MXPA97009686A - Method and apparatus for preparing tereftalicopurific acid - Google Patents

Abstract

The present invention relates to a method for purifying impure terephthalic acid from a liquid dispersion of this, which also contains impurities that are selected from: initial materials that do not react, solvents, products of side rections and / or other unwanted materials. The method consists of the steps of filtering the dispersion to form an impure terephthalic acid filter cake, dissolving the filter cake in a solvent for selective critalization, at an elevated temperature to form a solution, crystallizing the purified terephthalic acid from the solution in the crystallization solvent, I reduce the temperature of the solution and separate the crystallized, purified terephthalic acid solution from the solution. According to the invention, the solvent for selective crystallization is non-aqueous, non-corrosive and essentially non-reactive with terephthalic acid. Preferably, the solvent for selective crystallization is N-methyl pyrrolidone. The method and apparatus produces purified terephthalic acid having a desired purity for use in the formation of polyester resin and other products in an economically attractive proportion and in operating conditions of reduced stringency requiring less capital investment and simplified processing.

Description

METHOD AND APPARATUS FOR PREPARING PURIFIED TEREFTAL ACID The present invention relates to a method and apparatus for preparing purified terephthalic acid. It also relates to the methods and apparatus for purifying impure terephthalic acid to produce a purified terephthalic acid product, which is a useful initial material for producing polyester resin, which in turn is useful for the production of fibers, films, bottles of plastic and polyester resin structures, often reinforced by other materials such as fiberglass.

BACKGROUND OF THE INVENTION Purified terephthalic acid (ATP) is an initial material for the formation of polyester resin, which in turn is used to make various materials that have multiple uses in commerce. The purified terephthalic acid is formed from "impure" terephthalic acid for convenience by various purification methods, often with the aid of catalysts. The methods for purifying the impure terephthalic acid hitherto available are not completely satisfactory from a technical point of view, or from an economic point of view, still the purity of the purified terephthalic acid is an important determinant of the acceptance of the processes by which the polyester resin is formed. Different reaction systems are known for forming impure terephthalic acid from a variety of starting materials. The aspects of the purification of the present invention can be used with substantially any of these reaction systems, but, in accordance with the invention, the use of a reaction system including the oxidation of paraxylene (p-xylene) and the use of this synthesis system forms part of the present invention. The problems of existing and previous systems of purified terephthalic acid are focused er. around the difficulties in operating the reaction systems to produce economically good yields of impure terephthalic acid, compounded by the difficulties of refining impure terephthalic acid to remove impurities and unwanted components to produce purified terephthalic acid of adequate quality as initial material to produce polyester. The concomitant problems in the systems of the prior art include the high capital investment required for the ATP plants, the severity of the operating conditions of previous processes, both for the production of the impure terephthalic acid and for its purification, and the need to handle catalyst systems and reaction solvents, as well as the reaction by-products in such a way that environmental problems are minimized and the loss of the material can also be controlled.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a method and apparatus for producing purified terephthalic acid. In one aspect the method includes the production of impure terephthalic acid by the oxidation of p-xylene. The oxidation step produces not only terephthalic acid but through side reactions p-toluidic acid and 4-carboxybenzaldehyde (4-CBA). The product produced in the oxidation step is a liquid dispersion containing initial materials that did not react, solvents, if some are used, the products of the side reactions, particularly those just mentioned and other materials that are not desired in the search for purified terephthalic acid. The oxidation step of the present invention is carried out so that the conversion to the impure terephthalic acid should be at least about 30% by weight of p-xylene. Further, according to the invention, impure terephthalic acid from the oxidizer is first separated from the other oxidizer materials and then redissolved in a solvent for selective crystallization and, optionally, in one or more additional solvents of the invention described below. The impure terephthalic acid redissolved is then crystallized in the solvent for selective crystallization and additional solvents of the invention in one or, preferably, two crystallization steps. Preparation is made to separate the crystallized terephthalic acid, and progressively purified, from the solvents of the invention and the filter cake of the purified terephthalic acid finally obtained is washed with other solvents of the invention and finally dried for storage or for ~ to other processing. The invention also contemplates that the steps to recover and recycle the solvents of the invention be included in each stage of crystallization and washing, including the recycling of some recovered materials to the oxidizer. Steps are also taken to closely control the release of any of the materials harmful to the environment. In an important aspect, the present invention is based on various discoveries related to the solvents that are effective to carry out the purification of terephthalic acid: __? By the steps of crystallization and separation. These findings can be summarized in several ways as follows: The solvents for selective chromilization, useful in the practice of the present invention, include those in which: (a) the desired impurities to be separated from the terephthalic acid to purify it they are relatively more soluble in the solvent than the terephthalic acid at substantially every temperature within the desired range of temperatures at which the solvent containing the terephthalic acid can be handled and, (b) the terephthalic acid is more soluble at an elevated temperature and less soluble at a reduced or lower temperature. It should be understood that the term "solvent for selective crystallization" is proposed for solvents useful in the selective crystallization of terephthalic acid as described above and as described in more detail below and as shown in the figure. 2. In this context, it should be noted that US Patent No. 3,465,035 mentions that certain organic solvents (pyridine, dimethylsulphoxides, dimethylformamide and the like) have been used to purify terephthalic acid, but these are unstable in the air and easily form addition products with terephthalic acid. This same patent, along with some others, also shows the use of "acid" and water as purification solvents for terephthalic acid, On the contrary, the solvents for selective crystallization according to the present invention are (a) non-aqueous, (b) non-corrosive and (c) essentially non-reactive with terephthalic acid and do not include those prior practices already described, Specifically, water, acetic acid (and other alkyls) and the aforementioned organic solvents are excluded from the solvents for the selective crystallization contemplated in the present invention According to the invention, the solvent for selective crystallization is especially preferred is N-methylpyrrolidone (NMP), for various reasons described below, and for its superior performance.

