>As organic substances of low weight, including acetic acid, are removed through cooling crystallization and pressure filtering during the purification process, the content of by-products such as acetate-based ester compounds or diethylene glycol ester compounds can be effectively controlled.

Hereinafter, each step of the process for preparing a polymerization raw material according to the present invention will be described in detail.

Depolymerization of Waste Polyester

First, waste polyester is depolymerized by glycolysis to obtain a crude bis(2-hydroxyethyl) terephthalate solution.

The waste polyester raw material may be obtained from a polyester material product discarded after use. Specifically, the waste polyester may be obtained by pretreating waste products such as beverage bottles, fabrics, films, cases, boxes, partitions, shelves, protective panels, packaging materials, building materials, and interior and exterior materials, which comprise various polyester materials (e.g., polyethylene terephthalate (PET) material) discarded after having been used by consumers.

The pretreatment may be carried out by removing other plastics, metals, and foreign substances mixed in the waste, washing it, and then crushing it through a crusher. As a result of the pretreatment, the waste polyester raw material may have a flake form. In addition, the waste polyester raw material may have a fine structure like a fiber.

The waste polyester pretreated in this way is then subjected to a depolymerization process. The depolymerization process may comprise, for example, a glycolysis reaction. As is well known, the glycolysis reaction refers to a chemical reaction in which a polymer chain or the like is severed by a glycol such as ethylene glycol. The total weight of the glycol added may be 1, 2, or 3 times the weight of the waste polyester resin or more and may be 7, 5, or 4 times or less. For example, the weight of the glycol added may be 1 to 7 times, specifically, 2 to 5 times, more specifically, 3 to 4 times, relative to the weight of the waste polyester resin.

A catalyst may be used in the glycolysis reaction. The catalyst may be a metal catalyst, for example, a metal salt catalyst or a metallic organic catalyst. Specifically, the catalyst may be an acetate, carbonate, oxide, or hydroxide of a metal, and the metal may be an alkali metal, an alkali earth metal, or a transition metal. As a specific example, the catalyst comprises a metal acetate, or an anhydride or a hydride thereof. More specifically, it may be at least one selected from the group consisting of zinc acetate, sodium acetate, cobalt acetate, and manganese acetate, or in the form of a hydrate or anhydride thereof. In addition, the weight of the catalyst added may be 0.01 part by weight or more, 0.1 part by weight or more, 0.2 part by weight or more, or 0.3 part by weight or more, and may be 5 parts by weight or less, 1 part by weight or less, 0.7 part by weight or less, 0.5 part by weight or less, or 0.4 part by weight or less, relative to 100 parts by weight of the waste polyester resin. For example, the weight of the catalyst added may be 0.1 part by weight to 1 part by weight, specifically, 0.2 part by weight to 0.7 part by weight, relative to 100 parts by weight of the waste polyester resin. More specifically, the catalyst may be used in an amount of 0.2 part by weight to 0.4 part by weight relative to 100 parts by weight of the waste polyester.

As a specific example, the glycolysis in step (a) may comprise a reaction of waste polyester and ethylene glycol in the presence of an acetate-based catalyst.

The depolymerization may comprise, for example, a multi-stage depolymerization reaction at low temperatures. According to an embodiment, the depolymerization comprises subjecting waste polyester to depolymerization through a first glycolysis reaction at a high temperature; and subjecting the product to depolymerization at a low temperature through a second glycolysis reaction.

The temperature during the first glycolysis reaction may be 170° C. or higher, 180° C. or higher, or 190° C. or higher, and may be 205° C. or lower, 200° C. or lower, 195° C. or lower, or 190° C. or lower. For example, the temperature during the first glycolysis reaction may be 180° C. to 200° C., specifically, 180° C. to 195° C., more specifically, 180° C. to 190° C.

In addition, the temperature during the second glycolysis reaction may be 140° C. or higher, 150° C. or higher, or 160° C. or higher, and may be 170° C. or lower or 160° C. or lower. For example, the temperature during the second glycolysis reaction may be 150° C. to 170° C., specifically, 150° C. to 160° C., more specifically, 150° C. to 155° C.

