Method and plant for recycling polystyrene waste containing at least one organic halogen-containing flame retardant

Abstract

The present invention relates to a method for recycling polystyrene waste containing at least one organic halogen-containing compound by converting polystyrene to styrene, the method comprising: a) providing a molten polystyrene waste containing at least one organic halogen-containing compound; b) subjecting the molten material provided in step a) to at least one defloration step to obtain a defloration composition; c) subjecting the defloration composition obtained in step b) to at least one pyrolysis step to obtain a pyrolysis composition; and d) subjecting the pyrolysis composition to at least one distillation step to obtain a purified styrene composition.

Description

Technical Field

[0001] The present invention relates to a method for recycling polystyrene waste containing at least one organohalogen-containing compound, such as, in particular, an organohalogen-containing flame retardant, such as 1,2,5,6,9,10-hexabromocyclododecane, by converting polystyrene into styrene, and to a plant for carrying out this method.

Background Art

[0002] Polystyrene is one of the most widely used plastics in the world because it can be excellently molded into a desired shape, especially by injection molding or extrusion molding. Polystyrene is used, for example, as a CD case, as food packaging, such as a yogurt cup, as a plastic housing in electronic components, such as a switch, and also as a dielectric layer, as a Petri dish, as a test tube, etc. In addition, polystyrene foam is widely used as a building insulation material, for example, in insulation formwork and structural insulation panel methods, because of its excellent heat insulation properties. One major drawback of polystyrene is its flammability, and as a result, polystyrene emits a large amount of black smoke when burned. Therefore, polystyrene, especially polystyrene foam, such as expanded polystyrene (EPS) or extruded polystyrene (XPS), usually contains a flame retardant that is essential for its use as a building material in order to dramatically reduce the flammability of each of polystyrene or polystyrene foam. In particular, hitherto and still at present, organohalogen-containing compounds, such as organobromine-containing compounds, such as 1,2,5,6,9,10-hexabromocyclododecane (HBCD), have been used as flame retardants.

[0003] The recycling of polystyrene waste is becoming increasingly important. First and foremost, polystyrene is a homopolymer of styrene or a copolymer containing styrene comonomers, and is usually produced by (co)polymerizing styrene. However, styrene is usually obtained from crude oil or hydrocarbon flows derived from crude oil, and therefore, recovering raw styrene from polystyrene waste is advantageous not only from an economic standpoint but especially from an environmental standpoint. Furthermore, the processing of polystyrene, especially polystyrene foam, is troublesome and costly due to the presence of contaminants of varying properties within polystyrene waste. The same applies when recycling polystyrene back into styrene, because additives, particularly brominated flame retardants such as hexabromocyclododecane contained in brominated flame retardants, are difficult to completely remove from polystyrene, and therefore, the resulting styrene composition usually contains these flame retardants or their decomposition products generated during the recycling process. For example, several known processes for recycling polystyrene are based on the thermal decomposition of polystyrene into styrene, during which organic halogen-containing compounds contained in the polystyrene as flame retardants, such as hexabromocyclododecane, are decomposed into their respective halogens, e.g., bromine, hydrogen halides, and / or halogen-containing hydrocarbon by-products. However, the presence of these flame retardants or decomposition products, particularly halogens, e.g., bromine, or hydrogen halides, e.g., hydrogen bromide, in the recycled styrene composition is problematic because, even if only small amounts of each are present in the recycled styrene composition, they cause corrosion and / or fouling of piping and reactor equipment and / or poisoning of catalysts used during the use of the recycled styrene composition, for example, in (co)polymerization reactions. Although the use of such organic halogen-containing flame retardants, e.g., HBCD in particular, has been restricted until now, large quantities of polystyrene products generated before legal regulations were implemented are still in use and will need to be recycled in the future.Furthermore, there are country-specific exemptions that enable the application of HBCD in EPS and XPS used in building insulation materials. Generally, the efficient recycling of polystyrene waste containing organic halogen-containing compounds as flame retardants is important, and such efficient recycling of polystyrene waste requires the complete removal of organic halogen-containing compounds contained in the polystyrene waste as flame retardants, as well as the complete removal of possible decomposition products formed from the flame retardants during the recycling process, such as halogens, hydrogen halides, and / or halogen-containing hydrocarbons. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2010 / 066457(A1) [Patent Document 2] European Patent Application Publication No. 1206962(A1) [Patent Document 3] European Patent No. 2158027(B1) [Patent Document 4] European Patent No. 0655275(B1) [Patent Document 5] U.S. Patent No. 3,743,250(A) [Patent Document 6] European Patent No. 2548634(B1) [Patent Document 7] European Patent No. 0815929(B1) [Patent Document 8] European Patent No. 1510247(B1) [Patent Document 9] U.S. Patent No. 4,093,188(A) [Patent Document 10] U.S. Patent No. 4,296,779(A) [Overview of the project] [Problems that the invention aims to solve]

[0005] In view of this, the fundamental object of the present invention is to provide a method for recycling polystyrene waste containing at least one organic halogen-containing compound by converting polystyrene to styrene, which yields pure styrene containing, if any, only a minimal, non-impactful amount of the organic halogen-containing compound and its decomposition products, such as bromine and / or hydrogen bromide, and which nevertheless features low operating costs and capital expenditures. [Means for solving the problem]

[0006] According to the present invention, the objective is a method for recycling polystyrene waste containing at least one organic halogen-containing compound by converting polystyrene to styrene, a) A step of providing a molten polystyrene waste containing at least one organic halogen-containing compound, b) A step of subjecting the molten material provided in step a) to at least one defoliation step to obtain a defoliated composition, c) A step of subjecting the defoliated composition obtained in step b) to at least one thermal decomposition step to obtain a thermal decomposition composition, d) A step of subjecting the pyrolysis composition to at least one distillation step to obtain a purified styrene composition. This is solved by providing a method that includes [this].

