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Extended impregnation in the kraft cook: an approach to improve the overall yield in eucalypt kraft pulping
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
2010 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, 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.

Place, publisher, year, edition, pages
2010. Vol. 25, no 1, 7-14 p.
Keyword [en]
Yield, Kraft pulping, Eucalyptus, Oxygen delignification, Bleaching, Carbohydrate, Xylan, Cellulose
National Category
Paper, Pulp and Fiber Technology
URN: urn:nbn:se:kth:diva-27568DOI: 10.3183/NPPRJ-2010-25-01-p007-014ISI: 000278043500001ScopusID: 2-s2.0-84855522574OAI: diva2:377923
QC 20101215Available from: 2010-12-15 Created: 2010-12-13 Last updated: 2012-05-07Bibliographically approved
In thesis
1. Aspects of extended impregnation kraft cooking for high-yield pulping of hardwood
Open this publication in new window or tab >>Aspects of extended impregnation kraft cooking for high-yield pulping of hardwood
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.
Trita-CHE-Report, ISSN 1654-1081 ; 2012:21
Yield, pulp, kraft cook, carbohydrate, xylan, oxygen delignification, Hexenuronic acid, kappa number
National Category
Paper, Pulp and Fiber Technology
urn:nbn:se:kth:diva-94010 (URN)978-91-7501-336-7 (ISBN)
Public defence
2012-05-25, KTH sal F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
QC 20120507Available from: 2012-05-07 Created: 2012-05-04 Last updated: 2012-05-07Bibliographically approved

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