WO2013132507A1

WO2013132507A1 - An integrated process for the recovery of metal catalysts during the manufacture of purified terephthalic acid

Info

Publication number: WO2013132507A1
Application number: PCT/IN2012/000291
Authority: WO
Inventors: Parasu Veera UPPARA; Pavankumar ADURI; Thomas Mathew; Uday RATNAPARKHI; Ananth SHARMA; Tamagna UKIL; Rajaram S. GHADGE
Original assignee: Reliance Industries Ltd.,
Priority date: 2012-03-05
Filing date: 2012-04-20
Publication date: 2013-09-12
Also published as: PT2013132507W; CN104144906B; CN104144906A; BR112014021765A2; HK1203479A1

Abstract

The present disclosure provides a process for recovering PTA (purified terephthalic acid) oxidation catalysts during the manufacture of PTA, using an acid catalyst containing reaction stream resulting from the hydrolysis of an organic ester.

Description

AN INTEGRATED PROCESS FOR THE RECOVERY OF METAL CATALYSTS DURING THE MANUFACTURE OF PURIFIED TEREPHTHALIC ACID

This application is a patent of addition to Indian Patent Applicatio No. 1585/MUM/2001 1 filed on May 27, 201 1, the entire contents of which, is specifically incorporated herein by reference.

FIELD OF THE DISCLOSURE:

The present disclosure relates to an integrated process that involves the recovery of metal catalysts during the manufacture of purified terephthalic acid (PTA), using an acid catalyst containing reaction stream resulting from the hydrolysis of an organic ester.

BACKGROUND:

Methyl acetate is produced in large amounts as a by-product rom purified terephthalic acid (PTA) plants and polyvinyl alcohol (PVA) plants. Since methyl acetate is a less valuable solvent, methyl acetate is sold at a low price or hydrolyzed to methanol and acetic acid, which are more valuable, by a hydrolysis reaction followed by conventional distillation processes. The conversion is unfavorable at equilibrium and a. large amount of unconverted methyl acetate needs to be separated and recycled as the low equilibrium constant and presence of interfering azeotrope inhibit the hydrolysis of methyl acetate via conventional processes.

Acetic acid (HOAc) and methanol (MeOH) can be made by the liquid-phase reaction of methyl acetate (MeOAc) and water. The MeOAc hydrolysis reaction is reversible and the reaction equilibrium constant, K, is relatively small. Therefore, H+ ion is employed as the catalyst in order to increase the reaction rate. Strong acids such as sulfuric acid and hydrochloric acid can be used to produce an H+ ion and catalyze the reaction. However, strong acids cause corrosion in the whole process. In order to eliminate the mentioned disadvantages, a strongly cation- exchange resin containing sulfonic acid (SO3H) is usually used as an acid catalyst for hydrolysis.

Various processes for hydrolysis of methyl acetate have been reported in the prior art. For Example, TWI322801B describes a process that involves hydrolyzing methyl acetate solution in a reactive distillation system to generate two mixtures in the presence of heterogeneous catalyst. The mixtures are condensed and refluxed to the reactive distillation system via a reflux system filled with a heterogeneous catalyst.

Korean Patent KR888065B1 teaches a hydrolysis method of the methyl acetate wherein the hydrolysis is done in a reactor filled with solid catalyst. Hydrolysis of methyl acetate in the presence of a hydrolysation catalyst at a pressure above atmospheric and an elevated temperature below the boiling-point is described in GB687902. The catalyst employed is a strong acid, e.g. sulphuric acid equivalent to 0.05-2, preferably about 1 per cent by weight of the reactants while the other reaction parameters are temperature of 48-140°C and pressure up to 10 atm.

A process for catalytically hydrolyzing side product methyl acetate in production of pure terephthalic acid is disclosed in CN101481293A. The catalyst used for the hydrolysis of methyl acetate in accordance with the disclosure made in CN101481293A is heterogeneous catalyst. Though the process is integrated CN101481293A as such does not deal with problem associated with the recovery of metal catalysts used in the synthesis of the PTA.

