KR100787767B1 - Improved Process for Producing Carboxylic Acids - Google Patents

Abstract

The reactants are fed to a high pressure and high solvent ratio of the first zone of oxidation, where the absorption of oxygen is limited to a value of less than 50% of the oxygen required to completely convert the precursor to its corresponding carboxylic acid, and then the reaction obtained An improved process for preparing carboxylic acids or their esters by catalytic liquid phase oxidation of the corresponding precursors in a suitable solvent, comprising feeding the medium to the second oxidation reaction zone.

Oxidation zone, carboxylic acid, catalytic liquid phase oxidation

Description

Improved Process for Producing Carboxylic Acids

Cross Reference to Related Application

This application claims the benefit of currently pending US patent application Ser. No. 09 / 481,811, filed Jan. 12, 2000 and US patent application Ser. No. 09 / 757,458, filed Jan. 10, 2001.

The present invention relates to an improved process for producing carboxylic acids or esters thereof by catalytic liquid phase oxidation of the corresponding precursors in a suitable solvent. More specifically, the present invention provides an oxidation reaction by supplying a reactant to a first reaction zone in which the absorption of temperature and oxygen at elevated pressure is controlled, the formed terephthalic acid is maintained in solution, and then the resulting reaction medium is supplied to a second reaction zone. An improved method of catalytic liquid phase oxidation of paraxylene for preparing terephthalic acid, comprising a continuous step of completing.

Practically all terephthalic acid is produced on a commercial scale by the catalytic, liquid, air oxidation of paraxylene. Conventional methods use acetic acid as a solvent and polyvalent heavy metals or metals as catalysts. Cobalt and manganese are the most widely used heavy metal catalysts, and bromine is used as regeneration source of free radicals in this process.

Acetic acid, air (molecular oxygen), paraxylene and catalyst are continuously fed into the oxidation reactor maintained at a temperature of 175-225 ° C. and 1000-3000 kPa (ie 10-30 atm). The feed acetic acid: paraxylene ratio is typically less than 5: 1. In order to minimize the formation of undesirable byproducts such as color formers, air is added in excess of the stoichiometric requirement for paraxylene to be fully converted to terephthalic acid. The oxidation reaction is exothermic and heat is removed by evaporating the acetic acid solvent. The corresponding vapor is condensed and most of the condensate is refluxed to the reactor where some condensate is recovered to adjust the reactor water concentration (2 moles of water are formed per mole of paraxylene reacted). The residence time is typically from 30 minutes to 2 hours depending on the method. Depending on the oxidation reactor operating conditions, for example temperature, catalyst concentration and residence time, significant decomposition of the solvent and precursor can occur, which in turn can increase the operating cost of the process.

The effluent from the oxidation reactor, ie the reaction product, is a slurry of crude terephthalic acid (TA) crystals, which is recovered from the slurry by filtration, washed, dried and stored for transport. They are then fed either in separate purification steps or directly by the polymerization process. Although p-tolualdehyde and p-toluic acid may be present with undesirable color formers, most impurities in the crude TA are 4-carboxybenzaldehyde (4-CBA), which is incompletely oxidized paraxylene. By carrying out the oxidation reaction according to the present invention as described in detail below, it is possible to substantially reduce the formation of impurities in the final TA product and to effectively control solvent and precursor decomposition.

Summary of the Invention

The present invention provides a continuous step of supplying reactants including a suitable solvent to a first reaction zone in which temperature and oxygen uptake are controlled at elevated pressure, the formed terephthalic acid is retained in solution, and then the resulting reaction medium is supplied to a second reaction zone. An improved method of catalytic liquid phase oxidation of paraxylene for producing terephthalic acid. The method,

(a) forming a feed stream comprising a solvent and an oxidation catalyst in the range of about 2,000 kPa to 10,000 kPa or more at elevated pressure;

(b) a feed stream, (2) paraxylene, and (3) a supply of oxygen continuously and simultaneously to the first reaction zone, such that the solvent: paraxylene mass ratio is in the range of 10-25: 1. To form;

(c) limiting oxygen uptake in the reaction medium in the first reaction zone to a value of less than 50% of the oxygen needed to completely convert paraxylene to terephthalic acid;

(d) reducing the pressure of the reaction medium to a value in the range of 1,200 kPa to 2,000 kPa while simultaneously supplying the reaction medium to the second reaction zone.

