- •Recovered Paper and Recycled Fibers
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •2006, Isbn 3-527-30997-7
- •Volume 1
- •Isbn: 3-527-30999-3
- •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
- •1 Introduction 1149
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •150.000 Annual Fiber Flow[kt]
- •1 Introduction
- •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
- •Viscosity
- •Influence on Bleachability
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Introduction
- •International
- •Impregnation
- •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]:
- •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
- •In 1950, about 50% of the global paper production was produced. This proportion
- •4.0% Worldwide; 4.2% for the cepi countries; and 4.8% for Germany.
- •1150 1 Introduction
- •1 Introduction
- •1 Introduction
- •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
Introduction
It is useful to start with some definitions and to develop a basic understanding of
the mechanical set-up of a pressure screen before looking at the mostly mechanistic
and semi-empirical models that are used to describe the process phenomena
in a pressure screen.
Figure 6.1 shows the process streams around a screen. The pulp suspension
containing the impurities is fed with the feed stream QF at the pulp concentration
cF. The clean pulp passes through the apertures in the screen plate and is discharged
with the accept stream QA at the concentration cA. The reject stream QR is
rich in impurities. More in general, one fraction of a certain property is concentrated
in the reject stream QR, while the other fraction of different property leaves
the screen with the accept stream QA.
QF, cF QA, cA
QR, cR
Fig. 6.1 Streams around a screen.
In addition to the streams shown in Fig. 6.1, most industrial pressure screens
have an internal dilution by filtrate near the reject outlet. While dilution is often
needed from an operational perspective to avoid plugging of the screen’s reject
side, we can skip internal dilution during the present theoretical considerations
without losing insight.
The solid-solid separation with a pressure screen can be categorized by the type
of physical separation mechanism. In barrier separation, particles are rejected
from passage through an aperture because they are physically larger than the
aperture size in any orientation. In probability separation the particle can pass
through the aperture in at least one orientation and it depends on how the particle
approaches the aperture whether it passes, or not [1,2].
The distinction between barrier screening and probability screening is of utmost
importance for all screening applications. As for screening efficiency, probability
separation is apparently much more challenging than barrier separation.
The basic mechanical elements in control of separation in a simple pressure
screen are the screen and the rotor. In the most common design, the screen has
the shape of a cylindrical basket and is fixed in the screen housing. The apertures
in the screen basket have the form of either holes or slots and the screen surface
may be smooth or contoured.
6.2 Screening Theory 563
Pulp is fed axially to the screening zone. The accepted material passes through
the apertures in the screen basket, while the rejected material proceeds along the
inside of the screen basket towards the reject outlet. The rotor revolves inside the
screen basket. Over the years, a large number of different rotor designs has been
developed. Most of these are based on the closed bump rotor and open foil rotor
design principle (Figs. 6.2 and 6.3).
Reject
Accept
Reject
Accept
Reject
Accept
Rotor
Screen basket
Feed Feed Feed
Fig. 6.2 Rotor categories: closed rotor (left), open rotor (center), semi-open rotor (right).
Fig. 6.3 Examples of cross-sections of simple rotors:
left, bump rotor;center , step rotor; right, foil rotor.
Besides providing the tangential velocity near the screen and generating turbulence,
the most important task of the rotor is to keep the screen apertures clear.
This is accomplished by the regular backflush through the apertures caused by
the pumping action of the rotor’s pulsation elements.
6.2.2
Flow Regime
As the pulp suspension passes through the screening zone, the flow pattern near
the screen can be broken down into an axial flow vector from the feed side to the
reject side, a tangential flow vector induced by the rotor, and a radial flow vector
through the apertures to the accept side of the screen (Fig. 6.4).
