Supercritical reaction device with external circulation for degrading polyethylene glycol terephthalate by methanol and reaction method

Abstract

The invention provides a supercritical reaction device with external circulation for degrading polyethylene glycol terephthalate by methanol and a reaction method. The supercritical reaction device comprises a high-pressure reaction kettle, a circulating pump and a heater. A PET (Polyethylene Terephthalate) feeding hole, a circulating material flow outlet, a steam condensate outlet and a steam inlet are formed in a reaction cylinder of the high-pressure reaction kettle; a guide cylinder, a spiral coil pipe and a circulating material flow jet flow inlet which are coaxial with the cylindrical cylinder are arranged in the reaction cylinder; an inlet of the circulating pump is connected with a circulating material outlet at the bottom of the high-pressure reaction kettle, an outlet of the circulating pump is connected with an inlet of the heater, an outlet of the heater is connected with a circulating material inlet in the high-pressure reaction kettle, the inlet of the circulating pump is provided with a methanol inlet, and the outlet of the circulating pump is provided with a reaction product outlet. The supercritical reaction device with external circulation for degrading polyethylene glycol terephthalate by methanol has the characteristics of simple reactor structure, high production efficiency and good economic benefit.

Description

Technical Field

[0001] The invention belongs to the technical field of polyethylene terephthalate (PET) degradation, and in particular relates to a supercritical reaction device with external circulation for degrading polyethylene terephthalate with methanol, and also relates to a corresponding reaction method. Background Art

[0002] With the rapid development of the polyester industry, the amount of waste has increased dramatically. Polyethylene terephthalate, the main raw material of polyester, is widely used in beverage bottles, fibers, films, sheet bases and electrical insulation materials due to its advantages of high strength, good air tightness and high transparency. According to statistics, about 3% to 5% of polyester will become waste during the production and processing of polyester, and a large number of polyester beverage bottles will become waste after drinking. Waste polyethylene terephthalate has a large discharge volume, low density, large space occupation, and difficulty in natural decomposition. If it is not reasonably recycled, it will not only cause a huge waste of resources, but also cause serious environmental pollution.

[0003] There are two main methods for recycling waste polyethylene terephthalate: physical recycling and chemical recycling. Physical recycling refers to the recycling technology of reprocessing waste polyethylene terephthalate through processes such as cutting, crushing, heating and melting. Although physical recycling technology saves investment and has low processing costs, the performance of recycled plastics is greatly reduced compared with new materials, and contains a large amount of impurities. Generally, it can only be downgraded and is not suitable for use as food packaging materials. Chemical recycling is the depolymerization of solid polymer materials, converting them into smaller molecules, intermediate raw materials, or directly converting them into monomers, which can also be further polymerized to form new polymers. Therefore, chemical recycling is an ideal method for recycling polyethylene terephthalate.

[0004] According to the depolymerizing agent used, the chemical recovery of waste polyethylene terephthalate is currently divided into: hydrolysis, methanolysis, glycolysis, etc. The main products of polyethylene terephthalate hydrolysis are terephthalic acid (TPA) and ethylene glycol (EG), the main products of methanolysis are dimethyl terephthalate (DMT) and ethylene glycol (EG), and the main products of glycolysis are dihydroxyethyl terephthalate (BHET) and oligomers.

[0005] In recent years, the application of supercritical fluids in the treatment of waste plastics has received extensive attention. After using supercritical fluids (such as water, methanol), the depolymerization process is carried out rapidly, and polyethylene terephthalate is more easily degraded into monomers, and the operation is relatively easy, and the product purification is simple. The disadvantage of the supercritical hydrolysis method is that the reaction conditions are more harsh and are not conducive to industrialization, and the ethylene glycol yield is low. In contrast, the conditions of supercritical methanolysis are relatively mild. In addition, compared with low and medium pressure methanolysis, the supercritical methanolysis reaction time is significantly shortened, the single-pass conversion rate is high, and the product selectivity is good. Therefore, supercritical methanolysis becomes an important method for recycling waste polyethylene terephthalate.

