Supercritical reactor with external circulation for methanol degradation of polyethylene terephthalate and reaction method

Abstract

The application provides a supercritical reaction device with external circulation for methanol degradation of polyethylene terephthalate and a reaction method, which comprises a high-pressure reaction kettle, a circulating pump and a heater. A PET feeding port, a circulating flow outlet, a steam condensate outlet and a steam inlet are arranged on a reaction cylinder body of the high-pressure reaction kettle, and a flow guide cylinder coaxial with the cylindrical cylinder body, a spiral coil and a circulating flow jet inlet are arranged in the reaction cylinder body; the circulating flow outlet at the bottom of the high-pressure reaction kettle is connected with the inlet of the circulating pump, the outlet of the circulating pump is connected with the inlet of the heater, the outlet of the heater is connected with the circulating flow inlet in the high-pressure reaction kettle, meanwhile, 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 methanol degradation of polyethylene terephthalate has the characteristics of simple reactor structure, high production efficiency and good economic benefit.

Description

Technical Field

[0001] This invention belongs to the field of polyethylene terephthalate (PET) degradation technology, specifically relating to a supercritical reactor with external circulation for methanol degradation of polyethylene terephthalate, and also relating to the corresponding reaction method. Background Technology

[0002] With the rapid development of the polyester industry, the amount of waste generated has increased dramatically. Polyethylene terephthalate (PET), a major raw material for polyester, is widely used in beverage bottles, fibers, films, sheets, and electrical insulation materials due to its high strength, good airtightness, and high transparency. Statistics show that approximately 3% to 5% of polyester production and processing results in waste, and a large number of polyester beverage bottles become waste after consumption. Waste PET is generated in large quantities, has low density, occupies a lot of space, and is difficult to decompose naturally. Without proper recycling, it not only causes enormous resource waste but also serious environmental pollution.

[0003] The recycling of waste polyethylene terephthalate (PET) mainly involves two methods: physical recycling and chemical recycling. Physical recycling refers to the reprocessing of waste PET through processes such as cutting, crushing, and melting. While physical recycling saves investment and has low processing costs, the performance of recycled plastics is significantly reduced compared to virgin materials, and they contain a large number of impurities, generally limiting their use to a lower grade and making them unsuitable for food packaging. Chemical recycling depolymerizes solid polymer materials, converting them into smaller molecules, intermediate raw materials, or directly into monomers. These monomers can then be further polymerized to form new polymers. Therefore, chemical recycling is the ideal method for recovering PET.

[0004] Based on the depolymerization agent used, the chemical recycling of waste polyethylene terephthalate (PET) is currently divided into: hydrolysis, methanol hydrolysis, and glycolysis. The main products of PET hydrolysis are terephthalic acid (TPA) and ethylene glycol (EG); the main products of methanol hydrolysis are dimethyl terephthalate (DMT) and ethylene glycol (EG); and the main products of glycolysis are diethyl terephthalate (BHET) and oligomers.

[0005] In recent years, the application of supercritical fluids in waste plastic treatment has received widespread attention. Using supercritical fluids (such as water and methanol) allows for rapid depolymerization, making it easier for polyethylene terephthalate (PET) to degrade into monomers. The process is also relatively simple, and product purification is straightforward. However, supercritical hydrolysis suffers from more demanding reaction conditions, hindering industrialization, and results in low ethylene glycol yields. In contrast, supercritical methanol hydrolysis offers relatively milder conditions. Furthermore, compared to low- and medium-pressure methanol hydrolysis, supercritical methanol hydrolysis significantly shortens reaction time, achieves higher single-pass conversion rates, and exhibits better product selectivity. Therefore, supercritical methanol hydrolysis has become an important method for recycling waste PET.

[0006] Based on supercritical methanol hydrolysis, many beneficial explorations have been carried out in the existing technology. 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 under the conditions of temperature 240-340℃ and pressure 8.0-20.0MPa. However, under the aforementioned high temperature and high pressure conditions, the sealing structure of the stirring device is complex and difficult to manufacture, posing a significant risk of reactant leakage, especially making industrial-scale production difficult. Patent CN112646134B discloses a method for the continuous preparation of spinnable colorless recycled polyester from colored waste polyester textiles. This method involves continuously feeding waste polyester melt and an alcoholysis agent into a supercritical methanol alcoholysis unit for continuous supercritical methanol alcoholysis. The temperature in the supercritical methanol alcoholysis unit is 255-285℃, and the pressure is 8.5-12MPa. However, in the continuous supercritical methanol alcoholysis process, after the waste polyester melts, the melt is transported to the high-pressure supercritical methanol alcoholysis unit via a conveying device, resulting in a complex process and difficulties in industrial transportation. Typically, continuous methanol hydrolysis of polyethylene terephthalate uses atmospheric pressure (see patent US 5654470) and medium-pressure methods (see patent US 5432203), but these methods have low reaction rates and low raw material alcoholysis conversion rates.

