Pulping and Bleaching PSE 476/Chem E 471

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Pulping and BleachingPSE 476/Chem E 471

• Lecture 11
• Kraft Pulping Kinetics

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Agenda

• Effect of Variables on Kraft Pulping
• Wood Species
• Effective Alkali
• Sulfidity
• Kraft Cooking Models

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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

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Kraft PulpingEffect of Wood Species
Notes
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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
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Lignin RemovalEffect of Effective Alkali (2)
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Lignin RemovalEffect of Effective Alkali (3)
Higher levels of EA results in more lignin
removal but also lower screened pulp yield.
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Lignin RemovalEffect of Effective Alkali (4)
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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.

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Removal of GlucomannansEffect of Effective
Alkali
Why doesnt EA affect glucomannan removal?
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Removal of XylansEffect of Effective Alkali
Why does EA affect xylan removal?
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Lignin RemovalEffect 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

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Kraft Pulping KineticsLignin Removal Effect of
Sulfidity
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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