Drum Displacer

The Drum Displacer™ (DD) is a pressurized, mostly multi-stage washer which

can have as many as four washing stages in one single unit. A four-stage washer

may be efficient enough to serve as the only brownstock washing equipment after

cooking [35].

The pulp fed to the formation section of the DD washer can be of either low

consistency (3–5%) or medium consistency (8–10%). The circumference of the

5.6 Washing Equipment 549

rotating drum is divided into compartments, which are filled with pulp in the formation

section and proceed to the washing stages. At each washing stage, the

pulp is washed countercurrently with the filtrates from the succeeding washing

stage. In the discharge zone, the pulp is dewatered by means of vacuum and eventually

removed from the compartments by pressurized air to drop into a shredding

screw conveyor (Fig. 5.27).

Fig. 5.27 Andritz two-stage Drum Displacer™ washer. a = pulp feed;

b = wash liquor inlet; c = pulp discharge; d = filtrate discharge;

1 = formation section; 2 = first washing stage; 3 = second washing

stage; 4 = pulp discharge [35].

The DD washer features the specialty of fractional washing. Unlike other

washers, where in one washing stage the liquor volume in the pulp is overdisplaced

(dilution factor larger than zero), the DD washer has two or three liquor

cycles for each stage, each operating at a negative dilution factor. As the overall

washing efficiency of fractional washing is superior to the washing efficiency of other

units with the same number of stages, DD washers generate rather high E factors.

The E factors for two-stage, three-stage, and four-stageDD washers are 6–9, 9–12, and

11–14, respectively. The outlet consistency is usually in the range of 12–16%.

The DD washer has the smallest footprint of all multi-stage washers. The handling

of wash filtrate is very compact as it is brought from stage to stage by pumps

mounted on the washer itself. In the simplest arrangement, the multi-stage unit

needs just one filtrate tank. More tanks are required in case the filtrate from the

first stage needs to be divided into fractions of different properties (pH, temperature,

chemical concentration).

The DD washer’s enclosed design and mostly submerged, gas-free operation

guarantee good tolerance for high-temperature pulp feed, minimum foaming and

high standards with regard to emissions to the air.

550 5 Pulp Washing

5.6.3