[0007] Existing supercritical alcoholysis reactors have the following problems in terms of structure, feeding method, and heat exchange method:

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

[0009] 2. The continuous supercritical methanol alcoholysis method is used to transport the high-temperature molten waste polyester to the high-pressure reactor, which makes the manufacturing of industrial conveying equipment difficult.

[0010] 3. Existing supercritical alcoholysis reactors do not discuss the heating method, and use conventional reactor jacket heating or coil heating methods, which have limited heat exchange area and low heating efficiency. Summary of the Invention

[0011] To address the aforementioned shortcomings, the present invention proposes the following solution:

[0012] (1) A supercritical reactor with external circulation is provided for the degradation of polyethylene terephthalate by methanol. The reactor adopts an intermittent operation mode. Polyethylene terephthalate and methanol are first added to a high-pressure reactor. Then, the reaction stream is heated by a heater connected to the circulating pump and by a steam spiral coil of the high-pressure reactor. The reaction stream is heated by the vaporization and heating of methanol in a closed space to reach the supercritical state of methanol. The degradation of polyethylene terephthalate is completed under a certain reaction time.

[0013] (2) A supercritical reactor with external circulation is provided for methanol degradation of polyethylene terephthalate. A guide tube is set in the high-pressure reactor. The circulating material is mixed by jet, which plays a stirring role for methanol and polyethylene terephthalate, and promotes the uniformity of temperature and concentration of the reaction material in the high-pressure reactor.

[0014] (3) A supercritical reaction device with external circulation is provided for the degradation of polyethylene terephthalate by methanol. The reaction stream is circulated by a circulation pump and heated by a heater to promote the rapid temperature and pressure rise of the reaction stream in the high-pressure reactor and reduce the time to reach the supercritical reaction conditions.

[0015] Accordingly, the specific technical solution of the present invention is as follows:

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

[0017] The high-pressure reactor includes a reaction cylinder, which consists of a cylindrical body, an upper head, and a lower head. The upper head has a PET feed inlet at its top, and the lower head has a circulating fluid outlet and a spiral coil heating steam condensate outlet at its bottom. The cylindrical body has a spiral coil heating steam inlet at its top side, and inside it are a guide tube, a spiral coil, and a circulating fluid inlet, all coaxial with the cylindrical body. The guide tube is an open cylinder with a guide hole at the top and bottom, and its bottom is aligned with the bottom of the cylindrical body. Part of the spiral coil is located inside the guide tube. The circulating fluid inlet is located on the central axis, with its opening facing upwards and aligned with the bottom of the cylindrical body.

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

[0019] Furthermore, in the high-pressure reactor provided by the supercritical reactor with external circulation for methanol degradation of polyethylene terephthalate according to the present invention, the ratio of the height H of the cylindrical body to its diameter D 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 vessel 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.

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

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

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

[0023] Furthermore, in the supercritical reactor with external circulation for the methanol degradation of polyethylene terephthalate of the present invention, a canned pump or a magnetic pump is used as the circulation pump to prevent leakage of the reaction stream inside the circulation pump.

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

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

[0026] (1) Add a certain amount of PET fragments from the top of the upper head of the high-pressure reactor, add a certain amount of methanol from the methanol inlet, and then close the inlet;

[0027] (2) Start the circulating pump and use saturated steam to heat the reaction stream through the heater. The heated reaction stream is injected from the circulating stream inlet (fluid velocity is 10-20 m / s) into the high-pressure reactor. The reaction mixture of methanol and PET is mixed by jet. The reaction stream flows upward in the guide tube and then flows downward through the guide tube hole or the top of the guide tube from the annular gap between the cylindrical body and the guide tube.

[0028] (3) The reaction stream is heated by saturated steam through a spiral coil. After the reaction stream is heated to supercritical conditions (temperature 250-300℃, pressure 8-12MPa), 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 stream is heated by saturated steam through a heater when it is below 200°C, and by saturated steam through a spiral coil when it is above 200°C.

[0030] Preferably, within the lower head, a portion of the reactant flows into the guide tube and mixes with the injected reactant, while the other portion enters the external circulation system through the circulating material outlet, and returns to the high-pressure reactor after reheating.

[0031] The role and effect of invention

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

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

[0034] (2) The present invention adopts the external circulation method of reaction material, increases the external heater, improves the heat exchange capacity and improves the production efficiency; at the same time, it adopts low temperature saturated steam heating below 200℃ and high temperature saturated steam heating above 200℃, making full use of the temperature of the heat exchange medium, greatly reducing production energy consumption and improving production efficiency.

