WO2013041448A1

WO2013041448A1 - Removal of low molecular weight organic compounds from inorganic halide solutions

Info

Publication number: WO2013041448A1
Application number: PCT/EP2012/068016
Authority: WO
Inventors: Luca SAONER; Joao MARQUES
Original assignee: Solvay Sa
Priority date: 2011-09-19
Filing date: 2012-09-13
Publication date: 2013-03-28
Also published as: AR087965A1

Abstract

Use of nanofiltration membranes to reduce the content of low molecular weight organic compounds having a molecular weight of at most 200 Dalton in solutions containing at least 500 ppm of an inorganic halide, wherein the solutions exhibit a pH value higher than or equal to 2.

Description

Removal of low molecular weight organic compounds from inorganic halide solutions

[0001] The present application claims benefit of European patent application n° 11181852.2 filed on September 19, 201 1 the content of which is incorporated herein by reference for all purposes.

[0002] Should the disclosure of any of the patents, patent applications, and

publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0003] The instant invention relates to the use of certain membranes to reduce the content of low molecular weight organic compounds from inorganic halide solutions having a high halide content.

[0004] Inorganic halide solutions are useful for the manufacture of hydroxide solutions and elementary halogen by electrochemical processes, in particular by electrolysis. The most widely used commercial process of this type is the so called chlor-alkali electrolysis in which chlorine and caustic soda are prepared via the electrolysis of sodium chloride solutions (often also referred to as brine). The most economic process makes use of ion- exchange membranes which transport cations from the anodic side to the cathodic side and thus chlorine is obtained in the anodic chamber and sodium hydroxide in the cationic chamber. In order to operate the process economically, brine has to be recycled and quite frequently such brine contains organic compounds as contaminants. Such contamination negatively affects the energy consumption of the electrolysis process as well as the lifetime of the membrane and thus it is desirable to remove these components prior to recycling the brine to the electrolysis process.

[0005] US patent n° 5,445,741 of SOLVAY DEUTSCHLAND GmbH discloses the oxidation of the organic compounds by chlorine. A significant part of the organic compounds can be successfully removed but it remains the fact that the degradation respectively removal of some types of organic compounds is not fully satisfactory. Among these, low molecular weight organic compounds, e.g. low molecular weight organic acids may be particularly mentioned.

[0006] A suitable process would have to provide a reduced content in low

molecular weight organic compounds while not significantly influencing the halide content in the brine solution as the latter is important for an economic operation of the electrolysis process.

[0007] Nanofiltration is a membrane based pressure driven separation process.

The driving force of the separation process is the pressure difference between the feed and the filtrate. Generally, nanofiltration membranes have so-called molecular weight cut-off (MWCO) values of approximately 200 to 1000 Dalton. Compounds having a molecular weight exceeding the molecular weight cut-off are retained on the feed side of the membrane whereas lower molecular weight compounds permeate through the membrane.

[0008] Choi et al, Separation and Purification Technology 59(2008), 17-25

describe the results of a study of the removal of organic acids from wastewaters using nanofiltration membranes and report that compounds close to or exceeding the MWCO value are rejected to a degree of more than 90 % irrespective of the operating pressure. They could also observe a certain rejection of compounds having lower molecular weight, in particular acetic acid to name one specified example. They report that the rejection of lower molecular weight compounds can be negatively influenced by the presence of salts, in particular inorganic halides like e.g. sodium chloride. Fig. 7 shows this effect for concentrations of 0.4 and 4 mM NaCI.

[0009] The halide concentration of salt solutions used in electrolysis must be significantly higher (by orders of magnitude) than the salt concentrations tested in Choi et al. because otherwise the process could not be operated in an economically feasible manner. Taking into account the results of Choi, one would have assumed that the rejection of low molecular weight compounds in solutions with a high halide content would decrease significantly, [0010] The salt rejection of the available nanofiltration membranes typically is in the range of from 10 to 100 %, bivalent salts being rejected basically quantitatively and monovalent salts being usually rejected in an amount of from 10 to 80 %. This degree of rejection would be inacceptable in halide solutions used in electrolysis as the lower halide content of the purified solution would directly negatively impact the costs of the process. The broadest field of application for nanofiltration membranes is actually the preparation of potable water from salt water, i.e. the rejection of salts.

[0011] Overall, there still exists a need for a process to free halide solutions with high halide content from low molecular weight impurities without detrimentally affecting the halide content.

[0012] This object is achieved in accordance with the instant invention by the use of nanofiltration membranes to reduce the content of low molecular weight organic compounds, having a molecular weight of at most 200 Daltons , in solutions containing at least 500 ppm of an inorganic halide, wherein the solutions exhibit a pH value higher than or equal to 2.

