TWI865792B - Method for manufacturing bis-(2-hydroxyethyl)terephthalate and method for manufacturing recycled polyethylene terephthalate - Google Patents

Abstract

To provide a method for easily manufacturing high-purity bis-(2-hydroxyethyl) terephthalate (BHET) from colored polyester waste. The method for manufacturing BHET according to the present invention includes: a depolymerization step of mixing polyester waste colored with a colorant, monoethylene glycol and a depolymerization catalyst, and depolymerizing the polyester waste to obtain a depolymerized product containing colored crude BHET; a colorant-decomposition removal step of adding a decomposing agent for the colorant to the depolymerized product and cooling it, and then separating a decomposed product of the colorant from a precipitated solid content by centrifugation to remove at least a part of the decomposed product from the depolymerized product; a concentration step of removing low boiling point components containing the monoethylene glycol from the depolymerized product, from which the at least a part of the decomposed product is removed, to concentrate crude BHET; a collection step of collecting BHET from the concentrated crude BHET by distillation to obtain BHET having higher purity than the crude BHET.

Description

Method for producing bis-(2-hydroxyethyl) terephthalate and method for producing recycled polyethylene terephthalate

The present invention relates to a method for producing high-purity bis-(2-hydroxyethyl) terephthalate from colored polyester waste (particularly colored polyester clothing waste and colored plastic bottle waste), and a method for producing recycled polyethylene terephthalate using the bis-(2-hydroxyethyl) terephthalate.

Polyester (e.g. polyethylene terephthalate (PET)) is widely used for clothing, films, PET bottles, resin molded products, etc. due to its excellent properties. However, the effective utilization of polyester scraps (losses in the manufacturing process) such as fibers, films, and resin sheets generated in the manufacturing process of these products, as well as waste such as used PET bottles and used molded products, has become a big issue not only in terms of cost but also in terms of environmental issues. Material recycling, thermal recycling, chemical recycling, etc. have been examined and proposed as treatment methods for these.

Among them, in the recycling of materials, waste materials such as used transparent PET bottles and other polyester molded products are mainly collected by local governments and actively reused. In addition, fiber scraps are recycled by processing them into large grains of popcorn, and then remelting them to regenerate fiber. However, it is extremely difficult to apply material recycling methods to clothing and colored PET bottles, so there is no example of reuse.

In the heat recycling, polyester waste including clothing and fiber scraps is converted into fuel, which has the advantage of being able to reuse the combustion heat of polyester waste. However, since the heat release value of polyester is relatively low, a large amount of polyester waste must be burned if the combustion heat is to be utilized. This causes the loss of polyester raw materials and the generation of carbon dioxide, which is disadvantageous in terms of resource saving and environmental protection.

In contrast, in chemical recycling, polyester waste is recycled into raw material monomer bis-(2-hydroxyethyl)terephthalate (hereinafter also referred to as "BHET") or dimethyl terephthalate (hereinafter also referred to as "DMT"), and the recycled BHET or DMT is subjected to polycondensation reaction again to produce new polyester (see, for example, patent documents 1 to 3). As a result, the quality of the polyester is less degraded due to recycling, so it is excellent as a closed loop recycling. However, even in chemical recycling, most of the recycled materials are transparent plastic bottles, resin scraps, film scraps, etc. that are not colored and have few impurities.

When using polyester waste that has been colored by dyes as a recycling raw material, it is necessary to remove the coloring pigments such as dyes and pigments in order to regenerate the polyester from the raw material monomer. It is known that when a large amount of coloring pigments are mixed in the recycled raw material monomer, the recycled polyester will also be colored (especially yellowing). In addition, polyester clothing waste contains various compounds such as stabilizers, antistatic agents, dye-resistant agents, flame retardants, moisture absorbents, and gas barrier agents used to meet the characteristics required for clothing use; different materials such as nylon, urethane, cotton, and olefin used in clothing; additives such as titanium oxide used as a matting agent, and impurities such as zippers, buttons, metals, glass, and sand. Therefore, in order to recycle polyester clothing waste, it is also important to effectively remove the above substances.

For example, referring to the invention patent document 4 and the invention patent document 5, they propose a chemical recycling method for obtaining recycled polyester again through the raw material monomer DMT after removing the coloring agent such as dyes and pigments from the colored fibrous polyester. However, the methods described in the invention patent document 4 and the invention patent document 5 are methods for recovering DMT from the colored fibrous polyester. Since the recycling steps are long and require a large amount of energy, there are many problems in terms of cost. In addition, a major disadvantage of the refined recycled DMT is that it cannot be directly used in the polymerization process (terephthalic acid process) using terephthalic acid (terephthalic acid) as a raw material, which is currently widely used in the world. [Prior art document] [Invention patent document]

[Patent document 1] Japanese Patent No. 3715812. [Patent document 2] Japanese Patent No. 5189266. [Patent document 3] Japanese Patent No. 4067306. [Patent document 4] Japanese Patent No. 4537288. [Patent document 5] Japanese Patent No. 5134563.

〔Invent the problem you want to solve〕

The object of the present invention is to provide a method for producing bis-(2-hydroxyethyl) terephthalate by efficiently removing dyes, pigments, etc. from colored polyester waste (especially colored polyester clothing waste and colored plastic bottle waste) to easily produce bis-(2-hydroxyethyl) terephthalate, and a method for producing recycled polyethylene terephthalate using the bis-(2-hydroxyethyl) terephthalate as a raw material. [Technical Means for Solving the Problem]

The above-mentioned objects can be achieved by the present invention of the following (1) to (14). (1) A method for producing bis-(2-hydroxyethyl) terephthalate, comprising: a depolymerization step, which is a step of mixing polyester waste colored by a pigment with monoethylene glycol and a depolymerization catalyst, and depolymerizing the polyester waste to obtain a depolymerized product containing colored crude bis-(2-hydroxyethyl) terephthalate; a pigment decomposition and removal step, which is a step of adding a pigment decomposing agent to the depolymerized product and cooling it, and separating the precipitated solid component and the decomposed product of the pigment by centrifugal separation to remove at least a portion of the depolymerized product from the depolymerized product. a step of removing the low-boiling-point components containing the monoethylene glycol from the depolymerized product from which at least a portion of the decomposition product has been removed, thereby concentrating the crude bis-(2-hydroxyethyl) terephthalate; and a step of recovering the bis-(2-hydroxyethyl) terephthalate from the concentrated crude bis-(2-hydroxyethyl) terephthalate by distillation to obtain bis-(2-hydroxyethyl) terephthalate having a purity higher than that of the crude bis-(2-hydroxyethyl) terephthalate.

