Method for recovering phenol from waste polycarbonate plastic

Abstract

The application discloses a method for recovering phenol from waste polycarbonate plastic, and belongs to the field of plastic recycling. The method uses a fixed-bed continuous flow reactor and green water as a solvent. Without using a catalyst, the continuous directional rupture of C-O and C-C bonds in waste PC plastic particles is realized by high-temperature steam. Low-molecular chemical substances such as phenol and acetone are effectively recovered. Under optimal conditions, the selectivity of phenol is as high as 95.8%, and high-purity phenol is obtained through rotary evaporation separation and purification. The technology has the advantages of simple operation, low cost, environmental protection and the like, meets the green and sustainable development demand of phenol production, and has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of plastic recycling and relates to a depolymerization technology for waste polycarbonate plastics, specifically a method for continuously recovering phenol from waste polycarbonate plastics. Background Technology

[0002] Phenol is an important bulk chemical widely used in the synthesis of many value-added chemicals. As a key raw material for resins, bactericides, pharmaceuticals (such as aspirin), and cosmetics, it plays a vital role in modern industry. Traditionally, phenol production has relied mainly on primary energy sources, with common production methods including the coal tar process, petroleum-based process, and coal-based process. These primary energy sources are non-renewable, and overexploitation has a significant impact on the environment.

[0003] Polycarbonate (PC) plastic, as an important engineering plastic, has become the fastest-growing plastic among the five major engineering plastics due to its excellent mechanical properties, especially aromatic polycarbonates, which significantly outperform other types (such as aliphatic and aliphatic-aromatic hybrids). Therefore, the large-scale, harmless recycling of PC plastic has become an urgent issue. Traditional landfill methods not only introduce microplastics into ecosystems but also significantly increase greenhouse gas emissions, posing a serious threat to human health; while mechanical recycling leads to severe thermal degradation of plastics, damaging their optical, thermal, and mechanical properties. In contrast, thermochemical recycling methods, especially those utilizing high-temperature processing technology to convert waste plastics into more valuable chemicals, show broad application prospects. Structurally, PC plastic is formed by the condensation of the precursor bisphenol A via CO bonds, and bisphenol A is itself synthesized through the catalytic C-C bond condensation of phenol and pyruvic acid. Therefore, to recover phenol from waste PC plastic, it is essential to effectively break the CO and C-C bonds.

[0004] Currently, various methods have been proposed for the recycling of PC plastics, including hydrolysis, hydrogenolysis, alkaline catalysis, acid catalysis, and dealkylation. For example, patent CN 111116318 A discloses a solvent-free method for recycling waste polycarbonate, which uses a hydrogenation catalyst to react at 180-280°C for 1 to 24 hours to convert PC plastics into high-value low-molecular-weight chemicals such as phenol, cyclohexanol, 4-isopropylcyclohexanone, 4-isopropylphenol, 2-cyclohexanone, and 2-cyclohexanol. However, this process destroys the aromaticity of the products through hydrogenation. In addition, the literature "High-value products from the catalytic hydrolysis of polycarbonate waste" (PolymerJournal 42(6) (2010) 438-442) uses MgO and CaO as catalysts in a semi-batch reactor to hydrolyze PC plastics at 300°C-500°C. Bisphenol A is the main product of hydrolysis at 300 °C, while phenol and 4-isopropenylphenol can be obtained at 500 °C, but the yield of each product is only 5%-7%. Recently, the literature "Monomer recovery of waste plastics by liquid phase decomposition and polymer synthesis" (Journal of Materials Science 43(7)(2008) 2437-2441) reported the use of homogeneous inorganic alkali Na2CO3 to hydrolyze PC plastic at 250°C-300°C, achieving a phenol yield of up to 50%, but the problem of recycling and reusing the inorganic alkali remains unresolved. In addition, the selective breaking efficiency of C-C bonds in the methods reported above is low. To improve the selectivity and recovery efficiency of phenol, the literature "Breaking C-C Bonds and Preserving C-O Bonds in Aromatic Plastics and Lignin via a Reversing Bond Energy Cleavage Strategy" (Acs Catalysis 12(17) (2022) 10690-10699) uses a highly efficient Ru / Nb2O5 noble metal Lewis acid catalyst to recover phenol in mixed organic solvents (such as methanol and n-hexane). However, the use of noble metal catalysts and organic solvents not only increases operating costs but also reduces the overall economics of PC plastic recycling.

