Pulping and Bleaching
PSE 476/Chem E 471
Lecture #15
Kraft Pulping
Review of Reactions/Kinetics
Effective Alkali (g/l NaOH)
Kraft Pulping
60
70°C
70°C
137°C
50
40
30
Impregnation zone
170° C
20
Initial Phase
Bulk Phase
Residual Phase
10
0
0
5
10
15
20
Yield of Lignin (%)
25
30
Kraft Pulping Kinetics:
Initial Phase
Compound
Extractives
Cellulose
Glucomannans
Xylans
Lignin
% Removed
Majority
<1%
60-70%
20-30%
20-30%
Glucomannans
Reactions
1) Dissolution
Kinetics
1) Effective Alkali
a) A certain small %
a) No effect on rate of
of Glucommans are
degradation
soluble in alkali
2) Sulfide Charge
and will diffuse
a) No effect on reaction
from the wood.
2) Peeling
a) Peeling reaction
starts during this
phase @ ~ 100C
3) Deacetylation
Extractives
Reactions
Kinetics
1) Saponification of
1) Effective Alkali
Extractives
a) These reactions are
a) Fats
so rapid that the
b) Waxes
level of alkali has
c) Lignans (Lactones)
little effect on the
2) Neutralization
rate
a) Fatty Acids
2) Sulfidity
b) Resin Acids
a) Sulfidity has no
c) Monoterpene Acids
effect on these
d) Lignans
reactions
e) Phenolic Acids
3) Solubilization
a) Nearly all
extractives are
removed during the
initial phase of the
cook.
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.
Alkaline Hydrolysis:
Example Using Acetyl Groups
CH2OH
OH
HO
OH
CH2OH
O
OH
-
HO
OH
CH2OH
OH
HO
O C CH3
O
OH
O
OH
O C CH3
O
O
HO
+
OH
HO C CH3
O
• 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.
Saponification of Fats
(Review slide from PSE 406)
• Treatment of fats with alkali converts them to fatty acids and
glycerol through saponification.
OH
O
H2C O C
HOCH2CHCH2OH
R
-
OH
Glycerol (glycerine)
-
O
H2C O C
OH
R
H2O
O
H2C OH
R
-
O C
Once again this reaction
consumes part of the alkali
charge.
Removal of Glucomannans:
120
200
100
150
80
25% EA
60
100
15.8% EA
40
50
20
0
0
0
50
100
150
200
Time (minutes)
Why doesn’t 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
250
300
Temperature (C)
Glucomannan Yield (%)
Effect of Effective Alkali
Kraft Pulping Kinetics:
Initial Phase
Xylans
Lignin
Reactions
Kinetics
1) Cleavage of -O-4 1) Effective Alkali
a) No affect on
linkages
rate during this
a) Units with free
phase
phenolic hydroxyl
b) Rate diffusion
b) Very fast
limited in this
2) Cleavage of -O-4
phase
linkages
2) Sulfidity
a) Units with free
a) Sulfidity has no
phenolic hydroxyl
effect during
or  carbonyl.
this phase
b) Rapid reaction
3) Solubilization
a) Small amount of
lignin alkali soluble
4) Condensation
a) Limited
Reactions
Kinetics
1) Dissolution
1) Effective Alkali
a) Major reaction of
a) Higher EA increases the
Xylans during kraft
rate of dissolution
pulping
2) Sulfide Charge
2) Peeling
a) No effect on reaction
a) Very minor reaction
starts @ ~ 100C
3) Stopping
a) See 2a
4) Deacetylation
Cellulose
Reactions
Kinetics
1) Peeling
1) Effective Alkali
a) Loss of cellulose begins
a) ?
2) Sulfide Charge
@ ~ 120C - 130 C
2) Minor reaction as only 10% a) No effect on reaction
of all cellulose lost during
cook.
Lignin Removal:
Effect 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
40
KAPPA
30
It depends on:
•Wood species
•EA
•Cooking temp.
