JPH07309808A - Recovery of dimethyl terephthalate - Google Patents

Abstract

PURPOSE:To provide an efficient process for recovering the starting material for polyester fiber in the form of dimethyl terephthalate from a waste solution after the alkali thinning treatment of polyester fiber. CONSTITUTION:The terephthalic acid obtained by mineral acid precipitation of dialkali terephthalate from alkali thinning treatment of polyester fiber, poly(ethylene terephthalate), its oligomer or a mixture thereof with the polymer are fed into a reaction vessel and methanol is introduced into the vessel in an amount of 2-15 pts.wt. per l pt.wt. of the feed, as at least a part of the feed is kept molten, to conduct the reactions at 265-900 deg.C under a pressure of 1-15kg/cm<2>. The fraction containing the dimethyl terephthalate thus formed, ethylene glycol and the water generated by the reactions are evaporated along with unreacting methanol synchronously as the reactions proceed and separated from the unevaporating residue to recover dimethyl terephthalate. In order to immobilize the mineral acid residue in the recovered terephthalic acid, preferably, an adequate amount of an alkali carbonate is added to the reaction mixture prior to the reactions.

Description

Detailed Description of the Invention

[0001]

The present invention relates to acid precipitation, separation and recovery from an alkaline aqueous solution (hereinafter abbreviated as alkaline waste solution) containing a dialkali salt of terephthalic acid generated when a polyester fiber is reduced in weight with an alkaline solution. The present invention relates to a method of treating terephthalic acid (hereinafter abbreviated as TA) with methanol together with polyester scraps and recovering it as dimethyl terephthalate (hereinafter abbreviated as DMT).

[0002]

2. Description of the Related Art As a conventional example in which TA recovered by acidifying an alkali waste liquid and recovering it is treated with methanol and recovered as DMT, there is a method described in JP-A-60-163844. However, this method
The ash content is limited to 1200 ppm or less.

[0003]

The TA recovered by acid precipitation contains a large amount of inorganic impurities such as titanium oxide and metal salts. That is, the recovered T having an ash concentration of more than 1200 ppm
Most of A is presently treated as waste or incineration. Furthermore, these recovered TAs contain mineral acids such as sulfuric acid used in the acid precipitation treatment, and some of them can be removed even by washing with water as heavy metal sulfates derived from the polymerization catalyst. It exists as something that does not exist. These sulfuric acids and heavy metal sulfates are converted into corrosive gases such as dimethylsulfate and monomethylsulfate when treating the recovered TA with methanol, and diffuse not only to the reactor but also to auxiliary equipment such as a distillation column. Therefore, there is a risk that troubles such as corrosion of the entire plant may occur. It also affects the quality of recovered DMT and the like.

The present invention seeks to solve these difficult problems.

[0005]

Means for Solving the Problems As a result of intensive studies as a method for solving such a difficult problem, the present inventors have found the following effective means and arrived at the present invention.

That is, TA obtained by acidifying an alkaline waste liquid with a mineral acid is supplied to a reactor together with polyethylene terephthalate (hereinafter abbreviated as PET) containing a low polymer, and at least a part thereof is melted. A method of collecting DMT by blowing methanol into the solution and distilling a fraction containing DMT, ethylene glycol, and water generated by the reaction through reaction and evaporating together with unreacted methanol at the same time as the reaction, and separating from a residue that does not evaporate is there. Evaporating at the same time as the reaction means that TA or PET is reacted with blown methanol, and DMT produced by esterification or depolymerization reaction, ethylene glycol, and water generated by reaction are steam-distilled by an unreacted methanol in excess. It is to let out with methanol from the upper part of the reactor. At this time, the residual mineral acid in the acid-deposited TA is fixed in the reaction residue so that the mineral acid used in the acid-deposition does not escape into the mixed fraction such as DMT, ethylene glycol, water generated by the reaction and unreacted methanol. This is a method of performing the above-mentioned reaction by adding sufficient alkali carbonate for conversion.

Further, the contents of the present invention will be described in detail. The present invention is directed to (1) terephthalic acid obtained by acidifying a dialkaline terephthalic acid salt produced when a polyester fiber is weight-reduced with an alkaline solution, and (2) polyethylene terephthalate, a low polymer or a polymer. And a mixture of a low polymer to a reactor, while introducing at least a part of the feedstock in a molten state, introducing methanol into the melt of (3) (1) or / and (2). Then, it is reacted with methanol, and (4) the produced dimethyl terephthalate, ethylene glycol and the water generated by the reaction are evaporated together with the unreacted methanol at the same time as the reaction, and (5) not evaporated. It is a method of recovering dimethyl terephthalate, which consists of separating it into a residue.

A feature of the present invention is, for example, TA obtained by acidifying an alkaline waste liquid with a mineral acid in a reactor equipped with a distillation column, and waste PET, a low polymer or a mixture of a polymer and a low polymer. And superheated methanol is introduced into the melt and brought into contact with the melt efficiently while keeping at least a part of the melt in a molten state, and the esterification reaction of TA and the depolymerization reaction of PET are carried out. The DMT and ethylene glycol produced at the same time as the above-mentioned step are distilled outside the reactor through a distillation column together with unreacted methanol on the principle of so-called steam distillation.

