Transcript No Slide Title

Pulping and Bleaching
PSE 476
Lecture #16
Sulfite Pulping
1
Sulfite Pulping
Agenda
• Conditions
• Cooking Liquor
» Preparation
• Factors Affecting Pulping
• Lignin Reactions
• Carbohydrate Reactions
2
Sulfite Pulping
• Unlike alkaline pulping, the purpose of sulfite
pulping is not heavy lignin fragmentation.
• Acid sulfite
» Under acidic conditions, the object is to solubilize lignin
through the addition of hydrophillic sulfonate groups
(sulfonation)
• Neutral sulfite
» Most often neutral sulfite pulping is used in association
with mechanical pulping (Neutral Sulfite Semi ChemicalNSSC). The purpose here is mainly to “soften” the lignin.
• Alkaline sulfite
» The goal is sulfonation and fragmentation.
3
Sulfite Pulping
Conditions
Method
Acid
Bisulfite
pH
Range
“Base”
Active
Reagent
Max
Time @ Pulp Yield
Temp (°C) Temp
(%)
(hrs)
2+
2+
+
, H
, HSOdiffusion
125-145
45-55
Long1-2
cookingCa
time,isMg
necessary
to prevent
of SO2 ahead of 3-7
the base (avoid
lignin
3
+
+
condensation,
Na , “black
NH4 cook”); weak pulp; low hemis; acid hydrolysis; tissue paper
2+
+
Medium
writing
3-5 grades
Mgpaper:
, Nanewsprint
,
H+and
, HSO
150-170
3 paper
1-3
50-65
0.25-3
75-90
NH4+
Neutral
(NSSC)
Alkaline
+
+
2Good
high
corrugating
6-9stiffness
Napulp,
, NH
HSO
160-180
4 yield,
3 , SO3medium
Very9-13
similar pulps
brightness, bleachability
Na+ to kraft pulping
SOin 2-terms
, HOof- yield,160-180
3-5 and strength
45-60
3
4
Sulfite Pulping
Sulfur Oxides Equilibriums
• Sulfur dioxide exists in different forms depending
upon solution pH. This is demonstrated by the
chemical equilibrium's shown below and in figure
in the next slide.
SO 2 + H20
H2S0 3
HSO 3
H2SO 3 (SO 2 . H20)
+
+
H
HSO 3
+
H + SO 3
-
5
Effect of pH on Sulfur Dioxide
Species
% Sulfur Species
100
80
60
SO2
40
SO32-
HSO3-
20
0
0
2
4
pH
6
8
10
Ca pH=2 to stay in the solution,
Mg pH=4,
Na and NH4+ strongly alkaline without precipitation (any pH).
6
Acid Sulfite Cooking Liquor
Production
• Molten sulfur is burned with O2 at
1100°C to form SO2
• Controlled regulation of O2 – over
oxidation production of SO3
• The SO2 is cooled to 200°C and
adsorbed into water forming SO2•
H2O often referred to as H2SO3
• A base is added to the SO2
» Ca, Na, NH3, Mg
• Liquor is fortified with additional
SO2 from digester (relief) in pressure
system
• Brow or a red liquor is the waste
liquor
Reference
7
Sulfite Pulping
Variables
• Wood species
» Many softwoods (pines,etc) and hardwoods are difficult
to pulp particularly under acid conditions.
• General chip characteristics
» Size, Moisture, contaminants, etc.-very important to have
a uniform liquor penetration, if not chips will char not
enough base to supply the buffering action
• Cooking liquor
» Liquor/wood
• Digester variables
» Packing, venting, heating, etc.
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Sulfite Pulping
Factors Affecting Cooking Rate
•
•
•
•
pH, Rate 
SO2 Concentration, Rate
Temperature (most important), Rate 
Base (from slowest to fastest)
» Ca<Na<Mg<NH4
» Other factors influence choice of base
- Costs: Mg>Na>NH4, Ca
- Recovery systems: calcium cannot typically be recovered
- NH4 pulps have lower brightness.
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Sulfite Pulping
Lignin Reactions
Three Major Reactions
• Sulfonation
» Sulfonation on a carbon
(majority) and g carbon
increases the polarity of
lignin and solubilize
lignin
• Bond Cleavage*
At high temperatures a proton
attacks the benzylic oxygen
CH3O
Intermediatebenzylic carbocation
CH3O
HC
O
HC O
.. R
CH3O
HC
O
HC O R
(+)
H
+
H
OCH3
HC
OCH3
O (OH)
O
O
HC (+)
.. O
(OH)
OCH3
(OH)
CH3O
CH3O
HC
O
-
HSO3
HC SO3H
HC
O
HC
» Acid: a-O-4 linkage
OCH
OCH
O (OH)
O (OH)
only linkage affected.
(+)
» Neutral/Alkaline: b-O* This reaction does not reduce MW or improve delignification
4, a-O-4 linkages
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cleaved.
3
3
+ ROH
NSSC pulping
• Under neutral sulfite conditions:
» Sulfonation reactions are limited to β-O-4 ethers with free
phenolic hydroxyls only
» The net result of these reactions is light alpha-sulfonation
and some beta-ether cleavage
• Lignin is "softened" and most remains in the pulp
(higher temperatures it "gives" like thick molassas
and spreads out amongst the fibers)
• When made into a sheet and cooled down, the lignin
hardens again resulting in a stiff sheet
• This is the basis of cardboard type products
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Sulfite Pulping
Lignin Reactions
• Condensation
» These reactions are very prevalent in acid sulfite pulping.
The MW of the lignin is significantly increased. This is a
detriment to lignin removal.
OCH3
HC O H
HC (+)
(-)
HC
O
(OH)
β-1
+
O
O (+)
R (OH)
O
(OH)
HC (+)
(OH)
HC
H
+
(-)
OCH3
O
(OH)
OCH3
(+)
OCH3
O
OCH3
OCH3
OCH3
(OH)
OCH3
O
O
β-6
(OH)
(OH)
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Sulfite Pulping
Lignin Reactions ( What Really Happens)
• The unmentioned reaction.
» Six years ago it determined that there was basically
no free a-O-4 linkages in lignin. This means that the
proposed mechanisms for the reduction in lignin
molecular weight were incorrect.
» A forgotten proposed mechanism from Nakano
seems to explain this phenomena. In this
mechanism, lignin molecules are linked through
lignin-carbohydrate-lignin linkages. During acid
sulfite pulping, the carbohydrate is cleaved through
acid hydrolysis thus lowering the lignin molecular
weight.
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Sulfite Pulping
Acid Reactions of Carbohydrates
• Cellulose: cleavage of glycosidic linkages slow because of
macromolecular structure: crystalline nature.
• Xylans:
» Arabinofuranosyl linkages very acid labile
» Uronic acid linkages somewhat stable
» Acetyl groups easily cleaved
• Galactoglucomannans:
 a-D-(1galactosidic linkage very acid labile
» Acetyl Groups easily cleaved; this increases
resistance of glucomannan linkages to cleavage
» Glucomannan linkages more stable than xylan
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Sulfite Pulping
Carbohydrate Yield %
Acid Reactions of Carbohydrates
Cellulose Yield
100
80
60
40
20
0
40
45
50
55
60
65
70
75
80
85
90
95 100
Pulp Yield (%)
Carbohydrate degradation phenomena during sulfite cooking of Spruce
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Softwood Xylans

