WO2008057924A1

WO2008057924A1 - Oxidation reaction for producing aromatic carboxylic acids

Info

Publication number: WO2008057924A1
Application number: PCT/US2007/083310
Authority: WO
Inventors: Joan Fraga-Dubreuil; Samuel Duncan Housley; Walter Partenheimer; Martyn Poliakoff
Original assignee: Invista Technologies S.A R.L.
Priority date: 2006-11-03
Filing date: 2007-11-01
Publication date: 2008-05-15
Also published as: GB0621968D0; AR064594A1; TW200831456A

Abstract

A method of minimising or preventing catalyst loss and/or the precipitation of metal oxide in a process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, wherein (i)said method comprises the step of adding an acidic component comprising one or more acid(s) into the oxidation reaction zone; (ii) contact of at least part of said catalyst with said oxidant is in the presence of said acidic component; and (iii) said acidic component comprises one or more organic acid(s).

Description

OXIDATION REACTION FOR PRODUCING AROMATIC CARBOXYLIC ACIDS

Field of The Invention

This invention relates to the field of metal salt-catalysed synthetic oxidation processes in supercritical water, particularly the oxidation of alkyl-substituted aromatic hydrocarbons to the corresponding aromatic carboxylic acid. The invention is particularly concerned with catalyst stability in these systems, and particularly the maintenance of catalyst activity and/or efficiency, and the avoidance of fouling of the reactor.

Background of The Invention

The dielectric constant of water decreases dramatically from a room temperature value of around 80C²/Nm² to a value of 5C²/Nm² as it approaches its critical point (374⁰C and 220.9bara), allowing it to solubilise organic molecules. As a consequence, water then behaves like an organic solvent to the extent that hydrocarbons, e.g. toluene, are completely miscible with the water under supercritical conditions or near supercritical conditions. Terephthalic acid, for instance, is virtually insoluble in water below about 200⁰C. Dioxygen is also highly soluble in sub- and super-critical water.

Holliday R.L. et a! (J. Supercritical Fluids 12, 1998, 255-260) describe a batch process cauied out in sealed autoclaves for the synthesis of, inter alia, aromatic carboxylic acids from alkyl aromatics in a reaction medium of sub-critical water using molecular oxygen as the oxidant. The use of supercritical water as a medium for the production of aromatic carboxylic acids in a continuous flow reactor nevertheless presented significant problems.

WO-02/06201-A discloses a process for the production of an aromatic carboxylic acid comprising contacting in the presence of a catalyst, within a continuous flow reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions close to the supercritical point such that said one or more precursors, oxidant and aqueous solvent constitute a substantially single homogeneous phase in the reaction zone. In the process described in WO-02/06201 -A the contact of at least part of the precursor with the oxidant is contemporaneous with the contact of the catalyst with at least part of the oxidant.

The continuous process of WO-02/06201 -A is earned out with the reactants and the solvent forming a substantially single homogeneous fluid phase in which the components in question are mixed at a molecular level. The concentration of dioxygcn in water increases markedly as the supercritical point is approached and exceeded, which assists in improving the reaction kinetics. The reaction kinetics are further enhanced by the high temperatures prevailing when the water solvent is under supercritical or near supercritical conditions. The combination of high temperature, high concentration and homogeneity mean that the reaction to convert the precursor(s) to aromatic carboxylic acid can take place extremely rapidly compared with the residence times employed in the production of aromatic carboxylic acids such as terephthalic acid by conventional techniques using a crystallising three phase oxidation reactor. Under these conditions, the intermediate aldehyde (e.g. 4-carboxybenzaldehyde (4-CBA) in the case of terephthalic acid) is readily oxidised to the desired aromatic carboxylic acid which is soluble in the supercritical or near supercritical fluid thereby allowing a significant reduction in contamination of the recovered aromatic carboxylic acid product with the aldehyde intermediate. The process conditions of WO-02/06201- A substantially reduce or avoid autocatalytic destructive reaction between the precursor and the oxidant and consumption of the catalyst. The continuous process involves short residence times and exhibits high yield and good selectivity of product formation.

The preferred catalyst in the supercritical oxidation for the production of aromatic carboxylic acids comprises manganese salts (particularly MnBr₂), but it has been observed that manganese salts are oxidised irreversibly to manganese oxide(s) (including MnO₂, Mn₂Ch and MnO(OH)₂) during the strong oxidative conditions of the reaction. The manganese oxide(s) forms an insoluble precipitate which adheres to internal walls following the initial contact between the catalyst and the oxidant (typically molecular oxygen), resulting in the progressive fouling of the reactor and/or blockages in the pressure let-down equipment. Metal salts are known to oxidise in aqueous supercritical oxidation processes, and it is this feature which is exploited in the production of nano-particles, as described by, for instance, Viswanathan et al. (J.

Supercritical Fluids, 2003, 27(2), ppl 87-193). It is possible under certain conditions to solubilise metal oxides in isolation by reducing them to a more soluble lower oxidation state, and US-4645650 teaches that such a reduction may be effected at atmospheric pressure and relatively low temperature using coal or other reducing carbonaceous material in an amount of at least 50% by mass of the metal oxide and in the presence of mineral acids. However, it is also known that some metal oxides are stable in supercritical water oxidation conditions, which has been exploited by their use as catalysts in extremely oxidative conditions to enhance the total oxidation of certain organic materials, as described for instance by Kranjnc et al. (Applied Catalysis, B: Environmental (1997), 13(2), 93-103).

In the supercritical oxidation reaction for the production of aromatic carboxylic acids, the precipitation of manganese oxide(s) reduces or prevents the opportunity to recycle catalyst for effective operation of the process, and this loss of catalyst is economically undesirable. In addition, the precipitation reduces or prevents flow in a tubular reactor, and the channels in the apparatus need to be cleaned or unblocked in order to continue operation of the reactor, which is uneconomic and inefficient. Whilst the mixing configuration described in WO-02/06201-A minimises catalyst oxidation compared to other configurations, it is desired to make further improvements.

Summary of The Invention

It is an object of this invention to reduce or avoid one or more of the above- mentioned problems. In particular, it is an object of this invention to reduce the amount of precipitation of metal oxides and/or reduce loss of catalyst during a supercritical (or near-supercritical) water synthetic oxidation process in which a metal salt is used as the catalyst. Thus, it is an object of this invention to improve catalyst stability in these systems, particularly to maintain catalyst activity and/or efficiency, and to avoid fouling of the reactor. It is a further object to provide an improved process, particularly a continuous process, for the production of an aromatic carboxylic acid via catalytic oxidation of a precursor in supercritical water, wherein the amount of precipitation of metal oxides is reduced and/or loss of catalyst is reduced, particularly such a process having good selectivity for, and high yield of, the aromatic carboxylic acid.

According to a first aspect of the invention, there is provided the use of an acidic component comprising one or more acid(s) for the purpose of minimising or preventing catalyst loss and/or precipitation of metal oxide in a process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt (preferably a transition metal salt), within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, typically such that said one or more precursors, oxidant and aqueous solvent constitute a single homogeneous phase in the reaction zone, wherein:

(i) said acidic component is added into the reaction zone;

(ii) contact of at least part of said catalyst with said oxidant is in the presence of said acidic component; and (iii) said acidic component comprises one or more organic acid(s).

According to a second aspect of the invention, there is provided a method of minimising or preventing catalyst loss and/or the precipitation of metal oxide in a process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt (preferably a transition metal salt), within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, typically such that said one or more precursors, oxidant and aqueous solvent constitute a single homogeneous phase in the reaction zone, wherein:

(i) said method comprises the step of adding an acidic component comprising one or more acid(s) into the reaction zone; (ii) contact of at least part of said catalyst with said oxidant is in the presence of said acidic component; and (iii) said acidic component comprises one or more organic acid(s).

According to a third aspect of the present invention, there is provided an oxidation process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt (preferably a transition metal salt), within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursors) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, typically such that said one or more precursors, oxidant and aqueous solvent constitute a single homogeneous phase in the reaction zone, wherein

(i) an acidic component comprising one or more acid(s) is added to the reaction zone;

Preferably, the addition of the acidic component comprising one or more acid(s) should be effected such that the acid(s) is/are present in any location where the metal salt is in contact with the oxidant of the oxidation process. Thus, contact of at least part, and preferably substantially all, of said metal salt with the oxidant is effected in the presence of the acidic component, and preferably subsequent to the contact of said metal salt with said acidic component (i.e. metal salt and acidic component are mixed prior to contact with the oxidant such that said the acidic component is already present at the point of first contact between catalyst and oxidant). The acidic component is present in the preferred single homogeneous phase in the reaction zone, as described herein.

Brief Description of The Drawings Figure 1 is a schematic flowsheet illustrating the basic arrangement described for Embodiment I above. Dashed lines represent alternative pathways for the acid component.

Figures 2A and 2B are schematic flowsheets illustrating the basic arrangement described for Embodiment II above. Tn Figure 2B, the oxidant is introduced in a progressive manner along the reaction zone at multiple injection points. Dashed lines represent alternative pathways for the acid component.

Figure 3 is a schematic flowsheet illustrating an arrangement (such as in

Embodiment III above) where contact of the precursor and oxidant is non- contemporaneous with contact of catalyst and oxidant.

Figure 4 is a schematic flowsheet illustrating in more detail an arrangement wherein the precursor is added to a premixed stream of oxygen and water (i.e. an arrangement according to the process illustrated in Figure 1);

Figures 5A, 5B, 5C, 5D and 6 illustrate various premixer configurations that may be employed to effect mixing of at least one of the reactants with the aqueous solvent;

Figure 7 is a schematic view illustrating multiple stage injection of oxidant;

Figures 8 and 9 are schematic flowsheets illustrating mother liquor recycle and heat removal from a reactor for use in oxidising a terephthalic acid precursor in supercritical or near supercritical water, substantially pure oxygen being used as the oxidant in the embodiment of Figure 8 and air being the oxidant in the embodiment of Figure 9.

Figure 10 is a detailed illustration of the apparatus used for the laboratory-scale experiments.