[0007] This solution is based on the finding that a molten polystyrene waste material containing at least one organic halogen-containing compound is subjected to at least one defloration step, and the resulting deflorated composition is then thermally decomposed, thereby subjecting the resulting thermally decomposed composition to at least one distillation step, and optionally thereafter to a further purification step, such as one or more crystallization steps, to obtain a pure styrene composition having a styrene content of more than 98.8% by weight of styrene, at least 99.8% by weight, or even at least 99.94% by weight of styrene, and that this pure styrene composition contains, if any, only very small amounts, of at least one organic halogen-containing compound and possible decomposition products, such as halogens, hydrogen halides, and / or halogen-containing hydrocarbons, or contains none at all. While we do not wish to be bound by any theory, it is conceivable that organic halogen-containing compounds, such as hexabromocyclododecane (HBCD), contained in polystyrene as flame retardants, and decomposition products generated during defoliation, such as halogens (e.g., bromine), hydrogen halides (e.g., hydrogen bromide), and / or halogen-containing hydrocarbon by-products, are efficiently and completely, or at least substantially completely, removed from the polymer melt before the defoliated polymer melt is subjected to the thermal decomposition reaction. In particular, the use of solvents, additives, and / or catalysts used in conventional methods to remove these contaminants is not required in the method according to the present invention, thus eliminating the disadvantages, such as the need for additional separation equipment and catalyst / additive recovery systems that result in further operating and equipment costs. In addition, since organic halogen-containing compounds are removed in the method, maintenance costs due to corrosion in the regeneration plant are prevented, and the method can be carried out in a reactor with relatively inexpensive internal structures. In general, the method according to the present invention is characterized not only by yielding very pure styrene but also by low operating costs and capital expenditures.

[0008] According to this invention, thermal decomposition also means all types of thermal decomposition.

[0009] According to the present invention, step a) provides a molten polystyrene waste containing at least one organic halogen-containing compound. For example, the polystyrene waste used in step a) contains 0.5 to 15% by weight, preferably 2 to 4% by weight, of at least one organic halogen-containing flame retardant. The present invention is particularly suitable for using polystyrene waste containing at least one bromine-containing flame retardant, more preferably tetrabromobisphenol A, brominated polyacrylate, polybrominated diphenyl ether, brominated polybutadiene (PolyFR), or hexabromocyclododecane, for example, 1,2,5,6,9,10-hexabromocyclododecane, as the organic halogen-containing compound. The polystyrene waste may include non-foamed polystyrene or foamed polystyrene, for example, EPS or XPS.

[0010] The present invention is not particularly limited with respect to how the polystyrene waste is melted into a molten material in step a). Particularly good results are obtained when the polystyrene waste containing at least one organic halogen-containing compound is melted in an extruder, for example, a single-screw or twin-screw extruder. The extruder not only preheats and melts the polystyrene waste but also continuously mixes it. Particularly good results are obtained when the polystyrene waste is heated in the extruder to a temperature of 150-250°C, preferably 180-220°C. The polystyrene waste can be supplied to the extruder in any form, such as chips, flakes, pellets, or powder.

[0011] According to a particular preferred embodiment of the present invention, a molten polystyrene waste containing at least one organic halogen-containing compound, provided in step a), is subjected to at least one defoliation step b) in step b), which is preferably carried out at a temperature of 230 to 310°C, more preferably at 240 to 310°C. This ensures that at least one organic halogen-containing compound is evaporated and / or decomposed into one or more gaseous compounds, such as halogens and / or hydrogen halides, carbon dioxide, carbon monoxide, and brominated polycyclic aromatics, which are removed as gaseous compositions from the remaining defoliated polystyrene molten material.

[0012] Further development of the present invention suggests that the residence time of the molten material in at least one devolatilization step is 5 to 120 minutes, preferably 10 to 90 minutes if the temperature during at least one devolatilization step is 230 to 310°C, and preferably 20 to 60 minutes if the temperature during at least one devolatilization step is 240 to 310°C. Such residence times contribute to the sufficient decomposition and removal from the polystyrene molten material of at least one organic halogen-containing compound and the decomposition products formed therefrom. Residence time, in this regard, means the time spent in devolatilization step b) by the molten material provided in step a), which is the time spent in at least one devolatilization vessel used in devolatilization step b), as well as the time spent in a preferred static mixer, which will be described later.

[0013] The present invention is not particularly limited with respect to the pressure during at least one devolatilization step. Particularly good results are obtained when at least one devolatilization step is performed at ambient pressure or slightly below ambient pressure, for example, preferably at an absolute pressure of 0.1 to 20 kPa.

[0014] At least one devolatilization step can be carried out in a devolatilization vessel comprising at least one distributor and / or at least one internal structure selected from trays, structured packings and random packings. However, at least one devolatilization step also functions well if the devolatilization vessel contains no distributor and / or internal structure at all.

[0015] According to a particular preferred embodiment of the invention, the melt of the polystyrene waste containing at least one organohalogen-containing compound provided in step a) is mixed in at least one static mixer before or during at least one devolatilization step. Thereby, the polystyrene melt is homogenized before being devolatilized. Preferably, the polystyrene melt is mixed before at least one devolatilization step, or, if the polystyrene melt is mixed during at least one devolatilization step, it is mixed immediately after the start of at least one devolatilization step. Good results are particularly obtained if the melt of the polystyrene waste containing at least one organohalogen-containing compound provided in step a) is mixed in at least one static mixer having heat exchange capacity before or during at least one devolatilization step. Thereby, the temperature of the polystyrene melt can be increased rapidly, uniformly and in a very controlled manner before or at the start of devolatilization.