[0013] By low molecular weight organic compounds, one intends to denote ate least one low molecular organic compound.

[0014] The content of the inorganic halide and the pH of the solutions are to be understood as the content and the pH of the solutions before submission to nanofiltration.

[0015] This object is therefore achieved in accordance with the instant invention by the use of nanofiltration membranes to reduce the content of at least one low molecular weight organic compound, having a molecular weight of at most 200 Daltons , in solutions containing at least 500 ppm of an inorganic halide, wherein the solutions exhibit a pH value higher than or equal to 2.

[0016] The low molecular weight organic compounds have a molecular weight preferably of at most 150 Dalton, more preferably of at most 100 Dalton, yet more preferably of at most 80 Dalton and still more preferably of at most 70 Dalton. Low molecular weight organic compounds having a molecular weight of at most 50 Dalton are also convenient. [0017] Nanofiltration is commonly allocated between the separation limits of reversed osmosis and ultrafiltration. The pressure applied is generally within the range of from 0.2 to 5 MPa, in particular in the range of from 0.3 to 4 MPa, and more specifically in the range of from 0.3 to 1.1 MPa.

[0018] Generally, nanofiltration membranes comprise a support structure and a selective layer which may be a composite of several layers in itself. The selectivity of a nanofiltration membrane is governed by essentially two parameters - the molecular weight and the electric charge of the compounds. The permeability is higher for monovalent ions in diluted solution, bivalent ions being normally rejected to a very high degree.

[0019] Nanofiltration membranes are known to the skilled person and

commercially available from a number of suppliers. In principle any nanofiltration membrane may be used in accordance with the instant invention, there is no specific limitation as to composition or structure of the membrane. Dow Chemical through its subsidiary Filmtec and Koch Membrane systems are two suppliers offering a broader range of nanofiltration membranes. Other suppliers are Toray, Fluid systems or Nitto Denko Corporation and reference is made to the respective product leaflets of these suppliers.

[0020] In accordance with a preferred embodiment membranes as described in EP 355 188 may be preferably used and reference is made to this patent for further details. Other patent applications describing suitable

membranes are EP 1 080 777 and EP 2 269 717, the latter describing cross linked membranes on the basis of cellulose.

[0021] Good results have in certain cases been obtained with membranes ofr the DOW Filmtec^® NF series and NF-270 may be mentioned here as a specific example. NF 270 membranes have a structure comprising a semi- aromatic piperazin based polyamide layer on top of a polysulfone microporous support reinforced with a polyester non-woven backing layer.

[0022] The barrier layer of the nanofiltration membranes of Filmtec^® nanofiltration membranes is shown below. Basically it is an aromatic/aliphatic polyamide with amine and carboxylate end groups: [0023]

[0024] Nanofiltration membranes with a MWCO value of less than 1000,

preferably less than 500 and most preferably in the range of from 150 to 350 are can preferably be used in accordance with the instant invention.

[0025] The nanofiltration membranes are used in accordance with the instant invention to reduce the content of low molecular weight organic compounds, in particular compounds having a molecular weight of 200 Daltons , particularly preferred of less than 100 Daltons, in solutions containing at least 500 ppm of inorganic halide and exhibiting a pH value higher than or equal to 2.

[0026] As low molecular weight organic compounds with a molecular weight of at most 200 Dalton may be mentioned organic acids, alcohols, aldehydes and other organic impurities commonly present in inorganic halide solutions, in particular halide solutions used in electrolysis processes.

[0027] In the use according to the invention, the organic compound can be

selected from the group consisting of an acid, an alcohol, an aldehyde, and any mixture thereof.

[0028] The acid is more preferably an aliphatic acid. An acid selected from the group consisting of a monocarboxylic acid, a polycarboxylic acid, and any mixture thereof, is convenient. The monocarboxylic acid is preferably a fatty acid, more preferably selected from the group consisting of hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, and any mixture thereof. The monocarboxylic acid is also preferably selected from the group consisting of formic acid, acetic acid, propionic acid, butanoic acid, pentanoic acid, and any mixture thereof, and is more preferably acetic acid. The polycarboxylic acid is preferably selected from the group consisting of a dicarboxylic acid, a tricarboxylic acid, and any mixture thereof. The polycarboxylic acid is more preferably a dicarboxylic acid, yet more preferably selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and any mixture thereof, still more preferably from the group consisting of oxalic acid, adipic acid, and any mixture thereof and yet more preferably oxalic acid.

