JPS6241219B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for producing a catalyst from a mother liquor for terephthalic acid synthesis.
An improved method for recovering solvents and oxidized intermediates. The method is thus mainly based on the partial dehydration of the mother liquor.

According to US Pat. No. 3,170,768, p-xylene is oxidized by air in an acetic acid solution with the following reaction: C ₆ H ₄ (CH ₃ ) ₂ +3O ₂ →C ₆ H ₄ (COOH) ₂ +2H ₂ O The US patent describes a catalyst system consisting of cobalt, manganese and bromine and the crystallization of terephthalic acid followed by its separation from the mother liquor by centrifugation. The mother liquor thus contains, in addition to acetic acid, water and catalyst elements, oxidized intermediates, by-products and residual traces of terephthalic acid.
This patent also discloses a method of feeding the mother liquor to a reboiler of a column in which flash distillation (i.e., distillation caused by a rapid pressure drop) occurs. The steam then enters a second column with aqueous acetic acid exiting from other plant locations for acetic acid dehydration. The concentrate emerging from the bottom of the first column is sent to a set of equipment where the components of the catalyst system are separated from residual organic compounds and other impurities and then recycled to the oxidation process.

According to a recent technique described in US Pat. No. 3,970,696, the mother liquor is recycled directly into the synthesis process without being distilled or subjected to any treatment. However, the water content in the mother liquor must be below a predetermined level, and such low water content can be obtained with specific and complex performance synthesis equipment.

Both of the above two techniques have drawbacks. When implementing the first technique, the mother liquor is subjected to a prolonged concentration step, which changes the organic substances and deactivates the catalyst. Therefore, it is necessary to separate and regenerate the catalyst. The latter technique also avoids such changes, but is susceptible to the accumulation of organic substances and undesired metal ions, such as iron ions, coming from equipment as a result of corrosion phenomena. One object of the invention is thus to eliminate or alleviate these disadvantages.

The reaction water dilutes the acid and thus prevents oxidation. When the water content exceeds 30% by weight of the reaction mixture, and sometimes even when it is only 20% by weight, it cannot be recognized as a satisfactory synthesis from an industrial point of view. In addition to the need to keep the acetic acid anhydrous, there is a need to recover it from the more or less dilute solutions occurring at various points in the plant (e.g., the mother liquor coming from the centrifuge from which the solid terephthalic acid is isolated). Previously, various aqueous acetic acid solutions were fed to a rectification column where, by means of a large number of trays and a high reflux ratio, nearly anhydrous acetic acid was obtained (at the bottom of the column);
An acetic acid residue containing water was also obtained (at the top of the column). However, previously performed distillations have several drawbacks. In fact, such drawbacks include: (a) A large number (in some cases up to 80 ) trays and excessive steam consumption are required and (b) there is a complete loss of methyl acetate leaving the top of the column along with the water.

Another aim of the invention is to solve these problems. Other purposes will become apparent from the description below.

In its broadest aspect, ^the present invention provides p-
The acetic acid solution of xylene is oxidized, the by-produced water and methyl acetate are released from the top of the oxidation zone in vapor form, and after the oxidation reaction is completed, the solid terephthalic acid formed is separated from the mother liquor, and the released vapor is (A) The liquid obtained by condensing the emitted vapor is subjected to azeotropic distillation using isobutyl acetate as an azeotropic agent, together with the mother liquor after separation of terephthalic acid. (B) condensing the water-enriched light fraction containing isobutyl acetate and methyl acetate obtained by azeotropic distillation in the distillation system, and combining the resulting condensate with the aqueous phase; The lower aqueous phase containing isobutyl acetate and methyl acetate is then conveyed to a stripping zone where the isobutyl acetate and methyl acetate are recovered, while the organic phase is subjected to an azeotropic distillation system. The liquid obtained by condensing the vapors released during the oxidation and the mother liquor freed of suspended solids are advantageously fed to the same distillation zone. . According to a preferred embodiment of the invention, the distillation is first carried out in a partial dehydration column, and the water-enriched stream emerging from the top of said column is fed to a second column;
Distillation is then carried out in the presence of an azeotropic entrainer, preferably isobutyl acetate, so that the stream leaving said second column contains less than 5% by weight of water relative to the acetic acid, and The flow is recycled to the oxidation zone.

