Extraction of 1,6-hexanediamine using a combination of solvents

Abstract

The invention relates to the extraction of 1,6-hexanediamine from an aqueous mixture using a combination of solvents.

Description

[0001] This invention relates to the extraction of 1,6-hexanediamine from aqueous mixtures using solvent combinations. Technical Background

[0002] The depolymerization of polyamides and certain polyurethanes produces aqueous solutions containing diamines. This technical field describes various methods for removing diamines from water. An effective way to remove diamines from water and water-soluble impurities is to extract the diamine with a monovalent alcohol, followed by distillation. Because diamines such as 1,6-hexanediamine are highly polar, separating them with water-immiscible organic solvents is not easy, and therefore selecting a suitable organic solvent is difficult.

[0003] Monovalent solvents are known to be selective and sufficient, but due to the physicochemical properties of alcohols, the removal of water from diamines is limited, and a significant amount of water is transferred to the organic extraction phase, requiring distillation for removal. This increases the energy requirements of the method and reduces its advantages.

[0004] To minimize energy consumption during distillation, it is desirable to use low-polarity solvents. However, diamines such as 1,6-hexanediamine are typically highly polar, and their partition coefficients decrease with decreasing solvent polarity.

[0005] EP 2 806 026 A1 relates to the preparation of 1,5-pentanediamine. Within this method, a separation step from an aqueous solution is described. This separation can be carried out as an extraction method. The extractant can be an organic solvent, preferably an aliphatic alcohol.

[0006] US 2017 / 0369913 A1 describes the extraction of 1,6-hexanediamine from an aqueous solution using an organic solvent. Preferably, an alcohol is used. However, alkanes have also been tested and found to extract only small amounts of 1,6-hexanediamine, and also only small amounts of water. No combination of alkanes and alcohols is disclosed as more advantageous than using either an alcohol or an alkanes as a single compound.

[0007] Technical Purpose The problem to be solved by the present invention is to provide an improved method for extracting 1,6-hexanediamine from an aqueous mixture, which is economically efficient and, as far as possible, ecologically efficient compared with the prior art.

[0008] Technical solution This technical objective can be achieved by a method for separating 1,6-hexanediamine (HMDA) from an aqueous solution, the method comprising the following steps: a) Provide an aqueous solution containing HMDA by depolymerizing HMDA-based polymers. b) Extract the aqueous solution containing HMDA using a solvent mixture containing... (i) at least one monovalent alcohol having 4 to 7 carbon atoms, and (ii) at least one alkane having 5 to 7 carbon atoms, wherein the alkane content of the solvent mixture is in the range of 2% to 50% by weight based on the total weight of all alcohols and alkanes present in the solvent mixture; c) Obtain an HMDA-containing solvent mixture as the separation phase, and d) Distill the solvent mixture containing HMDA and separate HMDA from the solvent mixture.

[0009] It has been found that using a specific solvent mixture is beneficial for the extraction of HMDA. The first solvent is a monovalent alcohol having 4 to 7 carbon atoms, preferably 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, cyclopentanol, or cyclohexanol. As the second solvent, an alkane having 5 to 7 carbon atoms is added, preferably a n-alkane.

[0010] Typically, to reduce the energy requirements of distillation, the amount of water in the organic extract phase can be reduced by altering the properties of the organic solvent. To modify the properties of the organic solvent, additives can be introduced, such as nonpolar solvents like alkanes. However, diamines such as 1,6-hexanediamine are highly polar.

[0011] Therefore, it was expected that the partitioning into the modified organic solvent phase would be worse. Surprisingly, it was found that, with careful selection, modifying the organic extraction phase by adding additives not only led to a reduction in the water and energy requirements for distillation but also to improved partitioning. Even without an increase in the partition coefficient, the water content in the organic solvent and HMDA mixture was reduced, resulting in lower energy consumption during subsequent distillation. This effect must be balanced by the reduced HMDA concentration in the organic phase, which increases energy consumption.

[0012] Favorable technical effects First, a low water content in the extract can be achieved. Second, a surprisingly increased partition coefficient can be observed. As mentioned above, this technical effect of unipolar solvents on the extraction of polar compounds is unexpected. Invention Details According to the present invention, the expression "comprising", "containing" or "including" preferably means "consisting of substantially", and particularly preferably means "consisting of".

