Overview

In this section we will review the parameters that affect the operation and determine

the performance of a washing system. These include process conditions

such as the dilution factor, feed and discharge consistencies, pH, temperature, or

entrained air. They also include equipment-specific parameters, such as a particular

traveling speed, mechanical pressure, or fluid pressure or vacuum.

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 washing

parameters depends on the individual requirements of a washing application, and

is usually a compromise because the optimization of single parameters often

leads in opposite directions.

5.4 Washing Parameters

In addition to the equipment-specific and process-related parameters, it is

essential to observe the pulp characteristics, with drainage and sorption behavior

as the most important factors. The characteristic behavior of a pulp depends, inter

alia, on the species of wood, kappa number, preceding cooking or bleaching processes

and any mechanical treatment undergone. It is best determined in an

appropriate laboratory test.

Here, we will discuss mainly the qualitative influences of process parameters

on the washing efficiency and production capacity of pulp washing equipment.

Some reference will also be made to equipment-specific parameters.

Hakamaki and Kovasin have developed an empirical model for the capacity of a

rotary drum pressure washer [26]. The model was obtained from regression analysis

of a very large number of laboratory test results, and was verified on an industrial

scale:

G _ 0_ 675 n 0_62 N 0_65

in T 0_26_1 _ yair __0_24 D p 0_30 _15_

where G = specific washer surface load (i.e., pulp weight per cylindrical drum surface

area and day) (odt m2 day–1); n = washer drum rotations (min–1); Nin = pulp

feed consistency (%); yair = air content in pulp feed (vol. %);and D p = effective filtration

pressure (kPa).

The specific surface load is the ratio of the daily pulp production and the surface

area of the cylindrical drum – that is, the area of the filter medium. Once the

desired capacity and specific surface load for a washing application are known,

the adequate washer surface can be easily calculated by dividing the first through

the latter.

Although the validity of Eq. (15) is limited to pressure washers and to the tested

pine kraft pulp, the basic correlations have some general significance and can, at

least by tendency, also be transferred to other pieces of washing equipment. Thus,

we will use the model from time to time in this section to demonstrate the principal

effect of a washing parameter on washer capacity.

5.4.2

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