GB1593117A - Process for the recovery of catalyst and solvent from the mother lquor of a process for the synthesis of therephthalic acid - Google Patents

Description

(54)1MPROVED PROCESS FOR THE RECOVERY OF CATALYST AND SOLVENT FROM THE MOTHER LIQUOR OF A PROCESS FOR THE SYNTHESIS OF TEREPHTHALIC ACID (71) We, MONTEDISON S.p.A., an Italian body corporate of 31 Foro Buonoparte, Milan, Italy, 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:- The invention relates to the synthesis of terephthalic acid and in particular to a process by which the catalyst and solvent used may be recycled.

United States Patent Specification No. 3,170,768 discloses a reaction in which paraxylene is oxidized with air in an acetic acid solution, according to the reaction scheme: C6H4(CH3)2+3 02eC6H4(COOH)2+2 H2O.

That patent discloses a catalytic system consisting of cobalt, manganese and bromine, the crystallization of terephthalic acid and the successive separation by centrifugation of terephthalic acid from the mother liquor containing acetic acid, water and the catalytic elements, together with oxidation intermediates, byproducts and residual traces of terephthalic acid. The patent also discloses that the mother liquor is fed to the reboiler of a column in which a flash-distillation, i.e. a distillation caused by a sudden pressure reduction, takes place. The gases are then passed to a second column together with aqueous acetic acid coming from other parts of the plant for the concentration of the acetic acid. The concentrated liquid flowing from the bottom of the first column is passed to a set of apparatus, where the components of the catalyst system are separated from the residual organic compounds and from other impurities before being recycled to the oxidation reaction.

United States Patent Specification No. 3,970,696 discloses a modification of the above process in which the mother liquor is neither distilled nor subjected to any treatment, but it is all recycled to the synthesis stage; the water content in the mother liquor however, must be below a prefixed level, and this low percentage water content is obtained using complex equipment.

The two processes are not free from drawbacks. When the first process is adopted the mother liquor undergoes a lengthy concentration process, which alters the organic substances and inactivates the catalyst, and therefore it is necessary to carry out a separation and a regeneration of the catalyst. The latter process avoids such problems but facilitates the accumulation of organic substances and of undesired metal ions, for example iron ions which are formed as a consequence of corrosion of the apparatus.

The water formed in the oxidation reaction dilutes the acid and hinders the oxidation, which is deleterious to the synthesis from an industrial view-point when thc water content exceeds 30%, and sometimes even only 20% by weight of the reacting mixture. As well as the need to keep the acetic acid an hydros it is also necessary to recover the acetic acid from the dilute solutions which are obtained in the various parts of the plant, for example from the mother liquor coming from the centrifuges where solid terephthalic acid is isolated. According to one of the known processes the different aqueous-acetic solutions are fed to a rectification column, where a great number of trays and a high reflux ratio allow almost anhydrous acetic acid to be obtained from the column bottom and water containing acetic residues from the column top. Such distillations however are not free from drawbacks and involve: (a) a large number of trays (in some cases up to 80) and a large steam consumption, in order to obtain, at the column top, a water stream containing no more than from 1000 to 5000 ppm of acetic acid and, from the column bottom, a concentrated acid stream containing no more than 3% by weight of water; (b) a total loss of the methylacetate leaving the column top along with water.

The present invention has been made with the above points in mind.

According to the invention there is provided an improved process for the synthesis of terephthalic acid comprising oxidizing para-xylene in acetic acid solution in the presence of a catalyst system comprising manganese, cobalt and bromine the amount of water in the oxidation zone being maintained at less than 10% by weight with respect to the acetic acid, in which process solid terephthalic acid is separated from the mother liquor, and the solids content of mother liquor is reduced to less than 0.30% by weight and the mother liquor is then rectified by distillation to reduce the amount of water in the rectified liquor below 5% by weight with respect to acetic acid, a portion of the rectified liquor containing at least 50 /O by weight of the catalytic components present in the original mother liquor being recycled to the oxidation zone, the remaining portion of the rectified liquor being fed as a purge for the separation and regeneration of the catalyst.

