Method for recovering polyethylene glycol terephthalate through bipolar membrane electrodialysis

Abstract

The invention provides a method for recovering polyethylene glycol terephthalate by bipolar membrane electrodialysis. The method comprises the following steps: S1, providing a bipolar membrane electrodialysis device; s2, mixing and heating polyethylene glycol terephthalate and a sodium hydroxide solution, and carrying out an alkali depolymerization reaction to obtain a feed liquid containing sodium terephthalate; s3, feeding the feed liquid obtained in the step S2 into a feed liquid chamber; introducing water into the acid chamber; introducing water or a dilute alkali solution into the alkali chamber; s4, direct current is applied to the bipolar membrane electrodialysis device for electrodialysis, and terephthalic acid radical ions penetrate through the anion membrane to enter the acid chamber and are combined with hydrogen ions hydrolyzed by the bipolar membrane to form terephthalic acid; sodium ions penetrate through the positive membrane to enter the alkali chamber, and are combined with hydroxyl ions hydrolyzed by the bipolar membrane and the sodium ions to generate sodium hydroxide. According to the method, the sodium terephthalate solution generated by alkali depolymerization of polyethylene glycol terephthalate can be directly converted into high-purity terephthalic acid and recyclable sodium hydroxide in situ.

Description

Technical Field

[0001] This invention relates to the fields of organic and inorganic acid and base technology, and in particular to a method for recovering polyethylene terephthalate by bipolar membrane electrodialysis. Background Technology

[0002] Polyethylene terephthalate (PET) is one of the most widely used polyester materials in the world today, used extensively in the production of fibers, bottle flakes, and packaging materials. With the continuous growth in PET consumption, the amount of waste PET generated has also increased dramatically. Recycling and reusing waste PET not only meets the requirements of green, low-carbon, and sustainable development, but also has significant economic benefits.

[0003] Currently, there are three main methods for treating waste PET: physical depolymerization, biological depolymerization, and chemical depolymerization.

[0004] Physical depolymerization is simple, easy to implement, and relatively low in cost, but this method only changes the physical form of PET, limiting its scope of secondary use.

[0005] Biodepolymerization reactions are mild and environmentally friendly, but their rate is relatively slow and their depolymerization efficiency is low.

[0006] In existing technologies, chemical depolymerization, especially alkaline hydrolysis, is the mainstream technology for recycling PET monomers. This method typically reacts PET waste with sodium hydroxide solution at high temperatures to depolymerize and produce sodium terephthalate and ethylene glycol. However, to obtain high-purity terephthalic acid (TPA), subsequent acidification treatment is necessary, which involves adding an excess of a strong acid (such as sulfuric acid) to the sodium terephthalate solution to precipitate the terephthalic acid. This traditional acidification process has the following inherent drawbacks: (1) The consumption of acid is huge and the cost is high: the treatment of each ton of PET waste requires 0.4 to 0.5 tons of 98% sulfuric acid, resulting in significant raw material costs. At the same time, if the excessive use of acid is not properly controlled, it can easily lead to an excessively wide distribution of crystal size of terephthalic acid, which affects product quality.

[0007] (2) High-salinity wastewater is generated, causing serious environmental pollution: The acidification process produces a large amount of inorganic salt byproducts, such as sodium sulfate or sodium chloride. The total dissolved solids (TDS) of the wastewater often exceeds 50,000 mg / L, and direct discharge seriously violates environmental regulations. If the evaporation crystallization process is used to recover these inorganic salts, not only is the treatment cost per ton of wastewater as high as 80 to 120 yuan, but the purity of the obtained byproduct sodium sulfate is also low (usually below 95%), with limited economic value.

[0008] (3) The post-processing process is complex and there is secondary pollution: The mother liquor after precipitation of terephthalic acid usually contains residual acid (such as 1%~5% sulfuric acid). If the mother liquor is reused, impurities such as sodium ions and sulfate ions will accumulate in the system, affecting the crystallization purity and process stability of subsequent batches; if the mother liquor is neutralized, the sulfuric acid reacts with the alkali to generate sodium sulfate again, and the acid cannot be recovered, resulting in further waste of resources.

