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Extended Impregnation Kraft Cooking of Softwood: Effects on Reject, Yield, Pulping Uniformity, and Physical Properties

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Extended Impregnation Kraft Cooking of Softwood: Effects on reject, yield, pulping uniformity, and physical properties Katarina Karlström Licentiate thesis Royal Institute of Technology (KTH) Department of Fibre and Polymer Technology Division of Wood Chemistry and Pulp Technology Stockholm 2009 TRITA-CHE-Report 2009:59 ISSN 1654-1081 ISBN 978-91-7415-496-2 Extended impregnation kraft cooking of softwood: Effects on reject, yield, pulping uniformity, and physical properties Katarina Karlström AKADEMISK AVHANDLING Som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknologie licentiatexamen fredagen den 18:e december 2009, kl. 10.00 i STFI-salen, Innventia AB, Drottning Kristinas väg 61, Stockholm. Avhandlingen försvaras på svenska. © Katarina Karlström Stockholm 2009 Department of Fibre and Polymer Technology Teknikringen 56-58 SE-100 44 Stockholm Sweden Abstract Converting wood into paper is a complex process involving many different stages, one of which is pulping. Pulping involves liberating the wood fibres from each other, which can be done either chemically or mechanically. This thesis focuses on the most common chemical pulping method, the kraft cooking process, and especially on a recently developed improvement of the impregnation phase, which is the first part of a kraft cook. Extended impregnation kraft cooking (EIC) technique is demonstrated to be an improvement of the kraft pulping process and provides a way to utilize softwood to a higher degree, at higher pulp yield. We demonstrate that it is possible to produce softwood ( Picea abies ) kraft pulp using a new cooking technique, resulting in a pulp that can be defibrated without inline refining at as high lignin content as 8% on wood, measured as kappa numbers above 90. Lignin is the wood constituent that holds the wood fibres together in the wood matrix. The new cooking technique uses the differences in reaction rate between the diffusion and consumption of hydroxide ions; it is used to ensure a homogenous impregnation of wood chips at lower impregnation temperatures and longer impregnation times than are generally used in the industry. The applied cooking temperatures are also substantially lower than those used in conventional kraft pulping systems, promoting uniform delignification. This results in a narrower kappa number distribution than in lab-cooked conventional kraft pulp. High-kappa-number pulps were investigated for pulp sheet properties such as tensile strength, tensile stiffness, and compression strength. It was demonstrated that an EIC pulp of kappa number 95 has strength properties comparable to those of a conventional pulp of kappa number 82. Comparing the effects of starch multilayers on conventional and EIC pulps reveals similar effects. The use of the starch multilayer treatment increased the tensile index and decreased the tensile stiffness and short-span compression test (SCT) indices. The EIC technique has also been used to produce a series of bleachable-grade pulps. The results indicate the possibility of increasing the lignin content of the pulp entering the oxygen delignification stage, since the reject content of gently defibered pulp is lower than 0.1% at kappa number 49. In this thesis, we recommend that wood chips be impregnated for 2 h at 110 °C to neutralize acidic compounds in the wood and impregnate the chips with cooking chemicals, and that the ensuing cook be performed at 135–140 °C, depending on the target kappa number. We also recommend increasing the available amounts of cooking chemicals in the impregnation stage by using a higher liquor-to-wood ratio and keeping the alkali profile fairly high in the ensuing cook. This concept will reduce the amount of reject material, increase the pulping uniformity, and increase the selectivity towards lignin degradation in the kraft cook. Sammanfattning Omvandling av ved till papper är en komplicerad process som består av många olika steg där ett är massaframställningen (eng. pulping ). Massaframställning medför att vedfibrerna frigörs från varandra på kemisk eller mekanisk väg. Denna avhandling fokuserar på den vanligaste kemiska metoden, sulfatkokning och speciellt den nyligen utvecklade förbättringen av impregnerings fasen, som är den första delen av ett sulfatkok. Här visas att Extended Impregnation kraft Cooking (EIC) innebär en förbättring av sulfatkokningen och ett sätt att uppnå högre vedutnyttjande vid högre utbyte för barrved. Vi visar att det är möjligt att producera barrvedsmassa med en ny kokningsprincip som resulterar i en massa som är defibrerbar utan inline-raffinering vid så högt lignin innehåll som 8% (på ved), mätt som kappatal över 90. Lignin är den vedkomponent som håller