Pulp Discharge

The significant cooking reactions are terminated when the cool wash filtrate from

the first stage of brownstock washing brings the temperature of the digester contents

down to 85–95 °C. A rotating scraper reclaims the pulp from the cross-section

at the digester bottom to the outlet device. Besides cooling the pulp, the wash

filtrate provides the necessary dilution before the pulp discharge. The blowline

consistency is typically around 10%. Even at the end of the cook, the pulp in the

digester still exhibits the physical structure of the wood chips. This structure is

finally broken up as the pulp becomes defibrated during the turbulent pressure

reduction at the discharge control valve.

4.2 Kraft Pulping Processes 379

Heat Management

Typically, the heat exchangers use indirect steam to raise the temperature of circulation

liquors. Steam/liquor phase digesters may have direct steam addition to the

digester top. The heat in the extraction liquor is usually transferred to vapor which

is used for steaming of the chips, but it can also be exchanged with cooler process

liquors. Additional live steam may be used for chip steaming if necessary. Some

of the residual heat in the weak black liquor is spent for generation of hot water.

The hot water temperature achievable from the cooling of weak black liquor is

80–90 °C. The cooling of wash filtrate coming from brownstock washing yields

somewhat lower water temperatures, because the filtrate temperature must be

low enough to bring the digester contents safely beneath the boiling point.

Fiber Removal from Black Liquor

The slots in the screens installed in the digester need to be a few millimeters wide

to avoid plugging. As a consequence, the extraction liquor contains fibers which

are highly unwelcome in the evaporation plant. Hence, the liquor must be subjected

to fiber removal before being transferred to evaporation.

Gas Management

The gases vented from the chip bin and steaming vessel contain malodorous compounds,

and must be collected for reasons of emission control and maintaining

an acceptable workplace environment. Besides noncondensable constituents, the

vent gases carry certain amounts of moisture, and must therefore pass condensation

before proceeding to the mill’s gas collection and treatment systems. When a

digester plant processes softwood, the condensate also contains turpentine, which

is separated from the condensate by decanting.

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