N-mei ilpirroi - - '. MWWP) is the most preferred selective crystallization solvent in the practice of the invention. It is non-aqueous, thermostable, non-toxic, safe for the environment (non-corrosive and commercially available.) NMP is the preferred solvent for selective crystallization in the practice of the present invention because its solubility curve vs. temperature. for terephthalic acid it slopes up and to the right, which means that terephthalic acid can dissolve in it at elevated temperatures and precipitate or crystallize from it at low temperatures, although NMP is the solvent for selective crystallization more preferred, it should be understood that, in accordance with the present invention, it is possible to select other preferred selective crystallization solvents for the purification of impure terephthalic acid from various polar organic solvents including, but not limited to N-alkyl -2-pyrrolidone (such as N-ethylpyrrolidone), N-mercaptoalkyl-2-pyrrolidone (as N-mercaptoethyl-2-pyrrolidone) , N-alkyl-2-thiopyrrolidone (as N-methyl-2-thiopyrrolidone) and N-hydroxyalkyl-2-pyrrolidone (as N-hydroxyethyl-2-pyrrolidone) and the like and mixtures thereof. Still other solvents for selective crystallization contemplated for the present invention include, but are not intended to be limited to, s -:: -, methylsulpholane, sulfolanes, morpholines (such as morpholine and N-formylmorpholine), carbithols, alcohols from C1 to C12, ethers, amines, amides and esters and the like and mixtures thereof. It is preferred that the solvent for the desired selective crystallization can be used in a process for multistage crystallization, in combination with one or more additional solvents, preferably two of these additional solvents, particularly where the impure terephthalic acid is less than around 98% purity. Preferably, a washing solvent such as, but not intended to be limited to, p-xylene, acetone, methyl ethyl ketone (MEK) or methanol or the like are used in the washing of the initial filter cake obtained from the first separation of the terephthalic acid. impure from other materials originated in the oxidizer. In addition, a solvent having a low boiling point may be used for displacement, as it may be, but it is not intended to be limited to methanol, acetone, MEK and the like. Preferably methanol is used as the displacement solvent together with the third filtrate that follows the second crystallization step in the preferred process. The desired displacement solvent displaces the selective crystallization solvent from the resulting filter cake, means of which only the displacement solvent during the drying process will be present. It should be understood that the low boiling point of the displacement solvent facilitates the drying of the filter cake. As described in the above, NMP is the most preferred solvent for selective crystallization in the practice of the invention. This is a solvent non-aqueous, thermostable, non-toxic (safe for the environment), non-corrosive and available commercially. NMP is preferred as a solvent for selective crystallization for the present prac- tice, because, among other things, the solubility curve vs. The temperature for terephthalic acid slopes upward and to the right, which means that terephthalic acid can dissolve in it at elevated temperatures and precipitate or crystallize from it at low temperatures. However, the solubility curve vs. temperature for terephthalic acid is of a much milder slope than the solubility curves in NMP for other materials to be separated from impure terephthalic acid, such as benzoic acid, 4-carboxybenzaldehyde (4-CBA) and p-acid. toluidic Due, when the impure terephthalic acid, which contains or is associated with initial materials that did not react, solvents (if any) and side products, such as those mentioned above or other materials not desired, dissolves in NMP at an elevated temperature, substantially all materials dissolve, or at least are highly dispersed. Then, when removing the heat and cooling the NMP solution with these dissolved materials, the pure terephthalic acid preferably crystallizes, while the other more soluble materials that can be considered as impurities for the present purpose remain in solution in the NMP. In this way a separation is effected between the purified terephthalic acid and its impurities or "xa?". The NMP can be separated from the impurities in a regeneration column and recycled to the process, while impurities can be recycled to the oxidation step or otherwise disposed of. From the foregoing it can be seen that, according to one aspect of the present invention, there is provided a method for producing purified terephthalic acid from impure terephthalic acid, in which the impure terephthalic acid is dissolved in a solvent for the crystallization at an elevated temperature to form a solution and, further, to which a purified terephthalic acid crystallizes from this solution at a reduced temperature. According to another aspect of the invention, an apparatus method is provided -; to allocate impure terephthalic acid from a liquid dispersion thereof which also contains initial materials that did not react, solvents, products of side reactions and / or other unwanted materials in which the impure terephthalic acid is filtered from this dispersion to partially remove it of the other materials contained therein, by filtration, to produce a filter cake with impure terephthalic acid, to then dissolve this filter cake in a solvent for selective, desired crystallization at an elevated temperature to form a solution. The purified phthalic acid is crystallized from this solution by reducing the temperature thereof and is separated from the solvent after crystallization According to yet another aspect of the invention, a method and apparatus for producing terephthalic acid is provided. purified from impure terephthalic acid by dissolving the impure terephthalic acid in a solvent suitable for selective crystallization at an elevated temperature to form a first solution The terephthalic acid purified in the first stage is crystallized from this first solution at a reduced temperature The terephthalic acid purified in the first stage is separated from the solvent solution of other impurities and redissolved in the appropriate selective crystallization solvent at an elevated temperature to make a solution. crystallizes at a reduced temperature to form a purified terephthalic acid in the second stage, which is separated from the second solution. According to yet another aspect of the invention, impure terephthalic acid is synthesized by contacting para-xylene with oxygen in an oxidant reaction. The impure terephthalic acid is removed from the oxidizer and is separated in the crude from the side products of the reaction, and the initial materials that did not react. The separated impure terephthalic acid is dissolved in a solvent for the desired selective crystallization at an elevated temperature and crystallized therefrom as purified terephthalic acid at a reduced temperature. It is possible to perform more than one step of dissolving in a solvent for the desired selective crystallization at an elevated temperature followed by crystallization at a reduced temperature, with the separation and washing of the purified, crystallized terephthalic acid. From the foregoing it can be seen that, an object of the present invention is to provide an improved method and apparatus for producing purified terephthalic acid of a desired purity for use in the formation of polyester resin and other products, in an economical ratio. '"- - i ~ ^ Lractive and in operating conditions of reduced severity, which requires less capital investment and simplified processing. The manner in which these and other objects of the invention are achieved can be learned by taking into consideration the detailed description of the invention, which is shown below together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the method and apparatus of the present invention can be obtained in connection with the following Detailed Description when taken in conjunction with the accompanying drawings, wherein: Figures IA and IB are flow charts simplified of an apparatus with which the method according to the invention can be practiced, wherein Figure IA is the part of the apparatus for carrying out the steps of the first crystallization stage and wherein Figure IB is the portion of the apparatus for perform the steps of the second stage of crystallization; and Figure 2 is a graph of the solubility curve vs. temperature for terephthalic acid and for impurities or products of side reactions commonly associated with impure terephthalic acid.

