DE1925038B2 - Purification of arom polycarboxy acids using noble - Google Patents

Abstract

Aromatic polycarboxylic acids are purified by a process to make them pure enough to be esterified to superpolyesters by diols. The process is applied to acids as terephthalic, benzene tricarboxylic, isophthalic, naphthalene dicarboxylic, trimellitic, mellitic etc. The process consists of the catalytic oxidn. in the liquid state of polyalkyl hydrocarbons to remove undesirable aldehydes and other impurities. When the acid is made, an aq. soln. of it is brought into contact with a catalytic contng. a noble metal under mild hydrogenation conditions, and the purified acid is regained by crystallisation while cooling at a controlled rate of evapn.

Description

Corresponding studies have shown that evaporative cooling of an aqueous solution of terephthalic acid and small amounts of p-toluic acid, e.g. B. 6000 to 500 ppm p-toluic acid, with a temperature of 200 to 370 ⁰ C to a temperature of 182 to 166 ⁰ C with removal of water vapor and then leaving the mixture at 182 to 166 "C for one or more hours with subsequent Filtration at these temperatures gives a separation factor in the range from 2 to 5, so that the ratio of dissolved p-toluic acid to p-toluic acid in the washed and dried crystallized terephthalic acid is 2 to 5, even if the purification process according to the above US Pat. No. 2,838 is carried out 565 using evaporative cooling not disclosed therein does not give purified terephthalic acid having a p-toluic acid content of 200 to 100 ppm or less.

However, the above known processes for obtaining pure terephthalic acid can now be either do not use on aqueous solutions in which p-toluic acid is also dissolved, or do not produce terephthalic acid high purity or lead to a correspondingly pure product, but only with one Working with large-scale uneconomically long cooling and crystallization times.

The object of the invention is therefore to create a new process for the production of terephthalic acid, which runs rapidly and leads to a product of very high purity, and this object is achieved in accordance with the invention solved in the manner resulting from the claim.

The process according to the invention enables the production to be carried out at a rate that is sufficient on an industrial scale highly pure crystallized terephthalic acid with a p-toluic acid content of 200 to 100 ppm or less, that is to say with a separation factor which is at least in the range from 10 to 30, where such results are achievable even with removal of 55 to 65% of the evaporated water.

The usual, performed only by heat dissipation in batchwise operation cooling runs in aqueous solutions at such high temperatures and pressures, such as are described herein (118-150 ⁰ C ₁ about 1.4 to 3.8 bar), generally slow. The controlled evaporative cooling at a rate of 2.2 to 5.6 ° C. per minute, as used for the process according to the invention, on the other hand, achieves speeds which are suitable for industrial operation.

The inventive method starting from an aqueous solution having a temperature of 200 to 37o ° C, preferably 227-302 ⁰ C and in particular 240-288 ° C, and a pressure, which maintains the water in the liquid phase. The solution is practically saturated with terephthalic acid and contains from 100 to 500 ppm p-toluic acid, based on terephthalic acid. The expression "practically saturated" refers to a solution with a temperature which is about 6 ^° C. above the temperature at which the first terephthalic acid crystals form. This solution is batchwise or continuously subjected to evaporative cooling at a controlled rate in one or more crystallization zones with stirring devices, in which the temperature is expediently raised to an Rndtcmperatur of about 118 to 150 ° C, preferably 124 to 143 ° C, and in particular 135 to 14PC, and the pressure is reduced to values which correspond to the water vapor pressure at these temperatures. The resulting slurry of terephthalic acid crystals is subjected to the crystallization of a solid-liquid separation at the stated end temperatures and pressures, for example by centrifugation, filtration, decanting or with the aid of a filter centrifuge.

The moist crystalline terephthalic acid obtained as a product in the process according to the invention can be mixed with

ίο water to be washed. In some cases there is no Wash with water required. By using a plurality of alternately charged crystallizers and a slurry receptacle that holds slurry from all of them

As crystallizers accumulate, a supply of slurry is accumulated as a feed for continuous solid-liquid separation per kg of terephthalic acid at 118 to 150 ⁰ C and about 2.4 to 4.8 bar again slurried and then a second solid-liquid separation is carried out at 90 to 100 ⁰ C and atmospheric pressure. With the help of such a re-slurry process, large amounts of purified terephthalic acid can be washed and recovered practically continuously. The purified and washed terephthalic acid is then dried. The dried product

jo contains no more than about 100 ppm p-toluic acid and more often only 50 to 75 ppm down to 20 ppm (detection limit reached so far) p-toluic acid.

The following examples illustrate the advantages of the crystallization process according to the invention.

j5 explained.

