- •2006, Isbn 3-527-30997-7
- •Isbn-13: 978-3-527-30999-3
- •Isbn-10: 3-527-30999-3
- •Volume 1
- •1.1 Introduction 3
- •Isbn: 3-527-30999-3
- •2.2 Outlook 59
- •4.1 Introduction 109
- •4.2.5.1 Introduction 185
- •4.3.1 Introduction 392
- •5.1 Introduction 511
- •6.1 Introduction 561
- •6.2.1 Introduction 563
- •6.4.1 Introduction 579
- •Volume 2
- •7.3.1 Introduction 628
- •7.4.1 Introduction 734
- •7.5.1 Introduction 777
- •7.6.1 Introduction 849
- •7.10.1 Introduction 887
- •8.1 Introduction 933
- •1 Introduction 1071
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and
- •III Recovered Paper and Recycled Fibers 1147
- •1 Introduction 1149
- •2.2 Inorganic Components 1219
- •2.3 Extractives 1224
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •4680 Lenzing
- •Isbn: 3-527-30999-3
- •4860 Lenzing
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •1 Introduction
- •1.2 The History of Papermaking
- •1 Introduction
- •1.2 The History of Papermaking
- •1 Introduction
- •1.3 Technology, End-uses, and the Market Situation
- •1 Introduction
- •1.3 Technology, End-uses, and the Market Situation
- •1 Introduction
- •1.3 Technology, End-uses, and the Market Situation
- •1 Introduction
- •1.5 Outlook
- •150.000 Annual Fiber Flow[kt]
- •1 Introduction
- •1.5 Outlook
- •1 Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •Void volume
- •Void volume fraction
- •Xylan and Fiber Morphology
- •Initial bulk residual
- •4.2.5.1 Introduction
- •In (Ai) Model concept Reference
- •Initial value
- •Validation and Application of the Kinetic Model
- •Inititial
- •Volume.
- •Viscosity
- •Influence on Bleachability
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Introduction
- •International
- •Impregnation
- •4.3.4.2.1 Cellulose
- •Influence of Substituents on the Rate of Hydrolysis
- •140 116 Total so2
- •Xylonic
- •Viscosity Brightness
- •Xyl Man Glu Ara Furf hoAc XyLa
- •Initial NaOh charge [% of total charge]:
- •864 (Hemicelluloses), 2004: 254.
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Xylosec
- •Xylan residues
- •Viscosity
- •Introduction
- •Viscosity
- •Viscosity
- •Introduction
- •Initiator Promoter Inhibitor
- •Viscosity
- •Viscosity
- •Viscosity
- •Introduction
- •Viscosity
- •Introduction
- •Intra-Stage Circulation and Circulation between Stages
- •Implications of Liquor Circulation
- •Vid Chalmers Tekniska
- •Introduction
- •It is a well-known fact that the mechanical properties of the viscose fibers
- •Increase in the low molecular-weight fraction [2]. The short-chain molecules represent
- •Isbn: 3-527-30999-3
- •In the cooking process or, alternatively, white liquor can be used for the cold
- •Is defined as the precipitate formed upon acidification of an aqueous alkaline solution
- •934 8 Pulp Purification
- •8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution 935
- •Is essentially governed by chemical degradation reactions involving endwise depolymerization
- •80 °C [12]. Caustic treatment: 5%consistency ,
- •30 Min reaction time, NaOh concentrations:
- •8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution
- •80 °C is mainly governed by chemical degradation reactions (e.G. Peeling reaction).
- •Investigated using solid-state cp-mas 13c-nmr spectroscopy (Fig. 8.4).
- •Indicates cleavage of the intramolecular hydrogen bond between o-3-h and o-5′,
- •8 Pulp Purification
- •Interaction between alkali and cellulose, a separate retention tower is not really
- •In the following section.
- •3% In the untreated pulp must be ensured in order to avoid a change in the supramolecular
- •8.3 Cold Caustic Extraction
- •Xylan content [%]
- •8 Pulp Purification
- •Is calculated as effective alkali (ea). Assuming total ea losses (including ea consumption
- •Xylan content [%]
- •8.3 Cold Caustic Extraction
- •120 °C (occasionally 140 °c). As mentioned previously, hce is carried out solely
- •Involved in alkaline cooks (kraft, soda), at less severe conditions and thus avoiding
- •8.4Hot Caustic Extraction 953
- •954 8 Pulp Purification
- •120 Kg NaOh odt–1, 90–240 min, 8.4 bar (abs)
- •8.4Hot Caustic Extraction 955
- •956 8 Pulp Purification
- •Into the purification reaction, either in the same (eo) or in a separate stage
- •960 8 Pulp Purification
- •8.4.1.5 Composition of Hot Caustic Extract
- •8.4Hot Caustic Extraction 961
- •Isbn: 3-527-30999-3
- •Xyloisosaccharinic acid
- •Inorganicsa
- •Inorganic compounds
- •Value (nhv), which better reflects the actual energy release, accounts for the fact
- •968 9 Recovery
- •It should be noted that the recycling of bleach (e.G., oxygen delignification) and
- •9.1 Characterization of Black Liquors 969
- •9.1.2.1 Viscosity
- •9.1.2.3 Surface Tension
- •9.1.2.5 Heat Capacity [8,11]
- •9.2 Chemical Recovery Processes
- •Is described by the empirical equation:
- •9 Recovery
- •Vent gases from all areas of the pulp mill. From an environmental perspective,
- •9.2.2.1 Introduction
- •In the sump at the bottom of the evaporator. The generated vapor escapes
- •Incineration, whereas sulphite ncg can be re-used for cooking acid preparation.
- •9 Recovery
- •Values related to high dry solids concentrations. The heat transfer rate is pro-
- •9.2 Chemical Recovery Processes
- •9.2.2.3 Multiple-Effect Evaporation
- •7% Over effects 4 and 5, but more than 30% over effect 1 alone.
- •9.2 Chemical Recovery Processes
- •Increasing the dry solids concentration brings a number of considerable advantages
- •9.2.2.4 Vapor Recompression
- •Is driven by electrical power. In general, vapor coming from the liquor
- •Vapor of more elevated temperature, thus considerably improving their performance.
- •9 Recovery
- •Is typically around 6 °c. The resulting driving temperature difference
- •Is low, and hence vapor recompression plants require comparatively large heating
- •Vapor recompression systems need steam from another source for start-up.