[0005] In summary, current PC plastic depolymerization technologies are mostly carried out in batch reactors, making efficient continuous recovery difficult. Furthermore, the disordered breaking of CO / CC bonds under high temperature and catalyst conditions leads to complex product distribution, posing significant challenges to subsequent separation and purification. Therefore, developing an economical and mild hydrothermal conversion technology to achieve continuous directional breaking of CO / CC bonds in PC plastics is of significant theoretical value and has broad application prospects. This would not only enable efficient phenol recovery but also facilitate subsequent separation and purification. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for recovering phenol from waste polycarbonate plastics. Using waste PC plastic granules as raw materials, a fixed-bed continuous flow reactor and green water as solvent are used to continuously and directionally break CO and C bonds, obtaining a highly selective aqueous solution of phenol. Through subsequent rotary evaporation, the byproducts acetone and solvent water are separated, truly realizing the recovery of the high-value bulk chemical phenol from waste plastics.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] This invention provides a method for recovering phenol from waste polycarbonate plastics, comprising the following steps:

[0009] S1. Feed waste polycarbonate plastic into a high-pressure reactor;

[0010] S2. High-temperature steam is introduced into the high-pressure reactor from the top. Waste polycarbonate plastic undergoes hydrothermal decomposition under the action of high-temperature steam, and the decomposition products are carried away with the hydrothermal steam.

[0011] S3. The high-temperature steam carrying the decomposition products coming out of the bottom of the high-pressure reactor is condensed to obtain condensate.

[0012] S4. Phenol is extracted from the condensate using chemical separation technology.

[0013] Furthermore, the high-pressure reactor is a fixed-bed reactor, and the waste polycarbonate plastic is suspended inside the fixed-bed reactor through a hopper. The diameter of the high-pressure reactor is 2 cm.

[0014] Furthermore, the hopper is a conical container, a cylindrical container, or a hanging basket, with a large number of micropores on the container wall, which can both hold waste polycarbonate plastic and allow high-temperature steam to fully contact the waste polycarbonate plastic inside.

[0015] Furthermore, in step S1, when feeding waste polycarbonate plastic, an inert separating material is mixed in, preferably inert quartz wool, which does not participate in the reaction and can separate the waste polycarbonate plastic, allowing high-temperature steam to come into full contact with it.

[0016] Further, in step S2, the temperature of the high-temperature steam is 240-360°C; more preferably, the temperature is 280-360°C, and most preferably 300-360°C.

[0017] Furthermore, in step S2, a certain flow rate of inert gas is mixed into the high-temperature steam, and the pressure inside the high-pressure reactor is maintained at 0.5-10 MPa using a back pressure valve. More preferably, the pressure is 1-10 MPa.

[0018] Furthermore, the inert gas is one of nitrogen, argon, and helium.

[0019] Further, in step S2, the high-temperature steam is generated by setting up a preheater, introducing inert gas and water into the preheater to heat it to obtain high-temperature steam at the required temperature, and continuously introducing it into the high-pressure reactor.

[0020] Furthermore, the flow rate of the inert gas in the reactor ranges from 0.6 to 30 cm / min. -1 .

[0021] Furthermore, a heater is also installed in the high-pressure reactor to maintain the internal reaction temperature at 240-360°C.

[0022] Furthermore, the water is pumped to the heater via a horizontal flow pump with a flow rate of 0.001 mL / min. -1 .

[0023] Furthermore, in step S3, a condenser is installed at the bottom of the high-pressure reactor, and a liquid collection container is installed downstream of the condenser. The liquid condensed from the high-temperature steam after the reaction is collected in the liquid collection container.

