JPH07309810A - Recovery of dimethyl terephthalate - Google Patents

Abstract

PURPOSE:To provide an efficient process for recovering dimethyl terephthalate (DMT) from an alkaline waste solution after the alkali thinning treatment of polyester fiber. CONSTITUTION:(A) The terephthalic acid obtained by acidification precipitation of dialkali terephthalate resulting from the alkali thinning treatment of polyester fiber using a mineral acid (preferably sulfuric acid) and (B) p-xylene and/or methyl p-toluate are oxidized with a molecular oxygen-containing gas in the presence of a heavy metal catalyst. Then, the heavy by-product which is formed in the production of DMT by esterification of the oxidation reaction mixture with methanol is introduced into the reactor containing this reaction mixture and methanol is fed in an amount of 2-15 pts.wt., preferably 3-8 pts.wt. per 1 pt.wt. of the starting feed, as the mixture is kept at least partially molten, to conduct the reactions at 265-300 deg.C under a pressure of 1-15kg/cm<2>G. The fraction mainly containing the DMT thus formed is evaporated along with methanol to separate from the non-volatile residue and DMT is recovered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a terephthalate which is acid-deposited, separated and recovered from an alkali waste liquid (hereinafter abbreviated as alkali waste liquid) containing a dialkali salt of terephthalic acid generated when a polyester fiber is subjected to weight reduction treatment with an alkali solution. Acid (hereinafter abbreviated as TA) is a by-product having a higher boiling point than DMT generated in the so-called Witten-Hercules method (hereinafter abbreviated as WH method) dimethyl terephthalate (hereinafter abbreviated as DMT) production process. (Hereinafter abbreviated as RES) and a method of recovering as DMT by treating with methanol.

[0002]

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

[0003]

TA to be acid-deposited and recovered
Contains a large amount of inorganic impurities such as titanium oxide and metal salts. That is, most of the recovered TA has an ash concentration of more than 1200 ppm and is currently treated as waste or incineration. Further, these recovered TAs contain mineral acids such as sulfuric acid used in the acid precipitation treatment, and some of them are washed with water as heavy metal sulfates etc. due to heavy metals contained in the alkaline waste liquid. However, it exists as something that cannot be removed. 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.

[0004]

Means for Solving the Problems As a result of intensive investigations by the present inventors as a method for solving such problems,
The present invention has been accomplished by finding the following effective means.

That is, TA obtained by acidifying an alkaline waste liquid with a mineral acid is supplied to a reactor together with RES by-produced by the WH method, and at least a part of the TA is kept in a molten state. Methanol was blown into the touch fluid to react with it, and D formed
This is a method of recovering DMT by evaporating a fraction containing MT as a main component together with unreacted methanol at the same time as the reaction, and separating it from a residue that does not evaporate.

Further, according to the present invention, the acid-deposited T is used so that the mineral acid used in the acid-deposition does not escape into the DMT-containing fraction.
A method of performing the above reaction by adding sufficient alkali carbonate to immobilize the residual mineral acid in A in the reaction residue is included.

Further, the contents of the present invention will be described in detail. The present invention supplies (1) TA obtained by acidifying an alkaline waste liquid with a mineral acid and (2) RES, which is a by-product when producing DMT by the WH method, to a reactor, and supplies the RES. While maintaining the heating state so that at least a part of the raw material is in the molten state, (3)
Methanol was introduced into the melts of (1) and (2) to react with methanol, and (4) the produced DMT-based fraction was evaporated simultaneously with the reaction together with unreacted methanol. (5) A method for recovering DMT, which comprises separating from a residue that does not evaporate.

Here, the feedstocks refer to TA and RES supplied to the reactor.

A feature of the present invention is that, for example, TA and RES obtained by acidifying an alkaline waste liquid with mineral acid are supplied to a reactor equipped with a distillation column so that at least a part thereof is in a molten state. While maintaining the temperature of the melt, for example, superheated methanol is introduced into the melt to promote the esterification reaction of TA and the decomposition reaction of RES, and at the same time, the produced fraction containing DMT as a main component is subjected to so-called steam distillation. The principle is to distill out of the reactor through a distillation column together with unreacted methanol.

In the present invention, it is preferable that the RES to be used in the reaction is one in which the oxidation catalyst has already been extracted with water. On the other hand, the recovered TA used for the reaction may contain ash components such as titanium oxide, dyes, and general dust. That is, these items are separately disposed of together with the reaction residue. Further, the ratio of TA in the raw material used for the reaction is 50
A range of less than or equal to wt% is preferred. When it exceeds 50% by weight, the viscosity of the raw material is remarkably increased and the handling thereof becomes difficult.