[0029] The aliphatic acid can also be a halo carboxylic acid, preferably selected from the group consisting of monochloroacetic acid, dichloroacetic acid, trichloroacetic acid and any mixture thereof.

[0030] The aliphatic acid can also be an hydroxyl carboxylic acid preferably

selected from the group consisting of glycolic acid, lactic acid,

hydroxybutyric acid, and any mixture thereof.

[0031] The alcohol is preferably selected from the group consisting of methanol, ethanol, propanol, butanol, glycerol, and any mixture thereof.

[0032] The aldehyde is preferably selected from the group consisting of

formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, glyceraldehyde, and any mixture thereof.

[0033] In the use according to the invention, the organic compound is preferably an acid as described above.

[0034] In particular, low molecular weight organic acids such as formic acid, acetic acid, propionic acid, succinic acid or citric acid may be mentioned here, in particular acetic acid is frequently present in sodium chloride solutions used in chlor-alkali electrolysis, more specifically sodium chloride solutions recovered from the synthesis of chlorinated organics [which may be recycled in chlor-alkali electrolysis.

[0035] In the use according to the invention, the organic compound is preferably an acid as described above, and more preferably acetic acid

[0036] Thus, the use in accordance with the instant inventions is particularly preferably applied to sodium chloride solutions used for the production of caustic soda and chlorine via electrolysis. [0037] In the use in accordance with the invention, the solutions preferably exhibit a pH value higher than or equal to 3, more preferably higher than or equal to 4, yet more preferably higher than or equal to 5, still more preferably higher than or equal to 7, and most preferably higher than or equal to 9.

[0038] Depending on the type of low molecular weight compound the content of which is to be reduced it has been shown advantageous to further modify the pH of the salt solution. In case of organic acids which are to be removed, it may be advantageous if the pH of the solution exceeds the pK_a value of the organic acid as ions tend to be better retained by nanofiltration membranes than neutral compounds. Above their pK_a value organic acids are present predominantly in dissociated from, i.e. ionized.

[0039] When the organic acid is a mono carboxylic acid, it is advantageous that the solutions exhibit a pH value higher than or equal to the pKa value of the mono carboxylic acid. When the organic acid is a poly carboxylic acid, it is advantageous that the solutions exhibit a pH value higher than or equal to the lowest pKa value of the poly carboxylic acid, and more advantageous that the solutions exhibit a pH value higher than or equal to the highest pKa value of the poly carboxylic acid.

[0040] The concentration of the low molecular weight compounds in the inorganic halide solutions can vary over a wide range. Typically the content by weight is at least 10, at least 20, at least 50 or at least 100 ppm and typically at most 10 000, at most 5000, at most 500 and at most 200 ppm. These concentrations are understood to be the concentrations in the solutions before submission to nanofiltration.

[0041] The inorganic halide are usually selected from alkali metal halides or

alkaline earth metal halides, in particular selected from e.g. LiHal, NaHal, KHal, CsHal, Mg(Hal)2 and Ca(Hal)2 with Hal being selected from Fluorine, Chlorine, Bromine or Iodine. Chlorides are the preferred halides..

Particularly preferably the use in accordance with the instant invention can be applied to so-called brine solutions, i.e. aqueous sodium chloride solutions having a sodium chloride concentration of at least 500 ppm.

[0042] The halide content of the inorganic halide solution can vary over a wide range and may be at least 1000 ppm, at least 2000 ppm, at least 5000 ppm or at least 10 000 ppm. The upper concentration is in principle only limited by the solubility of the respective halide in water and can be preferably at most 300 000 ppm, at most 200 000 ppm, at most 100 000 ppm, at most 50 000 ppm or at most 25 000 ppm. A halide content of at least 200 000 ppm is also convenient.

[0043] The pressure applied in the use in accordance with the instant invention is typically at least 150 000 kPa, at least 300 000 kPa or at least 1 MPa. The upper limit is determined by the mechanical stability of the membrane and may be typically at most 5 MPa, at most 4 MPa, at most 2.5 MPa or at most 1.5 MPa.

[0044] There is no need for a special construction or equipment for the use in accordance with the instant invention. As long as a supply of the halide solution to the membrane and a take-off line for the purified product is present, the design is not critical. Of course, necessary equipment to apply the appropriate working pressure has to be provided.

[0045] The temperature again is basically only limited by the mechanical stability of the membrane; usually temperatures in the range of from 15 to 80 °C, in particular in the range of from 20 to 60 °C may be used, room temperature being particularly preferred as it eliminates the need for providing thermal energy to the system.

