Displacement Washing

Displacement washing is based on the idea of replacing the liquor in the pulp web

with wash liquor rather than mixing these two liquors. Appropriate displacement

washing is of primary importance in obtaining good washing efficiencies with all

types of washing equipment.

5.3 Principles of Washing

The principle of displacement washing is illustrated schematically in Fig. 5.12.

A volume of wash liquor is added to the pulp feed (1), in a way such that the wash

liquor pushes out the liquor coming along with the pulp feed as filtrate (2). Eventually,

the wash liquor occupies all the space originally held by the liquor in the

pulp feed (3).

Pulp feed

Filtrate

Wash liquor

1 2

Pulp discharge

Fig. 5.12 The principle of displacement washing.

Apparently, one single stage of displacement washing is much more efficient

than one single stage of dilution/extraction washing. Ideally, it would be possible

to remove all the liquor in the pulp feed, together with all the soluble substances,

by displacing it with the same volume of wash liquor.

The reality, however, is far from ideal: mixing occurs at the interface between

the wash liquor and displaced liquor;diffusion limits the mass transfer from

enclosed liquor;sorption plays its role;and inhomogeneities in the fiber web

cause channeling of the wash liquor flow.

5.3.3