[0035] (3) The present invention uses a supercritical reaction device with external circulation, which eliminates the need to preheat and melt the raw material polyethylene terephthalate, reducing complex pretreatment equipment and simplifying the production process.

[0036] In summary, the supercritical reactor 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 can also be applied to the supercritical degradation process of polyethylene terephthalate with ethylene glycol. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the supercritical reactor with external circulation used in the methanol degradation of polyethylene terephthalate according to the present invention.

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

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

[0040] The present invention will now be described in detail with reference to embodiments and accompanying drawings. However, the following embodiments should not be construed as limiting the scope of the present invention.

[0041] Example 1

[0042] according to Figure 1 The supercritical reactor 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 consists 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 material outlet 11 and a heating steam condensate outlet 9 of a spiral coil 7 at the bottom.

[0044] The upper side of the cylindrical body 4 is provided with a heating steam inlet 3 for a spiral coil 7. Inside, there is a guide tube 6, a spiral coil 7, and a circulating material inlet 8, all coaxial with the cylindrical body 4. The guide tube 6 is a cylinder with openings at the top and bottom. The lower end of the guide tube 6 is aligned with the lower end of the cylindrical body 4. The middle and lower ends of the spiral coil 7 are located inside the guide tube 6, and its diameter is smaller than that of the guide tube 6. The circulating material inlet 8 is located on the central axis, with its opening facing upwards, and its opening position is aligned with the lower ends of the cylindrical body 4 and the guide tube 6.

[0045] In terms of dimensions, the ratio of the height H to the diameter D of the cylindrical body 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 body 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] Furthermore, the flow deflector restricts the circulation path of the reactant stream, reducing the chance of fluid short-circuiting, especially in low-level conditions. Combined with... Figure 2 A circular hole with a diameter of 6-10 mm and an opening rate of 20%-40% is opened in the upper 1 / 3 of the guide tube to ensure that the fluid can flow into the annular gap between the cylindrical body and the guide tube when the liquid level is low.

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

[0048] In this embodiment, to prevent cavitation at the inlet of the circulating pump, the height difference H' between the outlet of the circulating reaction stream and the circulating pump is 1–3 m. To prevent leakage of the reaction stream within the circulating pump, a canned motor pump or a magnetic pump is used. To meet the heat exchange requirements of the reaction stream under high temperature and high pressure conditions, a U-tube heat exchanger is used as the heater.

[0049] The supercritical methanol hydrolysis reaction of polyethylene terephthalate (PET) using the above-described apparatus proceeds as follows: A certain amount of PET fragments are added to the high-pressure reactor 5 through the PET inlet 1 at the top of the upper head 2, and a certain amount of methanol is added to the high-pressure reactor 5 through the methanol inlet 13, after which the inlet is closed. The circulation pump 17 is started, and saturated steam is used to heat the methanol through the heater 14. The heated methanol is then injected into the high-pressure reactor 5 through the circulation outlet 11, where the methanol and PET are mixed using a jet. The reaction stream flows upward within the guide tube 6, and then downward through the annular gap between the cylindrical body 4 and the guide tube 6. Saturated steam is used to heat the reaction stream through the spiral coil 7. Within the lower head 10, a portion of the reaction stream flows into the guide tube 6 and mixes with the reaction stream entering through the circulation inlet 8, while the other portion enters the external circulation system through the circulation outlet 11, is reheated, and returns to the high-pressure reactor 5. The reaction stream is heated to supercritical conditions, reacts for a certain period, and then discharged through the reaction product outlet 16 for subsequent separation operations.

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

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

[0052] Specific application examples

[0053] The reaction volume is 1m 3 A supercritical reactor with external circulation for the methanol degradation of polyethylene terephthalate, employing methods such as... Figure 1 The structural form shown,

[0054] In the high-pressure reactor 5, the diameter D of the cylindrical body 4 is 800 mm, and the height H of the cylindrical body 4 is 1500 mm, with a height H to diameter D ratio of 1.88. The diameter d2 of the guide tube 6 is 480 mm, and the height h of the guide tube 6 is 900 mm, with a height h to height H of the cylindrical body 4 ratio of 0.6. The diameter d2 of the guide tube 6 to diameter D of the cylindrical body 4 ratio is also 0.6. The diameter d1 of the spiral coil 7 is 420 mm, with a diameter d1 to diameter d2 of the guide tube 6 ratio of 0.88. A circular hole with a diameter of 8 mm is opened in the upper 1 / 3 of the guide tube 6, resulting in an opening rate of 30%.

