The Dry Grind Corn to Ethanol Process

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National Corn-to-Ethanol Research CenterSouthern Illinois UniversityEducation and Workforce Development400 University Park DriveEdwardsville, Illinois 62025-3604Office: (618) 659-6737 ext. 239Fax: (618) 659-8762

www.ethanolresearch.com

A copyright statement is as follows:----------------------------Copyright © 2007, Board of Trustees of Southern Illinois University, National Corn to Ethanol Research Center, Edwardsville, Illinois 62026All rights reserved.These materials are protected by copyright and may not be reproduced, copied, displayed, or disseminatedin any media without the express consent of the National Corn to Ethanol Research Center----------------------------

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Yeast7

Hopper

Hammer Mill

Slurry Tank Jet

Cooker

CO2

Enzyme

4

Enzyme

4

Fermentation

Denatured Ethanol Molecular

Sieve 10

Distillation System

CentrifugeEvaporator

(Syrup)Drum Dryer

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Liquefaction 8

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Dried DistillersGrain with Solubles

The Corn to Ethanol ProcessNational Corn-to-Ethanol Research Center, SIUE

5% Gasoline

Added

(200 Proof )

(190 Proof )

(WholeStillage)

Start Here

Ammonia

Steam

(Recycled Water)

Urea

(Cooling)

( Wet Cake)

(ThinStillage)

Water

(Heating)

Enzyme

6

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© 2007 All Rights Reserved

The Dry-Grind Corn to Ethanol Process

The dry-grind ethanol fermentation process (from kernel to ethanol) requires approximately 54 to 58 hours. The time consuming step is the actual fermentation which requires almost 50 hours.

One bushel of corn (56 pounds) yields approximately 2.8 gallons of ethanol, 18 pounds of carbon dioxide, and 18 pounds of Dried Distillers Grain with Solubles (DDGS).

Midwest Corn typically contains: 70-72% starch, 9% protein, 4% oils, and 9% fiber on a dry basis, plus approximately 15% water.

1

Hopper

Most corn grown for ethanol production is #2 yellow dent corn, so named because of the indentation that occurs on top after the kernel is allowed to dry.

The corn kernels are milled to a coarse flour and passed through a fine mesh screen.

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Hammer Mill

The corn particle sizing is a compromise between:

(B) leaving large enough particles for drying the Distiller’s Grain at the end for animal feed.

(A) grinding fine enough to provide increased surface area to make starch granules available for reaction with water and enzymes and

Slurry Tank

3 Ammonia orSulfuric Acid

(Recycled Water)

Water

Corn flour from the hammer mill is mixed first in the slurry mixer with hot water and alpha-amylase enzyme.

The resulting slurry is adjusted to a pH of 5.6 to 6.0 with acid or base.

Hammer Mill

Slurry Mixer

(Corn flour)

SlurryTank

alpha-amylase

3

Only one third of the total alpha-amylase is added in the slurry Tank.

The remaining alpha-amylase will be added later in the liquefaction tank.

Temperature = 185 oF pH = 5.6 to 6.0 % Solids = 32%

Jet Cooker

Steam

Slurry Mash

The slurry mash is heated to above 220°F in a jet cooker using direct steam.

holding column

Upon exiting the jet cooker, the corn slurry enters a holding column where the mixture is held for 5 to 10 minutes.

Liquefaction Tank

alpha-amylase

5

From the holding column, the slurry enters a Liquefaction Tank (5).

The jet cooking and liquefaction steps break apart the long starch granules. The shorter molecules are called dextrin.

holding column

Temperature = 185 oF pH = 5.6 to 6.0 % Solids = 30%

Additional alpha-amylase is added and the mash is allowed to react (liquefy) for approximately 30 minutes.

Glucose is converted by yeast (7) through a series of multi-step reactions to ethanol and carbon dioxide in the fermentation tanks (8).

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CO2

Fermentation Tanks

SSFYeast

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8

Liquefaction Tank

A second enzyme, gluco-amylase (6), is added to complete the dextrin breakdown to glucose.

gluco-amylase6

Temperature = 90 oF pH = 3.5 to 4.0

Over the past 10 years, many ethanol plants have started adding the saccharifying enzyme (gluco-amylase) directly to the fermentation tank. This faster process is know as Simultaneous Saccharification and Fermentation (SSF).

CO2

Carbon dioxide is a major co-product in the fermentation process.

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Fermentation Tanks

Every bushel of corn (approximately 56 pounds) will produce approximately 18 pounds of carbon dioxide gas.