As a specific example, the depolymerization may comprise (1) subjecting waste polyester to depolymerization through a first glycolysis reaction at a temperature of 180 to 200° C. to obtain a first reactant; and (2) subjecting the first reactant to depolymerization at a temperature of 150 to 170° C. through a second glycolysis reaction to obtain a second reactant.

The period of time required for the first and second glycolysis reactions may be 1 hour or more or 2 hours or more, and may be 4 hours or less or 3 hours or less, from the point when the appropriate temperature is reached. For example, the period of time required for the first and second glycolysis reactions may be 1 hour to 4 hours, specifically, 1 hour to 3 hours, more specifically, 1 hour to 2 hours, from the point when the appropriate temperature is reached.

As a more specific example, the first glycolysis reaction may be carried out at a temperature of 180° C. to 190° C. for 1 hour to 3 hours. In addition, the second glycolysis reaction may be carried out at a temperature of 150° C. to 160° C. for 1 hour to 3 hours.

As an example, the first glycolysis reaction may be carried out in the presence of a zinc acetate anhydride catalyst. As a specific example, the first glycolysis reaction may be carried out at a temperature of 180° C. to 200° C. for 1 hour to 3 hours in the presence of a zinc acetate anhydride catalyst. The zinc acetate anhydride may be used in an amount of 0.2 part by weight to 0.4 part by weight relative to 100 parts by weight of the waste polyester. In addition, the second glycolysis reaction may be carried out at a temperature of 140° C. to 160° C. for 1 hour to 3 hours upon the further addition of ethylene glycol without an additional catalyst.

Cooling and Filtration

The crude bis(2-hydroxyethyl) terephthalate solution obtained by depolymerization may then be cooled. The cooling temperature may be, for example, 150° C. or lower, 140° C. or lower, 130° C. or lower, 120° C. or lower, 110° C. or lower, 100° C. or lower, and may be 50° C. or higher, 60° C. or higher, 70° C. or higher, 80° C. or higher, or 90° C. or higher.

As an example, the cooling may be carried out through a reduced-pressure flash process. Specifically, the temperature of the crude bis(2-hydroxyethyl) terephthalate solution may be lowered by evaporating ethylene glycol by applying a vacuum through the reduced-pressure flash process. For example, the crude bis(2-hydroxyethyl) terephthalate solution may be further subjected to a step of cooling to 120° C. or lower through reduced pressure flash prior to subsequent steps. More specifically, the temperature of the crude bis(2-hydroxyethyl) terephthalate solution may be lowered to 110° C. or lower or 100° C. or lower by the reduced-pressure flash process. The pressure condition of the reduced-pressure flash process may be, for example, 200 Torr or less, 100 Torr or less, or 50 Torr or less, specifically, 10 Torr to 200 Torr, 10 Torr to 100 Torr, or 10 Torr to 50 Torr.

Thereafter, insoluble foreign substances may be removed from the cooled crude bis(2-hydroxyethyl) terephthalate solution through filtration. As a specific example, a step of cooling the crude bis(2-hydroxyethyl) terephthalate solution to 120° C. or lower and filtering it with the addition of a filter aid may be further carried out. As a result, fine particles and insoluble organic substances present in the crude bis(2-hydroxyethyl) terephthalate solution can be filtered out by solid-liquid separation.

Known ingredients such as diatomaceous earth, perlite, and asbestos powder may be used as the filter aid. For example, 0.1 part by weight to 2.0 parts by weight of the filter aid may be added to 100 parts by weight of the crude bis(2-hydroxyethyl) terephthalate solution.

Since bis(2-hydroxyethyl) terephthalate (BHET) or oligomers obtained through the depolymerization reaction are present in a solid form at room temperature, it is difficult to separate foreign substances at room temperature. Thus, it is preferable to separate them at a temperature condition of 90° C. to 150° C., more specifically, 110° C. to 150° C. In addition, if the above temperature range is maintained, the removal of insoluble foreign substances may be facilitated thanks to good flowability.

Various methods and devices may be used in the removal of insoluble foreign substances through solid-liquid separation. For example, a device such as a pressurized filter, a centrifugal separator, a filter press, a belt press, or the like may be used. But it is not limited thereto as long as any method capable of separating foreign substances is used.

