Note: Descriptions are shown in the official language in which they were submitted.
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Recovery of carboxylic acids from their salts
The present invention relates to a process for recovering satu-
5 rated aliphatic or cycloaliphatic dicarboxylic acids having from
5 to 10 carbon atoms (I) and polybasic carboxylic acids of the
general formula II
HOOC - A- N - A COOH
A (II)
I
COOH
- n
where the A groups are each independently of the others a Cl- to
C4-alkylene group and n is from 1 to 4, by electrodialysis of
aqueous solutions of salts or partial salts of these carboxylic
20 acids.
~here are a number of these processes where polybasic carboxylic
acids are obtained in the form of aqueous solutions of their
salts.
For instance, the synthesis of nitrilotriacetic acid from formal-
dehyde, ammonia and hydrogen cyanide and subsequent alkaline hy-
drolysis of the tris(cyanomethyl)amine intermediate yields a
nitrilotriacetic acid salt.
Similarly, the hydrolysis of polyamides to recover the starting
materials frequently gives rise to dicarboxylic acids in the form
of aqueous salt solutions.
35 Carboxylic acids are also obtained in the form of their salts in
fermentation processes as described for example in US-A-3 086 928
for citric acid.
It is common knowledge that the underlying polybasic carboxylic
40 acids can be recovered by a~mixing such salt solutions with a
strong mineral acid, and indeed the carboxylic acids usually pre-
cipitate and are easy to separate off. However, appreciable
amounts of inorganic salts are produced as well.
45 It is also known to use electrodialysis to completely free poly-
basic carboxylic acids from their salts in aqueous solution and
to recover them from the resulting solution for example by
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crystallization. Since the free carboxylic acids frequently have
a lower solubility in water than their salts, the use of very
concentrated salt solutions will cause them to crystallize out in
the electrodialysis cell toward the end of the process, a
5 consequence of which can be that the cell becomes plugged and is
damaged if the flow of electric current continues.
According to DE-A-2 505 735, citric acid can be recovered from
aqueous solutions of its alkali metal salts by freeing it by
10 electrodialysis, preferably to from 90 to 99.9%, and crystalliz-
ing the free acid from the resulting solution. Owing to its high
solubility in water, citric acid does not precipitate in the
electrodialysis stage.
15 It is an object of the present invention to provide a widely
usable and economical process for recovering polybasic carboxylic
acids from their salts using electrodialysis.
We have found that this object is achieved by a process for
20 recovering saturated aliphatic or cycloaliphatic dicarboxylic
acids having from 5 to 10 carbon atoms (I) and polybasic carbox-
ylic acids of the general formula II
HOOC A - N - A - COOH
1 (II)
I
COOH
where the A groups are each independently of the others a Cl- to
C4-alkylene group and n is from 1 to 4, by electrodialysis of
aqueous solutions of salts or partial salts of these carboxylic
35 acids, which comprises
a) carrying on the electrodialysis until abput 90~ of the car-
boxylic groups are present in free form,
40 b) separating the carboxylic acid from the resulting solution
outside the electrodialysis cell as a whole or in part, and
c) a~m;x;ng the rPm~;n;ng solution with fresh salt and recircu-
lating it into the electrodialysis stage.
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Suitable saturated aliphatic or cycloaliphatic dicarboxylic acids
having from 5 to 10 carbon atoms (I) are preferably cycloali-
phatic representatives having 5-, 6- or 7-membered rings such as
1,2-cyclopentanedicarboxylic acid, 1,2-cyclohexanedicarboxylic
5 acid and 1,4-cyclohexanedicarboxylic acid and also especially
aliphatic representatives having unbranched alkyl chains such as
glutaric acid, adipic acid, subaric acid and sebacic acid.
The aliphatic or cycloaliphatic groups of said dicarboxylic acids
10 I can be interrupted by oxygen and/or sulfur and carry substitu-
ents, for example halogens, or nitro, cyano or ester groups.
