BACKGROUND OF THE INVENTION (a) Field of the Invention

The present invention relates to a process for refining of 2,6- naphthalene dicarboxylic acid with improved purity and color by repeating the re-crystallization step and with high yield by recovering further 2,6- naphthalene dicarboxylic acid amine salt dissolved in the filtrate remaining after the crystallization-recovery process. (b) Description of the Related Art

Polyester prepared by the polymerization of 2,6-naphthalene dicarboxylic acid and diol has been known to have several excellent properties such as thermal stability, tensile strength, gas permeability, etc., and is expected to be used as a good material for films, fibers, reservoirs, etc.

In particular, the polyethylene naphthalate (PEN) resin produced by the polymerization of 2,6-naphthalene dicarboxylic acid and ethylene glycol is expected to be greatly used for a high functional heat-resistant resin which cannot be made from polyethylene terephthalate (PET). 2,6-naphthalene dicarboxylic acid is largely prepared by oxidizing 2,6- dimethyl naphthalene with a gas including oxygen in the presence of catalysts of cobalt, manganese or bromine compounds, etc.

The crude 2,6-naphthalene dicarboxylic acid contains numerous impurities including acids having one functional group produced by the imperfect oxidation of 2,6-dimethyl naphthalene, such as formyl naphthoic l

acid and methyl naphthoic acid, etc.; trimellitic acid produced by opening naphthalene ring; naphthalene bromide; naphthoic acid; naphthalene tricarboxyl acid; colored organic impurities having unclarified structure; and metallic impurities including cobalt complex and manganese complex. Polyesters obtained by the polymerization of ethylene glycol and crude 2,6-naphthalene dicarboxylic acid containing many impurities as mentioned above show poor properties such as physical properties, heat stability, structural stability, and so on. Moreover, such polyesters are classified as low quality because they are tinged with color Of the impurities, monocarboxylic acids such as methyl naphthoic acid, naphthoic acid, etc. are especially problematic. If these monocarboxylic acids exceed a certain amount, the ratio of polymerization falls in the polyester generating process, and gelation and coloring occur. Particularly, formyl naphthoic acid has a serious effect on these problems. Therefore, it is important to reduce these impurities in order to obtain polyesters having high quality.

2,6-Naphthalene dicarboxylic acid cannot be refined by distillation because it is degraded at high temperature instead of becoming a vapor, and it is not easy to refine 2,6-naphthalene dicarboxylic acid by re-crystallization because it is not dissolved well in general solvents.

Until now, several refinement process of 2,6-naphthalene dicarboxylic acid have been known. One process is to dissolve 2,6-naphthalene dicarboxylic acid in conventional solvents and then re-crystallize it after hydrogenation. A second process is to convert 2,6-naphthalene dicarboxylic acid into its alkali salt and then dissolve and re-crystallize it. A third process

is to convert 2,6-naphthalene dicarboxylic acid into its amine salt and then dissolve and re-crystallize it.

Also, commercially pure dimethyl 2,6-naphthalene dicarboxylate is produced by reacting 2,6-naphthalene dicarboxylic acid with methanol to prepare dimethyl 2,6-naphthalene dicarboxylate 2,6-NDC), and then refining this through distillation to produce the commercial product. However, as can be seen in the preparation of polyethylene terephthalate resins, acid are superior to esters in processing and economics as raw materials for the synthesis of polyesters, and therefore studies on process capable of directly refining 2,6-naphthalene dicarboxylic acid in a convenient and economical manner are required.

US Patent No. 5,256,817 discloses a process for refining 2,6- naphthalene dicarboxylic acid by dissolving it in water or acetic acid and then hydrogenating and crystallizing it. However, in this process there is a problem that the production of naphthoic acid is increased since this process requires heating to a high temperature in order to dissolve the 2,6- naphthalene dicarboxylic acid, and also this process requires expensive metal catalysts for hydrogenation. Further, this process has another problem in that an initial investment expense is enlarged due to requiring a high temperature and high pressure reactor.

Japanese Patent Laid-open Publication Sho 62-230747 A discloses a process for refining 2,6-naphthalene dicarboxylic acid by dissolving in polar solvents, such as dimethylsulfoxide, dimethylformamide, dimethylacetamide, etc., adsorbing to activated carbons, hydrogenating , and then crystallizing. However, such a process requires a lot of solvents and activated carbons.

Moreover, the yield of the product is low because the solvents may be hydrogenated and formyl naphthoic acid is not eliminated.

Japanese Patent Laid-open Publication H05-32586 A discloses a process for refining 2,6-naphthalene dicarboxylic acid by dissolving in pyridine or pyridine derivatives and then crystallizing. However, this process has a problem that its yield is low because the solubility of 2,6-naphthalene dicarboxylic acid in the solvents is not sensitive to temperature.

As another process for refining 2,6-naphthalene dicarboxylic acid, there is a process of converting it into its metal salt and then dissolving and re-crystallizing it.

Japanese Patent Laid-open Publication S52-20993 A and Japanese Patent Publication S No. 48-68544 B disclose a process for refining 2,6- naphthalene dicarboxylic acid by dissolving in KOH or NaOH aqueous solution to thereby prepare its alkali metal salt, adsorbing with a solid adsorbent, and then crystallizing. Disproportionation of the thus-produced mono alkali salt with water generates a refined 2,6-naphthalene dicarboxylic acid. However, this process requires a quantity of solid adsorbents and solvents, and as all mono alkali salts, including salts generated by impurities such as naphthoic acid or formyl naphthoic acid, etc., are crystallized, it is difficult to separate salts generated by impurities.