Another Chinese Patent that also discloses hydrolysis of methyl acetate that is produced as a by-product in the process for preparation of refined terephthalic acid is CN100418939C. The technological process as described in CN100418939C employs cationic exchange resin inside the catalytic rectification tower.

Though interrelated processes for hydrolysis of methyl acetate and production of terephthalic acid have been taught, none of these processes address the specific problem associated with recovery of metal catalysts that are employed in the synthesis of terephthalic acid.

OBJECTS OF THE DISCLOSURE:

Some of the objects of the present disclosure are described herein below:

It is an object of the present disclosure to provide an integrated process that involves the hydrolysis of methyl acetate as well as the recovery of metal catalysts during the manufacture of PTA.

It is another object of the present disclosure to provide a simple and economic process for recovering the metal catalyst during the manufacture of PTA using an acid catalyst which is separated and recovered from the hydrolysis of an organic ester.

Other objects and advantages of the present disclosure will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present disclosure.

SUMMARY OF THE DISCLOSURE:

In accordance with the present disclosure there is provided a process for recovering PTA oxidation catalysts during the manufacture of PTA, using an acid catalyst containing reaction stream resulting from the hydrolysis of an organic ester; said process comprising the following steps : i) hydrolyzing an organic ester of a carboxylic acid into respective carboxylic acid and alcohol in presence of a first catalyst to produce an effluent stream containing said carboxylic acid, said alcohol, water and said first catalyst, wherein said first catalyst is an acidic catalyst; ii) optionally, removing water and alcohol from said effluent stream; iii) subjecting alkyl aromatics selected from the group consisting of p-xylene and meta-xylene to liquid phase oxidation in presence of a second catalyst comprising i) at least one metal salt selected from the group consisting of cobalt acetate and manganese acetate, and ii) hydrogen bromide to obtain a reaction mixture containing terephthalic acid (TA); iv) isolating said terephthalic acid from the reaction mixture to obtain a mother liquor comprising said second catalyst; v) treating said mother liquor with said effluent stream to displace acetate counter-anions of said second catalyst by oxalate ions from said first catalyst to obtain a mixture comprising respective insoluble salts of said second catalyst; and vi) separating the insoluble salts of said second catalyst by centrifugation and recovering the said second catalyst.

Typically, said first catalyst is oxalic acid.

Typically, the proportion of said carboxylic acid and said first catalyst in the effluent stream obtained in method step (i) is 9: 1.

Typically, the carboxylic acid is at least one selected from the group consisting of acetic acid, propanoic acid, and butanoic acid.

Typically, the alcohol is at least one selected from the group consisting of methyl alcohol, ethyl alcohol, iso-propyl alcohol, n-butyl alcohol, amyl alcohol and hexyl alcohol.

Typically, the ester is derived from any combination of carboxylic acids and alcohols.

Preferably, the ester includes but is not limited to methyl acetate, ethyl acetate, iso-propyl acetate, n-butyl acetate, amyl acetate and hexyl acetate. In accordance with one of the embodiments of the present disclosure, the ester is methyl acetate and the effluent stream contains acetic acid, methanol, oxalic acid and water.

BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1 is a flow diagram of a process and equipment for hydrolysis of methyl acetate according to the disclosure; and

Figure 2 illustrates an integrated process for methyl acetate hydrolysis and catalyst recovery.

DETAIL DESCRIPTION OF THE DISCLOSURE:

The process for hydrolyzing an organic acid ester in the presence of an acid catalyst has been disclosed by the present inventors in the co-pending Application 1585/MUM/2011 the entire contents of which, is specifically incorporated herein by reference.

The present disclosure provides an integration of the hydrolysis process with purified terephthalic acid (PTA) production process for recovering PTA oxidation catalysts during the manufacture. Accordingly, in accordance with the present disclosure there is provided a process for recovering PTA oxidation catalysts during the manufacture of PTA, using an acid catalyst containing reaction stream resulting from the hydrolysis of an organic ester.

