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Optimizing kraft cooking; pulp yield vs. HexA content and the effect of HexA content after cooking on the bleaching chemical requirement
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2007 (English)In: O Papel, ISSN 0031-1057, Vol. 68, no 6, 64-85 p.Article in journal (Refereed) Published
Abstract [en]

Using modern cooking concepts for the manufacture of hardwood kraft pulp, a higher pulp yield is obtained, mainly as a result of high xylan retention. This often leads to a somewhat higher HexA content of the pulp at a given kappa number. Pulp yield is the most important factor for cost-effective pulp manufacture. Hence, cooking to a low pulp yield in order to reduce the HexA content is not an attractive alternative. Questions have been raised as to whether a pulp with a high HexA content requires more bleaching chemicals to reach a given ISO brightness or shows a higher yellowing. The results of this study show that a high HexA content after cooking may have a positive effect in achieving a low bleaching chemical requirement in a contemporary bleaching sequence. The pulps having a high HexA content after cooking did not show a higher yellowing tendency than the pulps originally having a low HexA content. Optimization of the kraft cook in a hardwood pulp mill towards a high yield can therefore be made without penalties in the form of an increased bleaching chemical consumption, but instead often leads to lower bleaching chemical consumption.

Place, publisher, year, edition, pages
2007. Vol. 68, no 6, 64-85 p.
Keyword [en]
Bleaching, Hexenuronic acid content, Kraft cooking, Oxygen delignification, Pulp yield
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-6618Scopus ID: 2-s2.0-36348962469OAI: oai:DiVA.org:kth-6618DiVA: diva2:11375
Note

QC 20120316. Updated from manuscript to article in journal.

[Otimização do cozimento kraft; rendimento da polpa em relação ao teor de HexA e o efeito do teor de HexA pós cozimento na demanda de produtos químicos de branqueamento]

Available from: 2006-12-13 Created: 2006-12-13 Last updated: 2014-11-10Bibliographically approved
In thesis
1. 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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