- •9 Recovery
- •Its temperature is continuously falling to about 180 °c. After the superheaters,
- •In the furnace walls, and only 10–20% in the boiler bank. As water turns into
- •9.2.3.1.2 Material Balance
- •Is required before the boiler ash is mixed. In addition, any chemical make-up
- •In this simplified model, all the potassium from the black liquor (18 kg t–1
- •Values for the chemicals in Eq. (11) can be inserted on a molar basis, equivalent
- •9.2 Chemical Recovery Processes
- •Input/output
- •9 Recovery
- •9.2.3.1.3 Energy Balance
- •In the black liquor, from water formed out of hydrogen in organic material, and
- •9.2 Chemical Recovery Processes
- •9.2.3.2 Causticizing and Lime Reburning
- •9.2.3.2.1 Overview
- •9.2.3.2.2 Chemistry
- •986 9 Recovery
- •Insoluble metal salts are kept low. Several types of filters with and without lime
- •Is, however, not considered a loss because some lime mud must be
- •988 9 Recovery
- •In slakers and causticizers needs special attention in order to avoid particle disintegration,
- •9.2 Chemical Recovery Processes 989
- •Ing disks into the center shaft, and flows to the filtrate separator. There, the white
- •9.2.3.2.4 Lime Cycle Processes and Equipment
- •It is either dried with flue gas in a separate, pneumatic lime mud dryer or is fed
- •990 9 Recovery
- •Its temperature falls gradually. Only about one-half of the chemical energy in the
- •9.2.3.3.2 Black Liquor Gasification
- •Inorganics leave the reactor as solids, and into high-temperature techniques,
- •In the bed. Green liquor is produced from surplus bed solids. The product gas
- •992 9 Recovery
- •Incremental capacity for handling black liquor solids. The encountered difficulties
- •10% Of today’s largest recovery boilers. When the process and material issues are
- •9.2 Chemical Recovery Processes 993
- •9.2.3.3.3 In-Situ Causticization
- •Is still in the conceptual phase, and builds on the formation of sodium titanates
- •9.2.3.3.4 Vision Bio-Refinery
- •Into primary and secondary recovery steps. This definition relates to the recovery
- •994 9 Recovery
- •Is largely different between sulfite cooking bases. While magnesium and
- •Introduction
- •In alkaline pulping the operation of the lime kiln represents an emission source.
- •Isbn: 3-527-30999-3
- •Is by the sophisticated management of these sources. This comprises their collection,
- •Ions, potassium, or transition metals) in the process requires the introduction
- •Industry”. Similarly guidelines for a potential kraft pulp mill in Tasmania [3]
- •Initially, the bleaching of chemical pulp was limited to treatment with hypochlorite
- •In a hollander, and effluent from the bleach plant was discharged without
- •In a heh treatment and permitted higher brightness at about 80% iso (using
- •Increasing pulp production resulted in increasing effluent volumes and loads.
- •10.2 A Glimpse of the Historical Development 999
- •It became obvious that the bleaching process was extremely difficult to operate in
- •In a c stage was detected as aox in the effluent (50 kg Cl2 t–1 pulp generated
- •1% Of the active chlorine is converted into halogenated compounds (50 kg active
- •In chlorination effluent [12] led to the relatively rapid development of alternative
- •1000 10 Environmental Aspects of Pulp Production
- •10.2 A Glimpse of the Historical Development
- •In 1990, only about 5% of the world’s bleached pulp was produced using ecf
- •64 Million tons of pulp [14]. The level of pulp still bleached with chlorine
- •10 000 Tons. These are typically old-fashioned, non-wood mills pending an
- •In developed countries, kraft pulp mills began to use biodegradation plants for
- •10 Environmental Aspects of Pulp Production
- •Indeed, all processes are undergoing continual development and further improvement.
- •Vary slightly different depending upon the type of combustion unit and the fuel
- •10.3Emissions to the Atmosphere
- •Volatile organic
- •In 2004 for a potential pulp mill in Tasmania using “accepted
- •10 Environmental Aspects of Pulp Production
- •Is woodyard effluent (rain water), which must be collected and treated biologically
- •10.4 Emissions to the Aquatic Environment
- •Is converted into carbon dioxide, while the other half is converted into biomass
- •Into alcohols and aldehydes; (c) conversion of these intermediates into acetic acid and
- •10 Environmental Aspects of Pulp Production
- •In North America, effluent color is a parameter which must be monitored.
- •It is not contaminated with other trace elements such as mercury, lead, or cadmium.
- •10.6 Outlook
- •Increase pollution by causing a higher demand for a chemical to achieve identical
- •In addition negatively affect fiber strength, which in turn triggers a higher
- •Introduction
- •2002, Paper-grade pulp accounts for almost 98% of the total wood pulp production
- •Important pulping method until the 1930s) continuously loses ground and finds
- •Importance in newsprint has been declining in recent years with the increasing
- •Isbn: 3-527-30999-3
- •Virtually all paper and paperboard grades in order to improve strength properties.
- •In fact, the word kraft is the Swedish and German word for strength. Unbleached
- •Importance is in the printing and writing grades. In these grades, softwood
- •In this chapter, the main emphasis is placed on a comprehensive discussion of
- •1010 11 Pulp Properties and Applications
- •Is particularly sensitive to alkaline cleavage. The decrease in uronic acid content
- •Xylan in the surface layers of kraft pulps as compared to sulfite pulps has been
- •80% Cellulose content the fiber strength greatly diminishes [14]. This may be due
- •Viscoelastic and capable of absorbing more energy under mechanical stress. The
- •11.2 Paper-Grade Pulp 1011
- •Various pulping treatments using black spruce with low fibril
- •In the viscoelastic regions. Fibers of high modulus and elasticity tend to peel their
- •1012 11 Pulp Properties and Applications
- •11.2 Paper-Grade Pulp
- •Viscosity mL g–1 793 635 833 802 1020 868 1123
- •Xylose % od pulp 7.3 6.9 18.4 25.5 4.1 2.7 12.2
- •11 Pulp Properties and Applications
- •Inorganic Compounds
- •11.2 Paper-Grade Pulp
- •Insight into many aspects of pulp origin and properties, including the type of
- •Indicate oxidative damage of carbohydrates).
- •In general, the r-values of paper pulps are typically at higher levels as predicted
- •Is true for sulfite pulps. Even though the r-values of sulfite pulps are generally
- •Is rather unstable in acid sulfite pulping, and this results in a low (hemicellulose)
- •11 Pulp Properties and Applications
- •Ing process, for example the kraft process, the cellulose:hemicellulose ratio is
- •Increases by up to 100%. In contrast to fiber strength, the sheet strength is highly
- •Identified as the major influencing parameter of sheet strength properties. It has
- •In contrast to dissolving pulp specification, the standard characterization of
- •Is observed for beech kraft pulp, which seems to correlate with the enhanced
- •11.2 Paper-Grade Pulp
- •11 Pulp Properties and Applications
- •Is significantly higher for the sulfite as compared to the kraft pulps, and indicates
- •11.2 Paper-Grade Pulp
- •Xylan [24].
