PROCESS FOR RECOVERING AN AROMATIC MONOCARBOXYLIC ACID

TECHNICAL FIELD

The present invention relates to processes and apparatuses for recovering aromatic monocarboxylic acids from a residue stream.

BACKGROUND ART

Aromatic polycarboxylic acids are commonly manufactured by liquid-phase oxidation of an alkyl- substituted aromatic starting material, typically methyl-substituted benzene and naphthalene starting materials, in which the positions of the methyl substituents correspond to the positions of the carboxylic acid substituents in the desired end product. For example, terephthalic acid (TA), which is widely used in the manufacture of polyesters, is manufactured by liquid-phase oxidation of p-xylene with air or another source of oxygen using a bromine-promoted catalyst comprising cobalt and manganese and acetic acid as the solvent. Typically, a feed mixture containing p-xylene, acetic acid and the cobalt-manganese-bromide catalyst (a typical ratio of these compounds is 1 :1 :2) is fed to an oxidation reactor with compressed air and the reaction carried out at 150-230 °C.

The reaction mixture, which comprises crude TA along with a number of by-products such as p- toluic acid and 4-carboxybenzaldehyde (4-CBA), is cooled to yield crystals of TA, which are separated from the mother liquor by filtration. The mother liquor typically comprises water, acetic acid, organic impurities and by-products (e.g. isophthalic acid, benzoic acid, p-toluic acid, and trimellitic acid) as well as TA itself, and inorganic components (e.g. cobalt, manganese and bromide compounds). Although a portion of the mother liquor is returned directly or indirectly to the oxidation reactor to recycle these materials, another portion of the mother liquor is purged to a solvent recovery system to maintain the levels of impurities, by-products and water in the oxidation reactor within acceptable limits.

A portion of the acetic acid and a portion of the water are evaporated from the purge stream to leave a residue stream. A typical composition of such a residue stream is 2-25 weight percent (wt%) acetic acid, 10-50 wt% water, 50-60 wt% organic components (including 20-40 wt% benzoic acid, 5-20 wt% isophthalic acid, and 4-5wt% o-phthalic acid), and catalyst components comprising 0.2-1 .5 wt% cobalt, 0.2-2 wt% manganese, and 2-5 wt% hydrobromic acid (or its sodium salt). The quantity of these residues produced in TA manufacture is estimated to be of the order of several hundred thousand metric tonnes per year. Therefore, although these residues had previously been eliminated as waste (e.g. by burning), recovery of the useful chemicals present in these residues can bring significant environmental and financial benefits.

WO 201 1/1 19395 A1 , which is incorporated herein by reference in its entirety, describes a system for recovering aromatic monocarboxylic acids and catalyst components from this residue stream using a single extraction step, a simplified schematic of which is shown in Figure 1. The residue stream is fed to a collection vessel and combined with organic and aqueous solvents. The mixture is passed to a filtration unit to produce a filter cake and a filtrate, which is separated into an organic layer and an aqueous layer. The aqueous layer is passed to a concentrator and the aqueous solvent obtained from this unit is recycled to the collection vessel. The organic layer is passed to a solvent recovery section and the organic solvent obtained from this section is recycled to the collection vessel. The bottoms from the solvent recovery section are fed to a fractionation unit, with benzoic acid being recovered from the top product and paratoluic acid being recovered, via another distillation tower, from the bottom product.

However, the inventors have found that, even where good washing of the filter cake is achieved, a typical single-stage extraction process fails to recover significant portions of aromatic carboxylic acids into the organic phase. Specifically, the inventors have found that, for a typical residue stream from a process for manufacturing terephthalic acid, whilst most of the benzoic acid is extracted into the organic phase, approximately 1 1 % of the benzoic acid is extracted into the aqueous phase (a further component of benzoic acid remains in the solid phase). This constitutes a significant loss of yield of the valuable benzoic acid and a significant effluent treatment requirement. It is therefore an object of the present invention to provide a process and apparatus that provide improved recovery of the organic components, in particular aromatic monocarboxylic acids such as benzoic acid and p-toluic acid, from these residues.

