Batch Digester

With this process the chips and cooking liquor are heated under pressure according to a specified temperature-time programme. At the end of the programme the digester is emptied – blown  or pumped – out to the blow tank.

The spent cooking liquor containing chemicals and dissolved wood substance is called black liquor (or if diluted with wash water, “thin liquor”).

The steam released in the blow tank is recovered in a blow condenser where it is condensed. The condensate is then cooled in a heat-exchanger, creating a large volume of hot water which can be used for washing the pulp in the bleaching plant.

The batch cooking system has been developed significantly in recent years. New systems work in cycles and use energy from one digester to the next. The advantages are lower energy and liquor consumption. The disadvantages are a more complicated control system and greater wear on the equipment.

Batch digesters can be run using either the sulphate process or the sulphite process. In sulphate pulping the cooking liquor is alkaline (high pH) whereas with sulphite pulping, the cooking liquor is usually acid (low pH). Valve material selection is crucial.

The vast majority of new pulp mills make use of the sulphate pulping process.