The resulting terephthalic acid can be recovered by any convenient means from the reaction medium exiting the second reaction zone.

Herein, the present invention is described in terms of an improved oxidation scheme for converting paraxylene to terephthalic acid, but by catalytic liquid phase oxidation of the corresponding precursor in a suitable solvent of carboxylic acids or esters thereof, in particular phthalic acid or esters thereof It is understood that the present invention can also be applied to producing a range. The present invention

(1) forming a feed stream comprising a solvent and an oxidation catalyst at a elevated pressure of about 2,000 kPa or higher, and then simultaneously supplying the feed stream, the precursor and the oxygen feed to the high pressure first reaction zone so that the solvent: precursor mass ratio is relatively high; That is, to form a reaction medium in the range of 10-25: 1 (although the preferred ratio for cost reduction and workability is 10-20: 1);

(2) supplying the reaction medium from the first reaction zone to the second reaction zone, wherein the oxidation reaction proceeds and is completed,

It is based on the discovery that by carrying out the oxidation reaction in at least two stages or zones, the conversion of the precursor to its corresponding carboxylic acid can be substantially improved.

In addition to maintaining the solvent: precursor mass ratio as described, the oxygen uptake in the first reaction zone may be reduced to the corresponding carboxylic acid or ester thereof, which may have one, two, three or more acid groups, depending on the precursor. Is limited to a value of less than 50% of the oxygen required to convert completely. Oxygen uptake in the first reaction zone is controlled by one or more of the following methods: (i) maintaining an oxygen supply (ie, oxygen concentration) within a predetermined range; (ii) maintain the catalyst concentration within a predetermined range, and (iii) the residence time in the first reaction zone (defined as the reactor liquid volume divided by the reactor feed rate) is less than about 6 minutes, preferably 4 Limited to less than minutes, and (iv) optionally, heat is removed from the reaction zone to maintain the temperature of the reaction medium at a value below about 210 ° C. when the reaction medium exits the first reaction zone.

At the same time as supplying the reaction medium to the second reactor, the pressure of the reaction medium is reduced to a value in the range of 1,200 kPa to 2,000 kPa. The resulting carboxylic acid can be recovered from the final reaction medium by conventional methods, which is typically a slurry of acid crystals.

According to a preferred embodiment of the present invention, oxygen is directly dissolved in a feed stream comprising a solvent and an oxidation catalyst, and then the oxidized feed stream is fed into the first oxidation reaction zone, ie, the plug flow reaction zone, continuously and simultaneously with the precursor. . Upon entering the first reaction zone, the precursor, for example paraxylene, is thoroughly mixed with the oxygenated solvent to initiate the reaction. By adjusting the oxygen supply, catalyst concentration, residence time in the first reaction zone and / or temperature in the first reaction zone, the reaction medium is at a value of less than 50% of the oxygen required for the precursor to be fully converted to the corresponding carboxylic acid. It is possible to control, i.e., limit oxygen absorption within. As described above, the reaction medium is fed to a second, more conventional reactor.

The process of the present invention is particularly applicable for preparing terephthalic acid by catalytic liquid phase oxidation of paraxylene in a solvent comprising acetic acid and water.

1 is a simplified schematic diagram of a method of the invention according to one embodiment.                 

2 is a simplified schematic diagram of a method of the present invention in accordance with a preferred embodiment.

FIG. 3 is an alternative simplified schematic diagram of the process diagram shown in FIG. 2 and shows a back-mix reactor.

The present invention is directed to efficiently and stepwise performing oxidation reactions on a conventional scale in a first high pressure reaction zone, followed by a second more conventional reaction zone, when performing liquid phase catalytic oxidation of paraxylene in the presence of an acetic acid solvent. It is based on the finding that the process efficiency and product quality can be substantially improved.

