BACKGROUND OF THE INVENTIION

FIELD OF THE INVENTION

The present invention relates to a process for purifying crude terephthalic acid.

DESCRIPTION OF THE RELATED ART

A process for producing purified terephthalic acid (PTA) is known, for example, comprising the steps as described in the following. Namely, para-xylene (or p-xylene) is used as a starting material to be oxidized in a liquid phase of an acetic acid by oxygen-containing gas (for example, air) in the presence of a catalyst:. This oxidation reaction produces crude terephthalic acid (CTA). Then, after the resultant crude terephthalic acid is dissolved in an aqueous medium at elevated temperature and pressure, the aqueous solution is treated in the hydrogenation reaction.

The pressure and temperature of the hydrogenated aqueous solution at the high pressure and the high temperature after the hydrogenation reaction is gradually lowered by flash cooling, thereby crystals of terephthalic acid is recrystallized and a slurry of purified terephthalic acid crystals is produced. The resultant slurry of the crystals is treated in solid-liquid separation to recover the purified terephthalic acid crystals. The above process allows of production of purified terephthalic acid at high-purity (high grade terephthalic acid).

In the above mentioned method, large volume of an aqueous medium is not only needed in different steps but also a used aqueous medium is to be discharged outside. More specifically, the method includes the steps of: dissolving crude terephthalic acid, crystallizing purified terephthalic acid crystals (or a crystallization step) from hydrogenated aqueous solution, conducting solid-liquid separation of the resultant crystals slurry to recover the crystals after the separation; and drying the recovered crystals to prepare a dry powder product.

The aqueous medium used in those steps for producing purified terephthalic acid described above may mostly involve an aqueous medium that dissolves crude terephthalic acid (The aqueous medium needs 2-fold to 5-fold weight ratio to crude terephthalic acid dissolved therein). Further, another aqueous medium is mostly needed as washing water to wash purified terephthalic acid crystals when the purified terephthalic acid crystals are collected (The aqueous medium needs 0.5-fold to 2-fold weight ratio to the purified terephthalic acid crystals to be washed).

Moreover, fresh washing water is needed to be supplied to a scrubber (or gas absorber) for cleaning a discharged gas and a circulation gas. These gases are used for pressuring and sealing each apparatus, column and tank applied to the above- mentioned production steps. Washing water having 0.5-fold to 2-fold weight ratio to the purified terephthalic acid crystals is required. Further, fresh water as the using aqueous medium is needed high-purity water such as deionized water and condensed water supplied to a boiler. Such aqueous medium is different from general industrial water since the objective is to produce high-quality terephthalic acid. It is noted for such water to be generally needed high procurement costs.

Therefore, if discharged waters the respective treatment steps are cleaned and reused without discharging any aqueous media outside the manufacturing facilities in a single treatment step only, the aqueous media may be repeatedly reused and applied to other processes steps. This allows of the saving of a newly supplied aqueous medium, further enabling to decrease the amount of the treatment and reduce the load for treatment in waste water treatment. Moreover, this results in reduction of the costs for producing high-purity terephthalic acid.

Accordingly, a technique for reducing the used amount of the highly purified aqueous medium includes,
for example, a method disclosed in Japanese Unexamined Patent Application Publication No.H05-58948. The method comprises the steps of: treating an aqueous mother liquid (or a first mother liquid) thereby to produce low-purity precipitates composed of terephthalic acid and a second mother liquid; and conducting the following processes.

The patent document discloses a technique for producing terephthali'.. acid by using at least one step selected from a group comprising: (a) sending back the said lower-purity precipitates directly or indirectly to the reaction medium; (b) utilizing at least a part of the said second mother liquid directly or indirectly through treating the liquid in order to dissolve the crude solid substance; and (c) sending the second mother liquid to a fractional distillation device, recovering water treated by the fractional distillation device, and using the resultant water for washing the precipitates collected from the solution after the hydrogenation step.

Further, International Publication No. WO 2004/63136 discloses a method for producing high-purity terephthalic acid by using the steam and/or the condensed liquid generated from a crystallization tank when the pressure and temperature of the tank is gradually lowered in a crystallization step. Herein, the steam and/or the condensed liquid are/is reused as a part of the aqueous medium such that the concentration of para-toluic acid in the first separated mother liquid obtained in the first solid-liquid separation is adjusted at 800 ppm to 2000 ppm.

From the viewpoint as mentioned above, the present inventors proposed an improved method for treating a separated first aqueous mother liquid in PCT International Patent Application No. PCT/JP2011/0530701. The proposed method comprised the steps of: adding terephthalic acid crystals to the primary mother liquid (or first mother liquor), treating through the cooling and the like to the resultant aqueous mother liquid, and recovering the low-purity terephthalic acid crystals and a secondary mother liquid (or second mother liquor) with the reduced content of para-toluic acid.

SUMMARY OF THE INVENTION

When terephthalic acid is produced by oxidizing para-xylene, terephthalic acid is produced over the stepwise oxidation reaction. Thus, the key problem is to develop a method for reducing 4-formylbenzoic acid that is formed in the oxidation reaction as an intermediate by-produbt and remains in the crude terephthalic acid. Ir. particular, terephthalic acid is commonly applied as a starting material for manufacturing of polymers such as polyester materials like PET (polyethylene terephthalate). Hereby, contamination of a by-product that may terminate or inhibit such a polymerization reaction is not desirable.

