JPH0134257B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for efficiently recovering a terephthalic acid component from a distillation residue obtained when glycol distilled off during the production reaction of a polyalkylene terephthalate polyester is distilled and regenerated.

Polyalkylene terephthalate polyesters, especially polyethylene terephthalate, have many excellent properties and are therefore used in large quantities in a wide range of fields such as fibers, films, and resins.

The method for producing polyethylene terephthalate generally involves reacting terephthalic acid or terephthalic acid dialkyl ester with excess glycol to form a low polymer, and then forming a polymer by distilling off the glycol at high temperature and under reduced pressure. ing. The glycol distilled off at this time is recycled by distillation and used again for polyester production. On the other hand, the distillation residue produced during this distillation regeneration is colored due to decomposition because it is exposed to high temperatures during distillation, and contains a large amount of decomposed products with a low degree of polymerization and inorganic foreign matter. Moreover, the distillation residue after sufficient distillation recovery of glycol contains less than 5% by weight of free glycol, and this distillation residue is generally incinerated or buried without being effectively utilized. Since the distillation residue contains a large amount of terephthalic acid component, attempts were made to recover it by depolymerizing it with glycol or alcohol. However, the recovery rate of the terephthalic acid component was low, resulting in many production problems such as blockages in the overpurification process and deterioration in product quality. If a large amount of glycol or alcohol is used for depolymerization in order to avoid such problems, not only will equipment efficiency decrease, but the recovery rate of the terephthalic acid component contained in the distillation residue will be low, and moreover, Problems such as poor sex remained unresolved.

When recovering the terephthalic acid component from such distillation residue, the present inventors conducted intensive studies on a recovery method that is efficient and does not cause the problems of the recovery process described above. We have found that it is extremely effective to do so, and have arrived at the present invention.

That is, the present invention involves mixing a distillation residue obtained when distilling and regenerating the glycol distilled off during the production reaction of a polyalkylene terephthalate-based polyester with polyester in an amount more than twice the weight of the distillation residue, and then decomposing the glycol. This is a method for recovering a terephthalic acid component.

The distillation residue referred to in the present invention is mainly the distillation residue of ethylene glycol generated from the polyester manufacturing process, typified by polyethylene terephthalate, but other polyesters, such as polytrimethylene terephthalate, polytetramethylene terephthalate, or two types It may also be a distillation residue of glycol produced from the process of producing a copolyester consisting of the above-mentioned carboxylic acid component and/or two or more types of glycol. In addition, since the commonly employed glycol distillation conditions are high temperature and reduced pressure, the distillation residue obtained contains less than 5% by weight of free glycol, a number average degree of polymerization of 4 to 6, and a softening point. At temperatures above 150°C, most of the terephthalic acid components are yellow substances containing 60% by weight or more of terephthalic acid.

The most important thing in the present invention is that the amount of polyester mixed with the distillation residue is at least twice the weight,
Preferably, it is three times the weight or more. If the amount of polyester is less than twice the weight, problems such as a decrease in the recovery rate of the terephthalic acid component and clogging of the process will occur. The polyester used for mixing here is generally a polyalkylene terephthalate represented by polyethylene terephthalate, but it may also be a copolymerized polyester having two or more types of carboxylic acid components and/or two or more types of glycols. Further, these polyesters can be used even if they are generated as scraps, and it is preferable to use scrap polyesters. The shape of the polyester during mixing may be fibrous, block, or powdery. When scrap polyester is used, it may be pretreated, such as washing, if necessary.

In the glycol decomposition of the mixture of distillation residue and polyester carried out in the present invention, a metal salt such as an alkali metal compound and/or an alkaline earth metal compound or an amine is added to accelerate the decomposition, and the internal temperature is raised to 150.
Carry out at high temperatures above ℃. The glycol used here is preferably ethylene glycol;
Further, other glycol components such as diethylene glycol may be mixed therein. The amount used is per mole of terephthalic acid component in the mixture.
It is desirable to use 1.5 times the mole or more, preferably 3 times the mole or more from the viewpoint of decomposition yield and permeability.

