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Xylan Reactions in Kraft Cooking: Process and Product Considerations
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
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 [en]
Xylan, Kraft cooking, Paper strength, Sheet density, Methylglucuronic acid, Hexenuronic acid
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-4551ISBN: 978-91-7178-819-1 (print)OAI: oai:DiVA.org:kth-4551DiVA: diva2:12823
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
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, 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: 2010-07-02Bibliographically 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: 2016-12-21Bibliographically approved
3. An improved methodology for the quantification of uronic acid units in xylans and other polysaccharides
Open this publication in new window or tab >>An improved methodology for the quantification of uronic acid units in xylans and other polysaccharides
Show others...
2007 (English)In: Carbohydrate Research, ISSN 0008-6215, E-ISSN 1873-426X, Vol. 342, no 11, 1442-1449 p.Article in journal (Refereed) Published
Abstract [en]

Uronic acids can be quantified either by a colorimetric determination after treatment with concentrated sulfuric acid and carbazole or by gas chromatography after methanolysis and subsequent acetylation. Both methods suffer from incomplete hydrolysis, an unavoidable degradation of the products to be analysed, and an inability to separate and quantify different types of uronic acids. In the present work, the fundamental chemistry involved in the two methods has been evaluated, and some modifications to increase their accuracy are suggested. By combining the two methods, a complete quantification of all individual types of urome acids present in a sample can be achieved.

Keyword
uronic acid; carbazole; methanolysis; 4-O-methylglucuronic acid; galacturonic acid; glucuronic acid; hexenuronic acid; xylan
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-7714 (URN)10.1016/j.carres.2007.03.031 (DOI)000248370800007 ()2-s2.0-34250650112 (Scopus ID)
Note

QC 20100702

Available from: 2007-11-23 Created: 2007-11-23 Last updated: 2016-12-19Bibliographically approved
4. The effect of black liquor exchange in the kraft cook on the tensile properties of Eucalyptus urograndis kraft pulp
Open this publication in new window or tab >>The effect of black liquor exchange in the kraft cook on the tensile properties of Eucalyptus urograndis kraft pulp
2009 (English)In: O Papel cellulose, papel, impressão, ISSN 0031-1057, Vol. 70, no 4, 35-50 p.Article in journal (Refereed) Published
Abstract [en]

Xylan addition to kraft cooking has been shown to increase the tensile strength of the pulp but may also lead to a densification of the sheet. The main objective of this study was to correlate the molecular properties of black liquor xylan with its ability to increase tensile strength at a given sheet density. Black liquors were obtained from kraft cooks performed at different cooking temperatures and cooking times, which resulted in dissolved xylan with different degrees of substitution of uronic acids, different molecular weights, and connected to different amounts of lignin. These black liquors were added to the later part of subsequent kraft cooks. The results show that the degree of substitution of the black liquor xylan is of great importance when using it as an additive to the kraft cook in order to enhance pulp strength. Both, improved and deteriorated tensile strengths at given sheet densities were seen as a result of the black liquor exchange, depending on the charge of the added xylan. This illustrates that exchanging black liquor is an attractive tool in order to improve the physical properties of the produced paper if it is done wisely, but may cause impairments if the exchange is conducted in an incorrect manner. Furthermore, novel analytical tools made it possible to detect much higher uronic acid contents in xylan than previously reported.

Keyword
4-O-methyl-D-glucuronic Acid, Eucalyptus urograndis xylan, Hexenuronic acid, sheet density, Tensile strength
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-7715 (URN)2-s2.0-67649977326 (Scopus ID)
Note

QC 20100702

[O efeito da troca de licor negro em cozimento kraft nas propriedades de tração de polpa kraft de eucalyptus urograndis]

Available from: 2007-11-23 Created: 2007-11-23 Last updated: 2014-10-06Bibliographically approved
5. Adsorption of hardwood black liquor xylan on cellulose
Open this publication in new window or tab >>Adsorption of hardwood black liquor xylan on cellulose
(English)Manuscript (Other academic)
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-7716 (URN)
Note
QC 20100702Available from: 2007-11-23 Created: 2007-11-23 Last updated: 2011-02-04Bibliographically approved

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