[0006] Based on supercritical methanolysis, many useful explorations have been carried out in the prior art. For example, patent CN1304924A discloses a method for depolymerizing polyethylene terephthalate waste, in which waste polyester raw materials are added to a high-pressure reactor with a stirring device and degraded at a temperature of 240-340°C and a pressure of 8.0-20.0MPa. However, under the above-mentioned high temperature and high pressure conditions, the sealing structure of the stirring device is complex, the manufacturing is difficult, and the reactants have a large leakage risk, especially the industrial scale device is difficult to achieve; Patent CN112646134B discloses a method for continuously preparing a spinnable colorless recycled polyester from a colored waste polyester textile, wherein the waste polyester melt and the alcoholysis agent are continuously introduced into a supercritical methanol alcoholysis device for continuous supercritical methanol alcoholysis, the temperature in the supercritical methanol alcoholysis device is 255-285°C, and the pressure is 8.5-12MPa, but in the continuous supercritical methanol alcoholysis process, after the waste polyester is melted, the melt is transported to the high-pressure supercritical methanol alcoholysis device by a conveying device, and there are problems such as complex process and industrial transportation difficulties. Usually, the continuous methanolysis of polyethylene terephthalate adopts the normal pressure method (see patent US 5654470) and the medium pressure method (see patent US 5432203), but the reaction rate is low and the raw material alcoholysis conversion rate is small.

[0007] The existing supercritical alcoholysis reactor has the following problems in terms of structure, feeding method and heat exchange method:

[0008] 1. A high-pressure reactor with a stirring device is used to perform degradation under a pressure of 8.0-20.0 MPa. The sealing structure of the stirring device is complex and difficult to manufacture, and there is a safety hazard of reactant leakage;

[0009] 2. Continuous supercritical methanol alcoholysis is used to transport high-temperature molten waste polyester to the high-pressure reactor, and industrial transportation equipment is difficult to manufacture;

[0010] 3. The existing supercritical alcoholysis reactors do not discuss the heating method. The conventional reactor jacket heating or coil heating method is used, which has a limited heat exchange area and low heating efficiency. Summary of the invention

[0011] The present invention aims at solving the above-mentioned defects and proposes the following solutions:

[0012] (1) A supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol is provided, wherein the polyethylene terephthalate and methanol are first added to a high-pressure reactor, and then a circulating pump is used to heat the reaction stream through a heater connected to the circulating pump, and the reaction stream is heated through a steam spiral coil of the high-pressure reactor, and the supercritical state of the methanol is achieved through the gasification and temperature increase of the methanol in a closed space, and the degradation of the polyethylene terephthalate is completed within a certain reaction time;

[0013] (2) A supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol is provided, wherein a guide tube is arranged in a high-pressure reactor, and the jet mixing of the circulating flow is used to stir the methanol and the polyethylene terephthalate, thereby promoting the uniformity of the temperature and concentration of the reactant flow in the high-pressure reactor;

[0014] (3) A supercritical reaction device with external circulation for methanol degradation of polyethylene terephthalate is provided, wherein the circulation of the reactant flow is realized by a circulation pump, and the reactant flow is heated by a heater to promote rapid temperature and pressure increase of the reactant flow in the high-pressure reactor, thereby reducing the time to reach supercritical reaction conditions.

[0015] Accordingly, the specific technical solutions of the present invention are as follows:

[0016] In a first aspect, the present invention provides a supercritical reaction device with external circulation for degrading polyethylene terephthalate with methanol, comprising a high-pressure reactor, a circulation pump and a heater.

[0017] The high-pressure reactor has a reaction cylinder, which is composed of a cylindrical cylinder, an upper head and a lower head. A PET feed port is provided at the top of the upper head, and a circulating logistics outlet and a spiral coil heating steam condensate outlet are provided at the bottom of the lower head; a spiral coil heating steam inlet is provided at the top end of the side of the cylindrical cylinder, and a guide cylinder, a spiral coil and a circulating logistics inlet are provided inside the cylinder coaxial with the cylindrical cylinder; the guide cylinder is a cylinder with upper and lower openings, a guide hole is provided at the upper end, and the bottom end is aligned with the bottom end of the cylindrical cylinder, part of the spiral coil is located inside the guide cylinder, and the circulating logistics inlet is located on the central axis, with the opening facing upward, and the opening position is aligned with the lower end of the cylindrical cylinder.

[0018] The circulation pump and the heater form an external circulation; the inlet of the circulation pump is connected to the circulation flow outlet, the outlet is connected to the heater inlet, and the heater outlet is connected to the circulation flow inlet; at the same time, the inlet of the circulation pump is also provided with a methanol inlet, and the outlet is provided with a reaction product outlet.

