WO2023021347A1

WO2023021347A1 - Nickel and cobalt separation in a pta process

Info

Publication number: WO2023021347A1
Application number: PCT/IB2022/056897
Authority: WO
Inventors: Constantine COLLIAS; Keith Whiston; Peter Anthony GANNON
Original assignee: Koch Technology Solutions UK Limited; Koch Technology Solutions, Llc; Eco-Tec Inc.
Priority date: 2021-08-20
Filing date: 2022-07-26
Publication date: 2023-02-23

Abstract

A process for preparing an aromatic polycarboxylic acid is provided. The process includes the steps of: (a) providing a first liquid stream derived from an oxidation process of an aromatic starting material having two or more C1-C6 alkyl substituents, the first liquid stream comprising nickel cations and cobalt and/or manganese cations; (b) contacting the first liquid stream with a solid-phase chelating agent, wherein the solid-phase chelating agent has a higher binding affinity for nickel cations than cobalt and/or manganese cations; (c) allowing the nickel cations to bind to the solid-phase chelating agent; (d) removing from the solid-phase chelating agent a second liquid stream comprising a higher ratio of cobalt and/or manganese cations to nickel cations than the ratio of cobalt and/or manganese cations to nickel cations contained in the first liquid stream; and (e) recycling the second liquid stream into the oxidation process of aromatic starting material.

Description

NICKEL AND COBALT SEPARATION IN A PTA PROCESS

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 63/235,180, filed August 20, 2021, the entirety of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a process, especially a process for the preparation of aromatic polycarboxylic acids, and more particularly to a process involving a step of removing metal cation contaminants from a liquid stream derived from the process.

BACKGROUND

Aromatic polycarboxylic acids, such as terephthalic acid, are widely used as precursors for the manufacture of commercially important polymers (e.g. polyethylene terephthalate [PET]). They are generally produced by the catalytic liquid-phase oxidation of aromatic starting materials having two or more C1-C6 alkyl substituents, such as paraxylene. Cobalt or manganese or a combination of cobalt and manganese, e.g. in the form of their acetates, together with a source of bromide ions, such as hydrobromic acid, is known to provide an effective catalyst system for the oxidation process. The liquid phase oxidation is carried out using a short chain monocarboxylic aliphatic acid, such as acetic acid, as a solvent in which the catalyst system is dissolved.

The aromatic polycarboxylic acid produced by the oxidation process is discharged from the reactor in the form of a slurry of crystals in a mother liquor comprising mainly the aliphatic carboxylic acid. Further precipitation of the aromatic polycarboxylic acid is usually carried out by means of a crystallisation process in several vessels in series downstream from the reactor before separating the crystals from the mother liquor. The solid-liquid separation may be carried out by means of an integrated filtration and washing system, such as disclosed in EP0502628 and WO93/24440, the entire disclosures of which are incorporated herein by reference.

After separation of the aromatic polycarboxylic acid product from the mother liquor, conventional practice is to recycle a major portion of the mother liquor and its dissolved metal catalyst component to the oxidation reactor and to purge a minor portion, which is typically 10 to 40% by volume, to avoid undue build-up of primarily organic contaminants within the reaction system. The mother liquor purge stream is treated to recover the aliphatic carboxylic acid for recycle to the oxidation reaction, leaving a high melting point and viscous residue which contains, inter alia, metal and bromine catalyst components and organic acidic materials.

Efficient use of catalyst and process economics require this residue to be further processed to allow the catalyst metal to be recovered for reuse in the catalytic liquid-phase oxidation. Several methods for the recovery of the catalyst metals are known. These include contacting the residue with water so as to extract the desired metals. In such an approach, the residue is contacted with water such that the metal catalyst components dissolve while the organic contaminants remain largely undissolved. Following separation of the resultant extract solution from the undissolved components, the solution is contacted with an alkali metal carbonate or bicarbonate to precipitate the catalyst metals as carbonates or bicarbonates so that they can then be recovered for further treatment, if necessary, and recycled to the oxidation reactor. Such an approach is disclosed in, for example, British Patent No. 1,413,829, British Patent No. 1,413,488 and United States Patent No. 3,673,154.

Commercial plants that carry out the catalytic oxidation of aromatic polycarboxylic acid precursors at scale (for example, the purified terephthalic acid [PTA] process), have been in production for many decades. Over that time design improvements have seen the number of recycles within the process increase. Many of the process vessels and pipes are built from stainless steel, which although resistant to corrosion, do corrode slowly over time due to contact with the aliphatic carboxylic acid (typically acetic acid) and bromide ions in the reaction and processing streams. The main corrosion products of stainless steel are typically iron, chrome, nickel, manganese and molybdenum cations. As manganese is one of the catalysts used in the reaction process, its accumulation as a corrosion product is not of concern. In most early generation PTA plants there were purge routes for all of these minor components. As a result, corrosion metal build-up was not a concern as it did not manifest in the process.