[0016] A static mixer means any mixer that does not include any moving parts, and in particular, any rotating parts, and any such mixer can be used in this invention. Static mixers typically create a mixing action by generating turbulence through static elements, i.e., non-moving elements, such as plates, bars, crossbars, baffles, helically formed deflection means, grids, etc. If these elements are hollow, a heat transfer medium can pass through the elements so that each static mixer has heat exchange capacity. Suitable examples of static mixers are x-type static mixers, spiral / helical-type static mixers, quattro-type static mixers, baffle-plate-type static mixers, turbulator-strip-type static mixers, and any combination of two or more of the above-mentioned mixer types. An x-type static mixer comprises deflection means, such as bars, crossbars, or plates, having an x-shape in the plan view and / or side view and / or cross view. Such x-type static mixers are described, for example, in International Publication No. 2010 / 066457(A1), European Patent Application Publication No. 1206962(A1), European Patent No. 2158027(B1) and European Patent No. 0655275(B1), and are commercially available from Sulzer Chemtech (Winterthur, Switzerland) under the trade names SMX, SMXL and SMX plus, and from Fluitec (Neftenbach, Switzerland) under the trade name CSE-X. A spiral / helical static mixer has a helically formed deflection means and is described, for example, in U.S. Patent No. 3,743,250(A), while a quattro static mixer includes a deflection means that forms a chamber-shaped mixing section and is described, for example, in European Patent Nos. 2548634(B1) and 0815929(B1).Baffle plate type static mixers usually include longitudinal deflection means, for example, as described in European Patent No. 1510247 (B1) and US Patent No. 4,093,188 (A). In contrast, a turbulator strip type static mixer includes a plurality of elongated strips in a tube, and each of these strips is formed by a series of alternating deflection panels that are continuously joined together, for example, by substantially triangular bridging portions. The strips are held together and are substantially fixed to the axis of the tube by alternating bridging portions of the bridging portions, and the other bridging sections are arranged adjacent to the inner wall of the tube, for example, as described in US Patent No. 4,296,779 (A). Other suitable static mixers are sold under the trade names CompaX, SMI, KVM, SMV, and GVM from Sulzer Chemtech, and under the trade name GVM from Stamixco (Volra, Switzerland). In view of the above, at least one, more preferably all, of the static mixers of at least two suppliers are preferably selected from the group consisting of x-type static mixers, spiral / helical type static mixers, quattro type static mixers, baffle plate type static mixers, turbulator strip type static mixers, and any combination of two or more of the mixer types described above.

[0017] The gas composition generated during at least one devolatilization step is removed from the vessel in which the at least one devolatilization step is carried out. The gas composition can be treated by subjecting the gas composition to caustic washing with an acidic gas such as hydrogen bromide, for example, and / or by condensing it in a liquid light hydrocarbon composition.

[0018] To further reduce the amount of organic halogen-containing compounds in the polystyrene melt, if necessary, the devolatilized composition can be subjected to at least one additional devolatilization step. The at least one additional devolatilization step can be carried out, for example, under the same or similar temperature and pressure conditions and with the same or similar residence time as in the first devolatilization step described above.

[0019] According to the present invention, in order to obtain a thermally decomposed composition or a thermally decomposed composition, the defoliated composition is then subjected to at least one thermal decomposition step or thermal decomposition, respectively. When at least one thermal decomposition step is carried out at a temperature of 350 to 1,000°C, more preferably 450 to 650°C, particularly good results are obtained. This decomposes the polystyrene contained in the defoliated composition into styrene.

[0020] The residence time of the pyrolysis composition in the pyrolysis reactor is preferably 0.1 to 15 seconds.

[0021] Further development of the present invention proposes that at least one pyrolysis step be carried out in an isothermal or adiabatic reactor. For example, the pyrolysis step may be carried out in a rotary kiln, a pyrolysis reactor (batch), or a fluidized bed reactor. When a fluidized bed is used as an isothermal reactor, particularly good results can be obtained.

[0022] The thermal decomposition is preferably carried out in an inert gas atmosphere such as nitrogen gas or a noble gas, or in the atmosphere of the formed product.

[0023] The pyrolysis product or pyrolysis composition preferably contains 60 to 85% by weight of styrene, at most 15% by weight of a gas composition, and 5 to 20% by weight of a styrene dimer or trimer.

[0024] According to certain preferred embodiments of the present invention, the pyrolysis composition obtained in step c) is rapidly cooled to prevent further side reactions and to maximize the styrene monomer fraction in the pyrolysis composition, and then subjected to at least one distillation step in step d). More specifically, it is preferable that the pyrolysis composition obtained in step c) is cooled to a temperature of at least 200°C, preferably 40-200°C, at a cooling rate of at least 400°C / min, and then subjected to at least one distillation step in step d). Particularly good results are obtained when the cooling is performed in a spray condenser or a shell-and-tube heat exchanger.

[0025] According to the present invention, the pyrolysis composition is subjected to at least one distillation step in step d) to obtain a purified polystyrene composition. When the pyrolysis composition is subjected to 1 to 3, more preferably 1 or 2, distillation steps in step d), particularly good results are obtained.

