Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR EXTRACTING (METH)ACRYLIC ACID
10
The present invention relates to a process for extracting an aqueous solution
which
contains (meth)acrylic acid by bringing it into contact with a solution which
contains
at least one extracting agent which can be converted into (meth)acrylic acid
in one or
more stages and forms a miscibility gap with the aqueous solution.
The term "(meth)acrylic acid" used in this Application refers to both acrylic
acid and
methacrylic acid.
Owing to its very reactive monoethylenically unsaturated bond and the acid
function, methacrylic acid is a useful monomer for the preparation of
polymers, for
example for aqueous polymer dispersions suitable as adhesives. This also
applies to
acrylic acid.
Methacrylic acid is obtainable by, inter alia, gas-phase oxidation of 1-
butene,
isobutene, isobutyraldehyde, isobutyric acid, isobutene, MTBE and/or
methacrolein
with oxygen or oxygen-containing gases in the presence of catalysts, for
example
multimetal oxides which contain the elements molybdenum and vanadium in oxidic
form. The oxidation is carned out at elevated temperatures and, owing to the
considerable heat of reaction, preferably with dilution of the reactants with
inert
gases such as Nz, C02 and/or hydrocarbons and/or steam. However, these
processes
do nvt give pure methacrylic acid but a reaction gas mixture which, in
addition to
methacrylic acid, contains the starting materials, for example unconverted
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methacrolein, steam, inert diluent gas (eg. nitrogen) and byproducts (eg.
oxides of
carbon), lower aldehydes, eg. formaldehyde, high boilers, eg. citraconic acid,
and in
particular acetic acid, from which the methacrylic acid must subsequently be
isolated (cf. for example EP-A 253 409 and DE-A 19 62 431 ). However, other
possible starting compounds are those from which the actual Ca starting
compound,
eg. methacrolein, is formed initially as an intermediate during the gas-phase
oxidation. An example is the methyl ether of tent-butanol (MTBE).
For the preparation of methacrolein, it is also possible to subject
formaldehyde and
to propionaldehyde to a condensation reaction and to obtain methacrolein by
subsequent distillation. Such a process is described in EP-B 58 927. The
methacrolein thus obtained can then be converted into methacrylic acid in the
conventional manner by catalytic gas-phase oxidation. A reaction of this type
is
described, inter alia, in EP-B 297 445.
Acrylic acid can be obtained in a similar manner starting from the
corresponding C3
compounds, in particular propene and/or acrolein.
If it is intended to isolate (meth)acrylic acid from the resulting reaction
gas mixture
by extraction, the reaction gas mixture is first subjected to a condensation
stage and
then extracted. Thus, EP-B 345 083 describes such a process which comprises a
methacrylic acid extraction stage in which methacrylic acid is extracted with
a
saturated hydrocarbon of 6 to 9 carbon atoms.
According to EP-A 710 643, in a process for the purification of methacrylic
acid, an
aqueous methacrylic acid solution obtained by cooling and condensing the
reaction
gas is extracted from the aqueous solution by adding an organic solvent,
preferably
an aliphatic hydrocarbon of 5 to 9 carbon atoms, an aromatic hydrocarbon, an
ester
or a mixture thereof.
Japanese Patent JP 57 095 938 describes the extraction of acrylic acid from a
dilute
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aqueous solution by extraction by means of an oxygen-containing solvent and a
tertiary amine. Examples are trioctylamine and 2,6-dimethyl-4-heptanol.
However, all these processes are disadvantageous in that the extraction of
(meth)acrylic acid is effected in each case using an assistant which on the
one hand
is not obtainable without additional costs, and moreover, in such a process,
additional steps are required in order to separate off this assistant again,
entailing
additional costs and additional energy consumption.
1o None of the prior art processes describes the possibility of extracting
(meth)acrylic
acid by means of an assistant-free solution. The extracting agents used to
date are
merely assistants which cannot be converted into (meth)acrylic acid.
Thus, it is an object of the present invention to provide a process for
extracting an
aqueous solution which contains (meth)acrylic acid, which process can be
carried
out without assistants and furthermore can be effected using conventional
apparatuses and with comparatively low energy consumption.
We have found that this object is achieved by the process according to the
invention.
