Method and plant for recycling polymethyl methacrylate

Abstract

The present invention relates to a method for producing a purified methyl methacrylate composition from a crude composition containing methyl methacrylate, and the method comprises the following steps: a) providing a thermal decomposition composition obtained by thermal decomposition of a polymethyl methacrylate composition as the crude composition; b) subjecting the thermal decomposition composition to at least one distillation step to obtain a concentrated methyl methacrylate composition; and c) subsequently subjecting the concentrated methyl methacrylate composition to at least one melt crystallization step to obtain a purified methyl methacrylate composition.

Description

Technical Field

[0001] The present invention relates to a method and a plant for preparing a purified methyl methacrylate composition from a crude composition containing methyl methacrylate, for example, a crude composition prepared by thermal decomposition of a composition containing recycled polymethyl methacrylate. More specifically, the present invention can separate light components, heavy components and near-boiling components containing components that affect color from methyl methacrylate, thereby obtaining a high-quality methyl methacrylate particularly suitable for producing high-quality polymethyl methacrylate or optical grade polymethyl methacrylate, and relates to a method and a plant therefor.

Background Art

[0002] Polymethyl methacrylate, also known as acrylic glass, is a transparent thermoplastic known by trademark names and brand names such as Crylux®, Alfaplas®, Plexiglas®, Acrylite®, Lucite and Perspex® in addition to several other names, and is one of the most widely used plastics in the world, for example, because it can be very well molded into a desired shape by injection molding. Polymethyl methacrylate is used as a material for, for example, optical lenses, spectacle lenses, glazing, lamps, dental prostheses, medical prostheses, covers and lenses for external lamps of automobiles, and sanitary parts such as washbasins.

Summary of the Invention

[0003] The recycling of polymethyl methacrylate waste is becoming increasingly important. Firstly, polymethyl methacrylate is a homopolymer of methyl methacrylate, which is typically produced from starting materials obtained from crude oil or hydrocarbon flows derived from crude oil. Therefore, the possibility of recovering the raw material methyl methacrylate from polymethyl methacrylate waste and reproducing the polymer polymethyl methacrylate is advantageous not only from an economic standpoint but especially from an environmental standpoint. Furthermore, because polymethyl methacrylate waste contains pollutants with different properties, its disposal is difficult and costly.

[0004] Typically, the recycling of polymethyl methacrylate waste is based on the thermal decomposition of polymethyl methacrylate into methyl methacrylate in a thermal decomposition reactor. However, the thermal decomposition of polymethyl methacrylate yields numerous by-products, which must be separated from methyl methacrylate before it can be used as a monomer to produce new polymethyl methacrylate, compared, for example, polymethyl methacrylate from virgin monomers. This is particularly difficult because the resulting polymethyl methacrylate, and therefore the methyl methacrylate used as a monomer, needs to be of very high purity not only in most applications of polymethyl methacrylate, such as in dental prostheses or medical prostheses, but also in non-medical applications, such as optical-grade polymethyl methacrylate used as plastic covers for the rear and / or front lights of automobiles. In these applications, a very clear, i.e., virtually impurity- and colorless polymethyl methacrylate is required. Consequently, recycling polymethyl methacrylate via thermal decomposition to methyl methacrylate requires the near-complete removal of impurities such as carboxylic acid esters and diesters, alcohols, carboxylic acids, ketones, and others from the thermal decomposition composition. However, known purification methods based on several distillation steps are energy-intensive and involve high operating costs and capital expenditures.

[0005] In view of this, the fundamental object of the present invention is to provide a method for preparing a purified methyl methacrylate composition from a crude composition containing methyl methacrylate, such as a crude composition prepared by thermal decomposition of a composition containing recycled polymethyl methacrylate, wherein the method yields very high-purity polymethyl methacrylate, and even if this polymethyl methacrylate contains impurities, it contains only negligible amounts of impurities that do not cause problems, and the method is characterized by low operating costs and low capital expenditure. [Modes for carrying out the invention]

[0006] According to the present invention, this objective is achieved by providing a method for preparing a purified methyl methacrylate composition from a crude composition containing methyl methacrylate, the method comprising the following steps: a) A step of providing a thermal decomposition composition obtained by thermal decomposition of a polymethyl methacrylate composition as a crude composition, b) A step of subjecting the pyrolysis composition to at least one distillation step in order to obtain a methyl methacrylate concentrated composition, c) Next, the methyl methacrylate concentrated composition is subjected to at least one melt crystallization step in order to obtain a purified methyl methacrylate composition.