The rotor plays a most essential part in influencing the flow regime. Its motion
fluidizes the pulp suspension, provides the tangential fluid velocity along the
screen plate, and backflushes the screen apertures. Fluidization suppresses the
particle–particle interactions and provides for quickly changing particle orienta-
564 6 Pulp Screening, Cleaning, and Fractionation
6.2 Screening Theory
v vtangential axial
vradial
Feed
Reject
Accept
Fig. 6.4 Flow vectors near the screen basket.
tion relative to the screen aperture, thus increasing the probability for acceptable
particles to pass. As the rotor element passes cyclically over the aperture, it generates
a backflush through the aperture every time it passes by. The backflush removes
particles trapped in the narrow screen apertures and thus keeps them clear.
While the screen performance is influenced by all the three flow vectors, the
radial accept flow through the apertures is most critical for continuous operation.
When the accept flow becomes limited or even completely disrupted by fibers
blocking the screen apertures, the situation is referred to as “blinding” or “plugging”
of the screen. Blinding leads to the formation of a fiber mat on the screen
surface, and it can affect only part of the screen or the total screen. In the latter
case, it may take several minutes for the screen to be blinded, but typically it takes
only a few seconds. The triggering factor of blinding – that is, the build-up of
fibers at the edge of a screen aperture – occurs very rapidly, within several thousands
of a second [3]. Consequently, very frequent backflush is required to avoid
plugging. The typical pulse frequency provoked by the rotor in a pressure screen
is above 30 Hz [4].
The aperture velocity, or passing velocity, v, is often regarded as a fundamental
design parameter for a pressure screen. v (m s–1) is calculated from the accept
flow rate QA (m3 s–1) and the open area of the screen basket, AO (m2):
v _
QA
AO _1_
It is important to realize that the true flow velocity through the screen apertures
is considerably higher than the passing velocity calculated from Eq. (1). On the
one hand, pressure pulsation as induced by the rotor action leads to a backflow
from the accept side to the feed side, thus both increasing the volume to be transferred
from the feed side to the accept side and reducing the time for this transfer.
On the other hand, fiber accumulation at the screen apertures reduces the open
area [5].
565
6 Pulp Screening, Cleaning, and Fractionation
6.2.3
Fiber Passage and Reject Thickening
The essential part of the screening operation occurs in the annular gap between
the rotor and the screen basket.
Both a mixed-flow model and a plug-flow model have been used to describe the
flow pattern in a pressure screen [1,6]. Here, we will focus on the plug-flow model
because it is more flexible and seems to describe the actual flow regime in a pressure
screen more accurately.
The plug-flow model assumes ideal radial mixing between the rotor and the
screen basket without backmixing in axial direction. Let us define a parameter
called passage ratio, P:
P _
cs
cz _2_
where cs is the solids concentration in the stream through a screen aperture (kg m–3)
and cz is the solids concentration of the stream just upstream of the aperture at a
position z [1].
The passage ratio is a characteristic parameter of the screening system, which is
influenced by many variables including screen plate and rotor design, screen operating
conditions and pulp grade. P is best determined individually for a given
screening application based on field measurements. A passage ratio of zero
means that all the solids are retained on the screen and will be rejected. At P = 1,
the concentrations in the accept and reject are equal to the feed concentration and
there is no separation.
z = 0
z = L
z
dQz, P·cz
Rotor
Screen basket
Qz, cz
Qz- dQz, cz - dcz
Annular volume element
Fig. 6.5 Flows and concentrations around an annular differential volume element.
Considering Fig. 6.5, the mass balance for pulp over the annular differential element
between the screen plate and the rotor gives:
Qz cz _ dQz P cz _ _Qz _ dQz_ _cz _ dcz_ _ 0 _3_
566
6.2 Screening Theory
where Qz stands for the total flow rate (m3 h–1) entering the element in axial direction
and dQz for the flow rate leaving in radial direction, respectively.