The present invention accordingly relates to a process for extracting an
aqueous
solution which contains (meth)acrylic acid by bringing it into contact with a
solution
which contains at least one extracting agent which can be converted into
(meth)acrylic acid and forms a miscibility gap with the aqueous solution,
wherein an
organic phase, which contains (meth)acrylic acid and extracting agent, and an
aqueous phase are obtained.
As defined in, inter alia, the claims, the term extracting agent includes all
substances
which can be converted into (meth)acrylic acid in one or more stages and at
the
3o same time forms a miscibility gap with the aqueous solution which contains
(meth)acrylic acid.
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Particular examples are alkanes, alkanols, alkenes or alkenals of 3 or 4
carbon atoms
or mixtures of two or more thereof which can be converted into (meth)acrylic
acid
and form a miscibility gap with the aqueous solution defined above.
Substances suitable for this purpose are preferably (meth)acrolein, isobutene,
propene, propane, butane, isobutyraldehyde, the methyl ether of tert-butanol
(MTBE) or a mixture of two or more thereof. (Meth)acrolein is particularly
preferably used.
to
The solution which contains at least one extracting agent which can be
converted
into (meth)acrylic acid either consists completely of this extracting agent or
contains
it as a mixture with other substances, eg. water and/or acetic acid. The
solution may
also contain impurities arising from the preparation process of the components
and
therefore need not be purified before the extraction.
Additives which improve the extraction effect may also be added to this
solution.
Examples are antifoams, for example tallow fatty alcohol and other
polyalcohols,
demulsifiers, such as allcali metal chlorides, and surfactants and substances
which
2o extend the miscibility gap, for example higher alkanes, in particular those
of 4 to 14
carbon atoms.
There are in principle no restrictions with regard to the concentration of the
extracting agent which is contained in this solution and can be converted into
(meth)acrylic acid, but the concentration of this extracting agent in the
solution is
preferably from about 50 to 100, particularly preferably from about 70 to
about 99.9,
in particular from about 90 to about 97, % by weight.
The solution defined above must in any case have a miscibility gap with the
aqueous
3o solution which contains (meth)acrylic acid.
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Particularly if the novel process is integrated in a process for the
preparation of
(meth)acrylic acid or is operated in combination with such a process, it is
preferable,
if the solution which is used for extracting (meth)acrylic acid and contains
at least
one extracting agent which can be converted into (meth)acrylic acid is a
solution
5 which can be used as a starting material for the (meth)acrylic acid
synthesis and
preferably has a concentration of from 50 to 100% by weight of extracting
agent. It
is particularly preferably a solution which contains (meth)acrolein.
The reaction gas mixture which is obtained by oxidation of the C3~Ca compounds
in
l0 the first stage and still contains starting material may, after
condensation of said
mixture, also be used as the solution which contains at least one extracting
agent
which can be converted into (meth)acrylic acid, for extracting (meth)acrylic
acid.
The extracting agent which is contained in the solution and is present to a
certain
extent in the aqueous phase after the extraction is preferably recovered by a
thermal
separation method, for example by stripping with steam or an inert gas, eg.
nitrogen,
air, carbon dioxide, exit gas from the (meth)acrylic acid preparation or a
mixture of
two or more thereof, or by recovery by distillation. The extracting agent is
transferred virtually completely to the gas phase and can then be used for the
preparation of (meth)acrylic acid, if necessary after passing through further
(working
up) stages.
There are likewise no restrictions at all with regard to the concentration of
the
(meth)acrylic acid in the aqueous solution. The content of (meth)acrylic acid
in this
solution is preferably from about 0.01 to about 80, particularly preferably
from about
1 to about 40, in particular from about 5 to about 20, % by weight. In
addition to
(meth)acrylic acid and water, this solution, particularly if it is obtained
industrially
from the preparation of (meth)acrylic acid, may contain a small amount, as a
rule
less than about 3% by weight, of the starting material used for the
preparation of
(meth)acrylic acid and a small amount, as a rule less than 10% by weight, of
acetic
acid.
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As stated above, an organic phase which contains (meth)acrylic acid and
extracting
agent is obtained in the extraction. The extraction is preferably carried out
in such a
way that the total amount or virtually the total amount of the (meth)acrylic
acid used
is present in this phase. In addition to (meth)acrylic acid, the organic phase
contains
extracting agent and also small amounts of water, acetic acid and high
boilers. The
resulting aqueous phase may also contain small amounts of (meth)acrylic acid,
extracting agent and acetic acid, which however can be removed for the
predominant part by thermal separation methods.
