GB1589854A - Production of carboxylic acids - Google Patents

Abstract

Carboxylic acids are obtained by hydrolysing an aromatic or heterocyclic nitrile, or an intermediate hydrolysis product thereof, with water in the vapour phase in the presence of a solid acidic catalyst. The gaseous reaction product formed contains the desired carboxylic acid, which can be obtained therefrom by, for example, sublimation. The process yields very pure products.

Description

(54) IMPROVÉMENTS 1N OR RELATING TO THE PRODUCTION OF CARBOXYLIC ACIDS (71) We, THE LUMMUS COMPANY, a Company organised and existing under the laws lof the State of Delaware, United States of America, of 1515 Broad Street, Bloomfield, New Jersey 07003, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be par ticularly described in and by the following statement:- The invention relates to the production of carboxylic acids, and more particularly to the production of carboxylic acids from nitnles: At the present time aromatic or nonaromatic heterocyclic nitriles are generally converted to the corresponding carboxylic acid by aqueous hydrolysis which may be catalysed by an acid or base. Thus, 'for example, terephthalonitrile has been hydro lysed with aqueous ammonia to produce the corresponding ammonium salt, which is then converted to the acid by steam stripping. Although such hydrolysis reac tions are capable of producing the acid from the nitrile, there is a need for im provements in the production of carboxylic acids from acids from nitriles.

According to this invention there is pro vided a process for producing a carboxylic acid from a nitrile, comprising reacting a starting material comprising either a nitrile, or an intermediate hydrolysis pro duct thereof, with water in the vapour phase in the presence of a solid acid cata lyst to produce a gaseous effluent containing the corresponding carboxylic acid.

Conveniently the nitrile is aromatic, and has at least one cyano group substituted on a benzene ring or on a napthelene ring.

Preferably the carboxylic acid is re covered from the gaseous effluent by sub limation of the carboxylic acid.

One specific process, in accordance with the invention, for producing terephthalic acid comprises introducing water vapour, terephthalonitrite, an organic solvent for telephthalonitrite and recycle components, all in the vapour phase, into a terephthalic acid production zone, which contains a solid acid catalyst, wherein the terephthalonitrile is hydrolysed in the vapour phase to terephthalic acid, withdrawing from the terephthalic acid production zone a gaseous reaction effluent containing terephthalic acid, terephthalonitrile, an organic solvent for terephthalonitrile, water vapour, ammonia and reaction intermediates, intro ducing the gaseous effluent into a terephthalic acid recovery zone wherein terephthalic acid is separated from the gaseous effluent by sublimation at a temperature above the dew point of the remaining com nonets of the gaseous effluent, recovering solid terephthalic acid from the terephthalic acid recovery zone, recovering the remaining gaseous effluent from the terephthalic acid recovery zone, quenching the remaining gaseous effluent to condense a solution of unreacted terephthalonitrile in the organic solvent for terephthalonitrile, and an aqueous solution of reaction intermediates, and recovering and vaporising said terephtholonitrile solution and said aqueous solution for recycle to the terephthalic acid production zone.

Preferably a portion of the recovered aqueous solution is employed as a quench liquid for said quenching.

Advantageously said quenching is effected at a temperature of 1000F to 200"F and a pressure of from 1-3 atmospheres. Conveniently the hydrolysis is effected at a temerature of from 200"F to 10000F.

Preferably the catalyst is silica gel silicaalumina, supported phosphoric acid, a Group III metal phosphate, a Group III metal sulphate or a transition metal oxide. It will be apparent to a man skilled in the art that the term "acid" as used herein includes those compounds commonly termed "Lewis acids".

Advantageously the reaction is effected at a temperature at which there is a 50% conversion of nitrile at a contact time not exceeding one minute, and the reaction is effected at a temperature above the dew point of the nitrile and of the acid.

The process may conveniently employ a starting material comprising a heterocyclic nitrile, or an intermediate hydrolysis product thereof.

This invention also relates to a carboxylic acid, whenever made by such a process.