[0009] (4) Severe equipment corrosion and high investment costs: The strong acid environment at high temperature is extremely corrosive to the reaction equipment, requiring the equipment to be made of special materials such as duplex stainless steel or fluoropolymer lined. Its manufacturing cost is 50% to 80% higher than that of ordinary carbon steel equipment, which significantly increases the project investment.

[0010] Therefore, researching and developing a novel technology for recycling terephthalic acid from PET is of great significance for solving the problems of the aforementioned PET post-processing acidification process.

[0011] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art. Summary of the Invention

[0012] The purpose of this invention is to overcome the shortcomings of existing technologies, such as environmental pollution and complex processes in the recycling of PET and the preparation of TPA, to realize the resource-based recycling of PET waste and reduce the environmental burden and economic cost of the recycling process.

[0013] To achieve the above objectives, the present invention provides a method for recovering polyethylene terephthalate by bipolar membrane electrodialysis, comprising: Step S1: A bipolar membrane electrodialysis device is provided. The bipolar membrane electrodialysis device includes a membrane stack, the membrane stack includes at least one repeating unit, and the repeating unit includes a bipolar membrane, an anion membrane, a cation membrane, and another bipolar membrane arranged sequentially. An acid chamber is formed between the bipolar membrane and the adjacent anion membrane, a feed chamber is formed between the anion membrane and the adjacent cation membrane, and an alkali chamber is formed between the cation membrane and the adjacent bipolar membrane. Step S2: Polyethylene terephthalate is mixed with sodium hydroxide solution and heated to carry out alkaline depolymerization reaction, resulting in a liquid containing sodium terephthalate. Step S3: The liquid obtained in step S2 is introduced into the liquid chamber; water is introduced into the acid chamber; water or a dilute alkali solution is introduced into the alkali chamber. Step S4: Apply direct current to the bipolar membrane electrodialysis device to perform electrodialysis. In the feed chamber, sodium terephthalate ionizes into terephthalate ions and sodium ions. The terephthalate ions pass through the anion membrane into the acid chamber and combine with hydrogen ions hydrolyzed by the bipolar membrane to form terephthalic acid. The sodium ions pass through the cation membrane into the alkali chamber and combine with hydroxide ions hydrolyzed by the bipolar membrane and the sodium ions to form sodium hydroxide.

[0014] Optionally, in step S2, the concentration of the sodium hydroxide solution is 1 mol / L to 6 mol / L; and the heating temperature is 100℃ to 200℃.

[0015] Optionally, in step S4, the flow rate of the solution in the feed chamber, acid chamber, and alkali chamber is 100 mL / min to 500 mL / min.

[0016] Optionally, in step S4, the current density applied during electrodialysis is 20 mA / cm². 2 ~50mA / cm 2 .

[0017] Optionally, in step S3, the dilute alkali solution is a sodium hydroxide solution, and the initial concentration of the sodium hydroxide solution introduced into the alkali chamber is 0.1 mol / L to 0.3 mol / L.

[0018] Optionally, the mass concentration of sodium terephthalate in the liquid obtained in step S2 is 0.1% to 5%.

[0019] Optionally, conductivity sensors are installed in the feed chamber, acid chamber, and alkali chamber.

[0020] Optionally, step S4 further includes: monitoring the conductivity of the solution in the feed chamber, and when the conductivity is lower than 10 ms / cm, ending the electrodialysis and collecting terephthalic acid from the acid chamber.

[0021] Optionally, the bipolar membrane electrodialysis device further includes an anode plate and a cathode plate disposed on both sides of the membrane stack, wherein the anode plate forms an anode chamber with the adjacent bipolar membrane, and the cathode plate forms a cathode chamber with the adjacent bipolar membrane; step S3 further includes: introducing an electrolyte solution into the anode chamber and the cathode chamber.

[0022] Optionally, the electrolyte solution is a sodium sulfate solution with a concentration of 0.1 mol / L to 0.3 mol / L.

