Method for recovering phenol from waste polycarbonate plastic

Abstract

The invention discloses a method for recovering phenol from waste polycarbonate plastic, and belongs to the field of plastic recovery, the method adopts a fixed bed continuous flow reactor, uses green water as a solvent, and realizes continuous directional fracture of C-O and C-C bonds in waste PC plastic particles through high temperature water vapor under the condition of not using a catalyst. The low-molecular chemicals such as phenol, acetone and the like are effectively recovered, the selectivity of the phenol under the optimal condition is as high as 95.8%, and the high-purity phenol is obtained through rotary evaporation separation and purification. The technology has the advantages of simplicity in operation, low cost, environmental protection and the like, meets the green and sustainable development requirement of phenol production, and has a wide application prospect.

Description

Technical Field

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

[0002] Phenol is an important bulk chemical, widely used in the synthesis of many value-added chemicals. As a key raw material for resins, fungicides, drugs (such as aspirin), cosmetics and other products, it plays a vital role in modern industry. Traditionally, the production of phenol mainly relies on primary energy, and common production methods include coal tar method, petroleum-based method and coal-based method; these primary energy sources are non-renewable, and over-exploitation has a great impact on the environment.

[0003] Polycarbonate (PC) plastics are an important engineering plastic. Because of their excellent mechanical properties, especially aromatic polycarbonates, which are significantly superior to other types (such as aliphatic and aliphatic-aromatic mixed types), they have become the fastest growing plastic among the five major engineering plastics. Therefore, the harmless recycling of large-scale PC plastics has gradually become an urgent issue to be solved. Traditional plastic landfilling methods may not only introduce microplastics to pollute the ecosystem, but also significantly increase greenhouse gas emissions, posing a serious threat to human health; while mechanical recycling will cause severe thermal degradation of plastics, damaging their optical, thermal and mechanical properties. In contrast, thermochemical recycling methods, especially the use of high-temperature treatment technology to convert waste plastics into more valuable chemicals, have shown broad application prospects. In terms of structure, PC plastics are formed by the condensation of the precursor bisphenol A through the CO bond, and bisphenol A is synthesized by the catalytic CC bond condensation of phenol and pyruvic acid. Therefore, to recover phenol from waste PC plastics, the CO and CC bonds must be effectively broken.

[0004] At present, a variety of methods have been proposed for the recycling of PC plastics, including hydrolysis, hydrogenolysis, base 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 chemicals such as phenol, cyclohexanol, 4-isopropylcyclohexanone, 4-isopropylphenol, 2-cyclohexylcyclohexanone and 2-cyclohexylphenol. However, this process destroys the aromaticity of the product by hydrogenation. In addition, the literature: Preparation of high-value products from the catalytic hydrolysis of polycarbonate waste (Polymer Journal 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. Among them, bisphenol A is the main product of hydrolysis at 300 °C, while products such as 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 that the yield of phenol can reach 50% by hydrolyzing PC plastics at 250 °C-300 °C using homogeneous inorganic base Na2CO3, but the problem of recycling and reusing the inorganic base is difficult to solve. In addition, the selective cleavage efficiency of CC bonds in the above-mentioned technical reports is low. In order to improve the selectivity and recovery efficiency of phenol, the literature: Breaking C─C Bonds and Preserving C─OBonds in Aromatic Plastics and Lignin via a Reversing Bond Energy CleavageStrategy, Acs Catalysis 12(17) (2022) 10690-10699 uses efficient Ru / Nb2O5 noble metal Lewis acid catalysts 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 economic feasibility of PC plastic recycling.