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

[0056] In the supercritical reactor with external circulation used for the methanol degradation of polyethylene terephthalate (PET) of this invention, PET is added at a rate of 32 kg, and methanol at a rate of 240 kg. The reaction stream is heated below 200°C using saturated steam at 210°C via heater 14, and above 200°C using saturated live steam at 320°C via spiral coil 7. The supercritical methanol degradation reaction is carried out at a temperature of 300°C, a pressure of 11 MPa, and a reaction time of 90 minutes. The conversion rate of PET is 100%, the yield of dimethyl terephthalate is 93.5%, and the yield of ethylene glycol is 93.0%.

[0057] In summary, the supercritical reactor with external circulation used in this invention for the methanol degradation of polyethylene terephthalate has the following advantages:

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

[0059] (2) The present invention adopts the external circulation method of the reaction material, adds an external heater, improves the heat exchange capacity and increases production efficiency; at the same time, it makes full use of 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 eliminates the need to preheat and melt the raw material polyethylene terephthalate, reducing complex pretreatment equipment and simplifying the production process.

[0061] (4) The supercritical reactor 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 can also be applied to the supercritical degradation process of polyethylene terephthalate by ethylene glycol.

[0062] The foregoing has shown and described 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 embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. An apparatus for the degradation of polyethylene terephthalate with methanol in a supercritical reaction with external circulation, characterized in that It comprises a high-pressure reactor, a circulating pump and a heater outside the reactor, The high-pressure reactor has a reaction cylinder composed of a cylindrical cylinder, an upper head and a lower head, the upper head is provided with a PET feeding port at the top, the lower head is provided with a circulating stream outlet and a spiral coil heating steam condensate outlet at the bottom, the top end of the side surface of the cylindrical cylinder is provided with a spiral coil heating steam inlet, the inside is provided with a flow guide cylinder coaxial with the cylindrical cylinder, a spiral coil and a circulating stream inlet, the flow guide cylinder is an open-top and bottom cylinder, the upper end is provided with a flow guide hole, the bottom end is aligned with the bottom end of the cylindrical cylinder, part of the spiral coil is inside the flow guide cylinder, the circulating stream inlet is on the central axis, the opening is upward, and the opening position is aligned with the lower end of the cylindrical cylinder; The circulating pump and the heater constitute an external circulation, the inlet of the circulating pump is connected with the circulating stream outlet, the outlet is connected with the inlet of the heater, and the outlet of the heater is connected with the circulating stream inlet; meanwhile, the inlet of the circulating 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 degrading polyethylene terephthalate by methanol according to claim 1, characterized in that: wherein In the high-pressure reactor, the ratio of the height H of the cylindrical cylinder to the diameter D is 1-2; The ratio of the height h of the flow guide cylinder to the height H of the cylindrical cylinder is 0.3-0.7; The ratio of the diameter d2 of the flow guide cylinder to the diameter D of the reaction cylinder is 0.5-0.

7.

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

9.

4. The supercritical reaction device with external circulation for degrading polyethylene terephthalate by methanol according to claim 1, characterized in that: wherein, The flow guide holes are circular holes with a diameter of 6-10 mm and an opening rate of 20%-40% in the upper 1 / 3 of the flow guide cylinder.

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

6. The supercritical reaction device with external circulation for degrading polyethylene terephthalate by methanol according to claim 1, characterized in that: wherein The height difference H' of the circulating reaction stream outlet from the circulating pump is 1-3 m; the circulating pump is a shield pump or a magnetic drive pump.

7. A process for the supercritical methanolysis of polyethylene terephthalate, carried out using the apparatus according to any one of claims 1 to 6, characterised in that, It comprises the following steps: (1) A certain amount of PET fragments is added from the top of the upper head of the high-pressure reactor, and a certain amount of methanol is added from the methanol inlet, and then the inlet is closed; (2) Start the circulating pump, heat the reaction stream by using saturated steam through the heater, and then inject the heated reaction stream into the high-pressure reactor from the circulating stream inlet, so as to mix the reaction stream methanol and PET by using jet mixing, and then make the reaction stream flow upward in the flow guide cylinder, and then flow downward from the annular gap between the cylindrical cylinder and the flow guide cylinder through the flow guide holes of the flow guide cylinder or the top of the flow guide cylinder. (3) The reaction stream is heated by saturated steam through spiral coil, and after the reaction stream is heated to supercritical condition, it is discharged from the reaction product outlet after a certain reaction time, and then enters the subsequent separation operation.

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

9. The polyethylene terephthalate supercritical methanolysis reaction method according to claim 7, characterized in that: wherein The reaction stream is heated by saturated steam through a heater below 200 DEG C, and is heated by saturated steam through a spiral coil above 200 DEG C.

10. The polyethylene terephthalate supercritical methanolysis reaction method according to claim 7, characterized in that: wherein, In the lower head, part of the reaction stream flows into the guide tube and mixes with the injected reaction stream, and the other part enters the outer circulating system through the circulating stream outlet, and then returns to the high-pressure reaction kettle after being heated again.

Images