[0023] Compared with the prior art, the technical solution of the present invention has at least the following beneficial effects: This invention utilizes bipolar membrane electrodialysis technology to recover PET, directly converting the sodium terephthalate solution generated from PET alkaline depolymerization into high-purity TPA and reusable sodium hydroxide (NaOH) in situ. Specifically, this invention utilizes bipolar membrane water dissociation and ion migration driven by an internal electric field to directly generate TPA and NaOH, thus eliminating the need for traditional acidification processes and avoiding the introduction of additional acid radicals (such as SO42-). 2- This avoids the generation of high-salt wastewater and the waste of acid reagents, thus achieving clean production. Attached Figure Description

[0024] Figure 1 The flowchart of a method for recovering polyethylene terephthalate by bipolar membrane electrodialysis provided by the present invention is shown.

[0025] Figure 2 This is a schematic diagram of a bipolar membrane electrodialysis device.

[0026] Figure 3 This is a schematic diagram of the membrane stack in a bipolar membrane electrodialysis device.

[0027] Figure 4 This is a schematic diagram showing the change of NaOH concentration in the alkaline chamber over time during the electrodialysis process in Examples 1-3.

[0028] Figure 5 The image shows the 1H NMR spectrum of the acid chamber product obtained in Example 1.

[0029] Figure 6 The infrared spectra of the acid chamber products obtained in Examples 1-3 are shown.

[0030] Figure 7 The images show the Raman spectra of the acid chamber products obtained in Examples 1-3.

[0031] Figure 8 The image shows the sodium NMR spectrum of the acid chamber product obtained in Example 1.

[0032] Explanation of reference numerals in the attached figures: Bipolar membrane electrodialysis device 100; Membrane stack 10, bipolar membrane 11 (11'), anion exchange layer 111, cation exchange layer 112, anion membrane 12, cation membrane 13, acid chamber 14, feed chamber 15, alkali chamber 16; Anode plate 20, cathode plate 30, anode chamber 21, cathode chamber 31; Power supply 40, positive terminal 41, negative terminal 42, switch 43, display interface 44, adjustment button 45; Liquid storage tank 50 in the liquid storage chamber, R3 represents the liquid inlet route of the liquid storage chamber, and R3' represents the liquid outlet route of the liquid storage chamber; Acid chamber liquid storage tank 60, R2 represents the acid chamber liquid feed route, R2' represents the acid chamber liquid discharge route; Alkali chamber liquid storage tank 70, R4 represents the alkali chamber liquid feed route, R4' represents the alkali chamber liquid discharge route; The polar chamber liquid storage tank 80, R1 represents the polar chamber liquid feed route, and R1' represents the polar chamber liquid discharge route. Detailed Implementation

[0033] The following detailed description, in conjunction with the accompanying drawings and specific embodiments, provides a method for recovering polyethylene terephthalate (PET) using bipolar membrane electrodialysis, as proposed in this invention. The advantages and features of this invention will become clearer from the following description. It should be noted that the accompanying drawings are in a very simplified form and use non-precise proportions, intended only to facilitate and clarify the illustration of the embodiments of this invention. Please refer to the accompanying drawings to make the objectives, features, and advantages of this invention more apparent and understandable. It should be understood that the structures, proportions, sizes, etc., depicted in the accompanying drawings are only for illustrative purposes and to enable those skilled in the art to understand and read them. They are not intended to limit the implementation conditions of this invention and therefore have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to the size, without affecting the effects and objectives achieved by this invention, should still fall within the scope of the technical content disclosed in this invention.

[0034] The “cation exchange membrane” mentioned in this article refers to a cation exchange membrane that allows cations (positively charged ions) to pass through while blocking anions (negatively charged ions).

[0035] The “anion exchange membrane” mentioned in this article refers to anion exchange membranes, which allow anions (negatively charged ions) to pass through while blocking cations (positively charged ions).

[0036] To address the problems of severe environmental pollution, low resource utilization, high processing costs, and complex subsequent processes caused by existing chemical recycling methods for polyethylene terephthalate (PET) due to the use of acidification processes, this invention provides a green, efficient, and low-cost method for PET recycling and terephthalic acid (TPA) preparation. This method aims to avoid the use of large amounts of acid reagents, eliminate the generation of high-salt wastewater at the source, simplify the process flow, achieve high-value resource recycling of PET waste, and reduce the overall environmental burden and economic costs of the recycling process.

