On the interrelation between kraft cooking conditions and pulp composition
Catrin Gustavsson
Doctoral Thesis
Royal Institute of Technology Department of Fibre and Polymer Technology
Division of Wood Chemistry and Pulp Technology
Stockholm 2006
On the interrelation between kraft cooking conditions and pulp composition
Supervisors: Associate Professor Mikael E. Lindström
Adjunct Professor Martin Ragnar
Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknologie doktorsexamen fredagen den 15 december 2006 kl. 14.00 i Sal F3, KTH, Lindstedtsvägen 26. Avhandlingen försvaras på engelska. © Catrin Gustavsson 2006 TRITA-FPT-Report 2006:39 ISSN 1652-2443 ISRN KTH/FPT/R-2006/39-SE
Abstract 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. Keywords: Delignification, Kraft pulping, Residual phase lignin, Hydroxide, Hydrogen sulphide ion, Ionic strength, Temperature, Bleachability, Hexenuronic acid, Carbohydrates
Sammanfattning I början av 1990-talet utfördes mycket arbete med fokus på förlängd delignifiering av sulfatkoket till låga ligninhalter (lågt kappatal). Drivkraften bakom denna utveckling var en önskan om att ytterligare minska miljöpåverkan från blekningen eftersom mindre arbete skulle krävas där. Delignifieringen under sulfatkokets restfas är emellertid väldigt långsam och på grund av sin dåliga selektivitet, en begränsade faktor för borttagandet av lignin i koket. Om mängden lignin som reagerar enligt restfasen kunde reduceras skulle det vara möjligt att förbättra sulfatkokets selektivitet. I denna avhandling har speciellt intresse ägnats åt att bestämma aktiveringsenergin för restfasen vid sulfatkokning av barrved och för hur olika kokparametrar påverkar mängden restfaslignin. Aktiveringsenergin för restfasdelignifieringen under sulfatkoket visade sig vara högre än den för bulkfasdelignifieringen. För att kunna minska mängden restfaslignin var det nödvändigt att ha en hög vätesulfidjonkoncentration när kokningen utfördes med en låg hydroxidkoncentration. Det var också viktigt att undvika en hög koncentration av natriumjon när koket utfördes vid låga hydroxid- och vätesulfidjonkoncentrationer. Dessutom visar undersökningen att närvaron av utlöst vedsubstans påverkade delignifieringen positivt under sulfatkokets bulkfas. Hur olika kokparametrar i det förlängda barrsulfatkoket påverkar massans blekbarhet (d.v.s. hur lätt en massa kan uppnå en viss ljushet) har också undersökts. Ifall variationer i blekbarhet kunde påvisas var avsikten att försöka finna kemiska förklaringar till dessa. Det visade sig emellertid svårt att identifiera några tydliga korrelationer mellan restligninets kemiska struktur och massans blekbarhet. Dock kunde det konstateras att ju högre innehåll av β-aryleterstrukturer i restligninet efter koket desto bättre var QPQP*-blekbarhet. I mitten/slutet av 1990-talet skedde inom massaindustrin ett paradigmskifte i det att fokus flyttades från förlängd kokning till ett effektivt råvaruutnyttjande, d.v.s. att avbryta sulfatkoket vid högre kapptalsnivåer och välja lämpliga kokbetingelser för att maximera kokutbytet. Ett högt kokutbyte ger oftast en något högre mängd hexenuronsyra (HexA) i massan jämfört vid ett visst kappatal. Följaktligen lades extra vikt på hur mängden HexA och kolhydratssammansättnigen påverkades av ett antal kokparameterar. Vid genomförandet av dessa studier var det även viktigt att undersöka hur en strategi med låg HexA-halt (efter koket) påverkade centrala faktorer såsom kokutbyte, blekbarhet och eftergulning hos den framställda massan. Det visade sig vara svårt att markant minska mängden HexA i sulfatkoket genom att ändra kokbetingelserna för såväl barrved som lövveden Eucalyptus Globulus. En minskning av HexA-halten kunde åstadkommas genom att förlänga koket till låga kappatal eller genom att använda en hög hydroxidkoncentration, en låg vätesulfidjonkoncentration eller en hög koncentration av natriumjon. Ingendera av dessa strategier är emellertid attraktiva att tillämpa industriellt eftersom samtliga skulle leda till en omfattande förlust av utbyte och en försämrad viskositet liksom sämre styrkeegenskaper. Nyckelord: Delignifiering, Sulfatkokning, Restfaslignin, Hydroxid, Vätesulfid, Jonstyrka, Temperatur, Blekbarhet, Hexenuronsyra, Kolhydrater
Table of contents
1. LIST OF PAPERS .................................................................................................. 9
2. INTRODUCTION .................................................................................................. 11
2.1 Wood as raw material for pulp and paper.................................................................................................. 11
2.2 The composition of wood .............................................................................................................................. 13
2.3 Kraft Cooking................................................................................................................................................ 15 2.3.1 Delignification ......................................................................................................................................... 16 2.3.2 Degradation/dissolution of carbohydrates................................................................................................ 19 2.3.3 Hexenuronic acid ..................................................................................................................................... 20 2.3.4 Evaluation of mill cooking yield.............................................................................................................. 21
2.4 Bleaching of pulp........................................................................................................................................... 22 2.4.1 Bleachability ............................................................................................................................................ 23
2.5 The aim of this thesis..................................................................................................................................... 24
3. RESULTS AND DISCUSSION............................................................................. 25
3.1 Delignification kinetics of softwood (Paper I and II) ................................................................................. 25 3.1.1 A model that describes how the amount of residual phase lignin in spruce depends upon the cooking conditions (Paper I) ........................................................................................................................................... 26 3.1.2 Temperature-dependence of residual phase delignification (Paper II)..................................................... 30 ”Constant composition” cooks.......................................................................................................................... 31 ”Normal” cooks................................................................................................................................................. 32
3.2 The degradation of carbohydrates in kraft cooking and evaluation of mill cooking yield (Paper III, IV, V) .......................................................................................................................................................................... 35
3.2.1 The formation and dissolution/degradation of HexA in softwood kraft cooking (Paper III) ................... 36 3.2.2 The formation and dissolution/degradation of HexA in Eucalyptus kraft cooking (Paper IV) ................ 39 3.2.3 Dissolution/degradation of glucomannan, xylan, and cellulose in softwood kraft cooking (Paper III) ... 43 3.2.4 Estimation of mill cooking yield (Paper V) ............................................................................................. 46
3.3 Bleachability of softwood and Eucalyptus kraft pulps (Paper VI, VII, VIII).......................................... 49 3.3.1. The influence of cooking condition on the bleaching chemical requirement and chemical structure of softwood kraft pulps (Paper VI)........................................................................................................................ 51 3.3.6 Pulp yield vs. HexA content and the effect of HexA content after cooking on the bleaching chemical requirement (Paper VII, VIII) ........................................................................................................................... 59
4. CONCLUSIONS ................................................................................................... 69
4.1 General conclusions ...................................................................................................................................... 69
4.2 Industrial applicability ................................................................................................................................. 71
4.3 Looking into the future................................................................................................................................. 71
5. A GUIDE TO ABBREVIATIONS AND TECHNICAL TERMS .............................. 73
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6. NOMENCLATURE IN BLEACHING STAGES..................................................... 76
7. ACKNOWLEDGMENTS....................................................................................... 77
8. REFERENCES ..................................................................................................... 79
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1. List of papers
This thesis is based upon the following papers, referred to in the text by Roman numerals I-VIII:
I. A study of how the amount of residual phase lignin in kraft cooking depends upon the conditions in the cook Gustavsson, A-S.C. Lindgren, C.T. and Lindström, M.E., Nordic Pulp Paper Res. J.: 12(4), 225-229, (1997).
II. Temperature dependence of residual phase delignification during kraft pulping of softwood Blixt, J. and Gustavsson, C., Nordic Pulp Paper Res. J.: 15(1), 12-17, (2000).
III. The influence of cooking conditions on the degradation of hexenuronic acid, xylan, glucomannan and cellulose during kraft cooking of softwood Gustavsson, C. and Al-Dajani, W., Nordic Pulp Paper Res. J.: 15(2), 160-167, (2000).
IV. Formation and dissolution/degradation of hexenuronic acids during kraft pulping of Eucalyptus Globulus Ek, M., Gustavsson, C., Kadiric, J. and Teder, A., 7th Brazilian Symposium on the Chemistry of Lignins and other Wood Components, Belo Horizonte, Brazil, 99-106, (2001).
V. Estimation of kraft cooking yield Gustavsson, C., Näsman, M., Brännvall, E. and Lindström, M.E., 12th International Symposium on Wood and Pulping Chemistry (ISWPC), Madison, USA, Vol 2, 17-20, (2003).
VI. The influence of cooking conditions on the bleachability and chemical structure of kraft pulps Gustavsson, C., Sjöström, K. and Al-Dajani,W., Nordic Pulp Paper Res.J.: 14(1), 71-81, (1999).
VII. Optimising kraft cooking; pulp yield vs. HexA content and the effect of HexA content after cooking on the bleaching chemical requirement Gustavsson, C. and Ragnar, M., submitted to J. Pulp Paper Sci. (2006).
VIII. On the nature of residual lignin Backa, S., Gustavsson C., Lindström, M.E. and Ragnar, M., Cellul. Chem. Technol.: 38(5-6), 321-331, (2004).
These publications are appended to this thesis.
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Other related material is found in:
• Gustavsson, C., Sjöström, K. and Al-Dajani, W. (1998): The influence of cooking conditions on the bleachability and chemical structure of kraft pulps, International Pulp Bleaching Conference (IPBC), Helsinki, Finland, Book 1, 13-20.
• Gustavsson, C. and Ragnar, M. (2003): Brightness and HexA content after cooking and oxygen delignification – a statistical approach, 12th International Symposium on Wood and Pulping Chemistry (ISWPC), Madison, USA, Vol 2, 17-20.
• Gustavsson, C. and Ragnar, M. (2005): Bleaching chemical requirements and kappa number composition – optimising with regards to yield instead of HexA content, International Pulp Bleaching Conference (IPBC), Stockholm, Sweden, 244–247.
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2. Introduction
2.1 Wood as raw material for pulp and paper 105 A.D. is often cited as the year in which papermaking was invented by the Chinese, Ts’ai
Lun. Paper was first made from recycled materials such as rags made from linen and wool,
fishing nets, hemp and grass and it was not until the 19th century that paper began to be made
from wood, when the German Keller invented the Stone Groundwood pulping process (SGW)
and made it possible to liberate the wood fibres. In this method, the logs were soaked with
water and at the same time ground against a stone, one kind of mechanical pulping. Pulping is
the name given to any process by which wood (or other fibrous raw material) is reduced to a
fibrous mass. The cellulose fibres in wood are mainly bound together with lignin. The main
purpose of pulping is to liberate the fibres. This can be done either chemically or
mechanically, or by a combination of the two. During the chemical treatment, the lignin is
degraded and the degradation products are dissolved, a so-called delignifying process. Two
main chemical pulping processes exist, sulphite cooking and kraft cooking. The former
dominated until the mid-20th century, while the later is today completely dominant throughout
the world. Today, pulp for papermaking is produced mostly from wood fibres (more than 90
%). The rest is produced from non-wood fibres like bagasse, straw and bamboo.
According to the United Nations Food and Agriculture Organization (FAO), the total forest
area in the world in 2005 was estimated to be 3952 million hectares (ha) or 30 per cent of the
total land area. This corresponds to an average area of 0.62 ha per capita. However, the forest
is unevenly distributed. The ten most forest-rich countries account for two thirds of the total
forest area. Forest plantations make up about 3.8 per cent of the total forest area. Productive
forest plantations, primarily established for wood and fibre production, account for 78 per
cent of forest plantations, and protective forest plantations, primarily established for
conservation of soil and water, for 22 per cent. There is a wide variation in the number of
native tree species, from 3 in Iceland to 7780 in Brazil. Despite the large number of native
species in many countries, relatively few species account for most of the standing wood
volume. In most regions, the ten most common tree species (by volume) account for more
than 50 per cent of the total wood volume. Most of the world’s forests are publicly owned,
84%. The ownership structure in Sweden is quite different, 55 % being privately owned.
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Today, fast growing species, such as planted eucalyptus and acacia, are the most rapidly
growing wood raw material for pulp production in the world (www.fao.org).
The Swedish forest is part of the borelian zone and contains mainly softwood, 81 %, such as
pine and spruce. Only a minor part, 16 %, is hardwood such as birch and aspen. Accordingly,
the major raw material for the Swedish pulp industry is softwood and the industry’s products
are materials such as paperboard for packaging, linerboard and sacks, demanding high quality
and strong fibres. Mechanical pulp converted to newsprint and wood-containing printing
paper is also a large product. The forest industry is one of Sweden’s most important business
sectors. In terms of the net value exported, the forest products industry is the largest export
industry in Sweden. According to the annual summary prepared by Skogsindustrierna (the
Swedish Forest Industries Federation), the value of the Swedish forest industry’s exports was
114 billion kronor in 2005, equivalent to 12 % of the total exports from Sweden in that year.
The main markets for paper produced in Sweden are Germany, Great Britain and France,
whereas the pulp is primarily exported to Germany, France and Italy. The total production of
mechanical and chemical pulp in Sweden amounted to just over 12 million tonnes in 2005, 45
% of which was bleached kraft pulp. The total Swedish production of paper and paperboard in
2005 was 11.7 million tonnes, produced in approximately 45 mills. The raw material for paper
production consists to 45 % of chemical pulp and 29 % of mechanical pulp. The rest of the
raw material is mainly recycled fibre, coating and fillers (www.skogsindustrierna.org).
Paper is an essential part of our lives and satisfies many human needs. Paper embraces a wide
range of products with very different applications; communication (newspapers, books,
writing papers), cultural and artistic purposes, the transport and protection of food (packaging,
sacks, liquid containerboard), personal hygiene (tissues, napkins) etc. Each application is
associated with specific product demands. The end product properties are dependent on the
fibre species used, on the pulp manufacturing process and on the paper machine. In addition
to the demands on the product, the production of pulp has to comply with a variety of
environmental regulations. In this thesis, the process for the manufacture of kraft pulp is in
focus, scrutinising how the cooking conditions affect pulp properties such as yield, bleaching
chemicals demand and carbohydrate composition. Directly and indirectly, all these properties
will influence the environmental impact and the quality of the end product.
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2.2 The composition of wood Wood consists mainly of three polymers: cellulose, hemicellulose and lignin. These
macromolecules are not uniformly distributed within the wood cell wall, and their relative
concentrations vary between different parts of the tree. Wood also contains small amounts of
extractives and inorganic material. The rough compositions of spruce and Eucalyptus wood
can be seen in Table 1. However, it should be remembered that the relative proportions of
different components may vary within the species, depending on age and growth conditions.
Table 1. The relative chemical compositions (%) of Norway Spruce (Picea abies) and Eucalyptus Globulus according to Sjöström (1993).
Norway Spruce
Eucalyptus Globulus
Lignin 27.4 21.9
Cellulose 41.7 51.3
Glucomannan 16.3 1.4
Xylan 8.6 19.9
Other carbohydrates 3.4 3.9
Extractives 1.7 1.3
The main constituent, cellulose, is a linear homopolysaccharide composed of β-D-
glucopyranose units linked together by β-1,4-linkages. The cellulose chains, which in wood
consist of about 10 000 monomer units, are grouped together in bundles called microfibrils,
which form either ordered (crystalline) or less ordered (amorphous) regions. Microfibrils
build up fibrils and finally cellulose fibres. Cellulose is the main strength-bearing component
of the fibre.
