Fermentation, one of the oldest chemical processes known to man, is used to make a variety of products, including foods, flavorings, beverages, pharmaceuticals, and chemicals. At present, however, many of the simpler products such as ethanol are synthesized from petroleum feedstock at lower costs. The future of the fermentation industry, therefore, depends on its ability to utilize the high efficiency and specificity of enzyme catalysis to synthesize complex products and on its ability to overcome variations in quality and availability of raw materials.
Ethanol is made from a variety of agricultural products such as grain, molasses, fruit, whey and sulfite waste liquor. Generally, most of the agricultural products mentioned above command higher prices as foods, and others, ex, potatoes, are uneconomical because of their low ethanol yield and high transportation cost. The energy crisis of the early seventies may have generated renewed interest in ethanol fermentation, but its use still depends on the availability and cost of the carbohydrate relative to the availability and cost of ethylene. Sugar and grain prices, like oil prices, have risen dramatically since 1973.
Fermentation processes from any material that contains sugar can derive ethanol. The many and varied raw materials used in the manufacture of ethanol via fermentation are conveniently classified under three types of agricultural raw materials: sugar, starches, and cellulose materials. Sugars (from sugar cane, sugar beets, molasses, fruits etc) can be converted to ethanol directly. Starches (from grains, potatoes, root crops) must first be hydrolyzed to fermentable sugars by the action of enzymes from malt or molds. Cellulose from wood, agricultural residues, waste sulfite liquor from pulp and paper mills) must likewise be converted to sugars, generally by the action of mineral acids. Once simple sugars are formed, enzymes from yeast can readily ferment them to ethanol.
1. From Sugary & Starchy Crops
2. From Cellulosic Crops & Other Biomass
1 Sugary & Starchy Crops
The Sugary & Starchy Crops provide the major contribution in the production of Bioethanol. The basic principle involved in the production of Bioethanol from these feed stocks is Fermentation. Ethanol is produced by yeast fermentation of the sugar extracted from sugarcane or sugar beets. Ethanol produced by fermentation results in a solution of ethanol in water. During ethanol fermentation, glucose is evolved into ethanol and carbon dioxide. The equation is:
C6H12O6 → 2 CH3CH2OH + 2 CO2
Addition of a dilute acid speeds up the process.
For the ethanol to be usable as a fuel, water must be removed. Most of the water is removed by distillation, but the purity is limited to 95-96% due to the formation of a low-boiling water-ethanol azeotrope. The 96% ethanol, 4% water mixture may be used as a fuel, and it’s called hydrated ethyl alcohol fuel. For blending with gasoline, purity of 99.5 to 99.9% is required, depending on temperature, to avoid separation.
Production Facilities for Bioethanol production from these conventional crops are of two types:-
- Wet Mill Process
- Dry Mill Process.
- Wet Mill Process
About half of current ethanol production capacity is associated with wet mills, which are integrated facilities producing a range of products in addition to ethanol such as sweeteners, corn gluten meal, gluten feed, starch, and, increasingly, additional biologically-produced products such as feed supplements.
The entire corn kernel or other starchy grain is first ground into flour, which is referred to in the industry as “meal” and processed without separating out the various component parts of the grain. The meal is slurried with water to form a “mash.” Enzymes are added to the mash to convert the starch to dextrose, a simple sugar. Ammonia is added for pH control and as a nutrient to the yeast.
The mash is processed in a high-temperature cooker to reduce bacteria levels ahead of fermentation. The mash is cooled and transferred to fermenters where yeast is added and the conversion of sugar to ethanol and carbon dioxide (CO2) begins.
The fermentation process generally takes about 40 to 50 hours. During this part of the process, the mash is agitated and kept cool to facilitate the activity of the yeast. After fermentation, the resulting “beer” is transferred to distillation columns where the ethanol is separated from the remaining “stillage.” The ethanol is concentrated to 190 proof using conventional distillation and then is dehydrated to approximately 200 proof in a molecular sieve system.
