Continuous recovery method for styrene monomer from waste polystyrene

Abstract

The present invention relates to a method for continuously recovering styrene monomer from waste polystyrene, and more particularly to a method for continuously recovering styrene monomer from waste polystyrene, which can recover styrene monomer in high yield and high purity by mixing with potassium carbonate (K2CO3) catalyst and then continuously depolymerizing it.

Description

Technical Field

[0001] The present invention relates to a method for continuously recovering styrene monomer from waste polystyrene. More specifically, the present invention relates to a method for continuously recovering styrene monomer from waste polystyrene with high yield and high purity by mixing waste polystyrene with a potassium carbonate (K2CO3) catalyst and then continuously depolymerizing it.

Background Art

[0002] With the development of industries, a large amount of plastics are used worldwide. In the case of Korea, more than about 7 million tons of general-purpose plastic products were produced last year, making it one of the top four plastic-producing countries in the world. However, a large amount of plastics are discarded after use, causing many environmental problems. Currently, waste plastics are mainly treated by landfill, but their biodegradation time in soil is long, and the shortage of landfill sites and other problems cause serious environmental problems. Therefore, much attention is being paid to the development of technologies for recycling such waste plastics as resources.

[0003] Although various methods have been proposed for the treatment of waste plastics, methods of reusing them as value-added fuel oil and raw materials rather than simple physical addition or processing are considered the most desirable methods in terms of environmental problems or economics. Recycling methods for waste plastics can be divided into methods of recycling them as they are or after processing (material recycle), thermal recycling such as incineration (thermal recycle), and methods of recovering chemical substances such as resin raw materials (chemical recycle).

[0004] In the case of physical recycling methods, polystyrene is mainly recycled into the manufacture of recycled resin, lightweight concrete, and adhesives. However, this method has very little added value, and after repeated physical recycling, it becomes impossible to recycle, ultimately resulting in the generation of large amounts of waste polystyrene. Furthermore, large amounts of contaminated waste polystyrene discharged into agricultural and marine product markets or as construction waste are less clean than other types of waste polystyrene, making them difficult to use in physical recycling methods.

[0005] Furthermore, because large quantities of contaminated waste polystyrene have a volume approximately 50 times larger than other waste polystyrene, physical recycling is difficult, and they are disposed of through landfill or incineration. However, incineration has the problem of causing environmental issues such as the generation of dioxins.

[0006] As a result, chemical recycling methods have attracted attention, and the technology for recovering styrene monomer from waste polystyrene was first attempted by Nishizaki et al. in 1997, reporting that it was possible to recover approximately 50% of the monomer from polystyrene by depolymerization at 733K. Based on this technology, many researchers have investigated the effects of various catalysts to increase the yield of styrene monomer, and many catalysts have been developed.

[0007] Methods for recovering styrene monomer using such catalysts include techniques for recovering styrene monomer using metal oxides with high acidity, such as Co3O4, Fe2O3, Cr2O3, and CuO, as catalysts (Non-Patent Document 1), techniques for recovering styrene monomer using sulfate catalysts (Patent Documents 1 and 2), and binary catalyst technology (Patent Document 3) manufactured by supporting a metal oxide as the main catalyst and a basic catalyst as the co-catalyst on silica and alumina, respectively.

[0008] In particular, continuous depolymerization processes are more advantageous than batch processes for improving the economic efficiency of commercial processes for recycling waste polystyrene, but there has been little research on continuous depolymerization processes in the prior art. According to the present invention, when waste polystyrene is continuously depolymerized for a long period of time using the conventional technology, the activity of the depolymerization catalyst decreases as the reaction time progresses, which suppresses the depolymerization reaction of polystyrene, reduces oil production, lowers the yield of the monomer styrene, and significantly increases byproducts such as ethylbenzene, α-methylstyrene, benzene, and toluene. Ethylbenzene and α-methylstyrene have boiling points that are almost the same as those of the monomer styrene, and as their amount increases, the cost required to separate styrene increases, which worsens the economic efficiency.

[0009] Therefore, in order to commercially mass-produce recycled styrene from waste polystyrene, a new process technology is absolutely necessary that produces styrene in high yield and high purity through a continuous depolymerization process rather than a batch process. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Published Patent No. 2001-294708 (Publication Date: October 23, 2001) [Patent Document 2] Korean Published Patent No. 2001-87093 (Publication Date: September 15, 2001) [Patent Document 3] Korean Published Patent No. 2003-0081717 (Publication Date: October 22, 2003) [Non-patent literature]

[0011] [Non-Patent Document 1] Ind.Eng.Res., Vol.34, No.12, 1995, 4519 [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] The present invention aims to solve the above-mentioned problems, and its main objective is to provide a continuous recovery method for styrene monomer that can suppress the generation of ethylbenzene, α-methylstyrene, benzene, toluene, etc., which are produced as by-reactions in conventional continuous recovery processes for waste polystyrene, while increasing the yield of styrene, which is a monomer. [Means for solving the problem]

[0013] To achieve the above objective, one embodiment of the present invention provides a method for the continuous recovery of styrene monomer from waste polystyrene by continuous depolymerization of waste polystyrene, comprising the step of continuously feeding waste polystyrene and a potassium carbonate (K2CO3) catalyst into a depolymerization reactor and depolymerizing them to obtain a styrene monomer-containing product, wherein the yield of styrene monomer (SM) obtained by the depolymerization is 70% or more and satisfies one or more of the following formulas (1) or (2). (Styrene yield) / (Ethylbenzene yield) ≥ 90 [Equation 1] (Styrene yield) / (toluene yield + ethylbenzene yield + α-methylstyrene yield) ≥ 12 [Equation 2]

[0014] In a preferred embodiment of the present invention, the potassium carbonate (K2CO3) catalyst may be added in an amount of 0.1 to 10 parts by weight per 100 parts by weight of waste polystyrene.