Japanese Patent Publication S52-20994 B and Japanese Patent Publication S48-68555 B disclose a process for refining 2,6-naphthalene dicarboxylic acid with a diacid salt. This method is carried out by dissolving 2,6-naphthalene dicarboxylic acid in KOH or NaOH aqueous solution, adsorbing it to an adsorbent and then crystallizing the thus-produced diacid

salt. However, this method also requires a large amount of adsorbents. Moreover, not only is it difficult to eliminate a small amount of alkali salt impurities, but also it has a low yield.

Japanese Patent Laid-open Publication H02-243652 A discloses a method for refining by dissolving 2,6-naphthalene dicarboxylic acid in an alkali aqueous solution and then precipitating an alkali salt by adding a polar organic solvent that is well mixed with water thereto. However, when the purity of 2,6-naphthalene dicarboxylic acid from this method is high, it has a problem that its yield is low. A method for refining 2,6-naphthalene dicarboxylic acid by dissolving it into the form of its amine salt and then re-crystallizing it is proposed. The produced 2,6-naphthalene dicarboxylic acid in the form of an amine salt is heated to a temperature above the boiling point of the amine to obtain pure 2,6-naphthalene dicarboxylic acid. Japanese Patent Laid-open Publication S50-142542 A discloses a method of precipitating 2,6-naphthalene dicarboxylic acid in the form of an amine salt by dissolving 2,6-naphthalene dicarboxylic acid in an amine aqueous solution, and then distilling and concentrating the solution.

Japanese Patent Laid-open Publication S50-135062 A discloses a method for refining 2,6-naphthalene dicarboxylic acid by dissolving in an amine aqueous solution and then precipitating it through cooling or condensing the solution, and Japanese Patent Laid-open Publication H05- 294892 A discloses a method of obtaining pure 2,6-naphthalene dicarboxylic acid by dissolving 2,6-naphthalene dicarboxylic acid in a mixed solution of an alcohol and an amine, then precipitating it with an amine salt and heating it.

However, these methods have a problem that when the purity of 2,6- naphthalene dicarboxylic acid is high, its yield is low.

US Patent No. 5,859,294 discloses a method for obtaining pure 2,6- naphthalene dicarboxylic acid by mixing 2,6-naphthalene dicarboxylic acid with an amine, dissolving it in a mixed solution of water, a ketone, or acetonitrile, then cooling it to thereby precipitate the diamine salt of 2,6- naphthalene dicarboxylic acid, and then distilling it. However, this method has a problem of requiring high temperature and high pressure to obtain 2,6- naphthalene dicarboxylic acid with a high yield. Therefore, studies have been carried out on methods for refining 2,6- naphthalene dicarboxylic acid that are capable of obtaining 2,6-naphthalene dicarboxylic acid not only having excellent purity and color through a convenient process but also having excellent purity and high yield.

When the present inventors studied the refining method of 2,6- naphthalene dicarboxylic acid with high purity and high yield, they confirmed that excellently pure 2,6-naphthalene dicarboxylic acid is obtained with high yield by crystallizing crude 2,6-naphthalene dicarboxylic acid in the form of amine salt mixed together with a soluble solvent selected from the group consisting of water, an alcohol, and a mixture thereof, and a insoluble solvent like alkyl acetate. The present invention was developed from this.

Particularly, the present invention is characterized in that the purity and the color of 2,6-naphthalene dicarboxylic acid are improved by repeatedly crystallizing it more than 2 times, the recovery rate of 2,6- naphthalene dicarboxylic acid becomes high by reprocessing a remaining solvent after the crystallizing process, and the solvent used in the process is

reused.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for refining 2,6-naphthalene dicarboxylic acid which can be used for obtaining 2,6- naphthalene dicarboxylic acid having excellent purity and color with high yield in a short time.

It is another object of the present invention to provide a process for refining 2,6-naphthalene dicarboxylic acid which can be used for refining 2,6- naphthalene dicarboxylic acid in an easy, convenient, and economical manner in a shortened process time. In addition, the process is capable of saving energy and being environmentally friendly by re-using a solvent that is used during the refinement process and by using a by-product as a solvent.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flowchart showing refining steps according to a first embodiment.

Figure 2 is a flowchart showing refining steps according to a second embodiment.

Figure 3 is a flowchart showing refining steps according to a third embodiment. Figure 4 is a schematic drawing showing the purification system according to the first embodiment.

Figure 5 is a schematic drawing showing the purification system according to the second embodiment.

Figure 6 is a schematic drawing showing the purification system according to the third embodiment.

DETAILED DESCRITPION OF THE EMBODIMENTS

The present invention will hereafter be described in more detail, with reference to the figures.

For recrystallizing 2,6-naphthalene dicarboxylic acid, the present invention provides a process for refining 2,6-naphthalene dicarboxylic acid comprising the steps of: a) mixing crude 2,6-naphthalene dicarboxylic acid, amine compound, and a solvent mixture of solvent and nonsolvent; b) dissolving the crude 2,6-naphthalene dicarboxylic acid and amine compound by heating the mixture obtained in step a); c) crystallizing 2,6-naphthalene dicarboxylic acid amine salt by cooling and filtering the resultant solution of step b) ; and d) recovering 2,6-naphthalene dicarboxylic acid by deamination of the 2,6-naphthalene dicarboxylic acid amine salt in crystalline form through a thermal process, wherein the process is characterized in that the process further comprises at least a step of selected from the group consisting of the steps of: i ) after step c), dissolving the filtered 2,6-naphthalene dicarboxylic acid amine salt in crystalline form in the solvent mixture and inputting the resultant solution to step c); and ii ) after step c), concentrating the remaining filtrate, crystallizing 2,6- naphthalene dicarboxylic acid amine salt, and recovering by filtration, and then inputting the filtered 2,6-naphthalene dicarboxylic acid amine salt to step a).

