Method for producing styrene monomer

Abstract

This disclosure aims to reduce the amount of residue generated by the thermal decomposition of polystyrene resins containing styrene monomer units, and to recover styrene monomers in high yield. [Solution] This disclosure provides a method for producing a styrene monomer comprising a first pyrolysis step and a recovery step, The first thermal decomposition step is a step of supplying a polystyrene resin composition containing styrene monomer units to an atmosphere heated to 300°C or higher and thermally decomposing it to obtain a first thermal decomposition liquid. The recovery step is a step of recovering styrene monomers from the first pyrolysis liquid, The ambient temperature (Tb) of the first pyrolysis step and the temperature (Tc) of the supplied polystyrene resin composition are given by the following formula (1): [Mathematics 1] |Tb-Tc|≧150℃ (In the above formula (1), Tb represents the ambient temperature of the first pyrolysis step (hereinafter referred to as the first pyrolysis ambient temperature), and Tc represents the temperature of the polystyrene resin composition immediately before being supplied to the atmosphere.) This is a method for producing styrene monomers, characterized by satisfying the following relationship.

Description

[Technical Field]

[0001] This invention relates to a method for producing styrene monomers from a polystyrene resin composition containing styrene monomer units. [Background technology]

[0002] Polystyrene resins are broadly classified into colorless, transparent general-purpose polystyrene (hereinafter referred to as GPPS) and high-impact polystyrene (hereinafter abbreviated as HIPS), which has been improved to address the shortcomings of GPPS. ABS resin, composed of three monomers—acrylonitrile, butadiene, and styrene—is also known as a styrene-based resin. As a recycling method for these polystyrene-based resins, it is known that PS resin can be decomposed into low-molecular-weight compounds containing styrene monomers by methods such as thermal decomposition, and research on chemical recycling utilizing this property has been conducted. In the thermal decomposition of polystyrene-based resins, it is known that not only styrene monomers are selectively obtained, but several types of aromatic compounds and other by-products are also generated. In addition, carbides, which are thought to originate from the thermal denaturation of these by-products, are also generated. In particular, the amount of styrene monomers, by-products, and carbides generated is thought to differ depending on the type of polystyrene resin used for thermal decomposition, so there are issues that need to be addressed depending on the raw material.

[0003] Non-patent document 1 discloses the relationship between temperature and products in the thermal decomposition of HIPS, a resin containing conjugated diene monomer units and styrene monomer units. It shows that the higher the thermal decomposition temperature, the greater the carbide residue and the lower the yield of styrene monomer, and furthermore, there is a tendency for the number of cyclic products due to the acceleration of the Diels-Alder reaction or crosslinking reaction to increase. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Polyolefins Journal,2019,Vol.6,No.1,43-52.(DOI:10.22063 / poj.2018.2189.1114) [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] According to Non-Patent Document 1 mentioned above, it has been shown that adjusting the output of the heating device used for pyrolysis and increasing the heating rate reduces the amount of carbide residue and lowers the yield of styrene monomer. Furthermore, as mentioned above, the conventional pyrolysis of resins containing conjugated diene monomer units and styrene monomer units generates carbide residue and other by-products, posing a problem in terms of the generation of by-products in pyrolysis. Moreover, the conventional pyrolysis of resins containing styrene monomer units also generates carbide residue and other by-products, posing a problem in terms of the generation of by-products in pyrolysis.

[0006] Therefore, the object of this disclosure is to thermally decompose a polystyrene resin composition containing styrene monomer units, reduce the thermal decomposition residue generated by the thermal decomposition of the polystyrene resin composition containing styrene monomer units, and obtain a high concentration of styrene monomer units. In particular, when using a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units as raw materials, the objective is to convert styrene monomers into styrene monomers with good yield and to suppress the generation of thermal decomposition residues that occur due to thermal decomposition. [Means for solving the problem]

[0007] As a result of diligent research to solve the above problems, the inventors have found that by introducing a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin containing conjugated diene monomer units and styrene monomer units, into a thermal decomposition apparatus heated to a temperature higher than the decomposition temperature of the polystyrene resin, the residue generated in the thermal decomposition process can be suppressed, and the reduction in the amount of styrene monomer produced can be suppressed even under high thermal decomposition temperature conditions, thus completing the present invention.

[0008] In other words, the present invention is as follows: [1] A method for producing styrene monomers comprising a first thermal decomposition step and a recovery step, The first thermal decomposition step is a step of supplying a polystyrene resin composition containing styrene monomer units to an atmosphere heated to 300°C or higher and thermally decomposing it to obtain a first thermal decomposition liquid. The recovery step is a step of recovering styrene monomers from the first pyrolysis liquid, The ambient temperature of the first pyrolysis step and the temperature of the supplied polystyrene resin composition are given by the following formula (1): |Tb-Tc|≧150℃ (In the above formula (1), Tb represents the ambient temperature of the first thermal decomposition step (hereinafter referred to as the first thermal decomposition ambient temperature), and Tc represents the temperature of the polystyrene resin composition immediately before being supplied to the atmosphere.) A method for producing styrene monomers that satisfies the relationship.

[0009] [2] A method for producing styrene monomers comprising a first thermal decomposition step and a recovery step, The first thermal decomposition step is a step of supplying a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units to an atmosphere heated to 300°C or higher, and thermally decomposing it to obtain a first thermal decomposition liquid. The recovery step is a step of recovering styrene monomers from the first pyrolysis liquid, The ambient temperature (Tb) of the first pyrolysis step and the temperature (Tc) of the supplied polystyrene resin composition are given by the following formula (1): |Tb-Tc|≧150℃ (In the above formula (1), Tb represents the ambient temperature of the first thermal decomposition step (hereinafter referred to as the first thermal decomposition ambient temperature), and Tc represents the temperature of the polystyrene resin composition immediately before supply.) A method for producing a styrene monomer according to [1], characterized in that it satisfies the relationship.

[0010] [3] The method for producing a styrene monomer according to [1] or [2], wherein the temperature of the polystyrene resin composition supplied to the atmosphere is 0°C or higher and less than 300°C.

[0011] [4] A method for producing a styrene monomer according to any one of [1] to [3], characterized in that the polystyrene resin composition supplied to the first thermal decomposition step is a fluid.

[0012] [5] A method for producing a styrene monomer according to any one of [1] to [4], wherein the molten polystyrene resin composition is supplied to a thermal decomposition section, and the thermal decomposition products of the polystyrene resin composition are transferred from the thermal decomposition section without using an external pumping means until a thermal decomposition liquid is obtained.

[0013] [6] A method for producing a styrene monomer according to any one of [1] to [5], wherein the first thermal decomposition atmosphere temperature is 400°C or higher and less than 1200°C.

[0014] [7] A method for producing a styrene monomer according to any one of [1] to [6], wherein the atmospheric pressure in the first thermal decomposition step is 0 hPa or more and 1013 hPa or less.

[0015] [8] The method for producing a styrene monomer according to [2], wherein the content of conjugated diene monomer units in the polystyrene resin composition is more than 0.1% by mass.

[0016] [9] A method for producing a styrene monomer according to any one of [1] to [8], wherein the first pyrolysis step includes a cooling step of cooling a pyrolysis gas containing a styrene monomer obtained by pyrolyzing the polystyrene resin composition to obtain the pyrolysis liquid. [Effects of the Invention]

[0017] According to this disclosure, it is possible to reduce the amount of residue generated by the thermal decomposition of a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin containing conjugated diene monomer units and styrene monomer units, and to recover styrene monomers in high yield. According to this disclosure, by controlling the heat distribution in the pyrolysis process, the amount of residue generated by the pyrolysis of a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin containing conjugated diene monomer units and styrene monomer units, can be reduced, and styrene monomers can be recovered in high yield. According to this disclosure, the amount of residue discharged from the pyrolysis apparatus can be increased, and the amount of styrene monomer produced or recycled over a predetermined period can be increased. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 shows a schematic cross-sectional view illustrating an example of a preferred pyrolysis apparatus according to this embodiment. [Figure 2] Figure 2 shows a schematic cross-sectional view illustrating another example of a preferred pyrolysis apparatus according to this embodiment. [Figure 3] Figure 3 shows a schematic cross-sectional view illustrating another example of a preferred pyrolysis apparatus according to this embodiment. [Figure 4] Figure 4 is a schematic cross-sectional view showing another preferred embodiment of the pyrolysis apparatus of this embodiment, and shows an enlarged view of an example of the connection portion between the melting apparatus and the pyrolysis section. [Figure 5] Figure 5 shows a schematic cross-sectional view illustrating another example of a preferred pyrolysis apparatus according to this embodiment. [Figure 6] Figure 6 shows a schematic cross-sectional view illustrating another example of a preferred pyrolysis apparatus according to this embodiment. [Figure 7] Figure 7 shows a schematic cross-sectional view illustrating another example of a preferred pyrolysis apparatus according to this embodiment. [Figure 8] This flowchart shows an example of the overall method for producing styrene monomer according to this embodiment. [Modes for carrying out the invention]

[0019] The following describes in detail an embodiment for carrying out the present invention (hereinafter referred to as "this embodiment"), but the present invention is not limited to the following description and can be implemented in various modifications within the scope of its gist. Unless otherwise specified below, pressure should be expressed as absolute pressure.

[0020] [Method for producing styrene monomers] This disclosure relates to a method for producing styrene monomers comprising a first pyrolysis step and a recovery step, wherein the first pyrolysis step is a step of supplying a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, to an atmosphere heated to 300°C or higher and pyrolyzing it to obtain a first pyrolysis liquid, and the recovery step is a step of recovering styrene monomers from the first pyrolysis liquid, wherein the atmosphere temperature in the first pyrolysis step and the temperature of the supplied polystyrene resin composition are given by the following formula (1): |Tb-Tc|≧150℃ (In the above formula (1), Tb represents the ambient temperature of the first thermal decomposition step (hereinafter referred to as the first thermal decomposition ambient temperature), and Tc represents the temperature of the polystyrene resin composition immediately before being supplied to the atmosphere.) It satisfies the relationship. This makes it possible to efficiently produce styrene monomers from polystyrene resin compositions containing styrene monomer units, particularly resin compositions containing conjugated diene monomer units and styrene monomer units, while also reducing the amount of thermal decomposition residue generated by thermal decomposition.

[0021] In this embodiment, the method for producing styrene monomers is preferably such that a polystyrene resin composition containing styrene monomer units, particularly a resin composition containing conjugated diene monomer units and styrene monomer units, is supplied to an atmosphere heated to 300°C or higher, the resin composition is thermally decomposed, the thermal decomposition gas is cooled to prepare a first thermal decomposition liquid, and the styrene monomers are recovered from the first thermal decomposition liquid. In this case, if necessary, a first distillation step (S2) of distilling the first thermal decomposition liquid may be further included before the recovery step (see Figure 8). In the first distillation step (S2), it is preferable to separate the material into a first fraction mainly consisting of low-boiling components and a second fraction containing styrene monomers and having a higher boiling point than the first fraction.

[0022] The following describes the first thermal decomposition step, which is an essential requirement for the method of producing styrene monomer in this embodiment.

[0023] (First pyrolysis step: S1) The method for producing styrene monomers according to this disclosure includes a first thermal decomposition step as an essential step. The first thermal decomposition step comprises the steps of supplying a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units (hereinafter also simply referred to as a polystyrene resin composition), to an atmosphere heated to 300°C or higher, and thermally decomposing the polystyrene resin composition supplied to the atmosphere heated to 300°C or higher. This makes it possible to efficiently produce styrene monomers from polystyrene resin compositions containing styrene monomer units, particularly polystyrene resin compositions containing conjugated diene monomer units and styrene monomer units, while reducing the amount of thermal decomposition residue generated by thermal decomposition. More specifically, by exposing a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units to an atmosphere above a predetermined temperature, the temperature of the polystyrene resin composition itself rises rapidly. As a result, the time until the start of thermal decomposition of the composition is significantly shortened, and thermal decomposition becomes the main reaction rather than the crosslinking reaction, which tends to generate by-products. Therefore, it is believed that the amount of thermal decomposition residue generated by thermal decomposition can be reduced, and styrene monomers can be recovered in high yield.

[0024] In this embodiment, the ambient temperature (Tb) of the first pyrolysis step and the temperature (Tc) of the supplied polystyrene resin composition are given by the following formula (1): |Tb-Tc|≧150℃ (In the above formula (1), Tb represents the ambient temperature of the first pyrolysis step (hereinafter referred to as the first pyrolysis ambient temperature), and Tc represents the temperature of the polystyrene resin composition immediately before being supplied to the atmosphere of the first pyrolysis step.) It satisfies the relationship. It is preferable that the temperature of the first thermal decomposition atmosphere and the temperature of the polystyrene resin composition itself immediately before supply are within the above range, in order to reduce the thermal decomposition residue generated by thermal decomposition and / or improve the yield of styrene monomer. The temperature difference between Tb and Tc is more preferably 200°C or more, even more preferably 250°C or more, even more preferably 300°C or more, and even more preferably 400°C or more. The upper limit of |Tb-Tc| may be 950°C.

[0025] In this embodiment, the temperature of the polystyrene resin composition containing styrene monomer units supplied to an atmosphere heated to 300°C or higher, particularly the polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, is preferably 0°C or higher and less than 300°C. A temperature within this range is preferable because it reduces the amount of thermal decomposition residue generated by thermal decomposition and / or suppresses undesirable thermal deformation of the polystyrene resin composition. The temperature of the polystyrene resin composition is more preferably 20°C to 290°C, even more preferably 50°C to 280°C, even more preferably 100°C to 270°C, and even more preferably 200°C to 260°C.

[0026] Generally, polystyrene resin compositions containing styrene monomer units are known to undergo thermal decomposition, breaking down into styrene monomers and other low molecular weight components. However, at temperatures between 300°C and 450°C, the resulting components undergo secondary decomposition, consuming the generated styrene monomers and easily converting into carbide residues containing aromatic polycyclic compounds and aromatic fused ring compounds. Polystyrene resin compositions containing conjugated diene monomer units and styrene monomer units are more prone to crosslinking reactions at temperatures between 300°C and 450°C. It is believed that thermal decomposition and crosslinking reactions proceed simultaneously in this temperature range. Therefore, it has been confirmed that shortening the exposure time of the molten material to this temperature range tends to reduce residue. Furthermore, it has been confirmed that leaving the polystyrene resin composition at temperatures below 250°C for extended periods does not significantly affect the residue. Therefore, the process of supplying the polystyrene resin composition to an atmosphere heated to above 300°C is considered important from the standpoint of suppressing residue.

[0027] In this embodiment, it is preferable that a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, is supplied to an atmosphere heated to 300°C or higher, thereby transforming the polystyrene resin composition into a fluid. In the first pyrolysis step, it is preferable that the polystyrene resin composition immediately before pyrolysis is in a fluid state, as this makes it easier to control the amount per unit time when sending the polystyrene resin composition to the first pyrolysis step, according to the pressure or output of the drive equipment. Furthermore, when the polystyrene resin composition immediately before pyrolysis is in a fluid state, the bulk density of the fluidized polystyrene resin composition tends to increase, and the porosity when sending the polystyrene resin composition to the first pyrolysis step decreases. In other words, a decrease in porosity reduces the possibility of undesirable factors such as oxygen and moisture entering the first pyrolysis step. Specifically, this makes it easier to suppress oxidation and unintended deterioration of the polystyrene resin composition due to undesirable factors in the production of styrene monomers.

[0028] In this embodiment, the first thermal decomposition step is a step of thermally decomposing a polystyrene resin composition supplied to an atmosphere heated to 300°C or higher to obtain a first thermal decomposition liquid. A preferred form of the first thermal decomposition step of this embodiment is to supply the polystyrene resin composition, which has been heated to an atmosphere of 300°C or higher, under the following conditions (A) and (B): (A) The temperature of the first thermal decomposition atmosphere is 300°C or higher and less than 1200°C. (B) The first pyrolysis pressure is between 0 hPa and 1013 hPa. Preferably, the process involves preparing a first pyrolysis solution containing styrene monomers by thermal decomposition. In this embodiment, a method for thermally decomposing the polystyrene resin composition supplied to an atmosphere heated to 300°C or higher includes a thermal decomposition apparatus that heats the composition to 300°C or higher and less than 1200°C. Furthermore, the thermal decomposition apparatus used during thermal decomposition is not particularly limited as long as it is equipped with heating means and pressure adjustment means, as will be described later.

[0029] In the first pyrolysis step (S1), the temperature at which the polystyrene resin composition supplied to the atmosphere heated to 300°C or higher is firstly pyrolyzed (= first pyrolysis atmosphere temperature) is, for example, after filling the pyrolysis apparatus with the composition, preferably in a temperature range of 300°C or higher and less than 1200°C, more preferably in a temperature range of 400°C to 1100°C. In this case, the temperature of the composition to be filled is about 20 to 300°C, more preferably 150 to 280°C. Alternatively, the first pyrolysis step (S1) may be performed by filling a pyrolysis apparatus that has been preheated with the composition. In this case, the atmosphere temperature for preheating the pyrolysis apparatus is preferably 300°C or higher and less than 1200°C, more preferably 400 to 1100°C. By setting the ambient temperature of the above-mentioned pyrolysis apparatus to the specified temperature range, the majority of the generated pyrolysis gas (also referred to as pyrolysis vapor) can become pyrolysis products of polystyrene resin compositions containing styrene monomer units, particularly polystyrene resin compositions containing conjugated diene monomer units and styrene monomer units. Therefore, it is preferable to create an atmosphere with a temperature range of 300°C to less than 1200°C using, for example, a pyrolysis apparatus as described later. Furthermore, the polystyrene resin composition supplied to an atmosphere heated to 300°C or higher is preferably in a fluid state. In this case, the temperature of the fluid that is filled into the first pyrolysis step or pyrolysis apparatus is approximately 20 to 300°C, with 150 to 280°C being more preferable. In a preferred embodiment of this product, for example, the residue can be further reduced by controlling the molten portion to a temperature of 150°C to 300°C and controlling the thermal decomposition portion, which is fluidly connected to the molten portion, to a temperature range of 400°C to less than 1200°C. In this specification, “atmospheric temperature of the first pyrolysis step (first pyrolysis atmosphere temperature) (Tb)” means the temperature of the atmosphere in which the polystyrene resin composition is (first) pyrolyzed. More specifically, the first pyrolysis atmosphere temperature (Tb) means the ambient temperature of the polystyrene resin composition when it is supplied into the pyrolysis apparatus, and refers to either the temperature of the molten section (for example, molten section 4A in Figure 5 described later) or the temperature of the discharge port of the pyrolysis section (for example, pyrolysis section 1 in Figures 1 to 7 described later) in the pyrolysis apparatus.

