A method of obtaining a purified terephthalic acid

 

The invention relates to the purification of terephthalic acid which is a raw material for producing polyester resin. Purify the crude terephthalic acid from a liquid dispersion containing impurities selected from unreacted starting materials, solvents, products of side reactions and/or other undesirable impurities by filtering the specified dispersion to produce the filter cake of crude terephthalic acid. The filter cake is dissolved in a solvent selective crystallization at a temperature in the range of 50 - 250oWith the formation of a solution. Crystallized purified terephthalic acid from the solution by reducing its temperature. Allocate crystallizing purified terephthalic acid from the solution and re-dissolved in a solvent selective crystallization to obtain a second solution. Crystallized terephthalic acid of the second stage of the solution by reducing the temperature and pressure sufficient to instantaneous equilibrium evaporation (flash evaporation) of the solvent from the second solution of terephthalic acid, but without cooling the solution to a temperature below the 50oC. Produce purified terephthalic acid second STV interval 150-300oC. Soaked with water terephthalic acid is filtered and dried. Also variants of the method of purification of terephthalic acid. The technical result - the increase of the purity of the product, improving economic and environmental performance of the process. 4 C. and 30 C.p. f-crystals, 10 ill., 12 table.

The invention is a partial continuation of Application Serial No. 08/760890 declared December 6, 1996, entitled "Method and apparatus for producing purified terephthalic acid, which, in turn, is a partial continuation jointly considered Application Serial No. 08/477898, announced June 7, 1995, entitled "Method and apparatus for producing purified terephthalic acid", both applications assigned to the same assignee as this application, and content in its entirety introduced in the form of references to all the objects of the invention.

This invention relates to a method and apparatus for producing purified terephthalic acid. It also relates to methods and devices for purification of crude terephthalic acid to produce purified terephthalic acid, which is a useful raw material for producing polyester resins are useful, in turn, to obtain the ox is such as fiberglass.

Purified terephthalic acid (TCI) is the starting material for obtaining the polyester resin, which, in turn, is used to obtain a variety of commercial materials having various applications. Purified terephthalic acid is produced from crude terephthalic acid (JCC) usually with the help of a number of cleaning methods, often with the use of catalysts. Thus, the methods of purification of crude terephthalic acid, available up to the present time, are not quite satisfactory either from an engineering point of view, either from an economic point of view, in addition, the purity of the purified terephthalic acid is an important determinant of the ways in which obtain a polyester resin.

There are several reaction systems to obtain the crude terephthalic acid from various raw materials. Aspects of cleaning of the present invention can be used essentially with any of these reaction systems, but, in accordance with the invention, it is preferable application of the reaction system, comprising the oxidation of para-xylene (p-xylene), and the use of such synthesis system is part of this invention.

Problems existing in present systems the Rog problems getting working in the reaction system is good from an economic point of view outputs crude terephthalic acid, due to difficulties in purification of crude terephthalic acid to remove impurities and unwanted components and obtain purified terephthalic acid quality, acceptable for use as raw material in the production of polyester. Related problems of the systems of the prior art include the high capital costs required for production installations of QCD, the rigidity of the working conditions of the methods of the prior art as being received for crude terephthalic acid, and at the stage of purification, the need to use catalyst systems and reaction solvents and by-products of reactions in a way that minimizes environmental problems, as well as monitoring loss of material.

An important factor in obtaining purified terephthalic acid is to obtain crystals having a size and shape that gives it a good technological characteristics, the ability to undergo washing and filtration in the production method of the QCD and easier and better to be processed in the process of obtaining a complex polyester.

According to this invention provides a method and apparatus for producing purified terephthalic Cascadia oxidation leads to the production not only of terephthalic acid, but p-Truelove acid and 4-carboxybenzene (4-CBA) as an intermediate oxidation products, and other impurities from side reactions. The product obtained at the stage of oxidation, is a liquid dispersion containing unreacted starting materials, solvents, if used, the intermediate oxidation products, in particular products, and other substances that are undesirable in the resulting purified terephthalic acid. Oxidation step of the present invention is carried out in such a way that the conversion of crude terephthalic acid should be at least about 30 wt.% on passage p-xylene.

In addition, according to this invention the crude terephthalic acid from the oxidation apparatus is first roughly separated from the other materials of the device of oxidation and then re-dissolved in a solvent selective crystallization, and optionally one or more additional solvents of the invention discussed below. Re-dissolved crude terephthalic acid is then crystallized from a solvent selective crystallization and additional solvents of the invention by one or preferably two-stage crystal is of rastvoritelei of the invention, and finally obtained filter cake of purified terephthalic acid is washed out in other solvents of the invention and dried or alternative is dried (for example, using vacuum drying apparatus), is sent to coking-chamber to remove residual solvent and finally filtered and dried for storage or for further processing.

According to this invention also can be purified any TC obtained by this method or by the method of the prior art. An important advantage of this invention is that this method can be redesigned product with a higher content of intermediate oxidation products, such 4-CBA, and it allows you to soften the conditions of oxidation, which reduces losses during the combustion of p-xylene and acetic acid.

Also according to this invention of improvements in the methods of crystallization described only schematically, provide for the production of spherical crystals of large sizes, which are believed to contain a small amount of salt or do not contain salt, which can be formed by using as solvent(s) selective cu is they are resistant to destruction by washing with water and in other words, one is easier to remove the solvent, and they are easier to washing to remove residual impurities.

Improved methods of crystallization include instant (single equilibrium evaporation ("flash evaporation") crystallizing solvent from the acid by reducing the surface pressure, preferably before cooling and process cooling a saturated solution of acid. Preferably a progressive way to reduce pressure is to low levels in the mould periodic or continuous action, and the reduced pressure can be used in batch or continuous mode of operation. In addition, to increase the speed of removal of the solvent and its amount in crystallizing the acid in the process of applying a reduced pressure can be fed warm, observing, however, precautions to prevent significant temperature rise crystallizable acid, causing re-dissolving acid and therefore a waste of energy.

As described above, according to the invention, the crystallization can be carried out in several stages; when using this form of the invention, previsualise in the second or subsequent(their) stages(s), although these methods can be used to your advantage and at the first stage.

Also according to the invention for the purification of terephthalic acid by crystallization using flash evaporation of the solvent ("flash-crystallization) can be used as co-solvents. To reduce the temperature of the flash-crystallization and, consequently, the temperature of dissolution, can be used a co-solvent with a boiling point below the boiling point of the solvent. When implementing flash evaporation at a lower temperature flash crystallization may occur at a lower degree of vacuum.