(2) The method for producing bis-(2-hydroxyethyl) terephthalate as described in (1) above, wherein the pigment decomposing agent contains at least one of ozone, hydrogen peroxide, concentrated sulfuric acid, oxygen-based oxidizing agent and chlorine-based oxidizing agent. (3) The method for producing bis-(2-hydroxyethyl) terephthalate as described in (1) or (2) above, wherein the pigment decomposing agent is added until the depolymerization product is centrifugally separated. (4) A method for producing bis-(2-hydroxyethyl)terephthalate as described in any one of (1) to (3) above, wherein the content of ester components other than the bis-(2-hydroxyethyl)terephthalate remaining in the bis-(2-hydroxyethyl)terephthalate is 2.5 mass % or less, and the color b value of the bis-(2-hydroxyethyl)terephthalate is 0 or less.

(5) The method for producing bis-(2-hydroxyethyl) terephthalate as described in any one of (1) to (4) above, wherein the polyester waste contains 65% by mass or more of polyethylene terephthalate. (6) The method for producing bis-(2-hydroxyethyl) terephthalate as described in any one of (1) to (5) above, wherein the polyester waste is polyester clothing waste or plastic bottle waste. (7) The method for producing bis-(2-hydroxyethyl) terephthalate as described in (6) above, wherein in the depolymerization step, the polyester clothing waste is directly depolymerized in the form of clothing. (8) The method for producing bis(2-hydroxyethyl)terephthalate as described in (6) or (7) above, wherein the depolymerization step is followed by a solid removal step, wherein the solid removal step is to remove the polyester-insoluble solid components contained in the polyester clothing waste from the depolymerization product.

(9) A method for producing bis-(2-hydroxyethyl)terephthalate as described in any one of (1) to (8) above, wherein the pigment has a chromogenic group containing a nitrogen atom, and the pigment decomposition and removal step is carried out until the content of the nitrogen atom remaining in the depolymerized product becomes less than 900 ppm. (10) The method for producing bis-(2-hydroxyethyl)terephthalate as described in any one of (1) to (9) above, wherein a crystallization purification step is further provided after the recovery step, wherein the crystallization purification step is a step of improving the purity of the bis-(2-hydroxyethyl)terephthalate by cooling a solution prepared by dissolving the bis-(2-hydroxyethyl)terephthalate in a solvent to precipitate crystals of the bis-(2-hydroxyethyl)terephthalate, and solid-liquid separation of the crystals from a solvent component containing an ester component other than the bis-(2-hydroxyethyl)terephthalate.

(11) A method for producing bis-(2-hydroxyethyl)terephthalate as described in (10) above, wherein the solvent contains water and/or an ethylene glycol compound, and the ethylene glycol compound contains at least one selected from the group consisting of ethylene glycol monoethers or ethylene glycol diethers having 4 to 12 carbon atoms, and ethylene glycol having 2 to 6 carbon atoms. (12) A method for producing bis-(2-hydroxyethyl)terephthalate as described in (10) or (11) above, wherein the content of the aforementioned ester component remaining in the aforementioned bis-(2-hydroxyethyl)terephthalate after the aforementioned crystallization purification step is less than 1 mass %, and the color b value of the aforementioned purified bis-(2-hydroxyethyl)terephthalate after the aforementioned crystallization purification step is less than -1.

(13) A method for producing recycled polyethylene terephthalate, comprising: a step of obtaining the aforementioned bis-(2-hydroxyethyl) terephthalate by the method for producing bis-(2-hydroxyethyl) terephthalate as described in any one of the above (1) to (12); and a step of obtaining recycled polyethylene terephthalate by polycondensing the aforementioned bis-(2-hydroxyethyl) terephthalate. (14) A method for producing recycled polyethylene terephthalate as described in the above (13), wherein the aforementioned recycled polyethylene terephthalate contains more than 50% by mass of a structure derived from the aforementioned bis-(2-hydroxyethyl) terephthalate. [Effect of the invention]

According to the present invention, it is possible to efficiently decompose and remove dyes, pigments, and other pigments from colored polyester waste (particularly colored polyester clothing waste and colored plastic bottle waste) to easily produce high-purity bis-(2-hydroxyethyl) terephthalate (BHET). In particular, since high-purity BHET can be easily produced from colored polyester clothing waste, polyester clothing waste, which has been mostly treated by incineration, landfill, etc., can be recycled into polyester again through raw material monomers. As a result, a closed recycling cycle can be realized to convert polyester clothing waste into recycled polyester, and a great contribution can be made to the establishment of a recycling society. In addition, the high-purity recycled BHET produced by the present invention can be directly used in the polymerization process (terephthalic acid process) using terephthalic acid, which is currently widely used in the world, as a raw material, so it has a great advantage.

The following will be described in detail according to the preferred implementation of the diagram, with respect to the method for producing bis-(2-hydroxyethyl) terephthalate and the method for producing recycled polyethylene terephthalate of the present invention. In addition, the following description is used to illustrate the present invention, and is not used to limit the scope of the present invention. As long as it conforms to the purpose of the present invention, other implementations shall also fall within the scope of the present invention. For example, the "step" in the present invention does not only mean a step that can be distinguished and identified from other steps, but also refers to a step combined with other operations, a step that is actually divided into multiple steps, a step that includes other step elements, and an operation that can combine multiple steps in one step. As long as it conforms to the purpose of the invention, it belongs to the scope of the present invention.

FIG1 is a flow chart showing a preferred embodiment of the method for producing bis-(2-hydroxyethyl) terephthalate of the present invention. The method for producing bis-(2-hydroxyethyl) terephthalate (BHET) of the present invention can be applied to various colored polyester wastes such as clothing, plastic bottles, films, and resin molded products (hereinafter also referred to as "colored polyester waste"). In addition, in the following description, the colored polyester waste is described with respect to the case where it is applicable to colored polyester clothing waste (colored polyester clothing waste).

The method for producing BHET of the present embodiment comprises (1) a depolymerization step, (2) a solid matter removal step, (3) a pigment decomposition and removal step, (4) a concentration step, (5) a recovery step, and (6) a crystallization and purification step. The steps are described in order below.

(1) Depolymerization step First, polyester clothing waste colored with dyes such as dyes and pigments (coloring pigments), monoethylene glycol (hereinafter also referred to as "MEG") and a depolymerization catalyst are added to a reaction tank and mixed, and the colored polyester clothing waste is depolymerized to obtain a depolymerization liquid (liquid depolymerized product) containing the colored crude BHET. Hereinafter, the depolymerization liquid containing the colored crude BHET is also referred to as "colored depolymerization liquid". Examples of polyesters that constitute polyester clothing include polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), and polybutylene naphthalate (PBN).

The polyester clothing used as the object of the present invention is preferably a clothing mainly composed of polyethylene terephthalate (PET) fibers. Such polyester clothing may include different materials such as nylon, polyurethane, cotton, olefin, etc.; it may also include various compounds such as stabilizers, antistatic agents, dye-resistant agents, flame retardants, moisture absorbents, gas barriers, etc. used for the purpose of satisfying the characteristics required for the use of the clothing; additives such as titanium oxide used as a matting agent, and solids such as zippers, buttons, metals, glass, sand, etc.