The anhydrous ethanol is then blended with about 5% denaturant (such as natural gasoline) to render it undrinkable and thus not subject to beverage alcohol tax. It is then ready for shipment to gasoline terminals or retailers.
The stillage is sent through a centrifuge that separates the coarse grain from the solubles. The solubles are then concentrated to about 30% solids by evaporation, resulting in Condensed Distillers Solubles (CDS) or “syrup.” The coarse grain and the syrup are then dried together to produce dried distillers grains with solubles (DDGS), a high quality nutritious livestock feed. The CO2 released during fermentation is captured and sold for use in carbonating soft drinks and beverages and the manufacture of dry ice.
2. Dry Mill Process
The remainder of production capacity, and most of recently-added capacity, is in dry mills. Dry mills are typically smaller than wet mills, and produce ethanol and a single animal feed co product (distiller’s dried grains). Basic steps for dry mill production of ethanol from corn are: refining into starch, liquification and saccharification (hydrolysis of starch into glucose), yeast fermentation, distillation, dehydration (required for blending with gasoline), and denaturing (optional).
In wet milling, the grain is soaked or “steeped” in water and dilute sulfurous acid for 24 to 48 hours. This steeping facilitates the separation of the grain into its many component parts.
After steeping, the corn slurry is processed through a series of grinders to separate the corn germ. The corn oil from the germ is either extracted on-site or sold to crushers who extract the corn oil. The remaining fiber, gluten and starch components are further segregated using centrifugal, screen and hydroclonic separators.
The steeping liquor is concentrated in an evaporator. This concentrated product, heavy steep water, is co-dried with the fiber component and is then sold as corn gluten feed to the livestock industry. Heavy steep water is also sold by itself as a feed ingredient and is used as a component in Ice Ban, an environmentally friendly alternative to salt for removing ice from roads.
The gluten component (protein) is filtered and dried to produce the corn gluten meal co-product. This product is highly sought after as a feed ingredient in poultry broiler operations.
The starch and any remaining water from the mash can then be processed in one of three ways: fermented into ethanol, dried and sold as dried or modified corn starch, or processed into corn syrup. The fermentation process for ethanol is very similar to the dry mill process described above.
II Cellulosic Crops & Other Biomass
Cellulosic feedstock (composed of cellulose and hemicellulose) are more difficult to convert to sugar than are carbohydrates. There are four technologies for bioethanol production as given below. The first three are based on producing sugars from biomass and then fermenting the sugars to ethanol. The fourth is a very different approach involving thermal processing of biomass to gaseous hydrogen and carbon monoxide, followed by fermentation to ethanol.
- Concentrated Acid Hydrolysis This process is based on concentrated acid decrystallization of cellulose followed by dilute acid hydrolysis to sugars. Separation of acid from sugars, acid recovery, and acid reconcentration are critical unit operations. Fermentation converts sugars to ethanol. The robustness of this process makes it well suited to complex and highly variable feed stocks like municipal solid waste to take advantage of relatively high tipping fees available in the area for collection and disposal of municipal solid waste.
- Dilute Acid Hydrolysis Hydrolysis occurs in two stages to maximize sugar yields from the hemicellulose and cellulose fractions of biomass. The first stage is operated under milder conditions to hydrolyze hemicellulose, while the second stage is optimized to hydrolyze the more resistant cellulose fraction. Liquid hydrolyzates are recovered from each stage, neutralized, and fermented to ethanol.
Both the dilute and concentrated acid processes have several drawbacks. Dilute acid hydrolysis of cellulose tends to yield a large amount of byproducts. Concentrated acid hydrolysis forms fewer byproducts, but for economic reasons the acid must be recycled. The separation and reconcentration of the sulfuric acid adds more complexity to the process. In addition, sulfuric acid is highly corrosive and difficult to handle. The concentrated and dilute sulfuric acid processes are performed at high temperatures (100 and 220 oC) which can degrade the sugars, reducing the carbon source and ultimately lowering the ethanol yield.