In order that the invention may be more readily understood and so that further featurcs thereof may be appreciated, the invention will now be particularly described by way of example, and with refer ence to the accompanying drawing which is a simplified schematic flow diagram of one embodiment of the present invention.

The organic materials which are employed as starting materials for producing carboxylic acids by a process in accordance with the present invention are nitriles, or intermediate hydrolysis products thereof.

The nitrile, if aromatic, contains one or more cyano- groups, preferably one or two cyano- groups, and can be otherwise unsubstituted, or substituted with, for example, an alkyl group. The aromatic nucleus is preferably benzine or napthalene.

As representative examples of such aromatic nitriles, there may be mentioned: phthalonitrile, terephthalonitrile, isophthalonitrile, tolunitrile, I-cyanonapthalene, and 2,6-cyanonapthalene. Similarly, the nitrile, if heterocyclic, can contain one or more cyano- groups. with the hetcrocyclic nucleus generally being pyridine. The preferred starting materials are nicotinonitrile, isophthalonitrile, terephthalonitrile and phthalonitrile. As hereinabove noted, the starting material may be a nitrile intermediate hydrolysis product, and for example may be selected from the imides, amides, cyanoacids, cyano- amides and amide- acids, which can be employed alone or in combination with each other or the nitrile starting material.

A solid acid catalyst is employed for the vapour phase hydrolysis of the above described starting materials. As representative catalysts, there may be mentioned: silica gel, silica-alumina, supported phosphoric acid, Group III metal phosphates and sulphates for example phosphates and sulphates of aluminium, boron and gallium and transition metal oxides for example one or more oxides of vanadium, chromium, mangancse, iron, cobalt or nickel. The catalysts are of the type generally employed for hydration, dehydration, and esterification reactions. The most preferred catalyst is supported phosphoric acid.

The starting material and water are reacted in the vapour phase in contact with the catalyst at temperatures which are generally of from 2000F to 1,0000F.

and preferably of from 400"F to 8000F.

The temperatures which are employed are generally above the dew point of both the feed and product components. The temperatures are most preferably selected to provide at least a 50% conversion of the nitrile at a contact or reaction time of no greater than one minute.

The water may be employed in at least stoichiometric proportions; however, an excess of water is preferably employed, because reaction kinetics are more favourable at higher water partial pressures. The stoichiometric excess of water can be as much as to provide a water to nitrile mole ratio of up to 500:1, with the water to nitrile mole ratio generally being from 5: 1 to 50:1. The use of an excess of water also functions to maintain vapour phase conditions; however, an inert gaseous diluent, such as nitrogen, can also be used for such purposes.

The total reaction pressure is generally selected to provide the desired water partial pressure, with such pressures generally being from 1 to 10 atm.

The catalytic vapour phase reaction may be effected by any one of a wide variety of reaction techniques, including fixed bed, fluidised bed and dilute phase transport, and the selection of a specific technique is deemed to be within the scope of those skilled in the art from the present teachings.

The gaseous reaction effluent contains the carboxylic acid, unreacted starting material, ammonia, and some reaction intermediates.

Thus, for example, in the production of terephthalic acid from terephthalonitrile the gaseous reaction effluent includes terephthalic acid, ammonia, unreacted terephthalonitrile, reaction intermediates, such as cyanobenzoic acid or terephthalmic acid, as well as any unreacted water. The reaction mixture can be cooled to condense the entire effluent and produce a water solution and slurry of the ammonium salts of terephthalic acid and intermediate hydrolysis products.

The carboxylic acid and the intermediate hydrolysis products can be separated from the ammonia by any suitable means, for example by the addition of hydrochloric acid or of another suitable acid. In the preferred embodiment of a process in accordance with the invention, the carboxylic acid may be recovered from the effluent by sublimation of the acid.

The temperature at which the sublimation of the acid from the effluent is effected varies with the particular carboxylic acid.

