Pulping and Bleaching PSE 476/Chem E 471
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PSE 476: Lecture 10 1 Pulping and Bleaching PSE 476/Chem E 471 Lecture #10 Kraft Pulping Carbohydrate Reactions
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Pulping and Bleaching PSE 476/Chem E 471. Lecture #10 Kraft Pulping Carbohydrate Reactions. Agenda. Carbohydrate Reaction Mechanisms Glycosidic Cleavage Peeling Stopping Glucomannan Reactions Xylan Reactions Other Hemicellulose Reactions Cellulose Reactions. Carbohydrate Reactions. - PowerPoint PPT Presentation
Transcript of Pulping and Bleaching PSE 476/Chem E 471
PSE 476: Lecture 10 1
Pulping and BleachingPSE 476/Chem E 471
Lecture #10Kraft Pulping
Carbohydrate Reactions
Lecture #10Kraft Pulping
Carbohydrate Reactions
PSE 476: Lecture 10 2
Agenda
• Carbohydrate Reaction Mechanisms» Glycosidic Cleavage» Peeling» Stopping
• Glucomannan Reactions• Xylan Reactions• Other Hemicellulose Reactions• Cellulose Reactions
• Carbohydrate Reaction Mechanisms» Glycosidic Cleavage» Peeling» Stopping
• Glucomannan Reactions• Xylan Reactions• Other Hemicellulose Reactions• Cellulose Reactions
PSE 476: Lecture 10 3
Carbohydrate Reactions
• Carbohydrates react slower than lignin under alkaline conditions. Overall, however, just as much carbohydrates react as does lignin.
• The main alkaline reactions of carbohydrates are:» Glycosidic cleavage.» Peeling.» Stopping.
• Carbohydrates react slower than lignin under alkaline conditions. Overall, however, just as much carbohydrates react as does lignin.
• The main alkaline reactions of carbohydrates are:» Glycosidic cleavage.» Peeling.» Stopping.
PSE 476: Lecture 10 4
Alkaline Pulping : CarbohydratesGeneral Aspects
• There are considerable carbohydrate losses during kraft pulping due to alkaline degradation reactions.» Acetyl groups are very quickly cleaved.» Carbohydrates undergo “peeling”
- Peeling is the process in which sugars are removed one by one from the reducing end of the polymer.
- Hemicelluloses highly degraded through “peeling”- Monosaccharide fragments from peeling are highly
degraded to acidic compounds.- This reaction is stopped by “stopping” reactions.
• Glycosidic linkages in carbohydrates are cleaved through hydrolysis reducing overall molecular weight and creating new reducing ends.
PSE 476: Lecture 10 5
“Peeling Reaction”
CHO
HCOH
HOCH
HCOR
HCOH
CH2OH
HOCH
HCOR
HCOH
CH2OH
C O
CH2OH
HOC
HC
HCOH
CH2OH
C O-
CH2OH
OR
- H+- RO- C O
CH2OH
HOC
HCOH
CH2OH
CH
C O
CH2OH
HCOH
CH2OH
CH2
C O
CO2H
C(OH)CH2OH
CH2
HCOH
CH2OH
+ H2O
glucoisosaccharinic acid
CH2OH
C O
CH2OH
CHO
HCOH
CH2OH
CHO
COH
CH2
COOH
CHOH
CH3
Lactic Acid
CHO
C O
CH3
Formation of new reducing end group
A. Isomerization
B enediol formation
C. -alkoxy elimination
D. Tautomerization
E. Benzilic acid rearrangement
(A) (B) (C) (D) (E)
Notes
PSE 476: Lecture 10 6
Benzilic acid rearrangement
Nuclephilic addition
Bond rotation
α-hydroxy-carboxylic acid Proton transfer
PSE 476: Lecture 10 7
“Stopping Reaction”
CHO
HCOH
HOCH
HCOR
HCOH
CH2OH
HCOR
HCOH
CH2OH
CHHO
COH
HC O(-)
CHO
HCOR
HCOH
CH2OH
CH
COH-HO-
CHO
HCOR
HCOH
CH2OH
CH2
C O
CO2H
HCOR
HCOH
CH2OH
CH2
HCOH+ H2O
glucometasaccharinic acid
A. 1,2 Enediol formation
B. -hydroxy elimination
C. Tautomerization
D. Benzilic acid rearrangement
A. 1,2 Enediol formation
B. -hydroxy elimination
C. Tautomerization
D. Benzilic acid rearrangement
(A) (B) (C) (D)
(will not “peel”)
Notes
PSE 476: Lecture 10 8
Cleavage of Glycosidic Bonds
O
OMeHO
HO
HOCH2OH
O
CH2OH
OH
OH
O
OMe
(-)
- H+- MeO-
O
CH2
OH
OH
OH
O
O
CH2
OH
OH
O
O(-)
A. Inversion of ring confirmation, C2 OH ionizedB. Ionized hydroxyl groups attacks C1 eliminating methoxyl
group forming 3 membered epoxide (oxirane)C. Opening of epoxide forms new reducing end or if steric
conditions are correct a 1,6 anhydride. This compound is opened by alkali. This reaction requires elevated temperatures
A. Inversion of ring confirmation, C2 OH ionizedB. Ionized hydroxyl groups attacks C1 eliminating methoxyl
group forming 3 membered epoxide (oxirane)C. Opening of epoxide forms new reducing end or if steric
conditions are correct a 1,6 anhydride. This compound is opened by alkali. This reaction requires elevated temperatures
(A) (B) (C)
Notes
PSE 476: Lecture 10 9
Cleavage of Glycosidic Bonds
Methyl pyranosides Relative Rate of - Anomer -Anomer
D-glucose 1 2.5 D-mannose 2.8 1.1 D-galactose 1 5.7 D-xylose 1.2 5.8 L-arabinose 10 1 D-glucuronate 280 -
Notes
PSE 476: Lecture 10 10
Alkaline Reactions of Glucomanans/Cellulose