[0005] In summary, the current PC plastic depolymerization technology is mostly carried out in intermittent reactors, which makes it difficult to achieve efficient and continuous recovery. At the same time, under the action of high temperature and catalyst, the disordered cleavage of CO / CC bonds leads to complex product distribution, which poses a great challenge to subsequent separation and purification. Therefore, the development of an economical and mild hydrothermal conversion technology to achieve continuous and directional cleavage of CO / CC bonds in PC plastics can not only efficiently recover phenol, but also achieve subsequent separation and purification, which has important theoretical value and broad application prospects. Summary of the invention

[0006] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for recovering phenol from waste polycarbonate plastics, which uses waste PC plastic particles as raw materials, a fixed bed continuous flow reactor and green water as solvent, continuously and directionally breaks CO and CC bonds, and obtains a phenol aqueous solution with high selectivity. By subsequent rotary evaporation, by-product acetone and solvent water are separated, thereby truly realizing the recovery of high-value bulk chemical phenol from waste plastics.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows: The present invention provides a method for recovering phenol from waste polycarbonate plastics, comprising the following steps: S1, feeding waste polycarbonate plastic into a high-pressure reactor; S2, introducing high-temperature steam into the high-pressure reactor from the top, the waste polycarbonate plastic is hydrothermally decomposed under the action of the high-temperature steam, and the decomposition products are taken away with the hydrothermal steam; S3, condensing the high-temperature steam carrying the decomposition products coming out of the bottom of the high-pressure reactor to obtain a condensate; S4. Use chemical separation technology to extract phenol from the condensate.

[0008] Furthermore, the high-pressure reactor is a fixed bed reactor, the waste polycarbonate plastic is suspended in the fixed bed reactor through a hopper, and the diameter of the high-pressure reactor is 2 cm.

[0009] Furthermore, the hopper is a conical container, a cylindrical container or a hanging basket, and a large number of micropores are opened on the container wall, which can not only contain the waste polycarbonate plastic, but also allow the high-temperature steam to fully contact with the waste polycarbonate plastic inside.

[0010] Furthermore, in step S1, when the waste polycarbonate plastic is fed, an inert isolation material is mixed therein, preferably inert quartz wool, which does not participate in the reaction but can separate the waste polycarbonate plastic, facilitating full contact between the high-temperature steam and the plastic.

[0011] Furthermore, 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.

[0012] Furthermore, in step S2, a certain flow rate of inert gas is mixed into the high-temperature steam, and a back pressure valve is used to maintain the pressure in the high-pressure reactor at 0.5-10 MPa. More preferably, the pressure is 1-10 MPa.

[0013] Furthermore, the inert gas is one of nitrogen, argon and helium.

[0014] Furthermore, in step S2, the high-temperature steam is generated by providing a preheater, introducing inert gas and water into the preheater for heating to obtain high-temperature steam of a desired temperature, and continuously introducing the preheater into the high-pressure reactor.

[0015] Furthermore, the flow rate of the inert gas in the reactor is in the range of 0.6-30 cm min -1 .

[0016] Furthermore, a heater is also provided in the high-pressure reactor to maintain the reaction temperature therein at 240-360°C.

[0017] Furthermore, the water is delivered to the heater via a horizontal flow pump, and the water flow rate of the horizontal flow pump is 0.001 mL min -1 .

[0018] Furthermore, in step S3, a condenser is provided at the bottom of the high-pressure reactor, and a liquid collecting container is provided downstream of the condenser, and the liquid condensed by the high-temperature steam after the reaction through the condenser is collected in the liquid collecting container.

[0019] Furthermore, the cooling medium temperature of the condenser is -7-5°C and is delivered to the condenser through a condensation pump.

[0020] 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 an air bag or a gas harmless treatment system. The inert gas brought 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 air bag, or discharged or recycled after treatment by the gas harmless treatment system.

[0021] Furthermore, the high-pressure reactor is also provided with a high-pressure feeder for intermittently or continuously feeding waste polycarbonate plastic; the high-pressure feeder can be a screw feeder.

[0022] Furthermore, the waste polycarbonate plastic is a crushed granular material with a volume size of 0.5-20 cm 3 .

[0023] Further, in step S4, the technology for extracting phenol from the condensate is as follows: S41, evaporating acetone at low temperature; S42, evaporate the solvent water at high temperature to obtain concentrated phenol.

[0024] Furthermore, the temperature of evaporating acetone is 20-50° C., and the equipment used is a rotary evaporator.

