JPS6133015B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for obtaining terephthalic acid crystals by centrifuging a slurry liquid containing terephthalic acid crystals. Traditionally, polyester fibers were made from dimethyl terephthalate and ethylene glycol;
In recent years, so-called direct polymerization of terephthalic acid and ethylene glycol has been used to reduce product costs. High-purity terephthalic acid, which can be directly polymerized, is obtained by post-processing the crude terephthalic acid, such as purification. Since the cost becomes large and the price of terephthalic acid itself becomes high, the merits of using the direct polymerization method are ultimately halved. Therefore, there is a need for a method for producing high-purity terephthalic acid, which can be obtained by simply oxidizing paraxylene without performing any post-treatment such as purification, and then subjecting the resulting reaction product to solid-liquid separation, and which can be used directly for polymerization. , has been studied. Regarding the quality of terephthalic acid for direct polymerization, the metal content contained in terephthalic acid is a factor that determines the quality as a polyester fiber, and it is necessary that the metal content be as low as possible and constant. When producing high-purity terephthalic acid that can be directly polymerized without post-processing such as a purification process, terephthalic acid is produced by solid-liquid separation of metals that enter the system due to the heavy metal catalyst used for oxidizing paraxylene or corrosion of the plant. It is included in acid. In order to remove this metal content, conventionally the terephthalic acid slurry after the reaction is subjected to solid-liquid separation, a solvent is further added to the resulting cake to form a slurry, and solid-liquid separation is repeated two to several times. The need arose. Repeating the solid-liquid separation and re-slurry process many times like this
This is very disadvantageous because it complicates the plant, increases the construction cost of the plant, and complicates its operation. The method of separating a slurry solution into a mother liquor and crystals by centrifugal force involves the following operations: supplying the slurry to a basket of a centrifuge, separating the mother liquor by centrifugal force, washing with a solvent, and scraping off cake adhering to the inner wall of the basket with a knife edge. There are two types of centrifugation methods: a repeating batch centrifugation method and a continuous centrifugation method where a slurry is supplied to a centrifuge while simultaneously taking out the mother liquor and cake continuously.The present invention is applicable to the batch centrifugation method. However, the present invention provides a method for keeping the metal content contained in terephthalic acid as low as possible and constant, without repeating the steps of solid-liquid separation and re-slurrying many times. In the batch centrifugation method, the cake adhering to the inner wall of the basket is repeatedly scraped off using a knife edge. In order to avoid damaging the material placed on the inner wall of the basket with the knife edge, a small amount of cake is left on the material (this remaining cake is called a heel) before moving on to the next slurry supply process. is normal. This remaining cake not only protects the material, but also serves to filter the terephthalic acid slurry during solid-liquid separation of terephthalic acid, thereby reducing the leakage of terephthalic acid particles. have. However, when repeating this process to centrifuge terephthalic acid crystals, the heel resistance changes over time.
A phenomenon in which the shaking-off speed of the slurry mother liquor gradually worsens is observed. For this reason, when swinging becomes worse, the heel is washed with alkali to recover.
In order to extend the cycle of heel cleaning with alkali, when the slurry solution of terephthalic acid crystals is centrifuged and the terephthalic acid cake is scraped off,
A method of periodically changing the heel thickness has been proposed. Using this method, the time during which the shaking properties of the mother liquor deteriorate is extended, and the cycle of alkaline washing is reduced to 2.
4 hours, that is, about 60 cycles, but a considerable amount of metal remains in the terephthalic acid, and solid-liquid separation alone is too much and the metal content fluctuates greatly. , cannot be used as terephthalic acid for direct polymerization. It is possible to reduce the metal content in terephthalic acid by shortening the cycle of alkali washing, but this is not practical as it reduces the capacity of the centrifuge, and the alkali from the alkali washing reduces the metal content in terephthalic acid. It also has the disadvantage of increasing metal content. The present inventors have developed high-purity terephthalic acid that can be used directly in polymerization by simple solid-liquid separation to remove metals that enter the reaction system due to corrosion of heavy metal catalysts and plants used in the oxidation of para-xylene. As a result of intensive study on how to obtain this information, we obtained the following knowledge.
In other words, the amount of metal held inside the terephthalic acid particles varies slightly depending on the concentration of the catalyst used in the reaction, but in any case it is very small, and most of the reasons why the metal content increases in terephthalic acid are It was found that there are two cases: when the liquid containing the metal remains in the terephthalic acid, and when the metal is adsorbed on the very surface of the terephthalic acid. In other words, in order to reduce the metal content in terephthalic acid through solid-liquid separation, the most important things are to ``completely remove the metal-containing liquid'' and ``remove as much metal as possible adsorbed on the cake surface.'' It turns out that it is. Centrifugal separators that cannot perform acetic acid rinsing, that is, washing the cake by adding acetic acid to the solid-liquid separated cake, inevitably contain 3 to 10% of the cake on the cake side.
The mother liquor remains. The metal content in the mother liquor is 0.1-1
%, it is not possible to reduce the metal content in terephthalic acid to a few ppm by simple solid-liquid separation. As a method for removing as much metal as possible adsorbed on the surface of the cake, there is a method of sprinkling sufficiently heated acetic acid onto the solid-liquid separated cake, as shown in JP-A-52-133940. However, even when this method is used, the effect is not necessarily sufficient unless the reaction mother liquor is sufficiently removed from the cake. An object of the present invention is to completely remove metal-containing liquid from the terephthalic acid cake by preventing the over-resistance of the heel from changing over time. The purpose of this is to use a centrifugal separation method in which a slurry containing terephthalic acid crystals is passed through a batch centrifuge to separate the mother liquor, and the resulting terephthalic acid cake is repeatedly scraped off. This is achieved by gradually thinning the terephthalic acid cake, and prior to scraping the terephthalic acid cake, the cake is sprinkled with acetic acid, and the sprinkled acetic acid is removed from the cake by centrifugation. Metal content can be removed. Additionally, when thinning the heel, by making the heel thinner to a greater extent than the previous thinning, the overresistance of the heel can be made even more constant, making it possible to obtain terephthalic acid with a constant purity. becomes. The present invention will be explained in detail below. In the method of the present invention, a slight cake ( Heel)
This is done by changing the thickness of the In the case of terephthalic acid, the thickness of this heel is usually from 3 mm.
Approximately 15mm is used. The thinner the material, the better the processability.
The thicker it is, the faster the hypersensitivity will worsen. Furthermore, if the heel thickness is kept constant, hypersensitivity deteriorates rapidly. For example, if centrifugation is performed with the same heel thickness, the transient properties will worsen after about 20 cycles, and solid-liquid separation will no longer be possible after 60 cycles. Therefore, in the present invention, it is preferable to change the heel thickness so that a new heel surface is used every 20 cycles or so. If the heel is thicker than before,
The metal content in terephthalic acid becomes high. Therefore, by successively decreasing the thickness of the heel, it is possible to maintain a good state of hyperactivity. However, if the newly generated heel is made thinner from the beginning, the initial hyperactivity is very good, but the heel becomes thinner quickly and the total number of cycles from the beginning to the end becomes smaller. It is desirable to make it as thick as possible; it is thicker than the conventional 15 mm, and preferably from 18 mm to 25 mm. If it is too thick, the amount of terephthalic acid scraped off in one cycle will decrease and the performance of the centrifugal separator will decrease. Furthermore, since the newly generated heel has good hypersensitivity at the beginning, a method of postponing the number of cycles can be considered, but since the hyperactivity worsens when the heel is made thinner, the total number of cycles from the beginning to the end becomes smaller. After all, in order to keep the hypersensitivity constant at a good level and extend the period, it is preferable to reduce the thickness of the heel at regular intervals. As mentioned above, it is desirable to reduce the amount of thinning at first and gradually increase the amount of thinning. The frequency of thinning can be done continuously by a mechanical device, but it can also be changed automatically by a machine in 5 to 15 steps, or it can be changed manually. . When acetic acid rinsing is employed, the amount of acetic acid used for rinsing is usually 0.5 to 4 times the amount of terephthalic acid in the cake by weight. When using the conventional method, the metal content in the obtained terephthalic acid varies widely;
Even solid-liquid separation has become difficult after about 60 cycles, but the method of the present invention can stably reduce the amount of metal in terephthalic acid up to about 120 cycles. However, with the present invention, the alkali washing can be extended to 6 to 10 hours, and by using acetic acid rinsing, direct polymerization is possible. A metal content that can be used as terephthalic acid can be achieved. The method of the present invention can be applied to an acetic acid slurry of terephthalic acid obtained by liquid phase air oxidation of paraxylene in an acetic acid solvent using a heavy metal and a bromine compound as a catalyst. It is effective to apply it when The present invention will be explained below with reference to Examples. Example 1 Terephthalic acid (center particle size of terephthalic acid 150
μ) Feed 280 kg of acetic acid slurry containing 30% (containing metal concentration 0.5%) into an automatic batch centrifuge for 40 seconds, shake off the mother liquor after centrifugation for 40 seconds, and then
120 kg of acetic acid (1.5 times the amount of terephthalic acid) at 105° C. containing 50% water was supplied for 20 seconds for washing, the liquid was shaken off again for 50 seconds, and the cake was scraped off for 5 seconds. Repeat this operation. In this method, the thickness of the heel was initially scraped off to 18 mm, and the thickness of the heel was gradually thinned with the number of cycles, and the degree of thinning was gradually increased, as shown in Table 1, and the metal content was Stable at 2ppm. By carrying out alkaline cleaning for 120 cycles and repeating it again from the 1st cycle, a continuously stable metal component of terephthalic acid was obtained.

