Depolymerization method and depolymerization device

Abstract

The present invention provides a depolymerization method for depolymerizing a synthetic resin into a starting material monomer, comprising: a step for setting a target temperature inside a flow reactor (13e) and a target residence time of the synthetic resin and a solvent having a weight ratio to the synthetic resin of 1.5 times or more in the flow reactor (13e) so that the yield of the starting material monomer is 70% or more; and supplying the synthetic resin and the solvent to the flow reactor (13e) such that the temperature inside the flow reactor (13e) becomes a target temperature, the residence time of the synthetic resin and the solvent in the flow reactor (13e) becomes a target residence time, and the pressure inside the flow reactor (13e) becomes 10 MPa or more.

Description

TECHNICAL FIELD

[0001] The present application relates to a depolymerization method and a depolymerization apparatus for depolymerizing a synthetic resin into a raw material monomer. BACKGROUND

[0002] As an initiative to reduce waste, research and development related to recycling of waste is currently being conducted. As such a technique, a method for depolymerizing nylon 6, which is a synthetic resin (polymer), into epsilon-caprolactam, which is a raw material monomer, has been known in the past (for example, see Patent Literature 1). In the method described in Patent Literature 1, 15 parts by weight or less of an acidic substance is added to 100 parts by weight of nylon 6 (poly epsilon-caprolactam) in the waste, and heating treatment is performed at 220 to 400°C to reduce the solution viscosity, and after separating out non-melted matter, the poly epsilon-caprolactam is depolymerized.

[0003] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: Japanese Patent Application Laid-Open No. 2001-294571 (JP 2001-294571 A) SUMMARY

[0004] PROBLEMS TO BE SOLVED BY THE INVENTION However, in the method in which an acidic substance is added as described in Patent Literature 1, chemical decomposition takes a long time, and thus it is difficult to rapidly perform depolymerization.

[0005] SOLUTION TO PROBLEM One technical solution of the present application is a depolymerization method for depolymerizing a synthetic resin into a raw material monomer, including: a step of setting a target temperature of an inside of a flow reactor, a target residence time of a solvent in the flow reactor at a weight ratio of 1.5 times or more to the synthetic resin in a manner that a yield of the raw material monomer becomes 70% or more; and a step of supplying the synthetic resin and the solvent to the flow reactor in a manner that a temperature of the inside of the flow reactor becomes the target temperature, and a residence time of the synthetic resin and the solvent in the flow reactor becomes the target residence time, and a pressure of the inside of the flow reactor becomes 10 MPa or more.

[0006] Another aspect of the present application is a depolymerization apparatus that depolymerizes a synthetic resin into a raw material monomer, comprising: a flow reactor; a supply portion that supplies the synthetic resin and a solvent to the flow reactor in a manner such that the temperature inside the flow reactor becomes a predetermined target temperature, the residence time of the synthetic resin and the solvent in the flow reactor becomes a predetermined target residence time, and the pressure inside the flow reactor becomes 10 MPa or more, the weight ratio of the synthetic resin to the solvent being 1.5 times or more; and a back pressure valve provided downstream of the flow reactor and set to a set pressure of 10 MPa or more. The target temperature and the target residence time are predetermined in a manner such that the yield of the raw material monomer becomes 70% or more.

[0007] Effects of the Invention According to the present application, depolymerization can be performed quickly. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1 is a view schematically showing an example of the overall structure of a depolymerization apparatus of an embodiment of the present application; Figure 2 is a view for explaining the relationship between the reaction time and the yield when a depolymerization reaction from PA6 to ε-caprolactam is performed using subcritical water; Figure 3 is a view for explaining the appropriate reaction conditions when a depolymerization reaction from PA6 to ε-caprolactam is performed using the depolymerization apparatus of Figure 1 . DETAILED DESCRIPTION

[0009] Hereinafter, an embodiment of the present application will be described with reference to Figures 1 to 3 In the depolymerization method and the depolymerization apparatus of the embodiment of the present application, a synthetic resin (polymer) is depolymerized into a raw material monomer. The synthetic resin is, for example, PA (polyamide) 6 (nylon 6), PA66 (nylon 66), PA12, PA11, PET (polyethylene terephthalate), PBT (polybutylene terephthalate), POM (polyoxymethylene), a polyurethane resin, or the like. Hereinafter, an example in which PA6 as a synthetic resin is depolymerized into ε-caprolactam as a raw material monomer will be described.

