The washing efficiency of digester displacement: new aspects based on laboratory studies and improved calculation procedures.


Displacement batch cooking is nowadays a common practice in the production of kraft pulp. The cook is terminated by a cool filtrate displacement, in which the temperature of the digester is brought down. The terminal displacement is the first washing stage in the pulp mill. The displacement operation is widely practiced, but knowledge of the washing effect of the displacement is rather sketchy.

The terminal displacement operation in displacement batch cooking was investigated with laboratory scale cooking equipment. New calculation procedures were developed and utilized to give more detailed information of the washing effect of the terminal displacement.

When using a typical dilution factor (DF 3.0), the Nordén E washing efficiency in the displacement was 2.5 and 3.2 for dry solids and effective alkali, respectively. Channeling during displacement resulted in decreased washing efficiency. The laboratory experiments showed a good correlation with earlier industrial studies.