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Title:
EFFLUENT OFF-GAS TREATMENT
Document Type and Number:
WIPO Patent Application WO/2016/055542
Kind Code:
A1
Abstract:
The present invention provides a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, wherein off-gas generated by the oxidation reaction is vented to the atmosphere after treatment to remove HBr and/or Br2, wherein said process comprises scrubbing in a scrubbing column a gas stream derived from off-gas generated by said oxidation reaction and comprising HBr and/or Br2, characterised in that: (i) the gas stream is irrigated with liquid phase water and a wash stream comprising alkali metal ions within said scrubbing column; (ii) the scrubbed gas stream is withdrawn from a vent located in an upper region of the scrubbing column; and (iii)a stream comprising water, said alkali metal ions and bromide ions is withdrawn from an effluent outlet located in a lower region of the scrubbing column. The present invention further provides a scrubbing column for carrying out the process.

Inventors:
LIMBACH ANTONY PETER JOHN (GB)
URE ALAN MACPHERSON (GB)
LEONARD MARK CHRISTOPHER (GB)
ZHENG DONGHUI (GB)
Application Number:
PCT/EP2015/073184
Publication Date:
April 14, 2016
Filing Date:
October 07, 2015
Export Citation:
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Assignee:
INVISTA TECHNOLOGIES S À R L (CH)
International Classes:
C07C51/16; B01D3/00; C07C63/26
Domestic Patent References:
WO1996039595A11996-12-12
WO1997027168A11997-07-31
WO1998008605A11998-03-05
Attorney, Agent or Firm:
COCKERTON, Bruce Roger et al. (One Southampton Row, London WC1B 5HA, GB)
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Claims:
A process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, wherein off-gas generated by the oxidation reaction is vented to the atmosphere after treatment to remove HBr and/or Br2, wherein said process comprises scrubbing in a scrubbing column a gas stream derived from said off-gas generated by said oxidation reaction and comprising HBr and/or Br2, characterised in that:

(i) the gas stream is irrigated with liquid phase water and a wash stream comprising alkali metal ions within said scrubbing column;

(ii) the scrubbed gas stream is withdrawn from a vent located in an upper region of the scrubbing column; and

(iii) a stream comprising water, said alkali metal ions and bromide ions is withdrawn from an effluent outlet located in a lower region of the scrubbing column.

A process according to claim 1 wherein the off-gas generated by the oxidation reaction comprises organic by-products from the oxidation reaction wherein one of said organic byproducts is MeBr, and wherein said process further comprises the step of passing said off-gas to a catalytic combustion stage prior to said scrubbing column, wherein said catalytic combustion stage converts said MeBr to HBr and/or B¾ and further converts said organic by-products other than MeBr to C02 and H20.

A process according to claim 2 wherein said catalytic combustion stage converts at least 80%, preferably at least 90%, preferably at least 95% of said organic by-products other than MeBr to C02 and H20.

A process according to claim 2 or claim 3 wherein said off-gas is passed from said catalytic combustion stage to said scrubbing column without addition of liquid phase water thereto.

A process according to any one of claims 2-4 wherein said off-gas is passed from said catalytic combustion stage to said scrubbing column via an expansion means which effects a reduction in temperature of said off-gas.

A process of removing HBr and/or Br2 from a gas stream comprising HBr and/or Br2, said process comprising the step of scrubbing said gas stream in a scrubbing column characterised in that:

(i) the gas stream is irrigated with liquid phase water and a wash stream comprising alkali metal ions within said scrubbing column;

(ii) a scrubbed gas stream is withdrawn from a vent located in an upper region of the scrubbing column; and (iii) a stream comprising water, said alkali metal ions and bromide ions is withdrawn from an effluent outlet located in a lower region of the scrubbing column.

7. A process according to claim 6 wherein said gas stream is derived from an off-gas generated by the catalytic oxidation of a hydrocarbon precursor to an aromatic dicarboxylic acid in an organic solvent.

8. A process according to any preceding claim wherein said gas stream further comprises at least one of N2, CO2, O2 and H2O, preferably at least CO2 and H2O, preferably at least CO2, O2 and H20, and preferably all of N2, C02, 02 and H20.