Detailed Description of The Invention The use of an acid in an oxidation process in aqueous solvent comprising water under supercritical or near-supercritical conditions according to the present invention minimises or avoids the problems associated with the oxidative side-reaction of the metal salt to metal oxide in this oxidation process. While the inventors do not wish to bound by theory, it is believed that the presence of the acid minimises or prevents the oxidation of the metal salt to metal oxide under these supercritical or near-supercritical conditions. Thus, in accordance with the invention, the precipitation of the metal oxide from the metal salt is avoided because the formation of the metal oxide and/or the destruction of said metal salt is minimised or prevented, and therefore the stability of the metal salt is improved, thereby retaining catalyst activity and efficiency, reducing reactor fouling, and improving process economy and efficiency. It is surprising that the acid is capable of achieving such a result, because the metal salt is functional in the supercritical oxidation process and reaction pathway(s) associated therewith and these pathway(s) are in competition not only with reaction pathway(s) associated with the undesirable oxidative conversion of the metal salt to metal oxide, but also with reaction pathway(s) associated with the interaction of the acid with the metal salt. Thus, it is unexpected that the presence of acid minimises the undesirable production of metal oxide and solves the problem addressed by the present invention.

When compared with the continuous process of WO-02/06201-A, the process according to the present invention provides an unexpected improvement in reducing the oxidative side-reaction of the metal salt to metal oxide in this oxidation process, and a greater proportion of the catalyst is retained in, and/or recoverable from, the system. While the arrangement described in WO-02/06201 -A itself represented an unexpected improvement in this regard over other potential arrangements, loss of catalyst still occurs. The present invention shows that the addition of acid can improve catalyst stability and/or recovery. This effect is entirely distinct from any observed increase in yield and/or selectivity of the target molecule as a result of the optional addition of oxidation promoters, such as bromides.

The inventors of the present invention have found that the addition of acid, i.e. [H⁺], has the effect of improving the recovery of catalyst by reducing the amount of precipitation of metal oxide therefrom. Increasing amounts of acid in the reaction mixture increase the recovery of catalyst until a saturation point is reached, i.e. the catalyst recovery effect reaches a plateau and the addition of more acid has no further appreciable effect on catalyst recovery. In one embodiment therefore, the amount of acid added into the reaction mixture should be sufficient to achieve at least 60%, preferably at least 70%, preferably at least 80%, preferably at least 90% of the catalyst recovery effect at saturation, and in one embodiment the amount of acid added into the reaction mixture should be sufficient to achieve saturation of the catalyst recovery effect.

Preferably, organic acid is added to the reaction zone in an amount such that the molar ratio in the reaction zone of added organic acid to M, wherein M is the metal of said metal salt present during the oxidation reaction, is at least 0.5: 1, preferably at least 0.75: 1, preferably at least 1.0: 1, preferably at least 1.3: 1, and typically no more than about 30: 1 , typically no more than about 15: 1, typically no more than 10:1, typically no more than 5.0: 1, typically no more than about 3.0: 1, and more typically no more than about 2.6: 1.

As used herein, the term "acid" means any proton-donating species that has a pKa of less than that of water at ambient temperatures and pressures. In accordance with the Bronsted theory of acidity (see, for instance, Michael B. Smith, Jerry March; March's Advanced Organic Chemistry, 5th Edition, Chapter 8), the acidity of a proton- donor is measured on the pK_a scale that runs from approximately -12 for veiy strong acids, through 15.74 for water and to approximately 50 for very weak acids at ambient temperatures and pressures. The acid(s) chosen for the acidic component will depend on the stability of the acid at supercritical conditions. Preferably, the acid is substantially oxidation-stable at supercritical conditions or near-supercritical conditions. The acidic component according to the present invention comprises one or more organic acid(s). If more than one acid is used in the acidic component, at least one such acid must be an organic acid. Reference herein to an organic acid is to an acid of formula R(COOH)_n, wherein n is 1 or more (typically n = 1 or 2, and in one embodiment, n — 1) and wherein R is an organic moiety, such as a non-aromatic hydrocarbyl group (typically C i to C₆ hydrocarbyl; typically saturated hydrocarbyl; typically straight chain hydrocarbyl; optionally substituted) or, preferably, an aromatic group (substituted or unsubstituted). Where R is an aromatic group, the aromatic group may comprise a single aromatic ring or may comprise two or more aromatic rings, for instance two or more fused aromatic rings, the or each ring typically having 5, 6, 7 or 8 ring atoms, more typically 6 ring atoms. Typically, the aromatic group is mono-cyclic. The aromatic group may be a carbocyclic aromatic group or it may comprise one or more heterocyclic aromatic rings (for instance those containing 1, 2 or 3 heteroatoms (typically only 1 heteroatom) selected from N, O and S, typically N). In one embodiment, the aromatic group is phenyl. In an alternative embodiment, the aromatic group is pyridyl. Where R is a substituted aromatic group, one or more substituent group(s) may be present, and the or each substituent group is typically selected from alkyl, alcohol, alkoxyalkyl or aldehyde groups, particularly from C ₁.₄ alkyl, C ₁.₄ alcohol, (Ci_-4 alkoxy)-Ci_₄ alkyl or C_M aldehyde groups, and particularly from alkyl (particularly from C)_-4 alkyl, and particularly methyl). Suitable substituents where R is a non-aromatic hydrocarbyl group are typically selected from alcohol, alkoxyalkyl and aldehyde groups, particularly Cj_-4 alcohol, (C_M aIItOXy)-Ci_-4 alkyl and Cj_-4 aldehyde groups. The organic acid added to the reaction mixture is normally a different acid from the target aromatic carboxylic acid of the oxidation reaction. If there is more than one organic acid added to the reaction mixture, at least one of the organic acids is normally a different acid from the target aromatic carboxylic acid of the oxidation reaction.

The target aromatic carboxylic acid of the oxidation process typically has the formula Ar(CO₂H)_x, where Ar is an aromatic group as defined hereinbelow (preferably phenyl) and x is at least 2. In one embodiment, the acidic component comprises an organic acid added to the reaction mixture which has the formula Ar(CO₂H)_y. In a further embodiment, the acidic component comprises an organic acid added to the reaction mixture which has the formula Ar(CO₂H )_y(R¹)_z, wherein the or each R¹ is a substituent group typically selected from alkyl, alcohol, alkoxy or aldehyde groups, particularly from Ci_-4 alkyl, Ci_-4 alcohol, (Ci_-4 alkoxy)-Ci_-4 alkyl or C]_-4 aldehyde groups, and particularly from alkyl (particularly from C_M alkyl, and particularly methyl). It is preferred that y<x, and preferably y < (x-1), and typically y = 1. It is preferred that z<x, and preferably z < (x-1), and typically z ^~ 1. In one embodiment, y = 1. In one embodiment, z — 1. In a preferred embodiment, Ar is phenyl. In an alternative embodiment, the oxidation process to produce an aromatic carboxylic acid of formula Ar(COaH)_x may comprise the addition of Ar(CO₂H)_y and/or Ar(CO₂H)_y(R.')_z, where there is no limitation on the value y or z, and wherein for instance carboxylic acid groups may take up any, some or all substituent positions on the aromatic ring.

In a preferred embodiment, the organic acid is benzoic acid, particularly wherein the target aromatic carboxylic acid of the oxidation process claimed herein is a di-acid of foπnula CnHU(CO₂HTh, i.e. terephthalic acid, orthophthalic acid or phthalic acid, and particularly terephthalic acid. In a further embodiment, the organic acid is toluic acid, for instance p-toluic acid, particularly wherein the target aromatic carboxylic acid of the oxidation process claimed herein is a di-acid of formula C₆H₄(CO₂H)₂, i.e. terephthalic acid, orthophthalic acid or phthalic acid, and particularly terephthalic acid. In an alternative embodiment, the acidic component comprises benzoic acid or toluic acid or a mixture thereof.

In a preferred embodiment, the organic acid added into the reactor is a byproduct of the oxidation reaction, and this embodiment is particularly important for the oxidation process to produce a target aromatic carboxylic acid of foπnula Ar(COaH)_x wherein the organic acid added to the reaction mixture has the formula Ar(CO₂H)₇ or Ar(CO₂H)_y(R')_z, as described above. In the preferred embodiment of the present invention to produce di-acid(s) of formula CgH₄(CO₂H)₂, therefore, the supply of organic acid (e.g. benzoic acid) to the reactor is achieved by recycling any otherwise undesirable by-product of the synthetic oxidation reaction back into the oxidation reactor. An advantage of putting to use this otherwise undesirable by-product in processes where the acidic component comprises a corrosive material such as HBr is that the use of a second acid means that the amount of HBr passed into the system can be reduced.

A mixture of acids may be used to provide the acidic component. In particular, a mineral acid, preferably HX (where X is preferably halide, preferably bromide or chloride, more preferably bromide) can be used in combination with the organic acid(s). In a preferred embodiment, the acidic component comprises a mixture of the organic acid(s) (particularly benzoic acid) and one or more mineral acid(s) (particularly a hydrogen halide, and particularly HBr), and typically only one mineral acid. The mineral acid may be formed in situ; thus hydrogen halide for instance may be formed in situ by the addition of a proton source and a halide source to the reaction mixture.

Thus, in a fourth aspect of the present invention, there is provided an oxidation process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt (preferably a transition metal salt), within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, typically such that said one or more precursors, oxidant and aqueous solvent constitute a single homogeneous phase in the reaction zone, wherein

(ii) contact of at least part of said catalyst with said oxidant is in the presence of said acidic component;

(iii) said acidic component comprises one or more organic acid(s); and (iv) said acidic component further comprises one or more mineral acid(s), preferably hydrogen halide, preferably HBr.

The amount of mineral acid (preferably hydrogen halide) added is preferably such that the molar ratio ([X]: [M]) of the amount of total anion X (preferably halide) to the metal ion (M) of the catalyst is at least 1.0: 1, preferably at least 1.5: 1, preferably at least 2.0: 1, preferably at least 2.05: 1, preferably at least 2.1 : 1 , preferably at least 2.2: 1 , preferably at least 2.3: 1, and typically no more than about 12.0: 1 , more typically no more than about 7.0: 1 , more typically no more than about 6.0: 1 , more typically no more than about 5.0: 1, more typically no more than 4.5: 1 , more typically no more than about 4.0: 1, more typically no more than about 3.5: 1, and more typically no more than about 3.0: 1. In a preferred embodiment, the [X]:[M] ratio is in the range of from 1.5: 1 to 5.0: 1 , preferably 2.0: 1 to 3.5: 1 , and preferably 2.2: 1 to 3.0: 1. In a preferred embodiment, the metal catalyst comprises manganese, and the mineral acid is preferably hydrogen halide (HX) and preferably HBr.