[0024] Furthermore, the cooling medium temperature of the condenser is -7 to 5°C, and it is sent to the condenser by a condensation pump.

[0025] Furthermore, the liquid collection container is provided with a liquid outlet at the bottom and a gas outlet at the top. The gas outlet is connected to a gas bag or a gas harmless treatment system. The inert gas carried out by the high-temperature steam, the carbon dioxide produced by the reaction, and other non-condensable gases are collected and centrally treated through the gas bag, or discharged or recycled after being treated by the gas harmless treatment system.

[0026] Furthermore, the high-pressure reactor is also equipped with a high-pressure feeder for intermittent or continuous feeding of waste polycarbonate plastics; the high-pressure feeder can be a screw feeder.

[0027] Furthermore, the waste polycarbonate plastic is a pulverized granular material with a volume of 0.5-20 cm³. 3 .

[0028] Furthermore, in step S4, the technique for extracting phenol from the condensate is as follows:

[0029] S41, Acetone is evaporated at low temperature;

[0030] S42. Evaporate the solvent water at high temperature to obtain concentrated phenol.

[0031] Furthermore, the temperature at which acetone is evaporated is 20-50℃, and the equipment used is a rotary evaporator.

[0032] Furthermore, the temperature at which the solvent water is evaporated is 70-90℃, and the equipment used is a rotary evaporator.

[0033] Furthermore, in step S4, phenol can also be extracted from the condensate by extraction, with ethyl acetate as the extractant. During extraction, the extractant is added in multiple batches, such as in three batches, to improve the extraction efficiency.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] This invention achieves the directional conversion of waste PC plastic granules using water-based solvents in an inert atmosphere without the aid of any catalyst. It has low production costs, simple operation, and a safe, green, and environmentally friendly reaction process.

[0036] This invention uses a continuous flow fixed-bed reactor, which can continuously convert waste PC plastic particles into phenol through high-pressure feeding. The reaction process is simple and efficient.

[0037] This invention achieves directional cleavage of the CO / CC bond under mild hydrothermal conditions without causing other side reactions, and generates phenol with high selectivity; at the same time, acetone and water can be separated by rotary evaporation to obtain high-purity phenol chemicals. Attached Figure Description

[0038] Figure 1 This invention relates to an apparatus for recovering phenol from waste polycarbonate plastics.

[0039] Figure 2 This is a flowchart of the method for recovering phenol from waste polycarbonate plastics according to the present invention.

[0040] Figure 3This is a schematic diagram illustrating the PC depolymerization principle in the method for recovering phenol from waste polycarbonate plastics according to the present invention.

[0041] Figure 4 This is a mass spectrum of the phenol product obtained from the continuous recovery of phenol from waste polycarbonate plastic in Example 1.

[0042] Figure 5 This is the mass spectrum of the acetone product obtained from the continuous recovery of phenol from waste polycarbonate plastic in Example 1.

[0043] Figure 6 This is the mass spectrum of the bisphenol A product obtained from the continuous recovery of phenol from waste polycarbonate plastic in Example 1.

[0044] Figure 7 This is a gas chromatogram of the product from the continuous recovery of phenol from waste polycarbonate plastics in Example 14.

[0045] 100-High-pressure reactor, 110-Hopper, 120-Heater, 200-High-temperature steam generator, 210-Inert gas pipe, 220-Water inlet pipe, 221-Horizontal flow pump, 230-High-temperature steam pipe, 240-Preheater, 300-Condenser, 310-Condensation pump, 400-Liquid collection container, 410-Liquid outlet, 420-Gas outlet, 500-Feeder. Detailed Implementation

[0046] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0047] In the description of this invention, it should be noted that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for 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. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0048] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; 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. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0049] Example 1