- Enzymatic Hydrolysis The greatest potential for ethanol production from biomass, however, lies in enzymatic hydrolysis of cellulose. The enzyme cellulase can be used at lower temperatures, 30 to 50 oC, which reduces the degradation of the sugars. In addition, process improvements now allow simultaneous saccharification and fermentation (SSF). In the SSF process, cellulase and fermenting yeast are combined, so that as sugars are produced, the fermentative organisms convert them to ethanol in the same step. In the long term, enzyme technology is expected to have the biggest payoff.
The current high cost of cellulase enzymes is the key barrier to economical production of bioethanol from lignocellulosic material; research is on to achieve a tenfold reduction in the cost of these enzymes.
- Biomass Gasification and Fermentation Biomass can be converted to synthesis gas (consisting primarily of carbon monoxide, carbon dioxide, and hydrogen) via a high temperature thermal gasification process. Anaerobic bacteria are then used to convert the synthesis gas into ethanol.
Bioresource Engineering Inc. has developed synthesis gas fermentation technology that can be used to produce ethanol from cellulosic wastes with high yields and rates. The feasibility of the technology has been demonstrated, and plans are under way to pilot the technology as a first step toward commercialization. The conversion of a waste stream, the disposal of which is costly, into a valuable fuel adds both environmental and economic incentives. The yields can be high because all of the raw material, except the ash and metal, is converted to ethanol.
The above process flow diagram shows the basic steps in production of ethanol from cellulosic biomass. Note that there are a variety of options for pretreatment and other steps in the process and that several technologies combine two or all three of the hydrolysis and fermentation steps within the shaded box. Chart courtesy of the National Renewable Energy Lab.
Ethanol Fermentation with Bacteria
A great number of bacteria are capable of ethanol formation. Many of these microorganisms, however, generate multiple end products in addition to ethyl alcohol. These include other alcohols (butanol, isopropylalcohol, 2,3-butanediol), organic acid (acetic acid, formic acid, and lactic acids), polyols (arabitol, glycerol and xylitol), ketones (acetone) or various gases (methane, carbon dioxide, hydrogen).
Bacterial Species Which Produce Ethanol As The Main Fermentation Product
|Mesophilic Organisms||mmol Ethanol Produced perMmol Glucose Metabolized|
|Clostridium sporogenes||up to 4.15 a)|
|Clostridium indolis(pathogenic)||1.96 a)|
|Clostridium sphenoides||1.8 a) (1.8) b)|
|Zymomonas mobilis(syn. Anaerobica)||1.9(Anaerobe)|
|Zymomonas mobilisSsp. Pomaceas||1.7|
|Spirochaeta aurantia||1.5 (0.8)|
|Spirochaeta stenostrepta||0.84 (1.46)|
|Spirochaeta litoralis||1.1 (1.4)|
|Sarcina ventriculi(syn. Zymosarcina)||1.0|
- In the presence of high amounts of yeast extract
- Values in brackets were obtained with resting cells.
Many bacteria (i.e. Enterobacteriaceas, Spirochaeta, Bacteroides, etc.) metabolize glucose by the Embden-Meyerhof pathway. Briefly, this path utilizes 1 mol of glucose to yield 2 mol of pyruvate which are then decarbosylated to acetaldehyde and reduced to ethanol. Beside that the Entner-Doudoroff pathway is an additional means of glucose consumption in many bacteria.
As reported by Naim Kosaric et.al. (1983), for Z. mobilis on synthetics media containing either glucose, fructose or sucrose, the specific rates of sugar uptake and ethanol production are at a maximum when utilizing the glucose medium
Ethanol Fermentation with Yeast
The organisms of primary interest to industrial operations in fermentation of ethanol include Saccharomyces cerevisiae, S. uvarum, Schizosaccharomyces pombe, and Kluyueromyces sp. Yeast, under anaerobic conditions; metabolize glucose to ethanol primarily by way of the Embden-Meyerhof pathway. The overall net reaction involves the production of 2 moles each of ethanol, but the yield attained in practical fermentations however does not usually exceed 90 – 95% of theoretical. This is partly due to the requirement for some nutrient to be utilized in the synthesis of new biomass and other cell maintenance related reactions.
