Process Design Considerations for the Production of Ethanol from Corn Stover Alexandra Williams, Rheagan Chambers, Brandon Landry, Samuel Beck

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Table of Contents

Abstract………………………………………………………………………………………….3

Objective………………………………………………………………………………………...3

Introduction………………………………………………………………………………...……3

General Flowchart………………………………………………………………………….…....5

Detailed Flowchart………………………………………………………………….…………...5

Ethanol Production……………………………………………………………….…………...…6

Raw Materials………………………………………………………………..................6

Production Summary……………………………………………………..………...…...6

Pretreatment………………………………………………………………..…………....6

Hydrolysis…………………………………………………………………………….....7

Commercially Established Hydrolysis and Fermentation Strategies…………….….......8

Purification…………………………………………………………………..................10

Chosen Methods…………………………………………………………………………………12

Raw Material……………………………………………………………………………12

Pretreatment……………………………………………………………………………..12

Hydrolysis/Fermentation………………………………………………………………..13

Purification……………………………………………………………………………...14

Mass Balance…………………………………………………………………………………….14

Final Flowchart…………………………………………………………………………………..15

Cost Analysis…………………………………………………………………………………….16

Conclusion……………………………………………………………………………………….16

References………………………………………………………………………………………..17

Table of Figures

Figure 1…………………………………………………………………………………………..3

Figure 2…………………………………………………………………………………………..5

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Figure 3…………………………………………………………………………………………..5

Table 1…………………………………………………………………………………………...7

Table 2……………………………………………………………………………………….......8

Figure 4………………………………………………………………………………………......9

Figure 5………………………………………………………………………………..………....9

Figure 6…………………………………………………………………………………..……..10

Figure 7……………………………………………………………………………..............…..10

Figure 8………………………………………………………………………………................11

Figure 9…………………………………………………………………………………………11

Figure 10………………………………………………………………………………………..12

Figure 11………………………………………………………………………………………..14

Figure 12………………………………………………………………………………………..15

Table 3………………………………………………………………………………………….16

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ABSTRACT:

This paper details a proposed process design consideration for the conversion of corn

stover into fuel grade ethanol on an industrial scale. The production process includes

pretreatment using physicochemical methods of hammer milling and alkali treatment, hydrolysis

and fermentation using the Simultaneous Saccharification and Fermentation (SSF) method, and

purification using molecular sieves.

OBJECTIVE:

The objective is to evaluate current bio-ethanol production processes from corn, in order

to design an alternative efficient, continuous process.

INTRODUCTION:

Ethanol was first used in 1826 to power an engine developed by Samuel Morey. In 1876,

Nicolaus Otto, the inventor of the internal combustion engine, also chose ethanol as his fuel

source. Unfortunately for the citizens, a $2 tax was imposed on alcohol including ethanol during

the Civil War. Before the war, ethanol was readily used as illuminating oil in the United States.

After the tax was placed on the ethanol, it was too expensive to be used in this way. Henry Ford

once said, “There is fuel in every bit of vegetable matter that can be fermented. There's enough

alcohol in one year's yield of an acre of potatoes to drive the machinery necessary to cultivate the

fields for a hundred years” [1]. Henry Ford used ethanol to fuel the quadricycle, his first

automobile, in the late 1800’s. Henry Ford claimed that ethanol would be the fuel of the future.

His statement and use of ethanol opened multiple doors to the production and use of biofuels. In

order to make the production of ethanol more efficient, technology was created with the main

purpose of making it more cost effective, environmentally beneficial, and to have a greater

energy output to input ratio [2].

About fifty years after the tax on ethanol was imposed, Congress removed it and made

ethanol an alternative to gasoline. During World War I, the use of military vehicles increased;

therefore, the need for fuel increased. With that, the demand of ethanol rose to about fifty-five

million gallons per year. After World War I, the use of ethanol for fuel was reduced due to the

fact that there was no longer a need for war materials. At this time, foreign oil fuels were also

cheaper. From the late 1940’s to the early 1970’s, ethanol was essentially unused in the United

States. In 1974, the Solar Energy Research, Development, and Demonstration Act led to the

advancement of turning organic materials into useful fuels [3]. The United States began to

promote ethanol usage once again. In the Energy Tax Act of 1978, the term “gasohol” was

coined. Gasohol refers to a “blend of gasoline with at least 10% alcohol by volume, excluding

alcohol made from petroleum, natural gas, or coal” [4].