The content of PET (e.g., PET fiber) contained in the polyester clothing is not particularly limited, but is preferably 65% by mass or more, more preferably 75% by mass or more, even more preferably 85% by mass or more, and particularly preferably 95% by mass or more. In addition, the upper limit is 100% by mass.

Pigments used for coloring polyester clothing are roughly divided into dyes and pigments. As dyes, for example, disperse dyes, naphthol dyes, mordant dyes, vat dyes, etc. can be listed, but disperse dyes are preferred. Since disperse dyes are bonded to polyester by van der Waals force, they are most suitable for coloring polyester. Furthermore, according to the inspection of the inventors, it is known that if the present invention is used to decompose and remove disperse dyes, it has a good effect.

Examples of such disperse dyes include compounds classified as C.I. Disperse Black (Color Index Disperse Black), compounds classified as C.I. Disperse Blue (blue), compounds classified as C.I. Disperse Red (red), compounds classified as C.I. Disperse Orange (orange), compounds classified as C.I. Disperse Yellow (yellow), compounds classified as C.I. Disperse Green (green), compounds classified as C.I. Disperse Violet (purple), compounds classified as C.I. Disperse Brown (brown), and the like.

On the other hand, as pigments, for example, compounds classified as C.I. Pigment Black, compounds classified as C.I. Pigment Blue, compounds classified as C.I. Pigment Red, compounds classified as C.I. Pigment Orange, compounds classified as C.I. Pigment Yellow, compounds classified as C.I. Pigment Green, compounds classified as C.I. Pigment Violet, compounds classified as C.I. Pigment Brown, etc. can be cited.

Although the polyester clothing waste supplied for depolymerization may be in the form of uncut clothing, in the form of cut sheets, or in the form of granules granulated by any method, it is preferably in the form of clothing with a large bulk density. In the form of cut sheets, the operation is complicated and the bulk density is reduced, which is disadvantageous for depolymerization. Although the granular form after granulation is more advantageous in terms of ease of operation and the size of the bulk density, in order to make the polyester clothing waste into a granular form, there is a situation where the cost increases depending on the method adopted.

Furthermore, when polyester clothing waste and a predetermined content ratio of MEG are added to a reaction tank and the polyester clothing waste is depolymerized, it is preferred to perform the depolymerization reaction in a state where the polyester clothing waste is completely immersed in MEG. The overall density of the polyester fiber is 0.10 g/cm ³ to 0.14 g/cm ³ (uncompressed) in the form of clothing and 0.08 g/cm ³ to 0.10 g/cm ³ (uncompressed) in the form of sheets. That is, the overall density of the polyester fiber can be maintained at a higher density value in the form of clothing than in the form of sheets. Therefore, in order to effectively carry out the depolymerization reaction, even when the polyester clothing waste is completely immersed in MEG, the amount of MEG used can be reduced. When the polyester clothing waste in the form of clothing is completely immersed in MEG for depolymerization, the amount of MEG used is preferably about 4.5 to 7.0 times, more preferably about 5.0 to 6.5 times, relative to the mass of the polyester clothing waste.

When the amount of MEG used is too small, depending on the shape of the polyester clothing waste, the polyester clothing waste may not be fully immersed in MEG, or the time required for the depolymerization reaction may be prolonged, or the conversion rate from polyester (such as PET) to BHET by the depolymerization reaction may be low. In this case, a large amount of polyester oligomers are likely to exist in the depolymerization liquid, and the viscosity of the depolymerization liquid tends to increase. On the other hand, when the amount of MEG used is too large, the refining of MEG becomes economically disadvantageous, and depending on the reaction conditions, a large amount of by-products such as diethylene glycol (hereinafter also referred to as "DEG") and 1,4-dioxane may be generated in the reactants. When such BHET is used, there is a concern that the physical properties (especially the softening point) of the obtained recycled PET may be significantly reduced depending on the manufacturing conditions.

As depolymerization catalysts, for example, alkali metal hydroxides, alkali metal carbonates, alkali metal fatty acid salts, alkali metal alkoxides, alkali earth metal hydroxides, alkali earth metal carbonates, alkali earth metal fatty acid salts, alkali earth metal alkoxides, alkali earth metal oxides, transition metal hydroxides, transition metal carbonates, transition metal fatty acid salts, transition metal alkoxides, etc. can be listed, and one or more of them can be used in combination. By using these depolymerization catalysts, BHET can be effectively produced. Examples of the alkali metal include Li, Na, and K, examples of the alkaline earth metal include Mg and Ca, and examples of the transition metal include Ti, Zn, and Mn.

The temperature during depolymerization is preferably about 180°C to 210°C, more preferably about 185°C to 200°C. When the temperature is set in the above range, the depolymerization time is preferably about 1 hour to 10 hours, more preferably about 1.5 hours to 7 hours. The ambient pressure during depolymerization is preferably about 60 kPa to 160 kPa.

(2) Solid matter removal step As mentioned above, since polyester clothing waste mostly contains foreign matter other than polyester fibers (solid components insoluble in polyester), it is preferable to remove such foreign matter from the depolymerization liquid as needed. Such foreign matter can be listed as different materials such as cotton and olefin; solid matter such as zippers, buttons, and metal; and coarse solid matter such as glass and sand mixed in when collecting waste. Such foreign matter can be removed together with the depolymerization liquid by filtering using a coarse filter with a sieve hole of about 20 mesh to 40 mesh.

In polyester clothing, about 0.3% to 0.5% by mass of titanium oxide insoluble in polyester is added as a matting agent. Furthermore, in order to meet the characteristics required for clothing use, various additives insoluble in polyester are often used. It is certainly not good for these tiny solids to be mixed into recycled PET. Depending on the size of these tiny solids, they may precipitate or accumulate inside the container or piping during the processing process and block the flow of the liquid. In terms of the smoothness of the processing process, it is also better to remove these tiny solids. Specifically, long fiber filters with a size of several μm to tens of μm can be used for filtering or hot filtering to remove these tiny solids.

(3) Pigment decomposition and removal step Furthermore, the colored polyester clothing waste is colored by a dye, pigment or other pigment. When the degree of coloration of the colored polyester clothing waste is large, a pigment decomposition and removal step is performed in order to further improve the purity of the BHET finally obtained. In addition, the pigment of disperse dyes which are most suitable for coloring polyester will be described below as a representative. It is known that when disperse dyes contained in the colored polyester clothing waste are mixed into recycled polyester, the recycled polyester will be colored, and in particular, the yellowness will tend to increase. Therefore, when manufacturing BHET from the colored polyester clothing waste, it is preferable to decompose and remove the disperse dyes contained in the colored polyester clothing waste in advance by a pigment decomposing agent.

