Pulping and Bleaching PSE 476Chem E 471

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Pulping and BleachingPSE 476/Chem E 471

• Lecture 11
• Anthraquinone/Polysulfides

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Kraft Pulping AdditivesAgenda

• Anthraquinone
• Reactions with lignin and carbohydrates
• Use information
• Polysulfides
• Formation chemistry
• Reaction chemistry
• Use information

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Pulping AdditivesAnthraquinone

• Bach and Fiehn first reacted anthraquinone with
  hydrocellulose under alkaline conditions in 1972
• Degradation of cellulose was slowed
• When this additive was added to an alkaline cook,
  not only was the degradation of carbohydrates
  reduced, the rate of lignin removal was increased.

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Pulping AdditivesReactions of Anthraquinone

• It was discovered that anthraquinone (AQ) acts as
  a catalyst under pulping conditions. AQ oxidizes
  carbohydrates while the reduced form AHQ reacts
  with lignin.

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Pulping AdditivesAnthraquinone and Lignin
OR

• AHQ reacts with quinone methides this requires a
  free phenolic hydroxyl group. This is a rapid
  reaction.
• Slow alkali cleavage of non phenolic groups
  follows.

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Pulping AdditivesAnthraquinone and Carbohydrates

• AQ oxidizes the reducing end of carbohydrate to
  an aldonic acid.
• AQ is reduced to AHQ.
• Carbohydrate are stabilized against peeling.

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Pulping AdditivesAnthraquinone

• There are a large number of quinone chemicals
  which will function as catalysts.
• In United States, AQ is the most cost effective.
• AQ added at catalytic levels.
• 1987 FDA allows max addition of 0.11.
• AQ works better with hardwoods than softwoods.
• AQ compensates for reduction in
• EA, Sulfidity, H factor.

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Pulping AdditivesPolysulfides

• Sulfur polymers Na2Sx
• Polysulfides oxidize carbohydrate reducing end
  groups to carboxylic acid thus slowing peeling.
• Can be formed through the reaction
• Na2S So Na2S2
• Na2S2 So Na2S3
• The addition of elemental sulfur to white liquor
  forms polysulfides.
• Resulting increase in total sulfur causes
  sulfidity of liquor to rise.

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Pulping AdditivesPolysulfides

• It is best to generate polysulfides through the
  oxidation of white liquor forming elemental
  sulfur which will further react with Na2S.
• 2Na2S 2O2 H2O 2So 4NaOH
• Na2S (x-1)So Na2Sx
• This reaction can be catalyzed using activated
  carbon
• The oxidation of Na2S to thiosulfate is a
  competing reaction under these conditions
• 2Na2S 2O2 H2O Na2S2O3 2NaOH

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Pulping AdditivesPolysulfides

• Polysulfides are unstable at temperatures gt 130C
• 2Na2S3 6NaOH 4Na2S Na2S2O3 H2O
• The conditions used in a kraft cook must
  therefore be modified in order to gain a yield
  benefit from the reaction of polysulfides.
• Use thin or short chips.
• Allow for ample impregnation time.
• Use a slow rate of heat up.

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Pulping AdditivesPolysulfides

• The use of polysulfides gives a yield gain by
  reduction of peeling reactions.
• Most of the gain through reduced degradation of
  glucomannans.
• In 1987, a 3.2 yield increase in a kraft mill
  corresponded to a 1.7 million dollar savings.

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White Liquor Oxidation System
Chiyoda polysulfide process was developed by
Chiyoda and Mitsubishi Paper
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Chiyoda polysulfide process (2)

• This process is a very simple. Major equipment
  comprises the filter and the reactor.
• The unique up flow type filter can remove
  substantially all the suspended solid in feed
  white liquor.
• This process does not affect the operation of
  existing equipment.
• The catalyst has three to four-year life, and it
  maintains high polysulfide concentration during
  the life.
• An activated carbon with specific pore structure
  is used as the catalyst. The catalyst maintains
  optimum conversion to maximize polysulfide
  production and long catalyst life.

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Chiyoda polysulfide process (3)

• Oxidized carbohydrate becomes stable compound for
  alkaline liquor and the amount of dissolved
  carbohydrate in the black liquor is reduced. As a
  result
• Pulp yield is increased with insignificant
  changes to pulp properties.
• Black liquor viscosity is decreased.
• Recovery boiler load is reduced.