[0026] According to the first modification, the pyrolysis composition is subjected to two distillation steps, and in the first distillation step, C 7- The top composition containing hydrocarbons and the bottom composition are distilled into a bottom composition, and in the second distillation step, the bottom composition contains purified styrene and styrene dimers, etc., C9 + It is distilled into a bottom-of-the-column composition containing hydrocarbons.

[0027] Preferably, the temperature in each of the distillation columns is adjusted to 30-145°C, more preferably 40-110°C, while the pressure in each of the distillation columns is preferably adjusted to ambient pressure or less than ambient pressure, for example, an absolute pressure of 10-50 kPa.

[0028] The purified styrene composition obtained in this modified example preferably contains more than 98.8% by weight, more preferably at least 99.2% by weight, and even more preferably at least 99.5% by weight, for example, at least 99.8% by weight of styrene. Similarly, the content of organic halogen-containing compounds and their decomposition products, such as halogens, hydrogen halides, and halogenated hydrocarbons, in the purified styrene composition is at most 5 ppm, preferably at most 2 ppm.

[0029] According to an alternative second modification, the pyrolysis composition obtained in step c) is subjected to three distillation steps in step d), the first two of which are carried out as extractive distillation. Suitable extractants are sulfolane, silene, and any combination of two or more of the extractants described above.

[0030] The purified styrene composition obtained in this modified example also preferably contains more than 98.8% by weight, more preferably at least 99.2% by weight, and even more preferably at least 99.5% by weight, for example, at least 99.8% by weight of styrene. Similarly, the content of organic halogen-containing compounds and their decomposition products, such as halogens, hydrogen halides, and halogenated hydrocarbons, in the purified styrene composition is at most 5 ppm, preferably at most 2 ppm.

[0031] According to an alternative third modification, the pyrolysis composition obtained in step c) is subjected to a distillation step in a split-wall distillation column in step d). The present invention is not particularly limited in terms of the type of split-wall distillation column. Therefore, any distillation column can be used that includes a split wall that, in cross-sectional view, separates at least a longitudinally extending section of the split wall into two subsections. Preferably, the split wall is positioned at least substantially vertically downward within the split-wall distillation column. At least substantially vertically downward means, according to the present invention, that the angle between the split wall and the longitudinal axis or vertical direction of the split-wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5°, and most preferably 0°.

[0032] According to a particular preferred embodiment of the present invention, the composition to be pyrolyzed is supplied in step a) to an intermediate split-wall distillation column, and in the intermediate split-wall distillation column, C 7- A top-of-column composition containing hydrocarbons, a side-of-column composition of purified styrene, and styrene dimers and / or trimers, etc. 9+ The bottom composition containing hydrocarbons is distilled. An intermediate split-wall distillation column is defined according to the present invention as a split-wall distillation column and comprises a split wall extending at least substantially vertically downward from a point located below the top of the distillation column to a point located above the bottom of the split-wall distillation column, thereby subdividing the split-wall distillation column into a top section located above the split wall, a bottom section located below the split wall, and intermediate sections including a first intermediate subsection located on one side of the split wall and a second intermediate subsection located on the opposite side of the split wall. Favorable results are particularly obtained when the dividing wall extends from a point located at 10-45% of the distance from the bottom to the top of the dividing wall distillation column, to a point located at 50-90% of the distance from the bottom to the top of the dividing wall distillation column, preferably from a point located at 25-40% of the distance from the bottom to the top of the dividing wall distillation column, to a point located at 60-80% of the distance from the bottom to the top of the dividing wall distillation column. Furthermore, it is preferable that the dividing wall extends to more than 5-90%, more preferably more than 20-80%, and most preferably more than 30-70%, of the distance from the bottom to the top of the dividing wall distillation column, when viewed from the bottom to the top of the dividing wall distillation column.

[0033] A further development of the present invention suggests that the dividing wall subdivides the intermediate section of the dividing wall distillation column into approximately two equal-sized subsections or halves when viewed in cross-section. Thus, it is preferable that one of the first and second intermediate subsections covers at least 30%, more preferably at least 40%, even more preferably at least 45%, and most preferably 50% of the total cross-sectional area of ​​the intermediate section of the dividing wall column, while the other of the first and second intermediate subsections covers the remainder of 100% of the total cross-sectional area of ​​the intermediate section of the dividing wall column. For example, one subsection may cover 43% of the total cross-sectional area and the other 57%, or both subsections may each cover exactly 50% of the total cross-sectional area. If the total cross-sectional area of ​​the intermediate section varies along the axial length of the intermediate section, the above figures relate to the average total cross-sectional area of ​​the intermediate section.

[0034] Preferably, distillation is carried out in a split-wall distillation column at a temperature of 30 to 145°C.

[0035] The column side flow may or may not be subjected to the second distillation step (standard).

[0036] The purified styrene composition obtained in this modified example also preferably contains more than 98.8% by weight, more preferably at least 99.2% by weight, and even more preferably at least 99.5% by weight, for example, at least 99.8% by weight of styrene. Similarly, the content of organic halogen-containing compounds and their decomposition products, such as halogens, hydrogen halides, and halogenated hydrocarbons, in the purified styrene composition is at most 5 ppm, preferably at most 2 ppm.

[0037] To further increase the purity of the styrene product, the purified styrene composition obtained in at least one distillation step can be subjected to at least one crystallization step, particularly preferably at least one melt crystallization step.

[0038] The present invention is not particularly limited in terms of the type or number of melt crystallization steps. Particularly good results are obtained when at least one melt crystallization step is selected from the group consisting of static crystallization steps, suspension crystallization steps, and crystallization steps. Each of the crystallization techniques described above may include 1 to 12, preferably 1 to 10, more preferably 1 to 5, and most preferably 1 to 3 melt crystallization steps.

