How Ethanol is made
Basically, fermentation is a process in which micro-organisms such as yeasts convert simple sugars to Ethanol and carbon dioxide. Some plants directly yield simple sugars; others produce starch or cellulose that must be converted to sugar. The sugar obtained must be fermented, and the resulting "beer" must then be distilled to obtain fuel grade Ethanol.
Feedstocks can be selected from among many plants that either produce simple sugars directly (sugarcane, sweet sorghum) or produce starch (corn, grain sorghum). Feedstock preparation will vary with the feedstock, but some features are universal:
• sugarcane or sorghum must be crushed to extract their simple sugars.
• Starchy and cellulosic materials must be physically broken down by milling or grinding to break starch walls so that the material is available to water. Later steps break down the individual cell walls of the starch.
Starches are converted to sugars in two stages, liquefaction and saccharification, by adding water, enzymes, and heat (enzymatic hydrolysis).
From the corn plant to the production plant.
Most Ethanol is made from corn, but the production process is versatile. It works with most sugar-containing plant materials, such as sorghum, wheat, barley and potatoes! In fact, Ethanol producers in Brazil - the world's largest maker of Ethanol - use sugar cane to start the process.
The production process of Ethanol from corn plant is as under :
The corn is ground into small particles. This exposes the cornstarch, which will be used for the fermentation process. The cornstarch is removed and milled into a fine powder. The remaining grain material - protein, fiber, vitamins and minerals - is used for livestock feed.
2. Cooking (Liquefaction)
The cornstarch powder is mixed with water and alpha-amylase, an enzyme that helps break the starch into smaller particles. The resulting mash is cooked at 120 to 150 degrees to liquefy the starch and reduce bacteria levels, and then heated to 225 degrees to help break the starch down further.
3. Sweet (Saccharification)
The mash is removed from the cookers and cooled. Then a second enzyme, glucoamylase, is added to help convert the liquid starch into a sugar (dextrose) that can be fermented.
The mash is mixed with yeast, which changes the sugar to Ethanol and carbon dioxide. It takes about 48 hours for the mash to ferment.
The fermented mash contains about 10 percent Ethanol. The rest of the mixture is water and corn/yeast solids that couldn't be fermented. To separate the Ethanol, the mixture is heated once again - this time to a temperature at which Ethanol vaporizes, but the remaining materials do not. The Ethanol vapor is collected and cooled, where it condenses to its liquid form.
To purify the Ethanol and remove any remaining water, it's passed through a dehydration system, creating anhydrous Ethanol (anhydrous means "without water"). After this step, the Ethanol is approximately 200 proof ... which explains the need for step seven.
7. Potent, but not potable
To make the Ethanol unfit for human consumption - a requirement for all fuel-grade Ethanol - a small amount of gasoline is added (2 percent to 5 percent).
8. Distillers Grain and Carbondioxide
The leftovers, or co-products, of the process - distiller's grain and carbon dioxide - are saved. Distiller's grain is a highly nutritious livestock feed, and carbon dioxide is collected, purified, compressed, and sold for use by the carbonated beverage and dry-ice industries.
Production from Fermenting Sugar
Ethanol is produced both as a petrochemical, through the hydration of ethylene, and biologically, by fermenting sugars with yeast. Which process is more economical is dependent upon the prevailing prices of petroleum and of grain feed stocks.
Ethanol fermentation is the biological process by which sugars such as glucose, fructose, and sucrose, are converted into Ethanol and carbon dioxide. Yeasts carry out Ethanol fermentation on sugars in the absence of oxygen. Because the process does not require oxygen, Ethanol fermentation is classified as anaerobic. Ethanol fermentation is responsible for the rising of bread dough, the production of Ethanol in alcoholic beverages, and for much of the production of Ethanol for use as fuel.
The chemical process of fermentation
The chemical equation below summarizes Ethanol fermentation, in which one hexose molecule is converted into two Ethanol molecules and two carbon dioxide molecules:
The process begins with a molecule of glucose being broken down by the process of glycolysis into pyruvate:
This reaction is accompanied by the reduction of two molecules of Nicotinamide adenine to carbon dioxide
• Ethanol respiration is used to create bubbles in bread.
• Ethanol fermentation is responsible for the rising of bread dough. Yeast organisms consume sugars in the dough and produce Ethanol and carbon dioxide as waste products. The carbon dioxide forms bubbles in the dough, expanding it into something of a foam. Nearly all the Ethanol evaporates from the dough when the bread is baked.
• The production of all alcoholic beverages employs Ethanol fermentation by yeast. Wines and brandies are produced by fermentation of the natural sugars present in fruits, especially grapes. Beers, ales, and whiskeys employ fermentation of grain starches that have been converted to sugar by the application of the enzyme, amylase, which is present in grain kernels that have been germinated. Amylase-treated grain or amylase-treated potatos is fermented for the production of vodka. Fermentation of cane sugar is the first step in producing rum. In all cases, the fermentation must take place in a vessel that is arranged to allow carbon dioxide to escape, but that prevents outside air from coming in, as exposure to oxygen would prevent the formation of Ethanol.
