Principles of Continuous Cooking

The basic idea of continuous cooking is to close the chain of continuous processes

in the fiber line. Based on the original process, technologies have developed over

time which employ in-digester washing and advanced alkali and temperature profiling.

The outline of a typical single-vessel continuous cooking system is illustrated in

Fig. 4.137. Vapor recovered from the extraction liquor is used to remove air from

the chips and to preheat them. The chips are then continuously transported to the

digester with the help of liquor circulated between the chip feeding system and

the digester top. A percentage of the cooking chemicals is charged with white

liquor to the top circulation, while the remainder of the white liquor goes to the

digester.

The continuous digester itself is a huge vessel of vertical cylindrical design.

Chips move from the top of the digester to the bottom by gravity. The vessel is

equipped with a top separator, which separates the circulation liquor from the

chips, and with a scraper and outlet device in the bottom. The digester has strainers

at different levels, which hold back wood and pulp when liquor is extracted

from the digester. Several liquor circulation loops are used to change the chemical

regime and/or to adjust the temperature in the different zones of the digester. A

central pipe discharge brings the circulation liquor back to the center of the digester

near the corresponding set of circulation screens. Steam is used for liquor

heating, preferably in indirect shell-and-tube heat exchangers.

Spent liquor which is not circulated back to the cooking process is extracted for

vapor recovery. The weak black liquor is then subjected to fiber separation and

cooling before being transferred to the evaporation plant.

Continuous cooking systems can be categorized into single-vessel and two-vessel

systems, with hydraulic or steam/liquor phase digesters. Two-vessel systems have a

separate vessel where the impregnation of chips takes place. In such a system, the

chips are fed to the top of the impregnation vessel and then transferred from the bottom

of the impregnation vessel to the digester top by a separate circulation loop.

CHIP STEAMING,

CHIP FEEDING

SYSTEM

VAPOR

RECOVERY

SYSTEM

Extraction liquor

Pulp

Chips Top

circulation

liquor

Chips and liquor

Wash filtrate

Weak black liquor

Vapor

DIGESTER

White liquor

Steam

Fig. 4.137 Outline of single-vessel continuous cooking system.

4.2 Kraft Pulping Processes 377

Hydraulic digesters are completely filled with liquor, whereas steam/liquor phase

digesters have a vapor phase at the top. In a steam/liquor phase digester, the chips

reach above the liquor level. They can be heated to cooking temperature with

direct steam, which in most cases requires prior impregnation and therefore

makes sense mainly in a two-vessel system.

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