Process for producing terephthalic acid and terephthalic acid produced thereby

Abstract

The present invention relates to a method for producing terephthalic acid and the terephthalic acid produced therefrom, the method comprising the steps of: (1) washing waste polyester with a washing solvent; (2) pretreating the washed waste polyester with an alcohol having four or more carbon atoms; (3) alcoholystolysing the pretreated waste polyester to obtain a liquid composition; and (4) hydrolyzing the liquid composition.

Description

Technical Field

[0001] This invention relates to a method for preparing terephthalic acid using waste polyester in an energy-saving manner, and to a type of terephthalic acid prepared therefrom. Background Technology

[0002] Polyester resins are widely used in polymers as materials for containers for beverages or food, various packaging films or sheets, and internal and external materials such as panels, shelves, and partitions.

[0003] Due to the widespread use of polyester, waste polyester using polyester resins is generated globally at an uncontrollable level each year. Therefore, there is growing interest in recycling waste polyester or regeneration processes that utilize waste polyester. Specifically, methods have been developed for decomposing (depolymerizing) waste polyester to produce polymer feedstocks (recycled feedstocks); and for further using these polymer feedstocks to produce polyester resins (recycled polyester resins).

[0004] As an example of the above method, a method is provided in which waste polyester is washed and then alcoholyzed to produce terephthalic acid (recycled terephthalic acid, r-TPA) as a polymerization feedstock. However, the above method has the problem that the yield and purity of terephthalic acid are low, and process failures occur due to the washing process of waste polyester.

[0005] Therefore, there is a need for a technology that can improve the recycling process of waste polyester to produce high-purity terephthalic acid in high yield.

[0006] [Existing Technical Documents]

[0007] [Patent Literature]

[0008] (Patent Document 1) Korean Patent Application Publication No. 1997-0042469 Detailed Implementation

[0009] Technical issues

[0010] The inventors of this invention have discovered that when waste polyester is pretreated with a specific alcohol, the waste polyester depolymerization process for producing terephthalic acid can be carried out efficiently without undergoing a process (e.g., a drying process) to remove solvents contained in the waste polyester, thereby enabling the preparation of terephthalic acid with high purity in high yield.

[0011] Therefore, the object of the present invention is to provide a method for preparing terephthalic acid, wherein terephthalic acid with high purity can be prepared in high yield, and a terephthalic acid (recycled terephthalic acid) prepared therefrom.

[0012] Solution to the problem

[0013] To achieve the above objectives, the present invention provides a method for preparing terephthalic acid, the method comprising: (1) washing waste polyester with a washing solvent; (2) pretreating the washed waste polyester with an alcohol having four or more carbon atoms; (3) alcoholystolysing the pretreated waste polyester to produce a liquid composition; and (4) hydrolyzing the liquid composition.

[0014] In addition, the present invention provides terephthalic acid (recycled terephthalic acid), which is prepared by the above preparation method.

[0015] Furthermore, the present invention provides a polyester resin comprising a component derived from the aforementioned terephthalic acid.

[0016] Beneficial effects of the present invention

[0017] In the method for preparing terephthalic acid according to the present invention, waste polyester is pretreated with a specific alcohol and subjected to a depolymerization process including alcoholysis and hydrolysis to produce terephthalic acid. Therefore, even if the waste polyester contains moisture or solvents, the depolymerization process can be carried out efficiently without processes for removing moisture or solvents (e.g., drying processes). Thus, the present invention can help provide terephthalic acid with high purity in high yield.

[0018] Best way to carry out the invention

[0019] The present invention will be described in detail below. In this document, the invention is not limited to the disclosure given below; it can be modified in various forms without altering its essential elements.

[0020] In this specification, the term "comprising" is intended to specify a particular feature, region, step, process, element, and / or component. Unless expressly stated to the contrary, the presence or addition of any other feature, region, step, process, element, and / or component is not excluded.

[0021] Unless otherwise specified, all figures and expressions relating to the quantities of components, reaction conditions, etc., used herein should be understood to be modified by the term “about”.

[0022] In this specification, unless the context otherwise requires, singular expressions should be interpreted as also encompassing plural expressions.

[0023] The present invention is characterized in that terephthalic acid can be prepared in high yield even when the waste polyester used in the method for preparing terephthalic acid (recycled terephthalic acid, r-TPA) by separate alcoholysis and hydrolysis contains a large amount of moisture and alcohol components.

[0024] Waste polyester discarded after use may contain various contaminants by nature. In particular, biochemical contaminants remaining on polyester waste (such as food trays) can pose safety risks to workers if they come into contact with them. To prevent this in advance, raw materials used for chemical recycling are typically subjected to various washing processes beforehand.

[0025] Solvents such as water and alcohol are commonly used in the above washing processes. Methanol, ethanol, and isopropanol, which are known to have antibacterial and disinfecting properties, are mainly selected as alcohols.

[0026] Excessive use of solvents for washing purposes (rather than for depolymerization) can reduce the purity of the products formed by the depolymerization reaction and sometimes cause process failures.

[0027] Typical process failures are process interruptions caused by filter clogging. Problems can occur in the ethylene glycololysis process if one or both sides of BHET, a product of the ethylene glycololysis reaction, are replaced by water to form a carboxylic acid. Furthermore, the solvent components mentioned above have extremely low solubility for EG relative to BHET and are very likely to clog the insoluble filter installed downstream of the reactor, leading to a complete process shutdown.

[0028] Meanwhile, in methanololysis using methanol as a depolymerization agent, the same phenomenon may occur due to water.

