DE2024899A1 - Process for purifying isophthalic acid - Google Patents

Description

Telephone IMBBi 227051

May 21, 1970 R / Fi Applicant: Atlantic Richfield Company

717 Fifth Avenue - *

New York, New York 10022

United States

A 12 693

Process for purifying isophthalic acid

The invention relates to a method for purifying isophthalic acid to a relatively high level Purity. ·

Various aromatic dicarboxylic acids, particularly isophthalic have a considerable economic "ä Liehe gained importance for the formation of polymers and other products that nitteln for the production of coating <, films, fibers and threads as well as many other valuable products are useful. In polymerisation reactions of this type, however, it is necessary that the acids used in these reactions have a relatively high degree of purity, since the by-products and impurities formed during the preparation of the acids often have a disruptive influence on the achievement of the properties desired in the polymer to have.

009848/1976

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It has been shown that monomers »purity especially is important in the polymerizations of polyester resins to be used as textile fibers. The purity of such aromatic dicarboxylic acids necessary for use in these reactions is difficult to obtain in industrial plants for the production of isophthalic acid. So there is a big one There is a need for an isophthalic acid control process with high efficiency without uninteresting economic requirements such as extensive and expensive equipment, high operating costs} complex work process and expensive reagents.

Isophthalic acid is usually produced by oxidation of the appropriate xylene IsroEiG.es; ie meta-xylene can be oxidized with the formation of isophthalic acid. In general, the meta-xylene dissolved in an inert solvent such as a C ₂ to ^ carboxylic acid (often acetic acid) with molecular oxygen (often in the form of air) at elevated temperatures and at a pressure which is sufficient, the xylene and the solvent to keep in liquid phase, brought into contact. An oxidation catalyst is usually present. Such catalysts can be, for example, a soluble cobalt or manganese salt, such as cobalt acetate, or other oxidation catalysts. In some cases, an oxidizing agent such as a halogen or an aliphatic ketone or aldehyde, for example acetaldehyde, may be desirable.

In general, the acid product is recovered from the reaction mixture by cooling to remove the acid crystallize and then use the crystals

009848/1978

one of the well-known solid-liquid separation methods such as Filtration, centrifugal separation, decanting and the like. Separate. However, the raw acid product obtained with such processes which are customary in the art is generally contaminated with relatively large amounts of reaction by-products and other impurities such as catalyst. The properties of the impurities are the cause of serious difficulties which arise when considering the polymerization processes, especially the manufacture wants to achieve the necessary high levels of purity of polyester textile fibers.

The respective amount and type of impurities in the raw acid product often depend on the occurrence of their precursors in the xylene starting material and on the conditions of the oxidation reaction. In a meta-xylene starting material there are often negligible but still noticeable amounts of other isomers, such as ortho-xylene, para-xylene and ethylbenzene, and various large amounts of other impurities, such as paraffins and naphthalenes with approximately the same boiling range as the xylenes, which under the reaction conditions normally lead to the corresponding acid » Ren or oxidation intermediates are oxidized. The meta-xylene starting material itself can also be the The cause of reaction intermediates and other by-products · Additional impurities can include metal corrosion products from the system, catalyst and solvent residues and be like that. However, the main constituents of the impurities will be aromatic acids, including the isomeric forms of the isophthalic to be produced

00 9 848/1978

acid, toluic acid and benzoic acid; Farther Reducible oxidation intermediates, primarily the isomers of phthalic aldehyde, the isomers of toluylaldehyde and benzaldehyde, which from now on are mentioned under the collective term reducible substances, and traces of metal compounds caused by residues of the catalyst and corrosion in the reaction system, originate primarily from iron and the metal of the catalyst, e.g. cobalt and manganese.

Although there are quite a large number of conventional ones Purification methods to improve isophthalic acid purity have been suggested, but they either are generally ineffective for the production of an acid on an industrial scale with the desired low level of impurity, or else they involve such complex operations, extensive equipment and / or expensive materials that are economically unattractive to industry. So there is a great need for a cleaning system for this acid, which has a high degree of efficiency and is still economically interesting. Generally speaking, the invention provides an improved method for cleaning of isophthalic acid for the efficient manufacture of an acid product with low levels of impurities, the is economically interesting because of its simplicity and effectiveness.