Vapours released during the oxidation and comprising water vapour and acetic acid vapour are preferably condensed and the resulting liquid is fed together with the mother liquor to the same distillation zone. Preferably the distillation is initially performed in a rectification column and then the stream richer in water, coming from the top of the column, is fed to an azeotropic column, in which the distillation is carried out in the presence of an azeotropic entrainer, preferably isobutyl acetate, whereby the stream which leaves the bottom of the azeotropic column contains a maximum of 5%, preferably 3%, by weight of water, with respect to the acetic acid, and is recycled to the oxidation zone.

The invention will now be described with reference to the accompanying drawings in which: Figure 1 represents a flow diagram of a process in accordance with the invention, Figure 2 depicts details of apparatus for the azeotropic distillation and the recovery of methylacetate, Figure 3 represents a flow diagram of a simplified process to that of Figure 1, Figure 4 represents a diagram of the pretreatment of mother liquor before being fed to the concentrator column, and Figure 5 represents a ternary diagram of a water, methylacetate and isobutyl acetate system.

According to Figure 1, the process comprises the following steps: (a) feeding of the mother liquor (1) above at least one and preferably above five trays of a distillation column (6) and recycling to the oxidation zone a portion (4) of the bottom liquid containing at least 50 /n of the catalyst entering the column and an amount of water lower than the amount of water in the mother liquor; (b) passing the remaining portion (7) of the bottom liquid via a still pot (2) to a heat exchanger (3) and conveying the effluent from the exchanger to the top of the still pot; (c) passing the vapours released in the still pot to the bottom tray of the column, withdrawing a liquid purge (5) from the still pot and conveying the purge to conventional treatments for the catalyst isolation and regeneration.

Recycle (4) contains less than 5%, preferably less than 1% by weight of water, manganese in an amount ranging from 50 to 1000 mg/Kg of acetic acid (i.e. from 0.005 to 0100 /n by weight) and cobalt in an amount corresponding to a manganese:cobalt ratio from 2:1 to 4:1 by weight; the residence time of the mother liquor in the column (excluding the very long residence time in the still pot) is less than 30 minutes and preferably less than 10 minutes. The bromine:(manganese+cobalt) atomic ratio if between 0.5:1 and 2:1 by weight, preferably between 0.5:1 and 1.5:1. Iron is present in amounts never exceeding 50 mg/Kg of acetic acid.

A particular preferred method of preparing the catalytic system is disclosed in copending Patent Application No. 25950/78 (Serial No. pending).

Most of the aqueous-acetic solutions, free from catalyst, coming from different steps of the synthesis process (line 8 in Figure 1) can be advantageously fed to the top of the concentrator column; thus it is possible to recover almost all the acetic acid circulating in the plant, in a form suitable for re-employment in the oxidation reactors. A considerable portion of acetic acid passes from the top of the column along with water, and the gases from the column head are fed to a second column (9), where a thorough recovery takes place.

The advantages provided by the present invention are significant. The amount of water in the oxidation reactors may be kept at a reasonable level, and a good recovery of acetic acid at a high concentration as well as a lesser amount of organic substances in the process effluents are obtained. Another appreciable advantage is the lower degree of the acetic acid losses because of oxidation to CO2.

Furthermore, the vapours released above the mother liquor feeding point are so rich in water, that the recovery of the acetic acid contained therein can be effected by azeotropic distillation, thus markedly reducing the steam consumption; best results have been obtained by using isobutyl acetate as the azeotropic agent (boiling point about 117"C; latent heat about 74 Kcal/Kg; forms an azeotropic mixture with water containing 16.5 /n by weight H2O and boiling at 87.40 C).