[0007] This solution is based on the finding that a very high-purity methyl methacrylate composition can be obtained by subjecting a crude composition containing methyl methacrylate, such as a crude composition prepared by thermal decomposition of a composition containing recycled polymethyl methacrylate, to at least one distillation step, followed by at least one melt crystallization step, thereby obtaining a methyl methacrylate composition with a methyl methacrylate content of 99.8% by weight or more and a total impurity content of 50 ppm or less. In particular, at least one melt crystallization step makes it possible to efficiently remove impurities having a boiling point close to the boiling point of methyl methacrylate, which, even if removed, can only be done by distillation, which involves very high energy costs. Due to the high purity of the resulting purified methyl methacrylate, the produced purified methyl methacrylate is particularly suitable as a monomer for polymethyl methacrylate compositions used in dental prostheses, medical prostheses, plastic covers for rear and front lights of automobiles, and other applications requiring very high-purity polymethyl methacrylate. Another advantage of the method according to the present invention is that it has a high yield, i.e., a yield of more than 90% by weight of the mass of methyl methacrylate contained in the crude composition or the pyrolysis composition, respectively. Furthermore, by using crystallization, the total operating costs and capital expenditures of this method are relatively low. Overall, the method according to the present invention is characterized not only by yielding very high-purity methyl methacrylate but also by low operating costs and low capital expenditures.

[0008] In the present invention, the type of crude composition provided in step a) is not particularly limited, as long as the crude composition is a pyrolysis composition obtained by the pyrolysis of a polymethyl methacrylate composition, preferably a pyrolysis composition obtained by the pyrolysis of a recycled polymethyl methacrylate composition. Therefore, preferably, the method includes, as step a), a step of subjecting a polymethyl methacrylate composition to at least one pyrolysis step to obtain a pyrolysis composition. The (recycled) polymethyl methacrylate composition used for pyrolysis may be, for example, a virgin polymethyl methacrylate composition such as production waste. Preferably, the polymethyl methacrylate composition used for pyrolysis is a post-consumer polymethyl methacrylate composition. Before being subjected to pyrolysis, the recycled polymethyl methacrylate may be subjected to one or more preparation steps, such as automatic sorting, one or more crushing steps and / or one or more separation steps, in which non-plastic material is separated from plastic material.

[0009] For example, the (recycled) polymethyl methacrylate composition used in step a) contains at least 80% by weight of polymethyl methacrylate derived from mixed scrap. Particularly good results are obtained when the (recycled) polymethyl methacrylate composition used in step a) contains at least 90% by weight of polymethyl methacrylate, and most preferably 90-100% by weight.

[0010] Preferably, the polymethyl methacrylate composition is melted before the molten material is thermally decomposed in step a). The present invention is not particularly limited in terms of the type of method for melting the polymethyl methacrylate composition. Particularly good results are obtained when the polymethyl methacrylate composition is melted in an extruder, such as a single-screw or twin-screw extruder. The extruder not only preheats the polymethyl methacrylate composition to a molten state but also continuously mixes it. Particularly good results are obtained when the polymethyl methacrylate composition is heated in the extruder to a temperature of 190 to 250°C, preferably 210 to 240°C. The polymethyl methacrylate composition can be supplied to the extruder in any form, such as chips, flakes, pellets, powder, or similar forms.

[0011] According to the present invention, the polymethyl methacrylate composition (or its molten form) is then subjected to at least one thermal decomposition step to obtain a thermal decomposition composition as a crude composition. Particularly good results are obtained when at least one thermal decomposition step is carried out at a temperature of 400 to 600°C, more preferably at a temperature of 450 to 520°C. In this way, the polymethyl methacrylate is decomposed into methyl methacrylate in high yield.

[0012] In further development of the concept of the present invention, it is proposed that at least one pyrolysis step be carried out in an isothermal reactor or an adiabatic reactor. Particularly favorable results are obtained when a fluidized bed is used as the isothermal reactor.

[0013] Pyrolysis is preferably carried out in an inert gas atmosphere such as nitrogen or a noble gas, or in the atmosphere of the product.

[0014] Each pyrolysis product or pyrolysis composition preferably contains more than 80% by weight of methyl methacrylate, more preferably more than 90%, and most preferably more than 95%, with the remaining 100% by weight being impurities. The impurities include methyl esters of carboxylic acids, ethyl and butyl esters of carboxylic acids, hydrocarbons, alcohols, diesters of carboxylic acids, ketones, and carboxylic acids. Examples of methyl esters of carboxylic acids are methyl esters of acrylic acid, propionic acid, isobutyric acid, and benzoic acid, while examples of ethyl and butyl esters of carboxylic acids are those of acrylic acid. Furthermore, hydrocarbon impurities include pentene, 2-pentene, cyclopentene, toluene, benzene, propylbenzene, ethylbenzene, and / or similar substances, while alcohol impurities are mainly methanol and ethanol, while an example of a diester of carboxylic acid contained in the pyrolysis composition is ethylene glycol dimethacrylate, examples of phthalate esters are phthalic anhydride and dibutyl phthalate, an example of a ketone is dimethylcyclopentenone, and an example of a carboxylic acid is methacrylic acid. During the thermal decomposition reaction, methyl isobutyrate, ethyl acrylate, methyl acrylate, methyl propionate, methanol, water, and other by-products are formed, which have boiling points close to the boiling point of methyl methacrylate or form azeotropes containing methyl methacrylate. These by-products are removed from the thermal decomposition composition mainly during at least one melt crystallization step, according to the present invention.

