Process of producing xylose and dissolving pulp

1. A process for the production of xylose and dissolving pulp from a hardwood biomass, which has a xylan content of 10 to 35%, calculated on the dry substance (DS) of the biomass,

comprising the steps of:

(a) prehydrolyzing the hardwood biomass in a digester with a solution containing sulphur dioxide in a concentration of 0.1 to 2.5% based on the total liquid weight of the prehydrolysis liquid, at a temperature of 120-150° C., a liquid-to-biomass ratio of less than 6, and with a reaction time of 0.3 to 1 h, to provide (1) a dissolved hemicellulose hydrolyzate, wherein the xylose content is 25 to 55% of the dissolved solids of the hydrolyzate, the ratio of xylose to xylo-oligomers is more than 2 and the xylose yield is 30 to 80% on xylan in the biomass, and (2) a non-dissolved prehydrolyzed biomass,

(b) separating the dissolved hemicellulose hydrolyzate from the non-dissolved prehydrolyzed biomass,

(c) subjecting the dissolved hemicellulose hydrolyzate to chromatographic fractionation, nanofiltration or precipitation crystallization to obtain a xylose product having a xylose content of at least 55% on DS, and

(d) subjecting the non-dissolved prehydrolyzed biomass to sulphate cooking in a digester to obtain a dissolving pulp with a viscosity of more than 600 ml/g.

2. A process as claimed in claim 1 , wherein the concentration of sulphur dioxide in the prehydrolysis is 0.1 to 2.0%.

3. A process as claimed in claim 1 , wherein sulphur dioxide is added as a water solution or in a liquid or gas form.

4. A process as claimed in claim 1 , wherein sulphur dioxide is impregnated into the hardwood biomass.

5. A process as claimed in claim 1 , wherein the spent sulphur dioxide is recovered and returned to the prehydrolysis.

6. A process as claimed in claim 1 , wherein sulphur dioxide is returned to the prehydrolysis by dissolving into water after having been liberated from the digester.

7. A process as claimed in claim 1 , wherein the liquid/biomass ratio in the prehydrolysis is in the range of 1 to 6.

8. A process as claimed in claim 1 , wherein 15 to 40% of the dry substance of the hardwood biomass is dissolved in the prehydrolysis.

9. A process as claimed in claim 1 , wherein the prehydrolysis also comprises heating with direct steam.

10. A process as claimed in claim 1 , wherein the content of xylose in the dissolved hemicellulose hydrolyzate is 35 to 55% on DS.

11. A process as claimed in claim 1 , wherein the concentration of oligosaccharides in the dissolved hemicellulose hydrolyzate is less than 10% based on DS.

12. A process as claimed in claim 1 , wherein the content of monosaccharides in the dissolved hemicellulose hydrolyzate is more than 35% based on DS.

13. A process as claimed in claim 1 , wherein the content of xylose in the dissolved hemicellulose hydrolyzate is more than 65% based on total monosaccharides.

14. A process as claimed in claim 1 , wherein the ratio of xylose to xylo-oligomers in the dissolved hemicellulose hydrolyzate is more than 10.

15. A process as claimed in claim 1 , wherein the hemicellulose hydrolyzate has a dry substance content of more than 7% by weight.

16. A process as claimed in claim 1 , wherein the process also comprises evaporation of the prehydrolyzate after step (a), wherein acetic acid is partially removed from the prehydrolyzate by evaporation.

17. A process as claimed in claim 1 , wherein the process also comprises post-hydrolysis after step (a).

18. A process as claimed in claim 17 , wherein the post-hydrolysis is selected from enzymatic post-hydrolysis and hydrolysis with a hydrolyzing agent selected from sulphuric acid and sulphur dioxide.

19. A process as claimed in claim 1 , wherein the separation step (b) is carried out by washing selected from displacement wash, pressing and filtration.

20. A process as claimed in claim 19 , wherein the displacement wash is carried out in the digester with a liquid selected from water, a condensate, white liquor, chromatographic separation residue fraction and a nanofiltration retentate.

21. A process as claimed in claim 19 , wherein the yield of monomeric xylose dissolved from the biomass in the wash is 50 to 95%.

22. A process as claimed in claim 1 , wherein the process also comprises pH adjustment before step (c).

23. A process as claimed in claim 1 , wherein step (c) is carried out by chromatographic fractionation using an ion exchange resin as the column filling material.

24. A process as claimed in claim 23 , wherein the ion exchange resin is a cation exchange resin.

25. A process as claimed in claim 24 , wherein the cation exchange resin is a strongly acid cation exchange resin.

26. A process as claimed in claim 25 , wherein the cation exchange resin is in a monovalent metal cation form.

27. A process as claimed in claim 1 , wherein step (c) is carried out by nanofiltration, whereby the xylose product is recovered as the nanofiltration permeate.

28. A process as claimed in claim 1 , wherein the xylose product recovered from step (e) is subjected to crystallization to obtain crystalline xylose.

29. A process as claimed in claim 1 , wherein in step (d) dissolving pulp with a viscosity of more than 700 ml/g is obtained.

30. A process as claimed in claim 1 , wherein in step (d) dissolving pulp is obtained with a yield of greater than 35% on dry biomass.

31. A process as claimed in claim 30 , wherein in step (d) dissolving pulp is obtained with a yield of 76% or greater on dry biomass.