- •11 Pulp Properties and Applications
- •11.2 Paper-Grade Pulp
- •11 Pulp Properties and Applications
- •Introduction
- •Various cellulose-derived products such as regenerated fibers or films (e.G.,
- •Viscose, Lyocell), cellulose esters (acetates, propionates, butyrates, nitrates) and
- •In pulping and bleaching operations are required in order to obtain a highquality
- •Important pioneer of cellulose chemistry and technology, by the statement that
- •11.3 Dissolving Grade Pulp
- •Involves the extensive characterization of the cellulose structure at three different
- •Is an important characteristic of dissolving pulps. Finally, the qualitative and
- •Inorganic compounds
- •11 Pulp Properties and Applications
- •11.3.2.1 Pulp Origin, Pulp Consumers
- •Include the recently evaluated Formacell procedure [7], as well as the prehydrolysis-
- •11.3 Dissolving Grade Pulp
- •Viscose
- •11 Pulp Properties and Applications
- •11.3.2.2 Chemical Properties
- •11.3.2.2.1 Chemical Composition
- •In the polymer. The available purification processes – particularly the hot and cold
- •11.3 Dissolving Grade Pulp
- •In the steeping lye inhibits cellulose degradation during ageing due to the
- •Is governed by a low content of noncellulosic impurities, particularly pentosans,
- •Increase in the xylan content in the respective viscose fibers clearly support the
- •11.3 Dissolving Grade Pulp
- •Instability. Diacetate color is measured by determining the yellowness coefficient
- •Xylan content [%]
- •11 Pulp Properties and Applications
- •Xylan content [%]
- •11.3 Dissolving Grade Pulp
- •11.3 Dissolving Grade Pulp
- •Is, however, not the only factor determining the optical properties of cellulosic
- •In the case of alkaline derivatization procedures (e.G., viscose, ethers). In industrial
- •11.3 Dissolving Grade Pulp
- •Viscose
- •Viscose
- •In order to bring out the effect of mwd on the strength properties of viscose
- •Imitating the regular production of rayon fibers. To obtain a representative view
- •11 Pulp Properties and Applications
- •Viscose Ether (hv) Viscose Acetate Acetate
- •Xylan % 3.6 3.1 1.5 0.9 0.2
- •1.3 Dtex regular viscose fibers in the conditioned
- •11.3 Dissolving Grade Pulp
- •Is more pronounced for sulfite than for phk pulps. Surprisingly, a clear correlation
- •Viscose fibers in the conditioned state related to the carbonyl
- •1038 11 Pulp Properties and Applications
- •In a comprehensive study, the effect of placing ozonation before (z-p) and after
- •Increased from 22.9 to 38.4 lmol g–1 in the case of a pz-sequence, whereas
- •22.3 To 24.2 lmol g–1. The courses of viscosity and carboxyl group contents were
- •Viscosity measurement additionally induces depolymerization due to strong
- •11 Pulp Properties and Applications
- •Increasing ozone charges. For more detailed
- •11.3 Dissolving Grade Pulp
- •Is more selective when ozonation represents the final stage according to an
- •11.3.2.3 Supramolecular Structure
- •1042 11 Pulp Properties and Applications
- •Is further altered by subsequent bleaching and purification processes. This
- •Involved in intra- and intermolecular hydrogen bonds. The softened state favors
- •11.3 Dissolving Grade Pulp
- •Interestingly, the resistance to mercerization, which refers to the concentration of
- •11 Pulp Properties and Applications
- •Illustrate that the difference in lye concentration between the two types of dissolving
- •Intensity (see Fig. 11.18: hw-phk high p-factor) clearly changes the supramolecular
- •11.3 Dissolving Grade Pulp
- •Viscose filterability, thus indicating an improved reactivity.
- •11 Pulp Properties and Applications
- •Impairs the accessibility of the acetylation agent. When subjecting a low-grade dissolving
- •Identification of the cell wall layers is possible by the preferred orientation of
- •Viscose pulp (low p-factor) (Fig. 11.21b, top). Apparently, the type of pulp – as well
- •11 Pulp Properties and Applications
- •150 °C for 2 h, more than 70% of a xylan, which was added to the cooking liquor
- •20% In the case of alkali concentrations up to 50 g l–1 [67]. Xylan redeposition has
- •11.3 Dissolving Grade Pulp
- •Xylan added linters cooked without xylan linters cooked with xylan
- •Viscosity
- •In the surface layer than in the inner fiber wall. This is in agreement with
- •11 Pulp Properties and Applications
- •Xylan content in peelings [wt%]
- •Xylan content located in the outermost layers of the beech phk fibers suggests
- •11.3.2.5 Fiber Morphology
- •11 Pulp Properties and Applications
- •50 And 90%. Moreover, bleachability of the screened pulps from which the wood
- •11.3.2.6 Pore Structure, Accessibility
- •11.3 Dissolving Grade Pulp
- •Volume (Vp), wrv and specific pore surface (Op) were seen between acid sulfite
- •11 Pulp Properties and Applications
- •Irreversible loss of fiber swelling occurs; indeed, Maloney and Paulapuro reported
- •In microcrystalline areas as the main reason for hornification [85]. The effect of
- •105 °C, thermal degradation proceeds in parallel with hornification, as shown in
- •Increased, particularly at temperatures above 105 °c. The increase in carbonyl
- •In pore volume is clearly illustrated in Fig. 11.28.
- •11.3 Dissolving Grade Pulp
- •Viscosity
- •11 Pulp Properties and Applications
- •Increase in the yellowness coefficient, haze, and the amount of undissolved particles.
- •11.3.2.7 Degradation of Dissolving Pulps
- •In mwd. A comprehensive description of all relevant cellulose degradation processes
- •Is reviewed in Ref. [4]. The different modes of cellulose degradation comprise
- •11.3 Dissolving Grade Pulp
- •50 °C, is illustrated graphically in Fig. 11.29.
- •11 Pulp Properties and Applications
- •In the crystalline regions.
- •11.3 Dissolving Grade Pulp
- •Important dissolving pulps, derived from hardwood, softwood and cotton linters
- •11.3 Dissolving Grade Pulp 1061
- •Xylan rel% ax/ec-pad 2.5 3.5 1.3 1.0 3.2 0.4
- •Viscosity mL g–1 scan-cm 15:99 500 450 820 730 1500 2000
- •1062 11 Pulp Properties and Applications
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •1072 1 Introduction
- •Isbn: 3-527-30999-3
- •Inventor of stone groundwood. Right: the second version
- •1074 2 A Short History of Mechanical Pulping
- •In refining, the thinnings (diameter 7–10cm) can also be processed.
