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bioweb.sungrant.org » Technical » Biofuels » Technologies » Ethanol Production » Ethanol Wet Grind Processes

Ethanol—Wet Grind Processes	Printer Friendly

In 2006, more than 5 billion gallons of fuel ethanol were produced in the U.S. Most (> 90%) was produced from corn and used about 2.15 billion bushels (about 20% of the 2006 corn harvest) (RFA, 2006; USDA, 2006). Corn is largely composed of starch which can be easily hydrolyzed to sugars and fermented to ethanol using yeast. U.S. commercial production of ethanol from corn uses either a dry-grinding process or a wet milling process. The two processes 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).    Ethanol is produced by fermenting plant carbohydrates with yeast. Plant carbohydrates are grouped as soluble sugars (such as sucrose from sugarcane), storage carbohydrates (such as starch from grains and tubers), and structural carbohydrates (which make up the plant cell wall such as cellulose, hemicellulose, and pectin). The principal carbohydrate contained in corn grain and other grain crops (such as wheat, barley, rice, oat) is starch (table 1).              Two major forms of starch exist--amylose and amylopectin (BeMiller, 1996). Amylose is a straight chain polymer of glucose molecules joined by a (1-4) glycosidic bonds (figure 1). This primary structure results in the long polymers coiling into a helical conformation (BeMiller, 1996). Amylopectin is also primarily a straight chain of glucose but also includes branches occurring every 24 to 30 glucose units consisting of a (1→6) bonds (figure 2). Starch is semi-crystalline and transitions from an amorphous state at 60-70°C to a gel form through gelatinization where water molecules disrupt the hydrogen bonds within and between starch molecules. Starch, especially gelatinized starch, can be easily hydrolyzed to yield the individual glucose mollecules.                             Producing fuel ethanol from grains using the wet mill process involves four major steps—preparation of the grain, fermentation of the sugars, recovery of the ethanol, and handling of the coproducts. 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. But, the wet mill process is designed to fully fractionate the grain so that the major constituents (carbohydrates, lipids, and protein) can be efficiently recovered and purified for the production of value-added products (figure 3).

Grain preparation
The upstream processing steps of corn (steeping and fractionation) are common to all wet mills. 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 are soaked in water acidified with sulfur dioxide (SO2), typically for 22-50 hours at 52°C (125°F). Steeping softens the kernels by thoroughly wetting the grain and increaseing the moisture content from 15% to 45%. The sulfur dioxide (typically at 0.12 – 0.20% of the water) and possibly lactic acid produced by bacteria in the mix, break down the protein-starch matrix contained in the endosperm (figure 4) of the grain, which aids in the separation of the starch from the protein and other components of the grain.                    Source: May, 1987.   The corn kernel is further fractionated by first coarse grinding the grain followed by several separation steps. The coarse grinding mill consists of a stationary disk and a rotating disk, each with knobs that break up the kernels. They are adjusted so that few kernels exit without being broken while the seed germ remains undamaged (May, 1987). Following coarse grinding, the oil-rich germ is separated from the slurry based on differences in densities. Hydrocyclones are used to separate solids from a liquid slurry using centrifugal force. The slurry is fed tangentially at the top of a cylindrical entry section. The germ floats to the top and exits through a large opening located on the cylinder. Water and any remaining slurry exit through a smaller opening at the apex of a cone located at the bottom of the cylinder. The separated germ is dewatered and dried before removal of the corn oil by solvent extraction.   The material leaving the bottom of the cyclone passes through a wedge-wire screen to separate the hull and the fiber from the starch and protein (i.e., gluten). Approximately 30-40% of the starch is recovered in this step (May, 1987). The recovered fiber, containing attached starch, is next milled using either an entoleter or a disk mill. An entoleter mill forces the fiber against pins at high speed to shear the starch from the fiber. Disk mills, similar to the coarse mill, have knobs or grooves that remove the starch from the fiber. Both milling processes are adjusted to maximize starch seperation from the fiber, while keeping the fiber intract. The starch is separated a second time by passing it through a screen sized to prohibit passage of the fiber. The processed fiber is then pressed to remove water (to 40-60% moisture), mixed with evaporated stillage, and dried to produce corn gluten feed. Corn gluten feed contains less protein and has a lower energy content than distiller’s dried grains and solubles produced in the dry-grind process.   