Impregnation

COUNTERCURRENT

COOKING

CONCURRENT

COOKING

WASHING

Extraction

Liquor

White liquor

DILUTION /

DISCHARGE

STEAMING

Energy,

White liquor

Energy,

Wash filtrate

FLOW OF CHIPS

NET FLOW OF LIQUOR FLOW OF LIQUOR

Energy,

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.

Extended Modified Continuous Cooking (EMCC) and IsoThermal

Cooking (ITC)

Extended Modified Continuous Cooking [11]and IsoThermal Cooking [12]mark

the consequent prolongation of the ground broken by MCC related to the equalizing

of alkali profiles and co-utilization of washing zone volume for cooking and

washing.

A typical configuration of an EMCC/ITC single-vessel hydraulic digester is

shown in Fig. 4.144. The initial process steps up to countercurrent cooking corre-

Steam

Wash filtrate

Circulation transfer

White liquor

WASH

HEATER

COUNTERCURRENT

COOKING

HEATER

CONCURRENT

COOKING

HEATER

Pulp

Extraction

Liquor

Fig. 4.144 Typical EMCC/ITC single-vessel hydraulic digester [12,13].

spond to the MCC technology described above. The additional element of EMCC/

ITC lies in the extension of the cooking zone down to the lowest set of screens

(see also Fig. 4.145).

There is no more dedicated high-heat washing zone between strainers. White

liquor is added not only to the top circulation and countercurrent cooking circulation,

but also to the wash circulation. At the same time, the temperature of the

wash liquor is raised to a point where the cooking temperature is also reached in

the extended zone. In Fig. 4.144, this means that the cooking temperature of typically

150–165 °C is maintained in the digester from the first set of screens down

to the last.

The split of white liquor between the points of addition must ensure that a

minimum residual alkali concentration is maintained in all liquors at all times, so

that the detrimental re-precipitation of dissolved organic compounds is safely

avoided. From a process perspective, EMCC and ITC are widely similar. Installation-

wise, EMCC requires only one wash circulation, whereas ITC uses two sets of

wash circulation loops with individual heaters.

Дата добавления: 2015-10-21