Process for recovering styrene monomer from waste polystyrene

Abstract

The present invention relates to a method for recovering styrene monomer from waste polystyrene, and more particularly, to a method for recovering styrene monomer from waste polystyrene, which can inhibit the generation of by-products such as ethylbenzene by depolymerizing waste polystyrene using an environmentally friendly solvent and a potassium carbonate-containing depolymerization catalyst, and thus recover styrene monomer at a high yield.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for recovering styrene monomer from waste polystyrene, and more particularly, to a method for recovering styrene monomer from waste polystyrene, which is characterized by recovering styrene monomer in a high yield and high purity by inhibiting by-products generated in a side reaction, as a method for recovering styrene monomer by depolymerizing waste polystyrene in the presence of a catalyst. BACKGROUND

[0002] With the development of industries, a large amount of plastics is being used worldwide, and Korea has produced about 7 million tons or more of general plastic products last year, becoming the 4th largest plastic producer in the world. However, plastics are largely discarded after use, causing various environmental problems. Waste plastics are currently mainly treated by landfill, but can cause serious environmental problems due to long biodegradation time in soil and lack of landfill sites, and thus development of technology for recycling waste plastics as resources as described above is also attracting attention.

[0003] Various methods for treating waste plastics have been proposed, but from the aspects of environmental problems and economics, a method for recycling fuel oil and raw material substances having added value is considered to be the most desirable method compared to simple physical addition or processing. The recycling method of waste plastics is divided into a method for recycling as it is or after processing (material recycle), thermal recycling such as incineration, and a method for recovering chemical substances such as resin raw materials (chemical recycle).

[0004] As for the physical recycling method, recycling is mainly performed in the aspects of manufacturing of recycled resins, manufacturing of lightweight concrete, and manufacturing of adhesives, but the added value of such a physical recycling method is very low, and after a plurality of physical recycling, it cannot be recycled, and thus a large amount of waste polystyrene is finally generated. In addition, a large amount of waste polystyrene contaminated from agricultural and fishery markets or construction waste is not as clean as other waste polystyrene, and thus it is difficult to use in the physical recycling method.

[0005] In particular, a large amount of waste polystyrene contaminated is difficult to be physically recycled because it has a volume about 50 times or more than other waste polystyrene, and thus is treated by landfill or incineration. However, the incineration method can cause environmental problems because dioxin is generated.

[0006] Accordingly, a chemical recycling method has been focused on, and initially, Nishizaki et al. tried to develop a technology for recovering styrene monomers from waste polystyrene in 1997, and in the report, it was pointed out that about 50% of monomers could be recovered from polystyrene by thermal decomposition at 733 K. Based on the technology as described above, many researchers have reviewed the effects of various catalysts in order to improve the yield of styrene monomers and have developed various catalysts.

[0007] As the method for recovering styrene monomers using a catalyst, a technology for recovering styrene monomers using a metal oxide such as Co3O4, Fe2O3, Cr2O3, and CuO, which is strong in acidity, as a catalyst (non-patent document 0001), a technology for recovering styrene monomers using a sulfate catalyst (patent documents 0001 and 0002), and a binary catalyst technology (patent document 0003) in which a metal oxide is used as a main catalyst and an alkaline catalyst is used as an auxiliary catalyst, which are respectively supported on silica and alumina, are proposed.

[0008] However, in the case of using a metal oxide such as Co3O4, Fe2O3, Cr2O3, and CuO, which is strong in acidity, as a catalyst as in non-patent document 0001, etc., a carbon cation can be generated by the cleavage of a single (C-C) bond of polystyrene and a thermal decomposition reaction can occur with the chain separation of styrene, but since the carbon cation is in an unstable state in which two electrons are missing, side reactions such as an attack on a benzene ring around it can occur, thereby causing a problem of a decrease in the yield of styrene monomers.

[0009] On the contrary, in the case of using an alkaline catalyst such as BaO, K2O, MgO, ZnO, and CaO, or a sulfate catalyst, since an octet rule is satisfied, a stable carbon anion without an electron missing state can be generated and a thermal decomposition reaction can occur with the chain separation of styrene, but a problem of a decrease in the selectivity of styrene monomers can occur due to an increase in by-products, i.e., ethylbenzene and α-methylstyrene.