[0050] like Figure 1 As shown, the present invention first provides an apparatus for recovering phenol from waste polycarbonate plastics, which is a fixed-bed reaction system, including a high-pressure reactor 100, a high-temperature steam generator 200, a condenser 300, and a liquid collection container 400. The fixed-bed reactor 100 is a pressure vessel capable of withstanding a working pressure of 0.5-10 MPa. A hopper 110 is provided inside the fixed-bed reactor 100 for holding waste polycarbonate plastics. In this embodiment, the hopper 110 is a cylindrical container with numerous micropores on its sidewalls. The high-temperature steam generator 200 includes a preheater 240, and the inlet of the preheater 240 is provided with an inert gas pipe 210 and a water inlet pipe 2200. 20. A high-temperature steam pipe 230 is provided at the outlet of the preheater 240, and the other end of the high-temperature steam pipe 230 is connected to the high-pressure reactor 100. The bottom of the high-pressure reactor 100 is connected to the condenser 300 through a pipe. The bottom outlet of the condenser 300 is connected to the liquid collection container 400. The liquid collection container 400 is provided with a liquid outlet 410 at the bottom and a gas outlet 420 at the top. The gas outlet 420 is connected to a gas bag or a gas harmless treatment system. The inert gas carried out by the high-temperature steam, the carbon dioxide produced by the reaction and other non-condensable gases are collected and centrally treated through the gas bag, or discharged or recycled after being treated by the gas harmless treatment system.

[0051] The cooling medium of condenser 300 is chilled water or other cooling medium at -7 to 5℃, which is delivered to condenser 300 by condenser pump 310; preheater 240 can be an electric heater or other type of heater; inert gas pipe 210 is connected to an inert gas source, which is a gas cylinder or industrial gas pipeline, and the inert gas pipe is equipped with a valve and a flow meter. Figure 1 (Not shown in the diagram) is used to control the flow rate; the inlet pipe 220 is connected to the water source or the outlet of the horizontal flow pump 221, and the inlet pipe 220 is equipped with a valve and a flow meter to control the inlet water volume; the high-pressure reactor 100 should also be equipped with a pressure gauge or pressure sensor to detect the pressure inside the reactor, and a temperature gauge or temperature sensor to detect the temperature inside the reactor. The high-pressure reactor itself is also equipped with a heater.

[0052] The present invention also includes a rotary evaporator ( Figure 1 (Not shown in the diagram) The condensate collected from the liquid collection container is concentrated and purified.

[0053] like Figure 2 As shown, the present invention also provides a method for recovering phenol from waste polycarbonate plastics, comprising the following steps:

[0054] S1. Feed waste polycarbonate plastic into a high-pressure reactor;

[0055] S2. High-temperature steam is introduced into the high-pressure reactor from the top. Waste polycarbonate plastic undergoes hydrothermal decomposition under the action of high-temperature steam, and the decomposition products are carried away with the hydrothermal steam.

[0056] S3. The high-temperature steam carrying the decomposition products coming out of the bottom of the high-pressure reactor is condensed to obtain condensate.

[0057] S4. Phenol is extracted from the condensate using chemical separation technology.

[0058] like Figure 3 As shown, this invention utilizes high-temperature steam to hydrolyze waste polycarbonate plastic through fluid contact, continuously and directionally breaking CO and CC bonds. The reaction pathway is as follows: Due to the low bond energy of the CO bond, the CO bond breaks first during the reaction, yielding bisphenol A and carbon dioxide; subsequently, the CC bond in bisphenol A undergoes hydrolysis and breaks, yielding isopropylphenol and phenol, along with small amounts of isopropylphenol and 4-ethylphenol; the CC bond in isopropylphenol continues to be hydrolyzed to generate phenol and acetone; this invention, by rationally controlling the reaction parameters of high-temperature hydrolysis, achieves highly selective phenol aqueous solution, thereby purifying high-purity phenol.

[0059] Example 1: This example provides a method for recovering phenol from waste polycarbonate plastics using laboratory equipment. For industrial production, only a feeder and a rotary evaporator need to be added.

[0060] For example, screw feeders can replace intermittent feeding, thus enabling continuous industrial production.

