Ethanol is produced both as a petrochemical, through the hydration of ethylene, and biologically, by fermenting sugars with yeast.[80] Which process is more economical is dependent upon the prevailing prices of petroleum and of grain feed stocks.
[edit] Ethylene hydration
Ethanol for use as industrial feedstock is most often made from petrochemical feed stocks, typically by the acid-catalyzed hydration of ethylene, represented by the chemical equation
C2H4(g) + H2O(g) → CH3CH2OH(l).
The catalyst is most commonly phosphoric acid,[81] adsorbed onto a porous support such as diatomaceous earth or charcoal. This catalyst was first used for large-scale ethanol production by the Shell Oil Company in 1947.[82] The reaction is carried out with an excess of high pressure steam at 300°C.
In an older process, first practiced on the industrial scale in 1930 by Union Carbide,[83] but now almost entirely obsolete, ethylene was hydrated indirectly by reacting it with concentrated sulfuric acid to produce ethyl sulfate, which was then hydrolyzed to yield ethanol and regenerate the sulfuric acid:[19]
For more details on this topic, see Ethanol fermentation.
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 (e.g., Saccharomyces cerevisiae) metabolize sugar they produce ethanol and carbon dioxide. The chemical equation below summarizes the conversion:
C6H12O6 → 2 CH3CH2OH + 2 CO2.
The process of culturing yeast under conditions to produce alcohol is called fermentation. 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.[84]
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.[85]
[edit] Cellulosic ethanol
Main article: Cellulosic ethanol
Sugars for ethanol fermentation can be obtained from cellulose.[86][87] Until recently, however, the cost of the cellulase enzymes capable of hydrolyzing cellulose has been prohibitive. The Canadian firm Iogen brought the first cellulose-based ethanol plant on-stream in 2004.[88] Its primary consumer so far has been the Canadian government, which, along with the United States Department of Energy, has invested heavily in the commercialization of cellulosic ethanol. Deployment of this technology could turn a number of cellulose-containing agricultural by-products, such as corncobs, straw, and sawdust, into renewable energy resources. Other enzyme companies are developing genetically engineered fungi that produce large volumes of cellulase, xylanase, and hemicellulase enzymes. These would convert agricultural residues such as corn stover, wheat straw, and sugar cane bagasse and energy crops such as switchgrass into fermentable sugars.[89]
Cellulose-bearing materials typically also contain other polysaccharides, including hemicellulose. When hydrolyzed, hemicellulose decomposes into mostly five-carbon sugars such as xylose. S. cerevisiae, the yeast most commonly used for ethanol production, cannot metabolize xylose. Other yeasts and bacteria are under investigation to ferment xylose and other pentoses into ethanol.[90]
On January 14, 2008, General Motors announced a partnership with Coskata, Inc. The goal is to produce cellulosic ethanol cheaply, with an eventual goal of US$1 per U.S. gallon ($0.30/L) for the fuel. The partnership plans to begin producing the fuel in large quantity by the end of 2008. By 2011 a full-scale plant will come on line, capable of producing 50 to 100 million gallons of ethanol a year (200–400 ML/a).[91]
[edit] Prospective technologies
Ethanol plant in Turner County, South Dakota
The anaerobic bacterium Clostridium ljungdahlii, recently discovered in commercial chicken wastes, can produce ethanol from single-carbon sources including synthesis gas, a mixture of carbon monoxide and hydrogen that can be generated from the partial combustion of either fossil fuels or biomass. Use of these bacteria to produce ethanol from synthesis gas has progressed to the pilot plant stage at the BRI Energy facility in Fayetteville, Arkansas.[92]
Another prospective technology is the closed-loop ethanol plant.[93] Ethanol produced from corn has a number of critics who suggest that it is primarily just recycled fossil fuels
Leave a Comment
If you would like to make a comment, please fill out the form below.
