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

To digester

Liquor

From digester

CHIP BIN

AIRLOCK

CHIP METER

LOW-PRESSURE FEEDER

STEAMING VESSEL

HIGH-PRESSURE FEEDER

CHIP CHUTE

IN-LINE DRAINER

LEVEL TANK

SAND SEPARATOR

White liquor

Flash steam from

Flash tank 2

Flash steam from

Flash tank 1

Chips

CHIP CHUTE LEVEL PUMP TOP CIRCULATION PUMP

CHUTE CIRCULATION PUMP

Fig. 4.138 Conventional chip feeding system for continuous digester.

position, the liquor coming from the top circulation pulp pushes the chips out of

the pocket and transfers them to the digester (or impregnation vessel). There is a

certain intentional leakage from the high-pressure side of the feeder (i.e. the top

circulation side) to the low-pressure side (i.e. the chute side). Excess liquor in the

chute liquor loop is extracted via the in-line drainer and pumped back to the top

circulation by the chip chute level pump. White liquor can be added to the suction

side of the chute level pump, which is therefore also referred to as make-up liquor

pump.

More recently developed chip feeding systems attempt to reduce the amount of

equipment installed. For example, Kvaerner Pulping’s Compact Feed system

(Fig. 4.139) does not requires the sand separator, in-line drainer and level tank [6].

The Andritz Lo-Level Feed system skips the steaming vessel and chip chute by

replacing the low-pressure feeder with a helical-screw chip pump, which directly

feeds the high-pressure feeder. Recently, Andritz has developed the TurboFeed

system which also eliminates the high-pressure feeder (Fig. 4.140). Chips are

metered from the chip bin via twin screws into a chip tube. From there, they are

forwarded to the digester by a series of specially designed pumps. The feed circulation

cooler ensures that the liquor returned to the chip chute is below 100 °C.

Besides easing the feed process, the system also controls the digester pressure,

thereby allowing a more stable flow pattern of free liquor in the cooking zones

due to a constant wash filtrate feed rate to the digester [7,8].

4.2 Kraft Pulping Processes 381

Chips

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