- •In mechanical pulping as it causes foam; the situation is especially
- •In mechanical pulping, those fibers that are responsible for strength properties
- •Isbn: 3-527-30999-3
- •In mechanical pulping, the wood should have a high moisture content, and the
- •In the paper and reduced paper quality. The higher the quality of the paper, the
- •1076 3 Raw Materials for Mechanical Pulp
- •1, Transversal resistance; 2, Longitudinal resistance; 3, Tanning limit.
- •3.2 Processing of Wood 1077
- •In the industrial situation in order to avoid problems of pollution and also
- •1078 3 Raw Materials for Mechanical Pulp
- •2, Grinder pit; 3, weir; 4, shower water pipe;
- •5, Wood magazine; 6, finger plate; 7, pulp stone
- •Isbn: 3-527-30999-3
- •4.1.2.1 Softening of the Fibers
- •1080 4 Mechanical Pulping Processes
- •235 °C, whereas according to Styan and Bramshall [4] the softening temperatures
- •Isolated lignin, the softening takes place at 80–90 °c, and additional water
- •4.1 Grinding Processes 1081
- •1082 4 Mechanical Pulping Processes
- •1, Cool wood; 2, strongly heated wood layer; 3, actual grinding
- •4.1.2.2 Defibration (Deliberation) of Single Fibers from the Fiber Compound
- •4 Mechanical Pulping Processes
- •Influence of Parameters on the Properties of Groundwood
- •In the mechanical defibration of wood by grinding, several process parameters
- •Improved by increasing both parameters – grinding pressure and pulp stone
- •In practice, the temperature of the pit pulp is used to control the grinding process,
- •In Fig. 4.8, while the grit material of the pulp stone estimates the microstructure
- •4 Mechanical Pulping Processes
- •4.1 Grinding Processes
- •Is of major importance for process control in grinding.
- •4 Mechanical Pulping Processes
- •4.1.4.2 Chain Grinders
- •Is fed continuously, as shown in Fig. 4.17.
- •Initial thickness of the
- •75 Mm thickness, is much thinner than that of a concrete pulp stone, much
- •4 Mechanical Pulping Processes
- •Include:
- •Increases; from the vapor–pressure relationship, the boiling temperature is seen
- •4 Mechanical Pulping Processes
- •In the pgw proves, and to prevent the colder seal waters from bleeding onto the
- •4.1 Grinding Processes
- •In pressure grinding, the grinder shower water temperature and flow are
- •70 °C, a hot loop is no longer used, and the grinding process is
- •4 Mechanical Pulping Processes
- •Very briefly at a high temperature and then refined at high
- •4.2 Refiner Processes
- •4 Mechanical Pulping Processes
- •Intensity caused by plate design and rotational speed.
- •4.2 Refiner Processes
- •1. Reduction of the chips sizes to units of matches.
- •2. Reduction of those “matches” to fibers.
- •3. Fibrillation of the deliberated fibers and fiber bundles.
- •1970S as result of the improved tmp technology. Because the key subprocess in
- •4 Mechanical Pulping Processes
- •Impregnation Preheating Cooking Yield
- •30%. Because of their anatomic structure, hardwoods are able to absorb more
- •Is at least 2 mWh t–1 o.D. Pulp for strongly fibrillated tmp and ctmp pulps from
- •4 Mechanical Pulping Processes
- •4.2 Refiner Processes
- •1500 R.P.M. (50 Hz) or 1800 r.P.M. (60 Hz); designed pressure 1.4 mPa
- •1500 R.P.M. (50 Hz) or 1800 r.P.M. (60 Hz); designed pressure 1.4 mPa;
- •4.2 Refiner Processes
- •4 Mechanical Pulping Processes
- •In hardwoods makes them more favorable than softwoods for this purpose. A
- •4.2 Refiner Processes
- •Isbn: 3-527-30999-3
- •1114 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.2Machines and Aggregates for Screening and Cleaning 1115
- •In refiner mechanical pulping, there is virtually no such coarse material in the
- •1116 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.2Machines and Aggregates for Screening and Cleaning
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.3 Reject Treatment and Heat Recovery
- •55% Iso and 65% iso. The intensity of the bark removal, the wood species,
- •Isbn: 3-527-30999-3
- •1124 6 Bleaching of Mechanical Pulp
- •Initially, the zinc hydroxide is filtered off and reprocessed to zinc dust. Then,
- •2000 Kg of technical-grade product is common. Typically, a small amount of a chelant
- •6.1 Bleaching with Dithionite 1125
- •Vary, but are normally ca. 10 kg t–1 or 1% on fiber. As the number of available
- •1126 6 Bleaching of Mechanical Pulp
- •6.2 Bleaching with Hydrogen Peroxide
- •70 °C, 2 h, amount of NaOh adjusted.
- •6.2 Bleaching with Hydrogen Peroxide
- •Is shown in Fig. 6.5, where silicate addition leads to a higher brightness and a
- •Volume (bulk). For most paper-grade applications, fiber volume should be low in
- •Valid and stiff fibers with a high volume are an advantage; however, this requires
- •1130 6 Bleaching of Mechanical Pulp
- •6.2 Bleaching with Hydrogen Peroxide
- •Very high brightness can be achieved with two-stage peroxide bleaching, although
- •In a first step. This excess must be activated with an addition of caustic soda. The
- •Volume of liquid to be recycled depends on the dilution and dewatering conditions
- •6 Bleaching of Mechanical Pulp
- •6 Bleaching of Mechanical Pulp
- •Is an essential requirement for bleaching effectiveness. Modern twin-wire presses
- •Is discharged to the effluent treatment plant. After the main bleaching stage, the
- •6.3 Technology of Mechanical Pulp Bleaching
- •1136 6 Bleaching of Mechanical Pulp
- •Isbn: 3-527-30999-3
- •7.3 Shows the fractional composition according to the McNett principle versus
- •1138 7 Latency and Properties of Mechanical Pulp
- •7.2 Properties of Mechanical Pulp 1139
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •In Fig. 1.2, the development of recovered paper utilization and paper production
- •Is split into the usa, the cepi countries, and Germany. It is clear that since 1990,
- •5.8% For Germany and worldwide, and 5.9% for the cepi countries.
- •1150 1 Introduction
- •1 Introduction
- •Industry, environmentalists, governmental authorities, and often even the marketplace.