According to an embodiment, as the crude bis(2-hydroxyethyl) terephthalate solution is subjected to a solid-liquid separation step by filtering out the low-weight organic substances contained therein, most acetate-based ester compounds and diethylene glycol ester compounds can be discharged in a liquid form.

The process according to an embodiment comprises cooling the crude bis(2-hydroxyethyl) terephthalate solution to crystallize it; and separating the crystallized product into solid and liquid using a pressurized filter.

The temperature for cooling crystallization may be, for example, 70° C. or lower, 60° C. or lower, 50° C. or lower, 40° C. or lower, 30° C. or lower, or 25° C. or lower, and may be 0° C. or higher, 5° C. or higher, 10° C. or higher, 5° C. or higher, or 20° C. or higher. As a specific example, it may be room temperature.

Acetate-based compounds and diethylene glycol-based compounds may be removed through the cooling crystallization.

The operating pressure of the pressurized filter may be 0.1 bar to 21 bar, more specifically 1 bar to 5 bar. In addition, the operating temperature of the pressurized filter may be 5° C. to 35° C., more specifically 15° C. to 25° C.

The pressurized filter may be a Nutsche filter or a filter press.

Before use of the Nutsche filter, it is desirable to inactivate the inside of the container by nitrogen purging or the like. First, a mixture is put into a Nutsche filter, and an inert gas such as nitrogen is injected and pressurized to separate the solvent and the filter cake. Thereafter, water is injected into the Nutsche filter to wash out the residual solvent remaining in the filter cake, and inert gas is injected and pressurized to separate the filter cake and water. The obtained filter cake may be dried and cooled.

The filter press is a device that creates a filtration chamber with a filter plate having a filter surface, a filter cloth, and a cover, and separates solid and liquid by pressurizing the reactant between the filter cloth and the cover at a high pressure. Since a filter press increases the pressure difference before and after the filter medium as the solid-liquid separation surface as compared with a vacuum filter, more effective solid-liquid separation is possible.

Acetate-based compounds and diethylene glycol-based compounds may be removed through the pressurized filter.

Ion Exchange

In addition, the crude bis(2-hydroxyethyl) terephthalate solution may be further subjected to ion exchange through an ion exchange resin. As it is subjected to the ion exchange, ionic impurities present in the crude bis(2-hydroxyethyl) terephthalate solution, specifically, catalysts and foreign substances, may be removed.

As is well known, an ion-exchange resin refers to a resin or polymer that serves as a medium for ion exchange. The ion-exchange resin may comprise a cation-exchange resin, an anion-exchange resin, an amphoteric ion-exchange resin, a chelate resin, or the like.

The cation-exchange resin may comprise a strongly acidic cation-exchange resin having a sulfonic acid group (−SO₃H) and a weakly acidic cation-exchange resin having a carboxyl group (—COOH). The anion-exchange resin may comprise a strongly basic anion-exchange resin in the form of a quaternary ammonium salt and a weakly basic anion-exchange resin having a primary to tertiary amino group.

As a specific example, the ion-exchange resin may comprise at least one selected from the group consisting of a strongly acidic cation-exchange resin, a weakly acidic cation-exchange resin, and a chelate resin.

According to an embodiment, the ion exchange is carried out by adding an ion-exchange resin to the crude bis(2-hydroxyethyl) terephthalate solution.

The weight of the ion-exchange resin added may be 1, 3, or 5 times the weight of the catalyst added in the depolymerization reaction or more, and may be 20, 15, 10, or 8 times or less. For example, the weight of the ion-exchange resin added may be 1 to 20 times, specifically, 3 to 15 times, more specifically, 5 to 8 times, relative to the weight of the catalyst added in the depolymerization reaction.

In addition, the weight of the ion-exchange resin added may be 1 part by weight or more, 3 parts by weight or more, or 5 parts by weight or more, and may be 50 parts by weight or less, 20 parts by weight or less, 15 parts by weight or less, 10 parts by weight or less, or 7 parts by weight or less, relative to 100 parts by weight of the waste polyester resin employed in the depolymerization reaction.