Isobutyl acetate has exhibited an exceptional compatibility with the oxidation reaction and the reduction of the steam consumption exceeds that expectable on the basis of calculations. Furthermore, isobutyl acetate has proved extremely effective in allowing the recovery of methyl-acetate (boiling point about 57"C; latent heat about 98 kcal/Kg; forms an azeotropic mixture with water containing 3.5 by weight H2O and boiling at 56.50C); methyl-acetate can be thus recovered as an aqueous solution at 80% and even at 90 /n by weight. Simultaneously, the last traces of unconverted para-xylene, entrained through the various process steps, are recovered. A further advantage is the significant reduction of the apparatus dimensions, compared with the prior processes for obtaining comparable results.

This is particularly evident when the solutions to be distilled are very dilute.

In Figure 2, the effluent (20/I) from the top of the azeotropic distillation volumn (9) is condensed and separated into two phases, the aqueous phase being fed to a stripping column (19) for the recovery of isobutyl acetate. The effluent from the top of the stripping column is partially condensed and fed to a separator (15) and the liquid phase (20), consisting substantially of isobutyl acetate, is recycled to the azeotropic distillation volumn, while the vapour phase (16), consisting substantially of methyl-acetate, is condensed and cooled and then collected in a storage tank. The water leaving the stripping column bottom contains very low percentages or organic matter, far lower than the values obtained in prior processes of this type.

This excellent and unexpected result allows simple operations to be carried out downstream to protect the environment from the waste products. The reflux ratio between the amount of isobutyl acetate recycled through line (17/I) and the amount of water in the effluent (20/1) is preferably from 4:1 to 14:1 by weight and more preferably from 6:1 to 10:1.

The process of the invention may be modified in various ways. For example the purge (line 5 in Figure 2) is preferably highly concentrated and prior to the isolation and regeneration treatments it is advisable to use preferably thin layer heat exchangers, optionally equipped with rotary stirrers, coaxial with the downcoming layer.

Optionally, a vertical baffle can divide the bottom of the rectification column into two portions. This baffle must be arranged in such a way as to let the liquid coming from the column trays fall only into the first of the two portions, in which the bottom is divided, the second portion being filled by the liquid overflowing the baffle (see Figure 2). As an alternative, the still pot of Figures 1 and 2 may be coincident, as shown in Figure 3, with the entire bottom of the column; in this case, the liquid which is not recycled to oxidation through line (4) must be withdrawn by an overflow nozzle connected with pipe (21), diametrically opposed to the bottom downcomer, in order to decant the maximum amount of the suspended terephthalic acid, that it is advisable to recover and that therefore must be present in the purge (5) in very low amounts. This alternative is to be recommended only when the purge is minimum and when the liquid recycled to oxidation (4) contains at least 90% of the catalytic elements which have entered the column; in such a case the catalyst recovery column (6) and the solvent recovery column (9) can optionally be replaced, as shown in Figure 3, by a single column, which eliminates the reaction water from the top and supplies an acetic acid sufficiently concentrated and free from solid organic impurities or metal ions, through a suction line (22), a few trays above the bottom and in the vapour phase.

The bottom of the rectification column, where the catalyst is recovered, is equipped, with suitable devices or shaped parts to prevent an accumulation of solid deposits and the consequent risk of clogging; in fact, the mother liquor (see line I in Figure 4) contains solids in suspension, usually less than 1% by weight, in the form of very fine-grained particles, consisting for the most part of residual terephthalic acid. According to Figure 4, the mother liquor (1), before entering the column, flows into a tank equipped with a stirrer and kept at rest. When a sufficiently high amount of solids has accumulated on the bottom, the stirrer is started and a pump conveys (at irregular intervals) the resulting slurry to a centrifuge (23); a measurable reduction in the terephthalic acid losses is thus achieved. The cake that has formed in the centrifuge is discharged into a sloping wall hopper (24) arranged on a screw conveyor (25). If the hopper volume is large enough, neither incrustations nor bridges will form. It is advisable to line the hopper walls with polytetrafluoroethylene or with other material having a low friction coefficient.

The mother liquor entering the column through line (26) contains less than 0.10% by weight, and preferably less than 0 0.05 by weight of solids in suspension in order to avoid clogging on the column trays.