[0025] Furthermore, the temperature of evaporating the solvent water is 70-90° C., and the equipment used is a rotary evaporator.

[0026] Furthermore, in step S4, phenol can be extracted from the condensate by extraction, and the extractant is ethyl acetate. During the extraction, the extractant is added in multiple times, for example, in 3 times, to improve the extraction efficiency.

[0027] Compared with the prior art, the present invention has the following beneficial effects: The invention uses a water solvent in an inert atmosphere without any catalyst to achieve directional conversion of waste PC plastic particles, has low production cost, simple operation, and a safe, green and environmentally friendly reaction process.

[0028] The present invention uses a continuous flow fixed bed reactor and can realize uninterrupted and continuous conversion of waste PC plastic particles into phenol by means of high-pressure feeding, and the reaction process is simple and efficient.

[0029] The present invention directionally breaks 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 separation to obtain high-purity phenol chemicals. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The invention discloses a device for recovering phenol from waste polycarbonate plastics.

[0031] Figure 2 The present invention is a flow chart of the method for recovering phenol from waste polycarbonate plastics.

[0032] Figure 3 This is a schematic diagram of the PC depolymerization principle in the method for recovering phenol from waste polycarbonate plastics of the present invention.

[0033] Figure 4 This is the mass spectrum of the phenol product obtained by continuously recovering phenol from waste polycarbonate plastic in Example 1.

[0034] Figure 5 This is the mass spectrum of the acetone product obtained by continuously recovering phenol from waste polycarbonate plastic in Example 1.

[0035] Figure 6 This is the mass spectrum of the bisphenol A product obtained by continuously recovering phenol from waste polycarbonate plastic in Example 1.

[0036] Figure 7 This is the product gas chromatogram of the continuous recovery of phenol from waste polycarbonate plastic in Example 14.

[0037] 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-condensate pump, 400-liquid collecting container, 410-liquid outlet, 420-gas outlet, 500-feeder. DETAILED DESCRIPTION

[0038] The following embodiments of the present invention are described in further detail in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0039] In the description of the present invention, it should be noted that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance.

[0040] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "connected" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0041] Example 1 like Figure 1As shown, the present invention first provides a device 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, wherein the fixed bed reactor 100 is a pressure vessel that can withstand a working pressure of 0.5-10MPa, and a hopper 110 is arranged in the fixed bed reactor 100 for containing waste polycarbonate plastics. In this embodiment, the hopper 110 is a cylindrical container with a large number of micropores on the side wall; 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 220. 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 pipeline, and the bottom outlet of the condenser 300 is connected to a liquid collecting container 400, and the bottom of the liquid collecting container 400 is provided with a liquid outlet 410, and the top is provided with a gas outlet 420, and the gas outlet 420 is connected to an air bag or a gas harmless treatment system. The inert gas brought out by the high-temperature steam, the carbon dioxide generated by the reaction and other non-condensable gases are collected and centrally treated through the air bag, or discharged or recycled after being treated by the gas harmless treatment system.

[0042] The cooling medium of the condenser 300 is -7-5℃ chilled water or other cooling medium, which is sent to the condenser 300 through the condenser pump 310; the preheater 240 can be an electric heater or other types of heaters; the inert gas pipe 210 is connected to the inert gas source, which is a gas cylinder or an industrial gas pipeline, and a valve and a flow meter are provided on the inert gas pipe ( Figure 1 The water inlet pipe 220 is connected to the water source or the outlet of the horizontal flow pump 221, and a valve and a flow meter are provided on the water inlet pipe 220 to control the water inlet amount; the high-pressure reactor 100 should also be provided with a pressure gauge or a pressure sensor for detecting the pressure in the reactor, and a thermometer or a temperature sensor for detecting the temperature in the reactor. The high-pressure reactor itself is also provided with a heater.

[0043] The present invention also provides a rotary evaporator ( Figure 1 (not shown) the condensate collected from the liquid collection container is concentrated and purified.