[0037] like Figure 1 As shown, the present invention provides a method for recovering polyethylene terephthalate by bipolar membrane electrodialysis, comprising the following steps: Step S1: A bipolar membrane electrodialysis device is provided. The bipolar membrane electrodialysis device includes a membrane stack, which includes at least one repeating unit. The repeating unit includes a bipolar membrane, an anion membrane, a cation membrane, and another bipolar membrane arranged sequentially. An acid chamber is formed between the bipolar membrane and the adjacent anion membrane, a feed chamber is formed between the anion membrane and the adjacent cation membrane, and an alkali chamber is formed between the cation membrane and the adjacent bipolar membrane.

[0038] Among them, the bipolar membrane electrodialysis device 100 is as follows Figure 2 and Figure 3 As shown, an anode plate 20 and a cathode plate 30 are fixed on both sides of the membrane stack 10, respectively. The anode plate 20 is connected to the positive terminal 41 of the power supply 40, and the cathode plate 30 is connected to the negative terminal 42 of the power supply 40. In this embodiment, the power supply 40 also includes a switch 43, a display interface 44, and several adjustment knobs 45; the switch 43 is used to control the on / off state of the circuit; the display interface 44 is used to display the voltage and current values ​​output by the power supply 40 in real time; and the adjustment knobs 45 are used to set and adjust the constant current or voltage output by the power supply 40.

[0039] See also Figure 3 The membrane stack 10 includes at least one repeating unit, which includes a bipolar membrane 11, an anion membrane 12, a cation membrane 13 and another bipolar membrane 11' arranged in sequence; a sealing gasket (not shown in the figure) is provided between adjacent membranes to form and isolate the acid chamber 14, the feed chamber 15 and the alkali chamber 16, while preventing leakage or cross-contamination of the liquid flow between the chambers under pressure.

[0040] See also Figure 3 The bipolar membrane 11 (or 11') is composed of an anion exchange layer 111, a cation exchange layer 112, and an intermediate interface layer (not shown) tightly bonded between the anion exchange layer 111 and the cation exchange layer 112; wherein the anion exchange layer 111 faces the anode plate 20, and the cation exchange layer 112 faces the cathode plate 30. Under the action of a reverse bias electric field, in the intermediate interface layer of the bipolar membrane 11 (or 11'), due to a special chemical environment (e.g., introduced catalytic groups) and approximately 10 7 Under the combined effect of an extremely high electric field strength of V / cm, water molecules present in the intermediate interface layer rapidly dissociate into hydrogen ions (H+). + ) and hydroxide ions (OH) - ) ions, producing H + It passes through the cation exchange layer 112 and enters the chamber adjacent to the cation exchange layer 112 (i.e., acid chamber 14); the generated OH... -It passes through the anion exchange layer 111 and enters the chamber adjacent to the anion exchange layer 111 (i.e., the alkali chamber 16). In this embodiment, an acid chamber 14 is formed between the anion exchange layer 111 of the bipolar membrane 11 and the adjacent anion membrane 12, an anode chamber 21 is formed between the cation exchange layer 112 of the bipolar membrane 11 and the adjacent anode plate 20, an alkali chamber 16 is formed between the cation exchange layer 112 of the bipolar membrane 11' and the adjacent cation membrane 13, a cathode chamber 31 is formed between the anion exchange layer 111 of the bipolar membrane 11' and the adjacent cathode plate 30, and a feed chamber 15 is formed between the anion membrane 12 and the adjacent cation membrane 13.

[0041] In this embodiment, the anode plate 20 and cathode plate 30 are made of corrosion-resistant titanium coated with ruthenium; the thickness of the sealing gasket is 0.1mm~3mm; the anode membrane 13 is a CMX manufactured by Astom Corporation of Japan, with an effective area of ​​21cm² per sheet. 2 The anal membrane 12 is an AMX manufactured by Astom Corporation of Japan, with an effective area of ​​21 cm² per sheet. 2 The bipolar membrane 11 (11') is a BP-1 manufactured by Astom Corporation of Japan, with an effective area of ​​21 cm² per membrane. 2 .

[0042] In some embodiments, the number of repeating units can be set as needed, and the material throughput can be increased by increasing the number of repeating units. As an example, the number of repeating units is 1 to 5.