The first reaction zone of the process is a relatively high solvent: precursor mass ratio of 10-25: 1 or higher, and 2,000 kPa or more and 10,000 kPa or less, as described in more detail below in accordance with one embodiment. It features a high range of relatively high pressures. Higher solvent: precursor mass ratios can be used, such as, for example, 30: 1 or higher ratios, but when paraxylene is fed exclusively to the process over the first reaction zone or through the first reaction zone, Best results were observed for the whole process when the solvent: precursor mass ratio in the reaction zone was in the range of 10-20: 1. When the reaction medium exits the first reaction zone, the first reaction zone is optionally cooled to adjust the temperature of the reaction medium to a value below about 210 ° C. By adjusting the temperature, catalyst concentration, reactor residence time and / or maintaining the oxygen supply to the first reaction zone within a predetermined range, the oxygen uptake in the reaction medium is reduced to 50% of the oxygen required to completely convert paraxylene to terephthalic acid. It can be conveniently limited to a value below.                 

Temperature control can be achieved, for example, by placing an internal cooling coil or other cooling device within the first reaction zone, using a cooling jacket surrounding the reactor, or circulating the reaction medium through a heat exchanger located externally from the reactor. Can be.

Catalyst control can be achieved, for example, by passing a portion of the catalyst-containing mother liquor directly to the second reaction zone bypassing the first reaction zone.

It is important to prevent TA from settling on the cooling surface in the first reaction zone. TA formation is inhibited by limiting oxygen uptake and selecting appropriate cooling agents (e.g. boiling water) and cooling means to maintain a high solvent: precursor ratio in the reaction medium and to avoid the formation of cold spots anywhere in the reaction medium. This can prevent TA precipitation in the first reaction zone.

When exiting the first reaction zone, the pressure of the reaction medium is reduced while the reaction medium is fed to a more conventional oxidation reactor. For example, this reactor may typically be a stirred tank reactor or a bubble column reactor. Pressure reduction can be conveniently achieved by passing the reaction medium through one or more pressure drop valves located around the reactor. Best results are obtained if the reaction medium disperses quickly when entering the second reactor. Fast dispersion can be achieved by using existing methods for dispersing paraxylene-containing feeds in conventional reactors. In a stirred tank reactor, for example, this involves injecting the reaction medium into the reactor below the liquid line very close to the discharge from the stirrer impeller. By injecting the reaction medium in close proximity to the air supply, rapid dispersion of the reaction medium in the bubble column reactor can be achieved.

Referring now to the drawings, FIG. 1 is a simplified schematic diagram of a method of the present invention according to one embodiment. As mentioned above, the present invention may be applied to preparing a range of carboxylic acids or esters and mixtures thereof, but will be described in connection with the preparation of terephthalic acid.

The process is performed by first forming a feed stream 10 comprising an oxidation catalyst and a solvent such as acetic acid and water. In practice, the feed stream comprises (i) circulated solvent, circulated mother liquor and catalyst (line 11), (ii) reactor condensate from the second reactor (line 12) and (iii) fresh acetic acid constituent (line 13) It contains a mixture. The mixed feed stream contains typical catalyst components (eg, Co, Mn, Br) at concentrations that are generally diluted from the concentrations normally present when using one conventional oxidation reactor. Optionally, although not shown, the catalyst concentration in the first reaction zone can be adjusted by bypassing a portion of the catalyst-containing mother liquor (line 11) directly into the second reactor 20.

The mixed feed stream 10 generally has a temperature in the range of 130 ° C. to 160 ° C. based on the temperature of the various components forming the feed stream. However, the temperature of the feed stream is not critical.

Raise the pressure of feed stream 10 to a value of at least about 2,000 kPa, generally greater than 2,000 kPa, via a suitable pump 14, and feed stream to paraxylene via line 16, oxygen through line 17 It is continuously introduced into the first stirred tank reactor 15 simultaneously with the raw material of.

The feed of oxygen through line 17 may be air, oxygen-rich air, oxygen mixed with an inert gas, for example carbon dioxide, or essentially pure oxygen. When the source of oxygen comprises nitrogen or other inert carrier gas, the first reaction zone such that the vapor present in the first reaction zone is fuel-deficient, ie the hydrocarbon content of the vapor is below the lower explosion limit (LEL). The degree of cooling at and the working pressure thereof are preferably selected. When essentially pure oxygen is used as the source of oxygen, the degree of cooling in the first reaction zone and its working pressure are preferably selected so that no vapor phase is present in the first reaction zone. Optionally, although not shown in FIG. 1, some oxygen may be predissolved directly into feed 10 via a mixing device located downstream of the feed pump.