The starch/protein slurry that is fractionated from the fiber is further processed to separate the starch and the protein. Gluten (the protein) is lower in density than starch (1.06 specific gravity compared with 1.6 specific gravity for starch) permitting efficient separation by centrifugation or hydrocyclones (May, 1987). The fractionated protein is dewatered and dried to produce corn gluten meal which is used as livestock feed. The remaining starch contains 3-5% protein and may be further purified depending on the final use of the starch. For fuel ethanol production, this primary starch slurry usually undergoes liquefaction and saccharifaction processes similar to those used in the ethanol dry-grind process.    Liquefaction involves adding ammonia and lime to the starch slurry to adjust the pH to 6.5. The ammonia, which contains nitrogen, also serves as a nutrient for the yeast in the subsequent fermentation step. The slurry is heated to 88°C by direct steam injection using a “jet-cooker”. A thermostable enzyme (alpha-amylase) is added to cleave the starch molecules at random points along the middle of the polymer chain and to break the starch into smaller water soluble fragments called dextrins. After approximately one hour, the output from the first step of liquefaction is combined with “backset”, which is recycled water from the end of the ethanol distillation process. The backset accounts for approximately 15% of the final volume of the corn mash (McAloon, 2000). Critical nutrients for the yeast are also carried in the backset. As the liquefied slurry is cooled to 60°C, the heat is recovered and used to heat new, incoming slurry going to the jet-cooker (Kwiatkowski, 2006). A new enzyme technology developed by Genencor allows for the rapid hydrolysis of granular starch and eliminates the need for gelatination of the starch slurry by jet-cooking, thus significantly lowering the energy requirements for ethanol production from corn (Shetty, 2005).  Following liquefaction, sulfuric acid is added to the slurry to lower the pH to 4.5. An additional enzyme, glucoamylase (also called beta-amylase) is added to break down the starch and dextrins into glucose via a stepwise hydrolysis of glucose from the end of the molecules. The slurry is held at 60°C for 5-6 hours as the glucoamylase hydrolyzes the dextrins to glucose (Schenck, 2002). Most of the dextrins are converted to glucose during this step, however the glucoamylase remains active throughout the fermentation step and will continue to hydrolyze any residual dextrins during fermentation. After saccharification, the slurry (now called mash) is cooled to 32°C with the heat transferred to other process streams. The cooled mash then enters the fermentation tanks. A popular alternative to mash-presaccharification is to add the glucoamylase during the filling of the fermentor and to saccharify and ferment the starch simultaneously (SSF, Simultaneous Saccharification and Fermentation). An additional advantage to this approach is that reversion reactions (re-polymerization of glucose) are much less likely to occur (Power, 2003)

Sugar fermentation
Fermentation uses microorganisms (the yeast Saccharomyces cerevisiae, which is also used for brewing beer and baking bread) to convert sugars to ethanol and results in the production of ethanol and carbon dioxide, as well as additional yeast cells (from cell division) and heat.       On a mass basis, one kilogram of glucose will theoretically produce 0.51 kilogram of ethanol and 0.49 kilogram of carbon dioxide. However, glucose consumed to generate additional yeast cells (cell mass) does not result in the production of ethanol. Most industrial fermentation processes operate at 90 to 95% of the theoretical yield.    The fermentation of the saccharified starch in a wet mill is similar to the process used in dry-grind facilities with the only major differences being the presences of fewer insoluble solids in the fermentation liquid (due to the removal of the fiber and germ), and the use of steep water. The liquid used in the steeping process contains a significant amount of protein and micronutrients from the corn (Rausch, Thompson, Belyea, & Tumbleson, 2003) and is sometimes sold in a condensed form as a complex nitrogen/nutrient source for industrial fermentations (Kapen, 1996). For fuel ethanol production, the steep water is typically added to the saccharified starch to dilute it to the desired concentration (16 – 22 wt% sugar) before adding the yeast. The yeast, which is grown in seed tanks, is added to the saccharified starch to ferment the simple sugars (glucose) to ethanol.   