[0015] In one aspect of the present invention, step b) may comprise only one distillation step. In this embodiment, the pyrolysis composition is fed into a distillation column in step b), where it is distilled into an overhead fraction, a bottom fraction, and a side fraction, where the side fraction is led to at least one melt crystallization step c) as a (i.e., pre-purified) methyl methacrylate concentrate. Preferably, the temperature in the distillation column is adjusted to less than 95°C, more preferably less than 80°C, while the distillation column is connected to a vacuum system, and the pressure in the distillation column is adjusted to preferably 5 to 75 kPa in step b).

[0016] In a further development of the concept of the present invention, it is proposed that the pyrolysis composition is subjected to two or more consecutive distillation steps, more preferably two consecutive distillation steps, in step b). In this embodiment, the pyrolysis composition is supplied to a first distillation column in step b), where it is distilled into an overhead fraction and a bottom fraction, and the bottom fraction is led to a second distillation column, where it is distilled into an overhead fraction and a bottom fraction. The overhead fraction obtained in the first distillation column contains light components such as methanol and ethanol, and the bottom fraction obtained in the second distillation column contains heavy components such as phthalates and diesters, while the overhead fraction obtained in the second distillation column is led to at least one melt crystallization step as a methyl methacrylate concentrate composition.

[0017] In this embodiment as well, it is preferable that the temperature inside each distillation column is adjusted to less than 95°C, more preferably less than 80°C, in step b), while each distillation column is connected to a vacuum system, and the pressure inside each distillation column is preferably adjusted to 5 to 75 kPa in step b).

[0018] The methyl methacrylate concentrated composition obtained in this embodiment preferably contains more than 98% by weight of methyl methacrylate, more preferably at least 99% by weight, even more preferably at least 99.5% by weight, and most preferably 99.5% to 99.8% by weight of methyl methacrylate.

[0019] In another, particularly preferred embodiment of the present invention, the pyrolysis composition provided in step a) is subjected to a distillation step in a divided-wall distillation column in step b). This makes it possible to obtain a concentrated methyl methacrylate composition (i.e., unpurified) with low operating costs and low capital expenditure. The present invention is not particularly limited with respect to the type of divided-wall distillation column. Therefore, any distillation column equipped with a divided wall, wherein the divided wall separates at least a portion extending in the longitudinal direction of the divided-wall distillation column into two subsections when viewed in cross-section, can be used. Preferably, the divided wall is positioned at least essentially vertically downward within the divided-wall distillation column. “At least essentially vertically downward” according to the present invention means that the angle between the divided wall and the longitudinal axis or vertical direction of the divided-wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5°, and most preferably 0°.

[0020] According to a particularly preferred modification of this embodiment of the present invention, the pyrolysis composition is fed into a distillation column with an intermediate split wall in step b), where it is distilled into an overhead composition, a methyl methacrylate concentrate as a side composition, and a bottom composition, where the side fraction is led to at least one melt crystallization step c) as a methyl methacrylate concentrate.

[0021] In the present invention, a distillation column with an intermediate divided wall is a distillation column with a divided wall, wherein the divided wall extends at least essentially vertically downward from a point below the top of the distillation column to a point above the bottom of the distillation column, thereby dividing the distillation column into an upper section located above the divided wall, a lower section located below the divided wall, and an intermediate section, wherein the intermediate section is defined to include a first intermediate subsection located on one side of the divided wall and a second intermediate subsection located on the opposite side of the divided wall. Favorable results are obtained particularly when the divided wall extends from a point located 10-45% of the distance from the bottom to the top of the distillation column, viewed from the bottom to the top, to a point located 50-90% of the distance, preferably from a point located 25-40% of the distance from the bottom to the top, to a point located 60-80% of the distance. Furthermore, it is preferable that the dividing wall extends over 5-90%, more preferably 20-80%, and most preferably 30-70% of the distance from the bottom to the top of the distillation column with the dividing wall, when viewed from the bottom to the top.

[0022] In a further developed aspect of the present invention, when viewed in cross-section of the dividing wall column, it is proposed that the dividing wall subdivides the middle section of the dividing wall column into two sub-sections or halves of approximately the same size. Thus, one of the first intermediate sub-section and the second intermediate sub-section occupies 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 middle section of the dividing wall column, while the other of the first intermediate sub-section and the second intermediate sub-section preferably occupies up to the remaining 100% of the total cross-sectional area of the middle section of the dividing wall column. For example, one sub-section may occupy 43% of the total cross-sectional area and the other may occupy 57%, or both sub-sections may each occupy exactly 50% of the total cross-sectional area. When the total cross-sectional area of the middle section varies along the axial length of the middle section, the above numerical values relate to the average value of the total cross-sectional area of the middle section.

[0023] Preferably, the distillation is carried out in a dividing wall column at a temperature below 95 °C and a pressure below 75 kPa, more preferably at a temperature below 80 °C and a pressure below 55 kPa.