9. A process according to any preceding claim wherein said gas stream is passed into a lower region of the scrubbing column. 10. A process according to any preceding claim wherein said gas stream is irrigated with liquid phase water by spraying said liquid phase water into said gas stream within the scrubbing column.

1 1 . A process according to any preceding claim wherein said gas stream is irrigated with said liquid phase water simultaneously with irrigation of said gas stream with said wash stream comprising alkali metal ions.

12. A process according to any preceding claim wherein said alkali metal ions are sodium ions.

13. A process according to any preceding claim wherein said wash stream comprises sodium hydroxide, and optionally further comprises sodium formate.

14. A process according to any preceding claim wherein said gas stream, prior to entry into the scrubbing column, is at a temperature of about 70 °C to about 120°C, or about 75 °C to 100 °C, or about 80 °C to 90 °C.

15. A process according to any preceding claim further comprising maintaining a layer of liquid above said effluent outlet.

16. A process according to claim 15 wherein said gas stream is introduced into the scrubbing column at one or more inlets located above the level of said layer of liquid and said gas stream is irrigated with said liquid phase water such that said liquid phase water becomes entrained into said gas stream.

17. A process according to claim 16 wherein said gas stream is introduced into the scrubbing column below distributing means for spraying said liquid phase water into said gas stream. 18. A process according to claim 16 wherein distributing means spray said liquid phase water along the axis of said gas stream in the direction of flow of said gas stream.

19. A process according to claim 17 or claim 18 wherein said wash stream is introduced into the scrubbing column via one or more inlet points located above the location of said distributing means such that said wash stream passes from said inlet points towards the lower region of the scrubbing column.

20. A process according to claim 19 wherein said wash stream is introduced into the scrubbing column via one or more inlet points located above a packed bed positioned within the scrubbing column.

21 . A process according to claim 20 wherein a deflector baffle is positioned below the packed bed and is configured such that a portion of the wash stream exiting the bottom of the packed bed is directed onto the internal walls of the scrubbing column by the deflector baffle.

22. A process according to any preceding claim wherein irrigation of the gas stream with liquid phase water and the wash stream saturates the gas stream with water.

23. A process according to any preceding claim wherein the gas stream is not mixed with liquid phase water prior to entry of the gas stream into the scrubbing column.

24. A process according to any preceding claim wherein the gas stream is introduced into the scrubbing column via a pipe, preferably wherein said gas stream is not mixed with liquid phase water in the pipe.

25. A process according to claim 24 wherein the pipe comprises a distal portion that extends through a wall of the scrubbing column into the scrubbing column itself.

26. A process according to claim 25 wherein the distal portion of the pipe comprises an insulating material positioned to reduce the transfer of heat from the gas stream to the wall of the scrubbing column.

27. A process according to claim 25 or claim 26 wherein a shroud extends from a wall of the scrubbing column and is configured to prevent liquid running down the walls of the scrubbing column from entering and/or contacting the distal portion of the pipe. 28. A scrubbing column for removing HBr and/or B¾ from a gas stream derived from an off-gas generated by the catalytic oxidation of a hydrocarbon precursor to an aromatic dicarboxylic acid in an organic solvent, comprising

a gas stream inlet for introducing the gas stream into the scrubbing column;

a vent for withdrawing a scrubbed gas stream from the scrubbing column, wherein the vent is located in an upper region of the scrubbing column;

a wash stream inlet located above the gas stream inlet;

distributing means for spraying said liquid phase water into the gas stream; and

an effluent outlet located in a lower region of the scrubbing column and located below the gas stream inlet.

29. A scrubbing column according to claim 28 wherein the distributing means for spraying said liquid phase water into the gas stream is positioned such that liquid phase water from the distributing means becomes entrained into the gas stream entering the gas stream inlet. 30. A scrubbing column according to claim 28 or claim 29 wherein the distributing means for spraying said liquid phase water into the gas stream is located above the gas stream inlet.