DISCLOSURE OF THE INVENTION

A first aspect of the present invention provides a process and apparatus for recovering an aromatic monocarboxylic acid from a residue stream that comprises a solids extraction followed by an aqueous extraction. Therefore, this aspect of the present invention provides a process of recovering an aromatic monocarboxylic acid from a residue stream, which comprises the steps of:

(i) mixing a residue stream comprising an aromatic monocarboxylic acid with an aqueous liquid and an organic liquid to produce a first mixture comprising dissolved material and suspended material, wherein the dissolved material comprises the aromatic monocarboxylic acid,

(ii) filtering the first mixture from step (i) to produce a filtrate and a solid,

(iii) separating the filtrate from step (ii) into a first aqueous phase and a first organic phase, and

(iv) recovering the aromatic monocarboxylic acid from the first organic phase from step (iii), characterised in that the process further comprises the steps of:

(v) mixing the first aqueous phase from step (iii) with an organic liquid to produce a second mixture, and

(vi) separating the second mixture from step (v) into a second aqueous phase and a second organic phase, wherein the second organic phase comprises the aromatic monocarboxylic acid.

The aromatic monocarboxylic acid is suitably recovered from the second organic phase of step (vi). Preferably, this is achieved by recycling the second organic phase of step (vi) to step (i), thus providing a counter-current flow (relative to the residue stream) of organic liquid in the process. Thus, preferably the organic liquid in step (i) comprises (and preferably consists of) the second organic phase from step (vi). It will be appreciated that initiating the process requires the addition of fresh make-up organic liquid into step (i), but that at steady state operation fresh organic liquid is preferably introduced into step (v) only. The aromatic monocarboxylic acid from the second organic phase of step (vi) is thus recovered in step (iv).

The first aspect of the invention further provides an apparatus for recovering an aromatic monocarboxylic acid from a residue stream, which comprises:

(a) a mixer configured to receive and mix a residue stream comprising an aromatic monocarboxylic acid with an aqueous liquid and an organic liquid to produce a first mixture comprising dissolved material and suspended material, wherein the dissolved material comprises the aromatic monocarboxylic acid,

(b) a filtration unit in fluid communication with mixer (a) configured to receive the first mixture from mixer (a) and produce a solid and a filtrate,

(c) a separator in fluid communication with filtration unit (b) configured to receive the filtrate from filtration unit (b) and produce a first aqueous phase and a first organic phase, and

(d) a recovery means in fluid communication with separator (c) configured to receive the first organic phase from separator (c) and recover an aromatic monocarboxylic acid from the first organic phase from separator (c),

characterised in that the apparatus further comprises: (e) a mixer in fluid communication with separator (c) configured to receive and mix the first aqueous phase from separator (c) and an organic liquid to produce a second mixture, and

(f) a separator in fluid communication with mixer (e) configured to receive the second mixture from mixer (e) and produce a second aqueous phase and a second organic phase, wherein the second organic phase comprises the aromatic monocarboxylic acid.

The aromatic monocarboxylic acid is suitably recovered from the second organic phase from separator (f). Preferably, this is achieved by mixer (a) being in fluid communication with separator (f) and configured to receive the second organic phase from separator (f), thus providing a counter-current flow (relative to the residue stream) of organic liquid allowing the recycle of organic liquid from separator (f) to mixer (a). Thus, preferably the organic liquid that mixer (a) is configured to receive comprises (and preferably consists of) the second organic phase from separator (f). It will be appreciated that initiating a process for using the apparatus will require the addition of fresh make-up organic liquid into mixer (a), but that at steady state operation fresh organic liquid is preferably introduced into mixer (e) only. The aromatic monocarboxylic acid from the second organic phase from separator (f) is thus recovered in recovery means (d). Mixer (e) is suitably an in-line mixer. Separator (f) is suitably a decanter. Preferably, the organic liquid introduced in step (v) and/or mixer (e) is a clean organic solvent. Preferably, the aqueous liquid introduced in step (i) and/or mixer (a) is a clean aqueous solvent.

The residue stream is suitably derived from a process for manufacturing an aromatic polycarboxylic acid, typically a dicarboxylic acid, such as terephthalic acid. Typically, the residue stream is derived from a mother liquor from a separation step. The aromatic monocarboxylic acid may be dissolved in the residue stream, suspended in the residue stream, or present as a molten phase in the residue stream. The aromatic monocarboxylic acid is preferably selected from benzoic acid, p-toluic acid and mixtures thereof. Therefore, the first aspect of the invention further provides a process for the preparation of an aromatic polycarboxylic acid from the liquid-phase oxidation of an alkyl-substituted aromatic starting material, wherein the improvement comprises:

(i) mixing a residue stream comprising an aromatic monocarboxylic acid with an aqueous liquid and an organic liquid to produce a first mixture comprising dissolved material and suspended material, wherein the dissolved material comprises the aromatic monocarboxylic acid,

(ii) filtering the first mixture from step (i) to produce a filtrate and a solid, (iii) separating the filtrate from step (ii) into a first aqueous phase and a first organic phase,

(iv) recovering the aromatic monocarboxylic acid from the first organic phase from step (iii),

(v) mixing the first aqueous phase from step (iii) with an organic liquid to produce a second mixture, and

(vi) separating the second mixture from step (v) into a second aqueous phase and a second organic phase, wherein the second organic phase comprises the aromatic monocarboxylic acid.

