Note: Descriptions are shown in the official language in which they were submitted.
O.Z. 31,607
~L06S8~9
ISOLATION OF CARBOXYLIC ACIDS FROM THEIR AQUEOUS SOLUTIONS
. .
The present invention relates to a new process for isolat-
ing carboxylic acids of the general formula I
R - COOH
where R1 is hydrogen, methyl, ethyl or vinyl, from their aqueous
solutions.
A number of synthe~es result in carboxylic acids I being
obtained in the form of their dilute aqueous solutions.
Experience has 9hown th~t if it is desired to isolate the pure
or concentrated acids therefrom, substantial technical diffi-
culties are encountered. Removal of the water by distillationwhioh in the case of formic acid is in any case not possible
at commercially acceptable expense, because of azeotrope forma-
tion, re~uires in all other cases both a large amount of energy
and expensive distillation columns with numerous trays, since
the degree of separation of the acid~water system achieved by
a single tray is slight.
6 ~ ~ 8~
O.Z. ~1,607
It is true that the water can be removed more rapidly, and
with less expensive equipment, by azeotropic distillation with : :
a water-insoluble liquid such as ethyl acetate or benzene, but
this of course requires even more energy than a simple distil-
lation
For these reasons, numerous separation processes have been
developed, which are based on the extraction of the acid with a
liquid extractant such as isoamyl acetate or methyl isopropyl
ketone.
However, the efficiency of the extractants of the prior art
is not fully satisfactory, since the extractants take up insuffi-
cient acid and too much water. Hence, mixtures of extractants,
acid and water are obtained in every case and these, in turn,
require relatively expensiwe further processing since, in the
case o~ none of these extractants, the three components can be
separated by ~imple distillation.
It is an object of the present invention to increase the
e~ficiency of extraction of a carboxylic acid I from its dilute
agueous solutions, by providing more suitable extraçtants.
We have found that this object is achieved and that carb-
oxylic acids of the general formula I
R1 _ COOH
where R1 is hydrogen, methyl, ethyl or vinyl, can be obtained
by extracting their dilute aqueous solutions and then distilling
bhe mixtures obtained~ if the extractant used is a secondary
amide of the general formula II
R \ O
/ N - C - R II
where R2 and R3 are alkyl, cycloalkyl, aryl or aralkyl or con-
joinkly are 1,4- or 1,5-alkylene, in each case of 1 to 8 carbon
atoms, with the proviso that the sum of the carbon atoms of R2
- 2 -
~ 5~89 o.z. 31,607
and R3 is from 7 to 14 and that only one of these radicals is
aryl, and where R4 is one o~ the radicals R1.
Since a trans-amidation with the acid I can take place,
extractants where R4 and R1 are identical are always preferred.
Accordingly, formic acid is advantageously extracted with a form-
amide, acetic acid with an acetamide, propionic acid with a propion-
amide and acrylic acid with an acrylamide II, so that the occurrence
of the trans-amidation is outwardly not detectable I~ it is in-
tended to isolate mixtures of different acids 3 eO g. formic acid
and acetic acidg it is preferred to use the compounds of the form-
amide series, as being the most ef~icient compounds of the category
which has been defined~
From the point o~ view o~ the amide groupings, particularly
suitable compounds II, and their mixtures, are those derived
from N-ethyl-N-cyclohexylamine, N,N-dicyclohexylamine, N-methyl-N-
benzylamine, N-methylaniline, N-ethylaniline, N,N-diamylamine,
N-methyl-N-2~ethylhexylamine, N-n-butyl-N-cyclohexylamine, N-methyl-
N 2-heptylamine or N-propyl-N cyclohexylamine. The dibutylform-
amides,including above all di-n-bukylformamide, have proved best
for rOrmic acid, and N-n-butyl-N-2-ethylhexylacetamide and N-n-
butyl-N-cyclohexYlacetamide have proved best for acetic acid.
