What is Ethanol?
Ethanol, also known as ethyl alcohol or grain alcohol, is a renewable
alcohol fuel made from grain starch or cellulosic materials. The various
grains used include wheat, corn, and sorghum. Cellulosic materials used
include sugar cane, grass, and even old newspapers. Ethanol is the same
whether it is produced from grains such as corn, as it is in the United States,
or from cellulosic materials such as sugar cane, as it is in Brazil. Corn is the
most commonly used resource in the United States because it is very
abundant. In the United States ethanol is produced in over 110 ethanol
production facilities, with more than 75 currently under construction. This
growth in production shows that ethanol is becoming an important part of
the United States fuel supply, and is able to reduce the use of oil. Pure
ethanol cannot be used alone to fuel vehicles, it must be mixed with
gasoline. For example, E10 is a mixture that is composed of 10% ethanol
and 90% gasoline. Currently the use of ethanol as a fuel is common, but this
has not always been the case, as is shown in ethanol’s history.
History of Ethanol
Ethanol was first used by humans as the intoxication ingredient in
alcoholic beverages in China as far back as 9,000 years ago. Pure ethanol
was first obtained in 1796 by Johann Tobias Lowitz by filtering ethanol
through charcoal. In 1826, ethanol was prepared synthetically by Henry
Hennel and S.G. Serullas. Up to this point, ethanol had not been used as a
source of fuel for vehicles, but in 1908 the Ford Model T was designed to
run on pure alcohol. Ethanol continued to be used into the 1920’s and
1930’s, but then its use declined and interest was lost. In the early 1970’s
interest in ethanol was renewed due to the Arab Oil Embargo of 1973. This
embargo made the U.S. dependence on oil evident, as people lined up for
miles at gas stations. Eventually an economic recession occurred and the
stock market declined. It was clear at this time that the United States needed
to find an alternative to foreign oil and gasoline. Over the past 30 years the
U.S. government has passed many policies that have greatly increased the
production and use of ethanol as a fuel source. In 2007, 6.2 billion gallons
of ethanol was produced in the U.S., compared to only 1.3 billion gallons in
1997.
Terms
Distillation: a process used to separate the components of a liquid mixture
by boiling the liquid and then recondensing the resulting vapor
Fermentation: the breakdown of carbohydrates by microorganisms into
sugars, specifically yeast in the case of ethanol production
Denaturant: a substance added to ethanol to make it unfit for human
consumption
Enzyme: a large molecule, usually a protein, that helps to speed up a
chemical reaction
Stillage: composed of solids from the corn and yeast, as well as water,
separated from the ethanol during distillation
Meal: a fine flour that is produced after corn kernels are ground up by
passing through a hammer mill
Mash: a moist and soggy mixture composed of meal, water, and various
enzymes
Course Grain: leftover component of the corn kernel after the starch is
removed, contains high concentrations of protein and fat
*Terms are bolded in the text
Ethanol Co-Products
There are several other products, commonly known as co-products,
that are produced during the processing of corn into ethanol. These coproducts include condensed distillers solubles (CDS), wet distillers grains
(WDG), dried distillers grains (DDG), and dried distillers grains with
solubles (DDGS). They all contain high levels of protein, fat, minerals, and
vitamins because ethanol production only uses the starch portion of the corn.
Carbon dioxide is also produced, but is not considered a co-product. It is
usually captured and sold by many plants to be used to carbonate beverages
and produce dry ice.
Condensed distillers solubles (CDS), or syrup, is the liquid portion of
the stillage that is produced during distillation. CDS can be sold on its own
or mixed with dried distillers grains. Distillers grains are the grain part of
the stillage, and can be sold without being dried as wet distillers grain
(WDG), or it can be dried resulting in dried distillers grains (DDG). WDG
is more difficult to transport because it spoils easy, and thus has a much
shorter shelf life. Dried distillers grains is more common, and is sent
through dryers to remove some of the moisture, making it easier to store and
transport. The last type of distillers grain is dried distillers grains with
solubles (DDGS), which is dried distillers grains and condensed distillers
solubles mixed together. Due to their nutrient composition, co-products
make excellent feed sources for cattle, swine, poultry, and fish. They are
also usually cheaper than other feeds making them an economical choice for
many livestock producers.