Paraxylene feed 16 may optionally be premixed with acetic acid solvent and introduced into a system located upstream or downstream of feed pump 14. Although not shown, optionally, a portion of the paraxylene feed 16 may be fed directly to the second reactor 20 bypassing the reactor 15. The reaction medium obtained in the first reactor has an acetic acid: paraxylene ratio in the range of 10-25: 1, with no paraxylene bypassed to the second reaction zone. The best results in this embodiment are observed when the acetic acid: paraxylene molar ratio is 10-20: 1.

Part of the paraxylene feed, ie line 16 in FIG. 1 and line 31 in FIGS. 2 & 3, is arranged to bypass the first reactor and feed directly to the second reactor 20. The solvent: paraxylene mass ratio in the reaction medium in the reactor is correspondingly upregulated for the portion of paraxylene bypassing the first reactor and the resulting mass ratio will reach values in the range up to 100: 1. The paraxylene feed (line 16) must be quickly dispersed when entering the first reactor. This can be accomplished by using existing methods for rapidly dispersing paraxylene-containing feeds in conventional reactors. As shown in the embodiment of the present invention shown in FIG. 1, this involves injecting the feed in a stirred tank reactor 15 near the discharge from the stirrer impeller. Although the stirred tank reactor is shown in FIG. 1, satisfactory results can be obtained using other conventional oxidation reactor arrangements.

The process is carried out in the presence of an oxidation catalyst system which can be homogeneous or heterogeneous. Homogeneous catalysts are usually used and are selected from one or more heavy metals such as cobalt, manganese and / or zirconium compounds. In addition, the catalyst usually contains an oxidation promoter such as bromine. Catalytic metals and oxidation promoters are mostly maintained in solution throughout the process, recovered and circulated, and then recovered with the new catalyst constituents as a solution.

The feed stream to the first reaction zone (line 10) contains typical oxidation catalyst components (e.g. Co, Mn, Br) but is about 3 to 5 compared to the catalyst concentration in the mother liquor (line 11) circulated from product recovery. Dilute by the factor of. Then, when the solvent is evaporated in the second reaction zone 20 and removed at the overhead, the catalyst concentration rises to more conventional catalyst concentration levels. The total catalyst metal concentration in the first reaction zone is typically in the range of 150 to 1,000 ppm w / w, while the catalyst metal concentration in the second reaction zone is typically in the range of 500 to 3,000 ppm w / w. When using Co and Mn metal catalyst systems, the total catalyst metal concentration in the first reaction zone should preferably be adjusted to at least about 250 ppm w / w for good catalyst selectivity and activity.

Oxidation is a high exothermic reaction. Without the means to cool the reaction depending on the oxygen uptake and solvent ratio, the heat of reaction can raise the temperature of the first reaction medium to a value above the second reactor operating temperature and / or above 210 ° C. A relatively low first reactor discharge temperature is desirable to minimize solvent and precursor decomposition (eg, combustion) and to eliminate solvent flashing when the pressure of the reaction medium at the inlet to the second reaction zone is reduced. Thus, in order to control the discharge temperature of the reaction medium below 210 ° C. and preferably below the second reactor operating temperature, the first reaction zone may comprise a cooling coil 18, or the reactor (and reaction Other internal or external means for removing heat from the medium) may be used. In order to prevent cold spots from forming and local precipitation of terephthalic acid (TA), it is important that the temperature of the coolant is about 120 ° C or higher.

By maintaining the oxygen supply to the first reaction zone within a predetermined range and adjusting the discharge temperature, catalyst concentration and residence time of the reaction medium, the oxygen absorption in the reaction medium is 50% of the oxygen required to completely convert paraxylene to TA. It is possible to limit to values below. In other words, according to the present invention, paraxylene is mainly converted to TA intermediates such as p-tolualdehyde, p-toluic acid and 4-CBA in the first reactor 15. Under the process conditions described above, if the discharge temperature is efficiently controlled, the first reactor does not produce any solid TA.