Equation (3) can be rewritten to give:
dcz
cz _ _P _ 1_
dQz
Qz _4_
In a first approach, it is assumed that the fiber passage ratio P is independent
of the flow rate and consistency. Then, Eq. (4) can be integrated using the overall
screen boundary conditions as per Fig. 6.1; that is, cz = cF and Qz = QF for z = 0, and
cz = cR and Qz = QR for z = L. L is the length of the screening zone.
cR
cF _
QR
QF _ __P_1_
_5_
Note that the concentrations may relate to the totality of pulp as well as to a
fraction only, for example, to shives. Then, different passage ratios will apply for
total pulp and shives. When the concentrations in Eq. (5) refer to total pulp concentrations,
the quotient cR/cF is defined as the reject thickening factor, T. QR/QF is
termed the volumetric reject ratio, Rv. With these definitions we obtain a relationship
between the thickening factor, total fiber passage ratio and volumetric reject
ratio:
T _ R_P_1_ v _6_
The mixed-flow model of pressure screening assumes a completely mixed volume
inside the screen. In the mixed-flow model, the feed entering at the pulp concentration
cF is immediately mixed into the volume inside the screen which is at
the reject concentration cR. All accept passes through the screen apertures at the
accept concentration cA. The overall mass balance, pulp mass balance and passage
ratio of this system are:
QF _ QA _ QR _ 0 _7_
QF cF _ QA cA _ QR cR _ 0 _8_
P _
cA
cR _9_
Equations (7) to (9) can be combined and rewritten using the definitions of the
thickening factor and volumetric reject ratio to give the expression for the thickening
factor in the mixed-flow model:
T _
1
P_1 _ Rv__Rv _10_
567
6 Pulp Screening, Cleaning, and Fractionation
While the mixed-flow model seems worth considering for screens with open
rotors, it has proven to be second to the plug-flow model at lower reject rates for
virtually any screen configuration. Figure 6.6 shows, graphically, the comparison
of the reject thickening behavior predicted by the mixed-flow and plug-flowmodels.
0.5
1.0
1.5
2.0
2.5
0% 20% 40% 60% 80% 100%
Reject thickening factor, T
Volumetric reject ratio, Rv
Plug-flow model
Mixed-flow model
Fig. 6.6 Reject thickening in a pressure screen predicted by the mixed-flow
and plug-flow models at a constant fiber passage ratio of P = 0.7.
By examining the fit between experimental data and the curves calculated from
the plug-flow model in Fig. 6.7, it can be seen that the plug-flow model describes
the thickening behavior of an industrial screen quite well. Note that reject thickening
increases dramatically at reject ratios smaller than 10%, and that a lower passage
ratio generally leads to higher thickening.
Earlier, it was assumed that the fiber passage ratio is constant along the screening
zone, which implies that the single fibers do not interact with each other. This
holds true only for very low consistencies and, to a certain degree, for fiber suspensions
under high shear forces in a turbulent environment. Figure 6.8 shows
that the fiber passage ratio in a commercial pressure screen is fairly constant over
the first two-thirds of the screen length, but then can fall significantly at the reject
end of the screen [4].
568
6.2 Screening Theory
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0% 10% 20% 30% 40% 50%
Reject thickening factor, T
Volumetric reject ratio, Rv
1.8 mm holes
0.4 mm slots
Fig. 6.7 Example of reject thickening as a function of the volumetric reject ratio.
Comparison of experimental data [6] with calculation results from the plug-flow
model; P = 0.72 for 0.4-mm slots, P = 0.55 for 1.8-mm holes.
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.2 0.4 0.6 0.8 1.0
Fiber passage ratio, P
Normalised length of screening zone, z/L
Bump rotor
Step rotor
Fig. 6.8 Example of fiber passage ratio as a function of the screen length
and rotor geometry;smooth hole screen, eucalyptus pulp, Rv = 10% [4].
Figure 6.9 illustrates a typical consistency profile over the length of a pressure
screen. While the consistency of the accept remains fairly constant, the consistency
of the pulp flow passing along the screen increases disproportionately
towards the end of the screening zone. The profile explains why screens tend to
blind from the reject end.
569
6 Pulp Screening, Cleaning, and Fractionation
0.0
0.5
1.0
1.5
2.0
2.5
0.0 0.2 0.4 0.6 0.8 1.0
ed consistency, cz/cF
ed length of screening zone, z/L
Feed
Rejects
Accepts
Normaliz
Normaliz
Fig. 6.9 Example of consistencies as a function of the screen length;
smooth hole screen, bump rotor, eucalyptus pulp, Rv = 10% [4].