The present invention will now be described with reference to the accompanying drawings. However, the invention is not limited in any way by these attached figures, either individually or in combination.

According to FIG. 1, the process according to the invention comprises the following steps: (a) feeding the mother liquor 1 onto at least one, preferably 5, trays of a distillation column 6, preferably at least 20% of the catalyst entering said column; (b) recycling a portion 4 of the bottoms containing at least 50% water less than the water content in the mother liquor to the oxidation zone; (b) a remaining portion 7 of the bottoms being conveyed to the distillation kettle 2;
(c) the vapor released from the distillation column flows into the bottom tray of the distillation column and the liquid It consists of withdrawing the purge 5 from the distillation kettle and conveying the purge to conventional processing equipment for isolation and regeneration of the catalyst.

Recycle stream 4 contains less than 10% by weight (preferably less than 5%) water, 50 to 50% per kg acetic acid.
Manganese in amounts ranging from 1000 mg (i.e. 0.005 to 0.100% by weight) and manganese to cobalt ratio = 2:1 to
It contains cobalt in an amount corresponding to 4:1 (by weight). The residence time of the mother liquor in the distillation column (excluding very long residence times until distillation) is less than 30 minutes, preferably less than 10 minutes. Bromine to (manganese + cobalt) ratio is suitably 0.5 to 2, preferably 0.5 to 1.5 (by weight)
should be within the range of Iron is 50mg per 1kg of acetic acid.
should be present in an amount that never exceeds. A specific and useful method for preparing the catalyst system is disclosed in Italian patent application 26113A/77.

Most of the catalyst-free aqueous acetic acid solution (line 8 in Figure 1) coming from the various steps of the synthesis process can advantageously be fed to the top of the partial dehydration column. Thus, substantially all of the acetic acid circulating through the plant can be recovered in a form suitable for reuse in the oxidation reactor. Since a significant portion of the acetic acid exits from the top of the distillation column together with the water, the column head vapor is necessarily fed to the second column 9, where complete recovery takes place. The benefits provided by the present invention are significant. This means that the water in the oxidation reactor, which is always very harmful, can be retained in a moderate amount, acetic acid with a low water content can be recovered satisfactorily, and the amount of organic substances in the process effluent can be reduced. . Another measurable effect is
This means that the loss of acetic acid due to combustion to CO ₂ is small.

Furthermore, since the steam released above the mother liquor feed point is very rich in water, recovery of the acetic acid contained therein can be carried out by azeotropic distillation, thus significantly reducing the consumption of steam. . The best effect is with isobutyl acetate (bp approx. 117℃,
Latent heat approximately 74Kcal/Kg; together with water, H ₂ O16.5
(forming an azeotrope of bp 87.4°C) containing wt%.

Isobutyl acetate is exceptionally compatible with oxidation reactions,
Water vapor consumption is lower than would be expected based on calculations. Furthermore, isobutyl acetate has a bp of about 57°C and a latent heat of about 98 Kcal/Kg; together with water, it has a bp of 56.5°C containing 3.5% by weight of H ₂ O.
It has been found to be very effective in making it possible to recover azeotropes (forming azeotropic mixtures). Thus, methyl acetate is 80
It can be recovered as a %, sometimes 90%, aqueous solution by weight. Similarly, the last traces of unconverted p-xylene entrained through the various process steps are recovered. Yet another advantage is that the device method can be reduced significantly with equal effectiveness. This is particularly evident when the solution to be distilled is very dilute.

