Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
PRODUCTION OF AN AROMATIC DICARBOXYLIC ACID
Document Type and Number:
WIPO Patent Application WO/2016/055456
Kind Code:
A2
Abstract:
The present invention provides a method for raising steam in a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising the steps of: i) heating a crude aromatic dicarboxylic acid slurry to form a crude aromatic dicarboxylic acid solution; ii) transferring the crude aromatic dicarboxylic acid solution to a hydrogenation reactor; iii) transferring a purified aromatic dicarboxylic acid solution from the hydrogenation reactor to a series of one or more crystallisers; and iv) transferring heat from a crystalliser vent stream from the series of one or more crystallisers to a water stream in a steam raiser to generate steam. The present invention further provides an apparatus for carrying out the method, and a process for the production of an aromatic dicarboxylic acid incorporating the method.

Inventors:
LIMBACH ANTONY PETER JOHN (GB)
CHANDRA AKHILESH (GB)
MATHERS LEWIS ROBERT (GB)
GUPTA ANUJ (GB)
Application Number:
EP2015/073021
Publication Date:
April 14, 2016
Filing Date:
October 06, 2015
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
INVISTA TECHNOLOGIES S À R L (CH)
International Classes:
C07C51/265; F24V30/00
Attorney, Agent or Firm:
COCKERTON, Bruce Roger et al. (One Southampton Row, London WC1B 5HA, GB)
Download PDF:
Claims:
CLAIMS

1 . A method for raising steam in a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising the steps of:

i) heating a crude aromatic dicarboxylic acid slurry to form a crude aromatic dicarboxylic acid solution;

ii) transferring the crude aromatic dicarboxylic acid solution to a hydrogenation reactor;

iii) transferring a purified aromatic dicarboxylic acid solution from the hydrogenation reactor to a series of one or more crystallisers; and

iv) transferring heat from a crystalliser vent stream from the series of one or more crystallisers to a water stream in a steam raiser to generate steam.

2. The method of claim 1 , wherein the steam generated in step iv) is medium-pressure steam or intermediate-pressure steam.

3. The method of claim 2, wherein the steam generated in step iv) is intermediate-pressure steam.

4. The method of claim 2 or claim 3, wherein the intermediate-pressure steam is at a pressure of about 10-50 BarA, or about 12.5-40 barA, or about 15-35 BarA, or about 17.5-30 barA, or about

20 BarA.

5. The method of any one of claims 1 -4, wherein step i) comprises the step of:

v) transferring heat from a high-pressure steam to the crude aromatic dicarboxylic acid slurry in a first slurry heat exchanger.

6. The method of claim 5, wherein the high-pressure steam is at a pressure of about 50-150 BarA, or about 75-125 BarA, or about 100 BarA. 7. The method of claim 5 or claim 6, wherein a high-pressure condensate is generated in step v) and wherein step i) further comprises the step of:

vi) transferring heat from the high-pressure condensate to the crude aromatic dicarboxylic acid slurry in a second slurry heat exchanger positioned upstream of the first slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry.

8. The method of claim 7, wherein the crude aromatic dicarboxylic acid slurry is heated to at least 250 °C upstream of the second slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry. 9. The method of claim 8, wherein the crude aromatic dicarboxylic acid slurry is heated to at least 250 °C upstream of the second slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry without directly transferring heat from high-pressure steam or high-pressure condensate to the crude aromatic dicarboxylic acid slurry. 10. The method of any one of claims 7-9, wherein step i) further comprises the step of:

vii) transferring heat from a crystalliser vent stream from the series of one or more crystallisers to the crude aromatic dicarboxylic acid slurry in a third slurry heat exchanger positioned upstream of the second slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry.

1 1 . The method of claim 10, wherein step iii) comprises transferring the purified aromatic dicarboxylic acid solution from the hydrogenation reactor to a series of two or more crystallisers.

