Digester Single Vessel - Process info
CONTINUOUS digester - Introduction and background
Cooking in CONTINUOUS digesters
The purpose of the kraft cooking process is to chemically dissolve the glue (lignin) between the fibres so each fibre can move freely in the pulp.
The cooking process in a continuous digester is split in the following process stages:
• Chip steaming to remove air in the chips to enable good impregnation
• Chip impregnation with the strongly alkaline cooking liquor
• Cooking of the chips at alkaline conditions and at high temperatures (>140 °C) to remove the lignin
• Chip/pulp washing in the bottom zone of the digester to remove cooking liquor and dissolved wood substances.
Single Vessel Digester System
In this section a common type of continuous digester configuration and equipment from the 1970’s and onwards is described.
For a common type of a newly built digester system from About 2000-2005 and onwards go to next section.
In the continuous cooking process the wood chips are fed continuously to the chip bin (chip silo) for pre-steaming. The chips then enter the steaming vessel through a chip meter and a low pressure feeder. After the steaming vessel the chips are mixed with cooking liquor and are pumped to the high pressure feeder.
From the high pressure feeder chips and cooking liquor are pumped to the top separator in the digester inlet where the chips and part of the cooking liquor enters the digester vessel for impregnation, cooking and washing.
The digester is a pressurized vessel, where heating takes place by direct steam addition to the top of the digester and through different liquor circulation systems. Additional heated cooking liquor is pumped to the digester. The chip column moves slowly downwards in the digester vessel. In the bottom part of the digester (the Hi-Heat zone) the cooked chips are partly washed and cooled.
After this first wash the chips are continuously discharged and blown from the bottom of the digester. The pressure release and the temperature decrease in the blow line disintegrate the chips to separate fibres, and hence the chips have become pulp.
From the digester the pulp is blown to a blow tank and further washing.
If the pulp is washed in a pressure difuser (PD), the pulp passes the PD on its way to the blow tank. If the pulp is washed in an atmospheric diffuser (AD) the pulp is blown to the AD located at the top of the blow tank and is then discharged into the blow tank.
Note that the blow tank also serves as a pulp storage tower.