The co-solvents include water, C1-C5-alcohols, such as methanol or ethanol, With5-C10-hydrocarbons, such as p-xylene, and C1-C10organic acids such as formic acid or acetic acid, etc., Thus, can be included as co-solvents from approximately 1 to approximately 50% of inert solvents with boiling points in the range from 25 to 200oC.

The invention also assumes that in every stage of crystallization and washing included stage of the regeneration and recycling of the solvents of the invention, including the return of some of Wausau environment of any unwanted substances.

The important point is detected data related to the solvents that are effective for the implementation of the purification of crude terephthalic acid through stages of crystallization and separation. These detected data can be summarized several of the following methods.

The selective solvents of crystallization, useful in the practice of this invention include solvents in which (a) impurities, which are preferably separated from terephthalic acid to clean it, is relatively more soluble in the solvent than terephthalic acid (TC), essentially at each temperature within the desired temperature range at which the treated solvent containing terephthalic acid, and (b) terephthalic acid is more soluble at higher temperatures and less soluble at lower or cold temperature. The term "solvent selective crystallization should be understood solvents useful for the selective crystallization of terephthalic acid as described above and as described in more detail below and shown in Fig.1 and 2.

In this regard, it should be noted that in U.S. patent 3465035 indicated that some organic rastvoriteli unstable in air and it is easy to make with terephthalic acid addition products. In the same patent, among some other solvents also described the use of acetic acid and water as solvents purification of terephthalic acid. In contrast to the above, the selective solvents of crystallization according to this invention are (a) non-aqueous, (b) non-corroding cast and (C) essentially can't interact with terephthalic acid, and they are not included in the above solvents of the prior art. In particular, water, vinegar (and other alkyl) acid and the above organic solvents excluded from the selective solvents of crystallization, which is what is meant by this invention.

According to the invention the most important preferred solvents selective crystallization are N-organic (NMP) and N,N-dimethylacetamide (DMAC) for some reason, discussed below, and because of their excellent processing characteristics. In U.S. patent 2949483 dated 16 August 1960 (Ham) describes NMP used for crystallization of terephthalic acid, but it is not used in the same temperature range, which is preferred in this invention. Do not assume the flash-crystallization or her primary results. In PU is R. ) "Purification of aromatic dicarboxylic acids obtained by liquid-phase oxidation dialkylphosphate aromatic hydrocarbons" very briefly indicated in NMP as a solvent, but says nothing about the temperature of dissolution or crystallization or flash crystallization.

N-Organic (NMP) and N,N-dimethylacetamide (DMAC) are preferred solvents selective crystallization for the practice of the invention. These solvents are non-aqueous, thermally stable, non-toxic (environmentally safe), non-corroding cast and commercially available. NMP is the most preferred solvent selective crystallization for the practice of this invention, due to the fact that the graph of its ability to dissolve the terephthalic acid depending on the temperature goes up and to the right, and this means that terephthalic acid can be dissolved in it at elevated temperatures to precipitate or crystallize out of it at lower temperatures.

Although NMP is the most preferred solvent selective crystallization, it should be borne in mind that DMAC shows similar desired characteristics and that according to this invention each is to emiratesa from various polar organic solvents, including, but not limited list, N-alkyl-2-pyrrolidone (such as N-ethylpyrrolidin), N-mercaptoethyl-2-pyrrolidone (such as N-mercaptoethyl-2-pyrrolidone), N-alkyl-2-cooperalion (such as N-methyl-2-cooperalion) and N-hydroxyalkyl-2-pyrrolidone (such as N-hydroxyethyl-2-pyrrolidone), 1,5-dimethyl-pyrrolidone, N-methylpiperidine, N-methylcaprolactam, N,N-dimethylformamide and N-formylpiperidine, and mixtures thereof. Other selective solvents of crystallization, proposed by the present invention include, but are not limited to this list, sulfolan, methylsulfone, sulfones, morpholines (such as morpholine and N-formylmorpholine), carbitol, C1-C12-alcohols, ethers, amines, amides, esters, etc., and mixtures thereof.

Preferably, the desired solvent selective crystallization was used in multistage crystallization process in combination with one or more additional solvents, preferably with two additional solvents, particularly when crude terephthalic acid has a purity of less than approximately 98%. Preferably the solvent washing, such as (but not limited list) water, p-xylene, acetone, methyl ethyl ketone (MEK) or the th branch of the crude terephthalic acid from other materials, originating from the oxidation apparatus. In addition, there may be used a solvent displacement, which has a low boiling point, such as (but not limited list) water, methanol, acetone, MEK, etc., Preferably as a solvent displacement water is used in combination with the third filtering that follows the second stage of crystallization in the preferred way. The desired solvent displacement displaces the solvent selective crystallization of the obtained filter cake, resulting in the stage of maceration is only the solvent displacement. The process of soaking is preferable to remove any possible residual solvent trapped crystals TC before the product arrives at the final stage of filtration and drying.

As described above, NMP and DMAC are preferred solvents selective crystallization for the practice of the invention. They are non-aqueous, thermally stable, non-toxic (environmentally safe), non-corroding cast and commercially available. NMP is the preferred solvent selective crystallization for the practice of this invention because, among other things, the slot can be dissolved therein at elevated temperatures to precipitate or crystallize out of it at lower temperatures. However, a plot of the solubility of terephthalic acid from the temperature has a much smaller slope than the graphics solubility in NMP other substances allocation of crude terephthalic acid, such as benzoic acid, 4-carboxybenzene (4-CBA) and p-tolarova acid. As a result, when the crude terephthalic acid containing or associated with unreacted raw materials, solvents (if available), intermediate products of oxidation or other undesirable substances, dissolved in NMP or DMAC at elevated temperatures, essentially all materials dissolved in it, or at least well-dispersed. Then after the removal of heat and pressure and subsequent cooling of the solution of such substances in NMP or DMAC pure terephthalic acid is preferably crystallized from solution, while other, more soluble substances that can be considered as impurities for the purposes of this invention, remain in solution in NMP or DMAC. Thus, there is a division of pure terephthalic acid and associated with it of impurities. NMP or DMAC can be purified from impurities in the recovery column and return to the stage oxidation apparatus kiletki crude terephthalic acid from the liquid dispersion, also containing impurities selected from unreacted starting materials, solvents, products of side reactions and/or other undesirable substances. It includes obtaining purified terephthalic acid, the first stage of the specified crude terephthalic acid and re-dissolving the purified terephthalic acid, the first stage in the selective solvent crystallization to obtain a solution. Purified terephthalic acid second phase crystallizes from the solution by reducing the temperature and pressure sufficient to flash evaporate solvent from a solution of terephthalic acid, but without cooling the solution below about 50oC. Purified terephthalic acid of the second stage then is released from the solution and the dedicated purified terephthalic acid of the second stage is subjected to leaching by water. Washed allocated purified terephthalic acid of the second stage is soaked in water at a temperature of from about 150 to about 300oC, preferably at a temperature of from about 180 to about 250oWith, and then soaked with water purified terephthalic acid of the second stage is filtered and dried.