[0015] In a preferred embodiment of the present invention, a method for continuously recovering styrene monomer from waste polystyrene can be characterized by comprising the steps of: (a) continuously feeding waste polystyrene and potassium carbonate (K2CO3) catalyst into a depolymerization reactor consisting of one or more stages and depolymerizing them to obtain a styrene monomer-containing depolymerization product; (b) discharging the gaseous depolymerization product from the depolymerization product to a depolymerization gas discharge section using a sweep gas, and discharging the depolymerization residue other than the gaseous depolymerization product to a residue treatment section; and (c) collecting the discharged gaseous depolymerization product to obtain styrene monomer.

[0016] In a preferred embodiment of the present invention, step (b) is characterized by supplying a sweep gas to the depolymerization residue discharge flow in a direction opposite to the direction of the depolymerization residue discharge flow, thereby separating the styrene monomer contained in the depolymerization residue flow and discharged to the residue processing unit from the depolymerization residue flow and discharging it to the depolymerization gas discharge unit.

[0017] In a preferred embodiment of the present invention, if the depolymerization reactor in step (b) consists of two or more stages, the depolymerization reactor is characterized in that a depolymerization gas discharge section is connected between each stage to discharge gaseous depolymerization products from among the depolymerization products generated between each stage to the outside of the depolymerization reactor and move the depolymerization residues other than the gaseous depolymerization products to the next stage, and a sweeping gas is further supplied to the flow of depolymerization residues moving between stages in the opposite direction to the flow direction of the depolymerization residues, so that the styrene monomers remaining in the depolymerization residues are discharged to the depolymerization gas discharge section.

[0018] In a preferred embodiment of the present invention, step (c) may further include a step of reducing the content of dimer+ in the gaseous depolymerization product discharged from the depolymerization gas discharge unit.

[0019] In a preferred embodiment of the present invention, the step of reducing the content of the dimer + can be characterized in that it is carried out by a separation column section installed in the depolymerization gas discharge section.

[0020] In a preferred embodiment of the present invention, the sweep gas can be characterized in that it is supplied after being heated.

[0021] In a preferred embodiment of the present invention, the residence time of the waste polystyrene in the depolymerization reactor can be characterized in that it is 15 minutes to 2 hours.

[0022] In a preferred embodiment of the present invention, the method further includes the step of discharging the styrene monomer from the residue treatment section and contacting the remaining residue with water to recover the potassium carbonate (K2CO3) catalyst present in the remaining residue.

Advantages of the Invention

[0023] The continuous recovery method of styrene monomer from waste polystyrene according to the present invention can suppress the generation of ethylbenzene and the like generated as by-products in the recovery process of styrene monomer by depolymerizing waste polystyrene under specific catalysts and conditions, and can increase the yield and purity of the recovered styrene monomer.

Brief Description of the Drawings

[0024] [Figure 1] It is a process diagram for continuously recovering styrene monomer from polystyrene using the continuous recovery apparatus of styrene monomer according to an embodiment of the present invention. [Figure 2] It is a process diagram for continuously recovering styrene monomer from polystyrene using the continuous recovery apparatus of styrene monomer according to another embodiment of the present invention. [Figure 3] It is a graph measuring the yield of products according to the depolymerization reaction temperature in Experimental Examples 1-1 to 1-5 according to the present invention. [Figure 4]This graph shows the yield of the product measured by the residence time in the depolymerization reactor in Experimental Examples 2-1 to 2-4 according to the present invention. [Figure 5] This graph shows the yield of the product based on the catalyst content in Experimental Examples 3-1 to 3-4 according to the present invention. [Figure 6] This graph shows the selective productivity of styrene monomer measured by the sweep gas supply rate in experimental examples 6-9 to 6-11 according to the present invention. [Figure 7] This graph shows the selective productivity of styrene monomer based on the height of the packing material in experimental examples 6-5, 6-8, and 6-11 according to the present invention. [Modes for carrying out the invention]

[0025] Prior to a detailed description of the present invention, it should be noted that the present invention can be modified in various ways and has various embodiments. Therefore, the examples described below and shown in the drawings should be understood not to limit the present invention to any particular embodiment, but to include all modifications, equivalents, or substitutions that fall within the spirit and technical scope of the present invention.

[0026] When one component is described as being "linked" or "connected" to another component, it should be understood that it is either directly linked or connected to the other component, or that other components may exist between them. On the other hand, when one component is described as being "directly linked" or "directly connected" to another component, it should be understood that there are no other components between them.

[0027] Terms such as “equipped with,” “includes,” or “have” as used herein indicate the presence of features, figures, steps, actions, components, parts, or combinations thereof as described herein, and do not preclude the presence or addition of other features, figures, steps, actions, components, parts, or combinations thereof not mentioned herein.

[0028] Furthermore, when explaining with reference to the attached drawings, regardless of the reference numerals used in the drawings, identical components will be given the same reference numerals, and redundant explanations will be omitted. When explaining the present invention, if it is determined that a specific explanation of related known technologies may unnecessarily obscure the gist of the present invention, such detailed explanation will be omitted.

[0029] The present invention provides a method for the continuous recovery of styrene monomer from waste polystyrene by continuous depolymerization of waste polystyrene, comprising the steps of continuously feeding waste polystyrene and a potassium carbonate (K2CO3) catalyst into a depolymerization reactor and depolymerizing to obtain a styrene monomer-containing depolymerization product, wherein the yield of styrene monomer (SM) obtained by the depolymerization is 70% or more and satisfies one or more of the following formulas (1) or (2). (Styrene yield) / (Ethylbenzene yield) ≥ 90 [Equation 1] (Styrene yield) / (toluene yield + ethylbenzene yield + α-methylstyrene yield) ≥ 12 [Equation 2]

[0030] More specifically, in one embodiment of the present invention, a continuous recovery method for styrene monomer involves mixing waste polystyrene with potassium carbonate (K2CO3) and then continuously depolymerizing it under specific conditions. This suppresses the generation of by-products such as ethylbenzene, which are produced during the styrene monomer recovery process, thereby increasing the yield and purity of the recovered styrene monomer.

