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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.
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
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
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