CA2138113A1

CA2138113A1 - Recovery of carboxylic acids from their salts

Info

Publication number: CA2138113A1
Application number: CA 2138113
Authority: CA
Inventors: Hartwig Vos
Original assignee: Individual
Current assignee: BASF SE
Priority date: 1993-12-15
Filing date: 1994-12-14
Publication date: 1995-06-16
Also published as: JPH07216571A; EP0658371A1; DE4342668A1

Abstract

A process for recovering saturated aliphatic or cycloaliphatic dicarboxylic acids having from 5 to 10 carbon atoms (I) and poly-basic carboxylic acids of the general formula II

Description

` BASF Aktiengesellschaft 930460 O.Z. 0050/44514 213~ 13 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 BASF Aktiengesellschaft 930460 O.Z. 0050/44514 213811~

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.

BASF Aktiengesellschaft 930460 O.Z. 0050/44514 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 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

. BASF Aktiengesellschaft 930460 O.Z. 0050/44514 213Yli3 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.

BASF Aktiengesellschaft 930460 O.Z. 0050/44514 213~1 13 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.

BASF Aktiengesellschaft 930460 O.Z. 0050/44514 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 - BASF Aktiengesellschaft 930460 0.Z. 0050/44514 ` 213811~
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.

~ BASF Aktiengesellschaft 930460 0.Z. 0050/44514 21~8113 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.

BASF Aktiengesellschaft930460 O.Z. OOS0/44514 2 1 ~ 3 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 - BASF Aktiengesellschaft 930460 O.Z. 0050/44514 213~113 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).

BASF Aktiengesellschaft 93046~ 1 3 81 ~ 3 0050/44514 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.

` BASF Aktiengesellschaft 930460 2 1 3 8 1 ~z3 0050/44514 -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 ~ BASF Aktiengesellschaft 930460 213 8~ 0050/44514 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.

Claims

1. A process for recovering saturated aliphatic or cycloalipha-tic dicarboxylic acids having from 5 to 10 carbon atoms (I) and polybasic carboxylic acids of the general formula II

(II) where the A groups are each independently of the others a C1-to C4-alkylene group and n is from 1 to 4, by electrodialysis of aqueous solutions of salts or partial salts of these car-boxylic acids, which comprises a) carrying on the electrodialysis until about 90% of the carboxylic groups are present in free form, b) separating the carboxylic acid from the resulting solution outside the electrodialysis cell as a whole or in part, and c) admixing the remaining solution with fresh salt and recirculating it into the electrodialysis stage.

2. A process as claimed in claim 1 wherein the salts or partial salts of dicarboxylic acids I are those of saturated alipha-tic dicarboxylic acids having unbranched alkyl chains.

3. A process as claimed in claim 1 wherein the salts or partial salts of polybasic carboxylic acids II are those of the gen-eral formula IIa (IIa) where R is one of the following radicals:

-(CH2)m-COOH

where m is 1 or 2, or

4. A process as claimed in claim 1 wherein the starting salts are the sodium or potassium salts of the carboxylic acids.

5. A process as claimed in claim 1 wherein, in step (a), the carboxylic acid is freed to such an extent that in the case of a q-basic carboxylic acid the percentage of the carboxyl groups present in salt form is within the range from (100/q) 0.7 to (100/q) 1.3, where q is from 2 to 6.

6. A process as claimed in claim 1 wherein the electrodialysis is carried out at from 45 to 80 C.

7. A process as claimed in claim 1 wherein, in step (b), the carboxylic acid is separated off by crystallization of extraction.

8. A process as claimed in claim 1 wherein, in the electrodialy-sis cell, the compartment which is fed with the carboxylic acid in the form of its salt or partial salt is bounded on the anode side by a bipolar ion exchange membrane and on the cathode side by cation exchange membrane.

9. A process as claimed in claim 8 wherein the electrodialysis cell has the following construction: anode compartment/
bipolar ion exchange membrane/carboxylic acid compartment/
cation exchange membrane/base compartment.

10. A process as claimed in claim 9 wherein the electrodialysis apparatus comprises a side by side arrangement between the anode and the cathode of up to 300 cells of the construction:
bipolar ion exchange membrane/carboxylic acid compartment/
cation exchange membrane/base compartment.