Biomass, Biofuels, and BioChemicals: Dry-mill corn ethanol process

In a conventional dry-grind process, corn is ground and mixed with water to produce slurry. The slurry is cooked; starch in the slurry is liquefied, saccharified, and fermented to produce ethanol. The remaining nonfermentables in corn (germ, fiber, and protein) are recovered together at the end of the dry-grind process as an animal food coproduct called distiller dried grains with solubles(DDGS). Due to its lower capital and production investments than wet-mills, it is still the dominant process for corn ethanol production nowadays. Conventionally, the process involves the following steps:

Milling. The feedstock (Corn) passes through a hammer mill to break down the corn kernel into a fine powder.

Cooking. The fine powder flour is mixed with water to obtain ground corn slurry with ~27-37% solids content. The pH of the ground corn slurry can be adjusted with anhydrous ammonia to stabilize within the range 5.5 to 6.5. The slurry is then heated to 85 C (185_F) for 30-45 min, and subsequently cooked with high pressure steam in a jet cooker at 104 C (220 _F). After cooling to 85 C (185 F) and a holding period of 30–45 min, the slurry is reheated and cooked to gelatinize the starch and break down its crystalline structure. The resulting mixture of amorphous starch is called the ‘mash.’

Liquefaction. A unit operation involving an enzymatic digestion of the starch molecules in the mash into imonoeric sugar and oligosaccharides. In this step, alpha-amylase (an endoenzyme ) applied randomly hydrolyzes a-1,4-glucosidic bonds to reduce the viscosity of gelatinized starch, producing soluble dextrins and oligosaccharides.

• The total alpha-amylase dose:0.2 to 0.4 kg enzyme per metric ton ofvdry solids (0.02–0.04 %, w/w).

• The common practice: add one-third of the alpha-amylase prior to cooking and the remainder after the jet cooker step. The resulting liquefied corn mash has a typical composition as: maltotetraose and other soluble dextrins 24–35%; maltotriose 1.7–3.3%; maltose 0.4–2.7%; and glucose 0.1–1.3 %; i.e. a starch hydrolysate that has a dextrose equivalent ranging from 10.4 to 19.1 (The dextrose equivalent is a measure of the percentages of glucosidic bonds that are hydrolyzed during liquefaction.)

Saccharification. After the corn mash cools to 32 C, a second enzyme, glucoamylase (,an exoenzyme) is added (0.05–0.08 %, w/w) to catalyze the release of successive glucose units from the non reducing ends of soluble dextrins by hydrolyzing both linear a-1,4-glucosidic and branched a-1,6-glucosidic linkages, resulting in the fermentable sugars, mainly glucose.

Fermentation. The glucose generated by glucoamylase during saccharification step will be fermented into ethanol by yeasts (typically Saccharomyces cerevisiae) and carbondioxide as a by-product. Saccharomyces cerevisiae is the yeast species commonly selected because of its quick, efficient production of alcohol and its ability to withstand heat, osmotic stress and high alcohol concentrations. The fermentation process generally takes about 50 to 60 hours with a final ethanol concentrations of14%- 20% (v/v) depending on the corn slurry solids contents from improved fermentation process. Batch or continuous fermentation systems may be used, although batch processing is more common due to its simple, easily cleaning, and flexibility. Some new fermentation systems are designed to minimize dilution water, which reduces the evaporation requirements in the feed processing stages after fermentation.

Distillation. Distillation is the process of separating the ethanol from the solids and water in the mash. The fermented mash, now called “beer”, is pumped to the continuous flow, multi-column distillation system where the ethanol is removed from the solids and the water. Conventional distillation/rectification methods can produce 95% pure (190 proof) ethanol because the ethanol and water form an azeotrope so that further separation by heat cannot occur. Therefore, the ethanol leaves the top of the final column in the distillation syetem at about 95% concentration, and the residue mash (stillage), is transferred from the base of the column to the co-product processing area.

Dehydration. The ethanol from the top of the column passes through a dehydration system to remove remaining water to obtain anhydrous ethanol (pure, without water, and approximately 200 proof). Most modern dry-grind ethanol plants use a molecular sieve system to produce pure ethanol.

Stillage Processing. The solid and liquid fraction remaining after distillation is referred to as “whole stillage”, which includes fiber, oil and protein components of the grain, and the non-fermented starch.

The “thin stillage” is first separated from the insoluble solid fraction using centrifuges or presses/extruders and then sent to evaporator units to remove excess water to obtain the thick, viscous syrup, which is mixed back with the solids to create a feed product known as Wet Distillers Grains with Solubles (WDGS) with about 65% moisture.

Due to its low shelf life and high transportation cost, WDGS is usually dried to 10 to 12% moisture to produce a product known as Dried Distillers Grain with Solubles (DDGS). Although drying distillers grains is energy-intensive, consuming about one-third of the energy requirements of the entire dry-grind plant, , it is essential to produce a uniform, stable, high-quality feed co-product to the profitability of the plant, resulting in most plants producing DDGS rather than WDGS.

Typical Yield from a Bushel of Corn from a Dry-Mill Ethanol Plant