An examination of the commercially available isophthalic acid shows that the poor quality of the product and the poor color of such acids is primarily due to the relatively high levels of reducible substances and metals in the apparently pure product. Other, generally existing impurities

009848/1978

although it can sometimes be found that catalyst residues, e.g. cobalt, are insufficiently removed and incorporated into undesirable amounts are encountered. It has now been found, however, that isophthalic acid is easy according to the invention by recrystallization from an acetic acid-water mixture under controlled conditions to form a product of exceptional purity, color property, physical properties and lighter Handling can be cleaned. That obtained by the recrystallization process of the invention The product is particularly suitable where exceptional purity is required, namely for polymerization processes that are to lead to polyester fibers. The procedure described below is dependent on easily controllable parameters, is about a effective in large areas of contamination and is even able to easily deal with larger ones and / or fast Adjust contamination fluctuations.

Although various types of recrystallization processes for the purification of isophthalic acid have previously been proposed, such processes do not, ins special purity required for the production of polyester fibers created · Ef was for example proposed to use an aqueous xsophthalic acid solution in To form the form of the barium salt and to regenerate the acid crystals by treatment with a mineral acid or another acid. Such methods can be relatively effective, but are uninteresting, well one large · amounts of relatively expensive materials for converting the acid to salt and the salt back to Acid needed. The alternative for processes that require chemical conversions is

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recrystallize the dicarboxylic acid per se without the Change composition. To date, two such processes have been used and are believed to be used in industry. One process uses water as the solvent and another uses isopropanol or mixtures of isopropanol and water as the Solvent medium. However, there are many difficulties associated with these systems, including relatively poor product quality, especially that of the product Color properties and the crystal size of the recrystallized acid

In the process of the invention, isophthalic acid is recrystallized from an acetic acid water day under controlled conditions. The cationic reaction medium containing the isophthalic acid product and various other materials, such as the basic substance of the catalyst, onobasic acids and forms of the acid products, is generally first cooled to produce the product to crystallize, which is then separated by filtration, decanting or the like, and a crude product filter cake with noticeable amounts of impurities results. The raw filter cake is then combined with an acetic acid-water solution. This contains approx. 75 to 99 percent by weight, preferably approx. 95 to 99 percent by weight and in particular approx. 95 percent by weight acetic acid, the essentially remaining material of the solvent mixture being water. The solvent is contained in an amount which is at least sufficient to produce a pumpable slurry of the crude product in the solvent, but which is not greater than the amount which allows the acid product to be obtained easily

0098 ^ 8/1378

ft % * · C

» Λ M ♦ ■

A 12 693 - 7 -

admits from the solution by recrystallization. the resulting slurry is heated to dissolve the solids and form a solution, and is subjected to a two- or three-stage recrystallization, as will be described later, the recrystallized product subsequently by Filtration recovered, washed and dried =

Further features of the invention emerge from the following description in conjunction with the claims and the drawing. Λ

The figure shows a flow sheet of a three-step crystallization process for the purification of crude impurities such as isophthalic acid containing reducible materials, catalyst and metal traces, which is produced by air oxidation of a meta-xylene starting material with a small amount of para-xylene in the presence: an acetic acid medium and using a Heavy metal catalyst was produced _o

In the embodiment shown in the figure, the isophthalic acid and impurities containing reaction medium from the (not shown) oxidation _Λ

reactor withdrawn, cooled, for example on a ™

Temperature of approx. 38 ° C to 43 ° C (approx. 100 to HO ⁰ F.), passes through line 10 and is subjected to a filtering process in the filter 12. Das.Mutterlaugen-Filtrat is removed through line 14 and collected in vessel 16 for further processing to recover solvent and catalyst. The filter cake is washed with an acetic acid-water medium, which is made from sparing