If necessary, the solution terephthal is up to approximately 40 min after the temperature is reduced and before the allocation of a purified terephthalic acid of the second stage. In addition, washing of the purified terephthalic acid can be performed from one to three times. Similarly, the operation of lowering the temperature and pressure for the flash evaporation of the solvent can be Paladino in the form of from two to six stages, preferably two to four stages. Mainly, the proportion of the solvent is evaporated by flash evaporation in the first stage of evaporation is limited to increase the final size of the crystals.

In addition, according to the invention is provided a method of purification of crude terephthalic acid from a liquid dispersion also containing impurities selected from unreacted starting materials, solvents, products of side reactions and/or other undesirable substances. The method includes filtering the dispersion to obtain a filter cake of crude terephthalic acid, dissolving the filter cake in a selective solvent of crystallization at temperatures above approximately 60oTo obtain the solution, crystallization of purified terephthalic acid from the solution by lowering the temperature and/or pressure of the specified solution, but not below about 50oWith the allocation specified kristallizovannymi washed allocated purified terephthalic acid with water at approximately 150-300oWith, preferably in the range of 180-250oWith, and filtering and drying soaked in water purified terephthalic acid.

According to another aspect of the invention provides a method of purification of crude terephthalic acid from a liquid dispersion also containing impurities selected from unreacted starting materials, solvents, products of side reactions and/or other undesirable substances, which includes filtering the dispersion to obtain a filter cake of crude terephthalic acid, dissolving the filter cake in a selective solvent of crystallization at temperatures above approximately 50oTo obtain the solution, crystallization of purified terephthalic acid from the solution by reducing the temperature and/or pressure, the allocation of crystallizing purified terephthalic acid from the solution, re-dissolving the selected purified terephthalic acid in a solvent selective crystallization to obtain a second solution, the crystallization of purified terephthalic acid of the second stage from the second solution by reducing the temperature and pressure sufficient to flash evaporate solvent from the second solution Tere is phthalic acid of the second stage from the second solution.

Another variant implementation of the invention includes a method of purification of crude terephthalic acid from a liquid dispersion also containing impurities selected from unreacted starting materials, solvents, products of side reactions and/or other undesirable substances by filtering the dispersion to obtain a filter cake of crude terephthalic acid, dissolving the filter cake in a selective solvent of crystallization at temperatures above approximately 50oTo obtain the solution, crystallizing purified terephthalic acid from the solution by reducing the temperature and/or pressure, the allocation of crystallizing purified terephthalic acid from the solution, re-dissolving the selected purified terephthalic acid in a solvent selective crystallization to obtain a second solution, crystallization of purified terephthalic acid of the second stage from the second solution by reducing the temperature and pressure sufficient to flash evaporate solvent from the second solution of terephthalic acid, but without the specified cooling the solution below about 50oWith the selection of purified terephthalic acid, is makiwane water washed purified terephthalic acid of the second stage at about 150-300oWith, preferably in the range of 180-250oWith, and filtering and drying the soaked water purified terephthalic acid of the second stage.

From the foregoing it can be seen that the object of the present invention provides an improved method and apparatus for producing purified terephthalic acid purity required for use in preparation of polyester resins and other products with good from an economic point of view of speed and when working in high risk that require less capital expenditure and simplified technology. The way that made this and other objects of the invention, can be explored through consideration of the detailed description which follows in conjunction with the attached drawings.

A more complete understanding of the method and device of the present invention can be obtained by reference to the following description when considered in conjunction with the attached drawings.

Fig. 1 and 2 are graphs of the dependence of solubility on temperature, respectively, in NMP and DMAC terephthalic acid and impurities or products on the ti pH on the concentration of water to NMP, DMAC and the mixture of NMP and salt crystals.

Fig. 4 is a graph of the pH of the mass ratio JCC/solvent.

Fig. 5 is a graph of the solubility of the JCC in water temperature, indicating the points of flash evaporation for some methods of the prior.

Fig. 6 shows the distribution of particle sizes obtained by the flash method according to the invention.

Fig. 7 represents a graph flash profiles for different ways as flash and cooling type.

Fig. 8 is cooling curves for different ways on how flash and cooling type.

Fig.9 is a simplified vertical scheme of the mold, which can be used in the practice of the invention.

Fig. 10 is a block diagram of a preferred method of the invention.

This invention relates to the development of new production technologies Rev. Compared with the widely used nowadays technology QCD this technology provides a lower capital costs in new construction installation to obtain the TCI, as well as lower cost of installation. It also provides a means for current VRO more new installations receive Rev.

The success of this method is based on the development of technology nonaqueous highly selective crystallization using low pressure and low temperature. Technology crystallization can purify the crude terephthalic acid (TC) low purity in the range of from approximately 70% (from oxidation apparatus) to about 98+ % in the mould of the first stage and up to 99.99+ % in the mould of the second stage. This allows you to operate the apparatus oxidation when receiving TC in much less hard mode than in commonly used methods of the prior art. The selective solvent of crystallization used in the method of crystallization, is non-aqueous, thermally stable, non-toxic environmentally safe, non-corrosive and commercially available.

In the process of this invention using NMP or DMAC as a solvent selective crystallization of the authors of this invention have demonstrated a degree of purity of TC up to 99.9+ wt.% after the first crystallization process and up to 99.99+ wt.% after the second crystallization process. In particular, table 1 illustrates the allocation of TC with 99.95+ wt.% after the first crystallization and TC with a purity of 99.997 wt.% after the attraction of the first crystallization process and TC with purity 99,9933 wt.% after the second crystallization process from raw TC (89,89 wt.% TC) through increased as the saturation temperature and the crystallization temperature.

Table 3 illustrates the allocation of TC with purity 99,9960 wt.% (only the crystallization process from raw TC (98,99 wt.% TC). In addition, benzoic acid, p-tolarova acid, 4-CBA, MMT and other impurities was less than 10 million D.

Table 4 illustrates the allocation of TC with purity 99,63 wt.% (only the crystallization process from raw TC (83,91 wt.% TC) on a larger scale.

Table 5 illustrates the allocation of TC with purity 99,92 wt.% (only the crystallization process from raw TC (79,79 wt.% TC) on a larger scale.

Table 6 illustrates the allocation of TC with purity 99,15 wt.% (only the crystallization process from raw TC (83,90 wt.%) on a large scale at a higher temperature, saturation 190oC.

