Production of aromatic carboxylic acids

Production of aromatic carboxylic acids

PRODUCTION OF AROMATIC CARBOXYLIC ACIDS

Full description

Background of the invention:-

This invention relates to methods and systems for producing aromatic carboxylic acids, such as purified terephthalic acid (PTA). One aspect of this invention relates to a more efficient method of producing aromatic carboxylic acids. Another aspect concerns a method for reducing solvent loss in the production of aromatic carboxylic acids.

Aromatic polycarboxylic acids, such as terephthalic acid, are important chemical intermediates used to produce products of industrial interest, including polyester polymers, which can be used to produce fibers and in the manufacture of containers, bottles and other formed products.

Purified terephthalic acid (PTA) can be produced in a two-step method. Current technology for producing terephthalic acid involves the liquid-phase oxidation of an aromatic feedstock, such as paraxylene, using molecular oxygen in a solvent and the oxidation solvent includes a low-molecular-weight aliphatic carboxylic acid (C6-C2), typically Acetic acid and water, in the presence of a catalyst system containing a dissolved heavy metal which usually includes an activator, such as bromine. Acetic acid is useful as a solvent as it is relatively resistant to oxidation and increases the activity of the catalytic pathway for the oxidation of aromatic feed stock and reaction intermediates. The reaction is carried out in one or more stirred vessels under high temperature and pressure, in the range of about 150 to 250 C and 6 to 30 bar respectively, and ideally produces crude terephthalic acid (CTA) in higher yields, e.g. : at least 95%. Under these conditions, CTA precipitates from the solvent in an oxidation reactor to form a slurry paste of CTA solids in the oxidation solvent, which is kept in suspension by stirring in the reaction vessels. The temperature of the paste is reduced by passing it through a series of crystallizers, each at a lower pressure, before separating the CTA solids from the oxidation reaction solvent to obtain the oxidation mother solution. Separation of the CTA solids from the oxidation parent solution occurs at positive pressure or at Void.

Ideally, the liquid phase oxidation solvent is aqueous acetic acid and contains water produced from the oxidation of paraxylene and other starting compounds for the reaction. The oxidation reaction is hydrophobic and generates aromatic carboxylic acid, reaction intermediates from the partial oxidation of an aromatic feedstock, such as p-toluic acid (pTol), and byproducts (including color-forming compounds), volatile components, such as methanol, acetate. methyl acetate, methyl bromide, and breakdown products, such as carbon dioxide, carbon monoxide, and benzoic acid (BA).

The second stage of the production process is the purification of CTA by catalytic hydrogenation in aqueous solution. Ideally, CTA solids are dissolved in water at high pressure (70-90 bar) and high temperature (275-290°C), and hydrogenation takes place on a stable palladium substrate catalyst supported on carbon. The resulting solution is cooled as it passes through a series of crystallization compounds, where purified terephthalic acid (PTA) is crystallized. The resulting paste is fed at a temperature in the range of about 140-160°C to a device or devices for separating solids from liquids, such as a centrifuge or a rotary filter, where the PTA solids are separated from the mother solution stream for purification, washed and then dried. The PTA product is suitable for making polyester polymers for fibers, bottles, containers and other formed products.

The oxidation reaction is maintained at a constant temperature by evaporating the oxidizing solvent that exits the reactor and returning the condensed solvent, which can be further cooled, to the reactor. In this way, the latent heat of the oxidation solvent is used to cool the oxidation reaction mixture. The vapor phase leaving the reactor, as an off gas, typically includes evaporated acetic acid, water vapor and volatile reaction byproducts, as well as non-condensable components that include residual oxygen not consumed in the oxidation reaction, and nitrogen (when air is used as the source of molecular oxygen for oxidation reaction) and carbon oxides.