Preferred polybasic acids of the general formula II
- -
HOOC - A - N - A COOH
A (II)
I
COOH
where the A groups are each independently of the others a Cl- to
C4-alkylene group and n is from 1 to 4, are those in which the A
25 groups are each independently of the others a Cl- or C2-alkylene
group and n is from 1 to 3. Particular preference is given to
carboyxlic acids II of the general formula IIa
/ CH2-COOH
R-N (IIa)
~ CH2-COOH
where R is one of the following radicals:
--(CH2)m--COOH
where m is 1 or 2, or
/ CH2-COOH
- CH2 CH2 N - CH2 CH2 N
\ CH2-COOH
CH2 - COOH
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To recover said free carboxylic acids I and II, the process of
the present invention starts from the aqueous solutions of their
salts or partial salts, which are preferably ammonium salts and
particularly preferably alkali metal salts.
The ammonium salts preferably contain ammonium cations NX4+ in
which the X substituents are each independently of the others
hydrogen or Cl- to C5-alkyl groups, as in tetraethylammonium, te-
tra-n-butylammonium or triethylmethylammonium cations, and espe-
10 cially in ammonium (NH4+) or tetramethylammonium cations.
The preferred alkali metal salts include lithium, sodium and po-
tassium salts, of which sodium and potassium salts are very par-
ticularly preferred.
Also suitable are mixed salts of said carboxylic acids I or II,
ie. salts with different cations.
According to the present invention, the electrodialysis is car-
20 ried on until about 90% of the carboxylic groups are present in
free form.
Of particular importance for this is electrodialysis apparatus in
which the salt-forming cations pass through a cation exchange
25 membrane (KAM) from the carboxylic acid compartment (CK) into the
cathode compartment (KK). This cathode compartment acts as "base
compartment", since the cations passing into it generally combine
with the hydroxyl ions formed in the cathode reaction according
to equation (1)
1) 2 H2O > H2 + 2 OH+
to form bases:
CK KK
(anode side) F----KAM----K
CK = carboxylic acid compartment
KK = cathode compartment ("base compartment")
40 F = anode side bounding surface of carboxylic acid compartment
KAM = cation exchange membrane
K = cathode
The anode side bounding surface F of the carboxylic acid compart-
45 ment supplies further protons to compensate the loss of positive
charge carriers.
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The anode side bounding surface F can be the anode itself, where
protons are formed according to equation (2)
5 2) 2 H20 > 2 + 4 H+
and sequences which are preferably composed of anode, anode
compartment and cation exchange membrane and especially of anode,
anode compartment and bipolar ion exchange membrane (hereinafter
called "bipolar membrane") and where the carboxylic acid salt
10 solution does not come into contact with the anode. This circum-
vents the danger of the carboxylic acid corroding the anode and/
or being oxidized at the anode.
In the case of the sequence of anode, anode compartment and cat-
15 ion exchange membrane preferred for F, the charge in the carbox-
ylic acid compartment is normally balanced by protons entering
from the anode side compartment. There these protons can come for
example from the anodic oxidation of water as per equation (2).
20 It is also possible to guide the solution of the carboxylic acid
salt on leaving the carboxylic acid compartment CK through the
anode side compartment, for example through the anode compartment
AK (or generally through a second carboxylic acid compartment
CK', see below)l where a further depletion in the salt-forming
25 cations, now into the compartment CK, takes place:
AK CK KK
A----KAM----KAM----K
AK = anode compartment
30 A = anode
Protons which, in the course of this process, pass instead of the
salt-forming cations from the anode compartment AK into the CK
compartment come in useful for the first electrodialysis step
35 taking place there.
Of particular preference for the process of the present invention
are electrodialysis cells in which F represents a sequence of
anode, anode compartment and bipolar membrane, schematically:
AK CK KK
A----BPM----KAM----K
BPM = bipolar membrane
45 Such cells, being comparatively simple to construct, are particu-
larly economical.
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Bipolar membranes used in electrodialysis will usually produce
hydroxyl ions on the anode side and protons on the cathode side
through lysis of water.
5 For this reason the suitable cation exchange membrane-compartment
arrangements (cells) can also be arranged and operated in paral-
lel as cell stacks with interposition of bipolar membranes. The
cathode-nearest base compartment (BK) then also acts as the cath-
ode compartment.
In a preferred embodiment of the process of the present inven-
tion, the electrodialysis apparatus used comprises a membrane-
compartment arrangement as per
AK CK' CK BK
A----(BPM~ KAM----KAM----)nK
CK' = second carboxylic acid compartment
BK = base compartment
where n is from 1 to 500, preferably from 1 to 300. The carbox-
ylic acid salt solution generally flows in succession through the
compartments CK and CK' of the respective cell.