[0024] The side stream obtained in the dividing wall column may or may not be subjected to a second distillation step. Preferably, the side stream obtained in the dividing wall column is directly led to at least one melt crystallization step without being subjected to a second distillation step.

[0025] The methyl methacrylate enriched composition obtained in this embodiment also preferably contains more than 98% by weight of methyl methacrylate, more preferably at least 99% by weight, even more preferably at least 99.5% by weight, and most preferably 99.5 - 99.8% by weight of methyl methacrylate.

[0026] By removing by-products generated during thermal decomposition, such as methyl isobutyrate, ethyl acrylate, methyl acrylate, methyl propionate, methanol, water and other by-products that have a boiling point close to that of methyl methacrylate or form an azeotrope with methyl methacrylate, from methyl methacrylate, the purity level of methyl methacrylate is further increased. The methyl methacrylate concentrate composition obtained in at least one distillation step of step b) is subjected to at least one melt crystallization step c) according to the present invention. Good results are obtained when the methyl methacrylate concentrate composition is subjected to 1 to 10, more preferably 1 to 3, and most preferably 1 or 2 melt crystallization steps in step c), particularly for obtaining a purified methyl methacrylate composition.

[0027] The present invention is not particularly limited with respect to the type of at least one melt crystallization step. Preferably, at least one melt crystallization step b) includes at least one melt crystallization step selected from the group consisting of a static crystallization step, a suspension crystallization step, and a falling liquid film crystallization step.

[0028] Good results are obtained particularly when at least one melt crystallization step b) is a static crystallization step. Static crystallization is usually carried out in a crystallizer or vessel in which vertical plates heated or cooled by internal circulation of a heat medium are arranged. The melt to be crystallized, i.e., the methyl methacrylate concentrate composition, is actually filled in the voids between the vertical plates such that the vertical plates are immersed in the methyl methacrylate concentrate composition. By slowly cooling the heat medium until it is lower than the freezing point of methyl methacrylate, the formation of a layer of methyl methacrylate crystals on the surface of the vertical plates is initiated.

[0029] For example, the pyrolysis composition subjected to the static melt crystallization steps 1 to 10 in step c) is supplied to the first static melt crystallization step of the static melt crystallization steps 2 to 10, thereby generating a first methyl methacrylate concentrated crystallization fraction and a methyl methacrylate depleted residue fraction, the first methyl methacrylate concentrated crystallization fraction is supplied to the second static melt crystallization step of the static melt crystallization steps 2 to 10, and in the second and any third to tenth static melt crystallization steps, the methyl methacrylate concentrated crystallization fraction and A methyl methacrylate depletion residue fraction is generated, and each methyl methacrylate concentrated crystallized fraction generated in the second and any third to tenth static melt crystallization steps is supplied to the downstream static melt crystallization step, and each methyl methacrylate depletion residue fraction generated in the second and any third to tenth static melt crystallization steps is supplied to the upstream static melt crystallization step, and the methyl methacrylate concentrated crystallized fraction from the downstream static melt crystallization step is the purified methyl methacrylate composition.

[0030] For example, the generation of a methyl methacrylate concentrated crystallized fraction and a methyl methacrylate depleted residue fraction in a static melt crystallization process includes a sub-step of removing the remaining liquid from the crystallization process as a methyl methacrylate depleted residue fraction after the completion of crystallization in the static melt crystallization process, a sub-step of melting the crystal layer obtained in the static melt crystallization process, and a sub-step of removing the obtained crystalline melt as a methyl methacrylate concentrated crystallized fraction from the crystallization process.

[0031] Favorable results are obtained, in particular, when at least one, and more preferably all, of the melt crystallization steps are carried out at a crystallization temperature of -10°C to -80°C, preferably at a crystallization temperature of -25°C to -70°C, and even more preferably at a crystallization temperature of -35°C to -60°C.

[0032] To further increase the purity of the purified methyl methacrylate composition obtained in at least one melt crystallization step, it is preferable that at least one sweating sub-step be performed in at least one melt crystallization step. Sweating means gently heating the crystalline layer deposited on a cooled surface to a temperature near the melting point of methyl methacrylate in order to partially melt the crystals. Captured and adhering molten material containing impurities flows off during the partial melting of the crystals and is subsequently removed from the crystallizer. To perform such sweating, the surface on which the crystals are deposited is heated to a desired temperature with a heat transfer medium. Sweating may be performed once or multiple times before melting the crystalline layer deposited on the cooled surface. Thus, sweating results in one or more sweating fractions and a purified crystalline layer. Preferably, at least a portion of the resulting first sweating fraction is supplied to the remaining liquid removed as a methyl methacrylate depletion residue fraction.