[0043] In this embodiment, the bipolar membrane electrodialysis device 100 further includes a feed chamber liquid storage tank 50, an acid chamber liquid storage tank 60, an alkali chamber liquid storage tank 70, and an electrode chamber liquid storage tank 80. The feed chamber liquid storage tank 50 is connected to the feed chamber 15, the acid chamber liquid storage tank 60 is connected to the acid chamber 14, the alkali chamber liquid storage tank 70 is connected to the alkali chamber 16, and the electrode chamber liquid storage tank 80 is connected to the anode chamber 21 and the cathode chamber 31. The solution in each chamber is driven by a pump (not shown) and circulates between the chamber and the corresponding storage tank; the pump can be any one of a diaphragm pump, peristaltic pump, centrifugal pump, submersible pump, or piston pump. Figure 2 The connection relationship between each storage tank and each chamber and the circulation flow route are shown; wherein, R1~R4 represent the feed routes of the polar chamber solution, acid chamber solution, feed liquid chamber solution and alkali chamber solution from the corresponding storage tank into the corresponding chamber, respectively, and R1'~R4' represent the discharge routes of the polar chamber solution, acid chamber solution, feed liquid chamber solution and alkali chamber solution from the corresponding chamber back to the corresponding storage tank, respectively.

[0044] Step S2, Depolymerization: Polyethylene terephthalate is mixed with sodium hydroxide solution and heated to carry out alkaline depolymerization reaction, resulting in a liquid containing sodium terephthalate.

[0045] Polyethylene terephthalate (PET) chips are dissolved in a high-concentration sodium hydroxide solution and heated in an oil bath. In some embodiments, the concentration of the sodium hydroxide solution is 1 mol / L to 6 mol / L; the heating temperature is 100°C to 200°C. Under this reaction environment, the ester bonds in the PET macromolecular chain break, combining with sodium ions in the solution to form sodium terephthalate, and simultaneously generating ethylene glycol. It should be noted that, because the system is not completely sealed during the alkaline depolymerization process, a large amount of the generated ethylene glycol will volatilize and escape at the above heating temperature, and the amount of PET fed is limited. Therefore, the final ethylene glycol content in the solution is extremely low, and its impact on the subsequent electrodialysis process is negligible. After the reaction, the solution is cooled to a temperature suitable for subsequent electrodialysis treatment (e.g., room temperature to 60°C), finally obtaining a mixed solution mainly composed of sodium terephthalate and residual sodium hydroxide, which serves as the feed solution in the feed chamber.

[0046] In some embodiments, the mass concentration of sodium terephthalate in the feed solution is 0.1% to 5%.

[0047] Step S3: The liquid obtained in step S2 is introduced into the liquid chamber; water is introduced into the acid chamber; water or a dilute alkali solution is introduced into the alkali chamber.

[0048] In this embodiment, a feed solution containing sodium terephthalate is introduced into the feed chamber of the membrane stack, while deionized water and a dilute alkali solution (low-concentration sodium hydroxide solution) are introduced into the acid chamber and alkali chamber, respectively. The concentration of the low-concentration sodium hydroxide solution is 0.1 mol / L to 0.3 mol / L. By introducing a dilute alkali solution (i.e., a low-concentration sodium hydroxide solution) into the alkali chamber, the initial conductivity of the alkali chamber is improved, thereby reducing the initial resistance of the alkali chamber, and consequently reducing the initial resistance of the membrane stack, thus avoiding high energy consumption caused by excessively high initial resistance.

[0049] Step S4, Electrodialysis: Apply direct current to the bipolar membrane electrodialysis device to perform electrodialysis. In the feed chamber, sodium terephthalate ionizes into terephthalate ions and sodium ions. The terephthalate ions pass through the anion membrane into the acid chamber and combine with hydrogen ions hydrolyzed by the bipolar membrane to form terephthalic acid. The sodium ions pass through the cation membrane into the alkali chamber and combine with hydroxide ions hydrolyzed by the bipolar membrane and sodium ions to form sodium hydroxide.