Table 7 illustrates the selection of TC with purity 99,9915 wt.% from raw TC (98,50 wt.% TC) on a large scale. Oversaturation of the crystallization mixture leads to the formation of essentially large crystals TC than the crystals formed in the processes outlined above.

How should submit a qualified specialist, the size of the crystals TC it is important to consider from the point of view of separating them from solvents and impurities.

Table 8 shows the separation of the N-dimethylacetamide (DMAC) as solvent of crystallization. The content of 4-CBA was reduced from 6 wt.% to 3276 million D. the working temperature range was very moderate (45 to 120oC).

Although for many purposes are preferred relatively high temperature dissolution, for example, 140-200oFrom the experiments carried out at relatively low temperatures with a cooling crystallization, show what can be achieved clearance, similar to the complete dissolution at high temperatures (160oC) (see tab. 8A).

The use of lower temperature dissolution can make possible savings in energy consumption in the process.

As was discussed important aspects of this invention relate to the discovery of methods of crystallization of terephthalic acid from the organic solution, where the solvent tends to form an organic salt with TC. Salt is usually formed by cooling of a solution of an organic solvent or mixture of organic solvents, which is full of TC at higher temperatures. However, the crystalline structure of salt is destroyed by flushing with water or other solvents to remove solvent from the crystals. The washed crystals are very fine powders that are difficult filteredtree education such fine powders.

As is known, organic solvents easily form addition products with terephthalic acid, which leads to problems when cleaning and when the isolation of pure terephthalic acid. Some of these problems can be estimated from the summation of the various empirical observations. The salt crystals tend to be desirable large (>120-150 microns), but they are destroyed upon contact with water, methanol or acetic acid (i.e., the desired washing means) and re-dissolved in very thin crystals (<50 microns), which, as stated above, it is difficult to treat. The filter cake formed from crystals of these salts is soft and easily filtered, but after adding water, the sediment settles (its thickness is reduced), becoming very compact, and is difficult to remove water and washing to remove residual solvent. If you are trying to overcome this undesirable result by re-suspension of sediment after adding water (at room temperature), as installed, instead of the suspension formed a paste with a high viscosity, if the concentration of solids is not saved the loci for re-suspension in warm water (60oWith or above), as installed, the mixture tends to form a very stable foam (similar to foam for shaving), which is also a very difficult process. Due to the fact that after adding water to salt crystals are formed of particles of very small size, characteristics of the process of separation of the suspension is very bad, and the speed of technological processing in the wash column is very low, requiring very large equipment. Despite the above difficulties encountered when attempting to wash out the residual solvent from the salt crystals, the presence of residual solvent is critical, because such a solvent may discolor JCC when heated to temperatures between 120oC and above. This will affect the quality of the polyester made from such material. In addition, regeneration or water rinse is necessary to remove solvent for reasons of cost.

The salt crystals can be visually distinguished from crystals JCC, because the former are bright transparent and pale, and the latter are opaque and whiter. Researchers prior reported that salt contains 2 mol Rawley 45% and NMP is 55% in the salt. Empirically it was found that the solvent content in the salt crystals is limited 55-65%, and this confirms that derived by the authors prior formula is fair and is free solvent as the moisture in the sludge.

Measuring the pH of NMP and DMAC at room temperature show that both solvent are basic (pH equal to 10.3 and 11, respectively). Adding JCC pH is reduced to neutral, and adding water, the mixture becomes acidic. Fig.3 is a graph of pH on the concentration of water to NMP, DMAC and salt crystals NMP plus STK.

The study of the pH of various mixtures JCC with the solvent at 30oWith showed the same type of behavior with the change from basic to acidic with increasing content of the JCC. At about pH 6.5, JCC begins to precipitate, and this means that the solution is saturated. Fig.4 is a graph of the pH of the mass ratio JCC/solvent.

STK used for pH measurements, was a commercial STK-product (spherical crystals), and the precipitated crystals represented rods and was transparent (salt crystals), and this shows that the original crystals STK Transpower the acid-base interaction between the STC and the solvent with the formation of salts: STK+2NMP(1)-->STK:2NMP Study the melting temperature of the salt crystals NMP showed what a bright and clear crystals transformed into opaque and white crystals at 50-60oWith and remained such until the temperature was >300oSince then allocated some amount of visible vapor that demonstrated the possible sublimation. This same phenomenon was observed for crystals of salts formed from DMAC.

Experiments in which the salt crystals were subjected to drying to remove solvent, showed a weight loss of approximately 60% of the initial mass that is coordinated with the salt formula proposed by the authors of the preceding level. In the experiments of drying at temperatures above 120oThe material was brown.

Seasoning of salt crystals at elevated temperatures in the range of 80-100oWith some stirring suspension gives white, opaque, solid crystals (CLC) in a transparent liquid (NMP). The formation of the suspension was also observed at higher temperatures.

These observations show that at a temperature of 60oWith or above the decomposition of the salt at the JCC and NMP. At temperatures below the 50oSince salt is more stable. Therefore, the reaction of decomposition of the salt may be written as follows

Experiments involving either cooling or flash evaporation/cooling solutions to the crystallization temperature 60oWith (instead of 45oC) confirm the mechanism of formation/decomposition of the salt, postulated above, by obtaining the true crystals JCC, which are not destroyed by water.

In addition, according to the invention a solution of an organic solvent (or mixture of organic solvents), saturated TC and impurities, such as 4-carboxybenzene (4-CBA), p-tolarova acid and so on, served in a mold with supported reduced pressure (or vacuum) to allow the solvent to evaporate instantly in continuous or intermittent operation at temperatures above the 50oTo prevent the formation of salt. Then the solid particles (the nuclei of crystallization), formed as a result of instantaneous equilibrium evaporation of the solvent provided the opportunity for growth within a certain period of time under reduced pressure, and temperature. It is desirable to expose a saturated solution of multiple operations on the flash MEM is, each of which creates different negative pressure (or vacuum), to achieve greater yields of TC and large crystals TC. Unexpectedly, it was found that the structure of the crystals obtained by this method is not exposed to the adverse effects when using water or other solvents, which are capable of substantially dissolving the solvent of crystallization (or solvent mixture), or by vacuum drying the crystals to remove solvent. Thus, it is found that the salt has not happened, or at least the salt formation was minimized, so that the flushing water or other solvent, kotorye can dissolve the solvent of crystallization, or vacuum drying did not change the size and shape of crystals TC.

Flash-crystallization commercially used for STK using water as solvent. This method requires 3-5 flash evaporation to reduce the pressure and temperature, and achieve good particle size distribution (average 120 microns) particle size. This type of crystallization takes advantage of the form of the graph of solubility to create conditions of high supersaturation, especially in the first flash evaporation, ka is the value of STK in water temperature.