[0033] In addition, according to another preferred embodiment of the present invention, the purified methyl methacrylate composition obtained in at least one melt crystallization step contains at least 99.8% by weight of methyl methacrylate and impurities in amounts of 1000 ppm or less, preferably 600 ppm or less, more preferably 250 ppm or less, even more preferably 50 ppm or less, and most preferably 10 ppm or less. Particularly preferred is that the purified methyl methacrylate composition obtained in at least one melt crystallization step has an APHA (American Public Health Association) chromaticity of up to 5, where APHA chromaticity is a color standard named after the American Public Health Association, as defined by ASTM D1209.

[0034] In another embodiment, the present invention relates to a plant for preparing a purified methyl methacrylate composition from a (recycled) polymethyl methacrylate composition, wherein the plant includes: a) At least one pyrolysis reactor having an inlet for a polymethyl methacrylate composition and an outlet for a pyrolysis composition, b) At least one distillation column having an inlet, overhead outlet and bottom outlet connected to an outlet for the pyrolysis composition of at least one pyrolysis reactor, and c) At least one melt crystallizer having an inlet connected to one of the outlets of at least one distillation column and an outlet for the purified methyl methacrylate composition.

[0035] Preferably, the plant further includes an extruder, which is located upstream of at least one pyrolysis reactor, and the extruder has an inlet for the polymethyl methacrylate composition and an outlet for the molten polymethyl methacrylate composition, with the outlet for the molten extruder being connected to the inlet of at least one pyrolysis reactor. The extruder may be, for example, a single-screw extruder or a twin-screw extruder.

[0036] In a preferred embodiment of the present invention, the plant comprises two distillation columns, the first distillation column having an inlet connected to the outlet for the pyrolysis composition of at least one pyrolysis reactor, an overhead outlet and a bottom outlet, and the second distillation column having an inlet connected to the bottom outlet of the first distillation column, an overhead outlet and a bottom outlet for the methyl methacrylate concentrate composition.

[0037] According to another, particularly preferred embodiment of the present invention, the plant comprises a distillation column with split walls, the distillation column with split walls comprising an inlet connected to an outlet for the pyrolysis composition of at least one pyrolysis reactor, an outlet for the overhead composition, an outlet for the side composition which is a methyl methacrylate concentrate composition, and an outlet for the bottom composition.

[0038] In an embodiment that further develops the concept of the present invention, it is proposed that the distillation column with a divided wall is a distillation column with an intermediate divided wall, the distillation column having an intermediate divided wall that extends at least essentially vertically downward from a point below the top of the distillation column to a point above the bottom of the distillation column, thereby dividing the distillation column with a divided wall into an upper section located above the divided wall, a lower section located below the divided wall, and an intermediate section, the intermediate section comprising a first intermediate subsection located on one side of the divided wall and a second intermediate subsection located on the opposite side of the divided wall.

[0039] Favorable results are obtained particularly when the dividing wall extends from a point at 10-45% of the distance from the bottom to the top of the distillation column with the dividing wall, to a point at 50-90%, preferably from a point at 25-40% of the distance from the bottom to the top of the distillation column with the dividing wall, to a point at 60-80% of the distance from the bottom to the top of the distillation column with the dividing wall. Furthermore, it is preferable that the dividing wall extends over 5-90%, more preferably 20-80%, and most preferably 30-70%, of the distance from the bottom to the top of the distillation column with the dividing wall, when viewed from the bottom to the top of the distillation column with the dividing wall.

[0040] In a further development of the present invention, it is proposed that the dividing wall subdivides the intermediate section of the divided distillation column into two subsections or halves of substantially the same size when viewed in cross-section. Thus, it is preferable that one of the first and second intermediate subsections occupies 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 divided distillation column, while the other of the first and second intermediate subsections occupies the remaining 100% of the total cross-sectional area of ​​the intermediate section of the divided distillation column. For example, one subsection may occupy 43% of the total cross-sectional area and the other 57%, or both subsections may each occupy 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 value of the total cross-sectional area of ​​the intermediate section.

[0041] According to a further particularly preferred embodiment of the present invention, the plant comprises at least one static melt crystallizer as at least one melt crystallizer c).

[0042] In embodiments where the plant comprises at least one distillation column, it is preferable that the plant comprises 1 to 3, more preferably 1 or 2, static melt crystallizers.

[0043] The present invention will be described in more detail below with reference to the drawings. [Brief explanation of the drawing]

[0044] [Figure 1] Figure 1 schematically shows a plant for preparing a purified methyl methacrylate composition from a recycled polymethyl methacrylate composition according to one embodiment of the present invention.

