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On the Interrelation Between Kraft Cooking Conditions and Pulp Composition
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
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 [en]
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: urn:nbn:se:kth:diva-4232OAI: oai:DiVA.org:kth-4232DiVA: diva2:11377
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
List of papers
1. A Study of How the Amount of Residual Phase Lignin in Kraft Cooking Depends upon the Conditions in the Cook
Open this publication in new window or tab >>A Study of How the Amount of Residual Phase Lignin in Kraft Cooking Depends upon the Conditions in the Cook
1997 (English)In: Nordic Pulp Paper Research Journal, ISSN 0283-2631, Vol. 12, no 4, 225- p.Article in journal (Refereed) Published
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-12996 (URN)
Note
QC 20100521Available from: 2010-05-21 Created: 2010-05-21 Last updated: 2010-08-25Bibliographically approved
2. Temperature dependence of residual phase delignification during kraft pulping of softwood
Open this publication in new window or tab >>Temperature dependence of residual phase delignification during kraft pulping of softwood
2000 (English)In: Nordic pulp and paper research journal, Vol. 15, 12-17 p.Article in journal (Refereed) Published
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6613 (URN)
Note
QC 20100825Available from: 2006-12-13 Created: 2006-12-13 Last updated: 2010-08-25Bibliographically approved
3. The influence of cooking conditions on the degradation of hexenuronic acid, xylan, glucomannan and cellulose during kraft cooking of softwood
Open this publication in new window or tab >>The influence of cooking conditions on the degradation of hexenuronic acid, xylan, glucomannan and cellulose during kraft cooking of softwood
2000 (English)In: Nordic pulp and paper resarch journal, ISSN 0283-2631, Vol. 15, 160-167 p.Article in journal (Refereed) Published
Abstract [en]

The influence of hydroxyl ion concentration, hydrogen sulfide ion concentration, ionic strength and temperature on the dissolution/degradation of hexenuronic acid, xylan, glucomannan and cellulose in kraft pulping of Norweigan spruce :vas studied. A mathematical model of the dissolution/degradation of hexenuronic acid was developed. Approximate rate constants for glucomannan and xylan were also developed. The rate of dissolution/degradation of hexenuronic acid, xylan and glucomannan increases with increasing [HO-], increasing [HS-] and increasing temperature. A higher ionic strength, estimated as [Na+], leads to a slight decrease in the rate of degradation of xylan and an increase in the rate of degradation of hexenuronic acid. The lower hexenuronic acid content found when pulping with a high [HO-], a high ionic strength, a low temperature and a low [HS-] is mainly a result of the prolonged cooking time required to reach the same target kappa level, although the prolonged cooking time seems not to be the only explanation for the lower hexenuronic acid content at a given kappa number.