DETAILED DESCRIPTION OF THE MODES 1. Description of the process The present invention relates to the development of a new ATP manufacturing technology. Compared to the ATP technology that is currently widely used, this technology provides a substantially lower capital investment in the construction of a new ATP plant, as well as lower operating costs of the plant. Means are also provided for current DMT plants to co-produce ATP, to strengthen their competitiveness against the newer ATP plants.

Competence of the process' The success of this process is based on the development of a highly selective crystallization technology, with low pressure, low temperature and non-aqueous. The crystallization technology can purify impure terephthalic acid (AT) with purity as low as from about 70% (from the oxidizer) and about 98 +% in the first stage of crystallization, and about 99.99 +% in the second crystallization stage. This allows the AT oxidizer to be operated at a much lower severity than those prior art processes widely used. In accordance with the present invention, acetic acid (as a solvent / diluent) or bromo-crystallizer initiator is not necessary during oxidation. The solvent for the selective crystallization used in the ristali2jc? On process is non-aqueous, thermostable, non-toxic (safe for the environment), non-corrosive and available commercially. When carrying out the method according to the invention with the apparatus shown in Figures IA and IB, using NMP as a solvent for selective crystallization, the present inventors have demonstrated ATP purity levels of up to 99.9 +% by weight after a first crystallization process, and up to 99.99 +% by weight after a second crystallization process. In particular, Table 1 illustrates the recovery of ATP with a weight of 99.35. by weight after the first crystallization process and ATP with a purity of 99.997% by weight after the second crystallization process from impure ATP (89.89% by weight of ATP). Table 1 Crystallization 2a. Crystallization (a) Weight of ATP: 56.34 grams 31.81 grams (b) Weight of solvent for crystallization 400.02 grams 248.38 grams (c) Temperature of 60 ° C saturation: (d) Temperature of 15 ° C (one hour) crystallization (1) Composition of the product impure ATP: Benzoic p-toluidic 4-CBA ATP Other 0.3 ^^ by 4.49% by 2.49% by 89.89% by 2.74% or weight weight of weight (2) Product of the first crystallization 35 ppm 143 ppm 359 ppm 99.95% by no weight detected (3) Product of the second crystallization < 20ppm < 20ppm < lOppm 99,997 +% by weight Table 2 illustrates the recovery of ATP with 99.90% by weight in purity after the first crystallization process and ATP with 99.9933% by weight of purity after the second crystallization process from impure ATP (ATP 89.89 % by weight) increasing the saturation temperature and the crystallization temperature.

Table 2 la. Crystallization 2a. Crystallization (a) Weight of ATP: 138.08 grams 70.15 grams (b) Weight of 685.30 grams 247.46 grams solvent for crystallization: (c) Temperature of 110 ° C saturation: (d) Temperature of 40 ° C (one hour) crystallization: (1) Composition of the product impure ATP: Benzoic p- toluidic 4-CBA ATP Other 0.39% by 4.49% by 2.49% by 89.89% by 2.74% by weight weight of weight (2) Product of the first crystallization (recovery: 56.5% by weight) 35 ppm 143 ppm 359 ppm 99.95% by no weight detected (3) Product of the second crystallization (recovery: 47.5% by weight) < lOppm < 19ppm < 25ppm 99.9933% 13ppm by weight Table 3 illustrates the recovery of ATP with 99.9960% by weight purity (a single crystallization process) from impure ATP (98.99% by weight). In addition, benzoic, p-toluidic, 4-CBA, MMT and other impurities were less than 10 ppm.