Further variations of the process may be performed. For example, in the xylene oxidation, the use of oxidizing gases containing at least 7% by volume of oxygen, the use of temperatures ranging from 1000 to 2300C, at pressures from I to 30 atmospheres and with residence times between 0.5 and 3 hours, as well as the utilization of multi-inlet oxidation reactors, e.g. those described in United States Patent Specification No. 3,839,435.

The raw terephthalic acid may be purified according to various methods, for example, that disclosed in Italian Patent Specification No. 891,448. The oxidation reaction can be activated with small amounts of acetaldehyde, as disclosed, for example, in Japanese Patent Publication No. 66643/1974. The reactivation of the catalyst contained in the purge can be achieved for example, according to the teachings of United States Patent Specification Nos. 3,840,641 and 3,880,920, of German Offenlegungsschrift Nos. 2,260,491, 2,260,497 and 2,260,498, or of Italian Patent Specification Nos. 1,004,435 and 1,004,479.

The invention will now be illustrated by the following Examples.

Example 1 This Example adopted a process in accordance with Figure 1. Referring to Figure 1, the mother liquor (1), coming from the terephthalic acid centrifugation and containing about 85% by weight of acetic acid, was fed above the third tray from the bottom of a column (6), to the top of which was fed a stream (8) of aqueous acid at about 70 Cn by weight, free from catalyst, that had formed in other process steps.

The bottom of the column contained about 0.2% by weight of solids in suspension, consisting substantially of terephthalic acid that was suitably recovered. A portion of the bottom liquid, containing 94% by weight of acetic acid, 56% of the catalyst fed to the column (cobalt, manganese and bromine), 3% by weight of water and oxidation intermediates, was recycled to the synthesis through line (4); another portion of the bottom liquid flowed to the reboiler of same column, consisting of a still pot (2) equipped with a stirrer, and of a heat exchanger (3), arranged in series and steam-heated. A concentrated purge was sent, through line (5), to a thin layer evaporator, not shown in the Figure, and then to an incinerator; the remaining portion of the liquid coming from the still pot (2) passed through exchanger (3), flowed back to the still pot (2), where the vapours were released before entering again the column from the bottom. The residence time of the liquid on the bottom of the column, about 6 minutes, was not sufficient to case a decomposition of the organic substances of recycle (4). The vapours leaving column (6) from the top passed to a solvent recovery column (9), from whose bottom concentrated acetic acid (10) flowed out, that was utilized in the oxidation reactors together with the recycle (4) and with an acetic solution (11) containing regenerated catalyst and fresh catalyst to make up for the losses. The mixture was suitably prepared in a storage tank whereto the para-xylene feed was sent through line (11/a); line (11/b) coming from the tank led directly to the oxidation reactors.

Example 2 Referring to Figure 2, the mother liquor was fed, through pipe (1), to a rectification column equipped with 5 trays; the column had a bottom of enlarged diameter. This bottom was linked with the upper cylindrical section by means of a frusto conical surface and was divided into two portions by a vertical wall; the first portion was arranged under the downcomer of the bottom tray, and from the first portion the recycle (4) was directly drawn for the oxidation. Such recycle contained about 50% of the catalytic system entering the column (cobalt, manganese and bromine) and about 60% of the acetic acid necessary as a solvent for the oxidation; the water content in the recycle was approximately equal to 3% by weight.

Concentrated purge (5) contained the remaining portion of the catalytic system, but only a small amount of acetic acid, as exchanger (3) caused the release of the acetic acid vapours necessary for the distillation.

A liquid stream (8) of aqueous acetic acid at 70% (by weight), coming from other parts of the plant, entered the top of the column.