[0045] The plant 10 for preparing a purified methyl methacrylate composition from a recycled polymethyl methacrylate composition, as shown in Figure 1, comprises a pyrolysis reactor 12, a distillation column 14, and a static melt crystallizer 16. More specifically, the pyrolysis reactor 12 comprises an inlet line 18 for a feed, such as a molten recycled polymethyl methacrylate composition. To prepare the molten material, the plant 10 may further comprise an extruder (not shown) upstream of the pyrolysis reactor 12. Furthermore, the pyrolysis reactor 12 comprises an outlet line 20 for a pyrolysis liquid composition or a liquid crude composition, which is also the inlet line 20 for the pyrolysis composition in the distillation column 14. The distillation column 14 is a distillation column with a partial divided wall 14, which comprises a divided wall 28, the divided wall 28 extending vertically downward from about 25% to about 75% of the height of the distillation column 14, i.e., the straight-line distance between the bottom and top of the distillation column 14. The distillation column 14 with a split wall further includes an overhead outlet line 22 for light components such as ethanol, a bottom outlet line 24 for heavy components such as phthalates and diesters, and a side outlet line 26 for the methyl methacrylate concentrated composition. The side outlet line 26 for the methyl methacrylate concentrated composition in the distillation column 14 with a split wall is the inlet line 26 of the static melt crystallizer 16. Meanwhile, the static melt crystallizer 16 includes an outlet conduit 30 for the methyl methacrylate depletion residue fraction and an outlet conduit 32 for the purified methyl methacrylate composition.

[0046] During operation, for example, the molten recycled polymethyl methacrylate produced in a single-screw extruder is supplied to the pyrolysis reactor 12 via line 18, where the polymethyl methacrylate is pyrolyzed into methyl methacrylate and by-products. The pyrolysis composition, which contains methyl methacrylate and by-products such as carboxylic acid esters, alcohols, diesters, phthalates, ketones, carboxylic acids, and other hydrocarbons as impurities, is supplied as a crude composition to the distillation column 14 with an intermediate split wall via line 20. In the distillation column 14 with an intermediate split wall, the crude composition is obtained as an overhead fraction mainly containing ethanol.4- A hydrocarbon stream, a methyl methacrylate concentrated composition obtained as a side fraction, and a C obtained as a bottom fraction containing, for example, phthalate esters and diesters. 6+ The hydrocarbon stream is separated. The overhead fraction is removed from the distillation column 14 with an intermediate split wall via line 22, and the bottom fraction is removed from the distillation column 14 with an intermediate split wall via line 24, while the side fraction or methyl methacrylate concentrate is removed from the distillation column 14 with an intermediate split wall via line 26 and supplied to the static melt crystallizer 16 via line 26. The methyl methacrylate concentrate is further purified in the static melt crystallizer 16 by separating methyl isobutyrate, ethyl acrylate, methyl acrylate, methyl propionate, methanol, water, and other by-products generated during thermal decomposition and having boiling points close to the boiling point of methyl methacrylate or forming azeotropes containing methyl methacrylate from methyl methacrylate. The methyl methacrylate depleted residue fraction thus obtained is removed from the static melt crystallizer 16 via discharge conduit 30, and the purified methyl methacrylate composition thus obtained is removed from the static melt crystallizer 16 via discharge conduit 32. [Examples]

[0047] Next, the present invention will be further explained with reference to examples and comparative examples, all of which are illustrative of the present invention but do not limit it.

[0048] Comparative Example 1 (Purification of methyl methacrylate by distillation alone)

[0049] The purification of methyl methacrylate (MMA) was simulated using a two-column distillation system with a first distillation for removing light components and a second distillation for removing heavy components. An MMA-containing feed stream obtained by thermal decomposition of polymethyl methacrylate having the composition shown in Table 1 below was supplied to the first distillation column. The heat and mass balances of the distillation system were calculated using PRO / II process simulation software. Data on the thermophysical properties of the pure components and mixtures, such as vapor pressure and azeotrope, were obtained from the Dortmund Data Bank (DDB). The first distillation column used an NTS (Theoretical Stages) number of 88, and the second distillation column used an NTS of 22. The first distillation column was operated at a pressure of 0.5 bar and a temperature of 45-85°C, while the second distillation column was operated at a pressure of 0.5 bar and a temperature of 45-95°C. The specific energy requirement for pressurized vapor for the two distillation columns was calculated to be 1.1 MW (100%) for a mass flow rate of 1 ton of MMA supplied to the first distillation column.

[0050] Lighter components, such as methyl acrylate (MA), methylpropionate (MOPR), methanol, water, some methyl isobutyrate (MIBT), ethyl acrylate (EACR), and all other lighter components, were separated as an overhead flow from the top of the first distillation column, while MMA and heavier components were taken out as a bottom flow from the bottom of the first distillation column. The bottom flow was fed to the second distillation column, where heavier components were taken out as a bottom flow from the bottom of the second distillation column, while a purified flow with a purity of 99.77% was taken out as an overhead flow from the second distillation column. The composition of the purified product is shown in Table 1. [Table 1]

[0051] As shown in Table 1, only a small portion of methyl isobutyrate (MIBT) and ethyl acrylate (EACR), which are near-boiling point components with boiling points close to MMA and therefore most difficult to separate from MMA by distillation, were removed during the two distillations. Despite a relatively high number of theoretical stages (NTS) of 88 in the first distillation column and a relatively high NTS of 22 in the second distillation column, 50 ppm of MIBT and 2230 ppm of EACR remained in the purified product, making it impossible to achieve the required MMA purity of 99.80%.