[0030] In the first pyrolysis step (S1), the pressure conditions (=pyrolysis pressure) for pyrolyzing the polystyrene resin composition supplied to the atmosphere heated to 300°C or higher are such that the polystyrene resin composition containing styrene monomer units, particularly the polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, is pyrolyzed under an atmosphere of pressure between 0 hPa and 1013 hPa. For this reason, it is preferable to use, for example, a pressure adjustment means described later to create an atmosphere of pressure between 0 hPa and 1013 hPa. The pyrolysis pressure may more preferably be between 15 hPa and 700 hPa, even more preferably between 20 hPa and 500 hPa, and even more preferably between 30 hPa and 300 hPa. In this embodiment, for example, when thermally decomposing the polystyrene resin composition supplied to an atmosphere heated to 300°C or higher in a thermal decomposition apparatus, the thermal decomposition may be carried out at a pressure of 0 hPa or more and less than 1013 hPa, preferably 15 hPa or more and 700 hPa or less, more preferably 20 hPa or more and 500 hPa or less, and even more preferably 30 hPa or more and 300 hPa or less. By setting the pressure of the pyrolysis apparatus to the above-mentioned pressure range, the majority of the generated pyrolysis gas can become the pyrolysis products of a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units. Furthermore, it is possible to reduce the conversion of components in the raw material polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, into by-products that lead to a decrease in the yield of styrene monomers, and to reduce the generation of pyrolysis residue. In addition, since the decomposition of a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, occurs competitively with the vaporization of by-products (e.g., styrene dimer and / or styrene trimer, etc.) generated by pyrolysis, setting the pressure within the pyrolysis apparatus to the above-mentioned pressure range can suppress the vaporization of the residue before it decomposes.

[0031] Furthermore, the pyrolysis gas generated in the first pyrolysis step (S1) may be cooled to a temperature below the boiling point of the styrene monomer, liquefied by a liquefaction device, and returned to the pyrolysis device by being dropped back into the pyrolysis device, thereby repeating the pyrolysis step. Alternatively, the generated pyrolysis gas may be liquefied by cooling with a liquefaction device to obtain a pyrolysis liquid. The cooling temperature is preferably -30°C or higher and below the boiling point Tsb (°C) of the styrene monomer, and more preferably -20°C to 80°C. In this case, a known liquefaction device can be used. For example, various heat-removing solvents including water that can cool to the cooling temperature, or cooling tubes that utilize the heat-removing effect of heat-removing elements made of metals and various other inorganic materials can be used. If necessary, a reforming device for reforming the components of the pyrolysis gas (including, for example, dechlorination, adsorption of odor components and coloring components, removal of acidic or basic components, and heat treatment) may be fluidly connected between the pyrolysis furnace and the liquefaction device. In this specification, "thermal decomposition" refers to the chemical decomposition of organic matter, etc., by heating it in the absence of oxygen, etc. Furthermore, "rectification" refers to the separation of a mixture with multiple boiling points into multiple compounds by heating and then cooling it using the difference in the boiling points of each compound.

[0032] <Pyrolysis equipment> The pyrolysis apparatus used in the method for producing styrene monomer in this embodiment is not particularly limited as long as it is a pyrolysis apparatus, such as those described later, that serves as a heat source for thermally decomposing a polystyrene resin composition supplied to an atmosphere heated to 300°C or higher. Therefore, the pyrolysis apparatus of this embodiment can use a known pyrolysis apparatus. However, heating in the pyrolysis apparatus is preferably carried out under a reduced pressure environment or an atmosphere with a reduced oxygen concentration.

[0033] The configuration of the pyrolysis apparatus described above is not particularly limited, and it is preferable to have, for example, a pyrolysis furnace as described later and a pyrolysis section which is a cylindrical body that pyrolyzes the polystyrene resin composition supplied to the atmosphere heated to 300°C or higher and converts it into a pyrolysis gas containing styrene monomers. Furthermore, the pyrolysis apparatus of this embodiment may also include a melting apparatus having a melting section that transports the molten composition to the pyrolysis section or pyrolysis furnace. For example, pyrolysis furnaces can be shaft type, kiln type, fluidized bed type, tubular type, tank type, or extruder type equipped with a screw or static mixer. The pyrolysis apparatus of this embodiment may be configured such that a melting apparatus, a pyrolysis section, and a pyrolysis furnace are connected, or the melting apparatus equipped with the melting section may be provided separately, or the pyrolysis section and pyrolysis furnace and the supply section may be integrated or continuous. Furthermore, the pyrolysis apparatus of this embodiment may be a pyrolysis apparatus using microwaves. Microwaves are a type of electromagnetic wave, meaning electromagnetic waves with wavelengths ranging from approximately 1 meter to 1 millimeter (frequency range of approximately 0.3 GHz to 300 GHz). Preferred microwaves suitable for use in this embodiment are electromagnetic waves with frequencies of 1 GHz to 300 GHz. Examples of microwave sources include magnetron tubes, solid-state sources, gyrotrons, or traveling wave tubes, which are appropriately selected and used depending on temperature, reaction controllability, cost, and output. To initiate thermal decomposition using microwaves, additives may be added to the polystyrene resin composition subjected to the thermal decomposition treatment. Specific additives may be used to improve reaction efficiency or promote selective decomposition. The additive is not particularly limited as long as it absorbs microwaves, transfers heat to the polystyrene resin composition, and contributes to the thermal decomposition reaction of the polystyrene resin composition; it can be appropriately selected according to the purpose. However, it is preferable, for example, to use an additive containing a compound that has high dielectric loss at microwave frequencies. The additive may be a carbonaceous residue from the thermal decomposition reaction of the polystyrene resin composition, ceramic beads containing the additive, pellets containing the additive, or a catalyst consisting of a combination thereof. In this case, examples of additives include silicon carbide, boron nitride, activated carbon, graphite, zeolite, silicon carbide, and ceramic fillers. Oxides that are solid at room temperature and atmospheric pressure can also be used as additives, and examples include oxygen compounds that are elements from the 1st to 6th periods of the periodic table and from groups 1 to 17. The additive is preferably present in the raw materials subjected to the thermal decomposition treatment, which include the additive and the polystyrene resin composition, in an amount of 0% to 50% by mass, more preferably 0.01% to 5% by mass, and even more preferably 0.1% to 2.5% by mass. Using microwaves for a sufficient time to allow heat generation and a catalyst, the polystyrene resin composition is heated and decomposed in the thermal decomposition apparatus by the absorption of microwaves by the inner wall of the thermal decomposition apparatus and the microwave absorption by the additive.

[0034] <<Preferred configuration of a pyrolysis apparatus>> The pyrolysis apparatus of this embodiment is preferably a tubular type, a tank type, or a continuous type in which a screw or static mixer is installed in the pyrolysis path L described later, as shown in Figures 1 to 7. An example of a pyrolysis apparatus suitable for this embodiment will be described in detail below with reference to Figures 1 to 7. An example of the pyrolysis apparatus 10 of this embodiment is shown in Figure 1. Specifically, the pyrolysis apparatus 10 has a cylindrical pyrolysis section 1 with an opening 2 and an outlet 3 at its ends, and a pyrolysis furnace 5 surrounding the pyrolysis section 1. In the example of the pyrolysis furnace 5 shown in Figures 1 to 5, the pyrolysis furnace 5 (specifically, the pyrolysis furnace wall 5A in Figure 4) covers the tubular pyrolysis section 1, and there is a cavity between the pyrolysis furnace wall 5A (specifically, corresponding to the pyrolysis furnace wall 5A in Figure 4) and the tubular pyrolysis section 1, and various heating means are provided in the cavity. Therefore, the pyrolysis furnace 5 encompasses the pyrolysis section 1. As for the heating means, for example, a heating means can be used to heat the cylindrical pyrolysis section 1 by supplying a heat transfer medium (heating gas) into the cavity of the pyrolysis furnace 5, thereby controlling the temperature within a predetermined range. It is preferable from the viewpoint of suppressing the generation of pyrolysis residue if the inlet for the heat transfer medium (heating gas) sent into the cavity of the pyrolysis furnace 5 is located near the opening 2 to which the composition is supplied, because the ambient temperature to which the resin composition supplied to the pyrolysis section 1 from the opening is exposed becomes higher, allowing for a rapid temperature increase. When a raw material composition (for example, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, which are raw materials in a molten, fluid state, and a resin composition containing conjugated diene monomer units and styrene monomer units, which are raw materials in a solid state; the same applies hereinafter) (not shown) is supplied to the opening 2, the raw material composition moves along the process flow direction (arrow direction) inside the cylindrical pyrolysis section 1 heated by the pyrolysis furnace 5, and is converted into pyrolysis gas. The pyrolysis gas, in which the raw material composition has been pyrolyzed, is then discharged to the outside from the outlet 3. Furthermore, in the pyrolysis apparatus 10 of this embodiment, in order to adjust the amount of pyrolysis gas generated inside the cylindrical pyrolysis section 1 and discharge it to the outside, a vent hole (not shown) and a pipe through which the pyrolysis gas flowing out from the vent hole (not shown) can pass may be provided in the cylindrical pyrolysis section 1. The pipe can be extended, for example, to the vicinity of the discharge port 3 of the pyrolysis section 1. As a result, the pyrolysis gas flowing out from the vent hole (not shown) can be recovered by passing it through the pipe, which may be fitted with a valve or the like, if necessary. The vent hole and pipe described above can be applied to all embodiments shown in Figures 1 to 7.

[0035] This disclosure relates to a method for producing styrene monomers, comprising a series of steps: supplying a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, to a supply port (for example, an opening 2, or a hopper 9 if a melting device 4 is provided), converting the polystyrene resin composition supplied to the supply port (for example, an opening 2, or a hopper 9 if a melting device 4 is provided) into a pyrolysis gas containing styrene monomers obtained by pyrolysis by a pyrolysis unit 1, and discharging the pyrolysis gas from an outlet 3. The shortest distance L that passes through the inside of the thermal decomposition path L from the supply port through the thermal decomposition section 1 to the discharge port 3. mini The shortest distance L from the supply port side, which is 15% of the way from the supply port side. mini The external ambient temperature T is outside the thermal decomposition path at point . 15% And, the shortest distance L from the supply port side is 75% mini The ambient temperature T is outside the thermal decomposition path L at point L. 75% The relationship is as follows (2): |(T 15% -T 75% )|≧5 It is preferable that the following conditions be met. As shown in FIG. 1, when the raw material composition is supplied to the opening 2, the raw material composition is converted into a pyrolysis gas containing styrene monomers pyrolyzed by the pyrolysis unit 1, and follows a path to be discharged as the pyrolysis gas from the discharge port 3. In this specification, at this time, the path from the raw material to the conversion into the pyrolysis gas, from the opening 2 through the pyrolysis unit 1 to the discharge port 3, is defined as the pyrolysis path L. And, among the pyrolysis path L, the shortest distance from the opening 2 through the pyrolysis unit 1 to the discharge port 3 is defined as L mini When defined as such, the shortest distance L from the opening 2 through the pyrolysis unit 1 to the discharge port 3 mini becomes the straight dotted line shown in FIG. 1 Also, for the shortest distance L mini when the whole is taken as 100%, the shortest distance L mini from the side of the opening 2 is 15% of the whole (100%) of the shortest distance L 15% At the point of the external ambient temperature T of the pyrolysis unit 1 15% is defined as such Here, the "external ambient temperature of the pyrolysis unit 1 at the point of the shortest distance L 15% " means the temperature at a point on the line that is the shortest distance from the point of the shortest distance L 15% [[ID=2M]]to the inner wall of the pyrolysis furnace 5, and is the temperature at the midpoint (the black dot part in FIG. 1) between the surface of the pyrolysis unit 1 (in FIG. 1, the surface of the cylindrical body) and the inner wall of the pyrolysis furnace 5 that is the shortest distance from the surface

[0036] Similarly, for the shortest distance L mini when the whole is taken as 100%, the shortest distance L mini from the side of the opening 2 is 75% of the whole (100%) of the shortest distance L 75% At the point of the external ambient temperature T of the pyrolysis unit 1 75% is defined as such Also, here, the "external ambient temperature of the pyrolysis unit 1 at the point of the shortest distance L 75% " means the shortest distance L 75%The temperature at the point on the line that is the shortest distance from that point to the inner wall of the pyrolysis furnace 5 is the temperature at the midpoint between the surface of the pyrolysis section 1 (the surface of the cylindrical body in Figure 1) and the inner wall of the pyrolysis furnace 5 that is the shortest distance from that surface (the black dot in Figure 1). Similarly in this specification, "the external atmosphere temperature of the pyrolysis section 1 at the point..." is the temperature at the point on the line that is the shortest distance from that point to the inner wall of the pyrolysis furnace 5, and the temperature at the midpoint between the surface of the pyrolysis section 1 and the inner wall of the pyrolysis furnace 5 that is the shortest distance from that surface.

[0037] Another example of the pyrolysis apparatus 10 of this embodiment is the configuration shown in Figure 2. Specifically, the pyrolysis apparatus 10 has a cylindrical pyrolysis section 1 having a U-shaped bend, with an opening 2 and an outlet 3 at its ends, and a pyrolysis furnace 5 surrounding the pyrolysis section 1. The cylindrical pyrolysis section 1 having the U-shaped bend is heated by filling the inside of the pyrolysis furnace 5 (= the cavity between the surface of the U-shaped cylindrical pyrolysis section 1 having the bend and the inner wall of the pyrolysis furnace 5) with a heat transfer medium. Then, the shortest distance L that passes through the inside of the pyrolysis path L from the supply port (for example, opening 2, or hopper 9 if a melting device 4 is provided) through the pyrolysis section 1 to the discharge port 3. mini The shortest distance L from the supply port side, which is 15% of the way from the supply port side. mini The external ambient temperature T is outside the thermal decomposition path at point . 15% And, the shortest distance L from the supply port side is 75% mini The ambient temperature T is outside the thermal decomposition path L at point L. 75% The relationship is as follows (2): |(T 15% -T 75% )|≧5 It is preferable that the following conditions be met. In the configuration shown in Figure 2, similar to Figure 1, the path from the opening 2 through the pyrolysis section 1 to the outlet 3, through which the raw material is converted into pyrolysis gas, is defined as the pyrolysis path L. Furthermore, the external ambient temperature T of the pyrolysis section 1 in Figure 2. 15% The shortest distance L in a U-shape is mini When the whole is considered 100%, the shortest distance L in the U-shape miniThe shortest distance L from the opening 2 side is 15%. 15% At this point, and midway between the surface of the pyrolysis section 1 (the surface of the U-shaped cylindrical body in Figure 2) and the inner wall of the pyrolysis furnace 5, which is the shortest distance from that surface (the black dot in Figure 2), the external ambient temperature T of the pyrolysis section 1 is 15% It is stipulated as follows. Similarly, also, the shortest distance L mini When the whole is considered 100%, the shortest distance L in the U-shape mini The shortest distance L from the opening 2 side is 75%. 75% The temperature at the midpoint between the surface of the pyrolysis unit 1 (the surface of the U-shaped cylindrical body in Figure 2) and the inner wall of the pyrolysis furnace 5, which is the shortest distance from that surface (the black dot in Figure 2), is defined as the external ambient temperature T of the pyrolysis unit 1. 75% It is stipulated as follows.

[0038] Another example of the pyrolysis apparatus 10 of this embodiment is the configuration shown in Figure 3. Specifically, the pyrolysis apparatus 10 has a cylindrical pyrolysis section 1 having an S-shaped bend, with an opening 2 and an outlet 3 at its ends, and a pyrolysis furnace 5 surrounding the pyrolysis section 1. The cylindrical pyrolysis section 1 having an S-shaped bend is heated by filling the inside of the pyrolysis furnace 5 (= the space between the surface of the S-shaped cylindrical pyrolysis section 1 having the bend and the inner wall of the pyrolysis furnace 5) with a heat transfer medium. When a raw material composition (not shown) is supplied from the supply unit 4 to the opening 2, the raw material composition moves along the process flow direction (arrow direction) inside the cylindrical pyrolysis unit 1 heated by the pyrolysis furnace 5, and is converted into pyrolysis gas. The pyrolysis gas, in which the raw material composition has been pyrolyzed, is then discharged to the outside from the outlet 3. Then, the shortest distance L that passes through the inside of the pyrolysis path L from the supply port (for example, opening 2, or hopper 9 if a melting device 4 is provided) through the pyrolysis section 1 to the discharge port 3. mini The shortest distance L from the supply port side, which is 15% of the way from the supply port side. mini The external ambient temperature T is outside the thermal decomposition path at point . 15% And, the shortest distance L from the supply port side is 75% mini The ambient temperature T is outside the thermal decomposition path L at point L.75% The relationship is as follows (2): |(T 15% -T 75% )|≧5 It is preferable that the following conditions be met.

[0039] In the configuration shown in Figure 3, similar to Figure 1, if we define the path from the opening 2 through the pyrolysis section 1 to the outlet 3, where the raw material is converted into pyrolysis gas, as the pyrolysis path L, then the shortest distance from the opening 2 through the pyrolysis section 1 to the outlet 3 is L. mini This corresponds to the straight dotted line shown in Figure 3. Furthermore, the external ambient temperature T of the pyrolysis section 1 in Figure 3. 15% The shortest distance L in an S-shape is mini When the whole is considered 100%, the shortest S-shaped distance L mini The shortest distance L from the opening 2 side is 15%. 15% At this point, and midway between the surface of the pyrolysis section 1 (the surface of the S-shaped cylindrical body in Figure 3) and the inner wall of the pyrolysis furnace 5, which is the shortest distance from that surface (the black dot in Figure 3), the external ambient temperature T of the pyrolysis section 1 is 15% It is stipulated as follows. Similarly, the shortest distance L mini When the whole is considered 100%, the shortest S-shaped distance L mini The shortest distance L from the opening 2 side is 75%. 75% The temperature at the midpoint between the surface of the pyrolysis unit 1 (the surface of the S-shaped cylindrical body in Figure 3) and the inner wall of the pyrolysis furnace 5, which is the shortest distance from that surface (the black dot in Figure 3), is defined as the external ambient temperature T of the pyrolysis unit 1. 75% It is stipulated as follows.

[0040] As an example of another embodiment of the pyrolysis apparatus 10 of this embodiment, the apparatus (continuous type) shown in Figures 5 to 7 can be cited. In particular, the pyrolysis apparatus 10 shown in Figures 5 to 7 is equipped with a mechanism for sending a polystyrene resin composition containing styrene monomer units, which is the raw material composition, to the pyrolysis furnace 5 side, or for sending gas and other components in the pyrolysis furnace 5 to the discharge port 3 side. As a result, the amount of styrene monomer produced or recycled over a predetermined period is greater than that of the pyrolysis apparatus 10 described in Figures 1 to 3. Consequently, there is an advantage in being able to increase the amount of residue discharged from the pyrolysis apparatus 10. The following describes each pyrolysis apparatus 10 shown in Figures 5 to 7. In other words, the pyrolysis apparatus 10 shown in Figure 5 has a cylindrical pyrolysis section 1 with an opening 2 and an outlet 3 at each end, and a space protruding to the outside between the opening 2 and the outlet 3; a pyrolysis furnace 5 surrounding the cylindrical pyrolysis section 1; and a melting device 4 equipped with a hopper section 9 for supplying the raw material composition, a cylindrical melting section 4A for melting the raw material composition, and a screw section (the sawtooth-shaped section on the upper left side in the figure) for sending the molten raw material composition to the pyrolysis furnace 5. The cylindrical melting section 4A (or the same as the cylindrical cylinder 6 shown in Figure 6) and the cylindrical pyrolysis section 1 are connected so as to communicate with each other. Furthermore, the pyrolysis apparatus 10 shown in Figure 5 may also have an opening 11 for discharging residue etc. sent from the melting device 4 or residue etc. generated when the raw material composition is pyrolyzed in the pyrolysis furnace 5, as shown in Figure 5. Furthermore, the opening 11 may be provided with an extrusion device 12 equipped with a screw section (sawtooth-shaped section in the lower right of the figure) for sending the residue and the like towards the residue discharge port 13. This screw section for sending the residue and the like towards the residue discharge port 13 may be the same as the cylindrical cylinder 6 shown in Figure 6 or the screw section or cylinder provided in the melting device 4. Figures 4 and 5 show the cylindrical melting section 4A and the cylindrical pyrolysis section 1 separated for ease of explanation. The melting section 4A is composed of, for example, a cylindrical cylinder through which the screw section is inserted (corresponding to the cylinder in Figure 5 or Figure 6) and a heater wound around the surface of the cylindrical cylinder. Figure 5 shows an example of a melting apparatus 4 equipped with a screw section (the sawtooth-shaped section in the upper left of the figure) that feeds the raw material composition to the pyrolysis section 5, as a particularly preferred configuration. However, if the raw material composition is a fluid, the melting apparatus 4 does not need to be equipped with a screw section.