The vapours leaving the column top entered a second tray column (9), equipped with a reboiler, a reflux condenser and a separating tank (12). Through pipe (13), an amount of an azeotropic agent (isobutyl acetate), sufficient to make up for the losses, was added, a vertical baffle arranged in tank (12) facilitated easy separation of the organic phase (that flowed back to the column top) from the aqueous phase. Successively isobutyl-acetate, methyl-acetate and other organic compounds were recovered from the aqueous phase by means of a stripping with direct steam in a column (19), supplied with a partial condenser. A tank (15) separated the uncondensed vapour phase (16), consisting from the 92% of methylacetate, from a liquid phase predominantly consisting of isobutyl-acetate, that passed to the separating tank (12). The vapour phase, consisting predominantly of methyl-acetate, was condensed in exchanger (18) and sent to storage. The water flowing out from the stripping column bottom was discharged and had very low concentrations of organic products; it contained only 30 ppm of acetic acid and 20 ppm or isobutyl acetate, against 10,000 ppm of various other organic products usually present in this type of wastes, if distillation is of the conventional type.

The recovered concentrated acetic acid flowed out from the bottom of column (9) for the recycle, and contained only 3% by weight of water. The ternary diagram reported by Figure 5, experimentally determined at 300 C, is concerning the miscibility and immiscibility of the compositions comprising water, methylacetate and isobutyl acetate.

Examples 3 and 4 Examples 1 and 2 were repeated subjecting the liquor (1) to a decantation before entering the column, as shown in Figure 4, the stirrer of the settling tank was started at invervals and the accumulated slurry was recycled to centrifugation (23).

The amount of solids suspended in the mother liquor decreases thus to less than 0.03% by weight. The recycle (4) to the oxidation contained less than 0.02 /" by weight of suspended solids. The results obtained were superior to those of Examples 1 and 2.

The yield of Example 3 was 95 /n (moles of terephthalic acid per mole or xylene) and the properties of the terephthalic acid were the following: carboxy-benzaldehyde 1450 ppm colour (colour of a 15% by weight solution of terephthalic acid in 2N KOH) 35 APHA light transmittance (340 millimicron light through 15% by weight solution of terephthalic acid in 2N KOH) 58.7% The amount of carboxy-benzaldehyde was then reduced, by an hydrogenating treatment, to less than 20 parts per million; the purified terephthalic acid (PTA) obtained by this way showed a light transmittance higher than 95% and was "fibregrade".

Example 5 Example 4 was repeated in substantially the identical way, but increasing the amount of recycle (4), as to bring back to the oxidation 86% of the catalytic elements entering the partially anhydrifying column. The results were substantially comparable with the results of Example 4.

WHAT WE CLAIM IS: 1. A continuous process for the synthesis of terephthalic acid comprising oxidizing para-xylene in acetic acid solution in the presence of a catalyst system comprising manganese, cobalt and bromine the amount of water in the oxidation zone being maintained at less than 10% by weight with respect to the acetic acid, in which process solid terephthalic acid is separated from the mother liquor, and the solids content of mother liquor is reduced to less than 0.30% by weight and the mother liquor is then rectified by distillation to reduce the amount of water in the rectified liquor below 5% by weight with respect to acetic acid, a portion of the rectified liquor containing at least 50% by weight of the catalytic components present in the original mother liquor being recycled to the oxidation zone, the remaining portion of the rectified liquor being fed as a purge for the separation and the regeneration of the catalyst.

2. A process as claimed in Claim 1 in which vapours released during the oxidation and comprising water vapour and acetic acid vapour are condensed and the resulting liquid is fed together with the mother liquor to the same distillation zone.

3. A process as claimed in Claim I or Claim 2 in which distillation is initially performed in a rectification column and the stream richer in water, coming from the top of said column, is fed to an azeotropic column, in which the distillation is carried out in the presence of an azeotropic entrainer, whereby the stream which leaves the bottom of the second column contains a maximum of 5% by weight of water with respect to the acetic acid and is recycled to the oxidation zone.

4. A process as claimed in Claim 3 in which the stream leaving the azeotropic column contains a maximum of 3% by weight of water with respect to the acetic acid.

5. A process as claimed in Claim 3 or Claim 4 in which the azeotropic entrainer is isobutyl acetate.

6. A process as claimed in any one of claims 3 to 5 in which the rectification column contains trays and the mother liquor is fed above at least the third tray from the bottom.