Amoco Oil Company was the first to market alcohol-blended fuel. Congress proposed tax

incentives to encourage companies to produce and blend ethanol. Congress also placed an import

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fee on ethanol that was produced in foreign countries in order to minimize the number of citizens

that purchased foreign fuels. Around 1990, the production plants converted their power source

from coal to natural gas. This switch helped to reduce the cost of production. Automobile

manufacturers began to produce “flex-fuel” vehicles that would operate using E-85(mixture of

85% ethanol and 15% gasoline) or gasoline. Farmers began to alter their land to produce enough

crop such as corn and soybean to keep up with the demand for ethanol [3].

Ethanol is a clear, odorless chemical made through the process of fermentation using raw

materials such as sugars and starches from biomasses and agricultural crops. Because ethanol is

made from renewable, plant-based resources, it can be classified as an “eco-friendly” fuel.

Ethanol is non-toxic, water

soluble, and quickly

biodegradable [5]. Ethanol

is also known as ethyl

alcohol or grain alcohol

and can be abbreviated as

EtOH. It is an organic

molecule that falls under a

class of compounds

known as alcohols. It’s

structural formula is CH3CH2OH. Ethanol serves many different purposes. It can be used by

industries as a solvent in the synthesis of other organic chemicals. It can also be added to

gasoline to produce a fuel with higher-octane levels and fewer harmful emissions than unblended

gasoline [5]. Ethyl alcohol can also be found in cough suppressants, mouthwashes, and alcoholic

beverages.

Ethanol offers many benefits that gasoline does not. The need of a fuel alternative to

gasoline stems from the concern of preserving the environment. Using biofuels rather than fossil

fuels reduces the emissions of unwanted products in the engine exhaust such as carbon

monoxide, toxic emissions, and volatile organic compounds [6]. Because ethanol is about 35%

Oxygen, the combustion products of ethanol are considered to be “eco-friendly”. Anhydrous

ethanol, also known as absolute ethanol, is the standard ethanol used in vehicle fuels. It consists

of at least 99.5% ethanol by volume. Presently, Brazil and the United States are the top two

producers of ethanol. Brazil produces a large amount of sugarcane which is a considerable

feedstock for the production of ethanol. Due to Brazil’s large crop size and large bio-ethanol

program, it is not necessary for them to rely on importation of gasoline. The United States

produces more ethanol than any other country [7]. The main difference in ethanol production

between the two countries is the source of feedstock. The United States uses corn as their raw

material rather than sugar cane. Brazil produces fuel with 20% anhydrous ethanol, while the

United States produces mainly E-10(10% ethanol and 90% gasoline) [8]. Ethanol’s high octane

levels aid in cleaning out the vehicle fuel system; in turn, the vehicle will run with more

optimum performance. Overall, ethanol is a great alternative to gasoline due to it’s ability to

reduce toxic emissions; therefore, reducing pollution.

Figure 1. Four main steps involved in ethanol production from

lignocellulosic material [1].

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GENERAL FLOWCHART:

DETAILED FLOWCHART:

Figure 2. Main steps involved in ethanol production from corn stover [1].

Figure 3. Possible methods for each main step.

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ETHANOL PRODUCTION:

Raw Materials

The selection of feedstock starts with the consideration of the cost to extract and convert

the biomass to biofuel [9]. Ethanol can be produced from many raw materials including sugar-

containing feedstock, starchy materials, and lignocellulosic biomass. Considering it’s great

availability and relatively low cost, the largest potential feedstock for ethanol is lignocellulosic

biomass which includes agricultural materials such as corn stover [2]. Corn stover consists of the

leaves and stalks of maize plants.

Production Summary

Due to the complex structure of lignocellulosic biomass, the main challenge with using

these materials as feedstock is the pretreatment step [9]. Pretreatment is required for degradation

in order to expose the fermentable polysachharides such as cellulose. Next, hydrolysis is done to

break down the exposed cellulose into simpler sugars. Eventually, the sugars released in

hydrolysis will be converted to ethanol through fermentation. Various procedures are done to

separate the broth from the ethanol produced to ensure a purified product.

Pretreatment

The first major step in the production process of harvesting ethanol from corn stover

includes the disruption of the crystalline structure found in lignocelluloses to expose

fermentable, polysaccharides inside, such as cellulose and hemicellulose. More specifically,

pretreatment breaks the bonds of macro- and microfibrils. Pretreatment processes can be

subdivided into four major techniques: Physical, Physico-Chemical, Chemical, and Biological.

Physical treatment options encompass more simplistic, traditional methods which include

mechanical comminution, pyrolysis, and irradiation [10].