[0010] In particular, since the boiling point of ethylbenzene is similar to that of the final target substance, i.e., styrene monomers, expenses required for separating commercially usable high-purity (>99.6%) styrene monomers can increase, thereby causing a significant impact on economics.

[0011] Accordingly, there is a need for a new recycling method by which styrene monomers can be recovered from waste polystyrene at a high yield / high purity.

[0012] Prior Art Documents

[0013] Patent Documents

[0014] (Patent Document 1) Japanese Laid-Open Patent No. 2001-294708 (Publication Date: October 23, 2001)

[0015] (Patent Document 2) Korean Laid-Open Patent No. 2001-87093 (Publication Date: September 15, 2001)

[0016] (Patent Document 3) Korean Laid-Open Patent No. 2003-0081717 (Publication Date: October 22, 2003)

[0017] Non-Patent Document

[0018] (Non-Patent Document 1) Ind. Eng. Res., Vol. 34, No. 12, 1995, 4519 SUMMARY

[0019] The present application has an object to provide a method for recovering styrene monomers at a high yield by inhibiting the generation of ethylbenzene, α-methylstyrene, benzene, toluene, and the like, which are generated by side reactions in the conventional recovery process of waste polystyrene.

[0020] In particular, an attempt is made to inhibit the generation of ethylbenzene, which is difficult to separate because of a similar boiling point to that of styrene monomers, and thus to easily separate styrene monomers from depolymerization products of waste polystyrene.

[0021] To achieve the above object, one embodiment of the present application provides a method for recovering styrene monomers from waste polystyrene, characterized by comprising: step (a) of obtaining a mixture in which polystyrene is dissolved by adding one or more kinds of solvent selected from tetrahydrofuran and methyltetrahydrofuran and one or more kinds of depolymerization catalyst selected from potassium carbonate and sodium bicarbonate to waste polystyrene; step (b) of separating and recovering the solvent by distilling the mixture in which polystyrene is dissolved; and step (c) of obtaining a product containing styrene monomers by depolymerizing the mixture in which the solvent is separated and recovered in step (b), wherein the weight ratio (SM / EB) of styrene monomers (SM) to ethylbenzene (EB) in the product obtained in step (c) is 80 or more.

[0022] One preferable embodiment of the present application is characterized in that, in step (a), 100 to 200 parts by weight of tetrahydrofuran and / or methyltetrahydrofuran is added to 100 parts by weight of waste polystyrene.

[0023] One preferable embodiment of the present application is characterized in that step (a) is performed at normal temperature and pressure.

[0024] The preferred embodiment of the present application is characterized in that the distillation in step (b) is performed at 80°C to 250°C.

[0025] The preferred embodiment of the present application is characterized in that the depolymerization in step (c) is performed at 200°C to 600°C.

[0026] The preferred embodiment of the present application is characterized in that, in step (c), 1 part by weight to 10 parts by weight of potassium carbonate and / or potassium bicarbonate is present with respect to 100 parts by weight of the waste polystyrene.

[0027] The preferred embodiment of the present application is characterized in that the solvent, i.e., tetrahydrofuran and / or methyl tetrahydrofuran, separated in step (b) is reused in the dissolution of the waste polystyrene in step (a).

[0028] The method for recovering styrene monomers from waste polystyrene according to the present application can suppress the generation of by-products such as ethylbenzene by depolymerizing polystyrene using a specific solvent and a specific catalyst in combination, thereby improving the yield of the recovered styrene monomers.

[0029] Further, since the solvent used in the present application is an environmentally friendly solvent having a low boiling point, it is possible to prevent environmental pollution caused by the use of toxic organic solvents such as toluene, and it is also possible to reduce the recovery cost required for solvent reuse. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a schematic flowchart illustrating a method for recovering styrene monomers from waste polystyrene according to an embodiment of the present application.

[0031] Figure 2 is a graph illustrating the yield of by-products generated in Embodiments 1 and 2 and Comparative Examples 1 to 5 according to the present application.

[0032] Figure 3 is a graph illustrating the weight ratio of styrene monomers to ethylbenzene generated in Embodiments 1 and 2 and Comparative Examples 1 to 5 according to the present application.