• Similar yeast fermentation of various carbohydrate products is used produce much of the Ethanol used for fuel.
Feedstocks for fuel production
Ethanol for use in alcoholic beverages, and the vast majority of Ethanol for use as fuel, is produced by fermentation. When certain species of yeast, most importantly, Saccharomyces cerevisiae, metabolize sugar in the absence of oxygen, they produce Ethanol and carbon dioxide. The chemical equation below summarizes the conversion:
The process of culturing yeast under conditions to produce alcohol is called brewing. Ethanol's toxicity to yeast limits the Ethanol concentration obtainable by brewing. The most Ethanol-tolerant strains of yeast can survive up to approximately 15% Ethanol by volume.
The fermentation process must exclude oxygen. If oxygen is present, yeast undergo aerobic respiration which produces carbon dioxide and water rather than Ethanol.
In order to produce Ethanol from starchy materials such as cereal grains, the starch must first be converted into sugars. In brewing beer, this has traditionally been accomplished by allowing the grain to germinate, or malt, which produces the enzyme, amylase. When the malted grain is mashed, the amylase converts the remaining starches into sugars. For fuel Ethanol, the hydrolysis of starch into glucose can be accomplished more rapidly by treatment with dilute sulfuric acid, fungally produced amylase, or some combination of the two.
The product of either ethylene hydration or brewing is an Ethanol-water mixture. For most industrial and fuel uses, the Ethanol must be purified. Fractional distillation can concentrate Ethanol to 95.6% by weight (89.5 mole%). The mixture of 95.6% Ethanol and 4.4% water (percentage by weight) is an azeotrope with a boiling point of 78.2 °C, and cannot be further purified by distillation. Because of the difficulty of further purification, 95% Ethanol/5% water is a fairly common solvent. There are several methods used to further purify Ethanol beyond 95.6%:
Addition of an entrainer
The Ethanol-water azeotrope can be broken by the addition of a small quantity of benzene or cyclohexane. Benzene, Ethanol, and water form a ternary azeotrope with a boiling point of 64.9 °C. Since this azeotrope is more volatile than the Ethanol-water azeotrope, it can be fractionally distilled out of the Ethanol-water mixture, extracting essentially all of the water in the process. The bottoms from such a distillation is anhydrous Ethanol, with several parts per million residual benzene. Benzene is toxic to humans, and cyclohexane has largely supplanted benzene in its role as the entrainer in this process. However, this purification method leaves chemical residues which render the alcohol unfit for human consumption.
Alternatively, a molecular sieve can be used to selectively absorb the water from the 95.6% Ethanol solution. Synthetic zeolite in pellet form can be used, as well as a variety of plant-derived absorbents, including cornmeal,
straw, and sawdust. The zeolite bed can be regenerated essentially an unlimited number of times by drying it with a blast of hot carbon dioxide. Cornmeal and other plant-derived absorbents cannot readily be regenerated, but where Ethanol is made from grain, they are often available at low cost. Absolute Ethanol produced this way has no residual benzene, and can be used to fortify port and sherry in traditional winery operations.
Membranes can also be used to separate Ethanol and water. The membrane can break the water-Ethanol azeotrope because separation is not based on vapor-liquid equilibria. Membranes are often used in the so-called hybrid membrane distillation process. This process uses a pre-concentration distillation column as first separating step. The further separation is then accomplished with a membrane operated either in vapor permeation or pervaporation mode. Vapor permeation uses a vapor membrane feed and pervaporation uses a liquid membrane feed.
At pressures less than atmospheric pressure, the composition of the Ethanol-water azeotrope shifts to more Ethanol-rich mixtures, and at pressures less than 70 torr (9.333 kPa) , there is no azeotrope, and it is possible to distill absolute Ethanol from an Ethanol-water mixture. While vacuum distillation of Ethanol is not presently economical, pressure-swing distillation is a topic of current research. In this technique, a reduced-pressure distillation first yields an Ethanol-water mixture of more than 95.6% Ethanol. Then, fractional distillation of this mixture at atmospheric pressure distills off the 95.6% azeotrope, leaving anhydrous Ethanol at the bottoms.
Ethylene hydration or brewing produces an Ethanol-water mixture. For most industrial and fuel uses, the Ethanol must be purified. Fractional distillation can concentrate Ethanol to 95.6% by weight (89.5 mole%). This mixture is an azeotrope with a boiling point of 78.1 °C, and cannot be further purified by distillation.
In one common industrial method to obtain absolute alcohol, a small quantity of benzene is added to rectified spirit and the mixture is then distilled. Absolute alcohol is obtained in the third fraction, which distills over at 78.3 °C (351.4 K). Because a small amount of the benzene used remains in the solution, absolute alcohol produced by this method is not suitable for consumption, as benzene is carcinogenic.