[0044] like Figure 2 As shown, the present invention also provides a method for recovering phenol from waste polycarbonate plastics, comprising the following steps: S1, feeding waste polycarbonate plastic into a high-pressure reactor; S2, introducing high-temperature steam into the high-pressure reactor from the top, the waste polycarbonate plastic is hydrothermally decomposed under the action of the high-temperature steam, and the decomposition products are taken away with the hydrothermal steam; S3, condensing the high-temperature steam carrying the decomposition products coming out of the bottom of the high-pressure reactor to obtain a condensate; S4. Use chemical separation technology to extract phenol from the condensate.

[0045] like Figure 3 As shown, the present invention utilizes high-temperature water vapor to flow and contact with waste polycarbonate plastics to perform hydrolysis reaction on them, and continuously and directionally break CO and CC bonds. The reaction path is as follows: due to the low bond energy of the CO connection bond, the CO bond breaks first during the reaction to obtain bisphenol A and carbon dioxide; then the CC bond in bisphenol A is hydrolyzed and broken to obtain isopropenylphenol and phenol, which are also accompanied by a small amount of isopropylphenol and 4-ethylphenol; the CC bond in isopropenylphenol is further hydrolyzed to generate phenol and acetone; the present invention obtains a phenol aqueous solution with high selectivity by reasonably controlling the reaction parameters of high-temperature hydrolysis, and then purifies high-purity phenol.

[0046] Example 1: This example provides a method for recovering phenol from waste polycarbonate plastics using laboratory equipment. When industrial production is carried out, only a feeder and a rotary evaporator need to be added. For example, screw feeders can be used instead of intermittent feeding to achieve continuous industrial production.

[0047] The following steps are involved: First, fill a certain amount of quartz wool into the bottom of the high-pressure reactor (inner diameter of about 2 cm) to place the cylindrical container used as a hopper. Keep the material cylindrical container in the middle of the reactor. Weigh 0.05g of PC plastic particles and a small amount of quartz wool, mix them evenly and implant them into the hanging basket. Put the hanging basket into the high-pressure reactor, seal the reactor, fill the high-pressure reactor with nitrogen at a pressure of 2MPa, and use a back pressure valve to keep the pressure in the reactor at 2MPa. Use bubble water to pick up leaks and ensure that the reactor is sealed. Use a flow meter to control the nitrogen flow rate to 3cm min -1 (Based on the cross-sectional area of ​​the reactor, this is approximately equivalent to a nitrogen flow rate of 4 mL min -1 ). Use a high-pressure horizontal flow pump to fill water into the reactor, and the flow rate is controlled at 0.05 mL min -1Nitrogen and water are mixed in the heater, and the water is heated to steam to form high-temperature steam. The temperatures of the preheater and the heater are both set to 330°C to ensure that the reaction temperature is 330°C when the high-temperature steam contacts the PC plastic particles. The high-temperature steam after the reaction continuously enters the condenser from the bottom of the high-pressure reactor, and the liquid product is continuously recovered through the condenser and collected in a liquid collecting container. The liquid collecting container is provided with a liquid outlet at the bottom and a gas outlet at the top. The gas outlet is connected to an air bag, and the inert gas brought out by the high-temperature steam, the carbon dioxide generated by the reaction and other non-condensable gases are collected and centrally treated through the air bag. In this embodiment, the gas product, in addition to nitrogen, is mainly CO2 gas generated after the CO bond of polycarbonate is broken, and a gas chromatograph is used to quantitatively analyze the CO2 collected in the air bag.

[0048] The liquid product was collected from the liquid collection container through the liquid outlet every hour, and the liquid product was collected in a test tube. 6 mL of ethyl acetate was added to the collected liquid product three times for extraction. 1 mL of the upper ethyl acetate solution was added with 1 mg of internal standard dodecane for qualitative and quantitative analysis. The GC MS (gas chromatography-mass spectrometry) used an HP-5 high-temperature chromatographic column (0.25 mm × 30 m, 0.25 μm). The equipment was first maintained at 50 ° C for 2 minutes, and then at 5 ° C min -1 The heating rate was 0.040°C to 280°C, and finally the temperature was maintained at 280°C for 15 minutes for analysis. The total running time was 63 minutes. The mass spectra of the main products phenol, acetone and bisphenol A obtained are shown in FIG. 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.