[0019] Furthermore, in the high-pressure reactor provided by the supercritical reaction device with external circulation for methanol degradation of polyethylene terephthalate of the present invention, the ratio of the height H of the cylindrical barrel to the diameter D is 1-2, the ratio of the height h of the guide tube to the height H of the cylindrical barrel is 0.3-0.7, the ratio of the diameter d2 of the guide tube to the diameter D of the reaction barrel is 0.5-0.7, and the ratio of the diameter d1 of the spiral coil to the diameter d2 of the guide tube is 0.8-0.9.

[0020] Furthermore, in the high-pressure reactor of the supercritical reaction device with external circulation for methanol degradation of polyethylene terephthalate of the present invention, the guide tube limits the circulation path of the reactant flow, reducing the chance of fluid short circuit. At the same time, in view of the low liquid level, a circular hole is opened in the upper 1 / 3 of the guide tube as a guide hole with a diameter of 6-10 mm and an opening ratio of 20%-40%, thereby ensuring that the fluid can flow into the annular gap between the cylindrical barrel and the guide tube through the upper opening of the guide tube when the liquid level is low.

[0021] Furthermore, in the high-pressure reactor of the supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol of the present invention, in order to achieve a better jet mixing effect, the fluid velocity at the inlet of the circulating flow is 10-20 m / s.

[0022] Furthermore, in the supercritical reaction device with external circulation for methanol degradation of polyethylene terephthalate of the present invention, in order to prevent cavitation at the inlet of the circulation pump, the height difference H' between the outlet of the circulation reaction stream and the circulation pump is 1-3 m.

[0023] Furthermore, in the supercritical reaction device with external circulation for degrading polyethylene terephthalate with methanol of the present invention, in order to prevent leakage of the reactant flow in the circulation pump, the circulation pump adopts a shielded pump or a magnetic pump.

[0024] Furthermore, in the supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol of the present invention, in order to meet the heat exchange of the reactant flow under high temperature and high pressure conditions, the heater adopts a U-tube heat exchanger type.

[0025] In a second aspect of the present invention, a method for supercritical methanolysis of polyethylene terephthalate is provided, comprising the following steps:

[0026] (1) Add a certain amount of PET chips from the top of the autoclave head, add a certain amount of methanol from the methanol inlet, and then close the inlet;

[0027] (2) starting the circulation pump, using saturated steam to heat the reactant stream through the heater, the heated reactant stream is ejected from the circulation stream inlet (fluid velocity is 10-20 m / s) into the high-pressure reactor, and the reactants methanol and PET are mixed by the jet, the reactant stream flows upward in the guide tube, and then flows downward through the guide hole of the guide tube or the top of the guide tube from the annular gap between the cylindrical barrel and the guide tube;

[0028] (3) The reaction flow is heated by saturated steam through a spiral coil. After the reaction flow is heated to supercritical conditions (temperature of 250-300°C and pressure of 8-12 MPa), it is discharged from the reaction product outlet after a certain reaction time (30-100 minutes) and enters the subsequent separation operation.

[0029] Preferably, in the above reaction method, in order to achieve better energy utilization, the reaction flow is heated by saturated water vapor through a heater when the temperature is below 200°C, and the reaction flow is heated by saturated steam through a spiral coil when the temperature is above 200°C.

[0030] Preferably, in the lower head, a part of the reactant flow flows into the flow guide tube and mixes with the injected reactant flow, and the other part enters the external circulation system through the circulating flow outlet and returns to the high-pressure reactor after being heated again.

[0031] Functions and Effects of the Invention

[0032] Compared with the existing reactor structure and reaction mode for supercritical methanol degradation of polyethylene terephthalate, the present invention has the following technical advantages:

[0033] (1) The present invention adopts the circulation of reactant flow and utilizes the mixing effect of jet entrainment to promote the uniformity of temperature and concentration of reactant flow, eliminates the mechanical stirring equipment of the existing high-pressure reactor under supercritical conditions, reduces the difficulty of equipment manufacturing, and eliminates the potential safety hazard of reactant flow leakage;

[0034] (2) The present invention adopts an external circulation method for reactant flow, adds an external heater, improves heat exchange capacity, and improves production efficiency; at the same time, saturated water vapor at a low temperature below 200°C is used for heating, and saturated steam at a high temperature above 200°C is used for heating, which fully utilizes the temperature of the heat exchange medium, greatly reduces production energy consumption, and improves production efficiency;

[0035] (3) The present invention uses a supercritical reaction device with external circulation, which does not require the raw material polyethylene terephthalate to be heated and melted in advance, reduces complex pretreatment equipment, and simplifies the production process.