6.2.4
Selective Fiber Passage
The selective separation of the different types of solids contained in the feed
stream is of major importance for all contaminant removal and fractionation applications.
While the selectivity of barrier screening is essentially determined by
the chosen screen, the selective separation of particles is much more challenging
when screening is governed by the probability mechanism.
Several investigations have been made to evaluate the fiber length dependent
passage of fibers through pressure screen apertures (e.g., [7–10]). It has been
shown that the passage ratio can be approximated by the empirical equation
P _ e__l_k_b
_11_
where k is a size constant proportional to the size of the screen plate aperture and
l is the fiber length. k is to be determined experimentally for each screening application.
The second constant was found to be b = 0.8...1.1 for screen plates with
smooth holes, and b = 0.5 for contoured slotted screen plates. The different shapes
of the fiber passage ratio versus fiber length curves in Fig. 6.10 demonstrate the
divergent performance of holed and slotted screens reflected by b.
570
6.2 Screening Theory
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Fiber passage ratio,P
Fiber length [mm]
Slots
Holes
Fig. 6.10 Example of fiber passage ratio as a function of the fiber
length and screen type;smooth hole plate versus contoured slot plate,
bump rotor, softwood thermomechanical pulp (TMP) [8].
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Fiber passage ratio,P
Fiber length [mm]
Slots
Holes
Ideal
Fig. 6.11 Typical fiber passage ratio as a function of the fiber
length;comparison of ideal profile with typical profiles of
holed screen (b = 1) and slotted screen (b = 0.5) normalized
for a fiber passage ratio of 0.5 at 2-mm fiber length [8].
It is apparent from Fig. 6.11 that currently proven screening equipment is performing
far from ideally when it comes to fractionation. However, screening with
holed plates leads to better length-based fractionation because the holed screen
profile is closer to the ideal profile and the fiber passage ratio drops more quickly
571
6 Pulp Screening, Cleaning, and Fractionation
over the fiber lengths of main interest. Remember that P = 1 implies the distribution
of very short fibers between accept and reject according to the respective flow
rates. In contrast, very long fibers are selectively concentrated in the reject stream
as P approaches zero.
For a given combination of screen plate, rotor type and pulp furnish, the lengthbased
fiber passage ratio was shown to be independent of the reject ratio. While
for slotted screen plates the fiber passage ratio increases with the aperture velocity,
it is independent of the aperture velocity for holed screen plates. This behavior
marks another advantage of holed screens for fractionation, because it makes the
fractionation result independent of the production capacity [8]. Besides fractionation
for length, pressure screens separate fibers according to their coarseness
(weight per unit length) [11].
6.3
Screening Parameters
In this subsection we will review the parameters that affect the operation and
determine the performance of a screening system, and their qualitative influences
on screen capacity and screening efficiency.
These parameters include operating conditions, such as flow rates, feed consistency
or temperature. They also include equipment-specific parameters, such as
screen and rotor design or rotor tip velocity. In addition, it is necessary to observe
the furnish characteristics of both the pulp fibers and the contaminants.
Some of the above parameters can be adjusted, while some are intrinsic to a
special process step or piece of equipment. The chosen combination of adjustable
screening parameters depends on the individual requirements of the application,
and this is usually a compromise within performance limits and operating constraints,
because the optimization of single parameters often leads into opposite
directions.
Due to the complexity of the mechanisms involved and varying system performance
depending on the specific circumstances, the discussion of parameters
below will be often qualitative in nature.
6.3.1
Equipment Parameters
6.3.1.1 Screen Basket
The screen basket is fundamentally characterized by aperture size, aperture shape
and aperture spacing, as well as the character of its surface. There is a basic distinction
between perforated, or holed, screen plates and slotted screen plates.
Both types can be furnished with contours on the side of the screen surface which
faces the feed. Such contoured, or profiled, screens increase the turbulence near
the screen aperture and allow the screen to be operated at a higher capacity.
