1

Agenda

• Carbohydrate Reaction Mechanisms» Glycosidic Cleavage» Peeling» Stopping

• Glucomannan Reactions• Xylan Reactions• Other Hemicellulose Reactions• Cellulose Reactions• Viscosity

• Carbohydrate Reaction Mechanisms» Glycosidic Cleavage» Peeling» Stopping

• Glucomannan Reactions• Xylan Reactions• Other Hemicellulose Reactions• Cellulose Reactions• Viscosity

2

Carbohydrate Reactions

• The main alkaline reactions of carbohydrates :

» Glycosidic cleavage.

» Peeling.

» Stopping.

• The main alkaline reactions of carbohydrates :

» Glycosidic cleavage.

» Peeling.

» Stopping.

3

Peeling Mechanism

* The peeling reaction basically unzips the carbohydrates by removing terminal sugars one at a time. Reaction takesplace from reducing end of the molecule (aldehyde).

Reducing end group

Stable end group

4

Peeling Mechanism

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

* The peeling reaction basicallyunzips the carbohydrates by removing terminal sugarsone at a time. Reaction takesplace from reducing end of

the molecule (aldehyde).

Acids formed by peeling reaction responsible for most alkali consumption in kraft cook

5

Stopping Mechanism

(A) (B)

(C)

(will not “peel”)

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

* The stopping reaction stopsthe peeling process when anendgroup is formed which willmot peel.

6

Hydrolysis of Glycosidic Linkage

O

OMeHO

HO

HOCH2OH

O

CH2OH

OH

OH

O

OMe

(-)

- H+- MeO-

O

CH2

OH

OH

OH

O

O

CH2

OH

OH

O

O(-)

• 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

• 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

7

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

8

Effect of Effective Alkali on Glucomannan Loss

0

20

40

60

80

100

120

0 50 100 150 200 250 300

Time (minutes)

Glu

co

ma

nn

an

Yie

ld (

%)

0

50

100

150

200

Te

mp

era

ture

(C

)

25% EA

15.8% EA

0

20

40

60

80

100

120

0 50 100 150 200 250 300

Time (minutes)

Glu

co

ma

nn

an

Yie

ld (

%)

0

50

100

150

200

Te

mp

era

ture

(C

)

25% EA

15.8% EA

9

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

• 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

10

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

11

Effect of Effective Alkali on Xylan Loss

0

20

40

60

80

100

120

0 50 100 150 200 250 300

Time (minutes)

Xyl

an Y

ield

(%)

0

50

100

150

200

Tem

per

atu

re (C

)

25% EA

15.8% EA0

20

40

60

80

100

120

0 50 100 150 200 250 300

Time (minutes)

Xyl

an Y

ield

(%)

0

50

100

150

200

Tem

per

atu

re (C

)

25% EA

15.8% EA

12

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

• 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

13

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.

• 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.

14

Pulp Viscosity

• Modifying the hemicellulose content of the pulp won’t change the viscosity» Borohydride treatment

inhibits primary peeling which increase glucomannan content

• Modifying the hemicellulose content of the pulp won’t change the viscosity» Borohydride treatment

inhibits primary peeling which increase glucomannan content

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!

• 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

• 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!

• 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!

17

Pulp Viscosity

180

160

140

120

100

80

600 5 10 15 20 25 30 35 40 45

Ze

ro s

pa

n te

nsi

le(k

Pa

)

CED Viscosity (mPa.s)

Borohydride treated Untreated

18

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!

• 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!

19

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

• 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