KR20230080684A - Depolymerization method of polyester fiber - Google Patents

Abstract

The present invention relates to a method for depolymerization of polyester fibers. The method of the present invention can obtain high-purity monomer crystals in a process of recycling polyester fibers such as PET. The method of the present invention can efficiently proceed with the process of recycling polyester fibers.

Description

Depolymerization method of polyester fiber

The present invention relates to a method for depolymerization of polyester fibers.

Specifically, the present invention relates to a method for efficiently removing dyes in a recycling process of textile waste.

When a polyester fiber containing a polymer such as polyethylene terephthalate (hereinafter referred to as PET) is mixed with an organic solvent under a predetermined temperature condition, a monomer constituting a polymerization unit of the polymer can be obtained. The monomer thus obtained can be recrystallized through cooling at a predetermined temperature.

Patent Document 1 discloses depolymerization of polyester fibers including PET using ethylene glycol (hereinafter referred to as EG) as a method of recycling polyester fiber waste. Patent Document 1 introduces information on obtaining bis(hydroxyethyl) terephthalate (hereinafter referred to as BHET) by depolymerizing PET with EG. Subsequently, the obtained BHET is recrystallized at a predetermined temperature, and dyes and the like contained therein are removed through adsorption or the like.

Meanwhile, since BHET has high solubility in EG, when polyester fibers are depolymerized with EG, the recrystallization yield is low during the recrystallization process of BHET. In addition, EG used in the depolymerization process has a high viscosity and is not well separated from BHET crystals. As a result, in the method used in Patent Document 1, an excess of solvent and dye remain in the product solution having BHET. Here, although the dye can be removed using an adsorbent such as activated carbon, there is a problem in that the dye is not sufficiently removed.

Therefore, in the polyester fiber recycling process, research on how to obtain high-quality monomer crystals is required.

In the present invention, it is intended to obtain high-purity monomer crystals in a process of recycling polyester fibers such as PET.

In the present invention, it is intended to efficiently proceed with a process of recycling polyester fibers.

The method for depolymerizing polyester fibers of the present invention includes the steps of reacting raw materials including polyester fibers having polyethylene terephthalate with methanol to produce dimethyl terephthalate, and recrystallizing the same; Purifying the recrystallized dimethyl terephthalate with a decolorizing solution containing an organic solvent and an adsorbent; and reacting the purified dimethyl terephthalate with a polyhydric alcohol to prepare bis(hydroxyethyl) terephthalate and recrystallizing it.

The method of the present invention can obtain high-purity monomer crystals in a process of recycling polyester fibers such as PET.

The method of the present invention can efficiently proceed with the process of recycling polyester fibers.

The present invention relates to a method for depolymerization of polyester fibers. Specifically, the present invention is to obtain crystals of monomers constituting polyester fibers in high purity, and to proceed with the process more efficiently.

As mentioned above, EG, which is frequently used in the process of depolymerization of polyester fibers, should not be increased because of its viscosity and solubility with BHET. In the present invention, it is intended to reduce the amount of EG used in the depolymerization process of polyester fibers.

The method of the present invention proceeds in at least three steps. Specifically, the method of the present invention comprises the steps of obtaining recrystallized dimethyl terephthalate from raw materials including polyester fibers with polyethylene terephthalate (PET); Purifying the recrystallized dimethyl terephthalate; and obtaining recrystallized bis(hydroxyethyl) terephthalate from purified dimethyl terephthalate.

The present invention includes the step of reacting the polyester fiber raw material and methanol. Through the reaction of polyester fibers with methanol, a solution composed of monomers constituting polyester fibers can be obtained.

Here, the polyester fiber raw material includes polyester fibers. The polyester fiber raw material may include polyester fibers manufactured through a conventional process. Polyester fibers may be those that have been distributed in the market and then discarded and recovered. The polyester fiber raw material may include only polyester fibers as fibers, or may include blended fibers with other fibers. On the other hand, in order to efficiently proceed with the recycling process, it is preferable that the ratio of polyester fibers in the raw material is 90% by weight or more. Among waste polyester fiber raw materials, 100% waste polyester fiber raw materials may contain less than 2% by weight of impurities, preferably less than 1% by weight of impurities. Here, impurities refer to metals, other plastics, inorganic substances, and the like.

