GB1577019A - Process for producing high quality terephthalic acid - Google Patents

Description

(54) A PROCESS FOR PRODUCING HIGH QUALITY TEREPHTHALIC ACID (71) We, MITSUBISHI CHEMICAL INDUSTRIES LIMITED, a Japanese Body Corporate of 5-2, Marunouchi 2-chome, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for producing terephthalic acid, and more particularly to a process for producing high quality terephthalic acid suitable as a raw material for the production of polyethylene terephthalate by direct esterification, which process comprises oxidising p-xylene in the liquid phase under specifically chosen condition to such an extent that a conversion ofp-xylene into terephthalic acid of at least 95 % is achieved and subjecting the produced slurry of terephthalic acid obtained to post-oxidation.

In the past, polyethylene terephthalate has been produced from dialkyl terephthalates and ethylene glycol. Recently, more interest has been drawn to a process for polycondensation of terephthalic acid and ethylene glycol; however, the starting terephthalic used in this process should be of high purity, for example, the content of 4-carboxybenzaldehyde (hereinafter referred to as 4-CBA and one of the impurities present in terephthalic acid) must be less than 500 ppm.

In general, the terephthalic acid produced by the conventional liquid phase oxidation of a p-dialkylbenzene contains a large amount of impurities including various reaction intermediates, such as 4-CBA,p-toluic acid and others having an unknown chemical structure. Thus, in order to obtain a high quality product suitable for the production of polyethylene terephthalate, it is essential to subject the slurry of terephthalic acid thus produced to post-oxidation by passing molecular oxygen through the slurry (see, for example, British Patent Specification 983,677). Such post-oxidation removes the reaction intermediates and a trace of other impurities of mainly colouring substance not removed by the liquid phase oxidation, such undesirable materials being present in the mother liquor of the slurry, and by effecting post-oxidation the quality of terephthalic acid is thus improved. However, it has been noted that the amount of impurities removed by post-oxidation has a close relationship with conditions under which the liquid phase oxidation ofp-xylene is carried out and the intended effect of post-oxidation is often not achieved depending upon the conditions of the liquid phase oxidations.

In copending Application No. 32047/75 (Serial No. 1,454,478) we have described a process for producing terephthalic acid comprising catalytically oxidizing p-xylene in the liquid phase to obtain a slurry of terephthalic acid while maintaining the content of 4-CBA in the mother liquor of the slurry at less than 800 ppm and subjecting the resulting slurry to post-oxidation by passing a molecular oxygen-containing gas through the slurry thereby obtaining high purity terephthalic acid containing 4-CBA in an amount of less than 500 ppm and suitable for polycondensation with ethylene glycol. Among various processes for maintaining the content of 4-CBA in the mother liquor at less than 800 ppm, the most suitable one from the viewpoint of economy is to prolong the reaction period in the liquid phase oxidation.

However, this process involves an inherent disadvantage in that, in commercial operation, as a result of prolonged reaction period, so much acetic acid, which is the reaction solvent, is burned off that the process is rendered uneconomical. In view of the above, the inventors have conducted extensive investigation to find a process by which the content of 4-CBA in the mother liquor of the terephthalic acid slurry is reduced while oxidation or burning of the acetic acid solvent is prevented or decreased to a considerable extent and we have found that, in the liquid phase oxidation of p-xylene, a specific catalyst system serves to reduce the burning of the acetic acid solvent and shorten the reaction time while improving the quality of the terephthalic acid product.

Accordingly, the present invention provides a process producing terephthalic acid comprising the steps of: (a) subjectingp-xylene in acetic acid as solvent to a one-step oxidation in an oxidation vessel with a molecular oxygen-containing gas having an oxygen concentration of from 5 to 100% by volume in an amount of from 1 to 100 moles of oxygen per mole ofp-xylene in the presence of a heavy metal compound-containing oxidation catalyst at a temperature of 100 to 250"C under a pressure of from atmospheric to 200 atmospheres to achieve a conversion of at least 95% to obtain a slurry of terephthalic acid, and (b) subjecting the slurry of terephthalic acid to post-oxidation in a post-oxidation vessel or crystallizer tank by passing a molecular oxygen-containing gas in an amount of from 1/1000 to 1/10 that used in the liquid phase oxidation through the slurry at a temperature 0 to 300C below that of the liquid phase oxidation in the presence of the oxidation catalyst used in step (a); in which said liquid phase oxidation of step (a) is effected while the concentration of 4-carboxybenzaldehyde in the mother liquor of the slurry is maintained at less than 800 ppm and said catalyst consists essentially of (1) a cobalt compound present in an amount, calculated as Co, of from 200 to 6000 ppm based on the solvent, (2) a manganese compound present in an amount, calculated as Mn, of from 0.5 to 1.5 times that of Co by weight and (3) at least one bromine compound selected from hydrogen bromide, manganese bromide and cobalt bromide, present in an amount, calculated as Br, of from 400 to 2000 ppm based on the solvent and of more than two times that of Co by weight.