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for reasons of ability, preferably from a later stage of the process is recovered and fed through line 18 «, The wash filtrate is removed through line 20 and sent to the Further processing for the recovery of the acetic acid and the catalyst in the vessel 22 The washed Filter cake consists essentially of a crude isophthalic acid product. Approximate typical impurities in the isophthalic acid crude product are shown in the table 1 shown

* Table 1

Catalyst metal e.g. Co 250-600 ppm

Fe 5-15 ppm

Benzoic acid 300 ppm

total monobasic acids 15,000 ppm

reducible substances 0.01-0.5 percent by weight

Terephthalic acid 0.1 percent by weight

Isophthalic acid residue

The crude isophthalic acid can contain up to approx. 2.0% terephthalic acid and contain monobasic acids up to approx. * 30,000 ppm »*

The raw filter cake obtained in this way is slurried in an acetic acid-water medium fed in through line 24, which contains approx. 75 to 99, preferably approx. 95 to 99 percent by weight acetic acid and the remainder water. A particularly preferred medium contains about 95 percent by weight acetic acid and about 5 percent by weight Water. The crude acid and solvent medium can be in a solvent to solids weight ratio from approx. 10 to 40: 1, preferably approx. 20 to 30 t 1, are combined. A special »to strive for Solvent-to-solids ratio is approx. 24 s 1

0098 ^ 8/1970

4 · »ft *

A 12 693 - 9 -

In preparing the slurry, it is desirable to use acetic acid recovered from later stages of the process for at least a portion of the solvent medium. Sufficient fresh acetic acid is used to create the correct solvent-to-solids ratio and to compensate for the impurities in the raw acid. A ratio of 1: 1 between fresh and reused solvent is particularly effective in achieving the desired effect, although a ratio of 3 μ is often used

until l * s 1 can work between fresh and reused solvent. .

The slurry so formed is passed through line 26 to heat exchanger 28 where it is heated and mixed at a temperature and pressure sufficient to achieve substantially complete dissolution of the crude isophthalic acid in the solvent. The temperature is generally from about 120 ° C to 205 ° C (about 250 to 400 ° F. _# ), Preferably about 150 to 177 ° C (about 300 to 350 ° F.), And pressures of about 300 to 350 ° F 2.1 to 10.5 atmospheres (30 to 150 psig), especially about 2.8 to 5.6 atmospheres (4.0 to 80 psig) are suitable. At |

a solvent-to-Peststoff ratio of about 24: 1 · a temperature of about 160 ⁰ C is preferably (about 32O ° F.) is selected at about 4.6 atm (65 psig). Acetic acid is very corrosive at elevated temperatures and it is therefore desirable to maintain a relatively low temperature, but it should be noted that essentially all of the isophthalic acid must be dissolved in the solvent. Failure to ensure complete dissolution will drain the heater

009848/197

A 12 693 - 10 -

Vessel 30 supplied, where it is at the desired temperature mixing is carried out for a time sufficient to dissolve, e.g. approx. 5 to 15 minutes or even longer.

The solution is then fed through line 36 to a stepped crystallization zone which, in one embodiment of the invention, has three continuous crystallizers A, B and C arranged in series, one of which is provided by pumps 42, 52 and 62, respectively, in lines 40, 50 and 60 generated external circulation. The hot solution in line 36 is introduced into the circulation pipe 40 of the first crystallizer and in the first crystallizer A sufficient DES at a pressure of _9, a liquid phase maintain UNU preferably quick evaporation of the solvent to verhindern® The outer conduit 41, pump 42 and- Circulating stream passing through line 40 is a slurry of isophthalic acid crystals in acetic acid / water solvent at about the temperature of the first crystallizer , 5 ° C (0 to 15 ° F.) Higher than the temperature of the first crystallizer, and generally about 0 to 3 ° C (0 to 5 ° P.>, Preferably about 0 to 1 ° C (0 to The mixed feed is then introduced into the first stage crystallizer. D ₁ Ie feed is preferably tangential into the crystallizer The first stage was introduced. The tangentia-Ie introduction promotes mixing in the crystallizer vessel. The feed is also carried out below the surface level of the liquid in the crystallizer.