25 In a particularly preferred embodiment, the electrodialysis appa-
ratus used comprises a membrane-compartment arrangement as per
AK CK BK
A--------(BPM--------KAM--------)nK
where n is from 1 to 500, preferably from 1 to 300.
The n membrane-compartment arrangements can be bound to the elec-
trodes directly (see above) or with interposition of further ion
35 exchange membranes and compartments.
Suitable cation exchange membranes are those based on styrene-
divinylbenzene copolymers functionalized with sulfonic acid
groups or other anionic groups or on perfluorinated polymers
40 which carry such functional groups.
The suitable bipolar membranes can be of the single film type
(see for example US-A-4 057 481) or be put together from cation
exchange membranes and anion exchange membranes (produced for
45 example as described in EP-A-0 193 959 or J. Electrochem. Soc.
131 (1984), 2810-14. Suitable anion exchange membranes are those
based on styrene-divinylbenzene copolymers functionalized with
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quaternary ammonium groups. In the electrodialysis cell, the
bipolar membranes are arranged with the anion exchange side fac-
ing the anode.
5 Suitable cation and anion exchange membranes are commercially
available.
The membranes suitable for the purposes of the present invention
customarily range in thickness from 0.1 to 1 mm. The membrane
10 spacing is generally within the range from 0.4 to 3 mm.
Suitable anode materials are platinized titanium and platinum;
suitable cathode materials are stainless steel and platinum.
15 As for the rest, electrodialysis and in particular the construc-
tion of the cells used is known to the person skilled in the art
(see for example German Application P 42 19 758.9), so that there
is no need for further observations.
20 The electrodialysis of the present invention is carried on until
about 90% of the carboxyl groups are present in free form.
More particularly, the electrodialysis is carried on until in the
case of a q-basic carboxylic acid, where q is from 2 to 6, the
25 percentage of the carboxyl groups in salt form is within the
range from (100/q) 0.7 to (100/q) 1.3, ie. for example in the
case of a tribasic carboxylic acid (II) within the range from
23.3 to 43.3%.
30 The process is carried out in particular in such a way that no
significant amounts of precipitate are formed in the electro-
dialysis cell, but having regard to m~X; mi zing the economics of
the process the carboxylic acid salt solutions used are generally
very concentrated. The maximum concentration in a specific case
35 is determined in particular by the degree of freeing desired and
the solubility of the partially converted carboxylic acid salts
at the particular electrodialysis temperature.
In general, the partially converted carboxylic acid salts are
40 more soluble in hot water than in water at room temperature,
which is why for a higher space-time yield the electrodialysis
temperature is chosen as high as possible, preferably within the
range from 45 to 80C.
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The pressure has no discernible effect on the electrodialysis,
which is why it is preferably carried out at atmospheric pres-
sure.
5 The current density is generally from 20 to 200, preferably from
80 to 120, mA/cm2 in the case of the use of bipolar membranes.
Otherwise it is preferably from 50 to 1000, especially from 100
to 400, mA/cm2.
10 The flow velocity in the cell compartments of the solutions used
in the electrodialysis ranges customarily from 0.001 to 2 m/sec,
especially from 0.01 to 0.2 m/sec.
The degree of the electrodialytic replacement of the salt-forming
15 cations by protons can be monitored in a conventional manner, for
example by means of pH and/or conductivity measurements and their
analysis by means of calibrating curves.
The electrodialysis stage (a) can be carried out batchwise, but
20 is preferably carried out continuously by the known techniques.
As for the rest, the apparatus and the procedure of electrodialy-
sis are known to the person skilled in the art (cf. for example
H. Strathm~nn, Trennung von molekularen Mischungen mit Hilfe
25 synthetischer Membranen, Steinkopf Verlag, Darmstadt, 1979, pages
76 to 86, or German Application P 42 19 758.9).
The solution resulting from process stage (a) has all or some of
the carboxylic acid separated off outside the electrodialysis
30 cell, preferably by means of crystallization or extraction, par-
ticularly preferably by crystallization.