[0046] In accordance with the use in accordance with the invention, the content of the low molecular weight organic compounds can be reduced by at least 40, preferably at least 50 and particularly preferably at least 60 %.

[0047] Preferably the content of inorganic halide in the permeate is at least 70, preferably at least 80 and particularly at least 90 % of the content of the starting solution fed to the purification.

[0048] Since nanofiltration membranes are commonly and most often used to desalinate salt water it was completely unexpected and surprising that same could also be used to purify inorganic halide solutions having a high halide content without a detrimental influence on the halide content.

[0049] Another aspect of the invention relates to a process for reducing the

content of low molecular weight organic compounds having a molecular weight of at most 200 Dalton in inorganic halide solutions with a halide content of at least 500 ppm, wherein the solutions exhibit a pH value higher than or equal to 2, and wherein said process comprising the steps of feeding an inorganic halide solution to a pressurizable compartment with a nanofiltration membrane, preferably applying a pressure in the range of from 0.15 to 5 MPa and taking off the permeate from the system. Detailed information concerning the nanofiltration membranes and the other educts or reactants of the process as well as preferred embodiments concerning the final product can be taken from the previous detailed description relating to the use in accordance with the instant invention, to which reference is made.

[0050] According to a first preferred embodiment of the process in accordance with the instant invention, the low molecular weight compounds have a molecular weight of at most 100 Dalton and the content of said low molecular weight compounds having a molecular weight of at most 100 Dalton is reduced.

[0051] According to a further preferred embodiment a halide solution with a halide content of at least 1000 ppm is used.

[0052] In accordance with still another embodiment the content of aldehydes, acids or alcohols, in particular acids and especially particular acetic acid is reduced.

[0053] A still other preferred embodiment of the process in accordance with the instant invention uses sodium chloride solutions.

[0054] Example 1

[0055] An aqueous solution containing 15 000 ppm of sodium chloride and 100 ppm of acetic acid with a pH of 9 was fed at room temperature to a nanofiltration membrane available from DOW under the designation Filmtec NF 270-400 under a working pressure of 0.3 to 1.1 MPa.

[0056] The permeate contained acetic acid in an amount of from 26 to 49 ppm whereas the sodium chloride content remained at 15 000 ppm.

[0057] This result shows that the content of low molecular weight compounds in the halide solution was reduced by more than 50 % without diminishing the halide content.

[0058] Example 2 [0059] Example 1 was repeated but with a solution containing 25 000 ppm of sodium chloride. For working pressures exceeding 1 ,1 MPa, the acetic acid concentration in the permeate was stable at 31 ppm, i.e. nearly 70 % of the acetic acid was removed. No decrease in halide concentration was detected in the permeate.

[0060] The foregoing description and examples are explaining the instant

invention without limiting same and the skilled person can easily modify the type of membrane or the nature of the salt solution in a manner suitable for his purposes.

Claims

1. Use of nanofiltration membranes to reduce the content of at least one low

molecular weight organic compound having a molecular weight of at most 200 Dalton in solutions containing at least 500 ppm of an inorganic halide, wherein the solutions exhibit a pH value higher than or equal to 2.

2. Process for reducing the content of at least one low molecular weight organic compound having a molecular weight of at most 200 Dalton in inorganic halide solutions with a halide content of at least 500 ppm wherein the solutions exhibit a pH value higher than or equal to 2, said process comprising the steps of feeding an inorganic halide solution to a pressurizable compartment with a nanofiltration membrane, and taking off the permeate from the system.

3. Use in accordance with claim 1 or process in accordance with claim 2, wherein the low molecular weight compound has a molecular weight of at most 150 Dalton.

4. Use or process in accordance with claim 3 wherein the low molecular weight compound has a molecular weight of at most 100 Dalton.

5. Use or process in accordance with claim 4 wherein the low molecular weight compound has a molecular weight of at most 80 Dalton.

6. Use or process in accordance with claim 5 wherein the low molecular weight compound has a molecular weight of at most 70 Dalton.

7. Use or process in accordance with claim 6 wherein the low molecular weight compound has a molecular weight of at most 50 Dalton

8. Use in accordance with any one of claims 1 and 3 to 7 or process in

accordance with any one of claims 2 to 7, wherein the halide content is at least 1000 ppm.

9. Use or process in accordance with claim 8 wherein the halide content is at least 2000 ppm.

10. Use or process in accordance with claim 9 wherein the halide content is at least 5000 ppm.

11. Use or process in accordance with claim 10 wherein the halide content is at least 10000 ppm.

12. Use or process in accordance with claim 11 wherein the halide content is at least 50000 ppm.

13. Use in accordance with any one of claims 1 and 3 to 12 or process in

accordance with any one of claims 2 to 12, wherein the halide content is at most 300 000 ppm.