[0031] The present invention will now be described in detail with reference to the attached drawings. Figure 1 is a process diagram showing the continuous recovery of styrene monomer from polystyrene using a continuous styrene monomer recovery apparatus 100 according to one embodiment of the present invention, and Figure 2 is a process diagram showing the continuous recovery of styrene monomer from polystyrene using a continuous styrene monomer recovery apparatus 100 according to another embodiment of the present invention.

[0032] Referring to Figures 1 and 2, a preferred embodiment of the present invention provides a method for the continuous recovery of styrene monomer from waste polystyrene, which may include the steps of: (a) continuously feeding waste polystyrene and a potassium carbonate (K2CO3) catalyst into a depolymerization reactor consisting of one or more stages and depolymerizing them to obtain a styrene monomer-containing depolymerization product; (b) discharging the gaseous depolymerization product from the depolymerization product to a depolymerization gas discharge section using a sweep gas, and discharging the depolymerization residue other than the gaseous depolymerization product to a residue treatment section; and (c) collecting the discharged gaseous depolymerization product to obtain styrene monomer.

[0033] First, the method for continuously recovering styrene monomer from waste polystyrene according to the present invention involves continuously feeding waste polystyrene and a potassium carbonate (K2CO3) catalyst into a depolymerization reactor consisting of one or more stages, and carrying out a depolymerization reaction to obtain a styrene monomer-containing depolymerization product [(a) step].

[0034] The feeding device section 110 is generally a feeding device applied to a depolymerization reactor, and preferably includes a stirring screw (not shown) in a hopper section 111 equipped with an opening / closing device 112 to supply waste polystyrene P and catalyst S in a way that prevents air (especially oxygen) from being included during continuous feeding, and to prevent gaseous styrene monomer-containing products generated in subsequent processes from leaking to the outside.

[0035] The waste polystyrene P supplied to the input device includes all polystyrene discarded after use in various industrial fields, and can be applied regardless of its shape or use. Pretreatment steps such as washing and crushing can be performed before it is fed into the depolymerization reactor. At this time, the size of the crushed waste polystyrene can be adjusted according to the size of the depolymerization reactor.

[0036] On the other hand, potassium carbonate (K2CO3) catalyst C, which is introduced into the depolymerization reactor along with the waste polystyrene, is a catalyst for the depolymerization reaction of waste polystyrene. Compared to other alkali or alkaline earth carbonate catalysts, it has superior catalytic stability at high temperatures, is easy to recover from depolymerization residues, and when used as a catalyst for a continuous depolymerization reaction, it significantly increases the yield of styrene and drastically reduces by-products compared to a batch reaction, making it efficiently applicable to the continuous depolymerization reaction of waste polystyrene.

[0037] In this case, the potassium carbonate (K2CO3) catalyst can be added in an amount of 0.1 to 10 parts by weight, preferably 0.5 to 5 parts by weight, per 100 parts by weight of waste polystyrene. If the potassium carbonate (K2CO3) catalyst is present in an amount of less than 0.1 parts by weight per 100 parts by weight of waste polystyrene, the reactivity will decrease significantly, and the amount of ethylbenzene may increase significantly. If it exceeds 10 parts by weight, there will be no difference in reactivity, which may lead to increased costs.

[0038] Subsequently, the waste polystyrene, which is continuously fed into the depolymerization reactor 120 via the feeding device 110, undergoes a depolymerization reaction in the presence of a potassium carbonate (K2CO3) catalyst to obtain a styrene monomer-containing product.

[0039] The depolymerization reactor 120 can be any reactor capable of depolymerizing continuously fed waste polystyrene, but preferably it is an auger reactor consisting of one or more stages, connected to the feeding device section and equipped with at least one transfer screw inside for continuously transporting the waste polystyrene fed from the feeding device section from the feeding side to the discharge side.

[0040] Specifically, as shown in Figure 1, a depolymerization reactor according to one embodiment of the present invention comprises a box-type depolymerization reactor body 121 with an internal space, at least one heat source 124 provided around the depolymerization reactor body, and a depolymerization gas discharge unit 123 connected to the discharge port side of the depolymerization reactor body and used to discharge the depolymerization product generated from the depolymerization reactor. The reactor may also include a transfer screw 122 provided inside the depolymerization reactor body so as to generate a gaseous depolymerization product by heating the heat source while continuously transferring the waste polystyrene and catalyst that are fed into the reactor. In this case, the heat source 124 can be any ordinary heat source that can be applied to a depolymerization reactor, and examples include a burner, heater, heating element, etc.

[0041] As mentioned above, the depolymerization reactor according to the present invention can use a depolymerization reactor with a single depolymerization reactor body. However, the length of the body can be increased as needed. In such cases, considering the problem of excessive pressure generated by gas produced when waste polystyrene is heated, or the load on the transfer screw due to the increased length of the transfer screw that moves the waste polystyrene, it is preferable to have a multi-stage structure composed of multiple bodies. Therefore, the depolymerization reactor body can be composed of two or more bodies, and one can be adjusted and used according to the depolymerization conditions.