At this time, step a) and step b) can be sequentially performed or simultaneously performed in the same reactor.

In addition, the solution mixture of step c) can be concentrated by removing from the mixed solution before cooling and then is used in the method, or is cooled by addition of alkyl acetate solvent thereto.

Moreover, the solution mixture is additionally filtered for removing undissolved impurities between the steps b) and c).

The first embodiment

Figure 1 is a flowchart showing a process of refining 2,6-naphthalene dicarboxylic according to a first embodiment.

Referring to Figure 1 , a process for refining 2,6-naphthalene dicarboxylic acid according to the first embodiment of the present invention comprises: a) mixing crude 2,6-naphthalene dicarboxylic acid, amine compound, and a solvent mixture of solvent and nonsolvent; b) dissolving the crude 2,6-naphthalene dicarboxylic acid and amine compound by heating the mixture obtained in step a); c) crystallizing 2,6-naphthalene dicarboxylic acid amine salt by cooling the mixing solution obtained in step b) and filtering; d) dissolving the filtered 2,6-naphthalene dicarboxylic acid amine salt in crystalline form in the solvent and repeating step c); and e) recovering 2,6-naphthalene dicarboxylic acid by deamination of recrystallized 2,6-naphthalene dicarboxylic acid amine salt through a thermal process. Each step will hereafter be described in detail.

Step a) The step of mixing amine compounds In step a), 2,6-naphthalene dicarboxylic acid amine salt is prepared by mixing a crude 2,6-naphthalene dicarboxylic acid to be refined, amine compound, and a solvent mixture. The crude 2,6-naphthalene dicarboxylic acid reacts with the amine compound to produce its diamine form in order to increase the solubility to a solvent. Sequentially, the 2,6-naphthalene dicarboxylic acid can be recovered by performing the deamination of the 2,6-naphthalene dicarboxylic acid salt with thermal process, and cooling. In the step, the amine compound is required in an amount of a certain equivalent weight per functional group of 2,6-naphthalene dicarboxylic acid. Preferably, the amount is 0.9 to 1.5 equivalent weight, and more preferably 1.0 to 1.3 equivalent weight, but is not limited thereto. In consideration of cost, specific heat and so on of the amine compound, it is preferable to use ammonia, a trimethylamine, a triethylamine, a diethylamine, a dimethylamine, a methylamine or ethylamine.

In the step, the mixing process can be carried out by any means, and can be carried out under the condition of room temperature and atmospheric pressure. Step b) the step of dissolving

In step b), the crude 2,6-naphthalene dicarboxylic acid amine salt obtained in step a) is sufficiently dissolved in the solvent mixture of solvent and nonsolvent at a temperature of 25°C to 150°C.

There are several refining methods of compound using the crystallization step. In accordance with the present invention, it employs a

solvent/nonsolvent refining method, which a compound dissolves in some condition and precipitates as a crystal form in another condition. In this crystallization method, the purity and the yield of compound can be affected by choosing suitable solvent and nonsolvent, and a mixing ratio thereof. Suitable solvents used for the present invention include protic polar solvents such as alcohol, water and a mixture thereof, which can dissolve 2,6-naphthalene dicarboxylic amine salt. The nonsolvent can be aprotic solvents, including alkyl acetate, acetone and a mixture thereof. In a mixed solvent including solvent/nonsolvent, 2,6-naphthalene dicarboxylic acid be dissolved very well at high temperature due to high solubility, but in contrast, precipitates at low temperature due to low solubility.

As above mentioned, the mixing ratio of solvent/nonsolvent in a mixed solution affects the yield of crystallization-refining method. Preferably, the solvent and the nonsolvent are used in a mixing ratio of 1 :1 to 1 :20 by weight, and the solvent comprises an alcohol and water, or water and an alcohol in a mixing ratio of 1 :10 to 100:1 by weight. When the ratio is large excessively, the resultant 2,6-naphthalene dicarboxylic acid is recovered in low yield, due to low solubility of the crude 2,6-naphthalene dicarboxylic acid amine salt in the mixed solution. It is preferable to uses lower alcohols. More preferably, a alcohol can be at least one selected from the group consisting of methanol, ethanol, propannol, and iso-propanol.

Suitable nonpolar solvents include alkyl acetate, acetone and a mixture thereof, but are not limited thereto. For examples, the nonpolar solvent is selected from the group consisting of methyl acetate, ethyl acetate,

n-propyl acetate and isopropyl acetate, and preferably methyl acetate. The methyl acetate is produced as a by-product in oxidation process during synthesizing terephthalic acid. This means that the refining method of the present invention is environment-friendly and cost-effective. Additionally, to remove impurities from crude 2,6-naphthalene dicarboxylic acid amine salt, the mixed solution can be filtered. The filtering step can be carried out using a conventional filtering apparatus under the same temperature as that of the dissolving process, to precipitate 2,6- naphthalene dicarboxylic acid amine salt with a high yield in subsequent step. Step c) The step of crystallization

This step c) can be performed by cooling the mixing solution obtained at step c) to precipitate 2,6-naphthalene dicarboxylic amine salt as a crystal form, and filtering to recover 2,6-naphthalene dicarboxylic amine salt in a crystallized form. In the crystallization step, the amount of precipitated 2,6-naphthale dicarboxylic acid amine salt can be affected by the temperature of step b) that is the temperature of the mixing solution before cooling step. The large difference between the temperature of the mixing solution before and after cooling increases the precipitated content of 2,6-naphtalene dicarboxylic acid amine salt. Preferably, the step of crystallization can be carried out at the temperature of ranging from -10 ⁰ C to 50 °C .

