly soybean oil) and price. Developing alternative sources of feedstocks will become increasingly important as these plants attempt to increase their production to levels near their capacity.