[Table] Comparative Example 1 The same procedure as in Example 1 was performed except that the heel thickness was not changed. The results are shown in Table 2. After 40 cycles, the metal content increased, and after 60 cycles, solid-liquid separation was no longer possible. Summer. Since the alkaline cleaning was performed and the first cycle was repeated again, the metal content of the terephthalic acid changed and increased.

[Table] Example 2 The heel thickness was changed in the same manner as in Example 1 except that it was made thinner at equal intervals. The results are shown in Table 3, and although the results are good at first, the metal content gradually increases. After 120 cycles, alkaline cleaning was performed and the process was repeated from the 1st cycle, but the metal content increased before the alkaline cleaning.

[Table] Comparative Example 2 Example 1 except for periodically changing the heel thickness
The results were as shown in Table 4. The metal content has increased.

【table】

Claims (1)

[Scope of Claims] 1. In a centrifugal separation method in which a slurry containing terephthalic acid crystals is passed through a batch centrifuge to separate a mother liquor, and the resulting terephthalic acid cake is repeatedly scraped, A centrifugal separation method for a slurry liquid containing terephthalic acid crystals, which is characterized by gradually reducing the remaining thickness. 2. The centrifugal separation method according to claim 1, wherein the terephthalic acid cake is sprinkled with acetic acid before being scraped off, and the sprinkled acetic acid is removed from the cake by centrifugation. 3. The centrifugal separation method according to claim 1 or 2, wherein the thickness of the scraped residue is reduced to a greater extent than the previous reduction. 4. Claims 1, 2, or 3, wherein the terephthalic acid crystal-containing slurry liquid is a reaction liquid obtained by oxidizing paraxylene with an oxygen-containing gas in the presence of a heavy metal catalyst in an acetic acid solvent. Centrifugation method as described in section.