[0010] In the case where a synthetic resin such as PA6 is depolymerized into a raw material monomer for the purpose of industrial recycling, it is preferable to perform depolymerization quickly from an industrial viewpoint. Therefore, in the present embodiment, the depolymerization method and the depolymerization apparatus are configured in a manner such that depolymerization from a synthetic resin into a raw material monomer can be performed quickly by using a solvent in a subcritical state.

[0011] Figure 1 is a view schematically showing an example of the overall structure of a depolymerization apparatus 10 of an embodiment of the present application. As shown in Figure 1As shown, the depolymerization device 10 mainly has a hopper 11 for feeding the PA6 crushed material, a tank 12 for storing water as a solvent, and an extruder 13.

[0012] The hopper 11 is configured as a volumetric, a reduced weight control, or the like metering hopper. The hopper 11 is connected to the upstream side of the extruder 13 to continuously feed a certain amount of the PA6 crushed material at a predetermined rotational speed or vibration frequency to the extruder 13.

[0013] The tank 12 is connected to the extruder 13 at a downstream side from the hopper 11 via a pipe 14. The pipe 14 is provided with a pump 15 for pressure feeding water at a predetermined flow rate and a heater 16 for heating the water to a predetermined temperature (e.g., 240°C or higher). The pump 15 is set to a flow rate such that the weight of the water fed to the extruder 13 is 1.5 times or more, preferably 4 times or more, the weight of the PA6 fed to the extruder 13. At the tip of the pipe 14 on the extruder 13 side, a nozzle 14a is provided. The water stored in the tank 12 is pressure fed by the pump 15 via the pipe 14 and heated by the heater 16, and is injected into the extruder 13 at a high temperature and high pressure via the nozzle 14a.

[0014] The extruder 13 is configured as a single- or multi-shaft screw extruder having a steel cylinder 13a and a steel screw 13b penetrating the cylinder 13a. The PA6 crushed material from the hopper 11 and the water from the tank 12 are fed to a space 13c between the cylinder 13a and the screw 13b. The cylinder 13a is wound with a heater not shown, and the cylinder 13a is maintained at a predetermined reaction temperature by heating the heater as needed. The inner diameter of the cylinder 13a is set to be equal to or smaller than a predetermined value to uniformly heat the PA6 and the water fed to the space 13c. An electric motor M is connected to the upstream end of the screw 13b, and the screw 13b is rotated at a predetermined rotational speed by being driven by the electric motor M. A pipe 17 is connected to the downstream end of the cylinder 13a, and a back pressure valve 18 is provided to the pipe 17.

[0015] When the screw 13b is rotated, the PA6 crushed material fed to the space 13c is extruded to the downstream side of the extruder 13. At this time, the PA6 crushed material fed from the hopper 11 is compressed and sheared by the screw 13b to generate heat, and is rapidly melted with the shearing heat as a main heat source, and is extruded while being plasticized. The space 13c from the hopper 11 to the nozzle 14a in which the melting of the PA6 crushed material is performed is sometimes referred to as a melting section 13d. The volume of the melting section 13d is set to a volume required for the PA6 crushed material to be melted and plasticized.

[0016] In the space 13c, water is injected from the nozzle 14a at the downstream end of the melting section 13d. The plasticized PA6 is stirred by the rotation of the screw 13b, mixed with the water injected from the nozzle 14a, and extruded to the more downstream side of the extruder 13 while being mixed (kneaded). The water injected from the nozzle 14a is heated by the heater 16 to maintain a sufficiently high temperature, thereby preventing the plasticized PA6 from solidifying again.

[0017] The space 13c downstream of the melting section 13d forms a reaction field in which PA6 and water are continuously supplied and a depolymerization reaction is performed. The space 13c downstream of the melting section 13d in which the depolymerization reaction is performed is sometimes referred to as a "flow reactor 13e". In addition, the hopper 11, the screw 13b, and the motor M that supply PA6 to the flow reactor 13e, and the pump 15 that supplies water to the flow reactor 13e are sometimes referred to as a "supply section". Since the upstream side of the flow reactor 13e is blocked by the plasticized PA6, the pressure (reaction pressure) in the flow reactor 13e is increased to the set pressure of the back pressure valve 18 as the screw 13b rotates.

