Agenda
• Carbohydrate Reaction Mechanisms
» Glycosidic Cleavage
» Peeling
» Stopping
•
•
•
•
•
Glucomannan Reactions
Xylan Reactions
Other Hemicellulose Reactions
Cellulose Reactions
Viscosity
1
Carbohydrate Reactions
• The main alkaline reactions of carbohydrates :
» Glycosidic cleavage.
» Peeling.
» Stopping.
2
Peeling Mechanism
* The peeling reaction basically unzips the carbohydrates by
removing terminal sugars one at a time. Reaction takes
place from reducing end of the molecule (aldehyde).
Reducing end group
Stable end group
3
Peeling Mechanism
CHO
HCOH
HOCH
CH2OH
C O
HOCH
CH2OH
-
C O
+
-H
HOC
HCOR
HCOR
HC OR
HCOH
HCOH
HCOH
CH2OH
CH2OH
-
- RO
CH2OH
CH2OH
CH2OH
C O
C O
HOC
C O
CH
CH2
HCOH
HCOH
CH2OH
Acids formed by peeling reaction
responsible for most alkali consumption in
kraft cook
+ H2O
C(OH)CH2OH
CH2
HCOH
CH2OH
CH2OH
glucoisosaccharinic acid
* The peeling reaction basically
unzips the carbohydrates by
removing terminal sugars
one at a time. Reaction takes
place from reducing end of
the molecule (aldehyde).
CO2H
CH2OH
CHO
COOH
C O
C O
CHOH
CH2OH
CH3
CH3
Lactic Acid
CHO
CHO
HCOH
COH
CH2OH
CH2
4
Stopping Mechanism
CHO
HCOH
HOCH
(A)
HC O
(-)
COH
HO CH
(B)
-
-HO
CHO
(C)
CHO
COH
C O
CH
CH2
CO2H
HCOH
+ H2O
CH2
HCOR
HCOR
HCOR
HCOR
HCOR
HCOH
HCOH
HCOH
HCOH
HCOH
CH2OH
CH2OH
CH2OH
* The stopping reaction stops
the peeling process when an
endgroup is formed which will
mot peel.
CH2OH
CH2OH
glucometasaccharinic
acid
(will not “peel”)
5
Hydrolysis of Glycosidic
Linkage
• Cleavage of glycosidic bonds.
» This reaction cleaves the carbohydrate in the chain instead of at the
end of the chain as in the peeling reaction. This generates a new
reducing end which increases the rate of peeling.
• This reaction lowers the molecular weight of carbohydrates.
» Glycosidic cleavage of cellulose results in loss of pulp viscosity
and can lead to strength loss if too extensive
(-)
CH2OH
CH2OH O
HO
HO
OMe
+
-H
CH2
CH2 O
-
OH
- MeO
O
OH
OH
O
O
O
OMe
HO
OH
O
(-)
OH
O
OH
OH
6
100
200
80
150
60
Glucomannan
Temperature
40
100
50
20
0
Temperature (C)
Glucomannan Yield (%)
Loss of Glucomannans During
Kraft Pulping
0
0
50
100
150
200
250
Time (minutes)
7
120
200
100
150
80
25% EA
60
100
15.8% EA
40
50
20
0
Temperature (C)
Glucomannan Yield (%)
Effect of Effective Alkali on
Glucomannan Loss
0
0
50
100
150
200
250
300
Time (minutes)
8
Glucomannan losses
• Glucomannans are lost mainly through primary peeling.
» Responsible for much of yield loss, especially in softwoods
» Pulp yield can be increased by stabilizing glucomannans
- Oxidize reducing end group with either polysulfide or
anthraquinone
9
100
200
80
150
60
100
40
Xylan
Temperature
20
50
0
0
50
100
150
200
Temperature (C)
Xylan Yield (%)
Loss of Xylans During Kraft
Pulping
0
250
Time (minutes)
10
Effect of Effective Alkali on
Xylan Loss
200
100
150
80
60
100
40
25% EA
15.8% EA
20
50
0
0
50
100
150
200
250
Temperature (C)
Xylan Yield (%)
120
0
300
Time (minutes)
11
Xylan losses
• Xylans are lost mainly through glycosidic cleavage (and
some secondary peeling).
» Dissolve as macromolecule which can re-precipitate back on
to the pulp fibers if [OH-] becomes low enough – end of the
cook
- End group stabilization not very effective for hardwoods
» Responsible for substantial yield loss in hardwoods
» Presence of xylans on pulp have a significant effect on its
performance
- Refining is easier with xylans in the pulp
- Xylans appear to inhibit bleaching
12
Cellulose Reactions During
Kraft Pulping
• Cellulose undergoes peeling and glycosidic
cleavage reactions during kraft pulping.
» Because cellulose molecules are so long, peeling reactions
only cause small yield losses.
» Glycosidic cleavage is more of a problem because of
molecular weight losses that may cause strength
problems. This reaction also increases the rate of peeling
somewhat through generation of new reducing end
groups.
• Because cellulose molecules are so large
dissolution is not an issue.
13
Pulp Viscosity
• Modifying the
hemicellulose content of
the pulp won’t change the
viscosity
» Borohydride treatment
inhibits primary peeling
which increase
glucomannan content
14
Pulp Viscosity
• Pulp strength and
viscosity has a complex
relation
» A decrease in viscosity
may not correlate with
pulp strength until the
viscosity reaches a critical
level – then look out!
15
Pulp Viscosity
• Pulp strength and
viscosity has a complex
relation
» A decrease in viscosity
may not correlate with
pulp strength until the
viscosity reaches a critical
level – then look out!
16
Pulp Viscosity
Zero span tensile(kPa)
180
160
140
120
100
80
60
0
5
10 15 20 25 30 35 40 45
CED Viscosity (mPa.s)
Borohydride treated
Untreated
17
Pulp Viscosity
• Pulp strength and
viscosity has a complex
relation
» A decrease in viscosity
may not correlate with
pulp strength until the
viscosity reaches a critical
level – then look out!
18
Pulp Viscosity
• Pulp strength and viscosity has a complex relation
» The retention of hemicelluloses can, however, reduce the
strength of the pulp without any affect on the pulp’s
viscosity
19