The reaction medium exiting the first reactor 15 is fed via line 19 to the second reactor, ie the zone of oxidation 20, which is a conventional continuous stirred tank reactor as shown. At the same time, the pressure of the reaction medium is reduced to a value in the range of 1,200 kPa to 2,000 kPa. As described above, the pressure reduction is achieved by passing the reaction medium through one or more pressure drop valves or nozzles 21 located around the reactor 20, whereby the reaction medium is located below the liquid line of the reactor. Disperses quickly by being injected into the stirrer impeller zone. Process conditions in the reactor 20, ie temperature, pressure, catalyst concentration and residence time, are within the usual range, but reduce oxygen uptake for reduction of oxidative strength.

If the oxygen feed to the first reactor comprises nitrogen or other inert carrier gas, the spent or excess air from the first reactor 15 (line 22) is replaced with a fresh air feed or oxygen-containing gas (line 22a). And the resulting mixed feed gas stream are introduced into the reaction medium in the second reactor 20 by convenient means and quickly dispersed. Alternatively, condenser 24 as exclusively supplying fresh air or oxygen-containing gas to second reactor 20, as consumed or excess air from first reactor 15 is indicated by dashed line 22b. Can be supplied directly.

TA precipitates in the reactor 20 to form a slurry, which can be recovered from the reactor system via line 23 using conventional methods. Steam is condensed through the condenser 24 at the top from the reactor 20 which essentially contains some acetic acid and water, and most of the condensate is passed through the line 12 for the feed stream composition to the first reactor 15. Return to, i.e., circulates. To remove the reaction water, a portion of the acetic acid and water condensate stream (so-called drained water) is converted to a solvent dehydration system. Optionally, although not shown, the reaction is by returning a portion of the condensate to the reactor 20 or to the reactor headspace through a reflux slinger and / or through a separate feed line or by mixing with an existing feed stream (line 19). Can be returned to the band.

2 is a simplified schematic diagram of a preferred embodiment of the present invention. The first reaction zone, ie first reactor 30, according to this embodiment is a plug flow reactor. The term “plug flow reactor” is generally used to define an extended or tubular reaction zone, where rapid and thorough radial mixing of the reactants occurs when the reactants flow through the tube or conduit. However, the present invention is contemplated to include any reactor as long as it has a reactor arrangement similar to the plug flow reaction zone.

As described above in connection with FIG. 1, feed stream 10 comprises (i) circulated solvent, circulated mother liquor and catalyst (line 11), (ii) second reactor condensate through line 12 and (iii) ) Mixed feed stream comprising fresh acetic acid construct via line (13). Optionally, although not shown, the catalyst concentration in the first reaction zone can be adjusted by bypassing a portion of the catalyst-containing mother liquor (line 11) directly into the second reactor 20. The oxygen supply (line 17a) in this embodiment is essentially pure gaseous oxygen.                 

The mixed feed stream 10 generally has a temperature in the range from 130 ° C. to 160 ° C., depending on the temperature of the constituent stream. A temperature in the range of about 140 ° C. has been found to be suitable for initiating the oxidation reaction.

The pressure of the mixed feed 10 is raised by a suitable pump means 14 to a value in the range of at least 2,000 kPa, generally in excess of 2,000 kPa. The pressure is selected such that all gaseous oxygen introduced via line 17a is readily dissolved in the feed stream in front of the first reactor 30 shown. The mixed feed stream with dissolved oxygen is then fed into the plug flow reactor 30 simultaneously and continuously with paraxylene supplied via line 31 and the reaction is initiated. Paraxylene may optionally be pre-mixed with the acetic acid solvent and mixture supplied via line 31. Optionally, as described above, a portion of the paraxylene feed 31 is fed directly into the second reactor 20 bypassing the reactor 30. When a portion of the paraxylene feed 31 is fed directly to the second reactor 20, the resulting solvent: paraxylene mass ratio in the reaction medium in the first reactor is the portion of the paraxylene feed that bypasses the first reactor. Will be adjusted upwards corresponding to, and thus the mass ratio obtained will range from a value in the range of 80: 1 to a value in the range of 100: 1 or even higher.

In order to achieve dissolved oxygen concentrations of up to 3.0% w / w in the mixed feed stream, conventional in-line mixing device 33 is used to dissolve molecular oxygen in the mixed feed stream. Mixing apparatus 33 may be an in-line nozzle arranged for releasing oxygen directly into the feed stream. In-line static mixers (not shown) may be located upstream of the first reactor 30 to facilitate mixing.