[0014] Unless otherwise stated, in the context of this invention, statements regarding the content of organic compounds refer to values ​​determined by gas chromatography. Those skilled in the art are familiar with the quantitative evaluation of gas chromatograms, optionally using internal standards. Those skilled in the art are also familiar with any methods that may be needed to determine water content. Methods known in the art can also be used in the context of this invention. In cases of doubt, the water content determined by Karl Fischer titration is decisive. The presence of amines can cause tailing endpoints. In such cases, the value determined by Karl Fischer titration after buffering with anhydrous benzoic acid is decisive. Those skilled in the art are also familiar with methods using benzoic acid buffers. For a general overview of Karl Fischer titration, see Jander, Jahr, Massanalyse, 17th edition, de Gruyter, Berlin (2009), pp. 279-282.

[0015] In this context, "distillation" is understood to refer to a thermal separation method for recovering evaporable substances, preferably liquids, from a composition. The separated vapors are then condensed. The term specifically covers repeated evaporation and condensation using columns (distillation columns) with multiple separation stages. Strictly speaking, this method, in which multiple distillation steps are arranged in series in a column, should be called rectification, but for simplicity, it will be referred to as distillation herein as well. The advantages of these methods are strong separation effects and the possibility of continuous operation of the equipment. Therefore, a "distillation apparatus" is a suitable device for carrying out such thermal separation methods, such as sieve tray columns, packed columns, packed bed columns, bubble cap tray columns, or even single-stage evaporators, such as falling film evaporators, thin-film evaporators, flash evaporators, multiphase spiral evaporators, natural or forced circulation evaporators.

[0016] In this case, the relevant pressure of the distillation apparatus is the dominant pressure at the top of each column, i.e., the pressure at which the vapor exits the distillation apparatus. Therefore, this pressure will also be referred to as the top pressure below.

[0017] The steps of each method will be explained in more detail below.

[0018] Step a) In the first step a) of the method of the present invention, an aqueous solution containing HMDA is provided by depolymerizing an HMDA-based polymer or a mixture of polymers containing an HDMA-based polymer. The polymer may be coated or may contain organic or inorganic fillers. The HMDA content is freely selectable to allow for efficient extraction of HMDA. The aqueous composition contains at least HMDA and water. Typically, it will contain other depolymerization products of the polymer. In the case of depolymerization of polyamides obtained from HMDA and dicarboxylic acids, particularly polyamides 6.4, 6.6, 6.9, 6.10, and 6.12, such additional depolymerization products may be the corresponding dicarboxylic acids. Furthermore, the aqueous composition may contain residues of catalysts and / or additives used in the depolymerization reaction, particularly salts. It may also contain residues of any material used as a coating or filler for the original polymer.

[0019] In a preferred embodiment, additional depolymerization products are separated from the aqueous composition in step a), such that the composition present at the beginning of step b) contains HMDA, residues of catalysts and / or additives used for the depolymerization reaction, but not other depolymerization products.

[0020] In another preferred embodiment of the invention, method step a) includes additional sub-steps for reducing the water content of the composition after depolymerization. This sub-step may be performed in addition to separating additional depolymerization products, or as an alternative thereof. A preferred method is to remove water using a distillation portion as described in PCT / EP2022 / 075933.

[0021] Regardless of any other components in the aqueous composition containing HMDA at the end of step a), the composition comprises 0.1 to 50% by weight of HMDA, preferably 5 to 20% by weight, and most preferably 7 to 22% by weight of HMDA. The water content of the composition is preferably at least 50% by weight, more preferably at least 55% by weight. All percentages are based on a total weight of 100% by weight of the composition. Furthermore, the aqueous solution containing HMDA typically also contains other substances besides water with boiling points lower than HMDA. The content of these substances with boiling points lower than amines is preferably ≤10% by weight, particularly preferably ≤5% by weight, and most particularly preferably ≤3% by weight. Meanwhile, in the technical process, this content is typically ≥0.01% by weight, preferably ≥0.3% by weight, unless the low-boiling substances have been separated beforehand. Furthermore, the aqueous solution containing HMDA typically also contains other substances with boiling points higher than amines. In the first composition, the proportion of these substances with boiling points higher than amines is preferably ≥0.01% by weight and ≤10% by weight in total, particularly preferably ≥0.1% by weight and ≤5% by weight, and most particularly preferably ≥0.3% by weight and ≤3% by weight.