14. Use in accordance with any one of claims 1 and 3 to 13 or process in

accordance with any one of claims 2 to 13, wherein the content of the low molecular weight compound is at least 10 ppm.

15. Use or process in accordance with claim 14 wherein the content of the low molecular weight compound is at least 50 ppm.

16. Use or process in accordance with claim 15 wherein the content of the low molecular weight compound is at least 100 ppm.

17. Use in accordance with any one of claims 1 and 3 to 16 or process in

accordance with any one of claims 2 to 16, wherein the content of the low molecular weight compound is at most 10000 ppm.

18. Use in accordance with any one of claims 1 and 3 to 17 or process in

accordance with any one of claims 2 to 17, wherein the working pressure is at least 150 kPa.

19. Use in accordance with any one of claims 1 and 3 to 18 or process in

accordance with any one of claims 2 to 18, wherein the organic compound is selected from the group consisting of acids, alcohols, aldehydes, and any mixture thereof.

20. Use or process in accordance with claim 19, wherein the organic compound is an acid.

21. Use or process in accordance with claim 20 wherein the acid is an aliphatic acid.

22. Use or process in accordance with claim 20 or 21 , wherein the acid is selected from the group consisting of a monocarboxylic acid, a polycarboxylic acid, and any mixture thereof.

23. Use or process in accordance with claim 22, wherein the acid is a

monocarboxylic acid.

24. Use or process in accordance with claim 23, wherein the monocarboxylic acid is a fatty acid. PatXML

25. Use or process in accordance with claim 24, wherein the fatty acid is selected from the group consisting of hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, and any mixture thereof.

26. Use or process in accordance with claim 23, wherein the monocarboxylic acid is selected from the group consisting of formic acid, acetic acid, propionic acid, butanoic acid, pentanoic acid, and any mixture thereof.

27. Use or process in accordance with claim 26, wherein the monocarboxylic acid is acetic acid.

28. Use or process in accordance with claim 22, wherein the acid is a

polycarboxylic acid.

29. Use or process in accordance claim 28, wherein the polycarboxylic acid is selected from the group consisting of a dicarboxylic acid, a tri-carboxylic acid, and any mixture thereof.

30. Use or process in accordance with claim 29, wherein the polycarboxylic acid is a dicarboxylic acid.

31. Use or process in accordance with claim 30, wherein the dicarboxylic acid is selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and any mixture thereof.

32. Use or process in accordance with claim 31 , wherein the dicarboxylic acid is oxalic acid.

33. Use or process in accordance with claim 19, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol, and any mixture thereof.

34. Use or process in accordance with claim 19, wherein the aldehyde is selected from the group consisting of formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and any mixture thereof.

35. Use in accordance with any one of claims 1 and 3 to 34 or process in

accordance with any one of claims 2 to 34, wherein the inorganic halide is selected from the group consisting of an alkali metal halide, an alkaline earth metal halide, and any mixture thereof.

36. Use or process in accordance with claim 35 wherein the alkali metal halide is sodium chloride. PatXML

37. Use in accordance with any one of claims 1 and 3 to 36 or process in

accordance with any one of claims 2 to 36, wherein the solutions exhibit a pH value higher than or equal to 3.

38. Use or process in accordance with claim 37, wherein the solutions exhibit a pH value higher than or equal to 4.

39. Use or process in accordance with claim 38, wherein the solutions exhibit a pH value higher than or equal to 5.

40. Use or process in accordance with claim 39, wherein the solutions exhibit a pH value higher than or equal to 7.

41. Use or process in accordance with claim 40, wherein the solutions exhibit a pH value higher than or equal to 9.

42. Use or process in accordance with any one of claims 23 to 27, wherein the solutions exhibit a pH value higher than or equal to the pKa value of the mono carboxylic acid.

43. Use or process in accordance with any one of claims 28 to 32, wherein the solutions exhibit a pH value higher than or equal to the lowest pKa value of the poly carboxylic acid.

44. Use or process in accordance with claim 43, wherein the solutions exhibit a pH value higher than or equal to the highest pKa value of the poly carboxylic acid.

45. Process in accordance with any one of claims 2 to 44, comprising applying a pressure in the range of from 0.15 to 5 MPa.

46. Process in accordance with claim 45 wherein the organic compound is

selected from the group consisting of acids, alcohols, aldehydes, and any mixture thereof and wherein the content of aldehydes, acids or alcohols is reduced.