[0039] According to certain preferred embodiments of the present invention, at least one melt crystallization step comprises at least one flow-through thin-film crystallization step. During the flow-through thin-film crystallization step, the molten material to be crystallized flows downward along a cooling surface, for example, along the inside of a cooling tube, thereby enabling crystals to grow from the flow-through thin film of molten material on the inner surface of the tube, and the inner surface of the tube is cooled by the flow-through thin film of coolant flowing parallel to the outer surface of the tube. High and highly repeatable migration speeds are achieved on both sides of the tube, and the resulting shear at the crystal / liquid interface rapidly transports impurities into the bulk of the molten material. Preferably, the flow-through thin-film crystallization apparatus used in the melt crystallization step(s) comprises a plurality of vertical tubes, in which the crystal layer grows as a cylindrical shell in a collection container below the tubes and a circulation pump. Before the start of the crystallization step, the collection container is filled with a batch of molten material to be crystallized. The circulation pump then begins to inject water into the tubes, at which point the cooling water temperature begins to rise. The molten material circulation rate is adjusted to a value higher than the crystal deposition rate so that the temperature and composition conditions are approximately uniform over the length of the tube. The temperature is reduced at a constant rate until the collection container level drops to a preset value indicating that the desired amount of product has deposited in the tube. At this point, the molten material circulation is stopped.

[0040] During each crystallization step, a styrene-enriched crystallized fraction and a styrene-depleted residue fraction are obtained. The residual liquid is removed from the crystallization step as the styrene-depleted residue fraction after the crystallization is complete. Subsequently, the crystal layer obtained in the crystallization step is melted and removed from the crystallization step as the styrene-enriched crystallized fraction.

[0041] Further development of the present invention suggests that, before melting the crystalline layer obtained in the crystallization process, one or more sweating steps are performed on the crystalline layer to obtain one or more sweating fractions and a purified crystalline layer, preferably, at least a portion of the first sweating fraction thus obtained is supplied to the residual liquid removed as a styrene-depleted residue fraction. Sweating is achieved by increasing the temperature of the crystal to a value slightly below the melting point of styrene, for example, 0.1 to 2°C below the melting point of styrene, in order to liquefy impurities and aid in further excretion.

[0042] Favorable results are particularly obtained when at least one, preferably all, of the at least one melt crystallization steps is carried out at a temperature of -200°C to 30°C, preferably -140°C to 0°C, and more preferably -100°C to -30°C.

[0043] According to certain preferred embodiments of the present invention, the purified styrene composition obtained in at least one distillation step is subjected to at least one dynamic melt crystallization step and at least one static melt crystallization step, preferably first, at least one dynamic melt crystallization step is performed, followed by at least one static melt crystallization step. Particularly good results are obtained when the purified styrene composition obtained in at least one distillation step is subjected to 1 to 10, preferably 1 to 5, flow thin film melt crystallization steps, followed by 1 to 10, preferably 1 to 5, static melt crystallization steps.

[0044] According to certain preferred embodiments of the present invention, the purified styrene composition contains more than 98.8% by weight, more preferably at least 99.2% by weight, even more preferably at least 99.5% by weight, still more preferably at least 99.7% by weight, still still more preferably at least 99.9% by weight, and most preferably at least 99.94% by weight of styrene after at least one crystallization step. Similarly, the content of organic halogen-containing compounds and their decomposition products, such as halogens, hydrogen halides, and halogenated hydrocarbons, in the purified styrene composition is at most 5 ppm, preferably at most 2 ppm.

[0045] Further development of the present invention suggests that in at least one of the pyrolysis steps c), neither catalyst nor solvent is (intentionally) added to the composition being pyrolyzed. It is particularly preferable that neither catalyst nor solvent is (intentionally) added to the composition subjected to steps a) to d) throughout the entire process. In addition, it is preferable that no additives are (intentionally) added to the composition being pyrolyzed, and even more preferable that no additives are (intentionally) added to the composition subjected to steps a) to d) throughout the entire process.

[0046] In a further embodiment, the present invention relates to a plant for recycling polystyrene waste containing at least one organic halogen-containing compound by converting polystyrene to styrene, i) At least one defloration container having an inlet for molten polystyrene waste containing at least one organic halogen-containing compound, an outlet for a gas composition, and an outlet for a defloration composition, ii) At least one pyrolysis reactor having an inlet connected to an outlet for the devolatile composition of at least one devolatile container i), and an outlet for the pyrolysis composition, iii) At least one distillation column having an inlet directly or indirectly connected to an outlet for the pyrolysis composition of at least one pyrolysis reactor, a top outlet and a bottom outlet. Regarding a plant equipped with...

[0047] Preferably, the plant further comprises an extruder, preferably a single-screw or twin-screw extruder, located upstream of at least one daverter. The extruder has an inlet for polystyrene waste containing at least one organic halogen-containing compound and an outlet for molten polystyrene waste containing at least one organic halogen-containing compound, the outlet for molten polystyrene waste of the extruder being connected to the inlet of at least one daverter.

[0048] In addition, the plant further comprises a cooler having an inlet connected to an outlet for the pyrolysis composition of at least one pyrolysis reactor and an outlet for the cooled pyrolysis composition connected to an inlet of at least one distillation column, wherein the cooler is preferably embodied to allow the pyrolysis composition to be cooled to a temperature of at most 200°C, preferably 40-200°C, at a cooling rate of at least 400°C / min. Particularly good results are obtained when the cooler is a spray condenser and / or a shell-and-tube heat exchanger.