However, it is generally too difficult to remove 4-formylbenzoic acid contaminated in the crude terephthalic powder due to the highly chemical affinity thereof with terephthalic acid. Accordingly, for example, as described in International Publication No. WO 2004/63136, 4-formylbenzacid is selectively reduced by converting into para-toluic acid (or p-toluic acid). Then, para-toluic acid thus converted is removed by being dissolved into an aqueous medium, whereby the crude terephthalic acid is purified.
As mentioned above, by-products such as para-toluic acid are contained in an aqueous mother liquid (or first mother liquor) after purified terephthalic acid crystals have been recovered. Hereby, it is important to remove those by-products so as to decrease the volume of a newly using aqueous medium such as deionization water or the like. Therefore, after the primary mother liquid is treated with the ways such as cooling and the like, low-purity terephthalic acid crystals containing para-toluic acid (or precipitates) is recovered. Thus, a second mother liquid (or a secondary mother liquid) is produced. At least a part of the secondary mother liquid which is decreased in para-toluic acid content is directly reused or indirectly reused after treating as an aqueous medium to dissolve the crude terephthalic acid crystals. This procedure is disclosed in Japanese Unexamined Patent Application Publication No. H05-58948.

Therefore, it is preferable to remove by-products like para-toluic acid as much as possible from the aqueous medium which is to be reused when a used aqueous medium is reused, in order to decrease the volume of a newly using aqueous medium. Further, if the primary mother liquid is treated as mentioned above so as to maximally remove the by-products, this procedure leads to decrease in the volume of the newly using aqueous medium. Thus, those techniques are disclosed in Japanese Unexamined Patent Application Publication No.H05-58948 and International Publication No. WO 2004/063136.

That is, the technique described in Japanese Unexamined Patent Application Publication No. H05-58948 comprises, more specifically, the steps of: conducting fractional distillation of the secondary mother liquid by having a reflux ratio from 2 to 10 using a distillation column with the theoretical plate number from 25 to 125; and greatly decreasing the amount of para-toluic acid or the like. However, the technique has a drawback that large energy is required to remove para-toluic acid. Further, the technique has another disadvantage that additional costs and a construction place are required for installing the distillation system with the column.

Moreover, the technique as described in International Publication No. WO 2004/063136 is used to remove para-toluic acid from either of steam generated in the step ?or crystallizing terephthalic acid to give high-purity crystals (or crystallization step) in the purified aqueous solution and/or the resultant condensed water of the steam, or the secondary mother liquid by using a distillation process, a separation process by a membrane, and a synthetic adsorbent.

Eventually, those processes increase the costs in removing para-toluic acid. Further, those processes require additional costs for the usage and selection of the separation membrane and the usage and the after-treatment of the synthetic adsorbent in removing the para-toluic acid.

SUMMARY OF THE INVENTION

From the viewpoint of the respective techniques trying to overcome the above-mentioned difficulties and find a more improved treatment process of the secondary mother liquid, the present invention provides a process fo;: purifying crude terephthalic acid to be able to decrease the additional volume of the aqueous medium by using a simple processing construction and lower the content of para-toluic acid in the aqueous medium already used in the process effluent.

More specifically, the method of the present invention has been developed by concentrating on the aqueous medium already used (or the treated effluent) besides the primary and secondary mother liquids discharged in the process for purifying the crude terephthalic acid.

That is, the present inventors have condensed the efforts on improving the treatment of the treated aqueous medium including: 1) the condensed water derived from the steam generated in the flash cooling process by gradually ' lowering the pressure and temperature of the purified aqueous solution in the crystallizing step (Crystallization); 2) the scrubber effluent which is discharged by scrubbing the gas separated from the condensed water, discharged from the sealing gas and circulation gas used in solid-liquid separator, a dryer and the storage tank of the separated mother liquid (or the first mother liquor) the washing effluent supplied from the solid-liquid separator; and 3) the washing effluent which is recovered by washing the purified terephthalic acid crystals from the solid-liquid separation.

The present inventors have earnestly investigated to solve the above-mentioned problems. Eventually, a simply following treatment method for the aqueous media already used (or the treated effluents)as the objects of the problem has been developed; wherein the method comprise the steps: adding terephthalic acid crystals to the aqueous medium already used (or treated effluent) thereby to prepare an aqueous suspension or slurry; conducting solid-liquid separation with respect to the resultant suspension or slurry; and recovering the separated crystals and the separated water; further reusing the said separated crystals as a part of the starting material for the oxidation reaction to produce terephthalic acid reusing the said separated water as a part of the aqueous medium to dissolve the crude terephthalic acid. This allows supplied volume of the newly using aqueous medium (the fresh water) to be more decreased.
Here, the systematic routes for adding the terephthalic acid crystals may be multiply divided corresponding to the content of para-toluic acid contaminated in the respective effluents already used (the respective treated effluent)of the object. Further, a reasonably reused system of the separated water may be also provided. Based on those rational results or data, the present inventors have found that the above-mentioned drawbacks are effectively overcome to provide a more improved and effective method than the conventional techniques, whereby the present invention has been completed.

According to the present invention, a process for purifying crude terephthalic acid capable of decreasing the using volume of a fresh aqueous medium in a simple treatment construction is provided. Further, said separated crystals is recovered almost as terephthalic acid with para-toluic acid adsorbed on the crystals without any loss of said added terephthalic acid crystals by circulated into the oxidation reaction.

As a result, it contributes to the increase in the total yield of the purified terephthalic acid production.
Further, by the circulating reuse of the effluent already used (the treated effluent), the treatment load required in the waste water treatment step may be decreased;

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flowchart showing a reuse system of an aqueous medium; the system being used in addition to a process for producing high-purity terephthalic acid (PTA) crystals comprising the steps of producing crude terephthalic acid (CTA) and purifying the crude terephthalic acid (CTA).

FIG. 2 is a schematic diagram showing filtration test apparatus used in EXAMPLES.