In Embodiment A, which is preferred, the metal catalyst comprises manganese bromide MnBr₂, and the amount of hydrogen halide added is preferably such that the molar ratio [X]: [Mn] of the amount of total halide (X) to manganese (Mn) is greater than 2.0:1 and typically no more than about 12.0: 1. Preferably, [X]: [Mn] is at least 2.05:1, preferably at least 2.1 : 1, preferably at least 2.2: 1, preferably at least 2.3: 1, and typically no more than about 12.0: 1 , more typically no more than about 7.0: 1 , more typically no more than about 6.0: 1 , more typically no more than about 5.0: 1, more typically no more than 4.5: 1, more typically no more than about 4.0: 1, more typically no more than about 3.5: 1, and more typically no more than about 3.0: 1. The present inventors have found that the oxidation promoter effect is already saturated at a BnMn molar ratio of 2: 1 (i.e. the ratio which pertains using MnBr₂ as the catalyst in the absence of HBr) and that the catalyst recovery effect is not observed until acid is also added.

In an alternative embodiment, Embodiment B, the metal catalyst comprises manganese acetate Mn(OAc)₂ which, in conventional systems (i.e. non-SCW systems), is used in combination with HBr to form the catalyst MnBr₂ in situ. In this embodiment, the generic and preferred molar ratios [X]: [Mn] of the amount of total halide (X) to manganese (Mn) are as described above for the general metal M, i.e. at least 1.0: 1 and typically no more than about 12.0:1. The present inventors have found that the oxidation promoter effect becomes saturated at a Br:Mn molar ratio around 1.5: 1, and that the catalyst recovery effect docs not become saturated until a BnMn molar ratio of greater than 2: 1 is reached.

For the avoidance of doubt, reference herein to the term "transition metal" is to the conventional definition of a metal which can accept or donate electrons into its d- or f- orbitals and exhibit a plurality of oxidation states, and includes the lanthanide and actinide series of transition metals. The pressure and temperature of the process are selected to secure supercritical or near-supercritical conditions. In one embodiment, the term "near supercritical conditions" means that the solvent is at a temperature which is not less than 100⁰C below, preferably not less than 5O⁰C below, preferably not less than 35°C below, more preferably not less than 2O⁰C below the critical temperature of water at 220.9 bara. Thus, operating temperatures are typically in the range of 300 to 48O⁰C, more preferably 330 to 450⁰C, typically from a lower limit of about 350 to 37O⁰C to an upper limit of about 370 to about 42O⁰C. Operating pressures are typically in the range from about 40 to 350 bara, preferably 60 to 300 bara, more preferably 220 to 280 bara, and in one embodiment 250 to 270 bara. In an alternative embodiment, the operating pressures are in the range of 230 to 250 bara.

By the term "synthetic oxidation reaction" or "synthetic oxidation process" we mean the production of one or more target compound(s) from one or more oxidisable precursor(s) thereof by partial oxidation of said precursor(s). By the term "partial oxidation" we mean an oxidation reaction which consists of a degree of oxidation (or uptake of oxygen) less than that required for total oxidation of said precursor(s) to carbon oxides; such reactions are associated with controlled oxidant/precursor stoichiometry, selective reaction for the synthesis of a small number of compounds in high yield, and retention of chemical structure in the aromatic group of the precursor. By the term "total oxidation" we mean oxidation of a compound to carbon oxides (typically carbon dioxide), i.e. destructive oxidation.

In a preferred embodiment, the term "near supercritical conditions" means that the reactants and the solvent constitute a single homogeneous phase. In practice, this can be achieved under conditions below the critical temperature for water. By the term "single homogeneous phase" as used herein, we mean that at least 80%, typically at least 90%, typically at least 95%, more typically at least 98%, and most typically effectively all, by weight, of each of the precursor, oxidant, aqueous solvent, acidic component, catalyst and reaction product(s), and organic acid precursor P_OΛ if present (as described hereinbelow), are in the same single homogeneous phase in the reaction zone. By the term "aromatic carboxylic acid" as used herein we mean an aromatic compound in which a carboxylic acid group (-CO₂H) is attached directly to an aromatic group (Ar). The aromatic carboxylic acid may contain one or more carboxylic acid groups attached directly to an aromatic group, and the present invention is particularly directed to aromatic carboxylic acids which contain at least 2, and particularly only 2, carboxylic acid groups (CO₂H) attached directly to an aromatic group. The aromatic group (Ar) may comprise a single aromatic ring or may comprise two or more aromatic rings, for instance two or more fused aromatic rings, the or each ring typically having 5, 6, 7 or 8 ring atoms, more typically 6 ring atoms. Typically, the aromatic group is mono-cyclic. The aromatic group may be a carbocyclic aromatic group or it may comprise one or more heterocyclic aromatic rings (for instance those containing 1 , 2 or 3 heteroatoms (typically only 1 heteroatom) selected from N, O and S, typically N). In one embodiment, the aromatic group is phenyl. In an alternative embodiment, the aromatic group is pyridyl. Typical aromatic carboxylic acids which may be synihesised using the present invention include terephthalic acid, isophthalic acid, phthalic acid, trimellitic acid, naphthalene dicarboxylic acid and nicotinic acid.

By the term "precursor of an aromatic carboxylic acid" as used herein we mean an aromatic compound which is oxidisable to the target aromatic carboxylic acid with an oxidant under supercritical conditions or near-supercritical conditions. The precursor is selected from aromatic compounds having at least one substituent which is attached to the aromatic group (Ar; as defined hereinabove) and which is oxidisable to a carboxylic acid moiety. Suitable substituents are typically selected from alkyl, alcohol, alkoxyalkyl and aldehyde groups, particularly from alkyl, alcohol and alkoxyalkyl groups, and preferably from alkyl groups. An alkyl group is particularly selected from Ci-₄ alkyl groups, preferably methyl. An alcohol group is particularly selected from Ci_-4 alcohol groups. An alkoxyalkyl group is particularly selected from (C ₁.₄ alkoxy)Ci ₄ alkyl groups. An aldehyde group is particularly selected from Ci.₄ aldehyde groups. Where two or more substituent groups are present, these may be the same or different, and in a preferred embodiment are the same. For instance, a precursor of terephthalic acid may be selected from para-xylene, 4-tolualdehyde and 4-toluic acid, para-xylene being preferred. A precursor of nicotinic acid is, for instance, 3-methylpyridine. Unless otherwise stated, reference herein to the term "precursor" is a reference to the precursor of said aromatic carboxylic acid which is the target of the synthetic oxidation reaction claimed herein.

We do not rule out the possibility of adding at least part of the organic acid(s) of the acidic component to the reaction zone via in situ generation of at least part of the organic acid(s), and typically this is effected by the introduction of an oxidisable or hydrolysable precursor of the organic acid (referred to herein as precursor P_OA to avoid confusion with the precursor of said aromatic carboxylic acid which is the target of the synthetic oxidation reaction claimed herein) which is either oxidised in the reaction zone, or hydro lysed in (or prior to) the reaction zone, to the desired organic acid. Thus, as used herein the term "acidic component added into the reaction mixture" encompasses not only the addition of the acidic component itself into the reaction mixture, but the in situ generation of the acidic component within the reaction zone.

Thus, in one embodiment at least part of at least one of said organic acid(s) added into the reaction zone is generated in situ by addition of an oxidisable precursor P_OA of said organic acid, wherein said oxidisable precursor P_OA is different from the precursor(s) of the aromatic carboxylic acid(s) which is/are the target of the synthetic oxidation reaction claimed herein, and wherein said organic acid generated in situ is different from said target aromatic carboxylic acid. Typically, the acidic component comprises only one organic acid and in this embodiment at least part of said organic acid is generated in situ. By the term "oxidisable precursor PO_Λ of the organic acid" as used herein we mean a compound which is oxidisable to the organic acid R(COOH)_n with an oxidant under supercritical conditions or near-supercritical conditions. The oxidisable precursor PO_Λ is selected from compounds R-(Q)_n in which the substituent Q is oxidisable to a carboxylic acid moiety. Suitable substituents where R is an aromatic group are typically selected from alkyl, alcohol, alkoxy and aldehyde groups, particularly C_M alkyl, Ci ₄ alcohol, (C ₁.₄ alkoxy)-Ci_₄ alkyl and Ci_-4 aldehyde groups, and preferably alkyl groups (preferably Ci_-4 alkyl groups, preferably methyl). Suitable substituents where R is a non-aromatic hydrocarbyl group are typically selected from alcohol, alkoxyalkyl and aldehyde groups, particularly Ci_-4 alcohol, (Ci.₄ alkoxy)-Ci_₄ alkyl and C_M aldehyde groups. Where two or more substituent groups are present, these may be the same or different. In this embodiment, where the organic acid generated in situ is benzoic acid, for instance, the oxidisable precursor P_OA may be selected from toluene and benzaldehyde, toluene being preferred.

In an alternative embodiment, an organic acid is generated in situ by addition of a hydrolysable precursor P_OA- In that case, the hydrolysable precursor is typically selected from compounds RCOOR¹, wherein R is as defined herein and R is selected from alkyl groups (particularly Ci_-6 alkyl, and including benzyl) and aromatic groups (particularly phenyl), optionally substituted as defined herein. Thus, a hydrolysable precursor may be selected from alkylbenzoates.

The precursor PO_A may be added to the reaction zone in a manner similar to that described hereinbelow for the acidic component. In one embodiment, where the acidic component comprises organic acid generated in situ in combination with one or more further acid(s) (for instance a further organic acid and/or a mineral acid), the organic acid precursor (P_OA) and the or each further acid may be introduced into the reaction zone by the same or different pathways, as described hereinbelow. In a further embodiment of the process in which organic acid is generated in situ, the precursor P_OA may be added to the reaction zone in a manner similar to that described hereinbelow for the precursor(s) of the aromatic carboxylic acid(s) which is/are the target of the synthetic oxidation reaction claimed herein. In one embodiment, said precursor and said precursor P_OA are pre-mixed and added into the reaction zone in a single stream. In one embodiment, said precursor PO_A is pre-mixed with the catalyst. It will be appreciated that the means of addition of the precursor P_OA depends primarily on its miscibility with the other reactants. For instance, where the precursor P_OA is toluene then such a precursor P_OA is typically combined with the precursor of the target carboxylic acid (e.g. p-xylene in the case of the production of terephthalic acid). Where the precursor is miscible with water, it is typically added to any of the aqueous streams fed into the reaction zone, e.g. in a manner similar to that described below for the acidic component.

A reactor suitable for the performance of the present invention is preferably a continuous flow reactor. By "continuous flow reactor" as used herein we mean a reactor in which reactants are introduced and mixed and products withdrawn simultaneously in a continuous manner, as opposed to a batch-type reactor. For example, the reactor may be a tubular flow reactor (with either turbulent or laminar flow) or a continuous stirred- tank reactor (CSTR) although the various aspects of the invention defined herein are not limited to these particular types of continuous flow reactor. The invention defined herein is described primarily in terms of a continuous flow reactor because this is the most likely commercial and industrial embodiment, but the invention may also be conducted using a batch-type reactor.