12. The method of claim 1 1 , wherein step vii) comprises transferring heat from a crystalliser vent stream from the first crystalliser in the series to the crude aromatic dicarboxylic acid slurry in the third slurry heat exchanger.

13. The method of claim 12, wherein step iv) comprises transferring heat from a crystalliser vent stream from the second crystalliser in the series or a subsequent crystalliser in the series to the water stream in the steam raiser to generate steam.

14. The method of claim 13, wherein step iv) comprises transferring heat from a crystalliser vent stream from the second crystalliser in the series to the water stream in the steam raiser to generate steam.

15. The method of claim 14, wherein step i) further comprises the step of

viii) transferring heat from a crystalliser vent stream from the second crystalliser in the series to the crude aromatic dicarboxylic acid slurry in a fourth slurry heat exchanger positioned upstream of the third slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry.

16. A method for heating a gas stream in a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising the steps of:

I) raising steam by the method of any one of claims 1 -15; and

II) transferring heat from the steam to the gas stream in a first gas heat exchanger.

17. The method of claim 16, wherein the gas stream is a pressurised scrubber vent gas stream.

18. The method of claim 17 further comprising the step of:

III) transferring heat to the pressurised scrubber vent gas stream in a second gas heat exchanger positioned upstream of the first gas heat exchanger relative to the direction of flow of the pressurised scrubber vent gas stream.

19. The method of claim 18 further comprising the steps of:

IV) transferring the pressurised scrubber vent gas stream from the first gas heat exchanger to a third gas heat exchanger;

V) transferring the pressurised scrubber vent gas stream from the third gas heat exchanger to a catalytic combustor; and

VI) transferring a catalytic combustor exit gas stream to the third gas heat exchanger such that heat is transferred from the catalytic combustor exit gas stream to the pressurised scrubber vent gas stream.

20. The method of any preceding claim, wherein the aromatic dicarboxylic acid is terephthalic acid. 21 . An apparatus for raising steam in a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising: a) a first slurry heat exchanger configured to receive a crude aromatic dicarboxylic acid slurry; b) a hydrogenation reactor configured to receive a crude aromatic dicarboxylic acid solution from the first slurry heat exchanger;

c) a series of one or more crystallisers configured to receive a purified aromatic dicarboxylic acid solution from the hydrogenation reactor; and

d) a steam raiser configured to transfer heat from a crystalliser vent stream from the series of one or more crystallisers to a water stream to generate steam. 22. The apparatus of claim 21 , wherein the steam generated by steam raiser d) is medium-pressure steam or intermediate-pressure steam.

23. The apparatus of claim 22, wherein the steam generated by steam raiser d) is intermediate- pressure steam. 24. The apparatus of claim 22 or claim 23, wherein the intermediate-pressure steam is at a pressure of about 10-50 BarA, or about 12.5-40 barA, or about 15-35 BarA, or about 17.5-30 barA, or about 20 BarA.

25. The apparatus of any one of claims 21 -24, wherein the first slurry heat exchanger is configured to transfer heat from a high-pressure steam to the crude aromatic dicarboxylic acid slurry.

26. The apparatus of claim 25, wherein the high-pressure steam is at a pressure of about 50-150 BarA, or about 75-125 BarA, or about 100 BarA. 27. The apparatus of claim 25 or claim 26 further comprising:

e) a second slurry heat exchanger configured to transfer heat from a high-pressure condensate generated from the high-pressure steam to the crude aromatic dicarboxylic acid slurry and positioned upstream of the first slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry.

28. The apparatus of claim 26, wherein the apparatus is configured to heat the crude aromatic dicarboxylic acid slurry to at least 250 °C upstream of the second slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry. 29. The apparatus of claim 28, wherein the apparatus is configured to heat the crude aromatic dicarboxylic acid slurry to at least 250 °C upstream of the second slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry without directly transferring heat from high-pressure steam or high-pressure condensate to the crude aromatic dicarboxylic acid slurry.

