Separation process

1. A process of recovering xylose by nanofiltration from a solution of plant-based biomass hydrolysate containing xylose, comprising

subjecting the solution to nanofiltration to obtain a nanofiltration retentate and a nanofiltration permeate, and

recovering the nanofiltration permeate comprising a solution enriched in xylose,

further comprising regulating xylose flux in the nanofiltration permeate to a range of 0.2 to 3 kg xylose/(m 2 /h), whereby the xylose flux refers to the amount of xylose (kg) that permeates through the nanofiltration membrane during one hour calculated per one square meter of the membrane surface and is determined by measuring the flow (solution flux) and the content of the dry substance and xylose in the permeate,

by adjusting dry substance content (DS) of the solution of the plant-based biomass hydrolysate used as a nanofiltration feed to a range of 25-35% by weight and the xylose content of the nanofiltration feed to a range of 35-50% on DS, and

by further adjusting the nanofiltration temperature to the range of 55-70° C. and the nanofiltration pressure to the range of 30-40 bar, and

recovering a nanofiltration permeate having a dry substance content of 5 to 40% by weight and a xylose content of 35 to 85% on DS,

to provide a xylose enrichment factor of at least 1.2 in the nanofiltration permeate, a permeate dilution factor higher than 0.2 and a xylose yield of more than 60% on xylose in the nanofiltration feed.

2. A process as claimed in claim 1 , wherein the process provides a dry substance content in the range of 10 to 30% by weight in the nanofiltration permeate.

3. A process as claimed in claim 1 , wherein the process provides a xylose content in the range of 45-70% on DS in the nanofiltration permeate.

4. A process as claimed in claim 1 , wherein the process provides a xylose enrichment factor in the range of 1.2 to 6 in the nanofiltration permeate.

5. A process as claimed in claim 1 , wherein the process provides a permeate dilution factor higher than 0.5.

6. A process as claimed in claim 1 , wherein the nanofiltration is operated at a temperature of 55 to 70° C. at the start of the nanofiltration, followed by readjusting the temperature at a later stage of the nanofiltration.

7. A process as claimed in claim 6 , wherein the nanofiltration is carried out as a multi-phase nanofiltration and the temperature is readjusted in later phases to be different from that of the first phase.

8. A process as claimed in claim 6 , wherein the temperature at the later stage of the nanofiltration is readjusted to be lower than 90° C.

9. A process as claimed in claim 6 , wherein the temperature at the later stage of the nanofiltration is readjusted to be lower than 30° C.

10. A process as claimed in claim 1 , wherein the nanofiltration is carried out at a pH in range of 1-10.

11. A process as claimed in claim 10 , wherein the pH is adjusted with compounds containing divalent cations.

12. A process as claimed in claim 11 , wherein said divalent cations is Mg 2+ or Ca 2+ .

13. A process as claimed in claim 1 , wherein the xylose flux is regulated by adjusting the pH to the range of pH 3 to pH 6.

14. A process as claimed in claim 1 , wherein the nanofiltration is carried out with a nanofiltration membrane having a cut-off size in the range of 150-1000 g/mol.

15. A process as claimed in claim 1 , wherein the nanofiltration is carried out with a nanofiltration membrane selected from polymeric and inorganic membranes.

16. A process as claimed in claim 1 , wherein the hydrolysate of plant-based biomass comprises a hydrolysate of xylan-containing vegetable material.

17. A process as claimed in claim 16 , wherein the hydrolysate of xylan-containing vegetable material comprises a hydrolysate of lignocellulosic material.

18. A process as claimed in claim 17 , wherein the hydrolysate of lignocellulosic material comprises a hydrolysate of wood material.

19. A process as claimed in claim 18 , wherein the hydrolysate of wood material comprises a spent liquor obtained from a pulping process.

20. A process as claimed in claim 19 , wherein the spent liquor obtained from a pulping process is a spent sulphite pulping liquor.

21. A process as claimed in claim 20 , wherein the spent sulphite pulping liquor is an acid spent sulphite pulping liquor.

22. A process as claimed in claim 21 , wherein the spent sulphite pulping liquor is obtained from hardwood sulphite pulping.

23. A process as claimed in claim 1 , wherein the solution of plant-based biomass hydrolysate is a biomass-based distillation residue.

24. A process as claimed in claim 23 , wherein the distillation residue is obtained from the distillation of ethanol.

25. A process as claimed in claim 1 , wherein the solution of plant-based biomass hydrolysate contains further monosaccharides in addition to xylose.

26. A process as claimed in claim 25 , wherein the monosaccharides comprise one or more of glucose, arabinose, mannose, galactose and rhamnose.

27. A process as claimed in claim 1 , comprising recovering as the nanofiltration permeate a xylose solution having a content of monosaccharide impurities of less than 50 on DS.

28. A process as claimed in claim 27 , wherein the monosaccharides comprise one or more of glucose, arabinose, mannose, galactose and rhamnose.

29. A process as claimed in claim 28 , wherein the content of glucose impurity is less than 30% on DS.

30. A process as claimed in claim 28 , wherein the content of glucose impurity is less than 10% on DS.

31. A process as claimed in claim 1 , wherein biomass solution has been subjected to one or more pre-treatment steps.

32. A process as claimed in claim 31 , wherein the pre-treatment steps are selected from ion exchange, membrane filtration, chromatography, concentration, pH adjustment, filtration, dilution, crystallization and combinations thereof.

33. A process as claimed in claim 1 , wherein the process also comprises one or more post-treatment steps.

34. A process as claimed in claim 1 , wherein the post-treatment steps are selected from ion exchange, cystalization, chromatography, reverse osmosis, concentration by evaporation, colour removal, extraction and precipitation.

35. A process as claimed in claim 1 , wherein the nanofiltration is carried out as a batch process or a continuous process.

36. A process as claimed in claim 1 , wherein the process further comprises recovering a solution enriched in lignosulphonates as the nanofiltration retentate.

37. A process as claimed in claim 1 , wherein the process provides a xylose yield of more than 80% on xylose in the nanofiltration feed.

38. A process as claimed in claim 1 , wherein the process provides a xylose yield of more than 90% on xylose in the nanofiltration feed.

39. A process as claimed in claim 1 , wherein the process is carried out with a total permeate flux of 3-30 kg/m 2 /h.

40. A process as claimed in claim 1 , wherein the xylose content of the nanofiltration feed is 5-20 g xylose in 100 g of feed solution.