[0041] When the pyrolysis apparatus 10 shown in Figure 5 is equipped with a melting apparatus 4, the pyrolysis furnace 5 and the melting apparatus 4 are connected such that the internal space of the cylindrical cylinder 6 and the internal space of the cylindrical pyrolysis section 1 are in communication, as shown in Figure 7 and other figures described later. That is, Figure 7, described later, shows how the end of the cylindrical cylinder 6 on the side opposite to the hopper section 9 and the opening 2 of the cylindrical pyrolysis section 1 are connected so that the internal space of the cylindrical cylinder 6 and the internal space of the cylindrical pyrolysis section 1 are in communication (dotted line). Furthermore, although Figure 5 illustrates an example in which the cylindrical pyrolysis section 1 and the melting apparatus 4 are connected in the pyrolysis apparatus 10 shown in Figure 4, the pyrolysis apparatus 10 in Figures 1 to 3 may also have a melting apparatus 4, similar to Figures 5 to 7. Therefore, a preferred example of the pyrolysis apparatus 10 of this embodiment includes a cylindrical pyrolysis section 1 with an opening 2 and an outlet 3 at its ends, a pyrolysis furnace 5 surrounding the cylindrical pyrolysis section 1, a hopper section 9 for supplying the raw material composition, and a melting apparatus 4 equipped with a cylindrical melting section 4A for melting the raw material composition, with the cylindrical melting section 4A and the cylindrical pyrolysis section 1 connected to each other. Since the pyrolysis apparatus 10 has a melting apparatus 4, it becomes easier to maintain the atmosphere inside the melting section 4A at a temperature of 300°C or higher. This makes it easier to supply polystyrene resin compositions containing styrene monomer units, particularly polystyrene resin compositions containing conjugated diene monomer units and styrene monomer units, to an atmosphere heated to 300°C or higher.

[0042] As the melting device 4, it is preferable to use a known extruder, a mechanism capable of heating the raw material composition to 100°C or higher (such as a hot plate, electric heating wire, or hot air furnace), or a known devolatilization device. If the pyrolysis device 10 has a melting device 4 that heats the raw material composition to 100°C or higher, the raw material composition softens and can be stably supplied to the pyrolysis section 1 through the opening 2. The melting device 4 preferably has a mechanism (such as an extruder or pump equipped with a screw) that can pump the raw material composition heated to 100°C or higher. For example, if the melting device 4 is provided, the composition softened by the heat of the melting device 4 can be stably supplied to the pyrolysis section 1 through the opening 2. In the case where both the melting apparatus 4 and the pyrolysis section 1 have extruders equipped with tubular or screw-type pipes, both the melting apparatus 4 and the pyrolysis section 1 may consist of a common pipe (for example, a cylinder 6 and a cylindrical pyrolysis section 1) or screw, or they may consist of separate pipes or screws. For example, when the melting apparatus 4 and the pyrolysis section 1 consist of a common pipe or screw, it is advantageous in terms of manufacturing costs compared to when they consist of different pipes or screws, and also advantageous in terms of maintainability and safety due to the reduction in connection parts or drive parts. Furthermore, when the melting apparatus 4 and the pyrolysis section 1 consist of different pipes or screws, it is advantageous that different materials and shapes can be selected in terms of (first) pyrolysis atmosphere temperature and pressure resistance in each section.

[0043] Furthermore, the pyrolysis apparatus 10 shown in Figure 5 comprises a cylindrical pyrolysis section 1 having an opening 2 and an outlet 3 at each end, and a space protruding to the outside between the opening 2 and the outlet 3; a pyrolysis furnace 5 surrounding the pyrolysis section 1; an opening 11; a residue outlet 13 fluid-connected to the opening 11; and an extrusion device 12 equipped with a screw section (sawtooth-shaped section in the lower right of the figure) for sending residue to the residue outlet 13. In other words, the cylindrical pyrolysis section 1 shown in Figure 5 has a structure in which a recessed container, such as a reaction vessel, and a cylindrical body are joined so that their internal spaces are connected. In addition, the residue outlet 13 fluid-connected by the opening 11 may have a structure that allows it to be opened and closed as needed by attaching, for example, a sliding gate to the opening 11. This prevents the inflow of the raw material composition into the residue outlet 13 by closing the sliding gate until a certain amount of residue accumulates in the pyrolysis apparatus 10. Furthermore, if the residue can be discharged to the residue discharge port 13, the extrusion device 12 can be omitted. The cylindrical pyrolysis section 1 shown in Figure 5, like those in Figures 2-4, has a cavity between the pyrolysis furnace wall 5A surrounding the pyrolysis section 1 and the cylindrical pyrolysis section 1, and various heating means are provided within this cavity. One example of such a heating means is a heating means that, for example, supplies a heat transfer medium (heating gas) into the cavity in the pyrolysis furnace 5, thereby heating the cylindrical pyrolysis section 1 and controlling it to a predetermined temperature range. Furthermore, the extrusion device 12, which is equipped with a screw section (sawtooth-shaped section in the lower right of the figure) for sending residue and other materials to the residue discharge port 13, is preferably composed of a cylindrical body and a screw section (sawtooth-shaped section in the lower right of the figure) that passes through the cylindrical body, similar to the melting device 4, and may be equipped with a heating mechanism similar to the melting device 4 if necessary.

[0044] Furthermore, in the example shown in Figure 5, as an example of the pyrolysis section 1, an example of a structure in which a cylindrical container (omitted) having an opening and a cylindrical body are joined so as to communicate their internal spaces with each other is shown. The shape of the bottom surface of the (omitted) cylindrical body is preferably circular. The shape of the opening of the cylindrical body is also preferably circular. And, regarding the relationship between the bottom surface and the opening, it is preferable that both the opening of the cylindrical body and the bottom surface of the cylindrical body are circular, and that the circular center of the opening of the cylindrical body and the circular center of the bottom surface of the cylindrical body are coaxial centers. Figure 5 also shows an example of a preferred configuration in which a rotating shaft 8 with the stirring blades 7 attached is provided on the coaxial centerline. A rotary motor (not shown) may be attached to the rotating shaft 8 to provide power for rotating the rotating shaft 8 with the stirring blades 7 attached, and fixed to the rotating shaft 8.

[0045] In the example of the pyrolysis apparatus 10 shown in Figure 5, when the raw material composition is supplied to the hopper section 9 while the melting section 4A is kept at an atmosphere heated to 300°C or higher, the raw material composition moves toward the pyrolysis section 1 as its fluidity increases. When the molten raw material composition is transported into the pyrolysis section 1 from the opening 2, it fills a recess (cylindrical container) that houses a rotating shaft 8 to which stirring blades 7 are attached. Next, the molten raw material composition is heated while being stirred within the recess (cylindrical container), circulating within the recess (cylindrical container), and converted into pyrolysis gas, which is then discharged to the outside from the outlet 3 as pyrolysis gas. On the other hand, an opening 11 is provided for discharging residues etc. sent from the melting apparatus 4 or residues etc. generated when the raw material composition is pyrolyzed in the pyrolysis furnace 5. Therefore, the aforementioned residue is discharged through the opening 11 via a residue discharge port 13, which is fluidly connected to the opening 11, by an extrusion device 12 equipped with a screw section (sawtooth-shaped section in the lower right of the figure). Therefore, the amount of residue discharged from the pyrolysis apparatus can be increased, and the amount of styrene monomer produced or recycled over a predetermined period can be increased. In particular, when a large amount of rubber components having a conjugated diene structure are included, the presence of unsaturated bonds necessitates the cleavage of multiple bonds, making decomposition difficult and resulting in a tendency for residue to be generated due to factors such as the formation of by-products. However, the presence of a mechanism to discharge the residue to the residue outlet 13 allows the temperature inside the pyrolysis furnace 5 to be kept constant without reducing the heat transfer rate due to the accumulation of residue inside the pyrolysis furnace 5. Furthermore, the residue outlet 13, which is fluid-connected by the opening 11, may be equipped with a structure that can be opened and closed as needed by, for example, attaching a sliding gate to the opening 11. This prevents the inflow of the raw material composition into the residue outlet 13 by closing the sliding gate until a certain amount of residue accumulates in the pyrolysis apparatus 10. Also, if the residue can be sent to the residue outlet 13, the extruder 12 can be omitted. Then, the shortest distance L that passes through the inside of the pyrolysis path L from the supply port (for example, opening 2, or hopper 9 if a melting device 4 is provided) through the pyrolysis section 1 to the discharge port 3. mini The shortest distance L from the supply port side, which is 15% of the way from the supply port side. mini The external ambient temperature T is outside the thermal decomposition path at point . 15% And, the shortest distance L from the supply port side is 75% mini The ambient temperature T is outside the thermal decomposition path L at point L. 75% The relationship is as follows (2): |(T 15% -T 75% )|≧5 It is preferable that the following conditions be met.

[0046] In the configuration shown in Figure 5, similar to Figure 1, if we define the path from the opening 2 through the pyrolysis section 1 to the outlet 3, where the raw material composition is converted into pyrolysis gas, as the pyrolysis path L, then the pyrolysis path L traces a curve from the opening 2 along the inner wall of the recess (cylindrical container) to the outlet 3. Of the pyrolysis path L, the shortest distance from the opening 2 through the pyrolysis section 1 to the outlet 3 is defined as L.mini This is a straight line as shown in Figure 5. External atmosphere temperature T in the pyrolysis section 1 in Figure 5 15% The shortest distance L mini If the whole is considered 100%, the shortest distance L mini The shortest distance L from the opening 2 side is 15%. 15% The external ambient temperature T of the pyrolysis unit 1 at the midpoint between the surface of the pyrolysis unit 1 (the surface of the cylindrical body in Figure 5) and the inner wall of the pyrolysis furnace 5, which is the shortest distance from that surface (the black dot in Figure 5) 15% It is stipulated as follows. Similarly, the shortest distance L mini If the whole is considered 100%, the shortest distance L mini The shortest distance L from the opening 2 side is 75%. 75% The temperature at the midpoint between the surface of the pyrolysis unit 1 (the surface of the cylindrical body in Figure 5) and the inner wall of the pyrolysis furnace 5, which is the shortest distance from that surface (the black dot in Figure 5), is defined as the external ambient temperature T of the pyrolysis unit 1. 75% It is stipulated as follows.

[0047] Another example of the pyrolysis apparatus 10 of this embodiment is the configuration shown in Figure 6. Specifically, the pyrolysis apparatus 10 comprises a cylindrical pyrolysis section 1 with an outlet 3 at its end, a pyrolysis furnace 5 surrounding the cylindrical pyrolysis section 1, a hopper section 9 for supplying the raw material composition, a screw section (sawtooth-shaped section in the figure) for sending the raw material composition towards the outlet 3, an opening 11, a residue discharge port 13 fluid-connected to the opening 11, and an extrusion device 12 equipped with a screw section (sawtooth-shaped section in the lower right of the figure) for sending residue to the residue discharge port 13. In other words, the configuration of the pyrolysis apparatus 10 shown in Figure 6 is a structure in which a melting device 4, a cylindrical pyrolysis section 1 with an outlet 3 at its end, and a pyrolysis furnace 5 surrounding the cylindrical pyrolysis section 1 are integrated. In other words, as shown in Figure 7, a cylindrical pyrolysis unit 1 with discharge ports 3 at each end, a pyrolysis furnace 5 surrounding the cylindrical pyrolysis unit 1, a hopper unit 9 for supplying the raw material composition into the pyrolysis unit 1, a cylindrical melting unit for melting the raw material composition, and a screw unit (sawtooth-shaped part in the figure) for sending the molten raw material composition to the pyrolysis unit 5 are integrated into a single melting device 4, and the cylindrical melting unit and the cylindrical pyrolysis unit 1 are connected so as to be in communication with each other.

[0048] In the example of the pyrolysis furnace 5 shown in Figures 6 and 7, the pyrolysis furnace wall covers the tubular pyrolysis section 1, and there is a cavity between the pyrolysis furnace wall (corresponding to pyrolysis furnace wall 5A in Figure 6) and the tubular pyrolysis section 1, with various heating means provided within the cavity. As an example of such heating means, an electric heater or a heat transfer medium (such as a heating gas) is supplied into the cavity of the pyrolysis furnace 5 to heat the tubular pyrolysis section 1, and a heating means that can be controlled to a predetermined temperature range is provided. Furthermore, in the example of the pyrolysis furnace 5 shown in Figures 6 and 7, an opening 11 is provided for discharging residues and other materials generated when the raw material composition is pyrolyzed within the pyrolysis furnace 5.

[0049] When the raw material composition is supplied from the hopper section 9, it is melted in the cylindrical pyrolysis section 1, which is also the melting apparatus 4 directly below the hopper section 9 and maintained in an atmosphere heated to over 300°C. The raw material composition moves along the process flow direction (arrow direction) and is converted into pyrolysis gas, which is then discharged to the outside from the outlet 3 as pyrolysis gas. Furthermore, in the configuration of the pyrolysis apparatus 10 shown in Figures 7 and 8, a screw section (sawtooth-shaped section in the figure) is provided to send the raw material composition to the pyrolysis section in the pyrolysis furnace 5, so that the pyrolysis gas produced from the pyrolysis of the raw material composition can be easily moved to the outlet 3. On the other hand, there is an opening 11 for discharging residues sent from the melting apparatus 4 or residues generated when the raw material composition is thermally decomposed in the pyrolysis furnace 5. Therefore, the aforementioned residues are discharged through the opening 11 to a residue discharge port 13 which is fluidly connected to the opening 11 by an extrusion apparatus 12 equipped with a screw section (sawtooth-shaped section in the lower right of the figure). Therefore, the amount of residue discharged from the pyrolysis apparatus can be increased, and the amount of styrene monomer produced or recycled over a predetermined period can be increased. In particular, when a large amount of rubber components having a conjugated diene structure are present, decomposition is difficult because it is necessary to break the multiple bonds due to the presence of unsaturated bonds, and residue tends to be generated due to factors such as the formation of by-products. However, the presence of a mechanism to discharge the residue to the residue outlet 13 makes it possible to maintain a constant temperature inside the pyrolysis furnace 5 without reducing the heat transfer rate due to the accumulation of residue inside the pyrolysis furnace 5. Then, the shortest distance L that passes through the inside of the pyrolysis path L from the supply port (hopper 9) through the pyrolysis section 1 to the discharge port 3. mini The shortest distance L from the supply port side, which is 15% of the way from the supply port side. mini The external ambient temperature T is outside the thermal decomposition path at point . 15% And, the shortest distance L from the supply port side is 75% mini The ambient temperature T is outside the thermal decomposition path L at point L. 75% The relationship is as follows (2): |(T 15% -T 75% )|≧5 It is preferable that the following conditions be met. In the configurations shown in Figures 6 and 7, similar to Figure 1, if we define the path from the hopper section 9 corresponding to the opening 2 through the pyrolysis section 1 to the discharge port 3, where the raw material composition is converted into pyrolysis gas, as the pyrolysis path L, then the shortest distance from the hopper section 9 corresponding to the opening 2 through the pyrolysis section 1 to the discharge port 3 is L. mini This corresponds to the straight dotted line shown in Figures 6 and 7. External atmosphere temperature T of the pyrolysis section 1 in Figure 6 15% The shortest distance L mini If the whole is considered 100%, the shortest distance L mini The shortest distance L is 15% from directly below the hopper section 9. 15% At this point, and midway between the surface of the pyrolysis section 1 (the surface of the cylindrical body in Figure 6) and the inner wall of the pyrolysis furnace 5 at the shortest distance from that surface (the black dot in Figure 6), the external ambient temperature T of the pyrolysis section 1 is 15% It is stipulated as follows. Similarly, the shortest distance Lmini If the whole is considered 100%, the shortest distance L mini The shortest distance L is 75% from directly below the hopper section 9. 75% The temperature at the midpoint between the surface of the pyrolysis unit 1 (the surface of the cylindrical body in Figure 6) and the inner wall of the pyrolysis furnace 5, which is the shortest distance from that surface (the black dot in Figure 6), is defined as the external ambient temperature T of the pyrolysis unit 1. 75% It is stipulated as follows.

[0050] Furthermore, in Figures 1 to 7, the pyrolysis apparatus 10 can have a known temperature control mechanism, a known pressure control mechanism, a screw for transporting the raw material composition, a hopper section 9 which is also the opening 2, a heater, etc., installed in any desired position. In this embodiment, it is preferable to transfer the pyrolysis products of the raw material composition from the time the raw material composition is supplied to the pyrolysis unit 1 until the first pyrolysis liquid is obtained from the pyrolysis unit 1 without using an external pumping means. This improves the energy efficiency of the entire styrene monomer manufacturing process, thereby reducing the environmental impact. In the pyrolysis apparatus 10 shown in Figures 1 to 7, if the priority is to further reduce the amount of residue generated by the pyrolysis of polystyrene resin and to recover styrene monomers in a higher yield, it is preferable to use the pyrolysis apparatus 10 shown in Figures 1 to 3. On the other hand, if the priority is to balance the reduction of the amount of residue generated by the pyrolysis of polystyrene resin, a high yield of styrene monomers, and a large amount of styrene monomers produced or recycled over a predetermined period, it is preferable to use the pyrolysis apparatus 10 shown in Figures 5 to 7.

[0051] The pyrolysis apparatus 10 used for producing styrene monomers in this embodiment preferably includes an opening 2 for supplying a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units; a pyrolysis section 1 for heating and pyrolyzing the supplied polystyrene resin composition to 300 to 1200°C or higher; an outlet 3 for discharging pyrolysis gas containing styrene monomers obtained by pyrolysis of the polystyrene resin composition; and a cooling means for cooling the pyrolysis gas to prepare a first pyrolysis liquid. In the conventional technology described in Non-Patent Document 1 above, the generated carbide residue adheres and accumulates inside the decomposition apparatus during the thermal decomposition process, reducing heat transfer efficiency. This presents potential challenges in terms of equipment size, continuous operation, and maintenance frequency. Furthermore, it has been confirmed that the yield of styrene monomers obtained by the thermal decomposition of polystyrene resin compositions containing styrene monomer units, particularly polystyrene resin compositions containing conjugated diene monomer units and styrene monomer units, is low.