30. The apparatus of any one of claims 27-29 further comprising:

f) a third slurry heat exchanger configured to transfer heat from a crystalliser vent stream from the series of one or more crystallisers to the crude aromatic dicarboxylic acid slurry and positioned upstream of the second slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry.

31 . The apparatus of claim 30, wherein the apparatus comprises a series of two or more crystallisers.

32. The apparatus of claim 31 , wherein the third slurry heat exchanger is configured to transfer heat from a crystalliser vent stream from the first crystalliser in the series to the crude aromatic dicarboxylic acid slurry.

33. The apparatus of claim 32, wherein the steam raiser is configured to transfer heat from a crystalliser vent stream from the second crystalliser in the series or a subsequent crystalliser in the series to the water stream to generate steam.

34. The apparatus of claim 33, wherein the steam raiser is configured to transfer heat from a crystalliser vent stream from the second crystalliser in the series to the water stream to generate steam.

35. The apparatus of claim 34 further comprising

g) a fourth slurry heat exchanger configured to transfer heat from a crystalliser vent stream from the second crystalliser in the series to the crude aromatic dicarboxylic acid slurry and positioned upstream of the third slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry.

36. An apparatus for heating a gas stream in a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising:

A) an apparatus for raising steam according to any one of claims 21 -35; and

B) a first gas heat exchanger configured to transfer heat from the steam to the gas stream.

37. The apparatus of claim 36, wherein the gas stream is a pressurised scrubber vent gas stream.

38. The apparatus of claim 37 further comprising:

C) a second gas heat exchanger configured to transfer heat to the gas stream and positioned upstream of the first gas heat exchanger relative to the direction of flow of the gas stream.

39. The apparatus of claim 38 further comprising

D) a third gas heat exchanger configured to receive the pressurised scrubber vent gas stream from the first gas heat exchanger; and E) a catalytic combustor configured to receive the pressurised scrubber vent gas stream from the third gas heat exchanger,

wherein the third gas heat exchanger is further configured to receive a catalytic combustor exit gas stream and transfer heat from the catalytic combustor exit gas stream to the pressurised scrubber vent gas stream.

40. The apparatus of any one of claims 21 -39, wherein the aromatic dicarboxylic acid is terephthalic acid.

41 . A process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising the steps of:

i) oxidising the hydrocarbon precursor in the organic solvent in the presence of a metal catalyst to provide a crude aromatic dicarboxylic acid slurry;

wherein the process further comprises the steps of:

ii) heating the crude aromatic dicarboxylic acid slurry to form a crude aromatic dicarboxylic acid solution;

iii) transferring the crude aromatic dicarboxylic acid solution to a hydrogenation reactor;

iv) transferring a purified aromatic dicarboxylic acid solution from the hydrogenation reactor to a series of one or more crystallisers; and

v) transferring heat from a crystalliser vent stream from the series of one or more crystallisers to a water stream in a steam raiser to generate steam.

42. The process of claim 41 , further comprising the step of:

vi) transferring heat from the steam to a gas stream in a first gas heat exchanger.

Description:
PRODUCTION OF AN AROMATIC DICARBOXYLIC ACID

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).

Heat is typically introduced into an aromatic dicarboxylic acid manufacturing plant and transferred within it using steam as a carrier. Specifically, high-pressure steam at a temperature of, for example, about 300°C may be generated outside battery limits by burning heating fuel to heat water (e.g. in a fossil fuel-fired boiler) and directed towards one or more heat exchangers to transfer heat to process streams with the highest temperature demands. The resulting high-pressure condensate may be flashed to produce intermediate-pressure steam at a temperature of, for example, about 220°C that may then be used to heat process streams with lower heat demands, forming an intermediate- pressure condensate that may be flashed to produce a medium-pressure steam and so on until the energy of the steam is spent. The heating fuel required to generate high-pressure steam constitutes a significant variable cost of the aromatic dicarboxylic acid manufacturing process. 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 minimise the high-pressure steam requirement for an aromatic dicarboxylic acid manufacturing process. Further objects will be apparent from the description below.