Therefore, in the present invention, the collected TA and the waste PET, the low polymer or the mixture of the polymer and the low polymer, which are supplied as the raw materials, are mixed with the ash component such as titanium oxide, the dye and the general waste. It doesn't matter.
In addition, the ratio of TA in the raw material used for the reaction is 35% by weight.
The following range is preferable. If it exceeds 35% by weight, the viscosity of the raw material increases remarkably, and its handling becomes difficult.

Further, another feature of the present invention is that it shows a countermeasure against the mineral acid contained in TA recovered from the alkaline waste liquid. The mineral acids and their heavy metal salts, such as sulfuric acid and heavy metal sulfates, contained in these TAs are transformed into corrosive gases such as dimethylsulfate and monomethylsulfate when treating the recovered TA with methanol, and are distilled not only in the reactor. Spread to incidental equipment such as towers. Therefore, there is a risk that troubles such as corrosion of the entire plant including the reactor will occur. In addition, the quality of the recovered DMT and the like is affected to a considerable extent.

As a solution to this problem, the inventors of the present invention add an equivalent amount or more of alkali carbonate sufficient to immobilize the residual mineral acid in the recovered TA used by mixing with PET as a raw material to the TA before the reaction. It was found that the mineral acid used in the acid precipitation did not escape into the mixed fraction of DMT, ethylene glycol, water produced by the reaction and unreacted methanol obtained at the same time as the reaction even in the form of a low boiling point substance. That is, by adding alkali carbonate sufficient to fix the residual mineral acid in the recovered TA in the reaction residue to the recovered TA and then performing the reaction, the sulfate radicals are fixed in the residual residue in the reactor. It can be discarded out of the system together with the ash component and high boiling point component of. At present, the mineral acid used for the acid precipitation treatment of the alkaline waste liquid is generally sulfuric acid, and if the alkali carbonate for fixing it is inexpensive sodium carbonate, its effect is great.

The amount of methanol used in the present invention introduced into the reactor is in the range of 2 to 15 parts by weight, preferably 3 to 8 parts by weight, relative to 1 part by weight of the feedstock. That is,
If the amount of methanol introduced is less than 2 parts by weight, the depolymerization reaction of PET and the esterification reaction rate of TA and the amount of distillate of DMT are reduced, which is not preferable in terms of efficiency. On the other hand, if the amount exceeds 15 parts by weight, the amount of energy required for evaporation and heating of the introduced methanol increases, and the operating cost increases, which is not preferable. The temperature of the methanol introduced into the reactor may be a temperature at which the melt of the reaction system does not solidify, but 200
To 300 ° C is preferable.

In the present invention, the internal temperature of the reactor, that is, the reaction temperature is preferably 265 to 300 ° C. so that at least a part thereof is in a molten state, and more preferably 270 to 2
90 ° C. That is, if the reaction temperature is less than 265 ° C., the fluidity of the contents in the reactor may decrease, and efficient contact with methanol may become impossible. Therefore, the depolymerization reaction of PET and the esterification reaction rate of TA may be reduced. And the amount of distillate of DMT decreases. On the other hand, when the reaction temperature exceeds 300 ° C., on the contrary, the polymerization reaction and the thermal deterioration and thermal decomposition of the contents in the reactor are likely to occur, and the quality such as coloring of the recovered DMT is deteriorated and the yield of ethylene glycol is also decreased. .

The reaction pressure in the present invention is 1 to 15 kg.
/ Cm ² G. That is, when it exceeds 15 kg / cm ² G, DMT distilled out of the reaction system along with unreacted methanol.
Of less than 1 kg / cm ² G is not preferable because the depolymerization reaction of PET and the esterification reaction rate of TA are low. Further, by introducing an inert gas such as nitrogen gas together into the reaction system in order to accelerate the distillation of DMT and ethylene glycol, there is an advantage that the effect of the present invention is expanded.

In the present invention, since the alkali carbonate to be added and the polymerization catalyst contained in the raw material PET substantially act as a depolymerization / esterification reaction catalyst, a new catalyst is not always necessary, but general depolymerization is required. -Ester reaction catalyst can be used. Examples thereof include zinc, tin, titanium, antimony, manganese, cobalt or lead oxides and acetates.

The reactor used in the method of the present invention is, for example, (A) a tank reactor equipped with a distillation column and equipped with a stirring device,
Further, for example, a tower reactor in which several stages of multi-stage perforated trays and the like are installed inside (B) can be mentioned.

Further, in the present invention, for example, two reactors are connected in series, and a raw material obtained by esterifying or depolymerizing a part of the raw material with methanol or ethylene glycol in the first reactor is fed to the next distillation column. It is supplied to the attached second reactor, and progresses the esterification and depolymerization reaction with methanol, and at the same time, the produced DMT and ethylene glycol are passed through a distillation column attached to the second reaction tank together with unreacted methanol. It can also be distilled out of the system.