The inventors have found that the first aspect of the present invention, which comprises an aqueous extraction in addition to a solids extraction stage, increases recovery of aromatic monocarboxylic acid with no increase in the usage of solvent. For instance, the recovery of benzoic acid into the organic phase from a residue stream from a process for manufacturing terephthalic acid can be increased up to 95%. Furthermore, this significant increase in benzoic acid recovery is achieved with a relatively small capital investment (e.g. an in-line mixer and small decanter vessel) and a relatively small increase in process complexity. Therefore, the efficiency and economy of the overall manufacturing process is considerably improved.

A second aspect of the present invention provides a process and an apparatus for recovering an aromatic monocarboxylic acid from a residue stream that comprises a first solids extraction followed by a second solids extraction. Therefore, this aspect of the present invention provides a process of recovering an aromatic monocarboxylic acid from a residue stream, which comprises the steps of:

(I) mixing a residue stream comprising an aromatic monocarboxylic acid with an aqueous liquid and an organic liquid to produce a first mixture comprising dissolved material and suspended material, wherein the dissolved material comprises the aromatic monocarboxylic acid,

(II) filtering the first mixture from step (I) to produce a first filtrate and a first solid,

(III) separating the first filtrate from step (II) into a first aqueous phase and a first organic phase, and

(IV) recovering the aromatic monocarboxylic acid from the first organic phase from step (III), characterised in that the process further comprises the steps of:

(V) mixing the first aqueous phase from step (III) with an organic liquid and the first solid from step (II) to produce a second mixture and filtering the second mixture to produce a second filtrate and a second solid, and

(VI) separating the second filtrate from step (V) into a second aqueous phase and a second organic phase, wherein the second organic phase comprises the aromatic monocarboxylic acid. The aromatic monocarboxylic acid is suitably recovered from the second organic phase of step (VI). Preferably, this is achieved by recycling the second organic phase of step (VI) to step (I), thus providing a counter-current flow (relative to the residue stream) of organic liquid in the process. Thus, preferably the organic liquid in step (I) comprises (and preferably consists of) the second organic phase from step (VI). It will be appreciated that initiating the process requires the addition of fresh make-up organic liquid into step (I), but that at steady state operation fresh organic liquid is preferably introduced into step (V) only. The aromatic monocarboxylic acid from the second organic phase of step (VI) is thus recovered in step (IV).

The second aspect of the invention further provides an apparatus for recovering an aromatic monocarboxylic acid from a residue stream, which comprises:

(A) a mixer configured to receive and mix a residue stream comprising an aromatic monocarboxylic acid with an aqueous liquid and an organic liquid to produce a first mixture comprising dissolved material and suspended material, wherein the dissolved material comprises the aromatic monocarboxylic acid,

(B) a filtration unit in fluid communication with mixer (A) configured to receive the first mixture from mixer (A) and produce a first solid and a first filtrate,

(C) a separator in fluid communication with filtration unit (B) configured to receive the first filtrate from filtration unit (B) and produce a first aqueous phase and a first organic phase, and

(D) a recovery means in fluid communication with separator (C) configured to receive the first organic phase from separator (C) and recover an aromatic monocarboxylic acid from the first organic phase from separator (C),

characterised in that the apparatus further comprises:

(E) a mixer in communication with filtration unit (B) and in fluid communication with separator (C) configured to receive and mix the first solid from filtration unit (B), the first aqueous phase from separator (C) and an organic liquid to produce a second mixture,

(F) a filtration unit in fluid communication with mixer (E) configured to receive the second mixture from mixer (E) and produce a second solid and a second filtrate, and

(G) a separator in fluid communication with filtration unit (F) configured to receive the second filtrate from filtration unit (F) and produce a second aqueous phase and a second organic phase, wherein the second organic phase comprises the aromatic monocarboxylic acid.