Table 3 (a) weight of ATP: 152.67 grams (b) Weight of solvent 786.19 grams for crystallization: (c) Temperature of 100 ° C saturation: (d) Temperature of 40 ° C (one hour) crystallization: (1) Composition of the impure ATP product: Benzoic p-toluidic 4-CBA ATP MMT other < 10ppm < 10ppm 18ppm 98.99% for 303ppm 0.98% weight by weight (2) Product of crystallization (recovery 50.2% by weight) < 10ppm < 10ppm < 10ppm > 99.9960% for < 10ppm < 10ppm weight Table 4 illustrates the recovery of ATP with 99.63% by weight purity (a single crystallization process) from impure ATP (83.91% by weight ATP) in a large-scale process Table 4 (a) Weight of ATP: 1760 grams (b) Weight of solvent for 6162 grams crystallization: (c) Temperature of 160 ° C saturation: (d) Temperature of 50 ° C (one hour) crystallization: (1) Composition of the impure ATP feed product: Benzoic p-toluidic 4-CBA ATP Other 1. 03% 4.79% by 5.03% 83.91% by 5.24% by weight weight by weight weight (2) Product of crystallization (recovery 24.3% by weight) 38ppm 852ppm 0.23% by 99.63% per 500ppm weight weight Table 5 illustrates the recovery of ATP with 99.92% by weight purity (a single crystallization process) from impure ATP (79.79% by weight ATP) in a large-scale process.

Tao ± a 5 (a) Weight of ATP: 1700 grams (b) Weight of the solvent for the 5928 grams crystallization: (c) Saturation temperature: 160 ° C (d) Temperature of 45 ° C crystallization: (1) Composition of the impure ATP feed product: Benzoic p-toluidic 4-CBA ATP Other 1.59% 5.19% by 7.61% by 79.79% by 5.81% by weight by weight weight of weight (2) Product of crystallization (recovery 31.5% by weight) lOppm 203ppm 446ppm 99.92% by 184ppm weight Table 6 illustrates the recovery of ATP with 99.15% by weight purity (a single crystallization process) from impure ATP (83.90% by weight ATP) in a large scale process at a saturation temperature higher than 190 ° C .

Table 6 (a) Weight of ATP: 1965 grams (b) Weight of solvent for 5684 grams crystallization: (c) Temperature of 190 ° C saturation: (d) Temperature of 40 ° C (one hour) crystallization: (1) Composition of the product of the impure ATP feed: Benzoic p-toluidic 4-CBA ATP other 1.23% 5.25% for 5.34% for 83.90% for 3.28% for by weight weight weight weight (2) Product of crystallization (recovery 48.9o for weight) 0.41% for 0.61% for 99.15 % by 0.1% by weight weight weight Table 7 illustrates the recovery of ATP with 99.9915% by weight purity from impure ATP (98.50% by weight ATP) in a large-scale process. The supersaturation of the crystallization mixture resulted in the formation of ATP crystals substantially larger than the crystals that were obtained in the processes summarized in the above. As will be understood by those skilled in the art, the size of the ATP crystals is an important consideration for the separation of these from solvents and impurities.