31 . A scrubbing column according to claim 28 or claim 29 wherein the distributing means for spraying said liquid phase water into the gas stream comprises one or more spray nozzles, preferably wherein the one or more spray nozzles are directed along the axis of the gas stream entering the gas stream inlet in the direction of flow of the gas stream.

32. A scrubbing column according to any one of claims 28-31 further comprising a pump configured to transfer liquid removed from the effluent outlet to the wash stream inlet.

33. A scrubbing column according to any one of claims 28-32 further comprising a packed bed located below the wash stream inlet and above the distributing means for spraying said liquid phase water into the gas stream. 34. A scrubbing column according to claim 33 further comprising a deflector baffle positioned below the packed bed and configured such that a portion of the wash stream exiting the bottom of the packed bed is directed onto the walls of the scrubbing column by the deflector baffle.

35. A scrubbing column according to any one of claims 28-34 wherein the gas stream inlet comprises a pipe comprising a distal portion that extends through a wall of the scrubbing column into the scrubbing column itself.

36. A scrubbing column according to claim 35 wherein the distal portion of the pipe comprises an insulating material positioned to reduce the transfer of heat from the gas stream to the wall of the scrubbing column. 37. A scrubbing column according to claim 35 or claim 36 wherein a shroud extends from a wall of the scrubbing column and is configured to prevent liquid running down the walls of the scrubbing column from entering or contacting the distal portion of the pipe.

Description:
EFFLUENT OFF-GAS TREATMENT

TECHNICAL FIELD

The present invention relates to a process and apparatus for the production of an aromatic dicarboxylic acid.

BACKGROUND ART

Aromatic dicarboxylic acids are commonly manufactured by the catalytic oxidation of a hydrocarbon precursor in an organic solvent. An example is terephthalic acid (TA), which is widely used in the manufacture of polyesters, such as poly(ethylene terephthalate) (PET). The TA required as a reactant for PET production is known as "purified terephthalic acid" (PTA) and generally contains over 99.97 wt%, preferably over 99.99 wt%, of terephthalic acid, and less than 25 ppm 4- carboxybenzaldehyde (4-CBA). On the commercial scale, PTA suitable for use in PET production is generally prepared in a two-stage process. First, p-xylene is oxidized (e.g. using air) in the presence of a metal catalyst (e.g. a cobalt and/or manganese salt or compound) to provide "crude terephthalic acid" (CTA), as described in, for example, US 2,833,816. Second, the CTA produced by this oxidation reaction is then purified, as it is typically contaminated by impurities such as 4-CBA, p-toluic acid, and various coloured impurities that impart a yellowish colour to the TA. Purification of the CTA typically requires at least one chemical transformation (e.g. hydrogenation) in addition to at least one physical procedure (e.g. crystallization, washing, etc.) to yield PTA.

PTA is generally considered to be a commodity item, with several million tonnes being produced annually, and it is therefore desirable for manufacturers to reduce their costs to maximise the economy and efficiency of PTA production. This can be achieved both by reducing capital costs (e.g. equipment costs) and variable costs (e.g. costs associated with waste disposal, use of starting materials, organic solvent, heating fuel and demineralised water).

The oxidation reaction described above produces an off-gas from which water and organic solvent are recovered, followed by further treatment before it is, ultimately, vented to the atmosphere. The final stage of the treatment may involve scrubbing the off-gas in a scrubbing column to remove HBr and/or Br2, which are typically present in the off-gas due to the use of HBr as a catalyst promoter. In previous processes, the off-gas was mixed with water before entry of the off-gas into the scrubbing column, and this mixing was typically effected in the stainless steel pipe used to inject the off-gas into the scrubbing column, thus saturating the off-gas with water prior to its contact with NaOH to remove the HBr and/or Br2. However, it has been found that even though the concentration of HBr and/or B¾ in the off-gas is small, significant corrosion of this stainless steel pipe (due to the formation of a corrosive HBr dew) can occur in a relatively short space of time to such a degree that the stainless steel pipe requires replacement well before its expected operational lifetime is reached. Replacement of the stainless steel pipe not only has a capital cost associated with it, but also increases variable costs as it requires taking some or all of the PTA manufacturing plant offline whilst it is carried out. It is an object of the present invention to provide a more economic and efficient process and apparatus for the manufacture of aromatic dicarboxylic acids and, in particular, to provide a process and apparatus that overcome the aforementioned disadvantages. Further objects will be apparent from the description below. DISCLOSURE OF THE INVENTION

