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Aspects of extended impregnation kraft cooking for high-yield pulping of hardwood
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The long-term trend regarding wood is an increase in price. Because wood contributes to a large part of production costs, the efficient utilisation of wood is greatly desired to reduce production costs for kraft pulp producers. During the 1990s, the development of improved modified kraft cooking began, which led to higher yields. There was also a trend of terminating kraft cooking at a higher kappa number to maximise the overall yield. For hardwood, the defibration point became a critical setback in allowing this termination at a high kappa number. This thesis discusses how this issue has been tackled in the laboratory by using improved modified kraft cooking combined with extended impregnation to enable a decrease in reject content and shift the defibration point towards a higher kappa number for hardwood. This lab concept is referred to as extended impregnation kraft cooking (EIC), and this thesis reveals that EIC cooking efficiently reduces the reject content for both birch and eucalypt. By using EIC cooking, the defibration point was shifted to a kappa number of ca. 30 from ca. 20 using conventional kraft cooking. This study demonstrates the great potential for achieving a higher overall yield for eucalypt by terminating the EIC cooking at a high kappa number, but with the conditions used in this thesis, no improvement in yield was observed for birch.

 

An important issue is that the termination of kraft cooking at high kappa number increases the demand for extended oxygen delignification to reach a similar kappa number into bleaching, i.e., due to cost and environmental reasons. Extended oxygen delignification was shown to be possible for both birch and eucalypt EIC pulps (i.e., from kappa number 27 to 10) with an acceptable pulp viscosity number.

 

The other part of this thesis addresses aspects regarding the limitations in oxygen delignification. It has previously been shown in the literature that a high xylan yield of kraft cooking could negatively affect the efficiency of subsequent oxygen delignification. In this work, the increased xylan content in eucalypt kraft pulp within the range of 8–18% had only a marginally negative impact on the oxygen delignification efficiency after correcting for the HexA contribution to the kappa number. It is also desired to extend the oxygen delignification towards lower kappa number, i.e., below kappa number 10 to decrease the bleaching chemical requirement. In this study, the hypothesis that the reduced efficiency of oxygen delignification at low kappa numbers could partly be due to the formation of oxidisable carbohydrate-related structures (i.e., HexA and/or other non-lignin structures) was also tested. No formation was established. On the other hand, a final oxygen delignification stage in the bleaching could be an attractive alternative for reducing yellowing and enhancing brightness; in fact, this has led to the development of a patent (SE 528066).

Abstract [sv]

Ved står för en stor del av produktionskostnaderna vid framställning av sulfatmassa. Då vedpriserna har ökat genom åren är ett effektivt utnyttjande av veden önskvärt för att kunna sänka produktionskostnaderna. Under 1990-talet förbättrades den modifierade sulfatkokningen vilket innebar möjlighet till högre massautbyte. För att maximera massautbytet styrdes kokningsprocessen mot ett högre kappatal. Detta har visat sig vara svårare för lövved än för barrved, eftersom defibrerbarhetspunkten utgör ett kritiskt hinder. I denna avhandling har laborationsstudier utförts där förbättrad modifierad sulfatkokning kombinerats med förlängd impregnering för att kunna sänka spethalten och därmed förskjuta defibrerbarhetspunkten mot ett högre kappatal. Detta koncept kallas för extended impregnation kraft cooking (EIC). EIC-kokning visade sig vara en effektiv metod för att minska spethalten hos björk och eukalyptus. Med EIC-kokning kunde defibrerbarhetspunkten höjas från cirka 20 till cirka 30. I denna avhandling klarläggs att det finns stora möjligheter att öka massautbytet för eukalyptus genom att avsluta sulfatkoket vid ett högre kappatal. För björk kunde ingen ökning av massutbytet uppnås genom ovanstående metod.

 

Vid ett högre kappatal efter sulfatkoket ställs även krav på förlängd syrgasdelignifiering, för att kunna behålla samma kappatal in till blekeriet. Det visade sig vara fullt möjligt att förlänga syrgasdelignifieringen för de EIC-kokade björk- och eukalyptusmassorna (d.v.s. från kappatal 27 till 10) med accepterad massaviskositet.