As a specific example, the ion-exchange resin may be used in an amount of 1 part by weight to 20 parts by weight relative to 100 parts by weight of the waste polyester.

According to another embodiment, the ion exchange is carried out by using a column containing an ion-exchange resin.

Specifically, the column may be filled with particles of an ion-exchange resin, and ion exchange may be carried out while the crude bis(2-hydroxyethyl) terephthalate solution passes through the column.

The particle diameter of the ion-exchange resin particles may be, for example, 0.3 mm to 1.5 mm, more specifically, 0.6 mm to 0.9 mm.

The temperature for ion exchange may be, for example, 140° C. or lower, 130° C. or lower, 120° C. or lower, 110° C. or lower, 100° C. or lower, and may be 50° C. or higher, 60° C. or higher, 70° C. or higher, 80° C. or higher, or 90° C. or higher.

Further Purification Step

The polymerization raw material comprising recycled bis(2-hydroxyethyl) terephthalate as described above may be further subjected to a purification step as needed. According to an example, the polymerization raw material comprising recycled bis(2-hydroxyethyl) terephthalate may be prepared by further carrying out a step of removing an unreacted glycol by distillation.

Since an unreacted glycol still remains in the depolymerization product after filtration thereof in the previous step, it is necessary to remove the same from the product prior to the next step.

In addition, it is necessary to perform a step of recovering an unreacted glycol for an economical depolymerization process. That is, it is possible to recover and reuse a glycol, among glycols such as ethylene glycol, propylene glycol, diethylene glycol, or the like previously employed in the depolymerization, that remains without participating in the glycolysis reaction.

The distillation to remove the unreacted glycol may be carried out by, for example, vacuum distillation. A glass distillation apparatus or a rotary evaporator may be used for this purpose.

As the vacuum distillation to remove the unreacted glycol is carried out at a temperature of 150° C. or lower, the purity of BHET can be enhanced by further reducing the formation of diethylene glycol and impurities derived therefrom. For example, the vacuum distillation to remove the unreacted glycol may be carried out at a temperature of 150° C. lower, 130° C. lower, or 120° C. lower, and 80° C. higher, 90° C. higher, 100° C. higher, or 110° C. higher. Specifically, the temperature during the distillation to remove the unreacted glycol may be 80° C. to 190° C. or 90° C. to 150° C. As a more specific example, the distillation to remove the unreacted glycol may be carried out at a temperature of 100° C. to 130° C.

The pressure during the vacuum distillation to remove the unreacted glycol may be, for example, 0.1 Torr to 760 Torr, 0.1 Torr to 200 Torr, or 0.5 Torr to 30 Torr. More specifically, the vacuum distillation may be carried out under stepwise reduced pressure conditions from 760 Torr to 0.8 Torr.

According to another example, the polymerization raw material comprising recycled bis(2-hydroxyethyl) terephthalate may be prepared by further carrying out at least one of thin-film distillation under reduced pressure; and dissolution in water followed by adsorption-crystallization.

The thin-film distillation is a distillation method that makes a mixture to be separated into a thin film for increasing its surface area in contact with a heat source. Specifically, a mixture fed to the evaporator of the thin film evaporator forms a thin film on the inner wall of the thin film evaporator by the wiper rotor. Then, distillation is carried out under appropriate temperature conditions by heating. In addition, a condenser for recovering the evaporated material may be provided inside the thin film evaporator.

The thin film evaporation may be carried out by short path evaporation. Since such a short path and thin film evaporation has a short residence time and enables vacuum distillation using a high vacuum, it is possible to separate high-boiling or high-molecular-weight materials that are hardly separated by other distillation methods while minimizing the change of the reactants by heat. In addition, if the pressure inside a thin film evaporator is lowered, there is an advantage in that the vapor pressure of a material is reduced, which allows evaporation to take place at a lower temperature than its original boiling point.

As a specific example, the depolymerization product is fed to a short path and thin film evaporator, and a wiper for forming a thin film is rotated at 300 rpm or more. As a result, a vaporized material and a non-vaporized material can be separated from each other. The internal thin film temperature of the upper thin film evaporation apparatus during the thin film evaporation may be, for example, 150° C. to 250° C., 190° C. to 250° C., or 180° C. to 220° C. In addition, the internal pressure of the upper thin film evaporation apparatus during the thin film evaporation may be, for example, 0.005 Torr to 5.0 Torr, 0.05 Torr to 5.0 Torr, 0.05 Torr to 1.5 Torr, or 0.05 Torr to 1 Torr.