By carrying out the process in a continuous flow reactor, the residence time for the reaction can be made compatible with the attainment of conversion of the precursor(s) to the desired aromatic carboxylic acid without significant production of degradation products. The residence time of the reaction medium within the reaction zone is generally no more than 10 minutes, preferably no more than 8 minutes, preferably no more than 6 minutes, preferably no more than 5 minutes, preferably no more than 3 minutes, preferably no more than 2 minutes, and preferably no more than 1 minute.

The residence time may be controlled so that the precursor is converted to the aromatic carboxylic acid with high efficiency such that the aromatic carboxylic acid precipitated from the reaction medium following completion of the reaction contains no more than about 5000 ppm, preferably no more than about 3000 ppm, more preferably no more than about 1500 ppm, more preferably no more than about 1000 ppm and most preferably no more than about 500 ppm aldehyde produced as an intermediate in the course of the reaction (e.g. 4-CBA in the case of terephthalic acid production). Typically, there will be at least some aldehyde present after the reaction, and usually at least 5ppm.

The oxidant in the process of the invention is preferably molecular oxygen, e.g. air or oxygen enriched air, but preferably comprises gas containing oxygen as the major constituent thereof, more preferably pure oxygen, or oxygen dissolved in liquid. The use of air is not favoured, although not excluded from the scope of the invention, since large compression costs would arise and off-gas handling equipment would need to cope with large amounts of off-gas owing to the high nitrogen content of air. Pure oxygen or oxygen enriched gas on the other hand permits use of a smaller compressor and smaller off-gas treatment equipment. The use of dioxygen as the oxidant in the process of the invention is particularly advantageous since it is highly soluble in water under supercritical or near supercritical conditions. Thus, at a certain point, the oxygen/water system will become a single homogeneous phase.

Instead of molecular oxygen, the oxidant may comprise atomic oxygen derived from a compound, e.g. a liquid phase compound at room temperature, containing one or more oxygen atoms per molecule. One such compound for example is hydrogen peroxide, which acts as a source of oxygen by reaction or decomposition.

The oxidation reaction, particularly for the production of aromatic carboxylic acids, according to the present invention is carried out in the presence of a homogeneous oxidation catalyst soluble in the reaction medium comprising solvent and precursor(s). The catalyst is typically present in the single homogeneous phase in the reaction zone, as described herein. The catalyst typically comprises one or more heavy metal compounds, e.g. cobalt and/or manganese compounds, and preferably comprises manganese compounds. For instance, the catalyst may take any of the forms that have been used in the liquid phase oxidation of aromatic carboxylic acid precursors such as terephthalic acid precursor(s) in aliphatic carboxylic acid solvent, e.g. bromides, bromoalkanoates, alkanoates (usually Ci - C₄ alkanoates such as acetates) or benzoates (or other aromatic acid salts) of cobalt and/or manganese. Compounds of other heavy metals such as vanadium, chromium, iron, zirconium, hafnium molybdenum, a lanthanide such as cerium, and/or nickel may be used instead of cobalt and/or manganese. Advantageously, the catalyst system will include manganese bromide (MnBr₂). The catalyst can be added pre-prepared or it can be formed within the system by adding reagents which subsequently combine to form the catalyst. For instance, in the case of an MnBr₂ catalyst, it is possible either to introduce MnBr₂ itself into the system, or to introduce reagents such as manganese acetate and HBr into the system, which combine to form MnBr₂ under the reaction conditions.

The reactor system suitable for performing the process of the present invention may be generally configured as described below. The oxidation reaction is initiated by heating and pressurising the reactants followed by bringing the heated and pressurised reactants together in a reaction zone. This may be effected in a number of ways with one or both of the reactants being admixed with the aqueous solvent prior to or after attainment of supercritical or near supercritical conditions, such admixture being effected in such a way as to maintain the reactants isolated from one another until brought together in the reaction zone.

In the continuous process described herein, contact of at least part of said catalyst with said oxidant is effected in the presence of said acidic component. Preferably, contact of substantially all of said catalyst with said oxidant is in the presence of the acidic component.

In the continuous process for the production of carboxylic acids described herein, the reactor system is preferably configured such that the contact between the oxidant and at least part, and preferably substantially all, of the precursor is made at the same point in the reactor system as, and contemporaneous with, the contact between the catalyst and at least part, and preferably substantially all, of the oxidant, and such arrangements are shown in Figures 1 , 2A and 2B. However, other configurations for the production of carboxylic acids, in which the contact of at least part of said precursor with said oxidant is not contemporaneous with contact of said catalyst with at least part of said oxidant, are not excluded, and such arrangements are shown in Figure 3.

In Embodiment T, the oxidant is mixed with the aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state, with suitable pressurisation and, if desired, heating, of the oxidant prior to mixing with the aqueous solvent. The precursor is subjected to pressurisation and, if desired, heating. The catalyst-comprising component is subjected to pressurisation and, if desired, heating. The acid component may be subjected to pressurisation and, if desired, heating. The separate streams comprising precursor, catalyst, acid and the oxidant/solvent mixture may then be contacted simultaneously. In one arrangement, the acid is pre-mixed with the catalyst. A schematic flow diagram representing Embodiment I is presented in Figure 1. In Embodiment II of the invention, the precursor is mixed with the aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state, with suitable pressurisation and, if desired, heating, of the precursor prior to mixing with the aqueous solvent. In one arrangement, a homogenous catalyst component after pressurisation and, if desired, heating, is contacted with the aqueous solvent simultaneously with the contacting of the precursor with the aqueous solvent. The acid component after pressurisation and, if desired heating, may be contacted simultaneously with the contact of catalyst, precursor and aqueous solvent or may be pre-mixed with the catalyst. Alternatively, the acid component after pressurisation and, if desired heating may be fed directly into the reaction vessel (see Figures 2A and 2B). The oxidant after pressurisation and, if desired, heating, is mixed with aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state, and the oxidant/aqueous solvent mixture is then contacted with the mixture comprising the precursor, catalyst, acid component and aqueous solvent.

In Embodiment III, the oxidant is mixed with aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state, with suitable pressurisation and, if desired, heating, of the oxidant prior to mixing with the aqueous solvent. The catalyst may be mixed with oxidant with contemporaneous mixing of the acid component. Alternatively, the acid component may be pre-mixed with the catalyst prior to contact with the oxidant. The catalyst and/or acid component is subjected to pressurisation and, if desired, heating. The precursor is subjected to pressurisation and, if desired, heating, and then contacted with the mixture comprising the oxidant, catalyst and the acid component in the reaction zone. A schematic flow diagram representing Embodiment III is presented in Figure 3.

Where the acidic component comprises organic acid in combination with a mineral acid (for instance hydrogen halide), the organic acid and the mineral acid may be introduced into the reaction zone by the same or different pathways. Referring to Figures 1 to 3, the organic and mineral acids may independently be introduced via any of the pathways shown for the acid. Contact of the various streams may be effected by way of separate feeds to a device in which the feeds are united to form the preferred single homogeneous fluid phase thus allowing the oxidant and precursor to react. The device in which the feeds are united may for instance have a Y, T, X or other configuration allowing separate feeds to be united in a single flow passage forming the continuous flow reactor, or in some circumstances multiple flow passages forming two or more continuous flow reactors. The flow passage or passages in which the feeds are united may comprise a section of tubular configuration with or without internal dynamic or static mixing elements.

In a preferred embodiment, in-line or static mixers are advantageously used to ensure rapid mixing and homogeneity, for example to promote dissolution of oxidant into the aqueous solvent and the formation of a single phase.

The oxidant feed and the precursor feed may be brought together at a single location or the contact may be effected in two or more stages so that at least part of one feed or of both feeds are introduced in a progressive manner, e.g. via multiple injection points, relative to the direction of flow through the reactor. For instance, one feed may be passed along a continuous flow passage into which the other feed is introduced at multiple points spaced apart lengthwise of the continuous flow passage so that the reaction is carried out progressively. The feed passed along the continuous flow passage may include the aqueous solvent as may the feed introduced at multiple positions.

Similarly, the addition of catalyst may be effected in a progressive manner, e.g. via multiple injection points, relative to the direction of flow through the reactor.

In one arrangement, the oxidant is introduced to the reaction at two or more locations. Such locations are conveniently so positioned relative to the bulk flow of solvent and reactants through the oxidation zone that oxidant is introduced to the reaction at an initial location and at least one further location downstream of said initial location. There may be more than one reaction zone in series or in parallel. For instance, where multiple reaction zones in parallel are used, the reactants and solvent may form separate flow streams for passage through the reaction zones and, if desired, the product streams from such multiple reaction zones may be united to form a single product stream. Where more than one reaction zone is used, the conditions, such as temperature, may be the same or different in each reactor. The or each reactor may be operated adiabatically or isothermally. Isothermal or a controlled temperature rise may be maintained by heat exchange to define a predetermined temperature profile as the reaction proceeds through the reactor.

The heat of reaction may be removed from the reaction by heat exchange with a heat-accepting fluid, according to conventional techniques known to those skilled in the art, and described for instance in WO-02/06201-A the disclosure of which techniques is incorporated herein by reference. Conveniently the heat-accepting fluid comprises water.

After traversing the continuous flow reactor and upon completion of the oxidation process, the reaction mixture comprises a solution of aromatic carboxylic acid, which needs to be recovered from the reaction medium. Substantially the entire amount of aromatic carboxylic acid produced in the reaction is in solution at this stage. In the process of the invention, typically at least 80% wt, more usually at least 90% wt, preferably at least 95% wt, more preferably at least 98% wt and most preferably substantially all of the aromatic carboxylic acid produced in the reaction is maintained in solution during the reaction and does not begin to precipitate until the solution leaves the oxidation reaction zone and undergoes cooling. The solution may also contain catalyst, and relatively small quantities of by-products such as intermediates (e.g. p- toluic acid and 4-CBA in the case of terephthalic acid), decarboxylation products such as benzoic acid and degradation products such as trimellitic acid and any excess reactants. The desired product, aromatic carboxylic acid such as terephthalic acid, may be recovered by causing or allowing the aromatic carboxylic acid to crystallise from the solution in one or more stages followed by a solids-liquid separation in one or more stages. The product stream is subjected to a solids-liquid separation to recover the aromatic carboxylic acid and the mother liquor (which may but need not necessarily contain dissolved catalyst components) is recycled to the oxidation reaction zone. Preferably prior to re-introduction into the oxidation reaction zone, the mother liquor is heated by heat exchange with the product stream thereby cooling the latter.