[0036] In summary, the supercritical reaction device with external circulation for methanol degradation of polyethylene terephthalate of the present invention has the characteristics of simple reactor structure, high production efficiency and good economic benefits, and is also applicable to the ethylene glycol supercritical degradation process of polyethylene terephthalate. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 The present invention is a schematic structural diagram of a supercritical reaction device with external circulation for degrading polyethylene terephthalate with methanol.

[0038] Figure 2 for Figure 1 Schematic diagram of the opening of the middle guide tube.

[0039] In the figure: 1-PET feed port, 2-upper head, 3-steam inlet, 4-cylindrical barrel, 5-high-pressure reactor, 6-guide tube, 7-spiral coil, 8-circulating logistics inlet, 9-condensate outlet, 10-lower head, 11-circulating logistics outlet, 12-stop valve, 13-methanol inlet, 14-heater, 15-bypass valve, 16-reaction product outlet, 17-circulating pump. DETAILED DESCRIPTION

[0040] The present invention is described in detail below in conjunction with the embodiments and the accompanying drawings. However, the following embodiments should not be considered as limiting the scope of the present invention.

[0041] Example 1

[0042] according to Figure 1 The supercritical reaction device with external circulation for methanol degradation of polyethylene terephthalate provided in this embodiment includes a high-pressure reactor 5, a heater 14 installed outside the reactor, and a circulation pump 17.

[0043] The high-pressure reactor 5 has a reaction cylinder, which is composed of a cylindrical cylinder 4, an upper head 2, and a lower head 10. The upper head 2 of the reaction cylinder is provided with a PET feed port 1 at the top, and the lower head 10 is provided with a circulating flow outlet 11 and a heating steam condensate outlet 9 of the spiral coil 7 at the bottom.

[0044] The upper end of the side of the cylindrical body 4 is provided with a heating steam inlet 3 of a spiral coil 7, and the inside is provided with a draft tube 6 coaxial with the cylindrical body 4, a spiral coil 7 and a circulating flow inlet 8. The draft tube 6 is a cylinder with upper and lower openings, the lower end of the draft tube 6 is aligned with the lower end of the cylindrical body 4, the middle and lower end of the spiral coil 7 are located inside the draft tube 6, and its diameter is smaller than the diameter of the draft tube 6, and the circulating flow inlet 8 is located on the central axis, with the opening facing upward, and the opening position is aligned with the lower ends of the cylindrical body 4 and the draft tube 6.

[0045] In terms of size specifications, the ratio of the height H of the cylindrical barrel to the diameter D is 1 to 2, the ratio of the height h of the guide tube to the height H of the cylindrical barrel is 0.3 to 0.7, the ratio of the diameter d2 of the guide tube to the diameter D of the reaction barrel is 0.5 to 0.7, and the ratio of the diameter d1 of the spiral coil to the diameter d2 of the guide tube is 0.8 to 0.9.

[0046] In addition, the draft tube limits the circulation path of the reactant flow, reducing the chance of fluid short-circuiting and preventing low liquid level conditions. Figure 2 A circular hole is opened in the upper 1 / 3 of the guide tube as a guide hole with a diameter of 6-10mm and an opening rate of 20%-40%, to ensure that the fluid can flow into the annular gap between the cylindrical cylinder and the guide tube through the upper opening of the guide tube when the liquid level is low.

[0047] The inlet of the circulation pump 17 is connected to the circulation flow outlet 11 at the bottom of the high-pressure reactor 5, and the outlet of the circulation pump 17 is connected to the inlet of the heater 14. The outlet of the heater 14 is connected to the circulation flow inlet 8 inside the high-pressure reactor 4. At the same time, the inlet of the circulation pump 17 is provided with a methanol inlet 13, the outlet of the circulation pump 17 is provided with a reaction product outlet 16, the heater 14 is provided with a bypass pipeline, the inlet and outlet pipelines of the heater 14 are provided with a stop valve 12, and the bypass pipeline is provided with a bypass valve 15.

[0048] In this embodiment, in order to prevent cavitation at the inlet of the circulation pump, the height difference H' between the outlet of the circulating reactant flow and the circulation pump is 1 to 3 m. In order to prevent leakage of the reactant flow in the circulation pump, the circulation pump adopts a shielded pump or a magnetic pump. In order to meet the heat exchange of the reactant flow under high temperature and high pressure conditions, the heater adopts a U-tube heat exchanger.