As a result of the inventors' research on methods for effectively removing pigments, they have obtained the following insight: by adding a pigment decomposing agent to the colored depolymerization liquid obtained in the above-mentioned (1) depolymerization step to decompose and remove the disperse dye contained in the colored polyester clothing waste, and carrying out the treatment steps described below, a higher purity BHET can be obtained. The decomposition of the pigment can be appropriately carried out, for example, by adding a pigment decomposing agent containing at least one of ozone, hydrogen peroxide, concentrated sulfuric acid, oxygen-type oxidizing agent and chlorine-type oxidizing agent to the colored depolymerization liquid after the above-mentioned depolymerization step. Among them, from the perspective of excellent pigment decomposition ability and cheap and easy availability, a pigment decomposing agent containing a chlorine-type oxidizing agent is preferred.

Examples of oxygen-based oxidants include hydrogen persulfate, persulfate, percarbonate, perborate, and tetraacetyl ethylenediamine. Specific examples of hydrogen persulfate include alkaline metal salts such as lithium salt, sodium salt, and potassium salt; alkaline earth metal salts such as barium salt; and ammonium salt or hydrates thereof. In addition, hydrogen persulfate may also be hydrogen sulfate or a double salt with sulfate. Specific examples of persulfate include alkaline metal salts such as sodium salts and potassium salts; ammonium salts or hydrates thereof. Specific examples of percarbonate include alkaline metal salts such as sodium salts and potassium salts; ammonium salts or hydrates thereof. Specific examples of perboric acid include alkaline metal salts such as sodium salts and potassium salts; ammonium salts or hydrates thereof.

Examples of chlorine-based oxidants include chloramine compounds, chlorates, chlorinated isocyanuric acids, trisodium phosphate chloride, chlorine dioxide, etc. Specific examples of chloramine compounds include chloramine B (sodium N-chlorobenzene sulfonamide), dichloramine B (N,N'-dichlorobenzenesulfonamide), chloramine T (sodium N-chloro-P-toluenesulfonamide), and dichloramine T (N,N'-dichloro-P-toluenesulfonamide). Specific examples of chlorates include sodium chlorate, potassium chlorate, lithium chlorate, ammonium chlorate, calcium chlorate, sodium hypochlorite, potassium hypochlorite, calcium hypochlorite, sodium chlorite, disodium chlorite, potassium perchlorate, rubidium perchlorate, caesium perchlorate, tetraalkylammonium, ammonium perchlorate, potassium perchlorate, calcium perchlorate. perchlorate), silver perchlorate, sodium perchlorate, magnesium perchlorate, etc. Specific examples of chlorinated isocyanuric acids include chlorinated isocyanuric acid, sodium chlorinated isocyanuric acid, potassium chlorinated isocyanuric acid, dichloroisocyanuric acid, sodium dichloroisocyanuric acid, potassium dichloroisocyanuric acid, trichloroisocyanuric acid, sodium trichloroisocyanuric acid, potassium trichloroisocyanuric acid, and the like.

The addition of the pigment decomposition agent is preferably carried out until the coloring depolymerization liquid described later is centrifugally separated. The addition of the pigment decomposition agent can be carried out, for example, before, after, or both of the cooling of the coloring depolymerization liquid and the precipitation of the solid component. The amount of the pigment decomposition agent added is preferably 10 parts by mass or less relative to 100 parts by mass of BHET contained in the coloring depolymerization liquid, but it can also be 5 parts by mass or less or 2 parts by mass or less. On the other hand, although the lower limit of the amount of the pigment decomposition agent added is appropriately set according to the type of pigment and pigment decomposition agent, etc., and is not particularly limited, it is usually preferably 0.5 parts by mass or more. In addition, the content of BHET contained in the coloring depolymerization liquid is calculated according to the following formula: the mass of the colored polyester clothing waste × 254/192 × 100. By adding the pigment decomposing agent within the above range, the appropriate content of the disperse dye contained in the colored polyester clothing waste is obtained for oxidative decomposition, which has an advantage in terms of cost.

Here, it is known that most disperse dyes (many pigments) are compounds having a chromophore containing a nitrogen atom, and when disperse dyes contained in colored polyester clothing waste are mixed into recycled polyester, the recycled polyester is colored, and in particular, the yellowness tends to increase. As described above, even after the pigment (disperse dye) is oxidatively decomposed, nitrogen atoms derived from the disperse dye remain in the pigment decomposition product (hereinafter also referred to as "pigment decomposition product"). As one of the methods for confirming the degree of removal of nitrogen atoms derived from the disperse dye, a method of measuring the content of nitrogen atoms (N residual amount) remaining in the depolymerization liquid (depolymerization product) or BHET can be used.

As a result of the inventors' research on methods for effectively removing pigment decomposition products, they have obtained the following insight: when the content of nitrogen atoms from the pigment source contained in the colored polyester clothing waste exceeds 900 ppm, the content of residual nitrogen atoms in BHET after the pigment decomposition products are removed in advance (pre-removal) is reduced to below 900 ppm, and then the treatment steps described below are performed, thereby obtaining BHET with higher purity.

The removal of such pigment decomposition products can be carried out, for example, in the following manner. That is, first, the depolymerization liquid that has undergone the above-mentioned depolymerization step and solid matter removal step is cooled to 20°C to 25°C, and the microcrystals of BHET or polyester oligomers are precipitated as solid components. Thereafter, a centrifugal sedimentation centrifuge (separation plate type, horizontal spiral type, etc.) is used to set the centrifugal force to above 2000G to perform centrifugal separation on the depolymerization liquid. In this way, the precipitated solid components can be separated from the MEG in the depolymerization liquid and the pigment decomposition products dissolved in the MEG by solid-liquid separation. Through the above operation, a solid component (solid depolymerization product) of BHET or polyester oligomer having a residual nitrogen atom content of less than 900ppm can be obtained.

In addition, if the residual nitrogen atom content does not reach 900 ppm or less in one centrifugal sedimentation separation operation, MEG can be further added to the obtained solid component and centrifugal sedimentation separation operation can be performed again. In addition, although this step (pigment decomposition and removal step) can be omitted as long as the content of nitrogen atoms derived from pigments contained in polyester clothing waste is less than 900 ppm, it is better to perform this step in order to remove pigment decomposition products.

The operation of cooling the depolymerization liquid to separate BHET or polyester oligomer as a solid component is itself a crystallization operation, and a similar method is also described in the aforementioned invention patent document 1. However, in the depolymerization liquid of this step, there are various impurities that hinder the crystallization operation, and because crystallization occurs in MEG, the crystal shape of the solid component obtained by the crystallization operation is extremely poor, and becomes microcrystals with very small crystal size. Therefore, although the solid-liquid separation of crystals and liquid components is easily complicated in the generally known crystallization filtration separator or centrifugal filtration crystallization separator, the solid-liquid separation of very small microcrystals and liquid components can be easily performed by using a centrifugal sedimentation centrifugal separator.