DISCLOSURE OF THE INVENTION

This object may be achieved by raising lower-pressure steams (e.g. intermediate-pressure steam) within the process rather than by deriving these steams directly from high-pressure steam. A first aspect of the invention therefore provides a method for raising steam in a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising the steps of:

i) heating a crude aromatic dicarboxylic acid slurry to form a crude aromatic dicarboxylic acid solution;

ii) transferring the crude aromatic dicarboxylic acid solution to a hydrogenation reactor;

iii) transferring a purified aromatic dicarboxylic acid solution from the hydrogenation reactor to a series of one or more crystallisers; and

iv) transferring heat from a crystalliser vent stream from the series of one or more crystallisers to a water stream in a steam raiser to generate steam.

The steam that is raised is typically saturated steam. Preferably, the steam generated in step iv) is medium-pressure steam or intermediate-pressure steam. More preferably, the steam generated in step iv) is intermediate-pressure steam.

As used herein, "high-pressure steam" refers to saturated steam at a pressure of about 50-150 barA, or about 75-125 barA, or about 100 barA. Similarly, "high-pressure condensate" refers to condensate at a pressure of about 50-150 barA, or about 75-125 barA, or about 100 barA. As used herein, "intermediate-pressure steam" refers to saturated steam at pressure of about 10-50 barA, or about 12.5-40 barA, or about 15-35 barA, or about 17.5-30 barA, or about 20 barA. As used herein, "medium-pressure steam" refers to saturated steam at a pressure of about 5-10 barA, or about 6- 9 barA, or about 8 barA. As used herein, "low-pressure steam" refers to saturated steam at a pressure of less than about 5 barA.

The crude aromatic dicarboxylic acid slurry may be heated using high-pressure steam. Accordingly, step i) may comprise the step of v) transferring heat from a high-pressure steam to the crude aromatic dicarboxylic acid slurry in a first slurry heat exchanger. A high-pressure condensate may be generated in step v). The high-pressure condensate is typically at substantially the same temperature as the high-pressure steam, e.g. the high-pressure condensate may be at a temperature less than 10 °C, or less than 5 °C, lower than the high-pressure steam. For instance, if the high- pressure steam is at a temperature of 290-320 °C, the high-pressure condensate may be at a temperature of 280-310°C, or 285-315 °C. Step i) may thus further comprise the step of vi) transferring heat from the high-pressure condensate to the crude aromatic dicarboxylic acid slurry in a second slurry heat exchanger positioned upstream of the first slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry. The crude aromatic dicarboxylic acid slurry may be heated to at least 250 °C upstream of the second slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry, preferably without directly transferring heat from high-pressure steam or high-pressure condensate to the crude aromatic dicarboxylic acid slurry. Thus, the crude aromatic dicarboxylic acid slurry may be heated to a high temperature before heat from the high-pressure condensate and high-pressure steam is applied to the crude aromatic dicarboxylic acid slurry. The high-pressure condensate and high-pressure steam then provide the final increase in temperature required before the stream arrives at the hydrogenation reactor. Therefore, the present invention uses high-pressure steam in a more economic, efficient and advantageous manner in comparison to previous manufacturing processes and plants.

This aspect of the invention further provides an apparatus for raising steam in a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising:

a) a first slurry heat exchanger configured to receive a crude aromatic dicarboxylic acid slurry; b) a hydrogenation reactor configured to receive a crude aromatic dicarboxylic acid solution from the first slurry heat exchanger;

c) a series of one or more crystallisers configured to receive a purified aromatic dicarboxylic acid solution from the hydrogenation reactor; and

d) a steam raiser configured to transfer heat from a crystalliser vent stream from the series of one or more crystallisers to a water stream to generate steam.

The steam that is raised is typically saturated steam. Preferably, the steam generated by steam raiser d) is medium-pressure steam or intermediate-pressure steam. More preferably, the steam generated by steam raiser d) is intermediate-pressure steam.

