Dry grind ethanol plants are springing up everywhere and the latest spike in gasoline prices has done nothing to cool the optimism of investors. Because of the price volatility of both the corn and ethanol markets, producers are looking at ways to enhance their profitability, especially when ethanol prices are low relative to corn price. Dry grind ethanol plants have been shown to be the most economical means to produce ethanol, but profitability could be enhanced if additional coproducts could be gleaned from the corn (Singh et al, 1999). There currently is much rhetoric and confusion about “fractionation” of corn. What is the difference between dry fractionation and wet fractionation? Preseparation vs. post-separation? Are higher ethanol (gals/bu) yields always more profitable? What affect does corn composition have on the operation of the ethanol plant? These questions and more are addressed herein to help in understanding the differences between the technologies involved.
Recovering additional coproducts from corn prior to a dry grind process presents a dilemma. In order to maintain a low cost of operation, it is desirable to do minimal processing yet recover a maximum value from the coproducts. Components that are uniformly distributed throughout the kernel often have the highest cost for recovery because all of the kernel must be processed in order to achieve adequate coproduct recovery. Most desirable is to have a component concentrated in an area or structure of the kernel, thus minimizing the effort and making the recovery more profitable. The goal of preseparation or post-separation is to recover as “cleanly” as possible the non-fermentable components of the corn. “Clean” being defined as having no attached starch, sugar or intermediate products (for purposes herein we will just call it starch). It is a balancing act between recovery of non-fermentables and loss of ethanol production because if you increase recovery of non-fermentables you invariably increase the loss of starch. Generally as recovery increases, purity of the recovered fraction decreases.
Approximately 1/3 of the corn kernel is composed of the non-fermentable components of fiber, protein, ash, and oil. It is fortuitous for corn processors that the non-fermentables are concentrated in two major structural components of the corn kernel: pericarp (coarse fiber) and germ (also called embryo). In the 17.2% of the kernel weight that makes up the germ and coarse fiber exists 56 % of the non-fermentables. The rest of the non-fermentables (endosperm protein and cellular fiber) are more uniformly distributed throughout the remainder of the kernel and have higher recovery costs associated with them.
Table 1 shows that the pericarp and tip cap of standard dent corn contains approximately 7% starch (0.21 lb dry solids/bushel) while the germ contains 8.3% starch (0.44 lb ds/bu) and 10.8% sugars (0.57 lb ds/bu). In some processes most of the sugars and some of the starch can be recovered from the germ, but there almost always will be some residue. It is just not economically viable to wash out all of the sugars and starch.
Table 1. Average Composition (% and lb dry solids/bu) of Seven Midwestern Dent Hybrids
Source: Watson (1987)
* NDF is Neutral Detergent Fiber
**lb/bu of dry solids for corn at 14.5% w.b. moisture content
Preseparation is the recovery of coproducts prior to fermentation, whereas post-separation is the recovery of coproducts after fermentation. The advantage of post-separation, as touted by some dry grind ethanol producers, is that post-separation does not affect the standard dry grind fermentation and can be a mere add-on to the process. The disadvantage of post-separation processes is the same as the perceived advantage, that is, post-separation does not affect the dry grind fermentation and thus loses out on the advantages. Almost all of the impact of preseparation on fermentation and coproduct value has been positive. Removal of the nonfermentables increases the alcohol content of the beer, increases the amount of alcohol produced per fermenter turn, increases the speed of fermentation and decreases the likelihood of “stuck” fermentations (Muthy et al., 2004; Singh et al., 2004). The advantages of preseparation are so significant that even if the nonfermentables were routed from the front of the process and blended with the DDGS, it would be profitable. Other disadvantages of post-separation include the negative impact of fermentation and drying on germ oil quality and the effect of fermentation on separation efficiency.
