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Kinetic study of Hexenuronic and Methylglucuronic acid reactions in pulp and in dissolved xylan during kraft pulping of hardwood
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.ORCID iD: 0000-0002-2900-4713
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.

Place, publisher, year, edition, pages
2006. Vol. 45, no 7, 2174-2178 p.
Keyword [en]
Degradation; Dissolution; Mathematical models; Organic acids; Pulp; Reaction kinetics; Substitution reactions
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-7713DOI: 10.1021/ie051386vISI: 000236715600002Scopus ID: 2-s2.0-33646430308OAI: oai:DiVA.org:kth-7713DiVA: diva2:12819
Note

QC 20100702

Available from: 2007-11-23 Created: 2007-11-23 Last updated: 2016-12-21Bibliographically approved
In thesis
1. Xylan Reactions in Kraft Cooking: Process and Product Considerations
Open this publication in new window or tab >>Xylan Reactions in Kraft Cooking: Process and Product Considerations
2007 (English)Doctoral 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 takes place. The processes studied here are the following: dissolution, degradation, redeposition onto the fibres, side-group conversion, and cleavage of side groups off the xylan backbone. The side group in native xylan consists of methylglucuronic acid, which is partly converted into hexenuronic acid during kraft cooking. Hexenuronic acid affects the pulp in terms of increased brightness reversion and reduced bleachability. The kinetics of the side-group cleavage and conversion reactions were studied using various analytical tools. The study revealed that the most common methods for methylglucuronic acid quantifcation can be signifcantly improved in terms of accuracy. A modifcation and combination of two of the methods was suggested and evaluated.

In order to minimise the hexenuronic acid content, a common suggestion involves the use of a high cooking temperature. The kinetic study found 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 indicated a high temperature dependency for birch kraft cooking.

By collecting black liquors at different stages in the cook, different molecular properties of the dissolved xylan was obtained. The liquors were charged at later parts of the cook, making the dissolved xylan to reattach to the fibres. Depending on the molecular properties of the added xylan, the tensile strength properties of the produced paper were improved. These improvements in paper properties were correlated to the molecular behaviour of the added xylan in solution.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. [11], 73 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:78
Keyword
Xylan, Kraft cooking, Paper strength, Sheet density, Methylglucuronic acid, Hexenuronic acid
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-4551 (URN)978-91-7178-819-1 (ISBN)
Public defence
2007-12-14, F3, Lindstedtsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100702Available from: 2007-11-23 Created: 2007-11-23 Last updated: 2010-07-02Bibliographically approved
2. Xylan reactions in kraft cooking and their influence on paper sheet properties
Open this publication in new window or tab >>Xylan reactions in kraft cooking and their influence on paper sheet properties
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:nbn:se:kth:diva-3850 (URN)
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

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Lindström, Mikael

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