DETAILED DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments (or present embodiments) for carrying out the present invention will be explained in detail in reference to FIG.l.[1. Operational Flow of Producing High-Purity Terephthalic Acid]

As shown in FIG. 1, a process 100 for producing high-purity terephthalic acid mainly comprises two steps of (i) producing crude terephthalic acid, and (ii) purifying the crude terephthalic acid thereby to produce purified terephthalic acid (or high-purity terephthalic acid). Herein, bold solid lines in FIG.l indicate the operational flow from a starting material of para-xylene to a final product of purified terephthalic acid (PTA). Here, the respective steps will be explained in detail hereinafter, (i) Step of Producing Crude Terephthalic Acid

This step is comprised of oxidizing para-xylene in a liquid phase to produce crystalline powder of crude terephthalic acid. More specifically, para-xylene (or starting material), acetic acid (or solvent), and a catalyst or the like are added to an oxidation reactor 1 to oxidize para-xylene in the liquid phase by oxygen^containing gas (for example, air) . Then, the reaction mixture transferred to a •crystallization tank (not shown) is gradually cooled to sufficiently recrystallize terephthalic acid. Note that the oxidation reaction from para-xylene to terephthalic acid proceeds in a stepwise reaction. Hereby, 4-formylbenzcarboxilic acid or the like remains as an intermediate by-product, causing contamination of a trace amount of4-f ormylbenzcarboxilic acid or the like in the crude terephthalic acid.

After the step of oxidation reaction has been completed, a slurry thus formed, including terephthalic acid crystals in the crystallization tank (not shown), is supplied to a solid-liquid separator 2. Then, crude terephthalic acid crystals (CTA: or solid phase) and the acetic acid solvent containing the catalyst (or liquid phase) are separated in the solid-liquid separator 2. The crude terephthalic acid crystals thus separated are dried and subsequently supplied to the step of purifying crude terephthalic acid as described hereinafter. On the other hand, the acetic acid solvent thus separated is directly reused or circulated through an acetic acid recovery section (not shown) thereby to be reused in the oxidation reactor 1.

Note the separated crystals are additionally supplied together with the above mentioned para-xylene starting material to the oxidation reactor 1 as shown in FIG.l. The separated crystals will be explained in detail hereinafter, (ii) Step of Purifying Crude Terephthalic Acid

The step is comprised of purifying crude terephthalic acid (CTA) to produce high-purity (purified) terephthalic acid (PTA). The detailed procedure will be explained below.

Crude terephthalic acid (CTA) obtained in the step of producing crude terephthalic acid is supplied to a CTA slurry preparation tank 3 together with 2-fold to 5-fold weight of an aqueous medium (or deionized water) to the CTA weight, thereby to prepare the CTA crystals slurry. Then, the prepared CTA crystals slurry is hea:ed at 260°C to 300°C by a heater 4. Herein, the pressure at this process is at about 50 to about 90 kg/cm2G. (Note "G" represents a gauge pressure. The definition is the same as in the following descriptions.) Here, the crystals are dissolved in the aqueous medium to give a terephthalic acid solution. Note 4-formylbenzoic acid is also dissolved in the terephthalic acid solution.

Here, it should be noted that the aqueous medium supplied to the CTA slurry preparation tank 3 includes already used effluent produced in the steps described hereinafter (or the aqueous medium containing para-toluic acid or the like) which is to be reused directly as it is, or indirectly as the aqueous medium treated in the cleaning process for reusing the medium. The detailed procedure will be explained hereinafter.

Then, the terephthalic acid solution is supplied to a hydrogenation reactor 5 together with hydrogen (H2). A palladium catalyst supported by active carbon is placed in the hydrogenation reactor 5. Hereby, when the aqueous solution of the terephthalic acid is introduced into the hydrogenation reactor 5, an aldehyde functional group of 4-formylbenzoic acid in the aqueous solution of the terephthalic acid is hydrogenated to form para-toluic acid. That is, the aqueous solution of the terephthalic acid containing para-toluic acid is used in the following steps.

Next, the aqueous solution of the terephthalic acid hydrogenated in the hydrogenation reactor 5 is introduced into a crystallization tank 6. The crystallization tank 6 is a multistage series crystallization tank comprised of four to six tanks, that is, a plurality of crystallization tanks. Herein, the pressure of each multistage crystallization tank located in series is lowered stepwise. Thus, the aqueous solution of the terephthalic acid is stepwise made pressure discharge and cooled by flash evaporation (or that is flash cooling) in the series crystallization tanks 6, and the terephthalic acid crystals is gradually precipitated in the resultant aqueous solution. — Thereby a slurry of purified terephthalic acid crystals (or crystals slurry) is prepared at about 120°C to about 170°C.

The crystals slurry thus formed are introduced with keeping the temperature thereof to a solid-liquid separator 7 (for example, pressure filter or the like), thereby to separate and recover terephthalic acid crystals (in the solid phase) at the high temperature a:id pressure. Note para-toluic acid is dissolved at such high temperature. That is, because the para-toluic acid is dissolved in the liquid phase at the temperature, most of para-toluic acid is contained in the liquid phase.

Next, terephthalic acid crystals thus obtained in the solid-liquid separator 7, for example, in the type of a filtration apparatus in which a solid separation region and a washing region are integrated, are sufficiently washed with an aqueous medium. After the washing process, the purified terephthalic acid crystals are recovered as a wet cake.

Then, the wet cake is placed in a dryer 8 to be sufficiently dried up. After the drying process, purified terephthalic acid (PTA) crystals are obtained. Herein, aqueous mother liquor (or primary mother liquid) separated from the purified terephthalic acid crystals is discharged (not shown) from the solid-liquid separator 7, and an aqueous medium used for the washing process is also discharged as washing effluent. The discharged effluent is once stored in a washing effluent tank 11, and reused directly as it is and /or reused through treating step as shown FIG.l, for a part of the aqueous medium that dissolves crude terephthalic acid. More detailed operation will be explained hereinafter.

Further, the wet cake separated and recovered in the solid-liquid separator 7 (for example, a decanter that is a solid-liquid separator having no washing region) is converted again to the slurry by an aqueous medium in order to perform the washing process. Then, the slurry is separated to a wet cake made separation to the purified terephthalic acid crystals and the washing effluent by the solid-liquid separator (not shown). and recovered as the crystals and the washing effluent again. After that, the wet cake is transferred to the dryer 8 and the washing effluent is transferred to the washing effluent tank 11.