[0029] To date, the only way to prevent this is to use a separate energy source to completely dry the entire waste polyester. However, the aforementioned drying process also has several drawbacks.

[0030] First, if a sufficiently high temperature is maintained for a sufficiently long period to achieve complete drying, the water and alcohol used for washing may dry out, while the waste polyester may undergo solid-state polymerization, leading to an increase in molecular weight. As a result, depolymerization becomes more difficult.

[0031] If the waste polyester is not thoroughly dried to avoid this situation, there is a risk, as described above, that the filter will become clogged after the reaction due to the presence of carboxylic acid as a substituent, thereby halting the entire process itself.

[0032] However, in this invention, the depolymerization of waste polyester is carried out using higher alcohols having four or more carbon atoms. Therefore, the main product, such as a compound represented by Formula 1 (e.g., dibutyl terephthalate (DBTP)), acts as a solvent for the problematic monoacidic substance, allowing the entire product to be fed smoothly into the next step of hydrolysis without causing filter clogging.

[0033] Furthermore, even with high-temperature drying after the washing process, in typical processes, it is extremely difficult to prevent water entrainment due to the moisture content of the waste polyester. Therefore, the method of the present invention is extremely energy-efficient—where depolymerization can be carried out without drying the water, and the product (liquid composition) can be fed into hydrolysis to produce terephthalic acid (recycled terephthalic acid, r-TPA)—and can significantly reduce process failures.

[0034] Specifically, in this invention, processability is compared based on the water content in the waste polyester feedstock used in depolymerization in the following cases: filtering water without completely drying it; physically removing most of the water after filtration by immersing the waste polyester feedstock in an alcohol (such as butanol) to be used for depolymerization or by additional washing with the alcohol; and completely drying the water to completely control side reactions in the depolymerization reaction caused by water. Processability can be compared by measuring the time taken for the insoluble matter filtration process of the depolymerization reaction mixture to determine the feasibility of the process.

[0035] For example, waste polyethylene terephthalate (w-PET) must be washed and dried during the pre-screening process. This is because when using undried waste PET sheets, the residual moisture causes solid terephthalic acid (solid TPA) to form during the alcoholysis reaction instead of a liquid composition (or liquid intermediate), which may reduce the yield of terephthalic acid. In particular, if solid terephthalic acid is generated, side effects such as filter clogging in the post-reaction stage, reduced yield, and increased cost occur. However, in this invention, the water contained therein can be removed by immersing the undried waste PET sheets in an alcohol bath or spraying them with alcohol, and the alcohol used can be easily removed by azeotropic extraction.

[0036] Furthermore, a key feature of this invention is that the depolymerization process is carried out by controlling the water content in the waste polyester. Specifically, even when depolymerizing waste polyester with a water content of 0 to 20%, this invention can achieve a depolymerization reaction without process failure, even without a drying process. As a result, terephthalic acid can be produced with high purity and high yield.

[0037] Method for preparing terephthalic acid

[0038] The method for preparing terephthalic acid according to the present invention comprises: (1) washing waste polyester with a washing solvent; (2) pretreating the washed waste polyester with an alcohol having four or more carbon atoms; (3) alcoholystolysing the pretreated waste polyester to produce a liquid composition; and (4) hydrolyzing the liquid composition.

[0039] Step (1): Washing

[0040] According to the present invention, step (1) is a step of washing the waste polyester with a washing solvent. The washing is intended to remove foreign substances present in the waste polyester and can be carried out using generally known methods. The washing solvent is not particularly limited, but it can be water or an alcohol (e.g., methanol, ethanol, isopropanol, etc.).

[0041] Waste polyester can be waste polyester products or products obtained by crushing or melting waste. Specifically, waste polyester can include at least one selected from the group consisting of: waste polyethylene terephthalate fibers, waste polyethylene terephthalate containers, waste polyethylene terephthalate films, and polyester waste (post-industrial recycled materials; PIR), and it can be a product obtained by crushing waste (sheets) or converting waste into granular form (post-consumer recycled materials; PCR). Polyester waste (PIR) can refer to defective products or waste generated in the molding process of films, fibers, containers, etc.

[0042] Taking into account the yield of terephthalic acid, waste polyester may contain polyethylene terephthalate (PET) in amounts of 50% or more, specifically 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, or 95% or more (e.g., 50% to 100%, 60% to 97%, 70% to 95%, or 80% to 90% by weight) based on the total weight of the waste polyester.

[0043] Step (2): Preprocessing

[0044] According to the present invention, step (2) is a step of pretreating the washed waste polyester with an alcohol having four or more carbon atoms (first alcohol). The pretreatment method is not particularly limited, but it can be carried out by rinsing the waste polyester by immersing it in a bath containing an alcohol having four or more carbon atoms, or by spraying an alcohol having four or more carbon atoms onto the waste polyester.

[0045] Impregnation or spraying conditions are not particularly restricted and can be appropriately adjusted according to the amount and condition of the waste polyester. For example, the impregnation temperature can be 150°C or lower, 130°C or lower, 120°C or lower, 110°C or lower, or 100°C or lower.

[0046] Residual washing solvents in waste polyester can be removed through pretreatment. As a result, a liquid composition is successfully produced in the alcoholysis reaction instead of solid terephthalic acid (TPA), thus solving the problems of filter clogging in the post-reaction stage, reduced terephthalic acid yield, and increased costs.

[0047] Meanwhile, the alcohols with four or more carbon atoms used for pretreatment are azeotropic and can be easily removed by a separate distillation process. Specifically, the number of carbon atoms in the alcohol (first alcohol) can be four or more, six or more, eight or more, ten or more, twelve or more, or fourteen or more. More specifically, the number of carbon atoms in the alcohol (first alcohol) can be four to thirteen, four to ten, four to nine, four to eight, or four to six.

