JP3028100B2 - Process for producing dimethyl 2,6-naphthalenedicarboxylate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for producing dimethyl 2,6-naphthalenedicarboxylate useful as a raw material for high-performance polyesters, and more particularly, to an excess of 2,6-naphthalenedicarboxylic acid. The present invention relates to a method for recovering unreacted methanol in a process for producing dimethyl 2,6-naphthalenedicarboxylate by esterification with methanol.

[0002]

2. Description of the Related Art Conventionally, 2,6-naphthalenedicarboxylic acid (hereinafter referred to as 2,6-NDCA) is esterified with methanol to give dimethyl 2,6-naphthalenedicarboxylate (hereinafter referred to as 2,6-NDCM). ) Can be produced by a method using a mineral acid such as sulfuric acid as a catalyst (Japanese Patent Publication No. 49-1).
74), a method using a metal oxide, a salt, or the like as a catalyst (Japanese Unexamined Patent Publication No.
0-83360, 50-83361, 51-8252,
51-48641), and a method of reacting at a temperature higher than the critical temperature of methanol in the absence of a catalyst (Japanese Patent Laid-Open No. 50-95253). Also, from the reaction product liquid after esterification,
Regarding the method of recovering and purifying 6-NDCM, a method by a recrystallization method (Japanese Patent Publication No. 48-40349, Japanese Patent Application Laid-Open No.
14656, JP-A-50-84467), a method of performing a combination of distillation and recrystallization (JP-A-50-116461),
A method by distillation (JP-A-50-108248) and the like are known. However, on the other hand, it is generally assumed that unreacted excess methanol contained in the reaction product liquid after esterification would be appropriately recovered by distillation in order to be recycled for the reaction. However, at present, there is no known method for its recovery and purification in accordance with the actual situation.

[0003]

When 2,6-NDCA is esterified with methanol to produce 2,6-NDCM, 2,6-NDCA is hardly dissolved in methanol and the reaction rate is low. Methanol is generally used in excess because of its very slow and equilibrium point of view. Conventionally, it is conceivable that a reaction product liquid is subjected to a distillation operation to separate and recover unreacted methanol having a low boiling point, then to remove generated water, and then to separate and recover 2,6-NDCM.
Initially, this method was examined, and when the reaction product was directly subjected to a distillation operation for recovering methanol, flooding and / or clogging in the distillation column occurred, and stable operation over a long period of time was not possible. It turned out to be the situation. Therefore, regarding the recovery method by distillation of methanol, for example, the type of distillation tower such as a packed tower, a shed tray, a perforated plate tray without downcomer, and the operating conditions were examined, but all of them were industrially satisfactory. Was not something.

[0004]

Means for Solving the Problems The present inventors have proposed 2,6
Various investigations were continued to solve the above-mentioned problems relating to the recovery of unreacted methanol from the NDCA esterification reaction product liquid. The reaction product liquid contains 2,6-
In addition to NDCM, unreacted methanol and produced water, unreacted 2,6-NDCA and esterified intermediates, and various by-products and impurities are contained. Various by-products include, in addition to those produced by the esterification reaction,
Also included are those caused by DCA. That is, 2,6-NDCA is generally obtained by oxidation of dialkylnaphthalene, and various by-products at this time are included in the raw material 2,6-NDCA, though the degree thereof is different. If such a reaction product solution is directly passed through a methanol distillation column, stable distillation cannot be performed for a long period of time. This is because a substance that is hardly soluble in methanol is contained in a tray or packing in the column. It was found to be caused by adhesion and accumulation. This substance is mainly 2,6-NDCM or 2,6-NDCA, or an esterification intermediate, and also contains other aromatic acids and polymers. These causative substances were found to adhere and accumulate in the column accompanying the methanol distillation, albeit little by little. Above all, the main substance of the adhered substance is 2,6-NDCM, and the adhered substance is more likely to be adhered to the lower part than the feed stage. This is considered to be due to the fact that the boiling point of methanol is low, so that the solubility of 2,6-NDCM in the column is lowered, thereby causing precipitation.

The inventors of the present invention have attempted to separate and recover the desired 2,6-NDCM from the esterification reaction product by crystallization, and then tried to recover the mother liquor by distillation of unreacted methanol. . However, although the operable time of the distillation column tended to be long, it was found that the same problem could not be avoided. The cause of this is that the mother liquor contains 2,6-NDCM, 2,6-NDCA and various by-products in addition to methanol and produced water, and is accompanied by methanol during the distillation operation. This is due to adhesion and accumulation in the tower.

The present inventors have conducted intensive studies in order to completely solve this problem, and as a result, have found that the desired product, 2,6-NDCM
It has been found that, after crystallization and separation, prior to the distillation and recovery of unreacted methanol, a stable distillation operation can be continuously performed by previously performing an alkali treatment of the mother liquor, thereby completing the present invention. Reached. That is, the present invention
2,6-ND by crystallization from the esterification reaction product
After separating and recovering the CM, an alkali is added to the mother liquor and various by-products, impurities and dissolved 2,6-NDC as described above.
An object of the present invention is to provide a method for saponifying M and other aromatic acids, and thereafter recovering unreacted methanol by distillation.

