Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE OF THE INVENTION
PROCESS FOR DEHYDRATING ETHANOL
BACKGROUND OF THE INVENTION
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Ethanol is readily produced by fermentation
processes, which yield dilute aqueous ethanol
mixtures. However, the dehydration of ethanol from
its aqueous mixtures by traditional distillation
methods requires large amounts of heat energy.
Beverage grade ethanol is usually produced as an
azeotrope containing 5 percent water by weight. For
use in motor fuels, especially, gasohol, the ethanol
must be substantially anhydrous.
Various methods for producing anhydrous
ethanol, suitable for use in motor fuels are reviewed
in Hartline, "Lowering the Cost of Alcohol", Science,
VolO 206l 41-42 (1979). Hartline describes only one
adsorption process; usin~ zeolite molecular sieves to
selectively remove water from aqueous ethanol.
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Oulman et al., U.S. Patent No. 4,277,635
describe the use of a crystalline silica polymorph
(silicalite) for the adsorbtion of ethanol from an
aqueous ethanol mixture followed by recovery of the
adsorbed, dehydrated ethanol by passing carbon
dioxide gas through the silicalite bed.
Fornoff, U.S. Patent No. 4,273,621 describes
a gas phase distillation dehydration process using
crystalline zeolite molecular sieves, and a carbon
dioxide gas stream as a drying aid. This patent
teaches that zeolite sieves having a pore diameter of
three Angstroms are useful, because other adsorbents
such as molecular sieves, carbon, alumina and silica
would in addition to adsorbing water, coadsorb the
ethanol and the carbon dioxide drying aid.
Zeolite sieves have one major drawback when
used for the adsorption of water. They require a
great deal of heat energy for desorption of the
trapped water (i.e. regeneration).
None of the prior art discussed above, nor
any of the references cited therein, suggest that
carbon molecular sieves will be useful for
dehydrating aqueous lower alkanol mixtures. It has
been discovered that carbon molecular sieves having
an average effective pore diameter of from 2.0 to 5.0
Angstroms are suitable for producing absolute
ethanol, from an aqueous ethanol mixture having up to
60 percent water by weight. Moreover, carbon sieves,
unlike zeolite sieves, are easily regenerated by
methods such as those described herein.
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SUMMARY OF THE INVENTION
This invention is directed to the removal of
water from aqueous mixtures of lower alkanols, by
employing carbon molecular sieves.
Thus there is provided a process for
dehydrating aqueous lower alkanol mixtures which
comprises passing said mixtures through carbon
molecular sieves having an average effective pore
diameter in the range of about 2.0 to 5.0 Angstroms.
DETAILED DESCRIPTION OF THE INVENTION
The term "lower alkanol," as used herein
refers to those C~ to C5 straight and branched,
generally saturated alcohols that form azeotropes
with water. Examples include ethanol, n-propanol,
isopropanol, n-butanol, sec-butanol, tert-butanol and
sec-amyl alcohol. Methanol, is also to be included
as a lower alkanol, but it does not azeotrope with
water. The term "absolute" is defined as 100% and it
refers to an alkanol containing no water.
The term "carbon molecular sieve" as used
herein refers to those carbonaceous adsorbents that
have been manufactured under conditions which control
pore diameter. Typical carbon molecular sieves and
processes for their production are described in Mason
et al., U.S. Patent No. 3,222,412; Munzner et al.,
U.S. Patent No. 3,979,330; Yuki, U.S~ Patent No.
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4,046,709 and Vesterdal, U.S. Patent No. 2,556,859.
It is anticipated that any carbon molecular
sieve having an average effective pore diameter of
from about 2.0 Angstroms to about 5.0 Angstroms will
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be useful in this application. Pore diameters within
this range allow water to be more readily adsorbed
from a water-lower alkanol mixture than the alkanol.
The preferred method of dehydrating an
aqueous lower alkanol mixture is by distillation.