[0042] As an example, the depolymerization reactor having a multi-stage depolymerization reactor body, as shown in Figure 2, comprises: a first depolymerization reactor body 121a connected to an input device 110 at one end, which receives waste polystyrene and a catalyst from the input device 110 to perform a depolymerization reaction and produce a first depolymerization product; a first depolymerization gas discharge unit 123a connected to the discharge port side of the first depolymerization reactor body, which discharges gaseous depolymerization products from the first depolymerization reactor body to the outside and discharges depolymerization residues other than the gaseous depolymerization products to the next stage; a second depolymerization reactor body 121b connected to the other end of the first depolymerization gas discharge unit 123a, which receives the supply of depolymerization residues discharged from the first depolymerization gas discharge unit to perform a depolymerization reaction and produce a second depolymerization product; and the second The system may include: a second depolymerization gas discharge section 123b connected to the outlet side of the main body of the depolymerization reactor, which discharges gaseous depolymerization products from the second depolymerization product generated from the main body of the second depolymerization reactor to the outside and discharges depolymerization residues other than the gaseous depolymerization products to a downstream stage; a third depolymerization reactor main body 121c connected to the terminal end (other end) of the second depolymerization gas discharge section 123a, which generates a third depolymerization product by receiving a supply of depolymerization residues from the second depolymerization product discharged from the main body of the second depolymerization reactor and performing a depolymerization reaction; and a third depolymerization gas discharge section 123c connected to the outlet side of the main body of the third depolymerization reactor, which discharges gaseous depolymerization products from the third depolymerization product generated from the main body of the third depolymerization reactor to the outside and discharges depolymerization residues other than the depolymerization products to a downstream stage.

[0043] In the multi-stage depolymerization reactor configured in this way, waste polystyrene fed from the feeding device is transported in a zigzag flow and depolymerized by heating in the heat source to produce depolymerization products. The produced depolymerization products are separated into gaseous depolymerization products (dotted arrows) and depolymerization residues other than the gaseous depolymerization products (solid arrows) according to their boiling points. The separated gaseous depolymerization products (dotted arrows) are discharged to the outside or to the separation column section 140 described later, and the depolymerization residues (solid arrows) are discharged to the residue processing section.

[0044] In this configuration, the depolymerization reactor can control the depolymerization reaction temperature, residence time, and transfer screw rotation to adjust the yield of the styrene monomer produced. Similarly, in a multi-stage depolymerization reactor, the depolymerization reaction temperature, residence time, and transfer screw stirring speed of each individual depolymerization reactor can be independently controlled to adjust the yield of the styrene monomer.

[0045] In this case, the depolymerization reaction temperature in the depolymerization reactor is 420°C to 550°C, preferably 450°C to 500°C, and more preferably 450°C to 475°C. If the depolymerization reaction temperature is below 420°C, the amount of residual oil and high-boiling-point dimers and trimers produced will increase due to decreased reactivity, which may lead to a significant decrease in the amount of styrene monomer produced. If the temperature exceeds 550°C, the amount of benzene and toluene will increase significantly due to the cracking reaction, which may lead to a significant decrease in the amount of styrene monomer produced, and may lead to a problem where high-boiling-point dimers and trimers can be mixed into the product.

[0046] On the other hand, the residence time of polystyrene in the depolymerization reactor may be 15 minutes to 2 hours, preferably 20 minutes to 1 hour. If the residence time is less than 15 minutes or more than 2 hours, the yield of styrene monomer may be low, and the amount produced per unit time may decrease significantly.

[0047] The products obtained in the aforementioned depolymerization reaction include not only the final target component, styrene monomer (SM), but also high-boiling-point substances such as ethylbenzene (EB), toluene, cumene, alpha-methylstyrene, and dimer+, as well as depolymerization residues. Such depolymerization residues not only hinder the depolymerization reaction but also reduce the yield of styrene monomer. Dimer+ refers to a compound containing two or more benzene rings. The dimer+ may include substances such as 1,3-diphenylpropane, 4-diphenyl-1-butane, 1-phenylnaphthalene, 2-benzylnaphthalene, m-terphenyl, 1,3,5-triphenylcyclohexane, and 1,3,5-triphenylbenzene.

[0048] Therefore, in the continuous recovery method for styrene monomer according to the present invention, by depolymerizing under specific conditions using a potassium carbonate catalyst, the final target component, styrene monomer, can be continuously obtained in a yield of 70% or more, and ethylbenzene (EB), toluene, cumene, and alpha-methylstyrene can be recovered in yields of 3.5% or less, respectively.

[0049] In particular, the method for recovering styrene monomer according to the present invention yields 70% or more of styrene monomer and satisfies one or more of the following formulas (1) or (2), more preferably one or more of formulas (1) to (3), and even more preferably two or more of formulas (1) to (3). (Styrene yield) / (Ethylbenzene yield) ≥ 90 [Equation 1] (Styrene yield) / (toluene yield + ethylbenzene yield + α-methylstyrene yield) ≥ 12 [Equation 2] (Styrene yield) / (Dimer+ yield) ≥ 15 [Equation 3]

[0050] In other words, the styrene monomer recovery method according to the present invention allows for the suppression of the generation of ethylbenzene, α-methylstyrene, and toluene, particularly ethylbenzene, which are produced as by-reactions in conventional styrene monomer recovery processes, and facilitates the subsequent steps described below for recovering the final styrene monomer. Through these subsequent steps, the yield and purity of the final styrene monomer can be further improved.

[0051] As described above, of the depolymerization products generated via the depolymerization reactor, the gaseous depolymerization product (dotted arrow) is discharged to the depolymerization gas discharge section using sweep gas, and the depolymerization residue is discharged to the residue treatment section [(b) step].

[0052] At this time, the depolymerization reactor can be supplied with sweeping gas from the input side to the discharge side so that the gaseous depolymerization product produced by depolymerization can be easily discharged from the depolymerization reactor and moved to the outside or to the separation column section 140. The supply of the sweeping gas can be applied without limitation as long as it can be supplied to the depolymerization reactor, for example, it can be supplied together with waste polystyrene when it is fed into the depolymerization reactor at the input device section.

[0053] The sweep gas can be any non-reactive gas such as nitrogen, argon, or helium, and the sweep gas can be supplied after being heated. The temperature of the sweep gas is 20°C to 500°C, and it is preferably supplied after being heated to 200°C to 400°C in terms of depolymerization reaction efficiency.