For increasing the contents of precipitated 2,6-naphtalene dicarboxylic acid amine salt, there are two ways; one is to concentrate and agitate the mixing solution of step b) slowly as concentrated before cooling, and the other is to proceed step c) following the addition of a nonsolvent to

the mixing solution of step b).

In the crystallization step, the crystallized 2,6-naphtalene dicarboxylic acid amine salt is separated by being filtered with conventional filter apparatus . Step d) the step of re-crvstallization

In the step, after re-dissolving 2,6-naphthalene dicarboxylic acid amine salt recovered in step c) in the solvent mixture of step b), crystallization step in step c) is carried out for obtaining 2,6-naphthalene dicarboxylic acid having more excellent purity and color. The re-crystallization is carried out under the same condition of steps b) and c).

As mentioned above, repetitive crystallization makes 2,6- naphthalene dicarboxylic acid finally obtained have more excellent purity and color. Step e) the step of deamination

In the step, 2,6-naphthalene dicarboxylic acid and a amine compound were obtained by deamination of the filtered 2,6-naphthalene dicarboxylic acid amine salt in crystalline form recovered in step d) through the thermal process which is performed at the temperature of 50 °C to 120°C after washing, and the amine compound can be refined with a separate refining device and then reused.

2,6-naphthalene dicarboxylic acid refined through step a) to e) is able to be recovered with yield of 90% or more, purity of 99.6% or more, and color-b of 3.2 or less. Especially, the refined product include bromine compound of less than 3ppm, cobalt of less than 10ppm, and manganeseis

of less than 3ppm

The second embodiment

Figure 2 is a flowchart showing the steps of refining 2,6-naphthalene dicarboxylic according to a second embodiment. Referring to Figure 2, a method for refining 2,6-naphthalene dicarboxylic acid according to the second embodiment of the present invention comprises: a) mixing crude 2,6-naphthalene dicarboxylic acid, amine compound, and a solvent mixture of solvent and nonsolvent; b) dissolving the crude 2,6-naphthalene dicarboxylic acid and amine compound by heating the mixture obtained in step a); c) crystallizing 2,6-naphthalene dicarboxylic acid amine salt by cooling the mixing solution obtained in step b) and filtering; d) concentrating the filtrate remained after filtering of step c) and cooling to precipitate the 2,6-naphthalene dicarboxylic acid amine salt additionally; e) inputting 2,6-naphthalene dicarboxylic acid amine salt precipitated additionally in step d) in step a); and f) recovering 2,6-naphthalene dicarboxylic acid by deamination of 2,6- naphthalene dicarboxylic acid amine salt obtained in step c) through a thermal process.

The steps a) to c) and f) are followed in the first embodiment. Hereafter steps d) and e) will be described, in more detail.

In step of f), it can be performed by deaminating a 2,6-naphthalene dicarboxylic acid amine salt which is prepared by precipitation, and filtration.

The 2,6-naphthalene dicarboxylic acid amine salt can not be precipitated up to 100% even though the conditions is optimally adjusted. Therefore, the residual filtrate after filtering contains lots of 2,6-naphthalene dicarboxylic acid amine salt, which cannot be precipitated in previous step, namely crystallization step. The contents of 2,6-naphthalene dicarboxylic acid amine salt dissolved in residual filtrate depend on the condition of crystallization step.

In accordance with the present invention, to increase the yield of 2,6- dicarboxylic acid, it performs by concentrating the residual filtrate, and cooling to precipitate 2,6-naphthalene dicarboxylic acid amine salt, as the same manner in the step c) of the first embodiment.

The content of 2,6-naphthalene dicarboxylic acid amine salt depends on the concentrating of residual filtrate and the cooling temperature. However, when 2,6-naphthalene dicarboxylic acid amine salt recovers excessive, its quality is lower than crude 2,6-naphthalene dicarboxylic acid. Therefore, the excessively recovered 2,6-naphthalene dicarboxylic acid amine salt leads the quality of resultant 2,6-naphthalene dicarboxylic acid to lessen, including lots of impurities and color contamination, and so on. Therefore, it is very important to control the contents of recovery 2,6-naphthalene dicarboxylic acid amine salt, which its quality is similar with the crude 2,6-naphthalene dicarboxylic acid.

Because the residual filtrate is crystallized, 2,6-naphthalene dicarboxylic acid can recover in high yield over 95%, compared with 60 to 80% by the convention method. The third embodiment

Figure 3 is a flowchart showing the steps of refining 2,6-naphthalene dicarboxylic acid according to a third embodiment.

Referring to Figure 3, a method for refining 2,6-naphthalene dicarboxylic acid according to the third embodiment of the present invention comprises: a) mixing crude 2,6-naphthalene dicarboxylic acid, amine compound, and a solvent mixture of solvent and nonsolvent; b) dissolving the crude 2,6-naphthalene dicarboxylic acid and amine compound by heating the mixture obtained in step a); c) crystallizing 2,6-naphthalene dicarboxylic acid amine salt by cooling the mixing solution obtained in step b) and filtering; d) concentrating the filtrate remained after filtering of step c) and cooling to precipitate the 2,6-naphthalene dicarboxylic acid amine salt additionally; e) inputting 2,6-naphthalene dicarboxylic acid amine salt precipitated additionally in step d) in step a); f) adding the solvent mixture to the 2,6-naphthalene dicarboxylic acid amine salt in crystalline form precipitated in step c), dissolving and repeating step c); and g) recovering 2,6-naphthalene dicarboxylic acid by deamination of re- crystallized 2,6-naphthalene dicarboxylic acid amine salt through a thermal process.