The present invention provides a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, wherein off-gas generated by the oxidation reaction is vented to the atmosphere after treatment to remove HBr and/or Br2, wherein said process comprises scrubbing in a scrubbing column a gas stream derived from said off-gas generated by said oxidation reaction and comprising HBr and/or Br2, characterised in that:

(i) the gas stream is irrigated with liquid phase water and a wash stream comprising alkali metal ions within said scrubbing column;

(ii) the scrubbed gas stream is withdrawn from a vent located in an upper region of the scrubbing column; and

(iii) a stream comprising water, said alkali metal ions and bromide ions is withdrawn from an effluent outlet located in a lower region of the scrubbing column.

The off-gas generated by the oxidation reaction typically comprises organic by-products from the oxidation reaction, one of which may be MeBr. The process may therefore further comprise the step of passing said off-gas to a catalytic combustion stage prior to said scrubbing column, wherein said catalytic combustion stage converts said MeBr to HBr and/or B¾ and further converts organic byproducts other than MeBr to CO2 and water vapour. It is preferred that said off-gas is passed from said catalytic combustion stage to said scrubbing column without addition of liquid phase water thereto, thus minimising the risk of corrosion of the pipe(s) between said catalytic combustion stage and said scrubbing column.

The present invention further provides a process of removing HBr and/or B¾ from a gas stream comprising HBr and/or Br2, said process comprising the step of scrubbing said gas stream in a scrubbing column characterised in that:

(i) the gas stream is irrigated with liquid phase water and a wash stream comprising alkali metal ions within said scrubbing column; (ii) a scrubbed gas stream is withdrawn from a vent located in an upper region of the scrubbing column; and

(iii) a stream comprising water, said alkali metal ions and bromide ions is withdrawn from an effluent outlet located in a lower region of the scrubbing column.

The gas stream may further comprise at least one of N2, CO2, O2 and H2O, preferably at least CO2 and H2O, preferably at least CO2, O2 and H2O, and preferably all of N2, CO2, O2 and H2O.

The present invention further provides a scrubbing column for removing HBr and/or Br2 from a gas stream derived from an off-gas generated by the catalytic oxidation of a hydrocarbon precursor to an aromatic dicarboxylic acid in an organic solvent, comprising

a gas stream inlet for introducing the gas stream into the scrubbing column;

a vent for withdrawing a scrubbed gas stream from the scrubbing column, wherein the vent is located in an upper region of the scrubbing column;

a wash stream inlet located above the gas stream inlet;

distributing means for spraying said liquid phase water into the gas stream; and

an effluent outlet located in a lower region of the scrubbing column and located below the gas stream inlet. The irrigation of the gas stream with liquid phase water within the scrubbing column allows the saturation and cooling of the gas stream that is required for effective scrubbing with alkali metal ions without the need to mix the gas stream with liquid phase water in the pipe that supplies the gas stream to the scrubbing column, thus minimising the formation of HBr dew in this pipe. Accordingly, the corrosion of the pipe is reduced without compromising the effectiveness of the scrubbing process. As the gas stream is irrigated inside the scrubbing column, the local concentration and temperature of aqueous HBr on the internal walls of the scrubbing column can be kept low, thus the risk of corrosion of the walls of the scrubbing column is minimised. In addition, the irrigation process typically results in the walls of the scrubbing column being continuously washed with water, thus limiting the temperature and concentration of aqueous HBr on the walls of the scrubbing column. Thus, in the process of the present invention, it is possible to avoid an arrangement in which the gas stream is mixed with liquid phase water prior to entry of said gas stream into said scrubbing column, and hence to avoid an arrangement in which the gas stream is mixed with liquid phase water in the pipe which supplies said gas stream to said scrubbing column.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic of a process and apparatus according to the invention.