According to FIG. 2, the effluent 20/I from the top of the azeotropic distillation column 9 is condensed and stratified into two phases. The aqueous phase is fed to a stripping column 19 to recover isobutyl acetate. The effluent from the top of the stripping column is partially condensed and fed to separator 15. Thus, the liquid phase 20, consisting essentially of isobutyl acetate, is recycled to the azeotropic distillation column, and the gaseous phase 16, consisting primarily of methyl acetate, is first condensed and subcooled before being collected in a storage tank. .
The water leaving the stripping column bottom contains much lower organic content than previously determined for this type of process. This excellent and surprising effect makes it easier to carry out downstream operations in line with the protection of ecological living conditions. line 1
The reflux ratio between the amount of isobutyl acetate recycled by 7/I and the amount of water in the effluent 20/I is preferably from 4:1 to 14:1, more preferably from 6:1 to 10:1.
(weight).

Various useful modifications can be made to the invention. For example, purge (line 5 in Figure 2)
should preferably be highly concentrated.
Thus, before the isolation and regeneration process, it is expedient to use a preferably thin-layer heat exchanger, optionally equipped with a rotating stirrer in coaxial relationship with the descending layer.

Optionally, a vertical baffle can divide the bottom of the partial dehydration column into two parts. This baffle must be arranged in such a way that the liquid coming from the column tray falls only into one of these two parts. The other part is then filled by the liquid overflowing the baffle (see Figure 2). Alternatively, the stills of Figures 1 and 2 may be
It may correspond functionally to the entire bottom of the column shown in the figure.
In this case, the liquid which is not recycled to the oxidation process via line 4 is present in a very small amount in the purge 5 in order to maximize the decantation of the suspended terephthalic acid which is expedient to be recovered. It must then be drawn off by an overflow nozzle connected diametrically by a pipe 17 to the downcomer at the bottom of the column. This alternative is recommended only when purging is minimal and when the liquid 4 recycled to the oxidation process contains at least 90% of the catalyst elements that entered the column. In such a case, the catalyst recovery column 6 and the solvent recovery column 9 can optionally be replaced with a single column as shown in FIG. This single column rejects the water of reaction at the top, which also carries acetic acid in the gas phase, which is well dehydrated and free of solid organic impurities or metal ions, from a tray two to three stages above the bottom via suction line 22. bring.

At the bottom of the partial dehydration column from which the catalyst is recovered, suitable devices are of course installed to prevent the accumulation of solid precipitates and the resulting risk of blockage. In fact, the mother liquor contains suspended solids in the form of very fine particles, usually less than 1% by weight, consisting mostly of terephthalic acid. According to FIG. 4, before entering the column, the mother liquor 1 flows into a tank equipped with an agitator and is allowed to stay there. When a sufficiently large amount of solids has accumulated at the bottom,
The agitator is started and the resulting slurry is occasionally pumped to the centrifuge 23. Thus, losses of terephthalic acid are significantly reduced. The cake formed in the centrifuge is transferred to an inclined wall hopper 2 placed on a screw conveyor 25.
It is discharged at 4. If the capacity of the hopper is large enough, neither skinning nor crosslinking on the walls will occur, but it is advisable to line the hopper with polytetrafluoroethylene or other materials with a low coefficient of friction. The mother liquor entering the column through line 26 contains suspended solids of less than 0.10% by weight and preferably less than 0.05% by weight to avoid clogging or clogging on the trays of the column. Furthermore, various modifications can be made. For example, for the oxidation of xylene, an oxidizing gas with an oxygen content of at least 7% by volume, a residence time of 0.5 to 3 hours, and US Pat.
Mention may be made of using a multi-inlet oxidation reactor such as that described in No. 3,839,435.

The crude terephthalic acid can also be purified according to various methods (one example of which is given by Italian Patent No. 871,448) and the oxidation can be carried out according to e.g. can be activated with small amounts of acetaldehyde. Furthermore, reactivation of the catalyst contained in the purge is described, for example, in U.S. Pat.
3880920, German Patent Publication No. 2260491, German Patent Publication No.
2260479 and 2260498 and Italian Patent Nos. 1004435 and 1004479.

