Ethanol from Ethene

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

Using these conditions, around 5% conversion to ethanol occurs per pass through the plant. By continually recycling the unreacted ethene and steam, an overall yield of 95% is achieved in a continuous process.
Ethanol is produced as part of an integrated system of petrochemical production.

Ethanol is manufactured by the hydration of ethene using steam in the presence of a phosphoric acid catalyst.
chemical equation
diagram: phosphoric acid catalyst
The reaction has a theoretical atom economy (link to section on atom economy) of 100%, but some side reactions occur producing by-products such as methanol, ethanal, polythene and ethoxyethane.

diagram: The petrochemical route to ethanol

Purification
The reactions involved in the production of synthetic ethanol produce an ethanol and water mixture. Fractional distillation always results in a mixture of 96% ethanol and 4% water (instead of 100% 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 with calcium oxide, a dehydrating agent, or by mixing with benzene, which breaks up the azeotrope and produces pure ethanol when further distilled. Both these processes increase the energy costs of production, and benzene is also highly toxic and carcinogenic.

New Purification Methods
New purification techniques involve the use of zeolites, which have structures with holes that can absorb and hence remove water from the final mixture. Several zeolites have a particularly strong attraction, and they act as dehydrating agents at normal temperatures and pressures and can then be dried by heating and re-used. This produces considerable savings in energy and removes the need to use toxic substances like benzene.
The zeolites used are referred to as 3Å (angstrom) zeolites, as the holes are 3Å in diameter. The angstrom is a unit of length, 10-8 cm. 10Å = 1nm. The holes are larger than a water molecule, but considerably smaller than ethanol.


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