• Glucomannans:» Very unstable to peeling reactions» Galactose side chains fairly resistant
• Cellulose» Large Dp means that glycosidic cleavage
more important than peeling» Cellulose loss small but viscosity (Mw)
significantly reduced
PSE 476: Lecture 10 11
Loss of Glucomannans During Kraft Pulping
0
20
40
60
80
100
0 50 100 150 200 250
Time (minutes)
Glu
com
ann
an Y
ield
(%
)
0
50
100
150
200
Tem
per
atu
re (
C)
Glucomannan
Temperature
0
20
40
60
80
100
0 50 100 150 200 250
Time (minutes)
Glu
com
ann
an Y
ield
(%
)
0
50
100
150
200
Tem
per
atu
re (
C)
Glucomannan
Temperature
Notes
PSE 476: Lecture 10 12
Loss of Xylans During Kraft Pulping
0
20
40
60
80
100
0 50 100 150 200 250
Time (minutes)
Xyl
an Y
ield
(%
)
0
50
100
150
200
Tem
per
atu
re (
C)
Xylan
Temperature
0
20
40
60
80
100
0 50 100 150 200 250
Time (minutes)
Xyl
an Y
ield
(%
)
0
50
100
150
200
Tem
per
atu
re (
C)
Xylan
Temperature
Notes
PSE 476: Lecture 10 13
Xylan peeling Reactions
• Xylans much more resistant to peeling than glucomannans.
• Large % of xylans are dissolved during pulping instead of peeled. Much of these xylans precipitate on the fiber surface at the end of the cook as alkali is consumed.
• There appears to be two different temperature dependent mechanisms responsible for the protection from peeling.
• Xylans much more resistant to peeling than glucomannans.
• Large % of xylans are dissolved during pulping instead of peeled. Much of these xylans precipitate on the fiber surface at the end of the cook as alkali is consumed.
• There appears to be two different temperature dependent mechanisms responsible for the protection from peeling.
PSE 476: Lecture 10 14
Resistance of Xylan to Peeling Below 100°C
• Low temperature stability (below 100°C) is due to the resistance of galacturonic acid to peeling at this temperature.» Galacturonic acid groups are found in unique end group.
(see below)» Above 100°C, galacturonic acid groups undergo peeling.
O
O
O
OO
O
O
OH
OH
OHOH
COOH
HO
OH
HO CH3 OOH
OHO
OH
OHO
--Xly-14---D-Xly3--L-Rha-12--D-GalA-14-D-Xly
PSE 476: Lecture 10 15
Resistance of Xylan to Peeling above 100°C
• A theory has been presented that above 100°C glucuronic acid side chains on xylans slow the peeling reaction.» Above 120°C, glucuronic acids are somewhat
converted to hexenuronic acids which are much more stable to peeling. Hexenuronic acid formation is discussed in the next slide.
• A theory has been presented that above 100°C glucuronic acid side chains on xylans slow the peeling reaction.» Above 120°C, glucuronic acids are somewhat
converted to hexenuronic acids which are much more stable to peeling. Hexenuronic acid formation is discussed in the next slide.
4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly
4-O-Me--D-Glc
-L-Araf
PSE 476: Lecture 10 16
Formation of Hexenuronic Acids
• Hexenuronic acids are formed from uronic acids under alkaline conditions.» Method for identifying these compounds in pulps just developed.» Interfere with Kappa (lignin concentration) determination.» Attract metals (color problem/cause problems in bleaching)
• Hexenuronic acids are formed from uronic acids under alkaline conditions.» Method for identifying these compounds in pulps just developed.» Interfere with Kappa (lignin concentration) determination.» Attract metals (color problem/cause problems in bleaching)
OCO2H
OH
OHCH3O
OH OH- OCO2H
OH
OHOH
Uronic Acid Hexenuronic Acid
PSE 476: Lecture 10 17
Loss of Other Hemicelluloses During Kraft Pulping
• The minor hemicelluloses such as pectins, starches, etc. are supposedly completely destroyed during kraft pulping.
• This happens through dissolution and peeling.
• Many of these carbohydrates are water soluble so removal is easy. Once they are in the hot alkali solution they are easily destroyed.
• The minor hemicelluloses such as pectins, starches, etc. are supposedly completely destroyed during kraft pulping.
• This happens through dissolution and peeling.
• Many of these carbohydrates are water soluble so removal is easy. Once they are in the hot alkali solution they are easily destroyed.
PSE 476: Lecture 10 18
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 cause strength problems. This reaction also increases the rate of peeling through generation of new reducing end groups.
• Because cellulose molecules are so large dissolution is not an issue.
• 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 cause strength problems. This reaction also increases the rate of peeling through generation of new reducing end groups.
• Because cellulose molecules are so large dissolution is not an issue.