The polyester fiber referred to in the present invention may mean a fiber containing PET as a main component.

In the present specification, including a component as a main component may mean including 90% or more, 95% or more, or 99% or more of the component by weight, including about 100%, less than 100%, or 99% or less. can mean doing

When PET and methanol react, dimethyl terephthalate (DMT) is produced. That is, in the present invention, DMT is prepared by reacting a raw material including a polyester fiber having PET with methanol.

As such, in the present invention, the raw material and methanol are reacted differently from the conventional method of first mixing the raw material and EG. As will be described later, through this, the method of the present invention can reduce the amount of EG used.

During the reaction between the raw material and methanol, the application rate of methanol may also be appropriately adjusted. In one embodiment, the application rate of methanol may be within the range of 50 parts by weight to 4,000 parts by weight based on 100 parts by weight of the polyester fiber. In another example, the ratio may be 1,000 parts by weight or more and 2,000 parts by weight or less.

The reaction between the raw material and methanol may proceed in the presence of a catalyst. That is, the DMT can be prepared using a depolymerization catalyst. The depolymerization catalyst may be a metal acetate or a hydrate thereof. The catalyst may be, for example, zinc acetate, sodium acetate, tin acetate, antimony acetate, manganese acetate, or cobalt acetate. The catalyst may be any one of the above or a mixture of two or more of them.

When the DMT is prepared using a catalyst, the content of the catalyst may be 0.1 part by weight or more and 5 parts by weight or less based on 100 parts by weight of the raw material.

The reaction temperature of the raw material and methanol may also be controlled. In one embodiment, the reaction temperature of the raw material and methanol may be within a range of 140 °C or higher and 240 °C or lower.

The reaction pressure between the raw material and methanol may also be adjusted. This is because methanol having a relatively low boiling point was used as the solvent for the reaction. The reaction may proceed under so-called high-pressure conditions. In one embodiment, the reaction pressure between the raw material and methanol may be 10 bar or more and 80 bar or less. At pressures below 10 bar, it may be difficult to heat to the target temperature.

The reaction time of the raw material and methanol may also be adjusted. This is to prevent side reactions while sufficiently reacting the raw material and methanol. In one embodiment, the reaction time between the raw material and methanol may be 5 minutes or more and 2 hours or less.

The product of the reaction may include, in addition to the above-mentioned DMT, unreacted PET included in the raw material, other polyesters other than PET, fibers other than polyester fibers, and other impurities. Therefore, it is preferable to remove them from the products of the reaction. Since these exist in a solid phase, a filtration method using a filter or the like can be applied to their removal.

The method of the present invention then includes a step of separating and recovering DMT from the product of the reaction.

In the method of the present invention, following the reaction of the raw material with methanol, DMT, a reaction product thereof, is recrystallized. That is, the method of the present invention includes a step of preparing dimethyl terephthalate by reacting a raw material including a polyester fiber having polyethylene terephthalate with methanol, and recrystallizing the same.

Recrystallization of DMT may be performed by cooling the reaction product of the raw material and methanol. In the product solution, DMT may exist in a liquid phase, and solid DMT may be produced through recrystallization by cooling.

The recrystallization temperature of DMT can also be appropriately controlled. In one embodiment, the recrystallization temperature of the DMT may be within a range of 0 °C or more and 100 °C or less. In another example, the recrystallization temperature may be 5 °C or more and 25 °C or less.

In the method of the present invention, DMT thus recrystallized is purified. Purification, as the term implies, means a process of obtaining a target product of high purity. That is, in the method of the present invention, purified DMT is obtained by purifying the recrystallized DMT. Since recrystallized DMT contains unreacted methanol and dyes in polyester fibers, it is necessary to purify DMT by purifying the recrystallized DMT. Specifically, the method of the present invention includes purifying the recrystallized DMT with a decolorizing solution containing an organic solvent and an adsorbent. An adsorbent is necessary to remove the dye and the like by adsorption. The organic solvent is suitable for inducing solid-liquid phase separation in the mixture to which the decolorizing solution is applied. Specifically, the step of purifying the recrystallized DMT may be performed by mixing the recrystallized DMT and the bleaching solution and then stirring the same. Through the stirring, the adsorbent may adsorb the dye remaining in the DMT. Purified DMT crystals can be obtained by stirring the decolorizing solution and then removing the adsorbent and impurities using a commonly used filtering means such as a filter or centrifugal separator.