[0018] The reaction pressure in the flow reactor 13e is maintained at a pressure slightly higher than the saturated vapor of water. In addition, the temperature (reaction temperature) and the reaction pressure in the flow reactor 13e are maintained at subcritical conditions slightly lower than the critical point (374°C, 22.1 MPa) at which water changes its state between liquid and gas. In this view, the set pressure of the back pressure valve 18 is set to 10 MPa or higher, and preferably 13 MPa or higher. In addition, the set pressure of the back pressure valve 18 is set to an upper limit pressure (for example, about 20 MPa) corresponding to the pressure resistance of the device or lower. Thus, the water in the flow reactor 13e is maintained in a subcritical state, that is, in a state of subcritical water as a liquid. Subcritical water can hydrolyze synthetic resins such as PA6, PA66, PA12, PA11, PET, PBT, POM, and polyurethane resin, and rapidly perform a depolymerization reaction.

[0019] The plasticized PA6 is stirred by the rotation of the screw 13b, mixed with the subcritical water, and thus rapidly depolymerizes to generate ε-caprolactam. The volume of the flow reactor 13e is set to a volume required for the PA6 to be sufficiently mixed with the subcritical water, contacted, and perform a depolymerization reaction. The screw 13b does not necessarily have to be disposed to the downstream end of the flow reactor 13e as long as the PA6 is sufficiently mixed with the subcritical water in the flow reactor 13e. By actively stirring and mixing the PA6 with the subcritical water, the required amount of subcritical water can be reduced, and the entire device can be downsized. In addition, the PA6 pulverized material supplied from the hopper 11 is melted and plasticized in the melting section 13d and then supplied to the flow reactor 13e, thereby enabling the time required for the PA6 to be mixed with the subcritical water to be shortened (about 15 minutes) compared to a case in which the pulverized material is directly supplied to the flow reactor 13e.

[0020] When the reaction pressure in the flow reactor 13e exceeds the set pressure of the back pressure valve 18, the back pressure valve 18 opens, and the depolymerization liquid including ε-caprolactam generated by the depolymerization reaction, unreacted PA6, and water is discharged to the outside via the pipe 17. When the reaction pressure in the flow reactor 13e decreases to the set pressure of the back pressure valve 18, the back pressure valve 18 immediately closes, but at this time, the reaction pressure in the flow reactor 13e can temporarily be lower than the set pressure of the back pressure valve 18. When the reaction pressure in the flow reactor 13e deviates from the subcritical condition of water, the subcritical water required for the depolymerization reaction becomes insufficient due to the vaporization of water, and therefore the set pressure of the back pressure valve 18 is preferably set to be higher than the desired reaction pressure by a prescribed pressure or more (for example, 0.5 MPa or more). In this case, the reaction pressure in the flow reactor 13e is maintained at 10 MPa or more, and preferably 13 MPa or more, and therefore the set pressure of the back pressure valve 18 is set to 10.5 MPa or more, and preferably 13.5 MPa or more.

[0021] A recovery device for recovering ε-caprolactam from the depolymerization liquid is provided in the rear section of the depolymerization device 10. The depolymerization liquid discharged from the depolymerization device 10 to the normal pressure environment via the pipe 17 is separated into a solid component and a gas component. That is, the water in the depolymerization liquid vaporizes due to the deviation from the subcritical condition. In the recovery device, the solid component including ε-caprolactam and unreacted PA6 is directly recovered, and the gas component (water) including a trace amount of ε-caprolactam is collected and recovered by cooling. The high-temperature solid component recovered by the recovery device is kept warm for a prescribed period, and therefore the unreacted PA6 is decomposed into ε-caprolactam by the residual heat. The water recovered by the recovery device is returned to the tank 12 of the depolymerization device 10 and reused as a solvent for the depolymerization reaction.

[0022] Figure 2 is a graph for explaining the relationship between the reaction time and the yield when the depolymerization reaction from PA6 to ε-caprolactam is performed using subcritical water, and shows the test results of the yield corresponding to different reaction times when the reaction temperature T is varied in the range of 270°C to 370°C. As shown in Figure 2 , the higher the reaction temperature T, the shorter the reaction time required to make the yield maximum. When the reaction temperature T is set to 270°C or more, the maximum yield can be achieved in 2 hours or less. When the reaction temperature T is set to 350°C or more, the maximum yield can be achieved in 20 minutes or less.

[0023] Figure 3 is a graph for explaining the relationship between the reaction time and the yield when the depolymerization reaction from PA6 to ε-caprolactam is performed using Figure 1Figure showing appropriate reaction conditions when the depolymerization device 10 performs a depolymerization reaction from PA6 to ε-caprolactam. In the depolymerization device 10 that continuously performs a depolymerization reaction, from an industrial viewpoint, it is preferable to inhibit the reaction time of the depolymerization reaction, that is, the residence time τ of PA6 and subcritical water in the flow reactor 13e, to be the longest within 2 hours, preferably within 20 minutes. Therefore, as shown in Figure 2 and Figure 3 the target temperature To within the flow reactor 13e is set to be 270°C or higher, preferably around 350°C. In addition, the target temperature To within the flow reactor 13e is set to be below an upper limit temperature (for example, around 365°C) corresponding to the heat resistance of the device.