[0048] Step (3): Alcohololysis

[0049] According to the present invention, step (3) is a step of alcoholystomizing the pretreated waste polyester to produce a liquid composition. Specifically, the pretreated waste polyester can be alcoholystomized with an alcohol having four or more carbon atoms (a second alcohol) to prepare a liquid composition comprising a compound represented by Formula 1. Alcoholystomosis yields a liquid composition having a liquid phase rather than a solid phase and containing a high content of the compound represented by Formula 1 (as the substance to be hydrolyzed). As a result, terephthalic acid can be produced in high yield.

[0050] [Formula 1]

[0051] In Formula 1, R1 is an alkyl group having four or more carbon atoms.

[0052] The alcohol (second alcohol) used in the alcoholysis reaction can specifically have 4 or more, 6 or more, 8 or more, 10 or more, 12 or more, or 14 or more carbon atoms. Specifically, the alcohol can have 4 to 13, 4 to 10, 4 to 8, 4 to 7, or 4 to 6 carbon atoms. Since the alcoholysis of waste polyester is carried out using alcohols with the aforementioned number of carbon atoms, the alcoholysis can be conducted at lower temperatures and lower pressures than conventional processes; and a liquid composition containing only liquid components and no solid components can be obtained. Furthermore, the reaction rate of alcoholysis can be increased.

[0053] The boiling point of alcohols can range from 100°C to 290°C, specifically 110°C to 280°C, 120°C to 260°C, 130°C to 230°C, 140°C to 190°C, or 150°C to 180°C. When the boiling point of an alcohol is within the above range, byproducts formed during alcoholysis, such as ethylene glycol and diethylene glycol, can be more easily removed or recovered in subsequent processes, thereby further improving processability.

[0054] The reaction ratio of waste polyester to alcohol used for alcoholysis is not particularly limited, but it can be a weight ratio of 1:1 to 10. Specifically, the weight ratio can be 1:1 to 8, 1:1 to 6, 1:1 to 4, 1:1 to 3.5, 1:1.1 to 3.3, 1:2 to 4, or 1:2 to 3.5.

[0055] A liquid composition comprising a high content of the compound represented by Formula 1 above is produced by alcoholysis. Specifically, the liquid composition may contain the compound represented by Formula 1 above in an amount of 45% to 99.9% by molar, based on the total molar amount of the liquid composition. For example, based on the total molar amount of the liquid composition, the content of the compound represented by Formula 1 above may be 50% to 99.9% by molar, 55% to 99.5% by molar, 60% to 99.5% by molar, 65% to 99% by molar, 68% to 99% by molar, 70% to 98% by molar, 73% to 97% by molar, 75% to 96% by molar, 78% to 95% by molar, 80% to 93% by molar, 83% to 92% by molar, or 85% to 90% by molar. When the content of the compound represented by Formula 1 above is within the above range, terephthalic acid can be produced in high yield by the hydrolysis reaction of the liquid composition.

[0056] In the compound represented by Formula 1 above, R1 can specifically be butyl, n-butyl, isobutyl, tert-butyl, sec-butyl, 1-methyl-butyl, 1-ethyl-butyl, pentyl, n-pentyl, isopentyl, neopentyl, tert-pentyl, hexyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 4-methyl-2-pentyl, 3,3-dimethylbutyl, 2-ethylbutyl, hexyl, 1-methylhexyl, 2-ethyl-1-hexyl, heptyl, n-heptyl, 1-methylheptyl, octyl, n-octyl, isooctyl, tert-octyl, 2-propylpentyl, n-nonyl, 2,2-dimethylheptyl, 1-ethyl-propyl, 1,1-dimethyl-propyl, isohexyl, 2-methylpentyl, 4-methylhexyl, 5-methylhexyl, decyl, undecyl, dodecyl, tridecyl, or tetradecyl.

[0057] According to the present invention, alcoholysis can be carried out at a temperature of 160°C to 280°C for 0.5 to 24 hours. Specifically, alcoholysis can be carried out at temperatures of 165°C to 270°C, 170°C to 260°C, 175°C to 250°C, 180°C to 240°C, or 180°C to 230°C for 1 to 22 hours, 1.5 to 20 hours, 2 to 15 hours, 2.5 to 10 hours, 3 to 8 hours, or 3 to 6 hours. Meanwhile, the pressure of alcoholysis can be determined according to the reaction temperature and / or reaction time. Specifically, the pressure of alcoholysis can be 1 to 40 bar, 1 to 38 bar, 1.5 to 33 bar, 2 to 28 bar, 2.5 to 24 bar, 3 to 40 bar, 4 to 35 bar, or 5 to 30 bar.

[0058] A catalyst may or may not be added to the alcoholysis reaction. When the alcoholysis reaction is a non-catalytic reaction without a catalyst, the process of removing insoluble metals can be omitted, thus ensuring environmental friendliness while producing high-purity terephthalic acid. Furthermore, when the alcoholysis reaction is a catalytic reaction with a catalyst, the activity of the alcoholysis reaction increases, thereby improving processability (economic feasibility).