In the first step of the process according to this invention,p-xylene in acetic acid as solvent is subjected to oxidation in the presence of a heavy metal compound-containing catalyst to such an extent that a conversion ofp-xylene into terephthalic acid of at least 95%, preferably more than 98% and more preferably more than 99.5%, is achieved.

The liquid phase oxidation employed in this invention in whichp-xylene in acetic acid is oxidized in the presence of a heavy metal compound-containing catalyst with a molecular oxygen-containing gas is a technique well known as the "SD process". This process is described, for example, in USP 2833816, USP 2962361, USP 3092658, USP 3115520 and USP 3119860, and, therefore, any procedures disclosed in such U.S. patents may successfully be employed so far as the compounds of the catalyst and the oxidation conditions satisfy the requirements defined hereunder in detail. The process of this invention may be carried out as either a continuous process or a semi-continuous process. The continuous process will give the best results.

The amount of acetic acid is, in general, from 0.5 to 20 parts, preferably 1 to 10 parts per part by weight of the p-xylene to be oxidized. The acetic acid may contain less than 30% of water or any other organic material such as an aldehyde (other than 4-CBA) or a ketone.

The molecular oxygen-containing gas suitable for this invention contains from 5 to 100% by volume of oxygen. In general, air is conveniently used. The amount of molecular oxygencontaining gas to be supplied to the reaction system is, calculated as 02, from 1 to 100 moles, preferably from 3 to 100 moles per mole of p-xylene.

The pressure under which the oxidation reaction is carried out is not critical so far as the reaction system is maintained in a liquid state, and is usually from 10 to 50 kg/cm2.

The catalyst employed in this invention is a Co-Mn-Br system. The cobalt compound used according to this invention may be, for example, cobalt acetate, cobalt naphthenate or cobalt bromide. The amount of cobalt compound to be used is, calculated as Co on the basis of the solvent, from 200 to 600 ppm, preferably from 250 to 500 ppm and more preferably from 300 to 400 ppm.

The manganese compound used according to this invention may be, for example, manganese acetate, manganese naphthenate or manganese bromide. The amount of manganese compound to be used is, calculated as Mn on the basis of Co, from 0.5 to 1.5 times by weight.

An amount outside the above limitation leads to a deterioration in the quality of the terephthalic acid produced.

The bromine compound to be used according to this invention is hydrogen bromide, manganese bromide or cobalt bromide. It has been found that the other bromine compounds such as sodium bromide, potassium bromide, ammonium bromide and tetrabromoethane show low activity and in consequence they cannot achieve the intended effect.

The amount of bromine compound to be used is, calculated as Br on the basis of the solvent, from 400 to 2000 ppm, preferably from 700 to 1500 ppm. Too large an amount does not improve the quality of the terephthalic acid and is uneconomical since it makes it necessary to recover the bromine compound for reuse. On the other hand, too small an amount results in a deterioration of the quality of the terephthalic acid. In order to produce high quality terephthalic acid, the amount of bromine compound is to be chosen within the above-mentioned range so that the amount is, calculated as Br on the basis of Co, more than 2.0 times, preferably more than 2.5 times by weight. Where cobalt bromide or manganese bromide is used, it acts as both the bromine compound and the cobalt or manganese compound.

The temperature at which the liquid phase oxidation is carried out is, in general, from 100 to 250"C, preferably from 190 to 225"C and more preferably from 205 to 2200C. This higher reaction temperature is significant in this invention. In the prior art, the liquid phase oxidation ofp-xylene to terephthalic acid should be effected at a temperature below 200"C in order to avoid undue burning of the acetic acid solvent and deterioration of the quality of the terephthalic acid product. According to this invention, it is possible, by using the catalyst system mentioned above, to carry out the liquid phase oxidation at a higher temperature thereby shortening the time required and markedly reducing burning of the solvent.