However, in recent generations of PTA plants, in order to achieve improved environmental and economic performance, many streams within the process are recycled, and purge routes out of the process for minor components and PTA catalysts are minimised. As a result, the build-up of corrosion metal cations has increased. This has begun to be a problem. For example, accumulation of iron has been found to increase the level of colour forming components produced. Similarly, chrome, nickel and molybdenum cation accumulation has been found to increase both the formation of colour components in the system and undesirable side-products (e.g. by non-productive oxidation of acetic acid and paraxylene starting material), resulting in increased process costs.

Mechanisms within the PTA process appear to cause the deposition of chrome and molybdenum compounds and the partial deposition of iron, depending on its oxidation state, thereby minimising their deleterious effects. Nickel is the most problematic corrosion product since its chemical and physical behaviour in the form of Ni²⁺ salts is closely analogous to Co²⁺ salts (one of the components of the reaction catalyst), rendering its selective separation challenging. The problem is exacerbated by the very high molar ratio of Co: Ni within the process, which is typically between 50: 1 and 200: 1. As a result, buildup of nickel in the system over time causes an increase in reagent loss through sidereactions and product colouration.

Therefore, it would be desirable to remove nickel from modern processing plants operating the PTA process or other aromatic oxidation processes. However, since nickel behaves similarly to cobalt, removal of nickel in purge streams results in the unavoidable loss of cobalt. Cobalt is the most expensive element in the PTA catalyst mixture and its discharge to the environment is regulated in most areas of the world, with clppm in water returned to the environment being typical. Despite this long-standing problem, to date no means of selectively removing nickel from an oxidation process for producing aromatic polycarboxylic acids is known.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided a process for preparing an aromatic polycarboxylic acid, including the steps of: a) providing a first liquid stream derived from an oxidation process of an aromatic starting material having two or more C1-C6 alkyl substituents, the first liquid stream comprising nickel cations and cobalt and/or manganese cations; b) contacting the first liquid stream with a solid-phase chelating agent, wherein the solid-phase chelating agent has a higher binding affinity for nickel cations than cobalt and/or manganese cations; c) allowing the nickel cations to bind to the solid-phase chelating agent; d) removing from the solid-phase chelating agent a second liquid stream comprising a higher ratio of cobalt and/or manganese cations to nickel cations than the ratio of cobalt and/or manganese cations to nickel cations contained in the first liquid stream; and e) recycling the second liquid stream into the oxidation process of the aromatic starting material.

The process according to the first aspect of the invention achieves the advantage of removing nickel from the first liquid stream without proportionately removing cobalt and/or manganese, such that the second liquid stream can be recycled to the oxidation process with minimal loss of cobalt and/or manganese catalyst and with a reduced quantity of accumulated nickel. The process therefore minimises the likelihood of nickel cations accumulating in the system. Cobalt cations are preferably removed by the method over manganese cations due to the higher cost and value of cobalt.

The term 'ratio of cobalt and/or manganese cations to nickel cations' as used in the present disclosure refers to the molar ratio (Co+Mn):Ni. As such, if only one of cobalt or manganese is present, the molar ratio concerned is Co: Ni or Mn:Ni, respectively. The term 'ratio of nickel cations to cobalt and/or manganese cations' should be understood in a corresponding sense. Similarly, references herein to the 'cobalt and/or manganese content' refer to the total content of these cation species, regardless of whether one or both species are present.

The aromatic starting material may contain at least one methyl substituent, and may preferably contain at least two methyl substituents. Methyl substituents and other alkyl groups are oxidised to form carboxylic acid groups in the oxidation process. Where the aromatic starting material contains at least two methyl groups, at least two carboxylic acid groups will be produced in the polycarboxylic acid product. An advantage of using methyl substituted aromatic starting materials is that they are generally less expensive than aromatic starting materials having longer and more elaborate alkyl substituents. Minimising the costs of input materials is of particular importance to industrial scale oxidation processes, such as the PTA process.

The solid-phase chelating agent may be an ion-exchange resin. Ion exchange resins are well-known in the art, are commercially available, and are available for use with liquid process streams. The ion-exchange resin may comprise a styrene-divinylbenzene polymer support functionalised by amino groups. The amino groups may preferably be selected from one or more of aminodiacetic acid, aminophosphonic acid, thiourea and bis(2- pyridylmethyl)amine, preferably bis(2-pyridylmethyl)amine. Ion exchange resins containing groups of this type have been found to have good performance in preferential binding of nickel cations compared to cobalt and/or manganese cations. The solid-phase chelating agent may be present in a column having an inlet and an outlet. The column permits the first liquid stream to contact the solid-phase chelating agent and the second liquid stream to be removed from the solid-phase chelating agent in a controllable manner. This allows the contact time between the first liquid stream and the solid-phase chelating agent to be adjustable to a desired period of time.

The cobalt and/or manganese content of the second liquid stream may be at least 90% of the cobalt and/or manganese content of the first liquid stream, and preferably at least 95%. Having a significant proportion of the cobalt and/or manganese content from the first liquid stream present in the second liquid stream allows for efficient recycling of the cobalt and/or manganese catalyst back into the oxidation process.

The process may further comprise adjusting the pH of the first liquid stream to 1.5 or less, and preferably from 1.0 to 1.5. It has been found that nickel-binding solid-phase chelating agents, such as ion-exchange resins, exhibit particularly good selectivity for nickel over cobalt and/or manganese at these lower pH levels.

