Process for purifying alkanesulfonic anhydride and process for producing alkanesulfonic acid using the purified alkanesulfonic anhydride

1. A process for purifying alkanesulfonic anhydride, comprising:

(a) feeding a stream comprising alkanesulfonic anhydride, sulfuric acid, high boilers and residual low boilers into a melt crystallization to form crystals of the alkanesulfonic anhydride suspended in mother liquor;

(b) performing a solid-liquid separation to remove the crystals from the mother liquor and

(c) optionally washing the crystals to remove mother liquor adhering to the crystals.

2. The process according to claim 1 , wherein before feeding the stream comprising alkanesulfonic anhydride, sulfuric acid, high boilers and residual low boilers into the melt crystallization, a distillation for removing low boilers is carried out.

3. The process according to claim 2 , wherein a crude product stream is fed into a distillation apparatus, the crude product stream comprising alkanesulfonic anhydride, sulfuric acid, high boilers and low boilers.

4. The process according to claim 1 , wherein the mother liquor after removal of the crystals in (b) and/or the mother liquor generated in (a) is at least partly recycled into the melt crystallization.

5. The process according to claim 1 , wherein the low boilers comprise sulfur trioxide.

6. The process according to claim 1 , wherein the melt crystallization is carded out at a temperature in the range from 0 to 70° C.

7. The process according to claim 1 , wherein step (c) is carried out and the crystals are washed with molten crystals of the pure alkanesulfonic anhydride.

8. The process according to claim 7 , wherein the molten crystallizate for washing the crystals has a temperature 0.1° C. to 15° C. above the solidification temperature of the alkanesulfonic-anhydride-containing crystallizate.

9. The process according to claim 1 , wherein the alkanesulfonic anhydride is methanesulfonic anhydride.

10. A process for producing and purifying alkanesulfonic anhydride comprising:

(i) optionally distilling a melt comprising crude alkanesulfonic acid to completely or partly remove low boilers obtaining a material stream comprising alkanesulfonic acid, high boilers and residual low boilers;

(ii) feeding the material stream comprising alkanesulfonic acid, high boilers and residual low boilers as a first stream and sulfur trioxide as a second stream into a reaction stage where the alkanesulfonic acid reacts with the sulfur trioxide forming alkanesulfonic anhydride; and

(iii) purifying the alkanesulfonic anhydride in a process comprising

(a) feeding a stream comprising alkanesulfonic anhydride, sulfuric acid, high boilers and residual low boilers into a melt crystallization to form crystals of the alkanesulfonic anhydride suspended in mother liquor;

(b) performing a solid-liquid separation to remove the crystals from the mother liquor.

11. The process according to claim 10 , wherein the low boilers comprise sulfur trioxide.

12. A process for producing anhydrous alkanesulfonic acid, comprising:

purifying alkanesulfonic anhydride by a process comprising feeding a stream comprising alkanesulfonic anhydride, sulfuric acid, high boilers and residual low boilers into a melt crystallization to form crystals of the alkanesulfonic anhydride suspended in mother liquor; and performing a solid-liquid separation to remove the crystals from the mother liquor; and

solving the crystals in a water-containing alkanesulfonic acid, wherein the water of the water-containing alkanesulfonic acid reacts with the alkanesulfonic anhydride forming alkanesulfonic acid or mixing equimolar amounts of the alkanesulfonic anhydride and water wherein the water reacts with the alkanesulfonic anhydride forming alkanesulfonic acid.

13. The process according to claim 12 , wherein the water-containing alkanesulfonic acid is alkanesulfonic acid hydrate.

14. A process for producing peroxoalkanesulfonic acid, comprising:

purifying alkanesulfonic anhydride by a process comprising feeding a stream comprising alkanesulfonic anhydride, sulfuric acid, high boilers and residual low boilers into a melt crystallization to form crystals of the alkanesulfonic anhydride, suspended in mother liquor; and performing a solid-liquid separation to remove the crystals from the mother liquor; and

adding the purified alkanesulfonic anhydride to a mixture comprising water and hydrogen peroxide, wherein the hydrogen peroxide reacts with the alkanesulfonic anhydride forming peroxoalkanesulfonic acid and the water reacts with the alkanesulfonic anhydride forming alkanesulfonic acid.

15. A process for producing alkanesulfonic acid, comprising:

purifying alkanesulfonic anhydride by a process comprising feeding a stream comprising alkanesulfonic anhydride, sulfuric acid, high boilers and residual low boilers into a melt crystallization to form crystals of the alkanesulfonic anhydride suspended in mother liquor; and performing a solid-liquid separation to remove the crystals from the mother liquor; and

adding the purified alkanesulfonic acid to a mixture comprising sulfuric acid, wherein the sulfuric acid reacts with the alkanesulfonic anhydride forming alkanesulfonic acid and sulfur trioxide.

16. The process according to claim 15 , wherein the sulfur trioxide is recycled into a process for producing crude alkanesulfonic acid or into a process for producing alkanesulfonic anhydride.

17. The process according to claim 15 , wherein the alkanesulfonic acid is methanesulfonic acid.

18. The process according to claim 14 , wherein the peroxoalkanesulfonic acid is added as an initiator to sulphur trioxide and alkane to start a radical chain reaction for producing alkanesulfonic acid or alkanesulfonic anhydride.

19. The process according to claim 12 , wherein the alkanesulfonic acid is methanesulfonic acid.

20. The process according to claim 14 , wherein the alkanesulfonic acid is methanesulfonic acid.