[0024] The target residence time τ0 of PA6 and subcritical water in the flow reactor 13e is set in such a manner that the yield of ε-caprolactam reaches 70% or more and is below the maximum yield, depending on the target temperature To within the flow reactor 13e. When the residence time τ is too short, the yield of ε-caprolactam is insufficient, but even if the residence time τ is too long, ε-caprolactam is further excessively decomposed and thus the yield is rather reduced. In view of this, the target temperature To within the flow reactor 13e and the target residence time τ0 of PA6 and subcritical water in the flow reactor 13e are set in such a manner as to satisfy the following equations (i), (ii). That is, the target temperature To and the target residence time τ0 are set in such a manner as to be within the range defined by the equations (i), (ii) with the equation (i) as a lower limit and the equation (ii) as an upper limit.

[0025] τ0≥43200e^(-0.0244To) (i) τ0≤31800e^(-0.0211To) (ii) The equation (i) is an approximate equation based on the results of experiments on the reaction time when the yield of ε-caprolactam is 70% while changing the reaction temperature T within the range of 270°C to 365°C. The target temperature To and the target residence time τ0 are set in such a manner as to satisfy the equation (i), whereby the yield of ε-caprolactam can be 70% or more. Similarly, the equation (ii) is an approximate equation based on the results of experiments on the reaction time when the yield of ε-caprolactam is the maximum while changing the reaction temperature T within the range of 270°C to 365°C. The target temperature To and the target residence time τ0 are set in such a manner as to satisfy the equation (ii), whereby the yield of ε-caprolactam can be the maximum. Note that under the condition of τ0 > 31800e^(-0.0211To), the yield is rather reduced due to excessive decomposition of ε-caprolactam. In the case where the target temperature To is set to be around 350°C, the target residence time τ0 satisfying the equations (i), (ii) is around 10 to 20 minutes.

[0026] The reaction temperature T in the flow reactor 13e can be adjusted by the set temperature of the heater of the barrel 13a, the rotation speed (or the vibration frequency) of the hopper 11, and the rotation speed of the motor M that drives the screw 13b. As with the melting section 13d, the PA6 in the flow reactor 13e is also heated by compression and shearing by the rotation of the screw 13b, and is warmed up with the shearing heat as a part of the heat source. Therefore, the reaction temperature T in the flow reactor 13e can be adjusted not only by the set temperature of the heater of the barrel 13a, but also by the rotation speed of the motor M that defines the shearing speed of the PA6 by the screw 13b, and the rotation speed of the hopper 11 that defines the supply amount of the PA6.

[0027] The residence time τ of the PA6 and the subcritical water in the flow reactor 13e can be adjusted by the rotation speed of the hopper 11 and the rotation speed of the motor M that drives the screw 13b. Alternatively, the length of the flow reactor 13e can be adjusted by adjusting the length of the extruder 13 (barrel 13a), thereby adjusting the residence time τ.

[0028] The reaction pressure in the flow reactor 13e can be adjusted not only by the set pressure of the back pressure valve 18, but also by the set temperature of the heater of the barrel 13a, the rotation speed of the hopper 11, and the rotation speed of the motor M that drives the screw 13b.

[0029] The hopper 11, the screw 13b, and the motor M that are the supply sections that supply the PA6 to the flow reactor 13e are set and adjusted so that the reaction temperature T in the flow reactor 13e is the target temperature To, the residence time τ is the target residence time τ0, and the reaction pressure is 10 MPa or higher, preferably 13 MPa or higher. In addition, the set flow rate of the pump 15 that is the supply section that supplies water to the flow reactor 13e is set so that the rotation speed of the hopper 11 that defines the supply amount of the PA6 is 1.5 times or higher, preferably 4 times or higher, of the weight of the water supplied to the flow reactor 13e with respect to the weight of the PA6.

[0030] With the present embodiment, the following effects can be obtained.