The steps a) to c), and g) are followed by the steps in the first embodiment, and steps d), e) and f) including residual filtrate and re- crystallization are followed by that in the second embodiment and the first

embodiment.

There are variable ways for refining 2,6-naphthlene dicarboxylic acid, according to the first embodiment through the third embodiment, as above mentioned. Exemplarily, Figures 4 to 6 show the schematic drawing of refining system to perform methods proposed by the first to the third embodiments. The refining system can be preferably varied by skilled person in this technical art.

Figure 4 is a schematic drawing showing the purification system of 2,6-naphthalene dicarboxylic acid according to the first embodiment. The first embodiment of the present invention improves the purity and color of 2,6- naphthalene dicarboxylic acid obtained by recrystallizing 2,6-naphthalene dicarboxylic acid amine salt precipitated after the crystallization step.

Referring to Figure 4, the compounds of the reservoirs (not shown) in which crude 2,6-naphthalene dicarboxylic acid, amine compound, and a solvent mixture are stored are injected into the first dissolving bath 100 and the homogenous mixing solution is made by heating and stirring. The mixing solution injected into the first dissolving bath 100 above is transferred to the 1st filter 120 to remove impurity and the mixing solution obtained is transferred to the 1st crystallizing bath 140. The mixing solution is cooled by a cooling device equipped to 1st crystallizing bath 140 to precipitate 2,6- naphthalene dicarboxylic acid amine salt transferred from the 1st filter 120.

2,6-naphthalene dicarboxylic acid amine salt in crystalline form obtained in the 1st crystallizing bath 140 above is transferred to the 2nd dissolving bath 102 for re-crystallization and a solvent mixture for dissolving 2,6-naphthalene dicarboxylic acid amine salt is injected into the 2nd

dissolving bath 102.

The 2,6-naphthalene dicarboxylic acid amine salt in crystalline form recrystallized is recovered from the mixing solution obtained in the 2nd dissolving bath 102 above through the 3rd filter 122, the 2nd crystallizing bath 142 and the 4 ^th filter 162

The refined 2,6-naphthalene dicarboxylic acid is obtained from the 2,6-naphthalene dicarboxylic acid amine salt in crystalline form obtained in the 4 ^th filter 162 above through a washing step and a deamination step in a dryer and amine compound volatilized is recovered and reused by refining through a separate refining device.

For increasing yield for crystallization, a separate inputting device(not shown) is equipped toist crystallizing bath 140 above and 2nd crystallizing bath 142 to inject alkyl acetate as a solvent additionally or the mixing solution is concentrated before the crystallization step. The mixing solution above is concentrated between the 1st filter 120 and the 1st crystallizing bath 140 and between the 3rd filter122 and 2nd crystallizing bath142 by adding a solvent-evaporator (not shown) additionally. And the solvent evaporated is recovered and is reused by refining with a separate refining device or is returned to the first dissolving bath 100 or the 2nd dissolving bath 102. Addition to, the solvent above is recovered from the filtrate remained in the 2nd filter160 and 4th filter161 with a separate refining device and the other waste remained is removed.

Figure 5 is a schematic drawing showing the purification system of 2,6-naphthalene dicarboxylic acid according to the second embodiment. The second embodiment of the present invention increases the yield for

purification of 2,6-naphthalene dicarboxylic acid by recovering 2,6- naphthalene dicarboxylic acid amine salt from filtrate remained after crystallization.

Referring to Figure 5, the compounds of the reservoirs (not shown) in which crude 2,6-naphthalene dicarboxylic acid, amine compound, and a solvent mixture are stored are injected into the dissolving bath 200 and the homogenous mixing solution is made by heating and stirring in the dissolving bath 200. The mixing solution above is transferred to the 1st filter 220 to filter the mixing solution and the filtrate is transferred to the 1st crystallizing bath 240 equipped with a cooling device. The 2,6-naphthalene dicarboxylic acid amine salt in crystalline form obtained is filtered with the 2nd filter 260 which is linked to the 1st crystallizing bath 240. The 2,6-naphthalene dicarboxylic acid is obtained from the 2,6-naphthalene dicarboxylic acid amine salt in crystalline form filtered in the 2nd filter 260 through thermal process with the drye280r which is linked to the 2 ^nd filteri 60 above.

For recovering 2,6-naphthalene dicarboxylic acid amine salt with the 2nd filter 260 from filtrate remained, the filtrate separate from 2nd filter 260 above is transferred to the 1 ^st solvent-evaporator 210 and the solvent is removed for concentrating. And the solvent evaporated is recovered and is reused by refining with a separate refining device.

The mixture obtained In first solvent-evaporator 210 above is transferred to the 2nd crystallizing bath 230 and is cooled to precipitate 2,6- naphthalene dicarboxylic acid amine salt. A slurry obtained from the 2nd crystallizing bath 230 above is done solid-liquid separation in the 3rd filter 250. The 2,6-naphthalene dicarboxylic acid amine salt in crystalline form is

returned to the dissolving bath 200 above and the solvent is recovered from the filtrate with 2 ^nd solvent-evaporator 270 and the other waste remained is ejected.