[0052] <Preferred method for producing styrene monomers> A preferred method for producing styrene monomers in this embodiment preferably comprises a series of first thermal decomposition steps using a thermal decomposition apparatus 10, in which a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units (for example, a raw material composition), is supplied to an opening 2, and the polystyrene resin composition is thermally decomposed by a thermal decomposition section to convert it into a thermal decomposition gas containing styrene monomers, and the thermal decomposition gas is discharged from an outlet. As described above, in the pyrolysis apparatus 10 of this embodiment, the raw material composition is supplied from the opening 2 or hopper 9, converted into a pyrolysis gas containing styrene monomers that have been pyrolyzed by the pyrolysis unit 1, and then discharged as the pyrolysis gas from the outlet 3. If the path from the opening 2 or hopper 9 through the pyrolysis unit 1 to the outlet 3 is defined as the pyrolysis path L, then the shortest distance L that passes through the inside of the pyrolysis path L from the opening 2 through the pyrolysis unit 1 to the outlet 3 is... miniAt the shortest distance L of 15% from the opening 2 or the hopper 9 side mini The external ambient temperature T outside the thermal decomposition path L at the point 15% and the shortest distance L of 75% from the opening 2 or the hopper 9 side mini The ambient temperature T outside the thermal decomposition path L at the point 75% and satisfy the following relational expression (2): |(T 15% - T 75% )| ≧ 5 It is preferable to satisfy it.

[0053] In the thermal decomposition path L in the thermal decomposition apparatus 10, by controlling to the predetermined temperature distribution shown by the above relational expression (2), a polystyrene-based resin composition containing a styrene monomer unit, particularly a polystyrene-based resin composition containing a conjugated diene monomer unit and a styrene monomer unit is thermally decomposed, and the styrene monomer is converted into the styrene monomer with a higher yield, and at the same time, the generation of thermal decomposition residues generated by the thermal decomposition of a polystyrene-based resin composition containing a styrene monomer unit, particularly a polystyrene-based resin composition containing a conjugated diene monomer unit and a styrene monomer unit can be further reduced. In the thermal decomposition path L in the thermal decomposition apparatus 10, by applying the thermal history of the predetermined temperature distribution shown by the above relational expression (2) to the raw material composition, the thermal decomposition reaction can be further promoted, the amount of thermal decomposition residues generated by thermal decomposition can be further reduced, and it is considered that the styrene monomer can be recovered in a high yield.

[0054] The temperature difference of the said |(T 15% - T 75% )| is more preferably 6 ° C or more and less than 150 ° C, still more preferably 10 ° C or more and less than 150 ° C, even more preferably 20 ° C or more and less than 130 ° C, and even more preferably 25 ° C or more and less than 120 ° C.

[0055] In the present embodiment, the molten polystyrene-based resin composition and / or the thermal decomposition product of the polystyrene-based resin composition pass through the cylindrical thermal decomposition section 1 which is the thermal decomposition path L, and are transferred from the opening 2 of the thermal decomposition section (or directly below the hopper section 9) to the discharge port 3 through the thermal decomposition section 1. The shortest distance L from the opening 2 side, which is 15% of the way away. mini The outer surface temperature T on the gravity direction side of the cylindrical pyrolysis section 1 at point VdS15% The outer surface temperature T of the cylindrical pyrolysis section 1 on the top side facing the direction of gravity VuS15% The difference between the two is given by the following relationship (3): |(T VdS15% -T VuS15% )|≧0 It is preferable that the following conditions be met. In the cylindrical pyrolysis section 1, the molten polystyrene resin composition or the pyrolysis product of the polystyrene resin composition accumulates in contact with the cylindrical pyrolysis section 1 on the gravity side, but tends not to accumulate or accumulate in contact with the top surface of the cylindrical pyrolysis section 1. Furthermore, because the molten polystyrene resin composition or the polystyrene resin composition has an endothermic effect, a temperature difference occurs throughout the cylindrical pyrolysis section 1 between the gravity side (bottom surface) that is in contact with the molten polystyrene resin composition or the pyrolysis product of the polystyrene resin composition, and the top surface that is not in contact. By controlling this temperature difference within a predetermined range, the yield of styrene monomer can be further improved. Therefore, Tb > T VdS15% The relationship, T VuS75% ≧T VuS15% The relationship, and T VdS75% ≧T VdS15% A tendency to show a relationship between Tb and T is observed. VuS The curve tends to be convex upwards with respect to the distance from the supply port, T VdS T is a distance from the supply port. VdS0% More than T VdS75% The following shows a tendency towards an upward-convex curve relationship. Said “T VdS " represents the outer surface temperature on the gravity side of the cylindrical pyrolysis section 1 (in other words, the temperature of the outer wall surface at the bottom of the cylindrical body which is the pyrolysis section 1), and the above "T VuS " represents the outer surface temperature on the top side of the cylindrical pyrolysis section 1 (in other words, the temperature of the outer wall surface at the top of the cylindrical body which is the pyrolysis section 1). Said (T VdS15% -T VuS15%The temperature difference is more preferably 1°C or more and less than 110°C, even more preferably 10°C or more and less than 100°C, even more preferably 20°C or more and less than 90°C, and even more preferably 25°C or more and less than 80°C.

[0056] In this embodiment, the molten polystyrene resin composition or the pyrolysis product of the polystyrene resin composition is transferred through the cylindrical pyrolysis section 1, which is the pyrolysis path L, from the opening 2 of the pyrolysis section (or directly below the hopper section 9) to the discharge port 3 via the pyrolysis section 1. 75% of the shortest distance L from the opening 2 side mini The outer surface temperature T on the gravity direction side of the cylindrical pyrolysis section 1 at point VdS75% The outer surface temperature T of the cylindrical pyrolysis section 1 on the top side facing the direction of gravity VuS75% The difference between the two is given by the following relationship (4): |(T VdS75% -T VuS75% )|≧0 It is preferable that the following conditions be met. Said|(T VdS75% -T VuS75% The temperature difference is more preferably 1°C or more and less than 220°C, even more preferably 10°C or more and less than 210°C, even more preferably 20°C or more and less than 200°C, and even more preferably 30°C or more and less than 190°C. In the cylindrical pyrolysis section 1, the molten polystyrene resin composition or the pyrolysis product of the polystyrene resin composition accumulates in contact with the cylindrical pyrolysis section 1 on the gravity side, but tends not to accumulate or accumulate in contact with the top surface of the cylindrical pyrolysis section 1. Furthermore, because the molten polystyrene resin composition or the pyrolysis product of the polystyrene resin composition has an endothermic effect, a temperature difference occurs throughout the cylindrical pyrolysis section 1 between the gravity side (bottom surface) that is in contact with the molten polystyrene resin composition or the pyrolysis product of the polystyrene resin composition, and the top surface that is not in contact. By controlling this temperature difference within a predetermined range, the yield of styrene monomer can be further improved.

[0057] Furthermore, it is preferable to decompose the styrene monomer units in the raw material composition into styrene monomers by gasifying the raw material composition in a thermal decomposition section under a low-oxygen atmosphere. More specifically, the raw material composition is separated into a fraction (gas component) containing aromatic hydrocarbons such as styrene monomers, and a residue containing a solid-phase substance called char or tar and substances derived from impurities that may be contained in the composition. A preferred pyrolysis apparatus 10 of this embodiment includes a melting apparatus 4 for melting the raw material composition, and the raw material composition can be supplied from the melting apparatus 4. Furthermore, if necessary, the melting apparatus 4 for the raw material composition may be equipped with an on-off valve to adjust the supply amount of the raw material composition supplied to the pyrolysis section. Additionally, the pyrolysis apparatus 10 may be connected, if necessary, to a transport path for transporting a gas containing styrene monomers and a transport path for transporting the pyrolysis residue, and each of these transport paths may be provided with an on-off valve to adjust the respective transport amounts. The raw material composition can be supplied into the pyrolysis apparatus 10 via the supply path by a pump or other pressurized means. For example, if the pressurized means is a vacuum pump, it becomes possible to maintain negative pressure and / or a low-oxygen state inside the pyrolysis apparatus 10, thereby suppressing unintended oxidation of the raw material composition and its decomposition products under heating conditions.

[0058] The pyrolysis apparatus 10 of this embodiment preferably includes a heating means, a pyrolysis section 1 and a pyrolysis furnace 5, and a pressure adjustment means. By using the heating means, the ambient temperature inside the pyrolysis apparatus 10 can be raised to a predetermined range. Another example of the heating means is a heat exchanger, more specifically a heat exchanger configured as a heat transfer tube, or an electric heater. In the case of a heat exchanger configured as a heat transfer tube, the (first) pyrolysis ambient temperature can be adjusted to a temperature range of 300°C or more and less than 1200°C by the heat of the high-temperature gas passing through the heat transfer tube. The temperature of the gas passing through the heat transfer tube is preferably above the set value of the internal temperature of the pyrolysis apparatus described above, and can be, for example, 300°C or higher.

[0059] On the other hand, the pressure adjustment means is not particularly limited as long as it is a mechanism for adjusting the pressure inside the pyrolysis apparatus 10. Specifically, examples include pressure regulating valves, vacuum pumps, and pumps for introducing inert gas (e.g., nitrogen gas) into the pyrolysis furnace. Furthermore, the pressure adjustment means may further include, if necessary, means for detecting the pressure inside the pyrolysis furnace and means for adjusting the amount of inert gas introduced according to the detected pressure. By adopting the above configuration for the pressure adjustment means, the pressure conditions for pyrolysis can be maintained at a pressure of 0 hPa or more and 1013 hPa or less.

[0060] In the method for producing styrene monomers in this embodiment, if necessary, a polymerization inhibitor may be supplied intermittently or continuously using a device for supplying polymerization inhibitors to suppress the polymerization reaction of polymerizable components contained in the first or second pyrolysis liquid.

[0061] In this embodiment, the first pyrolysis step (S1) and the second pyrolysis step (S4) described later may be performed using a pyrolysis catalyst to prepare the first pyrolysis liquid or the second pyrolysis liquid described later. The pyrolysis catalyst is preferably a compound that contains an element from the second to seventh period of the periodic table, from group 1 to group 17, which has a stable isotope, exists as a solid at room temperature and pressure, and has a decomposition temperature of 300°C or higher. Examples of the aforementioned pyrolysis catalysts include BaO, TiO2, MgO, Al2O3, CaO, NiO, and Y2O3.

[0062] The first pyrolysis step (S1) of this embodiment may include a cooling step of cooling a gas containing styrene monomers. In other words, when a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, is thermally decomposed using the above-mentioned pyrolysis apparatus, the pyrolysis products can be fractionated into pyrolysis product gas, residual fluids that do not gasify, and solid matter. The fractionated pyrolysis product gas can then be cooled in a cooling tower to prepare a first pyrolysis liquid. The residual fluids that do not gasify and the solid matter are either discarded or used as fuel. Furthermore, if necessary, the system may include a compressor that compresses the cooled pyrolysis product gas to generate compressed gas, a second flow path that connects the cooling tower and the compressor and supplies the pyrolysis product gas from the cooling tower to the compressor, and a first flow path that connects the pyrolysis furnace and the compressor, or connects the pyrolysis furnace and the second flow path and supplies the pyrolysis product gas from the pyrolysis furnace to the compressor or the second flow path.

[0063] Specifically, the manufacturing apparatus used in the method for producing styrene monomers in this embodiment preferably comprises a pyrolysis apparatus for pyrolyzing a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, and a pyrolysis furnace which is an electrically heated furnace or a furnace that burns combustion gas or fuel oil. Optionally, it may also include a fractionation column fluidly connected to the pyrolysis furnace and fractionally distilling the pyrolysis product gas, residual fluids and solids that do not gasify, a cooling column for cooling the pyrolysis product gas, and a distillation column for distilling the pyrolysis product gas or the first pyrolysis liquid. Furthermore, if necessary, it may further include a compressor for compressing the cooled pyrolysis product gas and a distillation column for distilling the compressed pyrolysis product gas or the first pyrolysis liquid. The distillation column may also consist of multiple distillation columns.

[0064] The cooling tower is a device for cooling the fractionally distilled pyrolysis product gas, and a known cooling tower can be used. The cooling temperature in the cooling process can usually be between -30°C and 80°C. The compressor compresses the cooled pyrolysis product gas to produce compressed gas. The compressor can be a known type of compressor, such as a centrifugal compressor, an axial flow compressor, or a reciprocating compressor.

[0065] The distillation column can fractionate the generated compressed gas or first pyrolysis liquid into a second fraction containing styrene monomers (for example, a second fraction containing styrene monomers and high-boiling-point components) and a first fraction having a lower styrene monomer concentration than the second fraction (for example, a first fraction with a lower boiling point than the second fraction). A known distillation column can be used as the distillation column, for example, a tray column or a packed column.

[0066] Furthermore, the pyrolysis gas generated in the first pyrolysis step (S1) may be cooled to a temperature between the boiling point of the styrene monomer and 180°C, the high-boiling point components may be liquefied using a liquefaction device, and the gas may be returned to the pyrolysis furnace by dropping it back into the furnace, thereby repeating the pyrolysis step. Alternatively, the generated pyrolysis gas may be liquefied by cooling it with a liquefaction device to obtain the first pyrolysis liquid. The cooling temperature is preferably between -30°C and the boiling point Tsb (°C) of the styrene monomer, and more preferably between -20°C and 80°C. In this case, any known liquefaction device can be used. For example, various heat-absorbing solvents, such as water, that can cool to the cooling temperature, or cooling tubes that utilize the heat-absorbing effect of heat-absorbing elements made of metals and other various inorganic materials can be used. If necessary, a reforming device for reforming the components of the pyrolysis gas (including, for example, dechlorination, adsorption of odor or color components, removal of acidic or basic components, and heat treatment) may be fluidly connected between the pyrolysis furnace and the liquefaction device.

[0067] <Styrene monomer recovery process> The method for producing styrene monomers according to this embodiment includes a recovery step for recovering the styrene monomers. More specifically, it includes a recovery step for recovering the styrene monomers from the first pyrolysis liquid prepared in the first pyrolysis step (S1). The phrase "recovering styrene monomers from the first pyrolysis solution" includes both directly recovering styrene monomers from the first pyrolysis solution and indirectly recovering styrene monomers by performing a third distillation step (S5) or the like on the second pyrolysis solution described later (including rectification). If necessary, pre-thermal decomposition may be performed on the first thermal decomposition liquid before or during the recovery process. In addition, optional additives may be added to the first thermal decomposition process and / or the recovery process as needed to suppress the polymerization reaction of the resulting styrene monomer.

[0068] In this embodiment, known recovery methods can be used to recover the styrene monomer. For example, the first pyrolysis liquid may be distilled or rectified and fluidly connected to a rectification column capable of separating low-boiling-point components such as benzene or toluene from crude styrene monomer (styrene monomer with a purity of 90% or less). Furthermore, the rectification column may be fluidly connected to a distillation column for rectifying the separated crude styrene monomer to increase its purity. If necessary, a dechlorination device for dechlorinating components in the pyrolysis liquid may be fluidly connected between the pyrolysis furnace and the rectification column. Furthermore, in the recovery process, the recovered material may be the first pyrolysis liquid, or the second pyrolysis liquid, sixth fraction, and third fraction described later. Additionally, if necessary, a quantitative analysis using GC-FID as described in the examples may be performed in the recovery process to measure the styrene monomer concentration and determine whether recovery is possible. In this case, it is desirable that the recovered material contains 10% by mass or more, preferably 15% by mass or more, and more preferably 20% by mass or more of styrene monomer relative to the total recovered material. The recovered material may be shipped as a product.

[0069] (Polystyrene resin composition containing styrene monomer units as raw material) As a raw material for the method of producing styrene monomers in this embodiment, the polystyrene resin composition containing styrene monomer units only needs to contain styrene monomer units, and the said polystyrene resin composition contains styrene monomer units as its main component. In this specification, "containing A as a main component" means that the polystyrene resin composition contains 40% by mass or more of A, preferably 50% by mass or more. The polystyrene resin composition may contain a styrene polymer containing styrene monomer units, preferably a styrene polymer containing styrene monomer units and conjugated diene monomer units, or a resin containing a styrene polymer containing styrene monomer units and conjugated diene monomer units. A preferred embodiment of the polystyrene resin composition in this embodiment may be a composition comprising both a styrene polymer containing styrene monomer units and a resin containing conjugated diene monomer units, or a composition comprising a styrene polymer containing both styrene monomer units and conjugated diene monomer units. The content of styrene monomer units in the polystyrene resin composition used as a raw material for the thermal decomposition product is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 55% by mass or more, and even more preferably 60% by mass or more, based on the total polystyrene resin composition. Similarly, the content of conjugated diene monomer units is preferably 0% by mass or more and less than 15.0% by mass, more preferably more than 0% by mass and less than 14.7% by mass, even more preferably 0.05% by mass or more and less than 14.5% by mass, even more preferably 0.1% by mass or more and less than 13.0% by mass, and even more preferably 0.5% by mass or more and less than 12.5% ​​by mass, based on the total composition.

[0070] The polystyrene resin mentioned above includes used, discarded, or destined materials, pre-consumer materials such as factory recovered products, post-consumer materials such as market recovered products, long-term stock pellets, or off-spec pellets. The styrene resin composition may also contain additives such as phosphorus-based flame retardants, liquid paraffin, stabilizers, or colorants.

[0071] Furthermore, the polystyrene resin composition containing the styrene monomer units, in particular the polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, may contain impurities. The aforementioned impurities may include other resins that do not substantially contain styrene monomer units, such as olefin resins, polyether resins, polyester resins, or polyamide resins. Furthermore, the composition may be a product in which these other resins are laminated, or a mixed resin obtained by mixing other resins such as olefin resins, polyether resins, polyester resins, or polyamide resins with a styrene polymer containing styrene monomer units. In another embodiment of the styrene resin composition of this example, it may be a composition mixed with celluloses such as paper (e.g., paper labels) and monomers of cellulose components, known as thermal decomposition products of cellulose; thermosetting resins such as phenolic resins, polyurethane resins, epoxy resins, or melamine resins; or a styrene polymer containing styrene monomer units. It may also contain fillers such as silicate minerals such as talc, inorganic materials such as glass, and carbon fibers, glass fibers, and cellulose fibers used in fiber-reinforced plastics. It may also contain metals such as aluminum, iron, and stainless steel. The amount of impurities contained in the polystyrene resin composition containing the styrene monomer units used as a raw material, particularly the polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, is preferably 50% by mass or less, more preferably 40% by mass, even more preferably 30% by mass, and even more preferably 20% by mass or less, relative to the total composition.

[0072] In particular, the method for producing styrene monomers in this embodiment is beneficial when using post-consumer materials that generate residue during thermal decomposition. However, even if the resin composition containing conjugated diene monomer units and styrene monomer units in this embodiment is a virgin polystyrene resin composition containing unused styrene polymers, or a pre-consumer material such as a factory recovery product, it may contain the aforementioned additives or other resins or inorganic substances mixed in during processing. Therefore, the method for producing styrene monomers in this embodiment is beneficial as a method that can produce styrene monomers from these virgin polystyrene resin compositions or pre-consumer materials while suppressing the generation of thermal decomposition residue.