In addition, MEG separated by the centrifugal sedimentation centrifuge is preferably purified (recovered) and reused through evaporation operation, distillation operation, etc. On the other hand, the pigment component (pigment, its decomposition product or color base) used for coloring is preferably concentrated into evaporation residue, distillation residue, and treated as industrial waste. In addition, in this step, only a part of the pigment decomposition product can be removed, or all of it can be removed (that is, even when the N residue becomes below the detection limit).

(4) Concentration step Next, the solid is melted by heating, and a liquid depolymerization product (depolymerization liquid) is obtained in a molten state. Then, low-boiling point components having a boiling point lower than that of BHET are removed from the depolymerization liquid, and crude BHET having BHET and polyester oligomer as main components is obtained. In addition, since the obtained crude BHET is a viscous liquid in a molten state, it is also recorded as a crude BHET concentrate. Here, the low-boiling point components mainly include MEG and DEG. In this crude BHET concentrate, there are impurities such as pigment components, polyamide components, polyurethane components, polyester copolymer components, and ultrafine titanium oxide less than 5 μm that cannot be removed in the above-mentioned solid removal step (2).

The removal (evaporation/distillation) of low-boiling components in this step (4) can be performed using, for example, various evaporators. In particular, in order to prevent the polymerization of BHET and polyester oligomers during the evaporation operation, it is preferred to set the temperature of the crude BHET concentrate to below 130°C under reduced pressure. It is also preferred to select an evaporator having a structure (type) that allows the crude BHET concentrate to stay in the evaporator for less than 10 minutes. Specific evaporators include, for example, a down-flow film evaporator and a thin film evaporator.

(5) Recovery step Next, BHET and polyester oligomers are used as the main components, and BHET is recovered from the crude BHET concentrate containing impurities such as residual pigment components, residual polyamide components, residual polyurethane components, residual polyester copolymer components, and residual titanium oxide, as appropriate, to obtain BHET having a higher purity than the crude BHET concentrate. The recovery of BHET is smoothly carried out by distillation, preferably distillation under vacuum (reduced pressure) (hereinafter also referred to as "vacuum distillation"). In this case, although most of the components to be distilled are BHET, there are cases where trace amounts of organic compounds having a boiling point (hereinafter also described as "organic compounds having a boiling point") such as pigment components, polyester oligomer components, polyamide components, polyurethane components, and polyester copolymer components are distilled out together with BHET.

The amount of organic compounds with such boiling points to be distilled off depends on the temperature, pressure (vacuum degree) during the distillation operation and the residual molar concentration of organic compounds with boiling points in the crude BHET concentrate. Therefore, from the perspective of producing BHET with higher purity, it is preferable to remove organic compounds with boiling points as much as possible in the step before this step (5). Among organic compounds with boiling points, since pigment components (disperse dyes, etc.) have the characteristic of sublimating at a temperature below the boiling point, as long as the pigment components are removed until the residual nitrogen atom content in the crude BHET concentrate supplied to this step (5) reaches 900 ppm or less, the problem of a large amount of pigment components being mixed into the BHET obtained in this step (5) can be avoided.

When the distillation of BHET in step (5) is performed by simple distillation, the simple distillation can be performed in accordance with the temperature, pressure, retention time and other conditions of the thin film evaporator described in the invention patent document 1. However, in the case of simple distillation, in order to reduce the sublimation amount of the pigment component as much as possible and ensure the required quality of BHET, the temperature and pressure of the thin film evaporator must be further reduced, which inevitably leads to a tendency to reduce the production volume (recovery volume) due to the reduction in evaporation. In order to improve the above problems, it is better to use circulating distillation (refining distillation) in the distillation. By loop distillation, the required quality of BHET can be easily achieved by selecting an appropriate reflux ratio.

As for the quality of BHET obtained in this step (5), the content of the ester components other than BHET remaining in BHET (hereinafter also referred to as "other ester components") is preferably less than 2.5 mass % (preferably less than 2 mass %), and the color b value of BHET is preferably less than 0 (preferably less than -0.5). Here, as other ester components, mono-(2-hydroxyethyl) terephthalate (MHET), diethylene glycol ester, polyester dimer, polyester oligomer, etc. can be listed. In addition, these components are mixed into the obtained recycled polyethylene terephthalate when the recycled polyethylene terephthalate is produced.

In addition, in this specification, the content of other ester components remaining in BHET is a value (area %) obtained by the following method. That is, the obtained BHET is first dissolved (diluted) in an appropriate solvent to prepare an analytical sample. The analytical sample is analyzed using a high-performance liquid chromatography (HPLC) device (e.g., "LC-2010AHT" manufactured by Shimadzu Corporation), and the area A of the peak derived from BHET and the area B of the peak derived from other ester components are calculated from the obtained chart. Then, the ratio of area B to the sum of area A and area B is calculated as (B/A+B×100)%. As long as BHET that meets the above-mentioned quality is used as a raw material, the color b value (yellowness) of the recycled polyethylene terephthalate produced by polymerization (melt polymerization) can be made below 8, and recycled polyethylene terephthalate with a color b value equivalent to that of virgin polyethylene terephthalate can be produced.

BHET is recovered from crude BHET (crude BHET concentrate) by distillation to obtain BHET. Incompletely evaporated organic compounds with boiling points and solids such as titanium oxide without boiling points are discharged out of the system as evaporator residues. Preferably, the discharged residues are effectively utilized through additional treatment. That is, this step (5) can also be said to serve as the final separation step of solids without boiling points.

(6) Crystallization purification step Although recycled polyethylene terephthalate having a color b value equivalent to that of virgin polyethylene terephthalate can be produced by using BHET as a raw material, when BHET is used as a raw material for a reagent, etc., it is preferable to further improve the purity of BHET. The inventors of the present invention have devoted themselves to studying a method for further improving the purity of BHET obtained in the above-mentioned recovery step (5), and have found that the following method is the most suitable.

That is, it is preferable to perform the following operation: by using a solvent (hereinafter also referred to as "crystallization solvent") with a high solubility (solubility) for the purification target removal objects such as pigments, their decomposition products, chromophores, other ester components remaining in BHET, and other organic compounds with boiling points, cooling the solution in which BHET has been dissolved in the crystallization solvent, so that BHET crystals are precipitated, and the crystals and the crystallization solvent components containing the purification target removal objects are separated into solid and liquid (hereinafter also referred to as "crystallization purification operation"). In other words, the method for producing BHET of the present invention preferably has a crystallization purification step after the above-mentioned recovery step. During the crystallization purification, the mixing ratio of BHET and the crystallization solvent is adjusted according to the type of the crystallization solvent, so that the above-mentioned purification target removal object can be removed more effectively.

The results of solubility tests conducted by the inventors of the present invention on the object to be purified and removed confirmed that a solvent containing water and/or glycol compounds is more suitable as a crystallization solvent. The glycol compounds include glycol monoether, glycol diether, and glycol, and one or more of them can be used in combination.