The process may further comprise removing from the solid phase chelating agent a third liquid stream comprising a higher ratio of nickel cations to cobalt and/or manganese cations than the ratio of nickel cations to cobalt and/or manganese cations contained in the first liquid stream. By having a higher ratio of nickel cations than the first liquid stream, the third liquid stream can be treated as a purge stream and purged without significant loss of valuable cobalt and/or manganese catalyst.

The process may further comprise a step, prior to step (b), of filtering the first liquid stream. This step allows precipitated cobalt and/or manganese catalyst to be recovered and particulate impurities to be removed from the first liquid stream.

The aromatic polycarboxylic acid may be selected from the group consisting of terephthalic acid, isophthalic acid, phthalic acid, 1,2,3-benzenetricarboxylic acid, 1,2,4- benzenetricarboxylic acid and 1,3,5-benzenetricarboxylic acid, and may preferably be terephthalic acid. The process of the first aspect of the invention is thus of considerable use in the production of terephthalic acid, which can be used in the manufacture of polymers such as polybutylene adipate terephthalate (PBAT) and polyethylene terephthalate (PET). The aromatic starting material may be selected from the group consisting of ortho-xylene, meta-xylene, para-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene and 1,3,5- trimethylbenzene, and may preferably be para-xylene.

The aromatic polycarboxylic acid may be terephthalic acid and the aromatic starting material may be para-xylene.

Contacting the first liquid stream with the chelating agent may be performed at a temperature of from 20°C to 85°C, preferably from 50°C to 60°C.

The first liquid stream may have a residence time in the presence of the chelating agent of between 10 minutes and 6 hours, preferably from 1 hour to 3 hours, more preferably from 1.5 hours to 2.5 hours and most preferably 2 hours. Residence times lower than 10 minutes can lead to an ineffective removal of nickel cations from the first liquid stream, whereas residence times of longer than 6 hours, which result in effective removal of nickel cations, lead to non-optimal process times. In a process such as the type mentioned above, this is less desirable.

In accordance with a second aspect of the invention, there is provided a use of a solidphase chelating agent having a higher binding affinity for Ni cations than Co cations in a process for removing Ni cations from a first liquid stream derived from an oxidation process of an aromatic polycarboxylic acid.

In accordance with a third aspect of the invention, there is provided a method for removing nickel cations from a first liquid stream derived from a paraxylene oxidation process without a simultaneous proportionate removal of cobalt and/or manganese cations, comprising the steps of: a) providing the first liquid stream comprising nickel cations and cobalt and/or manganese cations; b) adjusting the pH of the first liquid stream to 1.5 or less, and preferably from 1.0 to 1.5; c) contacting the first liquid stream with a chelating ion-exchange resin in an ionexchange column; d) allowing the nickel cations to selectively bind to the ion-exchange resin; e) removing from the column a second liquid stream comprising a higher ratio of cobalt and/or manganese cations to nickel cations than in the first liquid stream; and f) recycling the second liquid stream into the paraxylene oxidation process. Advantages of the second and third aspects of the invention are similar to those described above in relation to the first aspect of the invention, as would be understood by the skilled person.

In accordance with a fourth aspect of the invention, there is provided a process for preparing terephthalic acid comprising the steps of: a) providing a first liquid stream derived from an oxidation of paraxylene, the first liquid stream comprising nickel cations and cobalt and/or manganese cations; b) contacting the first liquid stream with an ion-exchange resin, wherein the ionexchange resin has a higher binding affinity for nickel cations than cobalt and/or manganese cations; c) allowing the nickel cations to bind to the ion-exchange resin; d) eluting from the ion-exchange resin a second liquid stream comprising a higher ratio of cobalt and/or manganese cations to nickel cations than the ratio of cobalt and/or manganese cations to nickel cations contained in the first liquid stream; and e) recycling the second liquid stream into the paraxylene oxidation process.

As will be described in more detail herein below, the process of the fourth aspect is capable of providing efficient removal of nickel cations within a process for obtaining terephthalic acid from the oxidation of paraxylene, with advantageous retention of valuable cobalt and/or manganese catalyst but without retention of undesirable accumulated nickel cations.

The present invention will be better understood in light of the following examples and the accompanying figure, which are given in an illustrative manner only and should not be interpreted in a restrictive manner.

BRIEF DESCRIPTION OF THE FIGURES

In the accompanying Figures:

Figure 1 is a graph illustrating the effect of increased nickel cation concentration on the performance of the paraxylene oxidation process. The x axis gives the concentration of nickel in the reactor used in the experiments and the y axis shows the impact relative to a nickel-free standard reaction. The squares indicate the effect of nickel cations on burn (i.e. degradation of starting materials, calculated as total CO2 + CO formed in the oxidation), and the diamonds indicate the catalyst requirement to maintain a fixed paraxylene conversion rate. In both cases it can be seen that increasing the nickel concentration in the reactor results in a deterioration in the performance of the paraxylene oxidation reaction.

DETAILED DESCRIPTION

As used herein and in the accompanying claims, unless the context requires otherwise, the terms below are intended to have the definitions as follows.

