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Pulping and Bleaching PSE 476/Chem E 471

Pulping and Bleaching PSE 476/Chem E 471. Lecture #15 Kraft Pulping Review of Reactions/Kinetics. Kraft Pulping. 70°C. 70°C. Initial Phase Impregnation zone. 137°C. 170° C. Bulk Phase. Residual Phase. Kraft Pulping Kinetics: Initial Phase. Initial Reactions: Low Temperature.

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Pulping and Bleaching PSE 476/Chem E 471

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  1. Pulping and BleachingPSE 476/Chem E 471 Lecture #15 Kraft Pulping Review of Reactions/Kinetics

  2. Kraft Pulping 70°C 70°C Initial Phase Impregnation zone 137°C 170° C Bulk Phase Residual Phase

  3. Kraft Pulping Kinetics:Initial Phase

  4. Initial Reactions: Low Temperature • Carbohydrates • Alkaline hydrolysis of acetyl groups on xylan (see next slide). • Removal of certain soluble carbohydrates. • Certain galactoglucomannans. • Arabinogalactans. • Extractives • Alkaline hydrolysis of fats (saponification), waxes, and other esters. • Neutralization of extractives. • There are a number of acidic extractives which consume NaOH.

  5. Alkaline Hydrolysis:Example Using Acetyl Groups • Esters are cleaved in alkaline solutions through hydrolysis reactions forming carboxylic acids and alcohols. • Hydrolysis of acetyl groups occurs readily in alkaline solutions. • Reaction occurs rapidly even at room temperature. • Reaction consumes alkali.

  6. Saponification of Fats(Review slide from PSE 406) • Treatment of fats with alkali converts them to fatty acids and glycerol through saponification. Once again this reaction consumes part of the alkali charge.

  7. Removal of Glucomannans:Effect of Effective Alkali • Why doesn’t EA affect glucomannan removal? • Peeling reaction not sensitive to OH concentration • Rapid removal at the beginning of the cook (dissolving) • Over 100C greater rate of peeling • 70% removed rate drops again

  8. Kraft Pulping Kinetics:Initial Phase

  9. Lignin Removal:Effect of Sulfidity • Sulfidity effect zero order in initial delignification (this means that changing sulfidity has no effect on initial rate) • Sulfidity is important during bulk delignification • Effects final lignin/carbohydrate ratio • HS- does not react with carbohydrates • It depends on: • Wood species • EA • Cooking temp. • Hardwoods 20% • Softwoods 25%

  10. Removal of Xylans:Effect of Effective Alkali • Why does EA affect xylan removal? • Stable at the beginning • Hydrolysis and 2nd peeling

  11. Kraft Pulping Kinetics:Bulk Phase

  12. Kraft Pulping Kinetics:Bulk Phase

  13. Kraft Pulping Kinetics:Residual Phase

  14. Kraft Pulping Kinetics:Residual Phase

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