The aromatic monocarboxylic acid is suitably recovered from the second organic phase from separator (G). Preferably, this is achieved by mixer (A) being in fluid communication with separator (G) and configured to receive the second organic phase from separator (G), thus providing a counter-current flow (relative to the residue stream) of organic liquid allowing the recycle of organic liquid from separator (G) to mixer (A). Thus, preferably the organic liquid that mixer (A) is configured to receive comprises (and preferably consists of) the second organic phase from separator (G). It will be appreciated that initiating a process for using the apparatus will require the addition of fresh make-up organic liquid into mixer (A), but that at steady state operation fresh organic liquid is preferably introduced into mixer (E) only. The aromatic monocarboxylic acid from the second organic phase from separator (G) is thus recovered in recovery means (D). Separator (G) is suitably a decanter.

Preferably, the organic liquid introduced in step (V) and/or mixer (E) is a clean organic solvent. Preferably, the aqueous liquid introduced in step (I) and/or mixer (A) is a clean aqueous solvent.

The residue stream is suitably derived from a process for manufacturing an aromatic polycarboxylic acid, typically a dicarboxylic acid, such as terephthalic acid. Typically, the residue stream is derived from a mother liquor from a separation step. The aromatic monocarboxylic acid may be dissolved in the residue stream, suspended in the residue stream, or present as a molten phase in the residue stream. The aromatic monocarboxylic acid is preferably selected from benzoic acid, p-toluic acid and mixtures thereof.

Therefore, the second aspect of the invention further provides a process for the preparation of an aromatic polycarboxylic acid from the liquid-phase oxidation of an alkyl-substituted aromatic starting material, wherein the improvement comprises:

(I) mixing a residue stream comprising an aromatic monocarboxylic acid with an aqueous liquid and an organic liquid to produce a first mixture comprising dissolved material and suspended material, wherein the dissolved material comprises the aromatic monocarboxylic acid,

(II) filtering the first mixture from step (I) to produce a first filtrate and a first solid,

(III) separating the first filtrate from step (II) into a first aqueous phase and a first organic phase,

(IV) recovering the aromatic monocarboxylic acid from the first organic phase from step (III),

(V) mixing the first aqueous phase from step (III) with an organic liquid and the first solid from step (II) to produce a second mixture and filtering the second mixture to produce a second filtrate and a second solid, and

(VI) separating the second filtrate from step (V) into a second aqueous phase and a second organic phase, wherein the second organic phase comprises the aromatic monocarboxylic acid. The inventors have found that the process of this second aspect of the present invention, which comprises a second solids extraction in addition to the first solids extraction, can increase recovery of aromatic monocarboxylic acid with no increase in the usage of solvent. For instance, the recovery of benzoic acid from a residue stream from a process for manufacturing terephthalic acid into the organic phase can be significantly increased.

A third aspect of the present invention provides a method for the production of purified terephthalic acid comprising the catalytic oxidation of a hydrocarbon precursor in a reaction solvent, comprising the steps of:

• oxidising the hydrocarbon precursor in the reaction solvent in the presence of a metal catalyst to produce crude terephthalic acid; and

• purifying the crude terephthalic acid to yield the purified terephthalic acid, wherein the method further comprises the step of recovering an aromatic monocarboxylic acid from a residue stream by the process of the first aspect of the invention or the process of the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic of a conventional single-stage solids extraction according to the prior art.

Figure 2 is a schematic of a solids extraction combined with an aqueous extraction according to the first aspect of the invention.

Figure 3 is a schematic of a two-stage solids extraction according to the second aspect of the invention.

Figure 4 and Figure 5 are graphs showing the percent recovery of benzoic acid and p-toluic acid in the total organic (including organic liquid in the wet cake), aqueous and solid phases following a single-stage solids extraction process.

Figure 6 and Figure 7 are graphs showing the mass transfer of benzoic acid and p-toluic acid from the aqueous phase to the organic phase in an extraction at a watertoluene weight ratio of 1 .9 in a process according to the first aspect of the invention.

Figure 8 and Figure 9 are graphs showing the mass transfer of benzoic acid from the aqueous phase to the organic phase in an extraction at watertoluene weight ratios of 0.92 and 6 respectively in a process according to the first aspect of the invention.

Figure 10 and Figure 1 1 are graphs showing the mass transfer of p-toluic acid from the aqueous phase to the organic phase in an extraction at watertoluene weight ratios of 0.92 and 6 respectively in a process according to the first aspect of the invention. DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the invention are described herein. It will be recognised that features specified in each embodiment may be combined with other specified features to provide further embodiments.