[0054] On the other hand, the depolymerization residue remaining after depolymerization in the depolymerization reactor is continuously fed into the residue treatment unit 130, heated, and the styrene monomer remaining in the depolymerization residue is recovered. The remaining residue can be discharged to the outside or recycled back into the polystyrene inlet 110. The remaining catalyst can be recovered from the remaining residue discharged to the outside using water.

[0055] The residue processing unit 130 is connected to the depolymerization gas discharge unit 123 of the depolymerization reactor on one side, and receives the liquid depolymerization residue remaining after depolymerization in the depolymerization reactor. The supplied depolymerization residue is heated, and the gaseous styrene monomer remaining in the depolymerization residue is recovered and discharged to the outside or to the separation column unit 140 via the depolymerization gas discharge unit.

[0056] In this case, the temperature of the residue treatment section is preferably 100°C to 300°C from the viewpoint of improving the yield and selectivity of styrene monomer.

[0057] On the other hand, in one embodiment of the present invention, a continuous recovery method for styrene monomer from waste polystyrene involves injecting a sweeping gas into the discharge flow of the depolymerization residue in the opposite direction to the discharge flow of the depolymerization residue discharged from the depolymerization gas discharge section of the depolymerization reactor, in order to increase the recovery rate and selectivity of the styrene monomer, thereby separating the styrene monomer remaining in the depolymerization residue from the depolymerization residue and moving it back to the depolymerization gas discharge section.

[0058] The sweep gas injected into the depolymerization residue is injected in a countercurrent direction into the flow of depolymerization residue supplied to the residue processing unit using the sweep gas injection device 131. This helps separate the styrene monomer remaining in the depolymerization residue and moves it to the depolymerization gas discharge unit, thereby preventing polymerization due to stagnation of styrene monomer downstream of the depolymerization reactor and increasing the recovery rate of styrene monomer. The sweep gas injection device can be used without limitation as long as it is an injection device capable of injecting gas.

[0059] In this case, the sweep gas may be a non-reactive gas such as nitrogen, argon, or helium, or a light hydrocarbon such as methane or ethane, and may be supplied after being heated.

[0060] Furthermore, the sweep gas injected countercurrently into the residue flow can be supplied at a rate of 0.01 cm / min to 20 cm / min, preferably 0.5 cm / min to 10 cm / min. If the supply rate of the sweep gas is less than 0.01 cm / min, the countercurrent flow will be low, and polymerization may occur while the produced styrene monomer is stagnating. This may also lead to a problem where the separation efficiency of the styrene monomer remaining in the depolymerization residue is low, reducing the yield of styrene monomer. If the rate exceeds 20 cm / min, products with relatively higher boiling points than dimers remaining in the depolymerization residue may pass through the separation column section 140 together with the dimers and move to the gas discharge section, potentially reducing the selectivity of the styrene monomer.

[0061] Furthermore, in a continuous recovery method for styrene monomer from waste polystyrene according to one embodiment of the present invention, if the depolymerization reactor is a multi-stage depolymerization reactor consisting of two or more stages, as described above, a number of depolymerization gas discharge sections 123a and 123b are provided, connected to one side of each depolymerization reactor body, and a residue processing section 130 is connected only to the third depolymerization gas discharge section 123c of the last third depolymerization reactor body. By supplying sweep gas in the opposite direction to the flow direction of the depolymerization residue to the depolymerization residue flow moving to the depolymerization reactor body via the number of depolymerization gas discharge sections, the styrene monomer remaining in the depolymerization residue supplied to the subsequent depolymerization reactor body can be recovered without being discarded, and the last remaining depolymerization residue from which the styrene monomer has been recovered is discharged to the residue processing section 130.

[0062] On the other hand, a method for continuous recovery of styrene monomer from waste polystyrene according to another embodiment of the present invention may include the step of recovering styrene monomer from the residue treatment unit 130 and bringing it into contact with the remaining residue to recover the potassium carbonate (K2CO3) catalyst present in the remaining residue.

[0063] The potassium carbonate (K2CO3) catalyst used in the depolymerization reaction is continuously discharged from the residue treatment unit 130 along with the remaining residue. However, the potassium carbonate catalyst contained in the discharged remaining residue is water-soluble, unlike the other components of the residue. Therefore, by contacting the remaining residue containing the potassium carbonate catalyst with water, only the potassium carbonate catalyst can be dissolved and the potassium carbonate catalyst can be separated and recovered from the remaining residue. The separated and recovered potassium carbonate catalyst can then be recycled into the depolymerization reaction after removing the water.

[0064] Subsequently, the styrene monomer-containing gaseous depolymerization product that has been moved to or discharged to the depolymerization gas discharge section is collected to obtain styrene monomer [(c) step].

[0065] In this case, the present invention can further perform the step of reducing the content of dimer+ in the gaseous depolymerization product discharged from the depolymerization gas discharge section, and the step of reducing the content of dimer+ can be performed using a separation column section 140 connected to one side of the depolymerization gas discharge section.

[0066] In one embodiment of the present invention, the separation column section 140 is connected on one side to the depolymerization gas discharge section 123 of the depolymerization reactor, and receives the gaseous depolymerization product obtained in the depolymerization reactor 120 and the styrene monomer recovered from the residue treatment section 130, respectively. These are liquefied, and styrene monomer can be obtained by removing dimer+ with a low boiling point from the gaseous depolymerization product supplied from the depolymerization gas discharge section.