In one embodiment, the organic solvent may include at least one of toluene, xylene, acetone, and chloroform.

In one embodiment, the adsorbent may include activated carbon.

Therefore, in one embodiment, the step of purifying the recrystallized DMT comprises a decolorizing solution containing at least one of toluene, xylene, acetone and chloroform as an organic solvent and activated carbon as an adsorbent for the recrystallized DMT After mixing, it may be to stir it.

The content of the decoloring solution and the content ratio of the organic solvent and the adsorbent in the decolorization solution may also be adjusted.

In one embodiment, the content of the organic solvent may be 100 parts by weight or more and 4,000 parts by weight or less based on 100 parts by weight of the DMT. The content may be 1,000 parts by weight or more and 2,000 parts by weight or less in another example.

In one embodiment, the content of the adsorbent may be 1 part by weight or more and 40 parts by weight or less based on 100 parts by weight of the DMT. The content may be 10 parts by weight or more and 20 parts by weight or less in another example.

In one embodiment, the temperature of the purification step of the recrystallized DMT may also be controlled. In one embodiment, the temperature of the purification step of the recrystallized DMT may be within the range of 25 °C or more and 100 °C or less. The stirring temperature of the bleaching solution may be 40 ° C. or more and 80 ° C. or less in another example.

In one embodiment, before the process of removing the dye according to the use of the adsorbent, specifically, before applying and purifying the mixed solution, solid-liquid separation of the mixture including the recrystallized DMT, methanol, and the dye is performed good night. This is because liquid methanol and dyes dissolved therein can be separated, thereby improving the purity of DMT.

The solid-liquid separation may be performed by a commonly used solid-liquid separation means such as a filter or a centrifugal separator. As the filter, for example, a filter cloth having a filter size of 0.1 mm or more and 10 mm or less can be used.

The present invention further processes the thus purified DMT. Specifically, the present invention further depolymerizes the purified DMT. At this time, in the present invention, a polyhydric alcohol is applied as a solvent used for further depolymerization. Therefore, the present invention reacts the purified DMT with a polyhydric alcohol. In the present invention, a polyhydric alcohol means a compound having two or more hydroxyl groups in a molecule. When DMT reacts with a polyhydric alcohol, bis(hydroxyethyl) terephthalate (BHET) is produced through a transesterification reaction. Therefore, BHET is produced when DMT purified above reacts with a polyhydric alcohol. Therefore, the method of the present invention includes the step of preparing BHET by reacting the purified dimethyl terephthalate with a polyhydric alcohol. Then, the BHET thus prepared is recrystallized. That is, the method of the present invention includes preparing BHET by reacting the purified dimethyl terephthalate with a polyhydric alcohol and recrystallizing the same.

The type of polyhydric alcohol used as a solvent for depolymerization of the purified DMT is not particularly limited. In one embodiment, the polyhydric alcohol may be an alkylene diol having 2 to 6 carbon atoms.

Alkylene diols having 2 to 6 carbon atoms include, for example, ethylene glycol, diethylene glycol, tripropylene glycol, 1,2-propylenediol, 1,3-propylenediol, 1,2-butanediol, and 1,4-butanediol. and 2,3-butanediol. A representative alkylene diol is ethylene glycol.

The content of the polyhydric alcohol may also be appropriately adjusted. In one embodiment, the content of the polyhydric alcohol may be in the range of 100 parts by weight or more and 500 parts by weight or less based on 100 parts by weight of the purified DMT. In another example, the content may be within the range of 100 parts by weight or more and 400 parts by weight or less, or 100 parts by weight or more and 350 parts by weight or less. In the present invention, since the depolymerization reaction of PET with methanol proceeds before the depolymerization reaction of DMT, the amount of polyhydric alcohol used in the depolymerization reaction of DMT in the method of the present invention does not proceed with the depolymerization reaction with methanol, and the polyhydric alcohol is converted to PET It is much less than the case of proceeding with depolymerization of DMT and all depolymerization of DMT.

The transesterification reaction between the DMT and the polyhydric alcohol may proceed in the presence of a catalyst.