oottit / tm

A 12 693 - 11 -

leads · By introducing the feed below the surface of the liquid, uncontrolled evaporation of the solvent is avoided. The crystallizer is operated at a Temperc A 'ture of about 120 to 190 ° C (about 250 to 375 ° F.), Preferably about 130 to 140 ⁰ C (about 265 to 285 ° F.) At a residence time of at least about half an hour, preferably about half an hour to three hours, and usually at a pressure of about 1 to 7.7 ata (15 to 110 psia), preferably about 2 to 2.8 ata (approx .30 to 40 psia). A particularly desirable Arbeltsprozeß, wherein the feed solution has a temperature of about 160 ° C (320 ⁰ F _3), with the result that in the crystallizer A temperature of 135 ° C (about 275 ° F.) a pressure of about 2.4 ata (34 psia) and an operating time of about one hour is maintained. The acetic acid-water solvent is removed via an overhead line 44. The pressure is monitored in order to maintain the desired temperature in the crystallizer. The solvent is removed in a sufficient amount per unit of time to maintain the temperature in the crystallizer. The solvent vapors are condensed in heat exchanger 46 and collected in vessel 48.

In the crystallizer A, the crystallization is carried out by the The temperature and removal of the solvent are conveyed and monitored. At the operating temperature and the operating pressure of the first stage crystallizer can often remove about one third of the isophthalic acid from the Solution are crystallized out. The external circulation flow 41 ensures circulation speeds

009848/1978

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ten which are sufficient to achieve complete circulation of the material, ie a replacement of the crystallizer contents, in about 1/3 to 10, preferably 1/2 to 3 minutes. The crystallizer Jc can be conveniently increased to create the desired line pressure. The outer circulation passage 41 is continuously withdrawn from a current and swar so vielj that he the crystallizer "compensates ygeführte feed 'together with the gate from the -Kristallisa overhead remote solvent" This stream ₉ a slurry der.Isophthalsäurekristalle in the solvent, is passed through line 40 'fed to the second stage crystallizer B "

The crystallizer B of the second stage "arbeitet- under other operating conditions than the first but the working process is substantially in the same © _o the second crystallizer is operated at einfer temperature of ea" 1os to 130 ° C (about 220 "to 265 ^O F®) "or, about ;, preferably ca" 110 to 115 ⁰ C 240 ⁰ C23O big Po) operated slightly forward ■ at least about half an hour, preferably about one-half to three hours in the general- ataosphärischem with a residence time less than about Pressure and up to ce · 0.9 atmospheres (13 psig) ',' preferably about 0 ₉ 14 to Oj, 35 atmospheres Cca _e 2 to 5 psig) '. Removal of the solvent via the overhead line 54-' is sufficient quickly ₉ "in order to maintain the temperature, while the speed of circulation of the slurry in line 50 is sufficient - to circulate the material in about 1/3 to 10 minutes, preferably about 1/2 to 3 minutes A T is selected for the particularly preferred operation of the crystallizer 2 temperature of approx. 112 ° C approx. 234 ° P _e ), one

9 8 4 8/1978

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atmospheric pressure and a residence time of approximately one hour. The in the crystallizer B of the second stage from that of the first stage in line 40 incoming material is a slurry} in which the isophthalic acid is, for example, one third present as crystals and the remainder dissolved in acetic acid-water solvent β The size of the crystals ranged from approx. 0.15mm (approx; 100 mesh) to powder fineness, with approx. 50% a size between approx. 0.05 to 0.37mm (300 and 400 mesh). In the work process of the second stage, additional core formation occurs, but the chief effect is to grow the existing crystals to a size that some Treatments from about 0.15mm to 0.037mm (100 to 400 mesh), with about 50% being about 0.075 to 0.05 mm (about 200 to 300 mesh) «Die specific orders of magnitude may change, which, among other factors, depends on the size of the system, is pending. About a second third of the original isophthalic acid can often go into the crystallizer crystallize in the second stage, about a third remaining in solution. The solvent vapors are in the Heat exchanger 56 condenses and collected in vessel 58.