Further, the feature of the present invention is to show a countermeasure against the mineral acid contained in TA recovered from the alkaline waste liquid. The mineral acids and heavy metal salts thereof, such as sulfuric acid and heavy metal sulfate, contained in these TAs are converted into corrosive gases such as dimethylsulfate and monomethylsulfate when treating the recovered TA with methanol, and if only the reactor is used. Without diffusion to auxiliary equipment such as distillation columns. 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 can add an equivalent amount or more of alkali carbonate sufficient for fixing residual mineral acid in the recovered TA used as a raw material mixed with RES before the reaction. It was found that the mineral acid used in the acid precipitation did not escape even in the form of a low-boiling substance in the mixed fraction containing DMT as a main component simultaneously with the reaction. That is, the mineral acid is immobilized as an alkali salt in the residue remaining in the reactor by reacting after adding sufficient alkali carbonate to immobilize the residual mineral acid in the acid-deposited TA in the reaction residue. It can be discarded out of the system along with other ash components and high boiling point components. 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 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 rate of esterification reaction of TA and the rate of decomposition of RES 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,
0 ° C is preferred.

In the present invention, the internal temperature of the reactor, that is, the reaction temperature, is 265 to 300 ° C. so that at least a part thereof is in a molten state. It is preferably 270 to 290 ° C. That is, if the reaction temperature is less than 265 ° C., the fluidity of the contents in the reactor may be lowered, and efficient contact with methanol may be impossible. Therefore, the esterification reaction rate of TA and the decomposition rate of RES are lowered. However, the amount of DMT distilled out 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,
This is not preferable because it causes a decrease in recovery rate.

In the present invention, the reaction pressure is preferably 1 to 15 kg / cm ² G. That is, 15 kg / cm ²
If it exceeds G, the amount of DMT distilled out of the reaction system along with unreacted methanol decreases, which is not preferable, and 1 kg / cm ²
If it is less than G, the esterification reaction rate of TA and the decomposition rate of RES become small, which is not preferable. Further, in order to accelerate the distillation of DMT and other active ingredients, it is also possible to introduce an inert gas such as nitrogen gas into the reaction system in combination to enhance the effect of the present invention.

In the present invention, the added alkali carbonate substantially acts as a RES decomposition / TA esterification reaction catalyst, so that a new catalyst is not always necessary.

The reactor used in the method of the present invention is, for example, (A) a tank reactor with a stirring device equipped with a distillation column,
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.

In the present invention, either a continuous or batch reaction type can be carried out.

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

[0020]

Example 1 100 g of RES (excluding cobalt and manganese as an oxidation catalyst by water extraction, containing 17% by weight of DMT), 25.7 g of water-washed dried product of acid-deposited TA having ash content of 2000 ppm and 0.5 g of sodium carbonate were added. 300m
It was charged into an autoclave (1) equipped with a stirrer and mixed and melted at 280 ° C. Thereafter, while blowing methanol vapor superheated to 250 ° C. at a rate of 320 g per hour from a blowing nozzle dipped in the molten raw material, the reaction was carried out at 280 ° C. under the condition of 8.0 kg / cm ² G for 2.0 hours. The remaining 96 g of a mixture obtained by distilling methanol from the fraction containing methanol distilled at this point (DMT 67.0 wt%, methyl paratoluate 4.9)
% By weight, 17.0% by weight of methyl P-hydroxybenzoate, 3.0% by weight of P-toluyl alcohol, and 7.8% by weight of others) were obtained. About 56 g of residue remained in the reactor, but no scale adhered to the inner wall of the autoclave.

[0021]

Example 2 R of the same lot used in Example 1
100 g of ES, 25.7 g of a water-washed dried product of acid-deposited TA (sulfate ion concentration of 2610 ppm, ash content of 6000 ppm) and 2 g of sodium carbonate were charged into a 300 ml autoclave equipped with a stirrer and mixed and melted at 280 ° C. After that, 25
280 ℃, 8.0k while blowing methanol vapor superheated to 0 ℃ at a rate of 320g per hour
The reaction was carried out for 2.0 hours under the condition of g / cm ² G. The remaining 87 g of mixture A (DMT7) obtained by recovering methanol by distillation from the fraction containing methanol distilled at this point
3.6. % By weight, 5.4% by weight methyl paratoluate,
Methyl P-hydroxybenzoate 13.0% by weight, P
2.3% by weight toluoyl alcohol, and 5.7 other
Wt%) was obtained. About 60 g of residue remained in the reactor.

Claims (7)

[Claims] 1. A terephthalic acid obtained by acidifying an alkaline waste liquid containing a dialkali salt of terephthalic acid produced when a polyester fiber is subjected to weight reduction treatment with an alkaline solution with a mineral acid, and (2) P- Boiling point is higher than that of dimethyl terephthalate produced as a by-product when dimethyl terephthalate is produced by oxidizing xylene and / or methyl P-toluate with a molecular oxygen-containing gas in the presence of a heavy metal catalyst and then esterifying with methanol. And a high boiling point by-product are fed to the reactor, and while maintaining at least a part of the feedstock in a molten state, (3) introducing methanol into the melt of (1) and (2). Then, it is reacted with methanol, and (4) the produced fraction containing dimethyl terephthalate as a main component is evaporated simultaneously with the reaction together with unreacted methanol (5) ) A method for recovering dimethyl terephthalate, which comprises separating the residue from non-evaporating residues. 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 the feedstock 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.