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Prehydrolysis-kraft pulping

Prehydrolysis-kraft pulping. Ian Suckling APPI May 2012. Introduction. Prehydrolysis-kraft used for dissolving pulp production Recent interest in re-purposing kraft mills for this Prehydrolysis speeds up delignification in subsequent kraft cooking processes

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Prehydrolysis-kraft pulping

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  1. Prehydrolysis-kraft pulping Ian Suckling APPI May 2012

  2. Introduction • Prehydrolysis-kraft used for dissolving pulp production • Recent interest in re-purposing kraft mills for this • Prehydrolysis speeds up delignification in subsequent kraft cooking processes • Can the hemicelluloses be recovered from the pre-hydrolysis liquor?

  3. Overview • Process overview • Prehydrolysis chemistry • Carbohydrates • Lignin • Prehydrolysis-kraft pulping of softwoods • Prehydrolysis stage, incl severity factor • Effect of subsequent kraft pulping • Prehydrolysis-kraft pulping of hardwoods • Dissolving pulps - briefly

  4. Prehydrolysis-kraft pulping Steam Prehydrolysis Stage Kraft Pulping Bleaching Dissolving Pulp Chips Black liquor Alkaline Lignin & carbohydrate degradation compounds Prehydrolysate Acidic Mostly hemicelluloses Recovery Courtesy of John Lloyd

  5. Carbohydrate reactions during prehydrolysis • Formation of acetic acid – deacetylation of hemicelluloses • catalyses subsequent reactions (pH drops) • Hydrolysis of hemicelluloses and dissolution of oligomers • Degradation of oligomers to monomeric sugars • Decomposition of monomeric sugars to furfural and hydroxymethylfurfural • Further reactions of furfural and HMF

  6. Galactoglucomannan Hydrolysis – an example Ac Ac Ac Gal Ac k1 + M M M G M M M M G M M M Gal Ac Gal M M M G M M M k2 M M M M M M Severity G G G Ac Gal Ac Ac Gal + + M M M M M M G Gal M Ac + k3 Gal Ac Gal M M H+ M M G G M M M M M G M Ac Ac M Gal M Ac Ac k4 Ac Gal Courtesy of John Lloyd

  7. Lignin reactions during prehydrolysis • Reactions of lignin during prehydrolysis: • Cleavage of aryl ether bonds • Lower β-O-4 bonds • Generation of new phenolic units • Partially by a homolytic cleavage mechanism • Condensation reactions • limit the severity of prehydrolysis • Degradation products dissolved in prehydrolysate to limited extent due to limited solubility in water • Suggested higher MWt dissolved products precipitate on cooling of prehydrolysate but low MWt products remain soluble

  8. Reactions of β-ether units Li and Lundquist. Nordic P&P Resarch J. 15(4): 292 (2000)

  9. Prehydrolysis-kraft pulping of radiata pine • Pinus radiata from toplogs (402 kg/m3) • Prehydrolysis • Hot water • 5g/L Na2CO3 • Kraft pulping • 14.5% EA, 4:1L:W, 30% S, 60 min to 170ºC, 3 H-factors • pulping rate, yield & alkali consumption • Board properties • pulped to 50 kappa target J Lloyd et al. Proc. Appita Conf. Rotorua (2011)

  10. Prehydrolysis Severity • Several functions used but P-Factor (analagous to H-Factor in kraft pulping) is the most common P-Factor = ∫ exp (T – 100) . dt t 0 14.75 Severity Index (S.I.) = Log (P-Factor) P-factor: Brasch, D. J. and Free K. W. (1965). TAPPI, 48(4), 245-248. Eact = 126 kJ/mol

  11. Prehydrolysis Conditions - Radiata Pine

  12. Mass Extracted v’s Prehydrolysis Severity

  13. Mass Extracted v’s Final pH

  14. Analysis of Solids in Prehydrolysates • Hemicelluloses are extracted from the chips as: • soluble polymers • short chain oligomers (4-10 sugar units) • sugar monomers • Prehydrolysates also contains: • lignin • glucose from cellulose degradation • extractives • carbohydrate degradation products