[0033] Figure 4 is a graph of the X-ray diffraction (XRD) measurement results of K2CO3 obtained after evaporating the solvent from 5 g of K2CO3 in the original state and a mixture prepared by dissolving K2CO3 in 100 ml of tetrahydrofuran (THF) and 100 ml of toluene, respectively. DETAILED DESCRIPTION

[0034] Unless otherwise defined, all technical and scientific terms used in the present specification have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Generally, the nomenclature used in this specification is widely used and commonly understood by those skilled in the art.

[0035] Throughout the specification, when recited as a certain part "comprises" a certain component, unless otherwise explicitly recited to the contrary, it does not mean that other components are excluded, but it means that other components can also be included.

[0036] "Spent polystyrene" recited in the present specification is a meaning including spent polystyrene discarded after use in various industrial fields, which can be applied regardless of its shape and use, etc.

[0037] "Normal temperature" recited in the present specification means an atmospheric temperature under normal circumstances, which can be 20±10°C, and "normal pressure" means an atmospheric pressure under normal circumstances.

[0038] The terms such as "equipped with", "including", or "having" recited in the present specification are only used to indicate that the features, values, steps, actions, components, parts, or combinations thereof recited in the specification exist, and do not exclude the possibility that other features, values, steps, actions, components, parts, or combinations thereof not mentioned exist or are added in advance.

[0039] The present application relates to a method for recovering styrene monomer from spent polystyrene, characterized by comprising: step (a) obtaining a mixture in which polystyrene is dissolved by feeding to spent polystyrene one or more solvents selected from tetrahydrofuran and methyltetrahydrofuran and one or more depolymerization catalysts selected from potassium carbonate and sodium bicarbonate; step (b) separating and recovering the solvent by distilling the mixture in which polystyrene is dissolved; and step (c) obtaining a product containing styrene monomer by depolymerizing the mixture in which the solvent is separated and recovered in step (b), wherein the weight ratio of styrene monomer (SM) to ethylbenzene (EB) (SM / EB) in the product obtained in step (c) is 80 or more. In addition, the yield of styrene monomer according to the method of the present application exceeds 70 wt%.

[0040] Specifically, according to the method for recovering styrene monomers according to one embodiment of the present application, the generation of by-products such as ethylbenzene in the recovery process of styrene monomers can be inhibited by depolymerizing polystyrene using a combination of an environmentally friendly solvent derived from biomass, i.e., tetrahydrofuran and / or methyltetrahydrofuran, and a depolymerization catalyst, i.e., potassium carbonate and / or potassium bicarbonate, thereby improving the yield of recovered styrene monomers, and the waste polystyrene can be dissolved by adding the depolymerization catalyst together with the solvent when dissolving the waste polystyrene, thereby improving the depolymerization efficiency of the waste polystyrene by virtue of the uniform dispersion of the depolymerization catalyst in the solvent.

[0041] Next, a preferred embodiment of the method for recovering styrene monomers from waste polystyrene according to the present application will be described in detail with reference to the accompanying drawings.

[0042] Figure 1 is a schematic process diagram illustrating the method for recovering styrene monomers from waste polystyrene according to one embodiment of the present application.

[0043] Referring to Figure 1 , the method for recovering styrene monomers from waste polystyrene according to one embodiment of the present application, first in

step (a)

[0044] In the present application, the tetrahydrofuran (THF), methyltetrahydrofuran (MTHF), and mixtures thereof are used as solvents for dissolving the waste polystyrene.

[0045] The tetrahydrofuran (THF) and methyltetrahydrofuran (MTHF) have a high solubility for polystyrene, but a low solubility for other synthetic resins such as polypropylene and polyethylene, have a high vaporization point while being non-flammable, and have extremely high environmental properties because they are made from biomass, thereby allowing the waste polystyrene to be completely dissolved quickly at normal temperature and pressure without causing changes in the properties of the polystyrene itself, and thereby improving the volume reduction efficiency.