The slurry becomes the external circulation line 51 of the second crystallizer B continuously withdrawn and can through line 50 * a solid-liquid separation zone 70 where a high purity crystallized product is recovered, or the slurry stream can be fed directly to the crystallizer ede: third stage through line 50 ″, or it can the slurry can be separated, one part through line 50 x the separation zone 70 and a second part through

00 984 8 / 1.97 8

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Line 50 "the Krietullisates ¹ over the third stage is directly cited" We »the first from ues Trennsone 70

tor are led by line 72 ₉ ©© Fiss 74 and line 76 "of the ¥ © orxufsweis as a rotary filter or the like" ausgebideten Trennsone 70 for tohe purity supplied © material is separated in the feesperdtwr KristalIisaotre B of wide stage _8. is sufficient to keep all impurities in effect in the weeklong . The solid crystals can be mixed with a solution through the line 77 with a mixture of approx. _Q 9> 5% acetic acid and approx. 5% water in a solution >'hMltnis vpfs <s @ _e 5 feis 1 ι l _e ■ preferably ca · 2 _? 7 § 1 washed α ose Wasetofilte @ t Isajaa dwreli line 78 in vessel 8 © total wn ^ daara al © tfasehlösung through lines 82 and 18 The high purity crystals are "sent through a dryer 84 and cooled" to give isophthalic acid, the highest levels of which are, for example, those listed in Table 2 ". The product of high purity very often contains less than the specified degrees of impurity «, The. Values in Table 2 stütaen aus'f to a method in a dens Atssgencjemateslal ü®n ait in Table i ge "Verunreinigungegrad @ n hmnutzt was.

Cobalt Elsen

© ■ ₉ © t. © @ wiehtspr © s @ nt

A 12 693 - 15 -

The solvent vapors from dryer 84 will be removed by lines 86 and 88 and passed from condenser 90 to vessel 92.

The aspirator withdrawn from the second stage crystallizer slurry discharge and / or the filtrate removed from the filter 70 for high purity products can, as mentioned, a crystallizer C of the third stage are fed to the operating conditions in the third stage include a temperature * of about 40 to 77 ° C (about 170 ° F.), preferably about 40 ° C to 52 ° C (6a. 105 to 125 ° Fq), often sub-atmospheric pressures, preferably about 0.07 to 0.21 ata (about 1 to 3 psia) at a A residence time of at least about 1/2 hour, preferably 1 to 3 hours. The removal of the solvent The overhead line 64 is sufficient fast to maintain the temperature in the crystallizer while the circulation speed of the slurry in line 60 is sufficient, one circulation of material in about 1/3 to 10 minutes, preferably about 1/2 to 3 minutes to accomplish. One chooses one for a particularly desirable establishment Temperature of about 45 ° C (about 113 ° F ·), a pressure of approx. 0.077 ata (1.1 psia) and a residence time of approx. one hour. In the third stage crystallizer, crystal growth is again predominant and often results in a slurry in which substantially about 90 to 99 percent by weight of the total isophthalic acid is present present as crystals. The overhead portion from the crystallizer C can go to the condenser and then flow into vessel 68.

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The slurry becomes the external circulation line 60 of the crystallizer of the third stage continuously withdrawn through line 60 'and one second liquid-solid separation zone 100 ,. e.g. a fiI-tration, Decantation or centrifugation zone or the like · fed to the separation of the crystals » A particularly desirable separation can again be achieved by using a rotary drum filter or the like. "The separated crystals were ~

m the medium supplied through line 102

for example a mixture of 95 weight percent acetic acid and 5 weight percent water wherein _s washed, the washing mixture-to-Feststoff' ratio b _e i ca ο 5 to 1: 1, preferably from about "2.8 §. 1 Separate filtrate collecting vessels 104 and 106 are usually used for the mother liquor removed through line 108 and the faschinedium removed through line 110, the mother liquor being carried to a solvent treatment zone through line 112 and the wash filtrate being regularly returned to the reactor filter through lines -114 and 18 “To be used as a washing medium or at least as part of the washing medium

P serve. The acetic acid obtained overhead from each crystallizer in vessels 48, 58 and 68, together with the acetic acid obtained by dryers 84 and 118, can conveniently be recycled through lines 116 and 24 to, as already mentioned, at least part of the To form crude product solvent _e

The washed Kristallprodukt® are through the For example, with an inert gas or heat exchanger heated dryer 118 and cooled, the end product isophthalic acid of low purity

0098A8 / 1978

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with the approximate maximum contamination levels shown in Table 3. The overhead portion of the dryer can be fed to the vessel 92 via line 88. The contamination levels in Table 3 is using the method of the invention a starting material as shown in Table 1.