[0031] (1) The depolymerization method of PA6 into ε-caprolactam includes: a setting step of setting a target temperature T0 in the flow reactor 13e, a target residence time τ0 of subcritical water in the flow reactor 13e, and a weight ratio of PA6 to the subcritical water to be 1.5 times or more in a manner that the yield of ε-caprolactam is 70% or more; and a supply step of supplying PA6 and water to the flow reactor 13e in a manner that the reaction temperature T in the flow reactor 13e is the target temperature T0, the residence time τ of PA6 and the subcritical water in the flow reactor 13e is the target residence time τ0, and the reaction pressure in the flow reactor 13e is 10 MPa or more. The depolymerization requiring high temperature and high pressure is continuously performed using the flow reactor 13e, whereby the time required for temperature increase and decrease of the reactor can be shortened, and the depolymerization can be performed rapidly. In addition, the energy consumption amount required for temperature increase and decrease of the reactor can be reduced, and the depolymerization can be performed efficiently.

[0032] (2) In the setting step, the target temperature T0 and the target residence time τ0 are set in a manner that they are within the ranges defined by the formulas (i) and (ii). The target temperature T0 and the target residence time τ0 are set in a manner that formula (i) is satisfied, whereby the yield of ε-caprolactam can be 70% or more. In addition, the target temperature T0 and the target residence time τ0 are set in a manner that formula (ii) is satisfied, whereby the yield of ε-caprolactam can be maximized.

[0033] (3) In the setting step, the target temperature T0 is set to about 350°C, and the target residence time τ0 is set to a time within a range of 10 minutes or more and 20 minutes or less. In this case, the depolymerization can be performed rapidly within the heat-resistant temperature range of the apparatus.

[0034] (4) In the supply step, PA6 and water are supplied to the flow reactor 13e in a manner that the water in the flow reactor 13e is maintained in a subcritical state. That is, PA6 and water are supplied to the flow reactor 13e in a manner that the temperature and the pressure are maintained in a subcritical state which is slightly lower than the critical point which is a boundary value at which the state of water changes from liquid to gas. By maintaining the water in a subcritical state, the reaction rate of the depolymerization reaction can be increased, and the depolymerization can be performed rapidly.

[0035] (5) The depolymerization device 10 that depolymerizes PA6 into ε-caprolactam is provided with: a flow reactor 13e; a supply section (hopper 11, screw 13b, motor M, pump 15) that supplies PA6 and subcritical water in which the weight ratio to PA6 is 1.5 times or more to the flow reactor 13e in a manner such that the reaction temperature T in the flow reactor 13e becomes a target temperature To that is set in advance, the residence time τ of PA6 and subcritical water in the flow reactor 13e becomes a target residence time τ0 that is set in advance, and the reaction pressure in the flow reactor 13e becomes 10 MPa or more; and a back pressure valve 18 that is provided downstream of the flow reactor 13e and is set to a set pressure of 10 MPa or more Figure 1 ). The target temperature To and the target residence time τ0 are set in advance in a manner such that the yield of ε-caprolactam becomes 70% or more. This enables depolymerization that requires high temperature and high pressure to be performed continuously, reduces the time required for temperature increase and decrease of the reactor, and enables depolymerization to be performed quickly. In addition, it is possible to reduce the amount of energy consumption required for temperature increase and decrease of the reactor, and to perform depolymerization efficiently.

[0036] (6) The depolymerization device 10 is further provided with a melting section 13d that is provided upstream of the flow reactor 13e, melts PA6, and extrudes it to the flow reactor 13e Figure 1 ). This enables PA6 to be supplied to the flow reactor 13e in a molten state, and enables depolymerization to be performed quickly in the flow reactor 13e.

[0037] (7) The depolymerization device 10 is further provided with a screw 13b that stirs PA6 and subcritical water supplied to the flow reactor 13e by the supply section Figure 1 ). By actively stirring and mixing PA6 and subcritical water, it is possible to reduce the amount of subcritical water required, and to downsize the device as a whole.

[0038] (8) The depolymerization device 10 is further provided with an extruder 13 that melts PA6 and extrudes it to the flow reactor 13e, and stirs PA6 and subcritical water extruded to the flow reactor 13e Figure 1 ). That is, it is possible to perform melting of PA6, supply to the flow reactor 13e, stirring and mixing of PA6 and subcritical water, and discharge of ε-caprolactam obtained by depolymerization by a single extruder 13, and it is possible to simplify the structure of the device as a whole.