[0046] In addition, as the existing solvents for dissolving polystyrene, toluene, styrene, benzene, xylene, and limonene can be used, but the solubility of polystyrene is low, and the solvents are toxic to the human body and can cause environmental problems, and the boiling point of the solvents is similar to that of styrene monomers, so that the cost of separating the styrene monomers increases as the content of the solvents increases, and thus the economy is affected.

[0047] Therefore, in the present application, by dissolving the waste polystyrene with tetrahydrofuran, methyltetrahydrofuran, and a mixture thereof, the waste polystyrene can be completely dissolved quickly even at normal temperature and pressure, and thus the volume reduction efficiency is improved, and because the vaporization point is high, problems such as environmental pollution and solvent loss due to natural evaporation can be prevented during the recovery of the solvent, and the convenience of the subsequent operation process such as solvent recovery can be improved.

[0048] With respect to 100 parts by weight of the waste polystyrene, 100 to 200 parts by weight of the tetrahydrofuran and / or methyltetrahydrofuran can be added. In the case where the content of the tetrahydrofuran and / or methyltetrahydrofuran is less than 100 parts by weight with respect to 100 parts by weight of the waste polystyrene, the volume reduction of the polystyrene can be insufficient, and in the case where the content exceeds 200 parts by weight, the recovery cost of the solvent can increase.

[0049] In addition, potassium carbonate (K2CO3) and / or potassium bicarbonate (KHCO3) are thermal decomposition catalysts for promoting thermal decomposition, and because they are stable at high temperatures, they have high catalyst stability and are easy to recover.

[0050] In addition, the potassium carbonate (K2CO3) and / or potassium bicarbonate (KHCO3) are added to the polystyrene together with the tetrahydrofuran and / or methyltetrahydrofuran when the polystyrene is dissolved, so that the phenomenon of catalyst agglomeration that occurs when the catalyst is added in the existing depolymerization step can be prevented, and because the polystyrene is uniformly dispersed in the tetrahydrofuran and / or methyltetrahydrofuran, the depolymerization efficiency of the polystyrene can be improved.

[0051] At this time, with respect to 100 parts by weight of the waste polystyrene, 1 to 10 parts by weight of the potassium carbonate (K2CO3) and / or potassium bicarbonate (KHCO3) can be present. In the case where the content of the potassium carbonate (K2CO3) and / or potassium bicarbonate (KHCO3) is less than 1 part by weight with respect to 100 parts by weight of the waste polystyrene, the reactivity can be greatly reduced and the content of ethylbenzene can be greatly increased, and in the case where the content exceeds 10 parts by weight, the cost can increase but the reactivity can not be different.

[0052] To dissolve the waste polystyrene, the mixture added with tetrahydrofuran and / or methyl tetrahydrofuran and potassium carbonate (K2CO3) and / or potassium bicarbonate (KHCO3) as described above can be stirred at normal temperature and pressure. At this time, after the waste polystyrene is completely dissolved in the solvent, the foreign substances contained in the waste polystyrene, such as flame retardants, adhesives, and coating agents, other than the polystyrene, can be precipitated by temporarily standing, and the precipitated foreign substances can be removed from the mixture by filtering.

[0053] Further, the method for recovering styrene monomers according to one embodiment of the present application can further include a step of removing foreign substances, such as plastic, paper pieces, wood chips, food residues, and small stones, attached to the waste polystyrene by a method such as washing and pulverizing the waste polystyrene to an appropriate size suitable for dissolution before the step (a). At this time, the pulverized waste polystyrene can be pulverized to a size suitable for dissolution, i.e., an average particle size of 1.0 cm to 10 cm.

[0054] Next, in the step (b), since the mixture in which the polystyrene is dissolved, obtained by adding the tetrahydrofuran and / or methyl tetrahydrofuran and the potassium carbonate (K2CO3) and / or potassium bicarbonate (KHCO3), can be decomposed at a reaction temperature or can cause hindrance to a catalyst reaction, the solvent, i.e., the tetrahydrofuran and / or methyl tetrahydrofuran, is separated and recovered by performing distillation.

[0055] The distillation can be performed at 80°C to 250°C at normal pressure using a general distillation apparatus. In the case where the temperature at which the distillation is performed is less than 80°C, there can be a problem in that the tetrahydrofuran and / or methyl tetrahydrofuran cannot be effectively separated and recovered from the mixture in which the polystyrene is dissolved, and in the case where the temperature exceeds 250°C, there can be a problem in the recovery of the solvent due to thermal decomposition of the solvent.