The crude aromatic dicarboxylic acid slurry may be heated using high-pressure steam. Accordingly, the first slurry heat exchanger may be configured to transfer heat from a high-pressure steam to the crude aromatic dicarboxylic acid slurry. The apparatus may further comprise e) a second slurry heat exchanger configured to transfer heat from a high-pressure condensate generated from the high- pressure steam to the crude aromatic dicarboxylic acid slurry and positioned upstream of the first slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry. The apparatus may be configured to heat the crude aromatic dicarboxylic acid slurry to at least 250 °C upstream of the second slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry, preferably without directly transferring heat from high-pressure steam or high-pressure condensate to the crude aromatic dicarboxylic acid slurry. Thus, the crude aromatic dicarboxylic acid slurry may be heated to a high temperature before heat from the high-pressure condensate and high-pressure steam is applied to the crude aromatic dicarboxylic acid slurry. The high-pressure condensate and high-pressure steam then provide the final increase in temperature required before the stream arrives at the hydrogenation reactor.

The inventors have found that, surprisingly, heat from a crystalliser vent stream can be used to raise steam, particularly intermediate-pressure steam, without increasing the high-pressure steam requirement elsewhere in the process, e.g. no additional high-pressure steam (as compared to a process in which the crystalliser vent stream is not used to raise steam) is required to heat the crude aromatic dicarboxylic acid slurry. The present invention therefore provides a new source of steam, particularly intermediate-pressure steam, that may be used in place of steam derived from high- pressure steam, thus using high-pressure steam in a more economic, efficient and advantageous manner in comparison to previous manufacturing processes and plants and reducing the overall high-pressure steam requirement of the process, which in turn reduces the heating requirements and therefore reduces variable costs. Accordingly, a second aspect of the invention provides a method for heating a gas stream in a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising the steps of:

I) raising steam by the method according to the first aspect of the invention; and

II) transferring heat from the steam to the gas stream in a first gas heat exchanger.

This aspect of the invention further provides an apparatus for heating a gas stream in a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising:

A) an apparatus for raising steam according to the first aspect of the invention; and

B) a first gas heat exchanger configured to transfer heat from the steam to the gas stream. The gas stream may be a pressurised scrubber vent gas stream. It is necessary to heat this gas stream prior to passing it to a catalytic combustor in which organic compounds are removed from the gas stream. The present invention therefore provides a means for heating this stream without using high-pressure steam and without increasing the high-pressure steam requirement elsewhere in the process, thus reducing the overall high-pressure steam requirement of the process.

A third aspect of the invention provides a process for the production of an aromatic dicarboxylic acid comprising the catalytic oxidation of a hydrocarbon precursor in an organic solvent, comprising the steps of:

i) oxidising the hydrocarbon precursor in the organic solvent in the presence of a metal catalyst to provide a crude aromatic dicarboxylic acid slurry;

wherein the process further comprises the steps of:

ii) heating the crude aromatic dicarboxylic acid slurry to form a crude aromatic dicarboxylic acid solution;

iii) transferring the crude aromatic dicarboxylic acid solution to a hydrogenation reactor;

iv) transferring a purified aromatic dicarboxylic acid solution from the hydrogenation reactor to a series of one or more crystallisers; and

v) transferring heat from a crystalliser vent stream from the series of one or more crystallisers to a water stream in a steam raiser to generate steam.

The process may further comprise the step of transferring heat from the steam to a gas stream in a first gas heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 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 vent 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.