Unlike the graph of the solubility of the JCC in the water a graph of the solubility of the JCC in NMP is very gentle, but the speed of crystal growth is higher on the basis of the techniques of flash-crystallization using only 4 flash evaporation: get large crystals (average 160 and 170 microns). In addition, the crystals of the flash-crystallization larger than the crystals obtained only through cooling. Fig.6 shows the distribution of particle sizes obtained using this flash process.

Good control flash profile (the amount of solvent evaporated by flash evaporation after each flash evaporation) is important for maintaining the desired shape and size of the particles. Flash-crystallization is the only way to obtain crystals of spherical type, which have good speed filtering and washing efficiency, but, if during the first two flash evaporation evaporates too much solvent, then get a small spherical crystals, which are somewhat difficult to filter and wash. Thus, selection and control profile flash evaporation provides a way to create a desirable or optimal conditions pericallosal using NMP is carried out in vacuum due to the low vapour pressure of the solvent in comparison with the methods of the flash-crystallization in acetic acid or in water, which are held under pressure. The preferred conditions of the flash-crystallization are the following: 1st flash evaporation: 150 mm Od at 145-150oWith V/F*=0,26, 2nd flash evaporation: 80 mm Od at 120-125oWith V/F=0,12, 3rd flash evaporation: 40 mm Od at 110-115oWith V/F=0,07, 4th flash evaporation: 20 mm Od at 95-100oWITH V/F=0,07.

* The amount of flash solvent in the form of a fraction of the original solvent for each flash evaporation.

Experiments were conducted using both methods of crystallization by cooling or flash crystallization to a final temperature of 60oWith obtaining true crystals JCC by both methods, although they differ in shape and size. For flash-crystallization complete dissolution (29 g STK/100 g of NMP at 185oC) it is preferable for the initial conditions, but for cooling you need some way of making the seed, thus, the initial conditions are preferably 43,7 g STK/100 g of NMP at 185oC. the Crystals obtained by cooling, to form a mixture of rod and spherical crystals with a predominance of spherical shape, and were smaller than the crystals obtained flash evaporation, which is the result of a smaller saturation, to solid when cooled, compared with 49 g STK/100 g of NMP in the flash-crystallization). Fig.8 is graphs of cooling for different ways - flash-crystallization and crystallization with cooling.

According to the invention aging time for crystal growth is a very important factor. Minimum 15 minutes, maximum 60 minutes is the preferred time, and the optimal time is approximately 30 minutes. This time is sufficient to achieve a certain equilibrium state between liquid and solid phases and to minimize problems during filtration.

The results of several experiments crystallization are shown in table 9.

As mentioned above, organic solvents useful in this invention include, but are not limited to, N-organic (NMP), N, N-dimethylacetamide (DMAC), 1,5-dimethylpyrrole, N-methylpiperidine, N-methylcaprolactam, N, N-dimethylformamide, N-formylpiperidine, N-alkyl-2-pyrrolidone (such as N-ethylpyrrolidin), N-mercaptoethyl-2-pyrrolidone (such as N-mercaptoethyl-2-pyrrolidone), N-alkyl-2-cooperalion (such as N-methyl-2-cooperalion) and N-hydroxyalkyl-2-pyrrolidone (such as N-hydroxyethyl-2-pyrrolidone).

In order to remove the residual solvent trapped in crystals of finite have partial or complete dissolution of the crystals of the LC.

There have been several attempts to remove the residual solvent by means of vacuum or low-temperature drying of the salt crystals. The results showed that 0.5% of residual solvent remains, obviously, due to the location of the agglomeration of fine particles and ash certain amount of solvent between the agglomerates.

Steeping at 220oWith water at various ratios of water and JCC showed that the residual solvent can be effectively and satisfactorily removed without complete dissolution (see tab. 10).

The following examples illustrate the principles and features of the invention.

EXAMPLE 1
Cooling crystallization
9761 g of NMP is added to the crystallizer equipped with a jacket and a stirrer, together with 3028 g TC. This mixture is heated to 180oWith under atmospheric pressure until the dissolution of the entire TC.

After that, the mixture is subjected to surface cooling with circulating in the jacket of the cooling medium until the temperature reaches 45oC. Then, after 15 min, the suspension is filtered for separation of solid particles from the mother liquor, the filter cake washed with pure NMP at room temperature to displace all of the mother liquor from the point of their weapons size in the range of 120-150 microns.

Removal of the solvent from the precipitate precipitate should be washed with water or other appropriate solvent with high solubility. For washing the precipitate using warm water at 80oC. However, a rod-like crystals in the sediment were completely destroyed by water and turn into a fine powder, which is more like a powdery residue than the crystals obtained by crystallization. These fine powders are very difficult to flush and technological processing, and the removal of residual solvent is difficult.

EXAMPLE 2
Flash-crystallization
Use the same drug NMP and TC with the difference that the mixture also at the stage of cooling is subjected to flash evaporation of the solvent by reducing the pressure from atmospheric to vacuum 125 mm Od. In this way a certain amount of solvent evaporates and condenses in the refrigerator, the temperature of the mixture drops from 180 to 147oC. the Number is rapidly vaporized solvent creates a state of saturation, therefore, TC, dissolved in NMP, crystallized in the solid phase.

Although stage flash evaporation is performed instantly, crystallization TC requires some time, so the mixture of the event. The suspension was filtered to separate solids from the liquid phase, washed with pure NMP at room temperature and the sediment examined under a microscope. The crystals have a spherical shape, but not rod, as in the previous method of crystallization by cooling, and very uniform in size, but are smaller, approximately 40-60 microns.

Then the precipitate is washed with warm water at 80oAnd, suddenly, steropodon crystals are not impacted by water washing (their shape and size are not changed). These steropodon crystals have a very high filtering rate, and effective washing is much easier.

EXAMPLE 3A
The growth of crystals
To accelerate the growth of crystals repeat the experiment described in the previous example, with the difference that use 6174 g of NMP and 1952 terephthalic acid.

In addition, the pressure in the flash evaporation is 120 mm Od, not 125 mm Od, and the temperature is 145oC. Then the mixture is subjected to flash evaporation for the second time in 40 mm Od, as described in the previous example, and the temperature drops to 110oC. Thus, crystallizes a greater amount of terephthalic acid. Crystals have steropodon shape, and their size increases d the m using 7490 g of NMP and 2325 g of terephthalic acid. In addition, use a different pressure profile and add two flash evaporation.

The first flash evaporation: 150 mm Od at 154oC.