According to a preferred embodiment of the novel process, the organic phase
which
contains (meth)acrylic acid and extracting agent is subjected to a thermal
separation
method, for example stripping with steam or with an inert gas, eg. nitrogen,
air,
carbon dioxide, an exit gas formed in the oxidation of the starting materials
for the
preparation of (meth)acrylic acid or a mixture of these inert gases, or to
distillation
of the extracting agent/water mixture. In this process, on the one hand, the
(meth)acrylic acid, together with the high boilers and the acetic acid, is
separated
from the predominant part of the extracting agent and water.
2o There are no particular restrictions at all with regard to the temperature
at which the
novel process can be carned out. The only precondition is that, at the chosen
temperature and the chosen pressure, it is possible for two phases, an organic
and an
aqueous phase, to form. In general, the novel process is carried out at from
about 0
to about 150°C, preferably from about 30 to about 80°C, in
particular from about 50
to about 70°C, temperatures above about 68°C being employed at
superatmospheric
pressure when (meth)acrolein is used as the extracting agent. If other
extracting
agents are used, temperature and/or pressure must be modified depending on the
extracting agent.
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All apparatuses which are used in general for extraction, as are described,
for
example, in Ullmanns Encyklopadie der technischen Chemie, 4th Edition, Vol. 2,
page 546 et seq., in particular page 560 et seq. (1972), may be used for
carrying out
the novel process. Particular examples are mixer-settler apparatuses, packed
extraction columns, spray tower extraction columns, pulsed and unpulsed tray
columns and packed columns and stirred extraction columns or extraction
apparatuses which utilize centrifugal forces.
The aqueous solution which contains (meth)acrylic acid and the solution which
contains at least one extracting agent which can be converted into
(meth)acrylic acid
can be brought into contact with one another either cocurrent, crosscurrent or
countercurrent, the extraction by countercurrent being preferred.
The extraction can be earned out in one or more stages, and it is also
possible to use
combinations of extraction apparatuses.
The process according to the invention can be carried out continuously or
batchwise,
the continuous procedure being preferred.
It is also possible to carry out the novel extraction process as part of a
process for the
preparation of (meth)acrylic acid. The amount of (meth)acrylic acid to be
extracted
corresponds as a rule to the amount of (meth)acrylic acid which was not
condensed
in the condensation of the reaction mixture obtained directly in the
preparation of
(meth)acrylic acid by gas-phase oxidation.
3 0 As stated above, the extracting agent used for the extraction is
preferably recovered
after passing through the extraction. Particularly suitable methods for
recovering that
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part of the extracting agent which is dissolved in the aqueous phase are
thermal
separation methods, for example stripping with steam or with an inert gas,
e.g.
nitrogen, air, carbon dioxide, an exit gas formed in the oxidation of the
starting
materials for the preparation of (meth)acrylic acid or a mixture of these
inert gases,
or distillation of the extracting agent/water mixture.
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The stripping process is generally carried out at temperatures from about 30
to about
100°C, preferably from about 50 to about 80°C, and at a pressure
from about 1 to
about 1.5 x 105 Pa. For other pressures, the temperatures must be accordingly
modified.
The type of stripping apparatus used is not subject to any particular
restriction, and
any conventional stripping apparatuses which permit gas-liquid contact may be
used,
for example packed towers, sieve-tray towers, bubble cap towers or spray
towers.
Further readily usable spray apparatuses are described under the keyword
absorption
to column in EP-A 706 986, in column 3, lines 11 to 38, and in the prior art
cited
therein, which is hereby fully incorporated by reference in the present
Application.
As stated above, the (meth)acrylic acid contained in the organic phase is
likewise
recovered by a thermal separation method, preferably by subjecting the organic
i5 phase to a stage for the condensation of (meth)acrylic acid. This stage
gives the
condensate which contains the main amount of the (meth)acrylic acid present in
the
organic phase and the main amount of acetic acid, while the extracting agent
generally contained in the organic phase is converted into the gaseous state.
2o For this purpose, the organic phase is fed to a thermal separation method
which is
operated at a temperature at which the above object, namely the separation of
the
main amount of (meth)acrylic acid from the main amount of extracting agent, is
achieved.