Table 7: ATP weight: 2333 grams (b) Solvent weight for 5698 grams crystallization: (c) Temperature of 160 ° C saturation: (d) Temperature of 45 ° C (one hour) crystallization: (1) Composition of the impure ATP feeding product: Benzoic p-toluidic 4-CBA ATP Other 198ppm 0.15% by 1.23% by 98.50% by 989ppm weight of weight (2) Product of crystallization (recovery 69.7% by weight) < 10ppm 26ppm 38opm 99.9915% llppir. By weight According to the invention and as can be seen in the flow diagrams of the process of Figures IA and IB, a preferred embodiment of the process is divided into 5 sections: (1) Oxidation section: In this section p-xylene is oxidized according to the following main reactions: (a) p-xylene + oxygen terephthalic acid (b) p-xylene + p-toluid oxygen (c) p- xylene + oxygen »4-carboxybenzaldehyde (4-CBA) The HP time in the oxidizer is approximately 5 hours. Since the oxidizing effluent will contain up to about 30% of AT, mixing in the oxidizer will be very important to maintain the performance and selectivity, to avoid scale and blockages. The initial mixing of the feed streams can be carried out in a static mixer (outside the oxidizer). Other mixing can be provided by a spray tube with air and by external circulation. Depending on the perfection of the washing step of p-xylene in the filter (described below), the tereftá.1 acid. z ^? ) an-i bóJ can vary from ° n re to about 55% and close to 90 +%. (2) Crystallization Section: (A) First Crystallization After filtration, the solids from the oxidizer effluent are mixed with the mother liquor and the solvent wash liquor from the second stage crystallizer and with additional crystallization solvent. The mixed suspension is dissolved in a suspension tank at a predetermined temperature, preferably from about 140 ° C to about 190 ° C. The saturated solution is transferred to a resting tank to remove the p-xylene by means of evaporation. The saturated solution is then fed to the crystallizer in the first stage batch to recover the purified AT. After the crystallization step, the content of the crystallizer betweenIt is dropped into a tank containing the product and pumped continuously into a filter (or centrifuge) to collect the solids to be recrystallized in the second stage crystallizer for further purification. (B) Second crystallization The solids produced from the filter of the first crystallizer are redissolved in a solvent solvent with the crystallization solvent for the crystallizer of the second stage in a predetermined condition, such as at a temperature from about .? C and 190 ° C. The saturated solution is pumped into the crystallizer of the second stage for crystal growth and recovery. Then, the contents of the crystallizer are dropped into a standing tank for final filtration and drying steps. In the filtration step, the solid (cake) is first washed with the crystallization solvent to displace the mother liquor remaining in the cake. The solid is then washed with a low boiling solvent to displace the crystallization solvent in the cake. The wet cake is sent to the dryer to remove the final liquid of the ATP product. (3) Mother liquor / solvent recovery section: The mother liquor of the first crystallizer filter is transferred to a solvent recovery column to recover the crystallization solvent from the top of the column. The impurities, as they may be, but are not intended to be limited to p-toluidic acid, benzoic acid, 4-carboxybenzaldehyde (4-JSCA) and the like, are recovered from the bottom of the column. To ensure that the suspension in the lower part of the column can be transferred back to the oxidizer, diluent with a high boiling point is preferably added to the heat exchanger. II. Detailed description of the process and example The flow chart of the process of Figures IA and IB will be described in terms of the production and recovery of 65 kg per hour of terephthalic acid (AT) from the oxidation in air of p-xylene in presence of a solution of catalyst components in dimethyl terephthalate (DMT) or in a benzoic acid-water solvent system. The temperature of the oxidizer is preferably between about 150 ° C and about 250 ° C and the pressure is between about 5 and about 10 kg x cnr. Since the effluent of the oxidizer will contain up to 30. of AT, the mixing in the oxidizer is very important to maintain the yield and selectivity, and to avoid clogging and fouling. The initial mixing of the feed streams can be obtained in a static mixer (outside the oxidizer). Another mixing can be done through a spray tube with air and external circulation. In the preferred form of the process about 0.001-0.05 kg per hour of manganese acetate and 0.003-0.010 kg per hour of cobalt acetate in aqueous solution are fed to the oxidizer to catalyze the oxidation reactions. The oxidizer effluent (483.7 kg / h) at about 160 ° c is transferred through line 1 to the F-1 filter to separate the solid from the mother liquor (filtrate). In filter F-1 the solid cake is washed with 250.0 kg / h of p-xylene which is heated by heater E-1 from 30 to 100-150 * C. The liquid mother (281.4 kg / h) of filter F-1 is transferred through line 3 to tank T-1. The filter cake washing liquid F-1 (306.9 kg / h) is removed separately from the filter via line 4 to tank T-2. The washed cake of filter F-1 is emptied into tank T-3 by line 8 to mix it with the following streams: Current 19: 67.9 kg per hour of NMP wash liquor (solvent for selective crystallization) of filter F -3 (heated from 45 to 100-150 ° C by heater E-5). Current 9: 329.3 kg per hour of mother liquor from tank T-17 (heated from 50 to 100-150 ° C by heater E-3). Current 37: 43. kg per P: a.4P of tank T-19 (heated from 45 to 100-150 ° C by heater E-4). The above mixture (586.1 kg / h) of tank T-3 is then transferred from the bottom of the tank through line 10 to the dissolver T-4. The content in tank T-4 is indirectly heated from 100-150 ° C to 140-190 ° C by means of a heating coil for hot oil in the tank. About 75% of p-xylene (32.8 kg / h and 100% of the bubbling nitrogen (10 kg / h) in the mixture are vaporized from the dissolver and removed through line 11. The bubbling nitrogen is added to the tank T- 4 through line 47 to help eliminate p-xylene. "'• -The vapor currents 11 and 13 are combined in stream 5 (51.6 kg / h), they are condensed by the E-3 cooler and sent to the PX T-5 storage tank. The effluent from the lower part of the dissolver T-4 is transferred to the crystallizer S-1 as a batch. The content of the batch in the impure crystallizer S-1 is cooled from 140-190 ° C to 10-20 ° C through an external E-6 cooler to generate the desired supersaturation for the growth of the AT crystals. To improve the distribution of the size of the crystal and the recovery of the solids it can be useful to plant crystals. At the end of a batch crystallization cycle, the suspension is emptied into tank T-6 and transferred to the F-2 filter continuously at a rate of 5 l .6 kg per hour through line 15. In the F-2 filter, 42.7 kg per hour of NMP (from line 38) is used to wash the cake. The mother liquor plus the NMP scrubber are combined in stream 16 (432.6 kg / h) to be fed to the D-l column to recover the NMP. The washed cake (154.7 kg / h) is emptied into the dissolving tank T-8 where it is mixed with 241.3 kg per hour of NMP to form the supersaturation fed to the pure crystallizer S-2. The NMP is heated from 45 ° C to 140-190 ° C by the E-7 heater and fed to the T-8 tank at cic-? of the ii .. •• 1-8. The content of tank T-8 is transferred as batches to the pure crystallizer S-2 where the temperature is cooled from 140-190 ° C to 30-60 ° C to induce the growth of the AT crystals. Cooling is provided by circulating the contents of the crystallizer through an external E-8 cooler. Again, to improve the size distribution of the crystal and the recovery of the crystals it can be useful to plant crystals. At the end of the batch cycle, the suspension is emptied from crystallizer S-2 to tank T-10 (tank fed to filter F-3). The suspension is fed to the F-3 filter continuously through line 22 at a speed of 395, -; .g per hour. The mother liquor of the filter (301.8 kg / h, is transferred to tank T-17 through line 23. The cake is initially washed with NMP at 45 ° C to remove the remaining mother liquor from the cake and then the cake wash with the displacement solvent with low boiling point, such as methanol, to displace the NMP from the cake.From T-19, the washing of NMP is added through line 24 and, from T-18, the Displacement solvent is added through line 25 to F-3 (both at a rate of 64 kg / h) .The NMP washing liquid (67.9 kg./h) is sent to tank T-3 (suspension tank F-1; _., ÍV¡ < s from line 19, while the displacement solvent (64.1 kg / h) is transferred to tank T-13 through line 26.