[0056] The tetrahydrofuran and / or methyl tetrahydrofuran separated and recovered by the above-described method can be reused in the dissolution of the waste polystyrene of the step (a).

[0057] Next, in the step (c), the mixture in which the polystyrene is dissolved after the separation of the tetrahydrofuran and / or methyl tetrahydrofuran can perform a depolymerization reaction in the presence of the potassium carbonate (K2CO3) and / or potassium bicarbonate (KHCO3), thereby obtaining a product containing styrene monomers.

[0058] The depolymerization can be performed at 200 to 600°C for 0.5 to 5 hours. In the case where the depolymerization temperature is less than 200°C, there can be a problem in that the reactivity is greatly decreased because the depolymerization does not occur. In the case where the temperature exceeds 600°C, there can be a problem in that the contents of benzene and toluene are greatly increased because of cracking reaction, and thus the amount of styrene monomer produced is greatly decreased, and there can be a problem in that the higher boiling point dimers and trimers are mixed into the product. In addition, in the case where the depolymerization time is less than 0.5 hour, there can be a problem in that the amount of styrene monomer recovered is decreased because the reaction time is not sufficient. In the case where the time exceeds 5 hours, there can be a problem in that the productivity is decreased because the content of ethylbenzene is greatly increased and the energy consumption is increased.

[0059] Next, the product obtained in the depolymerization reaction can be recovered by capturing and cooling it. At this time, any cooling method suitable in the industry, such as an indirect cooling method in which a coolant is compressed and evaporated and the captured product is cooled using the evaporation heat of the coolant, or a direct cooling method, can be used without limitation.

[0060] In the product obtained by the depolymerization reaction, not only the final target component, i.e., styrene monomer (SM), but also high boiling point substances such as ethylbenzene (EB), toluene, cumene, and alpha methylstyrene, and depolymerization reaction residues, etc., are generated, and the residues as described above not only hinder the depolymerization reaction, but also decrease the yield of the styrene monomer, and currently cause a problem in that the yield of the styrene monomer is decreased when operated for a long time.

[0061] Therefore, in the method for recovering the styrene monomer according to the present application, by using the potassium carbonate-containing catalyst together with the environmentally friendly solvent to dissolve and depolymerize the waste polystyrene, the final target component, i.e., the styrene monomer, can be recovered at a yield of 70% or more, and the yields of ethylbenzene (EB), toluene, cumene, and alpha methylstyrene at the time of recovery can be controlled to 3.5% or less, respectively. In particular, in the method for recovering the styrene monomer according to the present application, the weight ratio (SM / EB) of the styrene monomer (SM) to ethylbenzene (EB) at the time of recovery can be 80 or more, and preferably 84 or more.

[0062] That is, in the method for recovering styrene monomer according to the present invention, the generation of ethylbenzene, α-methylstyrene, and toluene, especially ethylbenzene, which are generated by side reactions in existing styrene monomer recovery processes, can be suppressed, so as to facilitate the final execution of the separation process for recovering styrene.

[0063] The present invention will now be described in more detail through specific embodiments. The following embodiments are merely examples to aid in understanding the invention, and the scope of the invention is not limited thereto.

[0064] <Example 1>

[0065] 60g of pulverized waste polystyrene was added to a closable reactor at room temperature. Next, 100g of tetrahydrofuran and 3g of potassium carbonate (K₂CO₃) (Sigma-Aldrich, ACSreagent > 99%) as a catalyst were added. The mixture was then stirred for 10 minutes at room temperature and pressure to dissolve the waste polystyrene and disperse the catalyst. The tetrahydrofuran was separated by distillation of the mixture containing the dissolved waste polystyrene at 200°C for 30 minutes. The temperature was then raised to 375°C and a depolymerization reaction was carried out at this temperature for 1 hour. The products obtained by decomposition via the depolymerization reaction were obtained by liquefaction in a condenser, and the products were analyzed using a gas chromatography / flame ionization detector (GC / FID) (Yeonglin instrument) equipped with a capillary column (HP-5, 30m × 0.32mm × 1.0μm, Crosslinked 5% PH ME Siloxane). The results are shown in Table 1. Figure 1 as well as Figure 2 As shown.