[0049] The above-mentioned device is used to carry out the supercritical methanolysis reaction of polyethylene terephthalate, and the steps are as follows: a certain amount of PET fragments are added from the PET feed port 1 at the top of the upper head 2 of the high-pressure reactor 5, a certain amount of methanol is added to the high-pressure reactor 5 from the methanol inlet 13, and then the inlet is closed. The circulation pump 17 is started, and the methanol is heated by saturated steam through the heater 14. The heated methanol is sprayed into the high-pressure reactor 5 from the circulation flow outlet 11, and the methanol and PET are mixed by the jet. The reaction flow flows upward in the guide tube 6, and then flows downward from the annular gap between the cylindrical barrel 4 and the guide tube 6. The reaction flow is heated by saturated steam through the spiral coil 7. In the lower head 10, a part of the reaction flow flows into the guide tube 6 and mixes with the reaction flow coming from the circulation flow inlet 8, and the other part enters the external circulation system through the circulation flow outlet 11, and returns to the high-pressure reactor 5 after being heated again. The reaction flow is heated to supercritical conditions, and after a certain reaction time, it is discharged through the reaction product outlet 16 and enters the subsequent separation operation.

[0050] Specifically, the fluid velocity of the circulating stream inlet 8 in the high-pressure reactor 5 is 10-20 m / s. The conditions of the supercritical methanol degradation reaction are as follows: temperature 250-300°C, pressure 8-12 MPa, reaction time 30-100 minutes

[0051] Furthermore, the reaction flow is heated by saturated steam through the heater 14 when the temperature is below 200° C., and by saturated steam through the spiral coil 7 when the temperature is above 200° C.

[0052] Specific application examples

[0053] The reaction volume is 1m 3 A supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol is used. Figure 1 The structural form shown,

[0054] In the autoclave 5, the diameter D of the cylindrical body 4 is 800 mm, the height H of the cylindrical body 4 is 1500 mm, and the ratio of the height H to the diameter D is 1.88; the diameter d2 of the draft tube 6 is 480 mm, the height h of the draft tube 6 is 900 mm, the ratio of the height h of the draft tube 6 to the height H of the cylindrical body 4 is 0.6, the ratio of the diameter d2 of the draft tube 6 to the diameter D of the cylindrical body 4 is 0.6, the diameter d1 of the spiral coil 7 is 420 mm, and the ratio of the diameter d1 of the spiral coil 7 to the diameter d2 of the draft tube 6 is 0.88. A circular hole is opened in the upper 1 / 3 of the draft tube 6, the diameter is 8 mm, and the opening rate is 30%.

[0055] The fluid velocity of the circulating stream inlet 8 in the high-pressure reactor 5 is 15 m / s; the height difference H' between the circulating reaction stream outlet 11 and the circulating pump 17 is 1 m; the circulating pump 17 adopts a shielded pump; and the heater 14 adopts a U-tube heat exchanger.

[0056] In the supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol of the present invention, the amount of polyethylene terephthalate added is 32 kg, the amount of methanol added is 240 kg, the reaction flow is heated by saturated water vapor of 210 ° C through a heater 14 when it is below 200 ° C, and the reaction flow is heated by saturated steam of 320 ° C through a spiral coil 7 when it is above 200 ° C. The temperature of the supercritical methanol degradation reaction is 300 ° C, the pressure is 11 MPa, the reaction time is 90 minutes, the conversion rate of polyethylene terephthalate is 100%, the mass yield of dimethyl terephthalate is 93.5%, and the yield of ethylene glycol is 93.0%.

[0057] In summary, the supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol according to the present invention has the following advantages:

[0058] (1) The present invention adopts the circulation of reactant flow and utilizes the mixing effect of jet entrainment to promote the uniformity of reactant flow temperature and concentration, eliminates the mechanical stirring equipment of the high-pressure reactor under supercritical conditions, reduces the difficulty of equipment manufacturing, and eliminates the potential safety hazard of reactant flow leakage;

[0059] (2) The present invention adopts an external circulation method for reactant flow, adds an external heater, improves heat exchange capacity, and improves production efficiency; at the same time, it fully utilizes the temperature of the heat exchange medium, greatly reduces production energy consumption, and improves production efficiency;

[0060] (3) The present invention uses a supercritical reaction device with external circulation, which does not require the raw material polyethylene terephthalate to be heated and melted in advance, reduces complex pretreatment equipment, and simplifies the production process.