The liquid phase oxidation may be carried out as a continuous process or a semi-continuous process. It is essential to maintain the concentration of 4-CBA in the mother liquor of the terephthalic acid slurry produced at less than 800 ppm, preferably less than 700 ppm. At a concentration of 4-CBA of above 800 ppm, a large amount of impurities is entrapped in the terephthalic acid being precipitated; thus the final product which has been subjected to post-oxidation purification is inferior and is not suitable for direct polycondensation.

In order to reduce the concentration of 4-CBA in the mother liquor to the desired concentration of less than 800 ppm, it is convenient to control the dwell time of the raw material in the oxidation vessel; however, other measures may be employed, for example in any given oxidation system, by increasing the amount of solvent, lowering the water content in the reaction mixture, increasing the partial pressure of oxygen in the reaction vessel and improving the dispersion ofp-xylene and/or molecular oxygen-containing gas in the reaction mixture.

In particular, for example, the following measures are recommended: (1) In a continuous process, when the proportion of the solvent top-xylene is 3 : 1, by weight, the use of dwell time of from 1.5 to 2.5 hours; (2) In a continuous process, when the dwell time is one hour, the use of a proportion of solvent to p-xylene of about 6 : 1; and (3) In a continuous process, the use of a content of water in the mother liquor in the reaction mixture controlled to from 5 to 14% by weight.

The abovementioned figures vary depending upon other parameters of the reaction conditions and cannot be standardized. Accordingly, it should be understood that the above figures are given for the purpose of exemplification, and other conditions suitable for maintaining the specified 4-CBA content may be determined through experiments on the basis of the abovementioned guidelines.

The slurry of terephthalic acid thus produced is then subjected to post-oxidation treatment.

The primary object of the post-oxidation is to oxidize various reaction intermediate products, which are represented by 4-CBA, in the mother liquor, and, therefore, the presence of a large amount of unreacted raw materialp-xylene in the slurry should be avoided, although some of the p-xylene is also removed during the post-oxidation.

The post-oxidation may be effected by a continuous process or a semi-continuous process.

In the continuous process, the reaftion mixture is transferred into a post-oxidation vessel or a crystallizer tank and, before or during cooling the mixture to precipitate crystals of terephthalic acid, a molecular oxygen-containing gas is passed through the slurry. In the semicontinuous process, a molecular oxygen-containing gas is directly supplied to the oxidation vessel, after completion of the liquid phase oxidation.

In order to effect the post-oxidation as completely as possible, it is desirable to supply no more of the molecular oxygen-containing gas than is required to oxidize the reaction intermediates present in the slurry.

If too large an amount of gas is supplied, the solvent and water evaporate resulting in lowering of the post-oxidation temperature; this, in turn, requires additional heating with the result that the process becomes uneconomical. By maintaining appropriate control of the flow rate of oxygen-containing gas, the post-oxidation can be made to proceed smoothly without further heating.

Since the amount of impurities to be oxidized in the post-oxidation is small, the amount of molecular oxygen-containing gas to be supplied is from about 1/1000 to 1/10 as compared with the case of the liquid phase oxidation. In general, the flow rate of the gas is controlled so that the exhaust gas from the post-oxidation contains oxygen in a concentration of from 0.5 to 8%, preferably 1.5 to 4%, by volume. If the exhaust gas from the liquid phase oxidation contains enough oxygen, such exhaust gas may be supplied to the post oxidation step directly or after being made up with a fresh molecular oxygen-containing gas. The temperature at which the post-oxidation is effected is between the liquid phase oxidation temperature (which is designated as TOC) and (T-30)"C. If desired, an oxidation catalyst which is used in the first step may be added.

The terephthalic is suitably recovered from the final slurry by subjecting the slurry to crystallization, separating out the precipitated terephalic acid and washing it with acetic acid preferably by suspending the solid terephalic acid in acetic acid and subjecting the resultant slurry to a solid-liquid separation.

According to this invention, it is possible to produce terephthalic acid of quality comparable with the best product according to the prior art (for example, the product of Example 16) with a shorter dwell time, less burning of acetic acid and the use of less catalyst.