Hemicellulose is not one specific polymer but a family name for a group of
heteropolysaccharides built up of different types of monosaccharides. The chains of the
hemicelluloses are shorter than those of cellulose, with a degree of polymerisation of about
100 to 200 (Fengel, Wegener 1984). Like cellulose, most hemicelluloses function as
supporting material in the cell walls. Hemicelluloses are relatively easily hydrolysed by acids
to their main monomers consisting of glucose, mannose, xylose, galactose, arabinose and
rhamnose. In addition some hemicelluloses contain uronic acids. The compositions and
structures of the hemicelluloses prevailing in softwoods differ in a characteristic way from
those in hardwoods. The principal hemicelluloses in softwood are galactoglucomannan (O-
acetyl-galactoglucomannan) often referred to merely as “glucomannan” and
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arabinoglucuronoxylan (arabino-4-O-methylglucuronoxylan) often referred to merely as
“xylan”. They correspond to about two thirds and one third respectively of the total
hemicellulose content in softwood. The backbone of glucomannan is a linear chain built up of
1,4-linked β-D-glucopyranose and β-D-mannopyranose units. The α-D-galactopyranose
residue is linked as a single-unit side chain to the framework by 1,6 bonds. The hemicellulose
of hardwood consists mainly of glucuronoxylan (O-acetyl-4-O-methylglucuronoxylan) often
merely referred to as “xylan” and a minor amount of glucomannan. The xylan consists of a β-
D-xylopyranose backbone linked by 1,4 glucosidic bonds. In hardwood, most of the xylose
residue contains an acetyl group linked to the C2 or C3 position, about 7 acetyl residues per
10 xylose units. In addition, there is on average one 4-O-methyl-α-D-glucuronic acid side
group per 10 xylose units. In softwood xylan, there are, on average, two 4-O-methyl-α-D-
glucuronic acid side groups per 10 xylose units.
Lignin is the material that binds the fibres together in the wood and it differs from cellulose
and hemicelluloses in many ways. There is no obvious repeating unit building up the lignin
structure and the structure of lignin can in the broadest sense be described as three-
dimensional. The lignin is built up of hydroxyphenylpropane units and is phenolic in
character. The hydroxyphenylpropane units are connected by various types of bonds, of which
arylglycerol-β-arylethers (β-O-4) are the most frequent (about 50 % in softwood) (Adler
1977; Brunow 1998). The chemical structure of lignin is irregular in the sense that the
structural elements are not linked to each other in any systematic order. Lignin is a result of
the radical polymerisation of three hydroxyphenylpropane units, Fig. 1. Hardwood lignin is
built up of a combination of coniferyl alcohol and sinapyl alcohol whereas softwood lignin
consists almost entirely of coniferyl alcohol. para-Coumaryl alcohol is present in small
amounts in both hardwood and softwood lignins, but in larger amounts in grass lignin.
Figure 1. The phenyl propane units act as building blocks in lignin. From left to right, para-coumaryl, coniferyl alcohol and sinapyl alcohol.
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2.3 Kraft Cooking C.F. Dahl (1884) invented the kraft (or sulfate) process in 1879. The active chemical species
in the kraft cooking liquor (white liquor) are hydroxide and hydrogen sulphide ions. The kraft
process is today the dominating process for the production of chemical pulps in the world,
accounting for more than 90 % of the world’s total manufacture of bleached chemical pulps.
Kraft cooking was first carried out as a batch process. In the 1940’s, Richter and co-workers
developed a continuous cooking system called Kamyr cooking (Richter 1981). In both the
batch and continuous processes, the cooked chips are discharged from the digester under
pressure. When the chips are ”blown” from the digester, the mechanical force of ejection
breaks up the wood chips into individual fibres, forming the wood pulp, that can then be
further processed and utilised. The main advantages of the kraft process over other pulping
processes have traditionally been the production of valuable by-products, well-developed
methods for the regeneration of spent cooking chemicals, its relative insensitivity to variations
in wood properties and its applicability to all wood species. And the kraft process produces a
strong pulp, thereby the name – kraft – coming from the German and Swedish words for
strength. Some drawbacks of the kraft process compared to the sulphite process are the
formation of malodorous gases which cause environmental concern, lower yield and a much
darker pulp.
In conventional kraft cooking, the chips and white liquor were charged into the digester at the
same time, and heated under pressure for a certain time until the desired degree of
delignification was achieved. This meant that the alkali concentration was very high in the
beginning of the cook, causing severe carbohydrate degradation during the cook, and low at
the end of the cook, resulting in a low overall delignification rate. High selectivity during the
cook, i.e. a high ratio of delignification to carbohydrate degradation, allows extended
delignification. A more selective kraft process was developed by the introduction of several
modifications to the kraft process. The concept of the modified kraft process originated at
KTH, the Royal Institute of Technology, and STFI, the Swedish Pulp and Paper Research
Institute (Carnö, Hartler 1976; Hartler 1978; Nordén, Teder 1979; Teder, Olm 1981;
Johansson et al. 1984). The four rules of modified cooking developed in those days can be
summarised as:
(1) a levelled-out alkali concentration
(2) a high concentration of hydrogen sulphide ions, especially at the beginning of the bulk
phase
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(3) low concentrations of dissolved lignin and sodium ions, especially at the end of the cook,
(4) a low cooking temperature
Applying one or several of these rules in a modified cooking concept allowed the pulp
manufacturer to extend the cook to a lower kappa number, without affecting pulp strength or
yield. Since the establishment of the concept of the modified kraft process, several new
pulping technologies in both continuous and batch systems have been developed, such as
Modified Continuous Cooking (MCC), Extended Modified Continuous Cooking (EMCC),
Isothermal Cooking (ITC), Black Liquor Impregnation (BLI), Rapid Heating Displacement
(RDH) and SuperBatch. Further research and development during the last ten years have
resulted in Lo-solids cooking and Compact Cooking (CoC), which are the two dominating
continuous kraft cooking systems today.
2.3.1 Delignification The three-dimensional lignin network is largely insoluble in its original form, but it is
degraded by the cooking liquor to smaller and/or more soluble fragments. Reactions leading
to delignification take place between the active chemicals and the lignin during the kraft
cooking. The delignification in a softwood kraft cook can be divided into three phases
(Wilder, Daleski 1965; Kleinert 1966; LeMon, Teder 1973): an initial phase, a bulk phase,
and a residual phase. The lignins removed in the three delignification phases are called initial
lignin, bulk lignin, and residual phase lignin. In all three delignification phases, the
delignification rate is of an apparent first order with respect to the remaining lignin content in
the wood. This means that the delignification rate -dL/dt is proportional to the concentration
of lignin at any time during the reaction:
− = •dLdt
k L [1]
where L = lignin content of the wood residue calculated with respect to the original amount of
wood (%).
t = time (min)
k = rate constant (min-1)
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The rate constant k depends mainly on the temperature and on the concentrations of hydroxide
and hydrogen sulphide ions, but it is also influenced by the concentrations of dissolved lignin
and by the ionic strength.
About 20 % of the lignin is removed in the rapid initial phase of the kraft cook. This phase
was studied in detail by Olm and Tistad (1979) who found that it could be described as a first
order reaction with an Arrhenius activation energy of 50 kJ/mol. The low activation energy
indicates that the reactions during the initial phase are diffusion-controlled. The dissolution of
lignin was found to be independent of hydroxide and hydrogen sulphide ion concentrations if
the hydrogen sulphide ion concentration was above 0.1 mol/l. Kondo and Sarkanen (1984)
later found that the initial phase can be divided into two separate phases, the first constituting
13% of the original lignin and the second 11% of the lignin, and having an activation energy
of 73 kJ/mol. The initial stage delignification was attributed by Gierer and Norén (1980) and
Ljunggren (1980) to the cleavage of α- and β-aryl ether bonds in phenolic phenylpropane
structures. The conditions during the initial phase may influence the rate in subsequent phases
and also the proportion of the lignin reacting according to each of these phases (Wilder,
Daleski 1965; LéMon, Teder 1973; Teder, Olm 1981).
Most of the lignin is removed during the bulk delignification phase. A large number of
scientists have studied the kinetics of this phase, using different raw materials and different
techniques (Laroque, Maass 1941; Wilder, Daleski 1965; LéMon, Teder 1973; Olm, Teder
1978; Kondo, Sarkanen 1984; Kleinert 1966; Wilson, Procter 1970; Teder, Olm 1981). The
rate of delignification in the bulk phase increases with increasing hydroxide concentration
and/or increasing hydrogen sulphide ion concentration and decreasing ionic strength (LéMon,
Teder 1973; Lindgren, Lindström 1996). Gierer and Norén (1980) and Ljunggren (1980)
stated that the cleavage of β-aryl ether bonds in non-phenolic lignin units occurred through
the participation of a neighbouring hydroxyl group and that this was considered to be the rate-
determining reaction of the bulk phase.
The fact that the bulk phase reaction gave way to an even slower residual phase reaction of
poor selectivity was first reported by Kleinert (1966) who found that this phase was also of
first order with respect to lignin. Few studies have been published concerning the activation
energy of this residual phase reaction and the influence of liquor composition, although, it has
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been reported that the rate of delignification in the residual phase increases with increasing
hydroxide concentration (Teder, Olm 1981; Lindgren, Lindström 1996).
It is not known whether the residual phase lignin is present in the native lignin or whether it is
formed through unfavourable reactions during pulping. Kleinert (1966) has suggested that the
residual phase lignin is a modified lignin rather than a distinct lignin initially present in the
wood. In recent years, much attention has been devoted to the characterisation of the
remaining lignin, in order to better understand the difficulty of removing the last traces. The
following are some of the explanations suggested in the literature:
• Kraft cooking involves to a large extent the cleavage of phenylpropane-β-arylether
structures in the lignin. In addition, some of these structural units are converted into
alkali stable enol ether structures which may counteract the degradation and
solubilisation of the lignin (Gellerstedt, Lindfors 1987). The residual lignin contains
fewer β-O-4 linkages than the native lignin (Gellerstedt et al. 1984), although still in
detectable amounts even at very low kappa numbers (Froass et al. 1998).
• Condensation reactions between lignin fragments and lignin take place during cooking
(Gierer et al. 1976) and can be expected to reduce the ease of delignification. The
discussion concerning their extent and the types of structures is still continuing.
• Condensation reactions may occur between lignin and carbohydrates (Minor 1983;
Gierer, Wännström 1984; Gellerstedt, Lindfors 1991) leading to cross-links between
lignin and polysaccharide chains (LCC). Such alkali-stable linkages may be formed
during the cook but most of them are probably already present in the wood (Fengel,
Wegener 1984). Several studies have shown that most of the lignin is associated with
hemicelluloses and that a smaller part is associated with cellulose (Karlsson,
Westermark 1996; Karlsson et al. 2001; Tenkanen et al. 1999). Quantitative studies by
Lawoko et al. (2003a) suggested that about 90 % of the lignin in a softwood kraft pulp
is bound to carbohydrates. Xylan-lignin is the most frequent LCC at high kappa
numbers, while glucomannan-lignin is enriched later in the cook, as well as after
oxygen delignification (Lawoko et al. 2003b). Axelsson (2004) found that about 80 %
of the lignin in a birch kraft pulp after the cook was bound to carbohydrates, a greater
part to the hemicelluloses and a smaller part to cellulose.
19
2.3.2 Degradation/dissolution of carbohydrates Because of the alkaline degradation of polysaccharides, kraft cooking results in considerable
carbohydrate losses. In the earlier stages of the cook, the polysaccharide chains are peeled
directly from the reducing end groups present (primary peeling). As a result of the alkaline
hydrolysis of glycosidic bonds, occurring at high temperatures, new end groups are formed,
giving rise to additional degradation (secondary peeling). This chain cleavage also reduces the
DP of the cellulose (indirectly measured as viscosity), and this may reduce the pulp strength.
The yield of cellulose is somewhat reduced in kraft cooking (10-20 %), although to a lesser
extent than that of hemicelluloses, which are degraded more extensively due to their low
degree of polymerisation and amorphous character. The peeling reaction is finally interrupted
when the competing “stopping reaction” converts a reducing end group to a relatively stable
metasaccharinic acid. Only two kinetic phases of carbohydrate dissolution have been
observed during kraft cooking, with a very high rate of dissolution in the first and a lower rate
in the second. The loss of easily dissolved hemicelluloses in the first phase has a lower energy
of activation, 146 kJ/mol, than that in the second phase, 169 kJ/mol (Lindgren 1997), which is
to be expected since the first phase probably involves physical dissolution of hemicelluloses
and primary peeling.
It has been known for a long time that a large part of the glucomannan and xylan are
dissolved from the wood during the first part of the cook, and that the part that remains is
rather stable against further degradation (Aurell, Hartler 1965a). Later in the cook, the
carbohydrates are mainly lost as a consequence of alkaline degradation. At high temperatures,
the removal of xylan is more intensive due to dissolution (Saarnio, Gustafsson 1953;
Simonson 1963) and alkaline hydrolysis (Dryselius et al. 1958). Previous results (Sjöström
1977; Genco et al. 1989) have shown that 40 % of the arabinoglucuronoxylan and 70 % of the
galactoglucomannan were removed during the heating up to the cooking temperature. An
appreciable portion of the dissolved xylan appears in the cooking liquor as oligo- or
polysaccharides, whereas the dissolved glucomannan is degraded to a greater extent. In
softwood, the greater stability of xylan against peeling is due to the arabinose substituents in
the C-3 position, which allows the formation of the stable xylo-metasaccharinate end-group
(Whistler, BeMiller 1958). There is no arabinose unit substituted at the C-3 position in
hardwood xylan and the substitution of a glucuronic acid group at the C-2 position in xylose
gives only a partial stabilisation (Aurell 1963). The relatively high retention of xylan in
hardwoods may therefore be explained by factors such as a high DP, a lower degree of
20
substitution of uronic acids which affects the solubility of the xylan, and the possibility that
xylan may precipitate back onto the fibres towards the end of the cook (Yllner, Enström
1956). This re adsorption of xylan significantly contributes to the final pulp yield. According
to Aurell and Hartler (1965b), the xylan content in the pulp is greatly lowered and the
glucomannan content is slightly increased when the alkali charge in the kraft cook is
increased.
2.3.3 Hexenuronic acid 4-O-methyl-α-D-glucuronic acid groups exist in the xylan structure as a part of the
arabinoglucuronoxylan in softwood and glucuronoxylan in hardwood. In 1963, Clayton
(1963) proposed that removal of the 4-O-methyl-α-D-glucuronic acid groups could be
initiated by β-elimination of methanol during the alkaline pulping of wood. Later Johansson
and Samuelson (1977a) verified this with a dimeric model compound, 2-O-(4-O-methyl-α-D-
glucopyranosyluronic acid)-D-xylitol. Their experiments clearly showed the formation of 2-
O-(4-deoxy-β-L-threo-hex-4-enopyranosyluronic acid)-D-xylitol (hexenuronic acid-D-xylitol)
and its slow degradation with time. This structure is today commonly referred to as
hexenuronic acid and denoted HexA. Although it has been clear in the literature that 4-O-
methyl-D-glucuronic acid must undergo β-elimination during kraft cooking (Fig. 2), the
occurrence of HexA in kraft pulps or in the dissolved xylans was not verified until the mid-
1990’s (Maréchal 1993; Buchert et al. 1994; Teleman et al. 1995). Before this (re)discovery,
the acid-labile HexA had for many years escaped detection in the carbohydrate analysis,
which was based on a strong acid treatment.
OOHO
O
O
HOHO
CH3O
Xylan chain with a 4-O-Methyl-glucuronic acid unit
OOHO
O
O
HOHO
HOOC
Hexenuronic acid unit (HexA)
OH-
-CH3OH
HOOC
n n
Fig. 2 The formation of HexA from 4-O-methyl-D-glucuronic acid in xylan.
