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
Ethanol is produced as part of an integrated system of petrochemical
Ethanol is manufactured by the hydration of
ethene using steam in the presence of a 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.
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.