Method for recovering carboxylic acid and method for recycling polyester

Abstract

This method for recovering carboxylic acid comprises: preparing an electrodialysis tank which is provided with an anode, a cathode, and at least one electrodialysis unit, and has a bipolar membrane, a first cation exchange membrane, and a second cation exchange membrane, and in which a desalination chamber is formed between the first cation exchange membrane and the second cation exchange membrane; and circulating an aqueous carboxylate solution in a circulation path including the desalination chamber while having a voltage applied between the anode and the cathode, thereby causing a solid containing a free body of a carboxylic acid to be precipitated in the desalination chamber.

Description

Method for recovering carboxylic acid and method for recycling polyester

[0001] The present disclosure relates to a method for recovering a free carboxylic acid as a solid by electrodialysis, and a method for recycling polyester using the same.

[0002] Electrodialysis using a bipolar membrane is known as a method for recovering an acid component from an aqueous solution containing an inorganic or organic acid salt.

[0003] For example, Patent Document 1 discloses a method for recovering terephthalic acid and alkali components from alkaline processing wastewater from polyester fibers, which uses an ion exchange membrane electrodialysis cell partitioned by a bipolar membrane, an anion exchange membrane, and a cation exchange membrane, and having an anode chamber, an acid component concentrating chamber, a deionization chamber, an alkali component concentrating chamber, and a cathode chamber, and performs electrodialysis by feeding and circulating the treated alkaline processing wastewater through the deionization chamber, recovering the terephthalic acid component in the acid component concentrating chamber, and recovering the alkali component in the alkali concentrating chamber and the cathode chamber. Non-Patent Document 1 also discloses a method for recovering oily naphthenic acid by electrodialysis using an apparatus including a combination of a bipolar membrane and two cation exchange membranes.

[0004] Japanese Patent Application Publication No. 05-271150

[0005] Separation and Purification Technology, 212 (2019), 929-940

[0006] When a solid containing a free carboxylic acid is recovered by the conventional method disclosed in Patent Document 1, the carboxylic acid anion must pass through an anion exchange membrane. However, the free carboxylic acid may precipitate on the anion exchange membrane, causing the membrane to become clogged. To prevent this, it is necessary to constantly precisely control the pH of the acid component concentration compartment and the deionization compartment.

[0007] The present disclosure relates to a method for more efficiently recovering a solid containing a free carboxylic acid from an aqueous solution containing a carboxylic acid salt.

[0008] The present disclosure includes the following: [1] A method for recovering carboxylic acid, comprising: preparing an electrodialysis cell having an anode, a cathode, and one or more electrodialysis units disposed therebetween, the electrodialysis units having a bipolar membrane, a first cation exchange membrane, and a second cation exchange membrane arranged in this order from the anode side to the cathode side, wherein an acid component chamber is formed between the bipolar membrane and the first cation exchange membrane, a deionization compartment is formed between the first cation exchange membrane and the second cation exchange membrane, and an alkaline component chamber is formed on the cathode side of the second cation exchange membrane; and circulating an acid aqueous solution through a circulation path including the acid component chamber, an alkaline aqueous solution through a circulation path including the alkaline component chamber, and an aqueous carboxylate solution containing a carboxylate salt through a circulation path including the deionization compartment, while applying a voltage between the anode and the cathode, thereby precipitating a solid containing a free carboxylic acid formed from the carboxylate salt in the deionization compartment. [2] The method according to [1], wherein the solid comprises crystals of the free carboxylic acid. [3] The method according to [2], wherein the crystals are needle-shaped crystals. [4] The method according to any one of [1] to [3], wherein the aqueous carboxylate solution further contains an inorganic salt. [5] The method according to [4], wherein the inorganic salt contains one or more selected from sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, sodium nitrate, and sodium dihydrogen phosphate. [6] The method according to [4] or [5], wherein the concentration of the inorganic salt in the aqueous carboxylate solution at 25°C before application of a voltage between the anode and the cathode is 0.001 M or more and 1.0 M or less. [7] The method according to any one of [1] to [6], wherein the carboxylate is an alkali metal salt or alkaline earth metal salt of one or more dicarboxylic acids. [8] The method according to any one of [1] to [7], further comprising recovering an alkali component from the alkali component chamber. [9] A method for recycling polyester, comprising: forming an aqueous carboxylate solution containing an alkali metal salt of a dicarboxylic acid by depolymerizing a polyester; and recovering the dicarboxylic acid from the aqueous carboxylate solution by the method according to any one of [1] to [8].

[0009] A solid containing a free carboxylic acid can be recovered more efficiently from an aqueous solution containing a carboxylic acid salt.

