White liquor

Fig. 4.142 Typical Modified Continuous Cooking (MCC) process steps and flow regime.

extraction screens, as well as its integration into the two-vessel system are shown

in Fig. 4.143. If the digester in a two-vessel constellation is of hydraulic design, it

is equipped with a stilling well instead of the top separator.

Top circulation

liquor return

Chips and liquor

from feeding system IMPREGNATION

VESSEL

Chips and liquor

to digester

Bottom circulation

liquor return

BOTTOM Steam

CIRCULATION

HEATER

BOTTOM

CIRCULATION

PUMP

Fig. 4.143 Typical impregnation vessel in a two-vessel continuous cooking system.

386 4 Chemical Pulping Processes

In contrast to hydraulic digesters, steam/liquor phase digester have an inverted

top separator, where the chips are conveyed upwards inside the screen. The liquor

needed for top circulation flows back through the screen, while the chips and

excess liquor overflow from the separator into the steam phase. Since in the

steam/liquor phase digester the chips reach above the liquor level, direct steam

can be applied for chip heating. This has some disadvantages, such as dilution of

the extraction liquor, the related additional load on the evaporation plant, and a

reduced amount of live steam condensate returned to the boiler house. Steam/

liquor phase digesters allow compaction of the chip column to be influenced by

the height of chips standing above the liquor level.