Ideally, the water in the oxidation solvent in the oxidation reactor is maintained at a constant level by condensing the off-gas from the oxidation reactor to form condensate, separating the condensate from the remaining gas stream, and separating at least a portion of the water from the remainder of the liquid condensate, before returning the condensate to The remaining liquid condenses into the reactor as an oxidizing solvent. Excess water separated from the condensate can be fed to a treatment unit for an exhaust stream for disposal.

Water can be separated from the gas condensate leaving the reactor by distillation, with the aliphatic monocarboxylic acid-rich stream as the bottom product and the water-rich stream as the top product. A previous improvement in the production process was by eliminating the initial condensation step and then feeding the gas coming out of the oxidation reactor directly to the refining column. This column can be conveniently located above a stream oxidation reactor rich in low molecular weight aliphatic monocarboxylic acid for direct return to the oxidation reactor, although other distributions may be used.

Using distillation to separate reduced aliphatic carboxylic acid from the water extracted as a condensation product from the exhaust gas of the oxidation reactor requires a large number of distillation stages in the refining column and sufficient water reflux to the top of the column. However, the total flow of water reflux to the top of the column is governed by keeping the water concentration in the oxidation reactor at a low level, in order to maintain the activity of the oxidation reactor. Ideally, the reflux includes a portion of the overhead product after condensation of the water-rich vapor stream leaving the top of the refinery column. The remainder of the refining column condensate, including the reaction water, is then removed from the upper system of the refining unit.

The oxidation reaction works at high pressure and temperature, and the vent gas from the oxidation reactor can be used to extract energy after the refining column. Energy recovery can be either direct or indirectly, by heat exchange, by raising steam for use elsewhere in the process or by reducing the pressure of the gas stream through a machine, such as an expansion unit. The expansion unit can be used to extract energy, to direct the air produced by the compressor to the oxidation process or to generate electricity.

Methyl acetate is a byproduct of the oxidation reaction and is a volatile component that needs to be separated from the upper product vapor stream of the refinery, extracted and fed back to the oxidation reaction to avoid losses from the oxidation stage. However, the hydrolysis of methyl acetate to methanol and acetic acid occurs within the temperature and pressure ranges ideally used in a refining column and can reduce the separation efficiency of aliphatic carboxylic acid and water in the reactor exhaust gas. Oxidation and increases solvent loss from the manufacturing process.

There is an existing and alternative method to reduce acetic acid loss and energy consumption during PTA manufacturing, which is to extract acetic acid from wastewater streams using liquid extraction and feed the extracted stream to a distillation column and remove afterwater. Also, the extraction method can be used after an acetic acid water distillation column that operates at ambient temperature. Each of these choices is less energy efficient at higher method pressures. In addition, certain extraction techniques have detrimental effects on the operation of the oxidation reactor and on the CTA product that is produced, for example by using high-boiling solvents to extract a high-boiling liquid.

General description of the invention:-

All of these issues result in a reduction in the economic benefits of using a refining column to separate water from the oxidizing solvent and prevent the loss of acetic acid and methyl acetate from manufacturing processes. In addition, increasing the number of theoretical distillation stages necessary to separate the aliphatic carboxylic acid from the water extracted as a condensation product of the gas leaving the oxidation reactor results in an increase in the size of the refining unit, which will be a large vessel or several vessels, and high costs for this step in the production process. Because the PTA process uses corrosive solvents, such as acetic acid, and operates at high temperature and pressure, the refining unit must be designed from expensive, corrosion-resistant materials (such as titanium).

Therefore, there is a need for an improved method of operating a refinery unit as a distiller. In particular, there is a need to usefully separate water and the oxidation solvent from the gas produced by the oxidation reactor, to reduce the cost of separating water from the oxidation solvent, to improve the recovery of reaction by-products, such as methyl acetate and aliphatic carboxylic acid used in the oxidation reaction solvent, and to reduce the volume of Refining column, without increasing the resulting processing load.

