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A Study of How the Amount of Residual Phase Lignin in Kraft Cooking Depends upon the Conditions in the Cook
KTH, Superseded Departments, Fibre and Polymer Technology.
KTH, Superseded Departments, Pulp and Paper Technology.ORCID iD: 0000-0002-2900-4713
1997 (English)In: Nordic Pulp Paper Research Journal, ISSN 0283-2631, Vol. 12, no 4, 225- p.Article in journal (Refereed) Published
Place, publisher, year, edition, pages
1997. Vol. 12, no 4, 225- p.
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-12996OAI: oai:DiVA.org:kth-12996DiVA: diva2:320079
Note
QC 20100521Available from: 2010-05-21 Created: 2010-05-21 Last updated: 2010-08-25Bibliographically approved
In thesis
1. Some factors affecting the amount of residual phase lignin during kraft pulping
Open this publication in new window or tab >>Some factors affecting the amount of residual phase lignin during kraft pulping
1997 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The amount of lignin which needs to be removed according to the slow residual delignification phase in kraft pulping, i e residual phase lignin, has been studied as a function of the process conditions. The variebles studied were the concentrations of hydroxide ions, hydrogensulfide ions, sodium ions (ionic strength) and temperature. In addition, the amounts of residual phase ligning in the pulping of different lignocellulosic material were compared. Different additives can be applied to the kraft cook to improve the selectivity. Polysulfide sulfur is one of them and the effectt of polysulfide ions on the amount of residual phase lignin in kraft pulping was also investigated.

It is shown, that the amounts of residual phase lignin in constant composition cooks (highliquor-to-wood ratio) of softwood (spruce), hardwood (birch) andgrass (wheat straw) are affected by the same factors in theinvestigated range. The amount of residual phase lignin isstrongly reduced by higher hydroxide ion concentration during the kraft cook and is to some extent decreased by higherhydrogensulfide ion concentration and lower sodium ionconcentration. These effects were not, however, found to bepurely additive for spruce wood, instead interactions occurred.In order to decrease the amount of residual phase lignin, it isessential to have a high concentration of hydrogensulfide ionswhen cooking with a low hydroxide ion concentration. The temperature has no effect on the amount of residual phase ligninin the range investigated.

It is also shown that the lignin reacting according to theslow delignification phase is not a homogeneous lignin sincealteration of the conditions in the cook can make part of itreact as bulk phase lignin. This indicates that most of theresidual phase lignin is determined by the prevailing conditions,which determine how much of the native lignin reacts according tothe bulk and how much according to the residual delignificationmechanism.

Pulping experiments carried out under the same conditions onwheat straw,  birch wood and spruce wood also showed that theamount of residual phase lignin in wheat straw is only 30% of theamount of residual phase lignin in spruce wood, and that the amount of residual phase lignin in birch wood, is5 0% of the amount of residual phase lignin in sprucewood.

The amount of residual phase lignin was affected by apolysulfide treatment. The amount of residual phase lignin was lowered by 30% for spruce wood and 10% for birch wood with apolysulfide treatment, compared with cooks without polysulfide. An explanation for this may be the finding that polysulfide can introduce a carboxylic acid function into the lignin, which will increase its water solubility, and reduce the need for fragmentation. An additional explanation may be the ability of polysulfides to degrade enol ether structures, which are normally more or less stable during kraft pulping.

Place, publisher, year, edition, pages
Stockholm: KTH, 1997. 52 p.
Series
Trita-PMT, 1997:5
Keyword
Deligniflcation, Residual phase lignin, Hydrogensulfide ion, Hydroxide ion, Polysulfides, Ionic strength, Temperature, Lignin, Kraftpulping, Selectivity, viscosity, Kappa number, chemical degradation, Disproportionation
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-2502 (URN)99-2412642-4 (ISBN)
Public defence
1997-05-22, 00:00 (English)
Note
QC 20100521Available from: 2000-01-01 Created: 2000-01-01 Last updated: 2010-05-21Bibliographically approved
2. On the Interrelation Between Kraft Cooking Conditions and Pulp Composition
Open this publication in new window or tab >>On the Interrelation Between Kraft Cooking Conditions and Pulp Composition
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

In the early 1990’s, a lot of work was focused on extending the kraft cook to a low lignin content (low kappa number). The driving force was the need to further reduce the environmental impact of the bleaching, as less delignification work would be needed there. However, the delignification during the residual phase of a kraft cook is very slow and, due to its poor selectivity, it is a limiting factor for the lignin removal. If the amount of lignin reacting according to the residual phase could be reduced, it would be possible to improve the selectivity of the kraft cook. In the work described in this thesis, special attention has been given to the activation energy of the slowly reacting residual phase of a kraft cook on softwood raw material and to the influence of different cooking parameters on the amount of the residual phase lignin.

The activation energy of the residual phase delignification of the kraft cook was shown to be higher than that of the bulk phase delignification. In order to decrease the amount of residual phase lignin, it was essential to have a high concentration of hydrogen sulphide ions when cooking with a low hydroxide concentration. It was also important to avoid a high sodium ion concentration when cooking with low hydroxide and low hydrogen sulphide ion concentrations. Furthermore, it was demonstrated that dissolved wood components had a positive effect on the delignification rate in the bulk phase of a kraft cook.

The influence of different cooking parameters in the extended softwood kraft process on the bleachability (i.e. the ease with which the pulps can be bleached to a target brightness) of the manufactured pulp was also investigated. If variations in bleachability were seen, an attempt would also be made to find chemical reasons to explain the differences. It was difficult to establish clear relationships between the chemical structures of the residual lignin and the bleachability of the pulp. However, it was seen that the higher the content of β-aryl ether structures in the residual lignin after cooking, the better was the QPQP*-bleachability.

In the middle/end of the 1990’s, the focus moved from extended cooking to efficient utilisation of the wood raw material, e.g. by interrupting the kraft cook at higher kappa number levels and choosing appropriate cooking conditions to maximise the cooking yield. A high cooking yield often leads to a somewhat higher hexenuronic acid (HexA) content of the pulp at a given kappa number. Therefore additional attention was devoted to how the HexA content and carbohydrate composition were affected, e.g. by a set of cooking parameters. Performing these studies it was also important to investigate the effects of a low HexA (after cooking) strategy on such vital factors as the cooking yield, the bleachability and the yellowing characteristics of the pulp obtained. It proved to be difficult to significantly reduce the HexA content in a kraft pulp by altering the cooking conditions for both softwood and the hardwood Eucalyptus Globulus. A reduction in HexA content can be achieved by extending the cook to lower kappa numbers, or by using a high hydroxide concentration, a low hydrogen sulphide concentration or a high sodium ion concentration. However, neither of these strategies is attractive for industrial implementation since they would result in an extensive loss of yield, viscosity and strength.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 89 p.
Series
TRITA-FPT-Report, ISSN 1652-2443 ; 2006.39
Keyword
Delignification, Kraft pulping, Residual phase lignin, Hydroxide, Hydrogen sulphide ion, Ionic strength, Temperature, Bleachability, Hexenuronic acid, Carbohydrates
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-4232 (URN)
Public defence
2006-12-15, Sal F3, KTH, Lindstedtsvägen 26, Stockholm, 14:00
Opponent
Supervisors
Note

QC 20100825

Available from: 2006-12-13 Created: 2006-12-13 Last updated: 2016-12-21Bibliographically approved

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