The washed cake of filter F-3 (90.2 kg / h) is passed through line 27 to the dryer of product DR-1, where the displacement solvent in the cake is removed by heating and purging with a countercurrent flow of heated nitrogen. The dry ATP product (65.2 kg / h) is removed from the dryer through line 28 and stored in the product hopper. The displacement solvent, saturated with nitrogen (76.4 kg / h) of the DR-1 product dryer, is vented from the dryer through line 29 to the T-15 condenser, where it is mixed with gasses. 25-45 ° C circulating through the E-12 cooler. The condensed displacement solvent (21.2 kg / h) is transferred to the displacement solvent tank T-18, while the non-condensing gas (55.2 kg / h) is exited from the T-15 through line 30 to the ventilation tank T-16. Approximately 2.4 kg per hour of NMP are fed into the ventilation vessel T-16 of line 39 to trap the displacement solvent in stream 30 (3.8 kg / h). The trapped displacement solvent, plus NMP (6.1 kg / h), is transferred to tank T-13 through "- line 33." The nitrogen removed from the T-16 (51.4 kg / h) is sent back to the dryer by means of a fan through line 32 and through heater E-ll (to heat the current from 25 ° C to 80-120 ° C.) The lower stream of tank T-13 (70.3 kg / h -NMP mixture and displacement solvent) are transferred through line 34 and heater E-9 (to heat the current from 25 ° C to 80-120 ° C) to the evaporator of the displacement solvent T-14. Steam from the top-traveling solvent of T-14 (42.7 kg / h) is condensed by condenser E-10 and sent to the displacement solvent tank T-18 via line 35. The lower stream of T-14 (27.5 kg / h) is divided into two streams: stream 39 (2.4 kg / h) to the ventilation vessel T-16, and stream 40 (2 • "" ¿?:? T-17 tank of the mother liquor of the F-3 filter. The mother liquor and NMP of the F-2 filter wash are transferred to the T-7 tank through line 16 and then fed to the NMP recovery column D-1. This current (432.6 kg / h) is heated from 15-25 ° C to 130-170 ° C by means of heater E-13 before entering column D-1. The steam from the upper part (433.3 kg / h) is condensed by means of the condenser E-15 and sent to the condenser vessel T-20 through the line 41. A part of the condensate (39.4 kg / h) a 160-220 ° C is returned to the column as reflux through line 42. The rest of the product from the top of column D-1 (393.9 kg / h) is sent to the verification tank T-21 of the NMP through line 43. From the T-21 tank, the regenerated NMP is pumped to the storage tank T-19 of the NMP. To ensure that the suspension in the exchanger of column t-1 can be transferred back to the oxidizer, from 20 to 60 kg. per hour of high-boiling diluent, such as benzoic acid or DMT, are added to the exchanger via line 45. The suspension plus the high-boiling diluent (78.8 kg / h) are removed from the bottom of the column D-1 and are returned to the oxidizer via line 49. Although a preferred embodiment of the method and apparatus herein is illustrated in the accompanying drawings and has been described in the detailed description mentioned above, it will be understood that the The invention is not limited to the described embodiment, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.

Claims (32)