Table 3

Cobalt 1 ppm.

Iron 1 ppm

Benzoic acid 50 ppm

monobasic acids 100 ppm

reducible substances 0.02 percent by weight

Terephthalic acid 0.5 to 4 percent by weight

If all of the slurry removed from the second stage crystallizer B is fed directly to the third stage crystallizer C, relatively little terephthalic acid occurs in the individual end product in one example approx. 0.8%. On the other hand, when the crystals are recovered from the second stage slurry in the filter for a product high purity for all second stage effluent a higher purity product which is essentially does not contain terephthalic acid, and from the third crystallizer C a product of relatively low purity is obtained, e.g. about 3.8% terephthalic acid in the same example. The smallest ratio between that Low purity product and high purity product is in this example in which all of the crystallized material obtained in the second stage crystallizer is treated to obtain a high purity product is, at approx. 0.27: 1. The largest ratio occurs

009848/1978

If no product of higher purity becomes "onera", a branch ηηε @ in © @ T@i3.s of the outflow of the crystallizer of the wide stage directly to the "crystallizer of the third stage together" with the outflow of the Product of high purity © ns the filter has because of the amount of crystals in the Mebenstro "a reduced yield of the product of high purity as a result." while the amount of isophthalic acid increases · The variations in the relative amounts made possible by the process according to the invention and the "associated quality fluctuations of the product of low purity can be adapted to the respective requirements of the resin",

The following example serves for further explanation the invention"

EXAMPLE

A stream of high purity meta-xylene with a small amount of-pare-xylene is in the presence 'of an acetic acid medium using a Cobaltaeetet-Oxydationnkatalysatoes lisftoxy <äiert The _e from a rate of 1871 per minute Caoe C49 "Sgpra)" The medium escaping from the oxidation reaction is heated to approx. 42 ° C (approx. 7 ° P _e )
filter filtered »EIa

Waschfiltret deg Preäwkfefliter © as Maseteadiym in a solvent ratio 2.9 s i genofpeno Pte die ftettdrtewg © tan <ä di © filtrates are collected separately

/ 19?

A 12 693 - 19 -

The mother liquor is a separate catalyst and Solvent recovery system and the laundry liquid an acetic acid recovery system.

The crude isophthalic acid filter cake has a typical composition which is listed in Table 4 along with a typical analysis of the purified products Slurried 5 percent by weight of water, the solvent consisting of the overhead portion of the crystallizers, as described above, and fresh acetic acid in a ratio of approx. 1: 1. The slurry is heated by indirect heat exchange to 16O ° C (32O ⁰ F.) and applied at a rate of about 7201 per minute (190gpm) the solvent container to the solution of the phthalic acids. The solvent container is maintained to prevent formation of steam at 160 ⁰ C (32O ° F.) And about 3.9 ata (56 psia). The residence time is 10 minutes to ensure complete dissolution. The effluent from the solvent container is fed to the external circulation line of the first stage crystallizer where it is combined with an external circulation flow of about 230hl per minute (6000 gpm). The first crystallizer is operated at a temperature of about 135 ° C (275 ° F.) And a pressure of 2.39 ata (34 psia). The solvent is overhead at a steam flow of 42.5 normal m ³ / min. (1,500 SCF / M) and the crystallizer is raised to provide a line pressure of