[0010] 1 is a schematic diagram showing an example of an apparatus for recovering a solid containing a carboxylic acid. FIG. 2 is a schematic diagram showing an example of the action of electrodialysis in an electrodialysis unit. FIG. 3 is a scanning electron microscope image of an anion exchange membrane after electrodialysis in Comparative Example 1. FIG. 4 is a scanning electron microscope image of an anion exchange membrane after electrodialysis in Comparative Example 1. FIG. 5 is a scanning electron microscope image of an anion exchange membrane after electrodialysis in Comparative Example 1. FIG. 6 is an infrared absorption spectrum of the anion exchange membrane, free terephthalic acid, and disodium terephthalate after electrodialysis in Comparative Example 1, and a differential infrared absorption spectrum between the liquid-contacting portion and the non-liquid-contacting portion. FIG. 7 is a scanning electron microscope image of a first cation exchange membrane (non-liquid-contacting portion) after electrodialysis in Example 2. FIG. 8 is a scanning electron microscope image of a first cation exchange membrane (liquid-contacting portion) after electrodialysis in Example 2. FIG. 9 is an infrared absorption spectrum of the first cation exchange membrane, free terephthalic acid, and disodium terephthalate after electrodialysis in Example 2, and a differential infrared absorption spectrum between the liquid-contacting portion and the non-liquid-contacting portion. FIG. 10 is a graph showing the relationship between the pH and conductivity in a desalting compartment and the current-flow time in the electrodialysis test of Example 1. 1 is a graph showing the relationship between the pH and conductivity in the deionizing compartment and the current-flow time in the electrodialysis test of Example 2. FIG. 2 is a graph showing the relationship between the residual rate of disodium terephthalate and the current-flow time in the electrodialysis tests of Examples 1 and 2. FIG. 3 is a graph showing the relationship between the current flowing through the electrodialysis device and the current-flow time in the electrodialysis tests of Examples 1 and 2. FIG. 4 is a scanning electron microscope image of crystals of terephthalic acid recovered in the electrodialysis test of Example 2. FIG. 5 is a graph showing the relationship between the pH in the deionizing compartment and the current-flow time in the electrodialysis test of Example 3. FIG. 6 is a graph showing the relationship between the conductivity of the deionizing compartment and the current-flow time in the electrodialysis test of Example 3. FIG. 7 is a graph showing the relationship between the residual rate of disodium terephthalate and the current-flow time in the electrodialysis test of Example 3. FIG. 8 is a graph showing the relationship between the current flowing through the electrodialysis device and the current-flow time in the electrodialysis test of Example 3. FIG. 9 is a graph showing the relationship between the pH in the deionizing compartment and the current-flow time in the electrodialysis test of Example 4. FIG. 10 is a graph showing the relationship between the conductivity of the deionizing compartment and the current-flow time in the electrodialysis test of Example 4. 1 is a graph showing the relationship between the residual rate of disodium terephthalate and the energization time in the electrodialysis test of Example 4. 2 is a graph showing the relationship between the current flowing through the electrodialysis device and the energization time in the electrodialysis test of Example 4. 3 is a schematic diagram showing an example of a recovery device for the electrodialysis test of Comparative Example 1.

[0011] The present invention is not limited to the following examples.

[0012] Fig. 1 is a schematic diagram showing an example of a recovery apparatus used to recover a solid containing a carboxylic acid by electrodialysis. The recovery apparatus 100 shown in Fig. 1 includes an electrodialysis cell 50, an electrolyte tank 15, an acid component tank 25, a deionization compartment tank 35, an alkaline component tank 45, and piping connecting the electrodialysis cell 50 to the electrolyte tank 15, the acid component tank 25, the deionization compartment tank 35, or the alkaline component tank 45. The electrodialysis cell 50 is composed of an anode chamber 12 including an anode 11, a cathode chamber 14 including a cathode 13, and a plurality of electrodialysis units 5 disposed between the anode 11 (anode chamber 12) and the cathode 13 (cathode chamber 14). The plurality of electrodialysis units 5 are arranged in series from the anode 11 side toward the cathode 13 side. Each electrodialysis unit 5 includes a bipolar membrane 2, a first cation exchange membrane 3a, and a second cation exchange membrane 3b, arranged in this order from the anode 11 side toward the cathode 13 side. The bipolar membrane 2, the first cation exchange membrane 3a, and the second cation exchange membrane 3b are spaced apart from one another. An acid component chamber 20 is formed between the bipolar membrane 2 and the first cation exchange membrane 3a, a deionization chamber 30 is formed between the first cation exchange membrane 3a and the second cation exchange membrane 3b, and an alkaline component chamber 40 is formed on the cathode 13 side of the second cation exchange membrane 3b. The alkaline component chamber 40 of each electrodialysis unit 5 is formed between the bipolar membrane 2 and the second cation exchange membrane 3b of the electrodialysis unit 5 adjacent to the cathode 13 side. An additional bipolar membrane 2a is provided between the alkaline component chamber 40 of the electrodialysis unit 5 closest to the cathode 13 and the cathode chamber 14.

[0013] One electrodialysis unit 5 is a combination of one bipolar membrane 2, one first cation exchange membrane 3a, and one second cation exchange membrane 3b, arranged with either the acid component compartment 20 or the deionization compartment 30 interposed therebetween. The number of electrodialysis units included in one recovery device can be determined so that the total area of ​​the bipolar membrane 2, first cation exchange membrane 3a, and second cation exchange membrane 3b included in the recovery device falls within a range set in consideration of the processing capacity of the recovery device, etc. The number of electrodialysis units 5 included in one electrodialysis cell 50 may be, for example, 1 to 100, 1 to 10, or 10 to 100.