21
Since the (re)discovery of HexA, a significant amount of research has been focused on its
behaviour during cooking and bleaching. The content of HexA is in many ways an important
factor in the production of a bleached chemical pulp. The HexA consumes permanganate in
the kappa number analysis of pulps and thereby appears as “false lignin” in the kappa number
measurement (Vuorinen et al. 1996; Gellerstedt, Li 1996). Gellerstedt and Li reported that
typically 3-6 kappa number units of an unbleached hardwood pulp and 1-3 kappa number
units of an unbleached softwood pulp are due to HexA and not to lignin (Gellerstedt, Li
1996). HexA reacts with several bleaching chemicals, such as ozone, chlorine dioxide or
peracids (Buchert et al. 1995; Vuorinen et al. 1996; Bergnor-Gidnert et al. 1998), and thus
consumes these bleaching chemicals, whereas no significant degradation of HexA has been
detected in oxygen or peroxide stages (Buchert et al. 1995). Furthermore, the reaction
products formed from HexA during bleaching are a source of oxalate formation (Nilvebrant,
Reimann 1996; Elsander et al. 1997). If it is present in a fully bleached pulp, HexA has been
reported to decrease the brightness stability of the pulp (Buchert et al. 1996; Buchert et al.
1997a; Siltala et al. 1998; Granström et al. 2001). Recent studies show that HexA plays a
dominant role in the brightness reversion in bleached kraft pulps (Sevastyanova 2005). HexA
also has a strong affinity for transition metals (Devenyns, Chauveheid 1997; Devenyns et al.
1998; Vuorinen et al. 1996). However, according to Laine and Stenius (1995), a high surface
charge of the pulp, partly provided by the presence of HexA, leads to better paper strength
properties.
2.3.4 Evaluation of mill cooking yield Kraft cooking yield is one of the most important economic variables in the production of
chemical pulp. There are several ways to determine the pulp yield indirectly in a kraft mill.
The most common are measurements of wood consumption and black liquor solids in relation
to the pulp production. Measurement of the wood consumption requires a long testing period
to achieve an accurate yield determination and cannot be used to estimate the mill yield for
process changes during brief periods. Clearly, it is desirable to have a quicker and more
accurate method for determine the mill pulp yield. Much effort has therefore been devoted to
developing methods for determining pulp yield based on the chemical composition of the pulp
and/or the pulp properties. The yield has often been estimated from a kappa number–yield
relationship established in laboratory cooking trials. This relationship has proven to have a
very limited range of validity. In order to establish a more accurate relationship, it is
necessary to consider also the carbohydrate composition of the pulps. Marcoccia et al. (1998a,
22
1998b) developed a method for estimating the yield which assumes a linear relationship
between the lignin-free pulp yield and the term log10(V)/G'2, where V is the viscosity and G' is
the mass fraction of cellulose in the lignin-free pulp. Vaaler et al. uses the mannose or
glucomannan content in pulp to estimate softwood pulp yield (Vaaler et. al 2001a, 2001b,
2002). Easty and Malcolm (1982) published the "carbohydrate–lignin method". They estimate
the pulp yield based on the assumption of constant cellulose yield using the equation:
)/()(/)()( CLYCHCYyieldPulp cellcell •++•= [2]
where Ycell is the yield of cellulose based on oven-dried wood and C, H and L are weight
fractions based on oven-dried pulp of cellulose, hemicellulose and lignin respectively. Luthe
et al. (2003) have reviewed the above-mentioned methods and evaluated their suitability to
predict yield. According to this review article, the mannose method can predict yield well and
is not affected by pulping chemistry. The mannose method is, however, limited to softwood
pulps since mannose is a very minor constituent of hardwoods. The "carbohydrate–lignin
method" can predict yield fairly well, but it is to some extent process-dependent. The
Marcoccia et al. equation was found to be a poor predictor of yield gains achieved by altering
the process chemistry. Both the Marcoccia et al. equation and the "carbohydrate-lignin
method" have the advantage that they can be used to predict both softwood and hardwood
pulp yields.
2.4 Bleaching of pulp Bleaching is desirable for several reasons. Firstly, a bright pulp is necessary for good contrast
and printability for easy reading. Secondly, bleaching of the pulp makes it more resistant to
aging. Another purpose of bleaching is to improve the cleanliness of the pulp by removing
extractives, dots and shives. Chemically and biologically pure pulps are required especially in
the production of hygiene products and packages for food. The light absorption (colour) of
pulp is mainly associated with its lignin component. To reach an acceptable brightness level,
the residual lignin should thus either be removed from the pulp or, alternatively, freed from
strongly light-absorbing groups (chromophores) as completely as possible. Bleaching
effluents cannot be easily incorporated in a mill’s chemical recovery system. It is therefore
important from an environmental protection point of view that as much of the lignin in the
pulp as possible is removed before bleaching. However, the lignin cannot be removed from
the wood by cooking alone because of poor selectivity when the delignification is extended
23
too far in the cook. The cook is therefore interrupted after the dissolution of approximately 90
% of the lignin originally present in the wood. The cooking is then often followed by an
oxygen delignification stage. After the oxygen delignification stage, about 1.5 % (on wood) of
the lignin remains in the pulp. The final delignification must take place in a bleaching
operation. The bleaching is carried out in a number of consecutive stages using
predominately, chlorine dioxide, hydrogen peroxide, ozone and/or peracetic acid as bleaching
agents, with a minor use of the latter two.
In the mid-1980’s, environmental groups placed the issue of the use of elemental chlorine in
chemical pulp mills on the agenda. Unacceptable levels of chlorinated substances were found
in the pulp and in the pulp mill effluents. Complete substitution of chlorine dioxide for
chlorine, so-called ECF (elemental chlorine-free) bleaching processes were developed applied
by the mills in a response to the justified environmental concerns. Chlorine was last used in
bleaching in Sweden in 1993. Further developments led to bleaching processes that used
neither chlorine nor chlorine-containing compounds, denoted TCF (totally chlorine-free). The
development of TCF bleaching technologies have resulted in bleaching sequences using
oxygen, hydrogen peroxide, ozone and peracetic acid. TCF bleaching was expected to rapidly
increase all over the world, but reality has been different, and only a limited number of mills
now produce only TCF kraft pulp for papermaking purposes. Today, bleaching to full ISO
brightness (88-90 percent or above) is performed mainly in an ECF-process, although such
proceeses are, often significantly different from those used in the early days of ECF.
2.4.1 Bleachability The amount of lignin, the types of chemical structure and the metal content of the pulp
entering the bleaching stages determine the consumption of bleaching agents as well as the
overall result in terms of brightness and fibre strength. The term “bleachability” is used to
describe the ease of bleaching of a given pulp. There is no standard method to evaluate the
bleachability of pulps nor is there any standard definition associated with the notion of
”bleachability”, and this definitely makes it difficult to achieve a clear comparison of
conclusions from different studies in the literature (Ragnar 2004). One commonly used
method is to determine the consumption of bleaching chemicals per unit lignin for the pulp to
reach a certain ISO brightness. Several investigations have been undertaken to study the effect
of different cooking conditions on the bleachability of kraft pulps, both for softwood (Carnö
24
et al.1975; Svedman et al. 1995; Kettunen et al. 1997; McDonough et al.1997; Sjöström
1999b; Al-Dajani 2001; Björklund et al. 2004) and hardwood (Colodette et al. 2002; Pascoal
Neto et al. 2002; Axelsson, Lindström 2004). Furthermore, attempts have been made to relate
the chemical structure of the residual lignin to the bleachability of the pulp (Froass et al. 1996;
Colodette et al. 1998; Gellerstedt, Al-Dajani 2000; Rööst et al. 2003). The degree of
delignification in the cook has also been shown to be an important parameter for the
bleachability. In general, the consumption of bleaching chemicals required to reach a given
brightness decreases with decreasing kappa number, although the consumption per unit kappa
increases significantly. This has been reported for a range of different bleaching sequences
and wood species (Carnö et al.1975; Svedman et al. 1995; Rööst et al. 2000; Pascoal Neto et
al. 2002).
2.5 The aim of this thesis The overall aim of the work described in this thesis was to better understand how different
cooking parameters, i.e. the concentration of hydroxide ions, the concentration of hydrogen
sulphide ions, the ionic strength, and the temperature, affect the kraft process in terms of the
potential to delignify extensively, in terms of carbohydrate composition, and in terms of yield,
hexenuronic acid (HexA) content and bleachability. Special attention was given to the
activation energy of the slowly reacting residual phase of the kraft cook and to the influence
of different cooking parameters on the amount of the residual phase lignin. Another aim of
this work was to investigate the influence of different cooking parameters in the kraft process
on the bleachability of the manufactured pulp. If variations in bleachability were seen, an
attempt would also be made to find chemical reasons behind the differences. Another specific
goal was to develop a strategy of kraft cooking whereby extensive HexA formation could be
avoided or at least so that the unbleached pulp had a very small amount of HexA. Additional
attention was therefore devoted to how the HexA content was affected, e.g. by a set of
cooking parameters. In these studies, it was also important to investigate the effects of the low
HexA (after cooking) strategy on such vital factors as the cooking yield, the bleachability and
the yellowing characteristics of the pulp obtained.
25
3. Results and discussion
3.1 Delignification kinetics of softwood (Paper I and II) The complexity and the incomplete knowledge of the kinetics of kraft cooking prevent the
establishment of an exact mathematical model to describe the rates of the reactions occurring.
In spite of the extensive work of earlier investigators, several questions still remain. More
extensive knowledge would allow a better prediction of the effects of changes in the
temperature and/or chemical concentration profile on the results of the kraft cook. It would
also be expected to provide a good basis for efforts to modify and optimise the existing
process, or to guide the development of new ones.
In this work, the delignification process was analysed by assuming three parallel reactions, as
previously done by Lindgren and Lindström (1996), Dolk et al. (1989), and Chiang and Yu
(1989). They described the delignification in a kraft cook of softwood using models based on
the assumption that the overall delignification rate is the sum of three parallel reactions
(initial, bulk and residual) (Fig. 3), each being of first order with respect to lignin, and not as
three consecutive reactions, as had been assumed in most earlier studies. This approach, with
three parallel phases, is based on the assumption that all three types of lignin are present in the
wood from the beginning. The amounts of the native lignin that will react according to the
bulk and residual delignification mechanisms respectively are determined by the prevailing
cooking conditions. In other words, there is an equilibrium relationship between the bulk and
the residual phase mechanisms. The initial phase is of minor interest with regard to kinetics
and activation energy since it is so rapid that it passes long before the full cooking
temperature is reached.
26
Initial phase
Bulk phaseResidual phase
Fig 3. The amounts of lignin that react according to initial (⎯ ⎯ ⎯), bulk (- - -), and residual (⎯ - ⎯ - ⎯) delignification. The solid line is the sum of initial, bulk, and residual phase lignin. Adapted from Lindgren and Lindström (1996).
3.1.1 A model that describes how the amount of residual phase lignin in spruce depends upon the cooking conditions (Paper I) The delignification during the residual phase of a cook is very slow and it is therefore a
limiting factor for lignin removal, due to its poor selectivity. Consequently, the cook must be
interrupted when the residual phase is reached, in order to maintain a high pulp quality and
high yield. If the amount of lignin reacting according to the residual phase could be reduced,
it would be possible to prolong the bulk phase and thereby improve the selectivity of the kraft
cook. The amount of residual phase lignin is therefore of the utmost importance when
modelling kraft cooks to low kappa numbers that are of interest for pulp mills with very low
environmental impact.
Previous investigations have shown that the conditions in the earlier phases affect the amount
of residual phase lignin (Kleinert 1966; Teder, Olm 1981; Axegård, Wiken 1983; Pekkala
1983; Lindgren, Lindström 1996). It has been reported that the transition point between bulk
and residual delignification shifts to lower lignin contents when the cooking temperature, the
hydroxide concentration and the hydrogen sulphide ion concentration of the cooking liquor
are increased. Lindgren and Lindström (1996) reported that the amount of residual phase
27
lignin is reduced by a higher hydroxide concentration and to some extent by a higher
hydrogen sulphide ion concentration and by a lower ionic strength in the bulk phase, but that
it is unaffected by the temperature.
It is not known whether the lignin reacting according to residual phase kinetics is present in
the fibre at the start of the cook or whether it is formed through unfavourable reactions during
the cook. There are therefore two possible definitions of the amount of residual phase lignin.
If the residual phase lignin is assumed to be present in the native wood (or is formed very
early in the cook), the amount is determined by extrapolation of the delignification rate in the
residual phase to the start of the cook (Lindgren, Lindström 1996), see Fig. 3. If, on the other
hand, the residual phase lignin is assumed to be formed during the cook, the amount of
residual phase lignin is defined as the lignin content at the transition from the bulk to the
residual phase (Kleinert 1966; Teder, Olm 1981). In this study (Paper I), it has been assumed
that the residual phase lignin is present in the native wood (or is formed very early in the
cook).
The purpose of “Paper I” was therefore to develop an “equilibrium” model to show how the
amount of residual phase lignin in the kraft cooking of spruce chips (Picea abies), depends on
the conditions in the earlier phases of the cook. Such an “equilibrium” model would predict
how much of the lignin that reacts according to residual phase kinetics and bulk phase kinetics
respectively. The variables studied were hydroxide concentration, hydrogen sulphide ion
concentration and ionic strength. The liquor-to-wood ratio during the pulping was very high
to maintain approximately constant chemical concentrations throughout each experiment.
In this study, it was possible to describe the influence of [OH-] and [HS-] on the amount of
residual phase lignin, Lr, by the expression:
[ ]( ) [ ]−−− ••−= HSOHLr ln32.055.0 3,1 [3]
where Lr = residual phase lignin determined as extrapolation of the delignification rate in the
residual phase to the start of the cook.
This equation was derived for a constant sodium ion concentration of 2 mol/l and is valid for a
concentration of hydroxide concentration in the range from 0.17 to 1.40 mol/l, and a hydrogen
28
sulphide concentration from 0.07 to 0.60 mol/l. The effect of temperature was not evaluated
since Lindgren and Lindström (1996) had previously shown that the temperature had no effect
on the amount of residual phase lignin in the 150 °C to 180 °C range. No attempt was made to
find a chemical explanation for the form of the equation. It is merely the simplest equation
that fits the data with an acceptable degree of error. If there were a better understanding of the
chemistry involved in the partition of lignin between that reacting according to a bulk and that
reacting according to a residual delignification mechanism, it would probably be possible to
develop a better model using the same experimental data. Kubo et al. (1983) used a different
approach when modelling the amount of residual phase lignin, since they assumed that some
of the lignin is insoluble. In addition, Kubo et al. (1983) suggest that increasing the
temperature decreases the amount of insoluble lignin, contrary to the finding of Lindgren and
Lindström (1996). However, by fitting the proportionality constant in the model of Kubo et al.
(1983), their model is in close agreement with Eq. [3].
In Paper I, it was found that the amount of residual phase lignin was greatly influenced by the
hydroxide concentration during the cook. A high hydroxide concentration gave a low amount
of residual phase lignin, which is consistent with earlier results (Lindgren, Lindström 1996).
An increase in the concentration of hydrogen sulphide ion led to a decrease in the amount of
residual phase lignin (Fig. 4). The influence of hydrogen sulphide ions on the amount of
residual phase lignin was much greater when the cook was carried out with a low hydroxide
concentration, as was earlier reported by Pekkala (1983). One possible explanation for the
great influence of hydrogen sulphide ions in cooks at a low hydroxide concentration could be
that the fragmentation of the lignin to smaller parts becomes very important and that a high
hydrogen sulphide ion concentration is thus essential to achieve the desirable sulphidolytic
cleavage. At a higher hydroxide concentration, the liberation of new phenolic groups may be
sufficient to make the lignin soluble.
29
Fig 4. Amount of residual phase lignin plotted versus hydrogen sulphide ion concentration in cooks conducted at initial hydroxide concentrations of 1.4 (▼), 0.7 (■) 0.35 (●), and 0.175 (▲) mol/l. The solid lines are calculated using Eq. [3].