There is also an absolute alcohol production process by desiccation using glycerol. Alcohol produced by this method is known as spectroscopic alcohol — so called because the absence of benzene makes it suitable as a solvent in spectroscopy.
Other methods for obtaining absolute Ethanol include desiccation using adsorbents such as starch or zeolites, which adsorb water preferentially, as well as azeotropic distillation and extractive distillation.
Types of Ethanol
Denatured alcohol is Ethanol which has been rendered toxic or otherwise undrinkable, and in some cases dyed. It is used for purposes such as fuel for spirit burners and camping stoves, and as a solvent. Traditionally, the main additive was 10% mEthanol, which gave rise to methylated spirits. There are diverse industrial uses for Ethanol, and therefore literally hundreds of recipes for denaturing Ethanol. Typical additives are mEthanol, isopropanol, methyl ethyl ketone, methyl isobutyl ketone, denatonium, and even (uncommonly) aviation gasoline.
In the phrase denatured alcohol, denatured means "a specific property of Ethanol, its usefulness as a beverage, is removed". The Ethanol molecule is not denatured in the sense that its chemical structure is altered.
There is no duty on denatured alcohol in most countries, making it considerably cheaper than pure Ethanol. Consequently, its composition is tightly defined by government regulations which vary between countries. Different additives are used to make it both unpalatable and poisonous in such a way that is hard to rectify through distillation or other simple processes. MEthanol is commonly used for this in part because it has a boiling point close to that of Ethanol, and separating it by distillation is difficult, but not impossible as mEthanol and Ethanol form a zeotropic mixture (the opposite of an azeotropic mixture). In many countries, it is also required to be dyed blue or purple with an aniline dye.
The tax-exempt status for denatured alcohol dates from the mid-19th century. For instance the United Kingdom introduced legislation in 1855 to permit Ethanol containing 10% wood-naphtha to be exempt
Pure Ethanol and alcoholic beverages are heavily taxed. Ethanol has many applications that do not involve human consumption. To relieve the tax burden on these applications, most jurisdictions waive the tax when agents have been added to the Ethanol to render it unfit for human consumption. These include bittering agents such as denatonium benzoate, as well as toxins such as mEthanol, naphtha, and pyridine.
Absolute or anhydrous alcohol generally refers to purified Ethanol, containing no more than one percent water. Absolute alcohol not intended for human consumption often contains trace amounts of toxic benzene.
Pure Ethanol is classed as 200 proof in the USA, equivalent to 175 degrees proof in the (now rarely used) UK system.
Liquefaction, or the breakdown of starch to complex sugars, requires:
• thoroughly mixing prepared feedstock with water;
• adjusting pH of the mixture to a level suitable for the enzyme being used;
• thoroughly mixing in the appropriate proportions of liquefaction enzyme (alpha-amylase) for the quantity of starch to be converted; and
• heating the grain mash. This breaks the cell walls of the starch. The free starch will gelatinize as the temperature increases, forming a thick mash. As the mash reaches the enzyme's optimum temperature, the enzyme chemically breaks down the starch to complex sugars (dextrins). When this liquefaction stage is complete, the mash appears soupy, as it did before gelatinization.
Saccharification, or the breakdown of complex sugars to simple sugars involves:
• cooling the mash to the optimum temperature for the saccharifying enzyme;
• adjusting the pH of the mash to the level required by the enzyme;
• mixing the appropriate proportions of saccharifying enzyme (glucoamylase) needed to convert the
available sugar; and
• holding the pH and temperature (122 - 140°F) in the optimum range and stirring constantly until saccharification is complete, which is determined by testing for sugar content.
At this point the starch has been broken down to the simple sugar glucose and is now in a form which micro-organisms called yeasts can feed on. Yeasts, in metabolizing glucose, produce Ethanol and carbon dioxide. As with the enzymes, yeasts have an optimum temperature range.
• The mash is transferred to the fermentation tank and cooled to the optimum temperature (around 80 - 90°F). Care has to be taken to assure that no infection (other organisms that compete with the yeast for
the glucose) occurs.
• The appropriate proportion of yeast is added.
• The yeast will begin producing alcohol and should turn the mash into a "beer of 8-12 percent alcohol and then become inactive as the alcohol content becomes too high".
The mash is now ready for distillation. Separating the liquid beer from the solids of the mash stillage at this stage will help prevent possible clogging problems during distillation.
Distillation separates the Ethanol from the beer, which is mostly water and Ethanol. (in some alcohol plants, distillation takes place in one, very tall column; the process diagrammed above uses two separate columns, a stripper column and a rectifying column).
Ethanol boils at 172°F ( at sea level), while water boils at 212°F. By heating the beer to 172°F, the Ethanol can be boiled off and the vapor captured and condensed to produce 192-proof (96 percent) Ethanol concentration producible by conventional distillation. 200-proof (anhydrous) alcohol (which is required for blending gasohol) can be obtained through additional dehydration steps. Lower-grade Ethanol (170-190 proof) can be used by itself in vehicles modified for alcohol use.