The organic esters to be hydrolyzed in the first step are derived from any combination of carboxylic acids and alcohols. Preferably, the ester includes but is not limited to methyl acetate, ethyl acetate, iso-propyl acetate, n-butyl acetate, amyl acetate and hexyl acetate.

In accordance with one of the embodiments of the present disclosure mild acids are used as a first catalyst such as oxalic acid.

Preferably, the catalyst employed is 5-40% oxalic acid in aqueous solution or acetic acid or mixture of both.

In accordance with one of the exemplary embodiments, hydrolysis reaction of methyl acetate, in the presence of an acid catalyst, to produce acetic acid is described herein below.

MeOAc + H₂0 =i MeOH + HO Ac

The organic ester, methyl acetate (MeOAc) and water (H₂O) are reacted in a single continuous column reactor which acts as a combination of reactor and distillation column providing sufficient residence time to achieve high conversions of methyl acetate to methanol (MeOH) and acetic acid (HO Ac) in the presence of an oxalic acid as a catalyst. Fresh methyl acetate and water along with makeup oxalic acid is fed to mixer. Mixed stream from the mixer is fed into the middle of the column reactor (RD). Typically the overhead product stream from the RD consisting of about 88.2% of methyl acetate, 11.4% of methanol and 0.4% of water is continuously fed to the mixer. The bottom product stream from RD typically contains 29.5% of HO Ac , 45.2% of water, 15.7% methanol and 9.6% oxalic acid. Unless otherwise specified, all the percentages used herein are weight percentages. The separation of methanol from the bottom product stream of RD is achieved using the distillation column (Dl) with 98.3% purity. The fact that there is a huge difference in the boiling points of methanol, acetic acid and water and methyl acetate which forms azeotrope with methanol is absent in the RD bottom, makes the separation of methanol easier in Dl. The bottom product of Dl containing 53.4% water, 35.1% acetic acid, 0.1% methanol and 1 1.4% oxalic acid is fed into the distillation column D2. The overhead product of D2 contains 97.9% pure water with traces of acetic acid and methanol. The bottom product of D2 consists of 73.6% acetic acid and 24.6% of oxalic acid. The oxalic acid is crystallized from this stream using crystallizer (CI). The crystals are separated from acetic acid using filter F 1.

The column reactor is operated in such a manner that it provides intimate contact between methyl acetate, water, between acetic acid and methyl acetate / water azeotrope and between acetic acid and methanol / methyl acetate azeotrope. By providing intimate contact between the reactants and products, and continuously removing the products from the top and bottom of the column reactor, high conversions are achieved.

The methyl acetate and water are fed to the column reactor in the range of 1 : 1 to 1 :10 mole ratio, preferably 1 :1 to 1 :5 and even more preferably 1:3 to 1 :5 ratio.

The column rector is commonly operated at a temperature of about 40 to 90°C. However, the optimal temperature for a particular process is depends on the desired production rate, operating pressure. The common operating pressure is about 1 to 2 atmospheres. Typically, it is desirable to operate at low temperatures and low pressures. The residence time of the reaction mixture in the column reactor is also an important parameter of the process of the present disclosure. The minimum residence time which is required for higher conversion depends upon the catalyst and the catalyst concentration.

The catalyst provided to the column reactor in a concentration sufficient to provide the desired catalytic effect. The optimum way of providing the catalyst to the column reactor is feeding aqueous solution of oxalic acid at the upper part of the column. Optionally some amount of acetic acid or acetic acid/water mixture having oxalic acid dissolved in it is fed through the upper part of the column. With a 30% oxalic acid aqueous solution as a catalyst, the residence time for the process of the present disclosure is at least 30 minutes. Preferably, the residence time in the column reactor is about 30 to 45 minutes. The reflux ratio of the process of the present disclosure is at least about 1.32. The term reflux ratio is defined as the ratio of the overhead reflux flow rate to the overhead product flow rate.