Hereinafter, the contents of the present invention will be described in more detail with reference to examples.

[0019]

Example 1 70 g of waste PET, 25.7 g of a water-washed dried product of acid-deposited TA having an ash content of 2000 ppm and 0.5 g of sodium carbonate were added, and the mixture was charged into a 300 ml stirring autoclave, mixed and melted at 280 ° C., and dip into the melted raw material. The methanol vapor superheated to 250 ° C. was blown at a rate of 320 g per hour from the blowing nozzle, and reacted at 280 ° C. and 8.0 kg / cm ² G for 2.0 hours. At the same time, methanol was recovered by distillation from the fraction containing methanol, and the remaining 113 g of DMT was 81.0% by weight, methyl-, hydroxyethyl-terephthalate (hereinafter abbreviated as MHET) 2.7% by weight and ethylene. Glycol 16.
A mixture A containing 2% by weight was obtained. About 4g in the reactor
Remained, but this residue was dissolved in ethylene glycol and could be easily washed. When this result is expressed as a yield with the molar amount of the raw material unit present in the feedstock as the denominator, DM
It reached 92 mol% for T and about 83% for EG.

Even if this reaction was repeated several tens times under the same conditions, no scale that did not dissolve in ethylene glycol adhered to the inner wall of the reactor.

[0021]

Example 2 70 g of waste PET, a water-washed dried product of acid-deposited TA (sulfate ion concentration 2610 ppm, ash content 6000 pp
m) 25.7 g and sodium carbonate 2 g were added to 300 m
It was charged into an agitated autoclave (1), mixed and melted at 280 ° C, and 1 ml of methanol vapor superheated to 250 ° C from a blowing nozzle dipped into the melted raw material.
280 ℃ while blowing at a rate of 320g per hour,
The reaction was carried out for 2.0 hours under the condition of 8.0 kg / cm ² G.
80% of the remaining 110 g of DMT obtained by recovering methanol by distillation from the fraction containing methanol distilled at this point was obtained.
A mixture A containing 1% by weight, 3.0% by weight of methyl-, hydroxyethyl-terephthalate (hereinafter abbreviated as MHET) and 16.0% by weight of ethylene glycol was obtained. About 7 g of residue was obtained in the reactor. Expressing this result as a yield with the molar amount of the raw material unit present in the feedstock as the denominator, 88 mol% for DMT and EG
In the case of, it reached about 80%.

The total value (as SO ₄ ions) of dimethylsulfate, monomethylsulfate, sulfate ions, etc. in this mixture A was 1 ppm or less. The total value of dimethylsulfate, monomethylsulfate, sulfate ions and the like (as SO ₄ ions) in the recovered methanol fraction was 0.1 ppm or less (below the detection lower limit).

[0023]

Example 3 The reaction was carried out under the same conditions as in Example 1 except that 2.0 g of sodium carbonate was not added. 82.1% by weight of the remaining 105 g of DMT recovered by distillation of methanol from the fraction containing methanol distilled at this point,
A mixture A containing 4.0% by weight of methyl-, hydroxy-ethyl-terephthalate (hereinafter abbreviated as MHET) and 13.0% by weight of ethylene glycol was obtained. About 5 g of partially carbonized residue remained in the reactor. Expressing this result as a yield using the molar amount of the raw material unit present in the feedstock as the denominator, 86 mol% for DMT and E
In the case of G, it was about 80%.

The total value (as SO ₄ ions) of dimethylsulfate, monomethylsulfate, sulfate ions, etc. in this mixture A was 7.4 ppm. The total value (as SO ₄ ions) of dimethylsulfate, monomethylsulfate, sulfate ions, etc. in the recovered methanol fraction was 3.7 ppm.

Claims (7)

[Claims] 1. A terephthalic acid obtained by subjecting (1) a dialkali salt of terephthalic acid produced when a polyester fiber is subjected to a weight reduction treatment to an alkaline solution to an acid, and (2) a polyethylene terephthalate, a low polymer or A mixture of a polymer and a low polymer is supplied to a reactor, and while maintaining at least a part of a feed material in a molten state, methanol is added to the melt of (3) (1) or / and (2). (4) Distilled fraction containing dimethyl terephthalate, ethylene glycol and water generated by reaction is evaporated at the same time as reaction with unreacted methanol, and (5) Evaporate and evaporation A method for recovering dimethyl terephthalate, which comprises separating it into residual substances. 2. The recovery method according to claim 1, wherein the residual mineral acid in terephthalic acid and an equivalent amount or more of alkali carbonate are added to terephthalic acid and reacted. 3. The recovery method according to claim 1, wherein the amount of methanol introduced into the reactor is 2 to 15 parts by weight with respect to 1 part by weight of the feedstock. 4. The recovery method according to claim 1, wherein the reaction temperature is 265 to 300 ° C. 5. The recovery method according to claim 1, wherein the reaction pressure is 1 to 15 kg / cm ² G. 6. The recovery method according to claim 1, wherein the mineral acid is sulfuric acid. 7. The recovery method according to claim 2, wherein the alkali carbonate is sodium carbonate.