[0050] Step S4 further includes introducing an electrolyte solution into the anode chamber and the cathode chamber. In some embodiments, the electrolyte solution is a sodium sulfate solution with a concentration of 0.1 mol / L to 0.3 mol / L.

[0051] like Figure 3As shown, under the action of a DC electric field, sodium terephthalate (Na2TP) ionizes into terephthalate ions (TP) in the feed chamber 15. 2- ) and sodium ions (Na) + ), TP 2- Na migrates directionally through anion membrane 12 to acid chamber 14 + The hydrogen ions migrate directionally through the cation exchange membrane 11 into the alkaline chamber 16; in the acid chamber 14, the hydrogen produced by the hydrolysis of the bipolar membrane 11... + With TP from feed chamber 15 2- The combination forms terephthalic acid (TPA); in the alkaline chamber 16, the OH groups generated by the hydrolysis of the bipolar membrane 11' - With Na from feed chamber 15 + They combine to form NaOH.

[0052] It should be noted that the feed into the feed chamber 15 is a mixed solution obtained from the alkaline depolymerization of PET, which contains residual NaOH in addition to Na₂TP. NaOH will also ionize to generate Na₂. + and OH - Under the influence of a DC electric field, these Na... + It will interact with Na from Na2TP + They migrate together into alkaline chamber 16, jointly generating NaOH; while OH... - The OH groups will then migrate towards acid chamber 14. - H generated by the bipolar membrane + Neutralization produces water.

[0053] As electrodialysis proceeds, ions continuously undergo directional migration. Ions continuously migrate out of the feed chamber, causing the solution conductivity to gradually decrease; while Na+ continuously migrates into the alkali chamber. + The bipolar membrane continuously hydrolyzes to release OH- - The concentration of NaOH continued to rise; while TP continued to migrate into the acid chamber. 2- H, continuously hydrolyzed by the bipolar membrane + After binding, the insoluble solid precipitate is rapidly converted (i.e., TPA), so the conductivity of the solution remains essentially unchanged.

[0054] In some embodiments, to achieve efficient and stable operation of electrodialysis, the applied current density is 20 mA / cm². 2 ~50mA / cm 2 This invention discovers that when the current density is less than 20 mA / cm² 2 When the required electrodialysis time is too long, the production efficiency of TPA is low; while when the current density is greater than 50 mA / cm², the production efficiency is low. 2 At this time, the heat generated by the membrane stack increases sharply, posing a risk of damage to the cation membrane, anion membrane, and bipolar membrane.

[0055] In some embodiments, before performing step S4, the solution in each chamber is circulated by a drive pump for 5 to 30 minutes to remove air bubbles from the membrane stack.

[0056] In some embodiments, in step S4, the flow rate of the solution in the feed chamber, acid chamber, and alkali chamber is 100 mL / min to 500 mL / min.

[0057] In some embodiments, conductivity sensors are installed in the feed chamber, acid chamber, and alkali chamber to continuously monitor changes in the conductivity of the solution in each chamber.

[0058] In some embodiments, step S4 further includes: monitoring the conductivity of the solution in the feed chamber, and when the conductivity is below 10 ms / cm, ending the electrodialysis and collecting terephthalic acid from the acid chamber.

[0059] The technical solution of the present invention will be further described in detail below with reference to specific embodiments.

[0060] Example 1 This embodiment uses, as follows: Figure 2 and Figure 3 The bipolar membrane electrodialysis apparatus 100 and membrane stack 10 are shown; wherein the number of repeating units in the membrane stack 10 is 1. The anode chamber 21 and the cathode chamber 31 are connected in series, and the initial liquid inlet to each chamber in the membrane stack 10 is as follows: Liquid storage tank 50 in the material chamber: 100 mL of sodium terephthalate solution with a mass fraction of 0.5% is introduced; Acid chamber liquid storage tank 60: 100 mL of deionized water is introduced; Alkali chamber liquid storage tank 70: 100 mL of 0.1 mol / L sodium hydroxide solution is introduced; Electrode chamber liquid storage tank 80: 100 mL of 0.3 mol / L sodium sulfate solution is introduced.

[0061] During electrodialysis, the flow rate of each solution in membrane stack 10 is 300 mL / min, and the operation is constant current with a current density of 30 mA / cm². 2 Set the voltage limit to 200V.