"Comprise" or variations such as "comprises" or "comprising" will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

"Aromatic" means a 5- to 14-membered cyclic compound having a delocalised pi electron system extending continuously about at least one ring. Examples include benzene, naphthalene, anthracene, phenanthrene.

"Aromatic polycarboxylic acid" means an aromatic compound having two or more carboxylic acid substituents. Examples include phenyl, naphthyl, and anthryl polycarboxylic acids, with terephthalic acid being a preferred example.

"C1-C6 alkyl substituent" includes methyl, ethyl, propyl, isopropyl, butyl, isobutyl, secbutyl, tert-butyl, pentyl, isopentyl, neopentyl, 1-ethylpropyl, hexyl, isohexyl, 1,1- dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl and 2-ethylbutyl substituents, with methyl being a preferred example.

The present disclosure provides, in a first aspect, a method for the preparation of an aromatic polycarboxylic acid. The method includes the steps of: a) providing a first liquid stream derived from an oxidation process of an aromatic starting material having two or more C1-C6 alkyl substituents, the first liquid stream comprising nickel cations and cobalt and/or manganese cations (preferably cobalt cations); b) contacting the first liquid stream with a solid-phase chelating agent, wherein the solid-phase chelating agent has a higher binding affinity for nickel cations than cobalt and/or manganese cations; c) allowing the nickel cations to bind to the solid-phase chelating agent; d) removing from the solid-phase chelating agent a second liquid stream comprising a higher ratio of cobalt and/or manganese cations to nickel cations than the ratio of cobalt and/or manganese cations to nickel cations contained in the first liquid stream; and e) recycling the second liquid stream into the oxidation process of the aromatic starting material.

The aromatic starting material may be oxidised to form the aromatic polycarboxylic acid in a liquid-phase catalytic reaction. The catalyst may be selected from one or more of manganese, cobalt, vanadium, zirconium and hafnium, preferably in an acetate or oxide form. For example, the catalyst may be selected from a combination of manganese acetate and cobalt acetate, or zirconium acetate and hafnium acetate. In some preferred embodiments, the catalyst is cobalt acetate or a combination of manganese acetate and cobalt acetate. Preferably, the catalyst comprises cobalt cations, and preferably the method involves the separation of nickel cations from cobalt cations. The catalyst may be further combined with a source of bromide ions, which may be provided in the form of hydrobromic acid. The oxidation may be carried out using a lower monocarboxylic aliphatic acid, such as acetic acid, as solvent.

The oxidation may be performed in a reactor system in which one or more of the reactor walls, feed lines, recycle lines and purge lines are constructed from stainless steel. As a result, cations of Fe, Ni, Mn and other metals present in the stainless steel may be dissolved in the solvent after prolonged use and contaminate the oxidation reaction. Of these cations, it is the removal of Ni cations in particular which the present disclosure addresses.

The oxidation may be performed in a continuous process or a batch process. In both processes, the catalyst, solvent and soluble organic reaction intermediates are recycled back to the reactor once the product has been isolated and undesirable impurities removed (i.e. by purging from the reactor system). The aromatic polycarboxylic acid product may be collected in a number of ways, although it is typically collected by solid-liquid phase separation. This may be carried out by crystallising the product and filtering it to produce a mother liquor, part of which may be recycled to the reactor, and at least a part of which may be subjected to further processing to recover the catalyst and solvent, separately or in combination, in one or more recovery streams. For example, a catalyst-rich recovery stream, a solvent-rich recovery stream, and/or a reaction by-product rich recovery stream may be obtained by further processing. Trace quantities of metals corroded from the stainless steel apparatus walls may also be present in the mother liquor and in one or more of the recovery streams. Inevitably, one or more purge streams (also termed "residues") are generated during the recovery process. The purge streams may contain, among other things, impurities from the oxidation reaction, unrecovered catalyst and/or stainless steel-derived corrosion metals. In accordance with the invention, the first liquid stream may comprise the mother liquor, a recovery stream, or a purge stream, provided that whichever one it comprises contains nickel cations and cobalt and/or manganese cations.

Examples of catalyst recovery processes that generate purge streams which are suitable for use as the first liquid stream in the process of the present disclosure are disclosed in WO 2016/023958 Al, US3673154A, and WO 2011/119395 A2, the entire disclosures of which are incorporated herein by reference. Suitable first liquid streams can arise from dissolving precipitated cobalt:manganese carbonate salts, which have been produced by base treatment of an organic purge taken directly from the PTA process as described in US3673154A, in acetic acid with the optional addition of water. A suitable first liquid stream may also arise when the same cobalt and manganese carbonate base treatment and dissolution process is applied to an organic purge from the PTA process, in which the organic purge has been previously subjected to an extraction process for the recovery of organic by-products, particularly benzoic acid, using either water or an organic solvent or both, as described in WO2016023958A1.

The first liquid stream may be filtered to provide a substantially solids free liquid stream before being contacted with the solid-phase chelating agent. This may be carried out to ensure that any undissolved cobalt and/or manganese catalyst particles are recovered and to remove any particulates which may otherwise clog up the solid-phase chelating agent and hinder the process from being continuously operable. For example, the first liquid stream may be filtered through an inline filter, such as a cartridge microfilter or sintered metal filter element, or by a media filter. The solids collected by filtration may be recovered by washing or backwashing the filter.