The equipment Ml as illustrated in Fig 1 of the accompanying drawings comprises a mixer provided with a stirrer A. The mixer is marked Ml where lines 2 and 3 are connected. RD is a column reactor which is connected to the out let of Ml via line 4. RD Reflux lines are marked as 5 and 6 via condenser HI. Another recycle line 7 is connected to Condenser H2 and outlet line 8 connected to line 9. Line 9 is connected to Ml. Line 10 and 11 are connected in bottom of RD via re-boiler H3. Outline of RD, line 12 is connected to a distiller Dl. Line 13 is passed via condenser H4 and line 14 connected to the Dl. Top outlet of Dl is marked as 15. Bottom reflux lines are passes via re-boiler H5 and marked respectively 16 and 17. Similarly, bottom outline of Dl, 18 is connected to a distiller D2. Top reflux lines 19 and 20 are passed via condenser H6. Top outlet of D2 is marked as 21. Bottom reflux lines are passes via re-boiler H7 and marked respectively 22 and 23. Crystallizer CI is connected to the bottom of D2 via line 24. Filter Fl is connected with crystallizer via line 25. Lines 26 and 27 are also connected to filter Fl .

Methyl acetate is fed into the mixer Ml via line 9. Water is fed into mixer Ml via water inlet line 2 while catalyst is introduced via line 3. The mixture containing Methyl acetate, water and catalyst is fed into the column reactor RD via line 4. Reaction is carried out in column reactor at 30-120°C, preferably 90-100°C. Distiller Dl used for distilled out methanol (line 15) coming from bottom stream of column reactor via line 12. Similarly distiller D2 used for the separation of acetic acid and oxalic acid (line 24) coming from bottom Dl via line 18. The mixture of oxalic acid and acetic acid (line 24) is fed into the crystallizer CI, where oxalic acid is precipitated. The crystallizer temperature maintained 10-25° C preferably 15-18°C. The precipitated catalyst is separated by passing via filter Fl, where maximum catalyst and acetic acid is separated and come out via respectively line 26 and 27.

The effluent stream produced by the hydrolysis of the organic ester of carboxylic acid typically comprises respective carboxylic acid, an alcohol, water and the acid catalyst. Depending on the type of ester involved the carboxylic acid present in the effluent stream can be acetic acid, propanoic acid and butanoic acid while the corresponding alcohol may be methanol, ethanol, isopropanol, n-butanol amyl alcohol and hexyl alcohol. Optionally, water and methanol are removed from such stream and an effluent steam that contains the carboxylic acid and the acid catalyst is thus obtained. Typically, the proportion of the carboxylic acid and said first catalyst (acid catalyst) in the effluent stream is 9:1. This is stream 27 of Fig 1.

Terephthalic acid is manufactured by liquid-phase oxidation of alkyl aromatics selected from the group consisting of p-xylene and meta-xylene with air in the presence of a metal catalyst system (a second catalyst) that comprises cobalt- manganese-bromide catalyst system (cobalt acetate, manganese acetate, and hydrogen bromide) in acetic acid in the temperature range from 150 to 210 °C.

The relative ratio of cobalt, manganese and bromine are important, and the typical values are manganese: cobalt ratio 3:1 and cobalt: bromine ratio 1 :5. In reactor and crystallizer, most of the terephthalic acid crystallize out from the mother liquor and are separated by filtration.

The mother liquor comprises mainly acetic acid and organic compounds like isophthalic acid, benzoic acid and inorganic compounds like cobalt and manganese compounds along with iron, nickel, chromium and sodium.