A small concentration of oxygen must be provided to the fermenting yeast as it is a necessary component in the biosynthesis of polyunsaturated fats and lipids. Typical amounts of O2 maintained in the broth are 0.05 – 0.10 mm Hg oxygen tension.
The relative requirements for nutrients not utilized in ethanol synthesis are in proportion to the major components of the yeast cell. These include carbon oxygen, nitrogen and hydrogen. To leaser extent quantities of phosphorus, sulfur, potassium, and magnesium must also be provided for the synthesis of minor components. Minerals (i.e. Mn, Co, Cu, Zn) and organic factors (amino acids, nucleic acids, and vitamins) are required in trace amounts.
Yeasts are highly susceptible to ethanol inhibition. Concentration of 1-2% (w/v) is sufficient to retard microbial growth and at 10% (w/v) alcohol, the growth rate of the organism is nearly halted.
Distillation systems can either be batch or continuous. Choosing one system or the other is based on plant scale. Both types require heating systems, usually steam (which can be from low-pressure boilers), a distillation column, and a condenser.
Distillation column size and ethanol production rate are based on the concentration of ethanol in the fermented mash, fermentation capacity, and production schedules. Small-scale plants–up to about 100,000 liters annual ethanol production–can efficiently use batch distillation systems. In batch systems, the entire mash volume is passed, or charged, to a large vessel called a still, which is then heated. The vapors are allowed to pass into the distillation column. Though batch systems are less efficient than continuous feed distillation systems, they are much easier to build and operate.
In continuous feed systems, fermented mash is pumped at a controlled rate into the distillation column, with heat introduced at the bottom of the column. Provision is made at the top of the column to feed unprocessed mash back through the system. Continuous feed columns should be used in large-scale plants where the improved efficiency justifies the added complexity.
The intended use of the ethanol determines the need for dehydration systems to remove the five percent water that cannot be separated by distillation. If ethanol is to be blended with gasoline, dehydration is required. The presence of water in ethanol gasoline-blends results in phase separation in storage or fuel tanks. Dehydration is not required if ethanol is to be used to replace gasoline. Ethanol can be used directly in modified engines at concentrations of between 80 and 95 percent.
When 95% ethanol is heated, the proportion of water present in the vapor phase is the same as that in the liquid phase. It becomes azeotropic ethanol/water mixture. Therefore, simple distillation is no more helpful in further purification/concentration of ethanol. A small amount of benzene is added to 95% ethanol, which is then distilled.
The distillate is nearly 100% ethanol, and the benzene can also be recovered. Thus recovery of pure ethanol requires considerable amounts of energy, which raises its production cost; this has been the chief reason for the limited use of this valuable biofuel for transport purposes.
Solid by-products are recovered from stillage with solid/liquid separation equipment. This equipment can range from simple screens to such complex equipment as centrifuges or vacuum filters. Soluble protein in thin stillage can be recovered by evaporation. If by-products are to be stored or transported significant distances, drying is necessary. Stillage with high moisture content can often be fed directly to livestock at or near the site of ethanol production with minimal separation or processing.
Production of each volume of fuel ethanol will generate about nine volumes of effluent. A portion of the effluent can be recycled and used to dilute high concentration feedstocks. However, even if the effluent is recycled, it can still cause a significant pollution problem. To avoid pollution of surface water or ground water, the effluent must undergo microbiological degradation; that is, the harmful organic matter contained in the effluent must be broken down before the effluent is disposed of. This is done anaerobically, aerobically, or by a sequential combination of the two methods. Effluent degradation is usually done in a simple treatment pond, followed by a stabilization pond, if necessary. Alternatively, the effluent can be fed to biogas digesters, combining energy production with waste treatment.
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