[0014] An electrolyte aqueous solution is stored in the electrolyte tank 15, an acid aqueous solution is stored in the acid component chamber tank 25, a carboxylate aqueous solution is stored in the deionization chamber tank 35, and an alkaline aqueous solution is stored in the alkaline component chamber tank 45. The anode chamber 12, the cathode chamber 14, the electrolyte tank 15, and the piping connecting them form a circulation path C1 for circulating the electrolyte aqueous solution. The acid component chamber 20, the acid component chamber tank 25, and the piping connecting them form a circulation path C2 for circulating the acid aqueous solution. The deionization chamber 30, the deionization chamber tank 35, and the piping connecting them form a circulation path C3 for circulating a carboxylate aqueous solution containing a carboxylate. The alkali component chamber 40, the alkali component chamber tank 45, and the piping connecting them form a circulation path C4 for circulating the alkaline aqueous solution. A liquid delivery device such as a liquid delivery pump is usually provided on each circulation path.

[0015] The anode 11, cathode 13, bipolar membrane 2, first cation exchange membrane 3a, and second cation exchange membrane 3b may be those commonly used in electrodialysis. The first cation exchange membrane 3a and the second cation exchange membrane 3b may be the same as or different from each other. The additional bipolar membrane 2a may be the same as the bipolar membrane 2. The area (effective area) of the bipolar membrane 2, the first cation exchange membrane 3a, and the second cation exchange membrane 3b may be, for example, 20 cm ２ 4m or more ２The area here means the area when each ion exchange membrane is viewed from the thickness direction. The volume of the acid component chamber 20, the deionization chamber 30, and the alkaline component chamber 40 may be, for example, 2 cm ３ Over 8m ３ It may be the following:

[0016] By electrodialysis in which an electrolyte aqueous solution is circulated through circulation path C1, an acid aqueous solution is circulated through circulation path C2, an alkaline aqueous solution is circulated through circulation path C3, and a carboxylate aqueous solution is circulated through circulation path C3 while a voltage is applied between anode 11 and cathode 13, a solid containing a free carboxylic acid produced from the carboxylate is precipitated in deionization chamber 30.

[0017] 2 is a schematic diagram showing an example of the action of electrodialysis in an electrodialysis unit. － Na ＋ This example shows an example of electrodialysis in which a solid containing a free carboxylic acid XH is recovered from an aqueous carboxylate solution containing the acid. When a voltage is applied between the anode 11 and the cathode 13, protons H ＋ permeates the first cation exchange membrane 3a and moves to the deionization compartment 30, and the sodium cation Na ＋ permeates the second cation exchange membrane 3b and moves to the alkaline component chamber 40. As a result, a solid containing a free carboxylic acid XH is precipitated in the deionization chamber 30. － Since the first cation exchange membrane 3a does not substantially permeate the first cation exchange membrane 3a, clogging of the membrane due to precipitation of the free carboxylic acid is suppressed. In addition, the alkaline component (e.g., sodium hydroxide (NaOH)) concentrated in the alkaline component chamber 40 can also be recovered. According to the method of the present disclosure, the alkaline component can also be recovered efficiently.

[0018] The solid precipitated in the desalting compartment 30 can be a substance that is solid at 25°C. This solid may include crystals of a free carboxylic acid. According to the method of the present disclosure, solid particles (or crystalline particles) containing carboxylic acid crystals and having a relatively large particle size can be efficiently produced. Large particle sizes are advantageous in terms of easy disposal of waste liquid generated during the washing of the recovered solid. For example, the precipitated solid may include particles (particularly crystalline particles) having a long side length of 0.0001 mm or more. The upper limit of the long side length of the particles is not particularly limited, but may be approximately 1 mm. The long side length of the particles can be observed, for example, using an electron microscope (SEM). Here, the long side length refers to the maximum distance between two parallel lines sandwiching each particle in an electron microscope (SEM) image. Particles with a large length may be needle-shaped crystals. Needle-shaped crystals can be formed, for example, from terephthalic acid. Furthermore, particle size can also be quantitatively measured using a laser diffraction particle size distribution analyzer or the like.

[0019] The carboxylate contained in the aqueous carboxylate solution supplied to the deionization compartment 30 is water-soluble. The carboxylate may be a salt of a carboxylic acid that forms a poorly water-soluble free form. According to the findings of the present inventors, even if a poorly water-soluble free form precipitates in the deionization compartment 30, highly efficient electrodialysis can be continued for a long period of time. The carboxylic acid constituting the carboxylate may be a compound capable of forming a polyester by polycondensation with an alcohol compound (e.g., a diol), and may particularly be a dicarboxylic acid.