[0049] The steps for separation and purification of the product are as follows: The liquid product after the reaction is transferred to a round-bottom flask through the liquid outlet, and the acetone is evaporated by rotary evaporation in a 30°C water bath, and then the temperature is increased to 70°C to rotary evaporate the extractant ethyl acetate or the solvent water in the unextracted liquid product. Finally, the high-purity phenol is left in the round-bottom flask for recovery and sealed storage.

[0050] According to the analysis results of GC FID (flame ionization detector), the total yield of phenol monomer and other by-products (bisphenol A, isopropyl phenol, isopropenyl phenol, tetraethyl phenol) was calculated by external standard method and effective carbon number method, and the selectivity of phenol was calculated based on this. The effective carbon number of phenol was calculated to be 5.5, the effective carbon number of bisphenol A was 14, isopropyl phenol was 8.5, isopropenyl phenol was 8.4, and tetraethyl phenol was 7.5. Using dodecane as the internal standard, the specific calculation method is as follows: (1) (2) (3) (4) (5) In the above formula: (mg): mass of internal standard dodecane used in each sample, 1 mg; (mg mmol -1 ): relative molecular mass of dodecane, 170 g mol -1 ; (mmol): the amount of dodecane in each sample, 5.88 10 -6 mol; (mmol): the amount of substance of the product in each sample; : The peak area corresponding to each product in GC FID; : Peak area corresponding to internal standard dodecane in GC FID; : The effective carbon number of dodecane, ; : The effective carbon number of each product in the sample; (mg): mass of product in each sample; : The mass yield of all products in the sample; : Selectivity of each product in the sample.

[0051] After testing, under these conditions, the selectivity of phenol monomer was 92.2%, the selectivity of bisphenol A was 0.5%, the selectivity of isopropyl phenol was 4.4%, the selectivity of isopropenyl phenol was 2.0%, and the selectivity of acetone was 0.9%.

[0052] The patent with publication number CN103232326A discloses a continuous production process for preparing phenol by hydroxylation of benzene. The patent uses TS-1 molecular sieve as a catalyst and produces phenol by a continuous operation mode after intermittent operation. The selectivity of phenol can reach up to 92.6%. The patent with publication number CN118851884A discloses a method for directional chemical depolymerization of bisphenol A type polycarbonate plastic. The invention uses a hydrothermal reactor and utilizes acid catalysis to degrade PC. The selectivity of phenol in the final product is only about 30%. In addition to phenol, more bisphenol A, indane bisphenol, spirobiindane and other products are also produced, and the separation and purification of a single product cannot be achieved. However, the present invention, without using any catalyst, only uses high-temperature water vapor to achieve the directional conversion of various PC plastic particles, and uses a continuous flow production method to improve the depolymerization efficiency, which is more environmentally friendly and safe.

[0053] Embodiment 2-5 On the basis of Example 1, the feed amount of PC plastic particles 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 of Example 1, and the test results obtained are shown in Table 1.

[0054] Table 1 Effect of different PC plastic particle feed amounts on their directional conversion into phenol

[0055] According to Table 1, for a cylindrical container with an inner diameter of 2 cm, the selectivity of phenol gradually decreases and the selectivity of bisphenol A gradually increases when the amount of PC plastic particles added is changed. This shows that in the case of insufficient reaction, the CC bond is not easy to break, which reduces the selectivity of phenol and generates more other by-products. Compared with Example 1, Examples 2 and 3 have little change, so the mass of the material selected later is 0.05g.

[0056] Embodiment 6-9 On the basis of Example 1, PC plastic particles were made using plastic parts from different sources (goggles, CDs, washing machine shells, wheel covers), and these discarded plastic parts were cut into PC sheets with a side length of 0.5 cm, and the mass added to the reactor was 0.05 g, forming Examples 6-9; the subsequent experimental conditions and operating steps were the same as those in Example 1, and the test results obtained are shown in Table 2.

