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

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

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