Transcript No Slide Title

Pulping and Bleaching
PSE 476/Chem E 471
Lecture #11
Anthraquinone/Polysulfides
1
Kraft Pulping Additives
Agenda
• 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
O
-
SO3
O
Ant hraquinone 2 Sulfonat e
• 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 Additives:
Reactions 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.
Carbohydrate Reactions
O
O
O (-)
(-)
HO
O
OH
Anthrahydroquinone (AHQ)
Anthraquinone (AQ)
Lignin Reactions
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Pulping Additives:
Anthraquinone and Lignin
CH2OH
CH
CH
OH
O
(-)
H2COH
HC OR
(-)
O
CH
O
H2COH
HC OR
CH
OCH3
O
OCH3
-
O
+
OCH3
+OR
O
-
O
O
• 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 Additives:
Anthraquinone and Carbohydrates
(-)
CHO
(-)
-
C O
C OH
HO
CHOH
CHO
C O
-
HO
CO2
CHOH
(-)
O
O
O
OH
AQ
AHQ
• 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 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.
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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.
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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
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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.
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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.
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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.
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