[0057] Table 2 Effects of different waste sources of PC plastic particles on their directional conversion into phenol

[0058] According to Table 2, the use of PC plastic particles from different waste sources does not have a significant impact on the distribution of the products, which shows the versatility of this method.

[0059] Examples 10-14 On the basis of Example 1, the heating temperatures of the preheater and the heater were adjusted to 240°C, 260°C, 280°C, 300°C, and 360°C to form Examples 10-14; the subsequent experimental conditions and operating steps were the same as those of Example 1, and the test results obtained are shown in Table 3.

[0060] Table 3 Effect of different reaction temperatures on the directional conversion of PC plastic particles into phenol

[0061] According to Table 3, the selectivity of phenol increases with increasing temperature, while the selectivity of bisphenol A decreases, proving that under high temperature conditions, CC bonds are almost completely broken. The phenol selectivity obtained at a reaction temperature of 360°C corresponding to Example 14 is as high as 95.6%. After subsequent rotary evaporation purification, the purity of phenol crystals can reach 98.1%. Therefore, the subsequent examples are carried out under the same conditions as Example 14 to explore the effect of water flow rate on the depolymerization of PC plastic particles into phenol.

[0062] The gas phase product in Example 14 was subjected to gas chromatography analysis, and the results were as follows: Figure 7 As shown, from Figure 7 It can be seen that at a reaction temperature of 360°C, bisphenol A disappears, and only a small amount of isopropyl phenol is not converted.

[0063] Examples 15-19 Based on Example 14, the liquid inlet of the horizontal flow pump was changed to 0.01, 0.03, 0.1, 0.15, and 0.2 mL min -1 , forming Examples 15-19. The subsequent experimental conditions and operating steps are the same as those of Example 14, and the test results obtained are shown in Table 4.

[0064] Table 4 Effect of liquid flow rate on the directional conversion of PC plastic particles into phenol

[0065] When the pipe and reactor dimensions remain unchanged, the liquid inlet volume increases and the flow rate increases, which will shorten the contact time between the high-temperature steam and the PC plastic particles, and theoretically lead to incomplete reaction. According to Table 4, as the liquid inlet volume increases, the selectivity of phenol decreases, and the selectivity of isopropenylphenol as a by-product increases, proving that when the contact time between the PC plastic particles and the water vapor is short, the CC bond cannot be completely broken, and more isopropenylphenol by-products are generated. In order to extend the contact time, the following will continue to explore the effect of gas flow rate on the depolymerization of PC plastic particles into phenol.

[0066] Examples 20-24 Based on Example 14, the nitrogen inlet flow rate was changed to 0.6, 1.8, 6, 9, and 12 cm min -1 , forming Examples 20-24; the subsequent experimental conditions and operating steps are the same as those of Example 14, and the test results obtained are shown in Table 5 Table 5 Effect of air flow rate on the directional conversion of PC plastic particles into phenol

[0067] The flow rate of nitrogen is converted by flow rate, and the required flow rate is obtained by adjusting the flow rate. When the pipe and reactor size remain unchanged, the nitrogen flow rate increases proportionally when the flow rate is increased. Too fast a nitrogen flow rate will also shorten the contact time between high-temperature steam and PC plastic particles, which will theoretically cause incomplete reaction; according to Table 5, the change in the selectivity of phenol is consistent with the change in the liquid feed rate when the nitrogen flow rate is changed. The selectivity of isopropenylphenol as a by-product increases with the increase in gas flow rate, which proves that the contact time between PC plastic particles and high-temperature steam is short, the CC bond is not completely broken, and a shorter isopropenylphenol by-product is generated.