[0059] The catalyst added to the alcoholysis reaction can be a metal acetate, alkali metal salt, hydroxyl salt, etc. Specifically, the catalyst may contain at least one cation selected from the group consisting of alkali metal ions such as Li. + Na + K + and Cs + Alkaline earth metal ions such as Be 2+ Mg 2+ Ca 2+ and Ba 2+ NH 4+ and Zn 2+ ; and / or at least one anion selected from the group consisting of: OH- - OR - HCO3 - CO3 2- Benzoate ions (C7H5O2) - ), 4-alkoxycarbonylbenzoate ion, acetate ion and terephthalate ion.

[0060] For example, the catalyst may contain at least one selected from the group consisting of: Zn(OAC)2, Co(OAc)2, Mn(OAc)2, Mg(OAc)2, Ca(OAc)2, Pb(OAc)2, Ba(OAc)2, LiOAc, NaOAc, KOAc, Pd(OAc)2, Zn(OAC)2·2H2O, Co(OAc)2·4H2O, Mn(OAc)2·4H2O, Mg(OAc)2·4H2O, Ti(OBu)4, Ti(OiPr)4, GeO2, Al(OiPr)3, Na2CO3, K2CO3, dibutyltin oxide (IV), stannous octoate, titanium phosphate, and terephthalic acid.

[0061] For example, based on the total weight of waste polyester, the amount of catalyst added can be 10 to 10,000 ppm, 10 to 9,000 ppm, 15 to 8,000 ppm, 20 to 6,000 ppm, 50 to 3,500 ppm, 100 to 1,500 ppm, 150 to 1,000 ppm, 180 to 500 ppm, or 200 to 450 ppm.

[0062] Additionally, the liquid composition may contain 10% or less of terephthalic acid by weight based on the total weight of the liquid composition. Specifically, the liquid composition may not contain solid terephthalic acid (the content of terephthalic acid is 0% by weight), or it may contain 0.01% to 10%, 0.1% to 8%, 1% to 5%, or 1% to 3% of solid terephthalic acid by weight.

[0063] Furthermore, the liquid composition may further comprise at least one component selected from the group consisting of alcohol derivatives and oligomers. Specifically, the liquid composition may comprise alcohol derivatives but not oligomers, may comprise oligomers but not alcohol derivatives, or may comprise both alcohol derivatives and oligomers. The alcohol derivatives and / or oligomers may be present in the liquid phase of the liquid composition.

[0064] The content of each alcohol derivative and oligomer in the liquid composition may not be particularly limited. Specifically, the liquid composition may contain an amount of alcohol derivative from 0.01% to 50% by molar, based on the total molar amount of the liquid composition, and an amount of oligomer from 0.01% to 50% by molar. For example, based on the total molar amount of the liquid composition, the content of the alcohol derivative may be 0.1% to 40% by molar, 0.1% to 35% by molar, 0.5% to 30% by molar, 0.5% to 25% by molar, 1% to 23% by molar, 1% to 22% by molar, 1.5% to 21% by molar, 1.5% to 20% by molar, 2% to 18% by molar, 2.5% to 17% by molar, 3% to 15% by molar, or 5% to 10% by molar. Furthermore, based on the total molar number of the liquid composition, the content of the oligomer can be 0.05% to 40% by molar, 0.05% to 35% by molar, 0.1% to 30% by molar, 0.1% to 25% by molar, 0.5% to 20% by molar, 0.5% to 15% by molar, 1% to 10% by molar, 1% to 8% by molar, 1.5% to 5% by molar, 1.5% to 4% by molar, 2% to 4% by molar, or 2% to 3% by molar. When the contents of the alcohol derivative and the oligomer are each within the above ranges, the hydrolysis reaction of the liquid composition can proceed efficiently.

[0065] According to the present invention, alcohol derivatives may comprise compounds represented by Formula 2 below, and oligomers may comprise compounds represented by Formula 3 below.

[0066] [Equation 2]

[0067] [Formula 3]

[0068] In Formulas 2 and 3, R2 and R3 are each independently an alkyl group having four or more carbon atoms, and n is an integer of 1 or greater.

[0069] Specifically, in the compounds represented by Formulas 2 and 3 above, R2 and R3 are each independently an alkyl group having 4 to 12 carbon atoms, and n can be an integer from 1 to 3.

[0070] Furthermore, according to the present invention, the liquid composition may further comprise at least one selected from the group consisting of ethylene glycol and unreacted alcohols. Specifically, the liquid composition may comprise ethylene glycol but not unreacted alcohols, may comprise unreacted alcohols but not ethylene glycol, or may comprise both ethylene glycol and unreacted alcohols.

[0071] Ethylene glycol can refer to a byproduct produced by alcoholysis, and unreacted alcohol can refer to the residual alcohol remaining after an alcohol with four or more carbon atoms added for alcoholysis has not participated in the alcoholysis reaction.

[0072] The amount of unreacted alcohol contained in the liquid composition may not be particularly limited. Specifically, the liquid composition may contain unreacted alcohol in an amount of 0.01% to 50% (molar) based on the total molar number of the liquid composition. For example, based on the total molar number of the liquid composition, the content of unreacted alcohol may be 0.01% to 40%, 0.01% to 35%, 0.01% to 30%, 0.02% to 25%, 0.02% to 20%, 0.02% to 15%, 0.03% to 10%, 0.03% to 5%, 0.03% to 1%, 0.04% to 0.5%, 0.04% to 0.1%, or 0.05% to 0.07% (molar).

[0073] Furthermore, the liquid composition may be free of ethylene glycol and unreacted alcohols. Specifically, the liquid composition may optionally undergo additional steps of fractionation, adsorption purification, and concentration, which will be described below, prior to the hydrolysis reaction, to remove (or recover) ethylene glycol and unreacted alcohols; thus, the liquid composition may be free of ethylene glycol and unreacted alcohols (e.g., the content of ethylene glycol and unreacted alcohols is 0% each on a molar basis).