[0067] On the other hand, in another embodiment of the present invention, when the separation column section 140 is connected to a multi-stage depolymerization reactor, it can be configured in multiple units by being connected to one side of the depolymerization gas discharge section of each depolymerization reactor. For example, as shown in Figure 2, the separation column section 140 can be connected to each depolymerization gas discharge section of the multi-stage depolymerization reactor, thereby receiving a supply of gaseous depolymerization products discharged from the depolymerization gas discharge section. A residue treatment section is connected to the last stage's depolymerization gas discharge section, allowing for liquefaction by receiving a supply of gaseous depolymerization products discharged from the depolymerization gas discharge section together with styrene monomer recovered from the residue treatment section.

[0068] In this case, the separation column section 140 may be a separation column section that is packed with packing material or one that is not packed with packing material. By receiving the gaseous styrene monomer recovered in the residue treatment section and the gaseous depolymerization product obtained in the depolymerization reactor, a phase transition to a liquid state occurs as the gas passes through the separation column section, thereby further removing substances with high boiling points above dimer. As the packing material, glass (borosilicate), ceramic (porcelain), stainless steel, etc., can be used.

[0069] As a result, the purity of styrene monomer can be further improved as the proportion of the volume packed in the separation column (packing rate) increases, while the yield of styrene monomer decreases. Based on this, the yield and purity can be appropriately controlled.

[0070] Subsequently, the styrene monomer that has passed through the separation column can be collected and cooled for recovery. At this time, any cooling method used in the industry can be used without limitation, such as an indirect cooling method that compresses and evaporates a refrigerant and uses the heat of vaporization of the refrigerant to cool the collected product, or a direct cooling method. [Examples]

[0071] The present invention will be described in more detail below through specific examples. The following experimental examples are merely illustrative to aid in understanding the present invention, and the scope of the present invention is not limited thereto.

[0072] [Experimental Example 1: Measurement of yield and selectivity of depolymerization products based on depolymerization reaction temperature] 1 kg of waste polystyrene (PS) crushed to an average particle size of 0.5 cm and 50 g (5 wt%) of potassium carbonate (K2CO3) (Sigma-aldrich, ACS reagent > 99%) were mixed and then introduced into the auger reactor shown in Figure 1 via the input device along with nitrogen gas supplied at 5 cm / min. The mixture was then allowed to remain for 22 minutes at a stirring speed of 2.5 rpm under the reaction conditions described in Table 1 to carry out a continuous depolymerization reaction. At this time, a sweep gas injection nozzle was installed in the residue treatment section, and nitrogen gas (200°C), which is the sweep gas, was injected at a supply speed of 0.5 cm / min in the direction opposite to the inflow direction of the depolymerization residue, thereby liquefying the depolymerization product without the installation of a separation column. Subsequently, the liquefied product was measured using a GC / FID (Yonrin Instruments) with a capillary column (HP-5, 30 m × 0.32 mm × 1.0 μm, Crosslinked 5% PH ME Siloxane), and the results are shown in Tables 1 and 2 and Figure 3. In Table 1 below, α1, α2, and α3 were calculated using equations 1 to 3 below. Here, SM is styrene monomer, EB is ethylbenzene, and α-MS is alpha-methylstyrene. α1 = (Styrene yield) / (Ethylbenzene yield) [Equation 1] α2 = (Styrene yield) / (Toluene yield + Ethylbenzene yield + α-Methylstyrene yield) [Equation 2] α3 = (Styrene yield) / (Dimer+ yield) [Equation 3]

[0073] [Table 1]

[0074] [Table 2]

[0075] As shown in Tables 1 and 2 and Figure 3, when the depolymerization reaction temperature is 425°C or higher, as in Experimental Examples 1-3 to 1-5, the yield and selectivity of styrene monomer are higher compared to Experimental Examples 1-1 and 1-2, which have a depolymerization reaction temperature of 400°C or lower. In particular, in Experimental Examples 1-3 to 1-5, the values ​​of α1, α2, and α3 were 90 or higher, 12 or higher, and 15 or higher, respectively, confirming that there were fewer by-products.

[0076] [Experimental Example 2: Measurement of the yield of depolymerization products based on residence time] 1 kg of waste polystyrene (PS) crushed to an average particle size of 0.5 cm and 50 g (5 wt%) of potassium carbonate (K2CO3) (Sigma-aldrich, ACS reagent > 99%) were mixed and then introduced into the auger reactor shown in Figure 1 via the input device along with nitrogen gas supplied at 5 cm / min. Continuous depolymerization was carried out at 450°C under the reaction conditions described in Table 3. At this time, by equipping the residue treatment section with a sweep gas injection nozzle, nitrogen gas (200°C) was injected as sweep gas at a supply rate of 0.5 cm / min in the opposite direction to the inflow of the depolymerization residue, and the depolymerization product was liquefied without the installation of a separation column. Subsequently, the liquefied product was measured using a GC / FID (Yonrin Instruments) with a capillary column (HP-5, 30 m × 0.32 mm × 1.0 μm, Crosslinked 5% PH ME Siloxane), and the results are shown in Table 3 and Figure 4. α1, α2, and α3 in Table 3 below were calculated using equations 1 to 3 above.

[0077] [Table 3]

[0078] As shown in Table 3 and Figure 4, it was found that the yield of styrene monomer was higher in Experiments 2-2 to 2-4, where the residence time was longer than in Experiment 2-1. This was understood to be because the high rotation speed in Experiment 2-1 resulted in a short residence time in the depolymerization reactor, preventing sufficient depolymerization, thus lowering the yield of styrene monomer and reducing α3 due to an increase in unreacted dimer+.