The second flash evaporation: 80 mm Od with 135oC.

The third flash evaporation: 40 mm Od at 117oC.

The fourth flash evaporation: 20 mm Od with 101oC.

Examination under a microscope shows that the crystals are spherical in shape and significantly improves their size. The final sample contains crystals of a size in the range of 120-150 microns.

EXAMPLE 4A and 4b
Crystallization by flash evaporation/vaporization
Repeat the experiment described in example 3b, with the difference that the temperature of hot oil circulating in the jacket to keep 5 to 10oWith higher temperature of the mold so that some evaporation of solvent takes place simultaneously with the flash evaporation. This technique leads to the fact that more solvent is instantly vaporized/evaporated, and to the profile of lower temperatures, which increases the regeneration of crystals (see tab. 11).

In the study under the microscope the crystals look sharoobraznymi form, as described in example 2 above.

EXAMPLE 5
Crystallizatio only by cooling.

Flash-crystallization
In the crystallizer download 31 g TC/100 g solvent. 4-CBA add to start at a concentration of solids of approximately 2%. The mixture is heated to 185oC and stirred until then, until most of the crystals will not dissolve. Some crystals can be dissolved and they become a seed for crystal growth. The oil bath temperature was adjusted to 155oC. the First vacuum (150 mm) provide for the removal of approximately 15-20% of the liquid for about 15 minutes. Then flash the vacuum is brought to 80 mm Od, and 6-8% of the remaining fluid is removed within 5 minutes. In the third flash process 6-8% of the solvent is removed under vacuum of 40 mm Od for about 6-7 minutes. In the fourth flash process 12% of the solvent is removed with a vacuum of 20 mm Od, which will require approximately 10-15 minutes. Then the liquor quickly as possible, cooled to 50oWith, which takes approximately 30 minutes. Then the crystals are removed from the flask and filtered using a Buchner funnel and flask with a side outlet. Approximately 200 g of solvent at a temperature of 50oWith poured for washing crystals. Then the crystals make the press filter and dried by passing nitrogen pressure of 40 fontova get 500 million D.

Crystallization by cooling
In the crystallizer download 31 g TC/100 g solvent. 4-CBA type to start with a concentration of solids 2%. The mixture is heated to 185oC and stirred until then, until you dissolve a large part of the crystals. Some crystals may not dissolve, and they become a seed for crystal growth. Begin cooling the mixture for crystallization TC from the solution. The cooling rate is 2oC/min, Final temperature equal to 50oC. Then the crystals are removed from the flask and filtered using a Buchner funnel and flask with a side outlet. Approximately 200 g of solvent at a temperature of 50oSince then poured to the residue to wash the crystals. Then the crystals are transferred to a filter press and dried by passing nitrogen gas at a pressure of 40 pounds per square inch (275,790 kPa) for 30 minutes. The final crystals examined for the content of 4-CBA, the result is approximately 500 million D.

Experiments show that the flash-crystallization and crystallization by cooling have essentially the same ability to remove 4-CBA.

According to the invention the preferred embodiment of the method is divided into 5 sections.

(1) Section ocial + oxygen --> terephthalic acid + water
(d) p-xylene + oxygen --> p-tolarova acid + water
(C) p-xylene + oxygen --> 4-carboxybenzene (4-CBA) + water
The residence time in the oxidation apparatus is approximately 5 hours. Because the flow coming from the unit oxidation, will contain approximately 30% TC, the mixing apparatus oxidation is of great importance for the conservation of yield and selectivity and to prevent clogging and driving. The initial mixture of raw material flows can be obtained in a static mixer (outside of the apparatus oxidation). Additional mixing can be provided by an air bubbler and internal circulation. Depending on the thoroughness stage of leaching of p-xylene on the filter (discussed below), the content of terephthalic acid (TC) in the solid precipitate may vary in the range from about 55% to about 90+ %.

(2) Section crystallization
(A) First crystallization
After filtration of solids from the flow coming from the apparatus oxidation, mixed with mother liquor, and the solvent is washed with liquid from the second stage crystallizer and additional solvent crystallization. The mixed slurry is dissolved in the suspension tank PR>C. a Saturated solution is moved to a collector for removal of p-xylene through evaporation. A saturated solution is then applied to the mold periodic operation of the first stage to obtain purified TC by flash evaporation of the solvent under reduced pressure and/or by cooling. After the stage of crystallization content of the mold by gravity into the collection of the product and from there it is fed continuously by the pump to the filter (or in a centrifuge for separation of the solid substances recrystallization in the mould of the second stage for further purification.

(C) Second crystallization
Solids collected on the filter of the first mold, re-dissolved in the original containers for dilution with solvent of crystallization for the second stage crystallizer under predetermined conditions, such as temperature in the range of from about 140 to about 200oC. a Saturated solution pump serves in the second stage crystallizer for growing crystals and the selection again by flash evaporation of the solvent under reduced pressure and/or by cooling. Then the contents of the mold by gravity served in the collection for felt crystallization to displace the mother liquor, remaining in the sediment. Then the solid precipitate was washed with low-boiling solvent to displace the solvent of crystallization in the filtered precipitate, soaked in water at high temperature to remove any residual solvent and then dried to remove the final fluid from STK-product. The solvent of crystallization alternative may be supplanted by drying the solid precipitate using vacuum drying apparatus and subjecting the precipitate to a process of maceration. The process includes soaking the partial or complete dissolution of TC in the solvent, crystallization of the product in water at high temperature and high pressure to remove residual solvent trapped in the crystals and recrystallization, filtration and drying the precipitate TC.

(3) the regeneration Section of the mother liquor/solvent.

The mother liquor from the filter of the first mold is transferred into the column to regenerate the solvent for separation of the solvent and crystallization from top of shoulder strap columns. Impurities such as (but not limited to) p-tolarova acid, benzoic acid, 4-carboxybenzene (4-CBA), etc., separated from the lower part of the column. In order to ensure that the waste slurry columns can is eh.

Detailed description of the process and an example
The invention will be described on the example of the acquisition and allocation of terephthalic acid (TC) by oxidizing p-xylene with air in the presence of a solution of components of catalysis in the system solvent dimethyl terephthalate (DMT) or benzoic acid. The temperature in the oxidation apparatus is preferably in the range from approximately 150 to approximately 250oWith, the pressure is in the range from approximately 5 to approximately 30 kg per cm2(from about 490 to about 2940 kPa). Since the output of the apparatus oxidation thread will contain up to 30% TC, the mixing apparatus oxidation is very important to maintain yield and selectivity and to prevent contamination and zabivac. The initial mixing of the source streams can be achieved in a static mixer (outside of the apparatus oxidation). Further mixing may be provided by bubbling air or internal circulation. In the preferred form of the method of manganese acetate and cobalt acetate in aqueous solution are fed into the apparatus for oxidation catalysis of oxidation reactions.