The weight ratio of aqueous liquid to organic liquid used in step (v)/(V) can be optimised to maximise the ultimate recovery of the aromatic monocarboxylic acid. This weight ratio of aqueous liquid to organic liquid may be between 0.05:1 and 50:1 , more suitably between 0.1 :1 and 25:1 , 0.25:1 and 20:1 , 0.5:1 and 15:1 , 0.75:1 and 10:1 , 0.8:1 and 7.5:1 , or 0.9:1 and 6:1. Suitable weight ratios of aqueous liquid to organic liquid thus include about 1 :1 , about 2:1 , about 3:1 , about 4:1 , and about 5:1 . The solvent of the aqueous liquid (or the aqueous solvent) is preferably water. The solvent of the organic liquid (or the organic solvent) may be selected from toluene, benzene, methanol, cyclohexane, petroleum ether and mixtures thereof. Preferably, the solvent of the organic liquid is toluene. The mixture of aqueous and organic liquids may be heated in steps (i) and/or (v)/(l) and/or (V) (i.e. in mixers (a) and/or (e)/(A) and/or (E)) to facilitate dissolution of solids, for example up to a temperature of 75-80 °C. As mentioned above, the aqueous and organic solvents may be, and preferably are, "clean" when they are introduced into step (v)/(V) of the processes of the present invention. By "clean", it is meant that they have not been used elsewhere in the process (e.g. in an extraction), i.e. fresh or make-up solvent is used. The duration of step (v)/(V) can be optimised to maximise the ultimate recovery of the aromatic monocarboxylic acid whilst minimising the increase in the duration of the overall process. The duration of step (v)/(V) may therefore suitably be from 5 seconds to 60 minutes, or from 5 seconds to 30 minutes, or from 5 seconds to 15 minutes, or from 5 seconds to 5 minutes, or from 10 seconds to 150 seconds. A typical duration of step (v)/(V) may be about 15 seconds, or about 30 seconds.

A process of the first aspect of the invention comprises an aqueous extraction carried out in steps (v) and (vi). In one embodiment, the process further comprises one or more additional aqueous extractions performed on the second aqueous phase from step (vi). The one or more additional aqueous extractions may comprise mixing the second aqueous phase from step (vi) with an organic liquid to produce a further mixture, and separating the further mixture into a further aqueous phase and a further organic phase, wherein the further organic phase comprises the aromatic monocarboxylic acid. The further organic phase may be recycled to step (i). Similarly, an apparatus of the first aspect of the invention is adapted to carry out a first aqueous extraction in mixer (e) and separator (f). In one embodiment, the apparatus is adapted to carry out one or more additional aqueous extractions performed on the second aqueous phase from separator (f). The apparatus may therefore further comprise a further mixer in fluid communication with separator (f) configured to receive and mix the second aqueous phase from separator (f) and an organic liquid to produce a further mixture, and a further separator in fluid communication with the further mixer configured to receive the further mixture from the further mixer and produce a further aqueous phase and a further organic phase, wherein the further organic phase comprises the aromatic monocarboxylic acid. Mixer (a) may be in fluid communication with the further separator and configured to receive the further organic phase from the further separator.

A process of the second aspect of the invention comprises a first solids extraction carried out in steps (I), (II) and (III) and a second solids extraction carried out in steps (V) and (VI). In one embodiment, the process further comprises one or more additional solids extractions performed on the second aqueous phase from step (VI). The one or more additional solids extractions may comprise mixing the second aqueous phase from step (VI) with an organic liquid and the second solid from step (V) to produce a further mixture, filtering the further mixture to produce a further filtrate and a further solid and separating the further filtrate into a further aqueous phase and a further organic phase, wherein the further organic phase comprises the aromatic monocarboxylic acid. The further organic phase may be recycled to step (I). Similarly, an apparatus of the second aspect of the invention is adapted to carry out a first solids extraction in mixer (A), filtration unit (B) and separator (C) and a second solids extraction in mixer (E), filtration unit (F) and separator (G). In one embodiment, the apparatus is adapted to carry out one or more additional solids extractions performed on the second aqueous phase from separator (G). The apparatus may therefore further comprise a further mixer in communication with filtration unit (F) and in fluid communication with separator (G) configured to receive and mix the second solid from filtration unit (F), the second aqueous phase from separator (G) and an organic liquid to produce a further mixture, a further filtration unit in fluid communication with the further mixer configured to receive the further mixture from the further mixer and produce a further solid and a further filtrate, and a further separator in fluid communication with the further filtration unit configured to receive the further mixture from the further filtration unit and produce a further aqueous phase and a further organic phase, wherein the further organic phase comprises the aromatic monocarboxylic acid. Mixer (A) may be in fluid communication with the further separator and configured to receive the further organic phase from the further separator.