[0079] [Experimental Example 3: Measurement of the yield of depolymerization products based on the content of the depolymerization catalyst] 1 kg of waste polystyrene (PS) crushed to an average particle size of 0.5 cm and potassium carbonate (K2CO3) (Sigma-aldrich, ACS reagent > 99%) were mixed under the conditions shown in Table 4 below. This mixture, along with nitrogen gas supplied at 5 cm / min, was then introduced into the auger reactor shown in Figure 1 via the input device. Continuous depolymerization was carried out under the reaction conditions described in Table 4 at 450°C for 22 minutes (stirring speed 2.5 rpm). At this time, a sweep gas injection nozzle was provided in the residue treatment section, and nitrogen gas (200°C) was injected as a sweep gas at a supply rate of 0.5 cm / min in the opposite direction to the depolymerization residue inflow, liquefying the depolymerization product without the installation of a separation column. The liquefied product was then measured using a GC / FID (Yonrin Instruments) with a capillary column (HP-5, 30 m × 0.32 mm × 1.0 μm, Crosslinked 5% PH ME Siloxane), and the results are shown in Table 4 and Figure 5. α1, α2, and α3 in Table 4 below were calculated using equations 1 to 3 above.

[0080] [Table 4]

[0081] As shown in Table 4 and Figure 5, in Experimental Example 3-1, where no catalyst was added, the yield of styrene monomer was significantly lower compared to Experimental Examples 3-2 to 3-4, where a catalyst was added. In particular, in Experimental Example 3-2, where a catalyst content of 1 wt% was added, superiority was confirmed in terms of yield and byproduct formation. However, as the catalyst content increased to 3 wt% and 5 wt%, side reactions were accelerated, and a tendency for the yield of styrene monomer to decrease actually emerged.

[0082] [Experimental Example 4: Measurement of the yield of depolymerization products depending on the type of catalyst used in the depolymerization reaction] Experimental Examples 4-1 to 4-10 were carried out using the same method as in Experimental Example 1-1, but with the conditions described in Table 5 changed, and continuous depolymerization and depolymerization were performed in an auger reactor and a batch reactor, respectively. In Experimental Examples 4-7 to 4-10, 1 kg of waste polystyrene (PS) crushed to an average particle size of 0.5 cm and 50 g (5 wt%) of catalyst were mixed in a 3 L batch reactor, and the temperature was raised to 425 °C, where the depolymerization reaction was carried out for 1 hour. The products decomposed by the depolymerization reaction were liquefied in a condenser and analyzed using a GC / FID (Yonrin Instruments) with a capillary column (HP-5, 30 m × 0.32 mm × 1.0 μm, Crosslinked 5% PH ME Siloxane), and the results are shown in Table 6. α1, α2, and α3 in Table 6 below were calculated using Equations 1 to 3.

[0083] [Table 5]

[0084] [Table 6]

[0085] As shown in Table 6, in the case of carbonate catalysts, as in Experimental Examples 4-1 to 4-4, the yield of styrene monomer was found to be higher compared to Experimental Examples 4-5 and 4-6, which used hydroxide catalysts. Among the carbonate catalysts in Experimental Examples 4-1 to 4-4, the yield of styrene monomer in Experimental Examples 4-1 and 4-2, which used alkali metal carbonate catalysts, was found to be superior. In particular, in Experimental Example 4-1, which used a potassium carbonate (K2CO3) catalyst, the yield of styrene monomer and the α1 and α2 values ​​increased significantly in the continuous reaction compared to the batch reactor. Although this is not limited by theory, it can be inferred that this is due to the effect of mass transfer limitation between solid (catalyst)-liquid (reactant)-gas (product). In the case of potassium carbonate, which has a relatively high decomposition temperature at the depolymerization temperature, uniform mixing of reactants and catalyst can be maintained in a continuous reaction, but in a batch reaction, it is inferred that the contact between the catalyst and reactants is reduced uniformly due to the density difference in the case of a solid catalyst.

[0086] [Experimental Example 5: Measurement of the recovery rate of depolymerization products by temperature in the residue treatment area] Experimental Example 5 involved depolymerization in the same manner as in Experimental Examples 1-4 to measure the recovery rate of depolymerization products depending on the temperature of the residue treatment section. However, the process conditions after the auger reactor were changed to those described in Table 7, and continuous depolymerization was performed. At this time, a sweep gas injection nozzle was installed in the residue treatment section, and nitrogen gas (200°C) was injected as the sweep gas at a supply rate of 10 cm / min in the opposite direction to the inflow of the depolymerization residue. The depolymerization products were liquefied using a separation column section packed with packing material (borosilicate, Raschig ring, 6 mm × 6 mm) at a packing rate of 75%. Subsequently, the liquefied products were measured using a GC / FID (Yonrin Instruments) with a capillary column (capillary column, HP-5, 30 m × 0.32 mm × 1.0 μm, Crosslinked 5% PH ME Siloxane), and the results are shown in Table 7.

[0087] [Table 7]

[0088] As shown in Table 7, in Experimental Example 5-2, where the residue treatment section was heated to 200°C, the yield of recovered styrene monomer was found to be higher compared to Experimental Examples 5-1 and 5-3, where the residue treatment section was not heated or was heated to 400°C. In Experimental Example 5-1, where the temperature of the residue treatment section was not heated, it was confirmed that the high-temperature gas product was instantaneously cooled and moved to the residue treatment section, resulting in a decrease in the yield of styrene monomer. Furthermore, in Experimental Example 5-3, where the residue treatment section was heated to a high temperature of 400°C, it was confirmed that radical polymerization of styrene monomer occurred rapidly, resulting in a sharp decrease in yield.