Brief explanation of the drawings:-

Figure 1 is a diagram of one aspect of the method described, which shows a continuous oxidation process or shows the distribution of the refinery, water treatment column and extraction stage.

Figure 2 is a diagram of another manifestation of the described process, which shows a continuous extraction method, which can be used as an extraction stage for the described method.

Unless otherwise stated, all technical and practical terms used herein have the same meaning as would be understood by those with ordinary experience in the field to which this application relates. Any methods and materials that are the same or analogous to those described herein may be used in the testing of this application

Detailed description:-

In one aspect, a method for producing aromatic polycarboxylic acid is being prepared comprising:

a) Separating the vent gas stream of an oxidation reactor into a stream rich in acetic acid and a steam stream rich in water, as the water-rich steam stream includes volatile compounds and non-condensable gases, and the aforementioned separation process is carried out in a distillation device;

b) condensation of said water-rich vapor stream into a condensation stream and a vapor stream;

c) Feeding a first part of the said condensate stream to the aforementioned distillation device and feeding a second part of the said condensate stream to a water treatment column to form a stream of lower parts of the water treatment column without volatile compounds;

d) feeding a portion of the down stream of the water treatment column to a solvent extraction process including an extraction column; And

e) Separate the bottom stream of said water treatment column into an acetic acid-rich stream and a water-rich stream. The distillation device can be a refining unit. In addition, an acetic acid-enriched stream can be fed to an extraction column to separate the extraction product from acetic acid. Likewise, the water-rich stream can be fed to an extraction column to separate the extraction compound. All or part of the acetic acid stream, either before or after the extraction compound is removed, can be fed to the oxidation reactor.

In another aspect, a method is developed to reduce the hydrolysis of methyl acetate in the oxidation distillation process of an exhaust gas containing an aromatic polycarboxylic acid comprising:

a) Separation of an exhaust gas stream from the oxidation reactor into an acetic acid-rich stream and a water-rich steam stream, as the water-rich steam stream includes volatile compounds and non-condensable gases, and the separation process is carried out in a distillation device;

b) Condensation of said water-rich vapor stream into a condensation stream and a vapor stream; Whereas the hydrolysis of said methyl acetate is reduced by an amount ranging from about 30% to about 70% w/w and whereas the water-rich vapor stream from said distillation apparatus includes less than about 12% w/w of acetic acid. The distillation device can be a refining unit. Reducing the hydrolysis of methyl acetate occurs by reducing the residence time of the liquid in the distillation device, compared to the residence time in the distillation device without extracting the solvent.

In another aspect, a method for producing terephthalic acid is being prepared that includes:

a) Adding paraxylene, molecular oxygen, and acetic acid to a stirred oxidation reactor;

b) Removing the resulting gas from the aforementioned oxidation reactor, as the resulting gas from the reactor includes acetic acid and water vapor;

C) Feeding the aforementioned resulting gas from the reactor to the distillation column, where the resulting gas from the reactor is separated into: 1) a stream rich in acetic acid, which is fed back to the aforementioned oxidation reactor; 2) a stream rich in water vapor that is fed to the condenser;

d) The condensation of the said stream, which is rich in water vapor, into a condensation stream and a steam stream, where the first part of the said stream, the condensation product, is fed back to the aforementioned distillation column, and a second part of the said stream, the condensation product, is fed to the water treatment column to form a lower stream in the treatment column. column water;

e) feeding a portion of the downstream from said water treatment column to a solvent extraction process, which includes an extraction column;

f) feeding the said steam stream to an absorption unit to remove the volatile components present in the steam;

g) Feeding an extraction product to said extraction column in a direction opposite to the direction of the downstream part of said water treatment column; And

h) Extraction of acetic acid from the bottom stream of the water treatment column to form an aqueous stream and an organic stream containing acetic acid. The distillation column can be a refining unit. The organic stream containing acetic acid can be fed directly to the oxidation reactor. Alternatively, the organic stream can be fed to an extraction compound removal column to separate the extraction compound from the organic stream and extract the organic stream, including the acetic acid, before feeding the stream to the oxidation reactor. In addition, the aqueous product stream can be fed to the extraction compound column to separate the extraction compound from the aqueous stream and remove the aqueous stream.