The following examples are given to illustrate the invention, but are not intended to limit it thereby.

Example 1 According to FIG.
on the third tray from the bottom of the column, and also at the top of the column, about aqueous acetic acid formed in other process steps.
A 70% by weight catalyst-free stream 8 was fed.

The bottom of the distillation column contained approximately 0.2% by weight of solids in suspension, consisting primarily of terephthalic acid and to be recovered accordingly. A portion of this bottom liquor, containing 94% by weight of acetic acid, 56% of the catalyst fed to the column (cobalt, manganese and bromine), 3% by weight of oxidized intermediates and water, is recycled to the synthesis process via line 4; The other bottom liquid portion entered the reboiler of the same column, consisting of a stirred distillation kettle 2 and a heat exchanger 3 arranged in series, and was heated with steam. The purge to be concentrated was sent through line 5 to a thin layer evaporator (not shown) and then to an incinerator. The remaining liquid coming from the distillation kettle 2 passed through the heat exchanger 3 and returned to the distillation kettle 2 to release steam, and then flowed into the distillation column again from the bottom. The residence time of the liquid at the bottom of the column, about 6 minutes, was not long enough to alter the organic matter of recycle stream 4. Vapors exiting from the top of column 6 passed through solvent recovery column 9, and nearly anhydrous acetic acid 10 exited from the bottom thereof.
This was reused in the oxidation reactor along with recycle stream 4 and acetic acid solution 11 containing regenerated catalyst and fresh catalyst to replace what was lost. It should be noted that this mixture is suitably prepared in a storage tank into which the paraxylene feed is fed through line 11/a, and from which it is conveyed directly to the oxidation reactor via line 11/b.

Example 2 According to FIG. 2, mother liquor was supplied through pipe 1 to a partial dehydration tower equipped with five trays. The tower has a large diameter base, which is connected to the upper cylindrical part by a truncated cone and is divided into two parts by a vertical wall. One of these two sections was then placed below the downcomer of the bottom tray, from which the recycle stream 4 was drawn directly for oxidation. Such recycle stream accounts for approximately
Approximately 50% acetic acid and necessary as solvent for oxidation
Contained 60%. The water content of this recycle stream was approximately 3% by weight. The concentrated purge 5 contained the remaining catalyst system and also contained only a small amount of acetic acid. This is because the heat exchanger 3 releases acetic acid vapor necessary for distillation.

Additionally, 70% aqueous acetic acid comes from elsewhere in the plant.
(by weight) of liquid stream 8 entered the top of column 2.

The steam leaving the top of the column entered the second column 9, which was equipped with a reboiler, a reflux condenser and a layer forming layer 12. A sufficient amount of isobutyl acetate was added to the reflux condenser through pipe 13 to replenish the loss of the entrainer isobutyl acetate. Vertical baffles placed within vessel 12 allowed easy separation of the organic phase from the aqueous phase. The separated organic phase was returned to the column. In column 19 equipped with a partial condenser, isobutyl acetate, methyl acetate and other organic compounds were successively recovered by direct steam stripping. A liquid phase mainly consisting of isobutyl acetate was separated from an uncondensed gas phase 16 consisting of 92% methyl acetate by a separator 15 and passed to a layer forming tank 12.
The gas phase, consisting mainly of methyl acetate, was condensed in a heat exchanger 18 and sent to storage. Water was discharged from the bottom of the stripping tower. It was discovered that this material had very little organic content. In fact, it contained only 30 ppm and 20 ppm of acetic acid and isobutyl acetate, respectively, compared to the 10,000 ppm of various organic components normally present in this type of purge in conventional type distillations.

The recovered dehydrated acetic acid flows out from the bottom of column 9 and is recycled, but its water content is only 3% by weight. The ternary diagram in Figure 5 shows the miscibility and immiscibility of water, methyl acetate, and isobutyl acetate at 30°C.
The results of experimental measurements are shown.