According to the present invention, the introduction of oxygen can be carried out stepwise, that is, oxygen can be introduced at various positions along the length of the first reaction zone 30. By carrying out the oxygen injection stepwise, the maximum value of the local oxygen concentration is reduced, which in turn can lower the reactor operating pressure.

In practice, the feed stream 10 is fed into the plug flow reactor 30 continuously in parallel with the paraxylene, whereby the solvent obtained, although the solvent: paraxylene ratio can be maintained as high as 25: 1 or higher A reaction medium with a: paraxylene ratio of at least about 10: 1 is formed. In a preferred embodiment, the solvent: paraxylene ratio is in the range of 10-20: 1.

The residence time of the reaction medium in the plug flow reaction zone 30 is relatively short, ie less than 6 minutes.

The reactor 30 shown in FIG. 2 has a cylindrical tube structure. The reaction medium flows through the tube while coolant, for example compressed water (PW), is introduced into the cylinder, where it is boiled and removed as air stream (S). A small water septic tank (boiler discharge, BB) is used to control the accumulation of impurities / residues in the water system.

When the reaction medium exits the first reactor 30, the temperature of the reaction medium is maintained below about 210 ° C. by adjusting the pressure of the resulting vapor and hence its temperature. By adjusting the process parameters as described in accordance with the present invention, the oxygen uptake in the reaction medium in the first reaction zone can be limited to a value of less than 50% of the oxygen required to completely convert paraxylene to TA. That is, paraxylene in the first reactor 30 is mainly converted to TA intermediates such as p-tolualdehyde, p-toluic acid and 4-CBA. Under the described process conditions, if the temperature of the reaction medium is effectively controlled when the reaction medium exits the first reactor 30, the first reactor does not produce any solid TA.

Although the cylindrical tube reactor structure is shown in FIG. 2, any of the reactors 30 may be suitable as long as the reactor structure provides any heat removal and any multiple oxygen injections. For example, the reactor may have multiple tube passages for injecting oxygen into the reaction medium upstream of each tube passage. Alternatively, a back-mix reactor can be used, such as a pump circulating loop reactor, which injects oxygen into the loop and removes heat from the loop as shown in FIG. 3. The reactor may also include a series of cooled or non-cooled vessels which optionally inject oxygen upstream of each vessel.

The reaction medium exiting the plug-flow first reactor 30 is fed via line 19 to the second reactor, ie the oxidation zone 20, as described above in connection with the method embodiment shown in FIG. 1, which is shown As can be conventional continuous stirred tank reactor. At the same time, the pressure of the reaction medium is reduced to a value in the range of 1,200 kPa to 2,000 kPa. Pressure reduction is conveniently achieved by passing the reaction medium through one or more pressure drop valves or nozzles 21 located around the reactor 20 such that the reaction medium is injected into the stirrer impeller zone below the liquid line of the reactor. It quickly disperses. Process conditions in the reactor 20, ie temperature, pressure, catalyst concentration and residence time, are within the usual range, but reduce oxygen uptake for reduction of oxidative strength.

Fresh feed of air or oxygen-containing gas (line 22a) in the second reactor 20 is introduced into the reaction medium by a convenient method and quickly dispersed.

TA precipitates in the reactor 20 to form a slurry, which can be recovered from the reactor system via line 23 using conventional methods. Steam is condensed through the condenser 24 at the top from the reactor 20 which essentially contains some acetic acid and water, and most of the condensate is passed through the line 12 for the feed stream composition to the first reactor 30. Return to, i.e. circulated. To remove the reaction water, a portion of the acetic acid and water condensate stream (so-called drained water) is converted to a solvent dehydration system. Optionally, although not shown, the reaction is by returning a portion of the condensate to the reactor 20 or to the reactor headspace through a reflux slinger and / or through a separate feed line or by mixing with an existing feed stream (line 19). Can be returned to the band.