An improved method is demonstrated that increases the extraction rate of the method solvent in the oxidation stage of the method for the manufacture of aromatic carboxylic acids. In particular, the method combines a low-boiling separation of water and acetic acid in a distillation apparatus with an extraction of a low-boiling solvent to the refining unit above the condensate. Reduced separation in the distillation device reduces the load on the device and allows for fewer separation stages, thus reducing the length and density of the materials. The process of solvent extraction separates the remaining acetic acid and other organic compounds from an aqueous phase, which is usually water. This can be done in two steps. :(1) Water treatment step, where organic compounds, such as para-xylene, are removed from the aqueous phase; and (2): an extraction step where the aqueous phase from the bottom of the water treatment step enters a solvent extraction process to separate acetic acid and other organic compounds into an acetic/organic acid stream. Both the acetic acid/organic phase stream and the aqueous phase stream can be introduced into further separation steps to remove the extraction product. The acetic acid/organic stream can be fed, either before or after further separation of the extraction compound. The acetic acid/organic stream, either before or after further separation of the extraction compound, can be fed to an oxidation reactor. In addition, it can feed organic compounds from the water treatment step to the oxidation reactor. The acetic acid stream may contain additional organic compounds, such as methyl acetate, methanol and aromatic intermediates for the reaction. The method as a whole improves the acetic acid extraction rate in the PTA manufacturing process, while reducing the rate of hydrolysis of methyl acetate and also reducing the exhaust load entering the exhaust treatment unit. The loss of methyl acetate by hydrolysis occurs in the range of about 30% to about 70%, in comparison with the PTA manufacturing process using a refining compound without solvent extraction or reducing the liquid retention or residence time in the distillation device due to the reduction in the acetic acid-water separation load. In the distillation device. There is a reduction in the retention level of the liquid from about 20% to about 80% compared to retention in the distillation device without solvent extraction, which is by a percentage ranging from about 20% to about 70%, from about 20% to about 60%, from about 20% to about 50%, and from about 20% to about 40%.

Alternative separation techniques, such as membrane separation and adsorption, can also be used in conjunction with a refinery to reduce the number of theoretical stages required. To perform the membrane separation process, high pressures, large membrane surface areas, low temperatures, and high recirculation flow are required. Adsorption requires many currents, relatively large vessels, and an operation process to regenerate the adsorption balls.

Suitable solvents are aliphatic esters, including methyl acetate, ethyl acetate, isopropyl acetate, n-propyl acetate, methylpropyl ketone, methyl isobutyl ketone, and tert-butyl ketone. Methyl-tert-butyl ether and combinations thereof can be used as low-boiling extraction compounds.

The production of aromatic carboxylic acids, including terephthalic acid, can be done in a stirred oxidation reactor. Here, the aromatic feedstock, such as paraxylene, is reduced by molecular oxygen, ideally derived from air, in the presence of a reaction catalyst and an aqueous solvent of acetic acid, to produce the carboxylic acid. The reaction temperature can range from about 150°C to about 250°C, including 190°C, and the pressure can range from about 6 absolute bars to about 30 absolute bars, including 13 absolute bars. The CTA solids precipitate into the reactor as a product of the oxidation reaction and are maintained in suspension by a stirrer. Other feed streams to the oxidation reactor can include reflux solvent, recycled solvent, extracted para-xylene and extracted methyl acetate. Also, reaction intermediates can be extracted from the pure parent solution and recycled to the oxidation reactor. This increases the efficiency and conversion of the feed stock into the aromatic carboxylic acid product.