Fermentation processes involve either batch or continuous processing methods. In a batch process, a fermentation tank is filled and ferments completely before being drained and refilled with a new batch. The up-stream processes (grain preparation and separation, liquefaction, and saccharification) and downstream processes (distillation and recovery) occur continuously. Facilities using batch processing usually have three or more fermentors, with one or more fermentor filling, one or more fermenting (for approximately 46-68 hours), and one or more emptying and resetting for the next batch operating at the same time. While the exact size of each fermentor varies between plant designs, common fermentor sizes range between 300,000–500,000 gallons (1-2 million liters) each (Kwiatkowski, 2006). A major advantage of batch fermernations is that there are fewer opportunities for contamination, provided they properly santized between runs. Bacteria, especially species of Lactobacillus, can infect yeast fermentations and produce organic acids that lower ethanol yields and interfere with the Saccharomyces (Graves, 2006).   Continuous fermentation processes involve a series of cascading tanks where the liquid continuously flows through the process. New fermentation media is continuously added at the front end and fermented product is continuously removed from the back end. While continuous fermentation has greater reactor productivity because it is continuously operating with high yeast loads, much more care needs to be exercised to prevent contamination (Bayrock, 2001).   In addition to ethanol, carbon dioxide is also produced during fermentation. Carbon dioxide can be captured, cleaned, compressed and sold for carbonation of soft drinks or frozen into dry-ice for cold product storage and transportation. If the carbon dioxide is not recovered, it passes through a water scrubber to remove evaporated ethanol and other volatile organic compounds (VOCs) carried in the gas. The water from the scrubber, containing the recovered ethanol, is sent to the distillation system. The cleaned carbon dioxide is vented to the atmosphere.   Heat is generated during fermentation (approximately 12000 kJ per kilogram of ethanol; 516 BTU per pound) (Kwiatkowski, 2006). This heat must be continuously removed from the fermentors and is accomplished either by passing water through a cooling coil contained within the fermentor, or by continuously pumping the fermenting mash through a large heat exchanger where the heat is transferred to cooling water before the mash is returned to the fermentor. Failure to remove the heat causes a rise in temperature sufficient to kill the yeast. The optimal temperature for ethanol fermentation is between 27 and 32°C.   After the fermentation is nearly complete, the fermented corn mash (now called beer) is emptied from the fermentor into a beer well where it is stored enabling a continuous stream to be supplied to the ethanol recovery system. The beer contains 8-10% ethanol by weight.

Ethanol Recovery
Separation and recovery of the ethanol is accomplished through a continuous process involving several steps (figure 5). In the first step, the beer is processed through a beer column where steam is used to strip off almost all of the ethanol, along with some water, from the slurry. The ethanol and water vapor exit the top of the beer column and the whole stillage (containing less than 0.1% ethanol by weight) exits from the bottom. The overhead vapor flows to a rectifier column where the ethanol is concentrated from 45% to 91% through fractional distillation. The bottoms from the rectifier pass through a stripping column to remove residual ethanol.  Liquid exiting the bottom of the stripper has less than 0.1% ethanol by weight, and is recycled as process water for slurrying the ground corn.  The overhead vapor from the rectifier (91% ethanol by weight) is superheated and passes through molecular sieves. The final product from the molecular sieve system is ethanol vapor that is at least 99.6% pure. This vapor is condensed and mixed with a denaturant (e.g. gasoline) to render it as non-potable fuel ethanol. Generally, 8 to 12 days worth of denatured fuel ethanol production is stored on site (Kwiatkowski, 2006).               