[2. Reuse System of Aqueous Medium]
Next, the operational flow of the reuse system of the effluent (or used aqueous medium) discharged in the respective steps of (ii) Step of Purifying Crude Terephthalic Acid as shown in FIG.l will be explained below.

For example, treated aqueous medium is discharged, including the condensed water of the steam discharged from the crystallization tank 6, the scrubber effluent discharged by scrubbing the circulation and sealing gases separated from the condensed water, the solid-liquid separator 7, the storage tank of the separated aqueous mother liquor (or primary mother liquid)(not shown), the storage tank 11 of the washing effluent, the dryer 8 and so on.

Here, except for the aqueous mother liquor (or primary mother liquid) discharged from the solid-liquid separator 7, because an small amount of the impurity like para-toluic acid are evaporated in and accompanied with the circulation and sealing gases from the above-mentioned respective devices and storage tanks, those impurities are contained in the scrubber effluent discharged by scrubbing the gas from said circulation and sealing gases, of course, in the condensed water (or effluent) from the crystallization tank 6. But those effluents are generally transparent (that is an aqueous medium that is not precipitated in the reduced temperature).

As mentioned above, the aqueous medium required in (ii) the Step of Purifying Crude Terephthalic Acid has high purity, resulting in the necessity of the high procurement costs. Further, the used aqueous medium (or treated effluent) contains organic compounds such as terephthalic acid, para-toluic acid or the like. Accordingly, from the viewpoint of lowering the environmental load, it is preferable not to discharge as the industrial effluent outside as much as possible, and to reuse the effluent.

The treated effluents that are the object of the present invention are transparent effluents of the used aqueous medium treated in (ii) Step of Purifying Crude Terephthalic Acid.

In order to reuse the used aqueous medium, some amount of the terephthalic acid crystals is added and dispersed in a condensed water suspension tank 10 and a suspension tank 14 so as to reduce an amount of the contained para-toluic acid and purify the used aqueous medium, then the suspension liquid of terephthalic acid crystals is prepared. These liquids are separated with the solid-liquid separator 12 and 15 respectively. The respective separated crystals are supplied with para-xylene into oxidation reactor 1 and the respective separated liquids are supplied as a part of an aqueous medium to prepare the slurry with the crude terephthalic acid (CTA) into CTA slurry preparation tank 3.

For example, para-toluic acid is contained at about 500 ppm to about 600 ppm in the condensed water of the steam discharged from the crystallization tank 6. In contrast, as shown in FIG.l, the scrubber 13 is supplied with the sealing gas for the solid-liquid separator 7, a separated mother liquid tank (not shown), the washing effluent tank 11 and the like, and a discharged vent gas derived from the circulation gas for the dryer 8. Here, a used aqueous medium discharged from the scrubber 13 (described in detail hereinafter) contains para-toluic acid at about 100 ppm or less. Accordingly, it is not reasonable to treat those used water media all together in the same operational flow in order to reuse all the aqueous media.
Hereupon, the reuse system of the aqueous media is comprised of two systems of the reuse operational flow for treating the aqueous media, depending on the content level of contaminated para-toluic acid. A plurality of systems are provided with the reuse operational flow for the aqueous media corresponding to the amount of contaminated para-toluic acid, allowing the aqueous media to be reused as effectively recycled, thereby to more decrease the volume of the fresh introduced aqueous medium.

Hereinafter, referring to FIG.l again, the operational flow on the reuse system for the aqueous media will be explained. The bold dash lines in FIG.l indicate reuse operational flow for the used aqueous media (or separated water and washing effluent). Further, in FIG.l, the devices drawn with hutching are directly applied to the reuse treatment.

Firstly, the reuse flow for the condensed water using the separated condensed water will be explained below.

The steam discharged from the crystallization tank 6 is cooled by a condenser 9, to be separated into a condensed aqueous medium and a hydrogen containing gas. Herein, the condensed aqueous medium contains the high content of para-toluic acid (that is, about 500 ppn to 600 ppm) . After the steam is cooled, the condensed water is supplied to a condensed water suspension tank 10. In contrast, the discharged gas separated by the condenser 9 is supplied to the scrubber 13, washed, and discharged.

Further, heat generated in the condenser 9 (more specifically, heat derived from the steam) is applied to heating of crystals slurry in the heater 4.

Further, a used aqueous medium from the washing effluent tank 11 is supplied to the condensed water suspension tank 10. Herein, the detailed operation will be described hereinafter. The used aqueous medium having small content of para-toluic acid (or used aqueous medium discharged from the scrubber 13) is partially reused for washing purified terephthalic acid crystals as washing water in the slid-liquid separator 7.

Herein, it should be noted that the washing effluent recovered by separation from the mother liquor contains a certain degree of the separated mother liquid (or primary mother liquid) from the solid-liquid separator 7, resulting in the increase in a relatively high contamination of para-toluic acid.

Next, terephthalic acid crystals (TA). are added to the condensed water suspension tank 10. Here, the investigation of the present inventors demonstrates that the addition of terephthalic acid crystals to the used aqueous medium containing para-toluic acid makes para-toluic acid being adsorbed on the added terephthalic acid crystals. Therefore, the addition of the terephthalic acid crystals which are difficult to be dissolved in the aqueous medium allows para-toluic acid dissolved in the used aqueous medium to be adsorbed on the terephthalic acid crystals, resulting in the successful removal of para-toluic acid included in the used aqueous medium.

