Process for purifying a crude composition of dialkyl ester of 2,5-furandicarboxylic acid

1. A process for purifying an ester composition comprising at least 90% wt of the dialkyl ester of 2,5-furandicarboxylic acid as starting dialkyl ester concentration and up to 5% wt of the monoalkyl ester of 2,5-furandicarboxylic acid as starting monoalkyl ester concentration, the percentages being based on the weight of the ester composition, which process comprises subjecting the ester composition to melt crystallization to yield a purified dialkyl ester composition containing a dialkyl concentration higher than the starting dialkyl ester concentration and a melt residue containing a monoalkyl ester concentration higher than the starting monoalkyl ester concentration,

wherein the melt residue is subject to conversion into a monoalkyl ester-rich intermediate product containing a momoalkyl ester concentration of at least 10% wt, based on the weight of the intermediate product, and

wherein the monoalkyl ester-rich intermediate product that is subjected to melt crystallization has a temperature in the range of 180 to 250° C.

2. The process according to claim 1 , wherein the ester composition comprises up to 10% wt of contaminants.

3. The process according to claim 2 , wherein the contaminants include one or more compounds selected from the group consisting of 5-formyl-furan-2-carboxylic acid, an alkyl ester of 5-formyl-furan-2-carboxylic acid, 2,5-furandicarboxylic acid and coloration-causing compounds.

4. The process according to any one of claim 1 , wherein the melt crystallization of the ester composition is conducted in a static, falling film and/or suspension melt crystallization equipment.

5. The process according to claim 4 , wherein the melt crystallization of the ester composition is conducted in a static or falling film melt crystallization equipment, wherein crystallization takes place on cooled surfaces.

6. The process according to claim 5 , wherein melt crystallization is conducted in a falling film equipment, wherein the molten ester composition is passed along the walls of cooling tubes as cooled surfaces to effect crystallization of the dialkyl ester of 2,5-furandicarboxylic acid on the walls of the cooling tubes.

7. The process according to claim 5 , wherein, after the crystallization of the dialkyl ester of 2,5-furandicarboxylic acid on the cooled surfaces, the cooled surfaces are re-heated to a temperature below the melting point of the dialkyl ester of 2,5-furandicarboxylic acid, to effect the melting of low-melting components that are obtained as sweat, in addition to the purified dialkyl ester composition and the melt residue.

8. The process according to claim 1 , wherein the ester composition being subjected to melt crystallization has a temperature in the range of 110 to 200° C.

9. The process according to claim 1 , wherein the conversion of the melt residue is a transesterification of the dialkyl ester with 2,5-furandicarboxylic acid.

10. The process according to claim 1 , wherein the conversion of the melt residue is partial hydrolysis.

11. The process according to claim 1 , wherein the melt residue is subjected to evaporation to vaporize the dialkyl ester of 2,5-furandicarboxylic acid and recover the remaining liquid as monoalkyl ester-rich intermediate product.

12. The process according to claim 1 , wherein the monoalkyl ester-rich intermediate product is subjected to melt crystallization to yield a purified monoalkyl ester composition and a contaminants-rich residue.

13. The process according to claim 1 , wherein the melt crystallization of the monoalkyl ester-rich intermediate product is conducted in a static, falling film and/or suspension melt crystallization equipment.

14. The process according to claim 1 , wherein the ester composition has been obtained in a process, comprising:

contacting a vaporous stream of an alkanol counter-currently with a liquid starting material comprising 2,5-furandicarboxylic acid in a reaction zone to conduct an esterification reaction to a dialkyl ester of 2,5-furandicarboxylic acid and water;

withdrawing a reaction vapor comprising the alkanol and water from the reaction zone; and

discharging a liquid phase comprising at least the dialkyl ester of 2,5-furandicarboxylic acid, from the bottom part of the reaction zone, to obtain the ester composition comprising the dialkyl ester of 2,5-furandicarboxylic acid.

15. The process according to claim 14 , wherein the liquid phase obtained in the esterification reaction is separated into a first fraction rich in the monoalkyl ester of 2,5-furandicarboxylic acid and a second fraction rich in the dialkyl ester of 2,5-furandicarboxylic acid, said second fraction being subjected to melt crystallization as the ester composition.

16. The process for the preparation of 2,5-furandicarboxylic acid from an ester composition comprising at least 90% wt of the dialkyl ester of 2,5-furandicarboxylic acid as starting dialkyl ester concentration and up to 5% wt of the monoalkyl ester of 2,5-furandicarboxylic acid as starting monoalkyl ester concentration, the percentages being based on the weight of the ester composition, comprising

purifying the ester composition in a process according to claim 1 , to yield a purified dialkyl ester composition containing a dialkyl ester concentration higher than the starting dialkyl ester concentration and a melt residue containing a monoalkyl ester concentration higher than the starting monoalkyl ester concentration; and

hydrolysing the purified dialkyl ester composition to yield purified 2,5-furandicarboxylic acid.

17. The process according to claim 16 , which further comprises:

subjecting the melt residue to conversion into a monoalkyl ester-rich intermediate product containing a monoalkyl ester concentration of at least 10% wt, based on the weight of the intermediate product;

subjecting the monoalkyl ester-rich intermediate product to melt crystallization to yield a purified monoalkyl ester composition; and

hydrolysing the purified monoalkyl ester composition to yield purified 2,5-furandicarboxylic acid.

18. The process according to claim 16 , wherein purified dialkyl ester composition and purified monoalkyl ester composition are combined and hydrolysed simultaneously.