A decrease in the sodium ion concentration led to a decrease in the amount of residual phase
lignin, Fig. 5. When both the hydroxide and the hydrogen sulphide ion concentrations were
low, the effect of sodium ions was considerable. However, during pulping at a somewhat
higher hydroxide or hydrogen sulphide ion concentration, a large decrease in sodium ion
concentration was necessary to achieve a decrease in the amount of residual phase lignin. No
success was achieved in attempts to include the effects of the ionic strength in the equations
describing the amount of residual lignin. Sodium chloride was used to adjust the ionic
strength, measured as sodium ion concentration, throughout this work. The counter-ion,
chloride ion, is assumed not to affect the delignification rate. However, it should be noted that
chloride ions are not present in the mill white liquor since the only source of chlorine is the
wood. Lundqvist et al. (2006) have studied the effect of the addition of different anions, Cl-,
CO32- and SO4
2-, on the kappa number vs ionic strength (calculated on the basis of the added
sodium salts). Compared at a given ionic strength, the chlorine ion exerted the most negative
effect, sulphate being less detrimental and carbonate having a positive effect on the
dissolution of lignin. According to their results, adding sodium carbonate to a carbonate-free
laboratory white liquor resulted in a significantly higher rate of delignification of birch wood.
Moreover, the concentration of calcium in black liquor from the cooks performed without
added carbonate was twice that when carbonate was added from the beginning. The higher
30
rate of delignification in the kraft cook in the presence of carbonate was assumed to be due to
precipitation of calcium carbonate.
Fig 5. The amount of residual phase lignin as a function of the concentration of sodium ion. Initial conditions; (▼) 1.4 mol/l OH- and 0.6 mol/l HS-, (■) 1.4 mol/l OH- and 0.15 mol/l HS-, (●) 0.35 mol/l OH- and 0.6 mol/l HS-, (▲) 0.35 mol/l OH- and 0.15 mol/l HS-.
3.1.2 Temperature-dependence of residual phase delignification (Paper II) In Paper II, the focus was on further extending our knowledge of the behaviour of the residual
phase lignin in order to sort out question marks about the kinetics of the kraft cooking process
on softwood. One of the unresolved issues was why the activation energy of the residual
phase delignification should be lower than that of the bulk phase delignification, when lignin
solubilisation was more difficult during the residual phase than during the bulk phase
delignification. Since Arrhenius (1924) calculated the activation energy for the dissolution of
cellulose and the non-cellulosic material, much progress has been made in understanding the
kinetics of this process. Published values for activation energies of the bulk phase and residual
phase delignification are summarised in Table 2. Note that Lindgren and Lindström (1996)
found a higher activation energy for the residual phase delignification than for the bulk phase
delignification, 146 and 127 kJ/mol, which is surprising, since, in all previous studies, the
activation energy of the bulk phase delignification was found to be higher than that of the
residual phase delignification.
31
Table 2. Activation energy values reported for soda and kraft cooking.
Process Activation energy kJ/mol
Bulk phase Residual phase
References
134 Larocque, Maass 1941Soda 143 Wilder, Daleski 1965 131 117 Dolk et al. 1989 130 Wilson, Procter 1970 135 90 Kleinert 1966 134 Wilson, Procter 1970 Kraft 150 120 Teder, Olm 1981 140 Parming Vass 1994 127 146 Lindgren, Lindström 1996
Accordingly, in Paper II, the kinetics of delignification in kraft cooking was studied in order
to determine the activation energies of the bulk and residual phase delignification reactions. A
model for delignification in cooks with a high liquor-to-wood ratio (”constant composition”
cooks) was extended to include cooks with changing concentration profiles and significant
amounts of dissolved wood components (”normal” cooks).
”Constant composition” cooks In the ”constant composition” cooks, a single cooking temperature was used through the
initial and bulk phases to limit possible temperature effects (Kleinert 1966; Axegård, Wikén
1983), and the only temperature varied was the one in the residual phase. All previous studies
have used a different method in which the temperature used for the residual phase
delignification was the same as that used for the cooking through the initial and bulk phases.
This means that the lignin in these studies has not had the same history when the activation
energy of the residual phase delignification has been evaluated. The results of the ”constant
composition” cooks gave an activation energy of the residual phase delignification, see the
Arrhenius plot (Fig. 6), of 152 kJ/mol with a standard deviation of 7 kJ/mol. The results also
showed that the temperature did not affect the amount of residual phase lignin, in agreement
with Lindgren and Lindström (1996) but contradicting the findings of Kleinert (1966) and
Axegård and Wikén (1983). Kleinert suggested that if the residual phase lignin was present in
the wood then extrapolation of the residual phase lignin to time 0 at the various temperatures
investigated should lead to a single intercept, which was not the case. Extrapolation of
Kleinert's results (for 170-185 °C) to a point 10 min before the start of the cook does,
however, give a single intercept, and this may be taken to indicate that his correction for the
32
time to reach the correct cooking time was not correct. Axegård's and Wikén's (1983)
measurements where made at only two temperatures (160 and 170 °C) and the results are
somewhat noisy, making it possible to fit their data to a model with a single intercept, i.e. no
temperature effect.
Fig. 6 Arrhenius plot of the residual phase delignification rate constants for the ”constant composition” cooks.
”Normal” cooks If equilibrium and kinetic models are to be useful, it must be possible to show that they are
applicable to real systems. A kinetic study of “normal” cooks (liquor-to-wood ratio = 4:1) was
therefore made to determine the activation energy of the residual phase delignification and
attempts were also made to apply the previously determined “equilibrium” and kinetic
equations derived for ”constant composition cooks” (Paper I in this thesis and equations from
Lindgren, Lindström 1996) to ”normal” cooks. These equations were combined with models
for the concentration profiles of [OH-] and [HS-] (of the form )exp( 321 tkkk −+ with the
parameters fitted to the measured data) and then used to model our experimental results. In
order to obtain a good fit, it was necessary to adjust the models with appropriate correction
factors, taking into account that dissolved lignin has a positive effect early in the cook when
bulk phase delignification dominates, and a negative effect in the later part of the cook when
33
residual phase delignification starts to dominate (Sjöblom et al 1983; Sjöblom et al 1988;
Sjöblom 1996). With this addition to the model, the fit was acceptable, as seen in Fig. 7. The
activation energy was calculated to be 136 kJ/mol for the bulk phase delignification and 156
kJ/mol for the residual phase delignification. The correction factors were 1.2 for the bulk and
0.7 for the residual phase delignification, which means that the delignification rate in the bulk
phase is 20 % higher in the ”normal” than in the ”constant composition” cooks, while the
delignification rate in the residual phase is 30 % lower in the ”normal” than in the ”constant
composition” cooks. A possible explanation for the higher rate in the bulk phase could be that
the dissolved lignin fragments act as very effective nucleophiles. Recently, Sjödahl (2006) has
shown that the increase in delignification rate is related more strongly to the content of free
phenolic groups in the dissolved wood components (DWC) than to the total amount of DWC.
Moreover, when cooking in the presence of representative model substances, Sjödahl reported
that aromatic structures with free phenolic groups gave a rate-increasing effect while no
visible effect of the other structures could be seen. These results support the finding that the
delignification rate relates to the amount of free phenols in the cooking liquor and shows that
the phenolic functionality takes an active part in the delignification reactions. A suggested
mechanism for the retardation of delignification by dissolved lignin in the residual phase is
that condensation occurs between wood residue and dissolved lignin (Sjöblom 1996).
Fig. 7 Remaining lignin, on wood, as a function of time for ”normal” cooks at 160 °C (●), 170 °C (♦) and 180 °C (■).
34
As explained earlier, the modelling in this study was made assuming two parallel phases,
whereas the “traditional” way is to evaluate the activation energies assuming two consecutive
reactions. When the data were instead analysed using a model with two consecutive reactions,
values of 148 kJ/mol for the bulk phase and 135 kJ/mol for the residual phase delignification
were obtained i.e. a higher value for the bulk phase than for the residual phase delignification!
An analysis of the data in Figs. 2 and 3 in Kleinert (1966) using a model with two parallel
reactions does in fact give 127 kJ/mol for the bulk phase and 138 kJ/mol for the residual
phase delignification, in contrast to the values of 135 kJ/mol and 90 kJ/mol that Kleinert
obtained with a model using two consecutive reactions. This explains why some authors
(Kleinert 1966; Teder, Olm 1981; Dolk et al. 1989) have reported higher activation energies
for the bulk phase than for the residual phase delignification, although a higher activation
energy for the more difficult to degrade residual phase lignin seems more reasonable.
The successful use of a model with three parallel phases for both ”constant composition”
cooks and ”normal” cooks indicates that the residual phase lignin, or at least a major portion
of it, is native. The results obtained by Lindgren and Lindström (1996) also showed that the
residual phase lignin is not a homogeneous lignin, since an alteration in the conditions in the
cook can make part of it react as bulk phase lignin. Their results indicated that most of the
residual phase lignin is not created by condensation reactions during the bulk phase. The
amount of residual phase lignin is instead determined by the prevailing conditions that shift
the relationship between the amounts of the native lignin that react according to the bulk and
the residual delignification mechanisms. One earlier argument suggesting that residual phase
lignin is formed during the earlier stages of the cook was based on the observation that the
amount depended on the conditions during the initial phase of the cook (Teder, Olm 1981),
but a later study indicates that this is not the case (Axegård, Wikén 1983). The difference in
the effect of the cooking conditions in the initial phase on the amount of residual phase lignin
is explained by Axegård et al. (1983) as being due to carry-over of hydrogen sulphide ion
from the initial to the bulk phase in the work of Teder et al. (1981), probably due to a less
effective soaking procedure between the initial and bulk phases.
35
3.2 The degradation of carbohydrates in kraft cooking and evaluation of mill cooking yield (Paper III, IV, V)
The ultimate goal of chemical pulping is to liberate the cellulosic fibres from the wood
through delignification without degradation or removal of the carbohydrates. Unfortunately,
the kraft cooking process is not entirely selective for lignin, since it also causes some
degradation of the carbohydrates. The dissolution of carbohydrates during cooking takes place
through a number of different reaction paths such as primary peeling, secondary peeling, and
physical dissolution. Despite the great economic importance of kraft cooking, surprisingly
few investigations have been carried out to clarify how different variables affect the
degradation of carbohydrates in the pulping of wood. Published pulping studies deal with the
selectivity of delignification on the one hand and pulp yield or viscosity on the other, but the
loss of the individual carbohydrates is seldom treated separately.
Three different studies have here been performed to investigate how the cooking conditions
effect the degradation of carbohydrates during kraft cooking. The effect of hydroxide
concentration, hydrogen sulphide ion concentration, ionic strength and temperature on the
dissolution/degradation of HexA was investigated for both softwood (Paper III) and hardwood
(Paper IV). The effect of cooking conditions on the amounts of glucomannan, xylan and
cellulose in the unbleached pulp was studied for softwood (Paper III, Paper V). Moreover, a
method for the evaluation of mill yield that can be used for both hardwood and softwood is
presented in section 3.2.4 (Paper V). It should be pointed out that, in all three studies, the
liquor-to-wood ratio during the pulping was very high to maintain approximately constant
chemical concentrations throughout each experiment, so-called ”constant composition” cooks.
36
3.2.1 The formation and dissolution/degradation of HexA in softwood kraft cooking (Paper III) As mentioned in the introduction, the content of HexA is an important factor in the production
of bleached chemical pulps. It would therefore be valuable to develop a model based on the
cooking conditions that could be used to estimate the HexA content in the pulp. A low amount
of HexA groups in the unbleached pulp is, however, not the only important parameter and
attention must also, of course, be paid to pulp yield and selectivity when optimising the
cooking conditions.
The results of this study showed that the rate of dissolution/degradation of HexA fitted well to
a first order reaction of the form:
( )tk
t HexAHexA •−•= exp0 [4]
where HexA0=47 µmol/g pulp (% on wood) is the amount of HexA at the beginning of the
cook, and
HexAt = the amount of HexA at time t (min), and
[ ] [ ] [ ]( )+−−−−−− •+•+•+•= NakHSkOHkak NaHSOH
3333 10101010
•− −
⎛⎝⎜
⎞⎠⎟
⎛⎝⎜
⎞⎠⎟exp . .
EaT8 3141 1
443 15 [5]
where k (min-1) = the rate constant The values for the constants (a, kHO, kHS, kNa and Ea) in Eq. [5] are presented in Table 3. It has
been demonstrated that HexA groups are formed early in the kraft cook, i.e. during the
heating-up period (Buchert et al. 1995; Buchert et al. 1997b). The model, Eqs. [4]-[5],
presented in this paper is based on the assumption that the β-elimination of 4-O-methyl-α-D-
glucuronic acid which yields HexA occurs rapidly and more or less completely during the
pretreatment and heating up period of the kraft cook. Consequently, the model assumes that
the reaction rate for the formation of HexA groups is much greater than the reaction rate for
the dissolution/degradation of HexA groups. It should be pointed out that the model
developed in this paper is based on ”constant composition” cooks and may not be fully valid
for cooks with changing liquor composition.
37
Table 3. The values of the constants and apparent activation energies for HexA, xylan and glucomannan in Eq. [5].
HexA Xylan Glucomannan
a -2.5 0 -0.5 kOH 33.5 8.4 3.2 kHS 6.8 2.6 1.4 kNa 1.5 -0.9 0
Ea (kJ/mol) 123 134 119 HexA0=47 µmol/g pulp; *Xylan0=4.5 % on wood; *Glucomannan0=4.7 % on wood *These values were obtained when fitting our data to the model, Eqs. [4]- [5], and have no physical meaning.
0
5
10
15
20
25
30
35
40
45
50
0 50 100 150 200 250 300 350
Time (min)
mmol HexA/kg pulp, (% on wood)
Fig. 8 The effect of OH- concentration (▲=0.1, ●=0.23, ■=0.44 and ♦=0.9 mol/l OH-) on the dissolution/degradation rate of HexA. [HS-]=0.28 mol/l, [Na+]=1.3 mol/l and temperature 170 °C. The solid lines are calculated using Eqs. [4] and [5].
The amount of HexA in kraft pulp is affected by the pulping conditions (Buchert et al. 1995;
Buchert et al. 1997b; Kettunen et al. 1997; Achrén et al. 1998; Sjöström 1999a; Colodette et
al. 2000; Chai et al. 2001a; Chai et al. 2001b; Colodette et al. 2002; Pettersson et al. 2002
Pedroso, Carvalho 2003; Daniel et al. 2003; Axelsson et al. 2004). Fig. 8 shows how the
dissolution/degradation of HexA changed when the hydroxide concentration was changed at a
constant hydrogen sulphide ion concentration and a constant ionic strength. As can be seen in
Fig. 8, the model fits the experimental data well. The rate of dissolution/degradation of HexA
increased with increasing hydroxide concentration, increasing hydrogen sulphide ion
concentration, increasing sodium ion concentration, and increasing temperature. The
38
hydroxide ion dissolves/degrades the HexA groups more extensively than the hydrogen
sulphide ion; kOH>>kHS>kNa in Table 3.
At a given kappa number, there is a significant difference in the amounts of HexA in pulp
samples produced under different cooking conditions. The HexA content at a given kappa
number can be reduced by applying a high hydroxide concentration, a high sodium ion
concentration, a low cooking temperature, or a low hydrogen sulphide ion concentration. The
lower HexA content found when pulping with a high sodium ion concentration, a low
temperature and a low hydrogen sulphide ion concentration is mainly a result of the longer
cooking time required to reach the same target kappa level.