[0074] In other words, according to the present invention, the method for preparing terephthalic acid may further include fractionation of the liquid composition prior to the hydrolysis reaction described below. Fractionation can be carried out using commonly known fractionation methods, through which unreacted alcohols and ethylene glycol as a byproduct can be removed and recovered. In this case, the recovered unreacted alcohols can be reused as feedstock for the alcoholysis reaction, and the ethylene glycol can be reused as a polymerization feedstock for polyester resins or for other processes, thereby ensuring processability (economic feasibility). Simultaneously, byproducts such as diethylene glycol can also be removed by fractionation.

[0075] Furthermore, according to the present invention, the method for preparing terephthalic acid may further include adsorption purification of the liquid composition prior to the hydrolysis reaction described below. Specifically, adsorption purification may be carried out using at least one adsorbent selected from the group consisting of activated carbon, silica gel, alumina, zeolite, and activated clay, or by bed adsorption.

[0076] The adsorbent can specifically be activated carbon, or a mixture of activated carbon and silica gel. For example, the adsorbent can be a mixture of activated carbon and silica gel in a weight ratio of 1:0.5 to 1.5, or 1:0.8 to 1.2.

[0077] The amount of adsorbent added is not particularly limited, but it can be 0.1% to 20%, 0.1% to 18%, 0.2% to 15%, 0.2% to 10%, 0.3% to 5%, or 0.3% to 2% by weight based on the total weight of the liquid composition. Since adsorption purification occurs simultaneously with the addition of the adsorbent to the liquid composition in the above amounts, insoluble impurities such as metals, or impurities derived from waste polyester such as colorants and pigments, can be effectively removed. As a result, terephthalic acid with excellent purity and quality can be prepared.

[0078] Furthermore, according to the present invention, the method for preparing terephthalic acid may further include concentrating the liquid composition prior to the hydrolysis reaction described below. Specifically, once the liquid composition has been adsorbed and purified, a concentration step can be further performed. The concentration step can be carried out using commonly known concentration methods, through which unreacted alcohols and ethylene glycol as a byproduct can be removed and recovered.

[0079] Specifically, the concentration step can be carried out by stirring the adsorption-purified liquid composition at a temperature of 55°C to 115°C, 60°C to 110°C, 65°C to 105°C, or 75°C to 100°C for 1 to 5 hours, 1.5 to 4 hours, or 2 to 4 hours, and then filtering it.

[0080] According to Equation A below, the pigment residue (%) of the adsorption-purified liquid composition can be 15% or less, 13% or less, 11% or less, 10% or less, 8% or less, 6% or less, 5.5% or less, 5% or less, 4.3% or less, or 4% or less.

[0081] [Equation A]

[0082] In Equation A, A1 is the area of ​​the absorbance curve obtained using a UV-vis spectrophotometer at 400 to 800 nm after diluting the adsorption-purified liquid composition to a concentration of 5% in dimethyl sulfoxide (DMSO), dimethylformamide (DMF), or methylpyrrolidone (NMP).

[0083] A2 is the area of ​​the absorbance curve obtained for the unadsorbed and unpurified liquid composition in the same manner as described above.

[0084] According to the present invention, the filtration time of the liquid composition through a filter with a pore size range of 0.5 µm or smaller can be 30 minutes or less, specifically 20 minutes or less, 10 minutes or less, 5 minutes or less, 3 minutes or less, or 1 minute or less, thereby the yield and processability of terephthalic acid can be significantly superior.

[0085] Step (4): Hydrolysis reaction

[0086] According to the present invention, step (4) is a step of hydrolyzing the liquid composition. Specifically, the liquid composition can be hydrolyzed to produce terephthalic acid (recycled terephthalic acid).

[0087] Hydrolysis can be carried out by adding water to the liquid composition. Specifically, the hydrolysis reaction can be carried out by adding water to the liquid composition at temperatures of 180°C to 280°C, 185°C to 280°C, 200°C to 275°C, 220°C to 270°C, or 240°C to 265°C for 0.5 to 24 hours, 1 to 20 hours, 2.5 to 12 hours, or 3 to 8 hours.

[0088] The weight ratio of the liquid composition to water can specifically be 1:1 to 500, 1:1 to 450, 1:1 to 400, 1:1 to 250, 1:1 to 100, 1:1 to 50, 1:1.2 to 20, or 1:1.5 to 10.

[0089] A catalyst may or may not be added to the hydrolysis reaction. When the hydrolysis reaction is a non-catalytic reaction without a catalyst, the process of removing insoluble metals can be omitted, thus ensuring environmental friendliness while producing high-purity terephthalic acid. Furthermore, when the hydrolysis reaction is a catalytic reaction with a catalyst, the activity of the hydrolysis reaction increases, thereby improving processability (economic feasibility).

[0090] The catalyst added to the hydrolysis reaction can be a metal acetate, alkali metal salt, hydroxyl salt, etc. Specifically, the catalyst may contain at least one cation selected from the group consisting of alkali metal ions such as Li. + Na + K + and Cs + Alkaline earth metal ions such as Be 2+ Mg 2+ Ca 2+ and Ba 2+ NH 4+ and Zn 2+ ; and / or at least one anion selected from the group consisting of: OH- - OR -HCO3 - CO3 2- Benzoate ions (C7H5O2) - ), 4-alkoxycarbonylbenzoate ion, acetate ion and terephthalate ion.