[0020] Examples of dicarboxylic acids that can form carboxylate salts include malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid, eicosanedioic acid, pimelic acid, azelaic acid, methylmalonic acid, ethylmalonic acid, adamantanedicarboxylic acid, norbornenedicarboxylic acid, cyclohexanedicarboxylic acid, decalindicarboxylic acid, terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 5-sodiumsulfoisophthalic acid, phenylendanedicarboxylic acid, anthracenedicarboxylic acid, phenanthrenedicarboxylic acid, 9,9'-bis(4-carboxyphenyl)fluorene acid, and 2,5-furandicarboxylic acid.

[0021] The carboxylate salt may be an alkali metal salt, an alkaline earth metal salt, an ammonium salt, or a combination thereof. The alkali metal salt may be one or more selected from lithium salts, sodium salts, potassium salts, rubidium salts, cesium salts, and francium salts. The alkaline earth metal salt may be one or more selected from beryllium salts, magnesium salts, calcium salts, strontium salts, barium salts, and radium salts. The carboxylate salt may be, in particular, a sodium salt, potassium salt, ammonium salt, or a combination thereof.

[0022] Specific examples of the carboxylate include disodium terephthalate, sodium hydrogen terephthalate, dipotassium terephthalate, potassium hydrogen terephthalate, ammonium hydrogen terephthalate, diammonium terephthalate, magnesium terephthalate, and calcium terephthalate.

[0023] The concentration of the carboxylate in the aqueous carboxylate solution to be subjected to electrodialysis can be adjusted arbitrarily within a range not exceeding the saturated concentration. The molar concentration of the carboxylate in the aqueous carboxylate solution at 25°C before the start of electrodialysis may be, for example, 0.001 M or more, or may be 1.0 M or less, 0.90 M or less, 0.80 M or less, 0.70 M or less, 0.60 M or less, 0.50 M or less, 0.40 M or less, 0.30 M or less, 0.20 M or less, 0.10 M or less, 0.09 M or less, 0.08 M or less, 0.07 M or less, 0.06 M or less, 0.05 M or less, 0.04 M or less, or 0.03 M or less. 1 M is 1 x 10 ３ mol / m ３ is equivalent to

[0024] The aqueous carboxylate solution may further contain other components in addition to water and the carboxylate salt. Examples of other components that can be contained in the aqueous carboxylate solution include inorganic salts that are electrolytes, alkaline components (e.g., sodium hydroxide), and alcohol compounds (e.g., methanol and ethylene glycol).

[0025] The inorganic salt may be a compound that exhibits strong acidity in the presence of protons, and such a compound is sometimes referred to as a conductor. When the aqueous carboxylate solution contains such an inorganic salt, the pH of the aqueous carboxylate solution decreases during electrodialysis, thereby allowing electrodialysis to continue with high efficiency for a long period of time. The inorganic salt that can be added to the aqueous carboxylate solution may be, for example, a chloride, a sulfate, a phosphate, or a combination thereof. In particular, the inorganic salt may be one or more selected from sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, sodium nitrate, and sodium dihydrogen phosphate. The concentration of the inorganic salt in the aqueous carboxylate solution at 25°C before the start of electrodialysis may be, for example, 0.0005 M or more, 0.005 M or more, 0.01 M or more, 0.02 M or more, 0.03 M or more, 0.04 M or more, or 0.05 M or more, or may be 1.0 M or less, 0.90 M or less, 0.80 M or less, 0.70 M or less, 0.60 M or less, 0.50 M or less, 0.40 M or less, 0.30 M or less, 0.20 M or less, or 0.10 M or less.

[0026] The aqueous electrolyte solution circulating through the circulation path C1 including the anode chamber 12 and the cathode chamber 14 can be an aqueous solution containing water and an electrolyte. The electrolyte is not particularly limited and can be, for example, an inorganic alkaline component (e.g., sodium hydroxide) or an inorganic salt. The aqueous acid solution circulating through the circulation path C2 including the acid component chamber 20 can be an acidic aqueous solution containing water and an inorganic acid (e.g., sulfuric acid). The aqueous alkaline solution circulating through the circulation path C4 including the alkaline component chamber 40 can be an aqueous solution containing water and an alkaline component (e.g., sodium hydroxide).

[0027] During electrodialysis, the temperatures of the electrolyte aqueous solution, the acidic aqueous solution, the carboxylate aqueous solution, and the alkaline component aqueous solution may be, for example, 0° C. or higher and 95° C. or lower, or 5° C. or higher and 70° C. or lower. The voltage applied between the anode 11 and the cathode 13 is not particularly limited, and may be, for example, 1 V or higher and 300 V or lower, 15 V or higher and 300 V or lower, or 30 V or higher and 300 V or lower.

[0028] The aqueous carboxylate solution treated by the above-described method may be an aqueous solution formed by depolymerization of a polyester. In this case, the polyester can be efficiently recycled by a method including forming an aqueous carboxylate solution containing a carboxylate by depolymerization of a polyester and recovering carboxylic acid from the aqueous carboxylate solution by the method according to the present disclosure. The polyester to be recycled may be, for example, polyethylene terephthalate.

