Impregnation

HOT

DISPLACEMENT

COOKING

COLD

DISPLACEMENT

PUMP

DISCHARGE

CL

CL

IL

IL

Fig. 4.135 Steps of the CBC cooking cycle [4].

4.2.8.2.7 Batch Cooking Schedule Management

In the displacement cooking plant, a number of batch digesters share a common

tank farm and common supply and discharge systems. In order to make optimum

use of these systems, the digester operation follows a strict program. At a given

production rate, this program must account for scheduling the exclusive use of

certain systems (e.g., of chip supply or pulp discharge systems), for the management

of tank levels in the tank farm, and for the pulp quality.

The cooking schedule for a simple four-digester system is shown in Fig. 4.136.

The chip fill in digester 2 is offset from the chip fill in digester 1, then follows

digester 3, and after digester 4 it is again time for digester 1. The schedule for a

particular application depends on the number of digesters, the total length of the

cooking cycle as well as on the duration of the individual steps.

0 60 120 180 240 300

1

2

3

4

Digester #

Time, minutes

Chip Fill Impregnation Hot displacement Heating and cooking Cold displacement Discharge Space time

Fig. 4.136 Example for cooking schedule of four-digester displacement cooking plant.

376 4 Chemical Pulping Processes

4.2.8.3 Continuous Cooking Technology and Equipment

4.2.8.3.1 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.

4.2.8.3.2 Continuous Cooking Process Steps

While different continuous cooking technologies follow their individual concept,

the basic steps found in all these systems are chip steaming, chip feeding, impregnation,

cooking, washing, and pulp discharge. These steps are described in general

below, and are discussed later in connection with some special continuous

cooking techniques.

Chip Steaming

The continuous cooking process starts when the chips enter a steam environment.

The target of steaming is the elimination of air from the chips to a maximum

extent. As the chips are warmed up, the air is positively displaced from inside

them by the increasing partial pressure of wood moisture and by its own increasing

volume. The residual air removal must occur by counter-diffusion of water

vapor against air.

Adequate steaming of chips must be ensured at all times, because the buoyancy

of air entrapped in chips can critically influence chip column movement in the

digester. Other negative effects of poor air removal include pump cavitation, feed

line hammering, and inhomogeneous impregnation.

Traditionally, steaming was performed at elevated pressure in a steaming vessel

and/or near atmospheric pressure in the chip bin. Lately, it has been found that

pressurized steaming can be skipped when the duration of atmospheric steaming

is sufficiently long.

Chip Feeding

Once the air is removed from the chips, they must be brought to digester pressure,

which is done by a combination of rotary feeding devices and pumps. Liquor

is used as the transport medium for chips from the feeding system to the digester

top.