1. Agenda Carbohydrate Reaction Mechanisms Glucomannan Reactions
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.

3. 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

4. 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).
Acids formed by peeling reaction responsible for most alkali consumption in kraft cook

5. Stopping Mechanism * The stopping reaction stops
(B)
* The stopping reaction stops
the peeling process when an
endgroup is formed which will
mot peel.
(will not “peel”)

6. 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

7. Loss of Glucomannans During Kraft Pulping
As can be seen in this figure, almost 70% of glucomannans are lost during the kraft pulping of softwoods. This happens primarily through peeling reactions and dissolution. The majority of glucomannans are lost during the first seventy minutes of the cook. By time the reaction reaches temperature, the loss of glucomannans is nearly complete. It is not completely known why the remaining 30% of the glucomannans are not lost. It is assumed that it is because of physical restrains due to the bulky cellulose and perhaps because of linkages to residual lignin structures.
In a very interesting experiment, wood was reduced with sodium borohydride. The reducing end of the glucomannans was reduced to an alcohol group which would not peel (or would peel very slowly). When this material was subjected to kraft pulping, it was found that the large early loss of glucomannans was not due to peeling but rather to the NaOH dissolving part of the glucomannans (I.e. the rate of glucomannan loss was not changed). As the cook progressed, the rate of glucomannan loss significantly slowed (because the end groups had been reduced) showing that as the temperature increases in a kraft cook, glucommans are lost to peeling reactions.

8. Effect of Effective Alkali on Glucomannan Loss
As you can see by the figure, the effective alkali charge has no effect on the loss of glucomannans. You can draw a couple of conclusions from this.
The low temperature dissolution of glucomannans is not affected by the level of alkali.
The rate of the peeling reaction is not alkali dependent.
Two things that are not obvious from this slide but have been determined through other research.
The sulfidity of the cook does not affect the loss of glucomannans.
The rate of peeling is temperature dependent (higher temperature = faster rates)

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

10. Loss of Xylans During Kraft Pulping
You can see in this figure that the loss of xylans is both much less than glucomannans and also much slower. In the initial portion of the cook, xylans are lost very slowly through dissolution. As the reaction proceeds, the xylans are solubilized to a greater extent. As the cook nears completion, the amount of alkali in the reactor is much lower. At this point, the xylans begin to precipitate out of solution and to some extent back onto the surface of the fibers.
Sodium borohydride reduction studies have shown that the amount of xylans lost to peeling reactions is minimal. This is because xylans posses a unique end group which protects the xylans. Peeling does occur, however, if the xylan undergoes glycosidic cleavage first forming a new reactive reducing end. It is important to note that glycosidic cleavage is a high temperature reaction and therefore doesn’t significantly occur until almost maximum cooking temperature is reached.

11. Effect of Effective Alkali on Xylan Loss
You can see in the above figure that the level of effective alkali has a significant effect on the loss of xylans. As was discussed earlier, the loss of xylans is mostly through dissolution. The higher level of alkali serves to keep more of the xylans in solution reducing redeposition and also yield.
The higher level of EA also seems to increase the rate of glycosidic cleavage. This would tend to increase the rate of xylan losses through peeling. I have not found definitive evidence for this point so this is more of a theory at this point.

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

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.

14. Pulp Viscosity
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!

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!

17. Pulp Viscosity
180
160
140
120
100 80 60 5 10 15 20 25 30 35 40 45
Zero span tensile(kPa)
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!

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