manufacturing of ethanol from e then e

7/30/2019 Manufacturing of Ethanol From e Then e

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Manufacturing of Ethanol from Ethene

Ethanol is manufactured by the hydration of ethene using steam in the presence of a phosphoricacid catalyst.

The reaction has a theoretical atom economyof 100%, but some side reactions occur producingby-products such as methanol, ethanal, poly(ethene) and ethoxyethane.

Conditions

Low temperatures, high pressures and a moderately high steam concentration favour thisexothermic reaction. In practice, a temperature of 300C, a pressure of 60-70 atmospheres, anda steam:ethene ratio of 0.6:1 is used.

Using these conditions, around 5% conversion to ethanol occurs per pass through the plant. Bycontinually recycling the unreacted ethene and steam, an overall yield of 95% is achieved in acontinuous process.

Ethanol is produced as part of an integrated system of petrochemical production.

PurificationThe reactions involved in the production of synthetic ethanol produce an ethanol and watermixture. Fractional distillation always results in a mixture of 96% ethanol and 4% water (instead of100% pure or absolute ethanol). This is known as an azeotropic or constant boiling mixture.

Conventionally, this last 4% water is removed from the azeotropic mixture either by refluxing withcalcium oxide, a dehydrating agent, or by mixing with benzene, which breaks up the azeotropeand produces pure ethanol when further distil led. Both these processes increase the energy costs

of production, and benzene is also highly toxic and carcinogenic.

New Purification MethodsNew purification techniques involve the use of zeolites, which have structures with holes that canabsorb and hence remove water from the final mixture. Several zeolites have a particularly strongattraction, and they act as dehydrating agents at normal temperatures and pressures and canthen be dried by heating and re-used. This produces considerable savings in energy andremoves the need to use toxic substances like benzene.


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The zeolites used are referred to as 3 (angstrom) zeolites, as the holes are 3 in diameter. Theangstrom is a unit of length, 10-10 m. 10 = 1nm. The holes are larger than a water molecule, butconsiderably smaller than ethanol.

Feedstock CompositionFeedstock may include purpose-grown crops (including maize and corn), crop waste, paper millsludge, forest residues and household waste (including sewage). These are mostly lignocellulosicmaterials containing cellulose, hemicellulose and lignin. Cellulose and hemicellulose are longchain polymers that make up the bulk of plant material, and lignin is the chemical "glue" that holdsthem together.

HydrolysisThis process breaks the long cellulose and hemicellulose chains into simple sugars. Celluloseyields primarily glucose (a six-carbon sugar) whereas hemicellulose, in the region of 20% of thematerial, gives a mixture including several five-carbon sugars. Methods of hydrolysis includeusing enzymes and using dilute or concentrated acids. Whereas in the past hydrochloric or

hydrofluoric acid may have been used, sulphuric acid is found in newer processes.

Sugar SeparationAfter hydrolysis, the sugar for fermentation must be separated from the acid. A new processdeveloped by the company Arkenol in the United States, which is still at the pilot stage, makesuse of ion exchange to improve the separation, allowing a greater proportion of the acid to beconcentrated and re-used. Final traces of acid are precipitated as "gypsum" (calcium sulphate) byaddition of lime.


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FermentationFermentation is a complex series of reactions, which convert carbohydrates, mainly sugars andstarches, into ethanol and carbon dioxide. Several enzymes, such as zymase in yeasts, catalysethese reactions. Yeast is a living organism, and these are the products of anaerobic respiration.

Conditions

Fermentation with yeast works best at temperatures in the range 25 - 37C, in the absence ofoxygen (anaerobic) and will produce aqueous solutions of up to 14% ethanol.

Below 25C the reaction rate is too slow, but at higher temperatures the enzymes start todenature and lose efficiency.

If oxygen is present, aerobic respiration will occur producing carboxylic acids, in this caseethanoic acid (vinegar).

The toxicity of ethanol to the organisms used limits the ethanol concentration possible.

Solving the 5-carbon problemConventional yeasts cannot make use of five-carbon sugars that arise from the hydrolysis ofhemicellulose. Conventional methods for ethanol fermentation do not utilise this resource, whichmay count as 20% of the feedstock.

The process developed by Arkenol uses specially bred yeast (not genetically engineered) thatfeeds preferentially on C5 sugars, as well as on C6 sugars. In this way, a greater proportion ofthe feed is utilized.

Fermentation using bacteriaYeast is very good at converting glucose, and other six-carbon sugars into ethanol. Unfortunately,a significant proportion of waste biomass consists of complex natural polymers made from sugarsthat are not "digested" readily by yeast enzymes. These include hemicellulose, which onhydrolysis produces a range of sugars including mannose, xylose, arabinose and galactose,depending on the original source.


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KO11A genetically modified bacterium, developed by the microbiologist Lonnie Ingram, has enabledthese sugars to be converted to ethanol. The bacterium, referred to as KO11, would normallyroduce acids, but the modification means ethanol is produced instead.p

The advantage over yeast is that a wider range of sugars can be processed, enabling theutilization of biomass waste such as wood waste, corn stalks, rice hulls, and other organic waste,

which would otherwise require disposal by some other method, or which could only be partiallyutilized by conventional fermentation methods, making them uneconomic.

Commercial DevelopmentThe BC International Corporation is attempting to develop this approach commercially in theUnited States, focusing on:

areas where legislation prohibits burning of agricultural waste conventional ethanol plants suitable for conversion and refurbishment

Reference:

http://www.uyseg.org/greener_industry/pages/ethanol/ethanol6PM1.htm
http://www.uyseg.org/greener_industry/pages/ethanol/ethanol6PM1.htmhttp://www.uyseg.org/greener_industry/pages/ethanol/ethanol6PM1.htm

manufacturing of ethanol from e then e

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