572
6.3Screening Parameters
Only profiled screens in combination with the wedge-wire design have made
today’s narrow screen slots practical and widely accepted. Slotted screens with a
slot width around 0.15 mm have become state of the art for applications targeted
at the removal of smaller contaminants. Wedge-wire screens consist of solid bars
placed aside each other, forming long slots over the complete length of the screen
basket, while machined slots are milled out of a solid screen basket. Wedge-wire
slotted screens have considerable capacity advantages over machine slotted
screens due to their larger open area.
Holed screens have been traditionally preferred for their high capacity and reliable
operation and easy control under varying conditions. Their robustness makes
them first choice for the removal of larger contaminants. The advantages of holed
screens for fractionation have been discussed above. Typical hole sizes are
4–10 mm for larger contaminant removal, and about 1 mm for fractionation.
The aperture size is the most critical design variable of a screen. Holes of small
diameter and slots of narrow width have advantages with regard to the screening
efficiency. Their size actually determines whether a particle is rejected on the principle
of barrier separation, or whether it is subject to probability separation. On
the other hand, smaller apertures mean lower capacity at a given screen surface
area.
Similarly, the profile depth of the screen surface causes divergent screen performance.
By tendency, the additional turbulence created by a higher contour provides
a greater capacity but reduces the screening efficiency. If in turn the aperture
size is reduced to regain lost efficiency, the capacity of the contoured screen
still remains higher [12].
It has been shown that slot spacing is important, and that longer fibers require
wider slot spacing than shorter fibers. If the slots are too close, then stapling of
fibers occurs as the two ends of individual fibers enter adjacent slots at the same
time. Similar conclusions have been drawn for holed screens.
Note that the performance of a screen will deteriorate over time if the pulp furnish
contains an abrasive material such as sand. Especially with heavily contoured
screens, wear will significantly decrease both the capacity and the screening efficiency.
6.3.1.2 Rotor
There is a variety of different rotors available, with special shapes and sophisticated
local arrangements of bumps or foils. All of these are deemed to have their
individual advantages regarding screen capacity, screening efficiency or power
consumption.
The characteristic shape of the pressure pulse generated by a rotor depends on
the design of the pulsation element, for example on the shape, length and angle
of incidence of the foil, or on the shape and length of the bump. The intensity of
the pulse is determined again by the rotor shape, as well as by the rotor tip velocity,
the clearance between the pulsation element and the screen basket, as well as
the pulp consistency and pulp furnish parameters.
573
6 Pulp Screening, Cleaning, and Fractionation
-3
-2
-1
0
1
2
0 10 20 30 40 50 60
Dynamic pressure [bar]
Time [10-3 s]
Fig. 6.12 Example of pressure pulse profile for a short foil rotor [5].
-3
-2
-1
0
1
2
0 10 20 30 40 50 60
Dynamic pressure [bar]
Time [10-3 s]
Fig. 6.13 Example of pressure pulse profile for a contoured-drum rotor (S-rotor) [5].
Figures 6.12 and 6.13 show typical pressure pulses caused by the movement of
a foil rotor and a step rotor, respectively. At a random point on the screen surface,
there is in general a positive pressure pulse upstream of the rotor element, and a
negative pressure pulse right after the smallest clearance between the rotor tip
and the screen basket has passed by. The negative pressure is responsible for the
backflush through the screen apertures.
It is evident that the profile of the pressure pulse is very different between rotors.
Short negative-pressure pulses, as created by bump rotors and rotors with short foils,
keep the backflush flow low. At the same time, they ensure comparatively low true
574
6.3Screening Parameters
aperture velocity and low overall screen resistance. Longer negative-pressure
pulses, as created by rotors with long foils and step rotors, reduce reject thickening
by intensified backflushing. Higher feed consistencies require longer negative-
pressure pulses to keep the consistency at the reject end of the screening zone
low enough to avoid blinding. Note that the screen capacity decreases with the
magnitude and duration of the negative pressure pulse.