Physico-Chemical treatment options utilize both physical and chemical forces to initiate

the breakdown of macro- and microfibrils contained in lignocelluloses. Autohydrolysis,

Ammonia Fiber Explosion (AFEX), CO2/SO2 Explosion, and Hydro-thermal are all techniques

in this treatment option. In general, these methods rely on different superheated chemicals that

are allowed to mix with the lignocelluloses. A sudden drop in pressure of the system causes a

decompression explosion of the lignocelluloses, which causes inner cellulose to become exposed

or easily penetrable to enzymes [11].

Chemical treatments rely on chemical kinetics to provide substantial pretreatment. Most

commonly, dilute-acid treatment is used in this category. Other ways of chemically treating the

lignocelluloses are via ozonolysis, alkaline, oxidative delignification, and organosolv processes.

Biological pretreatment is another method that is quickly becoming a more viable way to

accomplish the same task done through the more typical methods that only rely on physical and

chemical properties. In biological treatment, use of certain fungi and actinomycetes are used to

act as biological catalysts [12].

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Hydrolysis

Following pretreatment processes, the now partially degraded fibrils of the

lignocelluloses and the accompanying complex sugars are broken down into more simple five

and six carbon sugar compounds. This process is known as hydrolysis. Hydrolysis is normally

done either by enzymatic or acid-base techniques. In enzymatic hydrolysis, it is possible to

achieve 100% hydroxylation. However, this method is more expensive and can take a number of

days to complete [9]. In addition to extended time necessary to hydrolyze, enzymatic hydrolysis

is negatively affected by inhibitory molecules which block the cellulase enzyme from properly

Table 1. Pretreatment Methods for Lignocellulosic Enzymatic Hydrolysis

[10].

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degrading sugars. Key factors that govern the yields in this process include: substrate

concentration, substrate quality, temperature, pressure, pH, applied pretreatment, and cellulase

activity.

Contrary to enzymatic hydrolysis, acid-base hydrolysis can take place in a matter of

minutes. Unfortunately, acid-base hydrolysis also faces some challenging set-backs which

include corrosive conditions onset by the inherent low pH’s and high temperatures which elevate

overall energy expenditure [10].

Commercially Established Hydrolysis and Fermentation Strategies

Hydrolysis and Fermentation steps in the processing pathway for ethanol production are

often linked into successive stages and are commonly designed to complement one another. Five

common strategies used for this purpose are Separate Enzymatic Hydrolysis and Fermentation

(SHF), Simultaneous Saccharification and Hydrolysis (SSF), Non-isothermal Simultaneous

Saccharification and Fermentation (NSSF), Simultaneous Saccharification and Co-fermentation

(SSCF), and Consolidated Bioprocessing (CBP). General differences amongst these five

commonly used processes are in the alterations of the number of reaction vessels required,

separation or consolidation of hydrolysis (cellulase) and fermentation (microorganism such as Z.

mobilis) [13], and the ability to ferment both pentose and hexose sugars together or individually.

All the processes include a centrifugation step, which removes solid biomass from the slurry

after leaving the reaction vessel(s).

The reactors used in all of the following can be of the following type: batch, plug flow,

percolation, countercurrent, and shrinking-bed.

Separate Enzymatic Hydrolysis and Fermentation (SHF) specify that the hydrolysis and

fermentation reaction steps occur in separate reactors. This design criterion is desired since the

optimum temperature ranges for the enzyme (45- 50 °C) and active microorganism (30-37 °C) lie

in different ranges. By allowing each step to take place under the best conditions, better yields

are possible and fewer enzymes are necessary [11].

Table 2. Comparison between Dilute-Acid and Enzymatic Hydrolyses [11].

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Simultaneous Saccharification and Hydrolysis (SSF) requires that the sugars produced

from the hydrolysis process be consumed directly by active microorganisms. In essence, SSF is a

one step process that only requires a single reaction vessel for either five or six carbon sugars.

Advantages of SSF over SHF are reduced contamination risk and less overall complexity. SSF is

the preferred method with lignocellulosic biomass. Complications with SSF include increased

inhibition of cellulase by ethanol. In concentrations as low as 30g/L, the cellulase enzyme is

slowed by 25% [13].

Figure 5. Simplified flow diagram for Simultaneous Saccharification and

Hydrolysis (SSF) [13].

Figure 4. Simplified flow diagram for Separate Enzymatic Hydrolysis and

Fermentation (SHF) [13].

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Non-isothermal Simultaneous Saccharification and Fermentation (NSSF) was first

implemented to address the issues seen in SSF with respect to the lowered temperatures negative

effect on the cellulase enzyme. In this process, saccharification and fermentation take place in

two separate reactors at the proper optimal temperature of 50°C. This change increases the

productivity of cellulase by a factor of 2-3. Since the enzyme is more effective, only 60-70% of

the total enzyme used in SSF is needed for similar yields [13].

