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Xylan reactions in kraft cooking and their influence on paper sheet properties
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2006 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

Xylan is the main hemicellulose in birch, eucalyptus, and most other hardwood species. During kraft pulping a series of chemical reactions and physical processes involving xylan take place. The processes studied in this thesis are the following: dissolution, degradation, redeposition onto the fibres, side group conversion, and cleavage of side groups off the xylan back bone. The side group in native xylan consists of methylglucuronic acid, which is mainly converted into hexenuronic acid during kraft cooking. Hexenuronic acid affect the pulp in terms of increased brightness reversion and deteriorated bleachability. The kinetics of the side group cleavage and conversion reactions were analysed using various analytical tools. The study revealed that the most commonly used methods for methylglucuronic acid measurements are not as accurate as has been claimed in the literature. A modification of two of the methods was suggested and evaluated.

A common practice used to minimise the hexenuronic acid content involves use of a high cooking temperature. The kinetic study showed that the degree of substitution of pulp xylan is only slightly affected by temperature, and that the observed effects are likely to be more associated with the xylan content of the pulp than with the hexenuronic acid content of the xylan. For the dissolved xylan, however, the degree of substitution showed a high temperature dependency and moreover it was always higher than in the pulp.

Xylan itself is known to have the capacity to increase the strength of the manufactured pulp. This knowledge was applied by exchanging cooking liquors between birch kraft cooks, in which a high amount of xylan is dissolved, and spruce cooks, which contain very small amounts of native xylan. This seems like an attractive alternative for softwood kraft cooking, since both the tensile strength and stiffness increased significantly. The magnitude of the strength increase was correlated with the molecular weight of the added xylan and with the increased surface charge of the fibres.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , 53 p.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2006:01
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-3850OAI: oai:DiVA.org:kth-3850DiVA: diva2:9706
Presentation
2006-02-17, STFI-salen, STFI, Drottning Kristinas väg 53, 100 44 Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101111Available from: 2006-02-14 Created: 2006-02-14 Last updated: 2010-11-11Bibliographically approved
List of papers
1. Influence of birch xylan adsorption during kraft cooking on softwood pulp strength
Open this publication in new window or tab >>Influence of birch xylan adsorption during kraft cooking on softwood pulp strength
2005 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 20, no 4, 436-441 p.Article in journal (Refereed) Published
Abstract [en]

The dissolution, degradation and redeposition of xylan in the kraft cooking of birch pulp were investigated. The molecular weight of the dissolved xylan was determined through gel permeation chromatography, and the loss in molecular weight could be correlated with the amount of degraded xylan in the initial stages of the kraft cook. This indicates that peeling is the only significant xylan degradation reaction taking place early in the cook. Two different birch black liquors containing xylan with molecular weights of 12.20 g/mol and 5.95 g/mol, respectively, were added to softwood kraft cooks in order to determine the effect of birch xylan on pulp strength properties. The results show an increase in both tensile strength and tensile stiffness. The magnitude of the strength increase was greatly affected by the molecular weight of the xylan added. Adding high-molecular-weight xylan increased the tensile strength by more than 10%, as measured at a beating degree of 1000 PFI revolutions. Tensile stiffness was also increased by xylan addition, though more so when the xylan was of high rather than low molecular weight.

Keyword
kraft pulping; hydroxyl ion concentration; xylan; hemicellulose; dissolution; degradation; precipitation; molecular weight; surface properties; paper strength; adsorption
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-7712 (URN)10.3183/NPPRJ-2005-20-04-p436-441 (DOI)000235258200010 ()2-s2.0-30344439844 (Scopus ID)
Note
QC 20100702Available from: 2007-11-23 Created: 2007-11-23 Last updated: 2017-12-14Bibliographically approved
2. Kinetic study of Hexenuronic and Methylglucuronic acid reactions in pulp and in dissolved xylan during kraft pulping of hardwood
Open this publication in new window or tab >>Kinetic study of Hexenuronic and Methylglucuronic acid reactions in pulp and in dissolved xylan during kraft pulping of hardwood
2006 (English)In: Industrial & Engineering Chemistry Research, ISSN 0888-5885, E-ISSN 1520-5045, Vol. 45, no 7, 2174-2178 p.Article in journal (Refereed) Published
Abstract [en]

During kraft pulping, the side group in the xylan backbone, 4-O-methyl-D-glucuronic acid, is partly converted to hexenuronic acid. Simultaneously, degradation reactions of these side groups take place. The rates of these reactions were studied during the kraft pulping of hardwood and were shown to be strongly affected by the location of the x Ian; dissolved xylan had markedly higher methylglucuronic acid and hexenuronic acid contents than pulp xylan did. The degree of substitution of methyl-lucuronic acid in dissolved xylan was found to be higher at reduced cooking temperatures; no such change was seen for pulp xylan. A kinetic model was developed that included the energies of activation for formation (129 U/mol) and degradation (143 U/mol) of hexenuronic acid and dearadation (141 kJ/mol) of methylglucuronic acid and bulk delignification (118 kJ/mol, in accordance with earlier studies). Decreased cooking temperatures thus increase the number of acidic charged groups in the pulp and in dissolved xylan.

Keyword
Degradation; Dissolution; Mathematical models; Organic acids; Pulp; Reaction kinetics; Substitution reactions
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-7713 (URN)10.1021/ie051386v (DOI)000236715600002 ()2-s2.0-33646430308 (Scopus ID)
Note

QC 20100702

Available from: 2007-11-23 Created: 2007-11-23 Last updated: 2017-12-14Bibliographically approved
3. Quantification of uronic acid compositions by modification and integration of carbazole colorimetric and methanolysis-GC methods
Open this publication in new window or tab >>Quantification of uronic acid compositions by modification and integration of carbazole colorimetric and methanolysis-GC methods
Show others...
(English)Manuscript (Other academic)
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-5361 (URN)
Note
QC 20101111Available from: 2006-02-14 Created: 2006-02-14 Last updated: 2010-11-11Bibliographically approved

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