The clearance between the pulsation element and the screen basket is quite different
between rotor designs. Common clearances are between 3 and 10 mm. Reducing
the clearance between the pulsation element and the screen basket leads
to some increase of the pressure pulse intensity [13,14].
6.3.2
Operating Parameters
6.3.2.1 Reject Rate
The reject rate is the most important operating parameter of a pressure screen. A
higher reject rate improves the screening efficiency and reduces the danger of
screen blinding caused by undue reject thickening (see Fig. 6.7; see also Figs. 6.23
and 6.24). The reject rate is also the only one parameter that really affects fractionation
efficiency (see Fig. 6.32).
However, there is an economic boundary on the reject rate, because large rejects
rates inflate subsequent screening stages and thus increase both investment and
operating costs. Typical volumetric reject rates for pressure screens range from
10% to 25%.
6.3.2.2 Accept Flow Rate
A pressure screen’s capacity is given by the accept flow rate, and is often expressed
in terms of the aperture velocity defined by Eq. (1). A higher aperture velocity
leads to a reduction in screening efficiency [15,16]. Figure 6.14 illustrates graphically
that a decreasing aperture velocity reduces the fiber passage ratio. This
means that reject thickening becomes more critical at lower aperture velocities,
and hence screen capacities. Note that the operation of a pressure screen below its
nominal capacity may soon lead to severe operating problems caused by reject
thickening (Fig. 6.15). The dependence of the fiber passage ratio on the screen
capacity of holed screens is less pronounced than that of slotted screens.
As mentioned in Section 6.2.2, the gross aperture velocity calculated from Eq. (1) is
a parameter of limited significance. Since the actual flow rate through the apertures
depends on many factors, the meaningfulness of the aperture velocity as a parameter
for evaluation of screen capacity or screening efficiency is restricted to systems
of similar mechanical design, pulp furnish and operating conditions.
For a given screen geometry, pulp furnish and pulp consistency, the pressure
drop across a screen is linearly related to the square of the accept flow rate, with
the slope determined by the hydraulic resistance of the screen plate [17].
575
6 Pulp Screening, Cleaning, and Fractionation
0.0
0.2
0.4
0.6
0.8
1.0
0 1 2 3 4 5 6
Passage ratio, P
Aperture velocity [m/s]
Slotted plate
Holed plate
Fig. 6.14 Example of fiber passage ratio as a function of the aperture
velocity;experimenta l data, bleached softwood kraft pulp [6].
0.5
1.0
1.5
2.0
2.5
3.0
0% 10% 20% 30% 40% 50%
Reject thickening factor, T
Volumetric reject ratio, Rv
2.000 l min-1
3.000 l min-1
4.000 l min-1
Fig. 6.15 Example of reject thickening in a pressure screen as a
function of the accept flow rate;3000 L min–1 nominal screen capacity,
experimental data, bleached softwood kraft pulp [6].
Optimum levels of fractionation in slotted screens occur at low aperture velocities
where the passage ratio of long fibers remains low, but that of short fibers is
significant [18]. In contrast, the fractionation performance of holed screens is
widely independent of the aperture velocity at the hole sizes of interest for fractionation
[9].
576
6.3Screening Parameters
6.3.2.3 Feed Consistency
The feed consistency determines the amount of liquor that has to pass the screen
at a given pulp production capacity. Pressure screens can operate at feed consistencies
up to 4% or 5%. The latter figure represents hardwood pulp, which generally
allows higher feed consistencies than softwood pulp. The limiting factor
defining the feed consistency ceiling is reject thickening. A screen’s pulp capacity
increases with rising feed consistency, until a point is reached when it rapidly
decreases due to blinding caused by excessive reject thickening.
A screening system with a high feed consistency is more compact and requires
less electrical energy than a low-consistency system due to the reduced amounts
of liquor pumped around. It is, however, also more demanding to control because
it operates closer to the critical point of reject thickening. At higher consistency,
blinding is favored not only by the increasing population of fibers but also by a
reduced backflush through the screen apertures. It has been shown that the intensity
of the pressure pulse goes down considerably with increasing pulp feed consistency
[13].
