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| Ethanol |
Ethanol, sometimes called grain alcohol, is an alcohol produced by fermentation of carbohydrates from a broad range of plant matter for many uses in the chemical industry. It has potentially significant use as a gasoline replacement and is also the primary alcohol component of alcoholic beverages.
Ethanol is produced by carbohydrate fermentation processes, hydration of ethylene, and, to a lesser extent, reduction of acetaldehyde obtained from acetylene. Also called ethyl alcohol, alcohol, and grain alcohol, ethanol is a colorless liquid with a mild and distinct aroma and taste.
It has a boiling point of 78.3 degrees Celsius and a melting point of 114.5 degrees Celsius. Ethanol is completely soluble in water and most organic solvents. It has a flash point of 8 degrees Celsius and is thus highly flammable.
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Ethanol undergoes numerous commercially important reactions and is thus a vital industrial chemical. It has been used as a partial replacement for gasoline (the hybrid fuel is called “gasohol”). As the major component of alcoholic beverages, ethanol has been known and recognized for thousands of years.
Primary Uses
Alcohol obtained from fermentation processes is generally included with other fermentation products and extracts from the carbohydrate-rich grains, fruits, and so on that are the raw materials for the many alcoholic beverages produced and consumed.
Alcohol produced by yeast fermentation is obtained at a maximum concentration of 14 percent; therefore, alcoholic beverages other than beer and nonfortified wines require the addition of concentrated alcohol, which is obtained by distilling dilute alcohol from the fermentation of molasses and other sugar sources. In the United States and other highly industrialized countries, the alcohol added to beverages is increasingly being produced by other methods.
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| Ethanol as replacement for gasoline |
Ethanol is also used in large quantities for chemical synthesis in the organic chemical industry. It is used for the preparation of numerous esters vital to many polymer industries and for the production of diethyl ether (also called ether or ethyl ether), a major solvent.
Other synthetic procedures lead to the manufacture of acetaldehyde, acetic acid, ethyl halides, and acetonitrile, which are in turn employed for the preparation of drugs, explosives, adhesives, pesticides,detergents, synthetic fibers, and other substances. Ethanol itself is used in vast quantities as a solvent.
Ethanol is added to gasoline to reduce air pollution, and it is frequently considered to be a likely replacement for gasoline when petroleum resources decline or drastically increase in price.
Gasohol, ethanol combined with varying amounts of gasoline, is being vigorously promoted and is already in use in Brazil and elsewhere. Biomass conversion (conversion of plant matter) to ethanol by cost efficient methods will speed the entry of gasohol into large-scale use in the industrial world.
Production Processes
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| Ethanol production process |
Until biomass conversion becomes more widely used, carbohydrate fermentation processes are destined to be a decreasingly important source of industrial ethanol. Beverage alcohol is produced from a great variety of sources, including grains, potatoes, and fruit, but fermentation-based industrial alcohol is almost entirely obtained by yeast fermentation of molasses.
Molasses (50 percent sucrose residue from sugar processing) is diluted with water to approximately 15 percent and under slightly acidic conditions is fermented by yeast to give 14 percent ethanol.
Fractional distillation of the solution yields the commercial product: 95 percent ethanol. Approximately two and one-half gallons of blackstrap molasses is needed to make a gallon of 190-proof ethanol. (Alcohol content is usually described in terms of its proof value, which is twice its ethanol percentage.)
Although ethylene hydration was known in the early part of the nineteenth century, it was not until 1929 that it became an industrial process. Today it is the dominant method of producing ethanol.
Ethylene, obtained from the thermal cracking of petroleum fractions or from natural gas separation processes, is treated with complex phosphoric acid-based catalysts at temperatures above 300 degrees Celsius and steam at pressures of thousands of pounds per square inch.
The ethanol can be fractionally distilled, and the residual ethylene can be recycled. Ethylene can also be passed into concentrated sulfuric acid, and after hydrolysis the ethanol can be distilled from the diluted sulfuric acid.
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| Research for better ethanol |
The 95 percent alcohol produced by any of these methods can be converted to nearly pure (absolute) alcohol by removal of the water by azeotropic distillation using benzene or trichloroethylene.
Trace amounts of the hazardous benzene or trichloroethylene remaining make the absolute alcohol undesirable for beverage purposes but useful for industrial purposes in which the 5 percent water interferes with use requirements.