All industrial fuel ethanol production uses continuous-feed distillation column systems. Distillation is a common chemical separation method that is based upon differences in volatility (Wankat, 1988). If a mixture of ethanol and water are placed in a container at a given temperature and pressure, after time the mixture will reach equilibrium. At equilibrium, some of the ethanol will be a vapor in the gas above the liquid and some will be in the liquid phase. Similarly, some of the water will be in the vapor phase and some will be in the liquid phase. Because ethanol is more volatile than water (boils at a lower temperature), the ratio of ethanol to water in the vapor phase is greater than the ratio of ethanol to water in the liquid phase. This characteristic allows for the separation of the ethanol from the water.   Through subsequent vaporization of the mixture, condensation, re-vaporization, and re-condensation the mixture becomes higher and higher in ethanol content because the vapor at each vaporization step has higher ethanol concentration than the liquid from which it was vaporized. Thus, multiple fractionation steps can be used to purify ethanol from water. However, the basic principle by which this occurs – difference in boiling points between water and ethanol – ceases to exist when the mixture is 95.6 wt% ethanol (4.4% water). At this point (the azeotrope), the ethanol and water both vaporize to the same degree and cannot be further fractionated by distillation. Either a third solvent can be introduced to break the azeotrope (e.g. beneze) or an alternate seperation method can be used such as adsorption of water using molecular sieves. Molecular sieves for drying ethanol are crystalline metal zeolites (aluminosilicates) with a 3-dimensional porous structure of silica and alumina tetrahedra. Zeolites strongly and preferentially adsorb water from vapor/gas mixtures. The adsorbed water can be removed by increasing the temperature of the zeolite and passing dry gas over the particles, thus allowing this rather expensive desiccant (~$10/lb) to be reused (Al-Asheh, 2004).   While this drying property was discovered with naturally occurring zeolites, commercial molecular sieves are synthetically produced to have highly uniform pores within a tight size distribution. Industrial molecular sieve drying systems consist of multiple columns each filled with a bed of uniform sized zeolite sieves. The ethanol/water vapor mixture leaving the fractional distillation system is super-heated and forced through the molecular sieve bed. The water vapor is selectively adsorbed to the particles while ethanol passes through the column, where it is recovered and condensed to liquid at high purity. After the capacity of the zeolites to adsorb water from the ethanol vapor is reached, feed vapor is stopped and the flow through the column is reversed. Dry gas (usually CO2 produced by the fermentation process) passes over the zeolites while the system is placed under a slight vacuum to drive the desorption of the water from the solid particles.  The water vapor and residual ethanol vapor exiting the column is condensed and returned to the stripper in the distillation system to re-vaporize the residual ethanol which improves the overall efficiency of ethanol recovery by the plant (Ladisch, 1979).   To achieve continuous processing, molecular sieve dehydration systems consist of pairs of beds. As the first bed in the pair processes wet ethanol, a second molecular sieve bed undergoes regeneration to remove the adsorbed water. When the capacity of the first column to remove water is filled, the duties of the columns are switched so that the wet column begins regeneration and the fresh column continues to process wet ethanol vapor.   An alternative to zeolite molecular sieves is using ground corn (corn grits) in a packed bed, similar in design to conventional molecular sieve beds (Chang, 2006; Neuman, 1986; Westgate, 1992). In this system, currently in commercial use by Archer Daniel Midlands, water is selectively adsorbed to corn starch from a water-ethanol vapor mixture (Beery, 1998; Beery, 2001; Ladisch, 1979). Similar in design to the zeolite molecular sieve beds, the corn grit system operates in pairs of beds with one drying ethanol vapor while the other(s) are undergoing regeneration to remove the adsorbed water. The major advantages of the bio-based adsorbents are that they are readily available, less expensive than molecular sieves, mechanically stable, and easily disposed (Ladisch, 1997).   Industrial fractional distillation to produce fuel ethanol is one of the major energy inputs for the production of fuel ethanol.  Process improvements that capture and recycle energy from the process has greatly reduced the cost of this step (Gulati, 1996; Ladisch, 1979).