Ethanol is produced both as a petrochemical, through the hydration of ethylene, and biologically, by fermenting sugars with yeast.[80] Which process is more economical is dependent upon the prevailing prices of petroleum and of grain feed stocks.
[edit] Ethylene hydration
Ethanol for use as industrial feedstock is most often made from petrochemical feed stocks, typically by the acid-catalyzed hydration of ethylene, represented by the chemical equation
C2H4(g) + H2O(g) → CH3CH2OH(l).
The catalyst is most commonly phosphoric acid,[81] adsorbed onto a porous support such as diatomaceous earth or charcoal. This catalyst was first used for large-scale ethanol production by the Shell Oil Company in 1947.[82] The reaction is carried out with an excess of high pressure steam at 300°C.
In an older process, first practiced on the industrial scale in 1930 by Union Carbide,[83] but now almost entirely obsolete, ethylene was hydrated indirectly by reacting it with concentrated sulfuric acid to produce ethyl sulfate, which was then hydrolyzed to yield ethanol and regenerate the sulfuric acid:[19]
C2H4 + H2SO4 → CH3CH2SO4H
CH3CH2SO4H + H2O → CH3CH2OH + H2SO4
[edit] Fermentation
For more details on this topic, see Ethanol fermentation.
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 (e.g., Saccharomyces cerevisiae) metabolize sugar they produce ethanol and carbon dioxide. The chemical equation below summarizes the conversion:
C6H12O6 → 2 CH3CH2OH + 2 CO2.
The process of culturing yeast under conditions to produce alcohol is called fermentation. 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.[84]
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.[85]
[edit] Cellulosic ethanol
Main article: Cellulosic ethanol
Sugars for ethanol fermentation can be obtained from cellulose.[86][87] Until recently, however, the cost of the cellulase enzymes capable of hydrolyzing cellulose has been prohibitive. The Canadian firm Iogen brought the first cellulose-based ethanol plant on-stream in 2004.[88] Its primary consumer so far has been the Canadian government, which, along with the United States Department of Energy, has invested heavily in the commercialization of cellulosic ethanol. Deployment of this technology could turn a number of cellulose-containing agricultural by-products, such as corncobs, straw, and sawdust, into renewable energy resources. Other enzyme companies are developing genetically engineered fungi that produce large volumes of cellulase, xylanase, and hemicellulase enzymes. These would convert agricultural residues such as corn stover, wheat straw, and sugar cane bagasse and energy crops such as switchgrass into fermentable sugars.[89]
Cellulose-bearing materials typically also contain other polysaccharides, including hemicellulose. When hydrolyzed, hemicellulose decomposes into mostly five-carbon sugars such as xylose. S. cerevisiae, the yeast most commonly used for ethanol production, cannot metabolize xylose. Other yeasts and bacteria are under investigation to ferment xylose and other pentoses into ethanol.[90]
On January 14, 2008, General Motors announced a partnership with Coskata, Inc. The goal is to produce cellulosic ethanol cheaply, with an eventual goal of US$1 per U.S. gallon ($0.30/L) for the fuel. The partnership plans to begin producing the fuel in large quantity by the end of 2008. By 2011 a full-scale plant will come on line, capable of producing 50 to 100 million gallons of ethanol a year (200–400 ML/a).[91]
[edit] Prospective technologies
Ethanol plant in Turner County, South Dakota
The anaerobic bacterium Clostridium ljungdahlii, recently discovered in commercial chicken wastes, can produce ethanol from single-carbon sources including synthesis gas, a mixture of carbon monoxide and hydrogen that can be generated from the partial combustion of either fossil fuels or biomass. Use of these bacteria to produce ethanol from synthesis gas has progressed to the pilot plant stage at the BRI Energy facility in Fayetteville, Arkansas.[92]
Another prospective technology is the closed-loop ethanol plant.[93] Ethanol produced from corn has a number of critics who suggest that it is primarily just recycled fossil fuels