For increasing yield for crystallization, theist crystallizing bath 240 above and 2nd crystallizing bath 230 is injected to alkyl acetate as a solvent additionally or the mixing solution is concentrated before the crystallization step by injecting solvent-evaporator(not shown) between the 1st filter 220 and 1 ^st crystallizing bath240 above by the same manner as used in the first embodiment above. Figure 6 is a schematic drawing showing the purification system of

2,6-naphthalene dicarboxylic acid according to the third embodiment. The third embodiment of the present invention increases the yield for purification of 2,6-naphthalene dicarboxylic acid and improves the purity and color of 2,6- naphthalene dicarboxylic acid obtained by recrystallizing 2,6-naphthalene dicarboxylic acid amine salt first-recovered with 2nd filter360 and recovering 2,6-naphthalene dicarboxylic acid remained in filtrate above

Referring to Figure 6, the compounds of the reservoirs (not shown) in which crude 2,6-naphthalene dicarboxylic acid, amine compound, and a solvent mixture are stored are injected into the dissolving bath 300 equipped with a heating device and a stirring device, and the homogenous mixing solution is made by heating and stirring in the dissolving bath 300. The mixing solution above is transferred to the 1st filter 320 which is linked to the dissolving bath 300 to filter the mixing solution and the filtrate is transferred to the 1st crystallizing bath 340 equipped with a cooling device, which is linked to the 1st filter 320. The 2,6-naphthalene dicarboxylic acid amine salt in

crystalline form obtained in the 2nd filter 360 above is transferred to 2nd dissolving bath 302 injected to the solution mixture. The 2,6-naphthalene dicarboxylic acid amine salt is recrystillized through the 3 ^rd filter322, the 2 ^nd dissolving bath 302 and the 4 ^th filter by the same manner as used in the first embodiment above. And the 2,6-naphthalene dicarboxylic acid is recovered from the 2,6-naphthalene dicarboxylic acid amine salt in re-crystalline form obtained in the 4th filter 362 by deamination with the dryer380 which is linked to the 4th filter 362.

For increasing yield for re-crystallization, a separate inputting device(not shown) is equipped to1 ^st crystallizing bath 340 above and 2 ^nd crystallizing bath 342 to inject alkyl acetate as a solvent additionally or the mixing solution is concentrated before the crystallization step. The mixing solution above is concentrated in between the 1 ^st filter 320 and the 1 ^st crystallizing bath 340 and in between the 3 ^rd filter322 and 2nd crystallizing bath342 by adding a solvent-evaporator (not shown) additionally. And the solvent evaporated is recovered and is reused by refining with a separate refining device or is returned to the first dissolving bath 300 or the 2 ^nd dissolving bath 302).

For recovering 2,6-naphthalene dicarboxylic acid amine salt from filtrate remained in 2 ^nd filter360 and recovered in 4 ^th filter362, the filtrate separated above is concentrated in the 1 ^st solvent-evaporator 310 and is crystallized in the 3 ^rd dissolving bath 300.

For increasing an effect of re-crystallization, arbitrary injection apparatus (not drawn) can further employ to the 1 ^st crystallizing bath 340 and the 2 ^nd crystallizing bath 342, to inject solvent or to concentrating a mixed

solution before crystallizing. The concentrating a mixed solution performs in solvent remover (not drawn) further equipped between the 1 ^st filter 320 and the 1 ^st crystallizing bath 340, the 3 ^rd filter 322 and the 2 ^nd crystallizing bath 342. The solvent distilled from the solvent remover can reuse after purification in a purifier or re-send to the first dissolving bath 300 or 2 ^nd dissolving bath 302.

Meanwhile, residual filtrate from the 2 ^nd filter 360 and the 4 ^th filter362 performs by crystallizing to recover 2,6-naphthalene dicarboxylic acid amine salt dissolved in filtrate. The crystallized 2,6-naphthalene dicarboxylic acid amine salt performs by a solid-liquid filtration passing though the 5 ^th filter350, and resending to first dissolving bath300. Through the 1 ^st solvent- evaporator 310, it recovers the distilled solvent. And a residual solvent obtained by passing through the 5 ^th filter 350 sends to the 2 ^nd solvent evaporator370 for recovering and the waste is removed. As mentioned above, it can be prepared 2,6-naphthalene dicarboxylic acid with improved purity and color property under controlling conditions, such as preferable solvent, crystallization step, in accordance with the present invention. Especially, due to using filtrate obtained after crystallization step, the yield of 2,6-naphthalene dicarboxylic acid is increased remarkably. In addition, because of reusing a used solvent during the process, the method of the present invention is very economic.

To facilitate the understanding of the invention, preferred examples thereof are provided. However, these examples are provided solely to illustrate the invention; the scope of the invention should not be construed to be limited thereto.

Example 1

To a 1-neck Erlenmeyer flask having a Pyrex-type lid, 50.0 g of crude 2,6-naphthalene dicarboxylic acid and 55.2 g of triethylamine were added at room temperature and atmospheric pressure. 400 g of a mixed solvent containing methanol, water and methyl acetate in a mixing ratio of 17.5:2.5:80.0 by weight were added thereto and the mixture was heated to 60 ⁰ C while stirring for 30 minutes to obtain a solution of the 2,6-naphthalene dicarboxylic acid amine salt

After filtering the mixing solution above using a filter with a 7-μm pore size at 60 ^° C, the filtrate is concentrated by removing the solvent. The filtrate obtained from filtration was then cooled at room temperature while slowly stirring for 1 hour to precipitate a crystal of the 2,6-naphthalene dicarboxylic acid amine salt.

