Pulping and Bleaching PSE 476/Chem E 471

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Pulping and BleachingPSE 476/Chem E 471

• Lecture 15
• Kraft Pulping
• Review of Reactions/Kinetics

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

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Alkaline HydrolysisExample 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.

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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.
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Removal of GlucomannansEffect of Effective
Alkali

• Why doesnt 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

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Kraft Pulping KineticsInitial Phase
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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

• It depends on
• Wood species
• EA
• Cooking temp.
• Hardwoods 20
• Softwoods 25

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Removal of XylansEffect of Effective Alkali

• Why does EA affect xylan removal?
• Stable at the beginning
• Hydrolysis and 2nd peeling

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Kraft Pulping KineticsBulk Phase
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Kraft Pulping KineticsBulk Phase
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Kraft Pulping KineticsResidual Phase
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Kraft Pulping KineticsResidual Phase