Hereinafter, the method of the present invention will be described in detail with reference to an example. Methano for 2,6-NDCA
And a reaction temperature of 120 to 150 moles.
The esterification reaction is carried out at a temperature of 10 ° C. under a pressure of 10 to ²⁰ kg / cm ² . After the esterification reaction, the product solution is cooled to 20 to 40 ° C., and the desired product, 2,6-NDCM, is crystallized, separated from the mother liquor and recovered. Next, the mother liquor is subjected to an alkali treatment. The alkali used for the mother liquor treatment in the method of the present invention is an alkali metal hydroxide, carbonate, or basic carbonate. Of the alkali metals, sodium and potassium are preferable. In particular, the use of caustic soda is preferred from an industrial point of view. For example, it is an essential requirement of the present invention to add an alkali such as caustic soda to the mother liquor and perform saponification treatment of 2,6-NDCM and other aromatic acids of various by-products, impurities and dissolved components. Becomes The saponification treatment of the mother liquor performed by the method of the present invention is carried out, for example, by adding an aqueous solution of caustic soda to the mother liquor and stirring it well under heating. In order to carry out this saponification treatment reliably, it is important to add an alkali so that the pH of the mother liquor becomes 10 or more, preferably 13 to 14, to complete the reaction. The processing temperature in this case is 20 to 60C. The operation of the saponification treatment with the alkali may be performed by providing a separate treatment step, or may be performed simultaneously with the methanol recovery distillation at the bottom of the distillation column.

For the recovery of methanol in the method of the present invention, ordinary distillation operations can be applied, but it is recommended to use a perforated plate tray type distillation column without downcomers. In particular, in the above-mentioned system, it is effective to adopt a system in which steam is blown into the bottom of the distillation column at the time of methanol distillation, which is also a feature of the method of the present invention. By injecting steam, the by-products and saponified impurities are made water-soluble to facilitate discharge from the bottom of the column, and high-purity methanol from the top of the column is recovered smoothly. As a result, stable operation of the distillation column is achieved. The amount of steam to be blown varies depending on the composition of the mother liquor, the amount of alkali added, and the type and amount of saponified product.
0 parts by weight. Further, the distillation of methanol may be either a batch system or a continuous system, but the latter system is industrially employed.

[0009]

According to the method of the present invention, 2,6-NDCA
Is esterified with excess methanol to give 2,6-NDCM
, And a series of processes for recovering unreacted methanol by distillation can be stably operated, and its industrial significance is great.

[0010]

Next, the present invention will be described more specifically with reference to examples and comparative examples. Note that the present invention is not limited by these examples. Example 1 NDC was placed in a reactor having an internal volume of 200 liters with a stirrer.
A15 kg, methanol 90 kg, and the reaction temperature 130
The reaction was carried out at 2 ° C. for 2 hours. The pressure in the system was adjusted to 12 kg / cm ² G by introducing nitrogen. After the reaction, the content of the reactor was cooled to 30 ° C., and the product was crystallized to form a slurry. The reactor was depressurized, the slurry was taken out, and separated into crystals and mother liquor by a centrifuge. In this mother liquor, under stirring,
At a temperature of 30 to 60 ° C., a 25% aqueous sodium hydroxide solution 5.0
Saponification treatment was performed by adding Kg to obtain PH13. The liquid after the saponification treatment was distilled in a 50 mm-diameter glass perforated plate distillation column (older-short type), methanol was collected from the top of the column, and water and salts were removed from the bottom of the column. . The feed stage is the middle stage, with 10 trays in the enrichment section and 10 trays in the recovery section. Although the treatment of all mother liquors was completed by the continuous distillation method, no problem such as deposition of deposits in the column occurred. The recovered methanol had a purity of 95% or more, only contained dimethyl ether, and was sufficiently reusable.

Comparative Example 1 Esterification reaction, crystallization, and centrifugation were performed in the same manner as in Example 1. The obtained mother liquor was directly subjected to the same operation as in Example 1 except that the saponification treatment was not carried out, and the same operation as in Example 1 was performed to recover methanol. However, about 1 /
At the time of processing 10, flooding of the distillation column occurs,
Distillation operation could not be continued. This was due to the fact that the holes in the tray of the recovery section in the distillation column were blocked by deposits. Comparative Example 2 Esterification reaction, crystallization, and centrifugation were performed in the same manner as in Example 1. To the obtained mother liquor, 2.0 kg of a 25% aqueous solution of caustic soda was added to carry out a saponification treatment to obtain PH9. This saponified solution was passed through a distillation column in the same manner as in Example 1 to recover methanol. About 1 / of mother liquor
Although distillation up to 3 was possible, some deposits were observed in the holes of the tray in the distillation column. At the stage where about 1/3 of the mother liquor was distilled, flooding of the distillation column occurred halfway, and the distillation operation could not be continued. This was due to blockage of the tray holes in the distillation column. Further, deposits were observed on the tray and the tower wall in the recovery section.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-140044 (JP, A) JP-A-5-339210 (JP, A) JP-A-3-223233 (JP, A) JP-A-3-3233 240750 (JP, A) JP-A-5-339209 (JP, A) JP-B-55-18699 (JP, B1) JP-B-48-40349 (JP, B1) (58) Fields investigated (Int. ⁷ , DB name) C07C 69/76 C07C 67/48 CAPLUS (STN) REGISTRY (STN) WPIDS (STN)

Claims (2)

(57) [Claims] 1. When producing 2,6-naphthalenedicarboxylic acid from dimethyl 2,6-naphthalenedicarboxylate by esterifying 2,6-naphthalenedicarboxylic acid with methanol, the dimethyl 2,6-naphthalenedicarboxylate is crystallized, separated and recovered. Upon recovering unreacted methanol contained in the mother liquor with distilled, pre a
The mother liquor whose pH has been increased to 10 or more by adding lukari to the distillation column
A process for producing dimethyl 2,6-naphthalenedicarboxylate, characterized in that it is supplied . 2. The method according to claim 1, wherein when unreacted methanol contained in the mother liquor is recovered by distillation, steam is blown into the bottom of the distillation column to discharge saponified impurities as an aqueous solution from the bottom of the column, and methanol is recovered from the top of the column. 2,6-naphthalene as described
A method for producing dimethyl dicarboxylate .