The mixing of the residue stream with the aqueous liquid and the organic liquid in step (i)/(l) may take place simultaneously or sequentially. For instance, the residue stream may be mixed with the organic liquid and then the aqueous liquid added subsequently. Alternatively, the residue stream may be mixed with the aqueous liquid and then the organic liquid added subsequently. Alternatively, the aqueous liquid may be mixed with the organic liquid and the residue stream added subsequently to this mixture. Alternatively, the residue stream, the aqueous liquid and the organic liquid may be mixed simultaneously.

The initial contacting of the streams may take place in mixer (a)/(A) or in a line or vessel upstream of mixer (a)/(A). Accordingly, mixer (a)/(A) may receive the residue stream, the aqueous liquid and the organic liquid individually, or two or more of these streams may be contacted prior to their being received by mixer (a)/(A). For instance, the residue stream may be contacted with the organic liquid prior to their being received by mixer (a)/(A). Alternatively, the residue stream may be contacted with the aqueous liquid prior to their being received by mixer (a)/(A). Alternatively, the aqueous liquid may be contacted with the organic liquid and the residue stream prior to their being received by mixer (a)/(A).

The weight ratio of residue stream : organic liquid : water may suitably be in the range of 0.25- 1 .5 : 0.5-3 : 0.75-7.5, or from 0.5-1 : 0.7-2 : 1 -5.

As noted above, the residue stream is suitably derived from a process for manufacturing an aromatic polycarboxylic acid, typically a dicarboxylic acid, such as terephthalic acid.

Terephthalic acid is typically produced by a process comprising the catalytic oxidation of a hydrocarbon precursor in a reaction solvent. The hydrocarbon precursor is a compound that may be oxidised to form the terephthalic acid. Thus, the hydrocarbon precursor is typically benzene substituted with groups such as Ci _-6 alkyl, formyl, or acetyl in the positions of the carboxylic acid substituents in terephthalic acid. Preferred hydrocarbon precursors are Ci_ ₆ alkyl-substituted benzene, in particular p-xylene. The reaction solvent is typically an aliphatic carboxylic acid, such as acetic acid, or a mixture of such aliphatic carboxylic acid(s) and water. The oxidation reaction may be carried out under any conditions wherein oxygen is available, e.g. the reaction can be carried out in air. The reaction catalyst typically comprises soluble forms of cobalt and/or manganese (e.g. their acetates), with a source of bromine, such as hydrogen bromide, used as a promoter. The temperature of the oxidation reaction is typically in the range of about 100-250 °C, preferably about 150-220 °C. Any conventional pressure may be used for the reaction, suitably to maintain the reaction mixture in a liquid state.

An oxidation stage performs the function of catalytically oxidizing the hydrocarbon precursor in the reaction solvent, thus forming a product stream and the vent gas. The product stream is typically transferred to a crystallisation stage to form a first slurry of crude terephthalic acid crystals and an overhead vapour. The first slurry of crude terephthalic acid crystals is typically passed to a separation stage in which a mother liquor is separated from the crude terephthalic acid crystals, which may then be mixed with an aqueous liquid to form a second slurry of crude terephthalic acid crystals. This second slurry of crude terephthalic acid crystals is typically transferred to a purification stage, heated and subjected to hydrogenation, before being cooled to form a slurry of purified terephthalic acid crystals.

The vent gas from the oxidation stage is typically separated in a distillation stage into an organic solvent-rich stream and a water-rich vapour stream. The organic solvent-rich stream from the distillation stage typically comprises 80-95 % w/w of the reaction solvent and is typically returned to the oxidation stage. The water-rich vapour stream from the distillation stage typically comprises 0.1 -5.0 % w/w of the reaction solvent and is typically condensed to form a condensate stream and an overhead gas in a condensing stage. A portion of the condensate stream is typically used as a source of the aqueous liquid used to form the second slurry of crude terephthalic acid crystals mentioned above. A portion of the condensate stream preferably forms a portion of the wash fluid for the purified terephthalic acid crystals from the purification plant.