In order to maintain the solubility of cobalt and/or manganese in the first liquid stream, the first liquid stream may be contacted with the solid-phase chelating agent at a temperature of from 20°C to 85°C, from 50°C to 75°C, or from 50°C to 60°C.

The first liquid stream may have a molar ratio of cobalt and/or manganese cations to nickel cations of greater than 1: 1, or from 1 : 1 to 1000: 1, from 10: 1 to 800: 1, from 20: 1 to 500: 1, from 50: 1 to 400: 1, or preferably from 50: 1 to 200: 1. In most instances, the ratio of cobalt and/or manganese to nickel is greater than 1: 1, and in many instances significantly greater. It will be appreciated that when cobalt and/or manganese is present in a molar excess compared to nickel, the selective removal of nickel from a solution containing both cobalt and/or manganese and nickel is made more difficult. The process of the present invention is surprisingly able to achieve a selective removal of nickel despite the existence of a high cobalt and/or manganesemickel ratio in the first liquid stream.

An additional useful feature of the present invention is that manganese ions, which may be present in the first liquid stream, will not be retained by the solid-phase chelating agent to any substantial degree and will therefore be returned to the reactor in the second liquid stream. This serves to retain the catalyst within the process and minimise catalyst loss.

The aromatic starting material may have one methyl substituent, preferably two methyl substituents, or preferably three methyl substituents, and may be selected from the group consisting of ortho-xylene, meta-xylene, para-xylene, 1,2,3-trimethylbenzene, 1,2,4- trimethylbenzene and 1,3,5-trimethylbenzene. The aromatic starting material may preferably be para-xylene.

The carboxylic acid groups of the product are formed in the oxidation process at the positions of the C1-C6 alkyl substituents on the aromatic starting material. The aromatic polycarboxylic acid has two or more carboxylic acid groups, preferably two or three carboxylic acid groups, and may be selected from the group consisting of terephthalic acid, isophthalic acid, phthalic acid, 1,2,3-benzenetricarboxylic acid, 1,2,4-benzenetricarboxylic acid and 1,3,5-benzenetricarboxylic acid. The aromatic polycarboxylic acid may preferably be terephthalic acid.

The solid-phase chelating agent preferentially binds to nickel cations in the first liquid stream through the formation of a chelate complex. Cobalt and/or manganese cations in the first liquid stream may also bind to the solid-phase chelating agent to a lesser extent. The first liquid stream is separated from the solid-phase chelating agent and the bound nickel and cobalt and/or manganese cations to produce the second liquid stream, which is recycled back to the reactor and the oxidation process. As a result of the solid-phase chelating agent's higher affinity for nickel cations, a higher proportion of nickel cations are removed from the first liquid stream resulting in the second liquid stream having a higher ratio of cobalt and/or manganese cations to nickel cations than the ratio of cobalt and/or manganese cations to nickel cations contained in the first liquid stream. The cobalt and/or manganese content of the second liquid stream may be at least 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% of the cobalt and/or manganese content of the first liquid stream, and preferably at least 95%, and the nickel content of the second liquid stream may be less than 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 99% of the nickel content of the first liquid stream, and preferably less than 40%.

In order to maximise the extent to which the nickel cations are bound to the solid-phase chelating agent without overly extending the duration of the process, the first liquid stream may be brought into contact with the solid-phase chelating agent for an optimised residence time of from 10 minutes to 6 hours, preferably from 1 hour to 3 hours, more preferably from 1.5 hours to 2.5 hours and most preferably about 2 hours.

The solid-phase chelating agent preferably comprises a polymer, such as an ion-exchange resin, which can be separated from the first liquid stream by solid-liquid separation. Where the chelating agent is an ion-exchange resin, it may comprise a styrene-divinylbenzene polymer support, and may be functionalised by amino groups. The amino groups may be selected from one or more of aminodiacetic acid, aminophosphonic acid, thiourea and bis(2-pyridylmethyl)amine, and may preferably be bis(2-pyridylmethyl)amine.

Although solid-phase chelating agents that selectively bind to nickel cations are known (for example, in the unrelated technical field of mineral processing), to date the use of such agents in the context of a process for producing an aromatic polycarboxylic acid by catalytic oxidation has not previously been disclosed. This may be because solid-phase chelating agents, such as ion exchange resins, are susceptible to contamination and clogging and the catalytic oxidation process generates high melting point viscous residues which may have been thought likely to contaminate the chelating agents. Furthermore, solid phase chelating agents, such as ion exchange resins, tend to be expensive and difficult to clean. As a result, it would be undesirable to have to dispose of or restore chelating agents that had become contaminated with viscous residues. Furthermore, the separation of nickel cations from cobalt and/or manganese cations in process streams comprising considerably higher concentrations of cobalt and/or manganese cations than nickel cations is difficult to achieve with most selective resins under most conditions (e.g. pH). However, the inventors of the present invention have surprisingly found that solidphase chelating agents are capable of being successfully used in the process of the present disclosure to selectively remove nickel cation contaminants from liquid streams derived from the oxidation process so that the liquid streams can be recycled into the process. Also it has not hitherto been commercially desirable or a requirement to separate nickel cations from cobalt and/or manganese cations within catalytic oxidation processes of aromatic starting materials. This requirement has arisen due to the much higher level of process recycle, particularly water recycle, practiced within the terephthalic acid manufacturing process.