[0091] For example, the catalyst may contain at least one selected from the group consisting of: Zn(OAC)2, Co(OAc)2, Mn(OAc)2, Mg(OAc)2, Ca(OAc)2, Pb(OAc)2, Ba(OAc)2, LiOAc, NaOAc, KOAc, Pd(OAc)2, Zn(OAC)2·2H2O, Co(OAc)2·4H2O, Mn(OAc)2·4H2O, Mg(OAc)2·4H2O, Ti(OBu)4, Ti(OiPr)4, GeO2, Al(OiPr)3, Na2CO3, K2CO3, dibutyltin oxide (IV), stannous octoate, titanium phosphate, and terephthalic acid.

[0092] Based on the total weight of the liquid composition, the amount of catalyst added can be 15 to 8,000 ppm, 20 to 5,500 ppm, 30 to 3,000 ppm, 50 to 1,600 ppm, 100 to 1,200 ppm, 150 to 1,100 ppm, 300 to 1,000 ppm, 350 to 950 ppm, 400 to 850 ppm, 420 to 700 ppm, or 450 to 650 ppm.

[0093] According to embodiments of the present invention, solid terephthalic acid, as a polymerization raw material, can be prepared by a hydrolysis reaction. Specifically, the method may further include filtering, washing, and drying the hydrolysis product prepared by the hydrolysis reaction after the hydrolysis step. That is, the hydrolysis product prepared by the hydrolysis reaction can be filtered, washed, and dried to produce solid terephthalic acid as a polymerization raw material.

[0094] For example, the hydrolysis product can be cooled to a suitable temperature at which water will not vaporize (e.g., from room temperature to below 100°C) to obtain a solution in slurry form, the solution can be filtered to obtain a solid, the solid can be washed and dried under vacuum to obtain solid terephthalic acid.

[0095] Washing can be performed using a mixture of alcohols and / or water having four or more carbon atoms, protic solvents such as isopropanol and acetic acid, or aprotic solvents such as acetone, dichloromethane, chloroform, tetrahydrofuran (THF), and toluene.

[0096] Washing effectively removes residual pigments or impurities produced during pigment decomposition, especially yellow impurities. As a result, the yellowness index or chromaticity characteristics can be enhanced. Furthermore, the use of water for washing removes inorganic salts, thereby improving the quality of the terephthalic acid prepared therefrom.

[0097] The yield of terephthalic acid can specifically be 65% or more, 68% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, or 95% or more.

[0098] Terephthalic acid (recycled terephthalic acid)

[0099] The terephthalic acid according to the present invention can be prepared by a method for preparing terephthalic acid.

[0100] According to the present invention, when measured by inductively coupled plasma atomic emission spectrometry (ICP-AES), terephthalic acid may have a total metal content of less than 100 ppm, 90 ppm or less, 80 ppm or less, 65 ppm or less, 50 ppm or less, 35 ppm or less, less than 30 ppm, 15 ppm or less, 9 ppm or less, 7 ppm or less, 5 ppm or less, or 1 ppm or less.

[0101] Furthermore, when measured by inductively coupled plasma atomic emission spectrometry (ICP-AES), the total content of Sb, Ti and Zn in terephthalic acid can be less than 30 ppm, 25 ppm or less, 20 ppm or less, 15 ppm or less, 10 ppm or less, 5 ppm or less, 3 ppm or less, or 1 ppm or less.

[0102] Specifically, when measured by inductively coupled plasma atomic emission spectrometry (ICP-AES), the Sb content in terephthalic acid can be 30 ppm or less, 25 ppm or less, 20 ppm or less, 15 ppm or less, 10 ppm or less, 5 ppm or less, 3 ppm or less, or 1 ppm or less, based on the total weight of terephthalic acid.

[0103] Furthermore, when measured by inductively coupled plasma atomic emission spectrometry (ICP-AES), the Ti content in terephthalic acid can be 30 ppm or less, 25 ppm or less, 20 ppm or less, 15 ppm or less, 10 ppm or less, 5 ppm or less, 3 ppm or less, or 1 ppm or less, based on the total weight of terephthalic acid.

[0104] Furthermore, when measured by inductively coupled plasma atomic emission spectrometry (ICP-AES), the Zn content in terephthalic acid can be 30 ppm or less, 25 ppm or less, 20 ppm or less, 15 ppm or less, 10 ppm or less, 5 ppm or less, 3 ppm or less, or 1 ppm or less, based on the total weight of terephthalic acid.

[0105] Meanwhile, when measured with a colorimeter, terephthalic acid can have a colorimetric index (b) of less than 2, 1.6 or less, 1.4 or less, 1.3 or less, or 1 or less. These ranges of colorimetric index (b) are comparable to those of raw terephthalic acid typically produced in petrochemical processes. When terephthalic acid meets these colorimetric index (b) ranges, it not only has a low yellowness index but also exhibits excellent quality due to proper purification.

[0106] Chromaticity b is a chromaticity coordinate system established by the International Commission on Illumination (CIE), where chromaticity is represented by L (lightness), a (green to red complementary color), and b (yellow to blue complementary color). It can be measured using a colorimeter.

[0107] Furthermore, when terephthalic acid is diluted to a concentration of 5% in dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and methylpyrrolidone (NMP) and measured, the yellowness index (YI) can be less than 2, 1.8 or less, or 1.7 or less. In methods for preparing terephthalic acid, the yellowness index of the terephthalic acid produced through purification steps, or purification and concentration steps, can be measured.

[0108] Polyester resin and its preparation method

[0109] The polyester resin according to the present invention is prepared using the above-mentioned terephthalic acid. Specifically, the polyester resin comprises a component derived from the above-mentioned terephthalic acid and a component derived from a diol compound.