In Simultaneous Saccharification and Co-fermentation (SSCF), there are both pentose

and hexose sugars being fermented within the same reaction vessel. Only a single reactor is used,

meaning hydrolysis is occurring at the same time of fermentation [13].

Consolidated Bioprocessing (CBP) is a new and emerging processing technique used to

harvest ethanol directly from thermophilic microorganisms that produce ethanol. Current

Figure 6. Simplified flow diagram for Non-isothermal Simultaneous

Saccharification and Fermentation (NSSF) [13].

Figure 7. Simplified flow diagram for Simultaneous Saccharification

and Co-fermentation (SSCF) [13].

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research is being done to genetically modify some of these microbes in order to create a specific

strain of bacteria that has increase yield values and reduce the amount of harmful by-products.

Since cultures of bacteria are the only necessary “vessel”, the economic and logistical impacts

for this method are a great advantage [13].

Purification

The level of purification necessary to meet the expectations of ethanol used in gasoline is

relatively high. Impurities such as water contaminants must be removed from the ethanol after

hydrolysis and fermentation are complete. To do this, the industry standard is to use a series of

molecular sieves which trap

water molecules based on

their unique molecular

weight property [14].

Other chemical

processes for purifying

ethanol are also feasible. For

example, treating the ethanol

with sulfuric acid and

calcium hydride during

multiple distillation events

can rid ethanol of virtually all

aldehyde, ketone, and water

impurities (less than 1ppm)

[15].

If the desired goal is

to remove only water contaminants so that an anhydrous ethanol product is formed, a saline

extraction process can be performed. This method is a refinement of the standard azeotropic

Figure 8. Simplified flow diagram for Consolidated Bioprocessing (CBP) [13].

Figure 9. Molecular Sieve Diagram [15].

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distillation process, which consumes considerable more energy when the saline extraction step is

not included in the feed. The reduction in energy consumed is contributed to the increased

separation of ethanol and water in the presence of a salt. The conservation of energy during the

final purification step is crucial since nearly 50-80% of all energy required is consumed in this

step [16].

CHOSEN METHODS:

Raw Material

Knowing that corn stover is the largest quantity of biomass residue in the United States,

we chose corn stover as our feedstock for ethanol production. There are about 120,000,000 tons

of biomass residue available for use each year which has the potential of supplying 23 billion to

53 billion liters of fuel ethanol [17]. Corn stover is composed of 70% cellulose and

hemicellulose. 15-20% of corn stover is lignin. Only cellulose and hemicellulose can actually be

converted to ethanol, but lignin can be burned to generate steam/electricity.

A vast majority of the cost to create biofuels comes from the price to ship the raw

materials. Therefore, to minimize costs, a biofuel factory should be located in the Corn Belt [18].

The Corn Belt is in the Midwest region of the United States including western Indiana, Illinois,

Missouri, Iowa, eastern Kansas, and eastern Nebraska. In this area, corn and soybeans are the

dominant crops. The nights are warm and the days are hot. There is well-distributed rainfall

during the growing season. These conditions are ideal for growing corn. As the production of

corn increases, the production of corn stover also increases.

Pretreatment

The process of pretreatment is considered to be one of the expensive steps in the

conversion of lignocellulosic biomass to ethanol. The first major step in the production process

of harvesting ethanol from corn stover includes the disruption of the crystalline structure found

in lignocelluloses to expose fermentable, polysaccharides inside such as cellulose and

hemicellulose. More specifically, pretreatment breaks the bonds of macro- and microfibrils.

Figure 10. Simplified flow diagram of extractive distillation

using salt (saline) as the solvent [16].

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Pretreatment processes can be subdivided into four major techniques: Physical, Physico-

Chemical, Chemical, and Biological. The chosen pretreatment approach for this process is

physico-chemical. The physical component of the pretreatment will include a hammer mill. The

hammer mill is composed of fixed cutting blades and floating hammers. The corn stover would

be deposited onto a conveyor belt that feeds the stover into the feed hopper of the mill. The

rotating hammers, usually going between 190 to 270 mph, hit the corn stover to crush them into

small particles [8]. The rotating hammers are covered with a screen that allows particles of a

specific size to pass through while the larger particles continue to rotate and be subjected to more

grinding. In order to increase the efficiency of this process, we require small particles that can

easily absorb water and gelatinize. If the large particles of corn stover are not reduced, they will

later become impurities in our final product. The hammer mill is suitable for reducing corn

stover size because the pore size of the interchangeable screens are no larger than 2 or 5 mm [8].