009848/1978

Create approximately 4.6 ata (65 psia) · The dwell time in the crystallizer is one hour and the external circulation rate for a complete Circulation in two minutes each »The one withdrawn from the external circulation line at approx. 135 ° C (approx. 275 ° P.) Drain slurry has, based on the total isophthalic acid, because of the removal of the solvent runoff, a solvent-to-solid ratio of 21: 1. The slurry is added to the crystallization Tor of the second stage at a rate of about 6451 per minute (170gpm), which is an operating temperature of approx. -112 ° C (approx. 234 ° F.), one Operating pressure of approximately 1.17 ata (16.7 psia), a solvent vapor drainage rate of 34 Normal m / min (1 200 SCF / M), an operating time of one hour and a circulation speed of has two minutes. The second crystallizer is increased to a line pressure of approx.2.4 ata (35 psia) on the inlet line for the unmixed supply. An external circulation rate of 230hl per minute (6000 gpm) is provided. The drain of the crystallizer of the second stage is separated into three in this example Species further processed. Xm one case the whole will be Current of about 6801 per minute (150 gpm) is fed to the filter for a high purity product and only that Mother liquor goes to the third stage crystallizer. In this way, 1 270 kg per hour (2,800 pounds) of the high purity product and 4 54 kg. (1000 pounds) of the lower purity product. With the other FsIl, the entire stream is fed directly to the crystallizer of the third stage, see above that no product of high purity and 1 720kg per hour

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(3,800 pounds) of the lower purity product will be produced * In the third case, 68% of the stream will be produced fed to the filter for a product of high purity and 32% goes directly to the crystallizer of the third Stage fed together with the mother liquor filtrate from the filter for a product of high purity, yielding 1,800 pounds per hour of each product grade.

The filter for a high purity product has an operating temperature of approximately 112 ⁰ C (234 ° F.), And the Mother liquor is collected and fed to the third crystallizer · The filter cake is filled with fresher 95 percent acetic acid with a solvent-too Solids ratio of 2.7 si washed and that Washing medium is fed to the reactor filter as washing medium. The xsophthalic acid crystals are dried and cooled · Table 4 shows a typical analysis «

The mother liquor filtrate and / or the drain from the crystallizer of the second stage come with approx. 1 * 17 ata (16.7 psia) into the outer circulation line at a rate of approximately 230 hectoliters per minute

(eooogpm). The third stage crystallizer has *

an operating temperature of approximately 45 ° C (113 ° F.) with a steam discharge rate of approx. 87.8 normal m / Min ·, a dwell time of one hour and a circulation rate of one minute. The third stage crystallizer effluent is fed to the lower purity product filter and the mother liquor filtrate is fed to a separate solvent recovery system. The filter cake is with 95 percent acetic acid with a washing medium too. Solids ratio of 2.7: 1 washed and that

009848/1978

Wash filtrate is fed to the reactor filter as a wash medium. The isophthalic acid crystals are dried and cooled to give the product of lower purity. Typical analyzes are shown in Table 4.

oosa ^ a / 1 ei

A 12 693 -. 23 -

Table 4

added material high purity low purity directly processed portion

of the outflow of the crystal! - - 100 0 63.5 0 100 36.5

2nd level sators in%

kg / hour (Lbs / Kr.) 1269 0 864 459 1728

(2794.5) (1902.5) (1010.5) (3805) (1902.5)

Cobalt ppm 316 <1 - -Cl 11 1

_o iron ppm 9 4 ^ 1 ■ - ^ i 1

co benzoic acid ppm 150 ^ -ν-Ό - * - * o <—O * · ~ ο ^ X)

*** Monobasic acid ppm. 4650 75-75 75 75 75

^ Reducible substances, weight pro

U5 ^z «nt 0.13 0.01 - 0 _v Cl 0.02 0.02 0.02

ο »terephthalic acid, percent by weight 0.14 ~~ Ό - vO 3.8 0.8 1.7

-Ο «OO

Claims (1)