The terephthalic acid crystals added to the condensed water suspension tank 10 includes dry crude terephthalic acid crystals, dry purified terephthalic acid crystals, or a wet cake of purified terephthalic acid crystals. Herein, from the viewpoint of more lowering in the content of para-toluic acid included in the used aqueous medium, it was shown that dry terephthalic acid was more useful than the wet terephthalic acid cake. Further, the lowering effect on the para-toluic acid content becomes more improved if the purified terephthalic acid crystals are used instead of the crude terephthalic acid crystals. In short, dry purified terephthalic acid crystals are particularly preferable as the added terephthalic acid material.
Note the dry crude terephthalic acid crystals, the dry purified terephthalic acid crystals, and the wet cake of purified terephthalic acid discharged from the respective devices in the process 100 for producing high-purity terephthalic acid are appropriately utilized as the terephthalic acid materials. For example, when dry purified terephthalic acid crystals are demanded, the dry purified terephthalic acid crystals obtained by being dried in the dryer 8 may be utilized. Alternatively, terephthalic acid crystals obtained from outside suppliers or in-house plant may be also utilized as the terephthalic acid crystals material.

Moreover, the investigation of the present inventors demonstrated that when water (or water molecule) as the solvent was evaporated from the surface and inside of the terephthalic acid crystals while the purified terephthalic acid crystals were dried up, the surface structure of the purified terephthalic acid crystals was converted, resulting in the improvement of the ability of adsorbing para-toluic acid. That is, the removal efficiency of para-toluic acid was improved by using the dry terephthalic acid crystals heated to be dried in the dryer 8 as mentioned hereinbefore.

The way prepared by adding the purified terephthalic acid crystals to an aqueous medium is not specifically limited. It is possible to prepare the suspension liquid by adding directly the purified terephthalic acid crystals or also the slurry form prepared beforehand by suspending purified terephthalic acid crystals (or crystalline powder) in the aqueous medium to the used aqueous medium in the condensed water suspension tank 10.

Furthermore, an additional amount of the purified terephthalic acid crystals is not specifically limited. However, the investigation of the present inventors demonstrated that if the para-toluic acid content was able to be reduced by half (for example, about 250 ppm to 300 ppm) compared to the concentration thereof in the steam derived from the crystallization tank 6, all of the condensed water supplied from the crystallization tank 6 to the condensed water suspension tank 10 was able to be reused as a part of the aqueous medium used for dissolving a crude terephthalic acid. From the viewpoint as mentioned above, the amount of the added purified terephthalic acid crystals into the condensed water suspension tank 10 is preferably 0.2 wt% or more to the amount of the used aqueous medium to which the purified terephthalic acid crystals were added. The addition amount in the range of these amounts allows the para-toluic acid content to be desirably lowered to at least a degree of the half content.

Note if crude terephthalic acid crystals are used as the added terephthalic acid crystals, the amount of the added crude crystals is preferably set in 0.5 wt% or more to the used aqueous medium.

Further, the more the upper-limited value of the added amount of the purified terephthalic acid is set, the more the adsorption amount of para-toluic acid is enhanced. However, this process increases the circulating amount for the whole procedure of producing high-purity terephthalic acid, and gives influence on the production costs of terephthalic acid. Hereby, the added amount of purified terephthalic acid is preferably set in 2 wt% or less to the amount of the used aqueous medium to which the purified terephthalic acid is to be added.

After the above determined amount of the purified terephthalic acid crystals has been added to the condensed water suspension tank 10, and sufficiently mixed and suspended therein, the resultant suspension is supplied to the solid-liquid separator 12. Then, in the solid-liquid separator 12, the resultant suspension separates into the separated condensed water and the separated crystals. The separated crystals are terephthalic acid crystals on which para-toluic acid is adsorbed. Herein, the solid-liquid separator 12 may include, for example, a continuous rotary filter or a decanter or the like. Among them, from the viewpoint of continuous separation, a disk type of a decanter or a screw bowl type of a decanter (or centrifuge decanter) is preferably applied to' the solid-liquid separation.
Next, the separated crystals thus obtained are supplied to the (i) Step of Producing Crude Terephthalic Acid, and used as a part of the starting material for the oxidation reaction. That is, the separated crystals contain a relatively large amount of para-toluic acid which is adsorbed on the terephthalic acid crystals.

Thus, the productivity (or yield) of terephtnalic acid may be increased by using the terephthalic acid crystals containing para-toluic acid as a part of the starting material reused in the oxidation reaction. Accordingly, terephthalic acid may be produced without any waste, allowing the costs of the starting materials to be reduced.

Further, the separated (condensed) water is supplied to a CTA slurry preparation tank 3 as a part of the aqueous medium. Note the excess volume of the separated (condensed) water is supplied into the steps of waste water treatment. This enables additional volume used as the aqueous medium and treatment load of the waste water to be decreases. Accordingly, production costs of terephthalic acid may be reduced.

Next, an operational flow of the aqueous medium reuse system will be explained about the scrubber treated effluent containing a small amount of para-toluic acid in astail.

First, the scrubber 13 is supplied with the following gases for the washing, including the discharged gas derived from the sealing and circulation gas for the solid-liquid separator 7 and the dryer 8, the separated gas derived from the condenser 9, and the discharged gas derived from the separated mother liquor (not shown) separated by the solid-liquid separator 7 and the sealing gas for the washing effluent tank 11.
Here, the above mentioned gases contain volatile para-toluic acid, and subsequently in the scrubber 13 the separated gas is washed to remove para-toluic acid by using the aqueous medium. After the washing, the washed gases are discharged to the outside. Therefore, the used aqueous medium discharged from the scrubber 13 contains a relatively very small amount of para-toluic acid. The content of para-toluic acid is smaller than the content thereof in the steam from the crystallization tank 6. More specifically, the content of para-toluic acid in the used aqueous medium discharged from the scrubber 13 is generally about 100 ppm or less.

Then, the used aqueous medium discharged from the scrubber 13 is supplied to a suspension tank 14. To the suspension tank1 14, the purified terephthalic acid crystals are added in the same manner as in the condensed water suspension tank 10. The amount and form of the added purified terephthalic acid crystals are not specifically limited. Herein, the amount and form of the added purified terephthalic acid crystals may be set in the same manner as in the condensed water suspension tank 10.