[0110] For example, the diol component may specifically include at least one selected from the group consisting of: ethylene glycol, diethylene glycol, 1,4-butanediol, 1,3-propanediol, 1,4-cyclohexanediol, neopentyl glycol, and isosorbide.

[0111] Furthermore, the polyester resin may further comprise a component derived from a dicarboxylic acid compound. Specifically, the dicarboxylic acid compound may comprise at least one selected from the group consisting of: isophthalic acid (IPA), 2,6-naphthalenedicarboxylic acid (2,6-NDA), dimethyl terephthalate (DMT), dimethyl isophthalate (DMI), and dimethyl 2,6-naphthalenedicarboxylic acid (2,6-NDC).

[0112] The method for preparing polyester resin according to the present invention includes esterification of a polymeric raw material composition, wherein the polymeric raw material composition is mixed with terephthalic acid, a diol compound and / or optionally a dicarboxylic acid compound; and polycondensation of the esterification product.

[0113] Esterification can be carried out at temperatures ranging from 200°C to 350°C, 220°C to 320°C, or 250°C to 290°C. Furthermore, esterification can be carried out at pressures above atmospheric pressure, from 0 to 10 kg / cm². 2 (0 to 7,355.6 mmHg), 0 to 5 kg / cm 2 (0 to 3,677.8 mmHg), or 0 to 2.0 kg / cm² 2 The reaction is carried out under pressure of 0 to 1,471.1 mmHg. Furthermore, the esterification reaction can be carried out for 1 to 24 hours, 1 to 10 hours, or 1 to 6 hours.

[0114] The polycondensation reaction can be carried out at temperatures ranging from 150°C to 400°C, 200°C to 370°C, 250°C to 350°C, or 270°C to 300°C. Furthermore, the polycondensation reaction can be carried out under reduced pressures of 0.01 to 400 mmHg, 0.05 to 100 mmHg, or 0.1 to 100 mmHg. The polycondensation reaction can proceed for the desired time until the desired intrinsic viscosity is reached. Specifically, it can proceed for 1 to 24 hours, 1 to 10 hours, or 1 to 4 hours.

[0115] At least one additive selected from the group consisting of antioxidants, branching agents, colorants, crystallizers, catalysts, stabilizers, and ultraviolet absorbers may be fed into the esterification and / or polycondensation reactions.

[0116] Antioxidants are not particularly limited, but may include at least one of the following: hindered phenol-based compounds, phosphite-based compounds, and thioether-based compounds.

[0117] Branching agents can be compounds having three or more functional groups. Specifically, they can contain at least one selected from the group consisting of trimellitic anhydride, trimellitic acid, pyromellitic dianhydride, glycerol, trimethylolpropane, pentaerythritol, citric acid, tartaric acid, and 3-hydroxyglutaric acid.

[0118] The colorant is not particularly limited, but it may contain at least one compound selected from the group consisting of: cobalt-based compounds, anthraquinone-based compounds, perylene-based compounds, azo-based compounds, and methine-based compounds. Specifically, the colorant may be cobalt acetate, cobalt propionate, PolysynthrenBlue RLS tint from Clariant, and Solvaperm Red BB tint from Clariant.

[0119] The catalyst is not particularly limited, but it may contain methoxides of sodium and magnesium; acetates, borates, fatty acid salts, and carbonates of Zn, Cd, Mn, Co, Ca, and Ba; or oxides or hydrates of Mg, Pb, Mn, Ti, Zn, Sb, Sn, Al, and Ge. Specifically, the catalyst may be tetraethyl titanate, acetyltripropyl titanate, tetrapropyl titanate, tetrabutyl titanate, 2-ethylhexyl titanate, octyl glycol titanate, triethanolamine titanate, acetylacetone titanate, ethyl acetoacetate titanate, isostearyl titanate, titanium dioxide, germanium dioxide, germanium tetrachloride, germanium glycolate, germanium acetate, or combinations thereof.

[0120] Stabilizers are not particularly restricted, but they may contain phosphorus-based compounds such as phosphoric acid, trimethyl phosphate, and triethyl phosphate.

[0121] Furthermore, if necessary, the method for preparing polyester resin according to the present invention may further include: subjecting the polymer product obtained by polycondensation reaction to a solid-state polymerization reaction in order to control the intrinsic viscosity (IV), molecular weight, etc., of the polymer product. The solid-state polymerization reaction conditions are not particularly limited and can be appropriately set according to the desired intrinsic viscosity, molecular weight, etc., of the polyester resin.

[0122] The invention will be described in more detail below with reference to embodiments. However, these examples are provided for illustrative purposes only, and the invention is not limited thereto.

[0123] Embodiments of the present invention

[0124] [Example 1]

[0125] Step (1): Washing of waste polyester

[0126] Waste polyethylene terephthalate (PET) bottles are fed into a crusher to produce waste PET flakes with dimensions ranging from 1 to 30 mm. The resulting waste PET flakes are then washed with water to obtain washed waste PET flakes.

[0127] Step (2): Preprocessing

[0128] One kg of washed waste PET flakes was immersed in a bath containing 1.5 kg of 1-butanol, and the waste PET flakes were pretreated at 100°C for 30 minutes to remove residual water from the waste PET flakes.