  15. Fractionation of 175˚C/60 min Hydrolysate

  16. Monomers and Oligo-/polymers in Prehydrolysates(% of total)

  17. Monomers and Oligo-/polymers in Prehydrolysates

  18. Arabinose and Galactose Extraction

  19. Glucose and Mannose Extraction

  20. Xylose Extraction

  21. Fractionation of Prehydrolysis Solids Prehydrolysate Activated carbon “Clean” Prehydrolysate Acetone:methanol (9:1) Solubles Sugar Monomers Insolubles Polysaccharides

  22. Size Exclusion Chromatography – MW Profile Polysaccharide Fraction from 175˚C/0m (SI = 2.75) hydrolysate 60K Da 200 Da 12K Da 1K Da Acids Neutrals

  23. Composition of 175˚C/0 min Hemicellulose Fragments Neutral partially-acetylated hexose polymers Between 5 and 79 hexose units with differing degrees of acetylation • Neutral non-acetylated hexose polymers • Between 5 and 30 hexose units. • Acidic pentose polymers • Non-acetylated.Between 5 and 30 pentose units with between 1 and 4 attached 4-O-methylglucuronic acid groups (DSMeGlcA ≈ 0.125 to 0.077). • Neutral non-acetylated pentose polymers • Small amounts. Fewer than 10 pentose units.

  24. EFFECT OF PREHYDROLYSIS ON KRAFT PULPING

  25. Effect of Prehydrolysis on Kraft Pulping Rate All Cooks used 14.5% EA

  26. Alkali Consumption

  27. Pulp Yields

  28. Pulp Compositions

  29. Sheet Strengths at 23oC /50% RH

  30. Conclusions • Up to 26% of chip mass removed by hot-water prehydrolysis. • Above 2.7 Severity Index get • excessive depolymerisation of hemicelluloses • formation of degradation products • problems with kraft pulping rate & pulp yield and strength • A 2.7 Severity Index extracts reasonable amounts of hemicelluloses with potential for further processing, but complex mixture • Faster kraft pulping and lower alkali consumption • Prehydrolysis-kraft pulps retain more strength at high RH • Buffering with 5 g/L Na2CO3 delivers higher MW hemicelluloses but reduced pulp strength at high RH. But still an option for hemicellulose extraction

  31. Yield and properties of spruce prehydrolysis pulps Prehydrolysis 170ºC, 0.5 h, water Kraft cooking 170ºC Numbers= active alkali in kg ptw NaOH

  32. PREHYDROLYSIS - KRAFT PULPING OF HARDWOODS

  33. Prehydrolysis of hardwoods Figure 2. Wood components remaining in Southern hardwood mixture (SHM) after autohydrolysis for 100 min. Tunc et al. Holzforschung 62:539 (2008)

  34. Prehydrolysis of hardwoods ctd Tunc et al. Holzforschung 62:539 (2008)

  35. Prehydrolysis-kraft cooking of E. globulus Duarte et al. Tappi J. 11(4): 47 (2012)

  36. Prehydrolysis-kraft cooking of E. globulus - ctd Duarte et al. Tappi J. 11(4): 47 (2012)

  37. Dissolving pulp • Bleached pulps containing low levels of hemicelluloses used for textiles and other cellulose derivatives • Total market 5-8 M t/yr • Properties: • Low yield (~35-38%) • High (> 95%) cellulose, low hemicellulose content • -cellulose typically > 94% • High brightness • Controlled viscosity (i.e. cellulose MWt) (10-15 MPa.s) • Manufactured by: • Acid sulfite process • Prehydrolysis-kraft – softwoods and hardwoods

  38. Dissolving pulp specification (Lenzing) http://www.lenzing.com/en/co-products/products/pulp/specifications.html

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