The passage ratio decreases as feed consistency goes up [6]. While different
opinions exist about the effects of feed consistency on screening efficiency, it is
likely that the latter is not significantly affected by the feed consistency [15]. However,
a more dilute feed is clearly improving the fractionation efficiency [19].
Several designs of modern washing equipment require feed consistencies between
3% and 4%. If such a piece of equipment is installed downstream of the
screen, only higher-end feed consistencies can provide the needed levels of accept
consistency. If the accept consistency is not critical, a good compromise between
screening efficiency, operability and power consumption for standard screening
applications may be found in the feed consistency range of 2.5% to 3.5%, with softwood
furnish at the lower end and hardwood furnish at the higher end of the range.
6.3.2.4 Temperature
The operating temperature affects the behavior of both liquor and solids. On the
one hand, the pulp fibers become more flexible at higher temperatures (see also
Section 6.3.3.1), while on the other hand the liquor viscosity decreases with higher
temperatures, improving the turbulence in the screening zone. Both of these
effects cause the screen capacity to rise [12].
6.3.2.5 Rotor Tip Velocity
As described above, the rotor is responsible for creating turbulence, providing the
tangential speed of the pulp along the screen plate, and for backflushing the
screen by pulsation. A higher tip velocity means a higher turbulence and a more
intense pressure pulse at basically unchanged pressure pulse profile. The intensity
of the pressure pulse increases with the square of the rotor tip velocity [13]. The
recommended operating range of rotors is varying significantly between rotor designs
and equipment manufacturers. Common tip speeds are between 10 and 40 m s–1.
577
6 Pulp Screening, Cleaning, and Fractionation
Increasing the rotor tip speed improves the screen capacity and allows higher feed
consistencies, while increasing the power demand of the screen. The power requirement
was found to be proportional to the cylinder area and to the tip speed cubed [20].
Within the ranges of velocities recommended by rotor suppliers for their products,
the screening and fractionation efficiencies are not notably affected [10,12,15].
6.3.3
Furnish Parameters
6.3.3.1 Pulp Fibers
With respect to screening, pulp fibers are characterized by a number of physical
properties such as fiber length, fiber flexibility, freeness and disruptive shear
stress of the fiber network. Together with the consistency, these properties determine
the performance of the furnish in a pressure screen.
The influence of fiber flexibility on passage ratio is secondary to the influence of
fiber length. Flexibility plays no role as long as the fibers are shorter than the
width of the slot or the diameter of the hole. As the fibers become longer, however,
the flexible fibers’ passage ratios are higher than those of stiff fibers [2,18]. Note
that fiber stiffness is a function of the temperature, with fibers becoming more
flexible as the temperature rises. Figure 6.16 exemplifies the fiber passage ratio as
a function of the fiber length and hole size.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Fiber passage ratio
Fiber length [mm]
1.00 mm holes
1.75 mm holes
Fig. 6.16 Example of fiber passage ratio as a function of the
fiber length and hole size;smooth hole screen, bump rotor,
softwood thermomechanical pulp (TMP) [8].
Regarding the pulpwood raw material, a distinct difference can be observed between
the long softwood and the short hardwood fibers. The capacity of a given
slotted pressure screen with hardwood pulp is 20–30% higher than its capacity
with softwood pulp.
578
6.4 Centrifugal Cleaning Theory
6.3.3.2 Contaminants
The nature of a contaminant decides the preferred technical solution for its
removal. The most important contaminant parameters for screening are the contaminant
size and shape, and its deformability.
Apparently smaller contaminants require a smaller aperture size to be removed
efficiently. Irregularly shaped contaminant can pose a challenge to reasonable
screening, as do deformable contaminants or contaminants that break up under
shear stress.
A categorization of contaminants and selective ways for their removal are highlighted
in Section 6.7.
6.3.3.3 Entrained Air
A small or moderate air content usually has no effect on the separation of pulp in
pressure screening under typical industrial conditions [16].
6.4
Centrifugal Cleaning Theory
6.4.1