- •It is accepted that recycling preserves forest resources and energy used for
- •1 Introduction
- •Incineration. The final waste (ashes) can either be discarded or used as raw
- •Virgin fibers
- •74.4 % Mixed grades
- •Indonesia
- •Virgin fibers
- •Inhomogeneous sample Homogeneous sample
- •Variance of sampling Variance of measurement
- •1.Quartile
- •3.Quartile
- •Insoluble
- •Insoluble
- •Insoluble
- •Integral
- •In Newtonion liquid
- •Velocity
- •Increasing dp
- •2Α filter
- •0 Reaction time
- •Increasing interaction of probe and cellulose
- •Increasing hydrodynamic size
- •Vessel cell of beech
- •Initial elastic range
- •Internal flow
- •Intact structure
- •Viscosity 457
- •Isbn: 3-527-30999-3
- •1292 Index
- •Visbatch® pulp 354
- •Index 1293
- •1294 Index
- •Impregnation 153
- •Viscosity–extinction 433
- •Index 1295
- •1296 Index
- •Index 1297
- •Inhibitor 789
- •1298 Index
- •Index 1299
- •Impregnation liquor 290–293
- •1300 Index
- •Industries
- •Index 1301
- •1302 Index
- •Index 1303
- •Xylose 463
- •1304 Index
- •Index 1305
- •1306 Index
- •Index 1307
- •1308 Index
- •In conventional kraft cooking 232
- •Visbatch® pulp 358
- •Index 1309
- •In prehydrolysis-kraft process 351
- •Visbatch® cook 349–350
- •1310 Index
- •Index 1311
- •1312 Index
- •Viscosity 456
- •Index 1313
- •Viscosity 459
- •Interactions 327
- •1314 Index
- •Index 1315
- •Viscosity 459
- •1316 Index
- •Index 1317
- •Xylose 461
- •Index 1319
- •Visbatch® pulp 355
- •Impregnation 151–158
- •1320 Index
- •Index 1321
- •1322 Index
- •Xylan water prehydrolysis 333
- •Index 1323
- •1324 Index
- •Viscosity 459
- •Index 1325
- •Xylose 940
- •1326 Index
- •Index 1327
- •In selected kinetics model 228–229
- •4OMeGlcA 940
- •1328 Index
- •Index 1329
- •Intermediate molecule 164–165
- •1330 Index
- •Viscosity 456
- •Index 1331
- •1332 Index
- •Impregnation liquor 290–293
- •Index 1333
- •1334 Index
- •Index 1335
- •1336 Index
- •Impregnation 153
- •Index 1337
- •1338 Index
- •Viscose process 7
- •Index 1339
- •Volumetric reject ratio 590
- •1340 Index
- •Index 1341
- •1342 Index
- •Index 1343
- •1344 Index
- •Index 1345
- •Initiator 788
- •Xylose 463
- •1346 Index
- •Index 1347
- •Vessel 385
- •Index 1349
- •1350 Index
- •Xylan 834
- •1352 Index
Impregnation
A uniform distribution of pulping chemicals within the wood chip structure is the
key step of any pulping process. The impregnation step is carried out immediately
after the chips have been immersed in the cooking liquor. Chemical transportation
into the wood structure is accomplished by two different mechanisms. The
first is the penetration of a liquid under a pressure gradient into the capillaries
and the interconnected voids of the wood structure. The second is the diffusion of
dissolved ions, which is governed by their concentration gradient and the total
cross-sectional area of accessible pores. Since diffusion takes place in a liquid saturated
environment, penetration must occur prior to diffusion.
Penetration is influenced by pore size distribution and capillary forces. Consequently,
the wood structure itself affects liquid penetration. In softwoods, the
impregnating liquor proceeds from one tracheid to the next through bordered
pits, while the ray cells provide ways for transport in the radial direction. In hardwoods,
the flow is greatly enhanced by the vessels. They are first filled with liquid,
which then penetrates into ray cells and libriform fibers. Difficulties are caused by
tylosis. Penetration is facilitated by a high moisture content, pre-steaming and
pressure impregnation. In sulfite cooking, the introduction of the Vilamo method
significantly improved the homogeneity of the cook [12–14]. Here, air is removed
from the chips by sudden pressure reductions in the liquor phase. First, a hydraulic
pressure of about 6 bar is applied immediately after liquor charge to full digester.
The pressure increase is followed by a pressure release to approximately 2 bar
by opening the top valve of the digester. Penetration is completed after several
pressure pulsations. However, later investigations have been shown that pressure
pulsations do not appear to give any important advantage over a constant hydrostatic
pressure [15]. A suitable combination of steaming and pressure impregnation
will be sufficient to complete impregnation allowing shorter cooking cycle
and more uniform pulping.
Unlike alkaline pulping, the resistance to radial and transverse diffusion of
cooking chemicals into the wood is much more pronounced in acid sulfite cooking.
It is reported that diffusion in the longitudinal direction at room temperature
is 50- to 200-fold faster than in the transverse and radial directions for softwoods
[16,17]. This finding suggests that hydrogen sulfite enters the wet chip almost
exclusively through the ends. Consequently, chips should be as short as possible
4.3 Sulfite Chemical Pulping 403
from the pulp quality point of view. In hot liquor, however, the wood structure is
opened up and diffusion across the grain is facilitated. Steaming at atmospheric
pressure may double the permeability in the tangential and radial directions. Chip
thickness is therefore as important as chip length. Scanning electron microscopy
(SEM) and energy dispersive X-ray analysis (EDXA) revealed that sodium sulfite
diffusion at slightly alkaline conditions occurred more rapidly into aspen than
into black spruce chips under comparable conditions [18]. The reason for the
higher diffusivity was clearly attributed to the higher porosity of aspen, as shown
by mercury porosity measurements. The reduction of interfacial energy by the
addition of wetting agents seems to help in the penetration of liquids into wood.
Preliminary studies confirmed that, in the presence of a surfactant in the sulfite
liquor, penetration into the wood structure improved. The degree of penetration
can thus be correlated with the contact angle of sulfite liquor drops on the crosssection
surface of the wood [19].
The active species in sulfite cooking show different diffusion constants. The
highest diffusion constant is given by hydrated sulfur dioxide, and the lowest by
magnesium hydrogen sulfite (Tab. 4.55). Interestingly, ammonium hydrogen sulfite
shows a rather high diffusivity, indicating a better penetration and a more uniform
cook. The results indicate that in acid hydrogen sulfite cooking, SO2 tends to
penetrate chips ahead of base.
Tab. 4.55 Diffusion coefficients, D, of various sulfur(IV)
species in pure aqueous solutions at 20 °C (according to [20]).
Sulfur species D at 20 °C
[m2 s . 109]
Sulfur dioxide 2.78
calcium hydrogen sulfite 1.02
magnesium hydrogen sulfite 0.96
ammonium hydrogen sulfite 1.92
Moreover, there is also some evidence that hydrogen sulfite ions migrate more rapidly
into the wood structure as compared to the corresponding cations (e.g., Na+) [21] .