[0066] <Example 2>

[0067] The product obtained by depolymerizing waste polystyrene according to the same method as in Example 1, wherein 3g of potassium bicarbonate (KHCO3) (Sigma-Aldrich, ACS reagent >99.7%) was added as a catalyst to replace potassium carbonate (K2CO3) for depolymerization. The obtained product was measured according to the same method as in Example 1, and the results are shown in Table 1. Figure 1 as well as Figure 2 As shown.

[0068] <Comparative Example 1>

[0069] The product obtained by depolymerizing waste polystyrene according to the same method as in Example 1 was obtained, wherein the depolymerization was performed without inputting a catalyst, and the obtained product was measured according to the same method as in Example 1, and the results are shown in Table 1, Figure 1 and Figure 2 .

[0070] <Comparative Example 2>

[0071] The product obtained by depolymerizing waste polystyrene according to the same method as in Example 1 was obtained, wherein 3 g of barium oxide (BaO) (Sigma-aldrich, ACS reagent > 97%) was inputted as a catalyst instead of potassium carbonate (K2CO3) to perform the depolymerization, and the obtained product was measured according to the same method as in Example 1, and the results are shown in Table 1, Figure 1 and Figure 2 .

[0072] <Comparative Example 3>

[0073] The product obtained by depolymerizing waste polystyrene according to the same method as in Example 1 was obtained, wherein 3 g of calcium oxide (CaO) (Sigma-aldrich, ACS reagent > 99.7%) was inputted as a catalyst instead of potassium carbonate (K2CO3) to perform the depolymerization, and the obtained product was measured according to the same method as in Example 1, and the results are shown in Table 1, Figure 1 and Figure 2 .

[0074] <Comparative Example 4>

[0075] The product obtained by depolymerizing waste polystyrene according to the same method as in Example 1 was obtained, wherein 3 g of strontium oxide (SrO) was inputted as a catalyst instead of potassium carbonate (K2CO3) to perform the depolymerization, and the obtained product was measured according to the same method as in Example 1, and the results are shown in Table 1, Figure 1 and Figure 2 .

[0076] <Comparative Example 5>

[0077] The product obtained by depolymerizing waste polystyrene according to the same method as in Example 1 was obtained, wherein 3 g of KNO3 was inputted as a catalyst instead of potassium carbonate (K2CO3) to perform the depolymerization, and the obtained product was measured according to the same method as in Example 1, and the results are shown in Table 1, Figure 1 and Figure 2 .

[0078] <Comparative Example 6>

[0079] The product obtained by depolymerizing waste polystyrene according to the same method as in Example 1 was obtained, in which depolymerization was performed using toluene instead of tetrahydrofuran as a solvent, and the obtained product was measured according to the same method as in Example 1, and the results are shown in Table 1, Figure 1 and Figure 2 .

[0080] [Table 1]

[0081]

[0082] As shown in Table 1, Figure 1 and Figure 2 , in Example 1 and Example 2, compared to Comparative Examples 1 to 6, a higher yield of styrene monomer (SM) was shown, and the yield of by-products, i.e., ethylbenzene (EB), toluene, cumene, and alpha-methylstyrene, was lower. In particular, in Example 1 and Example 2, the yield of styrene monomer (SM) was more than 70 wt%, and the weight ratio of styrene monomer (SM) to ethylbenzene (EB) was more than 84.

[0083] Comparing Example 1 and Example 2 with Comparative Examples 1 to 5, it can be found that the presence or type of the catalyst is also significantly related to the yield of styrene monomer (SM) and the amount of by-products generated. In particular, even in the case of using the same potassium salt, a large difference in activity was shown depending on the type of anion, in which the form of carbonate (CO3 2- ) or bicarbonate (HCO 3- ) showed a higher yield of styrene monomer (SM) and less by-products such as ethylbenzene (EB).