[0052] Example 1 (Purification of methyl methacrylate by distillation and crystallization)

[0053] The purification of methyl methacrylate (MMA) was simulated using a two-column distillation system that performed static crystallization after a first distillation to remove light components and a second distillation to remove heavy components. An MMA-containing feed stream with the same composition as the feed stream used in Comparative Example 1 was supplied to the first distillation column. Again, the heat and mass balances of the distillation system were calculated using PRO / II process simulation software, and data on the thermophysical properties of the pure components and mixtures, such as vapor pressure and azeotrope, were obtained from the Dortmund Data Bank (DDB). In this example, since the near-boiling point components MIBT and EACR were removed by static crystallization, the distillation did not need to be optimized to remove as much MIBT and EACR as possible. The first distillation column was operated at a pressure of 0.5 bar and a temperature of 45-85°C, and the second distillation column was operated at a pressure of 0.5 bar and a temperature of 70-86°C. Distillation was calculated under conditions where the product purity of the overhead flow from the second distillation column was 99.5% MMA. The composition of this overhead flow is shown in Table 2. This overhead flow was supplied to the static crystallization stage. For crystallization, a static crystallization process with one crystallization stage was simulated, and this simulation used a mass balance calculation method based on a database of experimental results. For each component, the mass balance for the crystallization fraction and the sweating fraction was determined. The operating conditions for the crystallization stage were as follows: During the 14 hours of the crystallization phase, the temperature was lowered from -50°C to -57°C, while during the 8 hours of the sweating phase, the temperature was raised from -53°C to -43°C. The purified product was recovered from the crystallizer by melting the crystals at a temperature of -30°C.

[0054] After static crystallization, a 99.95% high-purity MMA product with the composition shown in Table 2 was obtained. This exceeds the required purity of 99.8% for the MMA product, making it possible to further reduce the purity of the top product MMA from the second distillation to less than 99.5%. [Table 2]

[0055] The energy consumption for the purification method in Example 1 was compared with the energy consumption for the purification method in Comparative Example 1. The specific energy requirement for the distillation column as pressurized steam for the reboiler was 0.6 MW, calculated based on a mass flow rate of 1 ton of MMA supplied to the first distillation column. The specific energy requirement for the crystallizer was 0.17 MW (as electricity for the refrigerator), calculated based on a mass flow rate of 1 ton of MMA supplied to the first distillation column. Therefore, the total energy consumption calculated for Example 1 is lower than the energy consumption calculated for Comparative Example 1. [Explanation of symbols]

[0056] 10. Plant for preparing purified methyl methacrylate composition 12. Pyrolysis reactor 14 (Intermediate) Distillation Column with Split Walls 16. Static melting crystallizer 18 Inlet line for supplies 20 Outlet line for pyrolysis composition / Inlet line for pyrolysis composition 22 Overhead outlet line of the distillation column 24. Bottom outlet line of the distillation column 26. Side outlet line of distillation column / Inlet line of static melt crystallizer 28 Dividing wall of distillation column 30 Discharge conduit for methyl methacrylate depletion residue fraction 32 Discharge conduit for purified methyl methacrylate composition

Claims

1. A method for preparing a purified methyl methacrylate composition from a crude composition containing methyl methacrylate, a) The step of providing a thermal decomposition composition obtained by thermal decomposition of a polymethyl methacrylate composition as the crude composition, b) The step of subjecting the pyrolysis composition to at least one distillation step in order to obtain a methyl methacrylate concentrate composition, and c) Next, the methyl methacrylate concentrated composition is subjected to at least one melt crystallization step in order to obtain the purified methyl methacrylate composition, Methods that include...

2. The method according to claim 1, wherein step a) includes subjecting the polymethyl methacrylate composition to at least one thermal decomposition step to obtain the thermal decomposition composition.

3. The method according to claim 1 or claim 2, wherein the polymethyl methacrylate composition used in step a) comprises at least 80% by weight, preferably at least 90% by weight, and most preferably 90 to 100% by weight of polymethyl methacrylate.

4. The method according to any one claim, wherein step b) comprises two distillation steps, the pyrolysis composition being fed into a first distillation column and distilled therein into an overhead fraction and a bottom fraction, the bottom fraction being introduced into a second distillation column and distilled therein into an overhead fraction and a bottom fraction, and the overhead fraction being led to at least one melt crystallization step c) as a methyl methacrylate concentrated composition.

5. The method according to any one of claims 1 to 3, wherein step b) comprises one distillation step in a divided-wall distillation column, preferably one distillation step in an intermediate divided-wall distillation column, wherein the pyrolysis composition is distilled in the divided-wall distillation column into an overhead composition, a side composition and a bottom composition, and the side fraction is led to the at least one melt crystallization step as a methyl methacrylate concentrate.