The amides II are either known compounds or are readily
accessible by conventional methods. If their ~reezing point is
above the extraction temperature, it is necessary to use mixtures
of the extractants II, or to work in the presence of a solvent,
preferably an aromatic hydrocarbon such as p-diisopropylbenzene,
which does not form an azeotrope with the acids. It is true that
in this variant the efficiency of the extractant I~ is reduced, but
nevertheless sufficient advantages over conventional methods remain,
since the amount of solvent normally required to lower the ~reezing
point is low, namely from 10 to 40% by weight, hased on II.
The partition coe~ficient, which is defined in Example 3 and
-- 3 ~
~ 65~9 o. z . 31,607
is quoted for several acid/extractant systems is a measure of the
suitability of the extractants. The lower is this coefficient, the
greater the expense of the equipment required for the extraction~
However, it is not only the partition coeffi~ient for the acid but
also the partition coefficient ~or water which has to be taken into
account, since, naturally, the isolation of pure or concentrated
acid requires less energy if the extractant takes up very little
water; this latter condition is fulfilled satisfactorily by all
of the extractants of the invention.
The requisite amount of extractant II depends on various para-
meters, including, above all, the temperature, the amount and con-
centration of acidg the number of separation stages and the other
details of the equipment which affect equilibration and hence
af~ect the residence time. The amount of extractant II required
does not vary fundamentally with the particular extractant used
and the particular acid involved.
The preferred temperature range for the extraction is from
0 to 70 C. It is true that at the lower end of this range, the
extractant can absorb more acid than at higher temperatures, but
on the other hand the rate of equilibration is lower. The economic
optimum is in the range of from 20 to l10C.
In the range of fr~m 20 to 40C, from 1 to 10 kg of extractant
II are normally required for extracting 1 kg of acid at a contact
time Or from 1 to 5 minutes. The amount Or extractant is less ror
longer contact times and vice versa. The stated contact times
apply to the preferred embodiment of counter-current extraction,
in a simple extraction column without other auxiliaries such as
ba~les, trays or packing, and under Gonditions where the lighter
extractant forms the continuous phase. The efficiency is increased
by using multi-stage extraction apparatus, e.g. packed columns or
tray columns with, preferably, from 3 to 6 theoretical plates, so
that the amount of the extractant can be reduced in accordance with
the conventional rules.
~ 4 --
5 ~ ~ ~
OOZo 31,607
The above data-~elate to acid concentrations of from 5 to
50% by weight Or the aqueous solution, these being the most
commonly encountered concentrations in industry. The depletion
ratio, of from 95 to 99% by weight, is relatively constant, i.e.
if the initial solution is of 30% strength, from Oo l to 0O3
of acid remain in the aqueous medium, whilst if a 10% strength
solution is extracted, a solution containing from 0005 to 001%
of acid remainsO In general3 it is most economical to take the
exkraction to the point that a mixture of the extractant and
an acid of from 10 to 40 per cent strength by weight is obtained.
The water is then distilled from this mixture, after which the
acid is distilled off in a downstream columnO It is also
possible3 using conventional methods, to use an arrangement
where only a part of the water is removed in the first column,
whilst in the second column the residual water is distilled off
together with the acid. In that case, a commercial concentrated
acid is obtained instead of the p~re acidO
The above comments relate to the continuous manufacture
of carboxylic acids I, which in industry is virtually the only
embodiment of importanceJ However, the process can of course
also be carried out batchwise, if desired,in which case the
general sense of the conditions outlined above must be adhered
to.
It should be pointed out that the feature of the process
which is essential to the invention is the nature of the ex-
tractant, and not the extraction technique used, which is the
conventional tec~nique~ To that extent, the condil;ions outlined
merelY represent guidelines, from which it is possible to
deviate~ in individual cases, in accordance with conventional
rules and the conventional process technology, should this be
advisable; an example is the treatment of effluent, where the
purification of the water is more important than the isolation
3 651~8~
OOZ. 31,607
of the acidO Furthermore, it is posslble to use ~he stated
extractants for the extractive distillation of the aqueous
acidsO
The present process permits a substantial saving in energy
and investment costs, both in comparison to other extraction
processes and in comparison to distillative treatment~ It par-
ticularly represents an advance where the isolation of pure or
concentrated formic acid or acetic acid is concerned, and where
aqueous solutions which con~ain several acids I have to be
lQ worked up~
EXAMPLE 1
1 kg per hour of a 21 per cent strength by weight aqueous
formic acid, as obtained by industrial synthesis from methanol
and carbon monoxide, was fed, at from 20 to 25C, into the top of
a packed column into which 0.9 kg per hour of di-n-butylformamide
was fed, in counter-current, at the bottom. The extractant formed
the continuous phase.