[0029] The present invention is not limited to the following examples.

[0030] 1. Equipment and materials The following electrodialysis equipment and ion exchange membrane (effective membrane area: 550 cm) ２ An electrodialysis device, ASILYZER EX3B (product name, manufactured by ASTOM Corporation), and a bipolar membrane, NEOSEPTA BPU (product name, effective membrane area 550 cm), were used. ２ , manufactured by Astom Corporation) Cation exchange membrane Neocepta CBM (product name, effective membrane area 550 cm ２ , manufactured by Astom Corporation) Anion exchange membrane Neosepta AHA (product name, effective membrane area 550 cm ２, manufactured by Astom Co., Ltd.)

[0031] 2. Electrodialysis Test Comparative Example 1 An electrodialysis apparatus having the configuration shown in Fig. 22 was prepared as a carboxylic acid recovery apparatus. In the recovery apparatus 101 shown in Fig. 22, a total of ten sets of ion exchange membranes, each set consisting of a bipolar membrane 2, an anion exchange membrane 4, and a cation exchange membrane 3, were arranged in order from the anode 11 side toward the cathode 13 side between an anode chamber 12 provided with an anode 11 and a cathode chamber 14 provided with a cathode 13. A bipolar membrane 2 was also arranged between the cation exchange membrane 3 located closest to the cathode 13 and the cathode 13. An acid component chamber 20 was formed between the bipolar membrane 2 and the anion exchange membrane 4, a deionization chamber 30 was formed between the anion exchange membrane 4 and the cation exchange membrane 3, and an alkali component chamber 40 was formed between the cation exchange membrane 3 and the bipolar membrane 2 on the cathode 13 side. Ten electrodialysis units 5, each consisting of three compartments, an acid component compartment 20, a deionization compartment 30 and an alkaline component compartment 40, were arranged in series between the anode 11 and the cathode 13.

[0032] A 0.5M NaOH aqueous solution was circulated as an electrolyte aqueous solution through a circulation path C1 including the anode chamber 12 and the cathode chamber 14, and a 0.1M H ２ SO ４ Aqueous solutions were circulated, and a 0.1 M NaOH aqueous solution was circulated through circulation path C4 including alkaline component chamber 40, and a 0.5 M disodium terephthalate aqueous solution (containing 10% by volume of methanol and 0.5 M ethylene glycol) was circulated through circulation path C3 including desalting chamber 30. In this state, a voltage of 35 V was applied between anode 11 and cathode 13.

[0033] Immediately after the voltage was applied, the current value dropped to about 0.1 A, and electrodialysis did not substantially proceed. The anion exchange membrane 4 removed from the recovery device 101 was observed with a scanning electron microscope. Figure 3(a) is a scanning electron microscope image of the removed anion exchange membrane (magnification (at the time of observation): 30x). The liquid-contacting portion is the portion that had been in contact with the aqueous disodium terephthalate solution, and the non-liquid-contacting portion is the portion that had not been in contact with the aqueous disodium terephthalate solution. Figure 3(b) is an enlarged view of the non-liquid-contacting portion of the anion exchange membrane (magnification (at the time of observation): 500x). Figure 4 is a scanning electron microscope image of the liquid-contacting portion of the anion exchange membrane, with (a) being an enlarged view at 500x (at the time of observation) and (b) being an enlarged view at 1500x (at the time of observation). It was observed that crystals containing terephthalic acid had precipitated on and within the liquid-contacting portion of the anion exchange membrane, resulting in clogging of the membrane. This was thought to be because terephthalic acid ions were converted to the free form while passing through the anion exchange membrane 4 and precipitated inside the membrane. Figure 5 shows the infrared absorption spectra of the liquid-contacting and non-liquid-contacting parts of the anion exchange membrane, the differential infrared absorption spectrum between the liquid-contacting and non-liquid-contacting parts, and the infrared absorption spectra of the free form of terephthalic acid and disodium terephthalate. The differential infrared absorption spectrum of the liquid-contacting and non-liquid-contacting parts of the anion exchange membrane 4 closely matched the spectrum of the free form of terephthalic acid. This also supported the clogging of the membrane due to the precipitation of the free form of terephthalic acid. From the above, it was confirmed that it was difficult to perform electrodialysis operation according to conventional methods with the membrane configuration of Comparative Example 1.