The used aqueous medium to which terephthalic acid is added in the suspension tank 14, is separated into separated crystals and separated water in the solid-liquid separator 15 and recovered. Then, the separated crystals thus obtained are supplied to the oxidation reactor in which crude terephthalic acid is produced.

In constant, the separated water thus obtained is reused as a part of water for washing purified terephthalic acid crystals in the solid-liquid separator 7, or is supplied to the CTA slurry preparation tank 3 in order to have a part of the aqueous medium for dissolving crude terephthalic acid.

That is, the used aqueous medium contained relatively lower content of para-toluic acid from a scrubber 13 is added terephthalic acid crystals and made in suspension in t:he suspension tank 14, and the separated water obtained from a separator 15 is reused as the washing water of purified terephthalic acid crystals in the solid-liquid separator 7. Further, this washing effluent is recovered in a washing effluent tank 11 and then reused as a part of aqueous medium for dissolving crude terephthalic acid.

Further, in case of the separation of the washing effluent to the separated mother liquor (or primary mother liquid) in the solid-liquid separator 7 are mechanistically insufficient. The washing effluent containing an increased amount of para-toluic acid is supplied to the condensed water suspension tank 10, so as to be mixed with the condensed water derived from said condenser 9 thereby to be purified, allowing the washing effluent to be reused. [3. Modified Example of Aqueous Medium Reuse System]
As shown in FIG', 1, in the process 100 for producing high-purity terephthalic acid, the used aqueous medium from the scrubber 13 is supplied to the suspension tank 14, and the terephthalic acid crystals are added to the suspension. Herein, it should be noted that the amount of para-toluic acid contained in the used aqueous medium discharged from the scrubber 13 is a relatively lower as mentioned hereinbefore.

Therefore, the used aqueous medium from the scrubber 13 may be reused as it is for an aqueous medium of dissolving crude terephthalic acid without adding any terephthalic acid crystals. As mentioned above, such a process construction for producing high-purity terephthalic acid enables a process for improved purifying crude terephthalic acid to be provided. Herein, the method is capable of reducing the additional volume of the aqueous medium needed in the process even in the s:.mple processing construction.

Further, the suspension tank 14 and the solid-liquid separator 15 are not needed to be arranged, allowing installation sites to be decreased and more simplification of the treatment facilities to be achieved.
Further, the addition of terephthalic acid to the used aqueous medium is performed once to the respective aqueous media in the two flow systems corresponding to the para-toluic concentration in the respective aqueous media. However, additions of terephthalic acid to the used aqueous medium divided in multiple times may be performed.

Moreover, a fresh aqueous medium is used as the aqueous medium supplied to the scrubber 13 in FIG.l. However, purified separated water (or separated water) thus obtained .:. n the solid-liquid separators 12 and 15 may be applied to the aqueous medium.

[4. Modified Example of Operational Flow for Producing High-Purity Terephthalic Acid Crystals]
The process 100 for producing high-'purity terephthalic acid, to which the purification method of the present embodiment is applied, is not limited to a process construction shown in FIG.1. For example, the solid-liquid separator 7 is not limited to a single stage separation, while multistage separation may be conducted.

Further, the number of the scrubbers 13 used in the process is not limited to a single scrubber, while one or more scrubbers 13 may be arranged.

EXAMPLES
Hereinafter, referring to examples, the present invention will be more specifically explained.

In the present EXAMPLES and COMPARISON EXAMPLE, condensed water obtained by condensing steam derived from the crystallization tank 6 was collected to be used as a test sample, in the industrial manufacturing plant of a process 100 for producing high-purity terephthalic acid shown in FIG.l. Then, the condensed water was collected in four conical tubes filled with 50 ml thereof for a centrifugal separation respectively. The four conical tubes were prepared differently in the added amounts of the crystalline powder as shown below.

(EXAMPLE 1)
To a conical tube, purified terephthalic powder (crystals) was added in example 1 (0.05 g, that is, in 0.10 wt%) .

(EXAMPLE 2)
To a conical tube, purified terephthalic powder (crystals) was added in Example 2 (0.10 g, that is, in 0.20 wt%) .

(EXAMPLE 3)
To a conical tube, purified terephthalic powder (crystals) was added in Example 3 (0.25 g, that is, in 0.50 wt%) .

(COMPARATIVE EXAMPLE 1)
To a conical tube, was not added any purified terephthalic powder (crystals) was not added in Comparative Example 1(0 g, that is, in 0 wt%).

Then, the four conical tubes were vigorously shaken, thereby to sufficiently disperse the crystalline powder in the condensed water. The resultant conical tubes were set in a centrifugal separator.

The centrifugal separation was conducted for 5 min at a rotational speed of 7500 rpm (or 6200G) , and then the content of para-toluic acid in the supernatant liquid in each conical tube was measured.

The results are summarized in Table 1. Herein, each the content and the reduced ratio of the para-toluic acid are shown in Table 1. The percentage reduced to the content of para-toluic acid in COMPARATIVE EXAMPLE 1 is shown as a reduced ratio. Table 1.

The above results indicate that it is preferable that the terephthalic acid powder (crystals) with at least a degree of 0.10 g (that is, in about 0.20 wt%) is added to the condensed water 50ml in order to be reduced the content of para-toluic acid in a half.

The condensed water was collected in four conical tubes in the same manner as in EXAMPLES 1 to 3.

(EXAMPLE 4)
To one of the four conical tubes, the purified terephthalic acid powder (crystals) 0.5g (that is, in 1.0 wt%) was added. Then the conical tube was vigorously shaken to sufficiently disperse the crystalline powder, and subsequently set in the centrifugal separator.

(EXAMPLE 5)
To another conical tube, a wet cake of (or about 11 wt%) purified terephthalic acid 0.28g (that is, in 0.50 wt%) was added. The conical tube was vigorously shaken to disperse the crystalline powder in the condensed water, and subsequently set in the centrifugal separator.

