Displacement Ratio

Another simple, yet more meaningful, measure for the washing efficiency is the

displacement ratio. The washer is again treated as a black box (see Fig. 5.23). The

displacement ratio DR is then defined as the actual washing performance related

to the maximum possible washing performance at ideal displacement, expressed

in concentrations:

DR __

cin _ cout

cin _ cWL _22_

The displacement ratio depends heavily on the amount of liquor used for washing,

and is therefore only valid for the one dilution factor where the concentrations

have been measured.

An equivalent displacement ratio (EDR) has been established as a mathematical

tool for comparing washing equipment of different feed and discharge consistencies

[28]. However, this has not won the recognition of the standardized Norden

efficiency factor described below.

538 5 Pulp Washing

5.5 Washing Efficiency 539

Example: Determination of the displacement ratio

The measurements around a vacuum washer in the bleach plant have

given the following results:

COD concentration in pulp feed, c in= 7900 mg L–1

COD concentration in pulp discharge c out = 1500 mg L–1

COD concentration in wash liquor, c WL= 400 mg L–1

The displacement ratio is:

DR _

cin _ cout

cin _ cWL _

7 900 _ 1 500

7 900 _ 400 _ 85_

5.5.4