Expectations 1. A method for purifying isophthalic acid, which is produced by oxidation of meta-xylene with a small amount of para-xylene with an oxygen-containing gas in the liquid phase, characterized in that a solution of the solid Oxydationsrohprodukts in a water and about 75 to 99 percent by weight acetic acid-containing medium is fed to a first crystallization zone, the first crystallization zone to maintain a temperature of approx. 120 to 19O ° C (approx. 25 ° C ^; to 375 ° F.) And at one for maintaining a liquid phase for crystallization of the isophthalic acid sufficient pressure evaporated acetic acid is removed from the solution, the resulting slurry of isophthalic crystals from the first crystallization zone is fed to a second crystallization zone, the second crystallization zone to maintain a temperature of about 104 to 130 ° C (about 220 to 265 ° F.) and in one for the maintenance of a liquid phase a Sufficient pressure evaporated acetic acid is withdrawn, and isophthalic acid of higher purity is recovered from the slurry withdrawn from the second crystallization zone. 2 «Method according to claim 1, characterized in that the crystallized isophthalic acid solids from the slurry obtained from the second crystallization zone is withdrawn, are separated in a solid-liquid separation zone. 00 98A 8/1978 A 12 593 ■ - 25 - Process according to Claim 1, characterized in that at least part of the slurry from the second crystallization zone is fed to a third recrystallization zone from which evaporated acetic acid is removed ^{to maintain a temperature of about 40 ° C to 77 ° C (105 to 170 0 P.)} and which is under sufficient pressure to maintain a liquid phase, and the resulting slurry is fed to a solid-liquid separation zone in which the crystallized "isophthalic acid solids are separated from the slurry" Process according to claim 1, characterized in that part of the first crystallization zone withdrawn slurry is returned to the crystallization zone in an amount per unit time that is sufficient to achieve complete circulation of the material in the crystallization zone in about 1/3 to 10 minutes, and that part of the second crystallization zone withdrawn slurry to this crystallization zone, in an amount per unit of time is returned, which is sufficient for a complete circulation of the material in the second crystallization zone to accomplish in about 1/3 to 10 minutes. Method according to claim 4, characterized in that that the part of each slurry is the respective 'to' the first and second crystallization zones, with the feed material entering each crystallization zone prior to entering each of the crystallization zones is combined. BADORlGfNAL 009848/1978 A .12 693 - 26 - 6. The method according to claim 5, characterized in that that each crystallization zone is above the point at which the feed material is combined with the recycle material and the resulting common flow into each crystallization zone is below enters the liquid level 7. The method according to any one of the preceding claims, characterized in that the evaporated Acetic acid in the individual crystallization zones is withdrawn in such an amount per tent unit that the desired temperature is maintained. 8. Process according to one of the preceding claims, characterized in that the first crystallization zone is at a temperature of approx. Ϊ3Ο to 140 ⁰ C (approx. 265 to 285 ° P.), A pressure of approx. 1 to 77 ata (approx 15 to 11Opsia), in particular from about 2.1 to 2.8 ata (about 30 to 40 psia), and the dwell time is at least about half an hour, in particular half to three hours. 9. The method according to any one of the preceding claims, characterized in that the second crystallization zone is at a temperature of approx. 110 to 116 ° C (230 to 24O ° F.) And a pressure from about atmospheric pressure to about 0.9 atm (about 13 psig), especially from about 0.14 to 0.35 atmospheres (about 2 to 5 psig), and the dwell time at least about half an hour, in particular is about half an hour to three hours " 009848/1978 A 12 693 - 27 - 10. The method according to any one of the preceding claims »characterized in that the third order? crystallization zone at a temperature of approx. 40 to 52 ° C (about 105 to 125 ° F.) And under a sub-atmospheric pressure, especially one Pressure from about 0.07 to 0.21 ata (about 1 to 3 psia) is held and the residence time is about half an hour to three hours, in particular one to three Hours. 11. The method according to any one of the preceding claims, characterized in that the crystallized isophthalic acid solids of the The slurry removed from the second crystallization zone can be separated off by filtration to obtain solid isophthalic acid crystals in where less than 0.8 percent by weight of terephthalic . acid as an impurity. 12. The method according to any one of claims 3 to 11, characterized in that from the second Slurry recovered from the crystallization zone is transferred to the third crystallization zone is to produce a product with less than 3.8 percent by weight from the third crystallization zone Obtain terephthalic acid. 009848/1978 Blank page