Claims (16)

(a) forming a feed stream comprising a solvent and an oxidation catalyst at elevated pressure in the range of 2,000 kPa to 10,000 kPa; (b) (1) feed stream, (2) precursor, and (3) supply of oxygen continuously and simultaneously to the first reaction zone to form a reaction medium in which the solvent: precursor ratio is in the range of 10-30: 1. and; (c) limiting oxygen uptake in the reaction medium in the first reaction zone to a value of less than 50% of the oxygen required to completely convert the precursor to the corresponding carboxylic acid or ester thereof; (d) reducing the pressure of the reaction medium to a value in the range of 1,200 kPa to 2,000 kPa while simultaneously supplying the reaction medium to the second reaction zone, A process for producing a carboxylic acid or ester thereof by catalytic liquid phase oxidation of the corresponding precursor in a solvent selected from aliphatic carboxylic acids or non-aliphatic organic acids and which may comprise water. The method of claim 1, wherein reducing the pressure of the reaction medium to a value in the range of 1,200 kPa to 2,000 kPa while simultaneously supplying the reaction medium to the second reaction zone comprises: (a) present in the reaction medium within the second reaction zone; Evaporating a portion of the solvent; (b) removing steam from the reactor overhead; (c) condensing the steam; (d) circulating some or all of the condensate in the feed stream. The process according to claim 1 or 2, comprising the further step of recovering the carboxylic acid or ester thereof obtained from the second reaction zone. delete The method of claim 1 wherein the carboxylic acid is terephthalic acid, the precursor is paraxylene and the solvent is acetic acid. (a) forming a feed stream comprising solvent and oxidation catalyst at elevated pressure in the range of 2,000 kPa to 20,000 kPa; (b) oxygenate the feed stream; (c) (1) an oxygenated feed stream and (2) the precursors are continuously fed simultaneously into a first reaction zone to form a reaction medium in which the solvent: precursor ratio is in the range of 10-30: 1, and the reaction When the product is formed they remain in solution; (d) limiting oxygen uptake in the reaction medium in the first reaction zone to a value of less than 50% of the oxygen required to completely convert the precursor to the corresponding carboxylic acid or ester thereof; (e) feeding the reaction medium to the second reaction zone while simultaneously reducing the pressure of the reaction medium to a value in the range of 1,200 kPa to 2,000 kPa, A process for producing a carboxylic acid or ester thereof by catalytic liquid phase oxidation of the corresponding precursor in a solvent selected from aliphatic carboxylic acids or non-aliphatic organic acids and which may comprise water. The process of claim 6, wherein the first reaction zone is a plug flow reactor or a back-mix reactor. The method of claim 6 or 7, wherein the step of reducing the pressure of the reaction medium to a value in the range of 1,200 kPa to 2,000 kPa while simultaneously supplying the reaction medium to the second reaction zone comprises: (a) reacting in the second reaction zone; Evaporating a portion of the solvent present in the medium; (b) removing steam from the top of the reactor; (c) condensing the steam; (d) circulating some or all of the condensate in the feed stream. delete 8. The process according to claim 6 or 7, comprising the further step of recovering the carboxylic acid or ester thereof obtained from the second reaction zone. delete The method of claim 8 comprising the further step of recovering the carboxylic acid or ester thereof obtained from the second reaction zone. delete The method of claim 6 wherein the carboxylic acid is terephthalic acid, the precursor is paraxylene and the solvent is acetic acid. (a) forming a feed stream comprising solvent and oxidation catalyst at elevated pressure in the range of 2,000 kPa to 20,000 kPa; (b) oxygenate the feed stream; (c) (1) an oxygenated feed stream and (2) the paraxylenes are continuously fed simultaneously into a first plug-flow reaction zone such that the acetic acid: paraxylene ratio is in the range of 10-25: 1 The media are formed and they remain in solution when the reaction product is formed; (d) limiting oxygen uptake in the reaction medium in the first reaction zone to a value of less than 50% of the oxygen required to completely convert the paraxylene to terephthalic acid; (e) simultaneously supplying the reaction medium to the second reaction zone while reducing the pressure of the reaction medium to a value in the range of 1,200 kPa to 2,000 kPa, thereby evaporating a portion of the solvent present in the reaction medium; (f) continuously removing and condensing the vapor from the top of the reactor while recovering the terephthalic acid obtained from the second reaction zone, and circulating some or all of the condensate in the feed stream, Catalytic Liquid Phase Oxidation of Paraxylene in a Solvent Containing Acetic Acid. 16. The acetic acid: para in the first reaction zone according to any one of claims 5, 14 and 15, by converting a portion of the paraxylene feed from the first reaction zone to the second reaction zone. Further comprising increasing the xylene ratio to a value greater than 25: 1.

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