1. CLAIMS A method for purifying impure terephthalic acid from a liquid dispersion thereof, which also contains impurities that are selected from unreacted starting materials, solvents, side reaction products and / or other unwanted materials, the method comprising: filtering the dispersion to form an impure terephthalic acid filter cake; dissolve the filter cake in a solvent for selective crystallization at an elevated temperature dec1 ^ between *. . .a of -.40 ° C and close to 190 ° C to form a solution; crystallize the purified terephthalic acid from this solution in the crystallization solvent by reducing the temperature of the solution; and separating the crystallized purified terephthalic acid from the solution.
2. The method according to claim 1, wherein the dispersion contains 4-carboxybenzaldehyde (4-CBA).
3. The method according to claim 1, wherein the temperature of the solution is reduced by between about 5 ° C and about 50 ° C.
4. The method according to claim 3 wherein the temperature of the solution is reduced from between about 10 ° C and about 20 ° C.
5. The method according to claim 1, wherein the crystallization of the purified terephthalic acid from the solution in the solvent for selective crystallization is repeated by redissolving the purified terephthalic acid and crystallized in the solvent for selective crystallization to form a solution Redisueite at a high temperature; and crystallizing the purified terephthalic acid from the solution redissolved in the solvent for the crystallization selectively reduced to the temperature of it.
6. The method according to claim 1, wherein the solvent for selective crystallization is N-methyl pyrrolidone.
7. The method according to claim 1, wherein the solvent for selective crystallization is selected from the group consisting of N-alkyl-2-pyrrolidone, N-ethyl pyrrolidone, N-mercaptoalkyl-2-pyrrolidone, N-mercaptoethyl-2 -pyrrolidone, N-alkyl-2-thiopyrrolidone, N-methyl-2-thiopyrrolidone, N-hi -.- :. ic._qu l-2-pyrrolidone and N-hydroxyethyl-2-pyrrolidone.
8. 8. The method according to claim 1, wherein the solvent for selective crystallization is selected from the group consisting of sulfolane, methylsulpholane, sulpholanes, morpholine, N-formyl morpholine, carbithols, Cl to C12 alcohols, acetonitrile, adiponitrile, butyronitrile , ethers, amines, amides and esters.
9. The method according to claim 1 and further comprising the step of washing the filter cake with a wash solvent which is selected from the group consisting of p-xylene, acetone, methyl ethyl ketone, and methanol.
10. 10. The method according to claim 9, in which the wash solvent is p-xylene.
11. The method according to claim 1 and further comprises the step of displacing the selective crystallization solvent subsequent to the crystallization step with a displacement solvent which is selected from the group consisting of methanol, methyl ethyl ketone and acetone.
12. 12. The method according to claim 1, wherein the displacement solvent is methanol.
13. 13. A method for producing purified terephthalic acid from acid; uro consisting of: dissolving impure terephthalic acid in a solvent for selective crystallization at an elevated temperature from about 140 ° C to about 190 ° C to form a solution; and crystallizing the purified terephthalic acid from the solution at a reduced temperature.
14. The method according to claim 13, wherein the solvent for selective crystallization is N-methyl pyrrolidone.
15. The method according to claim 13 further comprises the step of separating the purified terephthalic acid from the solution.
16. The method according to claim 15 wherein the step be? X tion is effected by filtering or centrifuging the purified terephthalic acid from this solution, wash it with the solvent for the selective crystallization and with the displacement solvent, and after that dry it.
17. The method according to claim 13, wherein the solvent for selective crystallization is selected from the group consisting of N-alkyl-2-pyrrolidone, N-ethyl pyrrolidone, N-mercaptoalkyl-2-pyrrolidone, N-mercaptoethyl-2. -pyrrolidone, N-alkyl-2-thiopyrrolidone, N-methyl-2-thiopyrrolidone, N-hydroxyalkyl-2-pyrrolidone and N-hydroxyethyl-2-pyrrolidone.
18. The method according to claim 13, wherein the solvent for selective crystallization is selected from the group consisting of sulfolane, methylsulpholane, sulfolanes, morpholine, N-formyl morpholine, carbithols, Cl to C12 alcohols, acetonitrile, adiponitrile, butyronitrile , ethers, amines, amides and esters.
19. The method according to claim 13, wherein the impure terephthalic acid is washed with a wash solvent which is selected from the group consisting of p-xylene, methanol, acetone and methyl ethyl ketone.
20. The method according to claim 19 wherein the wash solvent is p 'z:.
21. The method according to claim 13 consists in the step of displacing the selective crystallization solvent subsequent to the crystallization step with a displacement solvent which is selected from the group consisting of methanol, methyl ethyl ketone and acetone.
22. The method according to claim 21, wherein the displacement solvent is methanol.
23. A method for producing purified terephthalic acid from impure terephthalic acid comprising: dissolving the impure terephthalic acid in a solvent for selective crystallization * at an elevated temperature from about 140 ° C to 190 ° C to form a first solution; crystallizing the purified terephthalic acid from the first stage from the first solution at a reduced temperature; separating the purified terephthalic acid crystallized in the first step from this solution; redissolving the purified terephthalic acid in the first stage, separated in a solvent for selective crystallization at an elevated temperature to form a second solution; crystallizing the purified terephthalic acid in a second step from the second solution at a reduced temperature; and separating the crystallized purified terephthalic acid in the second stage from the second solution.
24. The method according to claim 23, wherein the separation step is carried out by filtering or centrifuging the purified terephthalic acid from the solution, washing it with the solvent for selective crystallization and with a displacement solvent and then drying it.
25. The method of agreement according to claim 23, wherein the crystallization solvent is N-methyl pyrrolidone. *
26. 26. The method according to claim 23, wherein the solvent for selective crystallization is selected from the group consisting of N-alkyl-2-pyrrolidone, N-ethyl pyrrolidone, N-mercaptoalkyl-2-pyrrolidone, N-mercaptoethyl-2. -pyrrolidone, N-alkyl-2-thiopyrrolidone, N-methyl-2-thiopyrrolidone, N-hydroxyalkyl-2-pyrrolidone and N-hydroxyethyl-2-pyrrolidone.
27. 27. The method according to claim 23 wherein the selective crystallization solvent is selected from the group consisting of sulfolane, methylsulpholane, sulpholanes, morpholine, N-formyl morpholine, carbitoles, ale' ". Cl to C12, acetonitrile, adiponitrile, butyronitrile, ethers, amines, amides and esters
28. 28. The method according to claim 23, wherein the impure terephthalic acid is washed with a wash solvent which is selected from the group consisting of p-xylene, methanol, acetone and methyl ethyl ketone
29. 29. The method according to claim 28 wherein the washing solvent is p-xylene