The reactants obtained by such ethylene glycol decomposition reaction are bis-β-hydroxyethyl terephthalate and its low polymer. These reactants can be purified by an overwashing step or, if necessary, a decoloring step, and then subjected to a polyester polycondensation reaction. Alternatively, the obtained reaction product can be converted into dimethyl terephthalate by transesterification with methanol, and can be used as a polyester raw material by filtering, washing, distillation or recrystallization.

Through the method described above, it has become possible to efficiently recover and reuse the terephthalic acid component from the glycol distillation residue generated from the polyester polymerization process, which was conventionally discarded as something that could not be recovered. .

Next, the present invention will be described in detail with reference to Examples. First, symbols and measurement methods used in Examples will be explained.

(a) Free ethylene glycol and free diethylene glycol in distillation residue A sample was dissolved in acetone and measured using gas chromatography (column packing material: 2% PEG20MON Chromosolve 102).

(b) Number average degree of polymerization of distillation residue Measured using gel permeation chromatography [column: polyethylene gel G-2000M8 (manufactured by Toyo Soda)].

(c) Terephthalic acid component Measured by dissolving the sample in an alkali and titrating with hydrochloric acid using phenolphthalein and bromophenolbul indicators.

(d) Conversion rate to dimethyl terephthalate The content of dimethyl terephthalate after the end of the transesterification reaction was measured by gas chromatography, and the conversion rate (%) was calculated using the following formula = dimethyl terephthalate after the end of the transesterification × 166 /194/Terephthalic acid component in distillation residue and polyethylene terephthalate x 100 (e) Overresistance Calculated using the following formula (Chemical Engineering Handbook 3rd Edition P930). It is necessary that the excess resistance is 10 ⁹ m/Kg or less.

α=2・△P・gc/K・μ・C However, α: Overresistance (m/Kg) △P: Differential pressure (Kg/m ² ) gc: Constant 9.8 (Kg・m/Kg・sec ² ) Use K: constant pressure over constant (m ² /sec)
[The slope of the straight line was read from the graph of (overtime)/(overspeed) vs. (overtime)] μ: Viscosity (Kg/m・sec) C: Slurry concentration (-) (f) Acid value of dimethyl terephthalate Dissolve dimethyl terephthalate by heating in a mixed solution of ethanol and xylene, and use a mixed indicator of phenol red and bromothymol blue.
It was measured by titration with alcoholic potassium hydroxide standard solution.

The acid value needs to be 0.05 KOH mg/g-dimethyl terephthalate or less.

(g) Hazen color Measured by melting dimethyl terephthalate at 175°C and comparing the color with Hazen standard solution.

The Hazen color value must be 20 or less.

Example 1 1.2 parts by weight of ethylene glycol distillation residue generated from the polyethylene terephthalate manufacturing process,
3.8 parts by weight of polyester filament waste and 6.2 parts by weight of ethylene glycol generated in the spinning process were placed in a decomposition tank equipped with a stirrer, a rectification column, and a condenser, and then 0.01 part by weight of sodium carbonate was added, and the mixture was heated at an internal temperature of 220°C. The mixture was heated for 2 hours to carry out a depolymerization reaction. The composition of the distillation residue at this time was 0.1% by weight of free ethylene glycol, 2% by weight of free diethylene glycol, a number average degree of polymerization of 5, and 70% by weight of the terephthalic acid component.

After the depolymerization reaction is complete, 1.6 parts by weight of excess ethylene glycol is distilled out of the system to lower the internal temperature, and after the internal temperature has decreased to 150°C, ethylene glycol is transferred to a methanol decomposition tank equipped with a total reflux condenser. The decomposition product was transferred. Next, 6 parts by weight of methanol and 0.02 parts by weight of sodium carbonate were added to the decomposition tank, and the mixture was heated for 2 hours while completely refluxing the methanol to cause a transesterification reaction to produce dimethyl terephthalate. Furthermore, dimethyl terephthalate was separated using a constant pressure filter, dried, and purified by distillation.