Typically, the residue stream is derived from the mother liquor separated from the crude terephthalic acid crystals (i.e. from the first slurry of terephthalic acid crystals) in the separation stage. Although a portion of the mother liquor is returned directly or indirectly to the oxidation reactor to recycle these materials, another portion of the mother liquor is purged to a solvent recovery system to maintain the levels of impurities, by-products and water in the oxidation reactor within acceptable limits. A portion of the reaction solvent and a portion of the water are evaporated from the purge stream to leave a residue stream. Thus, the aromatic monocarboxylic acid that is recovered is preferably selected from benzoic acid, p-toluic acid and mixtures thereof. More preferably, the aromatic monocarboxylic acid is benzoic acid. However, additional monocarboxylic acids, such as m-toluic acid and o-toluic acid may be recovered. Thus, the aromatic monocarboxylic acid may be selected from benzoic acid, m-toluic acid, o- toluic acid, p-toluic acid and mixtures thereof. Where the aromatic monocarboxylic acid comprises a mixture of two or more aromatic monocarboxylic acids, the step of recovering the aromatic monocarboxylic acid, step (iv)/(IV), may include one or more distillation processes such as those as described in WO 201 1/1 19395 A1 . For instance, a first distillation process may be employed to remove the organic solvent as the overhead product, a second distillation process may be employed to remove a first aromatic monocarboxylic acid (e.g. benzoic acid) as the overhead product, and a third distillation may be employed to remove a second aromatic monocarboxylic acid (e.g. p-toluic acid) as the overhead product. Recovery means (d)/(D) may therefore comprise one or more distillation columns. The solid obtained in the filtration step(s) (step (ii)/(ll) and step (IV)) of the present invention may be subjected to further processing to recover a dicarboxylic acid as described in WO 201 1/1 19395 A1 . The dicarboxylic acid typically comprises terephthalic acid, isophthalic acid or a mixture thereof.

The aqueous liquid obtained in the final separation step, step (vi)/(VI), may also be subjected to further processing to recover catalyst metals (e.g. by precipitation, suitably with sodium hydroxide and/or sodium carbonate, to recover cobalt and manganese carbonate) and/or a tricarboxylic acid (e.g. trimellitic acid) as described in WO 201 1/1 19395 A1 .

The process and apparatus of the first and second aspects of the invention provide significantly improved recovery of aromatic monocarboxylic acid(s) (such as benzoic acid and p-toluic acid) from the manufacture of aromatic polycarboxylic acids. The process and apparatus according to the first aspect of the invention are particularly advantageous, relative to those of the second aspect of the invention, because they provide comparably improved recovery of aromatic monocarboxylic acid(s) (such as benzoic acid and p-toluic acid) but in a more efficient and economical manner involving fewer unit operations.

The invention will be further described with reference to the figures.

Figure 1 shows a conventional single-stage solids extraction. Residues stream 1 , organic solvent stream 3 and water stream 4 are combined in an extractor (although, as indicated by the dashed lines, either or both of organic solvent stream 3 and water stream 4 may be combined with residues stream 1 upstream of the extractor). The resultant mixture is passed to a filter. The solid phase is separated as solids stream 6 and the liquid phase is passed to a decanter. The organic liquor is separated as stream 8 and the aqueous liquor is separated as stream 9.

Figure 2 is a schematic of a process according to the preferred embodiment of the first aspect of the invention comprising a solids extraction combined with an aqueous extraction. Residues stream 1 , water stream 4 and an organic liquor recycled from a downstream process step are combined in an extractor (although, as indicated by the dashed lines, either or both of the organic liquor and water stream 4 may be combined with residues stream 1 upstream of the extractor). The resultant mixture is passed to a filter. The solid phase is separated as solids stream 6 and the liquid phase is passed to a decanter. The organic liquor is separated as stream 8 and the aqueous liquor is passed to a mixer and combined with organic solvent stream 3. The resultant mixture is passed to a decanter. The aqueous liquor is separated as stream 9 and the organic liquor is recycled counter-currently to be mixed with residues stream 1 in the extractor.