[0034] Example 1 An electrodialysis apparatus having the configuration shown in Figure 1 was prepared. In the recovery device 100 shown in Figure 1, a bipolar membrane 2, a first cation exchange membrane 3a, and a second cation exchange membrane 3b were arranged as a set of electrodialysis units 5 between an anode chamber 12 equipped with an anode 11 and a cathode chamber 14 equipped with a cathode 13, and a total of 10 sets of ion exchange membranes (electrodialysis units) were arranged in order from the anode 11 side toward the cathode 13 side. An additional bipolar membrane 2a was arranged between the cathode 13 and the second cation exchange membrane 3b constituting the electrodialysis unit 5 located closest to the cathode 13. A 0.5M NaOH aqueous solution was circulated as an electrolyte aqueous solution through a circulation path C1 including the anode chamber 12 and the cathode chamber 14, and a 0.01M H O O solution was circulated through a circulation path C2 including the acid component chamber 20.２ SO ４ A 0.01 M NaOH aqueous solution was circulated through circulation path C4, which included the alkaline component chamber 40, and a 0.01 M disodium terephthalate aqueous solution (containing 0.01 M NaOH) was circulated through circulation path C3, which included the desalting chamber 30. Under these conditions, a voltage of 15 V was applied between the anode 11 and the cathode 13, and electrodialysis was performed. Even after 60 minutes of current application, no membrane blockage occurred, and crystals precipitated in the desalting chamber 30. This was thought to be due to the inability of terephthalate ions to penetrate the cation exchange membrane. Before and during electrodialysis, the pH and conductivity of the solution in the desalting chamber, the current flowing through the electrodialysis device, and the UV absorbance of the supernatant were measured. The concentration of terephthalate ions remaining in the supernatant was calculated from the ratio of the UV absorbance at each measurement time after the start of electrodialysis to the UV absorbance before the start of electrodialysis.

[0035] Example 2 A 0.01M aqueous solution of disodium terephthalate (containing 0.01M NaOH) was circulated through the circulation path C3 including the desalting chamber 30. ２ SO ４Electrodialysis was performed under the same conditions as in Example 1, except that sodium sulfate (sodium sulfate) was added. No membrane clogging occurred, and crystals precipitated in the desalting compartment 30. The first cation exchange membrane 3a removed from the recovery device 100 was observed with a scanning electron microscope. FIG. 6 shows scanning electron microscope images of the non-liquid-contact portion of the first cation exchange membrane that was not in contact with the disodium terephthalate aqueous solution, with (a) being an enlarged view at 50x (at the time of observation) and (b) being an enlarged view at 1000x (at the time of observation). FIG. 7 shows scanning electron microscope images of the liquid-contact portion of the first cation exchange membrane that was in contact with the disodium terephthalate aqueous solution, with (a) being an enlarged view at 50x (at the time of observation) and (b) being an enlarged view at 1000x (at the time of observation). No precipitation of terephthalic acid was observed in the membrane, even in the liquid-contact portion. FIG. 8 shows infrared absorption spectra of the liquid-contacting and non-contacting portions of the first cation exchange membrane, the differential infrared absorption spectrum between the liquid-contacting and non-contacting portions, and the infrared absorption spectra of free terephthalic acid and disodium terephthalate. The differential infrared absorption spectrum of the liquid-contacting and non-contacting portions of the first cation exchange membrane 3a did not suggest the presence of either free terephthalic acid or disodium terephthalate. Before and during electrodialysis, the pH and conductivity of the liquid in the desalting compartment, the current flowing through the electrodialysis device, and the UV absorbance (240 nm) of the supernatant were measured. The concentration of terephthalate ions remaining in the supernatant was calculated from the ratio of the UV absorbance at each measurement time after the start of electrodialysis to the UV absorbance before the start of electrodialysis.

[0036] Results of Examples 1 and 2: FIG. 9 is a graph showing the relationship between the pH and conductivity of the desalting compartment and the energization time in Example 1, and FIG. 10 is a graph showing the relationship between the pH and conductivity of the desalting compartment and the energization time in Example 2. FIG. 11 is a graph showing the relationship between the proportion of remaining disodium terephthalate (residual rate) estimated from UV absorbance and the energization time. FIG. 12 is a graph showing the relationship between the current value flowing through the electrodialysis device and the energization time. In Example 1, the residual rate of disodium terephthalate salt decreased to 65% after 60 minutes of energization. In Example 2, the residual rate of disodium terephthalate salt decreased to almost 0% approximately 10 minutes after the start of electrodialysis, and essentially the entire amount of terephthalate crystallized as a free form in the desalting compartment. In the case of Example 1, as shown in FIG. 9, after the start of electrodialysis, the pH stagnated at approximately 5 to 6 and the conductivity decreased, which is thought to be the reason for the relatively gradual decrease in the residual rate of disodium terephthalate salt. In the case of Example 2, the pH rapidly decreased to 3 or less about 10 minutes after the start of electrodialysis, and the conductivity decreased to 3 mS / cm, and then began to increase again. Thus, it was considered that the addition of the conductor promoted the decrease in pH and maintained high conductivity for a long period of time, thereby enabling the free form of terephthalic acid to be recovered with high efficiency.

[0037] Crystals were recovered by filtration from the liquid in the desalting chamber of Example 2. It was confirmed by infrared absorption spectroscopy that the recovered crystals were crystals of the free form of terephthalic acid. Figure 13 is a scanning electron microscope image of the crystals recovered in Example 2. It was revealed that large needle-shaped crystals with a maximum long side length (particle size) of 0.1 mm were obtained.