A number of crystallization attempts in batch operation were prepurified with aqueous solutions Terephthalic acid carried out by catalytic treatment of aqueous solutions crude Terephthalic acid with hydrogen. The cooling of the solutions with which the crystallization was carried out in batch mode, was carried out by evaporative cooling, the rate of which was set at about 2.2 ° C per minute. Of the

4 ^r> surface of the solution was evaporated in other words, the water, the water vapor was condensed, and the condensate was fed back into the solution. In this series of batch tests using controlled

) 0 ter evaporative cooling, cooling to temperatures of 100 to 150 ⁰ C was allowed. In some of these crystallization attempts, in which ^{cooling was carried out to a temperature above 100 °} C. and a pressure above atmospheric pressure, a flash evaporation to atmospheric pressure and 100 ^° C. was carried out ^{during r >> of the filtration.} The experiments in which controlled evaporative cooling to 100 ^° C. and atmospheric pressure and, during the filtration, a slight expansion cooling to 100 ^° C. and atmospheric pressure were carried out, are both not within the scope of the invention, but serve as comparative examples. The numbered examples illustrate the process according to the invention. The results of these crystallization experiments and their operating conditions are given in Table I, in which "TA" means terephthalic acid and "p-Ta" means p-toluic acid. The separation factor is the ratio of p-Ta in solution to D-Ta in the product.

Table I.

Crystallization carried out in sections

example Deployed solution P-Ta ppm ² ) Filtration pressure Relaxation product Separating Tempe 1860 Tempe P-Ta ² ) factor rature concentration 1860 rature bar C ¹ C. 500 C. 1.6 no ppm 93 1 249 TA kg ¹ ) 1500 118 3.1 no 20th 93 2 249 9.2 1500 135 3.1 no 20th 50 3 271 10 1500 135 3.1 no 20th 25th 4th 271 25th 1500 135 1.02 88 ³ ) 20th 8.8 Comparative example 1 271 25th 1500 149 1.02 23 ³ ) 170 19.0 Comparative example 2 271 25th 113 1.02 no 79 21.5 Comparative example 3 271 25th 100 2.3 no 70 52 5 271 25th 124 29 25th

') kg / 100 kg H ₂ O.

² ) ppm, based on TA.

³ ) Relaxation cooling to 100 C and 1.02 bar.

Comparative Example 1 shows that the controlled evaporative cooling will not be achieved by cooling to 150 ° C during crystallization and relaxing at 100 ⁰ C and atmospheric pressure (1.02 bar) during the filtration of the purpose. In Comparative Example 2, a certain improvement is through controlled evaporative cooling to 113 ° C and subsequent relaxation to only 13 ° C to 100 ⁰ C and atmospheric pressure (1.02 bar) over Comparative Example 1 obtained, but a better result than with controlled evaporative cooling is to 100 ⁰ C and atmospheric pressure (1.02 bar) reached before the filtration according to comparative example 3. In contrast, the process according to the invention gives excellent results by controlled evaporative cooling to 118 to 135 ° C. and 1.8 to 3.1 b. <R and filtration under these conditions.

According to the invention, the crystallization process is carried out continuously with a degassed aqueous solution of Purified terephthalic acid (after catalytic treatment with hydrogen) carried out at 268 ° C and 61 bar comes from a storage tank. This degassed solution is continuously converted into a first Crystallization device passed below the liquid level of the slurry contained therein, which at 131 to 154 ° C and 2.4 to 4.5 bar is maintained. Of the Water vapor is condensed and the condensate is returned to the first crystallizer.

The slurry from the first crystallizer is continuously introduced below the liquid level into a second crystallizer, which is operated at 100 ° C. and atmospheric pressure (1.02 bar) and is used for comparison purposes. A slurry of terephthalic acid in water flows continuously from the second crystallization device to a filter centrifuge for the recovery of terephthalic acid, which is operated at 99 ° C. and atmospheric pressure (1.02 bar). This terephthalic acid is washed and dried. The residence time in the first crystallizer is about 2 hours. While this process is being carried out, to explain the process according to the invention, part of the suspension of terephthalic acid in water is continuously withdrawn from the first crystallizer through a candle filter which is operated at 131 to 154 ³ C and 2.4 to 4.5 bar. The terephthalic acid obtained in this way is also washed and dried. Products obtained in other ways are washed and dried in the same way. The two dried terephthalic acid products obtained in different ways are referred to as “ ^{product filtered at 99 °} C.” and as “product filtered under pressure”. The different levels of p-toluic acid in these two differently obtained terephthalic acid products are shown in Table II.