This concludes that incomplete impregnation might occur in liquid-phase cookswith
a rapid temperature rise. As a consequence, the base concentration in an acid sulfite
cook is not sufficient to neutralize the sulfonic acids formed. Because of the sharp
drop in pH, lignin condensation reactions are favored over sulfonation reactions
in the interior of the chips, and this results in uncooked regions. Correct impregnation
is a prerequisite for a uniform cook. The conditions for satisfactory chip
impregnation for acid sulfite cooking comprise the following steps:
404 4 Chemical Pulping Processes
_ Preparation of uniform chip size with short length dimension. A
short chip length ensures a better penetrability because acid
liquors penetrate mainly from the cut ends. Deterioration of fiber
length has been observed when chips were cut below 19 mm in
length.
_ Steaming at a slight overpressure (100–110 °C) until the air is displaced.
Steaming at a higher temperature should be avoided due
to the danger of lignin condensation reactions in subsequent acid
sulfite cooking.
_ Pre-steamed chips are immersed in the cooking liquor at about
80–85 °C to condense the water vapor in the chips and to fill the
evacuated volume with liquor.
_ Hydrostatic pressurization of the completely filled digester to
700 kPa or more by a cooking liquor pump.
The time–temperature and time–pressure profiles must be individually adjusted
to the applied wood source. The permeability and anisotropy of wood is a highly
variable property, not only between different species, but also within one single
species. For example, heartwood is much more difficult to impregnate than sapwood.
This is especially true for conifers, where heartwoods are highly resistant to
penetration by sulfite liquor.
4.3.4
Chemistry of (Acid) Sulfite Cooking
Antje Potthast
The composition of the spent sulfite liquor depends to a large extent on the cooking
conditions chosen and the chemical composition of cooking chemicals – that
is, mainly the ratio of free and combined SO2 (for details, see Section 4.3.2). The
degree of delignification is directly related to the concentration of the product
[H+]·[HSO3
– ], while the concentration of [H+]directly affects the rate of cellulose
hydrolysis.
Depending on the progress of the sulfite cook, the composition of the cooking
liquor changes mainly due to consumption of bound SO2 (HSO3
–) and changes in
acidity [1].
SO2
H O 2 H2SO3
H
+
HSO+ 3 +
_
Scheme 4.31 Equilibrium of bound and free sulfur dioxide.
The composition of the cooking liquor in terms of free SO2 and combined SO2
(hydrogen sulfite) must be balanced in a way that assures sufficient delignification
while keeping the condensation reactions to a limit. Kaufmann [2](F ig. 4.155)
illustrated the borderline ratio between total SO2 and combined SO2, which will
4.3 Sulfite Chemical Pulping 405
either result in cooks with acceptable outcome or, if outbalanced, in so-called
“black cooks”, where condensation processes preponderated. Crossing the border
towards lower amounts of combined SO2 and lower total SO2 will yield pulp with
highly condensed lignin fractions impracticable to bleach. Keeping the appropriate
ratio is indispensable to minimize condensation effects and to allow sufficient
delignification.
0
2
4
6
8
10
12
bisulfite solution
area of black
cooks
area of
acceptable
cooks
total SO
2
(%)
combined SO
2
(%)
Black cooks
Normal or brownish cooks
0.00 0.25 0.50 0.75 1.00 1.25
Fig. 4.155 Kaufmann diagram, indicating areas of black
cooks in relation to the cooking liquor composition (adopted
from [1]).
Cooking close to conditions of black cooks results in:
_ Increasing dehydration (more free SO2) due to increasing temperature.
_ Decreasing concentration of hydrogen sulfite due to consumption
by lignin:
– Formation of strong acid anions (lignosulfonate anions), which in
turn reduce the available bound SO2
– Less available hydrogen sulfite prevents sulfonation of lignin
(delignification), but increases condensation reactions
– Buffer capacity decreases towards the end of the cook
_ Formation of new H+ ions from sulfur dioxide and water according
to Scheme 4.31, hence increasing in [H+].
406 4 Chemical Pulping Processes
The general reactions in a sulfite cook can be divided into sulfonation, hydrolysis,
condensation, and redox processes. Sulfonation reactions mainly occur with lignin
and to a minor extent also with carbohydrates and low molecular-weight degradation
products. Condensation is mainly observed between lignin units and lignin
intermediates and extractives, and to some extent also with degradation products
of carbohydrates. Carbohydrates (especially hemicelluloses) are affected by
hydrolysis, but lignin moieties are also partly fragmented by this reaction type.
Hydrolysis is especially important to cleave lignin–carbohydrate linkages. Redoxprocesses
are taking place with inorganic compounds, most often with participation
of the degraded carbohydrates and extractives.
4.3.4.1 Reactions of Lignin
The reactions of hydrogen sulfite/sulfur dioxide with lignin are highly dependent
on the pH of the reaction medium. On one hand, the pH determines the reactive
species and their nucleophilicity, while on the other hand the formation of reactive
intermediates within the lignin molecule is also governed by the pH. This
will be further illustrated by the reactions of different lignin units occurring under
acid sulfite and neutral sulfite conditions.
Lignin degrading reactions with lignin in the acidic sulfite process are characterized
by three reaction principles: sulfonation, hydrolysis and, to some extent, sulfitolysis:
Lignin
sulfonation
hydrolysis sulfitolysis
dissolution
degradation
condensation
Condensation reactions are the major undesired processes counteracting delignification.
In the following section, the major reaction pathways will be illustrated. Specific
reactions of different lignin units (i.e., b-O-4, phenylcoumaran, and pinoresinol)
are discussed exemplarily in more detail, and are used to illustrate the differences
in the lignin’s reaction behavior under neutral sulfite conditions.
4.3.4.1.1 Sulfonation
The sulfonation is the main reaction principle under acidic conditions, which renders
the lignin molecule sufficiently hydrophilic to be dissolved in the cooking
liquor. The sulfonation reaction is always the fastest reaction at low pH value, and
4.3 Sulfite Chemical Pulping 407
there is a strong dependence on the pH [3]. No significant influence was observed
whether the lignin units are etherified or not. However, a slightly faster rate of
sulfonation was shown for phenolic units [4].
4.3.4.1.2 Hydrolysis
Hydrolysis of linkages between lignin and carbohydrate, and to a smaller extent
also of inter-lignin bonds, is somewhat slower than the sulfonation process [5].
Only the a-benzyl ether inter-lignin linkages are cleaved to a larger extent, which
decreases the molecular weight of lignin.
Major Reaction Mechanisms
Under the prevailing acidic conditions, the oxygen of the a-ether or a-hydroxy
group is protonated. Subsequent release of the a-substituent (as water or as alcohol/
phenol), which is the rate-determining step, leaves behind a resonance-stabilized
benzylium cation. This intermediate immediately adds hydrogen sulfite by
nucleophilic addition. The electron density distribution of the benzylium cation is
shown in Scheme 3 (left), where areas of high electron density are marked in red,
and centers with a low electron density are marked blue. From theoretical calculations,
as well as from model reactions [6], the benzylium cation (3a) is favored
over the methylene quinone resonance form (3b). The latter resonance structure
can only come into play if the a-proton and the a-substituent are fully arranged in
the aromatic plane, which requires bond rotation around the benzylic carbon–carbon
bond. Rotation out of this plane breaks the resonance. This bond rotation
requires additional energy and time, and might be disfavored by steric factors
imposed by the surrounding lignin scaffold. All of these factors favor 3a over 3b.