The CTA paste in the oxidation solvent (parent solution) flows into the crystallization units after the oxidation reactor. The CTA solids are then separated from the parent solution for oxidation using a rotary filter, centrifuge, or other similar devices. The separation temperature ranges from about 90°C to about 160°C and the pressure from about 0.5 bar absolute to about 4.5 bar absolute.

The oxidation reaction is hydrophobic and the heat of reaction is removed by evaporating the solvent into the reactor exhaust gas which flows to a distillation apparatus, such as a refinery, which can be one or more vessels. The acetic acid and water in the reactor gas can be separated by distillation, which is operated by the upper temperature in the distillation column in the range from about 140°C to 200°C, including about 170°C.

Reflux is supplied to the top of the distillation column from upper condensers, which may include one or more heat exchangers. Additional aqueous reflux including pure mother solution may be fed to or below the top of the column. An acetic acid-enriched stream can be returned from the base of the column to the oxidation reactor. The column base can operate at the same temperature as the oxidation reactor.

The water-rich vapor from the top of the column includes acetic acid, ideally from about 0.1 to about 12% w/w, including from about 0.1 to about 5% w/w, from about 3 to about 5% w/w and about 5% w/w. / The weight of what condenses and a portion of the condensation product is fed, ideally at a temperature in the range of about 130°C to about 160°C, to the top of the column as water reflux. The remainder of the condensate is cooled in stages to a temperature in the range from about normal temperature (25°C) to about 100°C, including about 40°C. The pure parent solution may be fed into the unit, which includes oxidation intermediates, such as PTOL at or below the top of the distillation column, as an additional aqueous liquid feed stream at temperatures in the range from about 140°C to 160°C including about 149°C, and the overhead condensers may Include two or more heat exchangers, ideally with at least one used to generate steam to efficiently extract heat from the top of the column.

The non-condensing gas is passed to an absorption unit at about 6 to about 30 absolute bar to remove volatile components, such as paraxylene, methanol and methyl acetate that remain in the steam. Volatile components can be removed by contact with a liquid, first with an acetic acid-enriched stream, such as an oxidizing solvent, then with a water-enriched stream. The curettage solutions are fed to the oxidation reactor. The resulting gas that is scraped from the top of the absorption unit in the range from about 4 to about 28 absolute bars, including about 11 absolute bars, can be further processed, including energy recovery, for example by passing through an expansion unit, before exiting to the atmosphere. External.

To extract the organic components in the upper condensate from the distillation column, at least a portion of the overhead condensate from the distillation column may be fed to a water treatment column, which operates at a pressure close to atmospheric pressure and at least a portion of the condensate stream flashes upon its entry To the water treatment column. Volatile components including methyl acetate and p-xylene are separated from the water stream and can be returned to the oxidation column. In order to improve the separation of volatile components from the aqueous phase in the water treatment column, steam can be fed to the bottom of the column.

A first portion may be fed from the lower portions of the water treatment column at approximately 105°C to the solvent extraction process, including an extraction column to separate organic compounds, including acetic acid, from the lower portions of the water-rich water treatment column, and a second portion may be fed to the purification section.

Figure 1 describes an aspect of the method demonstrated using one of the refining units such as a distillation apparatus/distillation column. Here, paraxylene is oxidized to CTA using molecular oxygen in a stirred reactor including one or more stirred vessels under high temperature and pressure. In particular, the oxidation reactor 100 may be fed with air 200, an aqueous solvent of acetic acid 201, containing the reaction catalyst, and paraxylene 220. The CTA solids are deposited in the reactor, as a product of the oxidation reaction, and are maintained in suspension by the stirrer 101. Other streams of the oxidation reactor may include reflux solvent 204 from the refinery 102; Recycling solvent 213 from the absorption unit 105, and methyl acetate and methanol 225 from the water treatment column 108. The oxidation reaction is exothermic and the heat of reaction is removed by evaporating the solvent to the reactor gas 202. The reactor product 203, the CTA paste in the oxidizing solvent, flows to more than one crystallizer in a cascade following the oxidizing reactor, before the CTA solids are separated from the oxidizing parent solution using a suitable device.