ihop vedfibrerna i vedmatrisen. Kokningsprincipen utnyttjar skillnaderna i reaktionshastighet mellan diffusion och konsumtion av hydroxidjoner och nyttjas till att skapa en homogen impregnering av vedflisen vid lägre impregneringstemperatur och under längre tid än vad som vanligen används i industrin. De använda koktemperaturerna är också betydligt lägre än vid konventionell sulfatkokning vilket gynnar jämn delignifiering. Detta resulterar i en smalare kappatalsfördelning jämfört med laboratoriekokade konventionella massor. Massor med höga kappatal undersöktes med avseende på egenskaper hos handark, såsom dragstyrka, dragstyvhet och kompressionsstyrka Det visades att handark från EIC massa vid kappatal 95 hade jämförbara styrkeegenskaper med konventionell massa vid kappatal 82. Vid jämförelse av effekten av stärkelse multilager på konventionella och EIC massor avslöjar liknande effekter. Användningen av stärkelsemultilager ökade dragindex och minskade dragstyvhets- och kompressions index (SCT, short-compression test). Kokprincipen har även använts för att ta fram en serie blekbara massor. Resultaten visar på möjligheten att öka lignininnehållet i massan in till i syrgasdelignifierings-steget eftersom spetinnehållet för milt defibrerad massa var lägre än 0,1% vid kappatal 49. I den här avhandlingen rekommenderar vi att vedflis impregneras i 2 timmar vid 110 °C för att neutralisera sura komponenter i veden och impregnera flisen med kokkemikalier, samt att utföra det efterföljande koket vid 135–140 °C beroende på önskat kappatal. Vi rekommenderar även att öka den tillgängliga mängden kokkemikalier i impregneringssteget genom att använda högre vätske-ved förhållande och att hålla alkali profilen relativt hög i det efterföljande koket. Detta koncept reducerar spetmängden, ger jämnare kokning och ökar selektiviteten för nedbrytning av lignin i sulfatkoket. List of papers This thesis is based on the following papers and on unpublished data not presented in the papers. The included papers will be referred to in the text by their roman numerals. I Extended impregnation cooking of Norway spruce: effects on yield, reject, pulping uniformity, and strength properties. Katarina Karlström, Elisabeth Brännvall, and Mikael E. Lindström Manuscript II Applying a novel cooking technique to produce high kappa number pulps – the effects on physical properties Stefan Antonsson, Katarina Karlström and Mikael E. Lindström Accepted for publication in Nordic Pulp Paper Res. J. , 24 (4) (2009) III The relationship between hygroexpansion, tensile stiffness, and mechano– sorptive creep in bleached hardwood kraft pulps Stefan Antonsson, Iiro Pulkkinen, Juha Fiskari, Katarina Karlström, and Mikael E. Lindström Submitted to Appita Journal Author’s contribution to the appended papers: I Principal author – planned and performed the largest part of the experiments II Co-author – planned and performed roughly half of the experiments III Co-author – gave advice on the manuscript and rewrote part of the manuscript after comments from Appita Journal Relevant conference papers not included in this thesis: Karlström, K., and Lindström, M. E. (2008): Moving the defibration point towards higher kappa numbers with a novel cooking technique. The Second Workshop on Chemical Pulping Processes , Karlstad, Sweden, 11–15. Karlström, K., Brännvall, E., and Lindström, M. E. (2009): Higher degree of softwood resource utilisation. Johan Gullichsen Colloquium , Espoo, Finland . Contents INTRODUCTION...........................................................................................................................1 KRAFT PULPING .................................................................................................................................................. 1 Kraft liner pulp.............................................................................................................................................. 2 THE COMPOSITION OF WOOD .............................................................................................................................. 2 Cellulose........................................................................................................................................................ 3 Hemicelluloses............................................................................................................................................... 3 Lignin ............................................................................................................................................................ 3 Wood fibre morphology................................................................................................................................. 4 Wood chips and penetration of liquid...........................................................................................................
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