[0022] The term "HMDA-based polymer" refers to all polymers that can release HMDA in single-molecule form using appropriate chemical reactions. Depending on the type of polymer, additional compounds may be released as additional depolymerization products. For polyamides obtained by reacting HMDA with dicarboxylic acids, the additional depolymerization product will be the corresponding dicarboxylic acid, its salt, or a derivative of that acid. Preferred "HMDA-based polymers" are polyamides and polyurethanes. Preferred polyamides are polyamide 6.4, polyamide 6.6, polyamide 6.9, and polyamide 6.10. Particularly preferred "HMDA-based polymer" is polyamide 6.6, which is a reaction product of adipic acid and HMDA.

[0023] The appropriate chemical reaction for releasing HMDA from a polymer obviously depends on the type of bonds present in the polymer. In the case of polyamides, this chemical reaction requires the cleavage of amide bonds to release HMDA. In the case of polyamides, hydrolysis is preferred as a method for cleaving amide bonds.

[0024] Preferred methods for hydrolyzing polyamide include ammonolysis, acidic hydrolysis, alkaline hydrolysis, glycolysis, microwave hydrolysis, and enzymatic hydrolysis. Among these, acidic and alkaline hydrolysis are preferred, with alkaline hydrolysis being particularly preferred.

[0025] Preferred reagents for acid hydrolysis are aqueous solutions of HCl, formic acid, sulfuric acid, and nitric acid. Suitable reaction temperatures for acid hydrolysis are in the range of 80 to 120°C. Acid hydrolysis is preferably performed at pressures of 1 to 6 bar. Suitable acid hydrolysis methods are described in US 3,069,465 and EP 0 833809.

[0026] The preferred reagent for alkaline hydrolysis is a solution of an alkali metal hydroxide in water or a mixture of water and alcohol. Preferred alkali metal hydroxides are NaOH and KOH. Particularly preferred are aqueous solutions of NaOH and KOH. Suitable reaction temperatures for alkaline hydrolysis are in the range of 140 to 220 °C. Alkaline hydrolysis is preferably carried out at a pressure of 0.8 to 150 bar. Suitable alkaline hydrolysis methods are described in EP 0 646 106 or US 2,840,606.

[0027] Ammonolysis is preferably carried out at a temperature of 300 to 350°C and a pressure of 34 to 172 bar. A suitable method is described in US 5,302,756.

[0028] It should be understood that method steps a) and b) may be performed at different locations and / or with time delays. In a preferred embodiment of the invention, the distance between the location where method step b) is performed and the location where method step a) is performed is at least 1 km, preferably at least 5 km, more preferably at least 10 km, and most preferably at least 20 km. In another preferred embodiment of the invention, the time delay between the end of method step a) and the start of method step b) is at least 1 hour, preferably at least 12 hours, more preferably at least 24 hours, and most preferably at least 72 hours.

[0029] In some embodiments of the invention, a method step prior to step a) may be included to promote depolymerization by breaking the polymer article into smaller particles to increase the surface area to volume ratio of the polymer. This can be achieved by any suitable method known to those skilled in the art.

[0030] Substances with lower boiling points than HMDA that can exist in aqueous solutions containing HMDA, besides water, include, for example, ammonia, alkylamines or alkenylamines, alcohols, ethylene glycol, cyclic amines such as pyrrolidine, piperidine, aziridine, imines, cyclic imines such as tetrahydropyridine or tetrahydroazines, or solvent residues such as hydrocarbons or halogenated hydrocarbons. Furthermore, gases such as nitrogen or carbon dioxide may exist in solution in soluble or combined forms.

[0031] Substances with higher boiling points than HMDA include dimers or oligomers of HMDA, which can be formed simultaneously with the cracking of ammonia from HMDA.

[0032] Aqueous solutions containing HMDA derived from chemical cycles of polyamides such as PA66 may also contain adipic acid, its salts, or PA66 fragments as substances with higher boiling points. Unless otherwise stated, statements regarding relative contents refer to the sum of substances with higher boiling points compared to HMDA.