The HexA is removed during the kraft cooking by two different mechanisms. The first is
simply a dissolution of xylan, which hence decreases the cooking yield. Since a high cooking
yield is one of the most important targets for a pulp mill, this mechanism must be kept in
mind when trying to optimise the cooking, so that the optimisation does not lead to the
manufacture of a pulp indeed having a low HexA content, but at the expense of a low pulp
yield. Secondly, alkali can act by splitting the HexA group from the xylan back-bone and thus
decrease the HexA content in the pulp, as discussed by Johansson and Samuelsson (1977b).
As can be seen in Fig. 9, the dissolution/degradation of HexA was much greater than that of
the xylan. Extending the cook from kappa number 58 to 11 at a hydroxide concentration of
0.9 mol/l resulted in a decrease in the HexA content of approximately 90 % (≈17 µmol
HexA/g pulp, % on wood) while the decrease in the xylan content was only about 40 % (≈1.1
% on wood), Fig. 9. One explanation could be that the fractions of xylan containing HexA
groups are more soluble than those not substituted with HexA groups. The greater removal of
HexA could also be the result of an uneven distribution of uronic acid groups in the pulp
xylan, most of the uronic acid groups being located on the surface of the pulp fibres and
thereby more easily removed while another part of the xylan already in the wood is more
difficult to dissolve for morphological reasons. However, Lindquist and Dahlman (1998)
suggested a regular distribution pattern for the uronic acid groups linked to the spruce xylan
backbone. In the case of normal liquor-to-wood ratio cooks, the degraded xylan present in the
black liquor which contains uronic acid has a lower tendency to sorb onto on the pulp fibres
than those xylan molecules which have a low uronic acid content (Hansson 1968), and this
would give a higher xylan content than HexA content in the pulp. Danielsson (2006) has
developed a kinetic model of HexA and MeGlcA reactions in pulp and in dissolved xylan
during the kraft cooking of birch. He found that the dissolved xylans are richer in MeGlcA
39
and HexA than the xylan located in the pulp. This strongly suggests that the number of acidic
groups greatly affects the solubility of xylan in the kraft cook.
0
5
10
15
20
25
30
0 1 2 3 4 5
mmol HexA/kg pulp (% on wood)
Xylan (% on wood)
Slope ~ 17
Fig. 9 The amount of HexA versus the xylan content at two different [OH-] levels (▲=0.1 mol/l, ♦=0.9 mol/l).
3.2.2 The formation and dissolution/degradation of HexA in Eucalyptus kraft cooking (Paper IV) Hardwood contains substantially more xylan than softwood. Gellerstedt and Li reported that
typically 3-6 kappa number units of an unbleached HW pulp and 1-3 kappa number units of
an unbleached SW pulp are due to HexA and not lignin (Gellerstedt and Li, 1996). The HexA
content is therefore much more important regarding brightness reversion and consumption of
bleaching chemicals in a hardwood kraft pulp than in a softwood kraft pulp. To increase the
understanding of how and to what extent it is possible to alter the amount of HexA in an
unbleached Eucalyptus Globulus kraft pulp, a number of cooks were performed with different
cooking conditions.
As discussed in the introduction, HexA is formed during the kraft cook through β-elimination
of methanol from the native 4-O-methyl glucuronic acid side group (MeGlcA) in the xylan. In
Fig. 10, the amount of MeGlcA is plotted versus the kappa number for cooks performed at
different hydroxide concentrations. A normal kappa number for Eucalyptus Globulus after
40
cooking is in the range 14-17. As can been seen in Fig. 10, MeGlcA is still present in all the
pulps at this kappa number level, even in the pulp manufactured at a very high hydroxide
concentration. The fact that the conversion of MeGlcA to HexA is not complete during kraft
cooking has also been observed in other investigations of kraft cooking for both softwood and
hardwood (Buchert et al. 1995; Simão et al. 2005).
0
20
40
60
80
100
120
140
160
180
0 5 10 15 20 25 30 35 40 45 50 55 60 65
Kappa number
MeG
lcA
(mm
ol/k
g pu
lp)
Fig. 10 The effect of OH- concentration (♦=0.2, ■=0.5, ▲=1.0 and ×=1.5 mol/l OH-) on the amount of 4-O-Methylglucoronic acid content versus the kappa number. [HS-]=0.3 mol/l, [Na+]=2 mol/l and temperature 150 ° C.
HexA is simultaneously formed and removed during kraft cooking conditions, and the amount
of HexA at a given kappa number therefore depends largely on the kinetics of HexA
formation, degradation and dissolution, in relation to the kinetics of delignification. As a
result, easily delignified wood species such as Eucalyptus Globulus show an increasing
amount of HexA with decreasing kappa number (Chai et al. 2001b, Pettersson et al. 2002).
Wood species that need longer cooking times such as softwood show a decreasing amount of
HexA in the same kappa number interval. Hence the content of HexA in unbleached kraft
pulps is strongly dependent on the wood species used (Chai et al. 2001b, Pettersson et al.
2002). However, the content can also be affected by the process conditions.
41
0
10
20
30
40
50
60
0 5 10 15 20 25 30 35 40 45 50 55 60 65
Kappa number
Hex
A (m
mol
/kg
pulp
)
Fig. 11 The effect of OH- concentration (♦=0.2, ■=0.5, × =1.0 and ▲=1.5 mol/l OH-) on the amount of HexA versus the kappa number. [HS-]=0.3 mol/l, [Na+]=2 mol/l and temperature 150 ° C. In Fig. 11 the amount of HexA is plotted versus kappa number for cooks performed at
different hydroxide concentration. An increase in the hydroxide concentration led to a
decrease in the amount of HexA at a given kappa number. Eucalyptus Globulus showed a
peak amount of HexA located somewhere in the technically most interesting interval around
kappa number 13-19, Fig. 11. Other investigations have shown that the hydroxide
concentration is the most important variable for the HexA content in the pulp and that the
amount of HexA increases with increasing cooking time for Eucalyptus Globulus (Pedroso,
Carvalho 2003; Daniel et al. 2003). When kraft cooking Eucalyptus Grandis to kappa number
16-17, it was also shown that the pulps produced at high residual alkali showed lower
contents of xylans and HexA (Colodette et al. 2000; Colodette et al. 2002). Birch has also
been reported to show a peak amount of HexA around kappa number 15-20 (Axelsson et al.
2004). Axelsson et al. concluded that it is difficult to modify the HexA content in the birch
pulp by altering the cooking conditions, which was indicated by quite similar amounts of
HexA, about 4-5 kappa number equivalents, in all pulps with kappa numbers above 15. The
contribution of HexA to the kappa number was calculated according to the method of Li and
Gellerstedt (1997) where 1 µmol of HexA per gram of pulp corresponds to 0.086 kappa
number units. The same method was used in the work described in this thesis. According to
Fig. 11, the amount of HexA at kappa number 15 corresponds to about 2.0-5.5 kappa number
equivalents depending on the hydroxide concentration used.
42
0
10
20
30
40
50
60
0 5 10 15 20 25 30 35 40 45 50 55 60 65
Kappa number
Hex
A (m
mol
/kg
pulp
)
Fig. 12 The effect of temperature (♦=140 °C, ■=150 °C, and ▲=160 °C) on the amount of HexA versus the kappa number. [OH-]=0.5 mol/l,.[HS-]=0.3 mol/l, [Na+]=2 mol/l.
Fig. 12 shows the relationship between the HexA content and the kappa number for kraft
cooks performed at different temperatures. Formation and dissolution/degradation of HexA
went through a maximum and decreased when the cook was extended to lower kappa number
values. The amount of HexA at a given kappa number is lower for the pulps manufactured at
higher temperatures. However, the effect of changing the temperature is much smaller than
the effect of changing the hydroxide concentration. A slight decrease in the HexA content
with increasing temperature has earlier been reported for both Eucalyptus Globulus and
Eucalyptus Grandis (Daniel et al. 2003; Colodette et al. 2002). According to Danielsson
(2006), a change in temperature primarily affected the MeGlcA content of the dissolved
xylan, whereas the MeGlcA content of pulp xylan was not affected at a given kappa number.
Moreover, the HexA content of pulp xylan was only slightly affected by a change in
temperature, indicating that a high cooking temperature is not a suitable way of minimising
HexA in birch kraft pulp.
The influence of sodium ion concentration and hydrogen sulphide concentration on the
amount of HexA in the unbleached pulp was also studied in this project. According to the
results, the lowest HexA content at kappa number 15 was achieved with the highest sodium
ion concentration or the lowest hydrogen sulphide ion concentration. This is mainly a result of
43
the longer cooking time required to reach the same target kappa level when pulping under
these conditions.
The results indicate that it is difficult to significantly reduce the HexA content in an
Eucalyptus kraft pulp by altering the cooking conditions. The amount of HexA in the pulp at a
normal kappa number level was relatively high, 30-50 mmol/kg, for all cooks included in this
study. A reduction in HexA content can be achieved by forcing the cook towards lower kappa
numbers, using high alkali concentration, or by using a cooking liquor with a low hydrogen
sulphide ion concentration or a high sodium ion concentration. Nevertheless, these strategies
will not fulfil industrial expectations, since such cooking conditions will result in extensive
losses in pulp yield, viscosity, and pulp strength. Therefore, when evaluating different ways to
reduce the amount of HexA, it seems more promising to focus on the selection of the
bleaching sequence.
3.2.3 Dissolution/degradation of glucomannan, xylan, and cellulose in softwood kraft cooking (Paper III) A mathematical model that describes the content of glucomannan and xylan in the pulp as a
function of cooking variables was developed in the same way as was described earlier in
chapter 3.2.1. The rates of degradation of both glucomannan and xylan increased with
increasing hydroxide concentration, increasing hydrogen sulphide ion concentration, and
increasing temperature. It could clearly be seen that a high hydroxide concentration promoted
the degradation of xylan, Fig. 13. According to our results, the ionic strength did not affect
the rate of degradation of glucomannan. A negative value for kNa (-0.9) means that a higher
ionic strength decreased the rate of degradation of xylan, Table 3. This is probably due to a
solubility effect, but the delignification rate also decreases with increasing ionic strength
(Lindgren, Lindström 1996), so the practical implications are not obvious. Cellulose is more
crystalline than hemicellulose and is therefore more resistant to alkaline hydrolysis (Rydholm
1965). The rate of degradation of cellulose increased with increasing hydroxide concentration
or increasing cooking temperature. The hydrogen sulphide ion concentration had no or a very
limited effect on the rate of degradation of cellulose, and the ionic strength had no effect at
all.
44
Fig. 13 The effect of OH- concentration (▲=0.10, ●=0.23, ■=0.44 and ♦=0.90 mol/l OH-) on the degradation rate of xylan. [HS-]=0.28 mol/l, [Na+]=1.3 mol/l and temperature 170 °C.
At a given kappa number, only small differences were observed in the amount of
glucomannan as a result of changes in the cooking parameters. When the hydrogen sulphide
ion concentration was increased, the content of glucomannan was slightly increased. An
increase in the hydroxide concentration or a decrease in the ionic strength gave a pulp with a
higher content of glucomannan at a given kappa number, but varying the cooking temperature
had no effect. At a given kappa number, the xylan content in the pulp was decreased by
increasing the hydroxide concentration (Fig. 14) or decreasing the hydrogen sulphide ion
concentration. Neither the ionic strength nor the cooking temperature had a significant effect
on the content of xylan at a given kappa number. The hydrogen sulphide ion concentration
was the only parameter shown to affect the content of cellulose in the pulp; the higher the
hydrogen sulphide ion concentration, the larger the content of cellulose at a given kappa
number.
45
Fig. 14 The effect of OH- concentration (▲=0.10, ■=0.44 and ♦=0.90 mol/l OH-) on the amount of xylan versus the kappa number. [HS-]=0.28 mol/l, [Na+]=1.3 mol/l and temperature 170 °C.
A comparison of the constants in Eq. [5] for the degradation of HexA, glucomannan and
xylan in Table 3 reveals that, at a given concentration of hydroxide, the rate of
dissolution/degradation of HexA was much greater than that of xylan (Fig. 9), while that of
xylan was somewhat greater than that of glucomannan. The same ranking order for HexA,
xylan, and glucomannan was also valid when considering the hydrogen sulphide ion
concentration. The dissolution/degradation of HexA, xylan, and glucomannan was however
affected more by the hydroxide concentration than by the hydrogen sulphide ion
concentration, since kHO>>kHS, Table 3. The effect of the ionic strength was not only small but
also varying; with increasing ionic strength the dissolution/degradation rate of HexA slightly
increased, that of xylan slightly decreased and that of glucomannan remained unaffected,
Table 3. The activation energies lie within the same order of magnitude, indicating that the
cooking temperature influences the dissolution/degradation of HexA, xylan, and glucomannan
in a similar manner, Table 3.
46
3.2.4 Estimation of mill cooking yield (Paper V) Kraft cooking yield is one of the most important economic variables in the production of
chemical pulp. In a pulp mill, it has been found difficult to accurately measure the yield in a
simple way. As a result there are strong incentives for developing yield determination
methods that are easy to perform and give reliable results. Several models for measuring pulp
yield based on pulp properties have been published (Kleppe 1970; Easty, Malcolm 1982;
Marcoccia et al. 1998a; Vaaler et al. 2002; Çöpür et al. 2003; Van Heiningen et al. 2004). The
method developed within this project is based on the assumption that the cellulose content at a
given kappa number level is the same in a laboratory pulp and in a mill pulp if the laboratory
cook is conducted at the same temperature, hydroxide and hydrogen sulphide ion
concentration as the mill cook. The yield of the laboratory pulp is measured gravimetrically
and the kappa number is determined according to normal testing procedures. This method is
similar to the ”carbohydrate-lignin method“ developed by Easty and Malcolm (1982). The
method presented here is however based on the fact that the cellulose content will vary
depending on the cooking conditions and therefore the laboratory cook should be carefully
performed at the same conditions, i.e. temperature, hydroxide and hydrogen sulphide ion
concentration as in the mill.
The lignin fraction, as part of the original wood, (L) is related to the kappa number by the
approximate relationship:
100/)( ykappaxL ••= [6] where x = (0.9/5.91) and y is the gravimetrically determined yield of the laboratory pulp. The lignin-free yield can be calculated as:
Lyy freeL −=− [7] The cellulose and hemicellulose contents are calculated as:
freeLcellulose ycellulosey −•= [8]
freeLosehemicellul yosehemicelluly −•= [9] where cellulose and hemicellulose are expressed as relative percentage. The cellulose content
in the laboratory pulp is then used to calculate the mill-cooking yield as:
47
mill
cellulosemillcellulosemillmillmill cellulose
yosehemicellulyykappaxy
•++••= [10]
In this study the effects of hydroxide concentration, hydrogen sulphide ion concentration, and
temperature on the cellulose content in a softwood pulp were investigated. In Fig. 15, the
effect of hydroxide concentration on cellulose content at different kappa numbers is
presented. At a given kappa number, increasing the hydroxide concentration increases the
cellulose content. A higher hydroxide concentration would lower the amount of slowly
reacting residual phase lignin and thereby improve the delignification rate during the cook
(Lindgren and Lindström, 1996). This meant that the cooking time required to reach a given
kappa number would be shorter at a higher hydroxide concentration, reducing the cellulose
dissolution. The glucomannan content is also improved by a higher hydroxide concentration.
However, the alkali-sensitive hemicellulose, xylan, is decreased, so that the overall pulp yield
was the same in the studied range of hydroxide concentration, 0.5-1.0 mol/l.