Yeast can withstand extreme environmental stresses including high ethanol concentrations (final concentrations of 12-18% by volume), as well as organic acids produced by contaminating bacteria. Fortunately, most bacterial contaminants do not grow below pH 4.

Yeast7 Fermentation

Tanks

Contaminating microorganisms can lower the yield by converting glucose to some undesirable fermentation products like fusel oils (undesired alcohols), acetic acid, and lactic acid. Antibiotics maybe added to the fermentation process to minimize bacterial contamination.

Distillation System

At the end of fermentation, the product is called beer and contains 12% or higher concentration of ethanol. The beer is typically stored in a tank referred to as the beer well.

Beer Well

The distillation system typically consists of three columns.

(beer)

First, the beer is passed through a degassing column (gray) to remove carbon dioxide and other gases.

Distillation SystemBeer Well

Centrifuge

(gases) Molecular Sieve

(whole stillage)

Separation of ethanol/water from the non-converted solids mixture occurs on the beer column (blue). These non-converted solids (whole stillage) fall to the bottom and are sent to the centrifuge for separation.

Further separation of the ethanol/water mixture is accomplished using a rectifier column (red). The 190 proof ethanol is sent through a molecular sieve column to convert it to 200 proof.

Conventional distillation methods yield 190 proof (95% pure ethanol) because ethanol and water form an azeotrope. An azeotrope is simply a mixture of two substances that form a constant boiling point mixture.

Molecular Sieve 10

DenaturedEthanol Tank

from Rectifier Column

200 Proof

190 Proof

Temperature > 280 oF Pressure = 6 psig

The remaining 5% water is removed by molecular sieves, which rely on pore sizes to separate the smaller water molecules from ethanol.

Finally, anhydrous (100% or 200 proof) ethanol is denatured, typically with 5% gasoline to exempt the ethanol from beverage alcohol taxes.

5% Gasoline

AddedDenatured Ethanol

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Molecular Sieve

Centrifuge whole stillage

wet cake

thin stillage

The solids materials remaining after distillation of ethanol from the beer column is called whole stillage. Whole stillage contains 13-17% solids and is composed primarily of small particles of corn that did not get converted to ethanol. This whole stillage is separated in the centrifuge into wet cake and thin stillage.

Wet cake is a more concentrated form of the whole stillage and contains approximately 35% solids after leaving the centrifuge. Since it contains mostly solids, it must be augured or conveyed to the drum dryer.

Evaporator

(syrup)

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Centrifuge

Drum Dryer

backset

(wet cake)

(thin

stillage)

The thin stillage is primarily water with between 4 and 5% solids. The thin stillage is evaporated down to a concentrated syrup in the evaporator. The condensed water is recycled to the Slurry tank. This condensed water is called backset and helps to conserve total water usage.

The remaining liquid is concentrated (called syrup) by evaporation and mixed with the wet cake before entering the dryer. Syrup is approximately 28-30% solids and contains mostly protein and oils from the corn. The addition of syrup increases the nutritional value of the DDG.

Rotary Drum DryerDDGS Centrifuge (wet cake)

Evaporator (syrup)

The mixture of syrup and wet cake is dried to generate Dried Distillers Dried Grain with Syrup or Solubles (DDGS). DDGS is typically dried to a 10% moisture level.

The moisture content and correspondingly short shelf-life of wet distillers grains limit use of this feed product to the immediate vicinity of ethanol plants.

The shelf life of the DDGS can be lengthened by adding organic acids as preservatives and removing almost all the moisture content.

Inlet Air Temperature = 600-1200 oF

DDGS Residence time = approximately 30 minutes

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Distillers DriedGrain with Solubles

In the dry grind process, corn is ground whole and fermented to ethanol. The only major by-products are ethanol, carbon dioxide, and Distillers Dried Grain with Solubles.

DDGS is sold primarily as cattle feed.

NCERC

Hopper

Hammer Mill

Slurry Tank Jet

Cooker Fermentation

Ethanol Storage

Molecular Sieve

Distillation System

Evaporator(Syrup)Drum Dryer

Liquefaction

9

9

Centrifuge

DDGS

(200 Proof )

(190 Proof )

WholeStillage

(Recycled Water)

(Cooling)

( Wet Cake)

(ThinStillage)

(Heating)

TM

Centrifuge

DETS %pH

DEpH pH

HPLC

H2O %

ProximateAnalysis

TS%

Proximates

Amino AcidsTotal P

Ashetc.

HPLC

Particle Size Grain Qual