[0038] Example 3 A 0.01M aqueous solution of disodium terephthalate (containing 0.01M NaOH) was circulated through the circulation path C3 including the desalting compartment 30. The aqueous solution was added with a conductor containing 0.001M, 0.005M, 0.01M, 0.02M, 0.05M, or 0.1M NaOH. ２ SO ４Electrodialysis was performed under the same conditions as in Example 1, except that each of the above was added. Under all conditions, membrane clogging did not occur, and crystals precipitated in the desalting compartment 30. Before the start of electrodialysis and during electrodialysis, the pH and conductivity of the liquid in the desalting compartment, the current value flowing through the electrodialysis device, and the UV absorbance of the supernatant were measured. The concentration of terephthalate ions remaining in the supernatant was calculated from the ratio of the UV absorbance at each measurement time after the start of electrodialysis to the UV absorbance before the start of electrodialysis.

[0039] Results of Example 3 Figure 14 is a graph showing the relationship between the pH of the deionizing compartment and the energization time, Figure 15 is a graph showing the relationship between the conductivity of the deionizing compartment and the energization time, Figure 16 is a graph showing the relationship between the proportion of remaining disodium terephthalate (residual rate) estimated from UV absorbance and the energization time, and Figure 17 is a graph showing the relationship between the current value flowing through the electrodialysis device and the energization time. These graphs suggest that the higher the conductor concentration, the greater the effect of promoting the pH reduction in the deionizing compartment, the higher the conductivity, and the higher the conversion efficiency of disodium terephthalate to the free form of terephthalic acid. However, even with the lowest conductor concentration of 0.001 M, nearly 100% conversion of disodium terephthalate to terephthalic acid (free form) was achieved within 60 minutes of energization.

[0040] Example 4 Electrodialysis was performed under the same conditions as in Example 1, except that 0.01 M, 0.05 M, or 0.1 M NaCl (sodium chloride) was added as a conductor to a 0.01 M aqueous solution of disodium terephthalate (containing 0.01 M NaOH) circulated through circulation path C3, which included the desalting compartment 30. No membrane clogging occurred, and crystals precipitated in the desalting compartment 30. Before and during the electrodialysis, the pH and conductivity of the liquid in the desalting compartment, the current flowing through the electrodialysis device, and the UV absorbance of the supernatant were measured. The concentration of terephthalate ions remaining in the supernatant was calculated from the ratio of the UV absorbance at each measurement time after the start of electrodialysis to the UV absorbance before the start of electrodialysis.

[0041] Results of Example 4 Fig. 18 is a graph showing the relationship between the pH of the deionization compartment and the energization time in Example 4, Fig. 19 is a graph showing the relationship between the conductivity of the deionization compartment and the energization time, Fig. 20 is a graph showing the relationship between the proportion of remaining disodium terephthalate (residual rate) estimated from UV absorbance and the energization time, and Fig. 21 is a graph showing the relationship between the value of the current flowing through the electrodialysis device and the energization time. As can be seen from these graphs, even when NaCl was used as the conductor, Na ２ SO ４ It was confirmed that similar results were obtained when the conductor was used as the conductor. The results of Examples 3 and 4 suggest that when the pH of the desalting compartment 30 is reduced to 4 or less by adding a conductor, the conversion efficiency from disodium terephthalate to terephthalic acid (free form) increases to nearly 100%.

[0042] Example 5: The circulation path C3 including the desalting chamber 30 was charged with NaH ２ P.O. ４ (sodium dihydrogen phosphate) or NaNO ３ Electrodialysis was carried out under the same conditions as in Example 1, except that only sodium nitrate was circulated at the concentrations shown in Table 1.

[0043]

[0044] The pH of the deionization compartment after 60 minutes of current application is shown in Table 1. As shown in Table 1, under all conditions, the pH of the deionization compartment was reduced to 4 or less by adding conductor. ２ P.O. ４ or NaNO ３ It was suggested that the cation exchanger also functions as a conductor to improve the conversion efficiency of disodium terephthalate to terephthalic acid (free form).

[0045] Example 6 The concentration of disodium terephthalate in the aqueous solution of disodium terephthalate circulated through the circulation path C3 including the desalting chamber 30 was changed to 0.03 M (containing 0.01 M NaOH), and a conductor of 0.02 M NaOH was added to the aqueous solution of disodium terephthalate. ２ SO ４Electrodialysis was carried out under the same conditions as in Example 1, except that the terephthalate ion solution was added. No membrane clogging occurred, and crystals precipitated in the desalting compartment 30. Before the start of electrodialysis and during the electrodialysis, the pH and conductivity of the liquid in the desalting compartment, the current value flowing through the electrodialysis device, and the UV absorbance of the supernatant were measured. The concentration of terephthalate ions remaining in the supernatant was calculated from the ratio of the UV absorbance at each measurement time after the start of electrodialysis to the UV absorbance before the start of electrodialysis.

[0046]

[0047] The measurement results are shown in Table 2. After 30 minutes of energization, disodium terephthalate was converted to terephthalic acid at almost 100%, and the acid was crystallized.