Specific examples of the glycol monoethers include ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, ethylene glycol monoisobutyl ether, diethylene glycol monoisobutyl ether, ethylene glycol monohexyl ether, diethylene glycol monohexyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether. propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, propylene glycol monobutyl ether, etc.

Specific examples of the glycol diethers include ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol dibutyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol dibutyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, dipropylene glycol dimethyl ether, glycol dimethyl ether), dipropylene glycol diethyl ether, etc.

Specific examples of ethylene glycol include ethylene glycol (monoethylene glycol), diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-butylene glycol (butanediol), 1,4-butylene glycol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, and cyclohexanediole.

Furthermore, the carbon number of each of ethylene glycol monoether and ethylene glycol diether is preferably 4 to 12, more preferably 4 to 10, and even more preferably 4 to 8. On the other hand, the carbon number of ethylene glycol is preferably 2 to 6. That is, the crystallization solvent is preferably an ethylene glycol compound containing water, at least one selected from the group consisting of ethylene glycol monoether or ethylene glycol diether having carbon atoms of 4 to 12, and ethylene glycol having carbon atoms of 2 to 6. By using a solvent containing water and/or an ethylene glycol compound as a crystallization solvent, the effect of removing the refining object removal substance remaining in BHET can be further improved. Among them, from the perspective of having a high removal ability for the refining object removal substance, a crystallization solvent containing ethylene glycol monoether is more preferred.

When water is used, the water content in the crystallization solvent is preferably 100% by mass. When ethylene glycol compounds are used, the ethylene glycol content in the crystallization solvent is preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. In addition, the upper limit of the content of ethylene glycol compounds in the crystallization solvent is 100% by mass. The crystallization solvent containing the above-mentioned content of ethylene glycol compounds has excellent removal ability for the refining object removal. This type of crystallization solvent is also preferred from the perspective of being able to dissolve pigment components. In addition, regarding the cooling temperature of the solution, from the perspective of reducing the content of BHET dissolved in the crystallization solvent and increasing the yield of BHET, it is preferable to have a lower cooling temperature. However, considering the energy required for cooling, the operability of solid-liquid separation caused by the increase in viscosity of the crystallization solvent accompanying the temperature reduction, etc., the cooling temperature is preferably set at about 20°C to 25°C. Furthermore, although the solution can also be cooled quickly, from the perspective of obtaining BHET crystals of sufficient size, it is better to cool slowly (including natural cooling).

As for the quality of BHET obtained in step (6) (hereinafter also referred to as "purified BHET"), the content of other ester components remaining in BHET is preferably less than 1 mass % (preferably less than 0.5 mass %), and the color b value of the purified BHET is preferably less than -1 (preferably less than -1.5). Although there is no particular limitation on the number of times the crystallization purification operation is performed, when the quality of the purified BHET obtained in the first operation does not reach the above quality, it is preferred to perform the crystallization purification operation again. The inventors of the present invention have confirmed that as long as the crystallization purification operation is performed at least 3 times, the quality of the purified BHET will reach the above quality regardless of the degree of coloring.

The crystallization solvent containing the object to be removed, which is separated into solid and liquid by crystallization refining operation, is preferably purified and reused by evaporation operation, distillation operation, etc. On the other hand, the object to be removed is preferably recovered as evaporation residue or distillation residue and treated as industrial waste. In the above-mentioned step (6), the purification method of BHET of this embodiment is implemented.

BHET obtained in a molten state can be directly polymerized (melt polymerization) to produce recycled polyethylene terephthalate (recycled PET), or BHET obtained in a molten state can be pelletized and then polymerized (melt polymerization) to produce recycled PET. That is, the method for producing recycled PET of the present invention includes a step of obtaining BHET through the above-mentioned method for producing BHET, and a step of obtaining recycled PET by polymerizing BHET. Therefore, the obtained recycled PET is a polymer of BHET. When producing recycled PET by polymerization, BHET and terephthalic acid are mixed in an arbitrary ratio, thereby making it easy to produce recycled PET. In this case, in order to facilitate the establishment of a sustainable closed recycling cycle, the structure of the BHET source in the recycled PET preferably contains more than 50 mass %, more preferably more than 65 mass %, and even more preferably more than 80 mass %.

Although the above description is directed to the method for producing BHET and the method for producing recycled PET of the present invention, the present invention is not limited thereto. The method for producing BHET and the method for producing recycled PET of the present invention can be replaced with any steps that can exert the same effect, and steps for any purpose can be added. In addition, the steps of the present invention can also be batch type, continuous type or any combination thereof. In addition, the colored polyester waste that can be applied to the present invention is preferably colored plastic bottle waste. In the case of colored plastic bottle waste, for example, it is supplied to the depolymerization step after being crushed. Furthermore, the treatment conditions can be set to the same conditions as those for the colored polyester clothing waste. [Example]

The following is a more detailed description of the content of the present invention through embodiments. It should be noted that the present invention is not limited to the embodiments.

1. Production of distilled BHET, purified BHET and recycled PET from colored polyester clothing waste (Example 1) <(1) Depolymerization step> First, four pieces of uncut PET clothing waste colored with black disperse dye, blue disperse dye, red disperse dye and yellow disperse dye ( ^hereinafter also referred to as "colored clothing waste") totaling 400 g (total density: 0.12 g/cm3; total nitrogen content in the colored clothing waste: 1,150 ppm) were prepared.

Next, the colored clothing waste in the form of clothing was placed in a 5L flask, and 2,245g of MEG preheated to 195°C and 1g of sodium hydroxide as a depolymerization catalyst were added to the flask, and the reaction was carried out at 195°C and normal pressure without stirring for 5.0 hours. Thus, a depolymerization liquid (colored depolymerization liquid) containing colored crude BHET as the main component was obtained. In addition, the content (g) of BHET in the depolymerization liquid was calculated according to the following formula: the mass of the colored clothing waste (400g) × 254/192 × 100.

<(2) Solid removal step> Then, the coloring depolymerization liquid is subjected to heat filtration of coarse solids through a wire mesh strainer with a mesh size of 30, thereby obtaining a coloring depolymerization liquid from which coarse solids have been removed. In this stage, coarse solids such as different materials other than PET (cotton, olefins), zippers, buttons, metal, glass, sand, etc. are mainly removed. Second, the coloring depolymerization liquid is cooled to 95°C and hot filtered using a 10μm stainless steel long fiber filter (NASLON filter) to remove fine solids such as titanium oxide that are insoluble in polyester and are larger than 10μm.