The reactor gas 202 of the oxidation reactor 100 flows to the refining unit 102. The acetic acid and water in the reactor gas are separated by distillation in the refining unit. Water reflux is supplied to the top of the refinery by stream 216, which is part of the condensate produced in the upper condensers 103 and 104, including one or more heat exchangers. Additional hydroreflux 228 may be fed at or below the top of the refinery, including the pure mother solution to the unit, resulting in the separation of PTA solids after crystallization in the purification stage. A stream rich in acetic acid 204 can be returned from the refinery base to the oxidation reactor.

The steam stream 205, which contains acetic acid, flows from the top of the refining unit to the top condensers of the refining unit 103 and 104. The condensers 103 and 104 include two or more heat exchangers with those used to generate a current for efficient heat extraction. Steam 205 is condensed from the top of the refining unit and cooling takes place in stages. The condensate products 206 and 208 may be separated from the steam stream at each condensation stage and flow to the reflux pot 106, and may be pressure balanced, for example via line 214 to overhead steam line 207. A portion of the condensate collected in the reflux pot 106 flows to the top of the refinery as reflux water 215 and 216. The non-condensable gas 209 from the final heat exchanger passes to an absorption unit 105 to remove volatile components retained in the steam. Volatile components are removed by contact with liquids, including first contact with an acetic acid-enriched stream, such as oxidizing solvent 210, and then with a water-enriched stream 211. The scavenging solutions 213, which include acetic acid and water from the bottom of the absorption unit, flow into the oxidation reactor 100. The exhaust gas that is being scraped 212 from the top of the absorption unit passes forward to perform treatment, including energy recovery, before exiting to atmospheric pressure.

The condensate 217 and 218 collected in the reflux pot 106 may also flow to the water treatment column 108, where the volatile components are separated from the aqueous product and re-extracted into the upper vapor 223 which flows to the condenser 109 where condensation takes place. A portion of the condensate product 230 from the upper condenser of the water treatment column 109 is returned as reflux to the column, while the remainder 225 may be returned to the oxidation reactor 100. Non-condensable vapors 224 from the upper condenser of the water treatment column 109 can exit the system.

To oxidize the separation of volatile components from the aqueous phase, steam 222 may be fed to the bottom of the water treatment column 108. Water from the base of the water treatment column 227 may be used elsewhere in the production process, including the purification unit 110. A portion of the water from the base of the water treatment column 226 is fed to the solvent extraction system 111, where the remaining organic compounds, including acetic acid, are extracted and fed 233 to the oxidation reactor and at least a portion of the water stream extracted from the solvent extraction system is fed to the process 245 Purification 110. The excess water 231 may be fed to other users of the method or to the exhaust. The extraction compound 232 is fed to the solvent extraction system 111, as needed, such as for loss preparation.

The solvent extraction system 111 is described in Figure 2. The condensate from the upper refinery condensate collected in the reflux pot 106 includes acetic acid which can be extracted to reduce the amount of oxidation method solvent fed to the reflux treatment and purification process and reduce losses in the remainder of the PTA production process. .