The extraction is carried out in a conventional manner, for exam-
ple by repeatedly shaking up the electrodialysis exit stream with
35 the extractant, preferably with an organic solvent, especially an
ether, collecting the organic phases, and removing the extractant
by distillation.
In the case of a crystallization, some of the water can initially
40 be removed from the electrodialysis exit stream, especially by
means of distillation. This makes it possible in most cases to
increase the yield of free carboxylic acid in the crystallization
stage.
45 The crystallization is customarily carried out at from O to 20 C,
preferably at from 5 to 15 C.
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The electrodialysis solution resulting from process stage (a) can
be cooled to the final temperature of the crystallization in one
stage or preferably in more than one stage, care being taken to
ensure that no undesirable coprecipitation of carboxylic acid
5 salts occurs.
The precipitate can be separated from the liquid phase batchwise
or preferably continuously, advantageously by means of filtra-
tion, and is generally washed with water.
Crystallization and removal of the crystals can otherwise be car-
ried out in a conventional manner. Suitable apparatus is
described for example in U1lm~nn's Encyklopadie der technischen
Chemie, 4th edition, Verlag Chemie, Weinheim, Volume 2, page 679.
The separation according to the present invention by means of
crystallization or extraction makes it possible to recover the
carboxylic acids I and II with a residual level of salt-forming
cations of C0.05% by weight.
The solution r~in;ng following process stage (b), which usually
still contains small amounts of carboxylic acids I or II, is
admixed with fresh salt and recirculated into the electrodialysis
stage.
The fresh salt can be added as such or preferably in the form of
an aqueous solution in which it is usually obtained in the art,
in which case it is advantageous to withdraw some of the water
from the recirculating solution in order that a progressive
30 increase in the volume of the electrodialysis solution may be
avoided. The water can be removed using generally known processes
such as reverse osmosis or preferably distillation.
The carboxylic acid salt solution thus obtained is customarily
35 brought to the electrodialysis temperature while still outside
the cell and then recirculated into the electrodialysis stage.
The process of the invention can be carried out batchwise or
especially continuously, by the known techniques.
The process of the present invention is advantageous over those
processes where the carboxylic acids are completely freed by
electrodialysis because the normally higher conductivity of the
partially converted carboxylic acids compared with that of the
45 free carboxylic acid means that it has lower power requirements,
especially if operated continuously, and because the usually
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higher water solubility of the salts generally leads to higher
space-time yields.
The dicarboxylic acids I recovered by the process of the present
5 invention are suitable for use as monomers for polyamides and the
polybasic carboxylic acids II are suitable for use as complexing
agents in photographic developers.
Examples
Example 1
Recovery of adipic acid
A) Electrodialysis
a) Apparatus
The electrodialysis cell used had the following schematic
construction:
AK CK BK KK
A----BPM----KAM----BPM----K
AK = anode compartment
25 CK = carboxylic acid compartment
BK = base compartment
KK = cathode compartment
A = anode
BPM = bipolar membrane
30 KAM = cation exchange membrane
K = cathode
The anode and the cathode were each made of platinum. The
compartments CK and BK each had a separate outside cycle, while
35 compartments AK and KK had a common outside cycle. All outside
cycles contained pumps, heat exchangers and buffer vessels. The
outside cycles of CK and BK additionally contained pH and conduc-
tivity meters.
40 The cation exchange membrane used was a sulfonated styrene-
divinyl copolymer (SelemionR CMV, Asahi Glass). The bipolar mem-
branes were made as described in EP-A-0 193 959 from a cation
exchange membrane of the type SelemionR CMV and an anion exchange
membrane based on a styrene-divinylbenzene copolymer functional-
45 ized with quaternary ammonium groups (SelemionR AMV, Asahi Glass).
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The effective electrode and membrane areas were 37.3 cm2 and themembrane spacing was 3 mm.
b) Procedure
At the start the carboxylic acid cycle contained 220.9 g of an
aqueous solution with 2.15 mol/kg of adipic acid disodium salt,
while the base cycle contained 165.2 g of sodium hydroxide solu-
tion of the concentration 0.12 mol/kg. The common rinse cycle of
10 the compartments AK and KK contained sodium hydroxide solution of
the concentration 1 mol/kg.
The solutions were recirculated at an initial current strength of
3 A and 50 C for 8 hours. By the end of the run the current
15 strength had dropped to 1.7 A and the pH in the carboxylic acid
cycle was 5.2.