One or both reactants may be admixed with the mother liquor recycle stream or separate mother liquor recycle streams prior to re-introduction of the mother liquor into the reaction zone and the mother liquor recycle stream (or at least that fraction or those fractions thereof to be combined with the reactant or reactants) may be heated and pressurised to secure supercritical/near supercritical conditions before being admixed with the reactant or respective reactant.

Where the mother liquor is heated by heat exchange with the product stream before re-introduction into the oxidation zone, the reactant or reactants may be admixed with the mother liquor stream or a respective mother liquor stream prior to or after such heat exchange with the product stream.

If the provision of organic acid as at least part of the acidic component added into the reactor is achieved by recycle of a by-product (such as benzoic acid) of the oxidation reaction, then in order to achieve the desired levels of the organic acid in the reactor, the mother liquor is recycled back to the reactor by pressurising the stream to near-supercritical or supercritical pressure, heating the stream to near-supercritical or supercritical temperature and mixing the stream with the other catalyst and/or organic acid streams, the precursor and the oxidant. The increase in the temperature of the recycled mother liquor stream is preferably effected by heat interchange with the reactor product, thereby cooling the product stream from the reactor. The pressurised and heated mother liquor stream, containing catalyst and organic acid, may be fed directly into the mixer or reaction zone, as illustrated in Figures 1 to 4. The composition of the recycled mother liquor stream in this system is essentially determined by two factors: firstly by the rate of formation of by-product(s) (such as benzoic acid, in the case of the oxidation reaction of p-xylene to terephthalic acid), which itself is determined by the reactor conditions (including pressure, temperature, catalyst concentration etc.); and secondly by the rate of removal of, inter alia, byproducts) from the mother liquor recycle stream, i.e. the extent of the purge taken from this stream. The purge removes organic acid (such as benzoic acid) generated by the oxidation of the precursor(s), as well as other undesirable reaction by-products and catalyst, from the reactor feed. Sampling and analysis of the recycle stream enables the level of additional catalyst and/or organic acid required for recycling to the reaction zone to be estimated, and then fed to the reaction zone, in order to control the reaction. At the start of the reaction, for instance in the oxidation of p-xylene to terephthalic acid, a sufficiently high steady-state concentration of benzoic acid (i.e. the organic acid) may not be present in the mother liquor recycle stream, and so it may be necessary to add benzoic acid until the desired steady-state concentration is reached. Equally, it may also be necessary to periodically add fresh organic acid (such as benzoic acid) into the reaction zone, in addition to the organic acid (such as benzoic acid) generated as a byproduct of the oxidation reaction and recycled to the reaction zone via the recycled mother liquor, in order to achieve the desired concentration of organic acid in the reaction zone. Thus, in the aspect of the invention in which the addition of organic acid to the reaction zone is provided by recycling by-product(s) from the oxidation reaction, the method comprises adjusting the extent of purge taken from the recycled mother liquor stream in order to control the amount and concentration of organic acid (such as benzoic acid) fed into the reactor, and thus in the reaction zone, effective to achieve the catalyst recoveiy effect described herein.

The invention will now be described further by way of example only with reference to the accompanying drawings.

Referring to Figure 1, dioxygen, after pressurisation, is mixed with water after the water has been heated and the mixture pressurised and optionally further heated in preheater 1 to achieve the supercritical state. The precursor and catalyst are added, after pressurisation, to the CVwater stream at the beginning of or immediately before the reactor 2 and the mixture passed through the reactor. The acid component is added, after pressurisation, to the O₂/water stream contemporaneously with addition of the catalyst, or alternatively is added to the catalyst stream prior to contact with the oxidant stream. Upon exiting the reactor, the stream is cooled and depressurised at the backpressure regulator 3. The products are carried out in a stream of cooled water.

Referring to Figures 2A and 2B, the precursor and catalyst after pressurisation are added to water after the water has been pressurised and optionally heated. The acid component may be added to the catalyst before, contemporaneously or after mixing with the water. The mixture is optionally further heated in preheater 1 A to achieve the supercritical state. Alternatively, the acid component after pressurisation and optionally heating may be mixed with the catalyst/precursor stream at the point when the latter stream is mixed with the oxidant. The dioxygen gas, after pressurisation is mixed with water at a supercritical state and optionally further heated in preheater 1. In Figure 2A, the streams are mixed at the beginning of or immediately before the reactor 2 and the mixture passed through the reactor. In Figure 2B, the O₂/water stream is added to the reactor in a progressive manner at multiple injection points. Upon exiting the reactor, the stream is cooled and depressurised at the back pressure regulator 3. The products are carried out in a stream of cooled water.

Figure 3 corresponds to Figure 1 wherein the catalyst and oxidant are mixed prior to contact of either stream with the precursor. The dioxygen gas after pressurisation is mixed with water at a supercritical state and optionally further heated in preheater 1. The acid component is mixed with the catalyst prior to or contemporaneously with contact of the catalyst with the oxidant.

Referring to Figure 4, feedstock components comprising water, precursor (e.g. paraxylene in the process for the production of terephthalic acid) and dioxygen gas are pressurised to operating pressure and continuously supplied from respective sources 10, 12 and 14 through a preheater 16 where the components are heated to a temperature of 300 to 48O⁰C, more preferably 330 to 450⁰C, typically from about a lower limit of about 350 to 37O⁰C to an upper limit of about 370 to about 42O⁰C, the pressure and temperature being selected in order to secure supercritical or near supercritical conditions. Part of the heat used to preheat the feedstock components may be derived from the exotherm produced in the course of the subsequent reaction between the precursor and the oxidant. Heat from other sources may be, for example, in the form of high pressure steam and/or heating may be effected by direct fired heating of the water stream. The heat of reaction may be recovered in any suitable manner, e.g. by means of heat exchange between the fluid following reaction and a suitable heat-accepting fluid such as water. For instance, the heat-accepting fluid may be arranged to flow in heat exchange relation, counter-currently and/or co-cuiτently, with the reactants and solvent passing through the reaction zone. The passage or passages along which the heat- accepting fluid flows in traversing the reaction zone may be external to the reaction zone and/or may extend internally through the reaction zone. Such internally extending flow passage(s) may for instance extend generally parallel with and/or transversely of the general direction of flow of the reactant/solvent through the reaction zone. For example, the heat-accepting fluid may traverse the reaction zone by passage through one or more coiled tubes located within the interior of the reactor. The enthalpy of reaction can be used to recover power via a suitable power recovery system such as a turbine; for instance the heat-accepting fluid, e.g. water, can be used to raise high pressure saturated steam at for example temperature and pressure of the order of

300⁰C/ 100 bara which, in turn, can be superheated by external heat and fed to a high efficiency condensing steam turbine to recover power. In this way, the reactor can be maintained at an optimum temperature and effective energy efficiency can be achieved. In an alternative approach, the reactor may be operated under adiabatic conditions and a suitably high rate of water flow through the reaction zone may be employed in order to constrain the temperature rise across the reactor in operation. If desired, a combination of both approaches may be used, i.e. recovery of the enthalpy of reaction via a heat- accepting fluid coupled with a suitable water flow rate through the reaction zone.

Following heating of the feedstock components, oxygen is mixed with water which, as a result of preheating and pressurisation, will be under supercritical or near supercritical conditions and hence capable of solubilising the feedstocks. In the embodiment illustrated in Figure 4, oxygen and water are mixed in premixer 18A. The precursor is also mixed with water in premixer 18B. Of course, the precursor could also be separately premixed with water prior to entry into the preheater 16.

The premixer (or premixers where premixing of each reactant and water is undertaken) may take various forms such as Y, L or T piece, double T configurations or a static mixer, as illustrated in Figures 5A, 5B, 5C, 5D and 6 respectively. In Figures 5A to 5D and 6, reference A depicts the preheated water supply to the premixer, B depicts the reactant (precursor or oxygen) and P depicts the resulting mixed stream. In the double T configuration of Figure 5D, two mixed streams are produced Pl and P2. These may either be passed through separate continuous flow reactors or be combined into a single stream and then passed through a single continuous flow reactor. An X piece configuration may also be used, as known to those skilled in the art. It will also be appreciated that any suitable mixing apparatus may be used in the present invention. It will further be appreciated that the mixing apparatus referred to above are for use in a continuous process apparatus. In a batch system, there is of course no continuous flow and therefore no specific flow-related mixing requirements. In a continuous vessel reactor, reactants can also be fed into the vessel independently.

It will be appreciated that instead of premixing one or each reactant with water prior to introduction into the reaction zone, the reactants and water may be introduced separately into the reaction zone and mixed within the reaction zone with the aid of some form of mixing arrangement (e.g. a static mixer) whereby substantially all mixing of the components occurs within the reaction zone.

The homogeneous catalyst is added as a solution from source 19 to the premixed oxygen/water stream at the same time as the precursor is added to the premixed oxygen/water stream either immediately prior to entering the reactor or at the beginning of the reactor (i.e. as shown in Figure 1). In a preferred embodiment, the acid is included in the catalyst source 19, as shown in Figure 4.

Following preheating and premixing, the feedstock components are combined in a reaction zone 20 to form a single homogeneous fluid phase in which the reactants are brought together. The reaction zone 20 may consist of a simple mixer arrangement in the form of a tubular flow reactor, e.g. a pipe of a length which, in conjunction with the flow rate of the combined reactants, provides a suitable reaction time so as to secure conversion, for example, of paraxylene to tercphthalic acid with high conversion efficiency and low 4-CBA content. The reactants may be combined in a progressive manner by injecting one reactant into a stream containing the other reactant at multiple points along the length of the reactor. One way of implementing a multiple injection arrangement is shown in the continuous flow reactor of Figure 7 in which the reactor is constituted by a pipe or vessel P. In an embodiment wherein a premixed oxygen/water stream is added to a premixed precursor/water stream, a premixed precursor/supercritical or near supercritical water stream W is supplied to the upstream end of pipe or vessel P. Water stream W would also contain the homogeneous catalyst plus acid. The stream passes through the reactor pipe or vessel P and at a series of locations spaced at intervals along the length of the pipe or vessel P, preheated and compressed oxygen dissolved in supercritical or near supercritical water is supplied via multiple injection passages A to E to produce a product stream S comprising carboxylic acid product in supercritical or near supercritical aqueous solution. In this manner, the oxygen necessaiy to effect complete oxidation of, for example, para-xylene to terephthalic acid is injected progressively with the aim of controlling oxidation and minimising side reactions and possible burning of paraxylene, terephthalic acid or terephthalic acid intermediates.

