Batch Digesters

With a batch digester, the chips and cooking liquor are heated under pressure according to a specified time and pressure programme. At the end of the time / pressure programme the digester is then emptied out to the blow tank either by being blown or pumped.

The used or spent cooking liquor that contains chemicals and dissolved wood substance is referred to as black liquor. If it is diluted with wash water it is then referred to as thin liquor.

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

The batch cooking system has been developed and improved significantly over the last few years. New systems now work in cycles and will use energy from one digester to the other. The advantages offered by these improved systems, are lower energy and liquor consumption. The disadvantages however are that a more complicated control system is required and there is greater wear on the equipment.

Batch digesters can be run by using either the sulphate process or the sulphite process. In sulphate pulping the cooking liquor is alkaline (meaning high pH) whereas with sulphite pulping, the cooking liquor is usually acidic (meaning low pH). This means that the selection of the valve bod, seat and ball materials is critical.

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

Click on the image to enlarge.

Some examples of NAF valves successfully used in this process

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