[0068] Examples 25-29 Based on Example 14, the set pressure of the high-pressure reactor was changed to 0.1, 0.5, 1, 5, and 10 MPa. The subsequent experimental conditions and operating steps were the same as those of Example 14, and the test results obtained are shown in Table 6. Table 6 Effect of reaction pressure on the directional conversion of PC plastic particles into phenol

[0069] According to Table 6, when the pressure of the high-pressure reactor is low, the selectivity of bisphenol A is as high as 45.8%, proving that when the pressure is low, the CO bond is basically broken, but only part of the CC bond is broken, and the phenol selectivity is only 45.1%. When the reactor pressure is increased, the CC bond is broken, the phenol selectivity is improved, and bisphenol A disappears; and when the reactor pressure reaches more than 1MPa, the phenol selectivity is greatly improved. The above results show that the heat and mass transfer of the reaction process is affected when the pressure is low.

[0070] Examples 30-31 On the basis of Example 14, the nitrogen used was changed to other types of gases such as argon and helium. The subsequent experimental conditions and operating steps were the same as those of Example 14, and the test results obtained are shown in Table 7.

[0071] Table 7 Effect of inert gas type on the directional conversion of PC plastic particles into phenol

[0072] According to Table 7, the use of other inert gases (argon, helium) has almost no effect on the conversion of PC plastic pellets. Verification Example 1-3 In order to prove that the invention is effective in breaking the CC bond in bisphenol A, the bisphenol A model compound was used for verification. The bisphenol A model compound was dissolved in a 1:1 methanol and water solution, and the bisphenol A concentration was 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 the heater were changed to 250℃, 270℃, and 300℃. The effect of temperature on the breakage of CC bonds in bisphenol A was verified, and the results are shown in Table 8 Table 8 Effect of reaction temperature on the conversion of model compound bisphenol A to phenol

[0073] The effectiveness of the invention on CC bond cleavage was explored using a bisphenol A model compound. The results in Table 8 showed that increasing the temperature slightly increased the phenol selectivity and decreased the isopropenylphenol selectivity, indicating that the conversion of isopropenylphenol to phenol was slow at low temperatures and the resulting phenol yield was low.

[0074] The above embodiments are only used to illustrate the present invention, but not to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that various combinations, modifications or equivalent substitutions of the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and should be included in the scope of the claims of the present invention.

Claims

1. A method for recovering phenol from waste polycarbonate plastics, characterized in that: The following steps are involved: The waste polycarbonate plastic is fed into a high pressure reactor; High-temperature steam is introduced into the high-pressure reactor from the top, and the waste polycarbonate plastic is hydrothermally decomposed under the action of the high-temperature steam, and the decomposition products are taken away with the hydrothermal steam; condensing the high-temperature steam carrying the decomposition products coming out of the bottom of the high-pressure reactor to obtain a condensate; Phenol is extracted from the condensate using chemical separation technology.

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

3. The method for recovering phenol from waste polycarbonate plastic according to claim 1, characterized in that: The temperature of high temperature steam is 240-360°C.

4. The method for recovering phenol from waste polycarbonate plastic according to claim 1, characterized in that: A certain flow rate of inert gas is mixed into the high-temperature steam to maintain the pressure in the high-pressure reactor at 0.5-10 MPa.

5. The method for recovering phenol from waste polycarbonate plastic according to claim 4, characterized in that: The high-temperature steam is generated by providing a preheater, introducing inert gas and water into the preheater for heating to obtain high-temperature steam of a desired temperature, and continuously introducing the steam into the high-pressure reactor.

6. The method for recovering phenol from waste polycarbonate plastic according to claim 1, characterized in that: A condenser is arranged at the bottom of the high-pressure reactor, and a liquid collecting container is arranged downstream of the condenser. Liquid condensed from the high-temperature steam after the reaction through the condenser is collected in the liquid collecting container.

7. The method for recovering phenol from waste polycarbonate plastic according to claim 1, characterized in that: The high-pressure reactor is provided with a high-pressure feeder for intermittently or continuously feeding waste polycarbonate plastic.

8. The method for recovering phenol from waste polycarbonate plastics according to claim 1, characterized in that: The waste polycarbonate plastic is a crushed granular material with a volume size of 0.5-20cm 3 .

9. 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.

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

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