That is, the transesterification reaction between the DMT and the polyhydric alcohol may proceed in the presence of a depolymerization catalyst. The depolymerization catalyst used in the transesterification reaction between DMT and polyhydric alcohol may be metal acetate or a hydrate thereof. The catalyst may be, for example, zinc acetate, sodium acetate, tin acetate, antimony acetate, manganese acetate, or cobalt acetate. The catalyst may be any one of the above or a mixture of two or more of them.

When the transesterification reaction between the DMT and the polyhydric alcohol is performed in the presence of a catalyst, the content of the catalyst may be 0.1 parts by weight or more and 5 parts by weight or less based on 100 parts by weight of the purified DMT.

The temperature of the transesterification reaction between the DMT and the polyhydric alcohol may also be appropriately controlled. From the viewpoint of smoothly removing the applied solvent, it is preferably higher than the boiling point of methanol used in the depolymerization reaction of the caustic raw material and lower than the boiling point of the polyhydric alcohol (specifically, ethylene glycol) used in the transesterification reaction between the DMT and the polyhydric alcohol. .

That is, in one embodiment, the temperature of the transesterification reaction between the DMT and the polyhydric alcohol may be within a range of more than the boiling point of methanol and less than or equal to the boiling point of the polyhydric alcohol. In another embodiment, since ethylene glycol is usually used for depolymerization of DMT, the temperature of the transesterification reaction between DMT and polyhydric alcohol may be in the range of 64 °C to 197 °C. Meanwhile, the temperature of the transesterification reaction between the DMT and the polyhydric alcohol may be appropriately adjusted according to the ratio of methanol remaining in the reaction raw material. At this time, a condenser may be installed at the top of the reactor, the amount of methanol removed to the top during the reaction may be checked, and the reaction temperature may be adjusted according to the amount of methanol to perform the transesterification reaction between the DMT and the polyhydric alcohol.

In one embodiment, side reactions and/or discoloration may be prevented by appropriately adjusting the transesterification time between the DMT and the polyhydric alcohol. Specifically, the reaction time of the transesterification reaction between the DMT and the polyhydric alcohol may be within a range of 5 minutes or more and 1 hour or less.

When DMT reacts with a polyhydric alcohol, a solution containing BHET is formed, and BHET may exist in a liquid phase in the solution. In the present invention, high-purity BHET is intended to be recovered as a solid phase. Accordingly, the method of the present invention includes a step of reacting the purified dimethyl terephthalate with a polyhydric alcohol to prepare bis(hydroxyethyl) terephthalate and recrystallizing the same. . Recrystallization usually proceeds through cooling of the solution. Therefore, in the present invention, the BHET prepared by the transesterification reaction of the DMT with a polyhydric alcohol is cooled to recrystallize the BHET.

The temperature of the process of recrystallizing the BHET may also be appropriately controlled. In one embodiment, the temperature of the process of recrystallizing the BHET may be in the range of 0 °C or more and 100 °C or less. That is, in one embodiment, the BHET prepared by transesterification of the DMT with a polyhydric alcohol may be cooled to a temperature within a range of 0 ° C. to 100 ° C. to recrystallize the BHET. The temperature may be greater than or equal to 5 °C and less than or equal to 25 °C in other embodiments.

In the method of the present invention, purified BHET can be obtained by further purifying the recrystallized BHET. That is, the method of the present invention may further include purifying the recrystallized BHET. In the recrystallization process of BHET, a solution containing the recrystallized BHET is produced, and purified BHET can be obtained by separating the recrystallized BHET from this solution. When solid-liquid separation is performed in a solution containing recrystallized BHET, concentrated BHET separated from the solvent can be obtained. That is, the step of purifying the recrystallized BHET may include solid-liquid separation of the recrystallized BHET from a solution containing the recrystallized BHET.

The solid-liquid separation may proceed by conventional means. That is, the solid-liquid separation may be performed with equipment such as a filter and a centrifugal separator. As the filter, for example, a filter cloth having a filter size of 0.1 mm or more and 10 mm or less can be used.

In addition to the above, the method of the present invention may further include other known processes necessary for obtaining BHET crystals from polyester fiber raw materials including PET.

Hereinafter, the content of the present invention will be described in more detail with examples. However, the following examples do not limit the content of the present invention.

[conversion rate]

The conversion rate can be calculated by the weight measured after drying the unreacted solid polyester fiber particles. The detailed process is as follows.