[0048] Example 7 H circulated through the circulation path C2 including the acid component chamber 20 ２ SO ４ The concentration of the aqueous solution was changed to 0.1 M, and the concentration of disodium terephthalate in the aqueous solution of disodium terephthalate circulated through the circulation path C3 including the desalting chamber 30 was changed to 0.1 M. The aqueous solution of disodium terephthalate was mixed with NaOH having a concentration of 0.01 M and NaOH having a concentration of 0.02 M. ２ SO ４ An electrodialysis test was performed under the same conditions as in Example 1, except that a conductor (A) was added and the voltage applied between the anode 11 and the cathode 13 was changed as shown in Table 3. No membrane clogging occurred, and crystals precipitated in the desalting compartment 30. Before the start of electrodialysis and during electrodialysis, the pH and conductivity of the liquid in the desalting compartment, the current flowing through the electrodialysis device, and the UV absorbance of the supernatant were measured. The concentration of terephthalate ions remaining in the supernatant was calculated from the ratio of the UV absorbance at each measurement time after the start of electrodialysis to the UV absorbance before the start of electrodialysis.

[0049]

[0050] The measurement results are shown in Table 3. When the voltage was increased to 35.1 V after 30 minutes of current application, and current application was continued, the pH decreased, and disodium terephthalate was converted to terephthalic acid at almost 100%, followed by crystallization.

[0051] Example 8 H circulated through the circulation path C2 including the acid component chamber 20２ SO ４ The concentration of the aqueous solution was changed to 1 M, the concentration of disodium terephthalate in the aqueous solution of disodium terephthalate circulated through the circulation path C3 including the desalting chamber 30 was changed to 0.4 M, and the aqueous solution of disodium terephthalate was mixed with NaOH having a concentration of 0.01 M and NaOH having a concentration of 0.02 M. ２ SO ４ An electrodialysis test was performed under the same conditions as in Example 1, except that a conductor (A) was added and the voltage applied between the anode 11 and the cathode 13 was changed as shown in Table 4. No membrane clogging occurred, and crystals precipitated in the desalting compartment 30. Before the start of electrodialysis and during electrodialysis, the pH and conductivity of the liquid in the desalting compartment, the current flowing through the electrodialysis device, and the UV absorbance of the supernatant were measured. The concentration of terephthalate ions remaining in the supernatant was calculated from the ratio of the UV absorbance at each measurement time after the start of electrodialysis to the UV absorbance before the start of electrodialysis.

[0052]

[0053] The measurement results are shown in Table 4. When the voltage was increased to 35.1 V after 30 minutes of current application, and current application was continued, the pH decreased, and disodium terephthalate was converted to terephthalic acid at almost 100%, followed by crystallization.

[0054] 2, 2a...bipolar membrane, 3a...first cation exchange membrane, 3b...second cation exchange membrane, 5...electrodialysis unit, 11...anode, 13...cathode, 15...electrolyte tank, 20...acid component chamber, 25...tank for acid component chamber, 30...demineralization chamber, 35...tank for demineralization chamber, 40...alkaline component chamber, 45...tank for alkaline component chamber, 50...electrodialysis cell, 100, 101...recovery device, C1, C2, C3, C4...circulation path.

Claims

1. A method for recovering carboxylic acids, comprising: preparing an electrodialysis cell having an anode, a cathode and one or more electrodialysis units disposed therebetween, the electrodialysis units having a bipolar membrane, a first cation exchange membrane and a second cation exchange membrane arranged in this order from the anode side to the cathode side, an acid component chamber being formed between the bipolar membrane and the first cation exchange membrane, a desalting chamber being formed between the first cation exchange membrane and the second cation exchange membrane, and an alkaline component chamber being formed on the cathode side of the second cation exchange membrane; and circulating an acid aqueous solution through a circulation path including the acid component chamber, an alkaline aqueous solution through a circulation path including the alkaline component chamber, and a carboxylate aqueous solution including a carboxylate salt through a circulation path including the desalting chamber, while applying a voltage between the anode and the cathode, thereby precipitating a solid including a free carboxylic acid generated from the carboxylate salt in the desalting chamber.

2. The method of claim 1, wherein the solid comprises crystals of the free form of the carboxylic acid.

3. The method of claim 2, wherein the crystals are needle-shaped crystals.

4. The method of claim 1, wherein the aqueous carboxylate solution further comprises an inorganic salt.

5. The method of claim 4, wherein the inorganic salt comprises one or more selected from sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, sodium nitrate, and sodium dihydrogen phosphate.

6. The method according to claim 4, wherein the concentration of the inorganic salt in the aqueous carboxylate solution at 25° C. before a voltage is applied between the anode and the cathode is 0.001 M or more and 1.0 M or less.

7. The method of claim 1, wherein the carboxylate is an alkali metal or alkaline earth metal salt of one or more dicarboxylic acids.

8. The method of claim 1, further comprising recovering the alkaline component from the alkaline component chamber.

9. A method for recycling polyester, comprising: forming an aqueous carboxylate solution containing an alkali metal salt of a carboxylic acid by depolymerizing a polyester; and recovering the carboxylic acid from the aqueous carboxylate solution by the method of claim 1.

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