Chip Steaming and Chip Feeding Systems

A conventional chip steaming and feeding system for a continuous digester is

shown schematically in Fig. 4.138. The chips are fed through the airlock, a rotary

star or screw feeding device, into the chip bin. Flash steam from the second

extraction liquor flash tank enters the chip bin near the bottom and provides atmospheric

steaming, which typically lasts for 15–25 min. The chip meter, which

again is a rotary star or screw feeder, sets the pulp production rate of the digester.

It discharges into the low-pressure feeder, which isolates the pressurized steaming

vessel from the atmospheric chip bin. Pressurized steaming is usually continued

for 1–2 min at a pressure of ca. 1.5 bar(g). A screw in the steaming vessel conveys

the chips to the chip chute.

The high-pressure feeder’s plug-type rotor always keeps a vertical and a horizontal

flow path open for liquor circulation. When one particular pocket of the feeder

is in the vertical position, the chips are sucked from the chip chute into the pocket

by the chip chute circulation pump. The chips are retained in the pocket by a

screen mounted in the casing of the feeder. Liquor passes the screen and returns

to the chip chute via the sand separator. As the pocket turns to the horizontal

380 4 Chemical Pulping Processes

Chips