[0062] Electrodialysis was stopped when the conductivity of the solution in the feed tank (50) reached approximately 5-10 mS / cm. The final sodium hydroxide concentration obtained in the alkali tank (70) was 0.34 mol / L. The PET conversion rate during the entire electrodialysis process was 11%, and the purity of the produced terephthalic acid was higher than 99%. The PET conversion rate was calculated as follows: PET conversion rate = (actual mass of terephthalic acid obtained / theoretical mass of terephthalic acid to be obtained) × 100%. The purity of the terephthalic acid was determined using a combination of analytical techniques, including 1H NMR and sodium NMR, as detailed below.

[0063] Example 2 The difference between this embodiment and Embodiment 1 is that the initial liquid entering the liquid storage tank of the feed chamber is 100 mL of sodium terephthalate solution with a mass fraction of 1%.

[0064] In this embodiment, the final sodium hydroxide concentration obtained in the alkali chamber liquid storage tank is 0.64 mol / L. The PET conversion rate during the entire electrodialysis process is 63.1%, and the purity of the produced terephthalic acid is higher than 99%.

[0065] Example 3 The difference between this embodiment and Embodiment 1 is that the initial liquid entering the liquid storage tank of the feed chamber is 100 mL of sodium terephthalate solution with a mass fraction of 2%.

[0066] In this embodiment, the final sodium hydroxide concentration obtained in the alkali chamber liquid storage tank is 0.64 mol / L. The PET conversion rate during the entire electrodialysis process is 80.1%, and the purity of the produced terephthalic acid is higher than 99%.

[0067] The test results obtained from Examples 1 to 3 above are shown in Table 1 below.

[0068] Table 1. Results of sodium hydroxide concentration, terephthalic acid purity, and PET conversion rate prepared in Examples 1-3. Figure 4 The figures show the changes in NaOH concentration in the alkali chamber liquid storage tank over time during the electrodialysis process in Examples 1-3. Figure 3As can be seen, the NaOH concentration continuously increases during electrodialysis. Referring to Table 1, as the initial sodium terephthalate concentration in the feed tank increases, the final NaOH concentration in the alkali tank also increases, and the PET conversion rate improves, meaning the yield of terephthalic acid increases. On one hand, feed solutions with lower sodium terephthalate concentrations produce less terephthalic acid, resulting in a higher relative material loss in subsequent fixed operations such as collection and transfer, leading to a lower terephthalic acid yield. On the other hand, feed solutions with lower sodium terephthalate concentrations provide insufficient ion quantity. Experiments showed that a layer of terephthalic acid products adhered to the cathode-facing side of the anion exchange membrane, severely hindering normal ion migration and thus reducing the terephthalic acid yield, consequently lowering the PET conversion rate.

[0069] The acid chamber products obtained in Examples 1-3 were analyzed, and the results are as follows: Figures 5-7 As shown. Figure 5 As shown, 1 ¹H NMR (400 MHz, DMSO-d⁶) δ 13.35 (s, 1H), 8.05 (d, J=4Hz, 2H), 8.04. The characteristic peaks of the ¹H NMR spectrum of the obtained product are completely consistent with those of the terephthalic acid standard (standard TPA); Figure 6 As shown, infrared spectroscopy confirmed the presence of a carboxyl group (1671 cm⁻¹) in the molecular structure of the obtained product. -1 and 1290cm -1 ), benzene ring (1420cm) -1 The benzene ring is in a para-disubstituted configuration (730 cm⁻¹). -1 The analytical results were basically the same as those of the terephthalic acid standard; for example... Figure 7 As shown, Raman spectroscopy confirmed the presence of a carboxyl group in the molecular structure of the product (1629 cm⁻¹). -1 The analytical results were also basically consistent with those of the terephthalic acid standard. Furthermore, such as... Figure 8 As shown, sodium NMR spectroscopy analysis did not detect any sodium ion signal, only a TMS (tetramethylsilane) internal standard chemical peak, indicating that the terephthalic acid purity of the product is extremely high. These results demonstrate that the product obtained by this invention contains only terephthalic acid, proving that the method of this invention not only successfully recovers PET but also prepares high-purity terephthalic acid. Compared with traditional processes, the method of this invention can obtain high-purity products without post-treatment steps such as acid washing and water washing, fundamentally avoiding the consumption of acid reagents and the generation of saline wastewater.