Coproduct Production and Handling
Co-product production and handling. The wet milling of corn produces ethanol and three co-products—germ, corn gluten meal, and corn gluten feed. 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. This may be done in the same facility as the ethanol plant or sold to a secondary processor.   Corn gluten meal is the protein rich (about 60%) product separated from the gluten-starch slurry and sold as a high-protein animal feed (Ham, 1994). The residual corn fiber separated from the starch-gluten slurry after the second milling step is mixed with evaporated light stillage and dried to produce corn gluten feed, a medium-grade protein product (20-26% of dry matter), which is also used as animal feed (Loe, 2006). Corn gluten feed is similar in fiber content, lower in protein content, and much lower in oil content compared to Distiller’s Dried Grains and Solubles (DDGS) produced in the corn ethanol dry grind process (Ham, 1994). 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 (May, 1987).   References   Al-Asheh, S.; Banat, F., and Al-Lagtah, N. (2004). Separation of ethanol-water mixtures using molecular sieves and biobased adsorbents. Chemical Engineering Research and Design, 82(A7), 855-864. Bayrock, D., and Ingledew, W. M. (2001). Changes in steady state on introduction of a Lactobacillus contaminant to a continuous culture ethanol fermentation. Journal of Industrial Microbiology and Biotechnology, 27(1), 39-45. Beery, K. E.; Gulati; M., Kvam, E. P., and Ladisch, M. R. (1998). Effect of enzyme modification of corn grits on their properties as an adsorbent in a skarstrom pressure swing cycle dryer. Adsorption-Journal of the International Adsorption Society, 4(3-4), 321-335. Beery, K. E., and Ladisch, M. A. (2001). Chemistry and properties of starch based desiccants. Enzyme and Microbial Technology, 28(7-8), 573-581. BeMiller, J. N., and Whistler, R. L. (1996). Carbohydrates. In Food Chemistry (3rd ed., pp. 157-223.). New York, New York: Marcel Deker. Chang, H., Yuan, X. G.; Tian, H., and Zeng, A. W. (2006). Experimental investigation and modeling of adsorption of water and ethanol on cornmeal in an ethanol-water binary vapor system. Chemical Engineering and Technology, 29(4), 454-461. Graves, T.; Narendranath, N. V.; Dawson, K., and Power, R. (2006). Effect of pH and lactic or acetic acid on ethanol productivity by Saccharomyces cerevisiae in corn mash. Journal of Industrial Microbiology and Biotechnology, 33(6), 469-474. Gulati, M.; Westgate, P. J.; Brewer, M.; Hendrickson, R., and Ladisch, M. R. (1996). Sorptive recovery of dilute ethanol from distillation column bottoms stream. Applied Biochemistry and Biotechnology, 57-8, 103-119. Ham, G. A.; Stock, R. A.; Klopfenstein, T. J.; Larson, E. M.; Shain, D. H., and Huffman, R. P. (1994). Wet Corn Distillers by-Products Compared with Dried Corn Distillers Grains with Solubles as a Source of Protein and Energy for Ruminants. Journal of Animal Science, 72(12), 3246-3257. Kapen, W. H. (1996). Chapter 2: Nutritional requirements in fermentative processes. In H. C. Vogel and C. C. Todaro (Eds.), Fermentation and Biochemical Engineering Handbook: Principles, Process Design, and Equipment (2nd ed., pp. 122-160). Westwood, NJ: Noyes Publications. Kwiatkowski, J. R.; Mcaloon, A. J.; Taylor, F., and Johnston, D. B. (2006). Modeling the Process and Costs of Fuel Ethanol Production by the Corn Dry-Grind Process. Industrial Crops and Products, 23, 288-296. Ladisch, M. R. (1997). 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Power, R. F. (2003). Enzymatic conversion of starch to fermentable sugars. In K. A. Jacques; T. P. Lyons and D. R. Kelsall (Eds.), The Alcohol Textbook, 4th Edition (pp. 23-32). Bath, England: Nottingham University Press. Rausch, K. D.; Thompson, C. I.; Belyea, R. L., and Tumbleson, M. E. (2003). Characterization of light gluten and light steep water from a corn wet milling plant. Bioresource Technology, 90(1), 49-54. Renewable Fuels Association, Ethanol Industry Outlook, 2006. Schenck, F. W. (2002). Starch hydrolysates - An overview. International Sugar Journal, 104(1238), 82-+. Shetty, J. K.; Lantero, O. J., and Dunn-Coleman, N. (2005). Technological advances in ethanol production. International Sugar Journal, 107(1283), 605-+. U.S. Department of Agriculture. U.S. Grains Supply and Distribution: Wheat, Corn, Sorghum, Barley, Oats, Rye, and Rice. (2006). Wankat, P. C. (1988). Introduction to Column Distillation. In Equilibrium-Staged Separations. New York, NY: Prentice Hall PTR. 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