36
36,5
37
37,5
38
38,5
39
39,5
40
14 16 18 20 22 24 26 28 30 32 34 36 38
Kappa Number
[HO-]=0,5 mol/l
[HO-]=1,0 mol/l
Fig. 15 Effect of hydroxide concentration on cellulose content at different kappa numbers. Temperature 155 °C and [HS-] = 0.2 mol/l. The cellulose content at a given kappa number is somewhat increased by lowering the
temperature. The difference in cellulose content at kappa number 25 is less than 0.3 percent
units. However, the effect on the viscosity is much more pronounced. Decreasing the cooking
temperature from 165 °C to 155 °C resulted in a viscosity increase of 70 units at kappa
number 25. Carbohydrate degradation has a higher activation energy than lignin degradation
(Kubes et al. 1983; Lindgren and Lindström 1996; Lindgren 1997), and this means that
48
increasing the cooking temperature affects the lignin degradation less than the carbohydrate
degradation and thus leads to a more extensive cellulose degradation, i.e. a lower viscosity
and a lower cellulose yield.
At a given kappa number, the hydrogen sulphide ion concentration affects the cellulose
content in the pulp; the higher the concentration, the higher the content of cellulose. This
result was also achieved in this study where the hydrogen sulphide ion concentration was
evaluated at 0.2 and 0.4 mol/l. Hydrogen sulphide ions only affect the delignification and do
not affect the carbohydrate dissolution or degradation. Hence, it is always favourable to
increase the hydrogen sulphide ion concentration during the kraft cook in order to improve the
selectivity, i.e. viscosity or carbohydrate yield, at a given kappa number.
The cellulose content versus kappa number data for all cooks included in this study are shown
in Fig. 16. The results show the importance of using the same conditions in the laboratory
cook as in the mill cook to obtain the correct cellulose content for the calculation of mill
cooking yield. The cellulose content at a given kappa number varied by up to 1 percentage
point depending on the cooking conditions. This new mill yield evaluation method is suitable
for both softwood and hardwood mill pulps.
36,5
36,7
36,9
37,1
37,3
37,5
37,7
37,9
38,1
38,3
38,5
38,7
38,9
39,1
39,3
39,5
12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44
Kappa No. Fig.16 Effect of cooking parameters on cellulose content.
49
3.3 Bleachability of softwood and Eucalyptus kraft pulps (Paper VI, VII, VIII)
It is necessary to regard pulping and bleaching together and not as independent processes in
order to minimise the consumption of bleaching chemicals. Several investigations have been
undertaken to study the effects of different cooking conditions on the bleachability of kraft
pulps, both for softwood (Carnö et al.1975; Svedman et al. 1995; Kettunen et al. 1997;
McDonough et al.1997; Sjöström 1999; Al-Dajani 2001; Björklund et al. 2004) and hardwood
(Colodette et al. 2002; Pascoal Neto et al. 2002; Axelsson, Lindström 2004). Furthermore,
attempts have been made to relate the chemical structure of the residual lignin to the
bleachability of the pulp (Froass et al. 1996; Colodette et al. 1998; Gellerstedt, Al-Dajani
2000; Rööst et al. 2003). There is no standard method of evaluating the bleachability of a pulp
nor is there any standard definition of the notion of “bleachability”, and this definitely makes
it difficult to achieve a clear comparison of conclusions from different studies in the literature.
In the present work, two different definitions of bleachability were used.
In the first study (Paper VI), the bleachability of the pulps was evaluated as the consumption
of bleaching chemicals per kappa number to reach a given brightness. The total consumption
of bleaching agents in a bleaching sequence was expressed as the sum of oxidation
equivalents (OXE) divided by the ingoing kappa number for each stage in the sequence. The
OXE calculations for the AZQP*, the QPQP*, and DQP* sequences were:
AZQP*: OXE/kappa number=OXE Z
Kappa O
OXE P
Kappa AZ
( )
( )
( *)
( )2+ [11]
QPQP*: OXE/kappa number**=OXE P
Kappa O
OXE P
Kappa QP
( )
* *( )
( *)
* *( )2+ [12]
DQP*: OXE/kappa number=OXE D
Kappa O
OXE P
Kappa D
( )
( )
( *)
( )2+ [13]
(**=corrected for the HexA contribution to the kappa number)
For OXE calculations, values of 58.79 OXE/kg H2O2, 125.0 OXE/kg O3 and 28.2 OXE/kg
active chlorine have been used (Grundelius 1991). As the amount of HexA did not change
50
during the peroxide bleaching, it was assumed that the HexA did not consume any bleaching
chemicals in the peroxide stage. It could be concluded, however, that HexA consumed
bleaching chemicals in the ozone stage, as no HexA was found in the pulp after that stage
(Ragnar 2000). HexA has also been reported to react with chlorine dioxide (Buchert et al.
1995; Vuorinen et al. 1996).
The way of evaluating bleachability used in Paper VI is somewhat different from that used in
other investigations reported in the literature (i.e. brightness vs. OXE/kappa after the O2 stage,
brightness vs. OXE/∆kappa over the whole sequence). However, when the difference in kappa
number after the oxygen delignification stage is small between the pulps compared, as in this
study, the conclusions regarding the relative order between the pulps will be independent of
the evaluation method used. Different evaluation methods lead to different magnitudes of the
OXE values. The evaluation method used in this thesis takes into account the amount of
HexA (P-stages) and divides the consumed OXE in each stage by the actual lignin content
entering each bleaching stage, as shown in the Eqs. [11]-[13]. According to Ragnar (2004),
the OXE concept should be used with considerable caution. The OXE system only considers
the theoretical transfer of electrons in a completed redox reaction. Hence treatment of a pulp
with different bleaching chemicals having the same amount of OXE can lead to very different
kappa numbers and different ISO brightness values.
The kappa number measurement was first introduced as a standard test method by SCAN-test
in 1959 (C 1:1959) based on the proposal of Tasman and Berzins (1957). According to the
standard test method, one of the scopes was to measure the bleachability of pulp. Instead of
using the consumption of the different bleaching chemicals actually used for the
determination of the bleachability, the consumption of permanganate was to provide a
combined value. However, it was later shown, vide infra, that the material oxidised by
permanganate included not only lignin, but also other unsaturated structures. As the analytical
tools were sharpened, the term bleachability began to be used in the mid-1970’s to describe a
situation where two pulps having the same kappa number still required somewhat different
amounts of bleaching chemicals to reach a target brightness (Carnö et al. 1975). This way of
understanding the term bleachability is the basis for the second study (Paper VII).
51
3.3.1. The influence of cooking condition on the bleaching chemical requirement and chemical structure of softwood kraft pulps (Paper VI) The purpose of this investigation (Paper VI) was primarily to investigate how variations in
cooking conditions in the kraft cooking 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 concentration, hydrogen sulphide
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 bleachability was studied in an AZQP*, a QPQP*,
and a DQP* sequence.
The influence of hydroxide ions on bleachability Many researchers have shown that increasing the effective alkali charge in the cook improves
the subsequent bleachability (Kettunen et al. 1997; Svedman et al. 1995). McDonough et al.
(1997) have reported somewhat different results regarding the influence of EA charge on the
bleachability in a D0(EO)D1ED2-sequence. For pulps pulped to a kappa number of about 27,
an increase in the EA charge in the cook gave an improved bleachability, but the opposite was
obtained for pulps cooked to a kappa number of about 15, i.e. the pulp produced with a low
EA showed a better bleachability in case of pulps cooked to a low kappa number.
Unfortunately, most of the earlier investigations presented in the literature have been devoted
to cooks with a constant sulphidity, which means that the hydrogen sulphide ion concentration
varied with varying alkali charge. It is also important that the sodium ion concentration is kept
the same in the different cooks. Sjöström (1999a) has shown that a low ionic strength in the
cook improves the QPQP*-bleachability after oxygen prebleaching. Keeping the other
cooking parameters as constant as possible would make it easier to isolate the effect of the
hydroxide concentration.
The QPQP* bleachability of pulps manufactured with different hydroxide concentrations and
a constant hydrogen sulphide ion concentration of 0.5 mol/l is shown in Fig. 17. A
bleachability maximum could here be noted for the pulp manufactured with an intermediate
hydroxide concentration. The same result was obtained for the AZQP*-sequence. The
difference in bleachability between the intermediate and the high hydroxide concentration was
not, however, pronounced. For the whole process, from the cook to the fully bleached pulp,
the process selectivity expressed as viscosity at a given brightness was also best for the pulp
52
manufactured at the intermediate hydroxide concentration. These results are in agreement
with earlier results for QPQP*-bleached pulps (Sjöström 1998). At a low hydrogen sulphide
ion concentration ([HS-]=0.075 mol/l), the difference in QPQP* and AZQP* bleachability
between the pulps produced at a high or a low hydroxide concentration was more pronounced
than at [HS-]=0.5 mol/l, in favour of the pulp manufactured at a high hydroxide concentration.
The process selectivity for the pulps manufactured at [HS-]=0.075 mol/l was very low since
the viscosity was low already after the cook and after the oxygen delignification stage.
Fig. 17 ISO Brightness vs consumed OXE/kappa number** for QPQP*-bleached pulps, pulped at [HS-]=0.5 mol/l and at different [OH-].
The ISO brightness of pulps manufactured at a constant hydrogen sulphide ion concentration
of 0.5 mol/l and bleached in a DQP*-sequence is shown in Fig. 18. The pulps manufactured
with a low or intermediate hydroxide concentration tended to have a slightly better
bleachability than the pulp manufactured at the high hydroxide concentration. In the case of
pulps manufactured with a low hydrogen sulphide ion concentration ([HS-]=0.075 mol/l), a
high hydroxide concentration led to a better bleachability than a low hydroxide concentration,
i.e. the same result as after a QPQP*, or AZQP* sequence.
53
Fig. 18. ISO Brightness vs consumed OXE/kappa number for DQP*-bleached pulps, pulped at [HS-
]=0.5 mol/l and at different [OH-].
The influence of hydrogen sulphide ions on bleachability As can be seen in Fig. 19, the bleachability in a QPQP* sequence was improved by a higher
hydrogen sulphide ion concentration in the cook. In addition, the higher the alkali
concentration, the less was the bleachability negatively affected by a low hydrogen sulphide
ion concentration. In the case of pulp pulped with high hydroxide concentration ([OH-]=1.5
mol/l), the hydrogen sulphide ion concentration had an effect only at a very high brightness
level. The process selectivity was also improved by increasing the hydrogen sulphide ion
concentration.
Fig. 19 ISO Brightness vs consumed OXE/kappa number** for QPQP*-bleached pulps pulped at [OH-]=0.55 mol/l and at different [HS-].
54
Fig. 20 ISO Brightness vs consumed OXE/kappa number for DQP*-bleached pulps pulped at [OH-
]=0.55 mol/l and at different [HS-]. The same trend was seen in a DQP*-sequence, i.e a pulp manufactured with a high hydrogen
sulphide ion concentration had a better bleachability than a pulp manufactured with a low
hydrogen sulphide ion concentration, Fig. 20. According to McDonough et al. (1983), the
bleachability in a (C+D)ED-sequence for pulps in a kappa number range of 15-35 was almost
independent of the sulphidity in the cook. In contrast to QPQP*- and DQP*-bleaching, the
hydrogen sulphide ion concentration in the cook did not affect the bleachability in an AZQP*-
sequence, Fig. 21.
Fig. 21 ISO Brightness vs consumed OXE/kappa number for AZQP*-bleached pulps pulped at [OH-]=0.55 mol/l and at different [HS-].
55
The influence of temperature on bleachability The DQP* bleachability was improved when the cooking temperature was increased from 160
°C to 180 °C when pulping with [OH-]=0.55 mol/l and [HS-]=0.075 mol/l (Fig. 22).
Fig. 22 ISO Brightness vs consumed OXE/kappa number for DQP*-bleached pulps, pulped at [OH-
]=0.55 mol/l and [HS-]=0.075 mol/l and at different temperatures.
The differences in DQP* bleachability were small between the pulps manufactured at
different cooking temperatures when the chemical charges were increased in the cook. The
QPQP* bleachability was improved by increasing the cooking temperature for pulps
manufactured with low and medium hydroxide concentration, but was unchanged when the
hydroxide concentration was high. A slightly better AZQP* bleachability was seen in the case
of the pulps produced with a higher cooking temperature, but only at high brightness levels
(greater than 92 % ISO). The process selectivity decreased with increasing temperature.
Relationship between bleachability and the characteristics of the unbleached pulp It has for a long time been known that pulps with the same kappa number can differ a lot in
brightness after the kraft cook. The reason for this is not fully understood. Therefore, it was of
interest to study how pulps manufactured under different cooking conditions to the same
kappa number differ from each other, for example by looking for differences in metal profile,
chemical structure, and content of HexA. Moreover, it was of interest to try to relate detected
differences to differences in bleachability.
56
Fig. 23 The light absorption coefficient, k, (measured at 457 nm) per kappa number** after the cook and after the oxygen stage vs residual alkali concentration in the cook.
Differences in ISO brightness of unbleached pulps have earlier been explained as being due to
a difference in resorption of lignin during the cook. Lignin isolated from the black liquor was
found to have a higher light absorption coefficient than lignin isolated from the pulp. The
degree of resorption was found to increase with increasing concentration of lignin dissolved in
the black liquor and also with decreasing pH (Surewicz 1962; Hartler, Norrström 1969; Janson
et al. 1975). The results obtained in this project showed that the k/kappa number** ratio
(**=corrected for the HexA contribution to the kappa number) of the unbleached pulp, after
the cook and after the oxygen delignification, decreased with increasing residual alkali
concentration in the black liquor, Fig. 23. The same result was obtained with an increasing
hydrogen sulphide ion concentration, but to a much smaller extent. An increase in the
temperature did not affect the k/kappa number** ratio to any great extent, except for pulps
manufactured with very low chemical charges in the cook, in which case the k/kappa
number** ratio decreased with increasing cooking temperature. The pulps with the best
QPQP* bleachability had a lower k/kappa number** ratio already after the cook as well as
after the oxygen delignification stage. Exceptions were noted at very high hydroxide
concentrations and when the temperature was increased at very high chemical charges. In such
cases, no further improvement in the bleachability was seen.
Metal ions bound to the unbleached pulp can also be suspected to cause a difference in
brightness at a constant kappa number, and it would be an advantage to be able to reduce the
57
unwanted metals already in the cook and thereby make the metal management easier in the
bleaching stages. The metal ion contents in the unbleached pulps differed from each other, as
shown in Table 4, where some of the investigated pulps and metal ions are shown. There was
a distinct decrease in the content of calcium when cooking with a high hydroxide
concentration. One possible explanation is that most of the calcium is bound to dissolved
lignin fragments at a high hydroxide concentration and that the amount of calcium carbonate
precipitated on the pulp fibres is therefore reduced. In a recent study, it was shown that
calcium exerted a negative impact on the rate of delignification in the kraft cooking of birch
(Lundqvist et al. 2005). Hence, if the white liquor used in the laboratory contains carbonate,
as industrial white liquors do, one would assume that part of the wood-orginating calcium
would precipitate as calcium carbonate and that its negative effect would consequently be
reduced (Lundqvist et al. 2006). The composition of the white liquor should consequently be
important for the cooking results. The wood species used for kraft pulping and the structure of
the dissolved organic components in the black liquor also affect the propensity of calcium to
precipitate as calcium carbonate (Westervelt et al. 1982; Lidén et al. 1996). One of the main
questions of interest regarding calcium and carbonate is the problem caused by the
precipitation of calcium carbonate on hot surfaces in pulping and recovery operations.
According to Magnusson et al. (1998), this calcium carbonate scaling can be reduced if the
black liquor is deactivated by a heat treatment. An increase in the hydroxide concentration
also reduced the amount of barium and manganese in the pulp, Table 4. However, after QP
bleaching, the difference in metal content between the pulps was negligible. No simple
relationships were found between the contents of the other investigated metals and the pulping
conditions.