(EXAMPLE 6)
Further, to another conical tube, was added crude terephthalic acid powder (crystals) 0.20g was added (that is, in 0.40 wt%). The conical tube was vigorously shaken to disperse the crystalline powder in the condensed water, and subsequently set in the centrifugal separator.

Finally, to the remaining one conical tube, any crystals was not added, and subsequently set in the centrifugal separator (for balancing).

Centrifugal separation was conducted under the same conditions as in EXAMPLES 1 to 3. Then, the para-toluic acid content and the reduced ratio thereof in each supernatant were calculated. The above results together with the result of COMPARATIVE EXAMPLE 1 are summarized in Table 2.
Table 2.

As shown in Table 2, when the form of the added terephthalic acid crystals was different, the reduced ratio of para-toluic acid was changed associated the corresponding form. Among the results in Examples, when purified dry terephthalic acid powder (crystals) was used (EXAMPLE 4), it was shown that the reduced ratio of para-toluic acid was particularly improved.

Note the eluted amount of 4-formylbenzoic acid (or one of the by-products contaminated in the crude terephthalic acid) included in the crude terephthalic powder was trace (or 5 ppm or less) in the supernatant.

The used aqueous medium discharged from the scrubber 13 in the industrial manufacturing plant of the process 100 for producing high-purity terephthalic acid shown in FIG.l was collected in two conical tubes (each 50 ml).

(EXAMPLE 7)
To one conical tube, the purified terephthalic acid powder (crystals) 0.80 g (that is, in 1.6 wt%) was added. Then, the conical tube was vigorously shaken to sufficiently disperse the crystalline powder, and set in the centrifugal separator. (COMPARATIVE EXAMPLE 2)

To the other conical tube, any crystalline powder of terephthalic acid was not added and then the tube was set in the centrifugal separator as Comparative Example 2.

Table 3.
The above results indicated that the contaminated para-toluic acid was able to be significantly removed even from the used aqueous medium from the scrubber 13 by adding the terephthalic crystalline powder (crystals).

As described hereinbefore, the crystallization tank 6 in the industrial manufacturing plant of the process 100 for producing high-purity terephthalic acid is comprised of a plurality of crystallization tanks. In other words, a terephthalic acid solution converted via the hydrogenation reaction in the hydrogenation reactor 5 was stored thereby to gradually lower the pressure and temperature in the plural series crystallization tanks, to produce a slurry of purified terephthalic acid crystals in the final stage crystallization tank (at 4Kg/cm2G/, at 150°C). Then, the slurry of the purified terephthalic acid crystals was supplied to the solid-liquid separator 7 as described above, to be separated into purified terephthalic acid crystals and mother liquor. The crystals thus obtained were washed by an aqueous medium (or fresh aqueous medium or reuse aqueous medium), whereby the purified terephthalic acid crystals were recovered.

Here, a filtration test apparatus 200 made in SUS304 of a solid-liquid separator 7 shown in FIG.2 was manufactured, in order to check the effect of reusing the separated water. Herein the separated water was produced via adding the purified terephthalic acid crystals to be reused as the washing water of the terephthalic acid crystals in the solid-liquid separator 7. The separated water was prepared by additional treatment of terephthalic acid crystals to the treated effluent from the scrubber 13. More specifically, the operational flow of the separated water was shown in the following flow: "treated effluent from the scrubber 13 -> suspension tank 14 -> solid-liquid separator 15 -> separated water."

The filtration test apparatus 200 is constructed with devices capable of adjusting the pressure and temperature in high levels and degrees, comprising a-filtration apparatus 201 having 82.2 cm2 of the filter area, a receiver 202, and an injector of washing water 203.

Herein, the flow rate and pressure in the respective units may be controlled by opening or closing ball valves 204a to 204f. Further, the filtration test apparatus 200 includes two pressure gauges 206a and 206b and controls inert gas pressure via valves 207a and 207b, allowing the inside pressure of the filtration test apparatus 200 to be controlled. Moreover, the filtration test apparatus 200 has a thermometer 205, enabling the inside temperature of the filtration test apparatus 200 to be detected.

Furthermore, the inside temperature of the filtration test apparatus 200 may be controlled by surrounding the outer circumference of the apparatus 200 with an electric ribbon heater (not shown).

The filtration apparatus 201 is provided with a filter cloth 201a (NAKAO FILTER MEDIA CORP., TR9B) made of polyester, the filter cloth being supported on a porous stainless steel plate (not shown) held between flanges. Note FIG.2 shows the state after the filtration. The filter cloth 201a filters a slurry or suspension material to collect a filtered layer (or crystals layer) 201b, producing a filtered cake.

A crystals slurxy (or slurry of purified terephthalic acid crystals) was introduced from a top inlet of the filtration apparatus 201. Then, the introduced crystals slurry was filtered through the filter cloth 201a to discharge only a mother liquid (or liquid phase) into a bottom part of the apparatus 201 through the receiver 202. In contrast, the filtrated layer is deposited on the filter cloth 201a (see the filtrated layer 201b shown in FIG.2) . Then, the filtrated layer 20.1b is washed by spraying washing water thereon prepared by heated in the injector of washing water 203 together with inert gas.

Next, the operation of the filtration test apparatus 200 in EXAMPLES will be explained more specifically.

In the present EXAMPLE, the filtration test apparatus 200 is attached to a short pipe arranged to be distributed in the middle of the pipe between the crystallization tank 6 and the solid-liquid separator 7 in the production system shown in FIG.l. Then, the performance of the process 100 in FIG.l of the present EXAMPLE was evaluated. First, the inside of the filtration test apparatus 200 was adjusted at the same pressure (or 5Kg/cm2G) and temperature (or 150°C) as the above mentioned pipe (or pipe between the crystallization tank 6 and the solid-liquid separator 7). Next, the crystals slurry (200 to 300 ml) was introduced into the filtration test apparatus 201 from the top inlet thereof. After the introduction, a pressure difference between the pressure gauges 206a and 206b) was adjusted at 0.5 Kg/cm2 by controlling the valves 204b and 207b, to filter the crystals slurry by the filter cloth 201a. The separated mother liquor liquid was temporally stored in the receiver 202 and discharged to the outsr.de. The inside pressure of the filtration apparatus 201 was kept by closing the valve 204b and subsequently opening the valve 204c. The terephthalic acid crystals were deposited on the filter cloth 201a in a layered form (or crystals layer 201b) as shown in FIG.2.