As the temperature at which sublimation of the acid is effected decreases, there is an increase in the recovery of carboxylic acid from the vapour phase; however, as the temperature decreases below the dew point of the least volatile component, other than the acid product, in the reaction mixture, there is also an increase in the amount of other products sublimated from the vapour phase, which decreases the purity of the sublimated acid product.

In general, the sublimation temperature is not less than 500F below the dew point of the least volatile component; however it is preferred to effect sublimation of the acid at a temperature above the dew point of the least volatile component, other than the acid product, of the vapour effluent.

The selection of a particular temperature to co-ordinate sublimated acid product purity with quantity of acid recovery Is deemed to be within the scope of those skilled in the art from teaching of the present specification. In general, the temperature is selected to provide for at least 50% recovery of the acid product from the vapour phase.

The 'sublimation is preferably effected in the presence of solid carboxylic acid, with such solid carboxylic acid functioning both as a nucleus for crystal growth and a heat transfer agent for the sublimation.

The invention will be further described with reference to the following specific ex amples: EXAMPLE 1 A U-shaped, 1" O.D. stainless steel reactor, 40" total length, was charged with 200 grams of a pelleted 10% H3PO4 on silicaalumina catalyst. The reactor was heated to 250 C in an air bath and a vapourised 1% ammonia solution passed over the catalyst for one hour. The reactor was then flushed with nitrogen and heated to 3000C.

Subsequently terephthalonitrile was heated to 2300C in a steel cylinder, through which nitrogen was passed at a rate of 1 litre/min.

so that approximately 0.5g of terephthalonitrile (TPN) per minute was volatised and passed through heated tubing into the reactor. Water, at the rate of 1.3g/min., was also vapourised and passed into the reactor co-currently with the TPN/nitrogen.

The mole ratio H20 TPN was approximately 18:1.

The reactor effluent was condensed in an ice bath. The resulting product consisted of a white solid slurried in water. The mixture was acidified with hydrochloric acid to precipitate terephthalic acid (TPA) dissolved as the ammonium salt. The resulting solids were filtered, washed with water and analysed by liquid chromatography. The solid consisted of a mixture of 58% unreacted TPN, 5% terephthalamic acid/cyanobenzoic acid and 37% terephthalic acid.

EXAMPLE 2 The process described in Example 1 was repeated but the reactor was operated at a temperature of 3500C and a pressure of 35 psig and TPN was fed at a rate of 5 millimoles /min. and steam was fed at a rate of 83 millimoles/min. The run was effected for a period of 50 minutes.

The product contained 45.7wit.% TPA; 38.2wit.% cyanobenzoic acid; 9wt.% terephthalmic acid; and 7.1wit.% TPN.

Referring now specifically to the acompanying drawing, one embodiment of the invention relating specifically to the proportion of terephthalic acid will now be described; however, the process is not limited solely to the production of this particular end product.

Referring to the drawing, vaporised terephthalonitrite in line 10 is combined with water in line 11; vaporised recycle terephthalonitrile combined with a vaporised organic solvent therefor (such as xylene) in a line 12, obtained as hereinafter described; and vaporised intermediate hydrolysis products, in a line 13, obtained as hereinafter described. The combined gas stream in a line 14 is introduced into a terephthalic acid production reactor, schematically indicated at 15. Reactor 15 contains a suitable catalyst, such as phosphoric acid supported on silica-alumina, and in the reactor 15 the water and terephthalonitrile react, in the gas phase, to produce terephthalic acid.

A gaseous reaction effluent, containing terephthalic acid, unreacted terephthalonitrile, unreacted water, ammonia, and reaction intermediate (in particular, terephthalmic acid and cyanobenzoic acid), is withdrawn from the reactor vessel 15 through a line 16 and introduced into a terephthalic acid recovery vessel 17 wherein terenhthalic acid is separated from the gaseous effluent by sublimation at a temperature above the dew point of the remaining components of the gaseous effluent. Tn this manner, terephthalic acid is recovered as a solid without condensation of the remaining components.

The recovery vessel 17 preferably includes solid terephthalic acid in a fluidised state to function as nucleation points for crystal growth and a heat transfer agent.