Table II Implemented solution pressure p-Ta Filtratioi 1 pressure Candle filter pressure Filter centrifuge pressure product P-Ta ppm at bar ppm 1. Crystallizing device bar Temp. bar Temp. bar at 99 (- under game Temp. 61 380 Temp. 2.7 t 2.7 ( 1.02 filtered pressure
nitrided C. 61 1990 C. 3.1 134 3.1 lOÜ 1.02 268 61 1990 134 2.4 139 2.4 100 1.02 93 53 6th 268 139 131 100 301 -] η 7th 268 131 285 86 8th

Example 9

For comparison purposes, the same apparatus is used as in Examples 6 to 8, but the crystallizations are carried out in batches in the first and second crystallization apparatus. In the first with water vapor of 277 ° C and 68 bar filled crystallizer a hydrogen-free aqueous solution is introduced at 277 ° C and cooled by controlled evaporative cooling to 132 ⁰ C and 2.4 bar. The obtained slurry is separated into a major part and a minor part. The larger part is introduced into the second crystallization device and ^{cooled to 100 °} C. and atmospheric pressure by controlled evaporative cooling. At the same time, the smaller part of the slurry is passed through a candle filter at 132 ° C and 2.4 bar. The crystalline terephthalic acid product from the second crystallizer is recovered with a centrifuge, washed and dried. The crystalline terephthalic acid product obtained with the candle filter is washed and dried. The same weight ratio of water to crystalline product is used in each washing process. The feed used for crystallization contained about 2000 ppm p-toluic acid based on terephthalic acid. The dry terephthalic acid from the pressure filtration at 132 ° C and 2.4 bar has a p-toluic acid content of 41 ppm and the dry terephthalic acid, which from the second crystallizer at 99 ⁰ C and atmospheric pressure (1.02 bar) both in the crystallizer as was also obtained in the centrifuge, a p-toluic acid content of 161 ppm. These two different processes for the production of terephthalic acid have separation factors of about 50 and 12.5, which corresponds to a 4-fold difference.

A comparison of the results of Examples 6 to 8 with those of Examples 1 to 5 and 9 shows that the advantages the recovery of terephthalic acid under the temperature and pressure conditions for the final crystallization and solid-liquid separation can partially be achieved even if a first modified Relaxation cooling and pressure relief is applied. The separation factors are used in pressure filtration compared to the filtration at 99 ° C. increased as follows: Example 6: 1.75 times; Example 7: 3.9 times and Example 8: 3.3 times. It can also be seen that the higher values are achieved with the aqueous solutions used which have higher (about 5 times higher) weight ratios of p-toluic acid to terephthalic acid.

Example 10

Crystallization is carried out in batches with an aqueous solution which contains 20 kg of terephthalic acid per 100 kg of water and a p-toluic acid content of 3000 ppm, based on the weight of terephthalic acid. This solution is of 271 ⁰ C and 62 bar, such as is obtained by removal of hydrogen, by evaporative cooling at a controlled rate of 2.2 ° C per minute to 135 ° C 2.8 bar cooled and. The resulting slurry of terephthalic acid crystals is separated by solid-liquid separation at 135 ° C. and 2.8 bar. the

ίο Obtained terephthalic acid is made without washing with Water dried. The dried product has a p-toluic acid content of 26 ppm, which is a Separation factor of 115.

Example 11

The procedure of Example 10 is repeated with the exception that the cooling rate is 5.6 ° C. per minute, the final temperature is 138 ° C. and the final pressure is about 3 bar. The resulting slurry is ^{filtered at 138 °} C. and 3 bar. The dried product obtained has a p-toluic acid content of 47 ppm, which corresponds to a separation factor of about 64.

In contrast to the results of Example 10 and

11, the same aqueous solution provides output when cooled by controlled evaporative cooling at a rate of 5.6 ° C per minute and 2.2 ⁰ C per minute to 100 ⁰ C dried Terephthalsäureprodukte with a p-Toluylsäuregehalt of 200 and 130 ppm, which separation factors of 15 and 23.1.

Comparison of these results with those of Examples 10 and 11 shows that the two different types of Obtained using the same cooling rates of 2.2 ° C and 5.6 ° C separation factors result, which in the case of the final crystallization and separation at 135 to 138 ° C and 2.8 to 3 bar about 7.5 times and 2.9 times better.

The advantages of the crystallization and separation process according to the invention can be used in batches or batches Continuous operation can be achieved if an aqueous feed solution has already been introduced Pressure reducing valve is relaxed, while at the same time more feed solution with the high temperature and the high pressure below the level of the liquid introduced in the crystallizer

so will. Such an expansion can lead to a small pressure drop in the range of approximately 1.4 to 7 bar. Such a relaxation with simultaneous introduction of the hot pressurized solution into the Crystallization can be used to advantage to shorten the last cooling section somewhat.