(1) The conversion rate of polyester in the reaction between the raw material and methanol is calculated by the following formula 1. Here, the unreacted solid particles are the weight measured after washing and drying after filtering.

[Equation 1]

Polyester conversion rate (%) in the reaction of methanol as a raw material = {1-(dry weight of unreacted solid particles)}/(total weight of input polyester fibers) X 100

(2) The conversion rate of the transesterification reaction between purified DMT and polyhydric alcohol is calculated by Equation 2 below. Here, the weight of unreacted DMT is measured by LC (liquid chromatography).

[Equation 2]

DMT conversion (%) in transesterification of purified DMT with polyhydric alcohol = {1-(weight of unreacted DMT/weight of input DMT)} X100

(3) Separation/purification (crystallization, solid-liquid separation, decolorization) step yield is calculated by Equation 3 below. Here, the weight of the raw material (DMT or BHET) in each separation process is measured by LC.

[Equation 3]

Separation/purification yield (%) = (Weight of purified raw material)/(Weight of input raw material) X 100

[transference number]

The separation/purification yield can be calculated by the weight measured after drying the product obtained in each step.

[Chromaticity]

The chromaticity of the final product means the CIE Lab value (L: brightness, a: redness, B: yellowness) measured with a spectroscopic colorimeter. In the CIE Lab color space, L stands for brightness. If L = 0, it is black, and if L = 100, it is white. A negative value of a means green, and a positive value of a means red. A negative b-value means blue, and a positive b-value means yellow. The closer L is to 100, and the closer the value of a or b is to 0, the better the removal of the dye in BHET.

[Example 1]

100 g (5 cm * 5 cm) of waste polyester fiber, methanol, and zinc acetate catalyst were introduced into a high-pressure reactor in a weight ratio of 1:10:0.005 (fiber:methanol:catalyst).

The temperature and pressure of the reactor were set at 200 °C and 40 bar. After the temperature rose to 200 °C, depolymerization was performed for 2 hours to obtain a depolymerization solution containing DMT.

After completion of the reaction, filtration is performed to separate unreacted fibers, the depolymerization solution is cooled to 25° C. to obtain crystallized DMT, and filtered through a vacuum filter equipped with a 1 mm filter cloth until no more crystal filtrate comes out. As a result, DMT crystals were obtained.

Subsequently, a solution was prepared by mixing DMT, chloroform and activated carbon in a weight ratio of 1:20:0.2 (DMT:chloroform:activated carbon). After stirring the solution at 60 ° C. for 1 hour, the activated carbon is separated using a vacuum filter. Residual solvent was then removed using a rotary vacuum concentrator to obtain purified DMT.

100 g of DMT purified by the above method, ethylene glycol, and zinc acetate catalyst were added to the glass reactor in a weight ratio of 1:3:0.005 (DMT: ethylene glycol: catalyst).

The reactor temperature was set at 190°C and the top condenser temperature was set at 65°C. The reaction proceeded for about 1 hour. During the heating process, distillation of methanol started at an internal temperature of 64 °C. Methanol passed to the upper condenser and was collected, and the temperature was raised to 190 °C. When methanol was captured more than 80%, N ₂ was added at 50 cc/min to promote slow distillation of methanol. After about 1 hour, the reaction was terminated when methanol was no longer collected. After completion of the reaction, the solution was cooled to 25 °C to crystallize BHET.

The crystallized BHET was filtered through a vacuum filter equipped with a 1 mm filter cloth and filtered under reduced pressure until no more crystal filtrate came out. The filtrate was washed with water and dried in an oven to obtain BHET crystals.

[Comparative Example 1]

100 g (5 cm * 5 cm) of waste polyester fiber, ethylene glycol, and zinc acetate catalyst were added to a glass reactor in a weight ratio of 1:5.4:0.01 (fiber:ethylene glycol:catalyst). After raising the temperature of the reactor to 197° C., depolymerization was performed for 2 hours to obtain a depolymerization solution containing BHET.

Subsequently, the depolymerization solution was cooled to 25° C. to crystallize BHET, and then filtered through a dark filter equipped with a 1 mm filter cloth to obtain BHET crystals and a crystal filtrate. The obtained crystals were washed with water and dried in an oven to obtain dry BHET.