The solid-phase chelating agent may be present in a column having an inlet and an outlet. The first liquid stream may be directed through the inlet to contact the solid-phase chelating agent and allow the nickel cations to bind to the solid-phase chelating agent, and out of the outlet to remove the second liquid stream from the solid-phase chelating agent. The first liquid stream may be allowed to move through the column under gravity. Alternatively, the first liquid stream may be pressurised and urged through the column, for example, by a pump. The inlet and/or outlet may include a tap for controlling the rate of flow through the column so that the contact time of the first liquid stream with the solidphase chelating agent may be controlled.

Alternatively, the solid-phase chelating agent may be present in a vessel into which the first liquid stream is introduced to bring the first liquid stream and the solid-phase chelating agent into contact with each other in order to allow the nickel cations to bind to the solid-phase chelating agent. The second liquid stream may be removed from the solidphase chelating agent by decanting the second liquid stream from the vessel, optionally after the vessel has been centrifuged.

For the solid-phase chelating agent to bind a greater proportion of nickel cations than cobalt and/or manganese cations, it may be preferable that the pH of the first liquid stream be acidic. Therefore, the pH of the first liquid stream may be adjusted to less than 7, or less than or equal to 6.5, 6, 5.5, 5, 4.5, 4, 3.5, 3, 2.5, 2, 1.5, 1.4, 1.3, 1.2, 1.1, 1, or 0.5, or any combination of two of these values, preferably 1.5 or less (e.g. from 1.0 to 1.5). The pH may be adjusted by the addition of an acid, such as HBr, acetic acid or HCI, preferably HBr, to the first liquid stream.

After the second liquid stream has been removed from the solid-phase chelating agent, the solid-phase chelating agent may be contacted with an eluting agent to release the bound nickel (and cobalt and/or manganese) cations, which may be removed from the solid-phase chelating agent as a third liquid stream. The third liquid stream comprises a higher ratio of nickel cations to cobalt and/or manganese cations than the ratio of nickel cations to cobalt and/or manganese cations contained in the first liquid stream. The eluting agent may be a solution having a different salt content or pH to the first liquid stream (e.g. a higher salt content, or a lower pH), or a solution of cations having a higher binding affinity for the solid-phase chelating agent than nickel or cobalt and/or manganese cations. Typically the eluting agent will be a solution of a strong acid, such as a mineral acid for example hydrochloric acid or sulfuric acid, preferably sulfuric acid.

The present disclosure further provides a use of a solid-phase chelating agent having a higher binding affinity for Ni cations than Co and/or Mn cations in a process for removing Ni cations from a first liquid stream derived from an oxidation process of an aromatic polycarboxylic acid. The solid-phase chelating agent, first liquid stream, oxidation process, and aromatic polycarboxylic acid may be as defined for the process of the first aspect.

Another aspect of the present disclosure provides a method for removing nickel cations from a first liquid stream derived from a paraxylene oxidation process without a simultaneous proportionate removal of cobalt and/or manganese cations. The method comprises the steps of: a) providing the first liquid stream comprising nickel cations and cobalt and/or manganese cations; b) adjusting the pH of the first liquid stream to 1.5 or less, and preferably from 1.0 to 1-5; c) contacting the first liquid stream with a chelating ion-exchange resin in an ionexchange column; d) allowing the nickel cations to selectively bind to the ion-exchange resin; e) removing from the column a second liquid stream comprising a higher ratio of cobalt and/or manganese cations to nickel cations than in the first liquid stream; and f) recycling the second liquid stream into the paraxylene oxidation process.

The first liquid stream, oxidation process, ion-exchange resin, column, and second liquid stream may be as defined with respect to the process of the first aspect.

Another aspect of the present disclosure provides a process for preparing terephthalic acid, comprising the steps of: a) Providing a first liquid stream derived from an oxidation of paraxylene, the first liquid stream comprising nickel cations and cobalt and/or manganese cations; b) contacting the first liquid stream with an ion exchange resin, wherein said ion exchange resin has a higher binding affinity for nickel cations than cobalt and/or manganese cations; c) allowing the nickel cations to bind to the ion exchange resin; d) eluting from the ion exchange resin a second liquid stream comprising a higher ratio of cobalt and/or manganese cations to nickel cations than the ratio of cobalt and/or manganese cations to nickel cations contained in the first liquid stream; and e) recycling the second liquid stream into the paraxylene oxidation process.

The first liquid stream, oxidation process, ion-exchange resin, and second liquid stream may be as defined with respect to the process of the first aspect.

EXAMPLES

Isotherm Testing for Resin Selectivity

The purpose of the test is to determine to what degree a resin is selective (or not) for a given component of interest at a given condition, and how quickly the resin reaches steady state (a measure of the kinetics of the reaction).