Figure 2 is a schematic of a process and apparatus according to 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.

It will be appreciated that the general operation of a process and apparatus for the production of an aromatic dicarboxylic acid by the catalytic oxidation of a hydrocarbon precursor in an organic solvent is well known. For example, as discussed above, terephthalic acid suitable for use in PET production (i.e. purified terephthalic acid) is generally prepared in a two-stage process. First, p-xylene is oxidized (e.g. using air) in the presence of a metal catalyst (e.g. a cobalt and/or manganese salt or compound) to provide crude terephthalic acid. Second, the crude terephthalic acid produced by this oxidation reaction is then purified to remove impurities, such as 4-CBA and p-toluic acid, to yield purified terephthalic acid. Purification of crude terephthalic acid typically requires at least one chemical transformation (e.g. hydrogenation) in addition to at least one physical procedure (e.g. crystallization, washing, etc.).

Production of an aromatic dicarboxylic acid

The aromatic dicarboxylic acid produced in the process and apparatus of the present invention is preferably selected from terephthalic acid, orthophthalic acid and isophthalic acid. The aromatic dicarboxylic acid is preferably terephthalic acid. The hydrocarbon precursor is a compound that may be oxidised to form the aromatic dicarboxylic acid. Thus, the hydrocarbon precursor is typically benzene or naphthalene substituted with groups such as Ci-6alkyl, formyl, or acetyl in the positions of the carboxylic acid substituents in the desired end product. Preferred hydrocarbon precursors are Ci-6alkyl-substituted benzene, in particular p-xylene. The organic 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 organic solvent, thus forming a product stream and a vent gas. The product stream is typically transferred to a crystallisation stage to form a first slurry of crude aromatic dicarboxylic acid crystals and an overhead vapour. The first slurry of crude aromatic dicarboxylic acid crystals is typically passed to a separation stage in which a mother liquor is separated from the crude aromatic dicarboxylic acid crystals, which may then be mixed with an aqueous liquid to form a second slurry of crude aromatic dicarboxylic acid crystals. This second slurry of crude aromatic dicarboxylic acid crystals is typically transferred to a purification plant, heated and subjected to hydrogenation, before being cooled to form a slurry of purified aromatic dicarboxylic acid crystals.

The off-gas from the oxidation stage is typically separated in a distillation stage into an organic solvent-rich liquid stream and a water-rich vapour stream. The organic solvent-rich liquid stream from the distillation stage typically comprises 80-95 % w/w organic 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 organic 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 aromatic dicarboxylic acid crystals mentioned above. A portion of the condensate stream also typically forms a source of wash fluid for the purified aromatic dicarboxylic acid crystals from the purification plant.

Overhead gas

The overhead gas from the condensing stage, which is derived from the off-gas generated by the oxidation reaction, typically comprises N2, CO2, O2 and H2O, unreacted hydrocarbon precursor (e.g. p-xylene), organic solvent (e.g. acetic acid) and derivatives of the organic solvent (e.g. methyl acetate), MeBr and methanol. The overhead gas is typically treated in a pressurised scrubber with organic solvent and then with water to reduce the levels of unreacted hydrocarbon precursor and derivatives of the organic solvent in the gas. The pressurised scrubber vent gas stream is typically treated in a catalytic combustion stage in which MeBr is converted to HBr and/or B¾ and the organic by-products remaining in the gas other than MeBr (e.g. methyl acetate and methanol) are converted to CO2 and H2O. The catalytic combustion stage preferably converts at least 80%, preferably at least 90%, or preferably at least 95% of said organic by-products other than MeBr to CO2 and H2O.