Figure 3 is a schematic of a process according to the preferred embodiment of the second aspect of the invention comprising a two-stage solids extraction. Residues stream 1 , water stream 4 and an organic liquor recycled from a downstream process step are combined in a first extractor (although, as indicated by the dashed lines, either or both of the organic liquor and water stream 4 may be combined with residues stream 1 upstream of the extractor). The resultant mixture is passed to a filter. The solid phase is separated and the liquid phase is passed to a decanter. The organic liquor is separated as stream 8 and the aqueous liquor is passed to a second extractor and recombined with the solid phase from the previous separation and combined with organic solvent stream 3. The resultant mixture is passed to a filter. The solid phase is separated as solids stream 6 and the liquid phase is passed to a decanter. The aqueous liquor is separated as stream 9 and the organic liquor is recycled counter-currently to be mixed with residues stream 1 in the first extractor.

The invention is further illustrated by the following examples. The examples are not intended to limit the invention as described above. Modification of detail may be made without departing from the scope of the invention.

EXAMPLES

Comparative Example 1

A single-stage solids extraction (Figure 1 ) was studied by extracting residues (from a typical residues stream 1 derived from the liquid phase oxidation of p-xylene to terephthalic acid) with toluene for a range of batch times. The slurry was filtered and the cake was washed with water. Samples of the wet cake, organic layer and aqueous layer were taken and analysed to assess of the extent of extraction.

The recovery of benzoic acid in the organic liquid was found to hardly change with increasing batch time, suggesting that the extraction is complete in a very short time.

The concentration of organic liquid in the wet cake can be calculated from the concentration of toluene in the cake. It was found from this calculation that the benzoic acid present as a solid in the wet cake reduces significantly between 5 and 10 minutes and then remains fairly constant, suggesting that the extraction is complete within 10 minutes. It was also seen that the majority of the benzoic acid in the wet cake is present as organic liquid. The benzoic acid recovery data were then adjusted to take into account the organic liquid in the wet cake. These data are shown in Figure 4. It can be seen from these data that, after 10 minutes, approximately 86% of the benzoic acid was extracted into the organic phase, with 1 1 % remaining in the aqueous phase and 3% remaining in the solid phase. The recovery of p-toluic acid was assessed in a similar manner. These data are shown in Figure 5. Approximately 54% of the p-toluic acid was recovered into the organic phase, and approximately equal amounts into the solid and aqueous phases.

Example 1

An artificial aqueous feed (500g) was prepared to simulate the composition of the aqueous phase from the solids extraction step, i.e. the aqueous phase from step (iii)/(lll) of the process of the present invention (in other words, stream 9 from Comparative Example 1 and Figure 1 ). The feed was heated to 80 °C and any undissolved solids filtered out. The filtrate was then mixed with hot toluene (200g). Samples of the aqueous and organic layers were taken at intervals and analysed by HPLC to find the mass transfer of benzoic acid and p- toluic acid from the aqueous to the organic phase. These data are displayed in Figure 6 and Figure 7. The data show the extraction into the organic liquid of organic by-products from the aqueous stream derived from the solids extraction step.

The water:toluene weight ratio in the above experiment was 1.9. In practice, the amount of water present may vary depending on the amount of water added to the residues and the amount of water used for washing the cake in the first solid extraction stage. The impact of the water:toluene ratio was assessed by carrying out two further experiments.

Example 2

A first experiment simulated a process with a watentoluene weight ratio of 0.92. The results are shown in Figure 8 and Figure 10. Example 3

A second experiment simulated a process with a watentoluene weight ratio of 6. The results are shown in Figure 9 and Figure 1 1 .

The recovery data for benzoic acid and p-toluic acid in each of the experiments of Example 1 to Example 3 show that the extraction was complete within 30 seconds. These 30 seconds consisted of 15 seconds mixing and 15 seconds settling, so it can be concluded that a simple in-line mixer with 15 seconds residence time followed by a decanter would be more than sufficient to complete the extraction.

Under normal conditions (Example 1 ), 70% of the benzoic acid (Figure 6) and 90% of the p- toluic acid (Figure 7) were extracted into the organic phase. Thus, combined with the single- stage solids extraction of Comparative Example 1 , this equates to a total recovery over the two stages of approximately 94% benzoic acid and 68% p-toluic acid.

Under reduced water content (Example 2), the benzoic acid recovery was improved to approximately 80% (Figure 8) whilst under increased water content (Example 3) the benzoic acid recovery was reduced to 50% (Figure 9). Similarly, under reduced water content, the p- toluic acid recovery was slightly improved to 92% (Figure 10) whilst under increased water content the p-toluic acid recovery was reduced to 75% (Figure 1 1 ). These results suggest that it is advantageous to minimise the relative water content of the feed, but that improvements in aromatic monocarboxylic acid recovery are nevertheless achieved at both low and high water contents.