Coming from the unit oxidation flux at approximately 160oSince p is Jesse filtering the solid filter cake is washed with p-xylene, which is heated from 30 to 100 to 150oC. the mother Liquor is transferred into the first collection. The filter cake is removed separately from the first filter to the second collection.

The washed precipitate is fed into the first tank for sludge for mixing with the following streams: (1) NMP or DMAC (solvent selective crystallization) - wash liquid (heated to a temperature of from 45 to 100 to 150oC); (2) liquor (heated to a temperature of from 50 to 100 to 150o(C) and (3) NMP or DMAC (heated to a temperature of from 45 to 100 to 150oC).

When used raw TC of the oxidation system of the present invention or the prior art, flushing p-xylene is not necessary, and raw TC enters the first tank for sludge, as described above. Raw TC can contain up to 2-3% of 4-CBA; therefore, the requirements for oxidation can be reduced, which leads to a significant reduction of losses of combustion of p-toluene and acetic acid.

The above mixture is then transferred from the bottom of the first tank for sludge in the first tank of dissolution. The content of the first volume of dissolution then heated by indirect heating from 100 to 150oWith up to 140-200oUsing oil heating coil, which is supplied with the first capacity dissolution. AP Bubbling nitrogen is applied in the first volume of dilution to guarantee the removal of p-xylene. The vapor flows from the first tank of dissolution and mold crude acid connect to the stream, condensed with fridge, and sent to the first storage tank. The stream coming from the bottom of the first vessel of the dissolution, periodically transferred into the mold crude acid.

The contents of the mold crude acid is cooled from a temperature of 140-200oWith up to 30-50oWith the help of an external cooling device to obtain a desired saturation for growing TC crystals. To improve the size distribution of the crystals and the separation of the solid substances that may be useful in making crystal seed. At the conclusion of the cycle of crystallization from a solution suspension by gravity in the third collection and transferred to the second filter where it is filtered at a constant speed.

In the filtering process on the second filter NMP or DMAC are used for washing the filter cake. The mother liquor and wash NMP or DMAC connected to the feed to the column regeneration of solvent of crystallization. The washed filter cake is fed into the second tank of dissolution, where it is mixed with NMP or DMAC to obtain a saturated feedstock for cristallization second container dissolution periodically transferred to clean the mold, where the pressure is reduced in the manner described previously, and the temperature is reduced from 140-200oWith up to 50-60oWith to induce crystal growth of TC. Cooling to accelerate the saturation gain of the stages of flash evaporation. Again, to improve the distribution of crystal sizes and selection of crystals, can be useful for making crystal seed. At the end of the cycle the suspension comes from clean mold in a container of raw material for the third filter.

The suspension is continuously fed into the third filter. The mother liquor from the first filter is transferred to the fourth volume. The filter cake is first washed with NMP or DMAC at 45oWith to displace the remaining mother liquor from the precipitate, and then the precipitate is washed with a low-boiling solvent displacement, such as water, to displace NMP or DMAC from the precipitate or alternative comes in vacuum drying. Wash NMP or DMAC (from storage tanks solvent of crystallization) and the solvent displacement then added to the third filter. Wash NMP or DMAC is served in a first tank for sludge, while the solvent displacement is transferred to the fourth volume.

The washed precipitate from the third filter passes through a wash column or megastate from the wash column or contact the device then enters coking camera, where the temperature is raised to approximately 150-250oWith removal of the crystals trapped solvent. The suspension is finally filtered and enters into the drying apparatus, product, where the water (moisture) is removed from the precipitate by heating and purging with a countercurrent flow of heated nitrogen. Dried STK-product is removed from the dryer and stored in containers for the product.

The bottom stream from the fourth collection (mixture of NMP and a solvent displacement) together with the liquid from the wash column or multi-phase contact apparatus is transferred through the heater (to heat flow from 25 to 80-120o(C) in the evaporator solvent displacement. Solvent vapours displacement of the upper part of the evaporator solvent displacement are condensed and the solvent displacement tank solvent displacement. The bottom stream from the evaporator solvent displacement is divided into two streams: one stream into the output reservoir, and the second thread is in a container of solvent of crystallization.

The mother liquor and wash NMP or DMAC from the second filter come in a container of solvent of crystallization and then go to the column regeneration NMP or DMAC. This stream is heated from 15-45oWith up to 130-170oWith before the ensata at 160-220oWith returns in column regeneration in the form of phlegmy. The remainder of the head product of column regeneration is served in the capacity of the test of solvent of crystallization. Of the capacity check of the solvent and crystallization of the recovered NMP or DMAC is pumped into a collection of NMP or DMAC.

To ensure that the suspension in the regeneration reboiler of the column can be moved back into the machine oxidation in the reboiler is added to the high-boiling diluent, such as benzoic acid or DMT. Suspension together with high-boiling diluent is removed from the regenerator column and back into the apparatus oxidation.

In Fig.9 shows the device of the mold S-2, useful for practicing embodiments of the invention, in which heat is applied to the crystallizing acid mixture while reducing the pressure for flash evaporation of the solvent. As shown in Fig. 3, the mold S-2 is provided as a circuit coolant recirculation heat exchanger E-8, and the circuit of the heating recirculation heat exchanger E-8A. Heat is supplied to the mixture by means of E-8A during flash evaporation, and cooling is supplied to the mixture in the other periods through E-8. The solvent is evaporated by flash evaporation (neprieinama to the crystallizer through line 50.

Fig. 10 is a block diagram of the preferred purification method of the invention on the basis of certain considerations discussed above.

This new configuration using the soaking water instead recrystallization reduces capital costs and energy consumption.

Another improvement is the use of flash-crystallization with a single stage, if the 4-CBA in the raw raw materials is 0.3-0.8%, and the required content of 4-CBA in the final product is 200-400 million D.

Although the preferred embodiment of the method and device of the present invention is illustrated in the accompanying drawings and described above, it should be understood that the invention is not limited to the described variants, but may be subject to different sub-allocations, modifications and substitutions without separating from the nature of the invention, which are presented and further defined in the following claims.