The chemical component of the pretreatment step is the use of calcium oxide, which is an

alkali treatment. Cellulose digestibility and lignin solubilization are greatly increased by the use

of alkali pretreatments. The use of calcium oxide, or lime, increases the alkalinity which

improves biomass digestibility and contributes to better biomass conservation. Some benefits of

alkali pretreatment use are that it is very flexible with respect to time, and it only requires room

temperature. Calcium oxide was selected for this process because it is a cost effective

pretreatment strategy, and it yields high levels of glucose and xylose from corn stover especially

when also subjected to enzymatic hydrolysis [19].

Hydrolysis/Fermentation

Of the many hydrolysis and fermentation methods, Simultaneous Saccharification and

Fermentation (SSF) is one of the most successful methods for ethanol production from

lignocellulosic materials. This method is a combination of the enzymatic hydrolysis of pretreated

lignocelluloses and fermentation. Combining hydrolysis and fermentation proves to be more

cost-effective. One main advantage of SSF is that the glucose produced by the cellulolytic

enzymes (Trichoderma reesei) is immediately consumed by the fermenting microorganism

(Saccharomyces cerevisiae) that is present in the culture [20]. Keeping the sugars at a low

concentration in the media ensures that the inhibition effects of cellobiose and glucose to the

enzymes are minimized. SSF also produces more ethanol than Separate Enzymatic Hydrolysis

and Fermentation (SHF) and requires less energy. The presence of ethanol reduces risk of

contamination. Since SSF produces more ethanol, it will have a lower risk of contamination.

Optimum conditions such as temperature and pH are essential for the enzymatic

hydrolysis and fermentation. However, it is difficult to find an optimum temperature for both the

hydrolyzing enzymes and fermenting organisms. Between 45-50˚C is the optimum temperature

for Trichoderma reesei. For Saccharomyces cerevisiae, the optimum temperature is 30-35˚C. The

optimum temperature for SSF using these two microorganisms is around 38˚C [20].

A drawback of SSF is that the ethanol produced can inhibit cellulose. “It was reported

that 30g/L ethanol reduces the enzyme activity by 25%” [11]. Despite any drawback of the SSF

method, it is the preferred method in many laboratories and pilot scale studies for ethanol

production.

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Purification

The level of purification necessary to meet the expectations of ethanol used as fuel is

relatively high. Impurities such as water contaminants must be removed from the ethanol after

hydrolysis and fermentation are complete. For this process design, the industry standard was the

chosen method, which is to use a series of molecular sieves. The molecular sieves trap water

molecules based on their unique molecular weight property. Molecular sieves are hard, granular

substances, spherical or cylindrical extrudates manufactured from materials such as potassium

aluminosilicates [21]. They are graded based on nominal diameter of the internal pores. For

ethanol dehydration, the typical grade used is Type 3Å [21]. This means that the average

diameter of the interstitial passageways is 3 Angstroms. The water molecules have a mean

diameter less than 3Å, and the ethanol molecules have a mean diameter greater than 3Å. In

addition, the water molecules can be adsorbed onto the internal surface of the passageways in the

molecular sieve structure. It is this combination of physical properties that make molecular

sieves useful for the separation of mixtures of ethanol and water.

MASS BALANCE:

Figure 11. Mass balance for ethanol production.

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The ethanol conversion process can be modeled in a mass balance equation utilizing a

mass rate basis of flowing materials. Reactions include ethanol synthesis. Overall, the process

should be in steady state so there is no accumulation of ethanol in the process; therefore,

dE/dt=0. The mass of ethanol can be calculated by isolating the out rate and substituting

reactions shown in the Figure 11.

FINAL FLOWCHART:

Calcium oxide

Figure 12. Final detailed flowchart for ethanol production from corn stover.

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COST ANALYSIS:

The cost analysis for the production of ethanol from corn stover is shown in Table 3. The

analysis includes all necessary equipment and materials.

CONCLUSION:

There are many advantages to using ethanol as a fuel over gasoline. It is ecofriendly,

renewable, and it’s production has the potential to boost the economy by creating more jobs.

Corn stover for feed is readily available and abundant. Beginning this process with 57 tons/hr of

corn stover that goes through physico-chemical pretreatment, simultaneous saccharification

fermentation, and purification yields 16.9 tons/hr of fuel grade ethanol. The yield produced can

then be used in E10 or E85 blend for consumer utilization. Through creating efficient processes

for ethanol production, the United States can reduce it’s dependency on imported oil.

Greenhouse gas emissions will also be greatly reduced.

Calcium Oxide Table 3. Cost analysis for ethanol production.

Calcium Oxide Calcium oxide

Calcium oxide

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