For recrystallizing the crystal, after recovering the crystal, 400 g of a mixed solvent containing methanol, water and methyl acetate in a mixing ratio of 17.5:2.5:80.0 by weight were added thereto and the crystal of the 2,6- naphthalene dicarboxylic acid amine salt is precipitated by the above method. After filtering the resultant solution containing the crystal above and separating, the mixture obtained is placed at 90 ⁰ C for 1 hour for removing the solvent to yield a purified 2,6-naphthalene dicarboxylic acid

Comparative example 1

To a 1-neck Erlenmeyer flask having a Pyrex-type lid, 50.0 g of crude 2,6-naphthalene dicarboxylic acid and 55.2 g of triethyl amine were added at room temperature and room pressure.

315 g of a mixed solvent containing methanol, water and methyl acetate in a mixing ratio of 17.5:2.5:80.0 by weight were added thereto and the mixture was heated to 55 ⁰ C while stirring for 30 minutes to obtain a solution of the 2,6-naphthalene dicarboxylic acid amine salt. After doing vacuum filtration of the mixing solution above using a filter with a 7-μm pore size at 60 °C and the thus-obtained filtrate was heated at 55 ⁰ C for 30 minutes. After heating, the filtrate is cooled until room temperature and is placed at room temperature for 12 hours to precipitate a crystal of the 2,6-naphthalene dicarboxylic acid amine salt. After filtering the resultant solution containing the crystal and separating, the mixture obtained is placed at 90 °C for 1 hour for removing the solvent to yield a purified 2,6-naphthalene dicarboxylic acid

The contents of the bromine compounds, cobalt and manganese remaining in the 2,5-naphthalene dicarboxylic acid obtained in Example 1 and Comparative Example 1 above were detected, and the yield, purity, and color of the 2,6-naphthalene dicarboxylic acid were detected. The results are shown in Table 1 below. The purity was detected using Gas Chromatography (G. C).

In addition, characteristics of the first crystallization step and the re- crystallization step were detected separately.

Table 1

Category Example 1 Comparative crude 2,6-NDA Example 1

1 ^st crystallization re-crystallization 1 ^st crystallization -

Yield (%) 60.3 22.5 62.2 -

Purity (%) 99.48 99.62 99.57 98.19

Color (Color-b) 6.18 3.17 3.60 11.23

Referring to Table 1 above, the re-crystallization step according to the present invention does not improve the purity significantly, but does give excellent color and the amounts of metals like Co and Mn lower the quality of the final polyester polymer product are significantly reduced. In addition, the amount of Br, which causes corrosion of a reactor used in polymerization through re-crystallization is reduced significantly, to under 10 ppm.

There are 3 methods of concentrating used in Examples 2 to 9 below The process used in Examples 2 and 3 is that a solvent is removed with distilling at atmospheric pressure to recover the 2,6-naphthalene dicarboxylic acid. The process used in Examples 4 to 6 is that a nonsolvent of which amount is amount of a solvent distillated at atmospheric pressure is added to the resultant solution in the re-crystallization step. The process used in Examples 7 to 9 is that after adding pressure in the dissolving step, a solvent is distillated by decompression in the re-crystallization step.

Example 2

To a 1-neck Erlenmeyer flask having a Pyrex-type lid, 50.0 g of crude 2,6-naphthalene dicarboxylic acid and 60.0 g of triethyl amine were added at room temperature and atmospheric pressure.

400 g of a mixed solvent containing methanol, water and methyl acetate in a mixing ratio of 17.5:2.5:80.0 by weight were added thereto and the mixture was heated to 60 ⁰ C while stirring for 30 minutes to obtain a

solution of the 2,6-naphthalene dicarboxylic acid amine salt

After filtering the mixing solution using a filter with a 7-jum pore size at

60°C, the filtrate obtained from filtration was heated to remove a part of the solvent in the filtrate. The concentrated solution obtained was then cooled at room temperature while slowly stirring for 1 hour to precipitate a crystal of the

2,6-naphthalene dicarboxylic acid amine salt.

For recrystallizing the crystal above, after recovering the crystal, 400 g of a mixed solvent containing methanol, water and methyl acetate in a mixing ratio of 17.5:2.5:80.0 by volume were added thereto and the crystal of the 2,6-naphthalene dicarboxylic acid amine salt is precipitated by the above method.

After filtering the resultant solution containing the crystal and separating, the mixture obtained is placed at 90 °C for 1 hour for removing the solvent to yield a purified 2,6-naphthalene dicarboxylic acid

Example 3

The purified 2,6-naphthalene dicarboxylic acid is obtained by the same manner as used in Example 2 above, except increasing the amount of a solvent that is removed in the filtrate obtained after filtering. The contents of the bromine compounds, cobalt and manganese remaining in the 2,5-naphthalene dicarboxylic acid obtained in Example 2 and Example 3 above were detected, and the yield, purity, and color of the 2,6-naphthalene dicarboxylic acid were detected. The results are shown in Table 2 below. The purity was detected using G. C. In addition, characteristics of the first crystallization step and the re-crystallization step

were detected separately.

Referring to Table 2 above, the purity and color is improved by recrystallizing in refining 2,4-naphthalene dicarboxylic acid and the yield in re-crystallization is increased by removing the solvent of the mixing solution and concentrating.

Example 4

To a 1-neck Erlenmeyer flask having a Pyrex-type lid, 50.0 g of crude 2,6-naphthalene dicarboxylic acid and 55.2 g of triethyl amine were added at room temperature and atmospheric pressure.