Claims

1. The method of purification of crude terephthalic acid from a liquid dispersion containing impurities selected from unreacted starting materials, solvents, products of side reactions and/or other undesirable substances, including filtrating filtered precipitate in a solvent selective crystallization at a temperature in the range from approximately 50 to approximately 250oWith the formation of a solution; the crystallization of purified terephthalic acid from this solution by reducing its temperature; the allocation specified crystallizing purified terephthalic acid from the specified solution; re-dissolution of the specified allocated purified terephthalic acid in a solvent selective crystallization to obtain a second solution; crystallization of purified terephthalic acid of the second stage from the specified second solution by reducing the temperature and pressure sufficient to instantaneous equilibrium evaporation (flash evaporation) of the solvent from the specified second solution mentioned terephthalic acid, but without the specified cooling the solution to a temperature below the 50oWith; the selection of the indicated purified terephthalic acid of the second stage from the specified second solution; washing the specified allocated purified terephthalic acid of the second stage water; soaking specified washed allocated purified terephthalic acid of the second stage of water at a temperature in the range from approximately 150 to approximately 300oWith; and filtering and drying the specified soaked with water purified terephthalic acid, the second hundred of niteline involves reducing pressure.

3. The method according to p. 1 or 2, in which the indicated soaking with water is conducted at a temperature in the range of from about 180 to about 250oC.

4. The method according to p. 1 or 2, wherein said second solution is maintained for from about 15 to about 60 minutes after the specified reduction temperature and before the allocation of a specified purified terephthalic acid of the second stage.

5. The method according to p. 4, wherein said second solution is maintained for from about 20 to about 40 minutes

6. The method according to p. 1 or 2, in which the specified washing the purified terephthalic acid is from one to three times.

7. The method according to p. 1 or 2, in which the specified reduction temperature and pressure for the flash evaporation of the solvent is Paladino 2-6 stages.

8. The method according to p. 7, in which the specified reduction temperature and pressure is carried out in stage 2-4.

9. The method according to p. 7, in which the proportion of the solvent is evaporated by flash evaporation is limited to increase the final size of the crystals.

10. The method of purification of crude terephthalic acid from a liquid dispersion also containing impurities selected from unreacted starting materials, the th of terephthalic acid, the first stage of the specified crude terephthalic acid; re-dissolution of the indicated purified terephthalic acid, the first stage in a solvent selective crystallization with formation of a solution; the crystallization of purified terephthalic acid of the second stage from the specified second solution by reducing the temperature and pressure sufficient to flash evaporate solvent from the specified second solution mentioned terephthalic acid of the second stage, but without the specified cooling the solution below about 50oWith; the selection of the indicated purified terephthalic acid of the second stage from the specified second solution; washing the specified allocated purified terephthalic acid of the second stage water; soaking specified washed allocated purified terephthalic acid of the second stage of water at a temperature in the range from approximately 150 to approximately 300oWith; and filtering and drying the specified soaked with water purified terephthalic acid of the second stage.

11. The method according to p. 10, in which the indicated soaking with water is conducted at a temperature in the range of from about 180 to 250oC.

12. The method according to p. 10, wherein said second solution is maintained for from about 15 to about 60 minutes n the second stage.

13. The method according to p. 12, wherein said second solution is maintained for from about 20 to about 40 minutes

14. The method according to p. 10, in which the specified washing the purified terephthalic acid is from one to three times.

15. The method according to p. 10, in which the specified reduction temperature and pressure for the flash evaporation of the solvent is Paladino 2-6 stages.

16. The method according to p. 15, in which the specified reduction temperature and pressure is carried out in stage 2-4.

17. The method according to p. 15, in which the proportion of the solvent is evaporated by flash evaporation in the first stage of evaporation is limited to increase the final size of the crystals.

18. The method of purification of crude terephthalic acid from its liquid dispersive also containing impurities selected from unreacted starting materials, solvents, products of side reactions and/or other undesirable substances, comprising: filtering a specified dispersion to obtain a filter cake of crude terephthalic acid; dissolving the specified filter cake in a selective solvent of crystallization at a temperature in the range from approximately 50 to approximately 250oWith the formation of RA is URS specified solution, but not below about 50oWith; the allocation specified crystallizing purified terephthalic acid from the specified solution; washing the specified allocated purified terephthalic acid with water; soak the specified washed allocated purified terephthalic acid with water at a temperature in the range from approximately 150 to approximately 300oWith; and filtering and drying the specified soaked with water purified terephthalic acid.

19. The method according to p. 18, in which the crystallization of purified terephthalic acid from the specified solution further includes lowering the pressure.

20. The method according to p. 18 or 19, in which the indicated soaking with water is conducted at a temperature in the range of from about 180 to about 250oC.

21. The method according to p. 18 or 19, wherein said second solution is maintained for from about 15 to about 60 minutes after the specified reduction temperature and before the allocation of a specified purified terephthalic acid of the second stage.

22. The method according to p. 21, wherein said second solution is maintained for from about 20 to about 40 minutes

23. The method according to p. 18, in which the specified flushing purified terephthalic what Alenia for flash evaporation of the solvent is Paladino 2-6 stages.

25. The method according to p. 24, in which the specified reduction temperature and pressure is carried out in stage 2-4.

26. The method according to p. 24, in which the proportion of the solvent is evaporated by flash evaporation in the first stage of evaporation is limited to increase the size of the crystals.

27. The method of purification of crude terephthalic acid from a liquid dispersion also containing impurities selected from unreacted starting materials, solvents, products of side reactions and/or other undesirable substances, comprising: filtering a specified dispersion to obtain a filter cake of crude terephthalic acid; dissolving the specified filter cake in a selective solvent of crystallization at a temperature in the range from approximately 50 to approximately 250oTo obtain a solution; the crystallization of purified terephthalic acid from this solution by reducing its temperature; the allocation specified crystallizing purified terephthalic acid from the specified solution; re-dissolution of the specified allocated purified terephthalic acid in a solvent selective crystallization to obtain a second solution; crystallization of purified ia, sufficient to flash evaporate solvent from the specified second solution mentioned terephthalic acid, but without the specified cooling the solution below about 50oWith; and the selection of the indicated purified terephthalic acid of the second stage from the specified second solution.

28. The method according to p. 27, in which the crystallization of purified terephthalic acid from the specified solution further includes reducing the pressure.

29. The method according to p. 27 or 28, wherein said second solution is maintained for from about 15 to about 60 minutes after the specified reduction temperature and before the allocation of a specified purified terephthalic acid of the second stage.

30. The method according to p. 29, wherein said second solution is maintained for from about 20 to about 40 minutes

31. The method according to p. 27 or 28, in which the specified washing the purified terephthalic acid is from one to three times.

32. The method according to p. 27 or 28, in which the specified reduction temperature and pressure for the flash evaporation of the solvent is Paladino 2-6 stages.

33. The method according to p. 32, in which the specified reduction temperature and pressure is carried out in stage 2-4.

34. The method according to p. 32, W is I to increase the final size of the crystals.

 

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