The adsorption-crystallization may be carried out by, for example, adding an adsorbent using water as a solvent, filtering, and crystallization.

Various solvents may be used for the adsorption-crystallization, but a solvent capable of dissolving bis(2-hydroxyethyl) terephthalate is preferably used as a solvent. As a specific example, in order to obtain the final reactant, water as a solvent is added to the polymerization raw material comprising recycled bis(2-hydroxyethyl) terephthalate, which is dissolved by heating, and an adsorbent is added thereto, followed by subjecting the solution obtained by filtration to crystallization and final filtration. As a result, a polymerization raw material with high purity can be obtained.

Water may be added in an amount of 100 parts by weight to 500 parts by weight, specifically, 200 parts by weight to 400 parts by weight, more specifically, 300 parts by weight to 350 parts by weight, relative to 100 parts by weight of the polymerization raw material.

In addition, the dissolution temperature may be 50° C. to 95° C., specifically, 60° C. to 85° C., more specifically, 70° C. to 75° C.

The adsorbent added may serve to adsorb and remove other foreign substances. It may be added in an amount of 0.1 part by weight to 3 parts by weight relative to 100 parts by weight of the polymerization raw material. The type and form of the adsorbent are not particularly limited. For example, activated carbon may be used.

MODE FOR THE INVENTION

Hereinafter, a preferred embodiment is presented for the understanding of the present invention. However, the following examples are provided only to help easily understand the present invention, and the scope of the present invention is not limited thereby.

Example 1

A first reactor made of stainless steel (SUS) was charged with 1,000 g of a waste polyester resin, 2,000 g of ethylene glycol, and 5.0 g of zinc acetate anhydride. The temperature inside the reactor was raised to 180° C., and depolymerization (first glycolysis reaction) was carried out for 2 hours. The reactant (first reactant) thus obtained was transferred to a second reactor and cooled to 150° C. 2,000 g of ethylene glycol was further added thereto, and depolymerization (second glycolysis reaction) was carried out for 2 hours while the reactor temperature was maintained at 150° C.

The reactant (second reactant) thus obtained was cooled to 120° C. through reduced pressure flash, and 16 g of a filter aid (Celite™ 545) was added thereto, followed by pressurized filtration to carry out solid-liquid separation. The separated liquid reactant was passed through a column filled with an ion-exchange resin (BC107(H) of Bonlite) to remove ionic impurities to obtain a mixture (third reactant) containing bis(2-hydroxyethyl) terephthalate and ethylene glycol.

The mixture (third reactant) was cooled to room temperature over 2 hours under stirring at 100 rpm in a 10-liter crystallizer equipped with a cooling water circulation jacket. The obtained crystallized product was separated into solid and liquid using a pressurized Nutsche filter (jacket type, filtration area 0.2 m²) under a pressure of 3 bar to obtain a BHET cake (fourth reactant).

The BHET cake was transferred to a 10-liter distillation apparatus and heated to 130° C. again. Vacuum distillation was carried out under stepwise reduced pressure conditions from 760 Torr to 0.8 Torr to recover unreacted ethylene glycol. The reactant (fifth reactant) from which ethylene glycol had been removed was subjected to thin film evaporation at 220° C. and 0.08 Torr in a thin film evaporator (VKL70-4S of VTA) to obtain 1,040 g of a product from which dimers or higher oligomers had been removed. Thereafter, for adsorption-crystallization, 1,040 g of the above product and 3,120 g of distilled water were charged to a 10-liter glass reactor, dissolved at a temperature of 70° C., and then 5.2 g of activated carbon was added thereto, followed by stirring for 30 minutes and filtration thereof. The filtrate was cooled to room temperature for the crystallization thereof, filtered, and dried in a vacuum oven. As a result, 1,980 g of a polymerization raw material containing recycled bis(2-hydroxyethyl) terephthalate was obtained.