30. 30. The method according to claim 23 comprises the step of displacing the solvent for the selective crystallization after the passage of the crystallization of the second stage with a displacement solvent which is selected from the group consisting of methanol, methyl ethyl ketone and acetone
31. 31. The method according to claim 1 n 30, wherein the solvent is methanol displacement.
32. 32. A method for purifying impure terephthalic acid from a liquid dispersion thereof that also contains impurities that are selected from unreacted starting materials, solvents, side reaction products and / or other unwanted materials, the method consists of steps of: dissolving the impure terephthalic acid in N-methyl pyrrolidone at a temperature of between about 140 ° C and close; The first solution is to crystallize the impure terephthalic acid dissolved from the first solution at a temperature of between about 5 ° C and about 50 ° C to form the purified terephthalic acid in the first step: separating the purified terephthalic acid in the first stage from the first solution by filtration to form a filter cake of the purified terephthalic acid in the first step: washing the filter cake of the purified terephthalic acid in the first step, separated with "N-methyl pyrrolidone; redissolving the filter cake of the purified terephthalic acid in the first stage in NMP at a temperature of between about 140 ° C and about 190 ° C to form a second solution; crystallizing the purified terephthalic acid in the first stage, redissolved from the second solution at a temperature of between about 5 ° C and about 50 ° C to form purified terephthalic acid in the second stage; separating the purified terephthalic acid in the second stage from the second solution by filtration to form a filter cake of purified terephthalic acid in the second stage; washing the filter cake of the purified terephthalic acid in the second step with N-methyl pyrrolidone; washing with methanol the filter cake washed with N-methyl pyrrolidone; and drying the filter cake of the purified terephthalic acid in the second stage to provide purified terephthalic acid in the second stage. A method for preparing purified terephthalic acid consisting of the steps of: contacting p-xylene with oxygen to form tereic acid in a dispersion thereof which also contains impurities selected from starting materials do not react, solvents, products of side reactions and / or other unwanted materials; dissolve the impure terephthalic acid in N-methyl pyrrolidone at a temperature from about 140 ° C and about 190 ° C to form a first solution; crystallize from the first solution the impure terephthalic acid dissolved at a temperature between about 5 ° C and about 50 ° C to form the purified terephthalic acid in the first stage; separating from the first solution, by filtration, the purified lerephthalic ion in the first stage p-.tra forming a filter cake of the purified terephthalic acid in the first stage; washing with N-methyl pyrrolidone the filter cake of the purified terephthalic acid in the first step, separated; redissolve in N-methyl pyrrolidone the filter cake of the purified terephthalic acid in the first stage at a temperature of between about 140 ° C and close to 190 ° C to form a second solution; crystallize from the second solution the purified terephthalic acid in the first stage, redissue, at a temperature between about 5 ° C and about 50 ° C to form purified terephthalic acid in the second stage; separating the purified terephthalic acid in the second stage by filtration from the second solution to form the filter cake of the purified terephthalic acid in the second stage; washing with N-methyl pyrrolidone the filter cake of the purified terephthalic acid in the second stage; washing with methanol the filter cake washed with N-methyl pyrrolidone; and drying the filter cake of the purified terephthalic acid in the second stage to provide terephthalic acid. , or purifA._tac sr. xa second stage. An apparatus for purifying impure terephthalic acid from a liquid dispersion thereof which also contains impurities that are selected from unreacted starting materials, solvents, side reaction products and / or other unwanted materials, the apparatus consists of: to filter the dispersion to form an impure terephthalic acid filter cake; the means for dissolving the filter cake in a solvent for selective crystallization at an elevated temperature of between about 140 ° C and about 190 ° C to form a solution; the means for crystallizing the purified terephthalic acid from the solution in the crystallization solvent by reducing the temperature of the solution; and the means for separating the purified terephthalic acid, crystallized from this solution. An apparatus for producing purified terephthalic acid from impure terephthalic acid, consists of: the means for dissolving impure terephthalic acid in a solvent for selective crystallization at an elevated temperature from about 140 ° C to about 190 ° C to form a first solution; the means for crystallizing purified t-cyclic acid in the first stage of the first solution at a reduced temperature; the means for separating the crystallized terephthalic acid purified in the first stage from the solution; the means for redissolving the purified terephthalic acid in the first stage, separated in the solvent for selective crystallization at an elevated temperature to form a second solution; the means for crystallizing the purified terephthalic acid in the second stage from the second solution at a reduced temperature; and the means for separating the purified terephthalic acid in the second, crystallized stage from the second solution. An apparatus for purifying impure terephthalic acid from a liquid dispersion thereof, which also contains impurities that are selected from the non-reacting starting materials, solvents, side reaction products and / or other unwanted materials, the apparatus consists of: the means for dissolving impure terephthalic acid in NMP at a temperature of between about 140 ° C and about 190 ° C to form a first solution; the means for crystallizing the impure terephthalic acid dissolved from the first solution at a temperature of between about 5 ° C and about 50 ° C to form the purified terephthalic acid in the first stage; the means for separating the purified terephthalic acid in the first stage from the first solution by filtration to form a filter cake of the purified terephthalic acid in the first step; the means for washing with N-methyl pyrrolidone the filter cake of the purified terephthalic acid in the first step, separated; the meaxu to edise the filter cake of the purified terephthalic acid in the first stage in NMP at a temperature of between about 140 ° C and about 190 ° C to form a second solution; the means for crystallizing the purified terephthalic acid in the first stage, redissolved from the second solution at a temperature of between about 5 ° C and about 50 ° C to form the purified terephthalic acid in the second stage; the means for separating the purified terephthalic acid in the second stage from the second solution, by filtration to form a filter cake of purified terephthalic acid in the second stage; the means for washing with N-methyl pyrrolidone the filter cake of the a; purified ether in the second stage; the means for washing with methane the filter cake washed with N-methyl pyrrolidone; and the means for drying the filter cake of the purified terephthalic acid in the second stage to provide purified terephthalic acid in the second stage.