 

Den andra delen av avhandlingen tar upp aspekter på syrgasdelignifieringens begränsningar. Tidigare studier har visat att ett högre utbyte av xylan vid sulfatkokning kan vara negativt för syrgasdelignifieringens effektivitet.  I denna studie har det påvisats att en ökad xylanhalt i intervallet 8–18 procent i eukalyptusmassa endast har en marginell negativ inverkan på syrgasdelignifieringens effektivitet efter att kappatalet korrigerats för HexA. Det är önskvärt att förlänga syrgasdelignifieringen till ett lägre kappatal än 10 för att minska förbrukningen av blekkemikalier. I den här studien prövades hypotesen att syrgasdelignifieringens begränsningar vid låga kappatal, under 10, delvis skulle kunna bero på bildning av oxiderbara kolhydratrelaterade strukturer (d.v.s. HexA och/eller andra okända ”non-lignin”-strukturer). Ingen bildning kunde dock observeras. Däremot indikerades att ett syrgassteg i slutet av bleksekvensen skulle kunna vara ett eftersträvansvärt alternativ för minskad eftergulning och ökad ljushet, vilket ledde till ett patent (SE 528066).

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , 77 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:21
Keyword [en]
Yield, pulp, kraft cook, carbohydrate, xylan, oxygen delignification, Hexenuronic acid, kappa number
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-94010ISBN: 978-91-7501-336-7 (print)OAI: oai:DiVA.org:kth-94010DiVA: diva2:524998
Public defence
2012-05-25, KTH sal F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20120507Available from: 2012-05-07 Created: 2012-05-04 Last updated: 2012-05-07Bibliographically approved
List of papers
1. Extended impregnation in the kraft cook: an approach to improve the overall yield in eucalypt kraft pulping
Open this publication in new window or tab >>Extended impregnation in the kraft cook: an approach to improve the overall yield in eucalypt kraft pulping
2010 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 25, no 1, 7-14 p.Article in journal (Refereed) Published
Abstract [en]

A potential way to improve the overall yield is to terminate the kraft cook at higher kappa number. This method was investigated using Extended Impregnation kraft Cook (EIC). As a reference, kraft pulp manufactured by conventional kraft cooking was used. By utilising the EIC concept, the defibration point in the kraft cook is shifted towards higher kappa numbers, resulting in a lower reject content at a given kappa number. This enables a termination of the ETC cook at higher kappa number. The results show that an overall yield gain of 2.6 percentage units could be achieved using ETC with higher cooking kappa number followed by extended oxygen delignification and bleaching with a D*(OP)D sequence to full brightness. However, the EIC concept at higher cooking kappa number resulted in somewhat higher bleaching chemical requirement. Extended oxygen delignification was shown to be possible to reach as low a kappa number as 10, but a high cooking kappa number is clearly a disadvantage if a high limiting viscosity number after oxygen delignification is required.

Keyword
Yield, Kraft pulping, Eucalyptus, Oxygen delignification, Bleaching, Carbohydrate, Xylan, Cellulose
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-27568 (URN)10.3183/NPPRJ-2010-25-01-p007-014 (DOI)000278043500001 ()2-s2.0-84855522574 (Scopus ID)
Note
QC 20101215Available from: 2010-12-15 Created: 2010-12-13 Last updated: 2017-12-11Bibliographically approved
2. Further insights into extended-impregnation kraft cooking of birch
Open this publication in new window or tab >>Further insights into extended-impregnation kraft cooking of birch
Show others...
2012 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 27, no 5, 890-899 p.Article in journal (Refereed) Published
Abstract [en]

Extended-impregnation kraft cooking (EIC) is a cooking concept that combines prolonged impregnation with modern improved modified kraft cooking. In the current investigation, the EIC cooking of birch was studied in relation to conventional kraft cooking. Specifically, the reject content and carbohydrate yield retention when terminating at a high cooking kappa number were examined. It was demonstrated that EIC cooking reduced the reject content. Unexpectedly, a high cooking kappa number led to no increase in carbohydrate yield, possibly due to the chemical composition of birch wood and the EIC cook lab procedure. A large amount of liquor was withdrawn after the impregnation, resulting in a loss of dissolved xylan that otherwise could have redeposited on the fibres and contributed to the carbohydrate yield. The effects of EIC cooking on extended oxygen delignification, bleaching chemical requirement in a D(OP)DP sequence, and strength properties were also examined. Compared with conventional lab cooking, EIC cooking resulted in a lower bleaching chemical requirement and similar strength properties.