[Comparative Example 2]

100 g (5 cm * 5 cm) of waste polyester fiber, ethylene glycol, and zinc acetate catalyst were added to a glass reactor in a weight ratio of 1:5.4:0.01 (fiber:ethylene glycol:catalyst). After raising the temperature of the reactor to 197° C., depolymerization was performed for 2 hours to obtain a depolymerization solution containing BHET.

Subsequently, the depolymerization solution was cooled to 25° C. to crystallize BHET, and then filtered through a dark filter equipped with a 1 mm filter cloth to obtain BHET crystals and a crystal filtrate. The obtained crystals were washed with water and dried in an oven to obtain dry BHET.

Subsequently, a solution in which BHET, chloroform and activated carbon are mixed in a weight ratio of 1:20:0.2 (BHET:chloroform:activated carbon) is prepared. After stirring the solution at 60 ° C. for 1 hour, the activated carbon was separated using a vacuum filter. Residual solvent was then removed using a rotary vacuum concentrator to obtain purified BHET.

[Comparative Example 3]

The same procedure as Comparative Example 2 was performed except that a solution in which BHET, chloroform, and activated carbon were mixed in a weight ratio of 1:20:0.4 (BHET:chloroform:activated carbon) was prepared.

[Comparative Example 4]

100 g (5 cm * 5 cm) of waste polyester fiber and 1,000 g of ethylene glycol were put into the glass reactor. After raising the temperature of the reactor to 150 °C, dye extraction was performed for 1 hour. Ethylene glycol containing the dye was removed as much as possible from the mixture of waste fibers and ethylene glycol from which the dye was extracted using a filter press.

100 g of dye-free waste polyester fiber, methanol, and zinc acetate catalyst were introduced into a high-pressure reactor in a weight ratio of 1:10:0.005 (fiber:methanol:catalyst).

The temperature and pressure of the reactor were set at 200 °C and 40 bar. When the temperature reached 200 °C, depolymerization was performed for 2 hours to obtain a depolymerization solution containing DMT.

After completion of the reaction, unreacted fibers were separated by filtration, and the depolymerization solution was cooled to 25° C. to obtain crystallized DMT, which was filtered through a vacuum filter equipped with a 1 mm filter cloth until no more crystal filtrate came out. As a result, DMT crystals were obtained.

100 g of DMT obtained in the above step, ethylene glycol, and zinc acetate catalyst were added to the glass reactor in a weight ratio of 1:3:0.005 (DMT:ethylene glycol:catalyst).

The reactor temperature was set at 190 °C and the upper condenser temperature at 65 °C. The reaction proceeded for about 1 hour. During the heating process, distillation of methanol started at an internal temperature of 64 °C. Methanol passed to the upper condenser and was collected, and the temperature was raised to 190 °C. When methanol was captured more than 80%, N ₂ was added at 50 cc/min to promote slow distillation of methanol. After about 1 hour, the reaction was terminated when methanol was no longer collected. After completion of the reaction, the solution was cooled to 25 °C to crystallize BHET.

The crystallized BHET was filtered through a vacuum filter equipped with a 1 mm filter cloth and filtered under reduced pressure until no more crystal filtrate came out. The filtrate was washed with water and dried in an oven to obtain BHET crystals.

[Comparative Example 5]

In a high-pressure reactor, 100 g (5 cm * 5 cm) of waste polyester fiber and methanol and zinc acetate catalyst were charged in a weight ratio of 1:1:0.005 (fiber:methanol:catalyst).

The temperature and pressure of the reactor were set at 200 °C and 40 bar. When the temperature reached 200 °C, depolymerization was performed for 2 hours to obtain a depolymerization solution containing DMT.

After completion of the reaction, unreacted fibers were separated by filtration, and the depolymerization solution was cooled to 25° C. to obtain crystallized DMT, which was filtered through a vacuum filter equipped with a 1 mm filter cloth until no more crystal filtrate came out. As a result, DMT crystals were obtained.

100 g of DMT obtained in the above step, ethylene glycol, and zinc acetate catalyst were added to the glass reactor in a weight ratio of 1:3:0.005 (DMT:ethylene glycol:catalyst).

