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Pulping and BleachingPSE 476/Chem E 471

Lecture #11Anthraquinone/Polysulfides

Lecture #11Anthraquinone/Polysulfides

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Kraft Pulping AdditivesAgenda

• Anthraquinone» Reactions with lignin and carbohydrates» Use information

• Polysulfides» Formation chemistry» Reaction chemistry» Use information

• Anthraquinone» Reactions with lignin and carbohydrates» Use information

• Polysulfides» Formation chemistry» Reaction chemistry» Use information

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Pulping Additives:Anthraquinone

• 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.

• 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.

O

O

Anthraquinone 2 Sulfonate

SO3-

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Pulping Additives:Reactions of Anthraquinone

O

O

Anthraquinone (AQ)

O

HO(-)

O

OH

(-)

Anthrahydroquinone (AHQ)

Lignin Reactions

Carbohydrate Reactions

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

• 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 Additives:Anthraquinone and Lignin

• 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.

• 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.

O

OCH3

CH

HC OR

H2COH

OOH

(-)

O-OCH3

HC OR

H2COH

CHOO

(-)O-

OCH3

CH

CH

CH2OH

+

O

O

+OR

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Pulping Additives:Anthraquinone and Carbohydrates

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

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

CHO

CHOH

HO- C O(-)

C OH

O

O

CHO

C O

O

OH

(-)

HO- CO2

CHOH

(-)

AQ AHQ

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Pulping Additives:Anthraquinone

• 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.

• 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 Additives:Polysulfides

• 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.

• 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 Additives:Polysulfides

• 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

• 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 Additives:Polysulfides

• Polysulfides are unstable at temperatures > 130°C

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.

• Polysulfides are unstable at temperatures > 130°C

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 Additives:Polysulfides

• 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.

• 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.

• 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.

• 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.