[0061] Includes the following steps:

[0062] First, fill the bottom of the high-pressure reactor (approximately 2cm inner diameter) with a certain amount of quartz wool to hold the cylindrical container used as a hopper, ensuring the cylindrical container is in the middle of the reactor. Weigh 0.05g of PC plastic granules and a small amount of quartz wool, mix them thoroughly, and insert them into the basket. Place the basket into the high-pressure reactor, seal the reactor, and fill it with nitrogen gas at 2MPa. Use a back pressure valve to maintain the pressure inside the reactor at 2MPa. Use bubble water to check for leaks and ensure the reactor is sealed. Use a flow meter to control the nitrogen flow rate at 3cm / min. -1 (Based on the reactor cross-sectional area, this is approximately equivalent to a nitrogen flow rate of 4 mL / min) -1 Water was introduced into the reactor using a high-pressure horizontal flow pump, with the flow rate controlled at 0.05 mL / min. -1Nitrogen and water are mixed in the heater, and the water is heated into steam to form high-temperature steam. The temperature of both the preheater and the heater is set to 330°C to ensure that the reaction temperature is 330°C when the high-temperature steam comes into contact with the PC plastic particles. The high-temperature steam after the reaction continuously enters the condenser from the bottom of the high-pressure reactor. The liquid products are continuously recovered by the condenser and collected in a liquid collection container. The liquid collection container has a liquid outlet at the bottom and a gas outlet at the top. The gas outlet is connected to a gas bag. The inert gas carried out by the high-temperature steam, the carbon dioxide produced by the reaction, and other non-condensable gases are collected and centrally processed through the gas bag. In this embodiment, in addition to nitrogen, the gaseous products are mainly CO2 gas generated after the CO bonds of polycarbonate break. Gas chromatography is used to quantitatively analyze the CO2 collected by the gas bag.

[0063] Every hour, samples of the liquid product were collected from the liquid collection container through the liquid outlet and transferred to test tubes. Ethyl acetate was added in three separate 6 mL portions for extraction. 1 mL of the upper ethyl acetate solution was taken and 1 mg of dodecane (internal standard) was added for qualitative and quantitative analysis. GC-MS (Gas Chromatography-Mass Spectrometry) was performed using an HP-5 high-temperature column (0.25 mm × 30 m, 0.25 μm). The instrument was initially set at 50 °C for 2 minutes, then at 5 °C for 1 minute. -1 The temperature was raised to 280°C at a specific heating rate, and then maintained at 280°C for 15 minutes before analysis. The total run time was 63 minutes. The mass spectra of the main products, phenol, acetone, and bisphenol A, are shown below. Figure 4-6 As shown, the molecular weight of phenol is 94, the molecular weight of acetone is 58, and the molecular weight of bisphenol A is 228.

[0064] The steps for product separation and purification are as follows:

[0065] The liquid product after the reaction is transferred to a round-bottom flask through the liquid outlet. Acetone is evaporated by rotary evaporation in a 30°C water bath. Then, the temperature is increased to 70°C to remove the extractant ethyl acetate or the solvent water from the unextracted liquid product by rotary evaporation. Finally, high-purity phenol is recovered in the round-bottom flask and stored in a sealed container.

[0066] Based on the analytical results of GC-FID (flame ionization detector), the total yields of phenol monomer and other byproducts (bisphenol A, isopropylphenol, isopropenylphenol, and tetraethylphenol) were calculated using the external standard method and the effective carbon number method, and the selectivity of phenol was calculated accordingly. The effective carbon number of phenol was calculated to be 5.5, bisphenol A to 14, isopropylphenol to 8.5, isopropenylphenol to 8.4, and tetraethylphenol to 7.5. Dodecane was used as the internal standard. The specific calculation method is as follows:

[0067] (1)

[0068] (2)

[0069] (3)

[0070] (4)

[0071] (5)

[0072] In the above formula:

[0073] (mg): The mass of dodecane used as the internal standard in each sample, 1 mg;