Referring now back to Figure 4, following the reaction to the desired degree, the supercritical or near supercritical fluid is passed through a heat exchanger 22 through which heat exchange fluid is circulated via closed loop 24 so that heat can be recovered for use in the preheater 16. One scheme (not shown) for post-reaction cooling of the carboxylic acid product solution involves the use of heat exchanger networks to cool the stream to subcritical temperatures, e.g. of the order of 300⁰C to retain the carboxylic acid product in solution and thereby reducing fouling of heat exchange surfaces, followed by use of a train of flashing crystallisers (similar to those employed in conventional terephthalic acid purification by hydrogenation) to cool and precipitate the carboxylic acid product.

The cooled solution is then supplied to a product recovery section 26 in which the carboxylic acid product is precipitated from the solution. Any suitable method of product recovery known to those skilled in the art may be used. The product recoveiy section 26 may comprise one or more stages of cooling or evaporative crystallisation to crystallise the carboxylic acid product to form a slurry of crystals in aqueous mother liquor. Where the product recovery section 26 comprises one or more flashing crystallisers, the resulting flash streams from the crystallisers may be used to preheat the inlet water and precursor streams to the reactor, either indirectly via conventional heat exchangers or via direct injection of the flash into the water and/or precursor feeds to the reactor. The slurry obtained following crystallisation may be subjected to a solids-liquid separation process using for example filtration devices operating under super-atmospheric, atmospheric or sub-atmospheric conditions, with or without washing facilities, such as described in prior published International Patent Applications Nos. WO-A-93/24440 and WO-A-94/ 17982 (the disclosures of which are incorporated herein by this reference). Thus, for example the integrated solids separation and water washing apparatus may comprise a belt filter unit, or a rotaiy cylindrical filter unit operated with the slurry side, or a drum filter unit (e.g. a BHS-Fest pressure filter drum formed with a plurality of slurry receiving cells in which the mother liquor is displaced from filter cake by water under hydraulic pressure supplied to the cells). After filtering the slurry, the recovered carboxylic acid product may be used directly for the production of polyester, for instance, for packaging, such as bottles, or fibres. Similarly it can be dried. If not already at atmospheric pressure, the filter cake of carboxylic acid product may be transferred to a low pressure zone (e.g. atmospheric pressure) for drying via a suitable pressure let-down device such as a lock hopper arrangement, a rotaiy valve, a ram-type pump, a screw feed device or a progressive feed device such as a progressive cavity pump of the type used to pump cold pastes of high solids contents.

The temperature of separation and the level of washing required will be dependent on the levels of impurities generated in the reaction, the means of recovering the product and the required product specification. Although in general, it will be desirable to produce carboxylic acid, such as terephthalic acid, which is sufficiently pure to render further purification unnecessary (e.g. by oxidation and/or hydrogenation of an aqueous solution of the terephthalic acid to convert 4-CBA to terephthalic acid or to para-toluic acid, as the case may be), we do not exclude the possibility of carrying out such purification subsequent to the supercritical or near supercritical water oxidation reaction. Following recovery of the aromatic carboxylic acid product, at least part of the aqueous mother liquor (including soluble catalyst components) may be recycled for reuse in the oxidation reaction, e.g. by admixture with fresh water and/or the reactants. However, if the recycled mother liquor contains catalyst components, it is preferably not added to the O₂/water stream prior to addition of precursor. The amount recycled will usually be a major fraction of the recovered mother liquor, with a purge being taken in order to reduce standing concentrations of by-products in the process. The purge stream may be treated to recover its catalyst content where applicable and its organic content.

Referring now to Figure 8, in this embodiment liquid oxygen (line 30), liquid precursor (e.g. paraxylene in the process for the production of terephthalic acid) (line 32) and water (line 34) are supplied to a mixing unit 36. The oxygen and precursor supplies are pressurised by pumps 38, 38A and heated to elevated temperature, for example by high pressure steam, in heat exchangers 40, 4OA. The mixing unit 36 is configured, as shown in figure 4, to mix the reactants with the water supply to produce two streams 42, 44, one stream comprising a water/precursor mixture and the other stream comprising oxygen dissolved in water, which are fed to a continuous flow reactor 46 in the form of a pipe in which the streams are mixed, e.g. by an unshown static mixing arrangement within the pipe, to initiate the reaction. The homogeneous catalyst and acidic component as a solution in water may be added either into the precursor/water stream 42 immediately prior to entering the reactor, or on combination of streams 42 and 44 at the beginning of or immediately before the reactor, using rapid mixing, for example by the use of a static mixer or similar device.

The supply of fresh make-up water to the system may be effected at various points. One of the most convenient points is upstream of the main pressurisation pump 68, for instance via line 1 16 which is described in more detail below in relation to Figure 9. Water may also be fed after pressurisation in pump 38C and heating in heat exchanger 4OC via line 35A into line 74, or prior to the exchangers (50,70).

Alternatively, water may be fed, after pressurisation in pump 38B and heating in heat exchanger 4OB independently into the preheater 36 via line 35. Following reaction under supercritical or near supercritical conditions, the product stream 48 in the form of a solution of reaction product(s) (plus small amounts of unreacted reactants, intermediates etc) is cooled by passage through heat exchangers 50 and 52 and may be optionally flashed down to a lower pressure and temperature in flash vessel 54. The means of effecting such a step at this point or in the product recovery section 62 may involve known devices, singly or in multiples, but should be configured to avoid deposition of solids, by means such as localised heating, as known to those skilled in the art. Thus, as the stream from reactor 46 is passed through heat exchangers 50 and 52, the temperature of the stream is monitored and controlled so that the product does not precipitate; precipitation should not occur until flash vessel 54. A substantial amount of steam and some gaseous components such as nitrogen, oxygen, carbon oxides are supplied via line 56 to an energy recoveiy system 58 while the carboxylic acid product solution is supplied via line 60 to a product recovery section 62.

In the product recoveiy section, the solution of carboxylic acid product is processed through a multi-stage crystallisation train in which pressure and temperature are progressively lowered to crystallise the carboxylic acid product in crystal form. The product of the crystallisation process is a slurry of carboxylic acid product crystals in an aqueous mother liquor. After the final crystallisation stage, the slurry may be at any desired pressure, e.g. atmospheric pressure or above. The slurry is then subjected to a solids-liquid separation of any suitable form to separate the crystals from the mother liquor. The solids-liquid separation may be carried out using any device suitable for this puipose and arranged to operate under elevated pressure conditions or at atmospheric pressure depending on the pressure following the final crystallisation stage. As referred to previously, the solids-liquid separation can be earned out using an integrated solids separation and water washing apparatus such as a belt filter unit, a rotary cylindrical filter unit, or a dram filter unit (e.g. a BHS-Fest filter drum formed with a plurality of slurry receiving cells in which the mother liquor is displaced from filter cake by water under hydraulic pressure supplied to the cells).

In Figure 8, the carboxylic acid product crystals recovered are supplied via line 64 to a drier (not shown) or to the direct production of polyester. Where the solids- liquid separation is carried out under elevated pressure conditions, the crystals are conveniently let down to atmospheric pressure using a suitable device (e.g. as disclosed in International Patent Application No. WO-A-95/19355 or US Patent No. 5470473) before being transferred to diying equipment. The mother liquor from the solids-liquid separation is recovered via line 66, repressurised by pump 68 and recycled to the mixer unit 36 via heat exchanger 70, line 72, heat exchanger 50, line 74, start-up/trim heater 76 and line 34. Thus, under steady state operating conditions, the recycled mother liquor may contribute to the source of water for supply to the reactor 46 as well as a vehicle for the recycle of catalyst and organic acid by-product (such as benzoic acid in the case of terephthalic acid production from para-xylene) to the process. The mixture unit 36 is configured such that, where the recycled mother liquor may contain catalyst, i.e. homogeneous catalyst, the recycled mother liquor is preferably mixed with the precursor stream rather than the oxidant stream since the addition of catalyst to oxidant is preferably contemporaneous with the addition of precursor to oxidant. Thus, where the recycled mother liquor contains catalyst, the mixture unit is configured such that the oxidant stream 30 may be mixed with fresh water from line 35. Similarly, additional catalyst or said acidic component, as required, may be added to the mother liquor in line 34, or directly to the reaction zone 46.

Because water is generated in the course of the reaction, a water purge is taken from the system. This may be effected in several ways; for instance, the purge may be taken via line 78 or from a suitable flash condensate (for example as will be described below in connection with the energy recovery system). The latter may be more advantageous as it will be somewhat less contaminated with organics than a purge from the mother liquor recovered via line 66. The purge however recovered may be passed to effluent treatment, e.g. aerobic and/or anaerobic processing.

In the heat exchanger 70, the temperature of the mother liquor is increased by about 30 to 100⁰C, through heat transfer from steam flashed from one or more of the crystallisation stages, e.g. the first stage highest pressure and temperature crystalliser vessel. The flash (line 79) used for this purpose may, following passage through the heat exchanger 70, be returned as condensate to the product recoveiy section for use as wash water in washing the carboxylic acid product filter cake produced by solids-liquid separation. In the heat exchanger 50, the temperature of the mother liquor is increased still further, for instance by about 100 to 200⁰C, as a result of heat transfer from the high temperature product stream 48 from the reactor 46. In this manner, the product stream is subjected to cooling while significantly increasing the temperature of the mother liquor recycle stream. The trim/start-up heater 76 serves to boost the temperature of the mother liquor recycle stream, if necessaiy, to secure supercritical or near supercritical conditions. Under steady state operation of the process such boost may be optional since the mother liquor may be rendered supercritical or near supercritical following passage through the heat exchanger 50. The heater 76 may not therefore be necessary under steady state conditions and may be deployed purely for start-up operation, initially using pressurised water from a source other than mother liquor. In this embodiment, the water solvent is rendered supercritical or near supercritical prior to mixing with one or both reactants. However, it will be understood that raising of the temperature to secure the desired supercritical or near supercritical conditions may be effected prior to, during and/or following the mixing stage.