7. A process as claimed in any one of claims 3 to 6 in which the rectification column and the azeotropic column coincide with a single column, from which acetic acid is withdrawn, which contains less than 5% by weight of water and which is in the vapour form.

8. A process as claimed in any one of claims 5 to 7 in which the effluent from the top of the azeotropic column is condensed and separated into two phases the aqueous phase of which is conveyed to a stripping column for recovery of isobutylacetate and methyl-acetate.

9. A process as claimed in claim 8 in which the effluent from the top of the stripping column is partially condensed and passed to a separating tank, in which the vapours consisting predominantly of methyl-acetate are separated from a liquid phase containing isobutyl acetate, which is recycled to the azeotropic column.

10. A process as claimed in claim 9, in which at least 90 /n by weight of the methyl acetate leaving the top of the stripping column is recovered by condensation.

11. A process as claimed in claim I, in which the amount of water in the rectified liquor is below 1% by weight with respect the acetic acid.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. Example 5 Example 4 was repeated in substantially the identical way, but increasing the amount of recycle (4), as to bring back to the oxidation 86% of the catalytic elements entering the partially anhydrifying column. The results were substantially comparable with the results of Example 4. WHAT WE CLAIM IS:

1. A continuous process for the synthesis of terephthalic acid comprising oxidizing para-xylene in acetic acid solution in the presence of a catalyst system comprising manganese, cobalt and bromine the amount of water in the oxidation zone being maintained at less than 10% by weight with respect to the acetic acid, in which process solid terephthalic acid is separated from the mother liquor, and the solids content of mother liquor is reduced to less than 0.30% by weight and the mother liquor is then rectified by distillation to reduce the amount of water in the rectified liquor below 5% by weight with respect to acetic acid, a portion of the rectified liquor containing at least 50% by weight of the catalytic components present in the original mother liquor being recycled to the oxidation zone, the remaining portion of the rectified liquor being fed as a purge for the separation and the regeneration of the catalyst.

2. A process as claimed in Claim 1 in which vapours released during the oxidation and comprising water vapour and acetic acid vapour are condensed and the resulting liquid is fed together with the mother liquor to the same distillation zone.

3. A process as claimed in Claim I or Claim 2 in which distillation is initially performed in a rectification column and the stream richer in water, coming from the top of said column, is fed to an azeotropic column, in which the distillation is carried out in the presence of an azeotropic entrainer, whereby the stream which leaves the bottom of the second column contains a maximum of 5% by weight of water with respect to the acetic acid and is recycled to the oxidation zone.

4. A process as claimed in Claim 3 in which the stream leaving the azeotropic column contains a maximum of 3% by weight of water with respect to the acetic acid.

5. A process as claimed in Claim 3 or Claim 4 in which the azeotropic entrainer is isobutyl acetate.

6. A process as claimed in any one of claims 3 to 5 in which the rectification column contains trays and the mother liquor is fed above at least the third tray from the bottom.

7. A process as claimed in any one of claims 3 to 6 in which the rectification column and the azeotropic column coincide with a single column, from which acetic acid is withdrawn, which contains less than 5% by weight of water and which is in the vapour form.

8. A process as claimed in any one of claims 5 to 7 in which the effluent from the top of the azeotropic column is condensed and separated into two phases the aqueous phase of which is conveyed to a stripping column for recovery of isobutylacetate and methyl-acetate.

9. A process as claimed in claim 8 in which the effluent from the top of the stripping column is partially condensed and passed to a separating tank, in which the vapours consisting predominantly of methyl-acetate are separated from a liquid phase containing isobutyl acetate, which is recycled to the azeotropic column.

10. A process as claimed in claim 9, in which at least 90 /n by weight of the methyl acetate leaving the top of the stripping column is recovered by condensation.

11. A process as claimed in claim I, in which the amount of water in the rectified liquor is below 1% by weight with respect the acetic acid.

12. A process as claimed in claim 1 substantially as herein described with

reference to any one of the Examples.

13. A process as claimed in claim 1 substantially as herein described with reference to the accompanying drawings.