Examples 3 and 4 In these examples, Examples 1 and 2 were repeated and Liquid 1 was decanted and then placed in a column as shown in FIG. The settler agitator was started intermittently to recirculate the accumulated slurry to the centrifuge 23. Thus, the solid content suspended in the mother liquor is
It decreased to 0.03% by weight or less. Recycle stream 4 to the oxidation contained less than 0.02% by weight suspended solids. The results obtained were much more satisfactory than those of Examples 1 and 2.

In Example 3, the yield of terephthalic acid was 95 mole percent per mole of xylene. The properties of terephthalic acid were as follows.

-Carboxy-benzaldehyde 1450ppm -Color (color of 15% terephthalic acid solution in 2N KOH) 35APHA -Light transmission (340 mμ light through 15% wt terephthalic acid solution in 2N KOH) 58.7% Carboxybenzaldehyde is then hydrogenated It was reduced to below 20ppm by chemical treatment. Purified terephthalic acid (PTA) obtained by this method
It exhibited a light transmittance of over 95% and was "fiber grade".

Example 5 Example 4 was repeated, but the amount of recycle stream 4 was increased so that 86% of the catalyst elements entering the pre-dehydration tower were returned to the oxidation process. The results obtained were essentially the same as those of Example 4.

[Brief explanation of the drawing]

FIG. 1 shows an example of a possible flow sheet for the process according to the invention, FIG. 2 is a flow sheet showing the azeotropic distillation and recovery of methyl acetate in some detail, and FIG. This is a flow sheet that simplifies the process shown in FIG. 1, FIG. 4 is a flow sheet relating to pretreatment of the mother liquor before supply to the dehydration tower, and FIG. 5 is a ternary diagram. Explanations of the symbols representing main parts in the attached drawings are as follows. 1: mother liquor, 2: distillation kettle, 6, 6A: distillation column or partial dehydration column, 6B: partial dehydration column and azeotrope column,
9: azeotropic distillation column or solvent recovery column, 12: layer forming layer, 15: separator, 19: stripping column, 23: centrifugal separation column, 25: screw conveyor.

Claims (1)

[Claims] 1. Oxidation of an acetic acid solution of p-xylene at a pressure of 1 to 30 Kg/cm ² and a temperature of 100 to 230°C in the presence of a catalyst based on manganese, cobalt and bromine to produce a by-product. A method for the synthesis of terephthalic acid comprising discharging water and methyl acetate in vapor form from the top of an oxidation zone, separating the solid terephthalic acid formed from the mother liquor after the completion of the oxidation reaction, and condensing the discharged vapor. (A) the liquid obtained by condensing the released vapor together with the mother liquor after separation of the terephthalic acid;
(B) condensing the water-enriched light fraction containing isobutyl acetate and methyl acetate obtained by azeotropic distillation in the system; separating the condensate thus formed into an aqueous phase and an organic phase, and conveying the lower aqueous phase containing isobutyl acetate and methyl acetate to a stripping zone where the isobutyl acetate and methyl acetate are recovered; On the other hand, the method for synthesizing terephthalic acid, characterized in that the organic phase is recycled to the azeotropic distillation system. 2 The reflux ratio between the amount of isobutyl acetate recycled with the organic layer and the amount of water in the light fraction is 4:1 to 14:
1. The method according to claim 1, wherein the method is: 3. The method according to claim 2, wherein the reflux ratio is 6:1 to 10:1 (by weight). 4 Distillation is first carried out in a partial dehydration column, and the water-enriched stream emerging from the top of said column is fed to a second column, where the distillation is carried out in the presence of the azeotropic entrainer isobutyl acetate, thereby 2. The method of claim 1, wherein the stream leaving the bottom of the second column contains less than 5% water by weight relative to acetic acid, and wherein the stream is recycled to the oxidation zone. . 5. The method of claim 4, wherein the stream exiting the second column contains less than 3% by weight water.