The concentration of 4-CBA in the mother liquor, the transmittance and the burning of acetic acid referred to in the Examples are determined as follows: Concentration of 4- CBA in Mother Liquor The slurry of terephthalic acid is discharged from the liquid phase oxidation reaction vessel and is subjected to a liquid-solid separation to recover the mother liquor which is then subjected to esterification with ethylene glycol. Then, the concentration of 4-CBA is measured by gas chromatography.

Transmittance (T 340) The transmittance is measured with a solution of 15 parts of terephthalic acid in 100 parts of 2N aqueous potassium hydroxide in a spectrophotometer using a quartz cell having an internal size of 1 cm and a wave length of 340 m.

Burning The burning is calculated on the basis of the amounts of CO and CO2 (which are decomposition products of acetic acid) in the exhaust gas from the reaction vessel in each run and the results are compared with those of Example 1 as being 1.

This invention will be explained in detail by means of the following Examples. However, it should be understood that this invention is in no way limited by these Examples. In the Examples, "parts" is by weight.

Example I (Continuous Process) The process was carried out in an apparatus comprising, as 5 liquid phase oxidation vessel, an autoclave made of titanium and equipped with a reflux condenser, a stirrer, a heating means, an inlet for raw material, an inlet for solvent containing catalyst, an inlet for molecular oxygen-containing gas, an outlet for exhaust gas and an outlet for reaction product, as 5 post-oxidation vessel, an autoclave made of titanium and equipped with a reflux condenser, a stirrer, an inlet for gas, an inlet for slurry of terephthalic acid, an outlet for exhaust gas and an outlet for reaction product, and a crystallizer with cooling means.

To the liquid phase oxidation vessel, the following materials were continuously supplied in the proportion given below: p-xylene 1 part acetic acid 2.85 parts cobalt acetate tetrahydrate 0.004 part manganese acetate tetrahydrate 0.004 part 47% aqueous hydrogen bromide 0.006 part water 0.15 part The proportions of Co Mn and Br on the basis of acetic acid were 330 ppm, 330 ppm and 1000 ppm, respectively. The reaction conditions were a temperature of 210 C and a pressure of 25 kg/cm2, while passing air through the reaction mixture at such a rate that the exhaust gas contained oxygen ata concentration of 4% by volume and adjusting the dwell time to 120 minutes.

The slurry of terephthalic acid thus produced was transferred into the post-oxidation vessel and oxidized at a temperature of 1900C, under a pressure of 17.5 kg/cm2 for a dwell time of 85 minutes, while a mixture of air and the exhaust gas from the oxidation vessel ( 2 concentration of the mixture being 10% by volume) was supplied at such a flow rate that the exhaust gas contained 4% by volume of oxygen.

The slurry of the terephthalic acid was transferred into a crystallizer to effect crystallization at a temperature of 100"C.

The crude terephthalic acid crystals separated were washed by mixing with 4 times of acetic acid by weight at 800C for 20 minutes with agitation, and the washed terephthalic acid was recovered and dried.

The transmittance of the terephthalic acid thus produced, the concentration of 4-CBA in the mother liquor and the burning of acetic acid are shown in Table 1.

Examples 2 to 5 Procedures similar to those of Example 1 were repeated but the reaction temperature and/or the dwell time of the liquid phase oxidation were changed as shown in Table 1.

The results are given in Table 1.

Table I Dwell Example Temp. time 4CBA 4-CBA No. ("C) (min.) (ppm) T340(%) (ppm/TPA**) Burning 1 210 120 310 91.5 270 1.0 2 " 95 500 90.0 400 0.8 3 220 85 280 90.5 240 1.0 4* 210 45 1000 78.0 800 0.5 5* " 40 1300 68.0 1100 0.4 Note: * For comparison purpose.

** TPA: terephtharic acid.

Examples 6 to 13 Procedures similar to those of Example 1 were employed, but the active components in the catalyst system were changed as shown in Table 2.

The results are given in Table 2.