25 Such separation methods are known from the prior art and are described, for
example, inter alia in German Laid-Open Applications DOS 4,235,321 and DOS
3,721,865, for the condensation of the gaseous reaction mixture obtained in
the
preparation of (meth)acrylic acid.
30 To avoid or reduce the tendency of (meth)acrylic acid and/or extracting
agent to
polymerize, a stabilizer, for example phenothiazine, hydroquinone or a
derivative
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thereof, may also be added to the solutions used in the present process.
As is evident from the above, the problem of the azeotropic (meth)acrylic
acid/water
mixture can be overcome in a simple manner by means of the novel process by
adding extracting agents which can be converted into (meth)acrylic acid. This
dispenses with expensive, energy-intensive separation steps involving
distillation.
Furthermore, contamination problems due to working up the assistant are
avoided.
The Examples which follow illustrate the present invention.
EXAMPLES
Example 1
An aqueous solution containing 10.3% by weight of methacrylic acid and 2.2% by
weight of acetic acid was extracted with an aqueous methacrolein-containing
solu-
tion with a methacrolein content of 92% by weight by the countercurrent method
in a
three-stage mixer-settler cascade which was operated at 50°C. A
residence tire of 3
2 0 minutes was sufficient for phase separation.
The organic phase obtained contained the total amount of the methacrylic acid
introduced, the concentration thereof in said phase being 17.6% by weight. 361
g of
methacrylic acid were extracted.
Example 2
630 g of aqueous solution containing 10.3% by weight of methacrylic acid and
2.2%
3 0 by weight of acetic acid were extracted in one stage with 300 g of an
organic meth-
acrolein solution. The methacrolein solution contained 7% by weight of water.
580 g
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of aqueous phase containing 1.7% by weight of methacrylic acid and 1.2% by
weight of acetic acid were obtained.
In the extraction step, the methacrolein solution was laden with 14.4% by
weight of
methacrylic acid and 1.1% by weight of acetic acid.
Example 3
449 g of aqueous solution containing 1.7% by weight of methacrylic acid and
1.2%
by weight of acetic acid were extracted in one stage with 212 g of an organic
meth-
10 acrolein solution. The methacrolein solution contained 7% by weight of
water 434 g
of aqueous phase containing 0.37% by weight of methacrylic acid and 1.2% by
weight of acetic acid were obtained.
In the extraction step, the methacrolein solution was laden with 2.9% by
weight of
methacrylic acid and 0.73% by weight of acetic acid.
Example 4
328 g of aqueous solution containing 0.37% by weight of methacrylic acid and
1.2%
by weight of acetic acid were extracted in one stage with 156 g of an organic
meth-
acrolein solution. The methacrolein solution contained 7% by weight of water.
325 g
of aqueous phase containing 0.06% by weight of methacrylic acid and 0.68% by
weight of acetic acid were obtained.
In the extraction step, the methacrolein solution was laden with 0.65% by
weight of
methacrylic acid and 0.56% by weight of acetic acid.
Example 5
3.49 kg/h of a first aqueous solution containing 10.3% by weight of
methacrylic acid
3 0 ~d 2.2% by weight of acetic acid were extracted by the countercurrent
method in a
three-stage mixer-settler system with 1.63 kg/h of a first methacrolein
solution
which was introduced into the 3rd settler. The first methacrolein solution
contained
7% by weight of water. The first aqueous solution was brought into contact, in
the
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first extraction stage (first settler) with a third methacrolein solution
flowing out of a
second settler.
The second aqueous solution flowing out of the first settler was brought into
contact
with a second methacrolein solution flowing out of a third settler. The third
aqueous
solution flowing out of the second settler was brought into contact in the
third stage
with the first methacrolein solution.
3 kg/h of a fourth aqueous phase were obtained from the third settler. This
fourth
1o aqueous solution contained 0.8% by weight of methacrylic acid and 1.5% by
weight
of acetic acid. 2 kg/h of the fourth methacrolein solution were obtained from
the first
settler. This fourth methacrolein solution contained 17.6% by weight of
methacrolein and 1.6% by weight of acetic acid.
The phase separation between the aqueous solutions and the methacrolein
solutions
was complete after less than 3 minutes in all Examples.
All exemplary experiments were carried out at a temperature of 50°C
and at
atmospheric pressure.