The heat of desublimation is removed by the use of a cooling coil 18; however, it is to be understood that other heat exchange means, such as heat exchange jackets or direct water quench could also be employed.

In general, the recovery vessel is operated at a temperature of from 400"F to 6000F and at a pressure of from 1 to 3 atm. in order to effect recovery of the terephthalic acid by sublimation at above the dew point of the remaining components of the gaseous element.

A gaseous stream, containing water, ammonia, unreacted terephthalonitrile, p-xylene, cyanobenzoic acid, terephthalamic acid and a small amount of terephthalic acid, is withdrawn from the recovery vessel 17 through a line 19 and introduced into a quench vessel 21 to separate ammonia from the remaining components. In the quench vessel 21 the gaseous stream is directly quenched by an aqueous quench liquid introduced through a line 33 to condense a xylene solution of terephthalonitrile and an aqueous solution d the ammonium salts of terephthalic, cyanobenzoic and terephthalamic acid. By including a solvent for tercphthalonitrilc, such as p-xylene, in the system the quench circuit is maintained substantially free of solid terephthalonitrile.

Similarly, by allowing the acids present in the gas stream to combine with ammonia and form water soluble ammonium salts the system is also maintained free of solid:.

In general, the quench vessel 21 is operated at a temperature of from 1000F to 2000F and a pressure of from 1 to 3 atm.; however, it is to be understood that such conditions are merely illustrative.

A gaseous overhcad is withdrawn from the quench vessel 21 through a line 22, which includes a suitable cooler and separator, and ammonia is withdrawn from the system through a line 23.

Condensed liquid is withdrawn from the quench vessel 21 through a line 24 and introduced into a separator 25, with an organic phase of terephthalonitrile dissolved in p-xylene being withdrawn therefrom through a line 26 and vaporised in a heater 27 for recycle to the reactor 15 through the line 12.

An aqueous phase, containing the ammonium salts, is withdrawn from the separator 25 through a line 31, with a first portion thereof being passed through a line 32, Including a cooler 34, for use as quench liquid in line 33. The remaining portion of the aqueous phase is passed through a line 35 and vaporised in the heater 27 for introduction into the reactor 15 through the line 13.

Solid crude terephthalic acid, containing the partial hydrolysis product terephthalamic acid, is withdrawn from the recovery vessel 17 through a line 41 and introduced into a storage vessel 42 for ultimate purification.

As particularly shown in the accompanying drawing, crude terephthalic acid withdrawn from the storage vessel 42 through a line 43 is repulped in a vessel 44 in water introduced through a line 45. A slurry of crude terephthalic acid in water is withdrawn from the vessel 44 through a line 46, heated in a coil positioned in the upper portion of a flash crystalliser 48 and further heated in a heater 49 to effect dissolution of the crude terephthalic acid. The solution of crude terephthalic acid is introduced into a hydrolysis vessel 51 wherein the solution is maintained at a temperature and a time sufficient to hydrolyse the terephthalamic acid to terephthalic acid.

In general, the solution is maintained at a temperature of from 400"F to 6000F, with the time being in the order of from 30 minutes to 2 hours.

An aqueous solution of terephthalic acid is withdrawn from the vessel 51 through a line 52 and introduced into the flash crystalliser 48 to crystallise terephthalic acid. In general, the crystalliser 48 is operated at a temperature of from 1000F to 2000F and a pressure of from 1 to 5 atm. It is to be understood that such crystallisation could be effected in one or more stages.

A slurry of terephthalic acid is withdrawn from the crystalliser 48 through a line 53 and introduced into a suitable separation vessel, such as a centrifuge 54 to recover terephthalic acid.

A wet terephthalic acid cake is withdrawn from the centrifuge 54 through a line 55 and dried in a drier 56, with the final pure terephthalic acid product being recovered through a line 57'use into 57.

A water wash is introduced into the centrifuge 54 through a line 62 and a first portion thereof employed in the line 45 for repulping the crude terephthalic acid.

A second portion is passed through a line 63 and vaporised in the heater 27 for introduction into the reactor 15 through the line 11.

