- •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
- •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.
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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