The azeotropic gaseous mixture of water and lower
alkanol is distilled through a column of carbon
molecular sieves having average effective pore
diameters within the range of about 2.0 to 5.0,
Angstroms and, for ethanol, preferably 2.5 to 3.5
Angstroms. The water vapor is preferentially
adsorbed, and the dehydrated lower alkanol is
recovered by condensation.
It is also anticipated that carbon molecular
sieves will adsorb water from an aqueous lower
alkanol mixture in the liquid phase. Employing
standard adsorption techniques, a mixture of aqueous
lower alkanol would be passed through a column of
carbon molecular sieves, at a flow rate found to be
sufficient to allow adequate adsorption by the carbon
of the water in the mixture. Generally, a slow flow
rate is preferred, for example, about 100 mililiters
per hour. Repeated passages through carbon molecular
sieve columns may be necessary depending upon the
amount of water present and/or the degree of dryness
desired.
After the carbon molecular sieves have
become saturated with adsorbed water, they must be
replaced with virgin carbon molecular sieves or be
regenerated. Water may be removed from the carbon
sieves by numerous known methods. One common method
is to drive off the water by heating the carbon at a
temperature sufficient to volatilize the adsorbed
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water and thereafter passing a dry carrier gas such
as nitrogen or air through the sieve to aid in
removing the water vapor. Another method for removing
water from carbon is described in Convers et al.,
U.S. Patent No. 4,287,089. This method employs
1,2-dichloroethane both in the liquid and gas phase
to remove adsorbed water.
While the examples that follow are directed
to the preferred embodiment of this invention, namely
the dehydration of aqueous ethanol, nevertheless it
is to be noted that the present invention is not
limited solely to this preferred embodiment. Other
aqueous lower alkanol solutions may be dehydrated
using either a gas phase process or a liquid phase
process as described herein without departing from
the spirit o~this invention. The carbon molecular
sieve, NSC-4~has an average effective pore diameter
of about 4 Angstroms and is available from Calgon
Carbon Corporation, Pittsburgh, Pa.
EXAMPLE
A 100 ml sample of ethanol containing 9.0
weight percent water was distilled through a column
containing about 70 grams of carbon molecular
sieves. The resulting deh~drated ethanol was
collected from a condenser and analyzed for its water
content. The results were as follows:
Fraction No.Volume% Ethanol
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30 1 5 ml 100.0
2 25 ml 97.0
3 30 ml 95.0
4 25 ml 95.0
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EX~MPLE 2
A 100 ml sample of ethanol containing 20
weight percent water is distilled through a column
containing about 150 grams of carbon molecular
sieves. The resulting dehydrated ethanol is
collected from a condenser and analyzed for its water
content. The results are:
Fraction No. Volume % Ethanol
1 10 ml 99.5
2 10 ml 97.0
3 15 ml 96.0
4 10 ml 95.5
20 ml 95.0
6 20 ml 95.0
7 5 ml 95.0
EXAMPLE 3
A 200 ml sample of ethanol containing 60
weight percent water is distilled through a column
containing 250 g of carbon molecular sieves. The
resulting dehydrated ethanol is collected from a
condenser and analyzed for its water content. The
results are:
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Fraction No. Vol_me ~ Ethanol
1 2 ml 100
2 2 ml 99.5
3 5 ml 99O0
4 5 ml 99-0
5 ml 99.0
6 5 ml 98.5
7 10 ml 98.0
8 15 ml 98.0
9 15 ml 97.0
EX~MPLE 4
A 100 ml sample of ethanol containing 10
weight percent water is passed through a column
containing 250 grams of carbon molecular sieves at a
flow rate of 100 ml/hour. The resulting dehydrated
ethanol is analyzed for its water content. The
results areo
20Fraction No. Volume ~ Ethanol
1 2 ml 100
2 2 ml 99.5
3 5 ml 99.0
4 11 ml 99.0
15 ml 99.0
6 10 ml 99.0
7 10 ml 98.5
8 10 ml 98.5
9 20 ml 98.0
4 ml 97.5
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Following the above examples, other
dehydrated lower alkanols may be produced, including
absolute butanol, absolute isopropanol, absolute
sec-amyl alcohol and the like.
Claims to the invention follow.