Claims (1)

Claim: Process for obtaining terephthalic acid from aqueous solutions containing them at a temperature of 200 to 370 ° C which are practically saturated with terephthalic acid and also contain 500 to 6000 ppm of dissolved p-toluic acid, based on dissolved terephthalic acid, these solutions being used in the purification of terephthalic acid by catalytic hydrogenation of an aqueous solution of the acid containing 4-carboxybenzaIdehyde and other impurities at a temperature of 200 to 400 ° C and a pressure sufficient to maintain the liquid phase after separating the catalyst, by precipitating the terephthalic acid from the solution by evaporative cooling with controlled Speed against a counterpressure that is above that of the surrounding atmosphere to a temperature of 118 to 150 ° C and separation of the terephthalic acid from the mother liquor in this temperature range under a pressure sufficient to maintain the liquid phase, characterized thereby This means that evaporative cooling is carried out at a rate of 2.2 to 5.6 ° C. per minute. 30th From NL-OS 64 03 348 a process for purifying terephthalic acid by catalytic hydrogenation of an aqueous solution of this acid which contains 4-carboxybenzaldehyde (or formylbenzoic acid) and other impurities is known. The hydrogenation is carried out at a temperature of 200 to 400 ° C. and at a pressure which is sufficient to keep the solution in the liquid phase. The catalyst is then separated off, whereupon the terephthalic acid is crystallized or precipitated from the solution, that either slowly cooling the solution to room temperature and separating the precipitated acid at room temperature or the solution to 160 ° C to cool and separating the precipitate at 160 ° C, or the solution to 50 to 15O ⁰ C, preferably, 30 to 120 ° C cools and separates the precipitated acid at these temperatures, or finally cools the solution by evaporating water and separates the solid acid, which is then optionally washed with a dilute mineral acid such as dilute hydrochloric acid. This process can be improved with regard to its technical applicability. So is the slow cooling of the solution of 200 to 400 ° C to room temperature (22 to 25 ⁰ C) for an industrial process to be time consuming. Slow cooling to 160 ° C. and separation of the terephthalic acid at this temperature is also unsuitable for industrial use, even if the cooling time is reduced somewhat. The reason lies in the fact that significant amounts of the desired product as terephthalic acid remaining in the mother liquor with a temperature of 160 ⁰ C. Cooling the solution by evaporating water to form a solid residue, which must then be washed with a dilute mineral acid, is in certain cases suitable for removing the catalyst metals, but is not sufficient to increase the concentration of the reduction products of some of the impurities originally present reduce that a commercially viable product is obtained. Furthermore, this additional treatment makes the process more expensive From BE-PS 6 81 180 it can be seen that the method known from NL-OS improve and the Precipitation of terephthalic acid can be accelerated if the cooling is carried out by evaporating water against a counterpressure above atmospheric pressure to a temperature of 50 to 150 ° C causes and the terephthalic acid at a controlled rate of 1.5 parts by weight per 100 Parts by weight of the water crystallized out per minute This controlled rate of crystallization is but too small for a technical application. If the process is carried out continuously there are also considerable amounts of the undesired p-toluic acid, which is produced by catalytic hydrogenation represents the product formed originally as an impurity, 4-carboxybenzaldehyde, together with the desired terephthalic acid on the surface of the product crystals formed first crystallizes out, even if there is a sufficient amount of water that would in itself prevent the Saturation concentration of p-toluic acid at 50 to 150 ° C is reached. From US-PS 28 38 565 a process for the purification of terephthalic acid, which contains p-toluic acid in an order of magnitude of 5% by weight as an impurity, is known in which the crude terephthalic acid is in water at 230 to 280 ° C dissolves in the liquid phase under pressure. The terephthalic acid is obtained from such a solution by not ^{cooling the solution below 165 °} C. and separating the terephthalic acid from the suspension obtained by crystallization at this temperature and at a pressure sufficient to maintain the liquid phase from the aqueous mother liquor. In the example, the output of which acid contains 7% p-toluic acid, the purpose Applied filtration is carried out at 180 ⁰ C. In US-PS 25 72 575 and GB-PS 9 06 035 purification processes for terephthalic acid contaminated with p-toluic acid are also described. For this purpose, the crude terephthalic acid is slurried in water at a temperature in the range of 100 to 200 ⁰ C and 121 to 260 ° C under pressure in the liquid phase, with the terephthalic acid from the suspension without cooling then at this temperature and under pressure by filtration . is separated. For this purpose, the method of pressure filtration in the aqueous liquid phase, recognized as particularly advantageous, is used, according to Example 1 of US Pat. No. 1 with a terephthalic acid containing 6.7% p-toluic acid at 175 ^° C. and according to the example the GB-PS is operated at 165.5 ° C with a starting acid which contains about 1% impurities. In the latter, a temperature range from 149 to 204 ^° C. is mentioned as preferred. For this purpose, the technical teaching is thus conveyed to carry out the separation of a terephthalic acid originally contaminated with p-toluic acid from an aqueous suspension above 100 ^° C. in the liquid phase under pressure.