Then, washing water (100 ml), which was prepared in the injector 203 of washing water beforehand adjusted at about 110°C under the increasing pressure (about 5.5 Kg/cna2G) by the inert gas and the heated steam, was supplied to the filtration apparatus 201 by increasing the pressure of the inert gas. Here, the supernatant (100 ml) collected under the conditions in the above described EXAMPLE 7 and the supernatant (100 ml) collected under the conditions in COMPARATIVE EXAMPLE 2 were used to investigate the respective examples. Those tests were conducted in EXAMPLE 8 and COMPARATIVE EXAMPLE 3. Then, the pressure difference between the pressure gauges 206a and 206b was adjusted at 0.5 Kg/cm2 by the valves 204b and 207b to wash the crystals layer 201b. By similar way to the mother liquor in above-mentioned, the washing effluent was collected in the receiver 202, thereby to measure the content of para-toluic acid in the washing effluent.

Furthermore, the filtration test apparatus 200 was cooled and then disassembled to take out the crystals layer 201b. After the crystals were dried up, the weight and the para-toluic acid content of the dry crystals (or purified terephthalic acid crystals) were measured.

The above mentioned results are summarized in Table 4. Note "the washing water ratio" is a value representing the volume of washing water per the weight of the dry terephthalic acid crystals. Further, in REFERENCE EXAMPLE, the results obtained when deionized water (fresh water) was used as the washing water are also listed.

Table 4.

The results shown in Table 4 indicate that the para-toluic acid content in EXAMPLE 8 in which terephthalic acid was added in advance was smaller than the para-toluic acid content in CONPARATIVE EXAMPLE 3 in which no terephthalic acid crystals were added, in both para-toluic acid content in dry terephthalic acid crystals and the washing effluent. Further, in EXAMPLE 8, the para-toluic acid content contaminated in the dry terephthalic acid crystals was the same level as in REFERENCE EXAMPLE using deionized water.

That is, the data demonstrate that the aqueous medium, and the separated water from which para-toluic acid are removed by adding the purified terephthalic acid crystals thereto, is capable of preferably reused instead of conventionally used the deionized water in the prior techniques.

We claim:

1. A process for purifying crude terephthalic acid comprising the steps of:

producing crude terephthalic acid by oxidizing para-xylene as a starting material in a liquid phase using an acetic acid solvent in an oxidation reactor;

dissolving the resultant crude terephthalic acid in an aqueous medium at high temperature and high pressure;

hydrogenating the resultant aqueous solution in a hydrogenation reactor;

preparing resultant purified terephthalic acid crystals slurry with flash cooling by gradually lowering pressure of the resultant aqueous solution at high temperature and high pressure in crystallization tanks;

conducting solid-liquid separation of the resultant purified terephthalic acid slurry in the solid-liquid separator to recover the purified terephthalic acid crystals and aqueous mother liquor;

washing the recovered purified terephthalic acid crystals by an aqueous medium so as to discharge washing effluent into a washing effluent tank divided from the resultant aqueous mother liquor; and

drying the resultant terephthalic acid crystals in a dryer to produce purified terephthalic acid crystals,
wherein the process is performed:

a suspension solution is prepared by adding purified terephthalic acid crystals to at least one kind of treated effluents derived from said process and recovering separated water and separated crystals
after the solid-liquid separation of said suspension has been completed;

the said separated water is reused as at least a part of an aqueous medium to dissolve the crude terephthalic acid; and

said separated crystals are reused as a part of a starting material to produce said crude terephthalic acid,
the treated effluents thus obtained comprising:

condensed water which is derived from steam generated by flash cooling of said aqueous solution of the purified terephthalic acid at the high temperature and the high pressure in the crystallization tanks
scrubber effluent which is discharged by scrubbing the gases separated from said condensed water and discharged from circulation gas and sealing gas used for the solid-liquid separator, the dryer, and a storage tank of the aqueous mother liquor;

the washing effluent which is recovered by washing said purified terephthalic acid crystals derived from said solid-liquid separation.

2. The process for purifying crude terephthalic acid according to claim 1, wherein the process comprises the steps of:

preparing the separated water by adding the purified terephthalic acid crystals to at least either of the condensed water or the washing effluent; and

reusing the resultant separated water and the scrubber effluent without any further treatment as at least a part of an aqueous medium to dissolve the crude terephthalic acid.

3. The process for purifying crude terephthalic acid according to claim 1, wherein the process further comprises the steps of:

preparing the separated water respectively by adding the purified terephthalic acid crystals to the scrubber effluent and at least either of the condensed water or the washing effluent;

reusing the resultant separated water obtained from the condensed water and/or the washing effluent as at least a part of an aqueous medium to dissolve the crude terephthalic acid; and

reusing the resultant separated water obtained from the scrubber effluent as at least a part of the washing water for washing the purified terephthalic acid crystals.

4. The process for purifying crude terephthalic acid according to claim 3, wherein the process
comprises the step of:

reusing the washing effluent as at least a part of an aqueous medium to dissolve the crude terephthalic acid.

5. The process for purifying crude terephthalic acid according to any one of claims 1 to 4, wherein the addition rate of the purified terephthalic acid crystals are in the range of 0.2 wt% or more to 2 wt% or less to the treated effluent.

6. The method for purifying crude terephthalic acid powder according to any one of claims 1 to 4, wherein the addition crystals instead of the purified terephthalic acid crystals, are utilized at least one kind of crystals selected from a group of the dry crude terephthalic acid crystals and the wet purified terephthalic acid crystals.