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