The resultant gas stream may be passed from the catalytic combustion stage to the scrubbing column via an expansion means which effects a reduction in temperature of the gas stream. For instance, the temperature of the gas stream may be about 250 °C on entering the expansion means and about 70 °C to about 120°C, or about 75 °C to 100 °C, or about 80 °C to 90 °C on exiting the expansion means. Accordingly, the gas stream, prior to entry into the scrubbing column, may be at a temperature of about 70 °C to about 120°C, or about 75 °C to 100 °C, or about 80 °C to 90 °C. The expansion means may be used to generate mechanical power that can be used elsewhere in the manufacturing plant. Scrubbing column

The gas stream is typically passed into a lower region of the scrubbing column at one or more inlets in this region and the scrubbed gas stream is withdrawn from a vent located in an upper region of the scrubbing column. The gas stream is preferably delivered into the scrubbing column by a pipe that comprises a distal portion that extends through the wall of the scrubbing column into the scrubbing column itself in order that direct contact of the gas stream with the wall of the scrubbing column is minimised. This reduces heating of the wall of the scrubbing column by the gas stream and thus reduces the risk of corrosion of the wall of the scrubbing column. The walls of the scrubbing column, and the pipe, may be made from any suitable material, e.g. stainless steel. The distal portion may be made from the same material as the main body of the pipe or may be made from a higher- grade material (e.g. a material with higher corrosion resistance). The distal portion may be unitary with the main body of the pipe. Alternatively, the distal portion, or a component of the distal portion, may be detachable from the main body of the pipe, allowing its removal and replacement if required. The distal portion may comprise an insulating material positioned to reduce the transfer of heat from the gas stream to the wall of the scrubbing column, thus also reducing the risk of corrosion of the wall of the scrubbing column. A shroud (e.g. a section of pipe of wider diameter than the distal portion) may extend from the wall of the scrubbing column and be configured to prevent liquid running down the walls of the scrubbing column from entering or contacting the distal portion of the pipe. This shroud helps prevent any liquid running down the walls of the scrubbing column from entering and/or cooling the distal portion of the pipe, and thus reduces the risk of corrosion within the pipe.

The gas stream may be irrigated with liquid phase water by spraying said liquid phase water from a distributing means (e.g. one or more spray nozzles) into said gas stream. A layer of liquid may be maintained in the scrubbing column above said effluent outlet, which is useful in protecting any pump used to withdraw the stream comprising water, said alkali metal ions and bromide ions from the effluent outlet. Accordingly, the gas stream may be introduced into the scrubbing column at one or more inlets located above the level of said layer of liquid. The gas stream may be irrigated with said liquid phase water such that said liquid phase water becomes entrained into said gas stream. In one embodiment, the gas stream is introduced into the scrubbing column below the distributing means for spraying said liquid phase water into the gas stream. In another embodiment, the gas stream is introduced into the scrubbing column level with the distributing means for spraying said liquid phase water into the gas stream. Preferably, liquid phase water from the distributing means is directed into the gas stream such that liquid phase water from the distributing means becomes entrained into the gas stream. This minimises the risk of the liquid phase water entering the pipe used to deliver the gas stream into the scrubbing column and also cools the gas stream, limiting heating of the wall of the scrubbing column opposite the inlet(s) for the gas stream and thus minimising the risk of corrosion of this wall. It also necessary to cool the gas stream prior to the gas entering the packed bed to achieve effective scrubbing. The distributing means is preferably configured to spray said liquid phase water away from the inlet(s) for the gas stream, e.g. along the axis of the gas stream (towards the wall of the scrubbing column opposite the inlet(s) for the gas stream). Accordingly, the distributing means may comprise one or more spray nozzles that may be directed along the axis of the gas stream (in the direction of flow of the gas stream).

The water that is fed to the distributing means is preferably process water, i.e. water that has been recovered elsewhere in a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, rather than "raw" water, which typically comprises chloride ions that may cause stress corrosion cracking of stainless steel components of the scrubbing column at elevated temperature. Process water typically comprises minor amounts of some of the organic components mentioned above, e.g. acetic acid, methyl acetate and methanol. The stream withdrawn from the effluent outlet comprises water, said alkali metal ions and bromide ions and may further comprise components such as carbonates and bicarbonates of the alkali metal formed within the scrubbing column. A portion of this stream is typically sent to an effluent treatment system. A further portion of this stream is typically recycled to a higher region of the scrubbing column. Thus, a portion of the stream withdrawn from the effluent outlet may form the wash stream (either alone or after being combined with one or more wash feed streams comprising said alkali metal ions).