400 g of a mixed solvent containing methanol, water and methyl acetate in a mixing ratio of 17.5:2.5:80.0 by weight were added thereto and the mixture was heated to 60 ⁰ C while stirring for 30 minutes to obtain a solution of the 2,6-naphthalene dicarboxylic acid amine salt.

After filtering the mixing solution using a filter with a 7-μm pore size at 60 ⁰ C ₁ the filtrate obtained from filtration was concentrated by removing a solvent. After adding methyl acetate as much as the solvent removed to the concentrated solution above, the filtrate obtained from filtration was cooled at room temperature while slowly stirring for 1 hour to precipitate a crystal of the 2,6-naphthalene dicarboxylic acid amine salt.

After filtering the resultant solution containing the crystal and separating, the mixture obtained is placed at 90 °C for 1 hour for removing the solvent to yield a purified 2,6-naphthalene dicarboxylic acid.

Example 5

The purified 2,6-naphthalene dicarboxylic acid is obtained by the same manner as used in Example 4 above, except using ethyl acetate as a solvent added after concentrating.

Because the 2,6-naphthalene dicarboxylic obtained has low qualities about purity, amount of T-Br and metal and etc., but has the relatively improved purity In using ethyl acetate as a nonsolvent, ethyl acetate is suitable nonsolvent when not necessary for high purity in the aspect of economy.

Example 6

The purified 2,6-naphthalene dicarboxylic acid is obtained by the same manner as used in Example 4 above, except using acetone as a solvent added after concentrating.

The yield, purity, and color of the 2,6-naphthalene dicarboxylic acid obtained in Example 4 to Example 6 above were detected by the same manner as used in Example 1 above. The results are shown in Table 3 below. Table 3

Referring to Table 3 above, it was prepared 2,6-naphthalene dicarboxylic acid with a high purity color since after concentrating solvent a nonsolvent was added, and then re-crystallized. As the 2,6-naphthalene dicarboxylic acid contains low metal content such as Co, Mn ₁ up to ppm level, it can produce a polyester product having a high quality. When it uses methyl acetate, ethyl acetate, acetone, preferably methyl acetate as a nonsolvent, the same result was obtained.

Example 7 In a container (SUS-316, 10 L), a crude 2,6-naphthalene dicarboxylic acid (750 g), triethyl amine (900 g) added at room temperature under atmospheric pressure. To the container, it was added 2885 g of mixture of methanol, water, methyl acetate in a ratio of 17.5:2.5:80.0 by weight, and stirred at 70 °C , 3 atm. for 30 minutes to prepare a mixing solution of 2,6- naphthalene dicarboxylic acid amine salt.

The mixing solution was filtered through cartridge filter (pore size: 5 μm) at 70°C to eliminate undissolved impurities, and thus an obtained filtrate was added in a crystallizer (SUS-316, 20 L) and then crystallized it. A flashing method used to increase a crystalline yield of 2,6-naphthalene dicarboxylic acid amine salt, which it performs cooling to room temperature and concentrating at the same time.

After separating 2,6-naphthalene dicarboxylic acid amine salt by filtering, it was heated at 90 °C to remove amine compound to prepare 2,6-

naphthalene dicarboxylic acid.

In addition to this, after concentrating the resultant filtrate to remove solvent, an obtained residue added into the crystallizer. Afterwards, it was prepared 2,6-naphthalene dicarboxylic acid in the same manner as the above-mentioned.

Example 8

Prepared in a similar manner as Example 7 to give a purified 2,6- naphthalene dicarboxylic acid, except but a mixed solvent was used methanol:water:methyl acetate in a ratio of 17.5 : 4.5 : 78.0 by weight.

Example 9

Prepared in a similar manner as Example 7 to give a purified 2,6- naphthalene dicarboxylic acid, except but a mixed solvent was used methanol: water: methyl acetate in a ratio of 17.5 : 8.0 : 74.5 by weight.

The yield, purity, and color of 2,6-naphthalene dicarboxylic acid in the 2,6-naphthalene dicarboxylic acid obtained in Examples 7 to 9, were determined in the same manner as in Example 1 above, and they are shown in Table 4 below. The results was determined each case, performing a crystallization of 2,6-naphthaliene dicarboxylic acid and using the filtrate remained. Table 4

Seen from the Table 4, the yield is very high over 90% in Example to 9 and the purity is high 99.86% to 99.82%, as well. A purity of 99.92% means that 2,6-naphthalene dicarboxylic acid contains impurities about 800 ppm. This result leads molecular weight of polyester to increase more about 5 times than that in Comparative example 1 and to synthesize a copolymer comprising 2,6-naphthalene dicarboxylic acid in a high content.

Moreover, in terms of treating a residual filtrate, Table 4 shows that yield increases as an amount removed by concentrating, slightly. However, such an increasing value is not considerable, even though such a change in yield might affect on purity and color to decrease a quality of polymer product. 2,6-naphthalene dicarboxylic acid prepared in Example 9 has high yield of 72.9% yield in spite of removing 85.2%, however, has low purity of 98.41%, and Col-b value of 1.4, which such purity and Col-b value low can be improved by re-crystallization of residual filtrate. 2,6-naphthalene dicarboxylic acid having a high purity and low color contaminant is recovered with high yield according to the present invention. Moreover, because the solvent used in the refining process is re-used and the byproduct of the reaction process is used as a solvent, this invention is environmentally friendly and has energy-saving effects. Further, it enables to refine pure 2,6-naphthalene dicarboxylic acid in an easy, fast and economical way by eliminating the naphthoic acid, formyl naphthoic acid, catalyst compounds, etc. remaining in impure 2,6-naphthalene dicarboxylic acid.