[0074] (mg mmol -1 ): Relative molecular mass of dodecane, 170 g mol -1 ;

[0075] (mmol): Amount of dodecane in each sample, 5.88 10 -6 mol;

[0076] (mmol): The amount of product in each sample;

[0077] Peak area corresponding to each product in GC FID;

[0078] : The peak area corresponding to the internal standard dodecane in GC FID;

[0079] The effective carbon number of dodecane, ;

[0080] The effective carbon number of each product in the sample;

[0081] (mg): Mass of product in each sample;

[0082] : Mass yield of all products in the sample;

[0083] Selectivity of each product in the sample.

[0084] Tests showed that under these conditions, the selectivity of phenol monomer was 92.2%, the selectivity of bisphenol A was 0.5%, the selectivity of isopropylphenol was 4.4%, the selectivity of isopropylphenol was 2.0%, and the selectivity of acetone was 0.9%.

[0085] Patent CN103232326A discloses a continuous production process for phenol by hydroxylation of benzene. This patent uses TS-1 molecular sieve as a catalyst and produces phenol through a batch-to-continuous operation, achieving a phenol selectivity of up to 92.6%. Patent CN118851884A discloses a method for the directional chemical depolymerization of bisphenol A type polycarbonate plastics. This invention uses a hydrothermal reactor and acid catalysis to degrade PC, but the phenol selectivity in the final product is only about 30%. Besides phenol, it also produces a significant amount of bisphenol A, indanebisphenol, spirobisindane, and other byproducts, failing to achieve the separation and purification of any single product. However, this invention, without using any catalyst, achieves the directional conversion of various PC plastic particles using only high-temperature steam, and uses a continuous flow production method, improving depolymerization efficiency and making it more environmentally friendly and safer.

[0086] Examples 2-5

[0087] Based on Example 1, the feed amount of PC plastic granules was changed (0.02g, 0.1g, 0.15g, 0.2g) to form Examples 2-5; the subsequent experimental conditions and operating steps were the same as those in Example 1, and the test results are shown in Table 1.

[0088] Table 1. Effect of different PC plastic pellet feed rates on its directional conversion to phenol.

[0089]

[0090] According to Table 1, for cylindrical containers with an internal diameter of 2 cm, changing the amount of PC plastic granules gradually decreased the selectivity of phenol and gradually increased the selectivity of bisphenol A. This indicates that under incomplete reaction conditions, the C-C bonds are not easily broken, leading to a decrease in the selectivity of phenol and the generation of more other byproducts. Compared with Example 1, Examples 2 and 3 show little change; therefore, the selected material mass for the following examples is 0.05 g.

[0091] Examples 6-9

[0092] Based on Example 1, PC plastic granules were made using plastic parts from different sources (goggles, CDs, washing machine shells, wheel covers). These waste plastic parts were cut into PC sheets with a side length of 0.5 cm and added to the reactor at a mass of 0.05 g, forming Examples 6-9. The subsequent experimental conditions and operating steps were the same as in Example 1, and the test results are shown in Table 2.

[0093] Table 2. The Influence of Different Waste Sources on the Directed Conversion of PC Plastic Particles to Phenol

[0094]

[0095] According to Table 2, using PC plastic pellets from different waste sources does not significantly affect the distribution of the product, which indicates the versatility of the method.

[0096] Examples 10-14

[0097] Based on Example 1, the heating temperatures of the preheater and heater were adjusted to 240℃, 260℃, 280℃, 300℃, and 360℃, forming Examples 10-14; the subsequent experimental conditions and operating procedures were the same as in Example 1, and the test results are shown in Table 3.

[0098] Table 3. Effect of different reaction temperatures on the directional conversion of PC plastic particles to phenol.

