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The kinetics of residual delignification and factors affecting the amount of residual lignin during kraft pulping
KTH, Superseded Departments, Pulp and Paper Technology.
KTH, Superseded Departments, Pulp and Paper Technology.ORCID iD: 0000-0002-2900-4713
1996 (English)In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 22, no 8, J290-J295 p.Article in journal (Refereed) Published
Abstract [en]

Kraft pulping of softwood can be divided into three phases: the initial, the bulk and the residual delignification phases. The rate of delignification in the residual phase and the amount of lignin to be removed in this slow phase have been studied as a function of the conditions in the kraft cooking of Norwegian spruce chips, Picea abies. The variables studied were concentration of OH-, concentration of HS-, ionic strength and temperature. The amount of lignin remaining undissolved in the wood residue at the transition from bulk to residual delignification is decreased by increasing the concentration of OH- and by increasing the concentration of HS- in the bulk phase. The amount of residual lignin is also decreased by a lower ionic strength in the bulk phase. The temperature in the range of 150 to 180 degrees C affected the lignin content at the transition from bulk to residual delignification but not the amount of residual lignin extrapolated to the beginning of the cook. Only the hydroxide ion concentration and the temperature seem to have an influence on the rate of delignification in the residual phase. The rate of delignification in the residual phase is well described by the following equation: dL(r)/dt = (0.0032 + 0.0020 . [OH-]) (. e) -146 000/8.314 . (1/T - 1/443.15)(Lr).

Place, publisher, year, edition, pages
1996. Vol. 22, no 8, J290-J295 p.
Identifiers
URN: urn:nbn:se:kth:diva-12999ISI: A1996VD41400006OAI: oai:DiVA.org:kth-12999DiVA: diva2:320092
Note
QC 20100521Available from: 2010-05-21 Created: 2010-05-21 Last updated: 2017-12-12Bibliographically 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

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