Ethanol Processing
So how is a bushel of corn (56 lbs) transformed into 2.7 gallons of
fuel that makes our cars run? The process of making ethanol may seem very
complicated, but it is actually very similar to other well known processes,
such as the brewing of beer. Both ethanol and beer production use
fermentation and distillation, but there are differences between the two.
One difference is that there are two different processes that can be used to
extract ethanol from corn. There is the dry mill process which is the most
common, accounting for 82% of production, and there is the wet mill
process which is much less common, accounting for only 18% of
production. The main difference between these two process is how the grain
is initially treated. In dry milling, the entire corn kernel is ground into flour,
also called meal, and processed without separating out the different parts of
the kernel. In wet milling, the corn is soaked, or steeped, in water and
sulfuric acid which separates it into its various components. Since dry
milling is the most common way to produce ethanol that is the process that
will be described below.
Dry milling can be broken down into eleven different steps. These
steps are labeled one through eleven on Figure 1.
1.) Corn from local producers is delivered to the ethanol plant via truck
or railcar, and unloaded into large storage bins that are designed to hold
enough grain to supply the plant for 7-10 days. Many ethanol plants are
located along major highways, interstates, or railroads so they can take
advantage of the close transportation system, which reduces transport time
and the associated cost.
2.) The delivered corn is screened to remove any debris, such as stalks or
cobs. The clean corn then passes through a hammer mill which grinds the
kernels into a fine flour, which in the ethanol industry is referred to as meal.
3.) The meal is transferred to a mixing tank where water and an enzyme,
called alpha-amylase is added, producing what is called mash. The function
of the alpha-amylase enzyme is to convert the starch to dextrose, which is a
sugar. Ammonia is also added to the mash to control the pH and to be a
nutrient for the yeast that will be added in a later step.
4.) The mash is then pumped to a jet cooker where it is heated to 210o F
for five minutes. The function of the heat is to liquefy the starch and reduce
bacteria levels. After five minutes the mash is cooled.
5.) The cooled mash is transferred to saccharification or liquefaction
tanks. Here the mash is held for one to two hours at 180-190o F, giving the
alpha-amylase enzyme time to convert the starch to dextrose. A second
enzyme, glucoamylase, is added here to break the dextrose produced by the
alpha-amylase enzyme into simple sugars.
6.) The mash now travels to the fermentation tank through heat
exchangers, cooling it to 90o F. The temperature of the mash can be no
higher than this because it would kill the yeast added during fermentation.
7.) Once in the fermentation tank, yeast, a microorganism, is added to
the mash to ferment, or breakdown, the sugars into ethanol and carbon
dioxide. The carbon dioxide is often captured and sold for use in soft drinks
and dry ice. The fermentation process takes about 50 hours, during which
time the mash is agitated and kept cool to facilitate the activity of the yeast.
After the yeast has broken down all the sugars fermentation is complete, and
the end result is called beer, which is approximately 15% ethanol and 85%
solids that are unable to be fermented.
8.) The beer is transferred to distillation columns where the ethanol is
separated from the stillage (solids from corn and yeast, water) by
continuously pumping the beer through two or more columns. By the time
the ethanol is ready to leave the distillation columns, it is 95% ethanol by
volume, or 190 proof.
9.) The 190 proof ethanol still contains about 5% water so it is transferred
to a molecular sieve to dehydrate the ethanol, removing any remaining
water. After dehydration is complete the pure ethanol is 200 proof
(waterless). Before the ethanol is sent to storage tanks a small amount
(~5%) of denaturant, usually gasoline, is added to make it unfit for human
consumption and to avoid taxes on alcoholic beverages.
10.) The denatured ethanol is then pumped to large storage tanks where it
awaits shipment by truck or railcar to gasoline stations.
11.) The stillage that was left over from the distillation process in step 8
is now sent through a centrifuge that separates the coarse grain from the
solubles. The solubles, which are mostly liquid with 5-10% solids, are
concentrated to 25% solids by evaporation, producing condensed distillers
solubles (CDS), or syrup. The coarse grain can either be dried or left wet,
producing dried distillers grains (DDG) or wet distillers grains (WDG),
respectively. If the coarse grain and syrup are dried together the result is
dried distillers grains with solubles (DDGS). The CDS, WDG, DDG, and
DDGS all make excellent feed for livestock.
In the end, ethanol production provides us with energy that can be used
to fuel cars and feed livestock.