[0099]

[0100] According to Table 3, increasing the temperature increases the selectivity of phenol while decreasing the selectivity of bisphenol A, demonstrating that under high temperature conditions, almost all C-C bonds are broken. The phenol selectivity obtained at the reaction temperature of 360°C in Example 14 is as high as 95.6%. After subsequent rotary evaporation purification, the purity of phenol crystals can reach 98.1%. Therefore, subsequent examples were conducted under the same conditions as in Example 14 to investigate the effect of water flow rate on the depolymerization of PC plastic particles into phenol.

[0101] Gas chromatographic analysis was performed on the gaseous products in Example 14, and the results are as follows: Figure 7 As shown, from Figure 7 As can be seen, at a reaction temperature of 360℃, bisphenol A disappears, and only a small amount of isopropylphenol remains unconverted.

[0102] Examples 15-19

[0103] Based on Example 14, the inlet flow rate of the horizontal flow pump was changed to 0.01, 0.03, 0.1, 0.15, and 0.2 mL / min. -1 Examples 15-19 were thus formed. The subsequent experimental conditions and operating procedures were the same as in Example 14, and the test results are shown in Table 4.

[0104] Table 4. Effect of influent flow rate on the directional conversion of PC plastic particles to phenol.

[0105]

[0106] With the pipe and reactor dimensions remaining constant, increasing the influent flow rate leads to a faster flow velocity, which shortens the contact time between the high-temperature steam and the PC plastic particles, theoretically causing incomplete reaction. Table 4 shows that increasing the influent flow rate decreases the selectivity of phenol while increasing the selectivity of isopropylene phenol as a byproduct. This demonstrates that when the contact time between the PC plastic particles and water vapor is short, the C-C bonds cannot be completely broken, resulting in a higher concentration of isopropylene phenol byproducts. To further extend the contact time, the effect of gas flow rate on the depolymerization of PC plastic particles into phenol will be investigated.

[0107] Examples 20-24

[0108] Based on Example 14, the nitrogen inlet flow rate was changed to 0.6, 1.8, 6, 9, and 12 cm / min. -1 Examples 20-24 were formed; the subsequent experimental conditions and operating procedures were the same as in Example 14, and the test results are shown in Table 5.

[0109] Table 5. Effect of airflow rate on the directional conversion of PC plastic particles to phenol.

[0110]

[0111] The nitrogen flow rate is calculated using flow rate conversion. The desired flow rate is obtained by adjusting the flow rate. With constant pipe and reactor dimensions, increasing the flow rate proportionally increases the nitrogen flow rate. Excessive nitrogen flow rate can also shorten the contact time between the high-temperature steam and PC plastic particles, theoretically leading to incomplete reaction. Table 5 shows that changing the nitrogen flow rate results in the same change in phenol selectivity as changing the influent flow rate. Increasing the gas flow rate improves the selectivity of isopropylene phenol as a byproduct, demonstrating that the shorter contact time between the PC plastic particles and the high-temperature steam results in incomplete C-C bond breakage, leading to the formation of a shorter isopropylene phenol byproduct.

[0112] Examples 25-29

[0113] Based on Example 14, the set pressure of the high-pressure reactor was changed to 0.1, 0.5, 1, 5, and 10 MPa. Subsequent experimental conditions and operating procedures were the same as in Example 14, and the test results are shown in Table 6.

[0114] Table 6. Effect of reaction pressure on the directional conversion of PC plastic particles to phenol.

[0115]

[0116] According to Table 6, when the pressure in the high-pressure reactor is relatively low, the selectivity of bisphenol A reaches as high as 45.8%, proving that at low pressure, the CO bond is basically broken, but only some C-C bonds break, resulting in a phenol selectivity of only 45.1%. Increasing the reactor pressure leads to the breaking of C-C bonds, increasing phenol selectivity, and eliminating bisphenol A; furthermore, when the reactor pressure reaches above 1 MPa, the phenol selectivity increases significantly. These results indicate that low pressure affects heat and mass transfer in the reaction process.

[0117] Examples 30-31

[0118] Based on Example 14, the nitrogen gas used was changed to other types of gases such as argon and helium. The subsequent experimental conditions and operating procedures were the same as in Example 14, and the test results are shown in Table 7.