Crude aromatic dicarboxylic acid slurry

As described above, the purification stage of a typical aromatic dicarboxylic acid manufacturing process is carried out on a slurry of crude aromatic dicarboxylic acid crystals in an aqueous liquid. Accordingly, in such a process, the second slurry of crude aromatic dicarboxylic acid crystals described above forms the crude aromatic dicarboxylic acid slurry used in the present invention. This slurry typically comprises, in addition to the aromatic dicarboxylic acid, reaction by-products (e.g. p-toluic acid, benzoic acid and 4-CBA in TA production) as well as derivatives of the organic solvent (e.g. methyl acetate from acetic acid) and small amounts of the reaction catalyst. The crude aromatic dicarboxylic acid slurry is heated to form a crude aromatic dicarboxylic acid solution that is subjected to a hydrogenation reaction in a hydrogenation reactor, typically over a fixed bed catalyst of palladium supported on carbon, at high temperature (e.g. 275-290 °C) and pressure (e.g. 70-90 barA) to convert some of the impurities to forms that are more easily removed from the purified aromatic dicarboxylic acid crystals (e.g. 4-CBA is converted to p-toluic acid).

Heating the slurry

The crude aromatic dicarboxylic acid slurry is heated to dissolve the reaction by-products prior to hydrogenation of the resulting crude aromatic dicarboxylic acid solution, typically up to a temperature of about 280 °C or greater. This may be accomplished by transferring heat from a series of streams of progressively higher temperature to the slurry in a series of heat exchangers (e.g. shell and tube heat exchangers). Accordingly, as the crude aromatic dicarboxylic acid slurry is heated by the series of streams of progressively higher temperature, solids in the crude aromatic dicarboxylic acid slurry are dissolved such that a crude aromatic dicarboxylic acid solution is ultimately formed. The first heat exchanger (counting backwards from the hydrogenation reactor) applies the final heating step prior to hydrogenation and this is typically carried out using a source of high-pressure steam at a temperature of, for example, about 300°C, which is generated outside battery limits. Thus, heat is transferred from the high-pressure steam to the crude aromatic dicarboxylic acid slurry in the first slurry heat exchanger. As the generation of this high-pressure steam constitutes a significant variable cost of the aromatic dicarboxylic acid manufacturing process, it is desirable both to minimise the high-pressure steam requirement of the process and to maximise the use of the heat energy from this steam. Accordingly, a high-pressure condensate generated in this first heat transfer step may be passed to a second heat exchanger (positioned upstream of the first slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry) in which heat is transferred from the high-pressure condensate to the crude aromatic dicarboxylic acid slurry. The high-pressure steam requirement of the heating step may, in part, be reduced by pre-heating the crude aromatic dicarboxylic acid slurry prior to its arrival at the second heat exchanger. Therefore, the crude aromatic dicarboxylic acid slurry may be heated to at least 220 °C, or at least 230 °C, or at least 240 °C, or, preferably, at least 250 °C upstream of the second slurry heat exchanger relative to the direction of flow of the crude aromatic dicarboxylic acid slurry, preferably without directly transferring heat from high-pressure steam or high-pressure condensate to the crude aromatic dicarboxylic acid slurry. This pre-heating may be carried out by transferring heat from one or more crystalliser vent streams from the series of one or more crystallisers to the crude aromatic dicarboxylic acid slurry in one or more additional heat exchangers. Accordingly, the step of heating the crude aromatic dicarboxylic acid slurry may further comprise transferring heat from a crystalliser vent stream from the series of one or more crystallisers to the crude aromatic dicarboxylic acid slurry in a third slurry heat exchanger positioned upstream of the second slurry heat exchanger (relative to the direction of flow of the crude aromatic dicarboxylic acid slurry).

The present invention may comprise transferring the purified aromatic dicarboxylic acid solution from the hydrogenation reactor to a series of two or more crystallisers, or a series of three or more crystallisers, or a series of four or more crystallisers, or a series of five or more crystallisers.