The reactor temperature was set at 190 °C and the upper condenser temperature at 65 °C. The reaction proceeded for about 1 hour. During the heating process, distillation of methanol started at an internal temperature of 64 °C. Methanol passed to the upper condenser and was collected, and the temperature was raised to 190 °C. When more than 80% of methanol was captured, N ₂ was added at 50 cc/min to promote slow distillation of methanol. After about 1 hour, the reaction was terminated when methanol was no longer collected.

After completion of the reaction, a solution in which the product solution and activated carbon were mixed in a weight ratio of 1:0.01 (solution:activated carbon) was prepared. After stirring the solution at 60 °C for 1 hour, activated carbon was separated through a vacuum filter. The solution was then cooled to 25 °C to crystallize BHET.

The crystallized BHET was filtered through a vacuum filter equipped with a 1 mm filter cloth and filtered under reduced pressure until no more crystal filtrate came out. The filtrate was washed with water and dried in an oven to obtain BHET crystals.

The results of Example 1 and Comparative Examples 1 to 5 are shown in Table 1 and Table 2 below.

Example
One comparative example
One comparative example
2 comparative example
3 reaction process A B B B B catalyst Zn(OAc) ₂ Zn(OAc) ₂ Zn(OAc) ₂ Zn(OAc) ₂ Zn(OAc) ₂ hour
[hr] 2 One 2 2 2 Temperature [℃] 200 64 to 180 200 200 200 pressure [bar] 40 - - - - separation
refine crystallization
[℃] 25 25 25 25 25 activated carbon
[wt%] 1.0 - - 1.0 2.0 reaction
result Raw material
conversion rate
[%] 99 100 99 99 99 crystallization
transference number
[%] 94 91 76 76 76 decolorization yield
[%] 96 - - 96 91 entire
transference number
[%] 81 75 72 68 BHET
color L 97 63 85 86 a 0.46 14 7 6 b 2.12 18 14 14 A: depolymerization with methanol
B: transesterification reaction with ethylene glycol

Through Table 1, if the BHET purification process is performed in a manner including a depolymerization step with methanol and a transesterification step with ethylene glycol as in the example, the separation/purification efficiency can be increased, the product color can be improved, , it was found that the productivity of the process could also be increased.

Example
One comparative example
4 comparative example
5 reaction process A B A B A B catalyst Zn(OAc) ₂ Zn(OAc) ₂ Zn(OAc) ₂ Zn(OAc) ₂ Zn(OAc) ₂ Zn(OAc) ₂ hour
[hr] 2 One 2 One 2 One Temperature [℃] 200 64 to 180 200 64 to 180 200 64 to 180 pressure [bar] 40 - 40 - 40 - separation
refine crystallization
[℃] 25 25 25 25 25 25 activated carbon
[wt%] 1.0 - - - - 1.0 reaction
result Raw material
conversion rate
[%] 99 100 99 100 99 100 crystallization
transference number
[%] 94 91 94 91 94 91 decolorization yield
[%] 96 - - - - 90 entire
transference number
[%] 81 84 76 BHET
color L 97 87 95 a 0.46 2.15 1.12 b 2.12 7.21 4.01 A: depolymerization with methanol
B: transesterification reaction with ethylene glycol

Through Table 2, as described in Example, if the BHET purification process in which the decolorization process proceeds between the depolymerization reaction with methanol and the transesterification reaction with ethylene glycol is performed, the separation/purification efficiency can be increased, and the product It was found that the color could be improved and the productivity of the process could also be increased.

Claims (4)

preparing dimethyl terephthalate by reacting a raw material including polyester fiber having polyethylene terephthalate with methanol, and recrystallizing it;
Purifying the recrystallized dimethyl terephthalate with a decolorizing solution containing an organic solvent and an adsorbent; and
Reacting the purified dimethyl terephthalate with a polyhydric alcohol to prepare bis(hydroxyethyl) terephthalate and recrystallizing it;
A method for depolymerization of polyester fibers. According to claim 1,
Purifying the recrystallized bis (hydroxyethyl) terephthalate; further comprising,
A method for depolymerization of polyester fibers. According to claim 1,
The organic solvent includes at least one of toluene, xylene, acetone and chloroform,
The adsorbent is activated carbon,
A method for depolymerization of polyester fibers. According to claim 1,
The proceeding temperature of the step of purifying the recrystallized dimethyl terephthalate is in the range of 25 ℃ or more and 100 ℃ or less,
A method for depolymerization of polyester fibers.