[0049] Further development of the idea of ​​the present invention suggests that the plant comprises two distillation columns, the first distillation column having an inlet connected to an outlet for pyrolysis compositions of at least one pyrolysis reactor, a top outlet and a bottom outlet, and the second distillation column having an inlet connected to the bottom outlet of the first distillation column, a top outlet for purified styrene and a bottom outlet.

[0050] According to an alternative embodiment of the present invention, the plant comprises three distillation columns, at least the first two of which are extraction distillation columns.

[0051] According to an alternative embodiment of the present invention, the plant comprises a split-wall distillation column having an inlet connected to an outlet for the pyrolysis composition of at least one pyrolysis reactor, an outlet for the top composition, an outlet for the side composition of purified styrene, and an outlet for the bottom composition.

[0052] Preferably, the segmented-wall distillation column is an intermediate segmented-wall distillation column comprising a segmented wall that extends at least substantially vertically downward from a point located below the top of the distillation column to a point located above the bottom of the segmented-wall distillation column, thereby subdividing the segmented-wall distillation column into a top section located above the segmented wall, a bottom section located below the segmented wall, and intermediate sections including a first intermediate subsection located on one side of the segmented wall and a second intermediate subsection located on the opposite side of the segmented wall.

[0053] Favorable results are particularly obtained when the dividing wall extends from a point located at 10-45% of the distance from the bottom to the top of the dividing wall distillation column, to a point located at 50-90% of the distance from the bottom to the top of the dividing wall distillation column, preferably from a point located at 25-40% of the distance from the bottom to the top of the dividing wall distillation column, to a point located at 60-80% of the distance from the bottom to the top of the dividing wall distillation column. Furthermore, it is preferable that the dividing wall extends to more than 5-90%, more preferably more than 20-80%, and most preferably more than 30-70%, of the distance from the bottom to the top of the dividing wall distillation column, when viewed from the bottom to the top of the dividing wall distillation column.

[0054] A further development of the present invention suggests that the dividing wall subdivides the intermediate section of the dividing wall distillation column into approximately two equal-sized subsections or halves when viewed in cross-section. Thus, it is preferable that one of the first and second intermediate subsections covers at least 30%, more preferably at least 40%, even more preferably at least 45%, and most preferably 50% of the total cross-sectional area of ​​the intermediate section of the dividing wall column, while the other of the first and second intermediate subsections covers the remainder of 100% of the total cross-sectional area of ​​the intermediate section of the dividing wall column. For example, one subsection may cover 43% of the total cross-sectional area and the other 57%, or both subsections may each cover exactly 50% of the total cross-sectional area. If the total cross-sectional area of ​​the intermediate section varies along the axial length of the intermediate section, the above figures relate to the average total cross-sectional area of ​​the intermediate section.

[0055] According to a further specific preferred embodiment of the present invention, the plant further comprises at least one melt crystallization apparatus having an inlet connected to an outlet for purified styrene of the last of at least one distillation column, and an outlet for purified styrene.

[0056] At least one melt crystallization apparatus, A dynamic crystallization section comprising one or more dynamic crystallization steps, A static crystallization section comprising one or more static crystallization steps, At least two conduits that fluidly couple at least one of one or more dynamic crystallization steps to at least one of one or more static crystallization steps, It is even more preferable that the crystallized block is equipped with the following features.

[0057] Specific embodiments of the present invention will be described later by example with reference to the attached drawings. [Brief explanation of the drawing]

[0058] [Figure 1] This is a schematic diagram of a plant according to one embodiment of the present invention. [Figure 2] This is a schematic diagram of a plant according to another embodiment of the present invention. [Figure 3] This is a schematic diagram of a plant according to another embodiment of the present invention. [Modes for carrying out the invention]

[0059] The plant shown in Figure 1 for recycling polystyrene waste containing at least one organic halogen-containing compound by converting polystyrene to styrene comprises an extruder 10 having an inlet line 12 for polystyrene waste such as expanded polystyrene in the form of chips, flakes, pellets, powder, etc., and an outlet line 14 for molten polystyrene waste containing at least one organic halogen-containing compound. The outlet line 14 of the extruder 10 is connected to the inlet of a defloration vessel 16, which has an outlet line 18 for the gaseous composition and an outlet line 20 for the deflorational composition. The outlet line 18 for the gaseous composition is connected to the inlet of a washing tower 22, which is connected via line 24 to a condenser 26 having an outlet line 28 for the liquid light hydrocarbon composition, while the outlet line 20 for the deflorational composition of the defloration vessel 16 leads to a pyrolysis reactor 30. The pyrolysis reactor 30 has an outlet line 32 for the residue and an outlet line 34 for the pyrolysis composition, which are connected to a quencher or cooler 36, respectively. The cooler 36 has an outlet line 38 for light hydrocarbon compositions and an outlet line 40 leading to the first distillation column 42, the first distillation column 42 has a top outlet line 44 for light hydrocarbon compositions and a bottom outlet line 46 leading to the second distillation column 48, the second distillation column 48 is C 9+ It has a bottom outlet line 50 for hydrocarbons and a top outlet line 52 for purified styrene. The outlet line 52 for purified styrene leads to a crystallizer 54, such as a flow-down thin-film crystallizer, which has an outlet line 56 for pure styrene.

