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.

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