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Pulping and Bleaching PSE 476/Chem E 471

Pulping and Bleaching PSE 476/Chem E 471. Lecture #11 Kraft Pulping Kinetics. Agenda. Effect of Variables on Kraft Pulping Wood Species Effective Alkali Sulfidity Kraft Cooking Models. Kinetics?.

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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 #11 Kraft Pulping Kinetics PSE 476: Lecture 11

  2. Agenda Effect of Variables on Kraft Pulping • Wood Species • Effective Alkali • Sulfidity • Kraft Cooking Models PSE 476: Lecture 11

  3. Kinetics? • It is important as a pulping engineer to understand how to produce pulp with the desired properties in the most economical fashion. • In order to do this, the kinetics of the pulping reactions need to be understood. • Pulping variables: • Wood: species, source, etc (discussed previously) • Chemical charge (EA, sulfidity, liquor/wood, etc) • Temperature PSE 476: Lecture 11

  4. Kraft Pulping:Effect of Wood Species Notes PSE 476: Lecture 11

  5. Effect of Effective Alkali on Kraft Pulping (1) • Simple story: if there is enough alkali present (with a certain level of sulfidity), pulp can be made. • Increasing EA will increase rate and degree of delignification. • Two ways of changing EA charge: • Increasing concentration of EA in white liquor • Increasing volume of White Liquor charged to digester • Increasing EA using either method will increase the rate of delignification and change pulp properties. • Concentration change may have a larger effect than White Liquor volume. Notes PSE 476: Lecture 11

  6. Lignin Removal:Effect of Effective Alkali (2) PSE 476: Lecture 11

  7. Lignin Removal:Effect of Effective Alkali (3) Higher levels of EA results in more lignin removal but also lower screened pulp yield. PSE 476: Lecture 11

  8. Lignin Removal:Effect of Effective Alkali (4) PSE 476: Lecture 11

  9. Effect of Increasing Effective Alkali on Pulp Properties. • In the kraft pulping of Scandinavian Pine, and increase in EA from 13.6% to 15.5% had the following effects: • Cooking time to yield cut in half. • Pulp yield at given kappa reduced. • Xylan content of pulp reduced while glucomannan content was increased. • Brightness increased. • Longer beating time to reached CSF. • Pulp strength properties (tear, breaking length,etc). changed. PSE 476: Lecture 11

  10. Removal of Glucomannans:Effect of Effective Alkali Why doesn’t EA affect glucomannan removal? PSE 476: Lecture 11

  11. Removal of Xylans:Effect of Effective Alkali Why does EA affect xylan removal? PSE 476: Lecture 11

  12. 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 PSE 476: Lecture 11

  13. Kraft Pulping KineticsLignin Removal: Effect of Sulfidity PSE 476: Lecture 11

  14. dL = k[Lignin]a[OH-]b[HS-]c[other variables]d dt Kraft Pulping KineticsKinetic Models • Reaction models have been developed which simulate the pulping process. • Models range from simple to quite complex. • Typically use the following reaction variables: • Time, Temperature, Alkali, Sulfidity, Liquor/Wood, Etc • Models roughly derived from the following expression: PSE 476: Lecture 11

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