The present invention is particularly advantageous because the nitrile is converted to the acid in shorter reaction times and at lower pressures than achieved with conventional liquid hydrolysis. In addition, less expensive materials of construction can be employed for the apparatus utilised.

The carboxylic acids produced by processes in accordance with the invention have been found to be of high purity. For example, embodiments of the invention have been found to produce terephthalic acid of fibre grade.

WHAT WE CLAIM IS:- 1. A process for producing a carboxylic acid from a nitrile, comprising reacting a starting material comprising either a

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

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. In general, the recovery vessel is operated at a temperature of from 400"F to 6000F and at a pressure of from 1 to 3 atm. in order to effect recovery of the terephthalic acid by sublimation at above the dew point of the remaining components of the gaseous element. A gaseous stream, containing water, ammonia, unreacted terephthalonitrile, p-xylene, cyanobenzoic acid, terephthalamic acid and a small amount of terephthalic acid, is withdrawn from the recovery vessel 17 through a line 19 and introduced into a quench vessel 21 to separate ammonia from the remaining components. In the quench vessel 21 the gaseous stream is directly quenched by an aqueous quench liquid introduced through a line 33 to condense a xylene solution of terephthalonitrile and an aqueous solution d the ammonium salts of terephthalic, cyanobenzoic and terephthalamic acid. By including a solvent for tercphthalonitrilc, such as p-xylene, in the system the quench circuit is maintained substantially free of solid terephthalonitrile. Similarly, by allowing the acids present in the gas stream to combine with ammonia and form water soluble ammonium salts the system is also maintained free of solid:. In general, the quench vessel 21 is operated at a temperature of from 1000F to 2000F and a pressure of from 1 to 3 atm.; however, it is to be understood that such conditions are merely illustrative. A gaseous overhcad is withdrawn from the quench vessel 21 through a line 22, which includes a suitable cooler and separator, and ammonia is withdrawn from the system through a line 23. Condensed liquid is withdrawn from the quench vessel 21 through a line 24 and introduced into a separator 25, with an organic phase of terephthalonitrile dissolved in p-xylene being withdrawn therefrom through a line 26 and vaporised in a heater 27 for recycle to the reactor 15 through the line 12. An aqueous phase, containing the ammonium salts, is withdrawn from the separator 25 through a line 31, with a first portion thereof being passed through a line 32, Including a cooler 34, for use as quench liquid in line 33. The remaining portion of the aqueous phase is passed through a line 35 and vaporised in the heater 27 for introduction into the reactor 15 through the line 13. Solid crude terephthalic acid, containing the partial hydrolysis product terephthalamic acid, is withdrawn from the recovery vessel 17 through a line 41 and introduced into a storage vessel 42 for ultimate purification. As particularly shown in the accompanying drawing, crude terephthalic acid withdrawn from the storage vessel 42 through a line 43 is repulped in a vessel 44 in water introduced through a line 45. A slurry of crude terephthalic acid in water is withdrawn from the vessel 44 through a line 46, heated in a coil positioned in the upper portion of a flash crystalliser 48 and further heated in a heater 49 to effect dissolution of the crude terephthalic acid. The solution of crude terephthalic acid is introduced into a hydrolysis vessel 51 wherein the solution is maintained at a temperature and a time sufficient to hydrolyse the terephthalamic acid to terephthalic acid. In general, the solution is maintained at a temperature of from 400"F to 6000F, with the time being in the order of from 30 minutes to 2 hours. An aqueous solution of terephthalic acid is withdrawn from the vessel 51 through a line 52 and introduced into the flash crystalliser 48 to crystallise terephthalic acid. In general, the crystalliser 48 is operated at a temperature of from 1000F to 2000F and a pressure of from 1 to 5 atm. It is to be understood that such crystallisation could be effected in one or more stages. A slurry of terephthalic acid is withdrawn from the crystalliser 48 through a line 53 and introduced into a suitable separation vessel, such as a centrifuge 54 to recover terephthalic acid. A wet terephthalic acid cake is withdrawn from the centrifuge 54 through a line 55 and dried in a drier 56, with the final pure terephthalic acid product being recovered through a line 57'use into 57. A water wash is introduced into the centrifuge 54 through a line 62 and a first portion thereof employed in the line 45 for repulping the crude terephthalic acid. A second portion is passed through a line 63 and vaporised in the heater 27 for introduction into the reactor 15 through the line 11. The present invention is particularly advantageous because the nitrile is converted to the acid in shorter reaction times and at lower pressures than achieved with conventional liquid hydrolysis. In addition, less expensive materials of construction can be employed for the apparatus utilised. The carboxylic acids produced by processes in accordance with the invention have been found to be of high purity. For example, embodiments of the invention have been found to produce terephthalic acid of fibre grade. WHAT WE CLAIM IS:-