[0073] The polystyrene resin composition containing styrene monomer units usable in this embodiment, particularly the polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, may be used, discarded, or destined materials. The composition only needs to contain styrene monomer units and conjugated diene monomer units. In other words, the composition is preferably a polymer having styrene monomer units, or a polymer having styrene monomer units and conjugated diene monomer units. Specifically, examples include a rubber-modified styrene resin in which rubbery polymer particles are dispersed in a polymer matrix containing polystyrene or a polystyrene-based polymer (such as a polystyrene and / or polystyrene-unsaturated carboxylic acid polymer), and a polymer having styrene monomer units and conjugated diene monomer units. The styrene polymer containing styrene monomer units contained in the polystyrene resin composition usable in this embodiment only needs to contain 50% by mass or more of styrene monomer units relative to the total styrene polymer (100% by mass), preferably 60% by mass or more, and more preferably 70% by mass or more. The upper limit of the styrene polymer containing styrene monomer units contained in the polystyrene resin composition may be less than 100% by mass or less than 99% by mass.

[0074] <Polystyrene> In this embodiment, polystyrene refers to a homopolymer of styrene monomers, or a copolymer obtained by polymerizing styrene monomers and styrene-based monomers, and any generally available polystyrene can be appropriately selected and used. Examples of styrene-based monomers include styrene, as well as α-methylstyrene, α-methyl-p-methylstyrene, ο-methylstyrene, m-methylstyrene, p-methylstyrene, vinyltoluene, ethylstyrene, isobutylstyrene, and t-butylstyrene or bromostyrene and styrene derivatives such as indene.

[0075] <Rubber-modified styrene resin> In this embodiment, the rubber-modified styrene resin is a resin in which particles of a rubbery polymer are dispersed in polystyrene or a styrene copolymer (for example, a copolymer having styrene monomer units and unsaturated carboxylic acid monomer units) as a matrix phase, and has a sea-island structure in which the matrix phase is the sea phase and the particles of the rubbery polymer (= rubbery polymer particles) are the island phases. This rubber-modified styrene resin can be produced by polymerizing styrene monomers (and unsaturated carboxylic acid monomers added as needed) in the presence of a rubbery polymer. The unsaturated carboxylic acid monomers include (meth)acrylic acid monomers and (meth)acrylic acid ester monomers.

[0076] The rubber-like polymer particles contained in the rubber-modified styrene resin of this embodiment may, for example, contain a resin containing polystyrene obtained from the above-mentioned styrene monomer inside the rubber-like polymer particles, and / or have a resin containing styrene monomer units grafted onto the surface of the rubber-like polymer particles. More specifically, the rubber-like polymer particles may also include a form in which polystyrene and / or a styrene copolymer such as a styrene-unsaturated carboxylic acid copolymer is contained within. Similarly, polystyrene and / or a polystyrene-unsaturated carboxylic acid polymer may be grafted onto the surface of the rubber-like polymer particles.

[0077] As the rubbery polymer, for example, rubber components such as polybutadiene, polyisoprene, natural rubber, polychloroprene, styrene-butadiene copolymer, and acrylonitrile-butadiene copolymer can be used. Among these, polybutadiene or styrene-butadiene copolymer is preferred as the rubbery polymer. For polybutadiene, both high-cis polybutadiene with a high cis content and low-cis polybutadiene with a low cis content can be used. Furthermore, both random and block structures can be used for the styrene-butadiene copolymer. One or more of these rubbery polymers can be used. Also, saturated rubber obtained by hydrogenating butadiene-based rubber can be used.

[0078] Examples of such rubber-modified styrene resins include HIPS (high-impact polystyrene), ABS resin (acrylonitrile-butadiene-styrene copolymer), AAS resin (acrylonitrile-acrylic rubber-styrene copolymer), and AES resin (acrylonitrile-ethylene propylene rubber-styrene copolymer).

[0079] When the rubber-modified styrene resin is a HIPS resin, among these rubbery polymers, high-cis polybutadiene composed of 90 mol% or more cis-1,4 bonds is particularly preferred. In the high-cis polybutadiene, it is preferable that vinyl-1,2 bonds are composed of 6 mol% or less, and particularly preferable that they be composed of 3 mol% or less.

[0080] The content of isomers of the above-mentioned high-cis polybutadiene that have a cis-1,4 structure, a trans-1,4 structure, or a vinyl-1,2 structure can be calculated by measuring with an infrared spectrophotometer and processing the data using the Morello method. Furthermore, the above-mentioned high-cis polybutadiene can be easily obtained by polymerizing 1,3-butadiene using a known production method, for example, a catalyst containing an organoaluminum compound and a cobalt or nickel compound.

[0081] In this embodiment, the content of rubber-modified styrene resin in the polystyrene resin composition used as a raw material for the pyrolysis product is preferably 0% by mass or more and 100.0% by mass or less, more preferably more than 0% by mass and less than 100% by mass, even more preferably 0.05% by mass or more and less than 90% by mass, even more preferably 0.1% by mass or more and less than 85% by mass, and even more preferably 0.5% by mass or more and less than 80% by mass.

[0082] The content of the rubber-like polymer in the rubber-modified styrene resin is preferably 3 to 20% by mass, and more preferably 5 to 15% by mass, based on 100% by mass of the total amount of the rubber-modified styrene resin. If the content of the rubber-like polymer in the rubber-modified styrene resin is less than 3% by mass, the impact resistance of the styrene resin may decrease. Furthermore, if the content of the rubber-like polymer exceeds 20% by mass, the flame retardancy may decrease. In this disclosure, the content of rubbery polymer in the rubber-modified styrene resin is a value calculated using pyrolysis gas chromatography.

[0083] The average particle size of the rubbery polymer particles contained in the rubber-modified styrene resin is preferably 0.5 to 4.0 μm, and more preferably 0.8 to 3.5 μm, from the viewpoint of impact resistance and flame retardancy.

[0084] In this disclosure, the average particle size of rubbery polymer particles contained in the rubber-modified styrene resin can be measured by the following method. Ultrathin sections with a thickness of 75 nm were prepared from a rubber-modified styrene resin stained with osmium tetroxide, and photographs were taken at a magnification of 10,000x using an electron microscope. In the photograph, the black-stained particles are the rubbery polymer (a). From the photograph, the following formula (N1) was derived: [Mathematics 1] Average particle diameter=ΣniDri 3 / ΣniDri 2 (N1) (In the above formula (N1), ni is the number of rubbery polymer particles with particle size Dri, and particle size Dri is the particle size calculated as the equivalent circle diameter from the area of ​​the particles in the photograph.) The area-average particle diameter is calculated and used as the average particle diameter of the rubbery polymer particles. This measurement is performed by scanning a photograph at a resolution of 200 dpi and measuring it using particle analysis software on the image analysis device IP-1000 (manufactured by Asahi Kasei Corporation).

[0085] The reduced viscosity of the rubber-modified styrene resin (which serves as an indicator of the molecular weight of the rubber-modified styrene resin) is preferably in the range of 0.50 to 0.85 dL / g, and more preferably in the range of 0.55 to 0.80 dL / g. In this disclosure, the reduced viscosity of the rubber-modified styrene resin is the value measured in a toluene solution at 30°C and a concentration of 0.5 g / dL.

[0086] <Polymers containing styrene monomer units and conjugated diene monomer units> Examples of polymers having styrene monomer units and conjugated diene monomer units in this embodiment include styrene-butadiene copolymer (SBR), styrene-butadiene-styrene (SBS) block copolymer, styrene-ethylene-butylene-styrene (SEBS) block copolymer, and styrene-isoprene block copolymer. In particular, conjugated diene structures, due to the presence of carbon-carbon double bonds, are less susceptible to thermal decomposition than single-bonded structures, and tend to leave behind more thermal decomposition residue.

[0087] <Optional addition ingredients> In this embodiment, a polystyrene resin composition containing styrene monomer units, which are the raw materials, and in particular a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, may contain, in addition to the above components, any conventionally known additives, processing aids, and other optional additives as needed, to the extent that they do not impair the effects of the present invention. Examples of such additives, processing aids, etc., include antioxidants, weathering agents, lubricants, antistatic agents, and fillers.

[0088] Examples of the above-mentioned antioxidants include phenolic compounds, phosphorus compounds, and thioether compounds. As the weather-resistant agent mentioned above, ultraviolet absorbers and the like can be used. As the above lubricant, fatty acid amides, fatty acid esters, fatty acids, fatty acid metal salts, etc., can be used. As the above-mentioned antistatic agent, cationic, anionic, nonionic, amphoteric, and fatty acid partial esters such as glycerol fatty acid monoesters can be used. Examples of fillers that can be used include talc, calcium carbonate, barium sulfate, carbon fiber, glass fiber, cellulose fiber, mica, wollastonite, and whiskers. The mixture in this embodiment may also contain the above-mentioned additives and processing aids, as well as optional additives such as blocking inhibitors, colorants, blooming inhibitors, surface treatment agents, antibacterial agents, and eye discharge inhibitors (eye discharge inhibitors such as silicone oil described in Japanese Patent Application Publication No. 2009-120717, monoamide compounds of higher aliphatic carboxylic acids, and monoester compounds obtained by reacting higher aliphatic carboxylic acids with monovalent to trivalent alcohol compounds). The total content of optional additives such as additives and processing aids may be 0.05 to 5% by mass of the mixture. The above describes the main first thermal decomposition step and recovery step of the method for producing styrene monomer according to this embodiment. Hereinafter, an overview of a preferred embodiment of the method for producing styrene monomer according to this disclosure will be described with reference to Figure 8, and then each step other than the first thermal decomposition step and recovery step will be described in detail below.

[0089] [Preferred manufacturing method for styrene monomers] As shown in Figure 8, one example of a preferred method for producing the styrene monomer of this disclosure is to provide a raw material preparation step (P1) before the supply step and the first pyrolysis step (S1) in which a polystyrene resin composition is prepared by a step of collecting waste plastic (post-consumer products) and recycling it as a raw material polystyrene resin composition (P1: post-consumer) or a step of collecting scraps from the production of products that are not yet distributed to consumers or the market (pre-consumer products), such as scraps from the production process, and recycling them as a polystyrene resin composition (P2: pre-consumer).

[0090] Next, a polystyrene resin composition containing styrene monomer units, particularly a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, is supplied to an atmosphere heated to 300°C or higher, and then subjected to a first thermal decomposition step (S1) to thermally decompose it. Preferably, the polystyrene resin composition is thermally decomposed under the above conditions (A) and (B), and then cooled under known cooling conditions to prepare a first thermal decomposition liquid. If necessary, the first thermal decomposition liquid may then be distilled to separate it into a second fraction containing styrene monomers and high-boiling-point components with a higher boiling point than styrene monomers under normal temperature and pressure conditions, and a first fraction containing low-boiling-point components with a lower boiling point than styrene monomers under normal temperature and pressure conditions, in a first distillation step (S2).

[0091] Furthermore, if necessary, a second distillation step (S3) may be performed in which the second fraction is distilled to separate it into a third fraction mainly composed of styrene monomer and a fourth fraction mainly composed of high-boiling-point components that have a higher boiling point than styrene monomer under the aforementioned room temperature and atmospheric pressure conditions. On the other hand, as shown in Figure 8, if the first distillation step (S2) is not performed, the first pyrolysis liquid corresponds to the fourth fraction. Similarly, if the second distillation step (S3) is not performed, the second fraction corresponds to the fourth fraction. By subjecting the polystyrene resin composition to the above supply step and the first thermal decomposition step (S1), the amount of raw material residue in the system is reduced. Furthermore, by subjecting it to the first distillation step (S2) and / or the second distillation step (S3), the amount of styrene dimer and / or styrene trimer can be concentrated, resulting in the recovery of styrene monomer in higher yield and higher purity.

[0092] Then, as shown in Figure 8, a second pyrolysis step (S4) is performed in which the first pyrolysis liquid, the second fraction, or the fourth fraction is pyrolyzed under predetermined conditions and cooled under known cooling conditions to prepare a second pyrolysis liquid. Furthermore, if necessary, a third distillation step (S5) may be performed in which the second pyrolysis liquid is distilled to separate it into a sixth fraction containing styrene monomers and low-boiling-point components and a fifth fraction containing higher-boiling-point components than the sixth fraction. If necessary, a recycling step (S6) may be performed in which the fifth fraction is made part of the first pyrolysis liquid (or the sixth fraction is mixed with the first pyrolysis liquid) from the viewpoint of increasing the yield of styrene monomers.

[0093] If necessary, the amount of styrene monomer in the first pyrolysis liquid or the sixth fraction is analyzed, and if it is confirmed to be styrene monomer of a purity of a higher than desired level, a recovery step is performed to recover the first pyrolysis liquid or the sixth fraction as styrene monomer. This makes it possible to produce styrene monomer with high purity.

[0094] Each step is explained in detail below. (Preparation process (P)) The method for producing styrene monomers according to this disclosure preferably includes a raw material preparation step (P) for preparing a polystyrene resin composition. The raw material preparation process (P) includes a process (P1: post-consumer) in which styrene-based resin-containing waste plastics (post-consumer products) generated from the market or consumers are collected and recycled as a composition containing styrene dimer and / or styrene trimer, and / or a process (P1) (P2: pre-consumer) in which products that have not yet been distributed to consumers or the market, such as scraps from manufacturing, are collected and recycled from styrene-based resin-containing virgin material as a polystyrene-based resin composition. The raw material preparation step (P1) for preparing a polystyrene resin composition using post-consumer and / or pre-consumer products preferably includes a crushing step for crushing styrene resin-containing waste plastic, a washing step for washing the crushed waste plastic, or a sorting step for sorting the crushed waste plastic. On the other hand, as a raw material preparation step (P1) for preparing a polystyrene-based resin composition using materials such as virgin pellets from a pre-consumer, it is preferable to include a crushing step for crushing the styrene-based resin-containing virgin material.

[0095] <Crushing process> The crushing step involves crushing waste plastic containing styrene resin or virgin material containing styrene resin using a known crusher to obtain crushed waste plastic containing styrene resin or virgin material containing styrene resin. The maximum length of the crushed material is approximately 0.5 cm or more, preferably 0.5 to 5.0 cm. For example, the crushed material (shredder dust) can be transported to the next step using a conveyor such as a screw conveyor.

[0096] The crushing process is particularly suited to the size of the crushed material, which is suitable for removing foreign matter such as metals or soil attached to the crushed material in an optional subsequent washing process, thereby improving the sorting efficiency in an optional sorting process. Furthermore, if necessary, before and / or after the crushing process, waste plastics containing styrene resin or virgin materials containing styrene resin may be dissolved in a solvent to separate components that are soluble in the solvent from components that are insoluble or sparingly soluble in the solvent. The components dissolved in the solvent may then be deflated and used in the next process. Additionally, if necessary, manual sorting or dry sorting may be performed before the crushing process. The aforementioned manual sorting is the process of removing metals, glass, inorganic materials, paper, or wood chips from scrapped automobiles, discarded home appliances, discarded building materials, discarded plastic containers, etc. In manual sorting, to prevent problems in subsequent processes, for example, a person can manually remove materials other than the intended recovery material while visually inspecting them on a conveyor belt. The removed materials are treated as residues. Metals or glass may be recycled as is. As the dry sorting method, sorting may be performed using an optical sorter, a sieve sorter, or an air sorter, or by using a magnetic sorter. For sorting using an optical sorter or an air sorter, known methods can be employed. For example, wavelengths can be analyzed using near-infrared light to recognize specific materials, and these materials can be sorted by blowing them away with an air nozzle.

[0097] <Sorting Process> The sorting process involves separating the styrene-based resin component from the crushed waste plastic containing styrene-based resin, and removing the metal, inorganic matter, or soil. Examples of the sorting process include dry sorting processes such as magnetic separators and sieving machines, and specific gravity sorting processes described later. If it is not necessary to recover magnetic materials such as iron, metal, inorganic matter, or soil may be separated from the crushed material using a sieving machine. Furthermore, the sorting process involves using a sieve sorter to separate metals, inorganic materials, or soil from the crushed waste plastic containing styrene resin, and recovering the metals, inorganic materials, or soil as residue. As the sieve sorter, eddy current sorting, which uses eddy currents and magnetic field interactions generated by a rotating magnetic field to propel non-magnetic materials (aluminum, copper, etc.) forward, and dry gravity sorting, which uses a plate with an opening tilted at an angle and vibrated while blowing air from below, can be used.

[0098] The crushed styrene-based resin waste plastic processed in the aforementioned sorting step may be further sorted by dry sorting using an optical sorter, an air sorter, or manual sorting, as necessary, to remove metals, soil, glass, etc. contained in the crushed material, thereby removing impurities such as metals, inorganic materials, or soil.

[0099] <Specific gravity sorting process> The gravity separation step is a step of separating the waste plastic containing the crushed styrene resin into floating material and settled material. The gravity separation step is a separation step in which the floating material is separated into floating material and settled material in a liquid tank containing a liquid with a specific gravity of 0.9 to 1.50 g / cm3, such as water, aqueous solution, or solvent containing oil; or a heavy liquid containing salt, ferrochrome, or potassium nitrate, etc., in order to use the floating material as a raw material in the method for producing styrene monomer of this embodiment. The specific gravity separation process allows for the removal of metals, inorganic materials, soil, and other materials, as well as primarily halogen resins and polyester resins, from the crushed styrene-based resin-containing waste plastic. The halogen-based resins include vinyl chloride resins and bromine resins. By performing a washing process before the specific gravity separation process, the crushed styrene-based resin-containing waste plastic can be made wettable, thereby shortening the sorting time and improving sorting accuracy. For the waste plastic containing the crushed styrene-based resin, a solvent or heavy liquid with a specific gravity of approximately 0.9 to 1.5 is used. Since the specific gravity of halogen-based resins (e.g., PVC-based resins) is 1.3 to 1.4, a specific gravity suitable for separating the halogen-based resins is used. The heavy liquid is prepared using salts, ferrochrome, potassium nitrate, etc. In the gravity separation process, the solvent can be appropriately adjusted to a specific gravity between 0.9 and 1.5, depending on the specific gravity of the material to be removed. Furthermore, as the liquid tank used in the specific gravity separation process, a processing device can be used that is equipped with a plurality of paddles attached to the top surface and a spiral discharge machine attached to the bottom of the box-type separation liquid tank for discharging the settled pulverized material.

[0100] In the gravity separation step, gravity separation is performed using a heavy liquid to separate and recover suspended solids and settled solids. The manual separation may be further performed on the suspended solids from the gravity separation step.

[0101] <Washing process> The cleaning step involves cleaning the waste plastic containing the crushed styrene resin and removing any dirt adhering to the crushed material. The washing step may also be performed to wash the crushed styrene-based resin-containing waste plastic and simultaneously finely crush it to a size of 5 cm or less, preferably about 1 cm.

[0102] <First pyrolysis step (S1)> The first pyrolysis step (S1) of this embodiment is as described above, but a pretreatment step may be used if necessary. The pretreatment step may be performed before the first pyrolysis step (S1), during the first pyrolysis step (S1), or after the first pyrolysis step (S1), and it is preferable to perform the pretreatment step before the first pyrolysis step (S1).