[0084] In addition, comparing Example 1 with Comparative Example 6, it can be found that even in the case of the same type of catalyst, the type of solvent in which waste polystyrene is dissolved is related to the final yield of styrene monomer and the amount of by-products generated. In Comparative Example 6, toluene, which is currently the most commonly used solvent for volume reduction of waste polystyrene, was used as a solvent, and in Example 1, tetrahydrofuran (THF) was used as a solvent. In the case of using toluene as a solvent, the yield of styrene monomer was 62.3 wt%, which is only about 88% of the case in which tetrahydrofuran (THF) was used as a solvent, and the yield of ethylbenzene was higher.

[0085] To confirm the cause of the activity difference as described above, K2CO3 in a pristine state (pristine K2CO3) and mixtures produced by dissolving K2CO3 in 100 ml of tetrahydrofuran (THF) and 100 ml of toluene, respectively, were stirred for 24 hours, the solvent was evaporated at 120°C for 12 hours after which the obtained K2CO3 was analyzed using X-ray diffraction (XRD) (Rigaku Ultima IV), and the results are shown in FIG. 2. Figure 4

[0086] Referring to Figure 4 , K2CO3 after being dissolved in tetrahydrofuran (THF) and toluene showed a change in overall crystallinity compared to pristine K2CO3, and in particular, in the case of using toluene, the peak at 2θ = 32.5 degrees increased significantly and the peak at 2θ = 31.6 degrees decreased, indicating that K2CO3 in tetrahydrofuran (THF) showed higher stability and lower phase transformation tendency compared to toluene, thereby confirming that the structure of K2CO3 in a tetrahydrofuran (THF) solution is more stable than in toluene.

[0087] Therefore, it can be seen that in the case of depolymerizing waste polystyrene using a combination of a solvent and a catalyst by selecting one or more of tetrahydrofuran (THF) and methyltetrahydrofuran (MTHF) as the solvent and one or more of K2CO3 and KHCO3 as the depolymerization catalyst as described in the present application, styrene monomers can be recovered in high yield by depolymerization, and the generation of ethylbenzene, which has a boiling point similar to that of styrene monomers, can be inhibited, thereby saving costs in the product separation process, and thus the efficiency of the depolymerization process of waste polystyrene can be greatly improved in actual industry.

[0088] The preferred embodiments of the present application are described above, but the present application is not limited thereto, and various modifications can be made within the scope of the claims and the detailed description of the application and the drawings, and such modifications are also included in the scope of the present application.​

Claims

1. A method for recovering styrene monomer from waste polystyrene, characterized in that, include: Step (a) involves adding one or more solvents selected from tetrahydrofuran and methyltetrahydrofuran and one or more depolymerization catalysts selected from potassium carbonate and potassium bicarbonate to waste polystyrene to obtain a mixture containing dissolved polystyrene. Step (b) involves separating and recovering the solvent by distilling the mixture containing dissolved polystyrene; and Step (c) involves depolymerizing the mixture obtained in step (b) from which the solvent has been separated and recovered to obtain a product containing styrene monomers. In step (c), the weight ratio of styrene monomer SM to ethylbenzene EB in the product is SM / EB or higher.

2. The method for recovering styrene monomer from waste polystyrene according to claim 1, characterized in that, In step (a), 100 to 200 parts by weight of tetrahydrofuran and / or methyltetrahydrofuran are added relative to 100 parts by weight of waste polystyrene.

3. The method for recovering styrene monomer from waste polystyrene according to claim 1, characterized in that, Step (a) is performed at normal temperature and pressure.

4. The method for recovering styrene monomer from waste polystyrene according to claim 1, characterized in that, The distillation in step (b) is performed at 80°C to 250°C.

5. The method for recovering styrene monomer from waste polystyrene according to claim 1, characterized in that, The depolymerization in step (c) is performed at 200°C to 600°C.

6. The method for recovering styrene monomer from waste polystyrene according to claim 1, characterized in that, In step (c), there are 1 to 10 parts by weight of potassium carbonate and / or potassium bicarbonate relative to 100 parts by weight of waste polystyrene.

7. The method for recovering styrene monomer from waste polystyrene according to claim 1, characterized in that, The solvent separated in step (b) is reused in the dissolution of the waste polystyrene in step (a).

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