<(3) Pigment decomposition and removal step> Next, 176.4 g (hydrogen peroxide: 52.9 g) of 30% by mass hydrogen peroxide as a pigment decomposition agent was added to the coloring depolymerization liquid at 95°C and stirred for 30 minutes. Then, the coloring depolymerization liquid containing the pigment decomposition product was cooled to 25°C. This caused microcrystals of BHET or polyester oligomer to precipitate as a solid component, and the precipitated solid component was separated from MEG in the depolymerization liquid and the pigment decomposition product dissolved in MEG by a separation plate type (disc type) centrifugal sedimentation centrifuge. The centrifuge conditions were as follows: the centrifugal force was set to 4,000G, and the centrifuge time was set to 30 minutes. In addition, the ratio of MEG containing pigment decomposition products separated by the centrifuge was 56% by mass relative to the entire coloring depolymerization liquid, and the residual nitrogen atom content in the solid component was 425ppm.

<(4) Concentration step> Then, the solid component is heated to 125°C to melt it and then sent to a thin film evaporator. The conditions of the thin film evaporator are as follows: the jacket heating heat medium temperature is set to 140°C, and the internal pressure of the evaporator is set to 400Pa (3.0mmHg). In this way, low-boiling point components with a boiling point lower than that of BHET are evaporated and distilled out to obtain a crude BHET concentrate. The obtained crude BHET concentrate is a colored viscous liquid.

<(5) Recovery step> Next, the crude BHET concentrate was sent to a short-stroke thin film evaporator. The conditions of the thin film evaporator were as follows: the jacket heating medium temperature was set to 180°C, and the internal pressure of the evaporator was set to 10Pa (0.08mmHg). Thus, BHET was simply distilled from the crude BHET concentrate and BHET was recovered (hereinafter also referred to as "simple distillation BHET"). In addition, the content of other ester components remaining in the obtained simple distillation BHET was 3.5% by mass, and the color b value of the simple distillation BHET was 0.9. In addition, the content of nitrogen atoms remaining in the simple distillation BHET (N residual amount) was 40ppm.

Secondly, the simply distilled BHET is melt-polymerized according to a conventional method to obtain PET (recycled PET).

(Example 2) Except that the simple distillation in the recovery step was replaced by the refined distillation of the circulating distillation (reflux ratio: 2.0), BHET (hereinafter also referred to as "refined BHET") and recycled PET were obtained in the same manner as in Example 1.

(Example 3) Simply distilled BHET and recycled PET were obtained in the same manner as in Example 1, except that the temperature of the coloring depolymerization liquid in the pigment decomposition and removal step was replaced with 25°C and the pigment decomposing agent was replaced with 53g of 96% concentrated sulfuric acid.

(Example 4) Except that the simple distillation in the recovery step was replaced by a circulating distillation (reflux ratio: 2.0) distillation, distilled BHET and recycled PET were obtained in the same manner as in Example 3.

(Example 5) Except that the pigment decomposing agent in the pigment decomposition and removal step was replaced with 441.0 g of a 12 mass % sodium hypochlorite aqueous solution (sodium hypochlorite: 52.9 g), simple distillation BHET and recycled PET were obtained in the same manner as in Example 1.

(Example 6) Except that the simple distillation in the recovery step was replaced by a circulating distillation (reflux ratio: 2.0), refined BHET and recycled PET were obtained in the same manner as in Example 5.

(Example 7) Simply distilled BHET and recycled PET were obtained in the same manner as in Example 1, except that the pigment decomposing agent in the pigment decomposition and removal step was replaced with 441.0 g of a 12 mass % calcium hypochlorite aqueous solution (calcium hypochlorite: 52.9 g).

(Example 8) Except that the simple distillation in the recovery step was replaced by a circulating distillation (reflux ratio: 2.0), refined BHET and recycled PET were obtained in the same manner as in Example 7.

(Example 9) 1 part by mass of the simply distilled BHET obtained in Example 1 and 4 parts by mass of diethylene glycol monohexyl ether (carbon number: 10) as a crystallization solvent were placed in a 5L flask and heated to a liquid temperature of 80°C. The simply distilled BHET was dissolved in diethylene glycol monohexyl ether to obtain a solution. Thereafter, the solution was naturally cooled and the liquid temperature was reduced to 20°C to precipitate BHET crystals.

Next, the crystals of BHET precipitated by a Nutsche type solid-liquid separator were subjected to solid-liquid separation from diethylene glycol monohexyl ether. That is, purified BHET was obtained by performing a single crystallization purification operation. In addition, the content of other ester components remaining in the purified BHET was 0.5 mass %, and the color b value of the purified BHET was -1.5. In addition, the content of nitrogen atoms remaining in the purified BHET (N residual amount) was 9.8 ppm.

Secondly, the refined BHET is melt-polymerized according to a conventional method to obtain PET (recycled PET).

(Example 10) Except that the crystallization solvent was replaced with triethylene glycol monobutyl ether (carbon number: 10), purified BHET and recycled PET were obtained in the same manner as in Example 9.

(Example 11) Except that the crystallization solvent was replaced with ethylene glycol monohexyl ether (carbon number: 8), purified BHET and recycled PET were obtained in the same manner as in Example 9.

(Example 12) Except that the crystallization solvent was replaced with ethylene glycol monobutyl ether (carbon number: 6), purified BHET and recycled PET were obtained in the same manner as in Example 9.

(Example 13) Except that the crystallization solvent was replaced with ethylene glycol (carbon number: 2), purified BHET and recycled PET were obtained in the same manner as in Example 9.

(Example 14) Except that the crystallization solvent was replaced with diethylene glycol (carbon number: 4), purified BHET and recycled PET were obtained in the same manner as in Example 9.

(Example 15) Except that the simple distilled BHET obtained in Example 1 was replaced by the simple distilled BHET obtained in Example 5, and the crystallization solvent was replaced by water, purified BHET and recycled PET were obtained in the same manner as in Example 9.

(Example 16) Except that the simply distilled BHET obtained in Example 5 was replaced by the refined BHET obtained in Example 6, the refined BHET and recycled PET were obtained in the same manner as in Example 15.

(Example 17) Except that the crystallization solvent was replaced with ethylene glycol (carbon number: 2), purified BHET and recycled PET were obtained in the same manner as in Example 15.

(Example 18) Except that the simply distilled BHET obtained in Example 5 was replaced by the refined BHET obtained in Example 6, the refined BHET and recycled PET were obtained in the same manner as in Example 17.

(Example 19) Except that the crystallization solvent was replaced with ethylene glycol monobutyl ether (carbon number: 6), purified BHET and recycled PET were obtained in the same manner as in Example 15.

(Example 20) Except that the simply distilled BHET obtained in Example 5 was replaced by the refined BHET obtained in Example 6, the refined BHET and recycled PET were obtained in the same manner as in Example 19.

(Example 21) Except that the pigment decomposing agent in the pigment decomposition and removal step was replaced with 220.5 g of a 12 mass % sodium hypochlorite aqueous solution (sodium hypochlorite: 26.5 g), simple distillation BHET and recycled PET were obtained in the same manner as in Example 5.