4-b-D-Xly-14-b-D-Xly-14-b-D-Xly-14-b-D-Xly4-b-D-Xly




4-O-Me-a-D-Glc 

2
2
2
OH
O
HO
O
OH
HO
O
O
O
OH
O
O
1
H3CO
COOH
HO
O
O
OH
O
O
O
HO


a-L-Araf
HO
O
H
HO
OH
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HMFs and Furfurals
17
Sulfite Pulping
Reactions of Monosaccharides
• Monosaccharides react with bisulfite to form
aldonic acids under high temperature. Bisulfite is
converted to thiosulfate, thus severe lignin
condensation (black cook).
• In an acid sulfite cook, typically 10-20% of the
sugars are converted.
• Thiosulfate responsible for condensation reactions.
• Formation of thiosulfate is self catalyzing:
-
RCHO + HSO3
4 HS03
-
-
RCOOH + S2O32
-
(Thiosulfate)
+
S2O32- +2 SO42 + 2H + H2O
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Sulfite versus kraft
• Not used why:
1) New bleaching techniques made efficient bleaching
of kraft pulp possible
2) More pollution compared to kraft and high SO2 loss
to atmosphere
3) Applicable to small amount of wood species because
of limited capabilities to dissolve extractives (spruce,
balsam fir, and hemlock, aspen, poplar, birch, maple)
4) Long cooking process
5) Weaker pulp
6) Poor chemical recovery
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