Table 4. Metal contents in the pulps after the cook. Temp. (°C)
Charged [HS-] mol/l
Charged [OH-] mol/l
Ca ppm
Fe ppm
Mg ppm
Ba ppm
Cu ppm
Mn ppm
160 0.5 1.5 373 26 150 <0.20 0.56 46 154 0.5 1.0 610 12 250 3.6 0.94 68 160 0.5 0.55 703 28 194 5.1 1.09 89
160 0.075 1.5 367 20 252 3.1 1.11 50 160 0.075 0.55 719 28 183 4.9 3.25 49
178 0.5 1.5 405 10 198 3.4 0.24 40 179 0.5 0.55 733 10 221 4.7 0.76 87
58
The content of β-aryl ether linkages, the predominant lignin structure in softwood, was
determined in the residual lignin after the cook. A pulp containing a high content of β-aryl
ether structures was obtained when cooking with a high hydroxide and a high hydrogen
sulphide ion concentration. There was no clear relationship between the amount of phenolic
hydroxyl groups and the different cooking parameters. A high hydroxide concentration gave a
pulp having a high mannan content and a low xylan content, as earlier reported by Aurell et
al. (1965b). In this study, cooking with a low hydroxide concentration and/or a low
temperature led to a pulp with a low methoxyl content. A higher content of enol ethers was
present in the pulp after pulping with high chemical charges, i.e. after a short cooking time. A
high content of enol ethers is not a disadvantage for the subsequent bleaching result, since
they readily react in the oxygen delignification stage (Ljunggren, Johansson 1990). The HexA
content in a pulp could be greatly reduced by using a high hydroxide concentration, a low
hydrogen sulphide ion concentration and a long cooking time.
It was difficult to establish clear relationships between the chemical structure of the residual
lignin and the bleachability of the pulp. However, it could be seen that the higher the content
of β-aryl ether structures in the residual lignin after cooking, the easier it was to bleach in a
QPQP*-sequence. Froass et al. (1996) showed that a high content of aryl ether linkages and a
low content of condensed structures in the residual lignin gave a lignin which was particularly
reactive towards chlorine dioxide. Heuts et al. (1998) showed that β-aryl ether structures react
with hydrogen peroxide in such a way that lignin dissolution is promoted through cleavage of
the side chains and the formation of aliphatic carboxylic acids. Free phenolic structures are
believed to be the main site of attack by several bleaching agents, such as oxygen and chlorine
dioxide. Nevertheless, the content of free phenolic hydroxyl groups in the unbleached residual
lignin could not be related to the observed bleaching responses in this study. Neither the xylan
nor the mannan content of the pulp after cooking affected the QPQP* bleachability.
59
3.3.6 Pulp yield vs. HexA content and the effect of HexA content after cooking on the bleaching chemical requirement (Paper VII, VIII) It is generally believed that a low HexA content after cooking is favourable for bleaching and
for yellowing (Buchert et al. 1997; Evtuguin et al. 2002; Granström et al. 2000; Granström et
al. 2001; Kobayashi et al. 2005; Wennerström 2005), but the balance between optimal
cooking and optimal bleaching is a delicate issue still to be decided (Lindström and Larsson
2003). 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 perhaps 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 and whether it shows a higher yellowing
tendency. Here the aim was therefore to compare how single-species hardwood pulps cooked
to the same kappa number, but different HexA and lignin ratios (Fig. 24), responded to three
different bleaching sequences. Each of these bleaching sequences represented a different
approach dealing with the HexA content in the brown pulp, the alternatives being; accepting a
high final HexA, degrading HexA oxidatively and degrading HexA to a large extent
hydrolytically in addition to oxidatively.
0
2
4
6
8
10
12
14
16
18
20
High temperature cooking Low temperature cooking
kapp
a nu
mbe
r afte
r coo
king
lignin HexA
Fig. 24 When two hypothetical pulps are compared at the same cooking kappa number they may differ in the composition of the kappa number. A high cooking temperature typically yields a pulp with a somewhat lower HexA content and thus higher lignin content at a given kappa number than a pulp manufactured at a low cooking temperature.
60
Cooking results It is well known that the cooking conditions affect the HexA content of the cooked pulp. It is
thus easy to manufacture pulps with different HexA contents. However, in a scientific study it
is important that the pulps being compared have been manufactured in a manner relevant for
comparison. Charging significantly different amounts of alkali would give pulps with
significantly different HexA contents, but these pulps would also have very different
bleachabilities (Carnö et al. 1975; Svedman et al. 1995), also indicated by greatly different
ISO brightness levels (Axelsson and Lindström 2004). Instead it was here chosen to vary the
temperature to obtain pulps with different HexA contents, which does not lead to any
significant difference in ISO brightness. The residual alkali, which is well known to affect the
bleachability, was however kept constant. To achieve the same kappa number when the
temperature was changed, the cooking time had to be adjusted in relation to the cooking
temperature. Moreover, since modern kraft cooking concepts differ from older concepts in
terms of temperature (“modern cooking” is carried out at lower temperature than “old
cooking”), the cooking temperature is clearly the parameter to be varied to obtain different
HexA contents, although the differences obtained are rather limited compared to those
obtained by varying the alkali charge.
Kraft cooking experiments in the laboratory were performed using industrial hardwood chips
from different mills. In order to achieve the lower HexA content, a significantly higher
temperature had to be applied, 16–19 ºC more, for the different raw materials included in this
study. In Fig. 25 the HexA content is plotted versus kappa number for the Eucalyptus Grandis
pulps. Fig. 25 shows that at a constant kappa number, cooking at high temperatures of 160–
165 °C generated a pulp with a HexA content that was 10–13 % lower than did cooking at
low temperatures of 143–146 °C.
61
55
57
59
61
63
65
67
69
71
73
75
13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0
Kappa number
Hex
A [m
mol
/kg
BD
pul
p]
Low temperature 143–146 °C High temperature 160–165 °C
Fig. 25 HexA content plotted versus kappa number for E. Grandis pulps manufactured at low and high cooking temperatures. The pulps marked with circles were further investigated in oxygen delignification and bleaching trials.
The high and low HexA pulps of Eucalyptus Grandis differed in HexA content by 9 mmol/kg
pulp, which corresponded to a difference in HexA-related kappa number of 0.8 units (Li and
Gellerstedt 1997). The cooking yield was 0.5–1.0 percentage points lower for the pulps
manufactured at a high temperature. The trend of lower yields obtained at the higher cooking
temperature could also have been the result of less precipitation of dissolved xylan back to the
fibres with the shorter cooking time. At the same kappa number level, the xylan content was
approximately 0.5–1.0 relative % (relative xylan content of total carbohydrate content
analysed in a pulp sample) lower for the pulps manufactured at high temperature, which
indicates that the cooking yield was lower. The viscosity also suffered when the cooking
temperature was raised. Depending on the raw material, the viscosity was 70–150 dm3/kg
lower for the pulps manufactured at the high temperature.
62
Oxygen delignification results The Eucalyptus Grandis and Eucalyptus Globulus pulps were subjected to oxygen
delignification. In Fig. 26, the kappa number is plotted against the alkali charge in the oxygen
delignification for the Eucalyptus Grandis high HexA (after cooking)/high yield and low
HexA (after cooking)/low yield pulps.
7
8
9
10
11
12
13
14
15
16
0 5 10 15 20 25 30 35 40
Alkali charge [kg/BDMT]
Kap
pa n
umbe
r
E. Grandis, High HexA (after cooking) E. Grandis, Low HexA (after cooking)
Fig. 26 The results of the preliminary oxygen delignification trials;The kappa number after oxygen delignification is plotted against the alkali charge for the E. Grandis pulps. At a given alkali charge, the high HexA (after cooking) pulp tends to have a higher kappa number. Although the two pulps in Fig. 26 had the same cooking kappa number, 15.2, and were
manufactured from the same raw material, their behavior in the oxygen delignification
differed somewhat. In the cooking, it was seen that the high HexA pulp (after cooking) was
also a high yield pulp and vice versa. In Fig. 26, a trend could be seen that the high-HexA-
containing pulp cooked at the lower temperatures did not delignify in an oxygen-
delignification stage to the same degree as did a low HexA-containing pulp cooked at high
temperatures. The ISO brightness at a given kappa number after oxygen delignification was
however higher for the high HexA (after cooking)/high yield pulp. This is logical since HexA
is colourless whereas lignin is coloured. The same general pattern was obtained in the trial
with the Eucalyptus Globulus pulps. Traditionally, chlorine dioxide in the D0 or D* stage is
charged in relation to the kappa number according to the charge factor, assuming that a lower
kappa number would lead to a saving of bleaching chemicals. In the literature it has been
proposed that, at least for a hardwood kraft pulp, it is the ISO brightness after oxygen
delignification rather than the kappa number that determines the bleaching chemical
63
requirement (Axelsson, Lindström 2004). This indicates that the bleaching chemical
consumption of the high HexA pulp, with a slightly higher kappa number after oxygen
delignification, need not be higher than that of the low HexA pulp.
As seen in Fig. 26, there was a decrease in the efficiency of the oxygen delignification at
lower kappa numbers. This decrease is generally ascribed to fewer available free phenolic
groups (Gellerstedt et al. 1986) or to the presence of Lignin–Carbohydrate Complexes (LCC)
(Gellerstedt et al. 1991; Chen et al. 1996; Vikkula et al. 1997; Chirat et al. 1998; Tamminen
and Hortling 2001). To this should be added today’s understanding that the kappa number
reflects not only the lignin but also the amount of HexA and other structures in the pulp (see
e.g. Li et al. 2002), which are not reactive towards oxygen. Antonsson et al. combined these
two aspects and claimed that LCC’s play a central role in determining the reactivity of the
residual lignin (Antonsson et al. 2003). Several attempts have been made to relate the
bleachability of the pulp to the structure of the lignin remaining in the pulp after the cook, the
so-called “residual lignin”. So far, no unequivocal relation has been found. There are
innumerable reports in the literature relating to the structure of residual lignin after cooking
and various other treatments. However, most of them suffer from the problem of how the
residual lignin has been isolated. It is particularly complicated to isolate lignin without
affecting it chemically and especially so when it is in a matrix where the total lignin content
amounts to only one per cent or so of the pulp. In addition, the isolation of the lignin or
residual lignin is far from complete and the yield is usually only 30–80 per cent (Gellerstedt,
Al-Dajani 1996; Jääskeläinen et al. 2003). To summarise: there are no non-destructive
methods for the isolation of lignin, as has been pointed out by e.g. Sjöström et al. (1999). This
means that the lignin isolated from the pulp probably has a structure that differs at least partly
from the residual lignin in the pulp. This indicates that, in judging the reactivity of the
residual lignin, the model compound approach or an approach in which a specific structure
element of the residual lignin is quantified is a reasonable one. However, this pattern is
complicated by the existence of Lignin–Carbohydrate Complexes (LCC). For an up-to-date
review see e.g. (Lai 2001; Lawoko et al. 2003a). The existence of LCC in pulp is now
indisputable, although their significance is still a matter of discussion.
The results of the oxygen delignification trials on kraft, prehydrolysis kraft and magnesium
sulphite pulps published by Antonsson et al. (2003) allowed a further comparative analysis to
be made. Scrutinizing the data in fact presented an alternative path to explore information on
64
the nature of the remaining lignin in the pulp after cooking and particularly after oxygen
delignification, the so-called residual lignin (Paper VIII). The study resulted in a proposal to
organise the residual lignin into LCC-bound oligolignins after cooking and almost solely of
LCC-bound monolignins after oxygen delignification. The nature of the residual lignin
according to this hypothesis is given in Fig. 27.
L
L
L
L
L
L
L
L
L
L
L
OH
HO
L
HO
HO
HO
LOH
I
L
L
L
L
L
L
L
L
L
L
L
OH
HO
L
HO
HO
HO
LOH
II Fig. 27 Hypothetical generalised structures of residual sulphite (I) and kraft (II) lignin after cooking. The chain represents a part of a hemicellulose chain including the two end-groups of this polymer. In the sulphite case, the unfilled rings indicate labile end-groups, whereas the bars in the kraft case indicate end-groups having passed the stopping mechanism where no further alkaline peeling can occur. The structures indicate the nature of the residual lignin in terms of oligolignins bound to the hemicellulose backbone via LCC bonds. Note that no separate lignin macromolecule is indicated. In the case of oxygen delignification, the theory presented in Paper VIII implies that the two
kinetic phases of oxygen delignification of a kraft pulp could be given a chemical
interpretation; the initial phase being effectively “phenolate delignification” i.e. according to
the well-known route of the phenolate reaction with oxygen, and the final slow phase being
“peeling delignification” i.e. end-wise alkaline peeling of the carbohydrates to release LCC
bound lignin fragments.
Bleaching results The oxygen-delignified Eucalyptus Grandis pulps were bleached in three different bleaching
sequences, D*(OP)D, (DQ)(PO) and (ZD)(OP)D while the oxygen-delignified Eucalyptus
Globulus pulps were bleached in two different bleaching sequences, D*(OP)D and
(ZD)(OP)D.
65
The ISO brightness is plotted against the total chlorine dioxide charge for the D*(OP)D and
(ZD)(OP)D sequences for the high HexA (after cooking)/high yield and low HexA (after
cooking)/low yield pulps for Eucalyptus Grandis in Fig. 28.
89.5
90.0
90.5
91.0
91.5
92.0
92.5
93.0
2 3 4 5 6 7 8 9 10 11 12
Total chlorine dioxide charge [kg ClO2/ADMT]
ISO
Brig
htne
ss [%
]
OD*(OP)D, E. Grandis, High HexA (after cooking) OD*(OP)D, E. Grandis, Low HexA (after cooking)O(ZD)(OP)D, E. Grandis, High HexA (after cooking) O(ZD)(OP)D, E. Grandis, Low HexA (after cooking)
Fig. 28 ISO brightness plotted against total chlorine dioxide charge for E. Grandis pulps bleached in OD*(OP)D and O(ZD)(OP)D sequences. In the (ZD)(OP)D sequence, in which the HexA is oxidatively degraded mainly by the ozone,
there was no difference in bleaching chemical requirement to reach the target brightness
between the high HexA (after cooking)/high yield and the low HexA (after cooking)/low yield
Eucalyptus Grandis pulps (Fig. 28). This is logical, since the oxidising chemical pays little
attention to what it oxidises, so that the kappa number reduction due to oxidative reactions is
what governs the bleaching chemical requirement. A slightly different result was obtained for
the same sequence applied to the Eucalyptus Globulus pulps, where the high HexA (after
cooking)/high yield pulp required less chlorine dioxide to reach the target brightness, a saving
of 0.7 kg chlorine dioxide/ADt. It has been proposed that the oxidative degradation by ozone
of one kappa number unit originating in HexA requires significantly less ozone than the
degradation of one kappa number unit originating in lignin (Ragnar 2000).
In to the D*(OP)D sequence, where a substantial part of the HexA is degraded hydrolytically,
it was seen that high HexA (after cooking)/high yield pulps require less chlorine dioxide to
66
reach the target brightness than low HexA (after cooking)/low yield pulps (Fig. 28). The same
result was obtained for the Eucalyptus Globulus pulps. This follows from the fact that the
more HexA a pulp with a given fixed kappa number contains, the stronger is the driving force
for the hydrolysis to occur and the greater is the hydrolytic work of the D* stage, and
correspondingly the need for oxidative chemicals like chlorine dioxide is lower.
Fig. 29 shows the ISO brightness plotted against the hydrogen peroxide consumption in the
(PO) stage of a (DQ)(PO) bleaching sequence for the Eucalyptus Grandis pulps.
90.0
90.5
91.0
91.5
92.0
92.5
93.0
0 2 4 6 8 10 12 14
Hydrogen peroxide consumption [kg H2O2/ADMT]
ISO
Brig
htne
ss [%
]
O(DQ)(PO), E. Grandis, Low HexA (after cooking) O(DQ)(PO), E. Grandis, High HexA (after cooking)
Fig. 29 ISO brightness plotted against hydrogen peroxide consumption for E. Grandis pulps bleached in the (DQ)(PO) sequence.