Keyword
Bleaching, Carbohydrate, Oxygen delignification, Physical properties, Pulp, Reject, Xylan, Yield
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-94034 (URN)10.3183/NPPRJ-2012-27-05-p890-899 (DOI)000313375400008 ()2-s2.0-84871725288 (Scopus ID)
Note

QC 20130116. Updated from submitted to published.

Available from: 2012-05-07 Created: 2012-05-07 Last updated: 2017-12-07Bibliographically approved
3. Influence of extended-impregnation kraft cooking on the xylan structure of Eucalyptus urograndis
Open this publication in new window or tab >>Influence of extended-impregnation kraft cooking on the xylan structure of Eucalyptus urograndis
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-94035 (URN)
Note
QS 2012Available from: 2012-05-07 Created: 2012-05-07 Last updated: 2012-05-07Bibliographically approved
4. Influence Of Xylan Content On The Oxygen Delignification Performance Of Eucalypt Kraft Pulps As Studied Using Prehydrolysis And Xylanase Treatments
Open this publication in new window or tab >>Influence Of Xylan Content On The Oxygen Delignification Performance Of Eucalypt Kraft Pulps As Studied Using Prehydrolysis And Xylanase Treatments
2012 (English)In: BioResources, ISSN 1930-2126, E-ISSN 1930-2126, Vol. 7, no 4, 5527-5541 p.Article in journal (Refereed) Published
Abstract [en]

Common metrics for evaluating the efficiency of oxygen delignification include the kappa number and Klason lignin content. As a change in xylan content often leads to a change in HexA content, the kappa number must be corrected for the HexA contribution before evaluating the degree of oxygen delignification when trying to understand the process in detail. Questions could also be raised about the accuracy of the Klason lignin method for oxygen-delignified hardwood kraft pulps, since the amount of residual lignin is small in such pulp. This study investigates the influence of xylan content on oxygen delignification efficiency in Eucalyptus urograndis kraft pulps. Xylan content was varied using two methods: treatment with xylanase and with acid prehydrolysis for various times before kraft cooking. The degree of oxygen delignification, expressed as the HexA-corrected kappa number, indicated no significant trend with xylan removal, and no significant trend was evident when expressed as Klason lignin content.

Keyword
Pulp, Hardwood, Xylan, Xylanase, Prehydrolysis, Alkalinity, Oxygen delignification
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-94079 (URN)000311336800085 ()2-s2.0-84872760176 (Scopus ID)
Note

QC 20130109. Updated from submitted to published.

Available from: 2012-05-07 Created: 2012-05-07 Last updated: 2017-12-07Bibliographically approved
5. On the role of carbohydrates in oxygen delignification
Open this publication in new window or tab >>On the role of carbohydrates in oxygen delignification
2005 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 20, no 4, 448-452 p.Article in journal (Refereed) Published
Abstract [en]

The decrease in efficiency of the oxygen delignification of chemical pulps at low kappa numbers has been investigated. In order to study the role of the carbohydrates, five different fully bleached pulps were oxygen delignified and carefully analysed. The results show that no permanganate-consuming structures were formed in the carbohydrates during oxygen delignification, but that small amounts of lignin are slowly degraded. Unexpectedly, the ISO brightness was significantly increased, while the yellowing tendency was reduced, in some cases dramatically, making a final oxygen delignification treatment after a bleaching sequence an interesting option for market pulp producers with yellowing problems.

Keyword
oxygen delignification, carbohydrates, cellulose, hemicellulose, xylan, glucomannan, kappa number, holocellulose, bleached kraft pulp, cotton linters, brightness, yellowing tendency, softwood kraft pulp, residual lignin, chemical pulps, kappa number, cooking, removal, alkali, bonds
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-15321 (URN)000235258200012 ()2-s2.0-30344433761 (Scopus ID)
Note
QC 20100525 QC 20111205Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved

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