Table 2 Dwell Example Co Mn Br Mn/Co time 4-CBA T(%) 4-CBA Burning No. (min.) (ppm) (ppm/TPA**) 6 500 500 1500 1.0 120 300 91.0 270 1.1 7 330 300 1000 0.91 120 300 91.0 270 1.0 8 " 250 " 0.758 120 310 91.0 280 1.0 9 " 420 " 1.27 120 290 91.5 260 1.0 10* " 600 " 1.82 120 290 84.0 260 1.1 11* " 100 " 0.33 120 900 55.0 700 0.6 12* 1000 100 3000 0.1 120 450 91.0 400 1.5 13* " " 1500 " 120 500 85.0 450 1.1 Note: * For comparison purpose.

** TPA: Terephthalic acid.

Examples 14 to 16 Procedures similar to those of Example 1 were employed, but the active components in the catalyst system were changed as shown in Table 3.

The results are given in Table 3.

Table 3 Dwell Example Co Mn Br Bromine time 4-CBA T3400)4-CBA Burning No. Compound (min.) (ppm) (ppm/TPA**) 14 330 330 900 CoBr2 120 350 91.0 300 1.0 15 " " 960 MnBr2 " 340 " 290 16* " 600 1000 NaBr 175 300 93.0 300 1.6 Note: * For comparison purpose.

** TPA: Terephthalic acid.

Claims (1)

1. WHAT WE CLAIM IS:

   1. A process for producing high purity terephthalic acid comprising the steps of: (a) subjectingp-xylene in acetic acid as solvent to a one-step oxidation in an oxidation vessel with a molecular oxygen-containing gas having an oxygen concentration of from 5 to 100% by volume in an amount of from 1 to 100 moles of oxygen per mole ofp-xylene in the presence of a heavy metal compound-containing oxidation catalyst at a temperature of 100 to 250"C under a pressure of from atmospheric to 200 atmospheres to achieve a conversion of at least 95% to obtain a slurry of terephthalic acid, and (b) subjecting the slurry of terephthalic acid to post-oxidation in a post-oxidation vessel or crystallizer tank by passing a molecular oxygen-containing gas in an amount of from 1/1000 to 1/10 that used in the liquid phase oxidation through the slurry at a temperature 0 to 300C below that of the liquid phase oxidation in the presence of the oxidation catalyst used in step (a); in which said liquid phase oxidation of step (a) is effected while the concentration of 4-carboxybenzaldehyde in the mother liquor of the slurry is maintained at less than 880 ppm and said catalyst consists essentially of (1) a cobalt compound present in an amount, calculated as Co, of from 200 to 600 ppm based on the solvent, (2) a manganese compound present in an amount, calculated as Mn, of from 0.5 to 1.5 times that of Co by weight and (3) at least one bromine compound selected from hydrogen bromide, manganese bromide and cobalt bromide, present in an amount, calculated as Br, of from 400 to 2000 ppm based on the solvent and of more than two times that of Co by weight.

   2. A process according to claim 1, wherein said oxidation is effected at a temperature of from 190 to 2250C.

   3. A process according to claim 2. wherein said oxidation is effected at a temperature of from 205 to 220"C.

   4. A process according to any one of the preceding claims. wherein said bromine compound is hydrogen bromide.

   5. A process according to any one of the preceding claims wherein said cobalt compound is cobalt acetate and said manganese compound is manganese acetate.

   6. A process according to any one of the preceding claims, wherein said molecular oxygen-containing gas used in step (b) is the exhaust gas from step (a) alone or a mixture thereof with fresh molecular oxygen-containing gas.

   8. A process according to any one of the preceding claims wherein said oxidation step (a) in a continuous process is effected by controlling the dwell time of the raw material in the reaction vessel wherein the content of 4-carboxybenzaldehyde in the mother liquor of the terephthalic acid slurry is reduced to less than 800 ppm.

   9. A process according to any one of the preceding claims wherein said step (a) and said step (b) are effected continuously.

   10. A process according to any one of the preceding claims wherein the slurry containing solid terephthalic acid is subjected to crystallization, and the precipitated terephthalic acid is separated and washed with acetic acid.

   11. A process according to claim 10, wherein said washing of terephthalic acid with acetic acid is carried out by suspending said solid terephthalic acid in acetic acid and subjecting the terephthalic acid slurry to a solid-liquid separation.

   12. A process as claimed in claim 1 substantially as hereinbefore described with reference to the Examples.

   13. Terephthalic acid when obtained by a process as claimed in any one of the preceding claims.