Page 118 - Biofuels Refining and Performance

Bioethanol: Market and Production Processes 101

3.16 Ethanol Dehydration
In order to allow blending of alcohol with gasoline, the water content of
ethanol must be reduced to less than 1% by volume, which is not pos-
sible by distillation. Higher water levels can result in phase separation
of an alcohol–water mixture from the gasoline phase, which may cause
engine malfunction. Removal of water beyond the last 5% is called dehy-
dration or drying of ethanol. Azeotropic distillation was previously
employed to produce higher-purity ethanol by adding a third component,
such as benzene, cyclohexane, or ether, to break the azeotrope and pro-
duce dry ethanol [82]. To avoid illegal transfer of ethanol from the indus-
trial market into the potable alcohol market, where it is highly regulated
and taxed, dry alcohol usually requires the addition of denaturing agents
that render it toxic for human consumption; the azeotropic reagents
conveniently meet this requirement [82]. Except in the high-purity
reagent-grade ethanol market, azeotropic drying has been supplanted
by molecular sieve drying technology.

3.16.1 Molecular sieve adsorption
The molecular sieve is a more energy-efficient method than azeotropic
distillation. Furthermore, this method avoids the occupational haz-
ards associated with azeotropic chemical admixtures. In molecular
sieve drying, 95% ethanol is passed through a bed of synthetic zeolite
with uniform pore sizes that preferentially adsorb water molecules.
Approximately three-fourths of adsorbed material is water and one-
fourth is ethanol. The bed becomes saturated after a few minutes and
must be regenerated by heating or evacuation to drive out the adsorbed
water. During the regeneration phase, a side stream of ethanol/water
(often around 50%) is produced, which must be redistilled before return-
ing to the drying process [82].

3.16.2 Membrane technology
Membranes can also be used for ethanol purification. Reverse osmosis
(RO), which employs membranes impermeable to ethanol and perme-
able to water, can be used for purification of ethanol from water. Using a
membrane permeable to ethanol but not to water is another approach [9].
Pervaporation, a promising membrane technique for separation of
organic liquid mixtures such as azeotropic mixtures or near-boiling-point
mixtures, can also be used for separation of these azeotropes [81, 83].
It involves the separation of ethanol–water azeotrope or near-azeotropic
ethanol–water composition (from about 95 to 99.5 wt% ethanol) through
water-permeable (or water-selective) membranes to remove the rest of
the water from the concentrated ethanol solution [84].