Digester two vessel - process info

Two Vessel digester system

In this section a common type of a newly built continuous digester system from About 2000-2005 and onwards is described.
Applying new cooking technology the 
various process stages and then especially the chip feed system are simpler than with old technology and are also vendor dependant.
There are only two vendors and 
below one of these systems is described.

The chips are fed to a chip bin for both pre-steaming and steaming at atmospheric conditions. From the bin the chips are discharged to a chip tube through a chip metering screw. In the chip tube the chips are mixed with fresh cooking (white) liquor. There is no low pressure feeder or steaming vessel as in older systems. From the chip tube the chips are pumped with three pumps in series to the pressurized impregnation vessel, where the chips are impregnated with cooking liquor.

The three chip pumps substitutes the high pressure feeder and other equipment in older systems. 
Chips are transferred from the impregnation vessel to the top separator in the digester inlet with a transfer (bottom) circulation pump.

Heating to full cooking temperature takes place by direct steam addition to the top of the digester and through different liquor circulation systems. Additional fresh cooking (white) liquor is added to the second row of the upper digester screens and may also be added to the digester top.

While the chip column moves downwards the digester vessel cooking takes place. In the bottom part of the digester the cooked chips are partly washed and cooled. After this first wash the chips/pulp are continuously discharged and blown from the bottom of the digester. In new systems the washing in the digester is limited compared to older systems (Hi-Heat). Washing is moved to a separate washing unit in the blow line or after the blow tank.