(Example 22) Simply distilled BHET and recycled PET were obtained in the same manner as Example 5, except that the colored depolymerized liquid was first cooled to 25°C to precipitate the solid components and then a pigment decomposing agent was added in the pigment decomposition and removal step.

(Comparative Example 1) Simply distilled BHET and recycled PET were obtained in the same manner as Example 1 except that the pigment decomposition and removal step was omitted.

(Comparative Example 2) Except that the pigment decomposition and removal step is omitted, purified BHET and recycled PET are obtained in the same manner as in Example 12.

(Comparative Example 3) Except that three crystallization purification operations were performed, purified BHET and recycled PET were obtained in the same manner as in Comparative Example 2.

2. Measurement 2-1. Measurement of nitrogen atom content The residual nitrogen atom content (N residual) in the waste of colored clothing, solid matter after decomposition and removal of coloring, distilled BHET, and purified BHET was measured using a trace total nitrogen analyzer (Mitsubishi Chemical Analytical Tech Co., Ltd. "TN-2100H"). 2-2. Measurement of color b value The color b value of distilled BHET, purified BHET, and recycled PET was measured using a color-difference meter (Nippon Denshoku Co., Ltd. "SE-7700") 2-3. Determination of the content of other ester components The content of other ester components remaining in the distilled BHET and purified BHET was measured using a high performance liquid chromatography apparatus ("LC-2010AHT" manufactured by Shimadzu Corporation).

The above results are summarized in Table 1 and Table 2. [Table 1] [Table 2] In addition, the abbreviations in Table 1 and Table 2 are as follows. DEGHE: diethylene glycol monohexyl ether TEGBE: triethylene glycol monobutyl ether MEGHE: ethylene glycol monohexyl ether MEGBE: ethylene glycol monobutyl ether MEG: ethylene glycol DEG: diethylene glycol

From the results in Tables 1 and 2, it can be seen that the embodiment in which the pigment decomposition and removal step is performed can remove the pigment more efficiently than the comparative example in which the pigment decomposition and removal step is omitted. In particular, the effect is more significant when a chlorine-based oxidant is used as a pigment decomposition agent. It can be seen that the pigment is removed efficiently. Even when ozone or oxygen-based oxidants are used as pigment decomposition agents, the same results as above can be obtained.

[Figure 1] is a flow chart showing a preferred embodiment of the method for producing bis-(2-hydroxyethyl)terephthalate of the present invention.

Claims (14)

A method for producing bis-(2-hydroxyethyl) terephthalate comprises: a depolymerization step, in which polyester waste, which is colored by a pigment and contains polyester having a structure derived from terephthalic acid, is mixed with monoethylene glycol and a depolymerization catalyst, and the polyester waste is depolymerized to obtain a depolymerized product containing colored crude bis-(2-hydroxyethyl) terephthalate; a pigment decomposition and removal step, in which a pigment decomposition agent is added to the depolymerized product and the product is cooled, and the precipitated solid component and the decomposition product of the pigment are separated by centrifugation to remove the depolymerized product from the depolymerized product. A step of removing at least a portion of the aforementioned decomposition products; a concentration step, which is a step of removing the low-boiling point components containing the aforementioned monoethylene glycol from the aforementioned depolymerization product from which at least a portion of the aforementioned decomposition products have been removed, to concentrate the aforementioned crude bis-(2-hydroxyethyl) terephthalate; and a recovery step, which is a step of recovering the aforementioned bis-(2-hydroxyethyl) terephthalate from the aforementioned concentrated crude bis-(2-hydroxyethyl) terephthalate by distillation to obtain bis-(2-hydroxyethyl) terephthalate having a purity higher than that of the aforementioned crude bis-(2-hydroxyethyl) terephthalate. The method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 1, wherein the pigment decomposing agent contains at least one of ozone, hydrogen peroxide, concentrated sulfuric acid, oxygen-based oxidizing agent and chlorine-based oxidizing agent. The method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 1, wherein the addition of the pigment decomposing agent is carried out until the depolymerization product is centrifugally separated. A method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 1, wherein the content of ester components other than the aforementioned bis-(2-hydroxyethyl) terephthalate remaining in the aforementioned bis-(2-hydroxyethyl) terephthalate is less than 2.5 mass %, and the color b value of the aforementioned bis-(2-hydroxyethyl) terephthalate is less than 0. The method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 1, wherein the polyester waste contains more than 65% by mass of polyethylene terephthalate. The method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 1, wherein the polyester waste is polyester clothing waste or plastic bottle waste. The method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 6, wherein in the aforementioned depolymerization step, the aforementioned polyester clothing waste is directly depolymerized in the form of clothing. The method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 6, wherein a solid removal step is further provided after the depolymerization step, and the solid removal step is to remove the polyester-insoluble solid components contained in the polyester clothing waste from the depolymerization product. The method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 1, wherein: the aforementioned pigment has a chromogenic group containing a nitrogen atom; and the aforementioned pigment decomposition and removal step is performed until the content of the aforementioned nitrogen atom remaining in the aforementioned depolymerization product becomes less than 900 ppm. The method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 1, wherein a crystallization purification step is further provided after the aforementioned recovery step, wherein the aforementioned crystallization purification step is a step of improving the purity of the aforementioned bis-(2-hydroxyethyl) terephthalate by cooling a solution in which the aforementioned bis-(2-hydroxyethyl) terephthalate is dissolved in a solvent to precipitate crystals of the aforementioned bis-(2-hydroxyethyl) terephthalate, and performing solid-liquid separation between the aforementioned crystals and a solvent component containing an ester component other than the aforementioned bis-(2-hydroxyethyl) terephthalate. The method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 10, wherein: the aforementioned solvent contains water and/or ethylene glycol compounds; the aforementioned ethylene glycol compounds contain at least one selected from the group consisting of ethylene glycol monoethers or ethylene glycol diethers having 4 to 12 carbon atoms, and ethylene glycol having 2 to 6 carbon atoms. The method for producing bis-(2-hydroxyethyl) terephthalate as described in claim 10, wherein the content of the aforementioned ester component remaining in the aforementioned bis-(2-hydroxyethyl) terephthalate after the aforementioned crystallization purification step is less than 1 mass %, and the color b value of the aforementioned purified bis-(2-hydroxyethyl) terephthalate after the aforementioned crystallization purification step is less than -1. A method for producing recycled polyethylene terephthalate, comprising: a step of obtaining the aforementioned bis-(2-hydroxyethyl) terephthalate by the method for producing bis-(2-hydroxyethyl) terephthalate as described in any one of claim 1 to claim 12; and a step of obtaining recycled polyethylene terephthalate by polycondensing the aforementioned bis-(2-hydroxyethyl) terephthalate. A method for producing recycled polyethylene terephthalate as described in claim 13, wherein the recycled polyethylene terephthalate contains more than 50% by mass of a structure derived from the bis-(2-hydroxyethyl) terephthalate.