[0061] (4) The supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol of the present invention has the characteristics of simple reactor structure, high production efficiency and good economic benefits, and is also applicable to the ethylene glycol supercritical degradation process of polyethylene terephthalate.

[0062] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. The present invention may be subject to various changes and improvements without departing from the spirit and scope of the present invention. These changes and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol, characterized in that: It includes a high-pressure reactor, and a circulating pump and a heater arranged outside the reactor. The high-pressure reactor has a reaction cylinder, which is composed of a cylindrical cylinder, an upper head and a lower head. A PET feed port is provided at the top of the upper head, and a circulating logistics outlet and a spiral coil heating steam condensate outlet are provided at the bottom of the lower head; a spiral coil heating steam inlet is provided at the top end of the side of the cylindrical cylinder, and a guide cylinder, a spiral coil and a circulating logistics inlet are provided coaxially with the cylindrical cylinder inside. The guide cylinder is a cylinder with upper and lower openings, a guide hole is provided at the upper end, and the bottom end is aligned with the bottom end of the cylindrical cylinder, part of the spiral coil is located inside the guide cylinder, and the circulating logistics inlet is located on the central axis, with the opening facing upward, and the opening position is aligned with the lower end of the cylindrical cylinder; The circulation pump and the heater form an external circulation; the inlet of the circulation pump is connected to the circulation flow outlet, the outlet is connected to the heater inlet, and the heater outlet is connected to the circulation flow inlet; at the same time, the inlet of the circulation pump is also provided with a methanol inlet, and the outlet is provided with a reaction product outlet.

2. The supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol according to claim 1, characterized in that: in, In the high-pressure reactor, the ratio of the height H to the diameter D of the cylindrical shell is 1 to 2; The ratio of the height h of the guide tube to the height H of the cylindrical body is 0.3 to 0.7; The ratio of the diameter d2 of the guide tube to the diameter D of the reaction tube is 0.5 to 0.

7.

3. The supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol according to claim 1, characterized in that: in, The ratio of the diameter d1 of the spiral coil to the diameter d2 of the guide tube is 0.8 to 0.

9.

4. The supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol according to claim 1, characterized in that: in, A circular hole is opened in the range of 1 / 3 of the upper part of the guide tube as a guide hole, with a diameter of 6-10mm and an opening rate of 20%-40%.

5. The supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol according to claim 1, characterized in that: in, The heater is provided with a bypass pipeline, the inlet and outlet pipelines of the heater are provided with a stop valve, and the bypass pipeline is provided with a bypass valve.

6. The supercritical reaction device with external circulation for the degradation of polyethylene terephthalate by methanol according to claim 1, characterized in that: in, The height difference H' between the outlet of the circulating reaction flow and the circulating pump is 1 to 3 m; the circulating pump adopts a shielded pump or a magnetic pump.

7. A method for supercritical methanolysis of polyethylene terephthalate, using the device according to any one of claims 1 to 6, characterized in that: The steps include: (1) Add a certain amount of PET chips from the top of the autoclave head, add a certain amount of methanol from the methanol inlet, and then close the inlet; (2) starting the circulation pump, using saturated steam to heat the reactant stream through the heater, the heated reactant stream is ejected from the circulation stream inlet into the high-pressure reactor, the reactants methanol and PET are mixed by the jet, the reactant stream flows upward in the guide tube, and then flows downward from the annular gap between the cylindrical barrel and the guide tube through the guide hole of the guide tube or the top of the guide tube; (3) The reaction flow is heated by saturated steam through a spiral coil. After the reaction flow is heated to supercritical conditions and a certain reaction time has passed, it is discharged from the reaction product outlet and enters the subsequent separation operation.

8. The method for supercritical methanolysis of polyethylene terephthalate according to claim 7, characterized in that: in, The fluid velocity at the circulating flow inlet is 10-20 m / s. The temperature of the supercritical methanol degradation reaction is 250-300° C., the pressure is 8-12 MPa, and the reaction time is 30-100 minutes.

9. The method for supercritical methanolysis of polyethylene terephthalate according to claim 7, characterized in that: in, The reaction flow is heated by saturated steam through a heater when the temperature is below 200°C, and by saturated steam through a spiral coil when the reaction flow is above 200°C.

10. The method for supercritical methanolysis of polyethylene terephthalate according to claim 7, characterized in that: in, In the lower head, a part of the reactant flow flows into the flow guide tube and mixes with the injected reactant flow, and the other part enters the external circulation system through the circulating flow outlet and returns to the high-pressure reactor after being heated again.

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