The gas stream may be irrigated with said liquid phase water simultaneously with irrigation of said gas stream with said wash stream comprising alkali metal ions. This may be accomplished by said wash stream being introduced into the scrubbing column via one or more inlet points located above the location of said distributing means such that said wash stream passes from said inlet points towards the lower region of the scrubbing column. The wash stream may be introduced into the scrubbing column above a packed bed (e.g. a bed of random packing) included to maximise mass transfer between gaseous and liquid phases within the scrubbing column. A deflector baffle (e.g. a deflector ring) may be positioned below the packed bed and may be configured such that a portion of the liquid (e.g. the wash stream) exiting the bottom of the packed bed is directed onto the internal walls of the scrubbing column by the deflector baffle, thereby ensuring that the walls of the scrubbing column are kept wet and cool and reducing the risk of corrosion of the walls of the scrubbing column. In addition, the liquid phase water that is sprayed from the distributing means may comprise said alkali metal ions, which may neutralise any acid (e.g. acetic acid) present in the liquid phase water and reduce or prevent frothing within the scrubbing column when this water is introduced into it. The irrigation of the gas stream with liquid phase water and the wash stream preferably saturates the gas stream with water. The alkali metal ions may be selected from lithium, sodium and potassium ions, with sodium ions being preferred. Accordingly, the wash stream (and one or more wash feed streams) typically comprises sodium hydroxide. Preferably, the wash stream (and one or more wash feed streams) also comprises sodium formate, which has been found to improve the efficiency of Br2 removal. In addition, as the stream withdrawn from the effluent outlet may form the wash stream, the wash stream may further comprise sodium bicarbonate and/or sodium carbonate.

The invention will be further described with reference to the figures. Figure 1 is a schematic of a process and apparatus according to an embodiment of the present invention. Gas stream 10a, which is preferably an off-gas stream derived from the catalytic oxidation of a hydrocarbon precursor to an aromatic dicarboxylic acid (preferably terephthalic acid) in an organic solvent and comprises bromine and bromides (e.g. hydrogen bromide), is introduced via an inlet into scrubbing column 10. Distribution means 20, which comprises a series of spray nozzles, sprays liquid phase water stream 10b into gas stream 10a within scrubbing column 10. Effluent stream 10c is withdrawn from an outlet below the level of liquid 12 in scrubbing column 10 to pump 40. Wash stream 40a is introduced into scrubbing column 10 above packing 30. Wash feed stream 40c, which comprises alkali metal ions, is combined with wash stream 40a. Alternatively or additionally, a wash feed stream may be combined with liquid phase water stream 10b. Effluent stream 40b is withdrawn for further treatment. Scrubbed gas stream 10d is vented to the atmosphere.

Figure 2 is a schematic of a process and apparatus according to an alternative embodiment of the present invention. Gas stream 1 10a, which is preferably an off-gas stream derived from the catalytic oxidation of a hydrocarbon precursor to an aromatic dicarboxylic acid (preferably terephthalic acid) in an organic solvent and comprises bromine and bromides (e.g. hydrogen bromide), is introduced via an inlet into scrubbing column 1 10. Distribution means 120, which comprises a series of spray nozzles, sprays liquid phase water stream 1 10b into gas stream 1 10a within scrubbing column 1 10. The spray nozzles are preferably directed towards the wall of scrubbing column 1 10 opposite the inlet via which gas stream 1 10a enters scrubbing column 1 10. Effluent stream 1 10c is withdrawn from an outlet below the level of liquid 1 12 in scrubbing column 1 10 to pump 140. Wash stream 140a is introduced into scrubbing column 1 10 above packing 130. Wash feed stream 140c, which comprises alkali metal ions, is combined with wash stream 140a. Alternatively or additionally, a wash feed stream may be combined with liquid phase water stream 1 10b. Effluent stream 140b is withdrawn for further treatment. Scrubbed gas stream 1 10d is vented to the atmosphere.