[0103] <Pre-treatment process> The method for producing the styrene monomer in this embodiment preferably includes a pretreatment step. It is preferable that the method includes the first thermal decomposition step (S1) after the pretreatment step. The aforementioned pretreatment step includes a mixture preparation step of preparing a mixture of a polystyrene resin composition and a solvent, a purification step of purifying the mixture using a purification means, and a defloration step of deflorating the purified mixture to obtain a fluid.

[0104] <<Mixture preparation process>> The method for producing styrene monomers in this embodiment preferably includes a step of preparing a mixture by mixing a polystyrene resin composition with a solvent (=mixture preparation step). This step provides the effect of concentrating the styrene polymer using the solubility of the solvent.

[0105] -solvent- The solvent in this embodiment is preferably one that dissolves the styrene polymer in the polystyrene resin composition. This allows for the separation of components containing the styrene polymer that dissolves in the solvent from components that are insoluble or poorly soluble in the solvent. As a result, the styrene polymer can be concentrated. In this embodiment, the solvent solubility parameter (SP value ((cal / cm)) is used. 3 ) 1 / 2 The solubility parameter of the solvent is preferably 8.0 or more and less than 11.0, more preferably 8.3 or more and less than 10.5, and even more preferably 8.6 or more and less than 10.0. When the solubility parameter of the solvent is within the above range, it becomes easier to selectively dissolve styrene-based polymers (e.g., polystyrene, styrene-(meth)acrylic acid copolymers, etc.). The following solvents are preferred solvents in this embodiment. The values ​​in parentheses indicate the SP values ​​of the corresponding solvents. The solvent is preferably an organic solvent, and specifically, the organic solvent is one or more solvents selected from the group consisting of acetone (9.9), chloroform (9.3), methyl ethyl ketone (9.3), benzene (9.2), tetrahydrofuran (9.1), toluene (8.9), and ethylbenzene (8.8). The solvent may be a single solvent or a mixed solvent of two or more solvents.

[0106] The solubility parameter (SP value) defined in this embodiment is calculated using the cohesive energy density function shown in equation (1) below. SP value ((cal / cm) 3 ) 1 / 2 ) = (△E / V) 1 / 2 (1) (In equation (1) above, △E represents the intermolecular cohesive energy (heat of vaporization), V represents the total volume of the mixture, and △E / V represents the cohesive energy density.) Furthermore, the change in heat quantity ΔHm due to mixing is given by the following equation (2) using the SP value. △Hm=V(δ1-δ2)·Φ1·Φ2 (2) (In the above formula (2), δ1 represents the SP value of the solvent, δ2 represents the SP value of the solute, Φ1 represents the volume fraction of the solvent, and Φ2 represents the volume fraction of the solute.) From equations (1) and (2) above, the closer the values ​​of δ1 and δ2, the smaller ΔHm becomes, and the smaller the Gibbs free energy. Therefore, elements with a small difference in SP values ​​have a higher affinity for each other. The SP values ​​listed above are SP values ​​obtained using the Hilderbrand method (including the Hansen method), and refer to literature values ​​("Polymer Handbook 4th Edition," "J. Brandrup, E. Himmergut, E. Grulke," WILEY-INTERSCIENCE).

[0107] The solvent content in this embodiment is preferably 5 to 95% by mass of the total mixture, more preferably 35 to 90% by mass, and even more preferably 50 to 80% by mass. When the amount of solvent mixed is within the above range, the component containing the styrene polymer that dissolves in the solvent becomes easier to separate from the component other than the aforementioned component.

[0108] <<Purification process>> The method for producing styrene monomer in this embodiment preferably includes a purification step in which the mixture obtained in the above-mentioned mixture preparation step is purified by a purification means. The process includes a purification step in which the mixed solution prepared in the mixed solution preparation step is purified using a purification means, thereby effectively removing any impurities that may be present in the mixed solution. Furthermore, the term "purification" as used herein refers to a process in which impurities and other contaminants derived from the polystyrene resin composition are removed, so that the concentration of styrene polymer in the mixture after the purification process is higher than the concentration of styrene polymer in the mixture before the purification process. The purification means in this embodiment is not particularly limited as long as it is an apparatus that can be adapted to the mixture, and pre-purification may be performed using a known purification mechanism before or during the defolatorial step described later. The purification mechanism is not particularly limited, but a method that uniformly purifies each component in the mixture is preferred. Specific examples of the purification mechanism include filtration, decantation, centrifugation, centrifugal sedimentation, screw decanter, strainer, screen mesh, or filter. Among these, a purification mechanism that can process continuously (e.g., centrifugation, filtration, screw decanter, strainer, screen mesh, or filter) is more preferred. The time required for purification, from the time the mixture is introduced into the purification means to the time the purified mixture is obtained through the purification means, is preferably about 1 second to 240 minutes. In addition, inorganic powders such as diatomaceous earth or aluminum silicate may be added to the mixture to improve purification efficiency, and the amount added is preferably 0.5% to 50% by mass of the total mixture, more preferably 0.6% to 20% by mass, and even more preferably 1% to 10% by mass. By setting the amount added within the above range, impurities in the mixture can be efficiently removed, the operating time of the purification mechanism can be extended, and the time required for purification can be shortened. For example, if the input amount is less than the aforementioned range, the removal effect of impurities will not be sufficient, and as a result, impurities that should be removed in the purification process will remain in the purified mixture. Also, if the input amount is more than the aforementioned range, it may cause pressure loss in the filtration mechanism, for example, and the time required for purification will be prolonged. In the purified mixture obtained through the aforementioned purification means, the content of styrene polymer in the mixture is preferably 5% by mass or more and 100% by mass or less, and more preferably 5% by mass or more and 50% by mass or less, relative to the entire purified mixture. Furthermore, in the purified mixture obtained through the purification means, the content of impurities in the mixture is preferably 30% by mass or less, and more preferably 20% by mass or less, relative to the entire purified mixture. The content of impurities in the purified mixture is the value obtained by subtracting the sum of the content of styrene polymers and solvents from the total content of the mixture. In addition, the purified mixture may contain other resins that do not substantially contain styrene monomer units, such as olefin resins, polyether resins, polyester resins, or polyamide resins.

[0109] <<Devolatilization process>> The method for producing styrene monomer in this embodiment preferably includes a defloration step in which the mixture purified by the above purification step is deflorated to become a liquid. The defloration step is a process of deflorating the solvent contained in the mixed liquid purified by the purification step to make it a liquid, thereby achieving the effects of volatilizing low molecular weight components and distilling off the solvent. The devolatilization process in this embodiment is not particularly limited as long as it is carried out using equipment suitable for the mixed liquid, and preliminary devolatilization may be performed using a known devolatilization apparatus before or while the thermal decomposition process described later is being carried out. The devolatilization process is not particularly limited, but it is preferable that the volatile components in the purified mixture are uniformly removed. Specific examples of the devolatilization process include conventional devolatilization equipment such as a flash drum, twin-screw devolatilizer, thin-film evaporator, or extruder. Among these, a devolatilization equipment with minimal stagnation is preferred. The temperature during the devolatilization process (e.g., the temperature inside the devolatilization apparatus) is typically around 100 to 280°C, with 190 to 260°C being more preferable. The pressure during the devolatilization process (e.g., the pressure inside the devolatilization apparatus) is typically around 0.13 to 5.0 kPa, preferably 0.13 to 4.5 kPa, and more preferably 0.13 to 4.0 kPa. Preferred devolatilization steps in this embodiment include, for example, a method of removing volatile components by reducing the pressure inside a devolatilization apparatus while heating the purified mixture at 100 to 280°C inside the apparatus, and a method of removing volatile components by passing the mixture through an extruder or the like designed for the purpose of removing volatile components.

[0110] For example, if viscosity adjustment of the fluid is necessary, it may contain residual solvent as a result of the defoliation process, or the viscosity may be adjusted by adding a separate solvent using the mixing or kneading mechanism described above. In this case, the solvent to be added separately may be preheated, and the temperature of the prepared fluid is approximately 50 to 280°C, with 190 to 260°C being more preferable.

[0111] In the aforementioned fluid, the solvent content in the fluid is preferably 90% by mass or less, and more preferably 50% by mass or less, relative to the total fluid. Furthermore, in the fluid, the content of the styrene polymer in the fluid is preferably 10% by mass or more and 100% by mass or less, and more preferably 50% by mass or more and 100% by mass or less, relative to the entire fluid.

[0112] (First distillation step (S2)) The method for producing styrene monomer according to this embodiment may optionally include a first distillation step (S2). In the first distillation step (S2), it is preferable to use one or more distillation columns and carry out the process in one or more stages. Furthermore, it is preferable to fill these one or more distillation columns with an inert gas (e.g., nitrogen, noble gas, etc.) from an inert gas supply source and replace the air inside the distillation column with the inert gas during distillation. The distillation temperature in the first distillation step (S2) is preferably in the range of 50 to 200°C, more preferably 50 to 150°C. For example, if the first pyrolysis liquid is distilled in one step using one distillation column in the first distillation step (S2), the distillation temperature for the one-step distillation is preferably in the range of 50 to 200°C, more preferably 50 to 150°C. The pressure of the distillation atmosphere in the first distillation step (S2) (for example, the pressure inside the distillation column) is preferably 10 to 1070 Torr, more preferably 20 to 760 Torr. When the first pyrolysis liquid is distilled under the above distillation conditions, it is separated into a second fraction (so-called high-boiling-point component) containing styrene monomers and styrene dimers and / or styrene trimers, and a first fraction (so-called low-boiling-point component) having a lower styrene monomer concentration than the second fraction. Preferably, the difference between the styrene monomer concentration in the second fraction and the styrene monomer concentration in the first fraction is such that the concentration of styrene monomer in the second fraction is approximately twice or more than the concentration of styrene monomer in the first fraction. For example, in the first distillation step (S2), if the second pyrolysis liquid is distilled in one step using a single distillation column, components with low boiling points (such as toluene) become low-boiling-point components in the distillation column, while styrene monomers and / or styrene dimers / trimers become high-boiling-point components. In addition, a polymerization inhibitor may be used in the second distillation step to prevent polymerization of styrene monomers. This allows the second pyrolysis liquid to be separated into a second fraction containing styrene monomers and high-boiling-point components, and a first fraction containing lower-boiling-point components than the second fraction, thereby enabling the production of high-purity styrene monomers.

[0113] (Second distillation process (S3)) The method for producing styrene monomer according to this embodiment may optionally include a second distillation step (S3). In the second distillation step (S3), it is preferable to use one or more distillation columns and carry out the process in one or more stages. Furthermore, it is preferable to fill these one or more distillation columns with an inert gas (e.g., nitrogen, noble gas, etc.) from an inert gas supply source and replace the air inside the distillation column with the inert gas during distillation. Furthermore, the distillation temperature in the second distillation step (S3) is preferably in the range of 50 to 200°C, more preferably 50 to 150°C. For example, if the second fraction is distilled in one stage of a single distillation column in the second distillation step (S3), the distillation temperature for the one stage of distillation is preferably in the range of 50 to 200°C, more preferably 50 to 150°C. The pressure of the distillation atmosphere in the second distillation step (for example, the pressure inside the distillation column) is preferably 10 to 1070 Torr, more preferably 20 to 670 Torr. When the second fraction is distilled under the above distillation conditions, it is separated into a fourth fraction containing styrene dimer and / or styrene trimer (a so-called high-boiling-point component) and a third fraction with a higher styrene monomer concentration than the fourth fraction (a so-called low-boiling-point component). Preferably, the difference between the styrene monomer concentration in the third fraction and the styrene monomer concentration in the fourth fraction is such that the concentration of styrene monomer in the third fraction is approximately twice or more than the concentration of styrene monomer in the fourth fraction. For example, in the second distillation step (S3), if the second fraction is distilled in one step using a single distillation column, components with low boiling points (such as styrene monomers) become low-boiling-point components in the distillation column, while styrene dimers and trimers become high-boiling-point components. Furthermore, a polymerization inhibitor may be used in the second distillation step (S3) to prevent polymerization of the styrene monomer. Furthermore, since the third fraction has the highest purity of styrene monomer, it is preferable to recover this third fraction as styrene monomer. As a result, the second fraction is separated into a third fraction mainly composed of styrene monomer and a fourth fraction mainly composed of higher boiling point components than the third fraction, making it possible to produce styrene monomer in high yield.

[0114] <Second pyrolysis step (S4)> The method for producing styrene monomers according to this disclosure optionally includes a second thermal decomposition step (S4). The second thermal decomposition step (S4) is a step of preparing a second thermal decomposition solution containing styrene monomers by thermal decomposing the fourth fraction. In this embodiment, a method for thermally decomposing the polystyrene resin composition is to heat the fourth fraction to 400°C or higher and less than 1200°C. Furthermore, a known thermal decomposition furnace can be used during the decomposition process. For example, the configuration of the pyrolysis furnace is not particularly limited, and may be an electric heating means or a furnace that burns combustion gas or fuel oil, and may be configured as a kettle-type furnace, a tubular-type furnace, a hot-blast furnace, a shaft furnace, a kiln furnace, a fluidized bed furnace, or a gasification reformer. Furthermore, the pyrolysis furnace is preferably a fluidized bed type kettle-type furnace or a tubular-type furnace.

[0115] In the second pyrolysis step (S4), the temperature at which the fourth fraction is pyrolyzed (= pyrolysis temperature) may be, for example, a temperature range of 400°C or more and less than 1200°C after filling the pyrolysis furnace with the fourth fraction. In this case, the temperature of the fourth fraction to be filled is about 20 to 300°C, and more preferably 150 to 260°C. Alternatively, the second pyrolysis step (S4) may be performed in which the polystyrene resin composition is filled into a pyrolysis furnace that has been preheated. In this case, the temperature at which the pyrolysis furnace is preheated is 400°C or more and less than 1200°C, preferably 420 to 800°C, more preferably 450 to 600°C, and the temperature of the fluid to be filled is about 20 to 300°C, and more preferably 150 to 260°C. By setting the temperature of the pyrolysis furnace within the above temperature range, most of the generated pyrolysis gas can become pyrolysis products of the polystyrene resin composition. The components contained in the fourth fraction can reduce the conversion to by-products that would otherwise lead to a decrease in the yield of styrene monomers.

[0116] In the second pyrolysis step (S4), the pressure conditions for pyrolysis of the fourth fraction (= pyrolysis pressure) are preferably an atmosphere with a pressure of more than 10 hPa and less than 300 hPa. By setting the pressure range of the pyrolysis furnace as described above, the majority of the generated pyrolysis gas can become pyrolysis products of the polystyrene resin composition. Furthermore, the conversion of components of the raw material polystyrene resin composition into by-products that would lead to a decrease in the yield of styrene monomers can be reduced.

[0117] Furthermore, the pyrolysis gas generated in the second pyrolysis step (S4) may be cooled to a temperature between the boiling point of the styrene monomer and 180°C, similar to the first pyrolysis step, and the high-boiling-point components may be liquefied using a liquefaction device and dropped back into the pyrolysis furnace, thereby returning it to the pyrolysis furnace and repeating the pyrolysis step. Alternatively, the pyrolysis gas generated in the second pyrolysis step (S4) can be cooled in the same manner as in the first pyrolysis step to obtain the second pyrolysis liquid.

[0118] (Third distillation process (S5)) The method for producing styrene monomer according to this embodiment may optionally include a third distillation step (S5). More specifically, the method for producing styrene monomer according to this embodiment preferably includes a third distillation step (S5) in which the second pyrolysis liquid obtained in the second pyrolysis step (S4) is distilled. Furthermore, when the second pyrolysis liquid is distilled in the third distillation step (S5), it is separated into a sixth fraction containing styrene monomers and low-boiling-point components (e.g., components with a boiling point of 50-200°C) and a fifth fraction containing more high-boiling-point components (e.g., components with a higher boiling point than the low-boiling-point components) than the sixth fraction. Since styrene monomers are produced when the second pyrolysis liquid is distilled, styrene monomers can be recovered in a higher yield or with higher purity. The fifth fraction, which contains more high-boiling-point components than the sixth fraction (for example, mainly composed of high-boiling-point components), is either discarded or used as fuel in the pyrolysis process in the present invention (for example, the first pyrolysis process and / or the second pyrolysis process). The third distillation step (S5) can use a known distillation column, such as a tray column or a packed column. In the third distillation step (S5), it is preferable to use one or more distillation columns and carry out the process in one or more stages. Furthermore, it is preferable to fill these one or more distillation columns with an inert gas (e.g., nitrogen, noble gas, etc.) from an inert gas supply source before starting up the operation, and to replace the air inside the distillation column with the inert gas before performing the distillation. Furthermore, the distillation temperature in the third distillation step (S5) is preferably in the range of 50 to 200°C, more preferably 50 to 150°C. For example, if the sixth fraction is distilled in one stage of a single distillation column in the third distillation step (S5), the distillation temperature for the one stage of distillation is preferably in the range of 50 to 200°C, more preferably 50 to 150°C. The pressure of the distillation atmosphere in the first distillation step (for example, the pressure inside the distillation column) is preferably 10 to 100 Torr, more preferably 20 to 70 Torr.

[0119] <Recycling process (S6)> The method for producing styrene monomer according to this embodiment may optionally include a recycling step (S6). More specifically, the method for producing styrene monomer according to this embodiment preferably includes a step of recycling the sixth fraction obtained by the third distillation step (S5) and / or a recycling step (S6) of recycling the first pyrolysis liquid. The recycling process (S6) is a process of using the second fraction as part of the raw material, or as part of the first pyrolysis liquid described later, or a process of pumping part or all of the second pyrolysis liquid using a pump or the like to make it part of the first pyrolysis liquid. In other words, the recycling process (S6) is a process of mixing the sixth fraction obtained in the third distillation process (S5), or the second pyrolysis liquid, with the raw material composition, or mixing the sixth fraction with the first pyrolysis liquid described later.

[0120] <Styrene monomer recovery process> The method for producing styrene monomers in this embodiment may optionally include a recovery step for recovering styrene monomers from a fraction other than the first pyrolysis liquid. More specifically, it includes a recovery step for recovering styrene monomers from the second pyrolysis liquid, the sixth fraction, or the third fraction prepared in the second pyrolysis step (S4). The phrase "recovering styrene monomers from the second pyrolysis liquid" includes both directly recovering styrene monomers from the second pyrolysis liquid and indirectly recovering styrene monomers by performing a third distillation step (S5) or the like on the second pyrolysis liquid (including rectification). The second pyrolysis liquid only needs to contain components produced by the pyrolysis of a composition containing styrene dimer and / or styrene trimer, and pre-pyrolysis may be performed before or during the recovery step. In addition, optional additives may be added as needed during the pyrolysis step and / or the recovery step to suppress the polymerization reaction of the resulting styrene monomer.