A further stabilization of the intermediate is achieved by a 2-aryl substituent or by
a hydroxyl in para-position, the latter is favoring the formation of the oxonium
type resonance form (3b) [11].
Other nucleophilesmay also add to the benzyliumcation and competewith the sulfonation
reaction [5]. Such nucleophiles can either be ligninmoieties [6], carbohydrate
compounds, or extractives. The stereochemical outcome of the sulfonation reaction
was found to be consistent with a unimolecular SN1mechanism: the pure erythro and
threo forms of lignin-model compounds (e.g., b-O-4 ether models) always yielded a
mixture of the threo and erythro forms. The observed erosion of the stereochemistry
strongly supports the intermediacy of the carbonium ion – and hence the SN1
mechanism – and dismisses an SN2 mechanism with Walden inversion.
In the following, some examples on the reactions of different lignin units and
their conversion under acid sulfite conditions will be given.
Phenolic and non-phenolic b-O-4-lignin model compounds react exclusively by sulfonation
in the a-position; sulfonation of the c-carbon is not a relevant process [6]. No
free phenolic groups are required for reactivity. In alkaline pulping systems, a major
differencewas seen between phenolic and nonphenolic lignin substructures: the phenolic
groups were hereby always more reactive as compared to the nonphenolics.
This difference is practically absent under acidic sulfite conditions.
408 4 Chemical Pulping Processes
O
R
OMe
H
O R
H
+
O
R
OMe
H
O R
H
O
R
OMe
H
O
R
OMe
H
O
R
OMe
H
SO3H
HSO3
-
+
-ROH
+
+
R = Alkyl; Aryl; H
+
1 2
a 3 b 4
Scheme 4.32 Formation of the benzylium cation as the reactive
intermediate in acid sulfite cooking.
Scheme 4.33 Electron density distribution (left) and lowest
unoccupied molecule orbital (LUMO)-distribution (right) of
the benzylium cation intermediate (3).
The b-O-4-ether bond is rather stable under acidic conditions lacking strong
nucleophiles. Hence, no cleavage of the lignin macromolecule is accomplished at
this point, except for a-substituents (6–8% of all lignin links), although a-aryl-LCC
model compounds showed a high stability also in acid sulfite systems [7], as mentioned
earlier. A sulfidolytic cleavage of the b-O-4-ether bond can only be accomplished
at higher pH than acid sulfite conditions (e.g., neutral sulfite pulping [8–
10]), when stronger nucleophiles are present.
4.3 Sulfite Chemical Pulping 409
OR
OMe
O
HO
OMe
HO3S
OR
OMe
O
HO
OMe
OR
MeO
O
OH
OMe
OR OR
OMe
O
HO
OMe
OR
OMe
O
HO
OMe
RO
H
+
SO2 H2. O
+
sulfonation
condensation
-ROH
5 6
7
8
Scheme 4.34 Reaction of b-O-4 aryl ether structures (according to [11]).
Phenolic pinoresinol structures (Scheme 4.35) are opened and the intermediate
benzylium cation undergoes an intramolecular electrophilic aromatic substitution
at C6 of the adjacent aromatic ring. This intramolecular condensation process is
favored due to the close proximity of the adjacent ring, the a-carbon of the side
chain being subsequently sulfonated. Nonphenolic pinoresinols are less reactive.
HC
HC
H C 2
O CH
O CH2
CH
OH
OH
OMe
OMe
SO2
OH
OMe
SO3H
OH
OH
HO
OMe
H2. O
9 10
Scheme 4.35 Reaction of pinoresinol structures (according to [11]).
Phenolic phenylcoumaran (Scheme 4.36) structures also show the possibility for
condensation reactions if the reactive centers are close enough to the benzylium
cation. Possible routes for the formation of 12 by opening the hetero-ring, recyclization
and sulfonation are discussed in more detail by Gellerstedt and Gierer [6].
410 4 Chemical Pulping Processes
CH2OH
O
OH
OH
OMe
MeO
O
SO3H
OH
OMe
CH2OH
MeO
SO2
11 12
.H2O
Scheme 4.36 Reaction of phenolic phenylcoumarans under acidic sulfite conditions [5].
Sulfonation of other positions than Ca has been demonstrated with Ca
–-carbonyl
compounds (see Scheme 4.39) and 1,2-diarylpropane structures (b–1, cf.
Scheme 4.37). The latter is converted into stilbene structures upon elimination of
formaldehyde, or to the corresponding c-sulfonated product after elimination of
water and addition of sulfite to the allylic carbonium ion (Scheme 4.38) [5]. The
formaldehyde can be further oxidized by hydrogen sulfite to carbon dioxide and
water.
CH2OH
OH
OH
OMe
RO OMe
OH
OH
OMe
RO OMe
-HCOH
13 14
Scheme 4.37 Reaction of b–1 structures to stable stilbenes [5].
CH2OH
OH
OH
OMe
OMe
CH2+
OH
OH
OMe
OMe
-H2O
HSO3
-
H
+
CH2SO3H
OH
OH
OMe
OMe
15 16 17
Scheme 4.38 Reaction of stilbenes [5].
4.3 Sulfite Chemical Pulping 411
Phenylpropane a-carbonyl-b-arylether structures react also by elimination of
water from the c-hydroxyl group and addition of hydrogen sulfite to the generated
electrophilic center (Scheme 4.39).
OH
OMe
O
O
HO OH
OMe
H
+
OH
OMe
HO
O
CH2
OH
OMe
-H2O
OH
OMe
HO
O
OH
OMe
HO3S
HSO3
+
-
18 19 20
Scheme 4.39 Sulfonation of phenylpropane a-carbonyl-b-arylether structures [5].
Coniferylaldehyde units are sulfonated at the a-position via the allylic carbonium
ion, which is formed after addition of a proton, in a Michael-type addition.
Sulfonation of the c-carbon is only observed under neutral sulfite cooking conditions
with coniferyl alcohol and with coniferylbenzoate at a pH of 3–4 [12].
OR
OMe
CHO
OR
OMe
CHOH
OR
OMe
CHO
HO3S
H
+ + HSO3
-
+
+
21 22 23
Scheme 4.40 Sulfonation of the a-position of coniferyl aldehyde-type structures.