In the embodiment of Figure 8, the heat of reaction generated in the course of reacting the precursor with oxygen is removed at least in part by heat exchange with a heat-accepting fluid, preferably water, which is passed through the interior of the reactor 46 by means of a coiled tube 80 or a series of generally parallel tubes (as in a tube in shell heat exchanger design) or the like. The water employed is pressurised and heated to a temperature sufficiently high that, at the external surface of the conduit or conduits 80 conducting the water through the reactor, localised cooling which could otherwise cause precipitation of components, such as terephlhalic acid, in the reaction medium is avoided. The water for this purpose is derived from the energy recovery system 58. Thus, in Figure 8, water at elevated pressure and temperature is supplied via line 82 to heat exchanger 52 where it is used to cool the product stream further following passage through the heat exchanger 50. The water then passes via line 83 through the conduit(s) 80 with consequent raising of high pressure, high temperature steam which is fed to the energy recoveiy system 58 via line 84.

The energy recovery system 58 is also supplied with steam flashed from one or more stages of the crystallisation train. This is depicted by line 88. This steam may for example be used to preheat the water supplied via line 82 to the heat transfer conduit(s) 80. Condensate resulting from processing of the steam feeds supplied to the energy recovery system 58 may be passed via line 90 to the product recovery section for use for example in washing the carboxylic acid product filter cake produced in the solids- liquid separation. A water purge 92 may be taken from line 90 if desired, with the advantage that a purge taken at this point will be less contaminated than a purge taken from the mother liquor via line 78.

In Figure 8, the reactant(s) are shown as being introduced into the recycled mother liquor after the mother liquor has been heated by heat exchange with the product stream in heat exchanger 50. In a modification, a reactant may be admixed with the mother liquor recycle stream upstream of the heat exchange with the product stream. Where both reactants are so admixed with the mother liquor recycle stream, the latter is split into separate streams with which the reactants are respectively admixed so that the reactants are maintained isolated from each other until brought together for reaction. It will also be understood that the embodiment of Figure 8 may be modified in the manner indicated in Figure 7 by introducing one or even both of the reactants via multiple injection points along the flow path of the reaction medium so that the one or both reactants are introduced to the reaction progressively.

In the energy recovery system 58, various heat recovery processes may be carried out in order to render the process energy efficient. For instance, the high pressure steam raised following passage of water through the conduit(s) 80 may be superheated in a furnace supplied with combustible fuel and the superheated steam may then be passed through one or more steam condensing turbine stages to recover power. Part of the high pressure steam may be diverted for use in preheating the reactants (heat exchangers 40, 4OA and 40B) or for preheating stream 82 where this is necessary to effect a system of high thermal efficiency. The condensed water recovered from the turbine stages and from the heat exchangers 40, 4OA and 4OB may then be passed through a train of heating stages in order to preheat the water for recirculation to the reactor 46 via heat exchanger 52 thus forming a closed loop with make-up water being added as needed. The heating stages typically comprise a cascade of heat exchangers by means of which the recirculating water flow returning to the reactor 46 is progressively raised in temperature. In some heating stages, the heat-donating fluid may be constituted by the flash steam derived at different pressures and temperatures from different stages of the crystallisation train. In other heating stages, the heat-donating fluid may be combustion gases rising in the furnace stack associated with the furnace used to superheat the high pressure steam supplied via line 84.

The embodiment of Figure 8 employs substantially pure oxygen as the oxidant. Figure 9 illustrates a similar embodiment but using a supply of compressed air (which may be oxygen enriched) as the oxidant. The embodiment of Figure 9 is generally similar to that of Figure 8 and those parts which function in generally the same way are depicted by the same reference numerals in both Figures and will not be described further below unless the context requires otherwise. As shown, the air supply 100 is supplied via an air compressor 102. As a result of using air, a substantial amount of nitrogen is introduced into the process and must therefore be appropriately handled. In this case, the product stream following passage through the heat exchangers 50 and 52 is flashed down in flash vessel 103 to a lower temperature to condense water to a greater extent than in the embodiment of Figure 8 thereby reducing the water content of the overheads. As described in relation to Figure 8, temperature of the product stream through the heat exchangers 50 and 52 is controlled such that precipitation of product occurs only in flash vessel 103. The overheads stream is supplied via line 104, heat exchanger 106 and fuel-fired heater 108 to a gas turbine 110. The overheads stream is passed through heat exchanger 106 in order to transfer heat to the mother liquor recycle stream while knocking out further water which can be passed to the product recoveiy section 62 via line 112 for use, for example, as wash water. For reasons of energy efficiency, it is desirable to heat the gaseous overheads stream to a high temperature before introduction into the turbine 1 10, hence the reason for heating the overheads stream by means of heater 108. There may be more than one gas turbine stage, in which case the overheads stream will be heated to an elevated temperature upstream of each such turbine stage. Line 114 depicts the overheads stream exiting the turbine 110 at low pressure and temperature. Where the oxidation process leads to the generation of species such as carbon monoxide etc. which are undesirable, for example for corrosion and/or environmental reasons, provision may be made for treating the overheads stream to reduce/eliminate such components before or after passage through the turbine 110 and/or discharge. Such treatment may comprise subjecting the overheads stream to catalytic combustion and/or scrubbing with a suitable reagent, e.g. an alkaline scrubbing liquor. The turbine 110 may be mechanically coupled with the air compressor so that the latter is driven by the turbine.

In the embodiment of Figure 9, water exits the system via the overheads stream. At least part of this water may be recovered if desired and recirculated for use for example as wash water in the product recovery section 62. Alternatively or additionally, make-up water may be supplied via line 116 to the product recovery section to compensate for the water lost in handling the large volumes of nitrogen as a result of compressed air usage. Such make-up water may be preheated and used as wash water, preheating being effected for example by diverting part of the flash streams (collectively depicted by reference numeral 88) via line 1 16 to heat exchanger 120 and returning the water condensed from the flash stream to the product recovery section 62 as wash water.

Although the invention has been described mainly with reference to para-xylene as a precursor for terephthalic acid, it will be appreciated that other precursors may be employed instead or in addition to para-xylene for the production of the corresponding carboxylic acid, and such precursors include ortho-xylene, meta-xylene, A- tolualdehyde, 4-toluic acid and 3-methylpyridine. As noted above, the invention is also applicable to the production of other aromatic carboxylic acids such as isophthalic acid, phthalic acid, trimellitic acid and naphthalene dicarboxylic acid from the corresponding alkyl aromatic compounds (preferably the methyl compounds) or other precursors. The invention is further illustrated below by the following non-limiting Examples.

EXAMPLES

Experimental work was carried out on a laboratory scale by the continuous oxidation of ortho-xylene by O₂ in supercritical water at about 38O⁰C and 230 to 240 bara with a combined catalyst and acid solution. The exotherm was minimised by using relatively dilute solutions (<0.5% organic w/w). The basic configuration of the system is as set out in Figure 1. A more detailed illustration of the system used in these laboratory scale experiments is shown in Figure 10. 0₂ originates from heating an H₂O₂/H₂O mixture in excess of 400⁰C in the preheater 152. The H₂O₂ decomposes to liberate O₂. The O₂/H₂O fluid then passes through the cross-piece 154, where it is contacted with the ortho-xylene and solution of acid and catalyst, fed in from their own pumps. The reaction mixture is passed through the reactor 156.

Other components labelled in Figure 10 are as follows: cooling coil 158; 0.5μm filter 159; back-pressure regulator 160; valves 162 A to C; pressure-release valves 163 A and B; non-return valves 164 A to C; pressure transducers 165 A to D; thermocouple T (the aluminium heater blocks of preheater 152 and reactor 156 also contain thermocouples, not shown). The pumps were Gilson 302, 305, 306 and 303; the back pressure regulator was obtained from Tescom.

Maximum corrosion occurs in the region of the crosspiece 154 where O₂, feedstock and the catalyst solution meet, particularly at the incoming unheated catalyst feed pipe where a high temperature gradient coincides with bromide ions. Hastelloy was used for the final section of the catalyst feed-pipe and for the reactor, and 316 stainless steel for the other components.

Before each run, the apparatus is hydrostatically pressure-tested when cold, and is then heated with a flow of pure water (5-10mL/min). Once the operating temperature has been reached, H₂O₂/H₂O is fed and the pumps for ortho-xylene and acid/catalyst are started, typically in that order. The residence time for each run remains constant and is typically up to about 1 minute, but in most cases about 10-20 seconds. The yields of manganese were calculated from the flow-out of Mn divided by the flow-in of Mn (where the concentration of manganese is measured by atomic absoprtion).

In these Examples, the concentrations of catalyst and acid are given as concentrations in the catalyst feed stream. The actual concentrations in the reactor correspond to about one-third of these values because the catalyst feed represents about a third of the (mass) flow, with the other two-thirds mostly coming from the other streams. Example 1

An oxidation process was performed using an aqueous solution Of MnBr₂ (1700pm Mn), with and without HBr (1000 ppm Br) and 1500 ppm benzoic acid (i.e. the same concentration as the HBr, 0.0124M). The preliminary results indicate that catalyst recovery was almost complete when benzoic acid was used in combination with HBr, demonstrating that benzoic acid also plays a role in stabilisation of the catalyst. When the ortho-xylene feed is switched off, the benzoic acid-containing run (Run C) demonstrated a significant improvement in terms of catalyst recovery relative to Run B which contained no BA. The results are in Tables 1 and 2.

Table 1

Table 2

a: PHT = phthalic acid; CBA = ortho-carboxylic benzaldehyde; BA = benzoic acid; TALD = ortho-tolualdehyde; TA= ortho-toluic acid; Phth = phthalide. (Selectivities calculated (as a percentage) as the molar concentration of that product relative to the total molar concentration of the products.)

In the first part of the experiment in each of Runs A, B and C, the oxylene stream was initially supplied such that the oxidant/catalyst contact is contemporaneous with the oxidant/precursor contact, as required by WO-02/06201 -A. In Run A, for example, 75% catalyst recovery was observed, the remaining 25% being lost in the form of manganese oxide precipitate (primarily MnO₂, but also including some Mn₂θ₃ and MnO(OH)₂, as determined by X-ray diffraction). When the precursor stream is switched off, catalyst recovery drops dramatically, indicating that MnBr₂ catalyst is significantly more stable in the presence of precursor. The effect of acid is illustrated by Runs B and C, in which initial mixing was contemporaneous as in Run A, and in which catalyst recovery increases significantly in the presence of the precursor, but also after the precursor stream is switched off. Run C is particularly noteworthy in that it shows that the combined presence of HBr and benzoic acid provides significantly more catalyst stability than HBr alone, a very surprising effect which is demonstrated further in the experiments described below. Runs B and C therefore demonstrate that, because of the increased stability of the catalyst in the presence of acid but in the absence of precursor, the contemporaneous mixing regime of WO-02/06201-A is desirable but not required.