In one aspect of the method described, a liquid aqueous stream 226 comprising less than about 20% w/w acetic acid is cooled in an exchanger 102 and a balance cooler 113 to approximately 40°C before being fed to the extraction column 114, where the aqueous stream is fed to the top of the column and It flows downward, upstream of the organic phase, and is fed to the bottom of the extraction column. The aqueous phase product 235 exits the bottom of the extraction column and is heated in a heat exchanger 117 before feeding the extraction compound removal column 116, to remove the dissolved organic extraction compound. The water-enriched product stream 240 from the base of the extraction compound removal column is cooled from 109°C to about 50°C in the exchanger 117 before being used elsewhere in the manufacturing process or rinsed to the exhaust (245 and 231). The upper vapor rich in the extraction compound 237 exits from the top of the extraction compound removal column at a temperature ranging from about 71°C to 100°C, including about 93°C, condenses in an overhead condenser 119 and flows to a decanter unit 118. The separation of the organic and aqueous phases takes place in a decanter, with the aqueous phase as reflux 238, 239 flowing to the extraction compound refining column 115 and the extraction compound removal column 116. The organic phase 241 flows to the bottom of the extraction column 114, and flows upstream of the column to the aqueous feed stream. , and exits from the top of the extraction column 234 and is heated in a heat exchanger 112, feeding the extraction refining column 115. The upper steam, rich in extraction compound 236, emerges from the top of the extraction refining column at a temperature in the range from about 71°C to about 100°C, including about 79°C.

Condensation takes place in the overhead condenser 119 and flow to the decanter 118. The acetic acid-rich organic stream exits the base of the extraction refining column 115 at approximately 119 C and is recycled 233 to the oxidation reactor 100. The extraction compound may be an aliphatic ester, including an alkyl ester. C1-C3 alkyl ester, such as ethyl acetate, or C4-C5 alkyl ketone.

Examples

The following examples serve to further illustrate different manifestations of the methods described

A combination of physical measurements and adjustment to the optimal level gives the results shown in the examples.

Example 1

A refinery unit, as shown in Figure 1, is equipped to receive the gas leaving the CTA oxidation reactor and reflux an acetic acid-enriched stream back to the reactor. The upper parts of the refining unit condense completely, and part of the condensation product is returned to the top of the refining unit as reflux, and the rest is fed to the water treatment column. And the upper steam from the column. The water treatment is completely condensed and part of the condensation product is returned to the water treatment column as reflux and the rest can be recycled to the oxidation reactor. Water is fed from the base of the water treatment column to the solvent extraction system, where the acetic acid is completely separated from the water stream and extracted to the method.

Table 1 shows acetic acid concentrations at key positions in the PTA production process and methyl acetate hydrolysis data for several aspects of the method described as well as a comparative embodiment without a solvent extraction system

Example 2

A system is set up as in Example 1, but the refinery has a lower water-acetic acid separation capacity by reducing the number of stages (theoretical).

As shown in Table 1, the described method can result in a higher concentration of acetic acid in the vapor at the top of the refining unit, resulting in a higher load on the solvent extraction system. The smaller size of the refining unit and the increase in the size of the solvent extraction system allows the described method to produce Efficiently produces pure PTA at lower costs without loss of solvent in the oxidation process.

Comparative example 1

A system is provided as in Example 1 but without a solvent extraction system including an extraction column to remove acetic acid from the water stream portion of the base water treatment column.

Table 1

Example 1

Example 2

Comparative example 1

Concentration of acetic acid in the upper steam of the refinery, % w/w

0.9

6

0.9

Concentration of acetic acid in the stream entering the solvent extraction system, % w/w

1.5

10

Not specified

Acetic acid extraction by solvent extraction system%

93

Not specified

Concentration of acetic acid in aqueous exhaust, % w/w

0.1

0.1

1.5

Relative water load in the exhaust

15

Theoretical stages in the refining unit

46

16

46

Concentration of methyl acetate in the upper steam of the refinery, % w/w

0.95

0.96

0.95

Total hydrolysis of methyl acetate in kilomol/hour refining unit

14.9

10- 4.9*

14.9

Relative decomposition of methyl acetate

0.7- *0.3

It depends on the retention of liquid in the column.

While this invention has been described on the basis of certain embodiments thereof, it is clear that there are many alternatives, modifications and changes that will be apparent to those with experience in the art in light of the foregoing description. Accordingly, this invention includes all such alternatives, modifications and changes that are within the spirit and scope of the appended claims.