Keyword
carbohydrates; cellulose; glucomannan; hexenuronic acid; hydrogen sulfide ion concentration; hydroxyl ion concentration; Picea abies; ionic strength; temperature; xylan
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6614 (URN)000088199800013 ()
Note
QC 20100825Available from: 2006-12-13 Created: 2006-12-13 Last updated: 2010-12-06Bibliographically approved
4. Formation and dissolution/degradation of hexenuronic acids during kraft pulping of Eucalyptus Globulus
Open this publication in new window or tab >>Formation and dissolution/degradation of hexenuronic acids during kraft pulping of Eucalyptus Globulus
2001 (English)In: 7th Brazilian Symposium on the Chemistry of Lignins and other Wood ComponentsArticle in journal (Refereed) Published
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6615 (URN)
Note
QC 20100825Available from: 2006-12-13 Created: 2006-12-13 Last updated: 2010-08-25Bibliographically approved
5. Estimation of kraft cooking yield
Open this publication in new window or tab >>Estimation of kraft cooking yield
2003 (English)In: 12th International Symposium on Wood and Pulping Chemistry (ISWPC)Article in journal (Refereed) Published
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6616 (URN)
Note
QC 20100825Available from: 2006-12-13 Created: 2006-12-13 Last updated: 2010-08-25Bibliographically approved
6. The influence of cooking conditions on the bleachability and chemical structure of kraft pulps
Open this publication in new window or tab >>The influence of cooking conditions on the bleachability and chemical structure of kraft pulps
1999 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 14, no 1, 71-81 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this investigation was primarily td investigate how variations in cooking conditions in the kraft pulping of softwood influence the subsequent bleaching, and secondly to study the relationship between the bleaching response and the chemical structure of the pulp. The cooking variables studied were hydroxide ion concentration, hydrogen sulfide ion concentration and cooking temperature. The pulps had the same kappa number after the cook, about 20, and were oxygen delignified to about kappa number 8 before bleaching. The influence of the cooking variables on the TCF-bleachability was studied in an AZQP*- and in a QPQP*-sequence (A=acid treatment, Z=ozone stage, Q=chelating agent stage, P*=peroxide stage with the addition of magnesium ions). All three cooking variables studied influenced the TCF-bleachability, but to different extents. The bleachability was improved by increased temperature for low chemical charges, but not at higher chemical charges. When [HS-] was increased the QPQP*-bleachability was improved but the AZQP*-bleachability was not affected. When [HO-] was varied a bleachability maximum was seen for the intermediate hydroxide ion concentration. The content of hexenuronic acid in the pulp after cooking could be reduced by using high initial [HO-], low initial [HS-] and a long cooking time. A high content of beta-O-4 structures in the unbleached residual lignin was found to contribute to a better bleachability of the pulp. However, the phenolic hydroxyl content could not be related to the bleaching response.

Keyword
bleachability, chlorine-free bleaching, softwoods, kraft pulps, hydrogen sulfide ion, hydroxide ion, temperature, beta-O-4, hexenuronic acid, chemical structures, HEXENEURONIC ACID GROUPS, TCF-BLEACHABILITY, LIGNINS, XYLAN
National Category
Materials Engineering Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-13904 (URN)000079757500009 ()
Note
QC 20100701Available from: 2010-07-01 Created: 2010-07-01 Last updated: 2017-12-12Bibliographically approved
7. Optimizing kraft cooking; pulp yield vs. HexA content and the effect of HexA content after cooking on the bleaching chemical requirement
Open this publication in new window or tab >>Optimizing kraft cooking; pulp yield vs. HexA content and the effect of HexA content after cooking on the bleaching chemical requirement
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.

Keyword
Bleaching, Hexenuronic acid content, Kraft cooking, Oxygen delignification, Pulp yield
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6618 (URN)2-s2.0-36348962469 (Scopus ID)
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
8. On the nature of residual lignin
Open this publication in new window or tab >>On the nature of residual lignin
2004 (English)In: Cellulose chem. techmol., ISSN 0576-9787, Vol. 35, 321-331 p.Article in journal (Refereed) Published
Abstract [en]

In a recent study, the results of oxygen delignification trials on kraft, prehydrolysis kraft and magnesium sulphite pulps were compared, thus providing the basis for a theory on the nature of the lignin residual lignin remaining in pulp after cooking, and particularly after oxygen delignification, the so-called residual lignin. It is suggested that, at least after oxygen delignification, the residual lignin effectively consists of LCC-bound monolignins, a theory relating to the efficiency of oxygen delignification and bleaching, and indicating possible strategies for improving the efficiency of such processes. An efficient means of breaking the LCC bonds would lead to a more efficient bleaching, known as requiring either a complete oxidation of the LCC-bound monolignins or a peeling off the carbohydrate binding to the LCC. In any case, a mere breaking of one unsaturated bond in an aromatic ring is not sufficient to achieve depolymerisation of the residual lignin or an efficient kappa reduction.

Keyword
residual lignin; lignin-carbohydrate complex (LCC); oxygen delignification; bleaching; oxygen; chlorine dioxide; ozone; hydrogen peroxide; peeling
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-6619 (URN)000229625200002 ()2-s2.0-18944405373 (Scopus ID)
Note
QC 20100825 QC 20110923Available from: 2006-12-13 Created: 2006-12-13 Last updated: 2011-09-23Bibliographically approved

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