[0033] The HMDA present in the solution provided in step a) is preferably in an uncharged form. This is typically the case when the pH is at least 11.5. Therefore, the aqueous solution containing HMDA in step a) preferably has a pH of at least 11.5, more preferably 12.0.

[0034] Step b) In the second step b) of the method of the present invention, an aqueous solution containing HMDA is extracted with a solvent mixture comprising (i) at least one monovalent alcohol having 4 to 7 carbon atoms; and (ii) at least one alkane having 5 to 7 carbon atoms, wherein the alkane content of the solvent mixture is in the range of 2% to 50% by weight based on the total weight of all alcohols and alkanes present in the solvent mixture.

[0035] Preferably, at least one monovalent alcohol having 4 to 7 carbon atoms is selected from 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, cyclopentanol, cyclohexanol, and mixtures thereof. More preferably, at least one monovalent alcohol having 4 to 7 carbon atoms is selected from 1-butanol, 1-pentanol, and 1-hexanol, and mixtures thereof. Most preferably, at least one monovalent alcohol having 4 to 7 carbon atoms is 1-hexanol.

[0036] At least one alkane having 5 to 7 carbon atoms may be straight-chain, branched, or cyclic. More preferably, at least one alkane having 5 to 7 carbon atoms may be straight-chain or cyclic. Most preferably, at least one alkane having 5 to 7 carbon atoms may be straight-chain.

[0037] Preferably, at least one alkane having 5 to 7 carbon atoms is selected from n-pentane, n-hexane, n-heptane, and mixtures thereof.

[0038] Preferably, based on the total weight of all alcohols and alkanes present in the solvent mixture, the alkane content of the solvent mixture is in the range of 2% to 40% by weight, more preferably 2% to 30% by weight, and most preferably 2% to 20% by weight.

[0039] Step c) In the third step c) of the method of the present invention, an HMDA-containing solvent mixture is obtained as the separation phase. There are no particular limitations on the measures taken to obtain the HMDA-containing solvent mixture as the separation phase. Generally, once two different phases are formed, the HMDA-containing solvent mixture can be separated from the aqueous phase after step b). This method can be performed in a mixing settler, extraction column, or centrifuge at 1 to 20 stages, preferably 3 to 15 stages, and most preferably 5 to 10 stages.

[0040] Step d) In the fourth step d) of the method of the present invention, the solvent mixture containing HMDA is distilled and the HMDA is separated from the solvent mixture.

[0041] The distillation apparatus used in step d) is not limited, but any distillation apparatus known in the art for such separation steps may be used. The distillation apparatus may be a single distillation column. A single distillation column may have 5 to 100, 10 to 60, or 20 to 50 theoretical stages. If 1-hexanol is a monovalent alcohol, 20 or more theoretical stages may be used, for example, 20 to 50 theoretical stages. The distillation pressure may be 100-1,000 mbar, 150-800 mbar, 150-700 mbar, 200-600 mbar, 200-500 mbar, or 200-400 mbar. Therefore, it is below atmospheric pressure of 1,013 mbar.

[0042] Any combination of implementation schemes, particularly preferred, more preferred, most preferred ranges and / or embodiments are suitable and optional, and are particularly preferred.

[0043] Brief description of the attached figures Figure 1 This is a graph showing the dependence of the partition coefficient and the water content of the solvent mixture on the ratio of butanol and hexane, as analyzed in Example 1. Figure 2 This is a graph showing the dependence of the partition coefficient and the water content of the solvent mixture on the ratio of hexanol and hexane, as analyzed in Example 2.

[0044] The present invention will be described in more detail in the following embodiments. However, these embodiments are for illustrative purposes only. Example

[0045] Example 1: Extraction of HMDA using 1-butanol The aqueous phase was prepared by dissolving 10% by weight of HMDA in 1 mole of NaOH aqueous solution. The organic phase was prepared by mixing various concentrations of n-hexane and 1-butanol. The organic phase was saturated with water before being used as the extraction organic phase. 100 g of the aqueous solution was mixed with 35 g of the water-saturated organic phase, and the temperature was adjusted to 50 °C. Mixing was then continued for at least 10 minutes to reach equilibrium. Subsequently, the phases were allowed to settle by gravity until complete separation of the two phases. The concentration of HMDA in both phases was measured by HPLC-MS. For evaluation, the partition coefficient was calculated as the ratio of the concentration of HMDA in the organic phase to the concentration of HMDA in the aqueous phase, and normalized relative to the highest measured value. In addition, the dissolved water in the organic alcohol / alkane mixture was measured by Karl Fischer titration, and normalized relative to the highest measured value.