[0060] During plant operation, polystyrene waste containing at least one organic halogen-containing compound, such as expanded polystyrene in the form of chips, flakes, pellets, or powder, is fed into an extruder 10, where it is melted. The resulting polystyrene molten material is then fed into a defloration vessel 16, where the organic halogen-containing compound(s) contained in the polystyrene molten material are evaporated and at least partially decomposed into halogens, hydrogen halides, and / or halogenated hydrocarbons. The evaporated compounds, including the organic halogen-containing compound(s) and the decomposition products from these compounds, are removed from the defloration vessel 16 and treated in a washing tower 22 by contact with a base such as a sodium hydroxide solution. These gases are then concentrated into a liquid light hydrocarbon composition in a condenser 26, while the defloration composition obtained in the defloration vessel 16 goes to a pyrolysis reactor 30, where it is pyrolyzed into a pyrolysis composition containing styrene, styrene dimer, and by-products. The pyrolysis composition thus obtained is then rapidly cooled to a temperature of less than 200°C in a cooler 36, and then extracted via the outlet line 44 in two distillation columns 42 and 48. 7- A light hydrocarbon composition containing hydrocarbons and C extracted via the outlet line 50 9+ The heavy hydrocarbon composition containing hydrocarbons is separated from the purified styrene, which is withdrawn via the outlet line 52. The purified styrene is then further purified in the crystallizer 54, and from this purified styrene, pure styrene having, for example, a styrene content of 99.94% by weight is withdrawn via the outlet line 56.

[0061] The plant shown in Figure 2 for recycling polystyrene waste containing at least one organic halogen-containing compound by converting polystyrene to styrene corresponds to the plant shown in Figure 1, except that the crystallization apparatus 54 is omitted. The purified styrene obtained by this plant has a styrene content of, for example, 99.8% by weight and is removed via the outlet line 56.

[0062] The plant shown in Figure 3 for recycling polystyrene waste containing at least one organic halogen-containing compound by converting polystyrene to styrene corresponds to the plant shown in Figure 1, except that a third distillation column 58 is used instead of the crystallizer 54 and the first two distillation columns 42, 48 are implemented as extraction distillation columns connected by a solvent recirculation line 60. The purified styrene obtained by this plant has a styrene content of, for example, 99.8% by weight and is removed via the outlet line 56.

[0063] The present invention will be further explained here by illustrative but non-limiting examples.

[0064] "Example 1" For all experiments, the concentration was 1.04 g / cm³. 3 We used Styrolution PS 153F, a general-purpose polystyrene (GPPS) from INEOS, which has a density of . Hexabromocyclododecane (HBCD) was obtained from Chemtura (CD-75P), and brominated polybutadiene (PolyFR) was obtained from Chemtura (Emerald3000).

[0065] A polymer containing a specific amount of flame retardant was melted using an extruder. The extruder temperature was carefully controlled (below 195°C) to ensure complete melting of the polymer without premature decomposition of the flame retardant.

[0066] The molten polymer was continuously fed from the extruder into a series of SMX(L) static mixers. These mixers had a total length of 1.5 meters and a binding capacity of approximately 400 milliliters. Within the static mixers, the flame retardant was decomposed into hydrogen bromide (HBr) and the corresponding hydrocarbons. This decomposition was carried out at specific temperatures and residence times selected to ensure the selective decomposition of the flame retardant without affecting the polymer.

[0067] After exiting the static mixer, the polymer molten material containing the decomposed flame retardant was placed into a flash container. The flash container had a capacity of approximately 50 liters and an inner diameter of 30 centimeters. Inside the flash container, HBr was rapidly vaporized from the polymer molten material. The degassed polymer molten material, free of HBr, was removed from the flash container and prepared for further processing.

[0068] A general-purpose polystyrene (GPPS) composition containing 1% by weight of HBCD as a flame retardant and a GPPS composition containing 2% by weight of a block copolymer of polystyrene and brominated polybutadiene (PolyFR) as a flame retardant were melted and then subjected to a defoliation process in a defoliation vessel at the temperatures and residence times summarized in the table below. Subsequently, for each defoliated composition, it was determined how much bromo contained in the flame retardant of the starting composition was removed during the defoliation process.

[0069] As shown in the following table, the defolatorial step completely or at least substantially completely removes the flame retardant from the polymer molten material, so that this defolatorial polymer molten material can then be thermally decomposed in a state where there is no flame retardant at all or at least a dramatically reduced amount of flame retardant, so that the thermally decomposed composition is also free of flame retardant or contains at most a minimal amount of flame retardant. Thus, the purified styrene composition produced can be used in (co)polymerization reactions without causing any corrosion and / or contamination of piping and reactor equipment, for example, and / or poisoning of the catalyst used.

[0070] [Table 1] [Explanation of symbols]

[0071] 10 Extruder 12. Extruder inlet line 14. Extruder outlet line 16 Devolatilization vessel 18. Outlet line for gaseous composition of defoliation container 20. Outlet line for defoliated composition in defoliation container 22 Washing Tower 24 lines 26 Condenser 28 Outlet line for liquid light hydrocarbon composition 30 Pyrolysis reactor 32. Outlet line for pyrolysis reactor residue 34 Outlet line for pyrolysis composition of pyrolysis reactor 36 Cooler 38 Outlet line for light hydrocarbon composition in a cooler 40 Outlet line for cooled pyrolysis composition of the cooler 42 First distillation column 44 Top outlet line of the first distillation column for light hydrocarbon compositions 46. ​​Bottom outlet line of the first distillation column 48. Second distillation column 50. Bottom outlet line of the second distillation column 52. Top outlet line of the second distillation column 54 Crystallization equipment 56. Outlet line for pure styrene 58 Third distillation column 60 Solvent recirculation line

Claims

1. A method for recycling polystyrene waste containing at least one organic halogen-containing compound by converting polystyrene to styrene, a) A step of providing a molten polystyrene waste containing at least one organic halogen-containing compound, b) A step of subjecting the molten material provided in step a) to at least one defoliation step to obtain a defoliated composition, c) A step of subjecting the defoliated composition obtained in step b) to at least one thermal decomposition step to obtain a thermal decomposition composition, d) A step of subjecting the pyrolysis composition to at least one distillation step to obtain a purified styrene composition. Methods that include...