[0119] Table 7. Effect of inert gas type on the directional conversion of PC plastic particles to phenol.

[0120]

[0121] According to Table 7, the use of other inert gases (argon, helium) has almost no effect on the conversion of PC plastic particles.

[0122] Verification Example 1-3

[0123] To demonstrate the effectiveness of this invention in breaking the C-C bonds in bisphenol A, further verification was conducted using a bisphenol A model compound. The bisphenol A model compound was dissolved in a 1:1 methanol and aqueous solution, with a bisphenol A concentration of 20 mmol / L. -1 The reaction conditions are as follows: Bisphenol A flow rate 0.05 mL / min. -1 High-pressure reactor pressure 2MPa, nitrogen flow rate 3cm / min -1 The heating temperatures of the preheater and heater were changed to 250℃, 270℃, and 300℃. The effect of temperature on the breakage of C-C bonds in bisphenol A was verified, and the results are shown in Table 8.

[0124] Table 8. Effect of reaction temperature on the conversion of the model compound bisphenol A to phenol.

[0125]

[0126] The effectiveness of this invention in breaking C-C bonds was investigated using bisphenol A model compounds. According to the results in Table 8, increasing the temperature slightly improved the phenol selectivity, while decreasing the selectivity of isopropenylphenol. This indicates that the conversion of isopropenylphenol to phenol is slower at lower temperatures, resulting in a lower yield of phenol.

[0127] The above embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Although the invention has been described in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications, or equivalent substitutions of the technical solutions of the invention do not depart from the spirit and scope of the invention and should be covered within the scope of the claims of the invention.

Claims

1. A method for recovering phenol from waste polycarbonate plastics, characterized in that, Includes the following steps: Waste polycarbonate plastic is fed into a high-pressure reactor; High-temperature steam is introduced from the top into the high-pressure reactor, and the waste polycarbonate plastic undergoes hydrothermal decomposition under the action of high-temperature steam, with the decomposition products being carried away with the hydrothermal steam. The high-temperature steam carrying decomposition products exiting from the bottom of the high-pressure reactor is condensed to obtain condensate. Phenol was extracted from the condensate using chemical separation techniques. The high-temperature steam has a temperature of 280-360℃, and the pressure inside the high-pressure reactor is 1-10 MPa. The high-temperature steam is obtained by heating inert gas and water in a preheater, with the inert gas flowing at a velocity of 0.6 cm / min. -1 The high-temperature steam generation method is as follows: a preheater is set up, inert gas and water are introduced into the preheater and heated to obtain high-temperature steam at the required temperature, and then continuously introduced into the high-pressure reactor.

2. The method for recovering phenol from waste polycarbonate plastics according to claim 1, characterized in that, The high-pressure reactor is a fixed-bed reactor, and the waste polycarbonate plastic is suspended inside the fixed-bed reactor through a porous container.

3. The method for recovering phenol from waste polycarbonate plastics according to claim 1, characterized in that, A condenser is installed at the bottom of the high-pressure reactor, and a liquid collection container is installed downstream of the condenser. The liquid condensed from the high-temperature steam after the reaction is collected in the liquid collection container.

4. The method for recovering phenol from waste polycarbonate plastics according to claim 1, characterized in that, The high-pressure reactor is equipped with a high-pressure feeder for intermittent or continuous feeding of waste polycarbonate plastics.

5. The method for recovering phenol from waste polycarbonate plastics according to claim 1, characterized in that, The waste polycarbonate plastic is a pulverized granular material with a volume of 0.5-20 cm³. 3 .

6. The method for recovering phenol from waste polycarbonate plastics according to claim 1, characterized in that, The inert gas is one of nitrogen, argon, and helium.

7. The method for recovering phenol from waste polycarbonate plastics according to claim 1, characterized in that, The technique for extracting phenol from condensate is as follows: Acetone was removed by rotary evaporation at low temperature. The solvent water is removed by high-temperature rotary evaporation to obtain concentrated phenol.

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