1.2 kg per hour of extract phase were taken o~f a settling
zone at the upper end of the column; this phase contained vir-
tually all the formic acid (210 g), together with 90 g of water,
i.e. 300 g of 70% strength ~ormic acid. This acid was separated
~rom the extractant by simple continuous distillation in a
packed column at 45C (column top temperature) and Go mm Hgo
The extractant, which still contained traces of ~ormic
acid, was recycled from the bottom of the distillation column
to the extraction column.
In order to obtain ~ormic acid of about 90 per cent strength
by weight, 70 g per hour of water were distilled from the
extract phase first obtained, in a packed column (atmospheric
pressure, bottom temperature lL~3C), after which the mixture
~0 remaining in the bottom of this column was subjected to a second
distillation in a column with 25 bubble-cap trays, at 60 mm Hg
-- 6 --
.. . . . .
3~;1658~9
-~; OOZ~ 31,~07
and 42 C (column top temperature), giving a 90% strength acid
as the distillateO
Using a similar method and employing the two columns, vir-
tually anhydrous formic acid was obtained from the 90% acidO
EXAMPLE 2
1 kg per hour of a 15 per cent strength by weight aqueous
acetic acid was fed, at room temperature, into the top of a
column with 12 sieve tra~s, whilst 0.75 kg per hour of N-n-butyl-
N-2-ethylhexylacetamide were fed in at the bottomO 81 per cent
strength by weight acetic acid and anhydrous acetic acid were
produced, by the method described in Example 1~ from the extract
phase, which contained virtually all the acetic acid, together
with 4% by weight of water,
EXAMPLE 3
Since the suitability of the extractants depends above all
on the partition coefficient
concentration of the acid in the organic phase
C = ~
concentration of the acid in the aqueous phase
these coefficients were determined, for the practical require-
ments of the present process, by stirring 100 g of the extrac-
tant with 143 g of a 30 per cent strength by weight acid (cor-
responding to 100 g of water) at 25C, until equilibrium was
reached. The quotient C was then calculated from the acid con-
centrations in the organic phase and in the aqueous phase.
The Table which follows gives a survey of the partition
coefficients of some extractants according to the invention,
and, by way of comparisD~, of some conventional extractantsO
F = formic acid
A = acetic acid
P - propionic acid
Acr = acrylic acid
- 7 -
,: .
5~39 oO z o 31, 607
Extractant AcidPartition
coefficient C
according to the invention
N-Di-n-butylformamide F 1012
A 1039
P 4060
Acr 6.91
N-Di-n-butylacetamide F 1,33
N-Methyl-N-2-heptylformamide F 1018
N-n-Butyl-N-2-ethylhexylacetamide A 1007
N-n-Butyl-N-cyclohexylacetamide A 1039
50% by weigHt of N-di-n-butylformamide
50% by weight o~ N-di-cyclohexyl-
formamide F 1~ o4
67% by weight of N~di-n-butylacetamide
+ ~ .
33% by weight of N-dicyclohexylacet-
amide A1.4ll
80% by weight of N dl-n-butylformamide
+
20% by weight of p-diisopropylbenzene F o.98
N-Ethyl-N-cyclohexylformamide F 1.26
N-Ethylformanilide F oO96
N-Dibutylpropionamide P 4002
conventional
Benzene F 0.006
A 00115
Methylene chloride F 00014
A 0025
Trichloroethylene F 0O002
A 00074
Diisopropyl ether F 00267
A 0039
-- 8 -
8 ~
OOZo 31,607
Extractant Acid Partition
coeff'icient C
Isobutyl acetate F 0034
A oO56
Methyl isopropyl ketone F oO84
A 1007
P 2082
Acr 3049
Cyclohexyl formate F 0~31
Cyclohexanol F 0038
_ g _
