Impregnation

During impregnation, the cooking chemicals become distributed inside the chips.

Impregnation starts as soon as the chips are subjected to digester pressure in the

chip feeding system. At that time, impregnation is governed by liquor penetration

into the chip voids under a pressure gradient. Once the chips are in the digester,

diffusion takes control over the mass transfer as the chips and liquor move concurrently

through what is commonly referred to as the impregnation zone.

378 4 Chemical Pulping Processes

Good impregnation is a key to uniform pulp quality and optimal cooking time.

The aspects related to steaming and impregnation and their effects on continuous

cooking have recently been extensively described [5].

Cooking

The impregnation step passes into the cooking stage as the free liquor is displaced

from the chip column by hot cooking liquor. Note that the liquor in the digester

can be divided into a free portion moving between the chips and a bound portion

trapped within the chip volume. Only the free liquor is available to displacement,

whereas the bound liquor is accessible by diffusion and thermal conduction.

The cooking liquor is distributed by the central pipe discharge and flows radially

towards the cooking circulation screens. It is essential that the circulation flow

rate is sufficiently high to ensure uniform chemical and temperature profiles

across the digester cross-section. This is of critical importance also for the other

circulation operations in the digester. Otherwise, different radial temperature

and/or chemical levels prevail that lead to inhomogeneously cooked fibers.

Cooking usually occurs in more than one zone. As the chip column moves

down, the alkali, temperature and solid levels in the digester can be adjusted by

introducing white liquor or wash filtrate into circulation loops, by indirect heating

of the liquor in those loops, or by extracting spent liquor for chemical recovery.

Washing

The transition between cooking and washing in a continuous digester is blurred. At

the high temperature levels applied for washing, a considerable number of reactions

continue in the washing zone. The in-digester washing step – termed Hi-Heat washing

– was originally designed to last for up to 4 h, and this resulted in excellent washing

efficiencies due to the vast time allowed for diffusion. In many mills, however, indigester

washing has been compromised for higher production rates when a part

of the digester’s washing zone volume was converted for cooking.

In-digester washing is the first brownstock washing stage. This means that,

over time, there should be a balance between the wash filtrate collected from the

wash plant and the liquor pumped into the digester. For washing to be efficient,

there must be a net flow of liquor from the wash circulation located near the bottom

of the digester to the liquor extraction above.

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.

4.2.8.3.3 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

to digester

Liquor

from digester

HIGH-PRESSURE FEEDER

CHIP CHUTE

Chips

CHIP CHUTE LEVEL PUMP TOP CIRCULATION PUMP

CHUTE CIRCULATION PUMP

Fig. 4.139 The Kvaerner Pulping Compact Feed system [6].

Chips

to digester

Liquor

from digester

TWIN SCREW CHIP METER

COOLER

Chips

LIQUOR

SURGE TANK

CHIP TUBE

CHIP PUMPS

Fig. 4.140 The Andritz TurboFeed system [8].

382 4 Chemical Pulping Processes

4.2 Kraft Pulping Processes 383

4.2.8.3.4 Modified Continuous Cooking (MCC)

Modified Continuous Cooking [9,10]was the first in a string of alterations

imposed on the conventional continuous pulping process. A typical configuration

of an MCC single-vessel hydraulic digester is shown in Fig. 4.141. The chips enter

the top of the digester together with the top circulation liquor, and are fed to the

top separator, which is a screw conveyor surrounded by a cylindrical screen. The

vertical screw transports the chips downwards and also keeps the slots of the

screen clean. Circulation liquor is extracted through the screen and returned to

the chip feeding system, where the largest portion of the white liquor is added.

The excess liquor from the top circulation travels downwards concurrently with

the chips and enters the impregnation zone (see also Fig. 4.142).

Impregnation is typically performed at a temperature between 115 and 125 °C

and a pressure above 10 bar(g) for 45–60 min. As the chips approach the first

screen section, liquor is displaced horizontally from the central pipe discharge

through the chip column to the strainers, and is then circulated back to the central

pipe via the concurrent cooking heater. A small portion of white liquor is added to

the cooking circulation loop. The heater is operated with indirect steam and the

hot liquor introduced into the digester brings the temperature of the chip column

up to the cooking temperature of 150–170 °C.

Steam

Steam

Wash filtrate

Circulation transfer

White liquor

WASH

HEATER

COUNTERCURRENT

COOKING

HEATER

CONCURRENT

COOKING

HEATER

Pulp

Extraction

liquor

Fig. 4.141 Typical MCC single-vessel hydraulic digester [9,10].

Hot cooking liquor and chips then continue traveling downwards through the

concurrent cooking zone to the extraction screens. This is where the spent cooking

liquor is taken from the digester. Below the extraction screens starts the countercurrent

cooking zone, where the net flow of liquor is directed upwards. The

temperature in both cooking zones is roughly the same, with the countercurrent

cooking heater being responsible for the temperature in the lower zone. White

liquor is added to the countercurrent circulation liquor to increase the alkalinity

towards the end of the cook. Typically, the total cooking time of 90–150 min is

equally split between the concurrent and the countercurrent zones.

As the chips proceed into the washing zone, the countercurrent flow regime

persists. The temperature in the so-called Hi-Heat washing zone decreases gradually

to about 130 °C, and the dissolved wood components as well as spent cooking

chemicals are removed from the pulp by diffusion washing. The final temperature

in the washing zone is controlled by steam addition to the wash heater, which is

installed in the lowest of the circulation loops. At the digester bottom, the pulp is

cooled and diluted by wash filtrate, before it is eventually discharged from the vessel

through the blow valve. The wash filtrate flow usually controls the pressure in

the digester.

The major force driving behind movement of the chip column in the digester is the

weight of the wood material. Forces acting against the direction of the wood’s weight

are the buoyancy of gas entrapped in the chips, friction between themoving chips and

the digester wall, and – in zones of countercurrent flow – the drag induced by the

upward liquor movement. Efficient air removal and reasonable countercurrent liquor

velocities are therefore important prerequisites for smooth chip columnmovement.

The need to maintain high circulation flow rates brings about a considerable

risk of plugging screens or screen headers because fines and other small material

are carried through the chip column and accumulate at the screen surface, together

with chips, or in the header. This is why techniques must be applied to keep

the screens and headers clear. In a typical set of screens, profile bar screen plates

are arranged at two levels above each other, with independent headers and two

nozzles for each header which are positioned at opposite sides of the digester

shell. This arrangement allows the automated side-to-side switching of headers

and resting of screens – that is, temporary stopping of the extraction through one

level of screens. When a screen rests, the movement of the chip column wipes its

slots clear. When a header is switched to the other side, the flow direction is

inverted, which makes the formation of deposits more difficult. In addition, backflushing

of screens may be necessary at times.

There is always a temperature and concentration gradient from the central pipe

discharge along the radius of the digester to the strainers, even at high circulation

rates. In particular, in large-capacity digesters it can be a major challenge to maintain

gradients that are adequate for uniform cooking. Two-vessel systems provide

the opportunity of heating the bottom circulation liquor returning from the digester

to the impregnation vessel, thus allowing constant temperature and alkali profiles

over the digester cross-section at the beginning of the bulk delignification

phase. A typical impregnation vessel with top separator, outlet device and optional

384 4 Chemical Pulping Processes

4.2 Kraft Pulping Processes 385