Systems for Fractionation

The wide range of tasks achievable by fractionation has been repeatedly indicated

above. Because of the specialty of each case, general design principles for the fractionation

of chemical pulps are not yet established. Thus, the information in this

subsection is restricted to some general comments.

6.8 Systems for Contaminant Removal and Fractionation 599

The flow rates of the different fractions are defined by the particular application,

and a low reject ratio is not necessarily part of the fractionation requirements. It

may be advantageous to perform fractionation in a multi-stage system. In contrast

to contaminant removal, the efficiency of fractionation can be improved by multistage

systems. Two simple fractionation systems using screens are illustrated in

Fig. 6.31.

A

A

R

R

F A

R R

F A

(a) (b)

Fig. 6.31 Two-stage fractionation systems with feedback (a) cascade and (b) series.

Remember that holed screens fractionate better than slotted ones. While feedback

is clearly important for obtaining a higher fractionation efficiency, it has

been shown that both cascade and series arrangements may yield similar fractionation

results at a given mass reject rate [19]. According to the example shown in

Fig. 6.32, the best achievable fractionation occurs between about 30% and 60%

mass reject ratio. Note that the location of the fractionation index peak shifts

along the mass reject ratio axis dependent on of the relative amount of fractions

of interest in the feed pulp.

0.0

0.2

0.4

0.6

0.8

0.0 0.2 0.4 0.6 0.8 1.0

Fractionation index, Φ

Mass reject ratio, Rm

Single screen

Two-stage

cascade

feedback

Two-stage

series

feedback

Fig. 6.32 Fractionation index as a function of the mass reject

ratio for single-stage and two-stage fractionation with holed

screens;length-ba sed fractionation, simulation results [19].

600 6 Pulp Screening, Cleaning, and Fractionation

6.9