Sample feed solutions are prepared according to an expected feed condition, i.e. an acidic solution with Co, Ni, Mn, and Fe which matches the condition expected in operation. The pH of the samples is adjusted to vary across the possible operating pH range. The volume of feed solution is small enough that a mass of resin added to it will have enough uptake to make an appreciable (detectable) change in the concentration of the cations being evaluated. Once the samples are prepared with solution and resin, they are placed in a temperature bath to keep at expected operation temperature. Sampling is performed periodically (first sample within 4 hours, then again at 20 - 24 hours, then again at about 36 hours) by transferring enough solution by pipette to perform analysis via AAS (atomic absorption spectroscopy) and evaluate the change in concentration of the cations in solution. This gives the loading (usually expressed as a percentage) of the initial cation that has found its way onto the resin. Comparing the percentages of the various cations shows which cations the resin preferentially uptakes. The varying timescale also helps indicate how quickly the sample equilibrates, which points to the contact time required for operation. In this case, more than 4 hours was needed, but the difference between 24 and 36 hours was not deemed substantial. The results are summarised in Table 1 below. Table 1: Summary of results of isotherm testing

Isotherm testing for pH evaluation

Ion exchange resins were evaluated for their ability to selectively bind Ni cations in solutions comprising Co, Mn and Ni cations. Initial testing was conducted at a pH range known in operation in the literature (3.7 to 0.5), and also recommended by the resin manufacturer (1.5). Some precipitation of a white solid (likely organic material) was noted at pHs of 0.55 to 1.5. pH shifts were observed slightly after precipitation.

Table 2:

Samples were prepared as described above under Isotherm Testing for Resin Selectivity. The results shown in Table 2 indicate that the pH range from approximately 0.8 to 1.2, preferably 1.0-1.2, provides the best selective removal efficiency. For interpretation of this data series, the pH range corresponding to the highest ratio of Ni to Co loaded onto the resin is considered optimum. Given the disparity in feed concentrations where [Co] >> [Ni], this usually means looking at which pH values provide the least Co uptake and still have a high (>50-60%) Ni uptake.

Example 1

A liquid-phase oxidation of paraxylene in acetic acid is carried out in a stainless steel reactor in the presence of cobalt acetate, manganese acetate, and hydrobromic acid. Air is bubbled through the reactor. After completion of the reaction, the crude reaction mixture is cooled, which results in crystallisation of a major portion of the phthalic acid product. The crystals are collected by filtration and the filtrate diluted with water to produce an immiscible binary mixture of aqueous phase and organic phase components. The organic phase is separated and recrystallised to yield a further crop of phthalic acid product. The aqueous phase, which contains dissolved catalyst metals, bromide ions, and reaction impurities, is processed further to recover the catalyst. The pH of the aqueous phase, hereinafter referred to as the first liquid stream, is lowered to about 3.7 by the careful addition of hydrobromic acid. The first liquid stream, which is maintained at a temperature of approximately 50 - 60 °C, is passed through a filter and into a column containing an ion-exchange resin (TP 220 from Lanxess® or MTS9600 from Purolite®). The ionexchange resin has a styrene-divinylbenzene polymer support functionalised by bis(2- pyridylmethyl)amine groups. By manipulating a tap at an outlet of the column, the flow rate of the first liquid stream through the column is controlled at an approximate rate of one column volume per two hours to ensure sufficient contact between the aqueous phase and resin. The eluate, hereinafter referred to as the second liquid stream, exiting the column is collected and recycled to the reactor to be reused in a further oxidation reaction. Samples of first liquid stream and the second liquid stream are collected and analysed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) or Inductively Coupled Plasma- Atomic Emission Spectroscopy (ICP-AES) to determine their levels of nickel and cobalt and/or manganese. The results indicate that the second liquid stream has a higher ratio of cobalt and/or manganese cations to nickel cations than the ratio of cobalt and/or manganese cations to nickel cations in the first liquid stream.

Example 2

The process of Example 1 is carried out except that following separation of the aqueous phase from the organic phase, the aqueous phase is contacted with sodium carbonate to precipitate the catalyst metals as carbonates. The precipitated metal carbonates are filtered off, dissolved in acetic acid and returned to the reactor. The filtrate (i.e. the first liquid stream) is treated with HBr to lower its pH to 1.5 or less (from 1.0 to 1.5) before being filtered and passed into the column containing the ion-exchange resin of Example 1. The flow rate of the first liquid phase through the column is controlled at an approximate rate of one column volume per two hours. The eluate exiting the column, i.e. the second liquid stream, is collected and recycled to the reactor to be reused in a further oxidation reaction. Samples of first liquid stream and the second liquid stream are collected and analysed by ICP-MS or ICP-AES to determine their levels of nickel and cobalt and/or manganese. The results indicate that the second liquid stream has a higher ratio of cobalt and/or manganese cations to nickel cations than the ratio of cobalt and/or manganese cations to nickel cations in the first liquid stream.