[0121] In this embodiment, known recovery methods can be used to recover the styrene monomer. For example, the second pyrolysis liquid may be distilled or rectified and fluidly connected to a rectification column capable of separating low-boiling-point components such as benzene or toluene from crude styrene monomer (styrene monomer with a purity of 90% or less). Furthermore, the rectification column may be fluidly connected to a distillation column for rectifying the separated crude styrene monomer to increase its purity. If necessary, a dechlorination device for dechlorinating components in the pyrolysis liquid may be fluidly connected between the pyrolysis furnace and the rectification column. Furthermore, in the recovery process, the recovered material may be the second pyrolysis liquid, the sixth fraction, or the third fraction. Additionally, if necessary, a quantitative analysis using GC-FID as described in the examples may be performed in the recovery process to measure the styrene monomer concentration and determine whether recovery is possible. In this case, it is desirable that the recovered material contains 10% by mass or more, preferably 15% by mass or more, and more preferably 20% by mass or more of styrene monomer relative to the total recovered material. The recovered material may be shipped as a product. [Examples]

[0122] [Measurement and evaluation methods] The physical properties of the polystyrene resin compositions obtained in each example and comparative example were measured and evaluated based on the following methods.

[0123] <GC-MS analysis of the decomposition solution> [GC / MS analysis] Analysis of the raw material compositions used in the styrene monomer production method, as well as the pyrolysis solutions obtained in the examples and comparative examples, was performed by preparing a methyl ethyl ketone solution and using GC / MS Agilent 7890 and Agilent 5975 under the following conditions. • Column HP-5MS (L 30m, ID 0.250mm, Film 0.25μm) Helium carrier • Detector MSD Ionization method EI Oven temperature: 40°C (hold for 5 minutes) → 20°C / min → 320°C (hold for 10 minutes) ·Inlet temperature 250℃ Transfer temperature: 320℃ ·Mass range m / z 10-800 • Injection mode: Splitless ·Injection volume 1μL • Measurement mode: SIM

[0124] <GC-FID analysis of the decomposition solution> [GC-FID analysis] Analysis of the raw material compositions used in the styrene monomer production method, as well as the pyrolysis solutions obtained in the examples and comparative examples, was performed using a methyl ethyl ketone solution prepared and a GC-FID Agilent 8860 under the following conditions. • Column HP-5MS (L 30m, ID 0.250mm, Film 0.25μm) Helium carrier • Detector FID Oven temperature: 40°C (hold for 5 minutes) → 20°C / min → 320°C (hold for 10 minutes) ·Inlet temperature 250℃ • Injection mode: Splitless ·Injection volume 1μL

[0125] [Quantitative analysis using GC-FID] Analysis of the raw material compositions used in the styrene monomer production method, as well as the pyrolysis solutions recovered in the examples and comparative examples, was performed using methyl ethyl ketone solutions prepared under the GC-FID analysis conditions described above. Quantitative analysis was performed using the absolute calibration curve method, where standards purchased from reagent manufacturers were dissolved in methyl ethyl ketone at arbitrary concentrations to create a calibration curve.

[0126] <Styrene monomer concentration in recovered pyrolysis liquid> The concentration of styrene monomer in the recovered solution was determined by analyzing the pyrolysis solutions recovered in the examples and comparative examples under the GC-FID analysis conditions described above. Quantitative analysis was performed using the absolute calibration curve method, where a standard sample purchased from a reagent manufacturer was dissolved in methyl ethyl ketone at an arbitrary concentration to create a calibration curve.

[0127] <Thermogravimetric reduction rate> Analysis was performed using a Shimadzu TGA apparatus (TGA-50) and a TA60-WS. A 10 mg sample of the coarse residue obtained from the pyrolysis experiment was taken into a deep aluminum pan. Measurements were then taken from 25°C to 550°C under a 20 mL / min nitrogen or dry air flow, with a heating rate of 20°C / min, and the sample was held at 550°C for 60 minutes using a controlled program. The thermal weight loss rates obtained from this measurement were calculated as the ratio of the weight difference between the weight at the starting point (25°C) and the weight at the end of the measurement (550°C after the heating program, where the sample was held for 60 minutes), with thermal weight loss rates (C) and (D) being calculated respectively. Thermal weight loss rate (C)% = "Remaining weight in the deep aluminum pan after measurement under nitrogen conditions" / "Sample weight weighed into the deep aluminum pan before measurement" × 100 Thermal weight loss rate (D)% = "Remaining weight in the deep aluminum pan after measurement under air conditions" / "Sample weight weighed into the deep aluminum pan before measurement" × 100

[0128] <1st residue rate (X)> In the method for producing a styrene monomer according to this embodiment, the first residue ratio (X) can be calculated from the crude residue ratio (B) calculated from the weight ratio between the polystyrene-based resin composition charged into the pyrolysis apparatus and the solid matter remaining after pyrolysis or discharged by the discharge mechanism, the thermogravimetric reduction rate (C) measured under a nitrogen atmosphere, and the thermogravimetric reduction rate (D) measured under an air atmosphere. The relationship between the first residue ratio (X), the crude residue ratio (B), the thermogravimetric reduction rate (C), and the thermogravimetric reduction rate (D) is as follows. At this time, the first residue ratio (X) is preferably 0.4% or less, more preferably 0.35% or less. If it is more than 0.5%, the thermal efficiency of the pyrolysis furnace will decrease, the pyrolysis efficiency will decrease, and there is a risk of blockage of pipes and the like and a decrease in throughput. In addition, when the weight of the solid matter remaining after pyrolysis of the charged resin composition or discharged by the discharge mechanism becomes zero, the crude residue ratio (B) becomes zero, and the thermogravimetric reduction rate (C) and the thermogravimetric reduction rate (D) are regarded as zero. First residue ratio (X) = (Crude residue ratio (B) × (Thermogravimetric reduction rate (C) - Thermogravimetric reduction rate (D)) + Crude residue ratio (B) × (Thermogravimetric reduction rate (D)) / 100

[0129] <Second residue ratio (Y)> The second residue ratio (Y) is a value calculated from the first residue ratio (X) and the mass ratio (wt%) of the conjugated diene monomer unit in the polystyrene-based resin composition charged into the pyrolysis apparatus, and indicates the ratio of the residue to the conjugated diene monomer unit. Second residue ratio (Y) = (First residue ratio (X) / Mass ratio of conjugated diene monomer unit) × 100

[0130] <Crude residue ratio (B) (%)> Crude residue ratio (B) = {(Weight of solid matter remaining after pyrolysis or discharged by the discharge mechanism) / (Weight of polystyrene-based resin composition charged into the pyrolysis apparatus)} × 100

[0131] <SM production efficiency (Z)> The SM production efficiency (Z) was determined from the relationship between the SM concentration in the recovered pyrolysis liquid and the residue ratio Y. A higher SM production efficiency (Z) indicates a higher SM concentration in the recovered pyrolysis liquid relative to the residue ratio of conjugated diene monomer units, thus demonstrating superior SM production efficiency. SM production efficiency (Z) = SM concentration in recovered pyrolysis liquid / percentage of secondary residue (Y)

[0132] <Residue discharge efficiency per hour> The above crude residue rate (B) (%) is used to determine the efficiency of the residue generated per day during the pyrolysis operation (residue discharge efficiency per hour). ◎: Less than 0.20 ○: 0.20 or higher and less than 0.50 △: 0.50 or higher, less than 1.50 ×: 1.50 or higher

[0133] <Percentage of coarse residue (B) remaining in the device after 10 days of continuous operation> The percentage of coarse residue (B) (%) remaining inside the pyrolysis apparatus after 10 days of continuous operation is defined as the residual rate of coarse residue inside the apparatus after 10 days of continuous operation.

[0134] <Maximum operating time per month> The following model case illustrates this approach. Assuming a month has 31 days, and that it takes one day to start up the system and one day for shutdown and maintenance, we can estimate these two steps. In the case of an extruder-type pyrolysis apparatus 10 (the pyrolysis apparatus in Figures 5, 6, or 7), it can operate for 30 days excluding the period required for the apparatus to start up (1 day). If it operates 24 hours a day, this amounts to 30 days × 24 hours / day = 720 hours of operation. On the other hand, the tubular pyrolysis apparatus 10 (the pyrolysis apparatus in Figures 1-3) is assumed to require maintenance three times. That is, 31 days - (3 times × 2 days) = 25 days of continuous operation (meaning maintenance is required once every 200 hours). In that case, the total operating time would be 25 days × 24 hours / day = 600 hours. If maintenance is required when the residual rate (e.g., crude residue rate (B)) in the pyrolysis apparatus 10 reaches 0.1%, then, for example, Example 3 assumes that this rate is reached 0.1% three times in one month.

[0135] <Average monthly SM yield ratio (relative value)> The average monthly SM yield ratio (relative value) was calculated using the value from Example 9 as 100. Average monthly SM yield ratio (relative value) = {(maximum monthly operating time) / (maximum monthly operating time in Example 9)} × 100

[0136] <Relationship with manufacturing efficiency> The manufacturing efficiency was calculated using the following formula. Manufacturing efficiency = (Average monthly SM yield ratio) / (SM manufacturing efficiency (Z))

[0137] Examples 1-3 In Examples 1-3, pyrolysis experiments were conducted using a small reactor (a small laboratory pyrolysis apparatus, corresponding to the small laboratory in Table 1) for simple experiments. However, there are no particular restrictions on the size of the equipment, as long as it does not deviate from the purpose of conducting the desired experiment. Specifically, the following applies: <Example 1: Thermal Decomposition Experiment 1> A SUS reaction vessel was placed inside a cast heater, a SUS cover with a branch pipe was attached, and bolted with a wrench. A dedicated adapter with an O-ring was attached to the branch pipe, and a glass Liebig condenser, a distillation adapter with a branch for attaching a vacuum hose, and an eggplant flask for recovering the pyrolysis liquid were attached, each coated with silicone grease. A three-way stopcock was attached to the branch for attaching the vacuum hose, and a nitrogen balloon and a vacuum pump were connected. The temperature was monitored using thermocouples installed inside the cast heater and the vessel. The refrigerant was supplied at -10°C, the pressure was set with a vacuum pump, and the output of the cast heater was adjusted. A polystyrene resin composition containing conjugated diene monomer units and styrene monomer units was introduced into the SUS reaction vessel at an arbitrary supply temperature, and pyrolysis was carried out at each temperature as described in Table 1. After liquefaction in a Liebig condenser into which the refrigerant flowed, the first pyrolysis liquid was recovered using an eggplant flask. Then, the first pyrolysis liquid was distilled as the first distillation step under conditions of 60°C and 25 Torr, and the resulting second fraction was distilled as the second distillation step under conditions of 60°C and 25 Torr to obtain styrene monomer as the third fraction. On the other hand, the second fraction other than the third fraction was subjected to the second pyrolysis step to obtain the fourth fraction. Specifically, a SUS reaction vessel filled with the fourth fraction was placed inside a cast-in heater, a SUS cover with a branch pipe was attached, and bolted with a wrench. A special adapter with an O-ring was attached to the branch pipe, and a condenser, a distillation adapter with a branch for attaching a pressure-reducing hose, and a receiving container for recovering the pyrolysis liquid were attached, each coated with silicone grease. A three-way stopcock was attached to the branch for attaching the pressure-reducing hose, and a nitrogen balloon and a vacuum pump were connected. The temperature was monitored using thermocouples installed inside the cast-in heater and the vessel. The refrigerant was introduced at -10°C, the vacuum pump was set to 34 hPa to reduce the pressure, the output of the pouring heater was adjusted, and the (second) pyrolysis was performed at 450°C. The second pyrolysis liquid was then prepared in a receiving container by liquefaction in a condenser into which the refrigerant was introduced. The prepared second pyrolysis liquid was heated using a distiller at 13 hPa, increasing the temperature from 50°C to 80°C in 5°C increments every 5 minutes to remove the light boiling component (sixth fraction). Meanwhile, the heavy boiling component (fifth fraction) was discharged to the outside. The light boiling component (sixth fraction) may be recycled by adding it to the first pyrolysis liquid as needed.

[0138] <Example 2: Thermal Decomposition Experiment 2> Using the pyrolysis apparatus described in Example 1 above, HIPS (rubber-modified styrene resin) containing conjugated diene monomer units and styrene monomer units was charged at a predetermined supply temperature as described in the pyrolysis conditions in Table 1, and pyrolyzed at each temperature. After liquefaction in a Liebig condenser into which a refrigerant was introduced, the first pyrolysis liquid was recovered using an eggplant flask. The first pyrolysis liquid was then distilled as the first distillation step at 60°C and 25 Torr, and the resulting second fraction was distilled as the second distillation step at 60°C and 25 Torr to obtain styrene monomer as the third fraction.

[0139] <Example 3: Thermal Decomposition Experiment 3> Using the pyrolysis apparatus described in Example 1 above, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units was charged at a predetermined supply temperature as described in the pyrolysis conditions in Table 1, and pyrolyzed at each temperature. After liquefaction in a Liebig condenser into which a refrigerant was introduced, the first pyrolysis liquid was recovered using an eggplant flask. The first pyrolysis liquid was then distilled as the first distillation step at 60°C and 25 Torr, and the resulting second fraction was distilled as the second distillation step at 60°C and 25 Torr to obtain styrene monomer as the third fraction.

[0140] Examples 4, 6-8, 9, 12-17 Using the pyrolysis conditions shown in Examples 4, 6-8, and 12-17 of Table 1-1, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units was prepared and used as the raw material composition. The first pyrolysis step (S1) was carried out using the pyrolysis apparatus shown in Figures 1-4. The gas generated in the first pyrolysis step (S1) was condensed in a cooling tube cooled to -10°C to prepare the first pyrolysis liquid. Subsequently, the GC-FID of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Tables 1-1 and 1-2.

[0141] Example 5 Using the pyrolysis conditions shown in Table 1-1, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units was prepared and used as the raw material composition. The first pyrolysis step (S1) was performed using the pyrolysis apparatus shown in Figure 1. At this time, the melting apparatus 4 shown in Figure 5 was connected to the opening 2 in Figure 1. More specifically, a pyrolysis apparatus was used in which the pyrolysis furnace 1 and the melting apparatus 4 were connected so that the internal space of the melting section 4A and the internal space of the tubular pyrolysis section 1 were in communication. Using the melting apparatus 4, which had been preheated to 250°C inside the melting section 4A, the polystyrene resin composition was introduced into the hopper section 9 of the melting apparatus 4 and melted. After that, the molten polystyrene resin composition was pushed out and supplied to the opening 2 in Figure 1. Then, using the pyrolysis apparatus shown in Figure 1, the first pyrolysis liquid was prepared by condensing the gas generated by the first pyrolysis in a cooling pipe cooled to -10°C. Subsequently, the GC-FID analysis of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 1-1.

[0142] Examples 10 and 11 Using the pyrolysis conditions shown in Examples 10 and 11 of Table 1-1, polystyrene resin compositions containing conjugated diene monomer units and styrene monomer units, which are pre-consumer or post-consumer materials, were prepared as raw material compositions, and the first pyrolysis step (S1) was carried out using the pyrolysis apparatus shown in Figure 1. The gas generated in the first pyrolysis step (S1) was condensed in a cooling tube cooled to -10°C to prepare the first pyrolysis liquid. Subsequently, the GC-FID of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomers in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 1-1.

[0143] Examples 18 and 19 Using the thermal decomposition conditions shown in Examples 18 and 19 of Table 1-1, a polystyrene-based resin composition containing a prepared conjugated diene monomer unit and a styrene monomer unit was used as a raw material composition, and with the thermal decomposition atmosphere set to a nitrogen atmosphere by a nitrogen introduction tube, the first thermal decomposition step (S1) was carried out for each using the thermal decomposition apparatus described in FIG. 1. The first pyrolysis liquid was prepared by condensing the gas generated in the first pyrolysis step (S1) with a cooling tube cooled to -10°C. Thereafter, the first pyrolysis liquid was analyzed by GC-FID, and the thermogravimetric weight loss rate and residue rate of the solid matter remaining or discharged by the discharge mechanism after pyrolysis were analyzed respectively. The evaluation results such as the concentration (mass%) of the styrene monomer in the first pyrolysis liquid and its residue rate during pyrolysis are shown in Table 1-2.

[0144] "Example 20" Ethylbenzene was added and heated to 40°C and stirred for 3 hours to prepare a slurry so that the concentration of the prepared polystyrene-based resin composition containing the conjugated diene monomer unit and the styrene monomer unit was 10% by mass. Next, the slurry was devolatilized at 300°C to obtain a melt containing the polystyrene-based resin composition, and the first thermal decomposition step (S1) was carried out for this using the thermal decomposition apparatus described in FIG. 1. The first pyrolysis liquid was prepared by condensing the gas generated in the first pyrolysis step (S1) with a cooling tube cooled to -10°C. Thereafter, the first pyrolysis liquid was analyzed by GC-FID, and the thermogravimetric weight loss rate and residue rate of the solid matter remaining or discharged by the discharge mechanism after pyrolysis were analyzed respectively. The evaluation results such as the concentration (mass%) of the styrene monomer in the first pyrolysis liquid and its residue rate during pyrolysis are shown in Table 1-2.

[0145] "Example 21" A slurry was prepared by adding methyl ethyl ketone to a resin-containing polystyrene resin composition containing the prepared conjugated diene monomer units and styrene monomer units to a concentration of 10% by mass, heating to 40°C, and stirring for 3 hours. The slurry was defolarated at 300°C to obtain a molten product containing the polystyrene resin composition, which was then subjected to a first pyrolysis step (S1) using the pyrolysis apparatus shown in Figure 1. The first pyrolysis liquid was prepared by condensing the gas generated in the first pyrolysis step (S1) in a cooling tube cooled to -10°C. Subsequently, the first pyrolysis liquid was analyzed by GC-FID, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate. The evaluation results of the concentration (mass%) of styrene monomers in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 1-2.

[0146] Example 22 A slurry was prepared by adding ethylbenzene to a polystyrene resin composition containing the prepared conjugated diene monomer units and styrene monomer units to a concentration of 10% by mass, heating to 40°C, and stirring for 3 hours. Next, the slurry was placed in a pressure filter and filtered at 40°C under pressure of 0.5 MPa of nitrogen to obtain a filtrate. Subsequently, the obtained filtrate was deflated at 300°C to obtain a molten product containing the polystyrene resin composition, which was then subjected to a first pyrolysis step (S1) using the pyrolysis apparatus shown in Figure 1. The first pyrolysis liquid was prepared by condensing the gas generated in the first pyrolysis step (S1) in a cooling tube cooled to -10°C. Subsequently, the GC-FID of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomers in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 1-2.

[0147] "Example 23" A slurry was prepared by adding ethylbenzene to a polystyrene resin composition containing the prepared conjugated diene monomer units and styrene monomer units to a concentration of 10% by mass, heating to 40°C, and stirring for 3 hours. Then, 1% by mass of Celite #350 (diatomaceous earth) was added to the slurry and stirred for 1 hour. Next, the slurry was placed in a pressure filter and filtered at 40°C under pressure of 0.5 MPa of nitrogen to obtain a filtrate. Subsequently, the obtained filtrate was deflated at 300°C to obtain a molten product containing the polystyrene resin composition, which was then subjected to a first pyrolysis step (S1) using the pyrolysis apparatus shown in Figure 1. The first pyrolysis liquid was prepared by condensing the gas generated in the first pyrolysis step (S1) in a cooling tube cooled to -10°C. Subsequently, the GC-FID analysis of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 1-2.