[0070] In summary, this invention provides a method for recovering PET and preparing terephthalic acid through bipolar membrane electrodialysis. This method successfully converts waste PET into terephthalic acid. The method is simple, the process design is simplified, the product has high purity, and the cost is low, providing a new approach for the development of green PET processes.

[0071] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0072] In the description of this invention, it should be understood that the terms "center," "height," "thickness," "upper," "lower," "vertical," "horizontal," "top," "bottom," "inner," "outer," "axial," "radial," and "circumferential," etc., indicating orientation or positional relationships, are based on the orientation or positional relationships shown in the accompanying drawings and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0073] In the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0074] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0075] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A method for recovering polyethylene terephthalate by bipolar membrane electrodialysis, characterized in that, include: Step S1: A bipolar membrane electrodialysis device is provided. The bipolar membrane electrodialysis device includes a membrane stack, the membrane stack includes at least one repeating unit, and the repeating unit includes a bipolar membrane, an anion membrane, a cation membrane, and another bipolar membrane arranged sequentially. An acid chamber is formed between the bipolar membrane and the adjacent anion membrane, a feed chamber is formed between the anion membrane and the adjacent cation membrane, and an alkali chamber is formed between the cation membrane and the adjacent bipolar membrane. Step S2: Polyethylene terephthalate is mixed with sodium hydroxide solution and heated to carry out alkaline depolymerization reaction, resulting in a liquid containing sodium terephthalate. Step S3: The liquid obtained in step S2 is introduced into the liquid chamber; water is introduced into the acid chamber; water or a dilute alkali solution is introduced into the alkali chamber. Step S4: Apply direct current to the bipolar membrane electrodialysis device to perform electrodialysis. In the feed chamber, sodium terephthalate ionizes into terephthalate ions and sodium ions. The terephthalate ions pass through the anion membrane into the acid chamber and combine with hydrogen ions hydrolyzed by the bipolar membrane to form terephthalic acid. The sodium ions pass through the cation membrane into the alkali chamber and combine with hydroxide ions hydrolyzed by the bipolar membrane and sodium ions to form sodium hydroxide.

2. The method as described in claim 1, characterized in that, In step S2, the concentration of the sodium hydroxide solution is 1 mol / L to 6 mol / L; the heating temperature is 100℃ to 200℃.

3. The method as described in claim 1, characterized in that, In step S4, the flow rate of the solution in the feed chamber, acid chamber, and alkali chamber is 100 mL / min to 500 mL / min.

4. The method as described in claim 1, characterized in that, In step S4, the current density applied during electrodialysis is 20 mA / cm². 2 ~50mA / cm 2 .

5. The method as described in claim 1, characterized in that, In step S3, the dilute alkaline solution is a sodium hydroxide solution, and the initial concentration of the sodium hydroxide solution introduced into the alkaline chamber is 0.1 mol / L to 0.3 mol / L.

6. The method as described in claim 1, characterized in that, The mass concentration of sodium terephthalate in the liquid obtained in step S2 is 0.1% to 5%.

7. The method as described in claim 1, characterized in that, Conductivity sensors are installed in the feed chamber, acid chamber, and alkali chamber.

8. The method as described in claim 1, characterized in that, Step S4 further includes: monitoring the conductivity of the solution in the feed chamber; when the conductivity is below 10 ms / cm, ending the electrodialysis and collecting terephthalic acid from the acid chamber.

9. The method as described in claim 1, characterized in that, The bipolar membrane electrodialysis device further includes an anode plate and a cathode plate disposed on both sides of the membrane stack. An anode chamber is formed between the anode plate and the adjacent bipolar membrane, and a cathode chamber is formed between the cathode plate and the adjacent bipolar membrane. Step S3 further includes: introducing an electrolyte solution into the anode chamber and the cathode chamber.

10. The method as described in claim 9, characterized in that, The electrolyte solution is a sodium sulfate solution with a concentration of 0.1 mol / L to 0.3 mol / L.

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