There is no question as to whether wet or dry fractionation results in the “cleanest” germ and pericarp. Look in any reference book concerning corn milling and it will show a significant difference in the cleanliness of the coproducts. Table 2 is a representative comparison, showing that wet mill products are more diverse and concentrated with certain components. Starch is 99.3% starch, gluten meal is 68.9% protein, germ (Table 3) is 40% oil and gluten feed is 30% NDF. Dry mill fractions are basically the same compositionally except for the germ and hominy feed. But neither of them have any one component over 30%. Fiber can be progressively aspirated in dry milling to get a high NDF fiber but the yield will be low.
Table 2. Comparison of Dry and Wet Milling Coproducts
* Sources: Alexander (1987); Wright (1987)
Table 3. Comparison of Dry Milling and Wet Milling Germ*
* Rausch and
The issue of gallons of ethanol per bushel of corn can be confusing when comparing different separation processes. The highest gallons of ethanol per bushel will always be with the standard dry grind process because any separation process will remove some fermentables with the nonfermentables, as discussed in the previous section. An economic model with a mass balance to keep track of the removed sugars or starch is the easiest way to compare processes. The total mass used to calculate yield of ethanol per bushel needs to be 56 lb bushel of whole kernel dent corn at 15% wet basis moisture (= 47.6 lb ds). Using a lower moisture content yields more ethanol per bushel because there is more carbohydrate available. In the same way any comparison needs to be based on a blend of normal dent corn and not a high starch hybrid. Any mass which leaves as coproducts can not be added back in or compensated for. For example it is not appropriate to take 56 lbs or 47.6 lbs ds of degermed and debranned corn as the basis for calculating yield. In the same way it is not appropriate to report yield on a denatured gallon basis because the denaturant was not part of the original bushel of corn.
The maximum ethanol yield possible can be calculated by
assuming an average corn analysis such as shown in Table
1. First calculate the amount of sugar needed to produce
adequate yeast mass. It is generally recognized that 2%
of the available sugar is a realistic minimum level of
cell mass produced and hydrolysis is 98% efficient.
Don’t forget to add in the naturally occurring sugars
from the corn.
The selection of a prefractionation method should not be based on who has the best sales pitch but on a variety of information, including:
Alexander, R.J. (1987). Corn dry milling: Processes, products, and applications, In: Corn Chemistry and Technology (Stan Watson and Paul Ramstad, editors), AACC Press, 604 pp.
Blanchard, P. 1992. “Technology of Corn Wet Milling and
Associated Processes”, Academic Press,
Eckhoff, S.R. 2004. Maize wet milling, In: Encyclopedia of Grain Science (C. Wrigley, editor), Academic Press, Inc., Orlando, FL , pp. 225-242
Hurt, C., Tyner, W., and Doering, O. 2006. "Economics of Ethanol." Purdue University, Cooperative Extension, Available at: http://www.ces.purdue.edu/bioenergy, Accessed: May 16, 2007.
Li, C., Rodriguez, L., Khanna, M., Spaulding, A., Eckhoff, S. 2007. An Engineering Economic Evaluation of Quick Germ Quick Fiber Process for Dry-grind Ethanol Facilities. (Poster), Livestock Industry and Renewable Fuels Conference, University of Illinois, Urbana, IL May 23-24, 2007.
Rausch, K.D. and Belyea, R.L. (2006). The future of
coproducts from corn processing,
Singh, V., Moreau, R. A., Doner, L. W., Eckhoff, S. R., and Hicks, K. B. 1999. Recovery of fiber in the corn dry-grind ethanol process: A feedstock for valuable coproducts. Cereal Chem. 76:868-872.
Singh, V. (2006). Past, present and future of dry grind corn processes. Presented at the 2006 Bioenergy Symposium, Purdue University, February 23.
Watson, S.A. (1987). Structure and composition, In: Corn Chemistry and Technology (Stan Watson and Paul Ramstad, editors), AACC Press, 604 pp.
Wright, K.N. (1987). Nutritional properties and feeding value of corn and its by-products, In: Corn Chemistry and Technology (Stan Watson and Paul Ramstad, editors), AACC Press, 604 pp.
Entrance 2 Entrance 3