[0129] Step (3): Alcohololysis

[0130] 1 kg of pretreated waste PET flakes, 3.3 kg of 1-butanol, and 1 g (1,000 ppm relative to the total weight of waste PET) of Zn(OAC)₂·2H₂O (catalyst) were charged into a first high-pressure reactor with a capacity of 7 liters. Next, with all connections of the first high-pressure reactor tightened and sealed, the temperature of the reactor was raised to 250°C over 1 hour, and the alcoholysis reaction was carried out under stirring for 3 hours while maintaining a temperature of 250°C and a pressure of 24 bar. After the alcoholysis reaction was completed, the reactor was cooled to room temperature and concentrated under reduced pressure to remove the 1-butanol and ethylene glycol (EG) formed during the reaction. Insoluble substances and solid products contained in the waste polyethylene terephthalate (PET) were removed by reduced-pressure filtration to obtain a liquid composition.

[0131] Step (4): Hydrolysis

[0132] 60 g (0.22 mol) of liquid composition, 240 g (13.3 mol) of water, and 12 mg (based on a total weight of 200 ppm of the concentrated liquid composition) of Zn(OAC)₂·2H₂O (catalyst) were charged into a second high-pressure reactor with a capacity of 600 ml. Next, the temperature of the second high-pressure reactor was raised to 260°C and maintained at 260°C for 4 hours for hydrolysis. The reactor was then cooled to 90°C to obtain a slurry of the hydrolysis product. The slurry was filtered to obtain a solid, which was washed with 1-butanol and water at 90°C and dried under vacuum to obtain solid terephthalic acid (TPA).

[0133] [Example 2]

[0134] Solid terephthalic acid (TPA) was obtained using the same procedure as in Example 1, except that 1 kg of washed waste PET flakes was placed in a container and 1-butanol was sprayed into the container containing the waste PET flakes at a rate of 0.05 kg / min for 30 minutes to pretreat the waste PET flakes.

[0135] [Example 3]

[0136] Solid terephthalic acid (TPA) was obtained using the same procedure as in Example 1, except that 1-pentanol was used instead of 1-butanol for pretreatment of waste PET sheets, and 1-pentanol was used for alcoholysis.

[0137] [Comparison Example 1]

[0138] Solid terephthalic acid (TPA) was obtained using the same procedure as in Example 1, except that the step of pretreating the washed waste PET sheets with 1-butanol was omitted.

[0139] [Comparative Example 2]

[0140] Solid terephthalic acid (TPA) was obtained using the same procedure as in Example 1, except that toluene was used instead of 1-butanol to pretreat the waste PET sheets.

[0141] [Test Example 1]

[0142] The components contained in the liquid compositions obtained by alcoholysis were identified by NMR analysis (JEOL analytical equipment). The results are shown in Table 1 below.

[0143] [Table 1]

[0144] [Test Example 2]

[0145] The liquid compositions obtained by alcoholysis and vacuum concentration were filtered under reduced pressure to confirm the filtration rate (time). The results are shown in Table 2 below. Specifically, the reduced pressure was kept constant at 30 Torr, and the filter used was a product of Sigma-Aldrich (ZapCap-CR, PTFE membrane, 0.45 μm pore size). In addition, the time required to filter the entire volume of each liquid composition obtained by vacuum concentration after alcoholysis was measured.

[0146] [Table 2]

[0147] [Test Example 3]

[0148] The filtrate obtained from Test Example 2 was dissolved in 10 ml of dimethyl sulfoxide (DMSO) and then analyzed by HPLC. The TPA content in the filtrate was confirmed by substituting it into a pre-calculated calibration curve using pure TPA (purity > 99.9%). Subsequently, the yield of terephthalic acid (the amount of TPA produced in the hydrolysis reaction) was calculated according to Equation 1 below. The results are shown in Table 3 below.

[0149] [Equation 1]

[0150] Yield (%) = {Amount of filtrate (g) × TPA content in filtrate (%) / Molecular weight of terephthalic acid (g / mol)} / {Mass of polyethylene terephthalate (g) / Molecular weight of polyethylene terephthalate (g / mol)}

[0151] [Table 3] .

Claims

1. A method for preparing terephthalic acid, comprising: (1) Wash the waste polyester with a washing solvent; (2) The washed waste polyester was pretreated with an alcohol having 4 or more carbon atoms; (3) The pretreated waste polyester is subjected to alcoholysis to produce a liquid composition; as well as (4) Hydrolyze the liquid composition.

2. The method for preparing terephthalic acid according to claim 1, wherein, The alcohol contains 4 to 8 carbon atoms.

3. The method for preparing terephthalic acid according to claim 1, wherein, The pretreatment is carried out by impregnating the waste polyester in the alcohol or spraying the alcohol onto the waste polyester.

4. The method for preparing terephthalic acid according to claim 3, wherein, The impregnation is carried out at 150°C or lower.

5. The method for preparing terephthalic acid according to claim 1, wherein, The terephthalic acid content in the liquid composition is 10% or less by weight based on the total weight of the liquid composition.

6. The method for preparing terephthalic acid according to claim 1, wherein, The filtration time for this liquid composition through a filter with a pore size range of 0.5 µm or smaller is 30 minutes or less.

7. The method for preparing terephthalic acid according to claim 1, wherein, The alcoholysis is carried out by adding an alcohol with four or more carbon atoms to the waste polyester.

8. The method for preparing terephthalic acid according to claim 1, wherein, The liquid composition comprises a compound represented by Formula 1: [Formula 1] In Formula 1, R1 is an alkyl group having four or more carbon atoms.

9. The method for preparing terephthalic acid according to claim 1, wherein, The yield of terephthalic acid is 70% or more.

10. A terephthalic acid prepared by any one of claims 1 to 9.

11. A polyester resin comprising a component derived from terephthalic acid according to claim 10.

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