Processes for producing succinic acid

1. A process for producing succinic acid, the process comprising:

feeding a succinic-acid-containing liquid continuously or intermittently to a crystallization tank,

crystallizing succinic acid from the succinic-acid-containing liquid in the crystallization tank, to obtain crystallized succinic acid, and

discharging the crystallized succinic acid continuously or intermittently from the crystallization tank,

wherein:

the feeding and the discharging are performed so as to keep a liquid level of the crystallization tank within a given range;

in the crystallizing, seed crystals of succinic acid are introduced continuously or intermittently into the crystallization tank; and

the seed crystals comprise crystals obtained by pulverizing at least a part of the crystallized succinic acid that has been discharged from the crystallization tank.

2. The process according to claim 1 , wherein the crystallization tank comprises a stirrer, and during at least a part of the crystallizing the stirrer is operated such that a stirring power per unit volume of the succinic-acid-containing liquid is 0.4 to 3 kW/m 3 .

3. The process according to claim 1 or 2 , performed such that the succinic-acid-containing liquid in the crystallization tank has a temperature of 25 to 60° C.

4. The process according to claim 3 , wherein during the crystallizing, a difference between a temperature of the succinic-acid-containing liquid present in the crystallization tank and a temperature at which the succinic-acid-containing liquid introduced into the crystallization tank becomes a saturated succinic acid solution, is within a range of 10 to 45° C.

5. The process according to claim 1 or 2 , wherein the succinic-acid-containing liquid has an average residence time in the crystallization tank of 1 to 5 hours.

6. The process according to claim 1 or 2 , further comprising reducing an internal pressure of the crystallization tank, during the crystallizing, to a value lower than a pressure of an ambient atmosphere.

7. The process according to claim 6 , wherein the internal pressure of the crystallization tank is 0.5 to 20 kPa.

8. The process according to claim 1 , wherein the pulverizing is conducted with a wet pulverizer.

9. The process according to claim 1 , wherein the pulverizing is conducted with (i) a circulation unit that discharges a succinic-acid slurry present in the crystallization tank and returns the slurry to the inside of the crystallization tank, or (ii) a pump disposed in a unit that feeds the succinic-acid-containing liquid to the crystallization tank, or both (i) and (ii).

10. The process according to claim 1 , wherein the seed crystals have a volume-average particle diameter of 200 μm or less, and an amount of the seed crystals introduced is 0.001 to 20% by weight of an amount of the succinic acid recovered.

11. The process according to claim 1 or 2 , wherein the succinic-acid-containing liquid has a succinic-acid concentration of 10 to 45% by weight.

12. The process according to claim 1 or 2 , wherein the succinic-acid-containing liquid comprises a solvent having a relative permittivity of 10 or higher.

13. The process according to claim 1 or 2 , wherein during the crystallizing, the succinic-acid-containing liquid is cooled by at least 10° C. from a temperature that the succinic-acid-containing liquid had when introduced into the crystallization tank.

14. The process according to claim 1 or 2 , wherein the succinic-acid-containing liquid is fed to a liquid phase present in the crystallization tank.

15. The process according to claim 3 or 4 , wherein the liquid present in the crystallization tank has a temperature that is lower than a temperature of a wall surface of the crystallization tank.