[0148] "Example 24" Using the thermal decomposition conditions shown in Table 1-2, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units prepared by the method described in Example 22 was used as the raw material composition (27°C), and the first thermal decomposition step (S1) was carried out using the thermal decomposition apparatus shown in Figure 1. At this time, the melting apparatus 4 shown in Figure 5 was connected to the opening 2 in Figure 1. More specifically, a thermal decomposition apparatus was used in which the thermal decomposition furnace 1 and the melting apparatus 4 were connected so that the internal space of the melting section 4A and the internal space of the tubular thermal decomposition section 1 were in communication. Using the melting apparatus 4, which had been preheated to 250°C inside the melting section 4A, the polystyrene resin composition at approximately 27°C was introduced into the hopper section 9 of the melting apparatus 4 to melt the polystyrene resin composition, and then the molten polystyrene resin composition was pushed out and supplied to the opening 2 in Figure 1. Then, using the thermal decomposition apparatus shown in Figure 1, the first thermal decomposition liquid was prepared by condensing the gas generated by the first thermal decomposition in a cooling pipe cooled to -10°C. Subsequently, the first pyrolysis solution was analyzed by GC-FID, and the thermal weight loss rate and residue rate of the solids remaining after pyrolysis or discharged by the discharge mechanism were analyzed. The evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis solution and the residue rate during pyrolysis are shown in Table 1-2.

[0149] Examples 25 and 26 Using the pyrolysis conditions shown in Examples 25 and 26 of Table 1-2, polystyrene resin compositions containing conjugated diene monomer units and styrene monomer units, which are pre-consumer materials or post-consumer materials, were prepared, and a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units prepared by the method described in Example 22 was used as the raw material composition, and the first pyrolysis step (S1) was performed using the pyrolysis apparatus shown in Figure 3. The gas generated in the first pyrolysis step (S1) was condensed in a cooling tube cooled to -10°C to prepare the first pyrolysis liquid. Subsequently, the GC-FID of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 1-2.

[0150] Example 27 Using the pyrolysis conditions shown in the examples in Table 1-2, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units prepared by the method described in Example 22 was used as the raw material composition. The first pyrolysis step (S1) was carried out using the pyrolysis apparatus shown in Figure 1, with the pyrolysis atmosphere set to a nitrogen atmosphere via a nitrogen introduction tube. The first pyrolysis liquid was prepared by condensing the gas generated in the first pyrolysis step (S1) in a cooling tube cooled to -10°C. Subsequently, the first pyrolysis liquid was analyzed by GC-FID, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate. The evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 1-2.

[0151] Example 28 Using GPPS 680 manufactured by PS Japan Co., Ltd., prepared under the pyrolysis conditions shown in Example 28 of Table 1-2, the first pyrolysis step (S1) was carried out using the pyrolysis apparatus shown in Figure 1. The gas generated in the first pyrolysis step (S1) was condensed in a cooling tube cooled to -10°C to prepare the first pyrolysis liquid. Subsequently, the GC-FID of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 1-2.

[0152] "Example 29" Using the thermal decomposition conditions shown in Example 29 of Table 1-2, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units prepared by the method described in Example 22 was used as the raw material composition, and the first thermal decomposition step (S1) was carried out using a microwave thermal decomposition apparatus. The gas generated by the first thermal decomposition step (S1) was condensed in a cooling tube cooled to -10°C to prepare the first thermal decomposition liquid. Subsequently, the GC-FID of the first thermal decomposition liquid, the thermal weight loss rate of the solids remaining after thermal decomposition or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomer in the first thermal decomposition liquid and the residue rate during thermal decomposition are shown in Table 1-2.

[0153] "Example 30" Using the thermal decomposition conditions shown in Example 30 of Table 1-2, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units prepared by the method described in Example 22 was used as the raw material composition. The first thermal decomposition step (S1) was carried out using a microwave thermal decomposition apparatus under a nitrogen atmosphere created by introducing a nitrogen tube. The gas generated by the first thermal decomposition step (S1) was condensed in a cooling tube cooled to -10°C to prepare the first thermal decomposition liquid. Subsequently, the GC-FID of the first thermal decomposition liquid, the thermal weight loss rate of the solids remaining after thermal decomposition or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomers in the first thermal decomposition liquid and the residue rate during thermal decomposition are shown in Table 1-2.

[0154] Examples 31-38 Using the pyrolysis conditions shown in Examples 31-38 of Table 2-1, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units was prepared and used as the raw material composition. The first pyrolysis step (S1) was performed using the pyrolysis apparatus 10 (tank type) shown in Figure 5. The gas generated in the first pyrolysis step (S1) was condensed in a cooling tube cooled to -10°C to prepare the first pyrolysis liquid. Subsequently, the GC-FID of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed. The evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in the Examples of Table 2-1.

[0155] Examples 39 and 40 Using the pyrolysis conditions shown in Examples 39 and 40 of Table 2-2, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units was prepared and used as the raw material composition. The first pyrolysis step (S1) was performed using the pyrolysis apparatus shown in Figure 5, with the pyrolysis atmosphere set to a nitrogen atmosphere via a nitrogen introduction tube. The first pyrolysis liquid was prepared by condensing the gas generated in the first pyrolysis step (S1) in a cooling tube cooled to -10°C. Subsequently, the first pyrolysis liquid was analyzed by GC-FID, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate. The evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 2-2.

[0156] "Example 41" A polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, which are post-consumer materials, was prepared by adding ethylbenzene to a concentration of 10% by mass, heating to 40°C, and stirring for 3 hours to prepare a slurry. Celite #350 (diatomaceous earth) was then added at a concentration of 1% by mass relative to the weight of the slurry and stirred for 1 hour. Next, the slurry was placed in a pressure filter and filtered at 40°C under pressure of 0.5 MPa of nitrogen to obtain a filtrate. Subsequently, the obtained filtrate was deflated at 300°C to obtain a molten product containing the polystyrene resin composition, which was then subjected to a first pyrolysis step (S1) using the pyrolysis apparatus shown in Figure 5. The gas generated in the first pyrolysis step (S1) was condensed in a cooling tube cooled to -10°C to prepare a first pyrolysis liquid. Subsequently, the GC-FID analysis of the first pyrolysis solution, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the styrene monomer concentration (mass%) in the first pyrolysis solution and the residue rate during pyrolysis are shown in Table 2-2.

[0157] Examples 42-49 Using the pyrolysis conditions shown in Examples 42-49 of Table 2-2, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units was prepared and used as the raw material composition. The first pyrolysis step (S1) was performed using the pyrolysis apparatus shown in Figure 6. The gas generated by the first pyrolysis step (S1) was condensed in a cooling tube cooled to -10°C to prepare the first pyrolysis liquid. Subsequently, the GC-FID of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed. The evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 2-2.

[0158] Examples 50 and 51 Using the pyrolysis conditions shown in Examples 50 and 51 of Table 1-2, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units was prepared and used as the raw material composition. The first pyrolysis step (S1) was performed using the pyrolysis apparatus shown in Figure 6, with the pyrolysis atmosphere set to a nitrogen atmosphere via a nitrogen introduction tube. The gas generated in the first pyrolysis step (S1) was condensed in a cooling tube cooled to -10°C to prepare the first pyrolysis liquid. Subsequently, the GC-FID of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 2-2.

[0159] Example 52 A polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, which are post-consumer materials, was prepared by adding ethylbenzene to a concentration of 10% by mass, heating to 40°C, and stirring for 3 hours to prepare a slurry. Celite #350 (diatomaceous earth) was added at a concentration of 1% by mass relative to the weight of the slurry and stirred for 1 hour. Next, the slurry was put into a pressure filter and filtered under pressure of 0.5 MPa of nitrogen at 40°C to obtain a filtrate. Subsequently, the obtained filtrate was deflated at 300°C to obtain a molten product containing the polystyrene resin composition, and this was subjected to a first pyrolysis step (S1) using the pyrolysis apparatus shown in Figure 6. The gas generated in the first pyrolysis step (S1) was condensed in a cooling tube cooled to -10°C to prepare a first pyrolysis liquid. Subsequently, the GC-FID analysis of the first pyrolysis liquid, the thermal weight loss rate of the solids remaining after pyrolysis or discharged by the discharge mechanism, and the residue rate were analyzed, and the evaluation results of the concentration (mass%) of styrene monomer in the first pyrolysis liquid and the residue rate during pyrolysis are shown in Table 2-2.

[0160] "Example 53 (Example 1 of using the monomer obtained in this example)" By performing a two-stage distillation on the first pyrolysis solution recovered in Example 42, styrene monomer obtained from the polystyrene resin composition through the pyrolysis process (hereinafter referred to as recycled styrene monomer) was obtained. A polymerization raw material composition liquid consisting of 75 parts by mass of the recycled styrene monomer, 25.0 parts by mass of ethylbenzene, and 0.02 parts by mass of 1,1-bis(t-butylperoxy)cyclohexane was supplied to a completely mixed reactor with a volume of 3.6 liters at a rate of 0.8 liters / hour, and the polymerization liquid was continuously supplied to a devolatilization device connected to a single-screw extruder through a 3.9-liter tower reactor to remove volatile components such as unreacted monomers and polymerization solvents. The polymerization temperature of the completely mixed reactor was 121°C, and the temperature of the tower reactor was continuously changed from 125°C to 135°C to carry out the polymerization. The temperature of the single-screw extruder was set to 200 - 250°C and the pressure was set to 10 torr to devolatilize volatile components such as unreacted monomers and polymerization solvents. The devolatilized volatile components were condensed in a condenser through which a refrigerant at -5°C passed, recovered as an unreacted liquid, and the styrene-based composition (r-PS1) obtained from the recycled styrene monomer was recovered as resin pellets. Also, instead of using the recycled styrene monomer, a styrene-based composition (v-PS1) obtained from virgin styrene monomer, which was obtained by performing the same polymerization using a so-called petroleum-derived (virgin) styrene monomer, was recovered as resin pellets. Regarding the styrene-based composition (r-PS1) obtained from the recycled styrene monomer and the styrene-based composition (v-PS1) obtained from the virgin styrene monomer, no differences were found in the molecular weight distribution (Mn, Mw, Mz, Mz / Mw), MFR (g / 10 min), and HAZE (%) analyzed by the method described in Patent No. 7336840.

[0161] "Example 54 (Use Example 2 using the monomer obtained in this example)" The first pyrolysis liquid recovered in Example 42 was subjected to two-stage distillation to obtain a styrene monomer (hereinafter, recycled styrene monomer) obtained through a pyrolysis process from a polystyrene-based resin composition. A polymerization solution was prepared by mixing and dissolving 78.2 parts by mass of recycled styrene monomer as a styrene monomer, 9.8 parts by mass of polybutadiene rubber as a rubbery polymer, 12 parts by mass of ethylbenzene as a solvent, 0.003 parts by mass of 1,1-bis(t-butylperoxy)cyclohexane as a polymerization initiator, and 0.10 parts by mass of α-methylstyrene dimer as a chain transfer agent. This polymerization solution was continuously charged at a rate of 3.2 liters / hr into a 6.2-liter laminar flow reactor-1 equipped with a stirrer and temperature controllable, and the temperature was adjusted to 120°C. The stirrer speed was set to 120 revolutions per minute. Next, the reaction mixture from laminar flow reactor-1 was transferred to laminar flow reactor-2, a 6.2-liter laminar flow reactor-2 equipped with a stirrer and temperature controllable, which was connected in series with laminar flow reactor-1. The stirrer speed was set to 15 revolutions per minute, and the temperature was set to 138°C. Subsequently, the reaction mixture from laminar flow reactor-2 was transferred to laminar flow reactor-3, a 6.2-liter laminar flow reactor-3 equipped with a stirrer and temperature controllable. The stirrer speed was set to 10 revolutions per minute, and the temperature was set to 156°C. The polymer solution continuously discharged from the polymerization reactor (laminar flow reactor-3) was guided to an extruder with a vacuum vent, and defoliated and pelletized under a reduced pressure of 10 torr. The extruder temperature was set to 230°C. The styrene-based composition (r-PS2) obtained from recycled styrene monomer was recovered as resin pellets. Furthermore, instead of using recycled styrene monomer, a styrene-based composition (v-PS2) obtained from virgin styrene monomer, which was obtained by performing a similar polymerization using so-called petroleum-derived (virgin) styrene monomer, was recovered as resin pellets. No differences were observed in the molecular weight distribution (Mn, Mw, Mz, Mz / Mw) of a styrene-based composition obtained from recycled styrene monomer (r-PS2) and a styrene-based composition obtained from virgin styrene monomer (v-PS2), as analyzed by the method described in Japanese Patent No. 7336840, or in the rubber content (%), gel content (%), rubber particle size (μm), and expansion index, as analyzed by the method described in Japanese Patent No. 7622264.

[0162] "Comparative Examples 1-3" A polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, prepared using the thermal decomposition conditions shown in the comparative examples in Table 3, was used as the raw material composition for the thermal decomposition process. The gas generated by the thermal decomposition was condensed in a cooling tube cooled to -10°C to obtain the thermal decomposition liquid. Subsequently, the thermal weight loss rate and residue rate of the solids remaining after thermal decomposition or discharged by the discharge mechanism were analyzed using GC-FID, and the concentration (mass%) of styrene monomers in the thermal decomposition liquid and the residue rate during thermal decomposition were evaluated, respectively. The results of these evaluations are shown in Table 3.

[0163] "Comparative Example 4" Using the thermal decomposition conditions shown in the comparative examples in Table 3, a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units was prepared as the raw material composition. The first thermal decomposition step (S1) was carried out using the thermal decomposition apparatus shown in Figure 1, with the thermal decomposition atmosphere set to a nitrogen atmosphere via a nitrogen introduction tube. The first thermal decomposition liquid was prepared by condensing the gas generated in the first thermal decomposition step (S1) in a cooling tube cooled to -10°C. Subsequently, the first thermal decomposition liquid was analyzed by GC-FID, the thermal weight loss rate of the solids remaining after thermal decomposition or discharged by the discharge mechanism, and the residue rate. The evaluation results of the concentration (mass%) of styrene monomer in the first thermal decomposition liquid and the residue rate during thermal decomposition are shown in Table 3.

[0164] "Reference example" A polystyrene resin composition containing conjugated diene monomer units and styrene monomer units, prepared according to the thermal decomposition conditions shown in the reference example in Table 3, was used as the raw material composition, and a thermal decomposition process was carried out. The gas generated by the thermal decomposition was condensed in a cooling tube cooled to -10°C to obtain the thermal decomposition liquid. Subsequently, the thermal weight loss rate and residue rate of the solids remaining after thermal decomposition or discharged by the discharge mechanism were analyzed using GC-FID, and the concentration (mass%) of styrene monomers in the thermal decomposition liquid and the residue rate during thermal decomposition were evaluated, respectively. The results of the evaluation are shown in Table 3. This reference example was conducted in accordance with the following reference example of a thermal decomposition experiment.

[0165] <Example of a thermal decomposition experiment> A SUS reaction vessel is placed inside a cast heater, a SUS cover with a branch pipe is attached, and bolts are tightened with a wrench. A special adapter with an O-ring is attached to the branch pipe, and a glass Liebig condenser, a distillation adapter with a branch for attaching a pressure-reducing hose, and an eggplant flask for recovering the pyrolysis liquid are attached, each coated with silicone grease. A three-way stopcock is attached to the branch for attaching the pressure-reducing hose, and a nitrogen balloon and a vacuum pump are connected. The temperature is monitored using thermocouples installed inside the cast heater and the vessel. The refrigerant is supplied at -10°C, the pressure is set with a vacuum pump, and the output of the cast heater is adjusted. A polystyrene resin composition containing conjugated diene monomer units and styrene monomer units is charged into the SUS reaction vessel at 25°C, and pyrolysis is performed by adjusting the temperature from 25°C to reach 460°C in the first pyrolysis atmosphere, and the first pyrolysis liquid is recovered in an eggplant flask by liquefaction in a Liebig condenser into which the refrigerant flows. In the reference example in Table 3, the ambient temperature (Tb) for the pyrolysis process was 25°C. The temperature was then raised to 460°C for continuous operation. In contrast, the ambient temperature (Tb) for the pyrolysis process in the examples and comparative examples was maintained at the ambient temperatures (Tb) of the pyrolysis process immediately after supplying the polystyrene resin composition, as shown in the table, during continuous operation.

[0166] [Table 1-1]

[0167] [Table 1-2]

[0168] [Table 2-1]

[0169] [Table 2-2]

[0170] Table 3

Claims

1. A method for producing styrene monomers comprising a first thermal decomposition step and a recovery step, The first thermal decomposition step is a step of supplying a polystyrene resin composition containing styrene monomer units to an atmosphere heated to 300°C or higher, and thermally decomposing it to obtain a first thermal decomposition liquid. The recovery step is a step of recovering styrene monomers from the first pyrolysis liquid, The ambient temperature (Tb) of the first pyrolysis step and the temperature (Tc) of the supplied polystyrene resin composition are given by the following formula (1): [Mathematics 1] |Tb-Tc|≧150℃ (In the above formula (1), Tb represents the ambient temperature of the first pyrolysis step (hereinafter referred to as the first pyrolysis ambient temperature), and Tc represents the temperature of the polystyrene resin composition immediately before being supplied to the atmosphere.) A method for producing styrene monomers, characterized by satisfying the relationship.

2. A method for producing styrene monomers comprising a first thermal decomposition step and a recovery step, The first thermal decomposition step is a step of supplying a polystyrene resin composition containing conjugated diene monomer units and styrene monomer units to an atmosphere heated to 300°C or higher, and thermally decomposing it to obtain a first thermal decomposition liquid. The recovery step is a step of recovering styrene monomers from the first pyrolysis liquid, The ambient temperature (Tb) of the first pyrolysis step and the temperature (Tc) of the supplied polystyrene resin composition are given by the following formula (1): [Mathematics 2] |Tb-Tc|≧150℃ (In the above formula (1), Tb represents the temperature of the first thermal decomposition atmosphere, and Tc represents the temperature of the polystyrene resin composition immediately before being supplied to the atmosphere.) A method for producing a styrene monomer according to claim 1, characterized in that it satisfies the following relationship.

3. A method for producing a styrene monomer according to claim 1 or 2, wherein the temperature of the supplied polystyrene resin composition is 0°C or higher and less than 300°C.

4. A method for producing a styrene monomer according to claim 1 or 2, characterized in that the polystyrene resin composition supplied to the first thermal decomposition step is a fluid.

5. A method for producing a styrene monomer according to claim 1 or 2, wherein the molten polystyrene resin composition is supplied to a thermal decomposition section, and the thermal decomposition products of the polystyrene resin composition are transferred without using an external pumping means until a thermal decomposition liquid is obtained from the thermal decomposition section.

6. A method for producing a styrene monomer according to claim 1 or 2, wherein the first thermal decomposition atmosphere temperature is 400°C or higher and less than 1200°C.

7. A method for producing a styrene monomer according to claim 1 or 2, wherein the atmospheric pressure in the first thermal decomposition step is 0 hPa or more and 1013 hPa or less.

8. The method for producing a styrene monomer according to claim 1 or 2, wherein the content of conjugated diene monomer units in the polystyrene resin composition is more than 0% by mass.

9. The method for producing a styrene monomer according to claim 1 or 2, wherein the first thermal decomposition step includes a cooling step of cooling a thermal decomposition gas containing a styrene monomer obtained by thermal decomposing the polystyrene resin composition to obtain the thermal decomposition liquid.

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