[0089] [Experimental Example 6: Measurement of the recovery rate of depolymerization products by supplying sweep gas and packing material in the separation column] Experimental Example 6 involved depolymerization in the same manner as in Experimental Examples 1-4 to measure the sweep gas supply rate and the recovery rate of depolymerization products by the packing material in the separation column. However, the process conditions after the auger reactor were changed to those described in Tables 8 and 9, and continuous depolymerization was performed. At this time, a sweep gas injection nozzle was provided in the residue treatment section, and nitrogen gas (200°C) was injected as the sweep gas in the opposite direction to the inflow of the depolymerization residue. The depolymerization products were liquefied using the separation column section packed with packing material (borosilicate, Raschig ring, 6 mm × 6 mm) and the separation column section without packing material. Subsequently, the liquefied products were measured using a GC / FID (Yonrin Instruments) with a capillary column (capillary column, HP-5, 30 m × 0.32 mm × 1.0 μm, Crosslinked 5% PH ME Siloxane), and the results are shown in Tables 8 and 9 and Figures 6 and 7. At this time, the β-selectivity and the selective productivity of styrene monomer were calculated using the following equations 4 and 5. β selectivity (%) = [(Y styrene / Y dimer ) / (X styrene / X dimer)] = (Composition discharged from the separation column / Composition of the stock solution) [Equation 4] Selective productivity of styrene monomer (g / h) = Purity of styrene monomer × Productivity of styrene monomer [Equation 5]

[0090] In the above formula, the productivity of styrene monomer refers to the amount of styrene monomer produced in the reactor per unit time.

[0091] [Table 8]

[0092] [Table 9]

[0093] As shown in Table 8, by equipping the residue treatment unit with a sweep gas injection nozzle, as in Experimental Example 6-2, it was confirmed that both the yield and selectivity of styrene monomer improved when nitrogen gas was supplied as the sweep gas, compared to Experimental Example 6-1 where no sweep gas was supplied.

[0094] Furthermore, as shown in Table 9, it was confirmed that under the same conditions, a higher sweep gas supply rate resulted in a decrease in the selectivity of the recovered styrene monomer, while a lower sweep gas supply rate resulted in a decrease in the yield of recovered styrene monomer. In addition, compared with Experimental Examples 6-2 and 6-3, it was confirmed that the presence of a separation column significantly reduced the generation of dimer+ compared to the absence of a separation column, and that a higher packing density in the separation column resulted in a higher selectivity of the recovered styrene monomer.

[0095] Therefore, as shown in Figures 6 and 7, when considering the selective productivity of styrene monomer, selectivity and yield are inversely proportional, and must be appropriately adjusted according to the situation.

[0096] Although preferred embodiments of the present invention have been described above, the present invention is not limited thereto, and can be implemented in various ways within the scope of the claims, the detailed description of the invention, and the accompanying drawings, and these will naturally also fall within the scope of the present invention. [Explanation of Symbols]

[0097] 100 Continuous recovery system for styrene monomer 110 Loading device section 111 Hopper section 112 Opening / closing device section 120 Depolymerization reactor 121 Depolymerization reactor main body 122 Transfer Screw 123 Depolymerization gas discharge section 124 Heat source section 130 Residue Processing Section 131 Injection device 140 Separation column section

Claims

1. In a method for continuous recovery of styrene monomer by continuous depolymerization of waste polystyrene, (a) A step of continuously feeding waste polystyrene and potassium carbonate (K₂CO₃) catalyst into a depolymerization reactor and depolymerizing them to obtain a styrene monomer-containing depolymerization product, (b) Of the depolymerization products, the gaseous depolymerization products are discharged to the depolymerization gas discharge section using a sweep gas, and the depolymerization residues other than the gaseous depolymerization products are discharged to the residue processing section, (c) The step of collecting the discharged gaseous depolymerization product to obtain styrene monomer, The depolymerization reactor comprises a depolymerization reactor body equipped with a transfer screw for continuously transporting the introduced waste polystyrene from the input side to the discharge side, and a depolymerization gas discharge unit connected to the discharge port side of the depolymerization reactor body for discharging the depolymerization product generated in the depolymerization reactor body, and is an auger reactor consisting of one or more stages. The residue processing unit is connected to the depolymerization gas discharge section of the depolymerization reactor so that one side is in communication with it. In step (a) above, the depolymerization reaction temperature in the depolymerization reactor is 420°C to 550°C, and the residence time of the waste polystyrene in the depolymerization reactor is 20 minutes to 1 hour. In step (b), the temperature of the residue processing unit is 100°C to 300°C, and sweep gas is supplied to the depolymerization residue discharge flow in step (b) in a direction opposite to the direction of the depolymerization residue discharge flow, thereby separating the styrene monomer contained in the depolymerization residue flow and discharged to the residue processing unit from the depolymerization residue flow and discharging it to the depolymerization gas discharge unit. A method for continuous recovery of styrene monomer from waste polystyrene, characterized in that the yield of styrene monomer (SM) obtained by the depolymerization is 70% or more, and one or more of the following formulas 1 or 2 are satisfied. (Styrene yield) / (Ethylbenzene yield) ≥ 90 [Equation 1] (Styrene yield) / (Toluene yield + Ethylbenzene yield + α-Methylstyrene yield) ≥ 12 [Equation 2]

2. The aforementioned potassium carbonate (K ２ CO ３ The method for continuous recovery of styrene monomer from waste polystyrene according to claim 1, characterized in that the catalyst is added in an amount of 0.1 to 10 parts by weight per 100 parts by weight of waste polystyrene.

3. The (c) step further comprises a step of reducing the content of dimer+ in the gaseous depolymerization product discharged from the depolymerization gas discharge section, The method for continuous recovery of styrene monomer from waste polystyrene according to claim 1, wherein the dimer+ is a compound containing two or more benzene rings.

4. The method for continuous recovery of styrene monomer from waste polystyrene according to claim 3, characterized in that the step of reducing the content of the dimer+ is performed by a separation column installed in the depolymerization gas discharge section.

5. The method for continuous recovery of styrene monomer from waste polystyrene according to claim 1, characterized in that the sweep gas is supplied after being heated.

6. Styrene monomer is discharged from the residue treatment unit, and the remaining residue is brought into contact with water, and potassium carbonate (K) present in the remaining residue is removed. ２ CO ３ A method for continuous recovery of styrene monomer from waste polystyrene according to claim 1, further comprising the step of recovering a catalyst.

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