[0039] In the above embodiment, an example was described in which PA6 pulverized material was melted and plasticized in the melting section 13d and then supplied to the flow reactor 13e. However, the synthetic resin supplied to the flow reactor is not limited to the melted and plasticized synthetic resin. Furthermore, even when the melted and plasticized synthetic resin is supplied to the flow reactor, the melting and plasticization are not limited to being performed within the extruder 13 as described in the above embodiment; a device for melting and plasticizing the synthetic resin can also be provided at the front end of the extruder 13. In this case, waste materials such as glass fiber reinforced plastics can also be melted and plasticized, and after removing glass fibers and other foreign matter, supplied to the flow reactor.

[0040] In the above embodiments, an example of depolymerizing PA6 as a synthetic resin was described, but the present invention is also applicable to hydrolyzable synthetic resins such as PA66, PA12, PA11, PET, PBT, POM, and polyurethane resin. Furthermore, in the above embodiments, an example of using subcritical water as a solvent was described, but subcritical methanol or similar solvents can also be used.

[0041] In the above embodiments, it is explained that in Figure 1 Figure 1 Examples of using a single-shaft screw extruder 13 are given, but the flow reactor for the depolymerization reaction is not limited to the space 13c between the barrel 13a and the screw 13b of the screw extruder 13. Additionally, an example of using the screw 13b to stir the synthetic resin and solvent is described, but the stirring blades are not limited to this.

[0042] The above description is merely an example, and the above embodiments and modifications do not limit the invention as long as they do not destroy its features. One or more of the above embodiments and modifications can be combined in any way, and modifications can also be combined with each other.

[0043] Explanation of reference numerals in the attached figures 10: Depolymerization device; 11: Hopper; 12: Tank; 13: Extruder; 13a: Cylinder; 13b: Screw; 13c: Space; 13d: Melting section; 13e: Flow reactor; 14: Pipeline; 14a: Nozzle; 15: Pump; 16: Heater; 17: Pipeline; 18: Back pressure valve; M: Electric motor.

Claims

1. A depolymerization method, characterized in that, a method for depolymerizing synthetic resin into raw material monomers, wherein... comprises: a setting step of setting a target temperature of the inside of the flow reactor, a target residence time of the synthetic resin and a solvent in the flow reactor, the solvent being in a weight ratio of 1.5 times or more to the synthetic resin, in a manner that the yield of the raw material monomer becomes 70% or more; and a supply step of supplying the synthetic resin and the solvent to the flow reactor in a manner that the temperature of the inside of the flow reactor becomes the target temperature, the residence time of the synthetic resin and the solvent in the flow reactor becomes the target residence time, and the pressure of the inside of the flow reactor becomes 10 MPa or more.

2. The depolymerization method according to claim 1, wherein the synthetic resin is nylon 6, the raw material monomer is ε-caprolactam, when the target temperature is set to T and the target residence time is set to τ, in the setting step, the target temperature and the target residence time are set in a range defined by the following two formulas, τ ≥ 43200e^(-0.0244T), τ ≤ 31800e^(-0.0211T).

3. The depolymerization method according to claim 2, wherein in the setting step, the target temperature is set to 350°C.

4. The depolymerization method according to claim 2, wherein in the setting step, the target residence time is set to a time in a range of 10 minutes or more and 20 minutes or less.

5. The depolymerization method according to claim 1, wherein in the supply step, the synthetic resin and the solvent are supplied to the flow reactor in a manner that the solvent in the flow reactor is maintained in a subcritical state.

6. A depolymerization apparatus, characterized in that, a depolymerization apparatus for depolymerizing synthetic resin into raw material monomers, wherein... comprises: a flow reactor; a supply part that supplies the synthetic resin and a solvent to the flow reactor in a manner that the temperature of the inside of the flow reactor becomes a target temperature set in advance, the residence time of the synthetic resin and the solvent in the flow reactor becomes a target residence time set in advance, and the pressure of the inside of the flow reactor becomes 10 MPa or more; and a back pressure valve provided downstream of the flow reactor and set to a set pressure of 10 MPa or more, the target temperature and the target residence time are set in advance in a manner that the yield of the raw material monomer becomes 70% or more.

7. The depolymerization apparatus according to claim 6, further comprising: a melting part that is provided upstream of the flow reactor, melts the synthetic resin, and extrudes to the flow reactor.

8. The depolymerization apparatus according to claim 6, further comprising: a stirring blade that stirs the synthetic resin and the solvent supplied to the flow reactor by the supply part.

9. The depolymerization apparatus according to claim 6, further comprising: Further, a screw extruder melts the synthetic resin, extrudes it to the flow reactor, and stirs the synthetic resin and the solvent extruded to the flow reactor.

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