[0046] A hexane mass fraction of 5 to 10 wt% in the organic phase increases the partition coefficient. Furthermore, the water content of the organic phase decreases with increasing hexane concentration. The aqueous phase was prepared by dissolving 10 wt% HMDA in 1 mole of NaOH aqueous solution. The organic phase was prepared by mixing various concentrations of n-hexane and 1-hexanol. The organic phase was water-saturated before being used as the extraction organic phase. 100 g of the aqueous solution was mixed with 35 g of the water-saturated organic phase, and the temperature was adjusted to 50 °C. Mixing was then continued for at least 10 minutes to reach equilibrium. Afterward, the phases were allowed to settle by gravity until complete separation. The concentration of HMDA in both phases was measured by HPLC-MS. For evaluation, the partition coefficient was calculated as the ratio of the concentration of HMDA in the organic phase to the concentration of HMDA in the aqueous phase, and normalized relative to the highest measured value. Additionally, the dissolved water in the organic alcohol / alkane mixture was measured by Karl Fischer titration, and normalized relative to the highest measured value.

[0047] For 1-hexanol, the addition of hexane does not increase the partition coefficient. However, the reduction in the water content of the organic phase simultaneously reduces the energy requirement during the separation of HMDA from the solvent in the distillation process, provided that the content is not too high. Typically, a hexane mass fraction of 40% by weight is the limit, at which point the energy reduction due to the lower organic phase water content offsets the increased energy consumption due to the need to remove excessive organic solvent.

Claims

1. A method for separating 1,6-hexanediamine (HMDA) from an aqueous solution, comprising the following steps: a) Providing diamine-containing aqueous solutions by depolymerizing HMDA-based polymers. b) Extracting an aqueous solution containing HMDA using a solvent mixture, said solvent mixture comprising (i) at least one monovalent alcohol having 4 to 7 carbon atoms and (ii) at least one alkane having 5 to 7 carbon atoms, The alkane content of the solvent mixture is in the range of 2% to 50% by weight, based on the total weight of all alcohols and alkanes present in the solvent mixture. c) Obtain an HMDA-containing solvent mixture as the separation phase, and d) Distill the HMDA-containing solvent mixture and separate the HMDA from the solvent mixture.

2. The method according to claim 1, wherein the at least one monovalent alcohol having 4 to 7 carbon atoms is selected from 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, cyclopentanol, cyclohexanol and mixtures thereof.

3. The method according to claim 1 or 2, wherein the at least one alkane having 5 to 7 carbon atoms is a straight-chain alkane.

4. The method according to any one of the preceding claims, wherein the alkane content of the solvent mixture is in the range of 2% by weight to 40% by weight, based on the total weight of all alcohols and alkanes present in the solvent mixture.

5. The method according to any one of the preceding claims, wherein the alkane content of the solvent mixture is in the range of 2% by weight to 20% by weight, based on the total weight of all alcohols and alkanes present in the solvent mixture.

6. The method according to any one of the preceding claims, wherein the aqueous composition comprises 0.1 to 50% by weight of HMDA based on the total weight of the aqueous composition.

7. The method according to any one of the preceding claims, wherein the HMDA-based polymer is a polyamide obtained by (i) the condensation of HMDA and dicarboxylic acid, or (ii) a polyurethane.

8. The method according to claim 7, wherein the polyamide is selected from polyamide 6.4, polyamide 6.6, polyamide 6.9, polyamide 6.10 and polyamide 6.

12.

9. The method according to claim 7 or 8, wherein the depolymerization of the polyamide is achieved by a method selected from acid hydrolysis, alkaline hydrolysis, ammonolysis, glycolysis, microwave hydrolysis, and enzymatic hydrolysis.

10. The method according to any one of the preceding claims, wherein method step a) and method step b) are performed at different locations with a minimum distance of 1 km.

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