Claims

1. A process for preparing an aromatic polycarboxylic acid, including the steps of: a) providing a first liquid stream derived from an oxidation process of an aromatic starting material having two or more C1-C6 alkyl substituents, the first liquid stream comprising nickel cations and cobalt and/or manganese cations; b) contacting the first liquid stream with a solid-phase chelating agent, wherein the solid-phase chelating agent has a higher binding affinity for nickel cations than cobalt and/or manganese cations; c) allowing the nickel cations to bind to the solid-phase chelating agent; d) removing from the solid-phase chelating agent a second liquid stream comprising a higher ratio of cobalt and/or manganese cations to nickel cations than the ratio of cobalt and/or manganese cations to nickel cations contained in the first liquid stream; and e) recycling the second liquid stream into the oxidation process of the aromatic starting material.

2. The process according to claim 1, wherein the aromatic starting material contains at least one methyl substituent.

3. The process according to claim 1 or claim 2, wherein the solid-phase chelating agent is an ion-exchange resin.

4. The process according to claim 3, wherein the ion exchange resin comprises a styrene- divinylbenzene polymer support functionalised by amino groups, preferably wherein the amino groups are selected from one or more of aminodiacetic acid, aminophosphonic acid, thiourea and bis(2-pyridylmethyl)amine, preferably wherein the amino groups are bis(2- pyridylmethyl)amine.

5. The process according to any one of claims 1 to 4, wherein the solid phase chelating agent is present in a column having an inlet and an outlet.

6. The process according to any one of claims 1 to 5, wherein the cobalt and/or manganese content of the second liquid stream is at least 90% of the cobalt and/or manganese content of the first liquid stream, and preferably at least 95%.

7. The process according to any one of claims 1 to 6, further comprising adjusting the pH of the first liquid stream to 1.5 or less, and preferably from 1.0 to 1.5.

8. The process according to any one of claims 1 to 7, further comprising removing from the solid phase chelating agent a third liquid stream comprising a higher ratio of nickel cations to cobalt and/or manganese cations than the ratio of nickel cations to cobalt and/or manganese cations contained in the first liquid stream.

9. The process according to any one of claims 1 to 8, further comprising a step, prior to step (b), of filtering the first liquid stream.

10. The process according to any one of claims 1 to 9, wherein the aromatic polycarboxylic acid is selected from the group consisting of terephthalic acid, isophthalic acid, phthalic acid, 1,2,3-benzenetricarboxylic acid, 1,2,4-benzenetricarboxylic acid and 1,3,5- benzenetricarboxylic acid, preferably wherein the aromatic polycarboxylic acid is terephthalic acid.

11. The process according to any one of claims 1 to 10, wherein the aromatic starting material is selected from the group consisting of ortho-xylene, meta-xylene, para-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene and 1,3,5-trimethylbenzene, preferably wherein the aromatic starting material is para-xylene.

12. The process according to claim 11, wherein the aromatic polycarboxylic acid is terephthalic acid and the aromatic starting material is para-xylene.

13. The process according to any one of claims 1 to 12, wherein contacting the first liquid stream with the chelating agent is performed at a temperature of from 20°C to 85°C, preferably from 50°C to 60°C.

14. The process according to any one of claims 1 to 13, wherein the first liquid stream has a residence time in the presence of the chelating agent of between 10 minutes and 6 hours, preferably from 1 hour to 3 hours, more preferably from 1.5 hours to 2.5 hours and most preferably 2 hours.

15. Use of a solid-phase chelating agent having a higher binding affinity for Ni cations than Co cations in a process for removing Ni cations from a first liquid stream derived from an oxidation process of an aromatic polycarboxylic acid.

16. A method for removing nickel cations from a first liquid stream derived from a paraxylene oxidation process without a simultaneous proportionate removal of cobalt and/or manganese cations, comprising the steps of: a) providing the first liquid stream comprising nickel cations and cobalt and/or manganese cations; b) adjusting the pH of the first liquid stream to 1.5 or less, preferably from 1.0 to 1-5; c) contacting the first liquid stream with a chelating ion-exchange resin in an ionexchange column; d) allowing the nickel cations to selectively bind to the ion-exchange resin; e) removing from the column a second liquid stream comprising a higher ratio of cobalt and/or manganese cations to nickel cations than in the first liquid stream; and f) recycling the second liquid stream into the paraxylene oxidation process.

17. A process for preparing terephthalic acid, comprising the steps of: a) Providing a first liquid stream derived from an oxidation of paraxylene, the first liquid stream comprising nickel cations and cobalt and/or manganese cations; b) contacting the first liquid stream with an ion-exchange resin, wherein the ionexchange resin has a higher binding affinity for nickel cations than cobalt and/or manganese cations; c) allowing the nickel cations to bind to the ion-exchange resin; d) eluting from the ion-exchange resin a second liquid stream comprising a higher ratio of cobalt and/or manganese cations to nickel cations than the ratio of cobalt and/or manganese cations to nickel cations contained in the first liquid stream; and e) recycling the second liquid stream into the paraxylene oxidation process.

18. The process of claim 17, comprising adjusting the pH of the first liquid stream to 1.5 or less, preferably from 1.0 to 1.5, prior to contacting the first liquid stream with the ion-exchange resin.

19. The process of any of claims 1 to 14, the use of claim 15, the method of claim 16, or the process of claim 17 or 18, wherein the cobalt and/or manganese is cobalt.