Method and apparatus for separating and refining high purity 2,6-naphthalene dicarboxylic acid using crystallization

1. A method for separating and refining 2,6-naphthalene dicarboxylic acid from crude 2,6-naphthalene dicarboxylic acid having a purity of 98.0˜99.5%, comprising the steps of:

(a) mixing crude 2,6-naphthalene dicarboxylic acid and water to prepare slurry;

(b) dissolving the mixed slurry;

(c) performing crystallization to grow the dissolved mixture into crystals through multiple-step crystallization using a plurality of crystallization units;

(d) washing the crystals obtained;

(e) performing solid-liquid separation to separate 2,6-naphthalene dicarboxylic acid and water after washing; and

(f) drying 2,6-naphthalene dicarboxylic acid;

wherein:

the plurality of crystallization units linked in series is four crystallization units, and the temperature and pressure thereof are set at 260-280° C. and 60-85 kg/cm 2 , respectively, in a first crystallization unit, at 240-260° C. and 40-65 kg/cm 2 in a second, 220-240° C. and 30-45 kg/cm 2 in a third and 200-220° C. and 20-35 kg/cm 2 in a fourth;

the plurality of crystallization units linked in series is two crystallization units, and the temperature and pressure thereof are set at 250-260° C. and 60-75 kg/cm 2 , respectively, in a first crystallization unit and at 200-240° C. and 25-45 kg/cm 2 , respectively, in a second; or

the plurality of crystallization units linked in series is three crystallization units, and the temperature and pressure thereof are set at 260-280° C. and 60-85 kg/cm 2 , respectively, in a first crystallization unit, at 240-260° C. and 40-65 kg/cm 2 in a second and at 200-220° C. and 20-35 kg/cm 2 in a third.

2. The method for separating and refining 2,6-naphthalene dicarboxylic acid according to claim 1 , wherein the slurry mixed in the dissolving step is primarily preheated using heat generated at the upper part of a crystallization unit, and then the slurry is entirely dissolved after passing through the preheater to thereby have a constant temperature of 290˜310° C.

3. The method for separating and refining 2,6-naphthalene dicarboxylic acid according to claim 1 , wherein the concentration of the slurry produced in the mixing step is in the range of 5 to 10%.

4. The method for separating and refining 2,6-naphthalene dicarboxylic acid according to claim 1 , wherein the crystallization process lasts 30 to 90 minutes.

5. The method for separating and refining 2,6-naphthalene dicarboxylic acid according to claim 1 , wherein the crystal growth rate is at 1×10 −6 ˜1×10 −9 m/s.

6. The method for separating and refining 2,6-naphthalene dicarboxylic acid according to claim 1 , wherein the size of the separated and refined crystal is 35˜75 μm.

7. The method for separating and refining 2,6-naphthalene dicarboxylic acid according to claim 1 , wherein the value of chromaticity b of the crystals is 5 or below, and the value of chromaticity L of the crystals is 95 or above.

8. The method of claim 1 , wherein the product of step b) is fed into a first crystallization unit.

9. The method of claim 8 , wherein the product fed into the first crystallization unit is at a temperature of 290-310° C.

10. The method of claim 1 , wherein the crude naphthalene dicarboxylic acid has an average crystal size of 5-15 μm and the average crystal size of the product of step f) is 35-75 μm.

11. The method of claim 1 , wherein the crude 2,6-naphthalene dicarboxylic acid of step a) comprises:

0.05-0.30 weight % 2-naphthoic acid;

0.05-0.10 weight % 6-methyl-2-naphthoic acid;

0.15-0.50 weight % trimellitic acid;

0.05-0.20 weight % 6-formyl-2-naphthoic acid; and

98.00-99.50 weight % 2,6-naphthalene dicarboxylic acid.