Example 2

Runs A and C of Example 1 were repeated except that MnBr₂ was replaced with CoBr₂. (1842 ppm Co). A similar catalyst stabilisation and recovery effect was observed. The results are shown in Table 3.

Table 3

a: in presence of precursor

Example 3

To further investigate the effects of benzoic acid in a system using manganese bromide as the catalyst, experiments similar to those of Example 1 were conducted. In order to solubilise greater amounts of benzoic acid into the system, the apparatus of Figure 10 was modified, in particular by employing a hot-catalyst feed system. A IOOW band heater was placed around the catalyst feed pipe (1/8 inch), controlled by a Variac (100V needed to reach 80 ⁰C). Hot water (80⁰C) was circulated round the catalyst pump head. The experimental conditions were as follows: Reactor temperature: approx. 380⁰C; Reactor pressure: approx. 230 bar Flow rate of acid/catalyst and o-xylene = 4. 084 mL/min. Flow rate of o-xylene = 0.061 mL/min

Flow rate oxidant (H₂O₂ in H₂O) = 8.1 mL/min. (providing an amount Of [O₂] in aqueous H₂O₂ = 0.276 mol.L ' (1.5 molar equivalents of the stoichiometry required for complete oxidation of the organic precursor to the aromatic acid, the molar ratio for which in the case of o-xylene is 3O₂/ organic)).

Typically, the experiment was earned out by (i) achieving SCWO conditions reached with pure water and then (ii) pumping catalyst without oxygen and o-xylene, followed by (iii) supply of oxygen via decomposition Of H₂O₂ in the pre-heater and (iv) switching on of the o-xylene pump.

Initially, however, the stability Of MnBr₂ in the presence of benzoic acid under supercritical conditions was tested in the absence of o-xylene precursor, firstly in the absence of oxidant (i.e. steps (i) and (ii) above), and then with the oxidant switched on (i.e. steps (i) to (iii) above). The results for Runs A to E in this set-up are presented in Table 4 below. The experiments were conducted using a concentration OfMnBr₂ at

1700 ppm Mn at 8O⁰C. The concentration of benzoic acid was varied as shown in Table 4 below.

a: HBr added at lOOOppm HBr

The results show firstly that BA is very stable in water under supercritical conditions, with or without oxygen. The atomic adsorption analysis demonstrated that MnBr₂ is veiy stable in the presence of BA under supercritical conditions. At lOOOOppm BA, total recovery of Mn was achieved in absence of oxygen and o-xylene precursor. At a similar concentration of BA, 90% Mn is recovered when the oxygen was introduced even when the o-xylene precursor is absent (O₂ is 3 -fold excess when the organic is absent). This is a highly significant and unexpected improvement compared with results from earlier work using MnBr₂ or MnBiVHBr which, in the absence of o-xylene precursor, gave about 20-30% Mn recovery in the presence of oxidant. The results are also unexpected in view of the Mn recovery rate of about 60% using MnBr₂/HBr/BA with the BA at 1500 ppm. The results indicate that using higher concentration of BA plays an important role in the stabilisation of the catalyst. The results also confirm that, because of the increased stability of the catalyst in the presence of acid but in the absence of o-xylene precursor, the contemporaneous mixing regime of WO-02/06201-A is desirable but not required.

The experiment proceeded with stage (iv), i.e. by switching on the o-xylene pump. The results are presented in Table 5 below and show that use of benzoic acid in the catalyst feed leads to Mn recoveries which are nearly quantitative. BA selectivities were estimated by taking into account the BA concentration from the catalyst feed, which was measured by HPLC. The results confirm that BA can be used as an additive in the MnBr₂ catalyst feed in order to stabilise the catalyst and minimise precipitation of manganese oxides. The results also indicate that the presence of HBr is not required to benefit from the catalyst recovery effect, which is important since HBr is a known corrosive of the internal surfaces of the apparatus. Table 5 also includes results obtained for runs including NaBr or HBr in the catalyst feed, underlining the importance of acid (rather than bromide) to achieve the catalyst recovery effect. Table 5

A

KJ

Claims

1. The use of an acidic component comprising one or more acid(s) for the purpose of minimising or preventing catalyst loss and/or precipitation of metal oxide in a process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, wherein:

(i) said acidic component is added into the reaction zone;

2. A method of minimising or preventing catalyst loss and/or the precipitation of metal oxide in a process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, wherein:

(i) said method comprises the step of adding an acidic component comprising one or more acid(s) into the oxidation reaction zone;

(ii) contact of at least part of said catalyst with said oxidant is in the presence of said acidic component; and

(iii) said acidic component comprises one or more organic acid(s).

3. An oxidation process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, wherein

(i) an acidic component comprising one or more acid(s) is added to the reaction zone; (ii) contact of at least part of said catalyst with said oxidant is in the presence of said acidic component; and (iii) said acidic component comprises one or more organic acid(s).

4. A use, method or process according to any preceding claim wherein said one or more precursors, oxidant and aqueous solvent constitute a single homogeneous phase in the reaction zone.

5. A use, method or process according to any preceding claim wherein organic acid is added in an amount such that the molar ratio in the reaction zone of added organic acid to M, wherein M is the metal of the catalyst, is at least 0.5:1.

6. A use, method or process according to any preceding claim wherein organic acid is added in an amount such that the molar ratio in the reaction zone of added organic acid to M, wherein M is the metal of the catalyst, is no more than about 30: 1.

7. A use, method or process according to any preceding claim wherein the addition of said acidic component is effected such that the acidic component is present in any location where the metal salt is in contact with the oxidant of the oxidation process.

8. A use, method or process according to any preceding claim wherein the metal salt comprises a transition metal.

9. A use, method or process according to any preceding claim wherein the metal salt comprises manganese.

10. A use, method or process according to any preceding claim wherein the metal salt comprises manganese bromide.

1 1. A use, method or process according to any preceding claim wherein said aromatic dicarboxylic acid is selected from terephthalic acid, isophthalic acid, phthalic acid, trimellitic acid, naphthalene dicarboxylic acid and nicotinic acid.

12. A use, method or process according to any preceding claim wherein said aromatic dicarboxylic acid is terephthalic acid.

13. A use, method or process according to any preceding claim wherein said precursor is selected from the group consisting of aromatic compounds having at least one substituent selected from alkyl, alcohol, alkoxyalkyl and aldehyde groups.

14. A use, method or process according to any preceding claim wherein said precursor is selected from the group consisting of aromatic compounds having at least one substituent selected from alkyl, alcohol and alkoxyalkyl groups.

15. A use, method or process according to any preceding claim wherein said precursor is selected from the group consisting of aromatic compounds having at least one substituent selected from alkyl groups.

16. A use, method or process according to any preceding claim wherein said precursor is selected from the group consisting of aromatic compounds having at least one substituent selected from Ci_-4 alkyl groups.

17. A use, method or process according to claim 12 wherein said precursor is para- xylene.

18. A use, method or process according to any preceding claim wherein said organic acid has the formula R(COOH)_n, wherein n is at least 1 and wherein R is selected from non-aromatic hydrocarbyl groups and aromatic groups.

19. A use, method or process according to any preceding claim wherein the target aromatic carboxylic acid of the oxidation process has the formula Ar(CO₂H)_x, where Ar is an aromatic group and x is at least 2, and the organic acid added to the reaction mixture has the formula Ar(CO₂H)_y wherein y<x.

20. A use, method or process according to any preceding claim wherein said organic acid is benzoic acid.

21. A use, method or process according to any of claims 1 to 18 wherein the target aromatic carboxylic acid of the oxidation process has the formula Ar(CO₂H)_x, where Ar is an aromatic group and x is at least 2, and the organic acid added to the reaction mixture has the formula Ar(CO₂H)_y(R^!)_z, wherein the or each R is a substituent group selected from C _M alkyl, C ₁.₄ alcohol, (C _M alkoxy)-C]^ alkyl or C_M aldehyde groups, wherein y<x and wherein z<x.

22. A use, method or process according to any of claims 1 to 18 and 21 wherein said organic acid is toluic acid.

23. A use, method or process according to any preceding claim wherein said acidic component comprises an organic acid and a mineral acid HX wherein X is halide.

24. A use, method or process according to claim 23 wherein X is bromide.

25. A use, method or process according to any preceding claim wherein said organic acid is a by-product in the oxidation reaction of said precursor to said aromatic carboxylic acid.

26. A use, method or process according to any preceding claim wherein said organic acid is added into the reaction via recycle of the by-product from a waste stream to the reactor.

27. A use method or process according to any of claims 1 to 24 wherein at least part of said organic acid is generated in situ by addition into the reaction mixture of an oxidisable precursor P_OA of said organic acid, wherein said precursor P_OA is different from the precursor(s) of the aromatic carboxylic acid which is the target of the synthetic oxidation reaction claimed herein, and wherein said organic acid generated in situ is different from said target aromatic carboxylic acid.

28. A use method or process according to claim 27 wherein said organic acid is benzoic acid and said precursor P_OA is toluene.

29. A use method or process according to any of claims 1 to 24 wherein at least part of said organic acid is generated in situ by addition into the reaction mixture of a hydrolysable precursor P_OA of said organic acid.

30. A use method or process according to claim 29 wherein said organic acid is benzoic acid and said hydrolysable precursor P_OA is selected from alkylbenzoates.

31. A use, method or process according to any preceding claim wherein at least part of said catalyst and acidic component are mixed prior to contact with the oxidant.

32. A process, use or method according to any preceding claim wherein said contact of at least part of said precursor with said oxidant is contemporaneous with contact of said catalyst with at least part of said oxidant.

33. A process, use or method according to any preceding claim wherein at least 98% wt of the aromatic carboxylic acid produced is maintained in solution during the reaction.

34. A process, use or method according to any preceding claim wherein the aromatic carboxylic acid following reaction is precipitated from the reaction medium and contains no more than 5000 ppm by weight of aldehyde produced as an intermediate in the course of the reaction.

35. A process, use or method according to any preceding claim in which following the reaction the aromatic carboxylic acid-containing solution is processed to precipitate the aromatic carboxylic acid and the precipitate is separated from the mother liquor.

36. Λ process, use or method according to claim 35 in which at least part of the mother liquor is recycled to the reaction zone.

37. A process, use or method as claimed in any preceding claim wherein the reactor is a continuous flow reactor.

38. A process, use or method according to claim 37 wherein the residence time of the reaction medium within the reaction zone is no more than 10 minutes.

39. An aromatic carboxylic acid when produced by the process of any one of claims

3 to 38.