6. The method according to claim 5, wherein, when viewed from the bottom to the top of the distillation column with an intermediate divided wall, the dividing wall extends from a point located at 10% to 45% of the distance from the bottom to the top of the distillation column with an intermediate divided wall to a point located at 50% to 90% of the distance from the bottom to the top of the distillation column with a divided wall, and thereafter, when viewed from the bottom to the top of the distillation column with an intermediate divided wall, the dividing wall extends over 5% to 90%, more preferably 20% to 80%, and even more preferably 30% to 70% of the distance from the bottom to the top of the distillation column with a divided wall.

7. i) The method according to any one of the claims, wherein the methyl methacrylate concentrate is subjected in step b) to at least one, preferably 1 to 10, more preferably 1 to 3, and even more preferably 1 or 2, static melt crystallization steps to obtain the purified methyl methacrylate composition.

8. In step b), the pyrolysis composition subjected to the static melt crystallization steps 1 to 10 is supplied to the first static melt crystallization step of the static melt crystallization steps 2 to 10, thereby generating a first methyl methacrylate concentrated crystallized fraction and a methyl methacrylate depleted residue fraction. The first methyl methacrylate concentrated crystallized fraction is supplied to the second static melt crystallization step of the static melt crystallization steps 2 to 10, and in the second and any of the third to tenth static melt crystallization steps, the methyl methacrylate concentrated crystallized fraction and the methyl methacrylate depleted residue fraction are generated. The method according to claim 7, wherein a methacrylate depletion residue fraction is generated, each methyl methacrylate concentrated crystallized fraction generated in the second and any third to tenth static melt crystallization steps is supplied to the downstream static melt crystallization step, each methyl methacrylate depletion residue fraction generated in the second and any third to tenth static melt crystallization steps is supplied to the upstream static melt crystallization step, and the methyl methacrylate concentrated crystallized fraction furthest downstream of the static melt crystallization step is the purified methyl methacrylate composition.

9. The method according to claim 8, wherein the generation of a methyl methacrylate concentrated crystallized fraction and a methyl methacrylate depleted residue fraction in the static melt crystallization step includes the following substeps: removing the remaining liquid from the static melt crystallization step as a methyl methacrylate depleted residue fraction after the completion of crystallization in the static melt crystallization step; melting the crystal layer obtained in the static melt crystallization step; and removing the obtained crystalline melt from the static melt crystallization step as a methyl methacrylate concentrated crystallized fraction, wherein, preferably, one or more sweating steps of the crystal layer are performed before melting the crystal layer obtained in the static melt crystallization step to obtain one or more sweating fractions and a purified crystal layer.

10. The method according to any one of the claims, wherein at least one, preferably all, of the melt crystallization steps are carried out at a crystallization temperature of -10°C to -80°C, preferably at a crystallization temperature of -25°C to -70°C, and more preferably at a crystallization temperature of -35°C to -60°C.

11. The method according to any one of the claims, wherein the purified methyl methacrylate composition contains at least 99.8% by weight of methyl methacrylate and impurities in amounts of 1000 ppm or less, preferably 600 ppm or less, more preferably 250 ppm or less, even more preferably 50 ppm or less, and most preferably 10 ppm or less.

12. A plant for preparing a purified methyl methacrylate composition from a polymethyl methacrylate composition, wherein the plant is a) At least one pyrolysis reactor having an inlet for the polymethyl methacrylate composition and an outlet for the pyrolysis composition, b) at least one distillation column having an inlet, an overhead outlet and a bottom outlet connected to the outlet for the pyrolysis composition of at least one of the pyrolysis reactors, and c) At least one melt crystallizer having an inlet connected to one of the outlets of at least one distillation column and an outlet for the purified methyl methacrylate composition, A plant that includes this.

13. The plant according to claim 12, further comprising an extruder positioned upstream of the at least one pyrolysis reactor, preferably a single-screw or twin-screw extruder, wherein the extruder has an inlet for the polymethyl methacrylate composition and an outlet for the molten polymethyl methacrylate composition, and the outlet for the molten material of the extruder is connected to the inlet of the at least one pyrolysis reactor.

14. i) comprising 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, an overhead outlet and a bottom outlet, and the second distillation column having an inlet connected to the bottom outlet of the first distillation column, an overhead outlet and a bottom outlet for the methyl methacrylate concentrate composition, or ii) The plant according to claim 12 or 13, comprising a distillation column with an intermediate divided wall having an inlet connected to the outlet for the pyrolysis composition of at least one pyrolysis reactor, an outlet for an overhead composition, an outlet for a side composition which is the methyl methacrylate concentrate composition, and an outlet for a bottom composition, wherein the distillation column with an intermediate divided wall further comprises a dividing wall extending at least essentially vertically downward from a point located below the top of the distillation column to a point located above the bottom of the distillation column with a divided wall, thereby dividing the distillation column with a divided wall into an upper section located above the dividing wall, a lower section located below the dividing wall, and an intermediate section including a first intermediate subsection located on one side of the dividing wall and a second intermediate subsection located on the opposite side of the dividing wall.

15. The plant according to any one of claims 12 to 14, wherein the plant comprises 1 to 3, preferably 1 or 2, static melt crystallizers b).

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