In accordance with exemplary embodiment of the present disclosure, the mother liquor containing second catalyst (metal catalyst) is then treated with the effluent stream containing carboxylic acid and the acid catalyst (oxalic acid) i.e stream 27 of Fig 1 to recover the metals catalysts present in the mother liquor in the form of oxalate salts. Typically, acetate counter anions of metal catalyst are displaced by oxalate ions from the acid catalyst. This is illustrated in Figure 2 wherein the methyl acetate hydrolysis section illustrates the process of hydrolysis and separation of product and catalyst (oxalic acid) along with acetic acid (stream 27 in Fig 1). Stream 27 containing 90% acetic acid and 10% oxalic acid exiting hydrolysis section is used for catalyst recovery. The stream 27 of Fig 1 is mixed with mother liquor having inorganic metal catalyst in mixing zone. Oxalic acid in the stream reacts with the acetates of cobalt and manganese and form respective oxalates which are separated and recovered by centrifugation which may also be recycled back to the oxidation reactor.

The present disclosure will be further described with reference to the following non-limiting examples:

Example 1: Example 1:

In a typical equipment of Fig 1 of the accompanying drawings, methyl acetate and water with weight ratios of 1 :5 was added to the reactor in presence of oxalic acid under agitation. Reaction was continued for 30 minutes at 65°C. After reaction completed total mixture was distilled and oxalic acid was separated by chilling. The percentages of formation of Acetic acid and methanol were 55 and 45 respectively.

Example 2:

In a typical equipment of Fig 2 of the accompanying drawings methyl hydrolysis section and catalyst recovery unit of Purified terephthalic acid (PTA) process. This example was followed by example 1. 10% oxalic acid solution containing acetic acid was separated by chilling process and it was sent to the purified terephthalic acid catalyst recovery unit where cobalt and manganese were separated in the form of oxalates.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression "at least" or "at least one" suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.

Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.

The foregoing description of the specific embodiments will so fully reveal the general nature, of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

Claims

Claims:

1. A process for recovering PTA oxidation catalysts during the manufacture of PTA, using an acid catalyst containing reaction stream resulting from the hydrolysis of an organic ester; said process comprising the following steps : i. hydrolyzing an organic ester of a carboxylic acid into respective carboxylic acid and alcohol in presence of a first catalyst to produce an effluent stream containing said carboxylic acid, said alcohol, water and said first catalyst, wherein said first catalyst is an acidic catalyst; ii. optionally, removing water and alcohol from said effluent stream; iii. subjecting alkyl aromatics selected from the group consisting of p- xylene and meta-xylene to liquid phase oxidation in presence of a second catalyst comprising i) at least one metal salt selected from the group consisting of cobalt acetate and manganese acetate, and ii) hydrogen bromide to obtain a reaction mixture containing terephthalic acid (TA); iv. isolating said terephthalic acid from the reaction mixture to obtain a mother liquor comprising said second catalyst; v. treating said mother liquor with said effluent stream to displace acetate counter-anions of said second catalyst by oxalate ions from said first catalyst to obtain a mixture comprising respective insoluble salts of said second catalyst; and vi. separating the insoluble salts of said second catalyst by centrifugation and recovering the said second catalyst.

2. The process as claimed in claim 1, wherein said first catalyst is oxalic acid .

3. The process as claimed in any of the preceding claims wherein the proportion of said carboxylic acid and said first catalyst in the effluent stream obtained in method step (i) is 9: 1.

4. The process as claimed in claim 1, wherein said carboxylic acid is at least one selected from the group consisting of acetic acid, propanoic acid, and butanoic acid.

5. The process as claimed in claim 1, wherein said alcohol is at least one selected from the group consisting of methyl alcohol, ethyl alcohol, iso-propyl alcohol, n-butyl alcohol, amyl alcohol and hexyl alcohol

6. The process as claimed in claim 1, wherein said ester is derived from any combination of carboxylic acids and alcohols claimed in claims 4 & 5 respectively.

7. The process as claimed in claim in any of the preceding claims, wherein said ester is methyl acetate and said effluent stream contains acetic acid, methanol, oxalic acid and water.