•Hardwoods 20%
•Softwoods 25%
20
10
0
0
10
20
30
Sulfidity
40
50
60
Removal of Xylans:
Effect of Effective Alkali
200
100
150
80
60
100
40
25% EA
15.8% EA
20
50
0
0
50
100
150
200
Time (minutes)
Why does EA affect xylan removal?
•Stable at the beginning
•Hydrolysis and 2nd peeling
250
0
300
Temperature (C)
Xylan Yield (%)
120
Kraft Pulping Kinetics:
Bulk Phase
Compound
Extractives
Cellulose
Glucomannans
Xylans
Lignin
% Removed (total)
Majority
<10%
60-70%
30-50%
80-85%
Cellulose
Reactions
Kinetics
1) Peeling
1) Effective Alkali
a) Loss of cellulose
a) ?
accelerates at temperature 2) Sulfide Charge
due to glycosidic cleavage a) No effect on reaction
2) Stopping
a) Stopping reactions slow
loss of cellulose
3) Glycosidic Cleavage
a) See 1a
b) Starts to reduce
cellulose viscosity
Lignin
Reactions
Kinetics
1) Cleavage of non
1) Effective Alkali
a) Higher EA =
phenolic -O-4
faster rate of
followed by:
degradation
a) Cleavage of -O-4
2) Sulfide Charge
linkages in units
a) Higher sulfide
with free phenolic
charge increases
hydroxyl
the rate of lignin
b) Cleavage of -O-4
removal.
linkages in units
b) Higher lignin
with free phenolic
removal rate
hydroxyl Rapid
results in a
reaction
lower lig/carb at
c) Condensation
end of cook
d) Cleavage of C-C
bonds (double)
Kraft Pulping Kinetics:
Bulk Phase
Xylans
Reactions
Glucomannans
Kinetics
1) Dissolution
1) Effective Alkali
a) Major reaction of
a) Higher EA increases the
Xylans during this
rate of dissolution
phase
2) Sulfide Charge
2) Peeling
a) No effect on reaction
a) Minor reaction
3) Glycosidic Cleavage
a) Will increase rate of
peeling reaction
4) Precipitation on fibers
a) Starts as alkali
consummed
Reactions
1) Peeling
Kinetics
1) Effective Alkali
a) Peeling is
b) No effect on rate of
extensive until
degradation
about 70% of xylan 2) Sulfide Charge
is lost and then
a) No effect on reaction
almost stops
2) Gylcosidic cleavage
a) Increases the rate
of peeling
3) Stopping
a) Stops losses due to
peeling
Kraft Pulping Kinetics:
Residual Phase
Compound
Extractives
Cellulose
Glucomannans
Xylans
Lignin
% Removed (total)
Majority
~10%
70-80%
40-50%%
85-95%
Cellulose
Reactions
1) Glycosidic Cleavage
Kinetics
1) Effective Alkali
a) Major reaction resulting in a) ?
2) Sulfide Charge
lower molecular weight
a) No effect on reaction
and strength loss
2) Peeling/stopping
a) Minor reactions
Lignin
Reactions
Kinetics
1) Majority of ether
1) Effective Alkali
linkages that will
a) The rate slows
cleave are gone:
rapidly:
2) Cleavage of C-C
b) amount of lignin
bonds (double)
remaining a
3) Condensation
function of the
a) Increase the Mw
EA and sulfide
and lower reactivity
charge during
of remaining lignin
the cook
Kraft Pulping Kinetics:
Residual Phase
Xylans
Reactions
1) Peeling/stopping/
a) negligible
2) Precipitation on fibers
a) Significant amount
deposited as alkali
continues to be
consummed
Kinetics
1) Effective Alkali
a) Lower levels of alkali at end
of the cook casuse
deposition of xylans
2) Sulfide Charge
a) No effect on reaction
Glucomannans
Reactions
1) Peeling
Kinetics
1) Effective Alkali
a) Limited
b) No effect on rate of
2) Gylcosidic cleavage
degradation
a) Limited
2) Sulfide Charge
3) Stopping
a) No effect on reaction
a) Limited