The chlorine dioxide charge to the (DQ)-stage was fixed, whereas the hydrogen peroxide
charge was varied in the (PO)-stage. The figure shows no difference between the high HexA
(after cooking)/high yield pulp and the low HexA (after cooking)/low yield pulp in the
hydrogen peroxide requirement to reach the target brightness. This also seems likely since the
HexA has not been extensively degraded (Törngren and Ragnar 2002), leaving the fully
bleached pulps with a fairly high residual kappa number, mostly made up of HexA. This
residual kappa number was 3.7 for the low HexA (after cooking)/low yield pulp and 4.1 for
the high HexA (after cooking)/high yield pulp, corresponding to 28 and 32 mmol HexA/kg
pulp.
67
Yellowing characteristics As discussed earlier, there are numerous reports stating that HexA is an important contributor
to yellowing. The yellowing tendency (the brightness reversion obtained under well defined
laboratory conditions) for the Eucalyptus Grandis pulps bleached in the three different
sequences is given in Fig. 30 after yellowing for 4 h at 105 °C in an oven, and in Fig. 31 after
yellowing for 96 h at 70 °C at 92 % dry content.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
(DQ)(PO) D*(OP)D (ZD)(OP)D
Yello
win
g te
nden
cy [I
SO u
nits
]
E. Grandis, High HexA (after cooking) E. Grandis, Low HexA (after cooking)
32 28 8 3 2 1
Fig. 30 The yellowing tendency (4h, 105 ºC) interpolated at ISO brightness 91.5 %. The numbers printed inside the staples refer to the HexA content [mmol HexA/kg pulp] of the bleached pulps.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
(DQ)(PO) D*(OP)D (ZD)(OP)D
Yello
win
g te
nden
cy [I
SO u
nits
]
E. Grandis, High HexA (after cooking) E. Grandis, Low HexA (after cooking)
32 28 8 3 2 1
Fig. 31. The yellowing tendency (96 h, 70 ºC) interpolated at ISO brightness 91.5 %. The numbers printed inside the staples refer to the HexA content [mmol HexA/kg pulp] of the bleached pulps.
68
For the Eucalyptus Grandis pulp bleached in the (DQ)(PO) sequence, which leaves a high
residual HexA in the fully bleached pulp, the high HexA (after cooking)/high yield pulp
showed a higher yellowing tendency than the low HexA (after cooking)/low yield pulp.
However, for the sequences which degrade the HexA either oxidatively, (ZD)(OP)D, or to a
substantial extent hydrolytically, D*(OP)D, the pattern was the opposite. The pulp originally
having a high HexA after cooking (and high yield) showed a tendency to have a slightly lower
yellowing tendency when fully bleached, although it had a higher (but still small) residual
HexA content. The results were similar regardless of the yellowing procedure, although the
(DQ)(PO) bleached pulp showed a greater yellowing tendency in the long-time/humid
approach for an unclear reason. The same pattern has earlier been reported by (Ragnar and
Dahllöf 2002). This study could not reveal any clear correlation between the HexA content in
the bleached pulps and the yellowing tendency. Nor could any relation between the HexA
content in the brown pulps and the yellowing tendency after bleaching to full ISO brightness
be established.
69
4. Conclusions
4.1 General conclusions It has previously been shown that the delignification of wood chips in the kraft process can be
considered to involve three parallel phases. Using this approach, the activation energy of the
slowly reacting residual phase delignification was found to be higher than that of the bulk
phase delignification. If the data were instead analysed using a model with two consecutive
reactions, the opposite result was obtained, i.e. a higher value was obtained for the bulk phase
than for the residual phase delignification. This explains why some authors have reported
higher activation energies for bulk phase than for residual phase delignification, although a
higher activation energy for the more difficult to degrade residual phase lignin seems more
reasonable. The successful use of a model with three parallel phases for both constant
composition cooks (high liquor-to-wood ratio) and normal cooks indicates that the residual
phase lignin, or at least a major portion of it, is native. Due to the dissolved wood
components, the delignification rate in the bulk phase was 20 % higher in the normal cooks
than in the constant composition, while the delignification rate in the residual phase was 30 %
lower in the normal than in the constant composition cooks. To decrease the amount of
residual phase lignin, it was essential to have a high concentration of hydrogen sulphide ions
when cooking at 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.
It is 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 extensive losses in pulp yield, viscosity, and strength.
Brown softwood kraft pulps with the same kappa number can differ considerably in
brightness. Pulps manufactured under different cooking conditions to the same kappa number
were analysed with respect to their differences in light absorption coefficient (k), metal
70
profile, chemical structure and bleachability. The pulps with the best QPQP* bleachability
had a lower k/kappa number** ratio (**=corrected for the HexA contribution) already after
the cook, as well as after the oxygen delignification stage. Exceptions were noted at very high
hydroxide concentrations and when the temperature was increased at very high chemical
charges, where no further improvement in the bleachability was seen. It was difficult to
establish any clear relationship between the chemical structures of the residual lignin and the
bleachability of the pulp. However, it could be seen that the higher the content of β-aryl ether
structures in the residual lignin after cooking, the better was the QPQP*-bleachability.
Manufacturing hardwood kraft pulp in a low-temperature cooking process leads to a pulp with
a significantly higher yield, but also with a somewhat higher HexA content than kraft pulp
manufactured in a high-temperature cooking process. After oxygen delignificatuion and
bleaching in D*(OP)D, (ZD)(OP)D and (DQ)(PO) sequences, these pulps showed that the
bleaching chemical requirement to reach a target ISO brightness was dependent on the
sequence. The differences could be explained by the different ways in which the HexA in the
brown pulp was taken care of in the bleaching. In the case of the D*(OP)D sequence, a high
HexA content after cooking had a positive effect in achieving a low bleaching chemical
requirement. It should thus be possible to optimise the kraft cook in a hardwood pulp mill
towards a high yield without the risk of subsequent penalties in the form of an increased
bleaching chemical consumption. Nor could any clear correlation be established between the
HexA content in the unbleached pulp and the yellowing tendency after bleaching to full ISO
brightness.
The most important conclusions of the present work can be summarised as:
• The activation energy of the residual phase delignification of the kraft cook is higher
than that of the bulk phase delignification.
• Dissolved wood components increase the delignification rate in the bulk phase of a
kraft cook.
• The lignin reacting according to the residual phase kinetics in a kraft cook, or at least a
major portion of it, is probably native.
• It is difficult to significantly reduce the HexA content in a kraft pulp by altering the
cooking conditions for either softwood or the hardwood Eucalyptus Globulus.
71
4.2 Industrial applicability It is always a challenge to transform research results into industrial applications that can be
used in daily life or lead to process or system changes that decrease investment costs and/or
operational costs. I have tried to make such a transformation and, based on the present study,
it seems reasonable to offer the following industrial recommendations:
• Dissolved wood components (DWC) have a positive effect on the cooking yield since
DWC increases the delignification rate and the dissolved xylan in the black liquor
precipitates back onto the fibres. In other words, industrial cooking aiming for a high
yield should be performed in a cooking system where the DWC is extracted as late as
possible from the process.
• Optimisation of a kraft cook should be focused on a high cooking yield and
parameters such as the HexA content should be dealt with in the bleaching. The higher
content of xylan and HexA is also favourable for the strength properties of the final
pulp.
• Wood consumption must be measured over a long testing period to achieve an
accurate yield determination. The method presented in this work is an alternative way
of estimating the pulp yield in a mill for process changes during brief periods.
4.3 Looking into the future In the early 1990’s, a lot of work was focused on extended delignification in the kraft cook.
The driving force behind the extended kraft cooking was the need to further reduce the
environmental impact of the bleaching. At this time, the Nordic pulp industry was also highly
engaged in turning away from chlorine bleaching to ECF and TCF technologies where TCF
seemed to be the ultimate goal. However, the TCF bleaching chemicals are not as selective as
chlorine dioxide and a low kappa number was therefore necessary to reach high brightness
levels. After some years in the pulp and paper business, my beliefs have changed regarding
extended kraft cooking. Since the wood raw material is the main cost for a pulp mill, it is
more logical to focus on achieving a high cooking yield, i.e. not delignify extensively in the
kraft cook but instead remove the remaining lignin in the subsequent oxygen delignification
stage and in the bleaching. It is well established that using oxygen delignification instead of
extending the kraft cook is an efficient way of increasing the overall pulp yield. However,
72
industrial oxygen delignification is seldom carried out to a kappa number below 10-12, either
for softwood or for hardwood. The decrease in efficiency of the oxygen delignification of
kraft pulps at low kappa numbers has been investigated in several publications. To obtain a
further delignification, an improved understanding of the limitations of the oxygen
delignification process remains as an important area of research. A high kraft cooking yield in
combination with an extended and selective oxygen delignification stage would give both a
high overall yield and less environmental impact due to lower requirement of bleaching
chemicals. If an industrial system for extended oxygen delignification were a reality, i.e. a
system giving a kappa number significantly lower than 10 after the stage, it would be possible
to terminate the kraft cook at even higher kappa numbers and further increase the overall pulp
yield. However, increasing the kraft cooking kappa number has a limit since the reject content
also increases with increasing kappa number. Therefore, another area of research may well be
to further study the possibility of moving the defibration point to higher kappa number levels.
Today, enzymes are not in general use in mill-scale applications. If it were possible to develop
more selective enzymes which do not give significant yield losses, it might be interesting to
treat the pulp with such an enzyme prior to the oxygen delignification stage in order to break
up lignin-carbohydrate complexes.
73
5. A guide to abbreviations and technical terms
Black liquor is formed during the cook when the hydroxide ions are consumed and lignin
together with carbohydrates and extractives are dissolved in the cooking liquor.
Bleachability of a pulp refers to the effectiveness with which chromophores and/or lignin are
removed, measured as ISO brightness or light absorption coefficient value attained with a
certain bleaching agent charge or consumption.
”Constant composition” cooks, cooks in which the liquor-to-wood ratio is very high so that
approximately constant concentrations of cooking chemicals are maintained throughout the
cook and the concentration of dissolved lignin is minimised.
ECF, Elemental Chlorine Free bleaching is bleaching without the use of molecular chlorine
or hypochlorite but with the use of chlorine dioxide.
Effective alkali (%), EA, is a measure of the concentration of hydroxide ions in the white
liquor. Due to the complete hydrolysis of the sulphide ion, it is equal to the sum of sodium
hydroxide and half of the sodium sulphide.
EA NaOH Na S= +1 2 2/
where the amounts of NaOH and Na2S are expressed as weight of NaOH.
HexA, Hexenuronic acid (i.e. 4-deoxy-β-L-threo-hex-4-enopyranosyluronic acid).
H-factor, (Vroom, 1957), is used to express cooking time (t) and temperature (T) as a single
variable for delignification. When the relative reaction rate is plotted against the cooking time
in hours, the area under the curve is the H-factor. Vroom assumed an activation energy of 134
kJ/mol.
∫ −=− dtefactorH T )/161132.43(
74
ISO Brightness (%) is by definition the intrinsic reflectance factor measured at an effective
wavelength of 457 nm under specified conditions. The brightness is also a commonly used
parameter to study the progress of bleaching, where e.g. a pulp having an ISO brightness
exceeding 88-90 % is normally considered to have reached “full brightness”.
Kappa number, an analytical method developed to estimate the amount of lignin in the pulp.
A decrease in lignin content gives a decrease in Kappa number. It is defined as millilitres of
0.02 mol/l of potassium permanganate consumed by one gram of dry pulp according to a
standard procedure, ISO 302:2004. It has more recently been shown that pulps contain
significant amounts of structures that are not lignin but which also contribute to the kappa
number. This is especially true for hardwood pulps where sometimes as much as half of the
kappa number of a pulp after cook originates from non-lignin structures. Softwood also
contains these structures but usually in smaller amounts.
Kraft pulp, sulphate pulp
Light absorption coefficient (m2/kg), k, a measure of the ability of a material to absorb light
and thus contribute to the colour and opacity of the paper.
Light scattering coefficient (m2/kg), s, a measure of the ability of a material to scatter light
and thus contribute to the brightness and opacity of the paper.
MeGlcA, 4-O-methyl-α-D-glucuronic acid
OXE, oxidation equivalents. The OXE concept was developed in an attempt to enable
different bleaching sequences to be compared with respect to chemical consumption, even
when different bleaching agents are used in the same sequence. One OXE is defined as the
amount of oxidation chemical which consumes 1 mol of e- when it is reduced (Grundelius
1990). However, the OXE concept should be used with considerable caution. The OXE
system only considers the theoretical transfer of electrons in a completed redox reaction.
Hence treatment of a pulp with different bleaching chemicals having the same amount of
OXE can lead to very different kappa numbers and ISO brightness values.
75
Selectivity, relative extent to which active pulping chemicals attack the lignin and preserve
the carbohydrates, i.e. better selectivity is synonymous with higher yield or viscosity at a
given kappa number.
Sulphidity (%), is a measure of the hydrogen sulphide ion concentration related to the
concentration of active alkali (NaOH+Na2S) in white liquor and is defined as:
100][][
][2% •+
•= −−
−
HSHOHSSulphidity
TCF, Totally Chlorine Free bleaching is bleaching performed without chlorine-containing
compounds, i.e. with compounds such as oxygen, hydrogen peroxide, ozone and peracetic
acid.
Total yield (%), is the ratio of the amount of pulp with shives produced to the amount of
charged wood.
Limiting viscosity number, [η], is used to estimate the degree of degradation of cellulose,
i.e. the cleavage of carbohydrate chains during a cooking or a bleaching process. A high
viscosity should reflect a high degree of polymerisation, i.e. long carbohydrate chains. The
viscosity is measured in a capillary viscosimeter and calculated using the equation, (ISO
5351:2004):
cc ⋅−
=→
0
0
0lim][
ηηη
η
where η0 is the solvent viscosity, η is the sample solution viscosity and c is the concentration
of the pulp. Viscosity is often used to estimate pulp strength.
White liquor, kraft cooking liquor containing the active alkali components of sodium
hydroxide (NaOH) and sodium sulphide (Na2S).
76
6. Nomenclature in bleaching stages TAPPI recommendations A set of bleaching stages is called a bleaching sequence. Each bleaching stage is designated a
symbolic letter. The symbols used in this thesis refer, where possible, to TAPPI
”Recommended Method for Designating Bleaching Stages” (van Lee 1987):
A Acid treatment
D Chlorine dioxide
D*1 D-stage treatment at high temperature and with a long retention time
O Oxygen
P Hydrogen peroxide
P*1 Hydrogen peroxide bleaching with an addition of magnesium
Q1 Chelating agent
Z Ozone 1Letter symbols not according to TAPPI
77
7. Acknowledgments First I would like to express my sincere gratitude to my two supervisors; Associate Professor Mikael E. Lindström and Adjunct Professor Martin Ragnar. Thank you for excellent supervision, valuable discussions, good friendship and for helping me with numerous of things. Mikael and Martin, without your encouragement, never ending enthusiasm and support this thesis would not have been written. I also wish to thank my first supervisor at KTH, Prof. Em. Ants Teder for introducing me to the field of pulping, for all support and for valuable comments on the manuscript. I would also like to thank my co-authors for good cooperation, especially my informal supervisors Johan Blixt and Christofer Lindgren for fruitful discussions and guidance. I also feel great gratitude to all the people at KTH, Stora Enso, Kvaerner Pulping and Billerud who made it possible in one way or another to finish this project, no one mentioned, no one forgotten. The help of Dr. Anthony Bristow with the linguistic revision of the thesis is gratefully acknowledged. All remaining errors are not to be blamed on any one else but the author. Last, but certainly not least, I would like to thank; Ove, my parents Nils-Erik and Anita, my brother and sisters, and my closest friends for always being there.
78
79
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