2. The method according to claim 1, wherein the polystyrene waste used in step a) comprises 0.5 to 15% by weight of at least one organic halogen-containing flame retardant, preferably at least one bromine-containing flame retardant, more preferably tetrabromobisphenol A, brominated polyacrylate, polybrominated diphenyl ether, brominated polybutadiene or hexabromocyclododecane, most preferably brominated polybutadiene or 1,2,5,6,9,10-hexabromocyclododecane.

3. The method according to claim 1 or 2, wherein the at least one defolatory step b) is carried out at a temperature of 230 to 310°C, preferably 240 to 310°C.

4. The method according to any one of claims 1 to 3, wherein the residence time of the molten material in the at least one defolatorial step is 5 to 120 minutes, preferably 10 to 90 minutes when the temperature is 230 to 310°C, and preferably 20 to 60 minutes when the temperature is 240 to 310°C.

5. The method according to any one of claims 1 to 4, wherein the molten polystyrene waste containing at least one organic halogen-containing compound, provided in step a), is mixed in at least one static mixer, preferably at least one static mixer having heat exchange capacity, before or during the at least one defloration step.

6. The method according to any one of claims 1 to 5, wherein the at least one thermal decomposition step is carried out at a temperature of 350 to 1,000°C, preferably 450 to 650°C.

7. The method according to any one of claims 1 to 6, wherein the pyrolysis composition is cooled to a temperature of at least 200°C, preferably 40 to 200°C, at a cooling rate of at least 500°C / min, and then subjected to at least one distillation step in step d).

8. i) The pyrolysis composition is subjected to two distillation steps, and the pyrolysis composition is subjected to C in the first distillation step. ７－ The top composition containing hydrocarbons and the bottom composition are distilled, and in the second distillation step, the bottom composition contains purified styrene and C ９＋ The pyrolysis composition is distilled into a bottom composition containing hydrocarbons, or ii) the pyrolysis composition is subjected to a distillation process in a split-wall distillation column, preferably in an intermediate split-wall distillation column, and the pyrolysis composition is C ７－ A top-of-the-column composition containing hydrocarbons, a side-of-the-column composition of purified styrene, and C ９＋ The method according to any one of claims 1 to 7, wherein the bottom composition containing hydrocarbons is distilled.

9. The method according to any one of claims 1 to 8, wherein the purified styrene composition obtained in the at least one distillation step is subjected to at least one dynamic melt crystallization step and at least one static melt crystallization step, preferably, at least one dynamic melt crystallization step is performed first, followed by at least one static melt crystallization step.

10. The method according to claim 9, wherein the purified styrene composition obtained in at least one distillation step is subjected to 1 to 10, preferably 1 to 5, flow thin film melt crystallization steps, and then subjected to 1 to 10, preferably 1 to 5, static melt crystallization steps.

11. The method according to any one of claims 1 to 10, wherein no catalyst or solvent is added to the composition that is thermally decomposed in at least one thermal decomposition step c), and preferably, no catalyst or solvent is added to the composition throughout the method subjected to steps a) to d).

12. A plant for recycling polystyrene waste containing at least one organic halogen-containing compound by converting polystyrene to styrene, i) At least one defloration container having an inlet for molten polystyrene waste containing at least one organic halogen-containing compound, an outlet for a gas composition, and an outlet for a defloration composition, ii) At least one pyrolysis reactor having an inlet connected to the outlet for the defoliated composition of the at least one defoliation vessel i), and an outlet for the pyrolysis composition, iii) At least one distillation column having an inlet directly or indirectly connected to the outlet for the pyrolysis composition of the at least one pyrolysis reactor, a top outlet, and a bottom outlet. A plant equipped with these features.

13. The plant according to claim 12, wherein the plant further comprises an extruder, preferably a single-screw extruder or a twin-screw extruder, located upstream of the at least one davolt vessel, the extruder having an inlet for polystyrene waste containing at least one organic halogen-containing compound and an outlet for molten polystyrene waste containing at least one organic halogen-containing compound, and the outlet for molten material of the extruder is connected to the inlet of the at least one davolt vessel.

14. The plant according to claim 12 or 13, further comprising a cooler having an inlet connected to an outlet for the pyrolysis composition of the at least one pyrolysis reactor and an outlet for the cooled pyrolysis composition connected to the inlet of the at least one distillation column, wherein the cooler is preferably a spray condenser.

15. The plant according to any one of claims 12 to 14, wherein the plant comprises i) two distillation columns, the first distillation column having an inlet connected to the outlet for the pyrolysis composition of the at least one pyrolysis reactor, a top outlet and a bottom outlet, and the second distillation column having an inlet connected to the bottom outlet of the first distillation column, a top outlet for purified styrene and a bottom outlet, or ii) a split-wall distillation column having an inlet connected to the outlet for the pyrolysis composition of the at least one pyrolysis reactor, an outlet for the top composition, an outlet for the column-side composition of purified styrene and an outlet for the column-bottom composition.

16. The plant according to any one of claims 12 to 15, further comprising at least one melt crystallization apparatus having an inlet connected to an outlet for purified styrene of the last of the at least one distillation columns, and an outlet for purified styrene.

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