The "first" crystalliser in the series is the crystalliser that receives purified aromatic dicarboxylic acid solution from the hydrogenation reactor. Thus, heat may be transferred from a crystalliser vent stream from the first crystalliser in the series to the crude aromatic dicarboxylic acid slurry in the third slurry heat exchanger. Alternatively or additionally, heat may be transferred from a crystalliser vent stream from the second crystalliser in the series to the crude aromatic dicarboxylic acid slurry in the third slurry heat exchanger. The step of heating the crude aromatic dicarboxylic acid slurry may further comprise transferring heat from a crystalliser vent stream from the series of one or more crystallisers to the crude aromatic dicarboxylic acid slurry in a fourth slurry heat exchanger positioned upstream of the third slurry heat exchanger (relative to the direction of flow of the crude aromatic dicarboxylic acid slurry). Specifically, heat may be transferred from a crystalliser vent stream from the second crystalliser in the series to the crude aromatic dicarboxylic acid slurry in the fourth slurry heat exchanger. A single vent stream may be taken from each of the crystallisers mentioned above and passed to the heat exchangers mentioned above. Alternatively, a plurality of vent streams may be taken from each of the crystallisers mentioned above (either by employing crystallisers having a plurality of vents, or by dividing a stream from a single vent in each crystalliser) and one of the plurality of vent streams passed to the heat exchangers mentioned above and the other(s) used elsewhere in the process (e.g. to raise steam).

Steam raiser

The steam raiser is a heat exchanger, preferably a shell-and-tube-type heat exchanger (e.g. a kettle- type heat exchanger), in which heat is transferred from a crystalliser vent stream to a water stream (e.g. a boiler feed water stream) to raise steam, preferably intermediate-pressure steam. The water stream is typically pumped under pressure to the steam raiser and may also be pre-heated prior to being transferred to the steam raiser. Although heat from a crystalliser vent stream from the first crystalliser in the series may be transferred to the water stream in the steam raiser, this crystalliser vent stream is typically hotter than is required to generate intermediate-pressure steam and it is therefore more efficient to use this crystalliser vent stream to heat the crude aromatic dicarboxylic acid slurry. Accordingly, it is preferred that heat is transferred from a crystalliser vent stream from the second crystalliser in the series or a subsequent crystalliser in the series to the water stream in the steam raiser to generate steam. In particular, it is preferred that heat is transferred from a crystalliser vent stream from the second crystalliser in the series to the water stream in the steam raiser to generate steam. Although cooler than the crystalliser vent stream from the first crystalliser in the series, the crystalliser vent stream from the second crystalliser in the series is at a sufficiently high temperature to generate intermediate-pressure steam. It is further preferred if heat is transferred from a crystalliser vent stream from the second crystalliser in the series to the crude aromatic dicarboxylic acid slurry in the fourth slurry heat exchanger and heat is transferred from a crystalliser vent stream from the second crystalliser in the series to the water stream in the steam raiser to generate steam. This is useful in ensuring that the heating of crude aromatic dicarboxylic acid slurry is not comprised by the generation of steam, i.e. that generation of steam does not result in additional high-pressure steam being required to heat the crude aromatic dicarboxylic acid slurry.

Heating the gas stream

The steam (e.g. intermediate-pressure steam) raised in the first aspect of the invention may be used to heat other streams in a process or apparatus for the production of an aromatic dicarboxylic acid, thereby reducing the use of other steams, in particular valuable high-pressure steam. The second aspect of the invention therefore uses this steam by transferring heat from the steam to a gas stream in a first gas heat exchanger.

The steam, particularly intermediate-pressure steam, has been found to be particularly useful for heating a pressurised scrubber vent gas stream. The pressurised scrubber is one of a series of stages used to treat the overhead gas from the condensing stage before it is released to the atmosphere. This device scrubs the overhead gas first with organic solvent (e.g. acetic acid) and then with water, thus reducing the levels of materials such as unreacted hydrocarbon precursor (e.g. p-xylene) and organic solvent derivatives (e.g. methyl acetate) in the gas. The pressurised scrubber vent gas stream may then be subjected to further treatments. For instance, it may be treated in a catalytic combustor to remove carbon monoxide along with any remaining organic components and the resultant catalytic combustor exit gas may be further treated in an offgas scrubber before being released to the atmosphere. It is desirable to heat the pressurised scrubber vent gas stream before subjecting it to further treatments, such as catalytic combustion, to increase the effectiveness of these treatments. The pressurised scrubber vent gas stream may be pre-heated (e.g. by transferring heat to it from a low- pressure or medium-pressure steam) in a second gas heat exchanger positioned upstream of the first gas heat exchanger relative to the direction of flow of the pressurised scrubber vent gas stream. Alternatively or additionally, the pressurised scrubber vent gas stream may be heated by transferring heat to it in a third gas heat exchanger positioned downstream of the first gas heat exchanger relative to the direction of flow of the pressurised scrubber vent gas stream. Thus, the pressurised scrubber vent gas stream may be transferred from the first gas heat exchanger to the third gas heat exchanger and then may be transferred from the third gas heat exchanger to a catalytic combustor. A catalytic combustor exit gas stream from the catalytic combustor may be transferred to the third gas heat exchanger such that heat is transferred from the catalytic combustor exit gas stream to the pressurised scrubber vent gas stream.