At the end of transesterification with methanol, the conversion rate to dimethyl terephthalate was 97%.
%, which was good. Next, when the obtained reaction product was filtered with a constant pressure filter, the excess resistance was 6×
The transient resistance was as low as 10 ⁸ m/Kg and good. Also,
The quality of dimethyl terephthalate obtained by distillation was also good, with an acid value of 0.022 KOH mg/g-dimethyl terephthalate and a Hazen color of 10.

Comparative Example 1 2 parts by weight of distillation residue, 2 parts by weight of polyester filament waste, and 0.02 parts by weight of sodium carbonate were heated to a reaction temperature.
A decomposition reaction was carried out at 230°C for 4 hours. The transesterification reaction was otherwise carried out under the same conditions as in Example 1.

After completion of the transesterification reaction, the conversion rate to dimethyl terephthalate decreased to 50%. In addition, since the particle size of the generated reactant was so fine, the overresistance in a constant pressure filtration machine was extremely high, 2×10 ¹¹ m/Kg, and filtration was not possible. Therefore, subsequent operations were impossible.

Example 2 0.8 parts by weight of distillation residue containing 0.2% by weight of free ethylene glycol, 3% by weight of free diethylene glycol, number average degree of polymerization of 4, and 60% by weight of terephthalic acid component, and 3.2 parts by weight of block-shaped polyethylene terephthalate waste generated before melt spinning. parts by weight, 6 parts by weight of ethylene glycol, and 0.02 parts by weight of sodium carbonate were mixed. Depolymerization, transesterification, and percolation/purification were otherwise carried out under the same conditions as in Example 1.

After completion of the transesterification reaction, the conversion rate to dimethyl terephthalate was 98%, and the specific resistance in a constant pressure filter was 6×10 ⁹ m/Kg, which was good. The quality of dimethyl terephthalate purified by distillation was also good, with an acid value of 0.021 KOH mg/g-dimethyl terephthalate and a Hazen color of 12.

Example 3 A glycol depolymerization reaction was carried out under the same conditions as in Example 1 except that 5.2 parts by weight of ethylene glycol was used in the glycol depolymerization reaction. After the depolymerization reaction was completed, 0.2 parts by weight of activated carbon was added without distilling off excess ethylene glycol, the reaction mixture was heated without cooling, and the residue was washed with 0.7 parts by weight of hot ethylene glycol. I did this. Note that the heat aging process was easily carried out without any problems. Next, the obtained ethylene glycol solution of the depolymerized product was transferred to a polycondensation reactor, and a polycondensation reaction was carried out at a final internal temperature of 285° C. and a degree of vacuum of 1 mmHg. The quality of the obtained polymer is [η]=
0.638, softening point 259.3°C, colL=70.3, and colb=9.2, which were good.

Comparative Example 2 Depolymerization was carried out under the same conditions as in Example 3, except that the distillation residue was 1.5 parts by weight and the polyester filament waste was 2.7 parts by weight. Then, activated carbon was added and heat aging was performed. was blocked and could not be tolerated. This made further operations impossible.

Claims (1)

[Claims] 1. Glycol decomposition after mixing a distillation residue obtained when distilling and regenerating the glycol distilled off during the production reaction of polyalkylene terephthalate polyester with polyester in an amount of at least twice the weight of the distillation residue. A method for recovering a terephthalic acid component, characterized by: 2. The method for recovering a terephthalic acid component according to claim 1, wherein the free glycol in the distillation residue is 5% by weight or less. 3. The method for recovering a terephthalic acid component according to claim 1, wherein the polyester is polyethylene terephthalate.