Comparison to Sulfonation Reactions under Conditions of Neutral Sulfite Pulping
Sulfite and bisulfite ions are both strong nucleophiles, which are able to bring
about the cleavage of ether bonds. Hence, with increasing pH values the b-O-4-
ether groups become less stable and undergo a sulfitolytic cleavage. However,
under neutral conditions only phenolic structures are reactive so that the sulfonation
is more selective, proceeding moreover at a high rate. This leads to a much
lower degree of sulfonation and thus a lower rate of delignification (roughly 20%
of the lignin units react) [6]. Model reactions show the sulfonation to occur also at
other positions than the a-carbon atom (e.g., the c-C) [13]as well as the existence
412 4 Chemical Pulping Processes
of two sulfonic acid groups per phenylpropane unit [5,14], which are present in
different lignosulfonate fractions [13].
At higher pH values and long reaction times, phenolic b-O-4-ether groups can
be converted to styrene-a-sulfonic acids (Scheme 4.41).
OH
OMe
O
HO
OMe
HO
O
OMe
O
HO
OMe
OH
OMe
HO
O3S
-
HSO3
-
OH
OMe
O
HO
OMe
O3S
5 24 24 26
-
Scheme 4.41 Reactions of b-O-4 ether structures during neutral sulfite pulping.
The final products obtained upon sulfonation are often similar to the sulfonated
lignin fragments produced under acidic conditions (cf. b-O-4-units), but the
mechanism of their formation is quite different. In analogy to the lignin reaction,
under alkaline kraft conditions the reactive intermediate in neutral and alkaline
sulfite reactions is the quinone methide in contrast to the carbonium ion (benzylium
ion), which prevails under acidic conditions. The sulfite or bisulfite ions attacks
the quinone methide at the Ca as depicted in Scheme 4.41.
Phenolic a-ether bonds are most completely cleaved, but the nonphenolic a-aryl
ether units are stable, which supports the quinone methide being the reactive
intermediate. Phenolic phenylcoumarans yield the corresponding a-sulfonic acids
(Scheme 4.42), whereas the nonphenolic phenylcoumarans are again mostly
stable. Nonphenolic pinoresinol units are cleaved at the respective a-carbons and
sulfonated.
CH2OH
O OH
OH
OMe
MeO
CH2OH
R
OH
OMe
MeO
OH
CH2OH
R
OH
OMe
MeO
OH SO3H
R = H, CH2OH R = H, CH2SO3
-
11 27 28
Scheme 4.42 Reaction of phenolic phenylcoumarans under
neutral sulfite conditions.
4.3 Sulfite Chemical Pulping 413
Eliminated formaldehyde results in the formation of hydroxymethanesulfonic
acid. At neutral pH, methoxyl groups are also removed by mechanisms (SN2) similar
to the demethylation in kraft pulping, but with formation of the corresponding
methylsulfonic acid. The rate of delignification generally decreases with increasing
pH.
HC
HC
H C 2
O CH
O CH2
CH
OH
OH
OMe
OMe
CH
C
R CH
R1
C
OH
OH
OMe
OMe
CH
HC
R HC
R1
CH
OH
OH
OMe
OMe
SO3
O3S
9 29 30
R = R1=H
R=H, R1=CH2OH
R=R1=CH2OH
-
-
R = R1=H
R=H, R1=CH2SO3
-
R=R1=CH2SO3
-
Scheme 4.43 Reaction of phenolic pinoresinol structures
under neutral sulfite conditions.
4.3.4.1.3 Condensation
Condensations always compete with the sulfonation process, and counteract the
delignification by formation of new, stable carbon–carbon bonds.
Increasing acidity, for example, towards the end of cook, favors condensation
reactions between the benzylium cation (3a) and other weakly nucleophilic lignin
positions, which are present due to resonance at position C1 (Scheme 4.44) and
C6 (Scheme 4.45). Condensation decreases with increasing concentration of bisulfite
ions (bound sulfur dioxide, cf. also the Kaufmann diagram; Fig. 4.155). The
resulting stable carbon–carbon bonds cause an increased molecular weight and a
lower hydrophilicity, and therefore work against the delignification process. However,
the introduction of sulfonic acid groups considerably increases the solubility,
which can often compensate for the increase in molecular weight by the formation
of new carbon–carbon bonds [15].
Intramolecular condensations have been described for phenylcoumaran and
pinoresinol structures (cf. Scheme 4.35) [6]. Due to the additional methoxyl group
in the 5-position, lignins of hardwoods generally have a lower tendency for condensation
reactions as compared to softwoods. Also, the sulfonation reaction is
somewhat slower for hardwood than for softwood lignins. Methoxyl groups are
not cleaved under acid sulfite conditions to a large extent due to the too low
nucleophilicity of the cooking chemicals, whereas under neutral sulfite cooking
conditions a cleavage of methoxyl groups is observed.
414 4 Chemical Pulping Processes
O
OMe
OAr
HO
HO
O
OMe
OAr
HO
O
OMe
OAr
HO OH
MeO
H
+
-
O
OAr
OH
-
+
+
+
9 3a 32
Scheme 4.44 Condensation of the reactive benzylium ion
with weakly nucleophilic resonance form at C1.
Lig
O
OMe O
OMe
Lig
O
O
OMe
MeO
H
+
31 33
-
+
+
Lig = Lignin moieties
-
Scheme 4.45 Condensation of the reactive benzylium ion
with weakly nucleophilic resonance form at C6.
4.3.4.1.4 Structure of Lignosulfonates
In contrast to alkaline lignin, lignosulfonates are water-soluble and often contain
considerable amounts of carbohydrates – either dissolved in the liquor or still
attached to the lignin polymer – which must be removed prior to analysis [20].
The removal of carbohydrates from lignosulfonates is rather tedious. The linkages
between lignin and carbohydrates in bisulfite pulps have been analyzed by gel-permeation
chromatography (GPC) with multiple detection [16]. A structural model
for the high molecular-weight fraction of sulfite waste liquor was proposed by
Hachey and Bui [17].
Monomeric lignosulfonates actually identified in sulfite waste liquor are
1-(4-hydroxy-3-methoxyphenyl)-prop-2-ene-1-sulfonate [18,19]and its isomer
1-(4-hydroxy-3-methoxyphenyl)-prop-1-ene-3-sulfonate [19].
In addition to common lignin analytics 20,21], lignosulfonates can be characterized
by their degree of sulfonation (S/C9 or S/OMe), which varies for commercial
preparations from 0.4 to 0.7 sulfonate groups per phenylpropane unit [22]. Lignosulfonates
carry carboxyl groups [23], and lignosulfonates have distinct polyelectrolytic
characteristics, which often render chemical analysis more difficult.
Recent progress has been achieved in the more accurate determination of the mo-
4.3 Sulfite Chemical Pulping 415
lecular weight employing size-exclusion chromatography in combination with
light-scattering techniques [24].
4.3.4.2 Reactions of Carbohydrates: Acid Hydrolysis