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

Figure 1 is a schematic of a process and apparatus according to a preferred embodiment of the present invention. Heating stage 10 comprises first heat exchanger 12, second heat exchanger 14, third heat exchanger 16, and fourth heat exchanger 18. First heat exchanger 12 is supplied with high-pressure steam feed 12a at a pressure of about 100 barA, which is generated in a fossil fuel- fired boiler, to heat crude aromatic dicarboxylic acid (preferably terephthalic acid) slurry stream 14a. High-pressure condensate stream 12b is passed to second heat exchanger 14 to heat crude aromatic dicarboxylic acid slurry stream 16a. Crystalliser vent stream 30a from first crystalliser 30 is fed to third heat exchanger 16 to heat crude aromatic dicarboxylic acid slurry stream 18a. Crystalliser vent stream 40a from second crystalliser 40 is fed to fourth heat exchanger 18 to heat crude aromatic dicarboxylic acid slurry stream 10a. Thus, crude aromatic dicarboxylic acid slurry stream 10a is fed to heating stage 10 and heated sequentially in fourth heat exchanger 18, third heat exchanger 16, second heat exchanger 14, and first heat exchanger 12 to dissolve its solid components and form crude aromatic dicarboxylic acid solution stream 10b. The heating is such that crude aromatic dicarboxylic acid slurry stream 16a is at a temperature of at least 250 °C when it reaches second heat exchanger 14.

Crude aromatic dicarboxylic acid solution stream 10b is fed to hydrogenation reactor 20, in which it is subjected to a hydrogenation reaction over a fixed bed catalyst of palladium supported on carbon at a temperature of 275-290 °C and a pressure of 70-90 barA. Purified aromatic dicarboxylic acid solution stream 20a is fed to first crystalliser 30 to form purified aromatic dicarboxylic acid slurry stream 30b, which is fed to second crystalliser 40. Purified aromatic dicarboxylic acid slurry stream 40b is removed for further processing to recover purified aromatic dicarboxylic acid crystals.

Crystalliser vent steam 40c from second crystalliser 40 is fed to steam raiser 50 to heat boiler feed water stream 50a. Intermediate-pressure steam feed 50b, which is at a pressure of about 22 barA, is fed to first gas heat exchanger 60 to heat pressurised scrubber vent gas stream 70d. Pressurised scrubber vent gas stream 70a is heated in second gas heat exchanger 70 by low-pressure steam feed 70b, and pressurised scrubber vent gas stream 70d is passed to first gas heat exchanger 60. Pressurised scrubber vent gas stream 60a is fed to third gas heat exchanger 80. Pressurised scrubber vent gas stream 80a is fed to catalytic combustor 90 to produce catalytic combustor exit gas stream 90a, which is fed back to third gas heat exchanger 80 to heat pressurised scrubber vent gas stream 60a. Catalytic combustor exit gas stream 80b is passed through an offgas expander for power recovery and then discharged to an offgas scrubber before being released to the atmosphere.

Spent steams 16b, 18b, and 50c are collected and recycled back to the process plant. Spent steams/condensates 14b, 60b and 70c are fed to a steam recovery system for further use.