1. A process for producing a carboxylic acid from a nitrile, comprising reacting a starting material comprising either a

nitrile, or an intermediate hydrolysis product thereof, with water in the vapour phase in the presence of a solid acid catalyst to produce a gaseous effluent containing the corresponding carboxylic acid.

2. A process according to claim 1 wherein the nitrile is an aromatic nitrile, having at least one cyano group substituted on a benzene ring or a naphthalene ring.

3. A process according to claim 1 or claim 2, wherein the carboxylic acid is recovered from the gaseous effluent by sublimation of the carboxylic acid.

4. A process according to claim 1 for producing terephthalic acid, comprising introducing water vapour, terephthalonitrile, an organic solvent for terephthalonitrile and recycle components, all in the vapour phase, into a terephthalic acid production zone, which contains a solid acid catalyst, wherein the terephthalonitrile is hydrolysed in the vapour phase to terephthalic acid, withdrawing from the terephthalic acid production zone a gaseous reaction effluent containing terephthalic acid, terephthalonitrile, an organic solvent for terephthalonitrile, water vapour, ammonia and reaction intermediates, introducing the gaseous effluent into a terephthalic acid recovery zone wherein terephthalic acid is separated from the gaseous effluent by sublimation at a temperature above the dew point of the remaining components of the gaseous effluent, recovering solid terephthalic acid from the terephthalic acid recovery zone, recovering the remaining gaseous effluent from the terephthalic acid recovery zone, quenching the remaining gaseous effluent to condense a solution of unreacted terephthalonitrile in the organic solvent for terephthalonitrile, and an aqueous solution of reaction intermediates, and recovering and vaporising said terephthalonitrile solution and said aqueous solution for recycle to the terephthalic acid production zone.

5. A process according to claim 4 wherein a portion of the recovered aqueous solution is employed as a quench liquid for said quenching.

6. A process according to claim 4 or 5, wherein said quenching is effected at a temperature of from 1000F to 2000F and a pressure of from 1-3 atmospheres.

7. A process according to any one of claims 4 to 6, wherein the hydrolysis is effected at a temperature of from 2000F to 10000F.

8. A process according to any one of the preceding claims, wherein the catalyst is silica gel, silica-alumina, supported phosphoric acid, a Group III metal phosphate, a Group III metal sulphate or a transition metal oxide.

9. A process according to claim 8, wherein the catalyst is supported phosphoric acid.

10. A process according to any one of the preceding claims, wherein the reaction is effected at a temperature at which there is a 50% conversion of nitrile at a contact time not exceeding one minute.

11. A process according to any one of the preceding claims, wherein the reaction is effected at a temperature above the dew point of the nitrile and of the acid.

12. A process according to any one of the preceding claims, wherein the nitrile is a heterocyclic nitrile.

13. A process for producing a carboxylic acid substantially as herein particularly described by way of example.

14. A process for producing a carboxylic acid substantially as herein described with reference to the accompanying drawing.

15. A process for producing a carboxylic acid substantially as herein described with reference to Example 1.

16. A process for producing a carboxylic acid substantially as herein described with reference to Example 2.

17. A carboxylic acid whenever prepared by a process according to any one of the preceding claims.