The carboxylic acid cycle contained 179.2 g of a solution con-
t~in;ng 2.62 mol/kg of protons and 2.57 mol/kg of sodium ions,
20 and the base cycle contained 204.9 g of sodium hydroxide solution
of the concentration 2.44 mol/kg.
The current efficiency was 70.3% in respect of the protons and
73.3% in respect of the sodium ions.
B) Crystallization
0.5 kg of an electrodialysis exit stream as per the preceding
section with 2.62 mol/kg of sodium ions and 2.61 mol/kg of pro-
30 tons was cooled down from 50 to 20 C. The precipitate was filteredoff, washed with 270 ml of 10 C water and dried at 70 C under
reduced pressure. The 51.07 g of crystals obtained had a sodium
content of 0.02% by weight and a water content of 0.17% by
weight.
The mother liquor still contained 1.15 mol/kg of protons and
2.18 mol/kg of sodium ions.
0.5 kg of adipic acid solution prepared by electrodialysis from a
40 sodium adipate solution and cont~ining about 0.55 mol/kg of
adipic acid (- saturation condensation at 50 C) merely yielded
27.4 g of dicarboxylic acid under the same crystallization condi-
tions.
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C) Extraction
0.5 kg of an aqueous solution of an adipic acid salt prepared as
per section (b) and cont~ining 0.994 mol/kg of sodium ions and
5 0.998 mol/kg of protons was extracted three times with 0.5 kg of
tert-butyl methyl ether each time. The organic phases were com-
bined, and the extractant was distilled off, leaving 24.1 g of
crystals having a sodium content of 0.02% by weight.
10 Example 2
Recovery of ~-alanine-N,N-diacetic acid
A) Electrodialysis
a) Apparatus; see Example 1
b) Procedure
20 At the start the carboxylic acid cycle contained 176.6 g of an
aqueous solution of a partial ~-alanine-N,N-diacetic acid sodium
salt with 0.89 mol/kg of sodium ions and 1.92 mol/kg of protons,
while the base cycle contained 159.5 g of sodium hydroxide solu-
tion of the concentration 2.52 mol/kg, and the common rinse cycle
25 of the compartments AK and KK contained sodium hydroxide solution
of the concentration 1 mol/kg.
At a current strength of 1.4 A and 50 C, the carboxylic acid
compartment was fed with 92.9 g of an aqueous technical grade
30 solution cont~;ning 0.886 mol/kg of ~-alanine-N,N-diacetic acid
trisodium salt and 0.25 mol/kg of sodium hydroxide over 6 hours,
and 79.8 g of a solution having a sodium ion content of
0.92 mol/kg and a proton content of 1.99 mol/kg was removed.
35 At the same time, the base compartment cycle was continuously
supplied with 69.9 g of water and 79.7 g of sodium hydroxide
solution of the concentration 2.42 mol/kg were removed.
At the end of the run the carboxylic acid cycle still contained
40 170.5 g of a solution of ~-alanine-N,N-diacetic acid cont~;ning
0.92 mol/kg of sodium ions and 1.99 mol/kg of protons. The base
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cycle contained 166.1 g of a sodium hydroxide solution of the
concentration 2.42 mol/kg.
The current efficiency was 63% in respect of sodium ions and 58%
5 in respect of the protons (sodium hydroxide solution included in
the starting solution).
B) Crystallization
10 0.5 kg of a 50 C electrodialysis exit stream obtained as per sec-
tion (b) with 0.96 mol/kg of sodium ions and 1.84 mol/kg of pro-
tons was cooled down to 0 C. The precipitate was filtered off,
washed with 100 ml of 0 C water and dried at 70 C, yielding 24.6 g
of ~-alanine-N,N-diacetic acid having a sodium content of 0.003%
15 by weight.
The mother liquor still contained 1.13 mol/kg of sodium ions and
1.02 mol/kg of protons.
20 0.5 kg of an aqueous ~-alanine-N,N-diacetic acid solution prepared
by electrodialysis from the trisodium salt and cont~;n;ng about
0.13 mol/kg of the tricarboxylic acid (= saturation concentration
at 50 C) merely yielded 13.1 g of carboxylic acid under the same
crystallization conditions.
